NEUROENDOCRINOLOGY
The Integrated Hypothalamic Tachykinin-Kisspeptin
System as a Central Coordinator for Reproduction
Víctor M. Navarro, Martha A. Bosch, Silvia León, Serap Simavli, Cadence True,
Leonor Pinilla, Rona S. Carroll, Stephanie B. Seminara, Manuel Tena-Sempere,
Oline K. Rønnekleiv, and Ursula B. Kaiser
Tachykinins are comprised of the family of related peptides, substance P (SP), neurokinin A (NKA),
and neurokinin B (NKB). NKB has emerged as regulator of kisspeptin release in the arcuate nucleus
(ARC), whereas the roles of SP and NKA in reproduction remain unknown. This work explores the
roles of SP and NKA in the central regulation of GnRH release. First, central infusion of specific
agonists for the receptors of SP (neurokinin receptor 1, NK1R), NKA (NK2R) and NKB (NK3R) each
induced gonadotropin release in adult male and ovariectomized, estradiol-replaced female mice,
which was absent in Kiss1r⫺/⫺ mice, indicating a kisspeptin-dependent action. The NK2R agonist,
however, decreased LH release in ovariectomized-sham replaced females, as documented for NK3R
agonists but in contrast to the NK1R agonist, which further increased LH release. Second, Tac1
(encoding SP and NKA) expression in the ARC and ventromedial nucleus was inhibited by circulating estradiol but did not colocalize with Kiss1 mRNA. Third, about half of isolated ARC Kiss1
neurons expressed Tacr1 (NK1R) and 100% Tacr3 (NK3R); for anteroventral-periventricular Kiss1
neurons and GnRH neurons, approximately one-fourth expressed Tacr1 and one-tenth Tacr3; Tacr2
(NK2R) expression was absent in all cases. Overall, these results identify a potent regulation of
gonadotropin release by the SP/NK1R and NKA/NK2R systems in the presence of kisspeptin-Kiss1r
signaling, indicating that they may, along with NKB/NK3R, control GnRH release, at least in part
through actions on Kiss1 neurons. (Endocrinology 156: 627– 637, 2015)
U
nderstanding the central and peripheral mechanisms
that control kisspeptin release has become a major
avenue of research in reproductive endocrinology (1).
However, the precise neuroendocrine events that determine the action of Kiss1 neurons and translate their message into congruent GnRH secretion remain largely un-
known. Recently, Kiss1 neurons in the arcuate nucleus
(ARC) have been described to coexpress neurokinin B
(NKB) and dynorphin A, thereafter renamed KNDy neurons (2). A number of studies have since emerged to document a predominantly stimulatory action of NKB on gonadotropin release in multiple mammalian species in a
ISSN Print 0013-7227 ISSN Online 1945-7170
Printed in U.S.A.
Copyright © 2015 by the Endocrine Society
Received August 1, 2014. Accepted November 18, 2014.
First Published Online November 25, 2014
Abbreviations: aCSF, artificial cerebrospinal fluid; ARC, arcuate nucleus; AVPV/PeN, anteroventral periventricular and periventricular nuclei; E2, 17-estradiol; GDX, gonadectomy; icv, intracerebroventricular; ISH, in situ hybridization; KNDy, presence of kiss1, NKB
and dynorphin in the same neuron; NKA, neurokinin A; NKB, neurokinin B; NK1R, neurokinin receptor 1; NK2R, neurokinin receptor 2; NK3R, neurokinin receptor 3; OVX, ovariectomy; PMV, premammillary nucleus; POA, preoptic area; SP, substance P; VMN, ventromedial nucleus; WT, wild type.
doi: 10.1210/en.2014-1651
Endocrinology, February 2015, 156(2):627– 637
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Division of Endocrinology, Diabetes, and Hypertension (V.M.N., S.S., R.S.C., U.B.K.), Department of
Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts 02115;
Department of Physiology and Pharmacology (M.A.B., O.K.R.), Oregon Health and Science University,
Portland, Oregon 97239; Department of Cell Biology, Physiology, and Immunology (S.L., L.P., M.T.-S.),
University of Córdoba; Centro de Investigación Biomédica en Red Fisiopatología de la Obesidad y
Nutrición (S.L., L.P., M.T.-S.), Instituto de Salud Carlos III; and Instituto Maimónides de Investigaciones
Biomédicas and Hospital Universitario Reina Sofia (S.L., L.P., M.T.-S.), 14004 Córdoba, Spain;
Department of Obstetrics and Gynecology (S.S.), Pamukkale University School of Medicine, Denizli,
20020 Turkey; and Massachusetts General Hospital and Harvard Medical School (C.T., S.B.S.), Boston,
Massachusetts 02114
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Navarro et al
Interaction Tachykinin-Kisspeptin and GnRH Release
Materials and Methods
Mice
Adult wild-type (WT) male and female C57Bl6 mice were
purchased from Charles River Laboratories International, Inc.
All experiments were approved by the Harvard Medical Area
Standing Committee on Animals in the Harvard Medical School
Center for Animal Resources and Comparative Medicine. Mice
were maintained in a 12-hour light, 12-hour dark cycle and were
fed a standard rodent diet. Free-hand intracerebroventricular
(icv) drug administration was performed in these animals as described previously (23).
