Selective fimbria and thalamic lesions differentially impair forms of working memory in rats

Molecular cell identities in the mediodorsal thalamus of infant mice and marmoset

The mediodorsal thalamus (MD) is a higher-order nucleus located within the central thalamus in many mammalian species. Emerging evidence from MD lesions and tracer injections suggests that the MD is reciprocally connected to the prefrontal cortex (PFC) and plays an essential role in specific cognitive processes and tasks. MD subdivisions (medial, central, and lateral) are poorly segregated at the molecular level in rodents, leading to a lack of MD subdivision-specific Cre driver mice. Moreover, this lack of molecular identifiers hinders MD subdivision- and cell-type-specific circuit formation and function analysis. Therefore, using publicly available databases, we explored molecules separately expressed in MD subdivisions. In addition to MD subdivision markers, we identified several genes expressed in a subdivision-specific combination and classified them. Furthermore, after developing medial MD (MDm) or central MD (MDc) region-specific Cre mouse lines, we identified diverse region- and layer-specific PFC projection patterns. Comparison between classified MD marker genes in mice and common marmosets, a nonhuman primate model, revealed diverging gene expression patterns. These results highlight the species-specific organization of cell types and their projections in the MD thalamus.

Genetic Ablation of Neural Progenitor Cells Impairs Acquisition of Trace Eyeblink Conditioning

Adult-born neurons are believed to play a role in memory formation by providing enhanced plasticity to the hippocampus. Past studies have demonstrated that reduction of neurogenesis impairs associative learning, but these experiments used irradiation or neurotoxic substances, which may have had unintended off-target effects. Therefore, to investigate the role of these adult-born neurons more precisely, we used nestin-HSV-TK transgenic mice (Nes-TK) to selectively ablate newborn neurons. Nes-TK mice were fed a chow infused with valganciclovir to induce the ablation of neural progenitor cells. After being on this diet for 4 weeks, mice were trained on trace eyeblink conditioning, a hippocampus-dependent temporal associative memory task. Following the completion of training, brain sections from these animals were stained for doublecortin, a marker for immature neurons, to quantify levels of neurogenesis. We found that male transgenic mice on valganciclovir had significantly decreased amounts of doublecortin relative to male control animals, indicating a successful reduction in levels of neurogenesis. In conjunction with this reduction in neurogenesis, the male transgenic mice on valganciclovir learned at a significantly slower rate than male control mice. The female Nes-TK mice on valganciclovir showed no significant decrease in neurogenesis and no behavioral impairment relative to female control mice. Ultimately, the results are consistent with, and expand upon, prior studies that demonstrated that adult-born neurons are involved in the formation of associative memories. This study also provides a foundation to continue to explore the physiological role of newborn neurons with in vivo recordings during behavioral training.

The dynamics of disordered dialogue: Prefrontal, hippocampal and thalamic miscommunication underlying working memory deficits in schizophrenia

The prefrontal cortex is central to the orchestrated brain network communication that gives rise to working memory and other cognitive functions. Accordingly, working memory deficits in schizophrenia are increasingly thought to derive from prefrontal cortex dysfunction coupled with broader network disconnectivity. How the prefrontal cortex dynamically communicates with its distal network partners to support working memory and how this communication is disrupted in individuals with schizophrenia remain unclear. Here we review recent evidence that prefrontal cortex communication with the hippocampus and thalamus is essential for normal spatial working memory, and that miscommunication between these structures underlies spatial working memory deficits in schizophrenia. We focus on studies using normal rodents and rodent models designed to probe schizophrenia-related pathology to assess the dynamics of neural interaction between these brain regions. We also highlight recent preclinical work parsing roles for long-range prefrontal cortex connections with the hippocampus and thalamus in normal and disordered spatial working memory. Finally, we discuss how emerging rodent endophenotypes of hippocampal- and thalamo-prefrontal cortex dynamics in spatial working memory could translate into richer understanding of the neural bases of cognitive function and dysfunction in humans.

