Memory facilitation by posttrial hypothalamic stimulation and other reinforcers: A central theory of reinforcement

Probing the nature of episodic memory in rodents

Episodic memory (EM) specifies the experience of retrieving information of an event at the place and time of occurrence. Whether non-human animals are capable of EM remains debated, whereas evidence suggests that they have a memory system akin to EM. We here trace the development of various behavioral paradigms designed to study EM in non-human animals, in particular the rat. We provide an in-depth description of the available behavioral tests which combine three spontaneous object exploration paradigms, namely novel object preference (for measuring memory for "what"), novel location preference (for measuring memory for "where") and temporal order memory (memory for "when"), into a single trial to gauge a memory akin to EM. Most important, we describe a variation of such a test in which each memory component interacts with the others, demonstrating an integration of diverse mnemonic information. We discuss why a behavioral model of EM must be able to assess the ability to integrate "what", "where" and "when" information into a single experience. We attempt an interpretation of the various tests and review the studies that have applied them in areas such as pharmacology, neuroanatomy, circuit analysis, and sleep. Finally, we anticipate future directions in the search for neural mechanisms of EM in the rat and outline model experiments and methodologies in this pursuit.

The antipsychotic drug sulpiride in the ventral pallidum paradoxically impairs learning and induces place preference

Sulpiride, as a D2-like dopamine (DA) receptor (D2R) antagonist, is an important antipsychotic drug in the treatment of schizophrenia. Recently, we have shown that the activation of D2Rs in the ventral pallidum (VP) modulates the activity of the ventral tegmental area (VTA) DAergic neurons. According to our hypothesis, intra-VP sulpiride can influence the motivational and learning processes, pervasively modifying the behavior of examined animals. In the present study, sulpiride was microinjected into the VP of male Wistar rats in three different doses. Morris water maze (MWM) test was applied to investigate the effects of sulpiride on spatial learning, while conditioned place preference (CPP) test was used to examine the potential rewarding effect of the drug. In order to show, whether the animals can associate the rewarding effect with an area which can be recognized only on its spatial location, we introduced a modified version of the CPP paradigm, the spatial CPP test. Our results show that the intra-VP sulpiride dose-dependently impairs learning processes. However, the largest dose of sulpiride induces place preference. Results of the spatial CPP paradigm demonstrate that the animals cannot associate the rewarding effect of the drug with the conditioning area based on its spatial location. In the CPP paradigm, locomotor activity decrease could be observed in the sulpiride-treated rats, likely because of a faster habituation with the conditioning environment. In summary, we can conclude that intra-VP sulpiride has a dual effect: it diminishes the hippocampus-dependent spatial learning processes, in addition, it has a dose-dependent rewarding effect.

Analysis of memory modulation by conditioned stimuli

Conditioned stimuli (CS) have multiple psychological functions that can potentially contribute to their effect on memory formation. It is generally believed that CS-induced memory modulation is primarily due to conditioned emotional responses, however, well-learned CSs not only generate the appropriate behavioral and physiological reactions required to best respond to an upcoming unconditioned stimulus (US), but they also serve as signals that the US is about to occur. Therefore, it is possible that CSs can impact memory consolidation even when their ability to elicit conditioned emotional arousal is significantly reduced. To test this, male Sprague–Dawley rats trained on a signaled active avoidance task were divided into “Avoider” and “Non-Avoider” subgroups on the basis of percentage avoidance after 6 d of training. Subgroup differences in responding to the CS complex were maintained during a test carried out in the absence of the US. Moreover, the subgroups displayed significant differences in stress-induced analgesia (hot-plate test) immediately after this test, suggesting significant subgroup differences in conditioned emotionality. Importantly, using the spontaneous object recognition task, it was found that immediate post-sample exposure to the avoidance CS complex had a similar enhancing effect on object memory in the two subgroups. Therefore, to our knowledge, this is the first study to demonstrate that a significant conditioned emotional response is not necessary for the action of a predictive CS on modulation of memory consolidation.

Orexin-1 receptor blockade differentially affects spatial and visual discrimination memory facilitation by intracranial self-stimulation

Intracranial self-stimulation (ICSS) of the medial forebrain bundle is an effective treatment to facilitate memory. Performance in both explicit and implicit memory tasks has been improved by ICSS, and this treatment has even been capable of recovering loss of memory function due to lesions or old age. Several neurochemical systems have been studied in regard to their role in ICSS effects on memory, however the possible involvement of the orexinergic system in this facilitation has yet to be explored. The present study aims to examine the relationship between the OX1R and the facilitative effects of ICSS on two different types of memory tasks, both carried out in the Morris Water Maze: spatial and visual discrimination. Results show that the OX1R blockade, by intraventricular administration of SB-334867, partially negates the facilitating effect of ICSS on spatial memory, whereas it hinders ICSS facilitation of the discrimination task. However, ICSS treatment was capable of compensating for the severe detrimental effects of OX1R blockade on both memory paradigms. These results suggest different levels of involvement of the orexinergic system in the facilitation of memory by ICSS, depending on the memory task.

