No abstract

Several molecular forms related to the decapeptide LHRH were characterized and quantified in various brain structures of intact and castrated male and female rats. Distinct moieties were separated by high performance liquid chro-matography (HPLC) and radioimmunoassayed against anti-LHRH antibodies of different specificities. The hypothalamus contained the highest concentration of LHRH-like material detected by the antisera. The predominant (89%) molecular form recovered from that structure was LHRH itself; 9% of the material corresponded to [hydroxyproline⁹]LHRH ([Hyp⁹]LHRH), an endogenous posttranslational product of the LHRH precursor, and the residual immunoreactivity was accounted for by C-terminal fragments of both decapeptides, as assessed after labelling HPLC columns with appropriate synthetic or endogenous hypothalamic peptides. The proportions were the same in both sexes and were not affected by castration, in spite of a lesser overall LHRH activity in females and in castrates. LHRH and [Hyp⁹]LHRH were also detected in the olfactory bulb and the hippocampus. In these structures however, most (97%) LHRH-related molecules corresponded to C-fragments derived from [Hyp⁹]LHRH, whereas only very few fragments derived from the nonhydroxylated decapeptide were found. Sex or castration affected neither total nor relative concentrations of LHRH-derived molecules in the olfactory bulb and the hippocampus. Taken altogether, these observations are suggestive of a different LHRH metabolic regulation in neurons projecting to either the median eminence or extrahypothalamic areas. In the latter case, larger amounts of the LHRH precursor appear processed to [Hyp⁹]LHRH. Recovery of relatively high concentrations of [Hyp⁹] LHRH C-fragments in the olfactory bulb and the hippocampus reflects the higher resistance of the Hyp⁹-Gly¹⁰-NH₂ than the Pro⁹-Gly¹⁰-NH₂ peptidic bond to hydrolysis by the postproline cleaving enzyme. In view of reports that intracerebral administration of C-terminal fragments of LHRH are able to trigger sex behavior, our finding that extrahypothalamic structures contain relatively high concentrations of the [Hyp⁹]LHRH-derived, more stable C-fragments suggests that these catabolites may have a role in the regulation of sex behavior.

Long-term effects of subcutaneous melatonin implants on intrahypothalamic LHRH content and on pulsatile luteinizing hormone release have been investigated in ganglionectomized male mink. Animals were submitted to bilateral removal of the superior cervical ganglion in mid-April. A preliminary study revealed that plasma LH concentrations remain at a basal level throughout the year following ganglionectomy. In a second experiment, one month after ganglionectomy and transfer from the natural photoperiod environment to short daylengths (LD 4:20), melatonin pellets were subcutaneously implanted to overcome deafferentation of the pineal. Progressive effects of treatment were studied 7 days, 15 days, and one, two and three months after insertion of the melatonin implants. The intra-hypothalamic LHRH content in ganglionectomized mink was at a basal level similar to that observed during seasonally sexual quiescence, or after exposure to inhibitory long days (LD 20:4). A significant and transient elevation in LHRH content was observed already after fifteen days, and also one month after insertion of melatonin implants. This resulted in mean values similar to those observed during the breeding season, or after exposure to stimulatory short days (LD 4:20). A decrease in hypothalamic LHRH content started after two months. No pattern of pulsatile LH secretion was recorded in ganglionectomized untreated mink. A significant increase in all parameters of pulsatile LH secretion was observed fifteen days after the elevation of LHRH content induced by melatonin treatment, and maximum values were reached after two months. Pituitary activity tended to decrease after three months, characterized in particular by a significant decrease in the mean frequency of LH pulses. In addition, the increase in pulsatile characteristics of LH release occurred two months before the peripheral renewal of testicular activity. Apparently, the reproductive endocrine function in ganglionectomized mink treated with melatonin implants is restored more rapidly at the hypothalamic level than at the pituitary or testicular levels.

No abstract