Morphine but not Naloxone Enhances Luteinizing Hormone-Releasing Hormone Neuronal Responsiveness to Norepinephrine

Some axon terminals of hypothalamic opiate neurons directly synapse on luteinizing hormone-releasing hormone (LHRH) neurons. To determine whether such synaptic connections affect LHRH neuronal activity, we have examined the profiles and concentrations of LH released in response to intracerebroventricular (icv) norepinephrine (NE, 45 μg) infusions alone or following medial preoptic area (MPOA) electrochemical stimulation (ECS) in estrogen-treated ovariectomized rats. Similar studies were performed in rats treated with naloxone (5 mg/kg ip) or morphine (20 mg/kg sc) given 15 min prior to MPOA-ECS or 30 min prior to icv NE. Naloxone neither augmented nor suppressed the LH response obtained with NE alone. MPOA-ECS evoked a significant increase in plasma LH. When the preoptic area was stimulated (0 min) and NE was infused at 30 min, a significant amplification of LH release occurred. Prior treatment of rats (-15 min) with naloxone had no effect on LH responses elicited by either preoptic stimulation alone or combined with icv NE. In the second study, morphine was given sc and had no effect on basal plasma LH levels. However, when morphine was given (-15 min) and icv NE infusions were made (30 min), the rise in plasma LH induced by NE was significantly enhanced. Preoptic ECS (0 min) evoked a rise in plasma LH and this response was also enhanced by morphine pretreatment. The major effect on LH release occurred when sc morphine injections (-15 min) were combined with MPOA-ECS (0 min) followed by icv NE (30 min). In these rats, a remarkable and highly significant release of LH occurred which reached peak levels even greater than those observed during spontaneous LH surges (2,392 versus 16 to 1,800 ng/ml). Since morphine has profound effects on the serotonergic system, in the third series of studies, morphine was infused into the dorsal raphe nucleus (DRN) and LH responses to MPOA-ECS or icv NE alone or following combined ECS + NE were examined. DRN morphine did not affect basal LH release but it produced a rapid and highly significant rise in plasma prolactin. When DRN morphine was given (-15 min) and NE was infused icv (30 min), there was marked amplification in LH release compared to those values observed after only NE. However, there were no appreciable differences in LH values obtained after sc versus DRN morphine injections in response to NE. Similarly, the amplification of LH release which occurred following DRN morphine (-15 min) + MPOA-ECS (0 min) was not different from that obtained after sc morphine. In the final group of rats, DRN morphine was given (-15 min), the preoptic area was stimulated (0 min) and NE was infused (30 min). Following this treatment, plasma LH release was also markedly enhanced and did not differ appreciably (except at 60 and 120 min) from the levels of LH released after sc morphine. Prolactin concentrations rose slowly after icv NE to reach peak levels 75 min after treatment. Combinations of morphine + MPOA-ECS without or with NE neither augmented nor suppressed the high prolactin concentrations achieved after only DRN morphine infusions. We conclude from these data that: 1) those opiate neuronal terminals which synapse directly on LHRH neurons do not affect LHRH neuronal responsiveness to either NE, to MPOA-ECS or to combined preoptic stimulation+ NE, and 2) morphine has profound effects on LHRH neuronal responsiveness to both NE, to MPOA-ECS and, in particular, to combined ECS + NE. Since amplification of LH release occurs after treatment of rats with morphine either by sc injections or DRN infusions, the augmented LH and prolactin responses observed are most likely due to the morphine-induced release of serotonin and not to direct morphine effects on LHRH neurons.