The Mediodorsal Thalamus: An Essential Partner of the Prefrontal Cortex for Cognition

Deficits in cognition are a core feature of many psychiatric conditions, including schizophrenia, where the severity of such deficits is a strong predictor of long-term outcome. Impairment in cognitive domains such as working memory and behavioral flexibility has typically been associated with prefrontal cortex (PFC) dysfunction. However, there is increasing evidence that the PFC cannot be dissociated from its main thalamic counterpart, the mediodorsal thalamus (MD). Since the causal relationships between MD-PFC abnormalities and cognitive impairment, as well as the neuronal mechanisms underlying them, are difficult to address in humans, animal models have been employed for mechanistic insight. In this review, we discuss anatomical, behavioral, and electrophysiological findings from animal studies that provide a new understanding on how MD-PFC circuits support higher-order cognitive function. We argue that the MD may be required for amplifying and sustaining cortical representations under different behavioral conditions. These findings advance a new framework for the broader involvement of distributed thalamo-frontal circuits in cognition and point to the MD as a potential therapeutic target for improving cognitive deficits in schizophrenia and other disorders.

Functional heterogeneity of the limbic thalamus: From hippocampal to cortical functions

Today, the idea that the integrity of the limbic thalamus is necessary for normal memory functions is well established. However, if the study of thalamic patients emphasized the anterior and the mediodorsal thalamus as the critical thalamic loci supporting cognitive functions, clinical studies have so far failed to attribute a specific role to each of these regions. In view of these difficulties, we review here the experimental data conducted in rodents harboring specific lesions of each thalamic region. These data clearly indicate a major functional dissociation within the limbic thalamus. The anterior thalamus provides critical support for hippocampal functions due to its cardinal location in the Papez circuit, while the mediodorsal thalamus may signal relevant information in a circuit encompassing the basolateral amygdala and the prefrontal cortex. Interestingly, while clinical studies have suggested that diencephalic pathologies may disconnect the medial temporal lobe from the cortex, experimental studies conducted in rodent show how this may differently affect distinct temporo-thalamo-cortical circuits, sharing the same general organization but supporting dissociable functions.

Fimbria/fornix lesions facilitate the learning of a nonspatial response task

The spatial cognitive map theory of O'Keefe and Nadel (1978) predicts that lesions of the hippocampal system should impair learning on spatial tasks but not learning on nonspatial tasks. However, there is evidence that such lesions can facilitate learning on certain nonspatial tasks. Their theory does not predict such facilitation. Nevertheless, it is reasonable to expect that animals possessing a spatial cognitive map would have an inherent bias to engage a mapping strategy and thus be at a disadvantage on certain nonspatial tasks in comparison with animals without the mapping capacity and bias. In the present study, fimbria/fornix lesions impaired learning on a spatial task, but actually facilitated learning on a nonspatial task of equal difficulty. Thus, brain lesions that interfere with map functioning can facilitate learning on tasks for which a mapping strategy interferes with task solution. The results require a modification of the spatial cognitive map theory.

What does the mediodorsal thalamus do?

Dense amnesia can result from damage to the medial diencephalon in humans and in animals. In humans this damage is diffuse and can include the mediodorsal nuclei of the thalamus. In animal models, lesion studies have confirmed the mediodorsal thalamus (MD) has a role in memory and other cognitive tasks, although the extent of deficits is mixed. Anatomical tracing studies confirm at least three different subgroupings of the MD: medial, central, and lateral, each differentially interconnected to the prefrontal cortex (PFC). Moreover, these subgroupings of the MD also receive differing inputs from other brain structures, including the basal ganglia thus the MD subgroupings form key nodes in interconnected frontal-striatal-thalamic neural circuits, integrating critical information within the PFC. We will provide a review of data collected from non-human primates and rodents after selective brain injury to the whole of the MD as well as these subgroupings to highlight the extent of deficits in various cognitive tasks. This research highlights the neural basis of memory and cognitive deficits associated with the subgroupings of the MD and their interconnected neural networks. The evidence shows that the MD plays a critical role in many varied cognitive processes. In addition, the MD is actively processing information and integrating it across these neural circuits for successful cognition. Having established that the MD is critical for memory and cognition, further research is required to understand how the MD specifically influences these cognitive processing carried out by the brain.

Effects of ethanol on hippocampal function during adolescence: a look at the past and thoughts on the future

It has been demonstrated by several laboratories that ethanol, both acute and chronic, produces effects that are age dependent. Specifically, adolescent rats are less sensitive to the hypnotic and motor-impairing effects of ethanol but are more sensitive to the hypothermic effects of the drug. However, the results on hippocampal function are not as clear. For example, there have been mixed findings regarding adolescent sensitivity of hippocampal-dependent (spatial) memory in response to ethanol. The current review explores the present state of the field as it relates to ethanol's effects in the hippocampus, particularly as it relates to spatial memory. In addition, we review potential neurobiological mechanisms that might underlie the age-dependent effects of ethanol in the hippocampus. Finally, future directions are proposed that will advance the state of the field as it relates to ethanol's effect during this developmental period.