Cocaine, nicotine, and their conditioned contexts enhance consolidation of object memory in rats

To test the hypothesis that drugs of abuse and their conditioned stimuli (CSs) enhance memory consolidation, the effects of post-training exposure to cocaine and nicotine were compared to the effects of post-training exposure to contextual stimuli that were paired with the effects of these drugs. Using the object recognition (OR) task, it was first demonstrated that both 10 and 20 mg/kg cocaine, and 0.2 and 0.4 mg/kg nicotine, enhanced recognition memory when administered immediately after, but not 6 h after the sample phase. To establish the drug CSs, rats were confined for 2 h in a chamber (the CS+) after injections of 20 mg/kg cocaine, or 0.4 mg/kg nicotine, and in another chamber (the CS−) after injections of vehicle. This was repeated over 10 d (5 drug/CS+ and 5 vehicle/CS− pairings in total). At the end of this conditioning period, when tested in a drug-free state, rats displayed conditioned hyperactivity in the CS+ relative to the CS−. More important, immediate, but not delayed, post-sample exposure to the cocaine CS+, or nicotine CS+, enhanced OR memory. Therefore, this study reports for the first time that contextual stimuli paired with cocaine and nicotine, like the drugs themselves, have the ability to enhance memory consolidation.

Drugs of abuse as memory modulators: a study of cocaine in rats

RATIONALE

It has been proposed that drugs of abuse reinforce behavior partly, or wholly, because they facilitate learning by enhancing memory consolidation. Cocaine can clearly serve as a reinforcer, but its effect on learning has not been fully characterized.

OBJECTIVES

To explore the effects of different regimens of pre- and post-training cocaine administration on win-stay and object learning.

METHODS

Cocaine naïve and cocaine pre-exposed (30 mg/kg/day, × 5 days followed by 7 days drug-free) male Sprague-Dawley rats received cocaine (0, 1, 2.5, 7.5, or 20 mg/kg, i.p.) immediately following training on a win-stay task in a radial maze or following the sample phase of an object learning task. Win-stay performance was also assessed in tests of extinction and after a set shift.

RESULTS

Post-training cocaine did not improve accuracy on the win-stay task and produced performance deficits at 20 mg/kg. These deficits were attenuated by prior cocaine exposure. There was indirect evidence of facilitated learning in extinction and set shift tests, but the effective dosage was different (2.5 and 7.5 mg/kg, respectively). Post-training cocaine produced dose-dependent improvements in object learning.

CONCLUSION

Post-training cocaine administration can facilitate learning, but this effect is highly dependent on the dose and the type of task employed.

Effect of post-training administration of cocaine, diazepam and their combination on a win-stay task

According to the memory-enhancing hypothesis of addictive drugs, post-training administration of cocaine should enhance consolidation and thus facilitate learning. This hypothesis has not been tested in appetitive tasks reinforced by sucrose. The current study assessed the effect of post-training cocaine administration on the acquisition of a win-stay task, and modulation of this effect by co-administration of diazepam. Male Sprague-Dawley rats (n=63) were trained for 5 days on a win-stay task performed on an 8-arm radial maze, and were administered cocaine (0, 2.5, 7.5 or 20mg/kg), diazepam (1mg/kg), or cocaine (7.5mg/kg)+diazepam (1mg/kg) immediately following each training session. Post-training cocaine caused dose-dependent impairments that appeared linked to the development of cocaine-induced sucrose taste avoidance and/or cocaine-induced anxiety. When it was attempted to modify these learned side effects of cocaine by co-administration of diazepam, it was observed that the drug combination slowed task completion and reduced overall number of nose pokes. These findings suggest that post-training cocaine can alter behavior on appetitive tasks through learned motivational deficits rather than through a selective action on memory consolidation. The implications for the memory-enhancing hypothesis of addictive drugs are discussed.

Intracranial self stimulation upregulates the expression of synaptic plasticity related genes and Arc protein expression in rat hippocampus

Post-training lateral hypothalamus (LH) intracranial self stimulation (ICSS) has a reliable enhancing effect on explicit memory formation evaluated in hippocampus-dependent tasks such as the Morris water maze. In this study, the effects of ICSS on gene expression in the hippocampus are examined 4.5 h post treatment by using oligonucleotide microarray and real-time PCR, and by measuring Arc protein levels in the different layers of hippocampal subfields through immunofluorescence. The microarray data analysis resulted in 65 significantly regulated genes in rat ICSS hippocampi compared to sham, including cAMP-mediated signaling as one of the most significantly enriched Database for Annotation, Visualization and Integrated Discovery (DAVID) functional categories. In particular, expression of CREB-dependent synaptic plasticity related genes (c-Fos, Arc, Bdnf, Ptgs-2 and Crem and Icer) was regulated in a time-dependent manner following treatment administration. Immunofluorescence results showed that ICSS treatment induced a significant increase in Arc protein expression in CA1 and DG hippocampal subfields. This empirical evidence supports our hypothesis that the effect of ICSS on improved or restored memory functions might be mediated by increased hippocampal expression of activity-dependent synaptic plasticity related genes, including Arc protein expression, as neural mechanisms related to memory consolidation.

Effects of ventral tegmental area stimulation on the acquisition and long-term retention of active avoidance learning

The development of avoidance learning depends on dopamine release in forebrain regions. Previous studies indicated that rewarding brain stimulation facilitated two-way active avoidance learning. However, it is not clear whether the temporal relationship of brain stimulation to the training session (before, during or after) is important. To investigate the role of stimulation condition (no stimulation, self-stimulation only, or self-stimulation plus avoidance stimulation) and sequence of self-stimulation training (before or after avoidance training), we used a 3×2 factorial design, in which every level of stimulation was paired with every level of sequence for a total of 6 different groups. The results suggest that self-stimulation either before or after avoidance learning improved acquisition performance, but acquisition was maximal when stimulation was also given during acquisition trials. Importantly, the sequence of self-stimulation (before or after each acquisition session) was irrelevant to this beneficial effect. However, stimulation had no apparent effect on long-term retention when tested 10 days later under conditions of no stimulation, except that the performance of the group that had previously received avoidance-contingent stimulation deteriorated over the course of 60 trials. This may reflect frustration from the omission of expected reward. These results are relevant for optimizing brain stimulation to improve learning.

