The effect of pre- and post-trial amphetamine injections on avoidance responses of rats

Effects of monoaminergic drugs on training-induced motor cortex plasticity in older adults

Primary motor cortex (M1) plasticity is involved in motor learning and stroke motor recovery, and enhanced by increasing monoaminergic transmission. Age impacts these processes but there is a paucity of systematic studies on the effects of monoaminergic drugs in older adults. Here, in ten older adults (age 61+4years, 4 males), we determine the effects of a single oral dose of carbidopa/levodopa (DOPA), d-amphetamine (AMPH), methylphenidate (MEPH) and placebo (PLAC) on M1 excitability and motor training-induced M1 plasticity. M1 plasticity is defined as training related long lasting changes in M1 excitability and kinematics of the trained movement. At peak plasma level of the drugs, subjects trained wrist extension movements for 30min. Outcome measures were motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation at increasing intensity (stimulus response curve, SRC) and peak acceleration of the trained wrist extension movements. Measures were obtained before and after completion of training. The curve parameters plateau (MEPmax), inflection point, and slope were extracted from SRC. At baseline drugs had a differential effect on curve parameters, while kinematics remained unchanged. Training alone (PLAC) increased MEPmax but did not improve kinematics. Drugs affected training-related changes of the curve parameters differently, but did not enhance them or kinematics when compared to PLAC. The results demonstrate that in the older adults, MEPH, DOPA, or AMPH have differential effects on baseline M1 excitability and training-related M1 plasticity but fail to enhance them above the naïve level.

Memory enhancement produced by post-training exposure to sucrose-conditioned cues

A number of aversive and appetitive unconditioned stimuli (such as shock and food) are known to produce memory enhancement when they occur during the post-training period. Post-training exposure to conditioned aversive stimuli has also been shown to enhance memory consolidation processes. The present study shows for the first time that post-training exposure to conditioned stimuli previously paired with consumption of a sucrose solution also enhances memory consolidation. Male Long Evans rats were trained on a one-session conditioned cue preference (CCP) task on a radial arm maze. Immediately or 2 hours after training, rats consumed a sucrose solution or were exposed to cues previously paired with consumption of sucrose or cues previously paired with water. Twenty-four hours later, the rats were tested for a CCP. Immediate, but not delayed, post-training consumption of sucrose enhanced memory for the CCP. Immediate, but not delayed, post-training exposure to cues previously paired with sucrose, but not with water, also enhanced CCP memory. The possibility that rewarding and aversive conditioned stimuli affect memory by a common physiological process is discussed.

Enhancing effect of heroin on social recognition learning in male Sprague-Dawley rats: modulation by heroin pre-exposure

RATIONALE

There is evidence that pre-exposure to drugs of abuse can induce sensitization to several of their effects.

OBJECTIVE

Four experiments were conducted to investigate the effect of heroin pre-exposure on modulation of memory consolidation as indexed by heroin's action on rate of learning.

MATERIALS AND METHODS

Male Sprague-Dawley rats were tested on a social recognition learning task which assesses changes in investigation during repeated exposure to the same rat (habituation training: four sessions) and during exposure to a novel rat (dishabituation test). In the first experiment, rats received 0, 0.3, or 1 mg/kg heroin s.c. immediately following each training session, or 1 mg/kg heroin 2 h post-training. In experiments 2 and 3, rats received 1 mg/kg heroin post-training after a 7-day drug-free period from heroin pre-exposure achieved through conditioned place preference (1 mg/kg s.c., 1 injection/day x 4 days) or intravenous self-administration (0.05 mg/kg/infusion i.v., 3 h/day x 9 days) training. In experiment 4, rats received 0, 0.03, 0.3, or 3 mg/kg heroin post-training after a 7-day drug-free period from a regimen of heroin administration (i.e., 1 mg/kg heroin/day s.c. x 7 days) that induced locomotor sensitization.

RESULTS

Post-training administration of heroin enhanced social recognition learning in a dose- and time-dependent manner. Importantly, no regimen of heroin pre-exposure significantly altered this effect of heroin.

CONCLUSIONS

These results do not support the hypothesis that heroin pre-exposure leads to sensitization to its effect on memory consolidation of non-drug-related learning. However, this requires further testing using alternative heroin pre-exposure regimens, a wider range of post-training heroin doses, as well as other types of learning tasks.

Behavioural assessment of drug reinforcement and addictive features in rodents: an overview

Some psychoactive drugs are abused because of their ability to act as reinforcers. As a consequence behavioural patterns (such as drug-seeking/drug-taking behaviours) are promoted that ensure further drug consumption. After prolonged drug self-administration, some individuals lose control over their behaviour so that these drug-seeking/taking behaviours become compulsive, pervading almost all life activities and precipitating the loss of social compatibility. Thus, the syndrome of addictive behaviour is qualitatively different from controlled drug consumption. Drug-induced reinforcement can be assessed directly in laboratory animals by either operant or non-operant self-administration methods, by classical conditioning-based paradigms such as conditioned place preference or sign tracking, by facilitation of intracranial electric self-stimulation, or, alternatively by drug-induced memory enhancement. In contrast, addiction cannot be modelled in animals, at least as a whole, within the constraints of the laboratory. However, various procedures have been proposed as possible rodent analogues of addiction's major elements including compulsive drug seeking, relapse, loss of control/impulsivity, and continued drug consumption despite negative consequences. This review provides an extensive overview and a critical evaluation of the methods currently used for studying drug-induced reinforcement as well as specific features of addictive behaviour. In addition, comic strips that illustrate behavioural methods used in the drug abuse field are provided given for free download under http://www.zi-mannheim/psychopharmacology.de.

