RESUMO
OBJECTIVE: This study was designed to evaluate the effects of nonselective opioid antagonist naltrexone on the expression of tyrosine hydroxylase (TH) in variable areas of hypothalamus in rats with chronic ingestion of 5% ethanol using immunohistochemical measures. METHODS: To induce polydipsia with 5% ethanol, Spraque-Dawley rats were placed in automatic cage where a pellet dispenser automatically dispensed 90 mg pellets at fixed time 60 seconds (FT 60s) feeding schedule over 150-minute test session. After 4 weeks of daily exposure to the FT 60s feeding schedule, experimental rats were administered naltrexone (0.25 mg/kg, i.p), vehicle (1 cc/kg, i.p) for 3 weeks. After completing the 3 weeks of naltrexone and vehicle injections, the polydipsic rats were sacrificed. The brains were removed and postfixed in the same overnight fixation, then frozen sections of 40microM thickness were made in the coronal plane. Sections were stained for detection of tyrosine hydroxylase (H) according to the immunohistochemical method. RESULTS: 1) Both experimental animals with schedule-induced polydipsia (IP) and the bolus with 5% ethanol control showed significant increase in the amounts of 5% ethanol ingestion as compared with their baseline. The naltrexone treated group showed significant decrease in the amount of 5% ethanol ingestion at 2nd and 3rd week as compared with their baseline. Meanwhile, the vehicle control showed no changes in the amount of 5% ethanol ingestion for 3 weeks as compared with their baseline. 2) There was diffused and definite decreases in the TH immunoreactive cells in the bolus control with chronic ingestion of 5% ethanol. The SIP with water group showed marked increase in TH immunoreactive cells in the paraventricular nucleus and the periventricular hypothalamic nucleus. The SIP with 5% ethanol group showed definite decrease of TH immunoreactive cells in the paraventricular nucleus and the periventricular hypothalamic nucleus. The naltrexone treated group showed significant increase of TH immunoreactive cells in the paraventricular nucleus but no changes in the periventricular hypothalamic nucleus. CONCLUSION: These results suggest that the fixed time feeding procedure for schedule induced polydipsia as an animal model of alcoholism was not suitable. The author identified that naltrexone has suppressed the ingestion of ethanol. The chronic ingestion of 5% ethanol suppress the TH immunoreactive cells in the paraventricular nucleus and the periventricular hypothalamic nucleus. Naltrexone increases the TH immunoreactive cells which was suppressed by chronic ingestion of 5% ethanol in the paraventricular nucleus.
