Cardiovascular and behavioural components of conditioned fear to context after ganglionic and α-adrenergic blockade

This study investigates the contribution of the peripheral nervous system to the cardiovascular component of long lasting (40 min) conditioned fear responses to context. The conditioned fear response evoked by reexposure to a footshock chamber was tested 10 min after intravenous injection of either the nicotinic ganglion blocker chlorisondamine (0.6 mg/kg) or the α-adrenoceptor antagonist phentolamine (10 mg/kg) in six rats implanted with telemetric probes. Compared to saline controls, chlorisondamine did not change the behavioural component of the response (freezing, ultrasonic vocalizations) but almost completely abolished its cardiovascular component (mean arterial pressure and heart rate). Phentolamine also abolished the pressor response but increased the cardiac response, and ultrasonic vocalizations were reduced by half. The results indicate that the long lasting pressor response of conditioned fear to context is sympathetically mediated like the much shorter pressor response of conditioned fear to a discrete stimulus.