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

Aversive motivation and cognitive control

Aversive motivation plays a prominent role in driving individuals to exert cognitive control. However, the complexity of behavioral responses attributed to aversive incentives creates significant challenges for developing a clear understanding of the neural mechanisms of this motivation-control interaction. We review the animal learning, systems neuroscience, and computational literatures to highlight the importance of experimental paradigms that incorporate both motivational context manipulations and mixed motivational components (e.g., bundling of appetitive and aversive incentives). Specifically, we postulate that to understand aversive incentive effects on cognitive control allocation, a critical contextual factor is whether such incentives are associated with negative reinforcement or punishment. We further illustrate how the inclusion of mixed motivational components in experimental paradigms enables increased precision in the measurement of aversive influences on cognitive control. A sharpened experimental and theoretical focus regarding the manipulation and assessment of distinct motivational dimensions promises to advance understanding of the neural, monoaminergic, and computational mechanisms that underlie the interaction of motivation and cognitive control.

An investigation of the single and combined effects of hypogravity and ionizing radiation on brain monoamine metabolism and rats' behavior

BACKGROUND

Ionizing radiation and hypogravity can cause central nervous system (CNS) dysfunctions. This is a key limiting factor for deep space missions. Up until now, the mechanisms through which they affect the neural tissue are not completely understood.

OBJECTIVES

We studied how the combination of hypogravity (antiorthostatic suspension model, AS) and ionizing radiations (γ-quanta and ¹H⁺ together, R) affects the CNS.

METHODS

We applied separately and in combination AS and R to determine the influence of these factors on behavior and metabolism of monoamines in Wistar rat's brain.

RESULTS

We found out that R has a slight effect on both the behavior and metabolism of monoamines. However, when applied in combination with AS the former was able to reduce the negative effects of the latter. The combined effect of ionizing radiation and hypogravity led to the recovery of locomotor activity, orientation and exploratory behavior, and long-term context memory impaired under the impact of hypogravity only. These changes came together with an increase in the serotonin and dopamine turnover in all of the brain structures that were studied.

CONCLUSIONS

We received the first evidence of interferential interaction between the effects of ionizing radiation and hypogravity factors with regard to a behavior and monoamine turnover in the brain. Further studies with heavy nuclei at relevant doses (<0.5 Gy) are needed.

Combined effects of antiorthostatic suspension and ionizing radiation on the behaviour and neurotransmitters changes in different brain structures of rats

Space flight factors (SFF) significantly affect the operating activity of astronauts during deep space missions. In contrast to an orbital flight, leaving the Earth's magnetic field is fraught with the dangers of exposure to ionizing radiation and more specifically, the high-energy nuclei component of galactic cosmic rays. Microgravity, just another critical non-radiation factor, significantly affects the normal functioning of the CNS. Some morphological structures of the brain, such as the prefrontal cortex and the hippocampus, that are rich in monoaminergic and acetylcholinergic neurones, are the most sensitive to the effects of ionizing radiation and non-radiation spaceflight factors (SFF). In this work we have studied the combined effects of microgravity (in antiorthostatic suspension model, AS) and irradiation (γ-ray and protons in spread-out Bragg peak) on the behaviour, cognitive abilities, and metabolism of monoamines and acetylcholine in the key structures of the rat's brain. Irradiation (as independently as combined with AS) resulted in the decrease of thigmotaxis in rats. Learning problems, caused by the malfunctioning of the working memory but not the spatial memory, were observed in response to AS as well as to the SFF in combination. Analysis of monoamines metabolism showed that the serotoninergic system was the most affected by the SFF. Concentration of acetylcholine in the hippocampus significantly increased in the groups of irradiated rats, and in the groups which were exposed to the SFF in combination, compared to the rats exposed only to AS.

Effects of chronic intracerebroventricular 3,4-methylenedioxy-N-methamphetamine (MDMA) or fluoxetine on the active avoidance test in rats with or without exposure to mild chronic stress

In despite the similarity of mechanisms of action between both selective serotonin reuptake inhibitors (SSRI) and MDMA (main compound of "Ecstasy") there are relatively few reports on the effects of the later on animal models of depression. There are many animal models designed to create or to assess depression. Mild chronic stress (MCS) is a procedure designed to create depression. MCS includes the chronic exposure of the animal to several stressors. After that, rats show behavioural changes associated to depression. In the other hand, the active avoidance task (AAT) is an experimental situation in which an animal has to accomplish a particular behaviour in order to avoid the application of a stressor. Animals exhibiting depression fail to acquire avoidance responses as rapidly as normal animals do. In order to assess the effect of MDMA on the acquisition of an active avoidance response, forty-five rats were divided in two groups exposed or not exposed to MCS. Rats also received chronic intracerebroventricular MDMA (0.2microg/microl; 1microl), fluoxetine (2.0microg/microl; 1microl) or saline solution (0.9%; 1microl). Our results showed that the effect of MDMA depends upon the level of stress. MDMA treated animals showed better acquisition (F([2,37])=7.046; P=0.003) and retention (F([2,37])=3.900; P=0.029) of the avoidance response than fluoxetine or saline treated animals when exposed to MCS. This finding suggests that MDMA (and no fluoxetine) was able to change the aversive valence of the stressors maybe enhancing coping strategies. This effect could serve as a protective factor against helplessness and maybe post-traumatic stress.

Impaired object recognition memory following methamphetamine, but not p-chloroamphetamine- or d-amphetamine-induced neurotoxicity

Repeated moderate doses of methamphetamine (mAMPH) damage forebrain monoaminergic terminals and nonmonoaminergic cells in somatosensory cortex, and impair performance in a novelty preference task of object recognition (OR). This study aimed to determine whether the memory deficit seen after a neurotoxic mAMPH regimen results from damage to dopamine (DA) and/or serotonin (5-HT) terminals. Animals were given a neurotoxic regimen of mAMPH, p-chloroamphetamine (PCA, preferentially damages 5-HT terminals), d-amphetamine (d-AMPH, preferentially damages DA terminals), or saline. After 1 week, animals were trained and tested for OR memory. Rats treated with mAMPH showed no recognition memory during the short-term memory (STM) test, whereas both PCA- and d-AMPH-treated rats showed OR STM scores comparable to controls. After behavioral testing, the specificity of monoaminergic lesions was determined by postmortem [125I]RTI-55 binding to dopamine (DAT) and serotonin (SERT) transporter proteins. Tissue from a separate group of animals killed 3 days after drug treatment was processed for Fluoro-Jade (F-J) fluorescence histochemistry to detect damaged cortical neurons. mAMPH-treated rats showed reductions in striatal DAT and hippocampal (HC) and perirhinal (pRh) SERT, as well as degeneration of neurons in primary somatosensory cortex. In PCA-treated rats, HC and pRh SERT were substantially depleted, but striatal DAT and cortical neuron survival were unaffected. By contrast, d-AMPH-treated animals showed marked depletions in striatal DAT and cortical neurodegeneration, but HC and pRh SERT were unaffected. This pattern of results indicates that no single feature of mAMPH-induced neurotoxicity is sufficient to produce the OR impairments seen after mAMPH treatment.

