STIMULATORY EFFECT OF MELANIN-CONCENTRATING HORMONE ON LUTEINIZING-HORMONE RELEASE

Alpha-melanocyte-stimulating hormone (alpha-MSH) and melanin-concentra ting hormone (MCH) are two peptide neurotransmitters widely distribute d in the mammalian brain, the former originating mainly from cell bodi es in the arcuate nucleus and the latter from cell bodies in the zona incerta and lateral hypothalamus. Within the hypothalamus they innerva te the pre-optic area, median eminence (ME) and ventromedial nucleus ( VMN). Both peptides stimulate sexual behaviour and in this report we h ave investigated their effect on another gonadal steroid-dependent fun ction, luteinising hormone (LH) release. alpha-MSH, MCH or a combinati on of the two were injected bilaterally (100 ng/side) into either the medial pre-optic area (MPOA), ME, or VMN of anaesthetised (Saffan 3 ml /kg i.p.) rats that had previously been ovariectomised and adrenalecto mised (O+A) and then primed with 5 mu g/rat s.c. oestradiol benzoate ( OB), 48 h before peptide administration. MCH stimulated LH release whe n applied to the MPOA and ME; alpha-MSH was inhibitory in the ME and i n this model was ineffective in the MPOA. Neither peptide was effectiv e in the VMN. The two peptides were then injected into the MPOA of O+A rats primed with OB followed 48 h later by 0.5 mg/rat s.c. progestero ne, which normally induces an LH surge. alpha-MSH, but not MCH, inhibi ted this induced rise in LH. Administration of anti-MCH antiserum (0.5 mu g/side neat serum) also had an inhibitory effect on LH release in this model. These results show that MCH has a stimulatory effect on LH release when administered into the ME and MPOA. In the MPOA, this may be physiologically significant since blocking endogenous MCH with an anti-MCH antiserum inhibits LH release. On the other hand, alpha-MSH h as an inhibitory effect on LH release in the MPOA and ME. In the teleo st skin these two peptides are functionally antagonistic; it seems tha t a similar antagonism exists between their effects on LH release.