Object recognition impairment in Fmr1 knockout mice is reversed by amphetamine: involvement of dopamine in the medial prefrontal cortex

Fragile X syndrome is an X-linked form of mental retardation including, among others, symptoms such as stereotypic behaviour, hyperactivity, hyperarousal, and cognitive deficits. We hypothesized that hyperactivity and/or compromised attentional, cognitive functions may lead to impaired performance in cognitive tasks in Fmr1 knockout mice, the most widely used animal model of fragile X syndrome, and suggested that psychostimulant treatment may improve performance by acting on one or both components. Since hyperactivity and cognitive functions have been suggested to depend on striatal and prefrontal cortex dopaminergic dysfunction, we assessed whether amphetamine produced beneficial, positive effects by acting on dopaminergic corticostriatal systems. Our results show that Fmr1 knockout mice are not able to discriminate between a familiar object and a novel one in the object recognition test, thus showing a clear-cut cognitive impairment that, to date, has been difficult to demonstrate in other cognitive tasks. Amphetamine improved performance of Fmr1 knockout mice, leading to enhanced ability to discriminate novel versus familiar objects, without significantly affecting locomotor activity. In agreement with behavioural data, amphetamine produced a greater increase in dopamine release in the prefrontal cortex of Fmr1 knockout compared with the wild-type mice, while a weak striatal dopaminergic response was observed in Fmr1 knockout mice. Our data support the view that the psychostimulant ameliorates performance in Fmr1 knockout mice by improving merely cognitive functions through its action on prefrontal cortical dopamine, irrespective of its action on motor hyperactivity. These results indicate that prefrontal cortical dopamine plays a major role in cognitive impairments characterizing Fmr1 knockout mice, thus pointing to an important aetiological factor in the fragile X syndrome.

Modulation of use-dependent plasticity by d-amphetamine

Use-dependent plasticity, thought to contribute to functional recovery after brain injury, is elicited by motor training. The purpose of this study was to determine if administration of d-amphetamine facilitates the effects of motor training on use-dependent plasticity. Healthy human volunteers underwent a training period of voluntary thumb movements under the effects of placebo or d-amphetamine in different sessions in a randomized double-blind, counterbalanced design. Previous work in a drug-naive condition showed that such training causes changes in the direction of thumb movements evoked by transcranial magnetic stimulation and in transcranial magnetic stimulation-evoked electromyographic responses. The endpoint measure of the study was the magnitude of training-induced changes in transcranial magnetic stimulation-evoked kinematic and electromyographic responses in the d-amphetamine and in the placebo conditions. Motor training resulted in increased magnitude, faster development and longer lasting duration of use-dependent plasticity under d-amphetamine compared to the placebo session. These results document a facilitatory effect of d-amphetamine on use-dependent plasticity, a possible mechanism mediating the beneficial effect of this drug on functional recovery after cortical lesions.

Cocaine enhances retention of avoidance conditioning in rats

The effects of post-training cocaine administration were tested on retention of a one-way active avoidance task in rats. A 5.0 mg/kg IP dose of cocaine enhanced retention of the avoidance task, in three separate experiments, as indicated by an increase in the number of avoidances made when animals were tested 24 h after training, while both a lower (2.5 mg/kg) and a higher (7.5 mg/kg) cocaine dose had no effect. Lidocaine (4-8 mg/kg) administered post-training did not reliably affect retention in the same task. Cocaine's ability to enhance retention depended on the interval between training and drug injection such that only cocaine administered directly after training enhanced retention the following day. The results show that post-training cocaine administration enhances retention of an active avoidance task in rats, and that this effect is probably independent of the anesthetic properties of the drug.

Neuropsychopharmacological mechanisms of stimulant drug action in attention-deficit hyperactivity disorder: a review and integration

The psychostimulants, D-amphetamine (D-AMP) and methylphenidate (MPH), are widely used to treat attention-deficit hyperactivity disorder (ADHD) in both children and adults. The purpose of this paper is to integrate results of basic and clinical research with stimulants in order to enhance understanding of the neuropharmacological mechanisms of therapeutic action of these drugs. Neurochemical, neurophysiological and neuroimaging studies in animals reveal that the facilitative effects of stimulants on locomotor activity, reinforcement processes, and rate-dependency are mediated by dopaminergic effects at the nucleus accumbens, whereas effects on delayed responding and working memory are mediated by noradrenergic afferents from the locus coeruleus (LC) to prefrontal cortex (PFC). Enhancing effects of the stimulants on attention and stimulus control of behavior are mediated by both dopaminergic and noradrenergic systems. In humans, stimulants appear to exert rate-dependent effects on activity levels, and primarily enhance the motor output, rather than stimulus evaluation stages of information-processing. Similarity of response of individuals with and without ADHD suggests that the stimulants do not target a specific neurobiological deficit in ADHD, but rather exert compensatory effects. Integration of evidence from pre-clinical and clinical research suggests that these effects may involve stimulation of pre-synaptic inhibitory autoreceptors, resulting in reduced activity in dopaminergic and noradrenergic pathways. The implications of these and other hypotheses for further pre-clinical and clinical research are discussed.

