Evidence increases for a broad involvement of the endocannabinoid signaling employed by preoptic/hypothalamic neurons/e.g., for corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH), oxytocin (OT), and vasopressin (VP)—see (Di et al. 2003
); for proopiomelanocortin (POMC)—see (Hentges et al. 2005
); for gonadotropin-releasing hormone (GnRH)—see (Farkas et al. 2010
) to regulate the activity of their afferents and thereby to influence their own excitability at various physiological and/or pathological processes. Interfering with this endocannabinoid signaling, i.e., blocking of 2-arachidonoylglycerol (2A-G) synthesis or antagonizing CB1-mediated actions prevent the endocannabinoid-mediated inhibition of GABA release from the afferents, as it was reported for, e.g., GnRH (Farkas et al. 2010
) and POMC neurons (Hentges et al. 2005
). Very few data are available, however, about the physiological/pathophysiological conditions and target system of an enhanced endocannabinoid signaling, which may recruit other than the GABAergic afferents. This could be similar to the outcome of treatments with CB1 agonists resulting in a reduction both the inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs) in target neurons (Hentges et al. 2005
Kisspeptin (KP)-producing neurons are known to form afferents of multiple neuroendocrine cells (Clarkson and Herbison 2006
). They are located in three major sites, i.e., the preoptic area (POA), the arcuate nucleus (ARC), and the medial amygdaloid nucleus (ME) (Clarkson et al. 2009
; Lehman et al. 2013
), where they establish local connections (Comninos et al. 2016
; Stephens and Kauffman 2017
; Krajewski et al. 2010
; Qiu et al. 2018
), as well as project to distant target areas including the POA (Qiu et al. 2018
), supraoptic (SON), and paraventricular (PVH) nuclei of the hypothalamus (Yeo et al. 2016
). Their processes play a key role in mediating the positive and negative estrogen feedback to GnRH neurons (Ohkura et al. 2009
), which is based on their direct genomic and non-genomic (Mittelman–Smith et al. 2012
) responses to estrogen, and direct connections to GnRH neurons. They are also implicated in conveying circadian (Vida et al. 2010
; Williams et al. 2011
), metabolic (Clarke and Arbabi 2016
; Wahab et al. 2013
), and limbic (Comninos et al. 2016
; Stephens and Kauffman 2017
) signals to GnRH neurons. In addition, they provide a rich innervation of OT (Seymour et al. 2017
; Liu and Herbison 2016
; Scott and Brown 2013
), VP neurons (Liu and Herbison 2016
) in the SON and PVH, and POMC (Higo et al. 2017
; Qiu et al. 2018
), and tyrosine hydroxylase (TH) neurons (Sawai et al. 2012
) in the ARC, and the functional significance of these neuronal connections, however, is incompletely understood.
Besides producing various neuropeptides (neurokinin B, dynorphin, galanin) (Goodman et al. 2007
; Murakawa et al. 2016
; Kallo et al. 2012
; Porteous et al. 2011
) or biogenic amines (dopamine) (Clarkson and Herbison 2011
; Skrapits et al. 2015
; Bardoczi et al. 2018
), subpopulations of kisspeptin neurons use classical neurotransmitters like glutamate and GABA, which contribute to the excitatory and/or inhibitory innervation of the neuroendocrine cells. Contrasting the peptide-type transmitters, these classical neurotransmitters are released spontaneously, as well as at an evoked manner, which raises a potential role of endocannabinoid signaling to discretely influence the release of neurotransmitters/neuromodulators from the different kisspeptin subpopulations. The regulation could happen with different efficacy in the GABAegic and glutamatergic subpopulations of KP neurons, similarly to those observed for the G-protein-dependent signaling of CB1 in cortical principal versus interneurons (Steindel et al. 2013
Electrophysiological recordings indicate ovarian cycle-dependent alterations in the GABAergic and glutamatergic inputs to GnRH neurons (Balint et al. 2016
; Farkas et al. 2018
). It was also reported that the endocannabinoid signaling operates in a phase-dependent manner; in metestrus, endocannabinoids suppress the postsynaptic currents (PSCs) in GnRH neurons, whereas in proestrus, they do not seem to contribute to the increase of these events (Balint et al. 2016
; Farkas et al. 2018
). By establishing many critically important estrogen-sensitive pathways, it is of interest whether (a) GABAergic and/or glutamatergic subpopulations of KP neurons express CB1 mRNAs, (b) the expression is influenced by estradiol (E2), and (c) CB1-immunoreactivity is present in the KP afferents to GnRH neurons.
