A.M. Tielen

Dynamic characteristics of visual evoked potentials in the dog. II. Beta frequency selectivity in evoked potentials and background activity.

Abstract 1. 1. Sinusoidally modulated light (SML) evoked potentials recorded from the posterior marginal gyrus were described as generated by a beta frequency selective system. 2. 2. The beta resonance is related to the dominant mode of spontaneous activity recorded from the same brain area and characterized by means of power spectra. 3. 3. The resonance behaviour of cortical SML evoked potentials was dependent on the state of the animal and disappeared during light Nembutal anaesthesia. 4. 4. A linear model including resonance was devised to account for the cortical frequency selective system. The model was tested experimentally by obtaining the systems impulse response by means of cross-correlating the input and output of the system excited with noise modulated light. 5. 5. The implications of beta selectivity in terms of neural pathways and networks are discussed.

Changes in synaptic membrane phosphorylation after tetanic stimulation in the dentate area of the rat hippocampal slice

Slices of rat brain hippocampus were stimulated electrically in the perforant pathway. After electrical stimulation, known to produce long-lasting post-tetanic potentiation, endogenous phosphorylation of membrane proteins was measured in a crude mitochondrial fraction, prepared from stimulated and unstimulated slices. Tetanic stimulation specifically enhanced the incorporation of [32P]phosphate into a protein band with apparent molecular weight of 50K. When the same number of pulses were given, but at a much slower rate (1 pulse per 4 sec instead of 15 pulses per sec) no posttetanic stimulation and concomitantly, no enhanced incorporation of [32P]phosphate were observed into the 50K band. When the stimulation of the slices was performed in Ca2+-free medium, again no potentiation and no enhanced incorporation into the 50K protein band were observed. It is suggested that electrical stimulation enhances the activity of the protein kinase that phosphorylates the 50K protein.

Dynamic characteristics of visual evoked potentials in the dog. I. Cortical and subcortical potentials evoked by sine wave modulated light

Abstract 1. 1. Potentials evoked by sinusoidally modulated light (SML) were recorded from the lateral geniculate nucleus and occipital cortex of unanaesthetized dogs. The evoked potentials were analysed by discrete Fourier analysis. 2. 2. The harmonic components of SML evoked potentials were described by frequency response functions. The range of linear and non-linear behaviour was determined. 3. 3. A linear description was only possible regarding the SML evoked potentials recorded from the posterior marginal gyrus. 4. 4. Three types of non-linearities were characterized: saturation, non-linear oscillations responsible for the generation of subharmonics and essential non-linearities. 5. 5. The essential non-linearities correspond to rectification occurring in “on” and “off” neuronal populations; they were the dominant features of SML evoked potentials in the lateral geniculate nucleus and the calcarine region of occipital cortex, but not in the posterior marginal gyrus. 6. 6. The phase functions were shown to be determined in part by a delay time. 7. 7. The relations between SML evoked potential parameters and data obtained at the unit level are discussed.

Membrane phosphoproteins of rat hippocampus: Sensitivity to tetanic stimulation and enkephalin

Hippocampal slices are electrically stimulated in the perforant path with a pulse-train, which can lead to long-term potentiation (LTP). Of the thus stimulated slices, subcellular fractions are prepared and used in an endogenous protein phosphorylation assay. A phosphoprotein band which was reported earlier to be sensitive to electric stimulation as well as to methionine-enkephalin is now further analyzed: it consists of two phosphoproteins only slightly differing in molecular weight: 50,000 Mr (50 K) and 52,000 Mr (52 K), but having distinct biochemical properties and subcellular localization. Their IEP is dissimilar (3.5-4.3 and 5.3, respectively), they display different sensitivity towards calcium when tested in the phosphorylation assay, but are both cAMP-independently phosphorylated. Only one of them responds to tetanic stimulation with an increased phosphorylation post hoc. This protein, the 52 K component, is localized in synaptic membranes. Moreover, this protein also responds to incubation of slices with methionine-enkephalin. The phosphorylation of the 50 K component is not influenced by electric stimulation, nor by incubations with neuropeptides; its phosphorylation takes place in material sedimenting with the mitochondrial cell fractions and is strongly calcium- and calmodulin-dependent.

Quantitative relationship between post-tetanic biochemical and electrophysiological changes in rat hippocampal slices.

Slices of rat brain hippocampus were tetanized in the perforant path fibers. In individual slices long-term changes of the electrophysiological parameters were determined simultaneously with post hoc endogeneous phosphorylation of proteins in a lyzed crude mitochondrial/synaptosomal fraction. A quantitative relation between the electrophysiological parameters and the degree of phosphorylation of a 52K protein was found to follow a non-linear function.

