The effects of lesions to the fornix and dorsomedial thalamus on concurrent discrimination learning by rats

Disrupting direct inputs from the dorsal subiculum to the granular retrosplenial cortex impairs flexible spatial memory in the rat

In a changing environment, animals must process spatial signals in a flexible manner. The rat hippocampal formation projects directly upon the retrosplenial cortex, with most inputs arising from the dorsal subiculum and terminating in the granular retrosplenial cortex (area 29). The present study examined whether these same projections are required for spatial working memory and what happens when available spatial cues are altered. Consequently, injections of iDREADDs were made into the dorsal subiculum of rats. In a separate control group, GFP-expressing adeno-associated virus was injected into the dorsal subiculum. Both groups received intracerebral infusions within the retrosplenial cortex of clozapine, which in the iDREADDs rats should selectively disrupt the subiculum to retrosplenial projections. When tested on reinforced T-maze alternation, disruption of the subiculum to retrosplenial projections had no evident effect on the performance of those alternation trials when all spatial-cue types remained present and unchanged. However, the same iDREADDs manipulation impaired performance on all three alternation conditions when there was a conflict or selective removal of spatial cues. These findings reveal how the direct projections from the dorsal subiculum to the retrosplenial cortex support the flexible integration of different spatial cue types, helping the animal to adopt the spatial strategy that best meets current environmental demands.

Mediodorsal but not anterior thalamic nuclei lesions impair acquisition of a conditional discrimination task

The limbic thalamus is a heterogeneous structure with distinctive cortical connectivity. A recent review suggests that the mediodorsal thalamic nucleus (MD), unlike the anterior thalamic nuclei (ATN), may be involved in selecting relevant information in tasks relying on executive functions. We compared the effects of excitotoxic lesions of the MD or the ATN on the acquisition of a simple conditional discrimination in rats. When required to choose from two levers according to auditory or visual cues, ATN rats and sham-lesioned rats performed to the same levels and displayed similar acquisition curves. Under the same conditions, MD rats' acquisition of the task was markedly delayed. This group nevertheless attained nearly normal performances after more extensive training. Furthermore, all rats learned reversal of the original discrimination at the same rate. These results highlight functional specialization within the limbic thalamus and support the notion that MD contributes to the identification of relevant dimensions in conditional tasks during the initial stages of acquisition.

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.

The importance of the rat hippocampus for learning the structure of visual arrays

It has been assumed that the integrity of the rodent hippocampus is required for learning the spatial distribution of visual elements in an array. Formally assessing this assumption is, however, far from straightforward as standard tests are amenable to alternative strategies. In order to provide a stringent test of this ability rats were trained on three concurrent visual discriminations in a water tank in which the stimuli in each pair of discriminations contained exactly the same elements but they differed in their spatial arrangement e.g. A|B vs. its mirror image B/A. Such 'structural' discriminations are a specific subtype of 'configural' or 'nonlinear' tasks. Following acquisition half of the rats received hippocampal lesions and all rats were retrained on the structural discriminations. Hippocampal lesions impaired the ability to relearn these 'structural' discriminations. In contrast, two other groups of rats with similar hippocampal lesions showed no impairment on relearning two non-structural, configural discriminations: transverse patterning and biconditional learning. All three tasks used the same apparatus, the same stimulus elements, and similar training regimes. Superior performance by the rats with hippocampal lesions during a generalization decrement probe showed that hippocampal lesions had diminished sensitivity to 'structural' features on the biconditional task. While the rat hippocampus need not be required for all configural learning, it is important for the special case when the spatial arrangements of the elements are critical. This ability may be a prerequisite for the creation of mental snapshots, which underlie episodic memory.

Differential effect of thalamic and cortical lesions on memory systems in the rat

The behavioral effects of lesions of anterior thalamic and medio-dorsal thalamic nuclei, posterior cingulate (retrosplenial) and posterior parietal associative cortex were studied in rats performing a eight-arm radial maze, in which three/eight arms were baited. Lesions were made after rats reached a training criterion. Rats were tested 2 weeks later, in the same experimental apparatus. In the first experiment, performance was assessed by number of errors and time per trial, retention by comparing performance during training with retraining sessions (using specific scores for reference and working memory) and by an evaluation of the 'reminiscence' defined as daily improvement in performance. After the reacquisition of the task, the rats were tested in experiment II, in modified situations for an evaluation of flexible aspects of memory processing in this spatial task. Results showed dissociation between the effects of the different lesions, according to the specific demands of the different tasks. Posterior parietal lesions produced significant, but relatively mild deficits, in all situations, in accordance with the well-established function of this cortical region in spatial tasks. In contrast, after cortical lesion in the retrosplenial region, performance deficit was only observed on priming and attention to contextual change. Lesions of the thalamic associative nuclei also induced task-specific deficits. Medio-dorsal lesions induced mild and reversible deficits in complex tasks only, with preservation of working memory, of priming effect, and of a novel acquisition. Rats with anterior thalamic lesions had massive deficits across tasks, probably due to basic difficulties with reference and working memory, demonstrated no benefit from a priming session but considerable interference from the previous training to a new one. These results are discussed within framework of the specificity of different cerebral regions for behavioural adaptation and plasticity.

