Localization of dopamine D4 receptors in GABAergic neurons of the primate brain.

Dopamine receptors are the principal targets of drugs used in the treatment of schizophrenia. Among the five mammalian dopamine-receptor subtypes, the D4 subtype is of particular interest because of its high affinity for the atypical neuroleptic clozapine. Interest in clozapine stems from its effectiveness in reducing positive and negative symptoms in acutely psychotic and treatment-resistant schizophrenic patients without eliciting extrapyramidal side effects. We have produced a subtype-specific antibody against the D4 receptor and localized it within specific cellular elements and synaptic circuits of the central nervous system. The D4-receptor antibody labelled GABAergic neurons in the cerebral cortex, hippocampus, thalamic reticular nucleus, globus pallidus and the substantia nigra (pars reticulata). Labelling was also observed in a subset of cortical pyramidal cells. Our findings suggest that clozapine's beneficial effects in schizophrenia may be achieved, in part, through D4-mediated GABA modulation, possibly implicating disinhibition of excitatory transmission in intrinsic cortical, thalamocortical and extrapyramidal pathways.

Regional, cellular, and subcellular variations in the distribution of D1 and D5 dopamine receptors in primate brain.

The pathways governing signal transduction in the mesocortical and nigrostriatal dopamine systems of the brain are of central importance in a variety of drug actions and neurological diseases. We have analyzed the regional, cellular, and subcellular distribution of the closely related D1 and D5 subtypes of dopamine receptors in the cerebral cortex and selected subcortical structures of rhesus monkey using subtype specific antibodies. The distribution of D1 and D5 receptors was highly differentiated in subcortical structures. In the neostriatum, both D1 and to a lesser extent D5 antibodies labeled medium spiny neurons, while only D5 antibodies labeled the large aspiny neurons typical of cholinergic interneurons. In the caudate nucleus, D1 labeling was concentrated in the spines and shafts of projection neurons, whereas D5 antibodies predominantly labeled the shafts, and less commonly, the spines of these cells. The D1 receptor was abundantly expressed in the neuropil of the substantia nigra pars reticulata while the D5 antibodies labeled only a few scattered cell bodies in this structure. Conversely, D5 antibodies labeled cholinergic neurons in the basal forebrain more intensely than D1 antibodies. Within the cerebral cortex and hippocampus, D1 and D5 antibody labeling was prominent in pyramidal cells. Double-label experiments revealed that the two receptors were frequently coexpressed in neurons of both structures. Ultrastructurally, D1 receptors were especially prominent in dendritic spines whereas dendritic shafts were more prominently labeled by the D5 receptor. The anatomical segregation of the D1 and D5 receptors at the subcellular level in cerebral cortex and at the cellular level in subcortical areas suggest that these closely related receptors may be preferentially associated with different circuit elements and may play distinct regulatory roles in synaptic transmission.

Cholinergic synaptic circuitry in the macaque prefrontal cortex.

Surprisingly little is known about the synaptic architecture of the cholinergic innervation in the primate cerebral cortex in spite of its acknowledged relevance to cognitive processing and Alzheimer's disease. To address this knowledge gap, we examined serially sectioned cholinergic axons in supra- and infragranular layers of the macaque prefrontal cortex by using an antibody against the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT). The tissue bound antibody was visualized with both immunoperoxidase and silver-enhanced diaminobenzidine sulfide (SEDS) techniques. Both methods revealed that cholinergic axons make synapses in all cortical layers and that these synapses are exclusively symmetric. Cholinergic axons formed synapses primarily on dendritic shafts (70.5%), dendritic spines (25%), and, to a lesser extent, cell bodies (4.5%). Both pyramidal neurons and cells exhibiting the morphological features of GABAergic cells were targets of the cholinergic innervation. Some spiny dendritic shafts received multiple, closely spaced synapses, suggesting that a subset of pyramidal neurons may be subject to a particularly strong cholinergic influence. Analysis of synaptic incidence of cholinergic profiles in the supragranular layers of the prefrontal cortex by the SEDS technique revealed that definitive synaptic junctions were formed by 44% of the cholinergic boutons. An unexpected finding was that cholinergic boutons were frequently apposed to spines and small dendrites without making any visible synaptic specializations. These same spines and dendrites often received asymmetric synapses, presumably of thalamocortical or corticocortical origin. Present ultrastructural findings suggest that acetylcholine may have a dual modulatory effect in the neocortex: one through classical synaptic junctions on dendritic shafts and spines, and the other through nonsynaptic appositions in close vicinity to asymmetric synapses. Further physiological studies are necessary to test the hypothesis of the nonsynaptic release of acetylcholine in the cortex.