Separate controls for conditioned cephalic phases of acid secretion and drinking in the rat

Adult male Sprague-Dawley rats, surgically equipped with a stainless steel gastric cannula, were tested in a paradigm known to support conditioning of cephalic phase gastric acid secretion. Rats were tested first under (a) baseline conditions—prepared to sham feed, but no food was offered and none should have been expected; then (b) 5 conditioning trials—prepared and allowed to sham feed sweetened milk; and finally (c) extinction trials—prepared to sham feed milk, but milk was not offered. Combined antagonism of H 1 and H 2 histamine receptors (using intraperitoneal dexbrompheniramine plus cimetidine) prevented the acquisition of conditioned cephalic phase of acid secretion, but had no effect on acquisition of conditioned water intake. Such histaminergic antagonism also blocked the expression of an established conditioned acid secretory response without effect on conditioned water intake; peripheral cholinergic blockade using atropine methyl nitrate had virtually the same effect as did histaminergic antagonism. The converse was observed following blockade of angiotensin II synthesis using subcutaneous captopril: Established conditioned secretion was unaffected, but conditioned water intake was inhibited. These results dissociate the physiological controls for conditioned responses which occur simultaneously in rats anticipating eating: Peripheral histamine mediates the acquisition and expression of the conditioned cephalic phase of acid secretion, whereas angiotensin II appears to mediate at least the expression of conditioned water intake.