Brain activity and fatigue during prolonged exercise in the heat

We hypothesized that fatigue due to hyperthermia during prolonged exercise in the heat is in part related to alterations in frontal cortical brain activity. The electroencephalographic activity (EEG) of the frontal cortex of the brain was measured in seven cyclists [maximal O2 uptake (VO2max) 4.8 +/- 0.1 (SE) 1 min-1] cycling at 60% VO2max in a hot (H, 42 degrees C) and a cool (C, 19 degrees C) environment. Fast Fourier transformation of the EEG was used to obtain power spectrum areas in the alpha (8-13 Hz) and beta (13-30 Hz) frequencies. The ratio alpha/beta was calculated as an index of arousal level; an elevated alpha/beta index reflects suppressed arousal. In H, subjects fatigued after 34.4 +/- 1.4 min coinciding with an oesophageal temperature (Toes) of 39.8 +/- 0.1 degrees C, an almost maximal heart rate (HR 192 +/- 3 beats.min-1), a rating of perceived exertion (RPE) of 19.0 +/- 0.8 and significantly elevated alpha/beta index (188 +/- 71% of the value after 2 min of exercise; P < 0.05). In C, subjects cycled for a similar period while Toes was below 38 degrees C, HR and RPE were low, and the alpha/beta index was not significantly elevated (59 +/- 27% of 2 min value; P = NS). Increases in the alpha/beta index were strongly correlated to increases in Toes (r2 = 0.98; P = 0.0001).

Task-specific enhancement of short-term, but not long-term, memory by class I metabotropic glutamate receptor antagonist 1-aminoindan-1,5-dicarboxylic acid in rats

Pharmacological application of broad agonists and antagonists has supported the notion of a potential role of metabotropic glutamate receptors (mGluRs) in learning and memory formation, but the specific function of the different classes or individual subtypes remains elusive. Furthermore, our knowledge with respect to different learning mechanisms is still fragmentary. In an attempt to clarify further the function of mGluRs in learning, rats were trained in various paradigms in the presence/absence of the specific class I antagonist 1-aminoindan-1,5-dicarboxylic acid (AIDA). Intraperitoneal application of AIDA prior to training led to enhanced within-session performance in animals trained in a positively reinforced reference memory task in a three-choice maze. However, this enhancement did not result in increased retention as measured by the number of correct responses during the first four trials of each session on subsequent days. The increase was purely an enhancement in within-session performance, required doses higher than 2 mg/kg, and was not accompanied by an unspecific increase in activity as monitored in the open field. By contrast, AIDA animals trained in a combined shock-reinforced contextual and cue conditioning paradigm demonstrated a pronounced retention deficit compared with controls in conditioning to the context, but not the cue (a high-frequency tone). Although within-session performance during context and cue periods was slightly increased in the AIDA group, the difference did not reach significance. Drug-induced hyperactivity, which could account for the memory deficit, was excluded by recordings of activity in specific activity cages. These results shed new light on the possible function of class I mGluRs in learning and memory formation and imply that systemic blockade of class I mGluRs may enhance short-term memory under certain learning conditions.

The class I metabotropic glutamate receptor antagonist, AIDA, improves short-term and impairs long-term memory in a spatial task for rats

Effects of the class I selective metabotropic glutamate receptor antagonist, 1-aminoindan-1,5-dicarboxylic acid (AIDA), on spatial procedural learning and episodic short-term memory of rats were investigated in an appetitively reinforced 3-choice delayed match-to-position task. First, an acute intraperitoneal injection of AIDA (2 mg/kg) was given 20 min before a single training session of 20 trials using repeated reward position in one alcove out of three. AIDA caused facilitated short-term acquisition within such a session compared to saline treated controls. Secondly, injections were given before each of ten sessions (48 h intervals) also using constant reward position. The results showed AIDA induced inhibition of procedural between-session acquisition. Finally, the use of reward positions in a non-repetitive but trial-specific version of the 3-choice test revealed a facilitating effect of AIDA on episodic short-term memory.

