AI Summary
This article discusses the role of the cholinergic system in regulating Gonadotropin-releasing hormone (GnRH) neurons and luteinizing hormone (LH) secretion in male mice. The study reveals that activation of the cholinergic system increases LH secretion and that GnRH neurons are innervated by cholinergic axons originating from specific brain regions. The cholinergic neurotransmission has biphasic effects on firing rate in GnRH neurons, involving nicotinic and muscarinic acetylcholine receptors. Optogenetic techniques show that cholinergic axons can also release GABA in addition to acetylcholine. Overall, the study highlights the importance of cholinergic control in the central regulation of reproductive processes in male mice.
Gonadotropin-releasing hormone (GnRH)-synthesizing neurons orchestrate reproduction centrally. Early studies have proposed the contribution of acetylcholine (ACh) to hypothalamic control of reproduction, although the causal mechanisms have not been clarified. Here, we report that in vivo pharmacogenetic activation of the cholinergic system increased the secretion of luteinizing hormone (LH) in orchidectomized mice. 3DISCO immunocytochemistry and electron microscopy revealed the innervation of GnRH neurons by cholinergic axons. Retrograde viral labeling initiated from GnRH-Cre neurons identified the medial septum and the diagonal band of Broca as exclusive sites of origin for cholinergic afferents of GnRH neurons. In acute brain slices, ACh and carbachol evoked a biphasic effect on the firing rate in GnRH neurons, first increasing and then diminishing it. In the presence of tetrodotoxin, carbachol induced an inward current, followed by a decline in the frequency of miniature postsynaptic currents (mPSCs), indicating a direct influence on GnRH cells. RT-PCR and whole-cell patch-clamp studies revealed that GnRH neurons expressed both nicotinic (α4β2, α3β4, and α7) and muscarinic (M1–M5) AChRs. The nicotinic AChRs contributed to the nicotine-elicited inward current and the rise in firing rate. Muscarine via M1 and M3 receptors increased, while via M2 and M4 reduced the frequency of both mPSCs and firing. Optogenetic activation of channelrhodopsin-2–tagged cholinergic axons modified GnRH neuronal activity and evoked cotransmission of ACh and GABA from a subpopulation of boutons. These findings confirm that the central cholinergic system regulates GnRH neurons and activates the pituitary–gonadal axis via ACh and ACh/GABA neurotransmissions in male mice.