Localization Within the Central Nervous System

The cholinergic system of brain tissue can be divided into three different subsystems:

• Cholinergic motoneurons in the spinal cord: The collaterals of these neurons activate small interneurons in the ventral horn of the spinal cord (Renshaw cells), which express nicotinic receptors.

• Interneurons and local projection neurons: The most representative neurons of this type are interneurons in the striatum. These interneurons interact with the dopaminergic terminals of neurons which project from the substantia nigra into the striatum. In addition, sparsely distributed cholinergic interneu-rons are located in the cortex, the hippocampus and in the olfactory bulb.

• Projection neurons: Different groups of cholinergic projection neurons can be distuingished according to a nomenclature which was established in 1983 by Mesulam and coworkers. Group Ch1 and Ch2 correspond with cholinergic neurons in the region of the medial septal nucleus and with neurons in the diagonal band of Broca. These neurons project to the hippocampus. Group Ch3 is located in the horizontal band of Broca. Neurons of this group innervate the olfactory bulb. Members of group Ch4 are represented by neurons of the magnocellular region of the preoptic nucleus, the magnocellular region of the nucleus basalis of Meynert and in the substantia innominata. These neurons project to the cerebral cortex and to the amygdala. Members of groups Ch5 and Ch6 are located in tegmental areas of the brain. They possess ascending projections to the thalamus and to the hypothalamus as well as descending projections. The descending projections approach the pons, the nucleus vestibularis, the locus coeruleus and various raphe nuclei. The neurons of group Ch7 occur in the habenula. They project to the interpeduncular nucleus. Finally, neurons of group Ch8 are located in the parabigeminal nucleus and send projections into the superior colliculus (Fig. 3.2).

Acetylcholine has been shown to occur in colocalization with GABA, though the physiological relevances of this coexpression is not known.

Terminals of these neurons can form excitatory asymmetric synapses with dendrites and symmetric, inhibitory synapses with additional dendrites. This ar-

Fig. 3.2 Schematic representation of the central cholinergic projections in the brain of rats. Abbreviations: Am = amygdala; BO = bulbus olfactorius; Cer= cerebellum; Cs = superior colliculus; CPu = caudate-putamen; HI = hippocampus; IPN = inter-peduncular nucleus; Th = thalamus.

borization allows a single neuron to exert different effects via functionally differentiated synapses.

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