Enhancement of the acoustic startle response by stimulation of an excitatory pathway from the central amygdala/basal nucleus of Meynert to the pontine reticular formation.

The acoustic startle response (ASR) is a simple motor reaction to intense and sudden acoustic stimuli. The neural pathway underlying the ASR in rats is already fairly well understood. As the ASR is subject to a variety of modulations, this reaction can serve as a model for vertebrate neuroethologists to investigate the neural mechanisms mediating sensorimotor transfer and their extrinsic modulation. We report here on experiments in rats which were undertaken in order to investigate the neural mechanisms underlying the enhancement of the ASR. An increased amplitude of the ASR can be observed during states of conditioned and unconditioned fear. By employing neuroanatomical tract-tracing methods, we describe a pathway from neurons of the medial division of the central amygdaloid nucleus (cA) and the basal nucleus of Meynert (B) to the caudal pontine reticular nucleus (PnC), an important relay station in the acoustic startle pathway. Extracellular recordings from acoustically responsive neurons in the PnC showed that electrical stimulation of the cA/B facilitates the tone-evoked response of these neurons. Behavioural tests following chemical stimulation of the cA/B with NMDA (N-methyl-d-aspartate) in awake rats indicated that activation of this pathway increases the ASR. The lack of sufficient spatial resolution of our stimulation techniques did not allow us to differentiate the relative contributions of the cA and the B to this effect. As the amygdaloid complex has been implicated in emotional behaviour, particularly in the mediation of fear, these findings substantiate the concept that the amygdaloid complex plays a key role for the enhancement of the ASR by conditioned and unconditioned fear.