Kiss1r-deficient (Kiss1r⫺/⫺) mice and WT littermates were
generated as described previously (11) and bred in the vivarium
of the University of Córdoba, Spain. The mice were maintained
under constant conditions of light (14 h of light, from 7 AM) and
temperature (22°C) and were weaned at age postnatal day 21,
when they were housed in groups of 5 mice per cage and with free
access to standard mouse chow and water ad libitum. For hormonal tests involving icv cannulation, mice were caged individually from the day before cannula implantation until termination
of experiments. Correct positioning of the cannulae was checked
by visual inspection, in order to exclude animals showing obvious displacement or detachment, and was confirmed at necropsy. Experimental procedures were approved by the University of Córdoba Ethical Committee for animal experimentation
and were conducted in accordance with the European Union
normative for care and use of experimental animals.
Kiss1-CreGFP (C57BL6/J and S129 background) mice were
produced by Steiner and coworkers at the University of Washington (24) and Gnrh1-GFP (CBB6) mice were produced by Dr
Suzanne Moenter, currently at the University of Michigan (25),
and housed under constant temperature and light in a 12-hour
light, 12-hour dark cycle with lights on from 6 AM to 6 PM at the
Oregon Health and Science University. Transgenic Kiss1CreGFP mice were maintained as heterozygous by breeding with
WT C57BL6/J mice. Gnrh1-GFP mice were maintained as homozygous and used as such. Food and water were provided ad
libitum. All animal treatments at Oregon Health and Science
University are in accordance with institutional guidelines based
on National Institute of Health standards and were performed
with institutional Animal Care and Use Committee approval.
Drugs
The NK1R agonist (GR73632), NK2R agonist (GR64349),
and NK3R agonist (senktide) were purchased from Tocris, and
17-estradiol (E2) was purchased from Sigma Chemical. The
doses of GR73632, GR64349, and senktide, 600 pmol in 5 L
of physiological saline, 0.9% NaCl, were selected on the basis of
previous references as maximally effective for senktide in inducing gonadotropin responses in the rat (26). Doses of the NK1R
and NK2R agonists were selected to span from low (600 pmol)
to high (3 nmol) in order to identify secretory responses on gonadotropin release. For experiments involving adult intact WT
female mice, adult virgin female mice were monitored daily by
vaginal cytology to confirm the occurrence of regular estrous
cycles; only mice with at least 2 consecutive regular estrous cycles
were subsequently used for hormonal and molecular analyses. In
addition, for experiments involving ovariectomy (OVX) and steroid replacement, groups of adult female mice were subjected to
bilateral GDX via abdominal incision under light isofluorane
anesthesia 1 week before hormonal tests or tissue sampling. Immediately after GDX, capsules filled with E2 or vehicle (sesame
seed oil) were implanted sc via a small midscapular incision at the
base of the neck; wound clips were used to close the incision.
Silastic tubing (15 mm long, 0.078 in inner diameter, 0.125 in
outer diameter; Dow Corning) was used for capsule preparation.
Dilutions of crystalline E2 at a low dose (20 g/mL, in sesame oil)
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process sensitive to the circulating levels of sex steroids
(3– 6), consistent with the hypogonadotropic hypogonadism observed in humans and mice with deficient NKB signaling (7–10). Moreover, compelling evidence suggests
that NKB exerts this action in a kisspeptin-dependent
manner by acting directly on KNDy neurons through autosynaptic loops (11, 12). Nonetheless, although these
studies represent an important step forward in the understanding of the mechanisms governing GnRH release, further research is needed to fully decipher the complex hierarchy of neuronal factors that participates in the control
of kisspeptin/GnRH release.
Interestingly, NKB, encoded by the Tac2 gene in rodents, belongs to a family of closely related peptides
termed tachykinins, which also includes substance P (SP)
and neurokinin A (NKA), both encoded by Tac1 (13).
However, the action of these additional tachykinins in the
control of GnRH and gonadotropin release has not been
defined. Over the past 3 decades, numerous studies have
associated SP with nociceptive and inflammatory processes in the brain (14), as well as with psychiatric disorders (15), but only a few reports have demonstrated a
stimulatory action of SP (and NKA) in the central control
of reproductive function in rodents and men (16 –20). Importantly, an elegant study by de Croft et al (21) has recently documented the ability of SP and NKA to activate
the firing of Kiss1 neurons in the ARC, placing these tachykinins in the spotlight as possible neuromodulators of kisspeptin release. Moreover, they demonstrated cross-reactivity between the receptor for NKB (neurokinin receptor
3, NK3R) and those for SP (neurokinin receptor 1, NK1R)
and NKA (neurokinin receptor 2, NK2R), which appears
critical for the full action of NKB, in line with previous
experiments in rats indicating the involvement of the 3
tachykinin receptors in the compensatory rise of LH after
gonadectomy (GDX) (22).
The primary goal of this study was to assess the effects
of specific activation of the receptors for SP and NKA in
the central control of reproductive function in vivo, as well
as to determine the expression and regulation of Tac1
mRNA in the hypothalamus and localization of the tachykinin receptors, through a series of genetic, functional and
histological studies in the mouse.