Amnesia produced by pre-training infusion of serotonin into the substantia nigra

Interference with dopaminergic, GABAergic or cholinergic activity of the substantia nigra is sufficient to disrupt cognitive functions. In spite of the known interactions between these neurotransmitters and serotonin, there is a notable scarcity of data germane to the participation of nigral serotonin in learning and memory. It was important, therefore, to investigate the possible involvement of serotonin in cognition. Pre-training injections of serotonin into the nigra of rats produced strong amnesia of an aversively-motivated task (inhibitory avoidance) compared to similar injections into the cerebral cortex and zona incerta. The present findings support the hypothesis that nigral serotonergic activity is involved in memory functions.

Blockade of striatal 5-HT2 receptors produces retrograde amnesia in rats

We have recently reported that intrastriatal administration of the serotonin (5-HT) releasing drug p-chloroamphetamine, and of 5-HT itself, produces a significant retention deficit of inhibitory avoidance. It is not known which of the 5-HT receptors are involved in the amnesic effect of serotonin. The present experiment was aimed at determining whether 5-HT2 receptors within the striatum are involved in memory consolidation. Ketanserine (0.5, 1.0, 2.0, or 4.0 ng) was infused bilaterally into the striatum of Wistar rats immediately after training of inhibitory avoidance, and retention of the task was measured 24 h later. A dose-dependent retention deficit was found. Together with the results from appropriate control groups, the results strongly suggest that striatal 5-HT2 receptors participate in memory consolidation of this aversive task.

p-Chloroamphetamine blocks physostigmine-induced memory enhancement in rats with unilateral nucleus basalis lesions

The present experiment examined whether p-chloroamphetamine (PCA), a serotonergic releasing/depleting agent, would block the memory-enhancing effect of physostigmine in rats with N-methyl-D-aspartic acid (NMDA)-induced unilateral lesions of the nucleus basalis of Meynert (uni-nbM). Six groups of subjects with uni-nbM lesions in addition to an isolated sham-operated control group were included. Subjects were trained and tested 72 h later on a one-trial passive avoidance task. Thirty minutes before training, rats with uni-nbM lesions were injected with either 1.0 or 5.0 mg/kg PCA or saline. Immediately after training, approximately half the subjects in each group were injected with either saline or 0.06 mg/kg physostigmine. Animals in the sham group received saline injections. Saline-injected animals with uni-nbM lesions performed poorly at test, a deficit that was reversed with physostigmine. Pretraining injections of PCA blocked physostigmine's memory-enhancing effect, although motor impairment during training may have contributed to decrements in test performance in animals injected with 5.0 mg/kg. Subjects were killed about 10 days later and their frontal cortices examined for choline acetyltransferase (ChAT). Results from the neurochemical analysis revealed that the lesion decreased ChAT levels and that the injection of 1.0 mg/kg PCA exaggerated this lesion-induced depletion. Implications for the interaction between acetylcholine and serotonin are discussed.

Regional infusions of serotonin into the striatum and memory consolidation

Lesions, temporal inactivation, electrical stimulation and administration of drugs that antagonize synaptic activity of the striatum lead to significant deficits of memory. Also, it has been shown that interruption of dopaminergic, GABAergic, or cholinergic activity in discrete areas of this structure is sufficient to disrupt cognitive functions. In spite of the known interactions among dopamine, GABA, acetylcholine, and serotonin, there is a notable scarcity of data germane to the participation of striatal serotonin in learning and memory. It was important, therefore, to investigate the possible involvement of serotonin in cognition. In light of the differential distribution of serotonergic elements within the striatum, a prediction was made that focal injections of serotonin into distinctive regions would produce dissimilar effects on memory. Rats were trained in a one-trial step-through inhibitory avoidance task and a retention test was carried out 24 h later. Posttraining injections of serotonin into the dorsal and ventral aspects of the posterior region produced strong amnesia compared to similar injections into the dorsal and ventral aspects of the anterior region. The present findings support the hypothesis that striatal serotonergic activity is involved in memory functions and also provide further evidence of neurochemical heterogeneity within the striatum regarding memory consolidation.

Seed finding in golden hamsters: a potential animal model for screening anxiolytic drugs

Fasting hamsters overnight, followed by temporary isolation in a novel environment are stressful conditions that hamper their ability or motivation to find hidden sunflower seeds when returned to their home environment. Studies were done to test whether treatment with antianxiety drugs would reduce stress and shorten the latency to find the hidden seeds. Prior to testing, the animals were fasted for 18-20 h. Ninety minutes after intraperitoneal injection of test drugs (fluoxetine, buspirone, chlordiazepoxide, clozapine, desipramine, yohimbine), the animals were taken from their home cage and placed into a novel holding cage for 2 min. During their absence, 6 sunflower seeds were buried under the bedding in one corner of their home cage. The animals were placed back into their home cage and timed for their latency to find the seeds in a 5-min observation period. Treatment with anxiolytics caused a dose-dependent reduction in the latency to find the sunflower seed, while treatment with other psychotherapeutics were ineffective. Seed finding is an extremely sensitive bioassay, responding to anxiolytics given in doses as low as 1 microg/kg. These data provide evidence that seed finding in hamsters may serve as an animal model for screening potential anxiolytic drugs.

Effects of pretraining intrastriatal administration of p-chloroamphetamine on inhibitory avoidance

Pretraining systemic administration of p-chloroamphetamine (PCA) consistently produces retention deficits of inhibitory avoidance. This drug causes a widespread acute release of serotonin from cerebral neuronal terminals, but it is not known where in the brain PCA exerts its disruptive cognitive effects. The present experiment was aimed at determining whether the striatum is a site of action of this drug. PCA (5 microg) was infused, bilaterally, into the striatum of rats at 30, 15, or 5 min before training of inhibitory avoidance, and retention of the task was measured 24 h later. An inversely related time-dependent deficit was found. Together with the results from appropriate control groups, this result strongly suggest that systemic PCA produces its reported cognitive effects, at least in part, through its pharmacological action within the striatum.

Enhanced inhibitory avoidance training protects against the amnesic effect of p-chloroamphetamine

The contribution of acetylcholine (ACh) to memory processing is well documented, but it has been proposed that it is not necessary for memory consolidation after an enhanced learning experience. It has been suggested that serotonin (5-HT) interacts with ACh during memory consolidation, although the nature of this interaction is unknown in the case of strong learning. As an initial approach to the study of these interactions, we determined whether training of inhibitory avoidance using relatively high aversive stimulation protects against the typical retention deficits produced by pre-training administration of the 5-HT releaser p-chloroamphetamine (PCA). Rats were trained after intraperitoneal administration of PCA or isotonic saline, using 2.0, 2.5, 3.0 or 3.5 mA and retention of the task was measured 24 h later. A significant amnesic state was observed only in the PCA groups that had been trained with the two lower intensities. These results indicate that 5-HT systems behave similarly to ACh systems, in the sense that the amnesic effect produced by interference with their physiological activity may be cancelled when animals are submitted to an intense learning situation.