Spared place and object-place learning but limited spatial working memory capacity in rats with selective lesions of the dentate gyrus

We studied the cognitive performance of rats with colchicine-induced lesions of the hippocampal dentate gyrus (DG) on a range of spatial, non-spatial and mixed spatial/procedural tasks. Rats were assigned to three experimental groups receiving large colchicine lesions (7 microg per hippocampus), small colchicine lesions (1.75 microg per hippocampus) or sham lesions. Stereological estimates of cell density indicated that the colchicine treatments induced dose-dependent damage to the DG, while sparing in large part other hippocampal subfields. Remarkably, the behavioural results showed that the colchicine lesions did not affect the performance of rats in an object discrimination task, in an object-place associative task in which a familiar object was displaced from a given position nor in a spontaneous spatial discrimination task performed in the T-maze. However, rats in both lesion groups were severely impaired in a reinforced non-matching-to-position working memory task conducted in the T-maze. Importantly, performance in the working memory task correlated strongly with cell density in the DG but not with cell density in the CA1 and CA3 areas. Only rats with large-lesions showed a transient deficit in a reinforced rule-based conditional discrimination task. These data demonstrated that rats with selective lesions of the DG readily acquire and retain neural representations relative to objects and places but are specifically impaired in their ability to update rapidly and flexibly spatial information that is essential to guide goal-directed actions.

The thalamic reticular nucleus: structure, function and concept

On the basis of theoretical, anatomical, psychological and physiological considerations, Francis Crick (1984) proposed that, during selective attention, the thalamic reticular nucleus (TRN) controls the internal attentional searchlight that simultaneously highlights all the neural circuits called on by the object of attention. In other words, he submitted that during either perception, or the preparation and execution of any cognitive and/or motor task, the TRN sets all the corresponding thalamocortical (TC) circuits in motion. Over the last two decades, behavioural, electrophysiological, anatomical and neurochemical findings have been accumulating, supporting the complex nature of the TRN and raising questions about the validity of this speculative hypothesis. Indeed, our knowledge of the actual functioning of the TRN is still sprinkled with unresolved questions. Therefore, the time has come to join forces and discuss some recent cellular and network findings concerning this diencephalic GABAergic structure, which plays important roles during various states of consciousness. On the whole, the present critical survey emphasizes the TRN's complexity, and provides arguments combining anatomy, physiology and cognitive psychology.

Parafascicular electrical stimulation attenuates nucleus basalis magnocellularis lesion-induced active avoidance retention deficit

Previous experiments from our laboratory showed that retention of two-way active avoidance learning is improved by post-training intracranial electrical stimulation (ICS) of the parafascicular nucleus (PF) and impaired by pre-training electrolytic lesions of the nucleus basalis magnocellularis (NBM). The question investigated here was whether post-training PF ICS is able to attenuate the active avoidance retention deficit observed in rats lesioned pre-training in the NBM. To this goal, the following experimental design was used: rats bilaterally lesioned in the NBM and stimulated in the PF, rats lesioned in the NBM, rats stimulated in the PF, control rats implanted in the PF, and sham-operated rats were first trained in a shuttle-box for a single 30-trial session and tested again following two successive retention intervals (24 h and 11 days). The results showed that: (1) NBM lesions impaired the 11-day performance without affecting either the acquisition or the 24-h retention of the avoidance learning; (2) PF ICS treatment in unlesioned rats improved performance in both retention sessions only when the stimulation was applied in the posterior region of the nucleus; and (3) stimulation of the posterior PF compensated the 11-day retention impairment induced by NBM lesions. These results are discussed in relation to the interaction of arousal systems in the modulation of cognitive processes.

