A critical role for long-term potentiation mechanisms in the maintenance of object recognition memory in perirhinal cortex revealed by the infusion of zeta inhibitory pseudosubstrate

Object recognition, the ability to discriminate between a novel and a familiar stimulus, is critically dependent upon the perirhinal cortex. Neural response reductions upon repetition of a stimulus, have been hypothesized to be the mechanism within perirhinal cortex that supports recognition memory function. Thus, investigations into the mechanisms of long-term depression (LTD) in perirhinal cortex has provided insight into the mechanism of object recognition memory formation, but the contribution of long-term potentiation (LTP) to object recognition memory formation has been less studied. Inhibition of atypical PKC activity by Zeta Inhibitory Pseudosubstrate (ZIP) impairs the maintenance of LTP but not LTD, thus here infusion of ZIP into the perirhinal cortex allowed us to investigate the contribution of LTP-like mechanisms to object recognition memory maintenance. Infusion of ZIP into the perirhinal cortex of rats 24 h after the sample phase impaired performance in an object recognition but not an object location task, in contrast infusion of ZIP into the hippocampus impaired performance in an object location but not an object recognition task. The impairment in object recognition by ZIP was prevented by administration of the peptide GluA2_3y, which blocks the endocytosis of GluA2 containing AMPA receptors. Finally, performance in a perceptual oddity task, which requires perirhinal cortex function, was not disrupted by ZIP. Together these results demonstrate the importance of LTP-like mechanisms to the maintenance of object recognition memory in the perirhinal cortex.

Putting objects in context: A prefrontal-hippocampal-perirhinal cortex network

When we encounter an object, we spontaneously form associations between the object and the environment in which it was encountered. These associations can take a number of different forms, which include location and context. A neural circuit between the hippocampus, medial prefrontal cortex and perirhinal cortex is critical for object-location and object-sequence associations; however, how this neural circuit contributes to the formation of object-context associations has not been established. Bilateral lesions were made in the hippocampus, medial prefrontal cortex or perirhinal cortex to examine each region contribution to object-context memory formation. Next, a disconnection lesion approach was used to examine the necessity of functional interactions between the hippocampus and medial prefrontal cortex or perirhinal cortex. Spontaneous tests of preferential exploration were used to assess memory for different types of object-context associations. Bilateral lesion in the hippocampus, medial prefrontal cortex or perirhinal cortex impaired performance in both an object-place-context and an object-context task. Disconnection of the hippocampus from either the medial prefrontal cortex or perirhinal cortex impaired performance in both the object-place-context and object-context task. Interestingly, when object recognition memory was tested with a context switch between encoding and test, performance in the hippocampal and medial prefrontal cortex lesion groups was disrupted and performance in each disconnection group (i.e. hippocampus + medial prefrontal cortex, hippocampus + perirhinal cortex) was significantly impaired. Overall, these experiments establish the importance of the hippocampal-medial prefrontal-perirhinal cortex circuit for the formation of object-context associations.

Multi-level analyses of associative recognition memory: the whole is greater than the sum of its parts

Associative recognition memory depends on the integration of information concerning an item and the spatio-temporal context in which it was encountered. Such an integration depends on dynamic interactions across a brain-wide memory network. Here we discuss evidence from multiple levels of analysis, behavioural, cellular and synaptic which demonstrating the existence of multiple overlapping, subnetworks embedded within these large-scale networks. Recent advances have revealed that of these subnetworks, a distinct hippocampal-prefrontal networks are engaged by different representations (object-spatial or object temporal). Other subnetworks are recruited by distinct processing demands, such as encoding and retrieval which are supported by distinct cellular and synaptic processes. One challenge to multi-level investigations of memory continues to be that conclusions are drawn from correlations of effects rather than from direct evidence of causation.

Remembering the order of serially presented objects: A matter of time?