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.

Post-training self administration of sugar facilitates cognitive performance of male C57BL/6J mice in two spatial learning tasks

Spatial memory can be strengthened by adverse stimuli that activate the stress system, and administration of the stress hormone corticosterone in close-context with the learning task. Less is known about modulation of spatial memory by post-training positive reinforcers (reward). Cognitive performance was assessed in male C57BL/6J mice using two learning tasks: the water maze (WM) and circular hole board (CHB). Sugar was chosen as a post-training reinforcer. We expected that the free access to sugar immediately (0 h) after training would facilitate spatial memory; delayed access to sugar (4h after training) or no sugar served as controls. In both tasks, 0 h sugar mice showed superior performance, indicated by shorter latencies and distances to the trained spatial location. The memory facilitating effect of sugar became visible at distinct times during training: on the CHB from the first trial onwards, in the WM on training days 4 and 5. Sugar-rewarded mice kept their superior performance during the free exploration/swim trial, expressed by more persistent search strategies for the exit hole or platform. Post-training sugar reward in close-context with performance strengthens memory via modulation of consolidation. This finding supports the integrative theory of reinforcement and memory. We suggest that our experimental set-up will allow to differentiate between direct effects on memory and alterations in reward processes in animal models of stress-related diseases.

Post-training intra-amygdala amphetamine injections given during acquisition of a stimulus–response (S–R) habit task enhance the expression of stimulus–reward learning: Further evidence for incidental amygdala learning

The effect of post-training intra-amygdala amphetamine injections was examined on the acquisition and expression of a visual discrimination task. Rats were trained to enter four lit arms for food (stimulus-response) and avoid unlit arms on an eight-arm radial maze visual discrimination task. Post-training intra-amygdala amphetamine injections (10 microg) were given for 4 consecutive days during the mid-point of training (days 20-23). The number of lit arm entries was used as a measure of stimulus-response habit learning 24 h after each injection. Twenty-four hours after the last injection, a transfer test was run to assess the effect of the same post-training manipulation. This transfer test assessed the amount of time spent in the lit arms and was used as a measure of stimulus-reward learning. Compared to saline-injected rats, rats that received post-training amphetamine spent more time in lit as opposed to dark arms during the transfer test. This occurred in the absence of an increase in the number of correct arm entries during visual discrimination training. This suggests that post-training amphetamine strengthened a stimulus-reward association that did not immediately affect behavioral output. This association may reflect a mnemonic representation stored in an ensemble of amygdala neurons.

Amygdala inactivation blocks expression of conditioned memory modulation and the promotion of avoidance and freezing

Rats were exposed to shock-paired cues immediately after training on an appetitive preference task. Elevated levels of freezing in and active avoidance of the shock-paired compartment were observed, and memory for the appetitive task was improved when tested 24 hr later. Intra-amygdala muscimol injected before the posttraining exposure eliminated freezing, avoidance, and memory modulation. The blockade of both freezing and active avoidance, which involve competing behavioral tendencies, makes it unlikely that the amygdala itself generates either behavior. The elimination of conditioned memory modulation suggests that conditioned neurohormonal responses were blocked. These conditioned internal responses may comprise the intervening variable of "conditioned fear" and may promote observable behaviors, the form of which is determined by the environment in which they occur.

Behavioural parameters in aged rats are related to LTP and gene expression of ChAT and NMDA-NR2 subunits in the striatum

Striatal parameters were assessed for their relevance to age-related behavioural decline. Forty aged rats (28-30 months) were tested in the water maze and open field. Of these, seven superior and seven inferior learners were compared with each other in terms of levels of in vitro short- and long-term potentiation (STP and LTP), and gene expression of choline acetyltransferase (ChAT) as well as of the NMDA-NR2A-C subunits assessed by quantitative RT-PCR. Results revealed that the superior as compared with the inferior learners had higher levels of ChAT mRNA in the striatum. For the superior group, ChAT mRNA was correlated with escape on to the cued platform in the water maze, whereas level of LTP was predictive of place learning in the water maze and rearing activity in the open field. For the inferior group, expression of NR2A and NR2B was positively correlated with place learning and probe trial performance in the water maze. The results show that individual differences in various behaviours of aged rats were accounted for by variability in striatal parameters, i.e. LTP, ChAT and NMDA-NR2 subunit mRNA. Notably, the correlations found were heterogeneous amid the groups, e.g. variability in place learning was explained by variability in levels of LTP in the superior learners, but in levels of NR2A-B mRNA in the inferior group.

Posttraining intracranial self-stimulation ameliorates the detrimental effects of parafascicular thalamic lesions on active avoidance in young and aged rats

To evaluate whether intracranial self-stimulation (SS) ameliorates conditioning deficits induced by parafascicular nucleus (PF) damage in young and aged rats, the authors gave rats a daily session of 2-way active avoidance until a fixed criterion was achieved. Four experimental groups were established in both young and aged rats: SS treatment after every conditioning session (SS groups), pretraining PF lesions (lesion groups), PF lesions and SS treatment (L + SS groups), and controls. SS treatment not only canceled the detrimental effects of PF lesions, but also improved conditioning in lesioned rats (L + SS groups). This effect was more powerful in aged rats. SS treatment compensated for memory deficits generated by hypofunctionality of arousal systems such as that involving the PF.

