INSULIN-TREATMENT STIMULATES THE RAT MELANIN-CONCENTRATING HORMONE-PRODUCING NEURONS

Melanin-concentrating hormone (MCH) is involved in the regulation of b ody colour in teleost fish. A peptide highly homologous to salmon MCH has been found in the rat brain, but its physiological functions have not yet been precisely defined. The location of MCH neurons in the lat eral hypothalamus (LHT) of the rat suggests possible implication in fe eding behaviour. In the present study, immunohistochemical and in situ hybridization methods were used to investigate MCH gene expression fo llowing insulin injections. Five hours after insulin injection, a sign ificant increase in the abundance and staining intensify of MCH immuno reactive perikarya and fibres was observed. Concurrently the level of MCH mRNA significantly increased (50%). Insulin-treatment also induced a marked and progressive increase in the number and staining intensit y of nuclei detected by a Fos antiserum in LHT and other brain areas. Double labelling technique demonstrated that very few if any MCH neuro ns exhibited Fos-like immunoreactivity. These results demonstrate that an insulin-treatment stimulates MCH neuron activity without the media tion of the proto-oncogene c-fos. The mechanisms triggering this activ ation remain to be elucidated.