Rat diencephalic neurons producing melanin-concentrating hormone are influenced by ascending cholinergic projections

Innervation of diencephalic neurons producing melanin-concentrating hormone by choline acetyltransferase-containing axons was examined using double im munohistochemistry. In the rostromedial zona incerta and perifornical regio ns of the lateral hypothalamic area, many choline acetyltransferase-positiv e fibers were detected in the immediate vicinity of melanin-concentrating h ormone perikarya and their proximal dendrites. Putative contact sites were less abundant in the far lateral hypothalamus, and only scattered close to the third ventricle. After injections of the retrograde tracer FluoroGold, most of these projections appeared to originate in the pedunculopontine and laterodorsal tegmental nuclei. Finally, to determine the putative effect o f acetylcholine on the melanin-concentrating hormone neuron population, the cholinergic agonist carbachol was added to the medium of hypothalamic slic es in culture. Using competitive reverse transcriptase-polymerase chain rea ction, carbachol was found to induce a rapid increase in the melanin-concen trating hormone messenger RNA expression. This response was abolished by bo th atropine, a muscarinic antagonist, and hexamethonium, a nicotinic antago nist. Thus, the bulk of these results indicates that the diencephalic melan in-concentrating hormone neurons are targeted by activating ascending choli nergic projections. (C) 1999 IBRO. Published by Elsevier Science Ltd.