Sensory Response of the Locus Ceruleus: Neonatal and Adult Studies

Abstract

This chapter summarizes two topics from recent experiments concerning sensory response of locus ceruleus (LC) neurons: sensory response of LC neurons in fetal and neonatal rats and a long-lasting, GABAergic inhibition of LC neurons following caloric vestibular stimulation in adult rats. Despite less spontaneous activity of fetal and neonatal LC neurons, sensory stimuli have been very effective in producing the excitation of LC neurons in these immature animals. It has been shown that negative- and positive-feedback mechanisms operate to regulate the electrical activity of LC neurons. Activation of α 1 adrenoceptors causes excitation of LC neurons (positive feedback), while that of α 2 adrenoceptors results in the suppression of the neurons (negative feedback). Based on these findings, the tonic activation of fetal and neonatal LC neurons by sensory stimuli may be explained by the feedback mechanisms that are dependent on the spontaneous activity of the neurons. The sensory-evoked responses of LC neurons are mainly e citatory, despite the difference in response pattern, tonic or transient. It has been found that an atypical sensory response of LC neurons in adult rats, LC neurons revealed a long- lasting inhibition of neuronal firing following caloric vestibular stimulation or electrical stimulation of the medial vestibular nucleus. It is also indicated that the long-lasting suppression of LC neuronal firing following caloric vestibular stimulation is mediated by GABA. These results suggest that the caloric stimulation-induced LC inhibition is mediated by the ventrolateral medulla (VLM) and may be independent of the VLM-mediated pressor effect. As the VLM is the source of the major excitatory input to the LC, it is possible that VLM neurons receiving vestibular input exert excitatory influence on GABAergic neurons connecting to the LC.