INTERACTION BETWEEN THE ALPHA(2)-ADRENERGIC SYSTEM AND NURSING IN THE REGULATION OF GROWTH-HORMONE SECRETION IN THE NEONATAL RAT

Separation of neonatal rats from their mothers decreases, while a subsequent period of suckling (nursing) increases, serum growth hormone (GH) levels in neonatal rats. Milk-borne (humoral) factors and neural factors inherent in mother-offspring interaction have been implicated in these phenomena. Conflicting reports have demonstrated the alpha2-adrenergic agonist clonidine to increase and to decrease serum GH levels in 10-day-old rats. The present experiments were aimed at testing whether an interaction between the alpha2-adrenergic system and the nursing-induced changes in GH secretion could account for the discrepancy. Rat pups were treated with clonidine (150 mug/kg) or the alpha2-adrenergic antagonist yohimbine (10 mg/kg), and the drug treatment was combined with separation of the mothers and nursing. Yohimbine did not affect serum GH levels in separated two-day-old pups (i.e. basal levels of the hormone), but prevented the nursing-induced increase in serum GH concentration. In two-day-old pups, clonidine had no effect on basal GH levels but, like yohimbine, prevented the increase in serum GH normally associated with nursing. Both yohimbine and clonidine prevented active sucking behavior, i.e. the pups did not search for and/or attach to the nipples of their mothers. Moreover, the pups treated with yohimbine and clonidine were cooler to the touch than the littermate controls. In eight-day-old pups, yohimbine prevented the nursing-induced increase in serum GH and decreased GH levels below the saline-injected, separated control. As in two-day-old pups, clonidine prevented the suckling-induced release of GH and failed to induce GH-release above that of saline-injected, separated pups. By day 10 postpartum, clonidine became capable of stimulating GH release, but only in separated male pups. The effects of CLO and nursing in male pups were not additive: either treatment alone was as effective as the combined treatment. In female rats CLO prevented the increase in serum GH levels in response to nursing. It is concluded that (1) the alpha2-adrenergic agents yohimbine and clonidine inhibit nursing-induced GH secretion in an indirect (perhaps hypothermia-related) manner not involving the well-established alpha2-adrenergic-GH releasing hormone pathway; (2) the alpha2-adrenergic system becomes fully functional in terms of stimulation of GH secretion between days 8 and 10; (3) the GH-releasing effects of the alpha2-adrenergic system are sexually dimorphic in 10-day-old rats.