The involvement of the mediodorsal nucleus of the thalamus and the midbrain extrapyramidal area in locomotion elicited from the ventral pallidum

Motor activity is regulated by projections from the nucleus accumbens to the ventral pallidum, but it is unclear which efferents regulate behavioral output from the ventral pallidum. Motor activity was elicited pharmacologically by microinjecting either the mu opioid receptor agonist, Tyr-D-Ala-Gly-NmePhe-Gly-OH (DAMGO) or the glutamate receptor agonist, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) into the ventral pallidum. The involvement of efferent projections was determined by microinjecting the local anesthetic procaine into the mediodorsal nucleus of the thalamus (MD) or the midbrain extrapyramidal area (MEA) prior to administering DAMGO or AMPA into the ventral pallidum. The motor activity induced by DAMGO was blocked by procaine microinjected into either the MD or the MEA. In contrast, procaine microinjected into the MD did not block motor activity elicited by AMPA while procaine into the MEA abolished the behavioral activation. These data indicate that the involvement of efferent projections from the ventral pallidum to either the MD or MEA in motor activation depends upon the type of receptor stimulated in the ventral pallidum.

Glutamatergic and dopaminergic afferents to the prefrontal cortex regulate spatial working memory in rats

The integrity of the prefrontal cortex is critical for the expression of working memory. The prefrontal cortex is innervated by dopaminergic afferents from the ventral tegmental area and glutamatergic afferents from the mediodorsal thalamus. To determine the role of dopaminergic and glutamatergic afferents in the regulation of working memory, rats were trained to perform a spatial delayed alternation task in a T-maze. The microinjection of the ionotropic glutamate antagonists 6-cyano-7-nitroquinoxaline-2,3-dione or 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid into the prefrontal cortex impaired working memory. Consistent with a role for glutamate receptor activation, microinjecting the GABA(B) agonist baclofen into the mediodorsal thalamus produced a dose-dependent disruption of working memory. In contrast, inhibition of the mesocortical dopamine projection was without effect on working memory. The blockade of D1 and/or D2 dopamine receptors with SCH-23390 and sulpiride was without effect on working memory. Likewise, the microinjection of baclofen into the ventral tegmental area did not impair working memory. However, stimulating mu-opioid receptors in the ventral tegmental area with [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin produced a dose-dependent impairment of working memory that was reversed by blocking D1 dopamine receptors with SCH-23390 in the prefrontal cortex. These data demonstrate that increased dopamine tone or reduced glutamate tone in the prefrontal cortex disrupts working memory in a spatial delayed alternation task.

Involvement of the pallidal-thalamocortical circuit in adaptive behavior

Interconnections among the ventral mesencephalon, nucleus accumbens, and ventral pallidum are critical in the initiation of adaptive behavioral responses to environmental stimuli. Within this circuit are two highly topographically organized subcircuits that are differentially interconnected with limbic and motor circuitry in the brain. However, there is not a great deal of anatomical interconnection between the limbic and motor subcircuits. A polysynaptic connection between the two subcircuits involves projections from the limbic ventral pallidum to the mediodorsal thalamus to the prefrontal cortex back to the motor regions of the nucleus accumbens. In the present report we show that this connection is critical in the expression of motor behavior elicited by opioids and the capacity of a rat to perform in a task requiring spatial working memory.

An examination of the spatial working memory deficit following neurotoxic medial dorsal thalamic lesions in rats

The present study examined the performance of rats with neurotoxic lesions centred in the thalamic nucleus medialis dorsalis on standard and modified versions of the eight arm radial maze test. In Experiment 1, the thalamic lesions produced a borderline deficit in acquisition of the standard task, but subsequently had no effect when a delay was interposed after the first four arms had been entered. The same lesions had no effect on T-maze alternation, but they did impair radial-arm maze performance when intramaze and extramaze cues were set against each other. In Experiment 2, lesions of the dorsomedial thalamus impaired acquisition of the standard radial-arm maze task, but combining the results from Experiments 1 and 2 showed that this acquisition deficit was confined to those animals in which bilateral damage extended into the adjacent anterior thalamic nuclei. In addition, lesions of the dorsomedial thalamus disrupted radial-arm maze performance when the task was modified to compare working memory and reference memory and increased activity and exploration. These changes were not associated with anterior thalamic damage. Finally, the thalamic lesions did not affect performance on a test of spontaneous object recognition. It is concluded that lesions of medialis dorsalis do not disrupt spatial memory but do affect other processes that can interact with task performance. These include a failure of extramaze cues to overshadow intramaze cues, a change in activity and exploration levels and deficits in with-holding spatial responses.

