The effects of clonidine, prazosin, and propranolol on short-term and long-term habituation of the acoustic startle response in rats.

The unique effect of clonidine in facilitating habituation of the acoustic startle response [10] was replicated. However, clonidine had no effect on between-session habituation, showing a pharmacological dissociation between short- and long-term habituation. Systematic manipulation of ISI showed clonidine's habituation-facilitating effect to be most striking with longer within-session ISIs where habituation was relatively weak in controls. Comparing clonidine's effect to that of two other hypotensive agents, prazosin and propranolol, showed that the habituation-facilitating effect was not due to blood pressure effects. Prazosin, an alpha 1-adrenergic blocker, facilitated short-term habituation, but significantly less so than did clonidine, an alpha 2-agonist. Propranolol, a beta-adrenergic blocker, had no effect on short-term habituation. Both prazosin and propranolol impaired long-term habituation, but propranolol did so without suppressing initial response levels. The data suggest that a synapse with both alpha-1- and alpha 2-adrenoceptors may be critically involved in habituation of the acoustic startle response. A beta-adrenergic involvement in long-term habituation is tentatively suggested.