Levels of spontaneous activity and spike responses of cortical neurons to local administration of excitatory amino acids to their dendrites and bodies

Studies of cortical cortex slices showed that spontaneous neuron activity depended on the conditions of transmission of excitation from dendrites to the body. Studies using a measure of the efficiency of dendrosomatic conduction showed that cortical neurons constituted a significantly heterogeneous population. Spike reactions to direct excitation of cell bodies were relatively stable in neurons with different levels of spontaneous activity.

Characterization of formation and trace rhythm reproduction by rabbit hippocampal neurons during early and late ontogeny

The formation and reproduction of memory traces by hippocampal neurons were studied and a relationship between the number of presented series of periodical electrocutaneous stimulation and degree of trace acquisition of the rhythm, on the one hand, and rabbit age (6-30 days, 1, 4-5, and 7 years), on the other, was detected. The hippocampus of 6-7-day and 7-year-old rabbits is characterized by low neuron activity and inability to trace acquisition of rhythm. The pulse frequency and trace acquisition of the rhythm in animals aged 8-14 days and 4-5 years (middle age) formed slower than in adult animals (after 2-4 stimulation series on days 2-4 of experiment) and could not be reproduced on the next day without reminding. In rabbits aged 25-30 days and 1 year the basal activity reached the optimum level and trace acquisition of rhythm was observed after 1-2 series on days 1-2 of experiment and was reproduced without reminding on the next day. The detected physiological stages are in good correlation with the morphochemical organization of the rabbit hippocampus at the stages of early and late ontogeny.

Comparative roles of acetylcholine and noradrenaline in controlling the spontaneous activity of cortical neurons

The effects of acetylcholine and noradrenaline on the spike activity of neurons recorded in guinea pig parietal cortex slices were studied. Iontophoretic application of these two neurotransmitters to cortical neurons induced similar responses consisting of slowly developing and prolonged increases in spike activity. Differences in the temperature sensitivity of responses to acetylcholine and noradrenaline were identified. When the incubation medium temperature was increased from 32-34 degrees C to 35-36 degrees C, the effects of acetylcholine on neuron spike activity increased sharply, with the result that neurons which showed no spontaneous activity at 32-34 degrees C became sensitive to acetylcholine. The temperature-dependent increases in the extent of responses to acetylcholine were accompanied by stable increases in the level of spontaneous activity. Responses to application of noradrenaline showed no significant change when the temperature increased from 32-34 degrees C to 35-36 degrees C. Since neuron responses to the iontophoretic application of glutamate, the major excitatory neurotransmitter in the cortex, remained constant over this range of temperatures, the data obtained here lead to the conclusion that acetylcholine is the main regulator of the level of spontaneous activity of cortical neurons.

The temperature sensitivity of the cholinergic responses of cortical neurons in the guinea pig brain

Studies on slices of the parietal cortex of the guinea-pig brain showed that a change in temperature from 20 degrees C to 36 degrees C led to increases in responses to microintophoretic application of acetylcholine to individual nerve cells. The greatest changes occurred over two temperature ranges: 27-29 degrees C and 34-36 degrees C. Increases in the cholinergic response were accompanied by increases in the spontaneous activity of the neurons.

Age-Related Structural and Functional Characteristics of Rabbit Hippocampal Neurons During the Formation of Temporal Associations

Spectral analysis (spectral density histograms) of neuron spike activity in the hippocampus of rabbits of different ages revealed trace assimilation of rhythms--an analog of a conditioned reflex to time--after rhythmic stimulation. An association was established between the number of series of periodic electrocutaneous stimulation presented and the extent of trace assimilation of the rhythm, depending on the age of the animal. Trace assimilation of rhythms occurred more slowly in older animals (aged 54-65 months) than in young animals (aged up to one year), predominantly on days 2-3 of experiments after presentation of 2-4 series of rhythmic stimulation, and persisted in a small proportion of cases on the day following stimulation. Periodic stimulation in very old animals (aged 66-85 months) almost completely failed to induce plastic rearrangements of hippocampal neuron activity. Stimulation produced a non-specific trace increase in the frequency of hippocampal baseline neuron activity in old animals. Quantitative and qualitative morphological changes were identified in the nerve and glial cells of the hippocampus of aged rabbits, as compared to young rabbits. It is suggested that the change in the course of mnestic processes seen in animals with aging may be associated with age-related destructive changes in the hippocampus.

Effects of Temperature on the Spike Activity of Cortical Neurons in Guinea Pigs

Studies on parietal cortex slices from guinea pigs showed that a decrease in temperature from 37 degrees C to 24 degrees C significantly decreased the level of spontaneous neuron activity, while activatory reactions to iontophoretic administration of glutamate showed no significant change.

Cholinergic excitation of dendrites in neocortical neurons

Discharge patterns were studied in response to iontophoretic application of acetylcholine to the soma and dendrites of 128 neocortical pyramidal neurons of layer V. Extracellular recordings were obtained from slices of the guinea-pig parietal cortex. All responses found were excitatory and were better expressed in spontaneously firing cells than in silent ones. Sensitivity to acetylcholine was approximately the same at somatic and dendritic sites in all the cells. Activation of muscarinic receptors gave rise to firing patterns with equal latencies and intensities when applied to both soma and dendrites. The latter suggests that membrane excitation elicited in dendrites by binding of acetylcholine to muscarinic cholinoreceptors is likely to propagate towards the soma through intracellular biochemical processes. Modulating effect of acetylcholine on output firing patterns, elicited by dendritic application of excitatory amino acids, included shortening of the somatic response latency and increase of response intensity and duration. We propose that, in contrast to glutamatergic excitation, the spread of cholinergic excitation along dendrites involves intra-cellular chemical signalling and results in changing the electrical properties of dendrites all over their length.

Functional geometry of amino acid sensitive membrane of layer V neurons in the guinea-pig neocortex in vitro

On guinea-pig neocortical slices the spatial organization of dendrites sensitive to excitatory amino acids was studied. Extracellular recording were obtained from the the soma of layer V neurons. Responses of 135 neurons to iontophoretically applied glutamate or aspartate have been analysed. An increased firing rate to somatic and most of dendritic applications were of short latency not exceeding 500 ms. Dendritic applications caused somatic responses with far longer latencies (up to 2-3 s) in 18% of cases. Latencies of responses to excitatory amino acids applied to several dendritic sites of the same neuron had similar values. The greatest reactions were obtained in response to excitatory amino acids imposed to the soma and proximal dendrites. At a distance of 100 microm beyond the soma in the basal region and region and further than 300 microm in the apical region excitatory amino acid applications produced two to three times less intensive somatic response. The area where dendritic activation gave rise to change in neuronal firing was confined to 350 and 800 microm for basal and apical dendrites, respectively. Topography of effective dendritic sites fell into the area corresponding to anatomically known outline of dendritic tree of pyramidal neurons. This fact implies that in our experiments we basically dealt with layer V pyramids. The results obtained suggest that local activation of distal dendrites may elicit spike generation in the soma. Different electrical properties of somatic and dendritic membranes are discussed.