SUBSTANCE-P AND OTHER PUTATIVE TRANSMITTERS MODULATE THE ACTIVITY OF RETICULAR PONTINE NEURONS - AN ELECTROPHYSIOLOGICAL AND IMMUNOHISTOCHEMICAL STUDY

In this study we investigated the effects of possible modulatory trans mitters on acoustically responsive neurons of the caudal pontine retic ular nucleus (PnC). From previous work in our laboratory it has been s uggested that the acoustically responsive giant neurons of this nucleu s are the sensorimotor interface mediating the acoustic startle respon se. Furthermore they are the site of some of the modulatory influence impinging on this response. Besides a possibly glutamatergic excitatio n from the amygdala a cholinergic input from the midbrain has been des cribed which may use substance P as cotransmitter. Therefore we used e lectrophysiological and histochemical methods to study this possible m odulatory influence in the caudal pontine reticular nucleus. In the fi rst part of this study we recorded extracellularly from single units i n the PnC in vivo and studied the effects of iontophoretically applied transmitters. Substance P elicited a long lasting excitation. This ex citatory effect of SP was potentiated by acetyl-beta-methylcholine (AM Ch, an acetylcholine agonist), whereas single application of AMCh show ed no uniform response. Glutamate elicited a potent brief excitation, while application of GABA showed a potent brief inhibition of PnC neur ons. In the second part of this study we employed immunoperoxidase sta ining for substance P, which revealed a fairly dense network of substa nce P-immunoreactive (SP-ir) fibers in the lateral and ventral aspects of the PnC. Combining retrograde tracing and immunocytochemistry for substance P, we demonstrated that the SP-ir axons in the PnC originate mainly in the laterodorsal tegmental nucleus. We therefore conclude t hat activation of the laterodorsal tegmental nucleus may facilitate th e acoustic startle response by a long lasting excitation of neurons in the caudal pontine reticular nucleus.