A 'window of time' during which testosterone determines the opiatergic control of LH release in the adult male rat

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Abstract
Summary. Male rats castrated before puberty (when 26 days of age) showed a progressively decreasing susceptibility to the inhibitory effects of morphine (5 mg/kg) upon LH secretion for up to 28 days after gonadectomy (∼ 100%, 40% and 10% inhibition at 5, 12 and 28 days after castration), but thereafter morphine again caused ∼50% reduction in serum LH values; the minimum inhibition found at 28 days after castration (age 54 days) occurred at the time at which male rats normally reach puberty. When rats were castrated at 59 days of age, morphine maximally suppressed serum LH concentrations (to 100%) in castrated rats of all ages, regardless of the time elapsed after gonadectomy. These results indicate (1) a transient fall in the inhibitory opioidergic tone upon LH secretion as the normal age of puberty approaches, (2) that the ability of opiates to alter LH release in adulthood may depend upon testicular steroids secreted during the peripubertal period, and (3) that the LH responses do not reflect general changes in the neuroendocrine response to opiates after castration since the prolactin response to morphine remains intact in rats castrated before and after puberty.