THIP, a selective gamma-aminobutyric acid receptor agonist, alters flash-evoked potentials in rats

This study examined the effects of the GABA(A) agonist THIP on flash-evoked potentials (FEPs) recorded from the primary visual cortex (VC) and superior colliculus (SC) of chronically implanted hooded rats. Animals were given I.P. injections of saline, and of 8, 16, and 24 mg THIP/kg body weight on separate days. Evoked potentials were recorded at 5, 20, and 35 min following injection. Animals were tested at a standard (22.5 degrees C) room temperature. Most significant effects were observed at the 20- and 35-min recording intervals for both the 16 and 24 mg/kg doses, with effects at the 24 mg/kg dose the most pronounced. VC P1 amplitude remained unchanged, while N1 was reduced to such an extent that it became positive, ultimately blending into the rising phase of a positive component appearing between N1 and P2. This positive component had a latency of about 6 ms longer than N1, and became larger in amplitude than P1 at the 24 mg/kg dose. P2 amplitude was drastically reduced, becoming negative. In contrast, components N2 and P3 were augmented, while the amplitude of N3 was unchanged. In the SC, P1 was augmented while P3 was reduced in amplitude. A biphasic (increase/decrease) effect was observed in the N4 complex. In both the VC and SC, latencies of most components were increased, with the late components in the VC increased to the greatest extent. A mild hypothermia was observed at 16 and 24 mg/kg. The results suggest that the GABA(A) receptor plays an important role in the elaboration of the middle (N1-P2) components of FEPs recorded from the rat VC, and that GABAergic mechanisms can influence other components in the VC and SC as well.