Shock-prod burying test in rats: Autonomic and behavioral responses

Abstract

Heart rate (HR), core temperature (CT) and gross locomotor activity (GLA) were recorded in rats by a computerized telemetry system before, during and after 15-min exposure to a constantly electrified (2mA) or nonelectrified (0mA) prod which was presented through a hole in the home cage with woodshavings on the floor [shock-prod burying (SPB) test]. Measurement of both autonomic and behavioral responses continued up to 60 min after introduction of the prod into the home cage. Twenty-four h after the SPB test, rats were tested for retention. During the SPB test, significant increases in HR, respective to baseline values, were observed in both the 2mA and 0mA group, but the tachycardiac response in the former group significantly outweighed that in the latter group. In shocked rats, HR remained elevated during the entire 45-min posttest period, whereas HR declined immediately after removal of the prod in nonshocked animals. An onset in rise in CT was observed in the 2mA, but not in the 0mA group, during prod exposure time, which eventually resulted in an additional—more than two-fold—increase after termination of the test and outlasted total recording time. Marked behavioral activation was observed in the 2mA group, whereas the 0mA responded to a lesser extent. Approach and exploration of the prod was reduced when the prod was electrified. The total time spent by rats burying the prod was significantly longer in the shocked than in the nonshocked group. When exposed to the nonelectrified prod after 24 h, total burying time was reduced in the 2mA group but still significantly longer than in the 0mA group. On this occasion, shocked rats showed a similar overall pattern in HR, CT and GLA reponses as on day 1, with concomitant behavioral activation up to 1 h after the onset of the retention trial, all increases, however, being less pronounced. Conversely, the 0mA group showed no significant response when tested for retention. However, detailed analysis of the first minute of prod reexposure revealed a tendency towards bradycardia in both groups, with HR deceleration being most marked in the shocked rats. These results demonstrate that a mild novelty stress (0mA) as well as an aversive stimulus (2mA) induces simultaneous changes in autonomic and behavioral responses in rats, the magnitude and duration of which being dependent on the nature of the stimulus. A (conditioned) responding pattern of both autonomic and behavioral parameters, similar to that observed on day 1, was observed during the 24-h retention trial. In the present study, we have demonstrated differential involvement of the ANS in various stress-induced behavioral states in rats. The SPB test, used together with a wireless telemetry system, enables the evaluation of genuine stress responses under well-defined and controlled stress conditions.