Passive avoidance in rats: disruption by dopamine applied to the nucleus accumbens

Acute administration of a dopamine D2/D3 receptor agonist alters behavioral and neural parameters in adult zebrafish

The dopaminergic neurotransmitter system is implicated in several brain functions and behavioral processes. Alterations in it are associated with the pathogenesis of several human neurological disorders. Pharmacological agents that interact with the dopaminergic system allow the investigation of dopamine-mediated cellular and molecular responses and may elucidate the biological bases of such disorders. Zebrafish, a translationally relevant biomedical research organism, has been successfully employed in prior psychopharmacology studies. Here, we evaluated the effects of quinpirole (dopamine D2/D3 receptor agonist) in adult zebrafish on behavioral parameters, brain-derived neurotrophic factor (BDNF) and neurotransmitter levels. Zebrafish received intraperitoneal injections of 0.5, 1.0, or 2.0 mg/kg quinpirole or saline (control group) twice with an inter-injection interval of 48 h. All tests were performed 24 h after the second injection. After this acute quinpirole administration, zebrafish exhibited decreased locomotor activity, increased anxiety-like behaviors and memory impairment. However, quinpirole did not affect social and aggressive behavior. Quinpirole-treated fish exhibited stereotypic swimming, characterized by repetitive behavior followed by immobile episodes. Moreover, quinpirole treatment also decreased the number of BDNF-immunoreactive cells in the zebrafish brain. Analysis of neurotransmitter levels demonstrated a significant increase in glutamate and a decrease in serotonin, while no alterations were observed in dopamine. These findings demonstrate that dopaminergic signaling altered by quinpirole administration results in significant behavioral and neuroplastic changes in the central nervous system of zebrafish. Thus, we conclude that the use of quinpirole administration in adult zebrafish may be an appropriate tool for the analysis of mechanisms underlying neurological disorders related to the dopaminergic system.

Long-lasting behavioral effects of quinpirole exposure on zebrafish

The human brain matures into a complex structure, and to reach its complete development, connections must occur along exact paths. If at any stage, the processes are altered, interrupted, or inhibited, the consequences can be permanent. Dopaminergic signaling participates in the control of physiological functions and behavioral processes, and alterations in this signaling pathway are related to the pathogenesis of several neurological disorders. For this reason, the use of pharmacological agents able to interact with the dopaminergic signaling may elucidate the biological bases of such disorders. We investigated the long-lasting behavioral effects on adult zebrafish after quinpirole (a dopamine D2/D3 receptor agonist) exposure during early life stages of development (24 h exposure at 5 days post-fertilization, dpf) to better understand the mechanisms underlying neurological disorders related to the dopaminergic system. Quinpirole exposure at the early life stages of zebrafish led to late behavioral alterations. When evaluated at 120 dpf, zebrafish presented increased anxiety-like behaviors. At the open tank test, fish remained longer at the bottom of the tank, indicating anxiety-like behavior. Furthermore, quinpirole-treated fish exhibited increased absolute turn angle, likely an indication of elevated erratic movements and a sign of increased fear or anxiety. Quinpirole-treated fish also showed altered swimming patterns, characterized by stereotypic swimming. During the open tank test, exposed zebrafish swims from corner to corner in a repetitive manner at the bottom of the tank. Moreover, quinpirole exposure led to memory impairment compared to control fish. However, quinpirole administration had no effects on social and aggressive behavior. These findings demonstrate that dopaminergic signaling altered by quinpirole administration in the early life stages of development led to late alterations in behavioral parameters of adult zebrafish.

Partial lesion of dopamine neurons of rat substantia nigra impairs conditioned place aversion but spares conditioned place preference

Midbrain dopamine neurons play critical roles in reward- and aversion-driven associative learning. However, it is not clear whether they do this by a common mechanism or by separate mechanisms that can be dissociated. In the present study we addressed this question by testing whether a partial lesion of the dopamine neurons of the rat SNc has comparable effects on conditioned place preference (CPP) learning and conditioned place aversion (CPA) learning. Partial lesions of dopamine neurons in the rat substantia nigra pars compacta (SNc) induced by bilateral intranigral infusion of 6-hydroxydopamine (6-OHDA, 3μg/side) or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 200μg/side) impaired learning of conditioned place aversion (CPA) without affecting conditioned place preference (CPP) learning. Control experiments demonstrated that these lesions did not impair motor performance and did not alter the hedonic value of the sucrose and quinine. The number of dopamine neurons in the caudal part of the SNc positively correlated with the CPP scores of the 6-OHDA rats and negatively correlated with CPA scores of the SHAM rats. In addition, the CPA scores of the 6-OHDA rats positively correlated with the tissue content of striatal dopamine. Insomuch as reward-driven learning depends on an increase in dopamine release by nigral neurons, these findings show that this mechanism is functional even in rats with a partial lesion of the SNc. On the other hand, if aversion-driven learning depends on a reduction of extracellular dopamine in the striatum, the present study suggests that this mechanism is no longer functional after the partial SNc lesion.

