Ivan Antonio Izquierdo

The effect of pre- and post-trial amphetamine injections on avoidance responses of rats

Rats were trained in a shuttle-box to avoid shocks, and then re-tested 5 days later for retention. A pre-trial injection of 2 mg/kg amphetamine increased the performance of conditioned responses (CRs) on the first day, but it did not improve retention beyond control levels. A similar post-trial injection, on the other hand, caused a marked enhancement of retention, even in rats which had received a pre-trial amphetamine treatment. Atropine potentiated the effect of pre-trial amphetamine on performance during the first-day session, but it partly antagonised that of post-trial amphetamine on retention. The effect of pre-trial amphetamine, and its interaction with atropine were similar to previously reported observations on pseudoconditioning. Thus, amphetamine was considered to have a dual effect on behaviour: on one hand, an enhancing effect on pseudoconditioning, which could be potentiated by atrophie; on the other, another stimulant action on memory consolidation, which was counteracted by atropine. Under the conditions of the present experiment, it was to be expected that the effect of pre-trial amphetamine on the performance of avoidance responses on the first-day would be due to an increase of pseudoconditioned responses, and therefore would not improve retention of CRs over control levels.A higher dose (5 mg/kg) of amphetamine lacked all facilitatory action on learning. The effects of atropine by itself were dose-dependent on pre-trial injection (a low dose depressed, a higher dose enhanced performance on both sessions), and was stimulant upon retention upon post-trial treatment.

Effect of cannabidiol and of other Cannabis sativa compounds on hippocampal seizure discharges

The natural Cannabis sativa compounds, cannabidiol, cannabinol, δ9- and δ8-tetrahydrocannabinol, in that order of potency, decreased the susceptibility of rat dorsal hippocampus to seizure discharges caused by afferent stimulation. The drugs were effective following both intraperitoneal injection and topical application. They were more active, on a dose basis, than the well-known antiepileptic agents mysoline and diphenylhidantoin. Within the dose range effective in depressing hippocampal seizures, they had no effect on hippocampal evoked responses. This suggested that they might act by interfering with K+ release from hippocampal cells, which is known to be the causative factor in hippocampal seizures. This point was investigated using cannabidiol, which was found to effectively block the release of K+ from the hippocampus caused by afferent stimulation.

Effect of learning and of drugs on the ribonucleic acid concentration of brain structures of the rat.

Abstract Hippocampal RNA concentration of rats was increased by: (a) One intraperitoneal injection of saline solution; (b) ten daily intraperitoneal injections of saline solution; (c) one intraperitoneal injection of amphetamine or nicotine; (d) one session of 25-min duration, of avoidance conditioning; (e) six daily sessions of 15 min each, of avoidance conditioning. The increases observed upon single amphetamine or nicotine injections were significant when values were compared to those of rats receiving one saline injection, taken as controls. The acute conditioning procedure led, in addition, to an increase in neocortical RNA. An acute pseudoconditioning procedure was without effect on hippocampal and neocortical RNA. The injection of saline solution was considered to involve a sequence of stimuli characteristic of a conditioning procedure, and the results obtained upon such injections were correspondingly interpreted. Animals which received ten daily injections of strychnine or amphetamine had a lower hippocampal RNA content than those which received ten daily saline injections. The chronic treatment with nicotine or strychnine raised cerebellar RNA levels. The results suggest: (a) A link between the learning process and the action of drugs known to favor learning, on one hand, and hippocampal RNA synthesis, on the other; (b) the possibility of a cerebellar site of action for nicotine and strychniine.

Effects of Cannabidiol and of Diphenylhydantoin on the Hippocampus and on Learning

Cannabidiol (3.5 mg/kg, i.p.) depressed hippocampal facilitation and posttetanic potentiation of evoked responses in rats, such, as had been reported before for diphenylhydantoin. Both diphenylhydantoin (80 mg/kg, i.p.) and cannabidiol blocked the increase of hippocampal RNA concentration caused by afferent stimulation, and depressed the acquisition of a conditioned avoidance response in rats. Neither drug affected the retention of such response when given by posttrial injection, nor the spontaneous locomotor activity of mice. The effects of both agents may be explained by the interference they have been previously shown to produce with the release of K+ from the hippocampus during stimulation. In fact, hippocampal facilitation and posttetanic potentiation and the RNA response to stimulation have been shown to be phenomena which depend on this K+ release, and have been attributed a role in learning.

Hippocampal physiology: Experiments on regulation of its electrical activity, on the mechanism of seizures, and on a hypothesis of learning

A review is made of experiments which show that hippocampal electrical activity is dependent on synaptic facilitation and regulated by extracellular potassium levels, and the relationship this has to hippocampal seizures. Later, the relation of hippocampal RNA to electrical activity and to [K + ] 0 levels is analyzed. Finally, a hypothesis on the role of the hippocampus in learning, supported by these findings, is considered.

