Mosaic distribution of opiate receptors, parafascicular projections and acetylcholinesterase in rat striatum

Abstract

Although cell-stained sections have suggested heterogeneity in the neostriatum¹, more conclusive evidence has come from selective neurochemical staining for receptors, enzymes or neural connections. For example, opiate receptors in the rat striatum are concentrated in discrete islands and in a streak under the corpus callosum^2,3—regions that lack any visible morphological distinction. In the monkey, cortical afferent fibres terminate in intricate patterns⁴. In the cat striatum, variations in the intensity of acetycholinesterase (AChE) staining⁵ coincide with complex mosaic patterns of striatal efferent neurones, with cortical afferent terminations⁶ and with enkephalin-like immunoreactivity⁷. In both cats⁸ and monkeys⁹, thalamic afferent fibres, from the parafascicular–centre median complex, terminate in discrete clusters. Even the intensely studied nigrostriatal dopamine pathway, previously assumed to project diffusely throughout the caudate, can be shown to be heterogeneous by pharmacological methods¹⁰. To extend this evidence, we decided to examine, on serial sections of the same rat striatum, four different histological markers of striatal heterogeneity. We now report that there is a precise mosaic ‘fit’ in striatal ‘islands’ of closely packed opiate receptors, vacancies in the termination of parafascicular projections and AChE-poor zones.