Preprotachykinin A gene expression after administration of 3,4-methylene dioxymethamphetamine (Ecstasy)

This study tested the effects of 8 days of subchronic administration of 3,4-methylene dioxymethamphetamine (MDMA) (5 mg/kg b.w.) on preprotachykinin A mRNA levels in discrete rat brain regions. In situ hybridization examined preprotachykinin A mRNA levels in the core and shell of the nucleus accumbens, the islands of Calleja, the olfactory tubercle, the dorsal and ventral caudate-putamen, the bed nucleus of the stria terminalis, the medial preoptic area, the medial habenular nucleus and in the postero-dorsal part of the medial amygdala. Higher levels of preprotachykinin A mRNA were found in the core and shell of the nucleus accumbens, in the islands of calleja, in the olfactory tubercle, in the bed nucleus of the stria terminalis, in the medial habenular nucleus and the postero-dorsal part of the medial amygdala, compared to control animals. Conversely, increased preprotachykinin A mRNA levels were observed in the dorsal and ventral caudate-putamen in MDMA treated when compared to control rats. In the social memory test, MDMA significantly impaired rats' short-term working memory. These results show that chronic exposure to MDMA strongly affects preprotachykinin A mRNA levels in discrete rat brain regions. These changes occur in experimental conditions in which working memory is markedly reduced, suggesting that changes in gene expression of tachykinin mechanisms may contribute to the effects of MDMA on memory function.

Role of adrenoceptor subtypes in memory consolidation

Noradrenaline release in areas within the forebrain occurs following activation of noradrenergic cells in the locus coeruleus (LoC). Release of noradrenaline by attentional/arousal/vigilance factors appears to be essential for learning and is responsible for the consolidation of memory. Noradrenaline can activate any of nine different adrenoceptor (AR) subtypes in the brain and selectivity of action may be achieved by the spatial location and relative density of the AR subtypes, by different affinities of the different subtypes and by temporal selectivity in terms of when the different ARs are activated in the memory formation process. This review examines the use of selective agonists and antagonists to determine the roles of the AR subtypes in the one-trial discriminated avoidance learning paradigm in the chick. A model is developed that integrates noradrenergic activity in basal ganglia (lobus parolfactorius (LPO)) and association cortex (intermediate medial hyperstriatum ventrale (IMHV)) leading to the consolidation of memory 30 min after training. There is evidence that beta(2)- and beta(3)-ARs are important in the association area but require input from alpha(2)-AR stimulated activity in the basal ganglia for consolidation. On the other hand, alpha(1)-AR activation in the IMHV is inhibitory and prevents consolidation. While there is no role for beta(1)-ARs in memory consolidation, they play a role in short-term memory (STM). The use of the precocial chick has clear advantages in having a temporally discrete learning task which allows for discrimination memory and whose development can be followed at discrete intervals after learning. These studies reveal clear roles for AR subtypes in the formation and consolidation of memory in the chick, which have allowed the development of a model that can now be tested in mammalian systems.

Multiple parallel memory systems in the brain of the rat

A theory of multiple parallel memory systems in the brain of the rat is described. Each system consists of a series of interconnected neural structures. The "central structures" of the three systems described are the hippocampus, the matrix compartment of the dorsal striatum (caudate-putamen), and the amygdala. Information, coded as neural signals, flows independently through each system. All systems have access to the same information from situations in which learning occurs, but each system is specialized to represent a different kind of relationship among the elements (stimulus events, responses, reinforcers) of the information that flows through it. The speed and accuracy with which a system forms a coherent representation of a learning situation depend on the correspondence between the specialization of the system and the relationship among the elements of the situation. The coherence of these stored representations determines the degree of control exerted by each system on behavior in the situation. Although they process information independently the systems interact in at least two ways: by simultaneous parallel influence on behavioral output and by directly influencing each other. These interactions can be cooperative (leading to similar behaviors) or competitive (leading to different behaviors). Experimental findings consistent with these ideas, mostly from experiments with rats, are reviewed.

Intracranial self-stimulation facilitates memory consolidation, but not retrieval: its effects are more effective than increased training

To evaluate possible differential effects of lateral hypothalamic intracranial self-stimulation (ICSS) on memory consolidation and retrieval, independent groups of Wistar rats were trained in a single session of two-way active avoidance task (acquisition session) and tested 24 h later (retention session). The post-ICSS groups received an ICSS treatment immediately after the acquisition session, and the pre-ICSS groups received the same treatment immediately before the retention session. Because the ICSS effects on memory seem to be dependent on the initial performance level shown by the subjects, the possible influence of initial training (number of trials) on ICSS effects was also studied. Therefore, we used different control and experimental groups, which received either 30 or 50 trials in the acquisition session. Post-training ICSS facilitated the 24-h retention in both training conditions (30 and 50 trials). In contrast, pre-retention ICSS treatment did not facilitate performance in the retention test. We also observed that post-training ICSS was more effective for improving the 24-h retention than increasing the initial training from 30 to 50 trials. This findings confirm that ICSS treatment improves memory consolidation and suggest that it might not affect memory retrieval mechanisms.

Dorsal hippocampal function in unreinforced spatial learning

This study examined learning about the spatial environment by rats during a single 10 min period of exploration on an eight-arm radial maze. Because no specific behaviors were learned during this procedure, the existence of learned spatial information was inferred from its retarding effect on subsequent conditioned cue preference (CCP) learning on the same maze. Previous experiments have shown that this form of spatial learning, measured in this way, requires an intact fimbriafornix and functional N-methyl-D-aspartate receptors. However, in the present experiments, large neurotoxic lesions of the dorsal hippocampus that impaired win-shift learning failed to eliminate the retarding effect of exploration on CCP learning. This result was obtained in three independent replications. These findings fail to confirm the hypothesis that the hippocampus is involved in spatial learning when that learning occurs in the absence of reinforcers and does not produce any specific learned behaviors. Previous work showed that this form of "pure" spatial learning requires an intact fimbria-fornix for acquisition but not for expression; the present findings suggest that the hippocampus is not required for either of these processes. The fimbria-fornix may interact with other temporal lobe structures in mediating this form of learning. The function of the hippocampus may be limited in some way to situations that involve reinforcers and/or situations in which specific behaviors are learned.