Endocrinology, February 2015, 156(2):627– 637
doi: 10.1210/en.2014-1651
were used to fill capsules; this dose was selected to achieve moderate levels of circulating E2, as previously described (4, 27, 28).
Experimental design
LH and FSH responses to the selective agonists of
NK1R, NK2R, and NK3R in adult male mice (experiment 1)
LH responses to NK1R, NK2R, and NK3R agonists in
adult OVXⴙsham/E2-treated female mice (experiment 2)
Adult (8 wk) female mice were bilaterally OVX and sham or
E2 replaced for 1 week (n ⫽ 5/group). Central icv administration
of the specific agonists was performed as described above. A
group treated with senktide (600 pmol/5 L) was included as a
positive control based on the previously reported sex-steroiddependent effect of senktide on gonadotropin release. NK1R and
NK2R agonists where injected at 2 doses: 600 pmol/5 L (based
on the maximally ED of senktide) and 3 nmol/5 L, to determine
the ability of the system to respond to a higher concentration.
Blood samples were collected 25 minutes after injection. Animals
injected with vehicle (physiological saline, 0.9% NaCl) served as
controls.
LH responses to NK1R, NK2R, and NK3R agonists in
adult male and female kisspeptin receptor (Kiss1r)
null mice (experiment 3)
Each agonist was injected icv (600 pmol/5 L) as described
above. Blood samples were collected 30 minutes after injection.
Animals injected with vehicle (physiological saline, 0.9% NaCl)
served as controls (n ⫽ 8 –10/group).
Mapping of Tac1 expression and regulation by E2 in
the brain of female mice (experiments 4 and 5)
In experiments 4 and 5, we aimed to map the expression of
Tac1 in the brain of female mice and compare the effects of E2 on
the expression of Tac1 in the positive hypothalamic nuclei identified in experiment 4, by comparing expression in OVX female
mice, replaced with E2 or vehicle (n ⫽ 5/group). One week after
surgery, animals were decapitated in the morning (10 AM), and
trunk blood and brains were collected, frozen on dry ice, and
stored at ⫺80°C for in situ hybridization (ISH). Plasma levels of
LH were measured to determine the efficacy of the hormone
replacement.
Colocalization of Tac1 and Kiss1 mRNA in the ARC and
the anteroventral periventricular and periventricular
nuclei (AVPV/PeN) of female mice (experiment 6)
To determine the presence of coexpression between Tac1 and
Kiss1 mRNA in key areas (anterior hypothalamus, AVPV/PeN;
and posterior hypothalamus, ARC) double-labeled ISH was per-
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formed in additional tissue samples collected in experiment 4. Of
note, OVX⫹sham and OVX⫹E2 animals were used to determine Kiss1/Tac1 coexpression in the ARC and AVPV/PeN, respectively, in order to maximize the expression of Kiss1 mRNA
in each nucleus.
Tacr1, Tacr2, and Tacr3 mRNA expression in Kiss1 and
GnRH neurons (experiment 7)
Single cell transcriptomes of Kiss1 and GnRH neurons were
isolated from female Kiss1-CreGFP and Gnrh1-GFP mice between 3 and 10 months of age. The Kiss1-CreGFP mice were
OVX bilaterally. The mice used to collect Kiss1 arcuate neurons
were killed on days 6 –7 after OVX. Mice used to collect Kiss1
AVPV/PeN neurons were treated with 2 doses of estrogen benzoate (1 g on d 5 and 2 g on d 6) before experimentation on
day 7 after OVX. Intact diestrous Gnrh1-GFP mice were used to
collect GnRH neurons. See below for single neuronal harvesting
and PCR details.
Tissue preparation
In selected experiments (see experiments 4 – 6), brains were
removed for ISH, frozen on dry ice, and then stored at ⫺80°C
until sectioned. Five sets of 20-m sections in the coronal plane
were cut on a cryostat, from the diagonal band of Broca to the
mammillary bodies, thaw mounted onto SuperFrost Plus slides
(VWR Scientific), and stored at ⫺80°C. A single set was used for
each ISH experiment (adjacent sections 100 m apart).
Hormone measurements
Serum LH and FSH levels in experiments 1, 2, and 4 were
measured using a Milliplex MAP immunoassay (Mouse Pituitary
panel; Millipore) in the Luminex 200 (29, 30). The minimum
detectable concentration (pg/mL) for LH was 1.9 and for FSH
was 9.5. The intraassay coefficient of variation was less than
15%. LH levels in experiment 3 were measured in 50-L samples
using a double-antibody method and RIA kits supplied by the
National Institutes of Health (Dr A.F. Parlow, National Hormone and Peptide Program). Rat LH-I-10 was labeled with 125I
with the use of Iodo-gen tubes, following the manufacturer’s
instructions (Pierce). Hormone concentrations were measured
compared with the reference preparation LH-RP-3 as a standard.
Intra- and interassay coefficients of variation were less than 8%
and 10%, respectively.
Detection of Tac1 mRNA
Total RNA was extracted from mouse brain using an
RNAqueous kit (Ambion). RNA was reverse transcribed into
cDNA for subsequent PCR. Primers were designed based on the
published sequence of the mouse Tac1 gene (GenBank accession
number NM_009311.2) with forward primers starting at 101 bp
and reverse primers starting at 453 bp. Primers were custom
synthesized (Invitrogen). PCRs contained the following in a volume of 25 L: 2 L of reverse transcriptase reaction product, 0.2
m of each primer, 12.5 L of RediTaq polymerase (SigmaAldrich), and 8.5 L of water. Clamp polymerase sequences for
T7 or T3 polymerase were added for the final primer product
sequence and transcribed for ISH.
