In situ hybridization histochemistry and immunocytochemistry, including double immunofluorescence, were used to study the populations of neurons expressing the alpha subunit of type II calcium/calmodulin-dependent protein kinase (CAM II kinase-alpha) or glutamic acid decarboxylase (GAD) in the somatic sensory and motor areas of the macaque monkey cerebral cortex. Sections were subjected to in situ hybridization using radioactive, complementary RNA probes specific for monkey CAM II kinase-alpha or 67 kDa GAD mRNAs. Others were stained immunocytochemically for CAM II kinase-alpha and/or GABA. CAM II kinase-alpha and GAD-67 are expressed in different populations of cells, with no colocalization. CAM II kinase-alpha is expressed in pyramidal cells of layers II-VI, especially layers II and III, as well as in certain small nonpyramidal cells of layer IV in areas 3a, 3b, 1, and 2 and of middle regions of area 4. Both cell types produce excitatory amino acid transmitters. Therefore, as in subcortical regions, CAM II kinase-alpha will be found on the presynaptic side of excitatory synapses but on the postsynaptic side only when these synapses occur on excitatory neurons in the sensory-motor cortex. Quantitative examination showed that CAM II kinase-alpha immunoreactive cells form, on average, approximately 50% of the total neuronal population in each area, while GABA immunoreactive or GAD cRNA hybridized cells form approximately 25-30%. Thus, CAM II kinase-alpha expressing cells cannot account for the total population of non-GABAergic cortical cells, and a certain proportion of the pyramidal cells probably do not express it. In other cortical areas, gene expression for the two molecules is regulated by afferent activity. Therefore, the present results form a necessary basis for studies aimed at determining the role of activity-dependent changes in the balance of excitation and inhibition as a mechanism underlying plasticity of representational maps in the primate sensory-motor cortex.

The ventral posterior lateral nucleus (VPL) of the monkey thalamus was investigated by histochemical staining for cytochrome oxidase (CO) activity and by immunocytochemical staining for the calcium-binding proteins parvalbumin and 28 kDa calbindin. Anterograde and retrograde tracing experiments were used to correlate patterns of differential distribution of CO activity and of parvalbumin and calbindin cells with the terminations of spinothalamic tract fibers and with the types of cells projecting differentially to superficial and deeper layers of primary somatosensory cortex (SI). VPL is composed of CO-rich and CO- weak compartments. Cells are generally smaller in the CO-weak compartment. Parvalbumin-immunoreactive cells and parvalbumin- immunoreactive medial lemniscal fiber terminations are confined to the CO-rich compartment. Calbindin-immunoreactive cells are found in both the CO-rich and CO-weak compartments. The CO-weak compartment, containing only calbindin cells, forms isolated zones throughout VPL and expands as a cap covering the posterior surface of the ventral posterior medial nucleus (VPM). Spinothalamic tract terminations tend to be concentrated in the CO-weak compartment, especially in the posterior cap. Other CO-weak, parvalbumin-negative, calbindin-positive nuclei, including the posterior, ventral posterior inferior, and anterior pulvinar and the small-celled matrix of VPM are also associated with concentrations of spinothalamic and caudal trigeminothalamic terminations. Parvalbumin cells are consistently larger than calbindin cells and are retrogradely labeled only after injections of tracers in middle and deep layers of SI. The smaller calbindin cells are the only cells retrogradely labeled after placement of retrograde tracers that primarily involve layer I of SI. The compartmental organization of VPL is similar to but less rigid than that previously reported in VPM. VPL and VPM relay cells projecting to different layers of SI cortex can be distinguished by differential immunoreactivity for the two calcium-binding proteins. The small- celled, CO-weak, calbindin-positive zones of VPL and VPM appear to form part of a wider system of smaller thalamic neurons unconstrained by traditional nuclear boundaries that are preferentially the targets of spinothalamic and caudal trigeminal inputs, and that may have preferential access to layer I of SI.

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