Field potentials in rat hippocampus: Monosynaptic nature and heterosynaptic post-tetanic potentiation

Abstract Subicular, commisural, and fornical stimulation evoked large field responses in the dentate gyrus and dorsal hippocampus of rats. Hippocampal responses were negative above and positive below the upper third of apical dendrites in CA1, CA2, and CA3; the application of a pressing foot on the alveus resulted in a distortion of this polarity pattern. Since a compound action potential was found in the fornix during the rising phase of hippocampal field responses, it was concluded that the latter correlated with excitation of pyramidal cells. The following frequency of dentate and hippocampal potentials was high (up to 125/sec) and similar for both; thus, it was concluded that both were monosynaptic responses. On the basis of excluding several impossible mechanisms, hippocampal field potentials were attributed to a flow of cations, through the extracellular clefts, from the stratum pyramidale down to the stratum radiale. Hippocampal- and dentate-evoked potentials suffered post-tetanic potentiation; in the case of hippocampal responses, this was both homo- and heterosynaptic. The possible relevance of the latter to mechanisms of learning, and the possible genesis of both by (K+)0 accumulation were discussed.

Pharmacological evidence that hippocampal facilitation, posttetanic potentiation and seizures may be due to a common mechanism

Abstract Pieces of filter paper soaked in veratrine, tetraethylamnonium, and diphenylhidantoin solutions were placed on the alveus. Facilitation of hippocampal evoked responses was enhanced by the former and depressed by the latter two drugs. The number of 10/sec stimuli to either the fornix or the contralateral alveus needed to cause a seizure was reduced by veratrine and increased by tetraethylammonium and diphenylhidantoin. Diphenylhidantoin, besides, had a depressant effect on hippocampal homosynaptic and heterosynaptic posttetanic potentiation. These data correlate with those previosuly reported by us on the effect of veratrine and tetraethylammonium on posttetanic potentiation and on spike complications and support the hypothesis that the occurrence of hippocampal facilitation, posttetanic potentiation and seizures are all dependent on a (K+)0 accumulation brought about by the firing of afferent fibers within the local restricted extracellular space.

A physiclogical difference in the hippocampus of rats with a low inborn learning ability

Rats with a low ability to learn conditioned avoidance responses in a shuttle-box were in-bred. After five or six generations, a fairly homogeneous population was obtained consisting of bad learners both in the shuttle-box and in a Lashley III maze. When the hippocampus of these bad-learner rats was perfused with high potassium solutions, it was observed that more stimuli were needed, under each potassium concentration, in order to elicit a seizure, than in normal animals. This was apparently the result of a lower release of potassium per stimulus into the extracellular space, in as much as other possibilities (low sensitivity to the epileptogenic effect of potassium, large extracellular space, high Na-K-Mg-ATPase activity) were excluded. This reduced potassium release by stimulation in the hippocampus of bad-learner rats was not due to a lower potassium gradient across cell membranes, and therefore, could in principle be attributed to a defect in the property of local neural membranes (pre- and/or postsynaptic) to increase potassium conductance when stimulated.The present data fit with the hypothesis advanced previously, that the hippocampus plays a role in learning through heterosynaptic interactions mediated by the release, and subsequent accumulation of potassium.

Hippocampal facilitation and RNA build-up in response to stimulation in rats with a low inborn learning ability

Albino rats which were poor performers in a shuttle-box avoidance task were inbred for seven generations, at the end of which a fairly homogeneous population of animals that were poor learners not only in the shuttle-box, but also in a Lashley III maze was obtained. In a previous paper, it was reported that the hippocampus of these rats releases less potassium upon afferent stimulation than that of normal animals. When tested electrophysiologically under anesthesia, and with the dorsal hippocampus exposed by surgery, these “poor-learner” rats showed practically no hippocampal heterosynaptic facilitation, and homosynaptic facilitation in only half the cases; in addition, there were several failures to detect heterosynaptic posttetanic potentiation in them. In contrast, control rats from the general stock which were previously tested in the shuttle-box and found to perform well on this task, showed these electrophysiological properties in practically all cases. When the hippocampus was submitted to 25 min of a suitable afferent stimulation (4/sec pulses delivered to the fornix), “high-performance” rats featured an increase of their hippocampal RNA concentration, while “poor-learners” showed none. On the basis of previous reports, both hippocampal facilitation and posttetanic potentiation as well as the RNA response to stimulation are considered to be dependent upon an extracellular potassium buildup secondary to stimulation; transient in the first case, and longer lasting in the second. Thus, the present data are interpreted as a support of the previously advanced hypothesis that the hippocampus participates in learning through a mechanism involving potassium release and accumulation, heterosynaptic facilitation, and the production of RNA.

Effect of potassium on hippocampal ribonucleic acid concentration

Abstract A cannula with normal (3.2 mEq) or higher than normal (7, 11, 25 mEq) K + concentrations was placed on the hippocampus; this was considered artificially to cause corresponding modifications of (K + ) 0 . Up to 11 mEq, K + produced an increase of total hippocampal RNA concentration, and 25 mEq caused a decrease. The increase was attributed to an enhancement of spike-dependent transmitter release, as it was antagonized by 15 mEq Mg ++ and by 30 mEq procaine. The possibility was considered that the increase of RNA was dependent on pyramidal cell firing, which would be enhanced by increased synaptic efficiency. Procaine, in the presence of normal K + , lowered the hippocampal RNA concentration. The decrease of hippocampal RNA found with 25 mEq K + was related to the induction of seizures and to stimulation of the Na + -K + pump.