Arrested neuronal proliferation and impaired hippocampal function following fractionated brain irradiation in the adult rat

The generation of new neurons in the adult mammalian brain has been documented in numerous recent reports. Studies undertaken so far indicate that adult hippocampal neurogenesis is related in a number of ways to hippocampal function.Here, we report that subjecting adult rats to fractionated brain irradiation blocked the formation of new neurons in the dentate gyrus of the hippocampus. At different time points after the termination of the irradiation procedure, the animals were tested in two tests of short-term memory that differ with respect to their dependence on hippocampal function. Eight and 21 days after irradiation, the animals with blocked neurogenesis performed poorer than controls in a hippocampus-dependent place-recognition task, indicating that the presence of newly generated neurons may be necessary for the normal function of this brain area. The animals were never impaired in a hippocampus-independent object-recognition task. These results are in line with other reports documenting the functional significance of newly generated neurons in this region. As our irradiation procedure models prophylactic cranial irradiation used in the treatment of different cancers, we suggest that blocked neurogenesis contributes to the reported deleterious side effects of this treatment, consisting of memory impairment, dysphoria and lethargy.

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.

Mediodorsal thalamic lesions impair trace eyeblink conditioning in the rabbit

Rabbits received lesions of the mediodorsal nucleus of the thalamus (MDN) or sham lesions and were subjected to classical eyeblink (EB) and heart rate (HR) conditioning. All animals received trace conditioning, with a.5-sec tone conditioned stimulus, a .5-sec trace period, and a 50-msec periorbital shock unconditioned stimulus. Animals with MDN lesions acquired the EB conditioned response (CR) more slowly than sham-lesioned animals. However, previous studies have shown that MDN damage does not affect delay conditioning using either .5-sec or 1-sec interstimulus intervals. The lesions had no significant effect on the HR CR. These results suggest that information processed by MDN and relayed to the prefrontal cortex is required for somatomotor response selection under nonoptimal learning conditions.

Efferent connections of the anteromedial nucleus of the thalamus of the rat

The projections from the anteromedial nucleus of the thalamus (AM) were investigated using anterograde and retrograde tracing techniques. AM projects to nearly the entire rostrocaudal extent of limbic cortex and to visual cortex. Anteriorly, AM projects to medial orbital, frontal polar, precentral agranular, and infraradiata cortices. Posteriorly, AM projects to retrosplenial granular, entorhinal, perirhinal and presubicular cortices, and to the subiculum. Further, AM projects to visual cortical area 18b, and to the lateral and basolateral nuclei of the amygdala. AM projections are topographically organized, i.e., projections to different cortical areas arise from distinct parts of AM. The neurons projecting to rostral infraradiata cortex (IRalpha) are more caudally located in AM than the neurons projecting to caudal infraradiata cortex (IRbeta). The neuronal cell bodies that project to the terminal field in area 18b are located primarily in ventral and lateral parts of AM, whereas neurons projecting to perirhinal cortex and amygdala are more medially located in AM. Injections into the most caudal, medial part of AM (i.e., the interanteromedial [IAM] nucleus) label terminals in the rostral precentral agranular, caudal IRbeta, and caudal perirhinal cortices. Whereas most AM axons terminate in layers I and V-VI, exceptions to this pattern include area 18b (axons and terminals in layers I and IV-V), the retrosplenial granular cortex (axons and terminals in layers I and V), and the presubicular, perirhinal, and entorhinal cortices (axons and terminals predominantly in layer V). Together, these findings suggest that AM influences a widespread area of limbic cortex.

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.

Intraseptal infusions of a low dose of AP5, a NMDA receptor antagonist, improves memory in an object recognition task in rats

The present study describes the effects of intraseptal microinjections of 2 nmol of AP5 upon memory of rats subjected to a two trial object recognition task. This task allows us to detect either a disruption or an improvement of memory according to the duration of the interval between the sample trial (T1) and the choice trial (T2). AP5 injected before T1 did not disrupt memory in a schedule able to detect an amnesia. In a schedule able to detect an improvement of memory, AP5 injected either 10 min before or just after T1, or 10 min before T2, improved retention. These results suggest that microinjection of a low dose of AP5 in the septum improves the acquisition, the consolidation and the restitution of the information in a working memory task.

Differential site-specific effects of parafascicular stimulation on active avoidance in rats

To study the effects of parafascicular intracranial electrical stimulation (PF ICS) on two-way active avoidance acquisition (five training sessions of ten trials each, one session per day) and long-term retention (one session of ten trials), two experiments were carried out. Experiment I tested if posttraining PF ICS can differentially affect the conditioning, depending on the stimulated region of the nucleus. Results indicated that rats stimulated at the posterior region of the parafascicular nucleus (PF) showed a better acquisition than those stimulated at the central one. Experiment II evaluated the effects of the stimulation at the medial, lateral and posterior parts of the PF area on the same task. Results showed that medial and lateral PF ICS disrupted two-way active avoidance, and that posterior PF ICS enhanced the long-term retention of the conditioning. These results suggest a possible role of the PF in modulatory processes of learning and memory, confirming that this nucleus is functionally heterogeneous. Potential facilitative effects are discussed in terms of the relations of the PF to the arousal system and the subparafascicular thalamic nucleus. Disruptive effects are discussed based on the relations of the PF with the 'motor' and 'associative-limbic' basal ganglia circuits.

