Number and type of synapses on the distal dendrite of a rat striatal cholinergic interneuron: a quantitative, ultrastructural study

Knowledge of the innervation of interneurons within the striatum is critical to determining their role in the functioning of the striatal network. To this end, the synaptic innervation of a distal dendrite of a rat striatal cholinergic interneuron was quantified for the first time. These synaptic data were compared to three other dendrites from rat striatal interneurons and to published data from dendrites in the mammalian cerebral cortex. To label the cholinergic interneurons and their distal dendrites, a male Wistar rat was perfused and the striatum was double-immunolabelled with an antibody to choline acetyltransferase (ChAT) and an antibody to m2 muscarinic receptor. After processing for transmission electron microscopy, a cholinergic interneuron was located and an m2-labelled distal dendrite identified by tracing it through serial ultrathin sections to this double-immunolabelled soma. Two interneuronal distal dendrites in the same tissue, and another from a second rat, were used for comparison. The widths and lengths of the four distal dendrites, the total number and type of synapses, and the number of synapses per μm for each distal dendrite were measured. Symmetric synapses were the most common type on all four dendrites. There were 0.73 synapses per μm on the distal dendrite of the identified striatal cholinergic interneuron. Two other interneuronal dendrites that were positive for the m2 muscarinic receptor antibody showed similar synaptic densities of 0.62 and 0.83 synapses per μm of distal dendrite, respectively. On a third unlabelled interneuronal distal dendrite located in the lateral striatum, there were 2.17 synapses per μm. This interneuron was thought to be a parvalbumin interneuron rather than a calretinin interneuron, which would more likely be medially located. These data suggest that the number of synapses per μm on the distal dendrite of the cholinergic interneuron, and possibly two other cholinergic interneurons, is three times lower than that of a likely parvalbumin interneuron in the rat striatum. The number of synapses per μm of distal dendrite for a striatal cholinergic interneuron is also lower than the published 1.22–3.3 synapses per μm of dendrite for neurons in the mammalian cerebral cortex. Such anatomical data are important for the construction of new generation computer models that are better able to emulate the operation of striatal cholinergic interneurons.