These questions prompted us to investigate the site- and cell-specific expression of CB1 in female mice using multiple labeling in situ hybridization and immunofluorescent histochemical techniques. Ovariectomized and estradiol-replaced models were used in the experiments, supplemented with mice genetically altered to maximize the visualization of KP afferents to GnRH neurons.
Materials and methods
Adult, female CD1 mice (n
= 20) were used for the RNAscope in situ hybridization histochemistry (ISHH), and adult, female CD1 (n
= 10) and Kiss1-Cre-GFP mice (Kiss1tm1.1(cre/EGFP)Steiner)
, Jackson Laboratory, stock No. 017701, n
= 20) were used for the immunohistochemical studies. The animals were housed under controlled lighting [12:12 h light–dark cycle, lights on at 07:00 h, and temperature (22 ± 2 °C)] conditions, with access to food and water ad libitum. The CD1 mice were ovariectomized (OVX) with 3-week recovery time, and thereafter, 15 of them received a two-step subcutaneous estrogen substitution or vehicle, as described before (Bosch et al. 2013
). Briefly, the animals (n
= 30) received on the first and the second days 0.25 or 1.5 μg 17β-estradiol benzoate (EB) in 50 μl oil (n
= 15) or 50 μl oil vehicle (n
= 15) at Zeitgeber time (ZT) 4–5 (Table 1
). The uterus weight of the OVX + Oil mice differed significantly from those treated with EB (0.049 ± 0.014 g versus 0.16 ± 0.014 g, respectively, p
< 0.001). The Kiss1-Cre-GFP animals were all OVX and received a viral construct AAV-EF1a-DIOhChR2 (H134R)-EYFP injection [200 nl 1:2 solution of the viral stock) into the anteroventral periventricular nucleus (AVPe, Bregma AP 0.25 mm, ML 0.2 mm, DV 4.6 mm, n
= 5)], ARC (Bregma AP −2.15 mm, ML 0.25 mm, DV 5.25 mm, n
= 8), or posterodorsal subdivision of the medial amygdaloid nucleus (MePD Bregma AP −1.9 mm, ML 2.0 mm, DV 4.9 mm, n
= 7) to label the KP neurons. Three weeks after the surgery, these animals were also treated with EB similarly to the above protocol.
Primary and secondary antibodies used in concurrent detection of three different tissue antigens in sections of ovariectomized 17β-estradiol benzoate-treated wild-type and Kiss1-CRE-GFP mice
In sections of wild-type CD1 mice
Guinea pig anti-GnRH (#1018, gift from Dr. Erik Hrabovszky, 1:50,000)
Sheep anti-KP (#053, gift from Dr. Alain Caraty, 1:1000)
Rabbit anti-CB1 (ImmunoGenes-ABS Zrt, 1:1000)
FITC-donkey anti-guinea pig IgG (H + L) (#706-095-148, Jackson Immuno Research Laboratories, 1:500)
CY3-donkey anti-sheep IgG (Fab) 2 (#713-165-147, Jackson Immuno Research Laboratories, 1:3000)
CY5-donkey anti-rabbit IgG (H + L), (#711-175-152, Jackson Immuno Research Laboratories, 1:2000)
In sections of Kiss1-Cre-GFP mice injected with AAV-EF1A-DIO-hChR2 (H134R)-EYFP
Guinea pig anti-GnRH (#1018, gift from Dr. Erik Hrabovszky, 1:50,000)
Rabbit anti-GFP, (AB10145, Millipore, 1:2000)
Goat anti-CB1 (gift from Dr. Masahiko Watanabe, 1:600)
CY5- conjugated donkey anti-guinea pig IgG (#706-175-148, Jackson Immuno Research Laboratories, 1:2000)
FITC-conjugated donkey anti-rabbit IgG (#711-095-152, Jackson Immuno Research Laboratories, 1:1000)
CY3-conjugated donkey anti-goat IgG (#706-165-147, Jackson Immuno Research Laboratories, 1:2000)
RNAscope ISHH to detect KP, CB1, vesicular GABA transporter (VGAT), or vesicular glutamate transporter (VGLUT) 2 mRNAs
Tissue preparation for in situ hybridization
Four hours after the second EB or oil vehicle injections, the animals were sacrificed, and the brains were removed and frozen on dry ice. 14 µm thick coronal sections were cut on a Leica CM 3050 S cryostat (Leica Microsystems, Vienna, Austria), and mounted consecutively onto groups of ten Super Frost Ultra Plus glass slides (Thermo Fisher Scientific, Budapest, Hungary). Two coronal sections of OVX + EB mice (n = 5) 126 µm apart from each other were paired with two corresponding sections of OVX + oil mice (n = 5) on each slide. The slides were stored at −80 °C until processed.
