Nocistatin excites rostral agranular insular cortex-periaqueductal gray projection neurons by enhancing transient receptor potential cation conductance via Gαq/11-PLC-protein kinase C pathway

Abstract

Rostral agranular insular cortex (RAIC) projects to periaqueductal gray (PAG) and inhibits spinal nociceptive transmission by activating PAG-rostral ventromedial medulla (RVM) descending antinociceptive circuitry. Despite being generated from the same precursor prepronociceptin, nocistatin (NST) and nociceptin/orphanin FQ (N/OFQ) produce supraspinal analgesic and hyperalgesic effects, respectively. Prepronociceptin is highly expressed in the RAIC. In the present study, we hypothesized that NST and N/OFQ modulate spinal pain transmission by regulating the activity of RAIC neurons projecting to ventrolateral PAG (RAIC–PAG). This hypothesis was tested by investigating electrophysiological effects of N/OFQ and NST on RAIC–PAG projection neurons in brain slice. Retrogradely labeled RAIC–PAG projection neurons are layer V pyramidal cells and express mRNA of vesicular glutamate transporter subtype 1, a marker for glutamatergic neurons. N/OFQ hyperpolarized 25% of RAIC–PAG pyramidal neurons by enhancing inwardly rectifying potassium conductance via pertussis toxin-sensitive Gαi/o. In contrast, NST depolarized 33% of RAIC–PAG glutamatergic neurons by causing the opening of canonical transient receptor potential (TRPC) cation channels through Gαq/11-phospholipase C-protein kinase C pathway. There were two separate populations of RAIC–PAG pyramidal neurons, one responding to NST and the other one to N/OFQ. Our results suggest that Gαq/11-coupled NST receptor mediates NST excitation of RAIC–PAG glutamatergic neurons, which is expected to cause the supraspinal analgesia by enhancing the activity of RAIC–PAG–RVM antinociceptive pathway. Opposite effects of NST and N/OFQ on supraspinal pain regulation are likely to result from their opposing effects on RAIC–PAG pyramidal neurons.