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synaptic contacts mainly with the necks of dendritic spines of MSN (Bouyer et al., 1984;
Freund et al., 1984; Smith et al., 1994). The head of spines that receive the dopaminergic
input invariably receive glutamatergic input from terminals forming asymmetric synapses
(Freund et al., 1984), which are generally derived from the cortex (Bouyer et al., 1984; Smith
et al., 1994) (see Fig. 8). This anatomical arrangement is ideally suited for the dopamine
released from the nigrostriatal terminal, which is likely to act on dopamine receptors localized
both within the synapse and at extrasynaptic sites (Yung et al., 1996), to very selectively
modulate the response to the excitatory input at the head of the spine. Other inputs to spiny
neurons are the cholinergic, which exhibit a similar anatomical organization (Izzo and Bolam,
1988) and GABAergic terminals also observed in contact with the necks of spines (Bolam
and Bennett, 1995).
Figure 7. The major synaptic types of the striatal spiny neuron. The distal spiny dendrites receive inputs
with round synaptic vesicles, which form asymmetrical contacts primarily on the dendritic spines, but
occasionally on the shafts of the dendrites. These arise mostly from afferent fibers from the cerebral
cortex (Cx) and thalamus (Thal), which contact the head of the spines. Spiny cell collaterals, TH-
staining axons from substantia nigra compacta (SNc), and intrinsic intrastriatal connections (from
aspiny cholinergic neurons) form symmetrical contacts with pleomorphic and flattened vesicles on the
stalks of dendritic spines, on the proximal part of the spine heads, and on dendritic shafts. The inputs to
the soma or to the proximal surface of the spiny projection neurons are from interneurons and collateral
arborizations of the spiny neurons. They form symmetrical synapses with pleomorphic or flattened
synaptic vesicles at very low density on the aspiny initial portion of the dendrites, the soma and axon
initial segment (Modified from Gerfen, 1988 and Wilson, 1996).
For many years, much of the work devoted to the striatum has focused on the
dopaminergic nigrostriatal input (Björklund and Lindvall 1984). This interest has been
sustained by the realization, in the early 1960s, that loss of dopaminergic neurons in the pars
compacta is the hallmark of PD (Bernheimer et al., 1973). As a result, many studies have
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