A role for Melanin-Concentrating Hormone in learning and memory

Abstract

The neurobiological substrate of learning process and persistent memory storage involves multiple brain areas. The neocortex and hippocampal formation are known as processing and storage sites for explicit memory, whereas the striatum, amygdala, neocortex and cerebellum support implicit memory. Synaptic plasticity, long-term changes in synaptic transmission efficacy and transient recruitment of intracellular signaling pathways in these brain areas have been proposed as possible mechanisms underlying short- and long-term memory retention. In addition to the classical neurotransmitters (glutamate, GABA), experimental evidence supports a role for neuropeptides in modulating memory processes. This review focuses on the role of the Melanin-Concentrating Hormone (MCH) and receptors on memory formation in animal studies. Possible mechanisms may involve direct MCH modulation of neural circuit activity that support memory storage and cognitive functions, as well as indirect effect on arousal.

Figure 1. The Melanin-Concentrating Hormone system targets memory circuits of the brain

Schematic drawing of a saggital section through the rat brain showing the neuroanatomical organization of the MCH system. MCH-expressing cell bodies are restricted to the lateral hypothalamus and zona incerta. Arrows point out some of the more prominent terminal fields. Targets of the MCH neurons include brain region involve in explicit memory (medial temporal lobe) and implicit memory, in particular the neuronal circuits that participates in procedural and associative (classical/operant conditionning) and non-associative (habituation/sensitization) learning including the putamen, caudate nucleus, nucleus accumbens, amygdala, cortex with the exception of the cerebellum. Note that MCH neurons project to the main arousal center of the brain including LC (noradrenergic cells), histaminergic neurons of the posterior hypothalamus, cholinergic cells of the basal forebrain and brainstem and serotonine-producing neurons of the raphe. This suggests that MCH may modulate learning and/or memory processes by modulating arousal. Abbreviations used: Amy, amygdala; Ctx, cortex; H, Hypothalamus; Hipp, hippocampus; LC, locus coeruleus; OB, olfactory bulb; Sp Ch, spinal chord; Th, thalamus.