A puberty-related attenuation of opiate peptide-induced inhibition of LH secretion

The long-acting synthetic enkephalin FK 33-824 (FK) potently inhibits LH secretion in gonadectomized prepubertal male and female rats. The effect is reversible by coinjection of naloxone. We have observed an age-related decrease in the ability of FK to reduce LH secretion in the 48 h post-gonadectomized rat. Thus, the IC50 values (i.e. the dose required to give 50% reduction in LH levels) for 12, 23 and 33-day old female rats were 0.03, 0.04 and 0.17 mg/kg BW, respectively. The figures for pre- and post-pubertal male rats (ages 26 and 70 days) were 0.25 and 0.89 mg/kg BW, respectively. This attenuation of opiate-induced inhibition of LH release may provide a neurochemical basis for the well-described shift in sensitivity of gonadal steroid feedback which occurs with the approach of puberty.