Comparative analysis of the cholinergic system of the neocortex, consisting of the terminals of neurons from the magnocellular basal nuclei and intracortical neurons, in cats with strong and weak abilities to solve abstraction and generalization tasks was performed by isolating subfractions of synaptic membranes and synaptoplasm from "light" (C) and "heavy" (D) synaptosomes from associative field Ep and measuring choline acetyltransferase and acetylcholinesterase activities, protein content, and sulfhydryl group concentrations. These experiments showed that all measures were significantly lower in subfractions from C synaptosomes from cats with strong cognitive abilities. This leads to the conclusion that relatively small numbers of cholinergic synapses form in field Ep of the brains of cats with strong cognitive abilities, while their location in the C fraction demonstrates that they correspond to neurons of the magnocellular basal nuclei. The possible physiological significance of the "deficiency" of cholinergic inputs in field Ep from these nuclei as a correlate of the animal's cognitive ability is discussed. The D subfractions from able cats had significantly higher acetylcholinesterase activity, while choline acetyltransferase activity was not different; this identifies differences between groups of animals in the organization of non-cholinergic acetylcholinesterase-containing synapses in field Ep.
The role of the substantia nigra in cognitive processes of different levels of complexity was studied using an original method. Neurosurgical or neurochemical exclusion of the substantia nigra in cats led to significant impairment of conditioned reflex activity, generalization and abstraction processes; these recovered with pharmacological treatment directed to the dopaminergic system, with partial recovery after treatment directed to the GABAergic and cholinergic systems. Treatment directed to the serotoninergic system was ineffective.
The upper, middle, and lower subfractions of the synaptic membranes and subfractions of the total synaptoplasm were isolated from fractions of light and heavy synaptosomes of the Ep fields of the cat brain. The content of protein and protein sulfhydryl (SH-) groups was determined spectrophotometrically in the subsynaptic fractions. The maximal content of the reactive SH-groups was detected in the upper membrane subfractions; this characterizes the structure of the membranes of cholinergic synapses that are primarily concentrated in them, and the minimal content was detected in the lower subfractions, primarily noncholinergic. The brain of cats with well-developed and low capacities for the solution of problems involving generalization, gnosis, and abstraction was compared. A substantially lower content of protein in the upper and middle subfractions of the light synaptosomes and an increased reactivity of SH-groups in the membrane-bound proteins of the upper and middle subfractions of the light and heavy synaptosomes were found in animals with well-developed cognitive capacities. The hypothesis that the synaptic architectonics of the Ep fields in cats with strong cognitive capacities are distinguished by an increased relative proportion of the cholinergic structures is discussed.
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