TRANSCALLOSAL CIRCUITRY REVEALED BY BLOCKING AND DISINHIBITING CALLOSAL INPUT IN THE CAT

The purpose of this study was to obtain quantitative measures of the i nfluence of callosal input to cells at the area 17/18 border region wh ere transcallosal axons terminate most densely. Single-cell recordings were performed at the area 17/18 border region of the right hemispher e, while gamma-aminobutyric acid (GABA) or its antagonist, bicuculline , were applied to the transcallosal projecting regions of the left hem isphere to either block or overactivate the cells which projected to t he neurons at the recording site. The results showed that visually evo ked responses of the cells at the area 17/18 border were affected by a dministration of GABA or bicuculline to the contralateral hemisphere. Blockade of transcallosal input by application of GABA in the left hem isphere diminished the visually evoked responses of 51% of the neurons in the right hemisphere, and led to an increase in response magnitude for 17% of the neurons. Disinhibition of transcallosal input by appli cation of bicuculline increased the evoked activity of 40% of the neur ons and diminished the response magnitude of 20% of the neurons in the right hemisphere. GABA and bicuculline failed to show antagonistic ef fects on some cells. Thirty-two percent of the cells were affected by only one type of drug administration, and 13% of the cells showed eith er an increase or a decrease in responses after both GABA and then bic uculline administration. This study demonstrated complex interactions between neurons connected by the transcallosal pathway. A model of the transcallosal circuitry was proposed to explain the results.