RNAScope in situ hybridization
Five slides of the rostral periventricular area of the third ventricle (RP3V) [containing the AvPe and/or the periventricular hypothalamic nucleus (Pe)], and five slides of the ARC regions (altogether 10 sections of five OVX-EB and 10 sections of five OVX + oil mice for each regions) were selected for each hybridization. The pre-treatment (#3,20,513-USM) and hybridization (#3,20,293) protocols of the RNAscope technique (Advanced Cell Diagnostics Inc., Newark, CA, USA) were used, which allowed the application of the Multiplex reagent kit. The hybridization cocktail contained the following probes: for channel 1, it was Kiss1 (catalog #476291_O1, XM_006529679, region 121 – 1376); for channel 2, it was either SLC32A1 (VGAT, catalog #3,19,191-C2, NM_009508.2, region 894–2037) or SLC17A6 (VGLUT2; catalog #3,19,171-C2, NM_080853.3, region 1986–2998); and for channel 3, it was CNR1(CB1, catalog # 4,20,721-C3, NM_007726.3, region 530–1458). The “B” variant of Amp4 was used, which labeled channel 1–3 with Alexa488 (green), Atto550 (red), and Atto647 (far red), respectively. The sections were coverslipped with Prolong Antifade kit (Molecular Probes, Leiden, The Netherlands) after the amplification/labeling steps and counterstaining with DAPI.
Capturing and analyzing the RNAScope signals
The quadruple-labeled sections were scanned in a Nikon C2 confocal microscope (Nikon, Japan) using the 20 × objective. Multiple stacks of optical slices (1024 × 1024 pixels, z-steps 0.6 µm) were obtained from the −0.6 to −2.4 µm layer of the sections by scanning the full RP3V and ARC regions on one side. The fluorochromes were excited with laser lines 488, 561, and 641 nm. The DAPI nuclear staining was also detected using the 405 nm laser. Laser intensities and other acquisition parameters were kept the same during the whole scanning. Using the Image J software, the different channels in the image stack were combined into maximum intensity projections, and binarized and merged into single TIFF images. The DAPI channel was also saved as separate TIFF images. Lit pixels were counted within regional and cellular borders, which were determined manually based on the DAPI image and kisspeptin signals. A cell was deemed as positive for a given mRNA when the number of lit pixels within the cell area were higher (containing more than five pixels) than the number of lit pixels in identical sized area of the white matter (maximum five pixels). Data were analyzed by one-way ANOVA, and significant difference was determined by the Holm–Sidak method.
Tissue preparation for detecting the viral fluorescent tracer and immunofluorescence
The animals were perfused transcardially with phosphate-buffered saline (PBS 0.1 M) containing 4% paraformaldehyde (PFA). The brains were removed, post fixed for 24 h, and transferred into 30% sucrose for cryoprotection, and then, 30 µm thick coronal sections were cut on a freezing microtome by collecting every third sections into the same well.