Inverse relationship of hippocampal serotonin to avoidance behavior, serotonergic activation by emotional stress differentiated by estrous cycle and surgical stress

The hippocampus is involved in learning, affect, and the neurobiology of stress. After recording individual emotional reactivity (ER) during handling for vaginal smear screening (VSS), serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA), in the hypothalamus-preoptic area (HY-PA) and hippocampus were studied in two rat lines with different susceptibility to Freund's adjuvant arthritis (FAA), in relation to active avoidance behavior (AB), and under basal conditions in the FAA susceptible strain, in relation to ER, cycle stage and 20 h prior surgical stress. FAA susceptible rats compared to non-susceptible under equally low ER, showed higher AB, but lower 5-HIAA and 5-HIAA/5-HT ratio, under basal conditions, and upon termination of avoidance assay, only in hippocampus. In rats with different ER after prolonged VSS, high ER paralleled on diestrus-2 (DE-2), increased 5-HIAA and 5-HIAA/5-HT in the hippocampus and HY-PA, and adrenal weight, but decreased thymus and hippocampal weight. Under equal ER 5-HIAA and 5-HIAA/5-HT in the hippocampus and 5-HIAA/5-HT in the HY-PA were diminished on proestrus vs DE-2 but not estrus. Under equal distribution of proestrus and estrus higher ER paralleled higher hippocampal 5-HIAA. After equally high ER, combined with 20 h prior surgical stress, diminished hippocampal 5-HIAA/5-HT ratio, paralleled pronounced increase in adrenal weight compared to controls. Results suggest: negative association of hippocampal 5-HT to avoidance learning, a positive one to ER, modulated by cycle stage, and a role of differentiated 5-HT in homeostasis of HPA system.

Analysis of the 5-HT1A receptor involvement in passive avoidance in the rat

1. The effects of the 5-HT2A/2C agonist DOB, the selective 5-HT1A agonist NDO 008 (3-dipropylamino-5-hydroxychroman), and the two enantiomers of the selective 5-HT1A agonist 8-OH-DPAT (R(+)-8-OH-DPAT and S(-)-8-OH-DPAT) were studied in a step-through passive avoidance (PA) test in the male rat. 2. The 5-HT1A agonists injected prior to training (conditioning) produced a dose-dependent impairment of PA retention when examined 24 h later. R(+)-8-OH-DPAT was four times more effective than S(-)-8-OH-DPAT to cause an impairment of PA retention. Both NDO 008 and the two enantiomers of 8-OH-DPAT induced the serotonin syndrome at the dose range that produced inhibition of the PA response, thus, indicating activation of postsynaptic 5-HT1A receptors. 3. Neither NDO 008 nor R(+)-8-OH-DPAT induced head-twitches, a behavioural response attributed to stimulation of postsynaptic 5-HT2A receptors. In contrast, DOB induced head-twitches at the 0.01 mg kg(-1) dose while a 200 times higher dose was required to produce a significant impairment of PA retention. 4. The impairment of PA retention induced by both NDO 008 and R(+)-8-OH-DPAT was fully blocked by the active S(+)- enantiomer of the selective 5-HT1A antagonist WAY 100135 and the mixed 5-HT1A/beta-adrenoceptor antagonist L(-)-alprenolol. In contrast, the mixed 5-HT2A/2C antagonists ketanserin and pirenperone were found to be ineffective. Moreover, the beta2-adrenoceptor antagonist ICI 118551, the beta-antagonist metoprolol as well as the mixed beta-adrenoceptor blocker D(+)-alprenolol all failed to modify the deficit of PA retention by NDO 008 and R(+)-8-OH-DPAT. None of the 5-HT1A or 5-HT2A/2C receptor antagonists tested or the beta-blockers altered PA retention by themselves. 5. A 3 day pretreatment procedure (200+100+100 mg kg(-1)) with the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA) did not alter PA retention and did not prevent the inhibitory action of the 5-HT1A agonists, indicating that their effects on PA do not depend on endogenous 5-HT. 6. The effects of NDO 008 on PA were also studied using a state-dependent learning paradigm. NDO 008 was found to produce a disruption of PA when given either prior to training or retention or both prior to training and retention but it failed to affect PA retention when given immediately after training. .7 These findings indicate that the deficit of passive avoidance retention induced by the 5-HT1A agonists is mainly a result of stimulation of postsynaptic 5-HT1A receptors but not 5-HT2A receptors. The 5-HT1A receptor stimulation appears to interfere with learning processes operating at both acquisition and retrieval.

Ketanserin effects on rat behavioral responses: modifications by the estrous cycle, ovariectomy and estradiol replacement

The present investigation was designed to explore the influence of estrous cycle phase, ovariectomy, and estradiol replacement on the behavioral effects of the 5-HT2 receptor antagonist, ketanserin. The parameters under investigation were ketanserin-influenced acquisition of conditioning avoidance responses (CARs), and the performance of some spontaneous motor behaviors. Ketanserin (KET 3 mg/kg) injected subcutaneously 30 min before testing improved active conditioned avoidance in intact female rats at estrus, and in ovariectomized (OVX) rats with estradiol replacement. Furthermore, KET impaired performance in female rats at diestrus and after ovariectomy. In male rats, which were included in this study in order to compare their behavioral responses with those exhibited by female rats, KET administration enhanced acquisition of CARs. These results provide behavioral evidence for the hypothesis that central serotonergic activity is a function of the hormonal status of the animal. An additional segment of the present study focussed on motoric behaviors. Spontaneous motor activity, number of rears, and time spent in grooming behavior were significantly increased by KET in all groups studied. In contrast, blockade of 5-HT2 receptors failed to induce significant changes in the number of head shakes. Relationships between ovarian hormones and the central serotonergic system are discussed.

Impulsivity as a confounding factor in certain animal tests of cognitive function

Performance in cognitive tasks which require the subject to wait and/or to process a large amount of information can be disrupted by an increase in impulsive-like behaviour. Accordingly, a decrease in impulsive-like behaviour can improve performance in such tasks. Conversely, impulsive-like behaviour may improve performance in cognitive tasks where simple and fast responses and/or only little information processing is required. Thus, impulsivity constitutes a confounding factor in studies of cognitive function. Impulsive-like behaviour may be modified by serotonergic (5-HT) activity, with underactivity in 5-HT neurotransmission increasing impulsivity and vice versa. Drug- or lesion-induced alteration in 5-HT neurotransmission may, therefore, constitute suitable tools to investigate the role of impulsivity in animal tests of cognitive function. Benzodiazepines also increase impulsive-like behaviour, possibly by decreasing 5-HT neurotransmission. Hence, the effects of modulation of 5-HT systems and of the benzodiazepine-binding site on performance in animals tests of cognitive function will be discussed. It is predicted that the effects of manipulations of serotonergic activity or of benzodiazepine administration depend upon the nature of the response required, and that these effects may be mediated through changes in impulse control.

