Orexin facilitates GABAergic IPSCs via postsynaptic OX1 receptors coupling to the intracellular PKC signalling cascade in the rat cerebral cortex.

Orexin has multiple physiological functions including wakefulness, appetite, nicotine intake, and nociception. The cerebral cortex receives abundant orexinergic projections and expresses both orexinergic receptor 1 (OX1R) and 2 (OX2R). However, little is known about orexinergic regulation of GABA-mediated inhibitory synaptic transmission. In the cerebral cortex, there are multiple GABAergic neural subtypes, each of which has its own morphological and physiological characteristics. Therefore, identification of presynaptic GABAergic neural subtypes is critical to understand orexinergic effects on GABAergic connections. We focused on inhibitory synapses at pyramidal neurons (PNs) from fast-spiking GABAergic neurons (FSNs) in the insular cortex by a paired whole-cell patch-clamp technique, and elucidated the mechanisms of orexin-induced IPSC regulation. We found that both orexin A and orexin B enhanced unitary IPSC (uIPSC) amplitude in FSN→PN connections without changing the paired-pulse ratio or failure rate. These effects were blocked by SB-334867, an OX1 receptor (OX1R) antagonist, but not by TCS-OX2-29, an OX2R antagonist. [Ala11, D-Leu15]-orexin B, a selective OX2R agonist, had little effect on uIPSCs. Variance-mean analysis demonstrated an increase in quantal content without a change in release probability or the number of readily releasable pools. Laser photolysis of caged GABA revealed that orexin A enhanced GABA-mediated currents in PNs. Downstream blockade of Gq/11 protein-coupled OX1Rs by IP3 receptor or protein kinase C (PKC) blockers and BAPTA injection into postsynaptic PNs diminished the orexin A-induced uIPSC enhancement. These results suggest that the orexinergic uIPSC enhancement is mediated via postsynaptic OX1Rs, which potentiate GABAA receptors through PKC activation.

Menthol-induced facilitation of cerebrocortical excitatory propagation induced by air puff stimulation of the nasal cavity in the rat: An optical imaging study.

Temperature plays a critical role in the sensation of airflow in the nasal mucosa. Neural activities of the ethmoidal nerve, a trigeminal afferent, responding to airflow are suppressed by warm airflow, whereas cold airflow enhances the ethmoidal nerve activities, which is mimicked by application of menthol, a cold-sensitive TRPM8 receptor agonist. However, it has been an open issue how menthol modulates the spatiotemporal profiles of neural activities of somatosensory cortical neurons. In this study, we assessed neural responses to an air puff stimulation (100 ms) to the nasal cavity in the absence or presence of l-menthol using an optical imaging technique with a voltage-sensitive dye in the primary cortex (S1) of urethane-anesthetized rats. A weak air puff application (15 psi) without l-menthol induced neural excitation in a part of the contralateral S1. The air puff stimulation with l-menthol significantly increased the optical signal intensity, expanded the activated area, and shortened the latency, compared to those in the absence of l-menthol. These results suggest that activation of cold-sensitive TRPM8 receptors sharpens airflow sensation in the nasal cavity and expands the receptive field, especially toward the pharynx, which may contribute to enhanced flavor perception.