The localization of leucine-enkephalin immunoreactivity within the guinea pig hippocampus

SummaryThe distribution of enkephalin immunoreactive fibres has been studied in the hippocampus, subiculum and entorhinal cortex of the guinea pig. Two immunoreactive enkephalin fibre systems were found. One system corresponds to the mossy fibre system from the fascia dentata to CA3 and courses at the level of the mossy fibre end bulb in a longitudinal direction along the main axis of the hippocampus. Another system originates in the medial and lateral entorhinal cortex, traverses the subiculum, and then courses in the stratum molecu-lare/lacunosum to CA1 and CA3; part of these fibres crosses the hippocampal fissure and reaches the stratum moleculare of the fascia dentata. In the fascia dentata intense immunoreactivity was observed in the distal and middle one-third of the stratum moleculare at the side of the terminations of the lateral and medial perforant path fibres. Various types of immunoreactive cell bodies were found in the fascia dentata, CA3, CA1, subiculum and in the entorhinal cortex.

Differential effects of enkephalin within hippocampal areas

SummaryThe synthetic opiate D-Alaninemethionine enkephalin (D-Ala) affects neuronal activity of the fascia dentata (FD). D-Ala causes a depression of the population spike evoked by stimulation of the perforant path, which is antagonized by naloxone. In so called “Tandem” experiments it was shown that D-Ala had opposite effects on the CA1 pyramidal and FD granule cells. It is likely that in the FD the site of action of D-Ala is at the afferent synapses.

Differential conduction velocities in perforant path fibres in guinea pig

SummaryConduction velocities (c.vs) of different hippocampal fibre groups have been measured in slices in vitro. In the mossy fibres, Schaffer collaterals and Str. oriens fibres the c.v. varied from 0.36–0.38 m/s; in the alveus c.v. was 1.20 m/s. In the perforant path (p.p.) two groups of c.vs were found: in the medial p.p. the mean (± s.e) c.v. was 0.32 ± 0.02 m/s, whereas in the lateral p.p. it was 1.36 ± 0.14 m/s.

Evoked responses to sinusoidally modulated sound in unanaesthetized dogs.

Abstract 1. 1. Responses evoked by sinusoidally amplitude-modulated sound in unanaesthetized dogs have been recorded from inferior colliculus and from auditory cortex structures by means of chronically indwelling stainless steel wire electrodes. 2. 2. Harmonic analysis of the average responses demonstrated that the energy of most of the responses was concentrated in components at fundamental and second harmonic frequencies. 3. 3. The influence of various stimulus parameters was investigated, in particular the effect of variation of modulation frequency between 10 and 80 c/sec. 4. 4. The amplitude/frequency characteristics of the fundamental components obtained from the inferior colliculus were almost flat, with a slight amplitude decrease at frequencies above 60 c/sec. In some of the cortical areas the characteristics showed a maximum of amplitude at modulation frequencies between 15 and 30 c/sec. 5. 5. The phase/frequency characteristics showed a lag of phase with modulation frequency. From this phase lag delay times were calculated. Those at the inferior colliculus were shorter than those at the auditory cortex. At the latter structure the calculated delay times corresponded closely with latencies of the first positive deflection of responses evoked in these same structures by tone bursts. 6. 6. The amplitude modulation depth characteristics showed that the amplitudes of the responses were influenced by modulation depth. At the inferior colliculus an almost linear relation appeared to exist between modulation depth and response amplitudes. In the auditory cortex more complex relations between modulation depth and amplitude occurred. “Saturation” phenomena were observed in two cortical areas. 7. 7. Restricted investigations into the effects of the composition of the carrier signal revealed that change of carrier signal from sine wave at 4000 c/sec to band-limited noise at 3200–5000 c/sec caused change of the response amplitude distribution over the auditory cortex. Decrease of average intensity to 62 dB produced no change.

Plasticity in synaptic transmission and changes of membrane bound protein phosphorylation

Publisher Summary Neural plasticity is the ability of the nervous system of changing its activity as a result of previous experience or of making adaptive changes after destruction of certain of its parts. Learning processes which become apparent as adaptive changes in behavior are the most essential manifestations of the plasticity of the nervous system. A question of great interest for neurobiologists is: which are the plastic changes in the nervous system that are responsible for learning? An important line of research in this sense is represented by the studies of changes in neural activity in relatively simple nervous systems at both the behavioral and the physiological level, as reviewed by Kandel. This chapter discusses some aspects of another research line within the same field—namely, the investigation of the process of long-term potentiation (LTP) which may be considered to constitute a model of a learning process at the synaptic level. This phenomenon consists of the enhancement of synaptic activity that occurs for a long period of time—that is, for some hours, after a period of electrical stimulation at relatively high frequencies or tetanization.