The contribution of the amygdala to conditioned thalamic arousal

Previous research has demonstrated that thalamocortical neurons within the dorsal lateral geniculate nucleus (dLGN) are affected by an acoustic, fear-arousing, conditioned stimulus (Cain et al., 2000). This effect is reflected in an increase in activity and a tonic firing pattern, a pattern that assures the most accurate relay of information from the retina to the visual neocortex. Such an effect is considered to be indicative of a heightened state of arousal. The present research was designed to determine the extent to which the central nucleus of the amygdala (ACe) contributes to this effect. To this end, in experiment 1 extracellular recordings were made from single dLGN neurons in the awake rabbit during electrical stimulation of the ACe. Increased neuronal activity was observed in response to stimulation in the majority of neurons. Neurons that were in a burst firing pattern immediately before stimulation assumed a tonic firing pattern in response to stimulation. Experiment 2 was designed to determine whether inactivation of the ACe with muscimol would attenuate the response of dLGN neurons in the awake rabbit to the presentation of acoustic, fear-arousing, conditioned stimuli. Compared with vehicle injections, infusions of muscimol attenuated both the spontaneous activity and the response of dLGN neurons to the presentations of these stimuli. The results provide support for the hypothesis that the amygdala, and in particular the ACe, contributes to a heightened state of arousal during conditioned fear.