Postnatal naltrindole treatments induce behavioural modifications in preweanling rats

To investigate the physiological role of the delta-opioid receptor during t he preweanling period, we have studied the effects of chronic (daily inject ions from birth to postnatal day 19) and acute treatments with the selectiv e delta-antagonist naltrindole (1 mg/kg), on behavioural and nociceptive re sponses in 20-day old male rats. Behavioural testing was performed using an open field paradigm. Acute naltrindole induced significant decreases in ex ternal and total ambulation (horizontal activity) and rearing behaviour (ve rtical activity), as well as a significant increase in grooming frequency. In animals chronically treated with naltrindole there was an increase in to tal ambulation one day after the discontinuation of the treatment. In a tes t of nociception (tail immersion) no significant effect of chronic naltrind ole treatment on baseline latencies or of acute naltrindole on latency quot ients (post-treatment latency/pre-treatment latency) were found. However, c hronic naltrindole administration significantly decreased the latency quoti ents. The results show that the delta-opioid receptor participates in the t onic regulation of motor activity during the preweanling period and might b e involved in certain aspects of stress responsiveness. (C) 2000 Elsevier S cience ireland Ltd. All rights reserved.