Psychopharmacology of memory modulation: evidence for multiple interaction among neurotransmitters and hormones

MK-801-induced disruptions of one-trial inhibitory avoidance are potentiated by stress and reversed by naltrexone

Five experiments were carried out to investigate opioid and NMDA receptor-mediated responses to one-trial inhibitory avoidance training in CD1 mice. In the first experiment immediate posttraining intraperitoneal administration of the noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 impaired the performance of mice. The effects of MK-801 were time-dependent (they were absent in mice injected with the drug starting 120 min after training). No effect was evident in no-foot-shock groups, showing lack of proactive influence of the treatment on performance. In the second experiment preexposure of the mice to the testing apparatus decreased the effects of MK-801. In the the third experiment naltrexone antagonized the effects of MK-801, suggesting an involvement of opioid neurons. In the fourth experiment immediate posttraining immobilization stress exerted a potentiating effect on the performance of MK-801-injected animals. In the fifth experiment the potentiation of the impairing effect of MK-801 induced by immobilization stress was antagonized by naltrexone.

The "anxiety state" and its relation with rat models of memory and habituation

Rats selected as "anxious", "nonanxious," or normal according to their behavior in an elevated plus maze were submitted to memory tasks and the densities of central benzodiazepine receptors in the amygdala and the hippocampus were studied. Anxious rats exibited better retention scores in the inhibitory avoidance task while nonanxious rats exibited worse retention scores in inhibitory and two-way active avoidance tasks compared to normal rats. No significant differences were detected in the retention scores for habituation to an open field. Nonanxious rats presented a lower benzodiazepine receptor density in the hippocampus but not in the amygdala compared to the other groups. These data suggest that the benzodiazepine receptors are involved in the effect of "anxiety" or emotional states on memory storage processes.

Post-training glucose administration attenuates forgetting of passive-avoidance conditioning in 18-day-old rats

The study of memory modulation in infant rats has typically focused on reminder/retrieval treatments involving reexposure to components of the internal or external training context. Rarely have studies employed pharmacological treatments to investigate the neurochemical substrates of memory storage in preweanling rats. The present study investigated the effect of 100 mg/kg of glucose, a common memory modulator in adult mammals, on memory for passive-avoidance conditioning in 18-day-old Sprague-Dawley rats. Subjects that were administered an immediate post-training injection of glucose performed significantly better, on a retention test 24 h following training, than those animals that received saline. The glucose group also performed comparably to a control group that was tested 10 min following training. These results are consistent with those of the memory modulation literature in adults and suggest that the rapid rate of forgetting in immature organisms may be the result of a deficiency in a general memory modulatory system.

Enhanced learning by posttrial injection of H1-but not H2-histaminergic antagonists into the nucleus basalis magnocellularis region

The aim of this study was to examine the effects of histaminergic antagonists on memory upon injection into the region of the nucleus basalis magnocellularis (NBM). In experiment 1, rats with chronically implanted cannulae were trained on the uphill avoidance task, which involves a punishment of a high-probability turning response on a tilted platform (negative geotaxis). Immediately after the training trial, that is, after a tail shock was administered upon performing the response, rats received one microinjection (0.5 microliter) of H1-receptor blocker chlorpheniramine (dose range 0.1 to 20 microgram) or the H2-receptor blocker ranitidine (same dose range) or saline into the NBM region. When tested 24 h later, rats treated with chlorpheniramine (20 micrograms) had significantly longer uphill latencies than vehicle controls and ranitidine-treated animals, indicative of superior learning of the avoidance response. In experiment 2, a test for possible proactive effects of posttrial chlorpheniramine on performance during the retention trial was performed. Animals were injected with either 20 micrograms chlorpheniramine or saline immediately after the training trial of the uphill task. One chlorpheniramine control group was treated with a delay of 5 h. Additional groups which received chlorpheniramine or vehicle after the training trial but no trail shock were included. When tested 24 h later, rats injected with 20 micrograms chlorpheniramine again exhibited significantly longer uphill latencies than did vehicle-injected rats. Retention latencies for the rats of the chlorpheniramine 5-h delayed group did not differ from those of the vehicle-injected rats, ruling out proactive effects of chlorpheniramine on performance. In summary, the histaminergic H1-blocker chlorpheniramine can enhance mnemonic functioning in addition to its reinforcing effects upon NBM injection as reported previously.