Amygdala modulation of multiple memory systems: hippocampus and caudate-putamen

A series of five experiments examined the differential mnemonic roles of the hippocampus and caudate-putamen and the modulatory influence of the amygdala on hippocampal and caudate-putamen memory processes. Findings indicate that (a) posttraining intrahippocampal injections of amphetamine selectively enhance memory in a hidden platform water maze task, (b) posttraining intracaudate injections of amphetamine selectively enhance memory in a visible platform water maze task, (c) posttraining intra-amygdala injections of amphetamine enhance memory in both water maze tasks, (d) preretention intrahippocampal lidocaine injections block expression of the memory enhancing effects of posttraining intrahippocampal amphetamine injections in the hidden platform task, (e) preretention intracaudate lidocaine injections block expression of the memory enhancing effects of posttraining intracaudate amphetamine injections in the visible platform task, (f) preretention intra-amygdala lidocaine injections do not block the memory enhancing effect of posttraining intra-amygdala amphetamine injections on either task, (g) in the hidden platform task, posttraining intrahippocampal, but not intracaudate, lidocaine injections block the memory enhancing effects of posttraining intra-amygdala amphetamine, (h) in the visible platform task, posttraining intracaudate, but not intrahippocampal, lidocaine injections block the memory enhancing effects of posttraining intra-amygdala amphetamine. The findings indicate a double dissociation between the roles of the hippocampus and caudate-putamen in memory and suggest that the amygdala exerts a modulatory influence on both the hippocampal and caudate-putamen memory systems.

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

Experimental results are reviewed which indicate that memory storage can be altered by a number of post-training treatments that affect different hormones and neurotransmitters. Moreover, evidence was reported which suggests that the action of treatments effective on memory processes involves interactions among different systems, consistently with the complexity of brain systems. In the last decade, inbred strains have been exploited to investigate the role of neurotransmitter and hormone systems in learning and memory, leading to behavioural and neurochemical correlations based on strain differences that provide unique information on the biological systems underlying behaviour. Research carried out on the inbred strains of mice C57BL/6 (C57) and DBA/2 (DBA), demonstrates that the genetic makeup plays an important role in modulating response to drug administration. Thus, recent results have shown that in C57 mice, similarly to what occurs in outbred strains of mice or in rats, GABAergic agonists impair memory and antagonists improve it, whilst the opposite is evident in the DBA strain. By contrast, post-training administration of selective D1 or D2 agonists impairs and post-training administration of selective antagonists improves retention in DBA mice, whilst these agents have opposite effects in the C57 strain. Dose- and strain-dependent effects are evident also following post-training corticosterone as well as opioid agonists and antagonists administration. On the other side, these two strains react similarly to oxotremorine (improvement) and to atropine (impairment) administration, DBA mice being more responsive to the effects of both drugs than C57 mice. Data on the interactions between agents acting upon different neurotransmitter and/or hormonal systems in these strains indicate strain-dependent synergistic or antagonistic interactions among some of these systems, pointing to inbred strains of mice as an important methodological tool in the study of neural and hormonal factors involved in emotion and in its effects on cognition. In particular, these studies have been carried out on inbred strains of mice from which recombinant inbred (RI) strains are available that have recently been proposed as a choice experimental method in psychopharmacogenetics.

Amphetamine-induced disruptions of latent inhibition are reinforcer mediated: implications for animal models of schizophrenic attentional dysfunction

Latent inhibition (LI) is a phenomenon observed when repeated, non-reinforced presentation of a stimulus results in a retardation of subsequent conditioning to that stimulus. Several recent experiments have suggested that LI is abolished in conditioned suppression paradigms following acute, low doses of amphetamine given during pre-exposure and conditioning. Experiment 1 sought to increase the generality of this finding in an appetitive LI paradigm, using a dose of amphetamine previously shown to disrupt the LI effect in an aversive paradigm (Killcross and Robbins 1993). However, no evidence for any disruption of LI was found. Experiment 2 extended this investigation to additional, higher doses of d-amphetamine, and also examined the role of reinforcer magnitude in the effect. A non-significant trend towards an attenuated LI effect was found, which was reversed by decreases in the concentration of the sucrose reinforcer. Experiments 3 and 4 investigated the influence of systemic amphetamine in aversive paradigms, with specific attention to the increased response to the aversive foot-shock reinforcer found in amphetamine-treated animals. These experiments revealed that the influence of amphetamine on the LI effect in conditioned suppression paradigms could be reversed by reducing the intensity of footshock used in conditioning, thereby paralleling the effect found in the appetitive paradigm. Therefore it is unlikely that a simple attentional account of the abolition of the LI effect in previous experiments can be sustained.

Effects of cocaine and amphetamine on acetylcholine release in the hippocampus and caudate nucleus

The role of dopamine in the control of hippocampal acetylcholine release was evaluated by using in vivo microdialysis. The effects of the two psychostimulants, cocaine and d-amphetamine, were studied on acetylcholine release in the hippocampus and compared to effects observed in the caudate nucleus. Administration of cocaine (10 and 20 mg/kg i.p.) increased acetylcholine release by 130 and 190% in the hippocampus, whereas in the caudate nucleus the enhancement was 51 and 80% over basal values, respectively. After the injection of d-amphetamine (1 and 2 mg/kg i.p.) the enhancement of acetylcholine release was 110 and 210% in the hippocampus whereas it was 35 and 54%, respectively, in the caudate nucleus. As observed in the caudate nucleus, pretreatment with the dopamine D1 receptor antagonist, SCH 23390, antagonized the cocaine- and amphetamine-induced increase in hippocampal acetylcholine release. These results show that cocaine and d-amphetamine, by increasing dopaminergic transmission, enhance the extracellular concentrations of acetylcholine in both brain areas. The relative enhancement in the hippocampus was far greater than that in the caudate nucleus, suggesting that dopaminergic control of cholinergic function differs in these two brain areas. The results also suggest that endogenous dopamine, by facilitating the release of acetylcholine in the hippocampus, may participate in the regulation of hippocampal cognitive processes.

