The effects of simultaneous or separate infusions of some pro-opiomelanocortin-derived peptides (β-endorphin, melanocyte stimulating hormone, and corticotrophin-like intermediate polypeptide) and their acetylated derivatives upon sexual and ingestive behaviour of male rats

Abstract

Intraneuronal post-translational cleavage of pro-opiomelanocortin yields a variety of peptides including β-endorphin, melanocyte stimulating hormone and corticotrophin-like intermediate polypeptide, some of which are subsequently N-acetylated. Such peptides may be co-released from neuronal terminals, and so these experiments explored the effects of co-administration of some of them on sexual behaviour in the male rat, which is known to be sensitive to hypothalamic infusions of β-endorphin. Peptides were infused into the pre-optic-anterior hypothalamic area bilaterally in doses up to 320 pmol, and males allowed access to a sexually receptive female and/or a sweet solution (0.1% Acesulfame-K) for 15min, so that both sexual and ingestive behaviour could be studied. β-Endorphin(1–31) by itself inhibited sexual interaction, confirming our previous data. Acesulfame-K ingestion was inhibited in control-infused rats in the presence of a female, but this inhibition was released when sexual behaviour was itself diminished by β-endorphin(1–31). Both the acetylated and nonacetylated forms of melanocyte stimulating hormone (α-melanocyte stimulating hormone and des-acetyl melanocyte stimulating hormone) stimulate sexual behaviour; latencies both to ejaculation and to resumption of copulatory behaviour after an ejaculation (post-ejaculatory interval) were reduced. However, infusions of either corticotrophin-like intermediate peptide or N-acetylated β-endorphin(1–31) had no effect on either sexual or ingestive behaviour. Infusion of either acetylated melanocyte stimulating hormone or des-acetyl melanocyte stimulating hormone mixed with β-endorphin(1–31) prevented the inhibitory effect of the latter on sexual behaviour. Dose-response studies showed that the behavioural effect of such mixtures depended upon the molar ratios of the two peptides, rather than their absolute concentrations. The higher the ratio in favour of α-melanocyte stimulating hormone or des-acetyl melanocyte stimulating hormone, the greater the display of sexual behaviour. Infusing either corticotrophin-like intermediate polypeptides or N-acetyl β-endorphin(1–31) with β-endorphin(1–31) did not prevent the inhibition of sexual activity expected with β-endorphin(1–31) alone. These results are discussed in terms of the functional consequences of co-release of proopiomelanocortin peptides from hypothalamic nerve terminals.