Remembering the sequence, in which stimuli are encountered or events have occurred, is a key process in episodic memory and can also facilitate recognition memory. Rodents, when presented with a sequence of objects, will explore the object encountered first; yet, whether this behaviour is because the rodents spontaneously encode the order of stimuli presentation or because of relative familiarity or temporal decay is unknown. Here, we tested sequence memory in rats using a series of spontaneous preference tasks. Experiment 1 demonstrated that when rats are presented with a sequence of four objects, with an inter-sample interval of 5 min or 1 h, they preferentially explored the object presented earlier in the list irrespective of the inter-sample interval. We then demonstrated that such memory for order was not affected by increasing or decreasing the inter-sample interval between the middle objects (Experiment 2). Finally, we showed that memory for order is not a function of absolute object familiarity, as animals showed clear discrimination between the objects presented in the sample phases and a novel object, independent of the sample objects' position in the sequence (Experiment 3). These results show that animals are able to encode the order of objects presented in a sequence, and as such temporal order memory is not achieved using the process of relative or absolute familiarity or temporal decay.

Disruption of hippocampal-prefrontal cortex activity by dopamine D2R-dependent LTD of NMDAR transmission

Functional connectivity between the hippocampus and prefrontal cortex (PFC) is essential for associative recognition memory and working memory. Disruption of hippocampal-PFC synchrony occurs in schizophrenia, which is characterized by hypofunction of NMDA receptor (NMDAR)-mediated transmission. We demonstrate that activity of dopamine D2-like receptors (D2Rs) leads selectively to long-term depression (LTD) of hippocampal-PFC NMDAR-mediated synaptic transmission. We show that dopamine-dependent LTD of NMDAR-mediated transmission profoundly disrupts normal synaptic transmission between hippocampus and PFC. These results show how dopaminergic activation induces long-term hypofunction of NMDARs, which can contribute to disordered functional connectivity, a characteristic that is a hallmark of psychiatric disorders such as schizophrenia.

Neural circuitry for rat recognition memory

Information concerning the roles of different brain regions in recognition memory processes is reviewed. The review concentrates on findings from spontaneous recognition memory tasks performed by rats, including memory for single objects, locations, object-location associations and temporal order. Particular emphasis is given to the potential roles of different regions in the circuit of interacting structures involving the perirhinal cortex, hippocampus, medial prefrontal cortex and medial dorsal thalamus in recognition memory for the association of objects and places. It is concluded that while all structures in this circuit play roles critical to such memory, these roles can potentially be differentiated and differences in the underlying synaptic and biochemical processes involved in each region are beginning to be uncovered.

Mechanisms of synaptic plasticity and recognition memory in the perirhinal cortex

Learning is widely believed to involve synaptic plasticity, employing mechanisms such as those used in long-term potentiation (LTP) and long-term depression (LTD). In this chapter, we will review work on mechanisms of synaptic plasticity in perirhinal cortex in vitro and relate these findings to studies underlying recognition memory in vivo. We describe how antagonism of different glutamate and acetylcholine receptors, inhibition of nitric oxide synthase, inhibition of CREB phosphorylation, and interfering with glutamate AMPA receptor internalization can produce deficits in synaptic plasticity in vitro. Inhibition of each of these different mechanisms in vivo also results in recognition memory deficits. Therefore, we provide strong evidence that synaptic plastic mechanisms are necessary for the information processing and storage that underlies object recognition memory.

Object-in-place associative recognition memory depends on glutamate receptor neurotransmission within two defined hippocampal-cortical circuits: a critical role for AMPA and NMDA receptors in the hippocampus, perirhinal, and prefrontal cortices

Object-in-place associative recognition memory depends on an interaction between the hippocampus (HPC), perirhinal (PRH), and medial prefrontal (mPFC) cortices, yet the contribution of glutamate receptor neurotransmission to these interactions is unknown. NMDA receptors (NMDAR) in the HPC were critical for encoding of object-in-place memory but not for single-item object recognition. Next, a disconnection procedure was used to examine the importance of “concurrent” glutamate neurotransmission in the HPC-mPFC and HPC-PRH. Contralateral unilateral infusions of NBQX (AMPAR antagonist), into the HPC-mPFC, or HPC-PRH, either before acquisition or test, impaired object-in-place performance. Thus, both circuits are necessary for encoding and retrieval. Crossed unilateral AP5 (NMDAR antagonist) infusions into the HPC-mPFC or HPC-PRH impaired encoding, but not retrieval. Specifically crossed HPC-mPFC infusions impaired both short-term (5 min) and longer term (1 h) memory while HPC-PRH infusions impaired longer term memory only. This delay-dependent effect of AP5 in the HPC-PRH on object-in-place memory, accords with its effects in the PRH, on single item object recognition memory, thereby suggesting that a single PRH synaptic plasticity mechanism underpins different recognition memory processes. Further, blocking excitatory neurotransmission in any pair of structures within the networks impaired “both” encoding and retrieval, thus object-in-place memory clearly requires network interdependency across multiple structures.