The 5-HT2 receptor antagonist ketanserine prevents electroconvulsive shock- and clonidine-induced amnesia

The 5-HT2-selective antagonist ketanserine was examined for its ability to prevent electroconvulsive shock (ECS)- or clonidine-induced performance deficit in the passive avoidance situation (step-down) in rats. The posttrain intraperitoneal injection of ketanserine at doses of 3 and 10 mg/kg prevented the ECS- or clonidine-provoked amnesia upon retention tests given 3 h, 24 h, and 7 days after training. The present data favor the view that the selective modification of 5-HT2 receptors after training can prevent the performance deficit in step-down-trained rats and suggest a role of the 5-HTergic neurotransmitter system in memory. The results of this study further suggest that 5-HTergic receptor antagonists might be useful in treatment of cognitive disorders.

Ameliorating effects of sigma receptor ligands on the impairment of passive avoidance tasks in mice: involvement in the central acetylcholinergic system

Three sigma receptor ligands were examined for their ameliorating effects on p-chloroamphetamine-induced amnesia in mice. p-Chloroamphetamine was administered intraperitoneally 30 min before the training session of the passive avoidance response. Each sigma receptor ligand was administered 60 min before or immediately after the training session, or 60 min before the retention test. (+)-N-Allylnormetazocine ((+)-SKF-10,047), a prototype benzomorphan sigma receptor ligand, significantly reduced the p-chloroamphetamine-induced amnesia in these three administration schedules, as do acetylcholinesterase inhibitors. On the contrary, the significant anti-amnesic effects elicited by non-benzomorphan sigma receptor ligands, 1,3-di-(2-tolyl)guanidine (DTG) or (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperizine ((+)-3-PPP), were observed depending upon the timing of their administration. In addition, the ameliorating effect of (+)-SKF-10,047 against the p-chloroamphetamine-induced amnesia was superior to that of (-)-SKF-10,047. The (+)-SKF-10,047-induced anti-amnesic effect was significantly antagonized by the concurrent administration of either scopolamine, a muscarinic receptor antagonist, or hemicholinium-3, an inhibitor of the Na(+)-dependent high-affinity choline uptake site. These findings indicated that sigma receptor ligands had anti-amnesic effects against drug-induced memory impairment. In addition, the anti-amnesic effect of (+)-SKF-10,047 was superior to those of other sigma receptor ligands, and was mediated by both the sigma receptor and the central acetylcholinergic system.

Similar ameliorating effects of benzomorphans and 5-HT2 antagonists on drug-induced impairment of passive avoidance response in mice: comparison with acetylcholinesterase inhibitors

Mice were trained to avoid electric shocks by means of step-down type passive avoidance learning tasks, and memory retention was measured 24 h after the training session. Memory impairment (amnesia) was produced by administering either p-chloroamphetamine (PCA), a serotonin (5-HT) releaser or scopolamine (SCOP), a muscarinic cholinoceptor antagonist, 30 min prior to the training session. Benzomorphans, 5-HT2 antagonists and acetylcholinesterase (AChE) inhibitors were administered immediately after the training session. PCA- but not SCOP-induced amnesia was attenuated by the post-training administration of two benzomorphans, (+)N-allylnormetazocine ((+)SKF-10,047) and (+/- )pentazocine ((+/- )PTZ). Similarly, PCA-induced amnesia was reversed by the post-training administration of 5-HT2 antagonists, ritanserin (RIT) and mianserin (MIA), but SCOP-induced amnesia was not. However, the AChE inhibitors, tetrahydroaminoacridine (THA) and physostigmine (PHY) attenuated both PCA- and SCOP-induced amnesia when administered immediately after the training session. These results indicated that benzomorphans and 5-HT2 antagonists have antiamnestic effects in mice, as do AChE inhibitors. In addition, it is interesting that the patterns of ameliorating effect of benzomorphans were similar to those of 5-HT2 antagonists, which differ from those of AChE inhibitors.

Effects of repeated treatment with 5-HT1A agonists on active avoidance responding in the rat

The behavioural effects of the serotonin 1A receptor (5-HT1A) agonist anxiolytics are generally examined after acute administration. The present study examined the effects of these substances during repeated treatment in the two-way active avoidance (Conditioned Avoidance Response, CAR) procedure. Previously it has been found that the prototypical 5-HT1A receptor agonist, 8-OH-DPAT, increases avoidance, apparently by increasing general activity, after repeated administration but not on acute administration. In the present study, it was demonstrated that this increase in activity can be blocked by the 5-HT1A receptor antagonists (-)alprenolol (also beta adrenergic antagonist) and (S)-UH-301, but not by the non-selective 5-HT antagonist metergoline. The relatively full 5-HT1A agonist, flesinoxan, and the partial 5-HT1A agonist, ipsapirone, had qualitatively similar effects to 8-OH-DPAT, although the effect of ipsapirone was clearly smaller in magnitude. Buspirone, the 5-HT1A partial agonist/dopamine D2 antagonist, markedly decreased activity, and thus avoidance of the shocks, in a manner similar to the antipsychotic drug, haloperidol. However, when the hypothermic effects of these compounds were investigated after acute administration, buspirone induced a strong hypothermic response in rats, like 8-OH-DPAT, whereas haloperidol had no effect. With the exception of buspirone, the effectiveness of these compounds in increasing activity in the CAR test appears to be related to their agonist efficacy at the 5-HT1A receptor. Similarities between the effects of these compounds and previously reported results with serotonin-depleting agents (Tenen 1967; Breese et al. 1974) suggest that the net effect of 5-HT1A agonists after repeated administration is to produce a functional reduction in 5-HT activity. The activity suppressing action of buspirone indicates that the dopamine antagonist activity of buspirone predominates in this procedure.

Fluoxetine and 8-OH-DPAT in the lateral septum enhances and impairs retention of an inhibitory avoidance response in rats

The present study investigated the role of lateral septal serotonin (5HT) in memory consolidation and the subtype of 5HT receptors involved in this process. Rats with cannulae implanted bilaterally into the lateral septum were trained in an inhibitory avoidance task. Immediately after training, the septal serotonergic function was manipulated by pharmacological agents selectively blocking 5HT reuptake (fluoxetine and zimelidine), antagonizing 5HT2 receptors (ketanserin and ritanserin), or activating 5HT1A receptors, respectively. Results indicated that direct fluoxetine infusions into the lateral septum at a dose of 6 micrograms/0.5 microliter and zimelidine at a dose of 5 micrograms/0.5 microliter both markedly enhanced memory. Intralateral septal injections of ketanserin (0.3 microgram/0.5 microliter and 0.5 microgram/0.5 microliter) and ritanserin (0.3 microgram/0.5 microliter and 0.6 microgram/0.5 microliter) did not have a significant effect by themselves on memory, and neither did they attenuate the memory-facilitating effect of fluoxetine in the same area. Intralateral septal infusions of 8-hydroxy-2-(di-n-propylamino)tetralin at 5 micrograms/0.5 microliter significantly impaired memory retention. These findings altogether support the notion that the lateral septal nuclei of rats are involved in the memory processes of inhibitory avoidance learning. Furthermore, postsynaptic 5HT receptor activation (not the 5HT2 receptor subtype) probably exerts a facilitatory effect while presynaptic 5HT1A receptor activation exerts an impairing effect on the memory consolidation process, probably due to autoreceptor inhibition of 5HT release.

