Orexin-A Modulates Firing of Rat Rostral Ventromedial Medulla Neurons: An in Vitro Study

The rostral ventromedial medulla (rvm) acts a key role in the descending inhibitorypain modulation. neuropeptide orexin-a (orxa) is confined to thousands of neuronsin the lateral hypothalamus (lh). while rvm gets the orexinergic projections, theorexin receptors are also expressed in this structure. the aim of this study was tospecify the cellular effects of orxa on rvm neurons in vitro by using the whole cellpatch-clamp recording. rvm neurons were classified into three types based on theirelectrophysiological characteristics. type 1 neurons exhibited an irregular spontaneousactivity which was interrupted by periods of pause in 25% of recorded neurons.type 2 neurons did not show any spontaneous baseline activity (53.8% of recordedneurons). type 3 neurons fired repetitively without interruption (51.2% of recordedneurons). orxa had either inhibitory or excitatory effects on 53.8% (7/13) of type 1neurons. orxa excited 46.4% (13/28) of type 2 neurons and 27.3% (3/11) of type 3neurons. the excitatory effect of orxa observed in type 2 neurons was suppressedby an orexin 1 receptor (oxr1) antagonist, sb-334867. briefly, we hypothesizedthat the orxa mediated excitation and/or inhibition in rvm neurons might work as amechanism to modulate pain processing by orexinergic neurons.