Contributions of area Te2 to rat recognition memory

Ablations and local intracerebral infusions were used to determine the role of rat temporal association cortex (area Te2) in object recognition memory, so that this role might be compared with that of the adjacent perirhinal cortex (PRH). Bilateral lesions of Te2 impaired recognition memory measured by preferential exploration of a novel rather than a familiar object at delays ≥20 min but not after a 5-min delay. Local infusion bilaterally into Te2 of (1) CNQX to block AMPA/kainate receptors or (2) lidocaine to block axonal transmission or (3) AP5, an NMDA receptor antagonist, impaired recognition memory after a 24-h but not a 20-min delay. In PRH all these manipulations impair recognition memory after a 20-min as well as a 24-h delay. UBP302, a GluK1 kainate receptor antagonist, impaired recognition memory after a 24-h but not a 20-min delay, contrasting with its action in PRH where it impairs only shorter-term (20 min) recognition memory. Also in contrast to PRH, infusion of the muscarinic receptor antagonist scopolamine was without effect. The Te2 impairments could not readily be ascribed to perceptual deficits. Hence, Te2 is essential for object recognition memory at delays >5 or 20 min. Thus, at long delays both area Te2 and PRH are necessary for object recognition memory.

Unraveling the contributions of the diencephalon to recognition memory: a review

Both clinical investigations and studies with animals reveal nuclei within the diencephalon that are vital for recognition memory (the judgment of prior occurrence). This review seeks to identify these nuclei and to consider why they might be important for recognition memory. Despite the lack of clinical cases with circumscribed pathology within the diencephalon and apparent species differences, convergent evidence from a variety of sources implicates a subgroup of medial diencephalic nuclei. It is supposed that the key functional interactions of this subgroup of diencephalic nuclei are with the medial temporal lobe, the prefrontal cortex, and with cingulate regions. In addition, some of the clinical evidence most readily supports dual-process models of recognition, which assume two independent cognitive processes (recollective-based and familiarity-based) that combine to direct recognition judgments. From this array of information a "multi-effect multi-nuclei" model is proposed, in which the mammillary bodies and the anterior thalamic nuclei are of preeminent importance for recollective-based recognition. The medial dorsal thalamic nucleus is thought to contribute to familiarity-based recognition, but this nucleus, along with various midline and intralaminar thalamic nuclei, is also assumed to have broader, indirect effects upon both recollective-based and familiarity-based recognition.

A role for calcium-calmodulin-dependent protein kinase II in the consolidation of visual object recognition memory

The aim was to investigate the role of calcium-calmodulin-dependent protein kinase (CAMK)II in object recognition memory. The performance of rats in a preferential object recognition test was examined after local infusion of the CAMKII inhibitors KN-62 or autocamtide-2-related inhibitory peptide (AIP) into the perirhinal cortex. KN-62 or AIP infused after acquisition impaired memory tested at 24 h, indicating an involvement of CAMKII in the consolidation of recognition memory. Memory was impaired when KN-62 was infused at 20 min after acquisition or when AIP was infused at 20, 40, 60 or 100 min after acquisition. The time-course of CAMKII activation in rats was further examined by immunohistochemical staining for phospho-CAMKII(Thre286)alpha at 10, 40, 70 and 100 min following the viewing of novel and familiar images. At 70 min, processing novel images resulted in more phospho-CAMKII(Thre286)alpha-stained neurons in the perirhinal cortex than did the processing of familiar images, consistent with the viewing of novel images increasing the activity of CAMKII at this time. This difference was eliminated by prior infusion of AIP. These findings establish that CAMKII is active within the perirhinal region between approximately 20 and 100 min following learning and then returns to baseline. Thus, increased CAMKII activity is essential for the consolidation of long-term object recognition memory but continuation of that increased activity throughout the 24 h memory delay is not necessary for maintenance of the memory.