Evaluation of viral tracing of KP fibers
A group of sections were mounted from each brain with RP3V (n = 5), ARC (n = 8), or MEA (n = 7) injections of the viral tracer. Brains showing successful labeling of KP neurons in consecutive sections were selected for subsequent multiple-label immunofluorescence staining of YFP in KP cells, GnRH, and CB1.
After the endogenous peroxidase activity had been quenched with 0.5% hydrogen peroxide (20 min), sections were permeabilized with 0.5% Triton X-100 (23,472–9, Sigma, 20 min), and treated with 2% normal horse serum (20 min) to reduce non-specific antibody binding. All treatments and interim rinses in PBS (3 × 5 min) were carried out at room temperature, except for incubation in the primary antibodies and fluorochromes, which was carried out at 4 °C. Sections were incubated in a cocktail of the primary antibodies for 72 h and the fluorochrome-labeled secondary antibodies overnight with a 2 h rinse in TRIS at RT in between. Primary and secondary antibodies used in the immunohistochemical procedure are listed in Table 1
Thus, guinea pig anti-GnRH (#1018, gift from Dr. Erik Hrabovszky, 1:50,000) (Hrabovszky et al. 2011
), sheep anti-KP (#053, gift from Dr. Alain Caraty, 1:1000) and rabbit anti-CB1, ImmunoGenes-ABS Zrt, 1:1000) primary antibodies, and FITC-donkey-anti-guinea pig IgG (H + L) (#706-095-148, Jackson ImmunoResearch Laboratories, 1:500), CY3-donkey-anti-sheep IgG (Fab)2 (#713-165-147, Jackson ImmunoResearch Laboratories, 1:3000), and CY5-donkey-anti-rabbit IgG (H + L), 711-175-152, Jackson ImmunoResearch Laboratories, 1:2000) secondary antibodies were used to stain sections of CD1 mice. To maximize the visualization of the axonal boundaries of KP neurons, mice expressing YFP in KP cells of the virus injected KP-CRE animals were used for the immunofluorescent detection of CB1 in afferents of GnRH neurons. In this case, guinea pig anti-GnRH (#1018, 1:50,000) (Hrabovszky et al. 2011
), rabbit anti-GFP (for KP, AB10145 Millipore, 1:2000), and goat anti-CB1, 1:600 (Makara et al. 2007
), primary antibodies, and CY5-conjugated donkey anti-guinea pig IgG (#706-175-148, Jackson ImmunoResearch Laboratories, 1:2000, 2 h), FITC-conjugated donkey anti-rabbit IgG (#711-095-152, Jackson ImmunoResearch Laboratories, 1:1000, 2 h), and CY3-conjugated donkey anti-goat IgG (#706-165-147, Jackson ImmunoResearch Laboratories, 1:2000, 2 h) secondary antibodies were used. Sections were then rinsed in TRIS (2 h), mounted onto glass slides and cover slipped with Moviol.
Confocal microscopy and 3-D reconstruction of KP afferents to GnRH-IR cells
The triple-labeled sections were scanned in a Nikon A1R confocal microscope (Nikon, Japan) using the × 20 and the 60 × oil immersion objectives. Multiple stacks of optical slices (1024 × 1024 pixels, z-steps 0.15 µm) were generated, which contained the KP-IR fibers in apposition to the cell bodies and the processes of GnRH-IR neurons. The separately recorded green, red, and far-red channels were merged and displayed with the ImageJ software running on an IBM compatible personal computer. Orthogonal views from different planes (x/y, x/z or y/z) of the confocal microscopic images were used to analyze the KP-immunoreactive fibers for apposition to GnRH neurons and CB1-immunoreactivity. To enable three-dimensional (3D) analyses, the images were further processed using the software Amira (6.0, Visual Imaging Group). The stacks of the optical slices were loaded into the visualization program and rendered in three dimensions with surfaces generated from above threshold immunoreactivity. The threshold was set individually for each image and color channel to minimize any noise, while maintaining the proper cellular boundaries. The surfaces generated from the three channels in the same optical volume were visualized to check for cell-to-cell contacts, and the presence of CB1-immunoreactivity in KP fibers associated with GnRH neurons. This enabled verification of the findings from the two-dimensional confocal image analyses.
