MEAL PATTERNS AND MACRONUTRIENT INTAKE AFTER PERIPHERAL AND PVN INJECTIONS OF THE ALPHA(2)-RECEPTOR ANTAGONIST IDAZOXAN

Studies with idazoxan (IDA), a specific alpha2-noradrenergic receptor antagonist, demonstrate effects on feeding behavior opposite to those observed with norepinephrine in the paraventricular nucleus (PVN) and peripheral injection of the alpha2 agonist clonidine. Administration o f IDA, both intraperitoneally (IP) and into the PVN at the onset of th e nocturnal feeding cycle. caused a dose-related, selective suppressio n of carbohydrate intake 90 min after injection. To characterize furth er the impact of this antagonist on macronutrient intake, we examined in IDA-injected animals the macrostructure of feeding using computer-a ssisted analyses of meal patterns. Both IP and PVN administration of I DA produced a selective suppression of carbohydrate intake, primarily during the first meal of the feeding cycle. This effect occurred throu gh significant reductions in meal size, diet composition, feeding time , and feeding rate for this nutrient. Idazoxan administration into the PVN continued to decrease carbohydrate intake in the next two meals a nd reduced the satiating impact of this nutrient. In contrast to this immediate change in carbohydrate intake. PVN IDA reduced protein intak e after a latency of 4 h, although fat intake was suppressed only afte r a latency of 7 h. An increase in total meal number and a decrease in the average meal size across the 12-h dark cycle were seen after PVN IDA administration. These results, showing effects of peripheral and P VN-injected IDA on carbohydrate intake. suggest a possible physiologic al role of endogenous PVN alpha2-noradrenergic receptors in modulating natural patterns of carbohydrate feeding at the onset of the dark per iod.