The projection from the striatum to the nucleus basalis in the rat: an electron microscopic study

Previous studies have shown that the striatum provides synaptic inputs to the globus pallidus and entopeduncular nucleus in which GABA is co-localized with the peptides enkephalin and substance P. The aim of this study in the rat was to determine whether the striatal projections also make synaptic contact with the cholinergic neurons of the nucleus basalis, which lie near to the pallidal areas in the rat brain. The anterograde tracer biocytin was injected into different parts of the striatum, and brain sections were stained for biocytin and choline acetyltransferase immunoreactivity by using a dual colour method. Terminals labelled with biocytin by anterograde transport and which made synaptic contact with choline acetyltransferase-positive soma and dendrites were identified by light–electron microscopic correlation methods. In the cases where the biocytin injections had been made in the dorsal or lateral striatum, biocytin-labelled terminals made synaptic contact with cholinergic cells in the region between the main termination zones in the globus pallidus and the entopeduncular nucleus. In the cases where the injections had been made in the ventromedial and posterior striatum, there was greater overlap between choline acetyltransferase-positive structures and biocytin-labelled terminals in the main termination zones in the globus pallidus or entopeduncular nucleus, but relatively few of these terminals made synaptic contacts on to the cholinergic neurons. The results therefore indicate that the cholinergic nucleus basalis cells receive a relatively sparse synaptic input from all parts of the striatum. It has recently been shown that the cholinergic cells of the nucleus basalis selectively express high levels of substance P and opioid receptor messenger RNAs, while the non-cholinergic pallidal cells have much higher levels of GABAA receptor subunit messenger RNAs. It is concluded that the cholinergic neurons of the nucleus basalis in the rat may be selectively responsive to the peptidergic components of the striatal outputs, and that they are most likely to be influenced by both the limbic and sensorimotor parts of the striatum.