Neurochemical interactions in the parabrachial nucleus mediating visceral inputs to visceral thalamic neurons

Previously we demonstrated that glutamatergic and noradrenergic receptors mediate the relay of visceral information through the parabrachial nucleus (PBN) and that calcitonin gene-related peptide (CGRP), substance P (SP), somatostatin (SOM), neurotensin (NT), and cholecystokinin (CCK) may modulate these responses. The interactions of these neurotransmitters and neuropeptides were examined in male Wistar rats (17) that were anesthetized with chloral hydrate and ventilated and in which blood pressure and heart rate were continuously monitored. The left cervical vagus nerve was stimulated at submaximal current intensities to elicit changes in single and multiunit activity of visceral thalamic neurons (VTNs). Peristimulus-time and continuous-time histograms of VTN activity were made before and after 200-nl injections of peptides, neurotransmitter agonists or antagonists, or artificial cerebrospinal fluid into the PBN. Combined injection of CGRP and SP into the PBN produced a synergistic inhibition of spontaneous VTN activity and the vagally evoked VTN response. Combined injection of NT and phenylephrine (PE) into the PBN produced only an additive increase in the spontaneous activity of VTNs. Prior administration of SOM in the PBN blocked the excitatory action of an alpha-adrenergic agonist (phenylephrine) injection on the spontaneous activity of VTNs, whereas CGRP, SP, or CCK had no effect on the alpha-agonist-induced response. Prior injection of an alpha-adrenergic antagonist (phentolamine) prevented the excitatory effect of NT in the PBN. Injection of CGRP, SP, NT, or CCK into the PBN did not change the response of VTNs to application of glutamate. These results suggest mechanisms for peptide interaction with primary neurotransmitters in the PBN and indicate whether the neuropeptides are acting before the primary neurotransmitter synapse or postsynaptically.