Similarities in development of substance P and somatostatin in peripheral sensory neurons: effects of capsaicin and nerve growth factor.

Development of the two putative peptide neurotransmitters, substance P (SP) and somatostatin (SS), were compared in rat dorsal root ganglion (DRG) and spinal cord in vivo. The content of SS in the sixth cervical DRG increased 5-fold during the first 5 weeks of life, rising from 24 pg per ganglion at birth. SP content increased 4.5-fold during the first 5 weeks, from 56 pg per ganglion at birth. The developmental profiles for these two peptides were virtually parallel, suggesting that their respective neuronal populations developed in synchrony. Treatment with nerve growth factor (NGF) significantly increased the content of both SP and SS in the DRG and dorsal spinal cord. Conversely, treatment with capsaicin significantly decreased both SP and SS in the DRG and dorsal spinal cord. Consequently, experiments involving NGF or capsaicin treatment of sensory neurons must be interpreted with extreme care, because specificity is not limited to a single peptide phenotype. Although the mechanisms of action of NGF and capsaicin on SP and SS have not been defined, the similarity of the responses of the two peptides suggests that their development may be regulated by similar processes.