Immunocytochemical localization of somatostatin receptor sst2A in the rat spinal cord and dorsal root ganglia

Intrathecal administration of octreotide, a stable somatostatin analogue, provides pain relief in patients, and locally applied somatostatin inhibits firing of nociceptive dorsal horn neurons. In the present study, we have raised polyclonal antibodies that specifically detect the somatostatin receptor sst_2A and used these antisera for immunocytochemical localization of the receptor protein in the rat spinal cord and dorsal root ganglia. In the superficial layers of the dorsal horn, sst_2A-like immunoreactivity (Li) formed a dense network consisting of neuronal perikarya and dendrites which were often closely apposed by, but not co-contained within, somatostatin-14-immunoreactive nerve fibres and terminals. sst_2A-Li was resistant to dorsal rhizotomy and did not colocalize with either substance P or calcitonin gene-related peptide suggesting that sst_2A-Li was not located to primary afferents, but rather confined to second-order spinal neurons. The position of sst_2A-Li perikarya and dendrites in the dorsal horn appeared to be similar to those containing μ-opioid receptor-Li; however, double labelling experiments revealed no instances of coexistence of these two receptors. sst_2A-Li was also observed in the dorsal root ganglia predominantly targeted to the somatic plasmalemma of medium size neurons distinct from those expressing somatostatin-14 or δ-opioid receptors. Thus, the present results not only provide a morphological substrate for spinal octreotide analgesia but also show that somatostatin and opioids are poised to modulate nociceptive transmission by distinct anatomical systems.