The effects of frontal cortical lesions on remembering depend on the procedural demands of tasks performed in the radial arm maze

We trained 24 rats to perform an eight-arm radial maze task and then assigned them with a matching procedure to one of three treatments: sham surgery or lesions of the projection areas of the mediodorsal thalamic nucleus (MDn) in the medial wall (MW) or in both the MW and rhinal sulcal (RS) areas of frontal cortex. After recovery we trained the rats to perform six tasks, beginning with the standard eight-arm task, followed by two versions of a four forced choice procedure, and then three versions of a two-choice delayed-nonmatching-to-sample (DNMTS) task. The two lesion groups performed comparably on all tasks, showing that impairments were not exacerbated by extension of the MW lesion to include all cortical areas innervated by MDn. As in previous studies, frontal animals performed the radial maze task poorly immediately after surgery but improved with subsequent training. Controlling the order of the arm entries by opening the first four gates in a random sequence had little effect on performance, although frontal animals were impaired when lengthy delays (5 or 15 min) were imposed after the last of the four forced entries. Frontal animals were not impaired on two-choice DNMTS when the arms used for training were selected at random from the eight alternatives on a trial by trial basis, even when visual cues were eliminated by darkening the room and covering the maze. Frontal animals were significantly impaired when the selection of sample and choice arms was limited to the same two alternatives on every trial. This finding may explain the reported sensitivity of DNMTS to the effects of frontal lesions when training is carried out in operant chambers.

Effects of RU 52583, an alpha 2-antagonist, on memory in rats with excitotoxic damage to the septal area

The anti-amnesic action of RU 52583, an alpha 2-adrenergic receptor antagonist, was evaluated through performance of spatial tasks in a radial maze by rats with N-methyl-D-aspartic acid (NMDA) lesion of the medial septal (MS) nuclei. Memory performance of lesioned or sham-operated rats was evaluated by measuring reference memory as long-term maintenance of an acquired performance and working memory or memory for recent events. The lesion: a produced significant impairments of the animals' memory performance, b) significantly reduced the sodium-dependent high-affinity choline uptake in the hippocampal formation, and c) deeply disrupted cholinergic hippocampal theta waves. Oral administration of RU 52583 at 1 and 2 mg/kg (tested doses: 1-5 mg/kg) prior to performance of the task markedly reduced memory impairments, whereas idazoxan, another alpha 2-adrenergic receptor antagonist, had no effect at tested doses (2-5 mg/kg). Cholinergic drugs--arecoline at 0.1 and 1 mg/kg (tested doses: 0.05-1 mg/kg) and physostigmine at 0.02 and 0.1 mg/kg (tested doses: 1, 2, and 5 mg/kg)-administered intraperitoneally showed a tendency to alleviate memory deficits. The present results show that the alpha 2-adrenergic antagonist RU 52583 possesses cognition-enhancing properties in rats with damage to the septohippocampal system.

The effects of lesions to thalamic lateral internal medullary lamina and posterior nuclei on learning, memory and habituation in the rat

The behavioral effects of radiofrequency lesions to the lateral internal medullary lamina region (IML) or the posterior region (Po: containing the parafascicular and posterior nuclei) of the thalamus were compared to sham operated controls. Subjects were pre-operatively trained and then tested for post-operative retention of a NMTP task. Whereas the Po-lesion group was impaired only on long delays (60, 90 s), the IML-lesion group was impaired on retention and re-acquisition and demonstrated lower performance at all delays (5-90 s) of the NMTP task. Post-operative training and testing was conducted on three additional tasks: Morris water maze, acoustic startle, and passive avoidance. The IML-lesion group was impaired in finding a hidden and visual platform in the Morris water maze, demonstrated a blunted response but normal habituation to an acoustic startle stimulus, and showed normal retention of a passive avoidance task. On those three tasks, the performance of the Po-lesion group was similar to controls. In the IML-lesion group, neuronal loss resulting from axotomy and/or transneuronal degeneration was observed within nuclei of the midline and anterior thalamus and the mammillary body. These results suggest that lesions to the IML region disrupt a range of cognitive functions and produce pathological destruction in distant brain regions; whereas damage to the posterior thalamus causes spatial delay-sensitive deficits.

