Dopamine-induced synaptic depression in the parabrachial nucleus is independent of CTX- and PTX-sensitive G-proteins, PKA and PLC signalling pathways

Abstract

We have previously reported that dopamine (DA) depresses non-NMDA receptor-mediated glutamatergic transmission in the rat parabrachial nucleus (PBN), an interface between brainstem and forebrain that is implicated in autonomic regulation. This work examined cellular signalling pathways that might underlie this DA-induced synaptic depression. Direct activation of adenylyl cyclase with 10 μM forskolin increased the evoked EPSC but did not occlude DA-induced EPSC depression. Similarly, a preferential protein kinase A inhibitor, H-7 (10 μM), did not block DA's synaptic effects. Incubation of slices with cholera toxin (CTX; 1 μg/ml) or pertussis toxin (PTX; 0.5 μg/ml) for 20 h, procedures used to irreversibly activate or disable the G s and G i proteins, respectively, did not change DA's effects. The putative phospholipase C inhibitor, U-73122 (10 μM) and its inactive analogue U-73343 (10 μM) did not alter DA-induced reduction in the EPSCs. Alterations in signalling molecules downstream of phospholipase C including depleting internal calcium stores by thapsigargin and cyclopiazonic acid and blocking protein kinase C with chelerythrine, had no effect on DA-induced synaptic depression. Furthermore, DA's depression of the non-NMDA response was not blocked by APV, an NMDA receptor antagonist. Finally, DA depressed evoked, pharmacologically isolated NMDA receptor-mediated synaptic responses while increasing NMDA-induced inward currents in the PBN. These results indicate that DA-induced synaptic effects in the PBN are not through the activation of cholera or pertussis toxin sensitive G proteins. Furthermore, it does not employ the adenylyl cyclase–cAMP–PKA cascade, the phospholipase C signalling pathway and NMDA receptor-coupled mechanisms to depress excitatory synaptic transmission in the PBN.