Tonic control of peripheral cutaneous nociceptors by somatostatin receptors

Abstract

The peptide somatostatin [somatotropin release-inhibiting factor (SRIF)] is widely distributed in the body and exerts a variety of hormonal and neural actions. Several lines of evidence indicate that SRIF is important in nociceptive processing: (1) it is localized in a subset of small-diameter dorsal root ganglion cells; (2) activation of SRIF receptors results in inhibition of both nociceptive behaviors in animals and acute and chronic pain in humans; (3) SRIF inhibits dorsal horn neuronal activity; and (4) SRIF reduces responses of joint mechanoreceptors to noxious rotation of the knee joint. The goal of the present study is to show that cutaneous nociceptors are under the tonic inhibitory control of SRIF. This is accomplished using behavioral and electrophysiological paradigms. In a dose-dependent manner, intraplantar injection of the SRIF receptor antagonist cyclo-somatostatin (c-SOM) results in nociceptive behaviors in normal animals and enhancement of nociceptive behaviors in formalin-injected animals, and these actions can be blocked when c-SOM is coapplied with three different SRIF agonists. Furthermore, intraplantar injection of SRIF antiserum also results in nociceptive behaviors. Electrophysiological recordings using an in vitro glabrous skin-nerve preparation show increased nociceptor activity in response to c-SOM, and this increase is blocked by the same three SRIF agonists. Parallel behavioral and electrophysiological studies using the opioid antagonist naloxone demonstrate that endogenous opioids do not maintain a tonic inhibitory control over peripheral nociceptors, nor does opioid receptor antagonism influence peripheral SRIF effects on nociceptors. These findings demonstrate that SRIF receptors maintain a tonic inhibitory control over peripheral nociceptors, and this may contribute to mechanisms that control the excitability of these terminals.

Fig. 1.

Dose–response relationships. A, Animals receiving an intraplantar injection of 0.13 μm or 1.3 mm c-SOM demonstrate significant flinching behavior compared with animals receiving PBS. * indicates a significant difference from PBS group, and + indicates a significant difference between 0.13 and 1.3 mm doses (one way ANOVA;p < 0.05). B, The time course of the flinching behavior is presented in 5 min intervals. Note that the flinching behavior produced by 1.3 mm c-SOM is blocked by coinjection with OCT. * indicates a significant difference from PBS group (Kruskal–Wallis test; p < 0.05).C, Coinjection of c-SOM with 10 μm SRIF, 20 μm VAP, or 20 μm OCT results in a significant reduction in c-SOM-induced flinching behavior (Kruskal–Wallis test; p < 0.05).D, The time course of the flinching behavior is presented in 1 min intervals. * indicates a significant difference from VAP, and + indicates a significant difference from SRIF (Kruskal–Wallis test; p < 0.05).

Fig. 2.

Intraplantar injection of 30 μl of SRIF antiserum (1:500–1:1000; n = 6) or the SRIF antagonist c-SOM (n = 7) results in significant flinching behavior compared with animals receiving PBS (n = 6). * indicates a significant difference from PBS group (Kruskal–Wallis test; p < 0.05).

Fig. 3.

A time course study demonstrating that coinjection of 1.3 mm c-SOM with 1% FM significantly enhances flinching (A) and lifting/licking behaviors (B). * indicates a significant difference from 1% FM (Mann–Whitney U test; p < 0.05).

Fig. 4.

Analysis of the phase 1 and 2 FM data indicates that, compared with FM alone (n = 7), c-SOM plus FM (n = 8) enhances phases 1 and 2 flinching (A) and phase 2 L/L behavior (B). The c-SOM effect is not attributable to a systemic effect because animals injected with 1.3 mm c-SOM in one hindpaw and 1% FM into the contralateral hindpaw (Contra; n = 7) had nociceptive behaviors no different from those seen in animals injected with FM alone. * indicates a significant difference from the 1% FM group (Kruskal–Wallis test; p < 0.05).

Fig. 5.

Dose–response relationships. A, Application of c-SOM to the receptive field of nociceptors produces an increase in their discharge rate. An ascending series of c-SOM concentrations, ranging from 0.00013 to 3.9 mm, was applied for 2 min to the receptive fields of CMH units, and unit activity was recorded. * indicates significant difference from background (BG; Friedman's ANOVA followed by a Dunn's post hoc analysis; p < 0.05). B, Responses of the individual fibers at each dose.

Fig. 6.

Demonstration of the increase in discharge rate in a CMH fiber after application of 1.3 mm c-SOM for 2 min to its receptive field. c-SOM produced no change in heat sensitivity, indicating that it did not sensitize the nociceptor to heat (compareHeat₁ withHeat₂).

Fig. 7.

A, Application of 1.3 mm c-SOM to the receptive field of CMH fibers (n = 18) produced a significant increase in their discharge rate. ** indicates a significant difference from background (Wilcoxon signed ranks test; p < 0.01).B, c-SOM-induced activity is significantly blocked by coapplication of 20 μm OCT or 20 μm SRIF and attenuated by 20 μm VAP (background activity has been subtracted in each group). * indicates a significant decrease compared with c-SOM alone (Mann-Whitney U test;p < 0.05).

Fig. 8.

The time course of the mean activity of CMH units that met the criteria for c-SOM responders. (Responders showed an increase in their discharge rate that was greater that the mean ± 2 SDs of the total population.) A, Application of c-SOM for 2 min resulted in a significant increase in neural activity.B, In a separate population, OCT applied in the presence of c-SOM prevented this increase in activity.

Fig. 9.

A time course study demonstrating that coinjection of 3 μm naloxone (Nal) with 2% FM does not enhance flinching behavior (A) or lifting/licking behaviors (B), indicating that, in contrast to SRIF receptor, opioid receptors do not maintain a tonic inhibitory control over peripheral nociceptors (t test; not significant).

Fig. 10.

A, Dose–response relationship. Naloxone (Nal), ranging from 0.2 to 200 μm, was applied for 2 min to the receptive fields of CMH units (n = 8), and unit activity was recorded. Compared with background (BG), there was no significant change in neural activity in the presence of naloxone, indicating that opioid receptors do not maintain a tonic inhibitory control over CMH fibers (Friedman's ANOVA; p = 0.34).B, Compared with c-SOM alone, there was no difference in discharge rate when 2 μm naloxone was coapplied with c-SOM (Mann–Whitney U test; p = 0.77).