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 K_d or B_max of opiate binding sites in whole male rat brain, or in the hypothalamus, utilizing H- 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.