Piracetam inhibits Pavlovian extinction and reversal learning in a spatial task for rats

Young male rats, trained in a spatial three-choice test, showed improved task acquisition after chronic treatment with piracetam (250 mg kg(-1)). After reaching a learning criterion, one group of animals was observed during Pavlovian extinction of the task skill and another group was assigned to reversal learning. The rate of extinction was slowed down in piracetam treated specimens compared to control animals. During reversal training, a new choice had to be learned while the previously acquired choice was no longer reinforced. Acquisition of the new skill was significantly impeded by piracetam in contrast to acquisition of the first skill, which was facilitated. Also during reversal learning, the piracetam treated group persevered longer than the control group in repeating the first acquired choice at the expense of learning the new choice. It is therefore suggested, that the impediment of reversal learning was caused by inhibition of extinction. In an open-field test, the time spent exploring in motion was increased by piracetam while the velocity of locomotion was unaffected by the drug. In a novelty test, piracetam increased the rate of loss of interactions with the novel object.

Effects of piracetam on the performance of rats in a delayed match-to-position task

1. The influence of acute and chronic treatment with piracetam on spatial working memory of rats was examined. A new version of an operant chamber "delayed match-to-position task" was used, in which the animals had to visit one randomly baited box out of three boxes ("choice boxes") in a front panel. Hereafter a delay period began, in which the subjects had to visit an alcove in the back panel ("reference box"). At the end of the delay the animals should return to the front panel and choose the same choice box that was baited before the delay, thereby obtaining a reward. 2. Rats were trained to a stable level of performance, measured as per cent correct responses during sessions of 20 trials. Additionally, the time spent between leaving the choice box and entering the reference box was recorded. Results were obtained from a single group of rats tested repeatedly under different experimental conditions. 3. Injections of scopolamine (0.6 mg/kg) significantly reduced the percentage of correct choices and increased the time spent to reach the reference box. The impairment of correct choices was reversed after chronic treatment with piracetam (250 mg/kg). However, the same treatment did not reverse the effect of scopolamine on the time performance. 4. Scopolamine also induced an increase of repetitive errors (a measure of perseverance), and the chronic treatment with piracetam caused full reversal of this increase. These results represent the first observation of a piracetam induced reversal of scopolamine impairments in a working memory test. 5. In normal animals not treated with scopolamine, acute injection of piracetam had no effect compared to saline injected controls, but chronic treatment with the nootropic significantly enhanced working memory performance. The drug did not affect the time used to reach the reference box.

Habituation: events in the history of its characterization and linkage to synaptic depression. A new proposed kinetic criterion for its identification

Reports on habituation from the last part of the nineteenth and the first part of this century are reviewed. Publications are selected according to their significance for the understanding of the properties that characterize habituation. Reports that have contributed to the establishment of a causative link between habituation and synaptic depression are also reviewed. A kinetic analysis of experimental cases of habituation is made for different reflexes in a number of species. The results of the analysis indicate the existence of two distinct subcategories of habituation: a slow and a fast version. Each category has a remarkably narrow range of kinetic variability, regardless of species and type of reflex. In one particular reflex, the tentacle withdrawal of Helix pomatia, both versions of habituation are displayed in response to different stimulation paradigms. On the basis of the kinetic analysis, a new criterion for identification of habituation is suggested.

Synaptic long-term depression alters recovery from, and frequency dependency of, short-term depression in Helix pomatia

Kinetic properties of synaptic depression were analysed for excitatory postsynaptic potentials in neuron number 3 in the right pallial ganglion of Helix pomatia. It was observed that long-term depression did not only suppress the amplitudes of excitatory postsynaptic potentials but also changed the kinetics of short-term depression. Early during each experiment (before the induction of long-term depression), the degree of short-term depression was inversely related to the frequency of excitatory postsynaptic potentials (contrary to most earlier reports). Later in the experiments, after the formation of long-term depression induced by four spaced excitatory postsynaptic potential series, the inverse frequency dependency had changed into a direct proportionality between short-term depression and frequency. This change was associated with, and could be at least partially explained by, an alternation of recovery from short-term depression induced by long-term depression: early in experiments, recovery showed a transient phase of large recovery between excitatory postsynaptic potentials separated by 3 to 45 s. After the induction of long-term depression, this large and transient recovery was absent. The relationship between the amount of long-term depression formed and the number of preceding excitatory postsynaptic potentials inducing the depression was analysed, and it was observed that a significant long-term depression was present 10 min after only six excitatory postsynaptic potentials. The results point towards the implication that many previous investigations of the kinetics of short-term depression observed in experiments involving a high number of excitatory postsynaptic potentials have been performed under the influence of variable degrees of long-term depression. Since long-term depression (at least in Helix) induced substantial alterations of the kinetic properties of short-term depression, such changes may have significantly influenced the conclusions of earlier reports.

