Serotonin neurons are thought to exert a modulatory influence on the secretion of the gonadotropin hormones in mammals, but their mechanism of action remains unclear. We examined here the potential role of serotonin neurons in modulating the activity of the gonadotropin-releasing hormone (GnRH) pulse generator formed by the arcuate nucleus kisspeptin (ARNKISS) neurons. Acute brain slice electrophysiology revealed that ∼60% of ARNKISS neurons in diestrous female mice were activated by serotonin while less than 10% were inhibited. Pharmacological studies indicated that combinatorial patterns of 5-HT receptor sub-type activation were likely responsible for the excitatory actions. The role of serotonin in ARNKISS neuron synchronization behavior was assessed using GCaMP imaging in acute brain slices from diestrous female and male mice. In both sexes, serotonin evoked potent recurring bouts of synchronization activity amongst ARNKISS neurons. To evaluate the impact of serotonin in vivo, we used "fluidic" GCaMP fibre photometry in which serotonin was infused directly into the ARN while recording the ARNKISS neuron population activity in freely behaving diestrous female mice. In all cases, the infusion of serotonin evoked a robust ARNKISS neuron synchronization episode. These data demonstrate that serotonin exerts a direct, predominantly stimulatory action on ARNKISS neuron pulse generator through a variety of 5-HT receptors. Serotonergic inputs appear to provide a potent synchronizing influence on the ARNKISS neuron population and suggest considerable potential for 5-HT to control the frequency of pulsatile reproductive hormone secretion in mice and likely other mammals.
Morris et al. (Fri,) studied this question.