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

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

Involvement of the parafascicular nucleus in the facilitative effect of intracranial self-stimulation on active avoidance in rats

To evaluate whether parafascicular nucleus (PF) is involved in the facilitative effect of lateral hypothalamic intracranial self-stimulation (LH-ICSS) on two-way active avoidance acquisition (5 sessions, 10 trials each, one daily) and long-term retention (10 days), rats were lesioned bilaterally at the PF and implanted with an electrode aimed at the LH to obtain ICSS behavior. After each acquisition session rats were allowed to self-administer 2500 trains of LH-ICSS. The main results were: (1) LH-ICSS facilitated the acquisition and retention of conditioning; (2) PF lesions impaired both acquisition and retention of two-way active avoidance; (3) there was a positive relationship between PF lesions size and learning disruption, and (4) LH-ICSS failed to facilitate learning when PF was lesioned. We concluded that the lesion size is a critical variable to evaluate the effects of PF lesions on learning and memory, and that LH-ICSS treatment may exert their effects through the PF nucleus or, at least, the integrity of PF is required for LH-ICSS to improve clearly the task.

The H1- and H2-histamine blockers chlorpheniramine and ranitidine applied to the nucleus basalis magnocellularis region modulate anxiety and reinforcement related processes

This study examined the effects of the H1-antagonist chlorpheniramine and the H2-antagonist ranitidine on reinforcement and anxiety-parameters following unilateral injection into the vicinity of the nucleus basalis magnocellularis (NBM). In Experiment 1, rats with chronically implanted cannulae were injected with chlorpheniramine or ranitidine (each at doses of 0.1, 1, 10 and 20 microg) and were placed into one of four restricted quadrants of a circular open field (closed corral) for a single conditioning trial. During the test for conditioned corral preference, when provided a choice between the four quadrants, only those rats injected with 10 or 20 microg chlorpheniramine spent more time in the treatment corral, indicative of a positively reinforcing action. None of the other doses of chlorpheniramine or of the H2-antagonist influenced rats' preference behavior. In Experiment 2, the elevated plus-maze (EPM) was used to gauge possible anxiolytic or anxiogenic effects of intra-basalis injection of chlorpheniramine or ranitidine (each at doses of 0.1, 1, 10 and 20 microg). A single injection of chlorpheniramine at 0.1 or 20 microg as well as ranitidine at 20 microg was found to exert anxiolytic-like effects in the EPM. Both compounds elevated the time spent on the open arms and increased scanning over the edge of an open arm. None of the other doses of the H1- and H2-antagonist influenced rats' behavior in the EPM. In sum, these findings show that H1- and H2-receptor antagonists differentially modulate reinforcement and fear-related processes in the NBM and thus, provide the first evidence for a behavioral relevance for the histaminergic innervation of this brain site.

Unilateral lesion of dorsal hippocampus enhances reinforcing lateral hypothalamic stimulation in the contralateral hemisphere

Whereas convincing evidence exists for an important role of the hippocampus in mechanisms underlying memory and encoding of location in space, the contribution of the hippocampus to the system underlying central processes of reinforcement is less well established. Scattered data suggesting that hippocampal ablation increases the effectiveness of positive reinforcers have alternatively been interpretated in terms of general and unspecific behavioral disinhibition, which results in higher levels of activity and rates of responding. In the present experiment, 22 Wistar rats were either given a neurotoxic or a sham lesion in the CA1 region of the hippocampus, and the effect on lateral hypothalamic self-stimulation behavior was assessed. To control for nonspecific performance effects rates of lever pressing were assessed ipsi- and contralateral to the lesioned hemisphere as well as under condition of extinction (current set to zero). Following the neurotoxic lesion the animals displayed significant higher rates of self-stimulation at the electrode sites in the hypothalamus situated contralateral but not ipsilateral to the hemisphere with the lesion compared with controls. The increase in self-stimulation commenced on the third day postlesion and was maintained over the 8 days of testing. The lesion did not change the animals' behavior under extinction. Thus, the hippocampal lesion led to an amplification of rewarding lateral hypothalamic self-stimulation behavior, indicative of a lesion induced disinhibition of the brain's reinforcement system.

The tuberomammillary nucleus projections in the control of learning, memory and reinforcement processes: evidence for an inhibitory role

The tuberomammillary nucleus (TM), a cluster of magnocellular cells in the posterior hypothalamus, is the main source of neuronal histamine in the brain. Although this nucleus is well described in terms of anatomy and neurochemistry, only little is known about its function. Our earlier work showed that the TM projection system may be involved in behavioral asymmetries and behavioral recovery after unilateral manipulations of the brain. Using horseradish peroxidase (HRP) labeling we found an increase in strength (structure and/or activity) in the crossed and uncrossed tuberomammillary-striatal projections in the course of recovery from behavioral asymmetries produced by unilateral removal of the rats' vibrissae, which were in the same direction as the asymmetries found in projections from the substantia nigra to the striatum. Experiments performed with unilateral lesions of the TM region provide evidence for an involvement of the TM system in reinforcement mechanisms. Unilateral destruction of the TM with direct current (DC) or ibotenic acid was found to increase the rate of lateral hypothalamic self-stimulation ipsilateral to the lesion site, suggesting that the TM (particularly the E2 subgroup in its rostral part) may function as a reinforcement inhibiting neural substrate. Experiments performed with bilateral DC or ibotenic acid lesions of the TM region suggest a role of the nucleus in learning and mnemonic processes. A bilateral electrolytic or neurotoxic lesion of the TM region was found to facilitate the performance of adult and behaviorally impaired aged rats in a variety of learning tasks, including a habituation paradigm, aversively motivated learning tasks and water mazes. Examination of the site of the neurotoxic lesion in the TM region with immunohistochemical techniques revealed a marked decline of histamine-staining neurons mainly in the rostral part of the TM nucleus, suggesting that the facilitatory effects on reinforcement and mnemonic processes might be related to the destruction of TM intrinsic histaminergic cells. In summary, the present results indicate that the TM nucleus is involved in neural plasticity and functional recovery following damage to the CNS and may function as an inhibitory neural substrate in the control of reinforcement and mnemonic processes.

