Some neurochemical correlates of altered memory consolidation

Changes in learning and memory, acetylcholinesterase activity and monoamines in brain after chronic carbamazepine administration in rats

Groups of adult male Wistar rats were administered carbamazepine (CBZ) in doses of 5, 10, 20, 40 or 80 mg/kg/day intraperitoneally (i.p.) for 21 days. The learning and memory of the rats were assessed by the T-maze and passive avoidance tests. The CBZ plasma levels, the activity of acetylcholinesterase (AChE) in different brain regions, and the levels of monoamines in the hippocampus were also measured. None of the administered doses of CBZ impaired learning and memory. Rats with CBZ plasma levels of 2.5 and 4.5 micrograms/ml corresponding to the doses of 20 and 40 mg/kg, learned significantly better than controls. AChE activity was decreased in hippocampus and pyriform cortex (19%) in these groups. Simultaneously, an increase in the serotonin (5-HT) (36%) and dopamine (137%) levels in the hippocampus was noted in the 20-mg/kg CBZ group. 5-Hydroxyindole acetic acid (5-HIAA) and homovanillic acid (HVA) levels were increased at 10-, 20-, and 40-mg/kg CBZ doses. However, a dose of 80-mg/kg caused no change in learning performance as compared with that of controls. Correspondingly, no changes were evident in the AChE activity or monoamine levels. We postulated that the decreased AChE activity caused by CBZ in the therapeutic range may lead to increased ACh levels in brain, thus producing improvement in learning and memory. The increased turnover of 5-HT and dopamine (DA) in the hippocampus may play a role in long-term potentiation and improvement in memory.

Portacaval anastomosis attenuates the impairing effect of cyproheptadine on avoidance learning in rats--an involvement of the serotonergic system

Portacaval-anastomized (PCA) rats were used to demonstrate the involvement of the serotonergic system in long-term memory formation. Significant increases in the concentration of 5-hydroxyindoleacetic acid, a metabolite of 5-hydroxytryptamine (5-HT), in all regions examined and the turnover rate of this indoleamine transmitter in the hippocampus, hypothalamus, midbrain and medulla oblongata were observed in PCA rats in comparison with sham-operated controls. Cyproheptadine, a 5-HT receptor blocking agent, impaired the retention of two-way avoidance learning reinforced by light stimuli when the drug was intraperitoneally injected immediately after the completion of training. PCA treatment attenuated the impairing effect of cyproheptadine. When cyproheptadine was injected 2 h after the completion of training, the correct response in the retention test period was not decreased. The present results suggest that memory formation is a time-requiring process and is mediated by the central serotonergic mechanism.

New animal model of social behavioral deficit: reversal by drugs

A new model of social behavioral deficit and its reversal by drugs is described. Mice isolated for one week or more (isolated mice) behave differently from controls (grouped mice). When observed together under an inverted beaker, the isolated mice make significantly less escape attempts than the grouped mice. This behavioral deficit is a social one because it exists only when the isolated and grouped mice are tested together but not when they are tested singly. Several drugs impair this social behavioral deficit, particularly the 5-HT1B receptors agonists.

Evidence for a possible functional interaction between serotonergic and cholinergic mechanisms in memory retrieval

A total of three experiments were conducted. In Experiment 1, the dose-dependent effects of the pretest administration of the serotonergic agonist alaproclate and the selective muscarinic cholinergic agonist oxotremorine, alone and in combination, were assessed in a one-trial inhibitory avoidance task. A clear dose-dependent enhancement of performance was demonstrated as a result of all three treatment conditions, which could not be explained in terms of nonspecific effects of the drugs on behavior in general. In addition, the facilitation of retrieval performance produced by the combined treatment of alaproclate and oxotremorine was observed at dose levels well below those observed following administration of either compound alone. In Experiment 2 attempts were made to block the enhancements of retention resulting from the different treatment conditions (alaproclate, oxotremorine, or the combination of alaproclate and oxotremorine) by pretreating the mice with either scopolamine (a muscarinic cholinergic antagonist) or quipazine (a serotonergic agonist). The results of these experiments indicate that (a) quipazine completely blocked the enhancement of retrieval resulting from alaproclate but not that following oxotremorine or oxotremorine in combination with alaproclate, while (b) scopolamine blocked the enhancement of retrieval resulting from oxotremorine alone as well as that resulting from alaproclate plus oxotremorine but failed to block the memory enhancement resulting from alaproclate. The present results lend further support to the view that both serotonin and acetylcholine play important roles in memory retrieval. More importantly, the results of the present series of experiments provide additional support for a functional interaction between the serotonergic and cholinergic nervous systems in the mediation of behavior.

