The effect of 5-HT1A receptor stimulation on nociceptive dorsal horn neurones in rats.

Spinal 5-HT1A receptor subtypes are involved in regulation of nociception. This study was performed to investigate the effect of stimulation of these receptors on wide dynamic range neurones in the spinal cord. Extracellular single unit recordings of dorsal horn neurones were performed in intact urethane-anaesthetized female Sprague-Dawley rats. The receptive field distally on one hind paw was electrically stimulated with needle electrodes applied to the skin. The 5-HT1A receptor agonist, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) , and the 5-HT1A receptor antagonist, WAY100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl ]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride), were applied directly onto the spinal cord, and single unit responses were counted separately for Abeta-, Adelta-, C-fibre responses and post-discharge according to the latencies. Only 500 nmol 8-OH-DPAT caused a significant inhibition of all the neuronal responses. Cells with a pronounced wind-up, limited C-fibre response before drug application and relatively large receptive field for pinch in laminae III-IV were most powerfully inhibited by 500 nmol 8-OH-DPAT. 50 nmol WAY100635 alone did not affect the neuronal responses but blocked the effect of 500 nmol 8-OH-DPAT. These results suggest that stimulation of 5-HT1A receptors inhibits the activity in spinal wide dynamic range neurones after repeated electrical stimulation.