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NKB and the specific agonists were administered centrally
through icv injections into the lateral cerebral ventricle (600
pmol/5 L); the site of the injection was 1 mm posterior and 1.2
mm lateral to bregma. Blood samples were collected 25 minutes
after senktide injection. Animals injected with vehicle (physiological saline, 0.9% NaCl) served as controls (n ⫽ 5– 8/group).
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Quantification and analysis of Tac1 mRNA
Single cell PCR data analysis
Brain sections were analyzed unilaterally. Slides from all of
the animals were read under dark-field illumination and analyzed using ImageJ software to count the total number of cells.
Data are presented depicting the number of cells within the coronal sections containing the hypothalamic nucleus studied for
each set, not the total mRNA in this specific nucleus. The starting
and ending point of quantification was determined according to
the atlas of Paxinos and Franklin (33).
For determination of neuronal expression of a particular transcript, 12–28 cells/animal were harvested from 3–5 animals, with
totals of 36 –126 cells. The number of arcuate or AVPV/PeN
Kiss1 neurons or preoptic area (POA) GnRH neurons expressing
each transcript was counted for each animal and the mean number of neurons/animal was determined and used for further analysis of mean, SEM, and percentage expression.
The Kiss1 probe used for detection of Kiss1 mRNA was described previously (23). The Kiss1-specific sequence of the probe
spans bases 76 – 486 of the mouse cDNA sequence (GenBank
accession number AF472576). The procedure for ISH is outlined
below.
Single-label ISH of Tac1 mRNA
Tac1 mRNA sense and antisense probes were transcribed
with T7 or T3 polymerase (Fermentas), as described previously
(31). Briefly, radiolabeled probes were synthesized in vitro by
inclusion of the next ingredients in a volume of 20 L: 250 Ci
[33P] uridine triphosphate (PerkinElmer Life and Analytical Sciences), 1 g of PCR product, 0.5mM each ATP, CTP, and GTP,
and 40 U of polymerase. Residual DNA was digested with 4 U of
deoxyribonuclease (Ambion), and the deoxyribonuclease reaction was terminated by addition of 2 L of 0.5M EDTA (pH 8.0).
The riboprobes were separated from unincorporated nucleotides
with ProbeQuant G-50 Micro Columns (GE Healthcare). Slides
with mice hypothalamic sections from the different experimental
groups were processed as reported previously (4).
Double-label ISH of Kiss1/Tac1 mRNA
Statistical analysis
Quantification and analysis of Kiss1 and Tac1
mRNAs double labeling
The brain sections were analyzed unilaterally. Kiss1 mRNAcontaining cells were visualized under fluorescent illumination,
and ImageJ software was used to count the number of silver
clusters over each Kiss1 cell. The starting and ending point of
quantification was determined according to Paxinos and Franklin (33).
Tacr1, Tacr2, and Tacr3 expression in Kiss1 and
GnRH neurons
Individual Kiss1 and GnRH neurons were harvested from
dispersed hypothalamic slice preparations as previously de-
All data are expressed as the mean ⫾ SEM for each group.
One- or two-way ANOVA followed by Bonferroni’s post hoc
test were used to assess variation among experimental groups.
Significance level was set at P ⬍ .05. All analyses were performed
with GraphPad Prism Software, Inc.
Results
LH and FSH responses to selective agonists of
NK1R, NK2R, and NK3R in adult male mice
The ability of NKB and selective agonists for NK1R
(GR73762), NK2R (GR64349), and NK3R (senktide) to
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Antisense mouse Kiss1 probe was transcribed from linearized
pAMP1 plasmid as described previously (23). The cDNA template for the Tac1 riboprobe was generated by PCR with primers
that were designed to contain promoters for T7 RNA polymerase
in the antisense direction and T3 RNA polymerase in the sense
direction. Radiolabeled riboprobe for Tac1 was synthesized as
described above. Digoxigenin-labeled Kiss1 antisense riboprobe
was synthesized with T7 RNA polymerase and digoxigenin labeling mix (Roche) according to the instructions of the manufacturer. Slides were processed for double-labeled ISH as described previously (32). Slides were stored at 4°C and developed
7 days later.
scribed (34). Briefly, the microdissected slice was incubated in
protease and then washed in low Ca2⫹ artificial cerebrospinal
fluid (aCSF) and then in aCSF. The digested slice was triturated,
and the effluent containing the dispersed cells was plated on a
glass bottom dish. Under a constant flow of oxygenated aCSF,
individual neurons were identified, patched, and harvested into
a standard glass pipette with gentle suction using the XenoWorks
microinjector system (Sutter Instruments). The contents of the
pipette were expelled into a siliconized 0.5-mL tube containing
a solution of 1⫻ Invitrogen Superscript III buffer, 15 U of ribonuclease inhibitor (Promega), 10mM of dithiothreitol, and diethylpyrocarbonate-treated water in a total of 5 L and immediately frozen on dry ice. Each harvested cell and controls (aCSF
near the cells, tissue RNA and cells with and without reverse
transcriptase) were reverse transcribed as previously described
(34), and the cDNA was stored at ⫺20°C.
