Chemically specific circuit composed of vesicular glutamate transporter 3-and preprotachykinin B-producing interneurons in the rat neocortex

The third vesicular glutamate transporter, VGLUT3, is distributed in cell bodies of neocortical neurons and axon terminals mainly in the superficial part of layer II/III of the cerebral cortex. We examined the chemical characteristics of VGLUT3-expressing neurons by immunohistochemistry in the rat neocortex. Since the vast majority of VGLUT3-immunoreactive neurons showed immunoreactivities for GABA, preprotachykinin B (PPTB) and cholecystokinin, VGLUT3-immunoreactive neocortical neurons were considered to constitute a subgroup of GABAergic interneurons. VGLUT3-immunoreactive axon terminals were immunopositive for either vesicular GABA transporter (VGAT) or serotonin. These results together with anterograde tracer injection and chemical lesion experiments in the dorsal and median raphe nuclei revealed that the neocortex contains at least two kinds of VGLUT3-laden axon terminals: one is serotonergic and derived from the raphe nuclei, and the other is GABAergic and intrinsic in the neocortex. Furthermore, many VGLUT3/VGAT-immunoreactive terminals formed axon baskets and made axosomatic symmetric synapses on neocortical neurons, most of which were immunoreactive for PPTB. VGLUT3-immunopositive axon baskets surrounded about a half of PPTB-positive and almost all VGLUT3-positive neurons. Thus, VGLUT3-expressing GABAergic interneurons form a chemically specific circuit within the PPTB-producing interneuron group and it is likely that glutamate is used within the chemically specific circuit.