Morphological analysis of the amygdalostriatal pathway in the control of emotional movements

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It has been known that the ventrolateral part of the striatum (CPvl) receives cortical fibers from the orofacial motor area and send projection fibers to the dorsolateral part of the substantia nigra pars reticulata (SNdl), which contains many neurons projecting to the premotor neuron pools for the orofacial motor nuclei. Recently, we found that the CPvl also received amygdaloid fibers from the rostral part of the basolateral nucleus (BLr). In the present study, we examined the possible pathway from the BLr to the SNdl via the CPvl by using the combined anterograde and retrograde tracing technique, and the anterograde tracing combined with immunohistochemistry for chemical markers of the striatonigral neurons. After ipsilateral injections of biotinylated dextran amine (BDA) into the BLr and cholera toxin B subunit (CTb) into the SNdl, a prominent overlapping distribution of BDA-labeled fibers and CTb-labeled neuronal cell bodies was found in the CPvl at the level of the decussation of the anterior commissure. In this region, electron microscopic observations revealed that the BDA-labeled axon terminals containing densely-packed, round synaptic vesicles made asymmetrical synaptic contacts mostly with the dendritic spines of striatal neurons, some of which were labeled with CTb. Furthermore, we demonstrated that the BDA-labeled BLr fibers formed synapses with the CPvl neurons showing immunoreactivity for preprotachykinin A and preprodynorphin, both of which are chemical makers for the striatonigral neurons. Thus, the BLr-CPvl-SNdl pathway revealed by the present study may be involved in the control of emotional/orofacial movements.