Glycine produced pressor responses when microinjected in the pressor areas of pons and medulla in cats

Neural control of pancreatic peptide hormone secretion

Pancreatic peptide hormone secretion is inextricably linked to maintenance of normal levels of blood glucose. In animals and man, pancreatic peptide hormone secretion is controlled, at least in part, by input from parasympathetic (vagal) premotor neurons that are found principally in the dorsal motor nucleus of the vagus (DMV). Iatrogenic (insulin-induced) hypoglycaemia evokes a homeostatic response commonly referred to as the glucose counter-regulatory response. This homeostatic response is of particular importance in Type 1 diabetes in which episodes of hypoglycaemia are common, debilitating and lead to suboptimal control of blood glucose. Glucagon is the principal counterregulatory hormone but for reasons unknown, its secretion during insulin-induced hypoglycaemia is impaired. Pancreatic parasympathetic neurons are distinguishable electrophysiologically from those that control other (e.g. gastric) functions and are controlled by supramedullary inputs from hypothalamic structures such as the perifornical region. During hypoglycaemia, glucose-sensitive, GABAergic neurons in the ventromedial hypothalamus are inhibited leading to disinhibition of perifornical orexin neurons with projections to the DMV which, in turn, leads to increased secretion of glucagon.

Inhibitory actions of serotonin on glutamate release in dorsal medulla suppress systemic arterial pressure of cats

The role of serotonin and glutamate release in dorsal medulla (DM) for regulation of systemic arterial pressure (SAP) was examined with microdialysis and high performance liquid chromatograph in anesthetized cats. KCl-perfusion in DM increased serotonin and glutamate concentrations in DM. Perfusion of serotonin resulted in decreases in glutamate concentration and SAP. Perfusion of alaproclate, a serotonin reuptake inhibitor that produced an increase in serotonin concentration in DM, had the same results as perfusion of serotonin. In conclusion, serotonin and glutamate appeared to be tonically and endogenously released from nerve terminals in DM, and the decrease in SAP could be attributed to the decreased glutamate release resulting from inhibitory action of serotonin in DM. The putative roles of serotonin and glutamate in DM may be important in SAP regulation.

Glycine increases arterial pressure and augments NMDA-induced pressor responses in the dorsomedial and ventrolateral medulla of cats

The present study is designed to determine and characterize two neurobiological events. Firstly, we investigated whether increases of systemic arterial pressure (SAP) and sympathetic vertebral nerve activity (VNA) produced by microinjection of glycine (Gly) in the dorsomedial (DM) or rostral ventrolateral medulla (RVLM) are mediated by pressor neurons in DM or RVLM. Secondly, we assessed whether simultaneous microinjections of Gly and N-methyl-D-aspartate (NMDA) in DM or RVLM potentiate the NMDA-pressor effects. Changes in SAP and VNA were recorded in 33 cats under alpha-chloralose and urethane anesthesia. Microinjection of sodium glutamate (Glu, 0.25 M, 30 nl) or Gly (1.0 M, 30 nl) into the DM or RVLM increased SAP and VNA in similar magnitude. Latencies of changes in SAP and VNA induced by Gly, however, were longer (3 s) than those induced by Glu. Prior microinjection of the following antagonists blocked the Gly-induced pressor responses: 2-amino-5-phosphonopentanoate (AP-5, 25 mM, 30 nl), a specific NMDA receptor antagonist; or glutamate diethyl ester (GDEE, 0.5 M, 30 nl), a quisqualate receptor antagonist; or kynurenic acid (KYN, 10 mM, 30 nl), a broad spectrum competitive Glu antagonist. Prior treatment with strychnine (3 mM, 30 nl), a specific Gly antagonist, also blocked the Gly-induced pressor responses. Since Gly is believed to be an inhibitory neurotransmitter, these data suggest that Gly may produce pressor actions via an inhibition on specific inhibitory neurons synapsing with the pressor neurons. NMDA (0.1 M, 30 nl) and Gly (1.0 M, 30 nl) microinjected simultaneously in DM or RVLM produced a greater pressor action than NMDA alone. This potentiation was blocked by KYN, another known antagonist for such potentiation, but was only partially blocked by strychnine.