Selective brain noradrenaline depletion induced by the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP 4) does not prevent the memory facilitation induced by a muscarinic agonist in mice

These experiments investigated the effects of central noradrenaline (NA) depletion and its interaction with cholinergic and dopaminergic mechanisms upon retention of a passive-avoidance response in mice. The NA selective neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP 4) (50 mg/kg IP, 7 days) was injected into mice to produce depletion of NA in frontal cortex, hypothalamus, cerebellum, midbrain and brain stem without any significant change in dopamine (DA) levels in frontal cortex, striatum, hypothalamus and midbrain. Depletion of brain NA produced by DSP 4 was significantly but not completely prevented by the NA uptake inhibitor desmethylimipramine (DMI) (10 mg/kg IP, 30 min before DSP 4 injection). Despite the marked NA depletion, DSP 4 neither impaired the retention of a passive-avoidance response in mice nor prevented the enhancement of retention of this response induced by the central muscarinic agonist oxotremorine (OTM) (0.05 mg/kg IP, immediately after training. This lack of effect of DSP 4 on retention was prevented neither by DMI nor by the serotonin uptake inhibitor fluoxetine (5 mg/kg IP, 30 min before DSP 4 injection). The enhancement of retention induced by OTM in the groups of mice injected with either water or DSP 4 was prevented by atropine (0.5 mg/kg IP, 20 min before training) but not by methylatropine in the same experimental conditions. This suggests that both in controls and DSP 4-pretreated mice, the primary effect of OTM is due to an interaction with muscarinic brain receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of acute zimeldine and alaproclate administration on the acquisition of two-way active avoidance: comparison with other antidepressant agents, test of selectivity and sub-chronic studies

The dose-dependent effect of acute zimeldine and alaproclate treatment upon the acquisition of two-way and one-way active avoidance in the rat was studied in a single-session and in a repeated-sessions design. Zimeldine (5-20 mg/kg, IP), but not alaproclate, caused disruptions of two-way avoidance acquisition. Acquisition deficits were also caused by citalopram and fluoxetine but not the other antidepressant drugs tested. Zimeldine, but not alaproclate or desipramine, caused a slight but non-significant impairment of one-way active avoidance; neither zimeldine nor alaproclate produced any effects upon fear conditioning and retention testing. The long-term action of p-chloroamphetamine (2 X 10 mg/kg) antagonised the acute zimeldine effect totally, and chronic treatment with zimeldine (15 days, 1 X 50 mumol/kg) and chlorimipramine (15 days, 2 X 10 mumol/kg) also caused some partial blockade of the two-way avoidance deficit. These data seem to suggest some involvement of serotonin (5-HT) in the observed disruptions of two-way active avoidance caused by acute zimeldine treatment.

Role of serotonin in memory: facilitation by alaproclate and zimeldine

The effects of alaproclate and zimeldine on memory retrieval were examined in male Swiss-Webster mice using a one-trial inhibitory avoidance task. All drugs were administered IP prior to the retention test 24 h after training. Both drugs were found to facilitate memory retrieval significantly in a dose- and time-dependent fashion that could not be explained in terms of non-specific effects of the drug (illness, lack of motility, etc.) at the time of the test. The temporal effects of alaproclate and zimeldine on memory closely followed their course of concentration of the drug within the blood stream. The facilitation of retrieval induced by alaproclate and zimeldine was blocked by the putative serotonergic receptor agonist quipazine but not blocked by the antagonist cyproheptadine. Pretreatment with quipazine alone in a group of animals trained to a shock level which normally results in high levels of suppression was not sufficient to produce memory impairment, suggesting that quipazine was probably antagonizing the facilitative effects of alaproclate and zimeldine directly, rather than overriding the facilitation through an indirect action on retrieval in general. The present results lend further support to the suggestion that serotonin plays a significant role in memory.