Memory enhancing effects of granisetron (BRL 43694) in a passive avoidance task

Recent evidence suggests that serotonin plays an important role in learning and memory processes in animals. The present study examined the effect of the 5-HT3 receptor antagonist, granisetron (BRL 43694), on acquisition, retention and retrieval of a passive avoidance response in mice. Granisetron (1 and 10 micrograms/kg) administered 30 min before presentation of footshock increased the step-down latency when tested 24 h after footshock. The acquisition process was not affected by a dose of 100 micrograms/kg. Granisetron (10 and 100 micrograms/kg) produced a significant increase in latency to step out of the safety zone, when administered immediately after or 23.5 h after footshock. However, at 1 microgram/kg, granisetron had no effect. These results confirm the important role played by 5-HT in the process of learning and memory, and also suggest that memory enhancement may be possible with non-cholinergic treatments.

Manipulation of serotonin protects against an hypoxia-induced deficit of a passive avoidance response in rats

The 5-HT antagonists ketanserin, mianserin, methysergide, and cyproheptadine and the 5-HT uptake inhibitors fluoxetine and zimeldine were evaluated for their ability to protect against an hypoxia-induced performance deficit in a passive avoidance (PA) task. The ability to retain a PA response was found to decrease as the oxygen concentration decreased with the largest retention deficit occurring at 6.5% O2. The 5-HT2 selective antagonists ketanserin (0.01-10.0 mg/kg SC) and mianserin (0.05-10.0 mg/kg SC) administered one minute after PA training produced dose-dependent increases in retention latencies following exposure to a 6.5% oxygen environment. Peak effective doses (PED) for ketanserin and mianserin were 3.0 mg/kg SC and 0.05 mg/kg SC, respectively. In contrast, methysergide (0.05-30.0 mg/kg SC) and cyproheptadine (0.05-7.0 mg/kg SC), antagonists that show less affinity for the 5-HT2 receptor subtype, were not effective in preventing the hypoxia-induced amnesia. Inhibition of 5-HT reuptake by fluoxetine (0.01-1.0 mg/kg SC) produced dose-dependent increases in retention latencies with a PED of 0.05 mg/kg SC while zimeldine (0.1-10.0 mg/kg SC), another 5-HT reuptake inhibitor, had no effect on the amnesia. The results of this study suggest that modification of 5-HT after exposure to hypoxia can ameliorate a performance deficit in an animal model of learning and memory.

Tryptamine impairs the acquisition of a one-way active avoidance task

The effects of intraperitoneal administration of tryptamine to rats pretreated with iproniazid, on the acquisition of an unsignalled one-way active avoidance task, were examined. Tryptamine at 2.5 and 5 mg/kg significantly increased the number of trials required to perform this task to a 9/10 consecutive avoidances criterion, without affecting escape performance. The iproniazid pretreatment had no affect on acquisition, or any other performance variable, of the task. Tryptamine did not significantly affect the avoidance response, or escape response latencies; further tryptamine did not alter gross locomotor activity measured as photocell counts. These results suggest that the acquisition deficit was not the result of nonassociative effects such as changes in response capability, general activity level or nociception. The acquisition deficit induced by tryptamine may involve a direct stimulation of central 5-HT receptors since it was not induced by systemically administered 5-HT, was reversed by the 5-HT antagonists methysergide and metergoline, but was not affected by depletion of brain 5-HT, with PCPA, or by the dopamine antagonist haloperidol. Possible behavioural mechanisms for the action of tryptamine are discussed.

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.

Effects of prenatal 5-methoxytryptamine and parachlorophenylalanine on serotonergic uptake and behavior in the neonatal rat

Parachlorophenylalanine (pCPA) or 5-methoxytryptamine (5MT) was administered to pregnant Sprague Dawley rats from day 8 (D8) of gestation till D17 and from D12 until birth respectively. Birth weights of both drug groups of neonates were approximately 20% less than the saline-injected controls. 5MT neonates showed a significant reduction of high affinity 3H-5HT uptake in the brainstem at all three time points: D1, D15, D30, and a slight reduction in the forebrain reaching significance only on D30. The pCPA animals showed a significant reduction in the brainstem and forebrain on D1 and D30, but only a small nonsignificant reduction in both areas on D15. Behaviors measured on day 15 revealed that in general activity, spontaneous alternation, and passive avoidance both drug groups of neonates showed deficits: less activity, less alternation, and less avoidance.

Enhanced passive avoidance retention following posttrain serotonergic receptor antagonist administration in middle-aged and aged rats

The experiments examined the ability of posttrain administration of serotonergic receptor antagonists to attenuate the age-related deficits in avoidance retention normally exhibited by middle-aged (12 months) and aged (22 months) rats. Ketanserin (0, 1.0, 10.0 mg/kg) produced a significant dose-dependent increase in test step-through latencies in both age groups. The suppression of responding did not appear to be due to generalized learned aversion as nonshocked rats, injected with the highest dose of ketanserin, did not exhibit similar elevations in test latencies. In order to determine whether the ketanserin-induced effect could be generalized to other serotonergic antagonists, middle-aged and aged animals were injected with a single dose of mianserin (10.0 mg/kg). This antagonist also significantly increased step-through latencies in both age groups, while not affecting the performance of nonshocked animals. The results provide additional evidence for a role of the serotonergic nervous system in memory, and may have important implications in the development of effective treatment strategies for geriatric-related cognitive disorders.

Nigral 5-HT and substance P-induced enhancement of passive avoidance retention

Peripheral, posttraining injection of substance P (SP) has been shown to facilitate the retention of aversive and appetitive learning tasks, suggesting that SP may play a role in information processing. In addition, SP may modulate the release of nigrostriatal monoamines, which have also been linked with avoidance learning. This paper examines the interaction between SP and nigrostriatal monoamines by observing the behavioral effects of neurochemical lesions on SP-induced avoidance retention, and by measuring changes in nigrostriatal monoamine activity and receptor regulation following avoidance training and SP injection. In Expt. 1, 5,7-dihydroxytryptamine lesions of the substantia nigra, but not the caudate, attenuated the retention-enhancing effects of posttraining SP injection. Further, 6-hydroxydopamine lesions of the substantia nigra produced a deficit in avoidance conditioning that was reversed by posttraining SP injection. Expts. 2 and 3 demonstrated that although passive avoidance training and posttraining SP injections did not significantly alter nigral 5-hydroxytryptamine (5-HT) activity, SP increased 5-HT1 receptor density. It was concluded that SP may affect avoidance retention by modulating nigral 5-HT activity.

