Effects of several amnesic drugs on the habituation of exploratory activity in mice as indexed by horizontal and vertical activities

Sequential habituation to space, object and stranger is differentially modulated by glutamatergic, cholinergic and dopaminergic transmission

Novel object and social interaction tasks allow assessments of rodent cognition and social behavior. Here, we combined these tasks and defined unequivocal locations of interest. Our procedure, termed OF-NO-SI, comprised habituation to the open field (OF), novel object (NO) and social interaction (SI) stages. Habituation was measured within- and between-trials (10 minutes each, two per stage). Ambulation emerged as the appropriate proxy during the OF stage, but NO and SI trials were best quantified via direct exploration measures. We pharmacologically validated the paradigm using 5-month old C57BL/6J male mice, treated intraperitoneally with (1) 0.5 mg/kg scopolamine, (2) 0.05 mg/kg MK-801 and (3) 0.05 mg/kg SCH-23390 to block muscarinic (M1), NMDA, and D1 receptors, respectively, or (4) vehicle (distilled water). Activity and gross exploratory behavior were affected by all compounds cf. vehicle: scopolamine and MK-801 cohorts were hyperactive, while SCH-23390 caused hypo-locomotion throughout. Vehicle treated mice showed reliable habituation to all stages for time in interaction zone, directed exploration and number of visits. Exploration was severely impaired by scopolamine. MK-801 mostly affected within-session exploration but also increased exploration of the conspecific compared to the object. Interestingly, even though within-trial habituation was lacking in the SCH-23390 cohort, between-trial habituation was largely intact, despite reduced locomotion. Our data suggest that the OF-NO-SI task is a convenient and robust paradigm to measure habituation to different experimental settings and stimuli. It allows the dissociation of proxies related to activity and non-associative learning/memory, as revealed by distinct pharmacological treatment effects within- vs. between-trials.

Reciprocal activation/inactivation of ERK in the amygdala and frontal cortex is correlated with the degree of novelty of an open-field environment

RATIONALE

Phosphorylated extracellular signal-regulated kinase (ERK) has been used to identify brain areas activated by exogenous stimuli including psychostimulant drugs.

OBJECTIVE

Assess the role of the amygdala in emotional responses.

METHODS

Experimental manipulations were performed in which environmental familiarity was the variable. To provide the maximal degree of familiarity, ERK was measured after removal from the home cage and re-placement back into the same cage. To maximize exposure to an unfamiliar environment, ERK was measured following placement into a novel open field. To assess whether familiarity was the critical variable in the ERK response to the novel open field, ERK was also measured after either four or eight placements into the same environment. ERK quantification was carried out in the amygdala, frontal cortex, and the nucleus accumbens.

RESULTS

After home cage re-placement, ERK activation was found in the frontal cortex and nucleus accumbens but was absent in the amygdala. Following placement in a novel environment, ERK activation was more prominent in the amygdala than the frontal cortex or nucleus accumbens. In contrast, with habituation to the novel environment, ERK phosphors declined markedly in the amygdala but increased in the frontal cortex and nucleus accumbens to the level observed following home cage re-placement.

CONCLUSIONS

The differential responsiveness of the amygdala versus the frontal cortex and the nucleus accumbens to a novel versus a habituated environment is consistent with a reciprocal interaction between these neural systems and points to their important role in the mediation of behavioral activation to novelty and behavioral inactivation with habituation.

Post-training scopolamine treatment induced maladaptive behavior in open field habituation task in rats

The effects of scopolamine on memory consolidation are controversial and depend on several factors (i.e. site of administration, time of administration and testing, dose, cognitive task, experimental protocol, specie, strain, etc.). Generally, the range dose of systemic administered scopolamine, used in memory consolidation studies, has varied from 0.05 to 50 mg/kg. However, according to the literature, the most frequently used doses of scopolamine efficient on memory consolidation, are 1 and 30 mg/kg, low and high doses, respectively. In open field habituation studies only lower doses of scopolamine were used to test memory consolidation. Therefore, in the present study we compared the effects of low (1 mg/kg) and high (30 mg/kg) scopolamine dose, on the open field habituation task, in male Wistar rats. Scopolamine was administered immediately after the acquisition task and animals were retested 48 h later on. On the retested day, the ambulation and rearing in the open field decreased in the same manner in all tested groups. In saline- and 1 mg/kg scopolamine-treated animals, the time spent in grooming significantly decreased in the habituation task, while the same parameter significantly increased in animals treated with 30 mg/kg of scopolamine. The defecation rate significantly decreased (control group), maintained (1 mg/kg of scopolamine treated animals) or significantly increased (30 mg/kg of scopolamine treated group) on retention test. In conclusion, the present data suggest that post-training scopolamine administration does not affect locomotion neither exploration in the habituation to a novel environment, but increases defecation and grooming, two behaviours associated with fearful and stressful situations.

