Substance P levels differ in sympathetic target organ terminals and ganglion perikarya

Regulation of the putative neurotransmitter, substance P (SP), was examined in the rat iris and pineal, targets of the peptide-containing superior cervical sympathetic ganglion (SCG). Decentralization (denervation) of the SCG, which is known to increase ganglion SP, did not alter the peptide in either target. Conversely, surgical or pharmacologic (6-hydroxydopamine) ablation of the SCG actually increased SP in both targets. Moreover, destruction of the trigeminal sensory innervation reduced iris SP to virtually blank levels. Finally, trigeminal ablation abolished the rise in iris SP subsequent to sympathetic destruction with 6-hydroxydopamine. Our observations suggest that sympathetic terminals in targets, in contrast to ganglion perikarya and processes, contain negligible quantities of SP. Consequently, SP may not be transported from ganglion to periphery, but may serve an intraganglionic function. Our studies also suggest that sympathetic terminals modulate sensory peptidergic innervation of the iris.