Histamine plays no part in schedule-induced polydipsia in the rat

Twelve Sprague-Dawley male albino rats were tested with or without combined antagonism of peripheral H1 (using 2 mg/kg dexbrompheniramine IP) and H2 (using 32 mg/kg cimetidine IP) receptors for histamine prior to (a) drinking after 2.5 mg/kg histamine SC, (b) drinking after 24-hr water deprivation, and (c) drinking during the acquisition and maintenance of schedule-induced polydipsia (SIP) with a 45 mg Noyes pellet delivered every 90 sec. Such antagonism of histamine receptors abolished drinking elicited by exogenous histamine without inhibiting drinking after water deprivation. Moreover, histaminergic antagonism failed to prevent the acquisition and maintenance of SIP and failed to alter the distribution of contacts with the drinking spout during interpellet intervals. These findings demonstrate no role for endogenous systemic histamine in SIP.