Hippocampal theta rhythm after local administration of procaine or amphetamine into the ventral tegmental area in fear conditioned rats.

The ventral tegmental area (VTA) is thought to be an important component in the mesocorticolimbic system involved in the regulation of theta rhythm in the hippocampus. In this study we investigate the effect of pharmacological inactivation (local procaine infusion) or activation (local amphetamine infusion) of the VTA on theta rhythm parameters during task specific behavior in fear conditioned, freely moving rats. Animals were implanted with bilateral recording electrodes into the dorsal hippocampus (CA1) and bilateral injection cannulas into the VTA. Behavioral activities and hippocampal local field potentials (LFP) were recorded throughout the experiment, in pre- and post-injection conditions. We found that intra-VTA injection of procaine temporarily suppressed fear conditioned avoidance response (escape from the foot-shock arena) and also influenced hippocampal theta rhythm parameters during immobility linked with arousal and/or attention. Procaine infusion decreased the signal power (Pmax) of theta rhythm during immobility behavior, in comparison to the control group (water infusion), whereas administration of amphetamine had no effect on the behavior and hippocampal LFP. Our results indicate that temporal inactivation of neuronal activity in the VTA affects hippocampal theta rhythm linked with attentional immobility and suppresses avoidance response in fear conditioned animals.