INNOCUOUS HAIR DEFLECTION EVOKES A NOCICEPTIVE-LIKE ACTIVATION OF CATECHOL OXIDATION IN THE RAT LOCUS-COERULEUS FOLLOWING INTRATHECAL STRYCHNINE - A BIOCHEMICAL INDEX OF ALLODYNIA USING IN-VIVO VOLTAMMETRY

Blockade of spinal glycinergic inhibition with intrathecal (i.t.) stry chnine induces a reversible allodynia-like state in both conscious al and lightly-anaesthetized rats. Since the locus coeruleus (LC) is acti vated by noxious stimuli, we determined the effect of non-noxious hair deflection (HD) on noradrenergic neuronal activity in rile LC of rats treated with i.t. strychnine. Differential normal pulse voltammetry w as used to measure the catechol oxidation current (CA.OC), an index of LC activity. Rats were maintained in a Light plane of anaesthesia wit h i.v. urethane and i.t. strychnine (40 mu g) was injected near the L( 1)-L(2) segment. HD, applied to the caudal dermatomes affected by i.t. strychnine, evoked a significant increase (max. 141 +/- 7%, n=5, P<0. 05) in CA.OC and mean arterial pressure as compared to baseline (no st rychnine). In contrast HD had no significant effect on CA.OC or mean a rterial pressure in the saline-treated a rats (n=5), Pre-treatment wit h i.t. MK801 (30 mu g) significantly blocked the increase in CA.OC and mean arterial pressure evoked by HD in strychnine-treated rats, The r esults of this study indicated that HD, in the presence of i.t. strych nine but: not saline, can evoke noradrenergic activity in the LC of li ghtly anaesthetized rats, This effect on the LC is: (1) comparable to that observed with noxious stimulation without i.t. strychnine; (2) se gmentally localized, corresponding to the spinal silt of strychnine in jection, and (3) mediated by spinal NMDA receptors, consistent with th e role af excitatory amino acids in sensory transmission, These data p rovide the first neurochemical evidence that HD, iii tile presence of i.t. strychnine, is a nociceptive event supporting the use of this pre paration as an experimental model of allodynia.