Pharmacological investigations of neurotransmitter involvement in passive avoidance responding: a review and some new results

The roles of acetylcholine (ACh), noradrenaline (NA), dopamine (DA) and serotonin (5-HT) in passive avoidance responding are examined by reviewing previous studies of the effects on this task of drugs which alter the functioning of these neurotransmitter systems and also by presenting the results of a new study. This new study includes a number of drugs which do not seem to have been examined before, namely pilocarpine, pempidine, pentolinium, tetrabenazine, desipramine, clonidine, isoprenaline, pimozide, fluoxetine, L-tryptophan, methysergide and cyproheptadine. Because there is large variability in the effects of any one drug or class of drugs on passive avoidance responding, it is difficult to determine the exact involvement of the various neurotransmitter systems. There is also little good evidence that drug effects on performance of the passive avoidance response are caused by drug-induced changes in learning and memory processes or by state-dependent effects. Three other factors which may influence performance of the passive avoidance response-shock sensitivity, the biochemical response to stress and locomotor activity-are discussed and may be responsible for many of the drug-induced changes in passive avoidance responding.

Forebrain serotonin and avoidance learning: behavioural and biochemical studies on the acute effect of p-chloroamphetamine on one-way active avoidance learning in the male rat

The acute effects of p-chloroamphetamine (PCA) on one-way active avoidance learning and on central monoamine concentrations were examined in the male rat. The 5-HT specificity of the acute behavioural effect of PCA was examined in several experiments. PCA (0.08-5 mg/kg IP) injected 30-60 min before testing produced a dose-related impairment of both avoidance acquisition and retention. Pretreatment with the selective serotonin (5-HT) uptake inhibitors fluoxetine and zimelidine, but not the noradrenaline (NA) uptake inhibitor desipramine, resulted in a blockade of the avoidance deficit. Degeneration of brain 5-HT neurons by a high neurotoxic dose of PCA (2 x 10 mg/kg IP) 7 days prior to the administration of PCA also blocked the avoidance deficit. There was also a complete blockade of the PCA-induced avoidance deficit by pretreatment with metergoline, a central 5-HT receptor blocking agent. A 2.,5 mg/kg dose of PCA examined 60 min after injection produced regional changes in the 5-HT-levels preferentially in the forebrain region with significant reductions in the cerebral cortex, hippocampus and striatum while marginal effects were observed in the hypothalamus, midbrain and spinal cord. PCA failed to reduce dopamine and noradrenaline concentrations in the time- and dose-range of the avoidance deficit. Thus, the avoidance learning impairment appears to be specifically related to the acute release of endogenous 5-HT from presynaptic nerve endings possibly in the forebrain resulting in stimulation of postsynaptic 5-HT receptors. These findings indicate that 5-HT neurons in the forebrain play a role in active avoidance learning possibly by an involvement in memorial and/or retrieval processes.

Nonmonotonic age changes in susceptibility to hypothermia-induced retrograde amnesia in rats

The effects of post-training and/or pretesting body cooling on retention of Pavlovian discriminated fear conditioning were examined in preweanling (16-day) and weanling (23-day) rats. Twenty-four retention was assessed in 16- and 23-day-old rats receiving hypothermia after training, after training and prior to testing, or at neither time. Amnesia was present in the preweanling but not weanling rats. Recovery from amnesia was not observed in the preweanling rats followed a second cooling treatment. Control groups indicated the differential amnesia was not the result of differences in 24 hr baseline retention, depth of hypothermia cooling, rate of recovery from hypothermia treatment, or body temperature immediately post-testing. The results are discussed with respect to current views of infantile amnesia and the growing evidence for similar nonmonotonic functions during ontogeny.

The acute effect of p-chloroamphetamine on the retention of fear conditioning in the rat: evidence for a role of serotonin in memory consolidation

The acute effect of the 5-hydroxytryptamine (5-HT) releasing compound p-chloroamphetamine (PCA) on the acquisition and retention of shock-elicited fear conditioning to the contextual cues of a normal two-compartment shuttlebox in the rat was studied in three experiments. PCA (5 mg/kg) did not impair the acquisition of fear conditioning (Experiment 1). PCA, administered either 30 or 60 min before fear conditioning, caused a total blockade of fear retention when tested 24 h after acquisition. This retrograde amnesic effect was blocked by the 5-HT uptake blocker zimelidine (10 mg/kg) when PCA was injected 60 min before shock. These findings indicate that 5-HT neurones, possibly in the forebrain, may exert an inhibitory influence upon the long-term aspects of information consolidation in memory.

Modification of memory systems: some neurobiological aspects

The findings of clinical and experimental studies conducted over the past 25 years provide extensive evidence that in both laboratory animals and man memory can be modified by treatments which affect the central nervous system. Patients with head injuries may suffer from retrograde amnesia, a loss of memory for experiences which occur just prior to the onset of the injury. Findings of laboratory studies using animal subjects indicate that retrograde amnesia can be produced by a wide variety of experimental treatments.