Cocaine inhibits hippocampal long-term potentiation

Long-term potentiation (LTP) is a form of synaptic plasticity that may underlie learning and memory. The experiments reported here demonstrate that cocaine blocks the induction of LTP at the excitatory synapses in the CA1 region of the hippocampus, but does not appear to do so by blocking NMDA receptors or channels. Once LTP had been established, however, cocaine had no effect on the potentiated response. Cocaine was also able to block LTP initiated by superfusing slices with 25 mM TEA. The ability to block LTP was shared by the local anesthetics lidocaine and procaine, but not by tetrodotoxin, suggesting that the blockade of sodium channels alone did not disrupt LTP. Biochemical experiments demonstrated that cocaine can inhibit phosphorylation of purified Synapsin I by Ca2+/calmodulin-dependent protein kinase II. This effect, presumably mediated by effects on calmodulin, is a previously unreported action of cocaine, and suggests that cocaine at high dose levels might disrupt types of learning that are mediated by an LTP-like mechanism.

Cocaine and amphetamine facilitate retention of jump-up responding in rats

The effects of cocaine and d-amphetamine administration on the acquisition of an automated jump-up active avoidance task were examined in two separate experiments. On days 1 and 2, male Sprague-Dawley rats received one escape-only training trial, followed immediately by the intraperitoneal injection of cocaine, amphetamine, or saline. On day 3, subjects received eight escape/avoidance trials. The posttraining administration of cocaine (2.75 and 5.55 mg/kg) and amphetamine (0.3 and 1.0 mg/kg) on days 1 and 2 facilitated jump-up avoidance performance on day 3. Importantly, both cocaine and amphetamine enhanced learning and memory under experimental conditions that allowed for drug-free training and testing.

The learning capacity of high or low performance rats is related to the hippocampus NMDA receptors

The hippocampal synaptic plasticity of rats with an inborn high (HP) or low (LP) learning capacity to perform in a shuttle box is closely related to their percentage of conditioned responses (Crs). HP rats show less sensitivity to the blocking effect of 2-aminophosphonopentanoic acid (AP5) on the generation of long-term potentiation (LTP) than do LP rats. Results described in the present report are indicative of an increased density of N-methyl-D-aspartate (NMDA) receptors in HP rats compared to control and LP rats. We postulate that the differential pharmacological sensitivity of LTP in these rats is a reflection of this biochemical difference. Also, from these results we suggest that the learning capacity may be related to the density of glutamate NMDA receptors of HP, LP and control rats.

Behavioral microanalysis of spatial delayed alternation performance: rehearsal through overt behavior, and effects of scopolamine and chlordiazepoxide

Rats were trained in an operant spatial delayed alternation task utilizing retention intervals from 2 to 32 s. In addition to response accuracy, operations of the levers during the retention intervals were recorded and analyzed. Animals were tested following the administration of the muscarinic antagonists scopolamine hydrobromide and methylbromide, and the benzodiazepine receptor agonist chlordiazepoxide. In vehicle-treated animals, the relative number of correct responses and correct rehearsal operations (operation of the forthcoming correct lever during retention intervals) varied with the length of the retention intervals, and these measures were correlated. The response rate for rehearsal operations increased with the length of the retention intervals. It is speculated that the delay-dependent increase in response rate reflects an effect of delayed reward that was also associated with a delay-dependent increase in the tendency to alternate between levers. The effects of delay on the accuracy of rehearsal operations may have contributed to the delay-dependent correct responding. Scopolamine hydrobromide (0.01, 0.03, 0.1, 0.3 mg/kg) and methylbromide (0.1, 0.3 mg/kg) impaired correct responding, but did not seem to interfere with the relative number of correct rehearsal operations. As only the presentation of the panel light indicated trial onset, it is speculated that the cholinergic receptor blockade resulted in an increase in the probability of a repositioning response that was triggered by light onset. Chlordiazepoxide (1, 3, 5, 10 mg/kg) did not affect behavioral performance. These results suggest that in tasks that allow the development of rehearsal operations, delay-dependent response accuracy does not represent a sufficient condition for conclusions on task demands on memory.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in hippocampal synaptic plasticity in rats with inborn high or low learning ability may be related to different sensitivity of aspartate receptors

Rats with an inborn high (HP) or low (LP) learning capacity were used to study the sensitivity to the blocking effect of 2-amino-phosphonopentanoic acid (AP5; 10 and 20 microM) on long-term potentiation (LTP) produced in hippocampal slices by a 1-s tetanus at 200 Hz. The potential evoked by stimulation of the perforant path was recorded from the granule cell layer of the dentate gyrus in 400 microns slices perfused with standard Krebs' solution or the AP5. Under perfusion with 10 microM of AP5, in 100% of slices from HP rats, LTP generation was not blocked; when AP5 20 microM was used, in 85% of the cases LTP was not blocked. In 60% of slices from LP rats, AP5 10 microM and in 100% of the cases at 20 microM AP5 blocked LTP generation. These results are coherent with the hypothesis that the different inborn learning ability of HP and LP rats is related to the different population or sensitivity of N-methyl-D-aspartate (NMDA) receptors.