A temporally distinct role for group I and group II metabotropic glutamate receptors in object recognition memory

Recognition memory, involving the ability to discriminate between a novel and familiar object, depends on the integrity of the perirhinal cortex (PRH). Glutamate, the main excitatory neurotransmitter in the cortex, is essential for many types of memory processes. Of the subtypes of glutamate receptor, metabotropic receptors (mGluRs) have received less study than NMDA receptors; thus, the reported experiments examined the role of mGluRs in familiarity discrimination in the rat PRH. Experiments 1 and 2 assessed the effects of systemic administration of MPEP, a group I mGluR (specifically mGluR5) antagonist, and/or LY341495, a group II mGluR antagonist, on a spontaneous object novelty preference task. Simultaneous antagonism of both group I and II mGluRs impaired familiarity discrimination following a 24-h but not a 15-min delay, while antagonism of either mGluR subtype alone had no effect at either delay. The impairment was in acquisition, as in Experiment 3 coadministration of MPEP and LY341495 did not affect recognition memory performance when administered either after the sample phase or prior to test. The impairment in long-term recognition memory was mediated by mGluRs in the PRH, as localized intracortical antagonism of group I and II mGluRs also produced a deficit (Experiment 4). No evidence was found for an involvement of group III mGluRs in the acquisition of long-term familiarity discrimination (Experiment 5). These findings establish that glutamatergic neurotransmission in the PRH via group I and II mGluRs is crucial for the acquisition, but not for the consolidation or retrieval of long-term object recognition memory.

Benzodiazepine impairment of perirhinal cortical plasticity and recognition memory

Benzodiazepines, including lorazepam, are widely used in human medicine as anxiolytics or sedatives, and at higher doses can produce amnesia. Here we demonstrate that in rats lorazepam impairs both recognition memory and synaptic plastic processes (long-term depression and long-term potentiation). Both impairments are produced by actions in perirhinal cortex. The findings thus establish a mechanism by means of which benzodiazepines impair recognition memory. The findings also strengthen the hypotheses that the familiarity discrimination component of recognition memory is dependent on reductions in perirhinal neuronal responses when stimuli are repeated and that these response reductions are due to a plastic mechanism also used in long-term depression.

Galanin regulates spatial memory but not visual recognition memory or synaptic plasticity in perirhinal cortex

It has previously been shown that the neuropeptide galanin plays a role in the age-dependent regulation of hippocampal synaptic plasticity and spatial memory. Here, we further extend these studies by demonstrating that galanin knockout mice also have deficits in an object-in-place spatial memory task. In contrast however, there is no deficit in single item object recognition memory, a memory that depends on perirhinal cortex. Furthermore, in perirhinal cortex slices there are no differences in activity-dependent long-term potentiation or depotentiation, nor in muscarinic receptor-dependent long-term depression between galanin knockout mice and wild-type litter-mates. Therefore, these results suggest that galanin has a differential role in hippocampal-dependent and perirhinal cortex-dependent memory.

The conjoint importance of the hippocampus and anterior thalamic nuclei for allocentric spatial learning: evidence from a disconnection study in the rat

A disconnection procedure was used to test whether the hippocampus and anterior thalamic nuclei form functional components of the same spatial memory system. Unilateral excitotoxic lesions were placed in the anterior thalamic (AT) nuclei and hippocampus (HPC) in either the same (AT-HPC Ipsi group) or contralateral (AT-HPC Contra group) hemispheres of rats. The behavioral effects of these combined lesions were compared in several spatial memory tasks sensitive to bilateral hippocampal lesions. In all of the tasks tested, T-maze alternation, radial arm maze, and Morris water maze, those animals with lesions placed in the contralateral hemispheres were more impaired than those animals with lesions in the same hemisphere. These results provide direct support for the notion that the performance of tasks that require spatial memory rely on the operation of the anterior thalamus and hippocampus within an integrated neural network.