Serotonin does not mediate anxiolytic effects of buspirone in the fear-potentiated startle paradigm: comparison with 8-OH-DPAT and ipsapirone

The present study evaluated the role of various neurotransmitter systems in mediating buspirone's blockade of the fear-potentiated startle effect, where acoustic startle amplitude is normally increase in the presence of a light previously paired with a shock. Large lesions of the dorsal and median raphe nuclei or IP injections of the serotonin antagonists cinanserin (10 mg/kg) or cyproheptadine (5 mg/kg) did not alter fear-potentiated startle, nor did these treatments prevent buspirone (5 or 10 mg/kg SC) from blocking fear-potentiated startle. The 5-HT 1A agonist 8-OH-DPAT (2.5-10.0) did not block fear-potentiated startle even at doses that produced a marked "5-HT syndrome". Another 5-HT 1A agonist, ipsapirone (10-20 mg/kg), blocked potentiated startle only at a very high dose (40 mg/kg). p-Chlorophenylalanine and p-chloroamphetamine did not alter fear-potentiated startle. Finally, pretreatment with the benzodiazepine receptor antagonist RO-15-1788 (1 mg/kg); the opiate antagonist naloxone (2 mg/kg) or the alpha 2-adrenergic antagonist yohimbine (5 mg/kg) did not reduce fear-potentiated startle, nor did they prevent buspirone from blocking fear-potentiated startle. Taken together, the data do not support the hypothesis that buspirone's anxiolytic effects are mediated by actions at 5-HT 1A receptors and more generally indicate that serotonergic neurons do not play an important role in fear-potentiated startle.

Passive avoidance deficit following intracerebroventricular administration of cholecystokinin tetrapeptide amide in rats

The effect of cholecystokinin tetrapeptide amide (CCK-4) injected into the lateral cerebral ventricle on memory processes was examined by a one-trial passive avoidance test in the rat. CCK-4 injection 30 and 60 min before the first retention test caused a shortened latency to response, and its chronic infusion into the lateral ventricle at a rate of 2 micrograms/day shortened the latency of the response to the level of almost complete amnesia. CCK-4 also reduced arginine-vasopressin effect on memory processes when administered simultaneously 30 min before the first retention test, but its amnestic action is short-lasting and antagonized by relatively small amounts of cholecystokinin octapeptide (CCK-8). In addition, the shortened latency to response was admitted to be not always associated with the motility effect of CCK-4.

The inhibition of the cage-leaving response--a model for studies of the serotonergic neurotransmission in the rat

It was observed that rats that had been given drugs that enhance serotonergic neurotransmission, e.g. the serotonin releasing compounds p-chloroamphetamine (PCA) and fenfluramine, the MAO-A inhibitors and serotonin releasing agents amiflamine and alpha-ethyltryptamine and the serotonin agonists 5-methoxy-N, N-dimethyltryptamine (5-MeODMT), 8-hydroxy-2-(di-n-propylamino) tetraline (8-OH-DPAT), m-chlorophenyl piperazine (m-CPP) and 5-methoxy-3 (1,2,3,6-tetrahydropyridin-4-yl)1H-indole (RU 24969), did not leave their home-cages when the grid-covers were removed in contrast to normal rats who almost immediately left the cages. The association between the serotonin neurotransmission and the inhibitory effect of PCA on the cage-leaving response was indicated by the findings that 1. Serotonin uptake inhibitors (alaproclate and citalopram) antagonized the effect of PCA. 2. High, neurotoxic doses of PCA antagonized the effect of PCA when tested one week after the former administration. The serotonin uptake inhibitor zimeldine counteracted the effect of neurotoxic PCA. 3. Depletion of brain serotonin with p-chlorophenylalanine counteracted the effect of acute PCA. 4. Repeated treatment of rats for 7 days with zimeldine, amiflamine, alpha-ethyltryptamine or clorgyline plus a low dose of PCA counteracted the effect of acute PCA probably due to a functional down-regulation at postsynaptic receptors. Clorgyline or a low dose of PCA by themselves had no effect. 5. Compounds interacting with dopamine or noradrenaline mechanisms, e.g. alpha-methyltyrosine, N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP 4), pimozide, remoxipride and prazosin did not antagonize the effect of PCA nor did (+)-amphetamine inhibit the cage-leaving response. None of the serotonin receptor antagonists (cinanserin, ketanserin, metergoline, methysergide, metitepine, mianserin, pirenperone) blocked the inhibition of the cage-leaving response produced by PCA, indicating that the receptors involved may not be of the S1- and S2- types. Observation of the cage-leaving response may be a valuable technique in studies of drugs that enhance the serotonin neurotransmission in the rat brain.

Analysis of the avoidance learning deficit induced by the serotonin releasing compound p-chloroamphetamine

The effects of the serotonin-releasing compound p-chloroamphetamine (PCA, 2.5 mg/kg) on avoidance acquisition, retention and memory retrieval were examined in male Sprague-Dawley rats using a one-way active avoidance and a one-trial passive avoidance task. The drug was injected IP prior to training, following acquisition and prior to the retention test 24 hr after training using a state-dependent design. In the normal context situation pretraining administration of PCA markedly impaired active avoidance acquisition, but PCA-treated rats did not differ from controls in their retention performance when tested 24 hr after training. In the dark/light box test pretraining administration of PCA caused a dose-dependent impairment of both active and passive avoidance retention which could not be explained in terms of changes in locomotor activity or behavioural disinhibition at the time of testing or state-dependent retention. Post-training administration of PCA failed to affect avoidance retention in both tasks. The drug was found to impair memory retrieval in a dose- and time-dependent fashion in the one-way active but not in the passive avoidance test. Pretraining administration of PCA produced a progressive loss of passive and active avoidance performance at increasingly longer retention intervals. The present results suggest that serotonin has dual effects on processes underlying learning and memory involving effects on both associative and non-associative learning processes in the rat. The time-dependent loss of memory retention following 5-HT release indicates that serotonin has a role in the way information is processed in the brain.

Serotonin receptor involvement in the avoidance learning deficit caused by p-chloroamphetamine-induced serotonin release

The receptor involvement in the p-chloramphetamine (PCA, 2.5 mg kg-1) induced impairment of active avoidance acquisition was examined in the male rat. The avoidance deficit was blocked at low doses by serotonergic (5-HT)-receptor blocking agents but not by alpha-adrenergic-, beta-adrenergic-, opiate-, muscarinic- or dopamine D2-receptor antagonists. The potency of the 5-HT antagonists to block the PCA-induced deficit correlated with their affinity in displacing [3H]ketanserin but not [3H]5-HT binding in the frontal cortex. The potencies of the 5-HT antagonists to block the action of PCA could not be related to their action on muscarinic-, histaminergic H1- or dopaminergic D2-receptor binding in vitro. It is concluded that the avoidance learning deficit caused by PCA-induced 5-HT release is related to activation of 5-HT receptors in the frontal cortex having the characteristics of a 5-HT2 receptor.

