Preweanling naltrindole administration differentially affects clonidine induced antinociception and plasma adrenaline levels in male and female neonatal rats

1 The influence of a chronic treatment with the delta-selective opioid anta gonist naltrindole (1 mg kg(-1)) during the preweanling period (daily injec tions from birth to postnatal day 19), on the antinociceptive and sympathol ytic effects of the alpha(2)-adrenergic agonist clonidine in male and femal e rats of 20 and 25 days of age was investigated. 2 Nociception was assessed using the tail immersion test (water at 50 degre es C) and plasma levels of adrenaline were measured by high-performance liq uid chromatography. 3 The dose of clonidine (1.5 mg kg(-1)) and the time point at which nocicep tive responses were recorded (30 min after the administration of the drug) were chosen on the basis of dose-response (0.5, 1, 1.5 and 2 mg kg(-1)) and time-response (5, 10, 15, 30 and 60 min) curves which were previously carr ied out in naive control neonatal rats. 4 In females, the functional blockade of the delta-receptor by neonatal nal trindole treatment did not modify the sympatholytic effect of clonidine but prevented clonidine induced antinociception. Conversely, in males naltrind ole treatment allowed the appearance of clonidine antinociception and the s ympatholytic effect of clonidine. 5 The results indicate that the delta-receptor is involved in the modulatio n of antinociceptive and sympatholytic responses to clonidine in neonatal r ats and suggest the existence of sex differences in the interactions betwee n delta-opioid and alpha(2)-adrenergic receptors.