Ensemble codes involving hippocampal neurons are at risk during delayed performance tests

Multielectrode recording techniques were used to record ensemble activity from 10 to 16 simultaneously active CA1 and CA3 neurons in the rat hippocampus during performance of a spatial delayed-nonmatch-to-sample task. Extracted sources of variance were used to assess the nature of two different types of errors that accounted for 30% of total trials. The two types of errors included ensemble "miscodes" of sample phase information and errors associated with delay-dependent corruption or disappearance of sample information at the time of the nonmatch response. Statistical assessment of trial sequences and associated "strength" of hippocampal ensemble codes revealed that miscoded error trials always followed delay-dependent error trials in which encoding was "weak," indicating that the two types of errors were "linked." It was determined that the occurrence of weakly encoded, delay-dependent error trials initiated an ensemble encoding "strategy" that increased the chances of being correct on the next trial and avoided the occurrence of further delay-dependent errors. Unexpectedly, the strategy involved "strongly" encoding response position information from the prior (delay-dependent) error trial and carrying it forward to the sample phase of the next trial. This produced a miscode type error on trials in which the "carried over" information obliterated encoding of the sample phase response on the next trial. Application of this strategy, irrespective of outcome, was sufficient to reorient the animal to the proper between trial sequence of response contingencies (nonmatch-to-sample) and boost performance to 73% correct on subsequent trials. The capacity for ensemble analyses of strength of information encoding combined with statistical assessment of trial sequences therefore provided unique insight into the "dynamic" nature of the role hippocampus plays in delay type memory tasks.

Retrograde amnesia in rats with dorsomedial thalamic damage

The present research addresses the effects of dorsomedial thalamic lesions on remembering a delayed alternation problem learned before damage. Male rats were trained on a delayed alternation task. Successful performance of the test first required learning of the general principle that Go and No Go trials alternate with each other and, second, in every trial, recalling the specific consequences of the responses in the previous trial after a relatively short or long interval. The animals which had reached the learning criteria were paired by the learning level shown during the training and assigned semi-randomly to one unoperated control and one dorsomedial thalamic lesioned group. Lesions of the dorsomedial nucleus of thalamus produced a moderate and transient impairment of remembering of the delayed alternation learned before the damage for delays ranging from 0 to 40 s, and a severe impairment of the postoperative performance when delays were increased to 80 s. Detailed analysis of the animals' performance throughout the postoperative retraining confirms the role of the dorsomedial nucleus of the thalamus in memory and suggests that this thalamic region plays a role in recalling specific response-related events. Further research concerning the extension of retrograde amnesia associated with thalamic damage and the nature of the memory loss is proposed as necessary.

A comparison of the effects of frontal cortical and thalamic lesions on measures of spatial learning and memory in the rat

Two experiments were conducted to compare the effects of radiofrequency lesions of thalamus and frontal cortex on three measures of spatial learning and memory: delayed non-matching to sample (DNMTS), radial arm maze with imposed delays, and serial reversal learning. Thalamic lesions were aimed at the lateral internal medullary lamina (L-IML) and cortical lesions at the projection areas of the mediodorsal nucleus along the medial wall (MW) and dorsal to the rhinal sulcus (RS) in frontal cortex. In Experiment 1 rats were trained on DNMTS prior to surgery. After recovery, rats with MW lesions showed persistent deficits on DNMTS that were significantly greater than for RS lesions. The deficits observed in MW lesioned animals were comparable to the effects of L-IML lesions on this task that have been described in previous studies. In Experiment 2 animals were trained to perform the radial arm maze task prior to treatment. After recovery, animals with L-IML lesions were impaired on the radial arm maze and on subsequent acquisition of the serial reversal task. Rats with RS and MW lesions showed transient impairments on the radial arm maze task, but otherwise performed as well as controls on both these tasks.