Fos imaging reveals differential neuronal activation of areas of rat temporal cortex by novel and familiar sounds

To provide information about the possible regions involved in auditory recognition memory, this study employed an imaging technique that has proved valuable in the study of visual recognition memory. The technique was used to image populations of neurons that are differentially activated by novel and familiar auditory stimuli, thereby paralleling previous studies of visual familiarity discrimination. Differences evoked by novel and familiar sounds in the activation of neurons were measured in different parts of the rat auditory pathway by immunohistochemistry for the protein product (Fos) of the immediate early gene c-fos. Significantly higher counts of stained neuronal nuclei (266 +/- 21/mm2) were evoked by novel than by familiar sounds (192 +/- 17/mm2) in the auditory association cortex (area Te3; AudA). No such significant differences were found for the inferior colliculus, primary auditory cortex, postrhinal cortex, perirhinal cortex (PRH), entorhinal cortex, amygdala or hippocampus. These findings are discussed in relation to the results of lesion studies and what is known of areas involved in familiarity discrimination for visual stimuli. Differential activation is produced by novel and familiar individual stimuli in sensory association cortex for both auditory and visual stimuli, whereas the PRH is differentially activated by visual but not auditory stimuli. It is suggested that this latter difference is related to the nature of the particular auditory and visual stimuli used.

Rats' processing of visual scenes: effects of lesions to fornix, anterior thalamus, mamillary nuclei or the retrohippocampal region

We analysed the effects of lesions of hippocampal-diencephalic projections -- fornix (FX) mamillary bodies (MB) and anterior thalamic nuclei (AT) -- and retrohippocampal (RH) lesions including entorhinal cortex and ventral subiculum, upon scene processing. All lesions except FX were neurotoxic. Rats learned to discriminate among computer-generated visual displays ("scenes") each comprising three different shapes ("objects"). The paradigm was constant-negative; one constant scene (unrewarded) appeared on every trial together with a trial-unique variable scene (rewarded). Four types of variable scene were intermingled: (1) unfamiliar objects in different positions from those of the constant (type O+P), (2) unfamiliar objects in same positions as in the constant (type O), (3) same objects as the constant in different positions (type P), (4) same objects and positions as the constant but recombined (type X). Group RH performed like controls while groups FX, AT and MB showed (surprisingly) enhanced performance on types X and O. One explanation is that normal rats attempt to process all objects in a scene concurrently, while hippocampal-projection lesions disrupt this tendency, producing a narrower attention, which paradoxically aids performance with some variable types. The results confirm that the entorhinal cortex has a different function from other components of the hippocampal system.

Disconnecting hippocampal projections to the anterior thalamus produces deficits on tests of spatial memory in rats

A disconnection procedure was used to test whether projections from the hippocampus to the anterior thalamic nuclei (AT), via the fimbria-fornix (FX), form functional components of a spatial memory system. The behavioural effects of combined unilateral lesions in the AT and FX were compared when they were either in contralateral hemispheres (AT-FX Contra) or the same hemisphere (AT-FX Ipsi). Other groups received bilateral FX lesions and Sham surgeries. Expt 1 demonstrated that none of these lesions affected performance of an object recognition task, while performance of an object location task, which tests the subjects' preference for an object that has changed location, was impaired in the AT-FX Contra and FX groups. In a T-maze alternation task, however, the FX group was severely impaired while both the AT-FX Ipsi and AT-FX Contra lesion groups showed only a mild impairment. In order to test whether spared crossed projections might support spatial performance in the AT-FX Contra group we then examined the effects of a combined AT-FX Contra lesion coupled with transection of the hippocampal commissure. This combination of lesions produced a severe disruption in spatial memory performance in the water maze, radial arm maze and T-maze, which was significantly greater than that produced by ipsilateral and contralateral AT-FX lesions alone. These results support the notion that disconnection of the AT from their hippocampal inputs produces impairments on a range of spatial memory tasks, but indicate that there are an array of different routes that can subserve this function.

Does pretraining spare the spatial deficit associated with anterior thalamic damage in rats?

Rats that had been pretrained on 2 tests of allocentric memory (water maze and T maze) received bilateral cytotoxic lesions in the anterior thalamic nuclei (ATN) or transection of the fimbria-fornix (FF). After surgery, both groups of rats were impaired on both tasks, although the preoperative training resulted in a rapid initial reacquisition of the water maze task. Those rats with lesions largely restricted to the ATN were impaired at a level comparable to that produced by FF lesions. This finding is consistent with a close functional relationship between the hippocampus and the ATN, necessary for the acquisition and on-line processing of allocentric spatial information but not for the maintenance/retrieval of procedural information. The rats with more extensive thalamic lesions were more impaired in both tasks and did show a loss of procedural information.