Tuberomammillary nucleus lesion facilitates two-way active avoidance retention in rats

To evaluate whether the tuberomammillary nucleus might be involved in the acquisition and/or retention of a two-way active avoidance conditioning, rats were given a unilateral lesion of the tuberomammillary nucleus (E2 region) 24 h prior to the first conditioning session. Four learning sessions were performed: one acquisition and 3 retention sessions (short-term, 24 h; and long-term, 8 and 18 days). Results showed that the lesion facilitated the long-term retention of conditioning, but no effects were observed on acquisition and short-term retention. Since rewarding intracranial electrical stimulation seems to be a consistent way to facilitate learning and memory processes, and tuberomammillary lesion has been shown to improve intracranial self-stimulation behavior (ICSS), we suggest that lesions in the present experiment could have facilitated two-way active avoidance retention by enhancing the function of brain reward mechanisms.

The role of neuropeptides in learning: focus on the neurokinin substance P

The neurokinin substance P (SP) can have neurotrophic as well as memory-promoting effects. The study of its mechanisms may provide new insights into processes underlying learning and neurodegenerative disorders. Our work shows that SP, when applied peripherally (i.p.), promotes memory and is reinforcing at the same dose of 37 nmol/kg. Most important, however, is the finding that these effects seemed to be encoded by different SP-sequences, since the N-terminal SP1-7 (185 nmol/kg) enhanced memory, whereas C-terminal hepta- and hexapeptide sequences of SP proved to be reinforcing in a dose equimolar to SP. These differential behavioral effects were paralleled by selective and site-specific changes in dopamine (DA) activity, as both SP and its C-, but not N-terminus, increased extracellular DA in the nucleus accumbens (NAc), but not in the neostriatum. The neurochemical changes lasted at least 2 h after injection. Direct application of SP (0.74 pmol) into the region of the nucleus basalis magnocellularis (NBM) was also memory-promoting and reinforcing, and again, these effects were differentially produced by the N-terminus and C-terminus, supporting the proposed structure-activity relationship for SP's effects on memory and reinforcement. In addition, it was found that a single injection of SP into the NBM led to an increase of extracellular DA in the contralateral NAc. This effect of SP was observed only in those animals where SP was reinforcing, providing evidence for a lateralized relationship between reinforcement induced by injection of SP into the NBM and DA activity in the NAc. Furthermore, the outcome of a series of experiments suggests, that SP may not only be considered to have memory-promoting effects in normal animals, but can also improve functional recovery after unilateral 6-OHDA lesion of the substantia nigra and after lesions of the hippocampus, and can counteract age-related performance deficits.

Behavioral habituation to spatial novelty: interference and noninterference studies

Long-term behavioral habituation (LTH), that is activity decrement upon repeated exposures to spatial novelty, is a relatively simple and ubiquitous form of behavioral plasticity in the animal kingdom, that can be used as a model of nonassociative learning in the freely behaving organism. Several strategies can be followed to tackle upon it. (a) Interference studies pertain to manipulation of the between-exposure interval by a variety of agents of different nature, that are known to interfere with hypothesized "consolidation process(es)" in associative learning paradigms. This approach indicates that LTH is modulated by NMDA receptors, requires polysome aggregation and protein synthesis, a functioning neocortex and both slow wave and paradoxical sleep. Further, it is modulated by endogenously released or exogenously given vasopressin and is not affected by blockade of endogenous opioids, at least through the "mu" receptor type. Moreover, LTH is disrupted by bilateral, electrolytic lesion of the locus coeruleus, but it is only impaired by 6-OH-DA bilateral lesion of the dorsal noradrenergic bundle, and it is facilitated by electrolytic lesion of the medial septal nuclei. (b) Noninterference correlative studies: Individual differences in behavioral variables can be correlated to some components of the architecture of the hippocampus to reveal structure-function relationships. (c) Noninterference maturation studies pertain to the study of the maturation of LTH during postnatal development in a scaled-down Làt-maze in normally reared rats and in rats with deranged rate of body and brain growth by litter size technique, differential stimulation or by perinatal propylthiouracil-induced hypothyroidism. (d) Noninterference development studies pertain to the formation of LTH varying the between-exposure interval. It was studied in albino rats of a Sprague-Dawley, random-bred stock (NRB) and of the Naples High (NHE) and Low-Excitability (NLE) lines. The study was carried out during the light or the dark phase of a 12:12LD cycle, by retesting at different inter-exposure intervals. Multivariate analysis of variance showed significant effects of strain, inter-exposure interval and of postexposure sleep or wakefulness. Furthermore, analysis of the temporal pattern showed the formation of LTH to follow a non linear complex function. Further, behavioral habituation consists of emotional and cognitive components that can be separated across different approaches. In conclusion, long-term habituation to a novel environmental is a useful model to study experience-induced nonassociative behavioral modifications.

