Role of inhibition in cortical reorganization of the adult raccoon revealed by microiontophoretic blockade of GABA(A) receptors

Cortical reorganization was induced by amputation of the 4th digit in 11 ad ult raccoons. Animals were studied at various intervals, ranging from 2 to 37 wk, after amputation. Recordings were made from a total of 129 neurons i n the deafferented cortical region using multibarrel micropipettes. Several types of receptive fields were described in reorganized cortex: restricted fields were similar in size to the normal receptive fields in nonamputated animals; multi-regional fields included sensitive regions on both adjacent digits and/or the underlying palm and were either continuous over the enti re field or consisted of split fields. The proportion of neurons with restr icted fields increased with time after amputation and was greater than prev iously found in subcortical regions. A GABA(A) receptor antagonist (bicucul line methiodide), glutamate, and GABA were administered iontophoretically t o these neurons while determining their receptive fields and thresholds. Bi cuculline administration resulted in expansion of the receptive field in 60 % of the 93 neurons with cutaneous fields. In most cases (33 neurons) this consisted of a simple expansion around the borders of the predrug receptive field, and the average expansion (426%) was not different from that seen i n nonamputated animals. In some neurons (n = 4), bicuculline produced an ex pansion from one digit onto the adjacent palm or another digit, an effect n ever seen in control animals. Bicuculline also changed the split fields of seven neurons into continuous fields by exposing a responsive region betwee n the split fields. Finally, bicuculline changed the internal receptive fie ld organization of 10 neurons by revealing subfields with reduced threshold s. In contrast to the situation in nonamputated animals, iontophoretic admi nistration of glutamate also produced receptive field expansion in some neu rons (n = 6), but the size and/or shape of the change was different from th at produced by bicuculline, indicating that the effects of bicuculline were not due to an overall facilitation of neuronal activity. These results are consistent with the hypotheses that an important component of long-term co rtical reorganization is the gradual reduction in effective receptive field size and that intracortical inhibitory networks are partially responsible for these changes.