Does pretraining spare the spatial deficit associated with anterior thalamic damage in rats?

Rats that had been pretrained on 2 tests of allocentric memory (water maze and T maze) received bilateral cytotoxic lesions in the anterior thalamic nuclei (ATN) or transection of the fimbria-fornix (FF). After surgery, both groups of rats were impaired on both tasks, although the preoperative training resulted in a rapid initial reacquisition of the water maze task. Those rats with lesions largely restricted to the ATN were impaired at a level comparable to that produced by FF lesions. This finding is consistent with a close functional relationship between the hippocampus and the ATN, necessary for the acquisition and on-line processing of allocentric spatial information but not for the maintenance/retrieval of procedural information. The rats with more extensive thalamic lesions were more impaired in both tasks and did show a loss of procedural information.

Differential deficits in the Morris water maze following cytotoxic lesions of the anterior thalamus and fornix transection

Rats with complete fornix lesions or cytotoxic lesions placed in the anterior thalamic region were trained on an allocentric spatial memory test (the Morris water maze). While both lesions led to impairments in locating the hidden platform in this test of reference memory, the thalamic lesions led to a significantly greater deficit than that observed after fornix transection as measured by a number of performance indices. The lesions also led to different patterns of swim behaviour in the pool. The severity of the thalamic lesion deficit was associated with anterior thalamic nuclei damage but not with damage to the nucleus medialis dorsalis. Both the fornix and the thalamic lesions also severely impaired T-maze alternation. In contrast, neither set of lesions appeared to affect the recognition of small or large objects. While the study provides further evidence of a close functional relationship between the hippocampus and the anterior thalamic nuclei, it also shows that disconnection of the fornical inputs to the anterior thalamic nuclei does not provide a full explanation of the thalamic deficit.

Comparing the effects of selective cingulate cortex lesions and cingulum bundle lesions on water maze performance by rats

The ability of rats to learn the location of a hidden platform in a swim maze was compared in animals with excitotoxic lesions of the anterior or posterior (retrosplenial) cingulate cortex or radiofrequency lesions of the cingulum bundle or fimbria-fornix. Performance of this allocentric spatial task was unaffected by the posterior cingulate cortex lesions, while anterior cingulate cortex damage produced only a mild acquisition deficit. Transection of the fornix and lesions of the cingulum bundle produced similar patterns of impairment on initial acquisition, but the cingulum bundle lesions had less effect on reversal of the task. The results from the water maze, and from a subsequent T-maze alternation task, indicate that cingulum bundle lesions can produce a spatial deficit that is similar, but milder, to that observed after fornix transection. The results of the excitotoxic lesions suggest that previous studies examining conventional cingulate lesions may have been influenced by damage to adjacent fibre tracts, such as the cingulum bundle.

Assessing the magnitude of the allocentric spatial deficit associated with complete loss of the anterior thalamic nuclei in rats

The behavioural effects of complete lesions of the anterior thalamic nuclei (ANT), the anterior thalamic nuclei plus the lateral dorsal nucleus (ANT + LD), and fornix (FX) were compared using a series of tests of spatial memory. ALl three lesion groups were found to have an equally severe and long-lasting impairment in the acquisition of a T-maze alternation task when compared with the control animals (COMB SHAM). In Experiment 2, the control animals were able to perform the alternation task when the test trial was started from a different location to the sample trial, so demonstrating that they were able to use allocentric cues in order to differentiate the most recently visited arm. In contrast, all the lesion groups performed close to chance level. In fact, for this condition the ANT / LD group was significantly worse than the FX group. In contrast, none of the lesion groups was impaired on an egocentric discrimination and subsequent reversal task (Experiment 3). The control animals came from two different control procedures, a surgical control sub-group (SHAM) and a group of animals that received injections of N-methyl-D-aspartic (NMDA) into the fornix (NMDA SHAM). There were no differences in the performance levels of the NMDA SHAM group compared with the surgical control group in any of the experiments conducted, so showing that the anterior thalamic lesion effects were not due to non-specific damage to the fornix by NMDA. This series of experiments demonstrated that complete lesions of the anterior thalamic region impair the ability to process allocentric information, and provide evidence for a contribution from the lateral dorsal thalamic nucleus.

