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 GABAA and GABAB 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 (GABAA) but not baclofen (GABAB). 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 GABAA receptors may exert a gating effect on sensory transmission through the trigeminal ganglion by decreasing putative nociceptive input and enhancing innocuous sensory input.

α1 adrenergic receptor activation has a dynamic effect on masticatory muscle afferent fibers.

Temporomandibular Disorder (TMD) patients report amplification of pain in the masticatory muscles after psychological trauma or stressful conditions. The mechanisms underlying this phenomenon are yet to be elucidated. This study combined immunohistochemistry with single cell in vivo electrophysiology recordings of masticatory muscle afferent fibers to investigate the role of α1-adrenergic receptors in muscle nociception. It was found that a subset of trigeminal afferent fibers which innervate the masseter and temporal muscles expressed α1a, α1b and α1d receptors, including a smaller number of putative nociceptors which co-expressed TrpV1 receptors. Local injection of the selective α1 adrenergic receptor agonist phenylephrine into masticatory muscle decreased and increased the mechanical activation threshold of slow and fast conducting afferent fibers, respectively. This effect was reversed by co-administration of the α1 selective antagonist terazosin. To rule out the possibility that local ischemia was responsible for the observed effect of phenylephrine on masticatory muscle afferent fibers, additional experiments were conducted where blood flow to the masticatory muscle was reduced by common carotid artery occlusion. This investigation found that muscle blood flow occlusion increased the mechanical activation threshold of the majority of masticatory muscle afferent fibers unrelated to conduction velocity. These findings suggest that under conditions of increased sympathetic tone, such as those related to stress, noradrenaline may sensitize masticatory muscle nociceptors to increase pain and desensitize muscle proprioceptors to alter muscle tone, through activation of α1 receptors.

Evaluation of Anticonvulsant Effect of Aqueous and Methanolic Extracts of Seven Inula Species.

In Iranian traditional medicine, Inula species have been used for the treatment of seizure. In this study we decided to investigate the anticonvulsant activity of seven species from this genus to find an effective remedy for seizure with less adverse effects compared to the available medicines. Aqueous and methanolic extracts of Inula britannica, Inula helenium, Inula viscidula, Inula oculus-christi, Inula aucheriana, Inula thapsoides, and Inula salicina were prepared and their antiepileptic activity was investigated by maximal electroshock (MES) and pentylentetrazole (PTZ) tests on Male NMRI Albino mice. Diazepam was used as positive control in both tests. In addition, two extracts with the best anticonvulsant activities were selected and their sedative and hypnotic effects were evaluated using open field and righting reflex tests, respectively. The effects of the both extracts on memory and motor coordination were also assessed by step-through passive avoidance and rotarod tests, respectively. Aqueous extract of Inula britannica and Inula viscidula showed the best activity in MES model and their ED50 (with 95% confidence interval) was 19.5 (7.9~48.5) mg/kg and 12.7(10.0~16.3) mg/kg, respectively. None of the extracts showed noticeable anticonvulsant effects in the PTZ model. The active extracts also showed sedative-hypnotic effects in righting reflex and open field tests. Furthermore, both extracts did not affect the memory and motor coordination in the experimental models. The anticonvulsant and sedative activities of the extracts were antagonized by flumazenil, indicating that benzodiazepine receptors are probably involved in the effects. This study indicates that Inula britannica and Inula viscidula are good candidates for further phytochemical and mechanistic studies in order to find anticonvulsant and sedative-hypnotic compounds with less adverse effect on memory and motor coordination.