Prolonged GABAA-mediated inhibition following single hair afferent input to single spinal dorsal horn neurones in cats.

To study the central processing mechanisms of sensory input from low threshold afferents to the spinal cord, we examined the excitatory response of single lumbar dorsal horn neurones to stimulation of hairs in the receptive field using a mechanically driven probe, and to activation of single hair follicle afferents using an intracellular current pulse to the cell bodies in the dorsal root ganglion. Experiments were done on anaesthetized, paralysed cats, spinalized at the L1 lumbar level. Responses of spinal neurones to two types of hair afferent input were characteristically different. The excitatory response to input from a single group II hair afferent (A beta; innervating guard hair follicle receptors) was multimodal, characterized by a small early depolarization followed by a sharp, large component with a slow, prolonged decay phase, whereas the response to input from a single group III hair afferent (A delta; innervating down hair follicle receptors) was unimodal. The unitary EPSPs in response to activation of group III hair afferents had a slower rise time and longer decay time constant than those in response to activation of group II hair afferents. When the receptive field of the afferent was located in the centre of the receptive field of the dorsal horn neurone, the gain of the central response was greater for the input from a single group II afferent (> 1) than that for the input from a single group III afferent (< 1). In the case of single group II hair afferents, when pairs of single action potentials or pairs of trains of action potentials were generated at intervals of 20 ms to 3 s, the response in the dorsal horn neurone to the second volley was markedly depressed at intervals of less than 2 s, without any apparent inhibition of the on-going rate of firing. The response to the second volley in single group III afferents was less depressed. This inhibition of the response to the second of a paired volley in single group II hair afferents was attenuated by administration of bicuculline, but not strychnine or naloxone. This indicates that the inhibition involves a GABAA-receptor-mediated mechanism. Bicuculline did not affect the late component of the response to single group II hair afferent input, but unmasked a late component of the response to mechanical stimulation of hairs.(ABSTRACT TRUNCATED AT 400 WORDS)