Time-dependent effects of post-trial amphetamine treatment in rats: evidence for enhanced storage of representational memory

Two studies were conducted to test the ability of post-trial amphetamine treatment to improve later recall in a nonaversively motivated task. These studies utilized 8- and 12-arm radial mazes, respectively, with an 11-h retention interval imposed after the rat traversed half the arms of the maze (termed, the to-be-remembered-event, or TBRE). In Experiment 1, the rats were injected with amphetamine (0, .25, and .50 mg/kg) immediately after the TBRE. Because the drug treatment improved retention, a time dependency study was conducted in which the drug (0 and .33 mg/kg) was administered 0, 3, and 6 h after the TBRE. The finding that amphetamine injection at 0, but not 3, h post-trial improved later recall indicates that the benefit derived from the former treatment is not due to proactive influences at the time of the retention test. Drug treatment 6 h post-trial produced a borderline improvement of recall; possible mechanisms are discussed. Two conclusions can be drawn from these results: (1) amphetamine administration can improve recall under conditions in which this effect cannot be attributed to alterations in information processing during either the learning or the retention sessions, indicating that the drug modulates memory storage processes; and (2) amphetamine treatment can improve working memory, thus excluding an alternative interpretation for the previous reports of impaired short-term memory in animals, all of which entailed assessments of working memory. The possibility remains, however, that the impairment seen in these tasks reflects the requirement for erasure of information from previous trials within each daily session, rather than the duration of the retention interval.

Localized intracaudate dopamine D2 receptor activation during the post-training period improves memory for visual or olfactory conditioned emotional responses in rats

Rats with cannulas aimed at the posteroventral (PV) or ventrolateral (VL) areas of the caudate nucleus were trained on a conditioned emotional response (CER) task. Post-training microinjections of the indirect catecholamine agonist, d-amphetamine (5 micrograms), or of the dopamine D2 receptor agonist, LY171555 (1 microgram), into the PV area improved retention of a CER with a visual CS, but had no effect on a CER with an olfactory CS. Post-training injections of the same two drugs into the VL area improved retention of a CER with an olfactory CS, but had no effect on a CER with a visual CS. Post-training injections of the dopamine D1 receptor agonist, SKF38393 (0.5, 1.0, 2.0 micrograms), into either site had no effects on either CER. These findings suggest that different areas of the caudate nucleus mediate acquisition of CERs with different CSs, possibly implicating the topographically organized corticostriatal innervation in the acquisition of certain types of memories in the caudate nucleus. The findings also suggest that dopamine D2 receptors in the caudate nucleus are involved in the acquisition of these CERs.

Memory facilitation produced by dopamine agonists: role of receptor subtype and mnemonic requirements

The role of dopamine (DA) receptor subtypes in the acquisition of two memory tasks in the 8-arm radial maze was examined. The receptors were manipulated with posttraining, subcutaneous injections of an indirect DA receptor agonist (D-amphetamine), a selective D2 receptor agonist (LY171555), and a selective D1 receptor agonist (SKF-38393). On a win-stay task (sensitive to caudate nucleus lesions) a light cue signalled the location of food in 4 randomly selected arms on each trial. Rats were given one trial per day and injected after training on day 5. D-Amphetamine (2.0 mg/kg) and LY171555 (2.0 mg/kg) improved performance relative to controls; however SKF-38393 (1-4 mg/kg) had no effect on the acquisition of win-stay behavior. On a win-shift task (sensitive to fornix/hippocampal lesions) a delay of 18 hr was imposed between the first 4 and second 4 choices; drugs were injected after the first 4 choices. D-Amphetamine (1.0 mg/kg) and LY171555 (2.0 mg/kg) significantly improved retention relative to controls. SKF-38393 (1-4 mg/kg) had no effect on win-shift retention. These results suggest that the memory-improving properties of DA agonists on tasks sensitive to both hippocampal and caudate lesions are mediated by the D2 receptor.

Reward or reinforcement: what's the difference?

The histories of the terms "reward" and "reinforcement" are reviewed to show the difference in their origins. Reward refers to the fact that certain environmental stimuli have the property of eliciting approach responses. Evidence suggests that the ventral striatum (nucleus accumbens area) is central to the mediation of this behavior. Reinforcement refers to the tendency of certain stimuli to strengthen learned stimulus-response tendencies. The dorsolateral striatum appears to be central to the mediation of this behavior. Neuroanatomical and neurochemical data are adduced suggesting that reward may be mediated by a neural circuit including the neostriatal patch system, together with the hippocampus, limbic system (amygdala, prefrontal cortex) and ventral pallidum. The evidence also suggests that reinforcement, in the form of dopamine release in the striatal matrix, acts to promote the consolidation of sensori-motor associations. Thus, the matrix may mediate stimulus-response memory as part of a circuit including the cerebral cortex, substantia nigra pars reticulata and its projections to thalamic and brainstem motor areas.