Spatial deficits in a mouse model of Parkinson disease

RATIONALE

Accumulating evidence in humans demonstrated that visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson disease (PD). These deficits have been generally attributed to cortical dopamine degeneration. However, more recent evidence suggests that dopamine loss in the striatum is responsible for the visuo-spatial abnormalities in PD. Studies based on animal models of PD did not specifically address this question.

OBJECTIVES

Thus, the first goal of this study was to analyze the role of dopamine within the dorsal striatum in spatial memory. We tested bilateral 6-OHDA striatal lesioned CD1 mice in an object-place association spatial task. Furthermore, to see whether the effects were selective for spatial information, we measured how the 6-OHDA-lesioned animals responded to a non-spatial change and learned in the one-trial inhibitory avoidance task.

RESULTS

The results demonstrated that bilateral (approximately 75%) dopamine depletion of the striatum impaired spatial change discrimination. On the contrary, no effect of the lesion was observed on non-spatial novelty detection or on passive avoidance learning.

CONCLUSIONS

These results confirm that dopamine depletion is accompanied by cognitive deficits and demonstrate that striatal dopamine dysfunction is sufficient to induce spatial information processing deficits.

Distinct roles of the different ionotropic glutamate receptors within the nucleus accumbens in passive-avoidance learning and memory in mice

Research on the role of the nucleus accumbens in behaviour has been largely focused on the functions of this structure in conditioning to appetitive stimuli. It has been suggested that a network comprising the nucleus accumbens and its convergent inputs might mediate dissociable functions in the acquisition, the consolidation and the retrieval of information. However, findings related to a role of this structure in aversive conditioning are somewhat contradictory, and its involvement in this form of learning is still under debate. Moreover, very little evidence is available on the step of information processing mediated by the accumbens. Thus the purpose of this study was to investigate the effects of the blockade of the AMPA and NMDA glutamate receptors, which have been suggested to mediate the transmission of information from the limbic system to this structure, on a classical aversive conditioning task - the one-trial step through inhibitory avoidance paradigm (24 h interval between training and testing). Intra-accumbens focal injections of AP-5 and DNQX (NMDA and AMPA antagonists, respectively) were performed immediately after training, before training and before testing in mice. The NMDA antagonist (37.5, 75 and 150 ng per side) impaired animal performance only if administered immediately after but not before training or before testing. Conversely, DNQX (0.5, 1.0 and 5.0 ng per side) reduced the step through latencies when administered before training and before testing. These findings suggest that NMDA receptor activation within the accumbens is necessary in formation but not expression of memory for inhibitory avoidance. AMPA receptors, instead, are necessary for the acquisition and the expression but not consolidation of inhibitory avoidance memory.

Haloperidol and chlorpheniramine interaction in inhibitory avoidance in goldfish

The purpose of this study was to investigate a possible interaction between histaminergic and dopaminergic systems in learning and memory processes, in an inhibitory avoidance test in goldfish. Haloperidol, a dopaminergic antagonist, was administrated pre-training and the chlorpheniramine (CPA), a histaminergic antagonist, post-training. The inhibitory avoidance procedure was performed in 3 days, using a rectangular aquarium divided into two compartments (black and white), with a central door. On the first day, the animals were habituated for 10 min. On the second day, they were injected with 2 mg/kg of haloperidol or dimethyl sulfoxide (DMSO) 20 min before training. Then, the animals were placed in the white compartment, the central door was opened and the time spent for crossing between compartments was recorded. After the fish crossed the line between compartments a 45 g weight was dropped. This procedure was done five times in a row. Immediately after the fifth trial, the fish were injected intraperitoneally (i.p.) with either saline or CPA (0.4, 1.0, 4.0, 8.0 or 16 mg/kg). On the next day (test) the time to cross was recorded again. On the training trials, the animals treated with DMSO or haloperidol presented a significant increase in the latencies indicating learning (Friedman P = 0.0062 and 0.0001). The latencies in the test day showed that groups pre-treated with haloperidol and treated with CPA presented a dose-dependent increase in latencies, and those treated with the 16 mg/kg CPA group showed a significant increase (ANOVA two-way followed by Student-Newman-Keuls (SNK) P < 0.01). Thus, it can be suggested that the facilitatory action occurs due to an additive interaction between both systems, in a dose-dependent way.