Primers were designed to span at least one intron-exon
boundary using the Clone Manager software program (Scientific
and Educational software). Stringent PCR conditions were
tested to determine the optimal primer concentration, magnesium concentration and annealing temperature to produce a single clear band. The primer sequences are as follows: mouse Tacr1
(accession number NM_009313, 148-bp product) forward
primer 1720 –1738 nt, reverse primer 1849 –1867 nt; mouse
Tacr2 (accession number NM_009314, 152-bp product) forward primer 1197–1217 nt, reverse primer 1330 –1348 nt;
mouse Tacr3 (accession number NM_021382, 175-bp product)
forward primer 906 –925 nt, reverse primer 1059 –1080 nt. PCR
was performed on 3 L of cDNA in a 30-L final volume containing 1⫻ Go Taq Flexi buffer (Promega), MgCl2 (2.5mM
Tacr1, 1.5mM Tacr2, and 2.0mM Tacr3), 0.33mM deoxynucleoside triphosphate, 0.33M forward and reverse primers,
2-U Go Taq and 0.22-g TaqStart antibody (Clontech) for 50
cycles of amplification with specific annealing temperatures
(Tacr1, 57°C; Tacr2, 61°C; and Tacr3, 63.5°C). PCR products
were visualized with ethidium bromide on a 2% agarose gel.
Detection of Kiss1 mRNA
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LH responses to NK1R, NK2R, and NK3R selective
agonists in adult OVXⴙsham/E2 replaced female
mice
Experiment 1 showed significant stimulatory actions of
all 3 tachykinin receptor agonists on gonadotropin secretion in the male mouse. In the female, previous studies
have demonstrated either stimulatory or inhibitory responses to senktide, depending on the sex steroid milieu
(3–5). Therefore, we hypothesized that the gonadotropic
responses to NK1R and NK2R activation would similarly
be subjected to regulation by circulating sex steroids. The
aims of this experiment were to 1) determine whether females also respond to NK1R and NK2R stimulation with
changes in gonadotropin secretion and, if so, 2) determine
whether the effect on gonadotropin secretion is E2 dependent. Interestingly, the NK1R agonist did not reduce LH
release, and 600 pmol NK1R agonist was even able to
further increase the release of LH in OVX⫹sham-treated
mice. In OVX⫹E2-treated animals, the NK1R agonist induced a robust stimulation of LH release, by approximately 20-fold in the group treated with 3 nmol NK1R
agonist, compared with vehicle-treated controls (Figure
2A). The NK2R agonist, however, displayed a senktidelike action in terms of LH release, showing inhibition of
LH release in OVX⫹sham-treated animals but clear stimulation in OVX⫹E2-treated animals (Figure 2B). In both
cases for the NK2R agonist, 600 pmol and 3 nmol had
Figure 1. Serum LH (left panel) and FSH (right panel) values of adult
male mice 25 minutes after central injection of 600 pmol NKB,
GR73632 (NK1R-A), GR64349 (NK2R-A), and senktide (NK3R-A).
Statistical analysis was performed using one-way ANOVA with
Newman-Keuls post hoc test. Different letters indicate significant
differences between groups (P ⬍ .05).
Figure 2. Serum LH levels of adult OVX⫹sham and OVX⫹E2 female
mice 25 minutes after central injection of 600 pmol or 3 nmol of
GR73632 (NK1R-A) and GR64349 (NK2R-A) or 600 pmol of senktide
(NK3R-A). One-way ANOVA ⫹ Newman-Keuls post hoc test or t test
(NK3R-A). *, P ⬍ .05; **, P ⬍ .001 compared with vehicle-treated
controls.
similar effects, resembling previous reports for senktide
(26). Indeed, this previously documented action of senktide (4) was replicated in an additional group of animals,
which significantly reduced plasma LH levels in shamtreated OVX mice but had a clear stimulatory effect in
E2-treated OVX mice after central senktide administration (Figure 2C).
LH responses to NK1R, NK2R, and NK3R agonists
in adult male and female kisspeptin receptor
(Kiss1r) knockout mice
Previous studies have demonstrated a kisspeptin-dependent action of NKB/senktide to induce gonadotropin
release (11, 12). In order to test whether stimulation of
gonadotropin release by NK1R and NK2R agonists is
kisspeptin-dependent, adult intact WT (Kiss1r⫹/⫹) males
and females (in diestrus) and Kiss1r⫺/⫺ littermates were
studied. Male and female Kiss1r null mice were GnRH
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acutely modify LH and FSH secretion in adult (8 wk) intact male mice (n ⫽ 5– 8 mice/group) was explored. These
compounds (600 pmol) were injected centrally in adult
male mice. Significant increases in LH and FSH were detected 25 minutes after icv injection of NKB and of each of
the selective agonists (P ⱕ .05) (Figure 1). Of note, although all 4 compounds exerted similar stimulatory effects on FSH release and all increased LH release, the selective agonists showed a trend towards inducing greater
LH release than NKB, which reached significance in the
NK2R agonist-treated group.