Different levels of Fos immunoreactivity after repeated handling and injection stress in two inbred strains of mice

Expression of Fos and Fos-related antigens was immunohistochemically analyzed in DBA/2J and C57BL/6J inbred mice in response to acute or repeated handling and injection stress. Both strains showed a strong induction of Fos and Fos-related antigens in discrete areas of hypothalamus, amygdala, neocortex, septum, and thalamus 2 h after an acute intraperitoneal injection of normal saline. To habituate animals to this procedure, mice were subjected to repeated handling and injections during 2 weeks preceding the experiment. This procedure led to complete habituation of the immediate early gene response to injection stress in stress-responsive brain areas of C57BL/6J mice, such that no significant difference was found between expression of these proteins in brains of saline-injected animals after repeated stress vs. control animals. In contrast, many brain areas of saline-injected DBA/2J mice still showed elevated Fos and Fos-related antigen expression after repeated injections. These results indicate that identical habituation procedures do not necessarily lead to identical levels of gene expression in brains of inbred strains of mice. In turn, they suggest that genetic components for some behavioral and pharmacological traits identified using inbred strains could be related to different rates of habituation to experimental procedures.

Memory performance in tree shrews: effects of stressful experiences

Male tree shrews were exposed to alternating non-stressful and stressful conditions, and their memory performance was tested during three different stress periods and after rest periods of various lengths using a modified holeboard. This paradigm circumvents confounding factors, e.g. food or water deprivation, transport to a special testing arena, and excludes the development of automatic cognitive processes by complex memory tasks. In experimental tree shrews, psychosocial conflict caused elevated cortisol levels during the stress phases. While this resulted in an impairment of the stress-sensitive declarative memory during the second stress phase, no memory deficit was found during the first and third stress phase. Despite normalized cortisol levels, significant memory deficits in experimental animals were observed even 10 weeks after the last stressful experience. The alternating order of stressful events revealed that the negative correlation between the level of adrenal steroid hormones and memory performance does not account for the long-lasting effects of psychosocial stress in tree shrews.

Strain-dependent involvement of D1 and D2 dopamine receptors in muscarinic cholinergic influences on memory storage

These experiments examined the interaction of muscarinic and dopaminergic systems in influencing memory for one-trial inhibitory avoidance training in mice of the C57BL/6 and DBA/2 strains. In both strains, immediate post-training systemic administration of the muscarinic cholinergic agonist oxotremorine enhanced retention and the cholinergic antagonist atropine impaired retention. No effects were seen with injections 2 h post-training. Furthermore, the drugs did not affect retention performance of animals that received no footshock on the training trial. These results confirm previous findings indicating that muscarinic cholinergic drugs affect memory by influencing memory consolidation. In C57 mice, pretreatment with selective D1 or D2 dopamine (DA) receptor agonists (SKF 38393 or LY 171555, respectively) in otherwise non-effective doses (5 and 0.25 mg/kg, respectively) potentiated the effects of oxotremorine (0.04 mg/kg). Furthermore, in C57 mice pretreatment with selective D1 or D2 receptor antagonists (SCH 23390 or (-)-sulpiride) in otherwise non-effective doses (0.025 and 6 mg/kg, respectively) blocked the memory enhancing effects of oxotremorine. The memory impairing effects of atropine (3 mg/kg) were blocked by the D1 and D2 selective agonists and potentiated by the selective D1 or D2 antagonists. In contrast, in DBA mice, the D1 and D2 selective agonists antagonised the memory enhancing effects of oxotremorine (0.02 mg/kg) and potentiated the effects of atropine (2 mg/kg). Furthermore, the D1 and D2 antagonists potentiated the effects of oxotremorine and antagonised those of atropine. These findings indicate that although muscarinic cholinergic influences on memory storage are comparable in mice of these two strains, the cholinergic-dopaminergic interactions are opposite in the two strains. These results have implications for hypotheses of cholinergic and dopaminergic regulation of memory storage.