Nucleus accumbens dopamine and learned fear revisited: a review and some new findings

A role for the nucleus accumbens (NAcc) and its dopamine (DA) innervation in fear and fear learning is supported by a large body of evidence, which has challenged the view that the NAcc is solely involved in mediating appetitive processes. Unfortunately, due to conflicting findings in the aversive conditioning literature the role of the NAcc in aversive conditioning remains unclear. This review focuses on the results of recent in vivo microdialysis studies that have examined the release of NAcc DA during Pavlovian aversive conditioning. In addition, we present additional new findings, which re-examine the involvement of NAcc DA in aversive conditioning. DA release was measured in the NAcc core using in vivo microdialysis during discrete cue Pavlovian aversive conditioning in four experiments. In all cases no change in DA levels was observed either during training or in response to the CS presentations despite robust behavioural evidence of discrete cue Pavlovian aversive conditioning. These findings contrast with some previous studies that show that primary and conditioned aversive stimuli increase DA release in the NAcc. We suggest that the inconsistencies in the literature might be due to procedural differences in the measurement of aversive conditioning, and the precise location of the probe in the NAcc region. Hence, rather than discount an involvement of NAcc DA in affective processes, we propose that functionally dissociable sub-regions of the NAcc may contribute to different aspects of Pavlovian aversive learning.

Hypericum perforatum as a nootropic drug: enhancement of retrieval memory of a passive avoidance conditioning paradigm in mice

Depression, among other non-cognitive symptoms, is common in patients with dementia. The effect of Hypericum perforatum (St. John's Wort) extract, with well-documented antidepressant activity, was tested on memory retrieval 24 h after training on a one-trial passive avoidance task in mice. Acute administration of Hypericum extract (4.0, 8.0, 12.0, and 25.0 mg/kg i.p.) before retrieval testing increased the step-down latency during the test session. The same doses of Hypericum extract, on the other hand, failed to reverse scopolamine-induced amnesia of a two-trial passive avoidance task. The involvement of serotonergic, adrenergic, and dopaminergic mechanisms in the facilitatory effect of Hypericum extract on retrieval memory was investigated. Pretreatment of the animals with serotonergic 5-HT1A receptor antagonist (-)-pindolol (0.3, 1.0, and 3.0 mg/kg), serotonergic 5-HT2A receptor blocker spiperone (0.01, 0.03, and 0.1 mg/kg), alpha adrenoceptor antagonist phentolamine (1, 5, and 10 mg/kg), beta receptor antagonist propranolol (5, 7.5, and 10 mg/kg), dopaminergic D1 receptor antagonist SCH 23390 (0.01, 0.05, and 0.1 mg/kg), and dopaminergic D2 receptor antagonist sulpiride (5, 7.5, and 10 mg/kg) revealed the involvement of adrenergic and serotonergic 5-HT1A receptors in the facilitatory effect of Hypericum extract on retrieval memory. It is concluded that Hypericum extract may be a better alternative for treatment of depression commonly associated with dementia than other antidepressants known to have anticholinergic side effects causing delirium, sedation and even exacerbating already existing impaired cognition. In dementias of old age, Hypericum perforatum would, therefore, serve as one medication targeting both depression and amnesia with lower potential side effects.

Effect of olanzapine on behavioural changes induced by FG 7142 and dizocilpine on active avoidance and plus maze tasks

