Lateralized state-dependent learning produced by hippocampal kindled convulsions: effect of split-brain

Neurobiological mechanisms of state-dependent learning

State-dependent learning (SDL) is a phenomenon relating to information storage and retrieval restricted to discrete states. While extensively studied using psychopharmacological approaches, SDL has not been subjected to rigorous neuroscientific study. Here we present an overview of approaches historically used to induce SDL, and highlight some of the known neurobiological mechanisms, in particular those related to inhibitory neurotransmission and its regulation by microRNAs (miR). We also propose novel cellular and circuit mechanisms as contributing factors. Lastly, we discuss the implications of advancing our knowledge on SDL, both for most fundamental processes of learning and memory as well as for development and maintenance of psychopathology.

Memory dissociation: the approach to the study of retrieval processes

The phenomenon of dissociated memory retrieval is observed when some influences (for example, pharmacological) on the brain result in specific changes of long-term memory. The purpose of present paper is to reveal possibilities of the phenomenon for study of long-term memory retrieval. Pharmacologically-induced dissociated states could be identified when the retrieval of responses learned before treatment is temporarily blocked by the drug influence, but the ability of the animals to learn new tasks is intact. Furthermore, memory traces that were formed in drugged state are not accessible for the retrieval in normal state and only the same drug treatment allows retrieving them. In the present work, dissociated learning of food-motivated tasks was carried out in Wistar rats with cholinesterase inhibitor physostigmine (0.5 mg/kg, intraperitonealy) or general anaesthetic sodium pentobarbital (15 mg/kg, intraperitonealy.). The retrieval of dissociated responses was studied under the influence of various doses of the same drugs. The results revealed the asymmetry of memory dissociation with physostigmine in contrast to pentobarbital-induced memory dissociation. Gradual access for the retrieval of dissociated memory traces after pharmacological modulation of cholinergic and GABA-ergic brain systems was shown. It was suggested an important role of hippocampus in memory dissociation, as a structure-performing match-mismatch operations between different retrieved memory traces.

Retrieval of attention-dependent long-term memory traces

The stage of retrieving long-term memory traces is discussed; studies of this stage can be performed using the phenomenon of dissociated brain states. These states arise in response to various external influences and are accompanied by selective, reversible derangements of retrieval processes without alteration of the mechanisms involved in perceiving and fixing information. Analysis of the appearance of dissociated states induced by pharmacological treatments leads to several conclusion: a) reversible derangements affecting the retrieval function are typical of types of memory which depend on attention; b) the hippocampus plays a key role in selecting engrams for extraction from long-term memory; c) the brain's cholinergic and GABAergic mediator systems are involved in regulating the retrieval of long-term memory traces; d) stable derangements of the retrieval of long-term memory traces are accompanied by deep molecular-cellular changes in the hippocampus, including alterations in genetic elements and phosphorylation of synaptic proteins.

Reversible activation of GABA and L-glutamate uptake into synaptosomes isolated from the rat brain in response to a single carbacholine injection into hippocampus

In the present investigation the functional activity of transport systems mediating the GABA and L-glutamate uptake into nerve terminals of the rat brain cortex and hippocampus in response to a single carbacholine administration to hippocampus was studied. It has been established that synaptosomes isolated from the brain cortex and hippocampus of rats used in the experiments 24 h after a single carbacholine injection possess an increased capability of GABA and L-glutamate accumulation, and 48 h later the GABA and L-glutamate uptake begins to return to its control level and was equal to it on seventh day after injection.

Kindling antagonism: mapping of susceptible sites

Previous research has shown that concurrent alternating stimulation of paired limbic sites culminates in kindling of generalized seizures from 1 (dominant) site, whereas the other (suppressed) site supports only focal or partial seizures. This phenomenon has been referred to as kindling antagonism, and it has been proposed that antagonism reflects an arrest of kindling, which is therefore viewed as a non-continuous stepwise process. We have attempted to replicate these important observations in adult rats stimulated in various combinations of forebrain sites. Kindling antagonism was displayed by rats stimulated in the amygdala and the septal area, in the bilateral amygdala, the septal area and the splenium of the corpus callosum, and the amygdala and the cingulate cortex. We also found that antagonism between the amygdala and septal area as well as electrographic and behavioral correlates of alternating stimulation were sensitive to the hemispheric relation of the electrodes and to the order in which the sites received initial stimulations. That is, rats that carried ipsilateral amygdaloid and septal electrodes were less likely to display antagonism when the amygdala was the first site stimulated. On the other hand, we failed to obtain antagonism from rats stimulated in other limbic pairs (e.g. entorhinal cortex and septal areas.

Facilitation of secondary site kindling in the dorsal hippocampus following forebrain bisection

Bisection of the corpus callosum and hippocampal commissure, after the kindling of one dorsal hippocampus (primary site), had no effect on the rate of generalized kindled seizure development in the contralateral dorsal hippocampus (secondary site). The rate of kindling in the secondary site was very rapid in both intact and commissure-bisected rats, resulting in a positive transfer between the two sites of approximately 90%. In addition, the afterdischarge threshold response of the secondary site, after commissurotomy but before secondary site kindling, revealed mature local epileptogenesis. These observations support the suggestion that the positive transfer observed between the dorsal hippocampi is based on bilateral epileptogenesis during primary site kindling. Bisection of the anterior corpus callosum lateralized the forelimb convulsion while the hippocampal commissure independently mediated the laterality of the hippocampal afterdischarge. Commissurotomy had no significant influence on the latency to onset or offset of these motor and electrographic responses. These results are compared with those from the amygdala, and their implication for the laterality of epileptogenesis is discussed.

Behavioral lateralization in the T-maze and monoaminergic brain asymmetries

Recently we have reported a marked rat lateralization in the T-maze choice. The present study examines the relationship between the ascending monoaminergic systems and the T-maze behavioral asymmetry. There were no significant differences for serotonin or norepinephrine between the T-maze preferred and non-preferred brain sides in the s. nigra, ventral tegmental area, striatum, acumbens, frontal lobe or hippocampus. Only in the hippocampus was dopamine concentration significantly greater for the brain site ipsilateral to the T-maze choice side. Previously, we reported that both apomorphine, a dopamine receptor agonist, and 6-hydroxydopamine lesion in the medial forebrain bundle of the catecholaminergic neurons affect the T-maze asymmetry; we therefore suggested that the T-maze choice could be related with the ascending dopaminergic systems. The present data strongly support this hypothesis and suggest that the DA cells involved in the spatial asymmetry in the T-maze are included in the dopaminergic mesohippocampal system.