Decreased CCK_B receptor binding in rat amygdala in animals demonstrating greater anxiety-like behavior

Abstract

Rationale. The potentiation of the acoustic startle response (ASR) by stimuli associated with aversive events is mediated via the amygdala and is used as an index of "anxiety" and "fear". In laboratory animals, cholecystokinin_B (CCK_B) agonists increase anxiety and fear and activation of amygdala CCK_B receptors potentiates ASR. Additionally, antagonism of CCK_B receptors attenuates fear-potentiated ASR.

Objectives. To investigate the putative role of CCK_B receptors in individual differences in fear and anxiety, we examined individual differences in amygdala CCK_B receptor binding for animals demonstrating low versus high baseline and fear-potentiated ASR. Additionally, we examined individual differences in CCK_B binding for animals demonstrating low versus high anxiety-like behavior on the elevated plus-maze (EPM).

Methods. Male Wistar rats were tested in the ASR, fear-potentiated ASR, and EPM paradigms. Following testing, brain slices were mounted and incubated with 50 pM ¹²⁵I-CCK8 (non-sulfated), a selective CCK_B receptor ligand, in the presence or absence of 1 µM non-radioactively labeled CCK and then exposed on tritium-sensitive film for 2–3 days.

Results. Animals with high fear-potentiated ASR showed decreased CCK_B receptor binding in both the basolateral and central amygdaloid nuclei. Animals with high anxiety-like responses on the EPM showed decreased CCK_B binding in the basolateral, but not central, amygdala. There were no differences in amygdala CCK_B binding in animals demonstrating low versus high baseline ASR. None of the groups showed differences in CCK_B receptor binding in the nucleus accumbens.

Conclusions. These results show that there is a down-regulation of amygdala CCK_B receptor binding in animals demonstrating greater anxiety-like responding in the fear-potentiated ASR and EPM models of anxiety, possibly as a compensation for increased CCK activity.