Projections from the anterodorsal and anteroventral nucleus of the thalamus to the limbic cortex in the rat

The present study characterized the projections of the anterodorsal (AD) and the anteroventral (AV) thalamic nuclei to the limbic cortex. Both AD and AV project to the full extent of the retrosplenial granular cortex in a topographic pattern. Neurons in caudal parts of both nuclei project to rostral retrosplenial cortex, and neurons in rostral parts of both nuclei project to caudal retrosplenial cortex. Within AV, the magnocellular neurons project primarily to the retrosplenial granular a cortex, whereas the parvicellular neurons project mainly to the retrosplenial granular b cortex. AD projections to retrosplenial cortex terminate in very different patterns than do AV projections: The AD projection terminates with equal density in layers I, III, and IV of the retrosplenial granular cortex, whereas, in contrast, the AV projections terminate very densely in layer Ia and less densely in layer IV. Further, both AD and AV project densely to the postsubicular, presubicular, and parasubicular cortices and lightly to the entorhinal (only the most caudal part) cortex and to the subiculum proper (only the most septal part). Rostral parts of AD project equally to all three subicular cortices, whereas neurons in caudal AD project primarily to the postsubicular cortex. Compared to AD, neurons in AV have a less extensive projection to the subicular cortex, and this projection terminates primarily in the postsubicular and presubicular cortices. Further, the AD projection terminates in layers I, II/III, and V of postsubiculum, whereas the AV projection terminates only in layers I and V.

Mediodorsal thalamic lesions impair acquisition of an eyeblink avoidance response in rabbits

Rabbits received ibotenic acid lesions of the mediodorsal nucleus of the thalamus (MD) or sham lesions. These animals were compared on four sessions of instrumental avoidance conditioning, during which an eyeblink (EB) response during the presentation of a tone-conditioned stimulus prevented the occurrence of a paraorbital electric shock unconditioned stimulus. Lesions of MD retarded acquisition of the EB avoidance response, but did not affect asymptotic performance. Concomitant heart rate (HR) changes were little affected by MD lesions, although there was some evidence that such changes were slightly larger in MD-lesioned animals. These results suggest that MD participates in some general aspect of the learning process, perhaps by affecting other behavioral processes such as 'attention' or 'arousal'.

On the role of thalamic pathology in diencephalic amnesia

Although it is now accepted that medial diencephalic lesions can produce severe amnesia in humans, the specific nuclei and neural pathways that must be damaged to impair memory have not yet been identified. Recent studies have shown that pyrithiamine-induced thiamine deficiency (PTD) in the rat can produce a consistent pattern of pathology in the thalamus and mammillary bodies and result in permanent impairments on behavioral measures of working memory. Behavioral deficits comparable to the PTD model have been observed in rats with thalamic lesions involving lateral portions of the internal medullary lamina (the L-IML site). Such impairments are not observed following lesions of limbic-related pathways associated with the fornix, mammillary bodies, or midline thalamus. The L-IML lesion affects the mediodorsal nucleus (MDn) and both the intralaminar and paralaminar non-specific thalamic nuclei. The relationship between the non-specific thalamic nuclei and working memory is underscored by the limited behavioral effects of MDn lesions, as compared to either L-IML or PTD-induced lesions, and by anatomical analyses of PTD-related pathology, which seems to destroy the non-specific nuclei while sparing large portions of the MDn. Recent physiological studies of thalamocortical processes suggest that there are several possible mechanisms by which the non-specific nuclei might participate in memory and by which lesions in these pathways might interfere with the consolidation of memories within the cortex.

Effects of adrenal medulla grafts on memory capacities of rats after hippocampal lesions

Behavioral and immunocytochemical techniques were used to study the effects of adrenal medulla grafts implanted in hippocampus--after lesion of this structure--on the memory capacities of rats. Performances of the grafted rats in the radial maze test were significantly improved and, in some aspects, fully restored. On the other hand, grafts had no significant effects on a one-trial spatial recognition test and impaired object recognition. Immunocytochemical identification showed that the grafts contained chromaffin cells with a choline acetyltransferase stainings while, in parallel, phenylethanolamine-N-methyltransferase stainings seemed to be decreased. Cholinergic innervation was established between the graft and the host hippocampus. A likely interpretation of this complex pattern of results is that the functional effects of the grafts depended on the arousal level induced by the behavioral task. At the neurobiological level, these effects probably set into play an interaction between opioid, catecholaminergic and cholinergic factors. Our results may contribute to the clarification of the problem of specificity of functional effects of intracerebral grafts as well as the problem of hippocampal role in learning and memory.