The present study examined the effect of atypical antipsychotic olanzapine on FG 7142- (N-methyl-beta-carboline-3-carboxamide) and dizocilpine-induced cognitive impairment in active avoidance paradigm and elevated plus maze in mice. Both FG 7142 (5 mg/kg) and dizocilpine (0.1 mg/kg) increased the latency to reach shock-free zone (SFZ) both during training and retention session in active avoidance paradigm. This effect was reversed by olanzapine (0.063, 0. 125, 0.25 and 0.5 mg/kg). Similarly, FG 7142 (5 mg/kg) increased transfer latency (TL) on both first and second day in elevated plus maze. The lower doses of olanzapine (0.063 and 0.125 mg/kg) reversed the effect of FG 7142 on second day in elevated plus maze but higher doses (0.25 and 0.5 mg/kg) failed to modify the effect of FG 7142 both on first and second day. Dizocilpine (0.1 mg/kg) treatment did not affect TL on first day while on second day, it increased TL significantly. Olanzapine (0.063 and 0.125 mg/kg) reversed the effect of dizocilpine on elevated plus maze but the higher doses (0. 25 and 0.5 mg/kg) failed to reverse it. Even though olanzapine (0. 063, 0.125 and 0.25 mg/kg) failed show any effect per se in active avoidance task, the higher dose (0.5 mg/kg) increased the latency to reach SFZ on second day. Olanzapine (0.063, 0.125, 0.25 and 0.5 mg/kg) did not show any per se effect on TL in elevated plus maze on first day while on second day, olanzapine (0.125, 0.25 and 0.5 mg/kg) increased TL as compared to control group. The present study demonstrated olanzapine's reversal of dizocilpine- and FG 7142-induced behavioural changes in active avoidance paradigm and elevated plus maze. Although the precise mechanism of action is unknown, olanzapine might be acting by blocking excessive dopaminergic activity in the prefrontal cortex.

Memory-impairing effects of local anesthetics in an elevated plus-maze test in mice

Post-training intracerebroventricular administration of procaine (20 micrograms/microliter) and dimethocaine (10 or 20 micrograms/microliter), local anesthetics of the ester class, prolonged the latency (s) in the retention test of male and female 3-month-old Swiss albino mice (25-35 g body weight; N = 140) in the elevated plus-maze (mean +/- SEM for 10 male mice: control = 41.2 +/- 8.1; procaine = 78.5 +/- 10.3; 10 micrograms/microliter dimethocaine = 58.7 +/- 12.3; 20 micrograms/microliter dimethocaine = 109.6 +/- 5.73; for 10 female mice: control = 34.8 +/- 5.8; procaine = 55.3 +/- 13.4; 10 micrograms/microliter dimethocaine = 59.9 +/- 12.3 and 20 micrograms/microliter dimethocaine = 61.3 +/- 11.1). However, lidocaine (10 or 20 micrograms/microliter), an amide class type of local anesthetic, failed to influence this parameter. Local anesthetics at the dose range used did not affect the motor coordination of mice exposed to the rota-rod test. These results suggest that procaine and dimethocaine impair some memory process(es) in the plus-maze test. These findings are interpreted in terms of non-anesthetic mechanisms of action of these drugs on memory impairment and also confirm the validity of the elevated plus-maze for the evaluation of drugs affecting learning and memory in mice.

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.

Effects of nicotine on memory retrieval in mice

The effect of nicotine was tested on retrieval 24 h after training on a passive avoidance task. Intraperitoneal (i.p.) injection of nicotine (0.25-1.5 mg/kg) increased the step-down latency in mice dose dependently. Pretreatment with the nicotinic receptor antagonist mecamylamine (0.5-1 mg/kg) decreased, whereas pretreatment with the dopamine D1 receptor antagonist SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol maleate) (0.01, 0.05 and 0.1 mg/kg) and the beta-adrenoreceptor antagonist propranolol (10 mg/kg) increased the nicotine response. The dopamine receptor D2 receptor antagonist sulpiride (5-10 mg/kg), the anti-muscarinic agent atropine (2.5-10 mg/kg), the peripheral nicotinic receptor antagonist hexamethonium (0.01-0.5 mg/kg), the alpha-adrenoceptor antagonist phenoxybenzamine (1 and 10 mg/kg) and the peripheral dopamine D2 receptor antagonist domperidone (5 and 10 mg/kg) did not change the response induced by nicotine. Single administration of the antagonists did not cause response; however, a high dose of domperidone (10 mg/kg) and propranolol alone increased the step-down latencies. It may be concluded that a nicotinic receptor mechanism is involved in the nicotine-induced improvement of memory retrieval.

Effects of high doses of theophylline on memory acquisition

Intraperitoneal (IP) injection of high doses of theophylline decreased memory acquisition dose dependently. Low but not high doses of the dopamine receptor D2 antagonist sulpiride and the dopamine D1 receptor antagonist SCH 23390 decreased the theophylline response. Pimozide and the beta-adrenoreceptor propranolol also decreased the drug response. The anti-muscarinic agent atropine, the alpha-adrenoceptor phenoxybenzamine and the 5-HT antagonist metergoline did not affect the response induced by theophylline. It is concluded that dopamine and beta-adrenergic receptor mechanisms may be involved in the theophylline-induced attenuation of memory acquisition.

