Effects of different opiates on hypothalamic monoamine turnover and on plasma LH levels in pro-oestrous rats

Opiate inhibition of luteinizing hormone (LH) appears to involve changes in hypothalamic monoaminergic activity. Agonists of mu-, kappa- and sigma-opioid receptors and an opiate antagonist were administered at the onset of the preovulatory LH surge and their effects on hypothalamic monoamine turnover, and on plasma LH levels, investigated. The opiate antagonist, naloxone, significantly increased both noradrenaline (NA) turnover and plasma LH levels. Morphine (mu-agonist), significantly decreased NA concentration and plasma LH levels, but significantly increased dopamine (DA) and serotonin (5-HT) activity. Levorphanol (another mu-agonist) significantly decreased both NA and 5-HT concentrations and had no effect on circulating LH. Cyclazocine, ketocyclazocine and tifluadom (kappa-agonists) increased NA turnover but only tifluadom increased 5-HT turnover, also reducing LH levels significantly. N-Allylnormetazocine (SKF 10,047; sigma-agonist) increased 5-HT activity but did not alter LH levels. This study has confirmed the existence of a heterogenous group of opioid receptors within the hypothalamus which modulate monoamine neurotransmitters controlling LH release.