Hypotensive stress retards associative learning in rabbits

This study examined the effects of hypotensive stress on classical conditioning of the rabbit's nictitating membrane response. Hypotension, consisting of an approximately 45% decrease in blood pressure, was maintained for 30 min by the i.v. infusion of sodium nitroprusside. Twenty minutes later animals were exposed to a conditioning session consisting of 60 pairings of a 200 ms tone conditioned stimulus with a 100 ms airpuff unconditioned stimulus directed at the cornea. This procedure was repeated for four consecutive days. Animals exposed to the hypotensive stress demonstrated a significantly retarded acquisition of conditioned responses as measured by their frequency and onset latency as well as by an increase in the number of trials required to reach acquisition criteria of five and 10 consecutive conditioned responses as compared with controls. A separate group of animals received a nitroprusside infusion one day after the acquisition of conditioned responses to the tone conditioned stimulus. These animals demonstrated a normal retention of conditioned responses and a normal response to varying intensities of the conditioned stimulus. Hypotensive stress also had no effect on the frequency and topography of the unconditioned response. It was concluded that a decrease in blood pressure can serve as a physiological stressor. One of the reactions to this stress consists of a retardation in the formation of associations during a learning task, without any decrease in the ability to retrieve previously learned material.