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Interaction Tachykinin-Kisspeptin and GnRH Release
Mapping of Tac1 expression and regulation by E2
in the brain of female mice
Tac1 mRNA (encoding SP and NKA) has been identified previously in the hypothalamus of several species,
including rodents and humans. However, a detailed description of the distribution of Tac1 mRNA in the mouse
is lacking (33). Here, we aimed to map the expression of
Tac1 in the brain of female mice by ISH in OVX⫹shamtreated and OVX⫹E2 mice. Tac1 mRNA was expressed in
the cerebral cortex, caudate putamen, horizontal limb of
the diagonal band, olfactory tubercule, paraventricular
hypothalamic nucleus (posterior part), ARC, ventromedial nucleus, central amygdaloid nucleus, medial forebrain bundle, parasubthalamic nucleus, basomedial
amygdaloid nucleus, ventral part of the premammillary
nucleus (PMV), retromammillary decussation, and ventral tegmental area (Figure 4). Within the hypothalamus,
expression was found to be concentrated mainly in 2 regions: the ARC and the ventromedial nucleus (VMN). All
known cotransmitters present in ARC Kiss1 neurons
(Kiss1, NKB, and dynorphin) are inhibited by sex steroids
as part of their hypothesized role in the negative feedback
of sex steroids upon GnRH release. To test whether E2
regulates Tac1 expression in these nuclei, we compared
OVX animals treated with sc implanted empty (sham)
capsules or E2-filled capsules (OVX⫹sham treated vs
OVX⫹E2 treated) and found that the number of Tac1expressing neurons in the ARC and VMN (and the PMV)
were significantly reduced by E2 treatment in OVX mice
(P ⬍ .05) (Figure 5). No apparent differences in Tac1
expression in response to E2 treatment were found in the
rest of the nuclei described above (Supplemental Figure 1).
Colocalization of Tac1 and Kiss1 mRNA in the ARC
and the AVPV/PeN of female mice
Tac2 is known to be coexpressed with Kiss1 in the ARC
of the mouse (31). Based on the kisspeptin dependence of
gonadotropic stimulation by NK1R and NK2R agonists,
we hypothesized that Tac1 might be similarly coexpressed
with Kiss1. Coexpression of Tac1 and Kiss1 was assessed
in ARC and AVPV/PeN of adult female mice. In order to
maximize Kiss1 and Tac1 mRNA expression in the ARC
and the AVPV, OVX⫹sham-treated, and OVX⫹E2treated animals were used, respectively (27, 28, 35). Interestingly, there was no detectable Tac1 expression in the
AVPV/PeN. Furthermore, in the ARC, the Tac1-positive
neurons detected were near, but did not colocalize with,
Kiss1-positive neurons in OVX mice (Figure 6).
Figure 3. Serum LH levels in WT (Kiss1r⫹/⫹) and Kiss1r⫺/⫺ adult (A)
male and (B) diestrous female mice 20 minutes after central injection
of 600 pmol GR73632 (NK1R-A), GR64349 (NK2R-A), or senktide
(NK3R-A). Two-way ANOVA ⫹ Bonferroni’s post hoc test. Different
letters indicate significant differences between groups (P ⬍ .05).
Tacr1, Tacr2, and Tacr3 mRNA expression in Kiss1
and GnRH neurons
Single cell RT-PCR analysis of the expression of all 3
tachykinin receptors (Tacr1, Tacr2, and Tacr3 mRNA) in
Kiss1 (ARC and AVPV/PeN) and GnRH neurons showed
that almost half (48.9 ⫾ 3.0%, n ⫽ 5) of Kiss1 neurons in
the ARC (126 neurons assessed from 5 animals) and over
one-fourth (26.8 ⫾ 5.5%, n ⫽ 4) of Kiss1 neurons in the
AVPV/PeN (90 neurons assessed from 4 animals) express
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primed before the experiments, as described previously
(11). The effect of central icv administration of selective
tachykinin receptor agonists (600 pmol) was performed in
parallel with adult age-matched WT males and females.
Control females were monitored for regular estrous cycles
and selected for the experiment on the morning of diestrus.
Unlike WT mice, both male and female Kiss1r⫺/⫺ mice
showed complete absence of a stimulatory effect of any of
the tachykinin receptor agonists on LH secretion (Figure
3, A and B). Interestingly, the ability to induce LH release
after central administration of the agonists was greatest
for the NK2R agonist (Figure 3A) in the males but greatest
for the NK1R agonist in the females (Figure 3B).
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633
Tacr1 mRNA, which is also present in a subset of GnRH
neurons (22.9 ⫾ 5.1%, n ⫽ 4) (100 neurons assessed from
4 animals) (Figure 7). Tacr2, however, was absent from
both populations of Kiss1 neurons and GnRH neurons,
but present in basal hypothalamic RNA, used as a positive
control. Finally, Tacr3 was confirmed to be present in all
(100%) ARC Kiss1 neurons (36 neurons assessed from 3
animals) (Figure 7) but minimally present (10.0 ⫾ 4.9%,
n ⫽ 4) in AVPV/PeN Kiss1 neurons (81 neurons assessed
from 4 animals), as previously described (31). Of note,
Tacr3 mRNA was also detected in a small subset of GnRH
neurons (11.0 ⫾ 1.1%, n ⫽ 4) (100 neurons assessed from
4 animals).