Role of protein synthesis in the transition from synaptic short-term to long-term depression in neurons of Helix pomatia

The time course of the formation of synaptic long-term depression was analysed in neurons of Helix pomatia. Polysynaptic excitatory postsynaptic potentials were evoked in identified neurons by stimulation of the right pallial nerve. First, synaptic depression was observed during four consecutive series of excitatory postsynaptic potentials spaced by either 2, 10 or 60 min in the presence and absence of compounds inhibiting protein synthesis. Inhibition of RNA translation (but not of transcription) reduced the increasing depression of series spaced by 10 and 60 min but not by 2 min. This demonstration of translation-dependent depression present only 10 min after a previous excitatory postsynaptic potential series was analysed further by observations of the time course of excitatory postsynaptic potential recovery from short-term depression in experiments with intact versus inhibited protein synthesis. When synthesis was inhibited, recovery occurred along a double-exponential trajectory. However, when synthesis was intact, the exponential recovery was interrupted discontinuously 6-10 min after a series of excitatory postsynaptic potentials by the onset of a spontaneous and increasing depression lasting for at least 12 h. Puromycin, anisomycin and actinomycin D were used as inhibitors not affecting the short-term properties of excitatory postsynaptic potentials. Experiments with camptothecin, on the other hand, revealed that this inhibitor enhanced excitatory postsynaptic potential amplitudes and reduced their short-term depression. It is concluded that long-term depression, depending upon de novo protein synthesis, was initiated only 10 min after a series of excitatory postsynaptic potentials. Evidence was obtained indicating that this early initiated protein synthesis circumvented the transcription processes within the cell nuclei.

The kinetics of homosynaptic short-term depression in cell rpa3 of Helix pomatia depends upon the level of activity prior to depression

1. Following synaptic rest periods of 2 min, recovery from EPSP depression was dominated by a monoexponential phase completed within 2 min. After rest periods of 6 min, a two-phased recovery occurred: a transient phase followed by a slow phase lasting 10 min. 2. The shift of recovery kinetics was associated with a change of the time course of depression: following rest periods of 2, 6 and 12 min, EPSP trains of 1/10 Hz gave three different depressions. 3. The change of recovery inverted a logarithmic proportionality between frequency of EPSPs and degree of depression to an inverse relation.

Intracellular Ca2+ activity during slow synaptic hyperpolarizations in Helix pomatia

In the "bursting" cell RPal of Helix pomatia, intracellular Ca2+ activity increased during the slow synaptic hyperpolarization termed LLH. The Ca2+ increase had a double exponential time course and the kinetics provide evidence for an intracellularly located Ca2+ source. The Ca2+ wave could be mimicked by dopamine and inhibited by ergometrine. dB-cAMP also enhanced the Ca2+ activity. Intracellular K+ was measured during LLH. An appendix by J. D. C. Labmert demonstrates that a dominant mechanism underlying LLH is the increase of K+ permeability.

EEG and extracelluar K+ in rat brain during pentylenetetrazol seizures and during respiratory arrest

The relationship between changes in EEG and the extracellular potassium concentration (K+e) in brain cortex was studied in rats during petylenetetrazol seizures and during respiratory arrest. During seizures an increase of K+e occurred subsequent to the electrical discharges, while during postictal depression of EEG normal levels of K+e were measured. During respiratory arrest K+e rose considerably one minute after flattering of EEG. These findings are against a concept of K+e determining the changes of the EEG pattern. It is hypothetized that during seizures the increased K+e may play an important role in the coupling of neuronal activity to cerebral blood flow and metabolism.