Self-administration of neurokinin substance P into the ventromedial caudate-putamen in rats

There is evidence that the neurokinin substance P plays a role in learning and reinforcement processes. Reinforcing effects of substance P were found upon injection into several parts of the brain. The aim of the present study was to gauge possible reinforcing effects of microinjections of substance P into the ventromedial caudate-putamen in rats. Two different behavioral paradigms were employed. In the first experiment a two-compartment choice procedure was used and the rats could trigger substance P injections (500 pg per 5 nl injection volume) into the ventromedial caudate-putamen by entering one distinctive compartment. During the injection period, substance P-injected animals spent significantly more time in the drug-paired compartment than vehicle-injected controls. In the second experiment, nose-poking through a hole in one wall of the cage was used as the operant. Rats that could self-administer substance P (100 pg per 5 nl injection volume) into the ventromedial caudate-putamen emitted a significantly higher rate of operant responding on the first day of testing and a significantly lower rate on the third day compared to vehicle-injected animals. The experiments provide evidence that the administration of substance P into the ventromedial part of the caudate-putamen can have positive reinforcing effects, but that repeated injections can have aversive properties. These effects are discussed, firstly, with regard to the possible mechanisms of intrastriatal substance P on striatonigral and striatopallidal output systems and, secondly, with respect to their possible relevance in the study of the basal forebrain reinforcement system.

Contribution of the vagus nerve in mediating the memory-facilitating effects of substance P

The present study determined whether the effects of peripherally administered substance P on memory are mediated via activation of the vagus nerve. Rats were submitted to subdiaphragmatic vagotomy, sham vagotomy or non-operated, and trained in a step-down inhibitory avoidance task and tested 24 h later. Posttraining administration of 50 micrograms/kg of SP facilitated retention performance in non-operated and sham-operated groups. The facilitating effects of 50 micrograms/kg of SP was blocked by vagotomy, although vagotomy did not attenuate the memory-enhancing effects of larger doses (250 and 500 micrograms/kg). These results suggest that the mnemotropic effects of peripherally administered SP are sensitive to the functional integrity of the vagus nerve. Alternatively, the vagus nerve may be one pathway but not the only pathway by which systemic SP influences the memory storage processes in the brain.

Facilitation of learning in adult and aged rats following bilateral lesions of the tuberomammillary nucleus region

In the present experiment, adult (3-month-old) and aged (31-month-old) rats received bilateral DC or sham-lesions in the region of the tuberomammillary (TM) nucleus before they were trained and tested on a one-trial step-through inhibitory avoidance task. Bilateral lesions of the TM nucleus led to significantly longer latencies in the step-through response during retention test in both adult and aged rats, indicative of superior learning of the task. Thus, lesions of the TM nucleus may have a facilitatory effect on learning and mnemonic functioning which is possibly related to a lesion-induced disinhibition or facilitation of reinforcement processes ("stamping-in") as revealed in previous studies.

Evidence that cholecystokinin-enhanced retention is mediated by changes in opioid activity in the amygdala

Mice, partially trained to avoid footshock in a T-maze, showed enhanced retention relative to vehicle-injected mice when treated peripherally with arecoline, D-amphetamine, cholecystokinin octapeptide (CCK-8), epinephrine or naloxone. Both intra-amygdaloid and intraventricular injections of beta-endorphin resulted in amnesia. D-amphetamine and arecoline blocked the amnestic effect of beta-endorphin administered into the amygdala but it required higher doses for CCK-8, epinephrine and naloxone to block the amnestic effect of beta-endorphin. The effects of CCK-8, epinephrine and naloxone showed a differential ability to block amnesia induced by beta-endorphin intraventricularly with epinephrine and naloxone preventing amnesia but CCK-8 not improving retention. This data suggests that the memory enhancement produced by peripherally administered CCK-8 involves the amygdala and that both CCK-8 and epinephrine interact with opioid amnestic mechanisms within the amygdala to alter memory processing.

Improvement of shuttle-box avoidance with post-training intracranial self-stimulation, in rats: a parametric study

Rats were trained in a two-way active avoidance task followed immediately by a lateral hypothalamic intracranial self-stimulation (ICSS) treatment, during 5 consecutive days. The effects of the number of ICSS trains allowed (0, 500, 2500 or 4500) were studied upon acquisition and long-term retention (LTR, 10 and 30 days). The number of ICSS trains administered and the number of avoidances at the last acquisition session (5th) showed a positive lineal relation, that is, the more number of ICSS trains, the more number of avoidances. The level of learning achieved during the 5th session was maintained after the LTR periods in all experimental groups. It is concluded that the number of ICSS trains could be a critical parameter in the facilitatory effect of lateral hypothalamic ICSS upon learning, and it is suggested that the facilitatory effect of post-training lateral hypothalamic ICSS might be due to the activation of general activatory neural systems.

Reinforcing effects of peripherally administered substance P and its C-terminal sequence pGlu6-SP6-11 in the rat

Reinforcing effects of intraperitoneally (IP) administered substance P (SP1-11), its amino-terminal fragment SP1-7 (SPN) and an analog of the carboxy terminus (pGlu6-SP6-11: SPC) were studied in rats. Two conditioned place preference paradigms were used. After three pairings of the drug with a certain environment the effect of the treatment was evaluated in the drug-free state during a test trial. The reinforcing effects of SP (37 nmol) and the equimolar dose of SPC were expressed by a significant increase in the amount of time the animals spent in the treatment environment. Other doses of SP (3.7 and 185 nmol) and SPC (7.4 and 185 nmol) and none of the doses of SPN (37, 185, 370 nmol) influenced the place preference behavior of the rats. The reinforcing effects of SP parallel the known facilitating effects of peripherally administered SP on memory. The amino acids that encode the reinforcing effects of SP may lie within the C-terminal sequence of the SP molecule.