Discussion
The identification of a growing number of regulators of
kisspeptin release is adding to the complexity of the central
mechanisms governing reproduction, increasing the need
for further investigation. Mounting studies are expanding
on the action of the kisspeptin cotransmitter, the tachykinin NKB, on the control of GnRH release (36). However, the tachykinin family includes 2 additional neuropeptides, SP and NKA (13), whose potential actions in the
Figure 4. Schematic representation of the neuroanatomical
distribution of Tac1 mRNA in adult OVX female mice as assessed by
ISH. Red dots indicate areas where Tac1 mRNA neurons are detected.
Red shading depicts a higher concentration of Tac1 mRNA. bma,
basomedial amygdaloid nucleus, anterior part; CPu, caudate putamen;
DM, dorsomedial nucleus; CeM, central amygdaloid nucleus; HDB,
nucleus of the horizontal limb of the diagonal band; PVPo,
paraventricular thalamic nucleus, posterior part; VMH, ventromedial
nucleus.
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Figure 5. Representative microphotographs of Tac1 expression in the
hypothalamus of OVX and OVX⫹E2 adult female mice (upper left and
right panels, respectively) and number of Tac1-positive cells in the ARC
(lower left panel) and VMN/PMV (lower right panel) of adult OVX and
OVX⫹E2 female mice. *, P ⬍ .05 compared with OVX⫹sham, by t
test. 3V, third ventricle.
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Interaction Tachykinin-Kisspeptin and GnRH Release
control of reproduction have not been thoroughly explored. Early studies documented a robust stimulatory action of LH release by SP in rats, rabbits and humans (16 –
20, 37) and recent electrophysiological studies have
described potent stimulatory actions of SP and NKA on
ARC Kiss1 neurons in the mouse (21). Of note, this study
showed that, in vitro, the action of NKB requires not only
the presence of its putative receptor, NK3R, but also the
receptors for SP and NKA (NK1R and NK2R, respectively), in line with previous studies in vivo indicating that
blockade of all 3 tachykinin receptors (but not each one of
them individually) suppressed the compensatory rise of
LH after GDX in rats (22). These data extended the previously described cross-reactivity between tachykinins
and their receptors (38 – 40). In these studies, the affinities
or EC50 values of each tachykinin for NK1R, NK2R, and
NK3R, respectively, were reported as follows: SP ⫽ 2nM,
2200nM, and 18000nM; NKA ⫽ 16nM, 3nM, and
1300nM; and NKB ⫽ 70nM, 25nM, and 4nM (41). These
data suggest a likely interaction of NKA with NK1R as
well as NK2R, and of NKB with all 3 receptors, at relatively low concentrations. Therefore, in this study, we used
specific agonists to characterize specifically the effects of activation of the putative receptors of SP (NK1R-A: GR73632;
EC50 NK1R ⫽ 4nM; NK2R ⫽ 960nM; and NK3R ⬎
1000nM), NKA (NK2R-A: GR64349; EC50 NK1R ⬎
1000nM, NK2R ⫽ 3.7nM, and NK3R ⬎ 1000nM), and
NKB (NK3R-A: senktide; EC50 NK1R ⬎ 10 000nM,
NK2R ⬎ 10 000nM, and NK3R ⫽ 18nM) (41, 42).
Central administration of NK1R and NK2R agonists to
intact adult male mice induced clear stimulatory gonadotropin responses, similar in magnitude to the responses
evoked by senktide. In the female, however, previous studies described a dual inhibitory and stimulatory action of
senktide on LH release depending on the absence or presence of physiological levels of sex steroids, respectively
(3–5). Intriguingly, we show that central activation of
NK2R recapitulates this dual effect, whereas the activation of NK1R induces LH release in OVX animals and an
even greater stimulation at higher doses in OVX⫹E2treated mice, reminiscent of kisspeptin’s action (43). Of
note, at low physiological levels of E2, such as during diestrus, the induction of LH release by NK1R agonists is
similar to that evoked in intact males (Figure 3, A and B).
The present data indicate that NK2R and NK3R may converge on a common pathway to regulate GnRH release in
a sex steroid dependent manner, consistent with previous
reports in the rat indicating inhibition of LH release after
NK2R stimulation (18, 20). In this vein, the present data
showed lack of senktide-induced LH release in diestrous
females, supporting the contention of highly sensitive responsiveness of NK2R and NK3R activation to the circulating levels of E2. In contrast, NK1R appears to act
through different regulatory mechanisms. It is possible
that the additional stimulatory action of NK1R-A on LH
release comes from the action of SP on both populations
of Kiss1 neurons, because we have observed that a fraction
of AVPV/PeN Kiss1 neurons expresses Tacr1 but not
Tacr2 and virtually no Tacr3 mRNA. Moreover, recent
studies indicated that the inhibitory action of NKB on LH
release is opioid mediated (3), similar to what was previously suggested for the inhibitory action of NKA on LH in
the rat (20), further suggesting common regulatory pathways for NKA and NKB in the control of gonadotropin
release.
A critical aspect for the action of NKB on gonadotropin
secretion is its dependence on kisspeptin release (11, 12).