Safety pharmacology assessment of central nervous system function in juvenile and adult rats: effects of pharmacological reference compounds

INTRODUCTION

Recent EU/US pediatric legislation and FDA/EMEA guidelines recognize the potential differences in safety profiles of drugs in adults versus young patients. Hence safety studies are recommended to investigate key functional domains of e.g. the developing CNS.

METHODS

Selected psychoactive stimulants (caffeine, d-amphetamine, scopolamine) and depressants (baclofen, diazepam, haloperidol, chlorpromazine, imipramine, morphine) were characterized upon single administration with regard to behavioural parameters, locomotor activity, body temperature, pro-/anti-convulsive activity (pentylenetetrazole, PTZ), and nocifensive responses (hotplate) in neonatal (2 weeks), juvenile (4 weeks) and adult rats (8-9 weeks).

RESULTS

In vehicle-treated rats, behavioural patterns matured with age, locomotor activity and handling-induced rise in body temperature were enhanced, whereas PTZ convulsion threshold dose and nocifensive response latency decreased. Single test compound treatment elicited behavioural effects characteristic for psychoactive drugs with stimulating and depressing properties regardless of age. However, incidence of certain behaviours, and magnitude of effects on locomotor activity and body temperature varied with age and became generally more pronounced in adult rats. Pro-/anti-convulsive effects and delayed nocifensive responses did not differ between juvenile and adult rats.

CONCLUSION

CNS effects of selected psychoactive reference compounds were qualitatively similar, but quantitatively different in neonatal, juvenile and adult rats.

Injections of tacrine and scopolamine into the nucleus accumbens: opposing effects of immediate vs delayed posttrial treatment on memory of an open field

Using the paradigm of habituation learning in the open field, we tested the effects of microinjections of the nonspecific acetylcholine-esterase inhibitor tacrine (0.1, 1.0, 10.0 micrograms), and the muscarinic receptor antagonist scopolamine (0.1, 1.0, 10.0 micrograms) into the core of the nucleus accumbens. When injected immediately after the first exposure to the open field (posttrial), tacrine dose-dependently enhanced habituation of rearing behavior during the test on the following day, indicating a facilitation of memory. In contrast, scopolamine impaired habituation of rearing behavior at the two lower doses, but not at the highest dose. When scopolamine or tacrine (10.0 micrograms) was injected with a delay of 5 h after the learning trial, both drugs impaired habituation of rearing on the following day. The effects on locomotor activity differed from those on rearing behavior. Here, habituation on Day 2 was observed only in those animals which had received posttrial injections of vehicle or 10 micrograms of tacrine on the day before, whereas in animals which had received the two lower doses of tacrine, locomotor activity on Day 2 was not significantly decreased. In animals with posttrial treatment of scopolamine, locomotor activity on Day 2 was even enhanced, especially with the lower doses. No such effects were observed when scopolamine or tacrine (10.0 micrograms each) was injected with a delay of 5 h after the learning trial. These results show that cholinergic manipulations aimed at the nucleus accumbens can have substantial effects in this posttrial memory paradigm, which depend on drug, dose, and time of injection, and the specific kind of behavioral measure analyzed. Among others, the findings are discussed with respect to the role of muscarinic and nicotinergic cholinergic mechanisms in the nucleus accumbens on cognitive functions. They may be relevant, for example, for understanding the psychopathology of Alzheimer's disease, since the nucleus accumbens is one of the sites where cholinergic neurons are lost in this neurodegenerative disease.

Effects of the dopamine D3 receptor agonist, R(+)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin, on memory processes in mice

The putative dopamine D3 receptor agonist, R(+)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (R(+)-7-OH-DPAT) (0.1-100 microg/kg, s.c.), administered before training, immediately after training, and before retention significantly shortened step-down latency of passive avoidance learning, indicating the amnesic effects of R(+)-7-OH-DPAT. Neither the dopamine D1 receptor antagonist, R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzaz epine maleate R(+)-SCH23390) (2.5 and 5 microg/kg, i.p.), nor the dopamine D2 receptor antagonist, S(-)-sulpiride (10 and 30 mg/kg, i.p.), markedly influenced the R(+)-7-OH-DPAT (10 and 100 microg/kg, s.c.)-induced amnesia. In addition, only a 1000 microg/kg dose of R(+)-7-OH-DPAT decreased locomotor activity; 1 and 100 microg/kg doses of the drug were ineffective. These results suggest that the amnesic effects of the dopamine D3 receptor agonist, R(+)-7-OH-DPAT, are not mediated via dopamine D1 or D2 receptors in the brain.