Forebrain serotonergic involvement in avoidance learning

The one-way active avoidance deficit caused by the serotonergic (5-HT) releasing compound p-chloroamphetamine (PCA) was examined in rats after degeneration of 5-HT neurons in the forebrain. Injection of 5,7-dihydroxytryptamine (5,7-DHT) into the forebrain in desipramine (20 mg/kg)-pretreated rats resulted in a 65-70% decrease in 5-HT concentrations in the prefrontal cortex and hippocampus without any significant effect on striatal 5-HT. Slight reductions in noradrenaline (NA) (25%) and dopamine (DA) (34%) concentrations were observed in the prefrontal cortex only. The 5,7-DHT lesions markedly attenuated the impaired avoidance performance induced by PCA (2.5 mg/kg), suggesting that the avoidance deficit depends on intact 5-HT terminals in cortex and/or hippocampus. The 5,7-DHT lesion alone caused a slight but significant impairment of acquisition. The results suggest that 5-HT terminal systems in the forebrain play an important role in avoidance learning.

Ejaculations induced by p-chloroamphetamine in the rat

The ejaculatory response following acute injections of p-chloroamphetamine (PCA) and several other drugs was measured by weighing the compact seminal material accumulated over 2 hr. p-Chloroamphetamine caused a dose-dependent ejaculatory response that was inhibited by the inhibitor of the synthesis of 5-hydroxytryptamine (5-HT), p-chlorophenylalanine (PCPA), neurotoxic doses of PCA, reserpine, DSP 4 a selective noradrenergic neurotoxin given 48 hr before PCA, the inhibitor of synthesis of noradrenaline (NA) FLA 63, the specific inhibitors of uptake of 5-HT, alaproclate, fluoxetine and norzimeldine and the selective inhibitor of the uptake of NA, CPP 199, the E form of norzimeldine. The doses of several receptor antagonists producing a 50% decrease in the weight of seminal material were determined. The non-selective 5-HT receptor antagonists, metitepine and methergoline, the selective alpha 1-adrenoreceptor antagonists, prazosin and phenoxybenzamine and the non-selective alpha-adrenoreceptor antagonist, phentolamine, had strong effects, followed by the selective 5-HT2 antagonists, ketanserin and pirenperone. Yohimbine, an alpha 2-adrenoreceptor antagonist and atropine, a muscarinic receptor antagonist, only produced a partial blockade. The rank order of potency for some dopamine (DA) receptor antagonists was chlorpromazine, domperidone, haloperidol, pimozide. Remoxipride, a selective DA2 receptor antagonist and the selective DA1 antagonist, Sch 23390, had no effect. The following drugs had no effect: propranolol, naloxone, picrotoxin, cimetidine and mepyramine. The 5-HT receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT 3 mg/kg, i.p.) produced a small effect on the weight of seminal material, although 72% of the rats ejaculated. d-Amphetamine did not induce ejaculation at 5 mg/kg but had a marked effect at 15 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Central serotonin neurones in avoidance learning: interactions with noradrenaline and dopamine neurones

The effects of p-chloroamphetamine (PCA), a serotonin (5-HT) releaser, on acquisition and retention were examined in male Sprague-Dawley rats using a one-way active avoidance task. PCA was found to impair avoidance acquisition and retention in a time dependent fashion which followed closely the temporal effects of the drug on 5-HT release in the brain. Thus, the avoidance deficit is related to the rate of change and not to the steady-state levels of 5-HT. The 5-HT releasing effect was most pronounced in the forebrain with less effect in the spinal cord. PCA caused time dependent, regional variations in catecholamine content, which was not related to avoidance performance. The avoidance and retention impairment induced by PCA was blocked by the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA) but not by depletion of catecholamines with alpha-methyl-p-tyrosine (H44/68) or by the noradrenergic-selective neurotoxin DSP4. Analysis of the time dependent effects of PCA on monoamine content in saline or PCPA-treated rats indicated that the temporal effects of PCA on avoidance performance is not due to a direct or indirect action on catecholamine neurones. The present experiments support the view that the ascending serotonergic pathways play a significant role in aversive learning in the rat.

Evidence for selective serotonergic receptor involvement in p-chloroamphetamine-induced antinociception

Administration of the serotonin (5-HT) releasing compound p-chloroamphetamine (PCA; 2.5 mg/kg) induced potent analgesia in rats tested with the hot plate method. The analgesia was prevented by pretreatment with either of the 5-HT uptake inhibitors alaproclate (20 mg/kg) or fluoxetine (10 mg/kg). Taking into account that the noradrenergic uptake inhibitor desipramine in previous experiments failed to interfere with the effect of PCA, these results demonstrate that PCA selectively acts on 5-HT terminals. The analgesia was attenuated by administration of the 5-HT antagonists methiothepin (0.125-0.5 mg/kg) and danitracen (0.25-2.5 mg/kg) but not by a series of other 5-HT receptor antagonists or antagonists acting on noradrenergic, dopaminergic, GABAergic, histaminergic or muscarinic receptors. It is concluded that the analgesic effect of PCA is mediated via stimulation of a type of 5-HT receptors possibly belonging to the 5-HT-1 class. Further studies are, however, needed in order to firmly establish the relationship to any particular sub-type of 5-HT receptor as characterized in in vitro binding studies.

Manipulations of 5-HT activity and memory in the rat

The activity of 5-HT was manipulated by means of the peripheral injection of 5-HT reuptake inhibitors, fenfluramine and fluoxetine. These drug treatments, at doses higher than 1 mg/kg, produced retrograde amnesia in a one-trial appetitive learning task in rats. A non-specific inhibitor of 5-HT reuptake, imipramine, did not produce this amnesic effect, nor did the combination of fenfluramine with the MAOI tranylcypromine, although it produced, as expected, the "serotonergic syndrome." Results for the metabolic precursor of 5-HT, 5-HTP, also administered peripherally, were inconsistent, with amnesic effects seen at 5 and 20 mg/kg but none at 10 mg/kg.