Dissociation of visual and olfactory conditioning in the neostriatum of rats

In an experiment designed to demonstrate a double dissociation, the effects of bilateral electrolytic lesions of either the posteroventral or the ventrolateral regions of the neostriatum on the conditioned emotional response (CER) were examined. Posteroventral lesions impaired acquisition of the CER with a visual CS but not with an olfactory CS. Sham-operated posteroventral and ventrolateral lesioned animals acquired the visual CER normally. Ventrolateral lesions impaired acquisition of the CER with the olfactory CS but not with the visual CS. Sham-operated ventrolateral and posteroventral lesioned animals acquired the olfactory CER normally. In a second experiment the effect of post-training unilateral intrastriatal microinjections of (+)-amphetamine on acquisition of the visual and olfactory CERs was studied. Posteroventral injections improved retention of the visual, but not the olfactory CER. Ventrolateral injections improved retention of the olfactory, but not the visual CER. Saline and delayed (+)-amphetamine injection controls demonstrated that the improvement of retention in each case was a retroactive improvement of memory for the recently acquired CERs by (+)-amphetamine. These findings are consistent with previous reports of post-training memory facilitation mediated by dopaminergic function in the neostriatum. The results of both experiments are consistent with a regional functional heterogeneity hypothesis: the idea that anatomically linked areas of cortex and neostriatum process memories involving different stimuli in similar ways and that the integrity of these structures and their connections is necessary to establish and consolidate associative memory.

A functional hypothesis concerning the striatal matrix and patches: mediation of S-R memory and reward

Recent neurochemical and anatomical findings delineating two compartments, the patches and the matrix, in the mammalian striatum, are described. Anatomical and neurochemical aspects of manipulations affecting behaviors related to S-R memory and reward correspond in certain respects to the features of these striatal compartments. These coincidences lead to an hypothesis concerning the function of the striatal compartments and their participation in neural systems controlling both immediate and learned behaviors.

An hypothesis on the role of glucose in the mechanism of action of cognitive enhancers

This review presents evidence that some cognition enhancing drugs produce their beneficial effects on learning and memory by increasing the availability of glucose for uptake and utilization into the brain. The hypothesis further suggests that many cognition enhancing drugs act through a peripheral mechanism rather than directly on the brain. The general hypothesis is supported by four independent and converging pieces of evidence: 1) Some cognition enhancing drugs may not cross the blood-brain barrier, but can still facilitate memory; 2) Some cognition enhancing drugs are effective only when injected peripherally, but not when injected directly into the brain; 3) Many cognition enhancing drugs are not effective after adrenalectomy; 4) Cognitive function is correlated with glucose regulation in aged animals and humans. These four lines of research have implications for the role of glucose in the action of specific cognitive enhancers.

Differential threshold for long-term potentiation in the hippocampus of rats with inborn high or low learning capacity

Threshold of stimulation frequency in the perforant path to induce long-term potentiation (LTP) in dentate gyrus was determined in hippocampal slices obtained from two different lines of rats inbred for 30 generations according to their performance in an avoidance escape test in a shuttle box. High-performance (HP) rats were defined as those giving at least 70% conditioned responses (CRs) and low-performance (LP) rats as those giving less than 15% CRs. LTP was defined as a 30% or more increase in the amplitude of the evoked population spike (PS), lasting at least 20 min. Stimulation frequency threshold was determined by stimulating with a train of pulses of 0.5 ms duration during 1 s. The same slice was stimulated with trains of increasing frequency from 5 to 400 Hz, each train separated by an interval of at least 20 min. HP rats showed a lower threshold (13 +/- 4 Hz) than LP rats (92 +/- 42 Hz) for the induction of LTP; there were no differences in the magnitude of LTP. The greater learning ability of HP rats may be related to the plasticity of hippocampal synaptic transmission.

Effect of nigrostriatal dopamine depletion on the post-training, memory-improving action of amphetamine

Systemic, post-training injections of d-amphetamine improve memory in a variety of learning situations. Evidence from self-stimulation and intra-striatal injection studies suggests that this effect may be a result of the amphetamine-promoted release of endogenous dopamine from nigro-striatal neurons. In the present study this hypothesis was supported by the demonstration that destruction of these neurons with intra-nigral injections of 6-hydroxydopamine eliminate the memory-improving effect of systemic, post-training injections of amphetamine.

Improved recognition memory in monkeys following naloxone administration

The effects of naloxone on visual recognition were evaluated in five macaques trained in delayed nonmatching-to-sample with trial-unique objects. In four of the five monkeys, naloxone yielded an inverted U-shaped dose-effect curve. For each of these four animals, as well as for all five animals as a group, at least one dose within a narrow range (0.32-3.2 mg/kg) produced a significant increase in the number of objects correctly recognized. Lower doses had little effect, while the highest dose (10.0 mg/kg) tended to disrupt performance.

Facilitation or inhibition of memory by morphine: a question of experimental parameters

The effects of morphine on memory are highly controversial. According to some investigators post-trial injections of morphine facilitate memory. Others, however, have reported impairment of memory after morphine injections. To investigate the extent to which this may be due to different experimental parameters, foot-shock intensity and dosage of morphine were systematically varied in a passive-avoidance task. It was found that post-trial administration of medium and relatively high doses of morphine facilitate retention performance following moderate levels of foot-shock. Under other conditions of dose and shock intensity, the drug was not effective or even impaired retention.