Attenuation of memory with Tyr-D-Arg-Phe-beta-Ala-NH2, a novel dermorphin analog with high affinity for mu-opioid receptors

The involvement of mu-opioid receptors in memory retrieval was examined in mice by using Tyr-D-Arg-Phe-beta-Ala-NH2 (TAPA), a novel dermorphin analog with high affinity for mu-opioid receptors, and passive avoidance learning. TAPA was intracerebroventricularly administered to mice before retention tests of passive avoidance learning. A 0.3-ng dose of TAPA markedly shortened step-down latency of passive avoidance learning, and the shortening of step-down latency was reversed by treatment with beta-funaltrexamine (5 micrograms), a mu-opioid receptor antagonist, indicating that TAPA (0.3 ng) attenuates memory retrieval. Although the attenuating dose (0.3 ng) of TAPA failed to affect horizontal or vertical locomotor activity, a 3-ng dose of TAPA showed a tendency to decrease vertical locomotor activity. A 30-ng dose of TAPA produced a significant increase in horizontal locomotor activity accompanied by a marked reduction of vertical locomotor activity. TAPA (3 ng) produced a significant increase in step-down latency of passive avoidance learning with lower intensity of electroshock or without electroshock during training. These results suggest that the activation of mu-opioid receptors impairs memory retrieval.

A new methodological approach to the study of habituation: the use of positive and negative behavioral indices of habituation

This report details a new method to measure habituation in an open-field. In addition to the measurement of spontaneous locomotor activity, time spent per visit to the center zone (CZ) is also measured. Critically, a small object is placed in the CZ to modulate an animal's investigatory response. When an animal is first exposed to the open-field, the presence of the object does not affect the duration of its visits to the CZ but, if the animal is given one additional 10-min exposure to the open-field, then the presence of the object substantially increases the duration of its visits to the CZ. The presence of the object, however, has no effect on the rat's locomotor activity. Thus, habituation could be observed by two different measures: a decrease in locomotor activity and an increase in an animal's investigatory response to a stimulus object. A basic problem with a reliance solely upon a decrease in locomotor activity to measure habituation is that it represents a negative change in behavior. The present method circumvents this shortcoming by incorporating a positive behavioral measure of habituation in conjunction with the measurement of locomotor activity. This modification of the open-field test offers substantial utility for studies of neurotoxicology and memory because one can assess concurrently treatment effects on motor activity, attention to an object and memory.

Effects of galanin on passive avoidance response, elevated plus-maze learning, and spontaneous alternation performance in mice

The present study was done to characterize the effects of intracerebroventricular injection of galanin (GAL) on memory processes by observing passive avoidance response, elevated plus-maze learning, and spontaneous alternation performance in mice. An intermediate dose (3 micrograms) of GAL (3 micrograms) significantly decreased step-down latency of passive avoidance response when given 15 min before training, whereas in particular the middle doses (0.3 and 1 microgram) of GAL significantly decreased it when given 15 min before retention tests. However, GAL (0.3-10 micrograms) was ineffective when given immediately after training. Physostigmine (0.2 mg/kg), a cholinesterase inhibitor, and oxotremorine (0.01 and 0.03 mg/kg), a cholinergic agonist, significantly inhibited the shortening of step-down latency induced by GAL (0.3 microgram) administered 15 min before retention tests, indicating the involvement of cholinergic dysfunction in the GAL (0.3 microgram)-induced shortening of step-down latency. In contrast, GAL (0.3-3 micrograms) failed to influence transfer latency in elevated plus-maze learning or percent alternation in spontaneous alternation performance. These results suggest that the activation of cholinergic neurons improves memory dysfunctions induced by GAL, which primarily impairs retrieval processes of memory. The lack of effects of GAL on the elevated plus-maze learning and spontaneous alternation performance may result from the selective effects of GAL in different learning paradigms.

Effects of human epidermal growth factor on passive avoidance and habituation learning in mice

1. Human epidermal growth factor (hEGF:0.02-2.0 micrograms) did not affect passive avoidance response or habituation learning when given before or immediately after training, or before retention. 2. hEGF (0.02-2.0 micrograms) failed to influence the scopolamine- or electroconvulsive shock-induced shortening of step-down latency in passive avoidance response when given before or immediately after training, or before retention. 3. These results suggest that the acute administration of hEGF does not affect memory processes as indexed by passive avoidance or habituation learning in normal or amnesic mice.

Characterization of the effects of scopolamine on the habituation of exploratory activity: differential effects of oxotremorine and physostigmine

1. Habituation was regarded as a difference between exploratory activity measured first (session 1) and that measured second (session 2) in a novel environment. 2. Scopolamine (1.0 mg/kg) significantly increased the horizontal activity in sessions 1 and 2 when administered prior to session 1, resulting in the impairment of habituation. 3. Haloperidol (0.2 mg/kg) inhibited scopolamine-induced hypermotility in session 1, but it did not inhibit the scopolamine-induced impairment of habituation in session 2. 4. The direct cholinergic agonist oxotremorine (0.03 mg/kg), unlike the cholinesterase inhibitor physostigmine, significantly inhibited the scopolamine-induced impairment of habituation in the horizontal and vertical activities. 5. These results suggest that the direct stimulation of cholinergic receptors is more effective for scopolamine-induced amnesia than the indirect stimulation of cholinergic receptors by cholinesterase inhibitors in the habituation task.