GABAB receptors in the NTS mediate the inhibitory effect of trigeminal nociceptive inputs on parasympathetic reflex vasodilation in the rat masseter muscle

Attenuation of Sensory Transmission Through the Rat Trigeminal Ganglion by GABA Receptor Activation

While the trigeminal ganglion is often considered a passive conduit of sensory transmission, neurons and satellite glial cells (SGCs) within it can release neurotransmitters and express neuroreceptors. Some trigeminal ganglion neurons contain the neurotransmitter γ-aminobutyric acid (GABA) and express GABA receptors. There is behavioral evidence that increased GABA levels in the trigeminal ganglion decreases nociception, while a loss of GABA receptors results in hyperalgesia, although the neural mechanisms for this remain to be investigated. In this study, the expression of GABA receptors by trigeminal ganglion neurons that innervate rat labial skin and masseter muscle was compared using immunohistochemistry. The effect of intraganglionic administration of GABA receptor agonists was investigated by single unit recording of trigeminal brainstem and ganglion neuron responses to stimulation of the labial skin and/or masseter muscle in anesthetized rats. The mean frequency of expression of GABA_A and GABA_B receptors by masseter and labial skin ganglion neurons was 62.5% and 92.7%, and 55.4% and 20.3%, respectively. The expression of both GABA receptors was significantly greater in skin ganglion neurons. Masticatory muscle evoked brainstem trigeminal neuron responses were significantly attenuated by intraganglionic injection of muscimol (GABA_A) but not baclofen (GABA_B). The mechanical sensitivity of slow and fast conducting masticatory muscle afferent fibers was decreased and increased, respectively, by intraganglionic injection of both muscimol and baclofen. Activation of GABA_A receptors may exert a gating effect on sensory transmission through the trigeminal ganglion by decreasing putative nociceptive input and enhancing innocuous sensory input.

The Role of Spinal GABAB Receptors in Cancer-Induced Bone Pain in Rats

Cancer-induced bone pain (CIBP) remains a major challenge in advanced cancer patients because of our lack of understanding of its mechanisms. Previous studies have shown the vital role of γ-aminobutyric acid B receptors (GABABRs) in regulating nociception and various neuropathic pain models have shown diminished activity of GABABRs. However, the role of spinal GABABRs in CIBP remains largely unknown. In this study, we investigated the specific cellular mechanisms of GABABRs in the development and maintenance of CIBP in rats. Our behavioral results show that acute as well as chronic intrathecal treatment with baclofen, a GABABR agonist, significantly attenuated CIBP-induced mechanical allodynia and ambulatory pain. The expression levels of GABABRs were significantly decreased in a time-dependent manner and colocalized mostly with neurons and a minority with astrocytes and microglia. Chronic treatment with baclofen restored the expression of GABABRs and markedly inhibited the activation of cyclic adenosine monophosphate (cAMP)-dependent protein kinase and the cAMP-response element-binding protein signaling pathway.

PERSPECTIVE

Our findings provide, to our knowledge, the first evidence that downregulation of GABABRs contribute to the development and maintenance of CIBP and restored diminished GABABRs attenuate CIBP-induced pain behaviors at least partially by inhibiting the protein kinase/cAMP-response element-binding protein signaling pathway. Therefore, spinal GABABR may become a potential therapeutic target for the management of CIBP.

Interactions between β-adrenergic vasodilation and cervical sympathetic nerves are mediated by α2-adrenoceptors in the rat masseter muscle

Neural and humoral autonomic mechanisms may be important in the maintenance of blood flow in the masseter muscle (MBF). However, their interactions remain unclear. In this study, we examined interactions between neural and humoral regulation of MBF and investigated the mechanisms mediating these interactions in urethane-anesthetized rats. Stimulation of the adrenal nerve (AN) projecting to the adrenal medulla increased MBF, and this increase was mediated by β-adrenoceptors. Sectioning of the superior cervical sympathetic trunk (CST) significantly inhibited increases in MBF induced by AN stimulation during high activity in the CST, but not during low activity. AN stimulation with clonidine after CST sectioning induced a significant increased in MBF, however phenylephrine had no observable effect. Pretreatment with yohimbine or propranolol significantly inhibited the increase in the MBF. Our results suggest an interaction between β-adrenergic vasodilation evoked by circulating adrenaline and the cervical sympathetic nerves that is mediated by α₂-adrenoceptors in the masseter muscle.

Parasympathetic reflex vasodilation in the cerebral hemodynamics of rats

We investigated the role of parasympathetic reflex vasodilation in the regulation of the cerebral hemodynamics, and whether GABAA receptors modulate the response. We examined the effects of activation of the parasympathetic fibers through trigeminal afferent inputs on blood flow in the internal carotid artery (ICABF) and the cerebral blood vessels (rCBF) in parietal cortex in urethane-anesthetized rats. Electrical stimulation of the central cut end of the lingual nerve (LN) elicited intensity- and frequency-dependent increases in ICABF that were independent of changes in external carotid artery blood flow. Increases in ICABF were elicited by LN stimulation regardless of the presence or absence of sympathetic innervation. The ICABF increases evoked by LN stimulation were almost abolished by the intravenous administration of hexamethonium (10 mg kg(-1)) and were reduced significantly by atropine administration (0.1 mg kg(-1)). Although the LN stimulation alone had no significant effect on rCBF, LN stimulation in combination with a blocker of the GABAA receptor pentylenetetrazole increased the rCBF markedly. This increase in rCBF was reduced significantly by the administration of hexamethonium and atropine. These observations indicate that the increases in both ICABF and rCBF are evoked by parasympathetic activation via the trigeminal-mediated reflex. The rCBF increase evoked by LN stimulation is thought to be limited by the GABAA receptors in the central nervous system. These results suggest that the parasympathetic reflex vasodilation and its modulation mediated by GABA receptors within synaptic transmission in the brainstem are involved in the regulation of the cerebral hemodynamics during trigeminal afferent inputs.