Defensive reactions evoked by activation of NMDA receptors in distinct sites of the inferior colliculus.

The inferior colliculus (IC) is primarily involved in conveying auditory information to higher cortical structures. Recently we have shown that this structure may also be part of a brain system commanding defensive behaviour. There is evidence that the neural substrates responsible for defensive behaviour in the inferior colliculus are regulated by GABAergic, serotonergic and opioid mechanisms and that these substrates may also be depressed by benzodiazepines as part of their anxiolytic action. Here we present evidence for the involvement of excitatory amino acids in the IC in the expression of defensive reactions. Microinjections of NMDA (5-40 nmol)--an excitatory amino acid--into the ventrolateral division of the central nucleus of the IC of rats placed inside a circular arena induced aversive reactions, characterized by running, rearing, and jumping. This hyperactivity was interspersed by immobility states which often progressed to convulsive seizures. These reactions were inhibited by the NMDA specific antagonist AP7 previously microinjected into the IC. It is suggested that NMDA receptor mediated mechanisms are called into play during the display of the defensive behaviour.