Stimulatory effect of melanin-concentrating hormone on luteinising hormone release.

Alpha-melanocyte-stimulating hormone (alpha-MSH) and melanin-concentrating hormone (MCH) are two peptide neurotransmitters widely distributed in the mammalian brain, the former originating mainly from cell bodies in the arcuate nucleus and the latter from cell bodies in the zona incerta and lateral hypothalamus. Within the hypothalamus they innervate the pre-optic area, median eminence (ME) and ventromedial nucleus (VMN). Both peptides stimulate sexual behaviour and in this report we have investigated their effect on another gonadal steroid-dependent function, luteinising hormone (LH) release. alpha-MSH, MCH or a combination 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 adrenalectomised (O+A) and then primed with 5 microg/rat s.c. oestradiol benzoate (OB), 48 h before peptide administration. MCH stimulated LH release when applied to the MPOA and ME; alpha-MSH was inhibitory in the ME and in this model was ineffective in the MPOA. Neither peptide was effective 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. progesterone, which normally induces an LH surge. alpha-MSH, but not MCH, inhibited this induced rise in LH. Administration of anti-MCH antiserum (0.5 microg/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 has an inhibitory effect on LH release in the MPOA and ME. In the teleost skin these two peptides are functionally antagonistic; it seems that a similar antagonism exists between their effects on LH release.