The relationship between stereotypy and memory improvement produced by amphetamine

This study examined the possibility that amphetamine-induced stereotypy and facilitation of memory consolidation are both mediated by amphetamine's stimulation of dopaminergic activity in the caudate nucleus. In the first experiment, rats were given pairings of a tone and a shock followed by SC amphetamine (2 mg/kg). The amount of stereotypy and increased locomotor activity produced by the injection were measured immediately. Retention of the tone-shock association was evaluated 48 h later by observing the ability of the tone to suppress drinking. The degree of retention was significantly correlated with the amount of stereotypy but not with the amount of locomotion previously measured. In the second experiment, amphetamine was microinjected into the caudate nucleus (10 micrograms/microliters) and its ability to produce the same three behavioral effects was examined. These injections produced increased stereotypy and improved retention, but no increase in locomotion. The correlation of memory facilitation with stereotypy and the fact that both were produced by intracaudate amphetamine suggest that they may be mediated by the same neuropharmacological substrate, namely amphetamine-induced release of dopamine in the caudate.

Amphetamine, conditioned stimulus, and nondebilitating preshock effects on activity and avoidance: further evidence for interactions between associative and nonassociative changes

A literature survey and preliminary experiments with rats on the consequences of shock preexposure on subsequent activity and escape or avoidance showed the need for further work on the interactions between nondebilitating preshock and various test and treatment factors. The two main experiments used 16 preexposure conditions, namely, presence or absence of unavoidable punishment (36 shocks of 2.5 mA and 5 sec subdivided in three daily sessions), a light CS, a central partition in the shuttle-box, and dl-amphetamine sulfate (1 mg/kg ip 15 min before each session). In both experiments the four factors studied exerted more than additive effects on activity in preexposure sessions, leading to a very high frequency of crossing in the CS-shock-no-partition-drug condition. Upon retesting for activity (Experiment 1) suppression of locomotion by prior shock was less marked in animals preexposed to CS-US pairings in the absence of partition, while proactive amphetamine effects consisted mainly of a progressive increase of activity over successive retest sessions in the groups not preshocked. Upon retesting for light-cued, two-way avoidance acquisition (Experiment 2) the groups preexposed to US only were mostly retarded, while those preexposed to paired CS and US were mostly facilitated. Other changes, including drug pretreatment consequences, were negligible or unsystematic, but in general the data showed that the effects of various preexposure conditions on activity could not account for those on avoidance. Overall, it appears that the interactions between nondebilitating preshock and other test and treatment factors can be further exploited to clarify the respective roles of various associative and nonassociative mechanisms in modulation of activity and adaptive responding in aversive situations.

Effect of naloxone and amphetamine on acquisition and memory consolidation of active avoidance responses in rats

Pretraining IP injection of naloxone (0.3 mg/kg) or amphetamine (2 mg/kg) enhanced performance during acquisition, but did not improve retention of active avoidance responses in rats. Naloxone (0.1 or 3 mg/kg) had no effect on acquisition or on retention. The combination of naloxone (0.3 mg/kg) plus amphetamine (2 mg/kg) did not produce the facilitation observed when each of the two drugs was administered alone. Pretreatment with the higher dose of naloxone (3 mg/kg) blocked the facilitative effect of amphetamine on acquisition. Post-training administration of naloxone (0.3 mg/kg) or amphetamine (2 mg/kg) improved retention. Naloxone (0.1 or 3 mg/kg) had no effect. When naloxone and amphetamine were combined, at respective doses of 0.3 mg/kg and 2 mg/kg, the improvement did not occur, i.e., the higher dose of naloxone prevented the facilitative effect of amphetamine. In addition, an ineffective dose of amphetamine (0.5 mg/kg), given either pre- or post-training together with the lower dose of naloxone (0.1 mg/kg), produced a significant enhancement of acquisition or consolidation, respectively. The results are consistent with the possibility that naloxone might exert its facilitative action on acquisition and memory consolidation through the release of catecholaminergic systems from inhibitory influences of opioids.

Amphetamine effects on long term potentiation in dentate granule cells

Long term potentiation (LTP) has received considerable attention as a neurophysiological analog of memory. Amphetamine, as well as several other catecholamine agonists, can enhance behaviorally-assessed memory storage in a variety of training situations. The present experiments tested the effects of amphetamine on LTP produced by high frequency stimulation of the perforant path in rats. The results indicate that amphetamine can enhance the development of LTP under some but not all testing procedures. Studies of the neurobiological bases by which central and peripheral catecholamines modulate memory storage may be augmented by examinations of catecholamine effects on a specific form of long-lasting change in brain function. Similarly, the ability to manipulate LTP may prove to be an important aid in examinations of neurobiological correlates of this phenomenon.

Ontogenetic differences in response to d-amphetamine: two-way avoidance, intertrial responses, and locomotor activity

In Experiment 1, 15-, 17-, 21-, 36-, and 90-day-old rats were injected with either physiological saline, 0.5-, 1.0-, 4.0-, 8.0-, or 16.0-mg/kg of d-amphetamine sulfate and 20-min later they were allowed to explore a two-way avoidance apparatus for 8 min. Immediately following adaptation, they were given a single session of 100 two-way avoidance trials. In general, in all ages, there was a dose related increase in avoidance on the first block of trials. However, across trials avoidance of the two youngest ages decreased, avoidance responding by 21-day-old animals remained relatively constant, and avoidance of the oldest ages increased. In the three youngest ages, avoidance and intertrial responses had a similar pattern, but in older ages there was little relationship between avoidance and intertrial responses. Shuttle crossings during adaptation were increased more by higher doses in younger rats than adults. In Experiment 2, para-hydroxyamphetamine (1.0, 4.0, 16.0 mg/kg) did not alter two-way avoidance, intertrial responses, or crossings during adaptation in 15-, 17-, 21-, 36-, or 90-day-old rats. The age-dependent behavioral effects of d-amphetamine may be due to maturation of central nervous system catecholaminergic neurons.

