Medial Amygdala <b><i>Kiss1</i></b> Neurons Mediate Female Pheromone Stimulation of Luteinizing Hormone in Male Mice

Abstract

Background/Aims: The medial amygdala (MeA) responds to olfactory stimuli and alters reproductive physiology. However, the neuronal circuit that relays signals from the MeA to the reproductive axis remains poorly defined. This study aimed to test whether MeA kisspeptin (MeA^Kiss) neurons in male mice are sensitive to sexually relevant olfactory stimuli and transmit signals to alter reproductive physiology. We also investigated whether MeA^Kiss neurons have the capacity to elaborate glutamate and GABA neurotransmitters and potentially contribute to reproductive axis regulation. Methods: Using female urine as a pheromone stimulus, MeA^Kiss neuronal activity was analysed and serum luteinizing hormone (LH) was measured in male mice. Next, using a chemogenetic approach, MeA^Kiss neurons were bi-directionally modulated to measure the effect on serum LH and evaluate the activation of the preoptic area. Lastly, using in situ hybridization, we identified the proportion of MeA^Kiss neurons that express markers for GABAergic (Vgat) and glutamatergic (Vglut2) neurotransmission. Results: Male mice exposed to female urine showed a two-fold increase in the number of c-Fos-positive MeA^Kiss neurons concomitant with raised LH. Chemogenetic activation of MeA^Kiss neurons significantly increased LH in the absence of urine exposure, whereas inhibition of MeA^Kiss neurons did not alter LH. In situ hybridization revealed that MeA^Kiss neurons are a mixed neuronal population in which 71% express Vgat mRNA, 29% express Vglut2 mRNA, and 6% express both. Conclusions: Our results uncover, for the first time, that MeA^Kiss neurons process sexually relevant olfactory signals to influence reproductive hormone levels in male mice, likely through a complex interplay of neuropeptide and neurotransmitter signalling.

Optogenetic stimulation of kisspeptin neurones within the posterodorsal medial amygdala increases luteinising hormone pulse frequency in female mice

“…The present study demonstrates that optogenetic stimulation of Kiss1 neurones within the MePD increases LH pulse frequency and provides clear evidence that MePD Kiss1 neuronal activity can modulate the frequency of the hypothalamic GnRH pulse generator. These data extend our earlier neuropharmacological findings where intra‐MePD administration of kisspeptin dose‐dependently increased LH levels, as well as more recent studies showing chemogenetic activation of MePD Kiss1 neurones increased mean circulating levels of LH . The relationship between the amygdala and its regulation of gonadotrophic hormone secretion has received considerable attention, with evidence to support the notion that the medial amygdala has a negative output on the reproductive system.…”

“…These data extend our earlier neuropharmacological findings where intra-MePD administration of kisspeptin dose-dependently increased LH levels, 12 as well as more recent studies showing chemogenetic activation of MePD Kiss1 neurones increased mean circulating levels of LH. 25,26 The relationship between the amygdala and its regulation of gonadotrophic hormone secretion has received considerable attention, with evidence to support the notion that the medial amygdala has a negative output on the reproductive system. Stimulating the medial amygdala results in delayed pubertal onset, 27 whereas lesions advance the onset of puberty 28 and increase the secretion of LH.…”

Kisspeptin as a Behavioral Hormone

“…27 By comparison, in male mice exposed to female urine, a twofold increase in MeA kisspeptin neuronal activity was observed with associated LH surges, whereas female urine failed to activate AVPV and ARC kisspeptin neurons. 28 Taken together, these studies highlight a crucial role for kisspeptin signaling in the transmission of opposite-sex olfactory stimuli to modulate the HPG axis. Whether the findings in rodents reflect a true sexual dimorphism or methodological and species differences remains to be explored.…”

The Roles of the Amygdala Kisspeptin System

“…In male rats, amygdala kisspeptin neurons maintain reciprocal connections with the accessory olfactory bulb, 12 which has an established role in conveying pheromonal cues, 28 with evidence demonstrating these kisspeptin neurons are targeted directly by pheromonal pathways. 29 In addition, approximately 15% of GnRH neurons in the hypothalamic preoptic area receive inputs from amygdala kisspeptin neurons, 12 suggesting they act as a plausible relay between pheromonal inputs and the HPG axis. It is interesting to note that these findings in rats are in close agreement with a neuroanatomical study in Kiss1-CRE transgenic mice which revealed fibers projecting from amygdala kisspeptin neurons through the medial forebrain bundle to the preoptic area, as well as extending further into the accessory olfactory bulb.…”

“…Along these lines, it is notable that the median eminence lacks direct projections from the MeA in male rodents. 29 This raises the possibility that in male rodents, rather than direct GnRH release at GnRH nerve terminals, the amygdala kisspeptin system influences the HPG axis through complex interplay with other neurotransmitter systems (such as GABA 12 ) or via interneuronal pathways. 29 However, this remains to be fully explored and studied in female rodents.…”