Inhibitory actions of melanin-concentrating hormone in the lateral septum

Abstract

Melanin-concentrating hormone (MCH) neurons can coexpress several neuropeptides or neurotransmitters and send widespread projections throughout the brain. Notably, there is a dense cluster of nerve terminals from MCH neurons in the lateral septum (LS) that innervate LS cells by glutamate release. The LS is also a key region integrating stress- and anxiety-like behaviours that are also emerging roles of MCH neurons. However, it is not known if the MCH peptide acts within the LS or whether MCH target sites are localized. We analysed the projections from MCH neurons in male and female mice anteroposteriorly throughout the LS and found spatial overlap between the distribution pattern of MCH-immunoreactive (MCH-ir) fibers with MCH receptor Mchr1 mRNA hybridization or MCHR1-ir cells. This overlap was most prominent along the ventral and lateral border of the rostral part of the LS (LSr). Most MCHR1-labeled LS neurons laid adjacent to passing MCH-ir fibers, but some MCH-ir varicosities directly contacted the soma or cilium of MCHR1-labeled LS neurons. We thus performed whole-cell patch-clamp recordings from MCHR1-rich LSr regions to determine if and how LS cells respond to MCH. Bath application of MCH to acute brain slices activated a bicuculline-sensitive chloride current that directly hyperpolarized LS cells. This MCH-mediated hyperpolarization was blocked by calphostin C and suggested that the inhibitory actions of MCH were mediated by protein kinase C-dependent activation of GABAA receptors. Taken together, these findings defined potential hotspots within the LS that may elucidate the contributions of MCH to stress- or anxiety-related feeding behaviours. Key pointsO_LIRESEARCH QUESTION. Melanin-concentrating hormone (MCH) neurons have dense nerve terminals within the lateral septum (LS), a key region underlying stress- and anxiety-like behaviours that are emerging roles of the MCH system, but it is not known if the LS is a MCH target site. C_LIO_LINEUROANATOMY. We found spatial overlap between MCH-immunoreactive fibers, Mchr1 mRNA, and MCHR1 protein expression especially along the lateral border of the LS. C_LIO_LIELECTROPHYSIOLOGY. Within MCHR1-rich regions, MCH directly inhibited LS cells by increasing a chloride conductance in a protein kinase C-dependent manner. C_LIO_LISIGNIFICANCE. Electrophysiological MCH effects in brain slices have been elusive and even fewer have described the mechanisms of MCH action. Our findings demonstrated, to our knowledge, the first description of MCHR1 Gq-coupling in brain slices, which was previously predicted in cell or primary culture models only. Together, these findings defined hotspots and mechanistic underpinnings for MCH effects such as in stress- and anxiety-related behaviours. C_LI