Intrahippocampal GMP administration improves inhibitory avoidance performance through GABAergic and glutamatergic mechanisms in rats

Guanidino compounds play specific physiological and pathological roles in the central nervous system. We investigated the effect of an intrahippocampal infusion of GMP (a guanidino compound) administered immediately post-training on the inhibitory avoidance learning paradigm in rats. Bilateral intrahippocampal micro-injection of GMP (0-30 nmol) caused a dose-dependent increase in test step-down latencies which was completely reversed by intrahippocampal co-administration of muscimol or baclofen (GABA agonists) or preadministration (15 min pre-training, i.p.) of MK-801 (an NMDA antagonist). These results provide evidence for a participation of GABA and NMDA receptors in the GMP-induced increase in the test step-down latencies.

Psychopharmacology of dopamine: the contribution of comparative studies in inbred strains of mice

Comparative studies of behavioral responses to centrally acting drugs in inbred strains of mice which show differences in brain neurotransmitter activity represent a major strategy in the investigation of the neurochemical bases underlying behavioural expression. Moreover, these studies represent a preliminary stage in behavioral genetic research since they allow quantitative scales to be established and suggest correlations to be tested in recombinant inbred strains. The present review evaluates results obtained in mice of the C57BL/6 (C57) and DBA/2 (DBA) inbred strains which have been used for studies of the behavioral pharmacology of dopamine (DA) and investigated for the functional and anatomical characteristics of their brain DA systems. Differences between C57 and DBA strain involve susceptibility and sensitivity as well as qualitative differences in the type or direction of the behavioral effects of DA agonists. Moreover, data on strain-dependent differences for DA metabolism, release and receptor densities and distribution provide important indications about the relationship between behavioral and central effects of DA agonists and, more generally, about the involvement of brain DA in behavior. Comparative studies in C57 and DBA mice have also revealed differences in susceptibility to context-dependent, context-independent and stress-induced behavioral sensitization to psychostimulants. Consequently, they support the view that the term "behavioral sensitization" may define different phenomena in which different, independent genotype-related factors play a major role. Finally, studies on the behavioral and central effects of stressful experiences in C57 and DBA mice together with psychopharmacogenetic analyses, indicate that different symptomatological profiles may derive from genotype-dependent adaptation of brain DA receptors to environmental pressure.

Blockade of spontaneous posttraining performance improvement in mice by NMDA antagonists

We investigated the effects of immediate post-training systemic administration of gamma-L-glutamyl-L-aspartate (gamma-LGLA) and 3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate (CPP), antagonists at the N-methyl-D-aspartate receptor, in a lever-press task in two inbred strains of mice. When retention performance was tested in control animals 24 h after partial acquisition of the task. BALB/c mice exhibited a spontaneous performance improvement whereas C57BL/6J mice did not gamma-LGLA at doses of 2.5 and 25 mumol/kg and CPP at doses ranging between 0.025 and 2.5 mumol/kg blocked the spontaneous performance improvement found in BALB/c mice but had no apparent effects on the retention performance of C57BL/6J mice. These data suggest that retention impairment induced by CPP and gamma-LGLA in BALB/c mice results from an interference with posttraining memory processes.

Strain-dependent effects of cocaine on memory storage improvement induced by post-training physostigmine

Post-training administration of the inhibitor of cholinesterase enzymes, physostigmine, dose-dependently (0.025-0.4 mg/kg) improved retention of an inhibitory avoidance response in C57BL/6 (C57) as well as in DBA/2 (DBA) mice, the latter being more responsive than C57 mice. The effects on retention performance induced by physostigmine in C57 and DBA mice appeared to be due to an effect on memory consolidation. In fact, they were observed when drugs were given at short, but not long, periods of time after training, which is when the memory trace is susceptible to modulation. Moreover, these effects are not to be ascribed to a rewarding or non-specific action of the drugs on retention performance, as the latencies during the retention test of those mice that had not received a footshock during the training were not affected by the post-training drug administration. Post-training administration of cocaine (1-5 mg/kg) dose-dependently improved retention of an inhibitory avoidance response in C57 mice, while impairing it in the DBA strain, thus confirming previous results (Puglisi-Allegra et al. 1994b). Pretreatment with cocaine at ineffective doses as well as at an effective one potentiated the effects of an ineffective as well as of an effective dose of physostigmine in C57 mice, while it antagonized the effects of the inhibitor of cholinesterase enzymes on memory consolidation in DBA mice. The present results indicate that the indirect DA receptor agonist cocaine affects physostigmine action on memory consolidation in an opposite manner in the two inbred strains, pointing to genotype-dependent interaction between cholinergic and dopaminergic activity in memory consolidation.