Exogenous and endogenous opioid-induced enhancements of naloxone's effects on serum luteinizing hormone levels in the male rat.

We have previously shown that brief periods of exposure to opiate alkaloid drugs markedly enhance the subsequent effects of the opiate antagonist, naloxone, on serum luteinizing hormone (LH) levels in the male rat. In the present studies, we have found that this phenomenon is not simply a property of opiate drugs, but can be produced by a metabolically stable analog of an endogenously occurring opioid peptide, methionine enkephalin (FK 33-824). These findings suggest that alterations in the sensitivity of those opioid receptors involved in LH-releasing hormone (LHRH) generated in our experimental paradigm may occur under in vivo conditions, particularly since it now appears that endogenous opioids are released in an episodic manner like most neurotransmitter/neuromodulators. We also attempted to more fully characterize the factors responsible for the development of opiate-induced enhancements of naloxone's effects on LH. We found that this effect was produced only by those doses of morphine which initially suppressed serum LH levels, followed by a "rebound" increase in the gonadotropin 6-8 h later. A modest facilitation of LHRH-evoked increases in serum LH was also observed, but our data suggest that this represents only a minor component of opiate-induced enhancements of naloxone's effects. These data indicate that hypothalamic or suprahypothalamic sites are the major loci involved, but no differences in the uptake or regional distribution of naloxone in brain have been previously found, as a function of morphine pretreatment, nor were we able to demonstrate any alterations in opiate binding sites in the hypothalamus or whole brain. Thus, the mechanisms involved in this effect remain unclear.

Testosterone does not influence opiate binding sites in the male rat brain.

It has been reported previously that castration produces testosterone-reversible increases in the density of 3H-naltrexone binding sites in the male rat brain. Unfortunately, we were unable to replicate these observations in a comprehensive series of studies. Specifically, we found that castration failed to produce changes in the Kd or Bmax of opiate binding sites in whole male rat brain, or in the hypothalamus, utilizing 3H-dihydromorphine (a mu receptor ligand), 3H-D-alanine, D-leucine enkephalin (delta) or 3H-naltrexone (ubiquitous). Furthermore, we found that the relative proportion of mu and delta binding sites in brain was unchanged by castration. The reasons for the discrepancy between the present results and those previously reported are unclear, but it appears that the provocative hypothesis that testosterone influences opioid receptors in brain must be carefully reevaluated.

Ethanol inhibits the naloxone-induced release of luteinizing hormone-releasing hormone from the hypothalamus of the male rat.

It has been inferred that ethanol suppresses the secretion of luteinizing hormone (LH) in the male by depressing the release of LH-releasing hormone (LH-RH) from the hypothalamus. Direct support for this inference has been difficult to obtain, however, because of significant technical difficulties in measuring LH-RH release under in vivo conditions. To circumvent these problems, we made use of the opiate antagonist naloxone, as a neuroendocrine probe, to elicit the release of LH-RH under in vivo conditions. We found that ethanol was a potent suppressor of the increase in serum LH levels evoked by naloxone at extremely low blood ethanol concentrations ( less than 60 mg/dl). Furthermore, we observed that the antagonism between ethanol and naloxone appeared to be competitive in nature since a fixed dose of ethanol (1 g/kg, blood ethanol concentration 60 mg/dl) shifted the naloxone dose-response curve significantly to the right and high doses of the antagonist overcame ethanol's effects. Finally, we found that the interaction between ethanol and naloxone took place at the level of the hypothalamus. Our results, therefore, seem to provide the first in vivo evidence supporting the widely-held hypothesis that ethanol reduces serum LH levels by depressing the hypothalamically-medicated release of LH-RH. The mechanisms underlying ethanol's depression of naloxone-induced increases in the release of LH-RH are not fully understood at this time, but one prominent possibility is that ethanol enhances the synthesis or release of endogenous opioids which in turn override naloxone's effects.