Expansion of receptive fields in motor cortex by local blockade of GABA_A receptors

Experiments were done in α-choralose anesthetized cats to determine whether local disinhibition would expand the sensory receptive field (RF) of motor cortical neurons. Most of the neurons (n = 17) responded only to a rapid high velocity "tap" of the paw or forearm, often requiring movement of a joint, while four cells responded to light touch of the skin. The receptive field of single neurons was re-examined after microiontophoretic ejection of bicuculline (BIC). In all 21 neurons examined, BIC produced an expansion of the RF (mean 4 times before drug). Expansion was seen most often in the proximal-distal axis (17 neurons) but was also commonly seen in the mediolateral axis (9 neurons). The expansion was usually restricted to the dorsal or ventral surface that the original RF was on; in only three neurons in which the pre-drug RF was on the dorsal surface of the paw did the expansion include part or the entire ventral surface. Response thresholds could only be tested in those neurons with touch RFs and showed no evidence of a change within the original RF of these cells. Local disinhibition has previously been shown to allow for the functional linking of motor cortical points, a mechanism that may be involved in the recruitment of movement related muscle synergies. The present results suggest that this may be also accompanied by expansion of the receptive fields. Such a receptive field expansion may be of functional value since motor cortical output neurons would receive sensory input integrated over a larger area of the limb. The role of local inhibitory control of sensory inputs to motor cortex neurons may thus be different than that in sensory cortex where it is thought to restrict receptive field size.