Lesions of the central gray block the sensitization of the acoustic startle response in rats.

The amplitude of the acoustic startle response (ASR) in rats is increased after administration of footshocks, a phenomenon termed sensitization. The neural circuitry underlying this kind of modulation of the ASR is only partly understood. It has been shown that the central nucleus of the amygdala (cA) and its efferent pathway to the caudal pontine reticular nucleus (PnC), an essential part of the primary startle circuit, is important for the sensitization of the ASR. It was unclear, however, whether the amygdaloreticular pathway directly transfers the effects of footshocks onto the PnC, or whether there exists a relay nucleus within this pathway. The present study tested the hypothesis that the midbrain central gray (CG) is important for the sensitization of the ASR. Neuroanatomical tracing experiments indicate that a descending projection from the medial part of the cA might form synapses in the region of the midbrain CG, where a descending projection to the PnC takes its origin. We lesioned the dorsal and lateral part of the CG with the neurotoxin quinolinic acid and measured the effects of this lesion on the sensitization of the ASR by footshocks. Lesions confined to the dorsal and lateral parts of the CG totally blocked the sensitization of the ASR, without affecting the ASR amplitude in the absence of sensitizing stimuli. These findings suggest a crucial role of the CG for the sensitization of the ASR. The present data are reconciled with other findings from our laboratory and from the literature and we discuss possible mechanisms underlying the mediation of the sensitization of the ASR in rats.