H-1, H-2, AND H-3 RECEPTORS CONTRIBUTE TO DRINKING ELICITED BY EXOGENOUS HISTAMINE AND EATING IN RATS

Roles for H-1, H-2, and H-3 receptor subtypes for drinking elicited by exogenous histamine and drinking elicited by eating was examined in a dult male Sprague-Dawley rats. Drinking elicited by SC 5 mg/kg histami ne was: (a) inhibited approximately 30% by H-1 antagonism using IP 1 m g/kg dexbrompheniramine (DXB); (b) inhibited approximately 30% by H-2 antagonism using IP 16 mg/kg cimetidine (C); (c) inhibited approximate ly 40% by H-3 antagonism using SC 10 mg/kg thioperamide (Th); (d) inhi bited approximately 80% by combined H-1 and H-2 antagonism using IP DX B plus IP C; (e) inhibited approximately 85% by combined H-1 and H-3 a ntagonism using IP DXB plus SC Th; (f) inhibited approximately 70% by combined H-2 and H-3 antagonism using IP C plus SC Th; and (g) abolish ed by combined H-1, H-2, and H-3 antagonism using IP DXB plus IP C plu s SC Th. For rats eating pellets and drinking after 24-h food deprivat ion: (a) systemic injections of DXB, C, and Th, sufficient to abolish drinking elicited by SC histamine, inhibited water/food ratio (W/F) by approximately 20%; (b) ICV injections (through a chronic cannula in a lateral ventricle) of 50 mu g DXB plus 100 mu g C plus 60 mu g Th inh ibited W/F by approximately 20%. For rats drinking after IG infusion ( through a chronic gastric catheter) of 2 ml 1800 mOsm/kg NaCl: (a) sys temic injections of DXB, C, and Th, sufficient to abolish drinking eli cited by SC histamine, inhibited water intake by approximately 70%; (b ) IP DXB alone and IP C alone failed to inhibit water intake; (c) IP T h alone inhibited water intake by approximately 20%; (d) IP DXB combin ed with IP C inhibited water intake by approximately 55%. The results demonstrate the involvement of H-1, H-2, and H-3 receptors for drinkin g elicited by exogenous histamine, and our findings extend the evidenc e for a role for endogenous histamine and H-1, H-2, and H-3 receptor s ubtypes for drinking elicited by eating, including drinking elicited b y gastrointestinal osmotic consequences of eating that can increase sy stemic plasma osmolality.