Synaptic relationships between dopaminergic afferents and cortical or thalamic input in the sensorimotor territory of the striatum in monkey.

The cerebral cortex and the intralaminar thalamic nuclei are the major sources of excitatory glutamatergic afferents to the striatum, whereas the midbrain catecholaminergic neurones provide a dense intrastriatal plexus of dopamine-containing terminals. Evidence from various sources suggests that there is a functional interaction between the glutamate- and dopamine-containing terminals in the striatum. The aim of the present study was to determine the synaptic relationships between cortical or thalamic inputs and the dopaminergic afferents in the sensorimotor territory of the monkey striatum. To address this issue, anterograde tracing in combination with immunocytochemistry for tyrosine hydroxylase (TH) was carried out by light and electron microscopy. Squirrel monkeys received injections of biocytin in the primary motor and somatosensory cortical areas or injections of either Phaseolus vulgaris-leucoagglutinin (PHA-L) or biocytin in the centromedian nucleus (CM) of the thalamus. Sections that included the striatum were processed to visualize the anterograde tracers alone or in combination with TH immunoreactivity. The anterogradely labelled fibres from the cerebral cortex and CM display a band-like pattern and are exclusively confined to the postcommissural region of the putamen, whereas TH-immunoreactive axon terminals are homogeneously distributed throughout the entire extent of the striatum. Electron microscopic analysis revealed that the anterogradely labelled terminals from the cerebral cortex form asymmetric synapses almost exclusively with the heads of dendritic spines. The thalamic terminals also form asymmetric synapses, but in contrast to cortical fibres, predominantly with dendrites (67.4%) and less frequently with spines (32.6%). The TH-immunoreactive boutons are heterogeneous in morphology. The most common type (84% of the total population) forms symmetric synapses; of these the majority is in contact with dendritic shafts (72.1%), less with spines (22.5%) and few with perikarya (5.4%). In sections processed to reveal anterogradely labelled cortical fibres and TH-immunoreactive structures, individual spines of striatal neurones were found to receive convergent synaptic inputs from both cortical and TH-immunoreactive boutons. In contrast, anterogradely labelled thalamic terminals and TH-immunoreactive boutons were never seen to form convergent synaptic contacts on the same postsynaptic structure. These findings suggest that the dopaminergic afferents are located to subserve a more specific modulation of afferent cortical input than afferent thalamic input in the sensorimotor territory of the striatum in primates.