Contrasting effects of systemic and intracerebral infusions of the 5-HT1A receptor agonist 8-OH-DPAT on spatial short-term working memory in rats

The present study compared the effects of systemic 8-OH-DPAT (0.05, 0.1 and 1.0 mg/kg) with intra-raphe and intra-hippocampal infusions of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (10, 30 100 ng) on delayed non-matching-to-position (DNMP) performance in rats. The highest dose of 8-OH-DPAT administered systemically impaired DNMP performance in a delay-independent manner, increased premature responding and increased response bias. Infusions of 8-OH-DPAT (100 ng) into the median raphe nucleus improved performance accuracy, independent of delay whilst having no effect on any other response measure. Infusions of 8-OH-DPAT into the dorsal raphe nucleus had no effect on performance at any dose tested. Infusions of 8-OH-DPAT into the dorsal hippocampus produced a small impairment in performance which was also independent of delay. However, this decrement in performance accuracy was not accompanied by any changes in other response measures. These findings demonstrate a dissociation between the effects of stimulation of pre- and post-synaptic 5-HT1A receptors on performance of a DNMP task although the changes in performance cannot be accounted for by changes in mnemonic function.

Extensive cytotoxic lesions involving both the rhinal cortices and area TE impair recognition but spare spatial alternation in the rat

Rats with cytotoxic lesions of the perirhinal, postrhinal, and TE cortices (Rh+TE, n = 7) were compared with surgical control animals (n = 7) on a series of spontaneous object recognition tests. The Rh+TE group was associated with a failure to select the novel object. This recognition deficit contrasted with the apparently normal ability of the same animals to learn and perform a spatial working memory test (T-maze alternation). The animals were also tested on the acquisition of an automated visual discrimination task in which the stimuli were presented on a visual display unit (VDU) equipped with a touch screen. The animals with Rh+TE lesions showed only a borderline deficit on this task. These findings are consistent with other evidence implicating the rhinal region in recognition memory. More importantly, they also provide a dissociation between spatial working memory and object recognition and, hence, show that extensive rhinal lesions are not sufficient to disconnect the hippocampus functionally.

The role of mesolimbic dopaminergic and retrohippocampal afferents to the nucleus accumbens in latent inhibition: implications for schizophrenia

Latent inhibition (LI) consists in a retardation of conditioning seen when the to-be-conditioned stimulus is first presented a number of times without other consequence. Disruption of LI has been proposed as a possible model of the cognitive abnormality that underlies the positive psychotic symptoms of acute schizophrenia. We review here evidence in support of the model, including experiments tending to show that: (1) disruption of LI is characteristic of acute, positively-symptomatic schizophrenia; (2) LI depends upon dopaminergic activity; (3) LI depends specifically upon dopamine release in n. accumbens; (4) LI depends upon the integrity of the hippocampal formation and the retrohippocampal region reciprocally connected to the hippocampal formation; (5) the roles of n. accumbens and the hippocampal system in LI are interconnected.

Antagonism of amphetamine-induced disruption of latent inhibition in rats by haloperidol and ondansetron: implications for a possible antipsychotic action of ondansetron

Latent inhibition (LI) is a behavioural phenomenon whereby preexposure to a stimulus without reinforcement interferes with the formation of subsequent associations to that stimulus. Using preexposure to a tone stimulus which subsequently serves as a conditioned stimulus for suppression of licking, we have confirmed that LI is disrupted by a low dose of amphetamine. Haloperidol was able to prevent this effect of amphetamine. Ondansetron, a selective and potent 5HT3 receptor antagonist, was also shown to be effective at blocking the amphetamine-induced disruption of LI at a dose of 0.01 mg/kg, but not at 0.1 mg/kg. In addition, it was demonstrated that ondansetron could enhance LI; using only ten preexposures, no LI was obtained in the saline group, but was apparent in animals given ondansetron, an effect which has been previously shown with haloperidol. Haloperidol, at the higher dose used, reduced suppression of licking, however, ondansetron at the effective dose had no such effect. It is concluded that ondansetron is able to attenuate increases in dopamine activity, produced pharmacologically with amphetamine without affecting baseline dopamine activity. The implications of these findings for a possible antipsychotic action of ondansetron are discussed.