Present imperfect: a critical review of animal models of the mnemonic impairments in Alzheimer's disease

This paper reviews the current literature on animal models of the memory impairments of Alzheimer's disease (AD). The authors suggest that modeling of the mnemonic deficits in AD be limited to the amnesia observed early in the course of the disease, to eliminate the influence of impairments in non-mnemonic processes. Tasks should be chosen for their specificity and selectivity to the behavioral phenomena observed in early-stage AD and not for their relevance to hypothetical mnemonic processes. Tasks that manipulate the delay between learning and remembering are better able to differentiate Alzheimer patients from persons with other disorders, and better able to differentiate effects of manipulations in animals. The most commonly used manipulations that attempt to model the amnesia of AD are reviewed within these constraints. The authors conclude that of the models examined, lesions of the medial septal nucleus produce behavioral deficits that are most similar to the mnemonic impairments in the earliest stage of AD. However, the parallel is not definitive and more work is needed to clarify the relationship between neurobiology and behavior in AD.

Chronic intraventricular infusion with NGF improves LTP in old cognitively-impaired rats

Aged (21 months) cognitively-impaired male Sprague-Dawley rats received intraventricular infusion of nerve growth factor (NGF) or cytochrome C (Cit C) for 14 or 28 days using miniosmotic pumps and were evaluated either 1 week or 3 months after treatment. Groups of untreated young, aged-impaired and aged non-impaired rats were also evaluated. Under narcose recording and stimulating electrodes were stereotactically implanted in the dentate gyrus and the perforant path. The stimulation intensity was individually adjusted to obtain a half-maximal population spike (P) for test stimuli and a quarter-maximal for tetanization. The amplitude and latency of P and the slope (S) of the field EPSP were determined before and at 2, 5, 15, 30 and 60 min after tetanization at 400 Hz. Paired stimuli at 30 ms interval were also applied before and after tetanization. Aged, cognitively impaired rats showed an absent S potentiation and a delayed P potentiation, both in amplitude and latency, while non-impaired rats behaved like the young controls. Paired pulse inhibition showed no difference among groups before or after tetanization suggesting that the impaired potentiation is not due to an increased retroactive inhibition. NGF treatment ameliorates LTP deficits to levels equivalent to non-impaired rats, while Cit C controls showed no improvement. No differences appear among NGF treated groups, but evidence suggest that the animals evaluated 3 months after treatment developed a stronger potentiation.

Expression of the mRNAs encoding the limbic system-associated membrane protein (LAMP): I. Adult rat brain

The search for molecular markers common to neural structures that are functionally related has become an attractive strategy for neurobiologists interested in identifying mechanisms involved in the formation of patterned connections. One such molecule is the limbic system-associated membrane protein (LAMP), a 64-68 kDa glycoprotein that is expressed in the soma and dendrites of subpopulations of adult neurons in the brain that are functionally associated with classic limbic structures. Such patterned molecular specificity is established prenatally; LAMP is detected during development on the surface of neurons, axonal membranes and pathfinding growth cones. This molecule has now been cloned (lamp) and has been shown to be highly conserved in rat and human. It is a new immunoglobulin superfamily member that has three Ig domains and a glycosyl-phosphatidylinositol (GPI) anchor to the cell membrane. In this study, the distribution of the lamp transcript in the adult rat brain was determined by using in situ hybridization. Generally, the distribution of lamp corresponds well with that of the LAMP protein. Within the cerebral cortex, the transcript is more abundant in areas that are associated with learning/memory and viscerosensory tasks. It is less abundant in somatic sensory and motor areas. The lamp transcript is also ubiquitous in the basal forebrain, amygdala, and preopticohypothalamic areas. In short, the lamp transcript is expressed heavily in areas of the forebrain and diencephalon that have been classically considered limbic and sparsely or moderately in nonlimbic midbrain and hindbrain regions. Correlative analysis of the connectivity patterns of the regions that express greater amounts of the transcript is consistent with a stronger limbic-associated function relative to the regions expressing less lamp. These quantitative differences may be significant in determining the function of LAMP in the adult brain.

Effects of aspiration lesions of hippocampus or overlying neocortex on concurrent and configural object discriminations in rats

Rats with aspiration lesions of the hippocampus plus overlying neocortex or control lesions of this cortex alone were trained on five-pair concurrent object discriminations in an enclosed Y-maze and subsequently on an open maze. Acquisition of the former task was impaired only in rats with cortical lesions, but on the latter both groups were equally impaired. Recombining positive and negative stimuli into novel pairs did not disrupt performance. Acquisition of single-pair discriminations was normal or slightly impaired in lesion groups. The cortical, but not the hippocampal group, was impaired on the concurrent learning of a positive and a negative pattern configural task. Recombining stimuli did not impair performance of this task either. The study demonstrates that extensive damage to the hippocampal formation need not necessarily impair concurrent learning, and impairments seen in other studies may relate to details of experimental procedure.