GnRH neurons (n = 6 per brains with successful viral labeling of RP3V kisspeptin neurons) located in the medial preoptic area and seen to be a candidate of receiving KP-IR afferents at 20 × magnifications were selected for 60 × scans and analyses. An axon was positive for CB1 when immunoreactivity was detected inside or in association with the cell membrane of the YFP-positive KP processes. No such signal was detected in CB1-KO mice (kindly provided by A. Zimmer, University of Bonn and bred at the Medical Gene Technology Unit of the Institute of Experimental Medicine).
This study provides evidence that (a) CB1 mRNA is expressed by both GABAergic and glutamatergic subpopulation of kisspeptin neurons, (b) the receptor protein is present in KP afferents of GnRH neurons, and (c) the expression of CB1 mRNA shows estrogen-dependent regulation. The applied estrogen-treatment, known to induce proestrus in mice, reduced the level of CB1 transcripts in the RP3V and ARC, and differently influenced its co-localization with VGAT or VGLUT2 in kisspeptin neurons.
Experimental model to study whether CB1 is involved in presynaptic regulation of kisspeptin afferents
Levels of CB1 expression are highly variable among different brain locations and cell types. While the cerebral cortex and hippocampus contain a very high level of CB1 protein, expression levels are relatively low in hypothalamic regions (Wittmann et al. 2007
). Despite its low levels, a large body of evidence supports strong cannabinoid-dependent signaling in the regulation of hypothalamic functions (Gammon et al. 2005
; Pagotto et al. 2006
; Tasker 2006
; Brents 2016
). Concerning the reported role of CB1 in the regulation of reproduction, and the critical functions of kisspeptin neurons in this regard, in the current study, the KP cell-specific expression of CB1 was investigated using multiple labeling in situ hybridization and immunofluorescent histochemical techniques. During the ovarian cycle, the hypothalamic regulatory circuits operate under the influence of estrogens’ negative and positive feedback effects. Therefore, we have used the animal models with reduced serum ovarian hormone levels achieved by 3-week time gonadectomy (Baumgartner et al. 2019
), and the hormone-replaced pairs treated subsequently with a priming and a surge-inducing dose of estrogen (Bosch et al. 2013
). As expected, this treatment regime resulted in a significant difference between the uterine weights of the two models by increasing the weights fourfold compared to controls. In addition, the appearance of KP transcript levels showed high estrogen-dependency in the RP3V and ARC regions, as it was reported earlier (Lehman et al. 2010
, Navarro et al. 2009
, Smith et al. 2005
). Thus, in the RP3V, KP transcript levels were high in the estrogen-treated animals, whereas strong signals were detected in the ARC in the absence of estrogen (Fig. 3
A). The estradiol-treated model was chosen also to investigate the afferents on GnRH neurons based on the observation of Chan et al. (2011
). They reported that the positive feedback levels of estradiol stimulate a robust increase in spine density and most likely in synaptic inputs of GnRH neurons.
Distribution of CB1 in hypothalamic GABAergic and glutamatergic neurons in previous studies
Our previous mapping studies in male mice (Hrabovszky et al. 2012
) revealed a differential expression of mRNAs for CB1, the GABAergic marker GAD65, and the glutamatergic marker VGLUT2 in the RP3V and ARC regions, where the two major subpopulations of KP neurons are located. Relatively low abundance of the hybridization signal and high-to-moderate-to-low number of labeled cells were found for CB1 in the AVPe, the Pe, and the ARC nucleus, respectively. CB1 mRNA was detected in both GABAegic and glutamatergic neurons in these regions. In the current study, the mRNA signals for CB1, VGAT and VGLUT2 were relatively low in both the RP3V and ARC of female mice and their relative abundance compared to the neighboring brain areas (i.e., MPA or the VMH) was similar to the one detected previously in male mice (Hrabovszky et al. 2012
Estrogenic regulation of hypothalamic CB1, VGAT, and VGLUT2 expression
Estrogen treatment resulted in a down-regulation of CB1 expression in both regions (Fig. 3
A). This agrees with the observation of Riebe and Gorzalka, (Riebe et al. 2010
), showing that OVX females have higher amounts of hypothalamic cannabinoid receptor binding relative to both cycling and OVX + E2 females. The current result, therefore, might indicate a reduced involvement of CB1 mediating suppression in local circuit activity, which generate higher frequency postsynaptic events in target cells in the presence of estrogen (Glanowska and Moenter 2011
Similarly to CB1, expression of VGAT mRNA was lower in RP3V of the OVX + EB mice compared to the oil-treated ones. This seems to be congruent with the observation of Ottem et al. (Ottem et al. 2004
) showing a decrease of the VGAT-containing vesicles in the RP3V of female rats at the time of the surge.
