The expression and localization of somatostatin receptors in dorsal root ganglion neurons of normal and monoarthritic rats

Abstract

Somatostatin has antinociceptive effects by acting on somatostatin (sst) receptors in primary afferent neurons. Five sst receptor subtypes (sst 1–5) have been identified. In the present study we assessed the expression and localization of the sst receptor subtypes in lumbar dorsal root ganglia of normal rats and of rats with unilateral antigen-induced arthritis (AIA) in the knee joint. We used polymerase chain reaction (PCR) of material from dorsal root ganglia and immunohistochemistry in dorsal root ganglion paraffin sections. PCR data show that sst 1, sst 2(a), sst 2(b), sst 3, and sst 4 receptors are expressed in lumbar dorsal root ganglia of the rat. The sst 5 receptor was expressed in a few samples. Available antibodies revealed sst 2(a) and sst 2(b) receptor-like immunoreactivity in the vast majority of neurons, and sst 4 receptor-like immunoreactivity in about 40% of the dorsal root ganglion neurons and in some satellite cells. Real time PCR at 3, 10 and 21 days after induction of AIA did not reveal changes in receptor expression. Immunohistochemistry showed that a similar high proportion of neuronal profiles expressed sst 2(b) receptor-like IR in control and AIA rats, but the proportion of neuronal profiles with sst 2(a) receptor-like IR was significantly lower in acute and chronic AIA rats than in control rats. Although the proportion of neuronal profiles with sst 4 receptor-like IR was significantly higher at 21 days than at 3 days values at 3 or at 21 days were not significantly different from control. These data show that the majority of dorsal root ganglion neurons exhibit somatostatin receptor-like IR thus suggesting a high potential for inhibition by somatostatin. The reduction in the proportion of neuronal profiles with sst 2(a) immunoreactivity suggests that inhibition of neuronal activity by somatostatin is reduced during painful arthritis.