Opiate Receptors Modulate LHRH and SRIF Release from Mediobasal Hypothalamic Neurons

In order to investigate the effect of opiates on luteinizing hormone-releasing hormone (LHRH) and somatostatin (SRIF) release, mediobasal hypothalamic (MBH) slices of male adult rats were superfused at 37 °C with oxygenated Hepes-buffered Locke medium for 1 h. Bacitracin (2 × 10–5M) was added to prevent enzymatic degradation of neuro-peptides. 6 min pulse of K+ (56 mM) markedly stimulated the release of both neuropeptides. Opiates (β-endorphin 10–7M; D-ALA₂-Met-enkephalin amide 10–7M; Leuenkephalin 10–7 M; morphine 10–6 M) did not alter the spontaneous release of LHRH and SRIF, but significantly inhibited the K+-induced neuropeptides release. The effect was reversed by a specific opiate antagonist Naloxone 10–7M, which was ineffective when perifused alone. β-Endorphin and D-ALA₂-Met-enkephalin inhibited K+-evoked LHRH and SRIF release in a dose-dependent manner with a half maximal value of inhibition for concentrations of 7.6 ± 2.7 10–9 M and 9.4 ± 2.5 10–9M for LHRH release and 11 ± 2.2 10–8M and 5.2 ± 3.3 10–9M for SRIF, respectively. Addition of vasoactive intestinal peptide (VIP) (10-9 M) to the medium also inhibited significantly the K+-evoked release of SRIF, whereas that of LHRH was not affected. The data suggest: (1) that opiates, acting through specific opiate receptors located on LHRH and SRIF neurons, modulate the release of the neurohormones; (2) the inhibitory effect of opiates could be due to the inhibition of calcium influx through voltage-dependent calcium channels; (3) this interaction may account for the stimulation of growth hormone and the inhibition of luteinizing hormone observed after systemic administration of opiates; (4) VIP inhibits SRIF release, by acting on VIP receptors present on MBH SRIF terminals; the effect is consistent with the stimulation of GH reported after in vivo administration of the peptide.