The relationship between reinforcement and memory: parallels in the rewarding and mnemonic effects of the neuropeptide substance P

A theory of reinforcement is presented which accounts for the backward action of a reinforcer on operant behavior in terms of its effect on memory traces left by the operant. Several possible ways in which a reinforcer could strengthen the probability of recurrence of an operant are discussed. Predictions from the model regarding general memory-promoting effects of reinforcers presented posttrial in various learning paradigms are outlined. The theory also predicts a parallelism in reinforcing and memory-promoting effects of stimuli, including drugs. The second part of the chapter outlines experiments investigating memory modulating and reinforcing effects of the neuropeptide substance P. In general, injection of SP is positively reinforcing when injected into parts of the brain where it has been shown to facilitate learning. Peripheral injection of SP is also reinforcing at the dose known to promote passive avoidance learning when presented posttrial.

Improvement of shuttle-box learning with pre- and post-trial intracranial self-stimulation in rats

The effects of intracranial self-stimulation (ICSS) in the lateral hypothalamus upon the acquisition and long-term retention (LTR) of shuttle box avoidance conditioning were studied in Wistar rats. Two groups of subjects learned the avoidance task in 5 daily training sessions and were allowed to self-stimulate either before (Pre-ICSS group), or after (Post-ICSS group) each training session. A control group received training but no ICSS. Ten days following the last training session, LTR of the task was determined in one avoidance session without ICSS. A fourth group was added post-hoc which was allowed to self-stimulate before the training sessions as well as before the LTR test. Both the Post-ICSS and Pre-ICSS groups improved in acquisition of the learned response over the successive training sessions, as compared with Controls. In the LTR test, the animals of the Post-ICSS group maintained the response level achieved in the last acquisition session. In contrast, the subjects of the Pre-ICSS group showed a significant decrease of the same response, unless they were given ICSS treatment prior to the LTR test. This may indicate a 'state-dependent learning' effect being responsible for the decrease in the LTR observed in Pre-ICSS group. Because both pre- and post-training ICSS treatments improved the acquisition and the LTR of the learned response, it is suggested that the contingency of the treatment with training (that is, ICSS treatment immediately after the training sessions) is not a necessary condition to facilitate the acquisition and the consolidation of two-way active avoidance learning.

Reinforcing properties of substance P in the lateral hypothalamus revealed by conditioned place preference

Reinforcing properties of substance P (SP) were investigated in rats using a conditioned place preference paradigm. After three baseline trials one conditioning trial of 10 min duration was performed. Either SP (100 pg, 1 ng, 100 ng) or saline (SI) was injected unilaterally into the lateral hypothalamus or a sham injection (OC) was given. Pairing one compartment with SP (100 pg, 1 ng) significantly increased the time spent in this compartment. Microinjections of 100 ng SP, saline or sham injection had no effect. Locomotor activity was not influenced by either treatment. These data are discussed in terms of (a) the possibility that SP has a role in mediating reinforcement and (b) the relationship between reinforcing effects and post-trial effects on learning and memory of SP applied into the lateral hypothalamus.

Afferent projections to the periaqueductal gray in the rabbit

The afferents to the periaqueductal gray in the rabbit have been described following hydraulic pressure injection of horseradish peroxidase at various sites throughout this structure. Every third section was reacted with tetramethylbenzidine, for the localization of afferent neurons. At the site of the deposit alternate sections were reacted with tetramethylbenzidine, Hanker-Yates reagent, or diaminobenzidine, for comparative assessment of the injection site. A large number of retrogradely labelled cells, assessed by bright- and dark-field microscopy, were observed in a wide range of areas throughout the brain. Major labelled areas within the telencephalon were cortical areas 5, 20, 21, 32 and 40. Within the diencephalon, the hypothalamus contained quantitatively by far the largest number of labelled cells. Of these nuclei, the dorsal pre-mammillary nucleus contained the largest number of labelled cells. Considerable labelling was also found within medial and lateral preoptic nuclei, anterior hypothalamic area, and ventromedial hypothalamic nucleus. Another diencephalic region containing a significant number of retrogradely labelled neurons was the zona incerta. At midbrain, pontine and medullary levels, additional labelled regions were: the substantia nigra, cuneiform nucleus, parabigeminal nucleus, raphe magnus, and reticular areas. Heavy labelling was seen within the periaqueductal gray itself, rostral and caudal to deposits placed within each subdivision. In addition, a large number of other areas labelled throughout the brain (Tables 2A-D). Not only were some differences noted in the pattern of labelled cells with deposits placed rostrally or caudally within periaqueductal gray, but certain topographical differences with respect to the degree of labelling within nuclei were also seen with injection sites ventral, lateral or dorsal to the aqueduct. In addition, a further difference was noted, in that over one third of the areas labelled with deposits in just one or other of the "divisions" within periaqueductal gray. The results therefore suggest that the periaqueductal gray might be divisible to some extent on the basis of connectivity with intrinsic subdivisions of the complex. It is hoped that, with time, it might prove possible to resolve any such differential input in functional terms. The wide variety of afferent input to the periaqueductal gray, and its strategic location, would seem to place it in a unique position for integrating and modifying a diversity of motor, autonomic, hormonal, sensory and limbic influences.(ABSTRACT TRUNCATED AT 400 WORDS)