Somatosensory processing in the spinal cord is known to be under the tonic inhibitory control of local circuits of GABAergic and glycinergic neurons. However, very little is known about the spatiotemporal properties of this inhibitory control over the tactile information. We used multielectrode recordings at fixed spatial intervals to investigate the populational effects of GABAergic inhibitory removal in the firing properties of dorsal horn neurons. Neuronal activity was recorded extracellularly in deep dorsal horn (laminae III-V) using multielectrode matrices of 8 tungsten filaments, aligned in one rostrocaudal row at 240 micrometers intervals, (160 neurons in 12 rats). Experimental protocol consisted of series of tactile stimulations (0.5 Hz) delivered by a robotic arm. After 30 minutes of recording, 0.2 micrograms of bicuculline (1 microliter) were deposited on the surface of the cord using a Hamilton syringe and the responses to the following series of stimulation were tested throughout 1-2 hours. Bicuculline was administered immediately near the electrode array at either its rostral or caudal end. Spatiotemporal profile of changes was calculated using stimulus-evoked firing rates and latency. Administration of bicuculline was found to have a distinct spatial effect upon spontaneous and evoked spinal activity. Bicuculline induced changes in the evoked activity and response latencies in the majority of the recorded neurons; these effects had a spatial component, with stronger changes happening in the neurons closer the administration point. Here we show that bicuculline induces complex and non-homogeneous effects on the spinal neuronal populations. Overall, the spontaneous activity increased in a non spatial dependent manner, while both the evoked activity and the response latency were enhanced in spatial dependent manner. These observations suggest the presynaptic inhibition of primary afferents is under a stronger local modulation than the tonic central inhibition of the spinal interneurons.

Support: FCT-POCI / 59383 / 2004.