The Journal of Neurophysiology Vol. 86 No. 1 July 2001, pp. 94-103
Copyright ©2001 by the American Physiological Society

Role of Inhibition in Cortical Reorganization of the Adult Raccoon Revealed by Microiontophoretic Blockade of GABA_A Receptors

 ¹Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia B3H 4H7; and  ²Institute for Biological Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada

Tremere, Liisa, T. Philip Hicks, and Douglas D. Rasmusson. Role of Inhibition in Cortical Reorganization of the Adult Raccoon Revealed by Microiontophoretic Blockade of GABA_A Receptors. J. Neurophysiol. 86: 94-103, 2001. Cortical reorganization was induced by amputation of the 4th digit in 11 adult raccoons. Animals were studied at various intervals, ranging from 2 to 37 wk, after amputation. Recordings were made from a total of 129 neurons in 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 entire field or consisted of split fields. The proportion of neurons with restricted fields increased with time after amputation and was greater than previously found in subcortical regions. A GABA_A receptor antagonist (bicuculline methiodide), glutamate, and GABA were administered iontophoretically to these neurons while determining their receptive fields and thresholds. Bicuculline 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 in nonamputated animals. In some neurons (n = 4), bicuculline produced an expansion from one digit onto the adjacent palm or another digit, an effect never seen in control animals. Bicuculline also changed the split fields of seven neurons into continuous fields by exposing a responsive region between the split fields. Finally, bicuculline changed the internal receptive field organization of 10 neurons by revealing subfields with reduced thresholds. In contrast to the situation in nonamputated animals, iontophoretic administration of glutamate also produced receptive field expansion in some neurons (n = 6), but the size and/or shape of the change was different from that 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 cortical reorganization is the gradual reduction in effective receptive field size and that intracortical inhibitory networks are partially responsible for these changes.