Separation of the associative and non-associative effects of brain serotonin released by p-chloroamphetamine: dissociable serotoninergic involvement in avoidance learning, pain and motor function

p-Chloramphetamine (PCA, 0.63-5 mg/kg IP) injected 30-60 min before testing produced a dose-related impairment of avoidance acquisition, prolonged reaction time in the hot-plate test and increased locomotor activity. Pretreatment with the selective serotonin (5-HT) uptake inhibitor zimeldine (10 mg/kg IP) blocked these behavioral effects. Degeneration of brain 5-HT neurons by a high neurotoxic dose of PCA (2 X 10 mg/kg IP) or inhibition of tryptophan hydroxylase by p-chlorophenylalanine (300 mg/kg IP) also blocked the behavioral effects of PCA. There was a complete blockade of the PCA-induced avoidance deficit following pretreatment with metergoline, a central 5-HT receptor blocking agent. On the other hand, metergoline failed to block the hot-plate analgesia and the increased locomotion caused by PCA. Depletion of brain NA and DA by the tyrosine hydroxylase inhibitor H44/68 did not counteract the PCA effect on avoidance or hot-plate performance, but reduced the locomotor stimulating effect. The selective NA neurotoxin DSP4 (50 mg/kg IP) or the opiate antagonist naloxone (1 mg/kg) failed to affect the PCA-induced modulations of the behaviours studied. In addition, PCA administration in doses that caused avoidance deficits, did not result in motor impairment as assessed by the tread mill test. The above results support the hypothesis that the PCA-induced impairment of active avoidance acquisition does not involve changes in nociception or altered locomotor activity. It is concluded that behavioural processes related to serotonergic neurotransmission can be independently modified, suggesting differences in the underlying 5-HT mechanisms.

Involvement of spinal serotonergic pathways in nociception but not in avoidance learning

The effects of selective lesions of the descending serotonergic (5-HT) pathways on analgesia and avoidance deficit induced by the 5-HT releasing compound p-chloroamphetamine (PCA, 2.5 mg/kg) were investigated in male rats. Intrathecal injection of 5,6-DHT (20 micrograms/rat) reduced the uptake of labelled 5-HT into spinal synaptosomes by approximately 85% but did not significantly affect the uptake of noradrenaline. The lesions produced a significant hyperalgesia and strongly attenuated the analgesic effect of PCA in the hot-plate test. In the flinch-jump test 5,6-DHT lesioned rats receiving PCA did not differ from the saline control group. Spinal lesioning did not, however, affect one-way active avoidance performance and did not prevent the marked impairment of avoidance performance induced by PCA. Thus, the avoidance deficit caused by PCA is independent of the descending serotonergic pathways and of the analgesia induced by PCA. These results support the view of a differential involvement of the ascending and descending serotonergic projections in behavioural processes controlled by aversive stimuli.

Test-dependent variations in the antinociceptive effect of p-chloroamphetamine-induced release of 5-hydroxytryptamine

p-Chloroamphetamine (PCA), in doses that did not significantly impair motor performance in a rotating-wheel task, induced marked analgesia in rats tested with the hot-plate and flinch-jump methods. In the tail-flick test, moderate hyper- or hypo-analgesia was found to be dependent on dose. In hot-plate experiments the analgesia was attenuated by inhibition of uptake of 5-HT (with zimelidine), depletion of stores of 5-HT (with PCPA) and by lesioning of 5-HT-containing terminals (long-term PCA treatment). Blockade of serotonin receptors by metergoline produced hyperalgesia, but failed to reduce the analgesia induced by p-chloroamphetamine. Manipulation of catecholaminergic and opioid systems did not reduce the effect of p-chloramphetamine. It is concluded that induction of release of 5-HT by chloroamphetamine induces antinociception which varies in magnitude between tests, suggesting that different serotonergic mechanisms modulate complex and reflex responses to noxious stimulation. The failure of metergoline to antagonize the analgesia induced by p-chloroamphetamine suggests an involvement of 5-HT receptors different from the ones implicated in other types of behaviour mediated by 5-HT.

Selective lesions of the bulbospinal serotonergic pathways reduce the analgesia induced by p-chloroamphetamine in the hot-plate test

The antinociceptive effect of the serotonin releasing compound p-chloroamphetamine (PCA) was investigated in rats by means of the hot-plate method. Selective neurotoxic lesions of the descending serotonergic pathways induced by intrathecal injections of 5,6-dihydroxytryptamine (20 micrograms, 7 days before testing), produced hyperalgesia and significantly attenuated the analgesic effect of acutely administered PCA (2.5 mg/kg). The lesions reduced the spinal serotonin content by about 90% without significantly altering noradrenaline levels. It is concluded that the descending serotonergic pathways contribute to the expression of PCA-induced hot-plate analgesia.

Alteration of convulsive threshold and conditioned avoidance response in mice fed diets containing contraceptive steroids

Female CD1 mice given the contraceptive steroids mestranol and norethynodrel (1:10) in the diet (0.0033%) for 4 months had a growth reduction of 20% when compared with mice fed a normal diet, and had lower convulsive thresholds when tested with the vitamin B6 antagonists 2,4-dimethyl-5-methyl-hydroxypyrimidine and thiosemicarbazide. In conditioned avoidance response (CAR) tests, mice fed the steroid-containing diets showed a decreased acquisition performance during all six sessions; however, mice fed the same diet supplemented with vitamin B6 (0.04%) performed as well during the last three sessions of the CAR tests as mice fed the normal diet.

Retention deficits induced by acute p-chloroamphetamine following fear conditioning in the rat

Rats were given four inescapable shocks (1.0 mA) when confined to the right-hand corner of a modified shuttlebox. p-Chloroamphetamine (PCA) injected just before the retention test 24 h later completely blocked the immobile posture that was observed after saline injections. This retention deficit was shown to be selectively associated with 5-hydroxytryptamine (5-HT) release, since the administration of the 5-HT uptake inhibitors zimelidine and citalopram 60 min prior to PCA antagonized this effect. The 5-HT specificity of the deficit was further established by the findings that 5-HT-depleted rats (PCA, 2 x 10 mg/kg, and fenfluramine, 2 x 25 mg/kg), but not NA-depleted rats (DSP4, 1 x 50 mg/kg), or rats treated with zimelidine (2 x 20 mg/kg) 60 min before PCA (2 x 10 mg/kg), showed an almost complete blockade of the retention failure. The data presented may provide a useful experimental model for investigating the efficacy of functional 5-HT activity in the treatment of phobic anxiety.

Serotonin involvement in aversive conditioning: reversal of the fear retention deficit by long-term p-chloroamphetamine but not p-chlorophenylalanine

5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT), a serotonin (5-HT) agonist, fenfluramine and p-chloroamphetamine (PCA), which are 5-HT releasers, produce deficits in fear retention as indicated by a notable lack of the immobility resulting from inescapable shocks. Depletion of central 5-HT neurones after long-term PCA treatment (2 X 10 mg/kg) completely blocked the retention impairment resulting from acute PCA (2.5 mg/kg) and fenfluramine (5 mg/kg), and partially blocked the deficit produced by 5-MeO-DMT (4 mg/kg). 5-HT depletion after p-chlorophenylalanine (PCPA) treatment (200, 100, 100 mg/kg, 72, 48 and 24 h before) did not do so; this is in agreement with other findings which suggest the involvement of different 5-HT stores in the action of PCA and PCPA. These data further underline the importance of the ascending 5-HT pathway in aversive conditioning in the rat.

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.