Role of orexin in regulating synaptic transmission and spine morphology in central vestibular system

Abstract

Orexin is a neuropeptide produced by the lateral hypothalamus. In the hippocampus, it is known to modulate synaptic plasticity in the hippocampus and contributes to social memory in adult rodents. While orexin neurons in the lateral hypothalamus project to the vestibular nucleus (VN), the role of orexin in the maturation of vestibular functions remains unexplored. To understand the influence of orexin in the vestibular system, we targeted medial vestibular nucleus (MV) to study how orexin regulates synaptic transmission and morphology of dendritic spines and reaction to vestibular stimulus.

Using whole-cell patch-clamp recording, we studied the effect of orexin on miniature inhibitory postsynaptic currents (mIPSC) of MV neurons in P14 rats. Both the frequency and amplitude of GABAA receptor-mediated mIPSCs decreased significantly upon orexin treatment. Bath application of OX1R agonist or OX2R agonist allowed elucidation of the special effects of OX1R and OX2R, and the mIPSCs recorded showed a decrease in frequency but not in amplitude upon agonist application. These findings suggest that orexin modulates synaptic transmission in MV by suppressing the release of inhibitory transmitter GABA and the activity of postsynaptic receptor, while OX1R or OX2R suppress presynaptic transmitter release of GABA.

To figure out if orexin has a long-term impact on postnatal development of the vestibular system, we perturbed orexin receptors in the VN of rats at P1 by surgically implanting into the fourth ventricle an Elvax slice for slow release of drug to the VN. At P14, we used Golgi staining to exam the spine morphology in different treatment groups. There are fewer mature spines and more immature spine in MV sections of rats pre-treated with orexin at P1, as compared to those of rats treated with saline. In rats pre-treated with OX1R antagonist at P1, there were more mature spines and fewer immature spines in MV sections. It suggests that orexin treatment at postnatal stage may lead to a delay in the postnatal development in the VN.

We further addressed the impact of orexin on MV in response to vestibular stimuli. The expression of Fos in neurons within VN differed a lot between OX1R antagonist and OX2R agonist group. The sections of rats implanted with OX1R antagonist-loaded Elvax showed an abundant number of Fos-immunoreactive (Fos-ir) neurons, while much fewer Fos-ir neurons were located in VN sections of rats implanted with OX2R agonist, suggesting that orexin has inhibitory effect on VN by decreasing its sensitivity to vestibular stimuli.

Taken together, we demonstrated that orexin regulates synaptic transmission and spine morphology in VN, as well as the MV neuron sensitivity to vestibular stimulation.