Differential effects of l-amphetamine on ontogeny of active avoidance, intertrial responses, and locomotor activity

Two experiments examined the effects of various doses of l-amphetamine on locomotor activity, two-way avoidance, and intertrial responses in rats that were 15, 17, 21, 36, and 90 days of age. In the first experiment, testing began 20 min after drug administration and 100 avoidance trials were given. In the second experiment, testing began immediately after drug treatment and 200 trials were given. In the two youngest groups, shuttle crossings during adaptation were increased by the highest doses. The typical inverted U-shaped function between amphetamine dose and locomotor activity found in adults began to emerge at 36 days of age. Generally, at the two youngest ages, there was a dose-related increase in avoidance and intertrial responses on the first few blocks of trials and then a decrease across trials. In 21-day rats, the drug increased avoidance on the first few trial blocks but acquisition was altered only slightly. Acquisition of 36- and 90-day rats was increased, but this increase was greater in adults. It appears likely that development of CNS catecholaminergic neurons is responsible for the age-dependent behavioral effects of l-amphetamine.

The role of opioid peptides in memory and learning

Evidence is discussed which points to the existence of a physiologic amnesic mechanism mediated by beta-endorphin and perhaps by other opioid peptides as well. This mechanism is triggered by various forms of training and by either painful or painless stimulation. It may operate through the inhibition of central dopaminergic and beta-adrenergic systems that modulate the memory consolidation process. This amnesic mechanism in unrelated to the regulation of pain perception, and operates at opioid peptide levels several orders of magnitude below those that are needed to cause analgesia or other effects. In addition, shuttle avoidance and habituation learning seem to be dependent on a state induced by the release of beta-endorphin. It is possible that this may be related to the amnesic properties of this substance. Therefore, it appears that the endogenous opioid peptides may exert their primary function in the modulation of memory processes.

Effect of beta-endorphin and naloxone on acquisition, memory, and retrieval of shuttle avoidance and habituation learning in rats

Naloxone impairs acquisition of shuttle avoidance behavior (0.8 mg/kg IP) and habituation to a rearing response to a tone (1.6 mg/kg IP) in rats. beta-Endorphin (2 microgram/kg IP) has no effect on acquisition, but, when given prior to test sessions, facilitates retrieval of the two tasks. Naloxone has no effect of its own upon retrieval. In addition to these effects, the pretraining administration of beta-endorphin disrupts, and that of naloxone facilitates retention of the two tasks. The results are consistent with the hypothesis that these two forms of learning are state-dependent on the release of beta-endorphin (and, perhaps, of other opiate peptides as well), that this substance is released during training in a sufficient amount for this purpose, and that, in addition, there is a physiological amnesic mechanism mediated by opiate peptides. Furthermore, the results are also consistent with previous observations that beta-endorphin is released from the rat brain during training, but not during test sessions of the two tasks (Izquierdo et al., 1980b). The possibility is discussed that state-dependency and the amnesic effect comprise one single, rather than two separate mechanisms.

Effect of an inhibitor of dopamin-beta-hydroxylase on the acquistion and retention of four different avoidance tasks in mice

The effect of [bis-(4-methyl-1-homopiperazinylthiocarbonyl)-disulfide] (FLA-63) (40 mg/kg, i.p.) on acquisition and retention in four different avoidance tasks was evaluated in mice. In all tasks animals were submitted to a training session on one day and to a retest session 24 h later. The drug or its vehicle were given either 2 h prior to training and/or retest (pretrial treatments) or immediately after the end of the training session (posttrial treatments). Two hours after injection, FLA-63 was found to lower brain norepinephrine (NE) levels by 51% without affecting those of dopamine (DA). Pretrial administration of the drug resulted in poorer performance of step-through and step-down passive avoidance as well as of step-up active avoidance in retest sessions. There was no apparent posttrial effect of the drug nor any effect on acquisition in these tasks. The drug was without influence on either acquisition or retention in an any-way passive avoidance task in which the response required from the animals was immobility. No evidence for state-dependent learning was detected in any of the tasks. The data are consistent with the hypothesis of an involvement of NE either in memory processes or in events parallel and related to memory processes. The present results rule out, however, several such possible parallel events (effects on acquisition, motor disturbances, and effects on reactivity to shocks).

Effects of mescaline and amphetamine on simultaneous visual discrimination in two inbred strains of mice

The effects of mescaline and amphetamine were investigated in BALB/cJ (BALB) and C57BL/6J (C57) mice using the five-choice Yerkes--Thompson Bryant--Bovet-Nitti apparatus for patterns discrimination. Two sets of experiments were carried out. In the first set, pretrial administration of mescaline (5, 10, and 20 mg/kg) was followed by performance improvements in the C57 mice, while performances of the BALB strain were impaired by the treatment, as compared with those of the saline-injected (4 ml/kg) controls. The pretrial administration of amphetamine (0.1, 0.25, and 0.5 mg/kg) improved performances of both strains. In a second set of experiments, the same effects as in the pretrial experiments were observed in both strains following administration of mescaline (20 mg/kg) and amphetamine (0.5 mg/kg) immediately after each experimental session. No effect was evident when the drugs were injected 2 h after training, suggesting that effects of the pretrial treatments were due to influences of mescaline and amphetamine on the consolidation processes of the two strains of mice tested.