Gonadal hormones were reported to alter also the VGLUT2 expression in these regions. E2-treatment was shown to increase the VGLUT2 immunoreactive vesicles in the afferents of GnRH neurons in ovariectomized rats (Ottem et al. 2004
). An increase of VGLUT2 mRNAs by estrogen-treatment was reported also in the ARC of ovariectomized mice (Qiu et al. 2018
). The positive effect of gonadal hormones on VGLUT2 expression is, however, not unambiguous, since gonadectomy led to a significant elevation of VGLUT2 mRNA in male mice, compared to the levels observed in gonad intact conditions (Nestor et al. 2016
The comparison of current RNAscope signals of the EB or oil-treated animals, however, did not reveal a significant difference for VGLUT2 signals neither in the RP3V nor in the ARC. This may indicate the presence of mixed glutamatergic cell populations in these regions in which expression of VGLUT2 is differently or even antagonistically regulated by estrogen.
Expression of CB1 in preoptic and arcuate KP neurons
Analyses of the RNAscope data at cellular levels revealed the presence of CB1 transcripts in KP neurons in both the RP3V and ARC regions, but the hormonal conditions had a great impact on the level of co-expression. Nearly all preoptic KP neurons expressed CB1 in the OVX + OIL group (93.4 ± 2.7%), but this value fall to one-third in the OVX + EB group (32 ± 6.9%). The opposite was observable for ARC KP neurons, showing low levels of co-expression in the OVX + OIL group (15.5 ± 2%), whereas CB1 mRNA expression was detectable in nearly half of the KP neurons in the OVX + EB group (49.3 ± 4.4%). By involving data on the mRNA levels for VGAT and VGLUT2 in the analyses, the presence of CB1 mRNA was also examined separately in the GABAergic and glutamatergic subpopulations of KP neurons. Both subpopulations proved to be positive for CB1 mRNA and the co-localization levels varied again according to the hormonal status of the animals. The percentage of the GABAergic KP neurons expressing CB1 was significantly lower in the RP3V of OVX + EB-treated mice, indicating that CB1 signaling might be reduced or suspended in this cell population at high estrogen levels.
Conversely, the percentage of the glutamatergic KP neurons expressing CB1 (Fig. 3
F) was significantly higher in the ARC of the estrogen-treated animals compared to controls. The dramatic differences seen in the preoptic versus arcuate co-localization levels can be explained by the antagonistic regulation of KP transcript in these regions, which is combined with the reduced expression of CB1 mRNA in response to estrogen-treatment in both locations. Thus, lower co-localization levels can be found in RP3V in the OVX + EB model and in the arcuate nucleus in the OVX + Oil model, in which models the KP expression is high and the CB1 message is downregulated.
