Rat melanin-concentrating hormone messenger ribonucleic acid expression: marked changes during development and after stress and glucocorticoid stimuli.

Melanin-concentrating hormone (MCH) is a cyclic neuropeptide first isolated from fish and rats. MCH may be involved in the control of the hypothalamic-pituitary-adrenocortical axis and, more generally, of specific goal-oriented behaviors and homeostatic functions in mammals. In this paper we examine 1) the cellular distribution of MCH gene transcripts in the rat central nervous system, 2) the changes in neuronal expression of MCH mRNA during rat development, and 3) the effects of stress and hormonal stimuli on rat MCH (rMCH) gene activity. Northern blot analysis and in situ hybridization histochemistry show that mature rMCH mRNA (1.0 kilobase) is very abundant in the zona incerta and the dorsolateral hypothalamus. While this is in agreement with previous peptide mapping by immunohistochemical techniques, a surprising new result is that a few clusters of rMCH mRNA-containing cells are found outside the hypothalamus, in the olfactory tubercle and the pontine tegmentum. Developmentally, rMCH mRNA is detected on embryonic day 18; its level increases gradually during early postnatal life and rises abruptly at weaning to reach a constant value in adult rats. In addition, striking variations in rMCH mRNA length occur during postnatal development and are found to be variations in the polyadenylate tail. Interestingly, this structural modification appears to be independent of the increase in rMCH mRNA levels. The regulation of rMCH mRNA expression by glucocorticoids and chronic stress is examined by Northern blot analysis. Chronic intermittent footshock stress causes a 58% or 29% decrease in rMCH mRNA content in the whole hypothalamus after a 1- or 3-day regimen, respectively. In contrast, the rMCH mRNA level returns to normal after a 7-day regimen. Two weeks after adrenalectomy (ADX) the whole hypothalamus rMCH mRNA content decreases 2.5-fold, but rises close to the control value 3 weeks after ADX. Dexamethasone administration 2 weeks after ADX not only reverses the fall in rMCH mRNA, it even provokes a slight increase (123% of control). No change in rMCH mRNA length is observed after chronic stress or ADX and dexamethasone injection. These results provide evidence for a negative regulation of rMCH gene expression by stress and suggest a major role for glucocorticoids in a positive feedback control of rMCH gene activity.