Serotonergic modulation of slow inward rectification in mesencephalic trigeminal neurons

Epigenetic HDAC5 Inhibitor Reverses Craniofacial Neuropathic Pain in Mice

Identifying and resolving molecular complexities underlying chronic neuropathic pain is a significant challenge. Among the numerous classes of histone deacetylases, Class I (HDAC 1-3) and Class III (sirtuins) have been best studied in experimental pain models where inhibitor pre-treatments but not post-treatments abrogate the development of pain-related behaviors. Post-treatment here in week 3 with less well-studied Class IIa HDAC4/5 selective inhibitor LMK235 diminishes the trigeminal ganglia increases of HDAC5 RNA and protein in two chronic orofacial neuropathic pain models to levels measured in naïve mice at week 10 post-model induction. HDAC4 RNA reported in lower limb inflammatory pain models is not evident in the trigeminal models. Many other gene alterations persisting at week 10 in the trigeminal ganglia (TG) are restored to naïve levels in mice treated with LMK235. Important pain-related upregulated genes Hoxc8,b9,d8; P2rx4, Cckbr, growth hormone (Gh), and schlafen (Slfn4) are greatly reduced in LMK235-treated mice. Fold increase in axon regeneration/repair genes Sostdc1, TTr, and Folr1 after injury are doubled by LMK235 treatment. LMK235 reduces the excitability of trigeminal ganglia neurons in culture isolated from nerve injured mice compared to vehicle-treated controls, with no effect on neurons from naïve mice. Electrophysiological characterization profile includes a shift where ∼20% of the small neurons recorded under LMK235-treated conditions are high threshold, whereas none of the neurons under control conditions have high thresholds. LMK235 reverses long-standing mechanical and cold hypersensitivity in chronic trigeminal neuropathic pain models in males and females (5,10 mg/kg), preventing development of anxiety- and depression-like behaviors. PERSPECTIVE: Data here support HDAC5 as key epigenetic factor in chronic trigeminal neuropathic pain persistence, validated with the study of RNA alterations, TG neuronal excitability, and pain-related behaviors. HDAC5 inhibitor given in week 3 restores RNA balance at 10 weeks, while upregulation remains for response to wound healing and chronic inflammation RNAs.

Lysophosphatidic Acid and Several Neurotransmitters Converge on Rho-Kinase 2 Signaling to Manage Motoneuron Excitability

Intrinsic membrane excitability (IME) sets up neuronal responsiveness to synaptic drive. Several neurotransmitters and neuromodulators, acting through G-protein-coupled receptors (GPCRs), fine-tune motoneuron (MN) IME by modulating background K⁺ channels TASK1. However, intracellular partners linking GPCRs to TASK1 modulation are not yet well-known. We hypothesized that isoform 2 of rho-kinase (ROCK2), acting as downstream GPCRs, mediates adjustment of MN IME via TASK1. Electrophysiological recordings were performed in hypoglossal MNs (HMNs) obtained from adult and neonatal rats, neonatal knockout mice for TASK1 (task1^–/–) and TASK3 (task3^–/–, the another highly expressed TASK subunit in MNs), and primary cultures of embryonic spinal cord MNs (SMNs). Small-interfering RNA (siRNA) technology was also used to knockdown either ROCK1 or ROCK2. Furthermore, ROCK activity assays were performed to evaluate the ability of various physiological GPCR ligands to stimulate ROCK. Microiontophoretically applied H1152, a ROCK inhibitor, and siRNA-induced ROCK2 knockdown both depressed AMPAergic, inspiratory-related discharge activity of adult HMNs in vivo, which mainly express the ROCK2 isoform. In brainstem slices, intracellular constitutively active ROCK2 (aROCK2) led to H1152-sensitive HMN hyper-excitability. The aROCK2 inhibited pH-sensitive and TASK1-mediated currents in SMNs. Conclusively, aROCK2 increased IME in task3^–/–, but not in task1^–/– HMNs. MN IME was also augmented by the physiological neuromodulator lysophosphatidic acid (LPA) through a mechanism entailing G_αi/o-protein stimulation, ROCK2, but not ROCK1, activity and TASK1 inhibition. Finally, two neurotransmitters, TRH, and 5-HT, which are both known to increase MN IME by TASK1 inhibition, stimulated ROCK2, and depressed background resting currents via G_αq/ROCK2 signaling. These outcomes suggest that LPA and several neurotransmitters impact MN IME via G_αi/o/G_αq-protein-coupled receptors, downstream ROCK2 activation, and subsequent inhibition of TASK1 channels.

Membrane excitabilities in neonatal rat mesencephalic trigeminal neurons under dietary zinc deficiency

PURPOSE

The present study aimed to evaluate the effects of zinc deprivation on the properties of membrane and spike-discharge features of mesencephalic trigeminal neurons (MTNs), which are important sensory neurons for oral-motor reflexes and rhythmical jaw movements.

METHODS

Neonatal Sprague Dawley rats (P10-12) were distributed equally into a normal diet group and a zinc-deficient diet (ZD) group. Whole cell patch-clamp recordings were obtained from MTNs from coronal brain slices.

RESULTS

Passive membrane properties showed a modest depolarized membrane potential and decreased cell capacitance in the ZD group. Zinc deprivation decreased the minimal current amplitude, which induced an action potential and increased the amplitude of afterhyperpolarization following the action potential. Negligible changes were observed for other action potential properties. A decreased burst duration was observed, accompanied by hastened spike frequency adaptation in the burst discharge. There was no difference in the resonant properties at both the subthreshold depolarized potential and hyperpolarized membrane potential between the control and ZD groups.

CONCLUSION

These results suggests that neither the persistent sodium conductance nor slow inwardly rectifying conductance were altered; however, there appeared to be an increase in Ca²⁺-dependent K⁺ conductance in zincdeficient MTNs.