Colocalization of KP and VGAT or VGLUT2 mRNAs
Both VGAT or VGLUT2 mRNA-containing KP cells could be detected in the preoptic and ARC regions, which is in concordance with studies detecting VGAT/or VGLUT2 mRNA in KP-CRE mice (Cravo et al. 2011
) or KP-immunoreactivity in VGAT-CRE or VGLUT2-CRE mice (Cheong et al. 2015
). Thus, about half to two-third of the KP neurons were positive in the current study for VGAT mRNA, and about one quarter to one-third of KP neurons were positive for VGLUT2 mRNA in the RP3V. Most of the ARC KP neurons were also positive for VGLUT2 mRNA and three quarter of them also expressed the VGAT mRNA. Of note, the level of co-localization of KP and VGAT or VGLUT mRNAs may depend on the animal model used, since quantitative PCR studies were able to find relatively few, or no RP3V KP cells with Slc17a6 (VGLUT2) mRNA or no ARC KP cells with Slc32a1 (VGAT) mRNA (Qiu et al. 2018
) in Kiss1Cre:GFP mice (Gottsch et al. 2011
). Furthermore, translation of neither amino acid transporter markers has been proved to functional transporter proteins, respectively, in the RP3V or ARC KP cells of these animals (Qiu et al. 2018
KP neurons in all major locations are involved in the regulation of GnRH secretion. We have traced projections of KP neurons located in the RP3V, ARC, and MePD (Fig. 4
), and in concordance with the previous observations (Kallo et al. 2012
; Pineda et al. 2017
; Yeo 2013
; Yeo and Herbison 2011
; Yeo et al. 2019
; Wintermantel et al. 2006
), found KP fibers in apposition to GnRH cells to originate primarily from the RP3V cells.
CB1-IR was found in the majority of these KP neuronal processes independent of being in apposition to the perikaryon or proximal processes of GnRH neurons (Fig. 5
C and F). This suggests that neurotransmitter release from the KP afferents to GnRH neurons is also under regulation of endocannabinoids.
The endocannabinoid control is inhibitory by reducing the frequency of mPSCs and the firing activity of GnRH neurons in slices of metestrus female mice (Balint et al. 2016
). This phase of the cycle is characterized by low serum estrogen levels, which triggers the synthesis and release of 2-AG from GnRH neurons via activating membrane-associated, rapid ERβ-mediated processes in these neurons. Acute administration of a high dose of estrogen to the slice had no effect on this inhibition, indicating that rapid, non-genomic actions of estrogen might not interfere with the CB1 signaling. The inhibition, however, does not seem to operate at high serum estrogen levels in proestrus GnRH neurons (Farkas et al. 2018
). A possible explanation for this could be a longer exposure to and most likely genomic action of estrogen seen in the current study, which resulted in the down-regulation of CB1 expression in both the RP3V and ARC KP neurons. Whether it has involved both the GABAergic and glutamatergic KP afferents of GnRH neurons needs to be further investigated. However, the down-regulation of CB1 in the glutamatergic KP afferents may have contributed to the appearance of the glutamate receptor mediated PSC-es in the GnRH neurons seen on the proestrus afternoon (Farkas et al. 2018
Glutamate released by ARC KP neurons targets KP neurons in this brain region and may contribute to the synchronized activity increase in populations of KP neurons during pulse generation. Glutamate released by ARC KP neurons also targets preoptic KP neurons, which in turn can enhance stimulation of GnRH neurons leading ultimately to a GnRH surge (Qiu et al. 2016
). Therefore, a down-regulation of CB1 mRNA expression in the glutamatergic KP neurons by the preovulatory estrogen rise would facilitate an increased firing of KP afferents to other KP neurons, which in turn could enhance the activity of GnRH neurons.
KP neurons also innervate the Arc POMC neurons. The glutamatergic and GABAergic afferents of POMC neurons are under tonic, endocannabinoid-induced inhibition (Hentges et al. 2005
), which seems to be differentially regulated by estrogen. The estrogen-treatment, given 24 h prior to experimentation, increased the mEPSC frequency, and markedly decreased the potency of CB1 agonists to decrease mEPSC frequency in POMC neurons. In contrast, estrogen potentiated the cannabinoid-induced decrease in mIPSC frequency (Nguyen and Wagner 2006
). The opposing effects of estrogen on the cannabinoid regulation of amino acid neurotransmission lead ultimately to the excitation of POMC neurons. This shows similarities with the effect of estrogen on the tonic endocannabinoid signaling of GnRH neurons. As the GABAergic input of GnRH neurons is facilitatory due to their high intracellular chloride levels (DeFazio et al. 2002
), an estrogen-mediated down-regulation of CB1 in both GABAergic and glutamatergic afferents, including those originating from KP neurons, could potentially increase the excitation of GnRH neurons. These needs, however, to be confirmed by electrophysiological recordings.
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