Muscimol in the deep layers of the superior colliculus/mesencephalic reticular formation blocks expression but not acquisition of fear-potentiated startle in rats.

Deposits of the retrograde tracer Fluoro-Gold into the ventrolateral nucleus reticularis pontis caudalis labeled neurons in the deep layers of the superior colliculus/mesencephalic reticular formation (deep SC/Me). To test the involvement of this area in the fear-potentiated startle effect, rats were implanted with cannulas into the deep SC/Me and trained for fear-potentiated startle after infusion of the GABA(A) agonist muscimol (0.1 microg/0.5 microl). Two days later, they were tested for fear-potentiated startle. Rats then received a 2nd training session without any infusions, and 2 days later they were reinfused with muscimol (0.1 microg/0.5 microl) and tested for fear-potentiated startle. Local infusion of muscimol into the deep SC/Me completely blocked the expression but not the acquisition of fear-potentiated startle. These results indicate that a synapse in the midbrain is critical for the expression of fear-potentiated startle.