FEAR-POTENTIATED STARTLE AND ELECTRICALLY-EVOKED STARTLE MEDIATED BY SYNAPSES IN ROSTROLATERAL MIDBRAIN

Startle amplitudes are increased when acoustic startle responses are e licited in the presence of a stimulus that has previously been paired with shock. This ''fear-potentiated'' startle response appears to be m ediated via the caudal ventral amygdalofugal pathway to the brainstem. Electrical stimulation of this pathway evokes unconditioned startleli ke responses. Collision tests have shown that a monosynaptic connectio n from amygdala to midbrain mediates these responses. Collision tests here localize these synapses mediating electrically evoked startlelike responses to the rostrolateral midbrain in awake rats. To test whethe r rostrolateral midbrain synapses also mediate fear-potentiated startl e, we lesioned cells in these sites with ibotenic acid. These lesions completely blocked fear potentiation of acoustic startle. These same l esions did not block potentiation of startle by d-amphetamine (6 mg/kg ).