The disruptive effects of ketamine on passive avoidance learning in mice: involvement of dopaminergic mechanism

The involvement of dopaminergic mechanisms in ketamine-induced disruption of one trial step-through passive avoidance performance was assessed through the coadministration with the dopamine D1 antagonist SCH 23390, the dopamine D2 antagonist YM-091512 and the dopamine autoreceptor agonist at low doses, apomorphine, in mice. Pretraining (10 min before) administration of ketamine (0; saline, 2.5, 5 and 10 mg/kg SC) dose-dependently reduced the latency in the retention trial conducted 24 h after the training. However, ketamine did not affect the retention latency when administered immediately after the training or prior to retention. YM-09151-2 (0.01 and 0.03 mg/kg SC) and apomorphine (0.01 and 0.03 mg/kg SC), but not SCH 23390 (0.01 and 0.03 mg/kg SC), ameliorated the impaired reduction by ketamine (10 mg/kg) in a dose-dependent manner. These results suggest that ketamine obstructs the acquisition of the passive avoidance task, and that this effect is induced by stimulation of dopamine D2 receptors through dopamine release from the presynaptic terminals.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Cholinergic and dopaminergic agents which inhibit a passive avoidance response attenuate the paradigm-specific increases in NCAM sialylation state

The influence of cholinergic and dopaminergic agents on the acquisition of a passive avoidance response in the rat is demonstrated. Trifluoperazine (0.12 mg/kg), a dopamine antagonist, inhibited task acquisition when present during training or later, during consolidation, at the 10-12 h post-training period and at no other intervening time point. Induction of amnesia was dose-dependent and was not apparent when the dose exceeded 0.12 mg/kg. This effect appears to be due to an increase in dopamine release through presynaptic receptor antagonism as similar results could be obtained by the administration of apomorphine (0.5 mg/kg), a dopamine agonist, and this effect could be antagonized by the D1 receptor selective antagonist SCH-23390. Scopolamine (0.15 mg/kg), a muscarinic antagonist, impaired acquisition of the passive avoidance response when administered during training and, separately, at the 6 h post-training period. This could not be attributed to presynaptic antagonism as oxotremorine (0.2 mg/kg), a muscarinic agonist, had no amnesic action. Administration of apomorphine or scopolamine during training and at the appropriate post-training period prevented subsequent paradigm-specific increases of neural cell adhesion molecule sialylation state in hippocampal immunoprecipitates obtained at 24 h after task acquisition and 4 h following intraventricular infusion of the labelled sialic acid precursor - N-acetyl-D-mannosamine. Oxotremorine alone did not influence neural cell adhesion molecule sialylation state. These observations provide further evidence of a regulatory role for neural cell adhesion molecule sialylation state in information storage processes.

Effects of dopamine receptor agonists on passive avoidance learning in mice: interaction of dopamine D1 and D2 receptors

The present study examined the effects of dopamine D1 and D2 receptor agonists on the acquisition stage of passive avoidance learning and on locomotor activity in mice. The D2 agonist, RU 24213 (1-10 mg/kg s.c.), and the non-selective agonist, apomorphine (0.3-3 mg/kg s.c.), but not the D1 agonist, SKF 38393 (1-10 mg/kg s.c.), impaired learning and activated locomotion. RU 24213 (1 mg/kg s.c.) was more effective in impairing learning than in activating locomotion. The concurrent administration of SKF 38393 (10 mg/kg i.p.) and RU 24213 (1 and 3 mg/kg s.c.) produced a synergistic effect in both behavioral situations. The D1 antagonist, SCH 23390 (0.025 mg/kg i.p.), slightly inhibited the effects of apomorphine and of the combination of SKF 38393 and RU 24213 on learning but not on locomotion. The D2 antagonist, (-)-sulpiride (40 mg/kg i.p.), completely blocked these effects in both situations. These results suggest that dopamine receptor agonists impair passive avoidance learning through the D2 receptor, and that D1 and D2 receptors act synergistically in this impairment, as they do in their effects on locomotion. The involvement of D1 and D2 receptors is qualitatively similar in each of these behaviors, although some small differences may exist.

