UBC Theses and Dissertations
The effect of intraganglionic injection of GABA and GABA agonists on peripheral sensory transmission Ranjbar Ekbatan, Maryam
Due to the lack of chemical synapses in the trigeminal ganglion, it is widely believed that it does not play a role in transmission of sensory signals. However, it has been shown that neurons and satellite glial cells in trigeminal ganglion express neuroreceptors such as γ-aminobutyric acid (GABA)A and GABAB receptors and can release neurotransmitters like GABA. Increasing the levels of GABA in the trigeminal ganglion was showed to have anti-nociceptive effects, while blockade of GABA receptor expression in the trigeminal ganglion was demonstrated to increase pain behaviors. However, the neuronal mechanism underlying these effects remains to be examined. In the current study, the expression of GABA receptors in the trigeminal ganglion neurons that innervate labial skin and masseter muscle was evaluated by immunohistochemistry. Using single unit recording, trigeminal brainstem and ganglion neuron responses to stimulation of labial skin and/or masseter muscle were evaluated after intraganglionic injections of GABA receptor agonists in 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. In both skin and muscle ganglion neurons, the expression of GABAA was higher than GABAB. There was a higher frequency of GABAA as well as GABAB receptor expression in ganglion neurons that innervated the skin compared with those that innervated muscle. In ganglion neurons that innervated the skin, there was a higher expression of GABAA receptors and GABAB receptors in males compared to females. Masticatory muscle evoked brainstem trigeminal neuron responses were attenuated by intraganglionic injection of muscimol (GABAA), but not baclofen (GABAB). Compared to Phosphate Buffered Saline (PBS), all substances reduced mechanical threshold 30 minutes post injection. Further, GABA (500 mM) and baclofen 10 mM decreased mechanical threshold (MT) compared to PBS for the entire recording period. All these changes were statistically significant. 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. This study suggests that activation of peripheral GABA receptors may exert a selective gating effect on sensory input passing through trigeminal ganglion.
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