The activation of Kiss1 neurons by SP and NKA (21) suggests that all 3 tachykinins may require Kiss1 neurons as
mediators for their reproductive role. However, the study
by de Croft et al (21) did not address whether SP and NKA
may also act on GnRH neurons or, perhaps, through alternative kisspeptin-independent mechanisms. These possibilities are addressed in our studies using Kiss1r null
mice. These mice showed a conspicuous absence of LH
responses to any of the tachykinin receptor agonists,
which therefore limits their effect to kisspeptin/kiss1r-dependent mechanisms, either on or upstream Kiss1 neurons
or, potentially, on GnRH neurons in the presence of kisspeptin-Kiss1r signaling, because we have observed a subset of GnRH neurons that express SP and NKB receptors.
Intriguingly, control animals included in this study
showed a clear sexual dimorphism in terms of the effects
of NK1R and NK2R activation, with NK1R agonists being more potent than NK2R agonists to stimulate LH release in females (supporting the potential additional action
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Figure 6. Representative microphotograph of a double label ISH
depicting absence of colocalization between Kiss1-expressing neurons
(red cells) and Tac1-expressing neurons (silver grains, indicated by
white arrows) in the ARC (left panel) and AVPV/PeN (right panel) areas
of adult OVX and OVX⫹E2 mice, respectively. 3V, third ventricle.
Endocrinology, February 2015, 156(2):627– 637
doi: 10.1210/en.2014-1651
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635
of SP on AVPV/PeN Kiss1 neurons), and vice versa in
males. The action of NKA, however, remains mysterious,
because Tacr2 has been identified in neither Kiss1 nor
GnRH neurons, while showing a kisspeptin-dependent action, thus suggesting the presence of unidentified intermediate neurons upstream of Kiss1 neurons. Of note, our
present data are in keeping with the overall expression of
tachykinin receptors in the ARC of mice, where Tacr1
expression is approximately half that of Tacr3 and Tacr2
is almost undetectable (44).
Based on the above documented action of SP and NKA
receptors on gonadotropin release, deciphering the specific role(s) of these neurotransmitters in the control of
reproduction is crucial to understand reproductive physiology. Thus, sex steroids play a key role in the control of
GnRH release, acting on Kiss1 neurons, and may even
shift the biological action of specific ligands, as we observed for NK2R (present data) and NK3R agonists (3–5).
Of note, the mRNA expression of all neuropeptides coexpressed in Kiss1 neurons (kisspeptin, NKB, and dynorphin) is significantly inhibited by sex steroids, suggesting
their involvement in the negative feedback of sex steroids
upon GnRH release (31). Given the similarities in action
between NKB, SP, and NKA on gonadotropin release, we
hypothesized that Tac1 (encoding SP and NKA) would be
also expressed in the ARC of the mouse and likely inhibited by estradiol. Indeed, we observed a wide distribution
of Tac1 mRNA throughout the brain, which was particularly intense in the VMN/PMV and, to a lesser extent, in
the ARC, in keeping with previous reports of SP immunoreactivity in rats, monkeys, and humans (45–50). Both
populations were sensitive to the inhibitory action of cir-
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Figure 7. Representative gels illustrating the expression of Tacr1, Tacr2, Tacr3, and Kiss1 in dispersed and harvested Kiss1-GFP neurons in the
ARC (A) and AVPV/PeN (B) nuclei and Gnrh1-GFP neurons in the rostral POA (rPOA) (C). Expected sizes for the PCR products are 148 bp for Tacr1,
152 bp for Tacr2, 175 bp for Tacr3, and 120 bp for Kiss1. Summary bar graphs of the mean ⫾ SEM percentage expression of Tacr1, Tacr2, and
Tacr3 of ARC Kiss1 neurons (D), AVPV/PeN Kiss1 neurons (E), and GnRH neurons (F) expressing each of the transcripts per animal (n ⫽ 3–5
animals; 12–28 neurons/animal). MM, molecular markers. TC, tissue control for ARC Kiss1 neurons was ARC RNA for Tacr1, Tacr3 and Kiss1, and
basal hypothalamic RNA for Tacr2. TC for AVPV and GnRH neurons was POA RNA. Note: Tacr2 expression in AVPV/PeN Kiss1 and GnRH neurons
was also not detected.
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Acknowledgments
Address all correspondence and requests for reprints to: Víctor
M. Navarro, PhD, Division of Endocrinology, Diabetes, and
Hypertension, Department of Medicine, Brigham and Women’s
Hospital and Harvard Medical School, Boston, MA 02115. Email: vnavarro@partners.org.
This work was supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development
through cooperative agreement U54 HD028138 as part of the
Specialized Cooperative Centers Program in Reproduction and
Infertility Research grants from the National Institute of Health
and by National Institutes of Health (NIH) Grant R01
HD019938 (to U.B.K.); NIH Grant K99 HD071970, Charles H.
Hood Foundation for Child Health Research Program and the
Microgrant Program from The Biomedical Research Institute
and the Center for Faculty Development and Diversity’s Office
for Research Careers at the Brigham and Women’s Hospital
(V.M.N.); by The Scientific and Technological Research Council
of Turkey Grant 2219 (to S.S.); and by NIH Grants R01
NS043330 and R01 DK068098 (to O.K.R.).
Disclosure Summary: The authors have nothing to disclose.
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