Effects of haloperidol, sulpiride and SCH 23390 on passive avoidance learning in mice

The main purpose of the present study was to examine the effect of dopamine blockers on memory processes by means of a one-trial passive avoidance (PA) task with ddY mice. Haloperidol (0.025-0.4 mg/kg i.p.) did not affect the PA response when it was given before the training or retention test. Sulpiride (10-80 mg/kg i.p.) had different effects, depending on the doses employed: A lower dose (20 mg/kg) of sulpiride, which is thought to block presynaptic receptors, impaired the PA response but higher doses (40 and 80 mg/kg i.p.) did not affect it when sulpiride was given before the training or retention test. SCH 23390 (0.025-0.1 mg/kg i.p.) impaired the PA response only when it was given before the training. These results suggest that blocking of postsynaptic D-2 receptors does not impair memory processes but blocking of presynaptic D-2 receptors impairs both acquisition and retrieval stages of memory processes following an increase in dopamine release. The involvement of D-1 receptors in memory processes involved in the PA response may be essentially different from that of D-2 receptors, since the blocking of D1 receptors impaired only memory acquisition.

Opposite effects induced by low and high doses of apomorphine on single-trial passive avoidance learning in mice

The effects of apomorphine (0.0125-1 mg/kg, SC), a dopamine (DA) agonist, on passive avoidance learning were assessed in mice which received brief and long foot-shocks in a training test. At low doses, apomorphine stimulates DA autoreceptors. With a shock of brief duration, apomorphine at a low dose (0.05 mg/kg), enhanced the avoidance learning when it was administered 20 min before the training test or the retention test. At high doses, apomorphine stimulates postsynaptic DA receptors. With a shock of long duration, apomorphine at a high dose (1 mg/kg), impaired the avoidance learning when it was administered 20 min before the training test or the retention test. However, apomorphine (0.05 and 1 mg/kg) given immediately after the training test did not have any effect on the avoidance behavior with shocks of either brief or long durations. Apomorphine-induced enhancement of passive avoidance learning was antagonized by sulpiride, but not by haloperidol. These results show that apomorphine induced the opposite effects on the passive avoidance learning depending on the dose or on the reinforcement intensity and suggest that the central DA system may play an important role in modulating memory processes.

A delayed onset of haloperidol effects on learned escape and avoidance behavior

The effects of 0.3 mg/kg haloperidol (H) on the acquisition and maintenance of footshock escape behavior of rats in a one meter runway was investigated. In the acquisition phase, a group (N = 6) given H before testing (HB) showed severely retarded acquisition and performance of the escape response, as compared with a group (N = 6) given H after testing (HA). When the HB and HA treatments were reversed for the groups behavioral performance was initially unaffected. At first, the HA group switched to the HB condition continued to exhibit rapid escape behavior and the HB group switched to the HA treatment continued to have slow escape behavior. Over the course of 8 days of testing, however, the performances of the two groups gradually reversed. After completion of this testing the HB and HA treatments again were switched and the animals were tested for both avoidance and escape behavior. Again, the performance of the animals initially did not change after the treatment switch, but with repeated testing and treatments, the avoidance and escape behavior of the HB group slowed substantially and that of the HA group accelerated markedly. These findings support previous observations that over learned behaviors are much less sensitive to disruption by haloperidol treatment than behaviors which are undergoing learning. The important contribution of the present study was in demonstrating that this insensitivity is a transitional, transient phenomenon and that with chronic treatment and testing, over learned behaviors can be strongly affected by haloperidol.(ABSTRACT TRUNCATED AT 250 WORDS)

Deficits in inhibitory avoidance after neurotoxic lesions of the ventral striatum are neurochemically and behaviorally selective

Inhibitory avoidance (step-in type) was investigated in rats subjected to neurochemical lesions of the ventral striatum. The neurotoxins 6-hydroxydopamine or 5,7-dihydroxytryptamine were used to produce selective depletions of either dopamine, norepinephrine or serotonin. Only lesions which decreased the dopamine content of the ventral striatum impaired post-shock step-in behavior. Measurement of footshock reactivity by the Flinch-Jump technique indicated that only serotonin depletion altered reactivity to footshocks. Assessment of open-field locomotor behavior showed that the dopamine-denervated rats were hypoactive (fewer rearings) compared to controls, whereas serotonin-depleted rats were hyperactive. It is concluded that the deficit in inhibitory avoidance behavior following ventral striatal dopamine loss was dissociated from its effect on locomotor activity.