Local effects of octreotide on glutamate-evoked activation of Aδ and C afferent fibers in rat hairy skin

The aim of the present study was to investigate whether local application of octreotide, an analogue of somatostatin, suppresses the glutamate-evoked activities of Aδ and C primary afferent fibers innervating dorsal hairy skin of the rat in vivo. The single unit activity of Aδ and C afferent fibers was recorded in isolated filaments from the dorsal cutaneous branches of the T9–T12 spinal nerves. Changes in discharge relative to baseline during injection of glutamate (0.3 mM, 10 μL) into the receptive field with pretreatment by octreotide (20 μM, 10 μL) were compared with injection after pretreatment with normal saline. Most of Aδ fibers (21/27, 78%) and C fibers (21/26, 81%) in the dorsal cutaneous branches were significantly activated by local injection of glutamate following saline injection. The mean discharge rates increased from 2.39 ± 0.30 and 2.42 ± 0.37 impulses/min to 12.79 ± 2.04 and 13.56 ± 2.56 impulses/min during injection of glutamate for Aδ and C fibers, respectively. After octreotide pretreatment group, glutamate increased the mean discharge rates from 1.93 ± 0.38 and 2.25 ± 0.29 impulses/min to 6.11 ± 0.9 and 6.31 ± 1.18 impulses/min in Aδ fibers and C fibers, respectively. The discharge rates during injection of glutamate after octreotide pretreatment were significantly lower than after normal saline pretreatment. The suppressive effect of octreotide was reversed by the somatostatin receptor antagonist cyclo-somatostatin. These results suggest that interactions between excitatory amino acid and inhibitory neuropeptides may contribute to sensory signaling in the peripheral nervous system.