Covariations between hippocampal mossy fibres and working and reference memory in spatial and non-spatial radial maze tasks in mice

Male mice from nine inbred mouse strains were tested at the age of 3 months in either a spatial or a non-spatial version of the radial maze. Only four out of eight arms contained food rewards, permitting simultaneous assessment of working and reference memory in both situations. Other animals from the same strains were processed histologically to estimate the strain-specific extents of the mossy fibre projections. No significant between-task correlations were obtained for either working or reference memory. However, measures of working and reference memory correlated with each other within tasks. This suggests that these concepts may perhaps not be validly used in the mouse. Large, positive correlations of the size of the intra- and infrapyramidal mossy fibre projection with both working and reference memory were obtained in the spatial radial maze task, but not in the non-spatial one. We conclude that heritable variations of the hippocampal intra- and infrapyramidal mossy fibre projection influence processes related to spatial learning capabilities in radial mazes.

The effects of fornix and medial prefrontal lesions on delayed non-matching-to-sample by rats

The present study compared the effects of fornix lesions and medial prefrontal lesions on a test of object recognition memory, delayed non-matching-to-sample. Neither lesion impaired the acquisition of this non-spatial test of working memory, indeed there was clear evidence that fornix damage resulted in improved non-matching performance during initial acquisition. This improvement in performance could be related to the loss of a spatial bias during the early stages of training. A series of experiments then systematically increased the familiarity of the stimuli (i.e. testing recency rather than recognition judgements). Neither the fornix nor the prefrontal lesions disrupted performance under these conditions, even though this manipulation affected nonmatching in a predictable manner. The same animals were also tested on a spatial forced-alternation task in a T-maze (spatial delayed non-matching-to-sample). The animals with fornix lesions performed at chance while the prefrontal animals were mildly, but significantly, impaired. The present findings are considered in the light of a number of seemingly contradictory findings regarding the effects of hippocampal system damage on nonspatial tests of recognition memory.

Spatial but not object recognition is impaired by aging in rats

Four groups of male Wistar rats (4, 8, 18, and 24 months) were submitted to object recognition and spatial recognition tests. Object recognition was not significantly affected by aging even at a longer retention interval. On the contrary, spatial recognition was significantly impaired in 18- and 24-month groups. The existence of two dissociable neural systems, respectively involved in object and spatial recognition, is discussed.

The Roman strains of rats as a psychogenetic tool for pharmacological investigation of working memory: example with RU 41656

This study examined the effects of RU 41656, a dopaminergic D2 agonist, on the differential working memory performances and on the differential activities of the neurochemical systems of the Roman high (RHA) and Roman low (RLA) avoidance strains of rats. Compared with RLA, RHA performed worse in three tests of working memory (spontaneous alternation, radial maze and object recognition) and had higher levels of exploratory locomotor activity. Hippocampal and frontal cortex choline acetyltransferase (ChAT) activities were lower in RHA. Frontal cortex DA and DOPAC levels, hippocampal and striatal 5-HT and NA levels were higher in RHA. RU 41656 induced a significant improvement in working memory performance of RHA, whereas in RLA it had no effect. It decreased exploratory locomotor activity in both strains. ChAT activity in hippocampus was not affected by RU 41656 in either strain, whereas in frontal cortex it was increased in RHA but not in RLA. Hippocampal NA levels were decreased by RU 41656 in RHA but not in RLA. These results confirm previous data concerning the promnesic effect of RU 41656 and extend the finding that the Roman strains are a psychogenetic model for the behavioural, neurochemical and psychopharmacological study of the working memory in rats.

Roman strains as a psychogenetic model for the study of working memory: behavioral and biochemical data

Performances of male rats of the Roman High- (RHA), Roman Control- (RCA) and Roman Low- (RLA) Avoidance strains were compared in two working memory tests, a spatial one, the radial maze, and a nonspatial one, an object recognition test. The same rats were subjected to measures of emotional reactivity and of different forms of motor activity and finally to measures of cholinergic and aminergic activities in the hippocampus, frontal cortex and striatum. Compared to RHA, RLA performed better in the two working memory tests, displayed "anxiety" and had also lower levels of exploratory locomotor activity. Hippocampal ChAT activity was higher in RLA than in RHA. Levels of DA and DOPAC in the striatum were higher in RLA compared to RHA, whereas in the frontal cortex they were lower. For most of these measures, RCA were intermediate between RLA and RHA. These results confirm and extend the finding that the Roman strains are not only a genetic model for two-way avoidance conditioning but also for working memory.