The summation of excitatory postsynaptic potentials (EPSPs) generated in separate parts of the dendritic tree of hippocampal pyramidal cells has been investigated using the in vitro slice preparation. Two separate inputs with known synaptic location were used. The EPSP produced by simultaneous activation of the two inputs (observed sum) was compared to the algebraic sum of the individual EPSPs. Small-amplitude EPSPs (0.5-1.5 mV) added linearly. The shortest distance between the two synaptic groups was 75 micron. With larger amplitudes (greater than 2.5 mV), the EPSP summated nonlinearly. The nonlinear summation was reduced by moderate hyperpolarizations (2-10 mV) of the soma membrane. Also, large EPSPs (greater than 2.5 mV) summated linearly when the peak of the summed EPSP was brought close to the equilibrium potential for the inhibitory postsynaptic potential (IPSP) (EIPSP). When the EPSP peak was made more negative than the EIPSP, summation was again nonlinear but the algebraic sum was now smaller than the observed EPSP sum, i.e., the direction of the nonlinearity was reversed. EPSP summation was linear after the IPSP had been blocked by benzyl penicillin application. We conclude that separate EPSPs in hippocampal pyramids (minimal separation, 75 micron) add linearly but that the addition of an IPSP may complicate this picture. No evidence was found for interaction between the different populations of excitatory synapses.
A ndersen , P. Interhippocampal impulses. II. Apical dendritic activation of CAI neurons. C hakravarty , N. The occurrence of a lipid‐soluble smooth‐muscle stimulating principle (‘SRS’) in anaphylactic reaction. This work is an analysis of the commissural response of the field CAI of the hippocampus, a surface positive wave with a spike, followed by a negative wave. The major, later part of the surface positive wave was recorded negative in the layer of apical dendritic shafts. This deep negative wave is interpreted as a summated excitatory postsynaptic potential. It seems to have a causal relationship to the production of the spike. The spike most often showed its shortest latency in the apical dendritic shaft layer, increasing in both directions along the dendritic tree. The speed of propagation along the dendritic shaft was about 0.35 m/sec. The surface negative wave is confined to the basal layers and is probably partly due to the spread of the depolarizing wave along the dendrites and partly to the activity of alveus‐near located neurons. In the CAI apical dendritic membrane two mechanisms were found ‐ one was responsible for the production of the deep negative wave‐the other was responsible for the initiation and propagation of the spike along the dendrites.
Behaviorally induced brain temperature changes have significant effects on field potentials recorded in the hippocampal formation. All components of the field potential are slowed during cooling. Field excitatory postsynaptic potentials (f-EPSPs) are often reduced, while the population spike is increased in this state. To investigate whether such synaptic alterations affect hippocampus-dependent learning, we have compared the effects of reduced brain temperature on dentate field potentials and spatial learning in a Morris water maze. Rats were implanted with thermistors in the brain. A subset of the rats received electrodes for field potential recording in the perforant path-granule cell synapses of the dentate gyrus. After recovery, the rats were cooled by swimming in a pool of water. This invariably led to a brain temperature reduction of several degrees centigrade and a delay of the extracellular response. In addition, the field potential changed as described above. The effect of these changes on spatial learning in a second pool, the water maze, was determined by first cooling and then reheating each rat to a given level of brain temperature prior to each spatial training session. In spite of marked changes in dentate field potentials, all rats trained at brain temperatures above 30 degrees C learned to find the submerged platform similarly well. The speed of acquisition and the final precision of search behavior were also similar in these rats. Only rats that had been cooled below 30 degrees C failed to locate the hidden target. These animals also showed clear evidence of motor impairment.(ABSTRACT TRUNCATED AT 250 WORDS)
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