Efferent and afferent connections of GABAergic neurons in the supratrigeminal and the intertrigeminal regions

It has been reported in the cat and rat that inhibitory premotor neurons, which send their axons to motoneurons of the trigeminal motor nucleus (Vm) are distributed in the reticular regions around the Vm, especially in the supratrigeminal region (Vsup) and the intertrigeminal region (Vint). In the present study, we examined neuronal connections of GABAergic neurons in the Vsup and Vint in the mouse by utilizing the adult heterozygous GAD67-GFP knock-in mouse, in which green fluorescence protein (GFP) is expressed in GABAergic neurons under the control of the endogenous GAD (GAD67) gene promoter [Yanagawa, Y., Kaneko, K., Kanbara, N., Totsuka, M., Yagi, T., Obata, K., 2001. Development of mouse expressing GFP in GABAergic neurons. Neurosci. Res. Suppl. 25, S77; Tamamaki, N., Yanagawa, Y., Tomioka, R., Miyazaki, J.-I., Obata, K., Kaneko, T., 2003. Green fluorescent protein expression and colocalization with calretinin, parvalbumin and somatostatin in the GAD67-GFP knock-in mouse. J. Comp. Neurol. 467, 60-79]. The connections were examined light- and electron-microscopically by combining the anterograde or the retrograde tract-tracing method with the immunohistochemical method for GFP. The data indicated that the Vsup and Vint of the mouse contained GABAergic neurons, which received projection fibers from the marginal layer of the nucleus of the spinal tract of the trigeminal nerve (Vc) on the ipsilateral side and sent their axons to the Vm on the contralateral side. Some of these GABAergic neurons may represent Vm-premotor neurons that receive nociceptive input from the Vc to elicit jaw-opening reflex by inhibiting jaw-closing Vm-motoneurons.