The effects of radiofrequency lesion or transection of the fimbria-fornix on latent inhibition in the rat

Behavioral facilitation after hippocampal lesion: A review

When parts of the brain suffer from damage, certain functional deficits or impairments are the expected and typical outcome. A myriad of examples show such negative consequences, which afford the daily tasks of neurologists, neuropsychologists, and also behavioral neuroscientists working with experimental brain lesions. Compared to lesion-induced deficits, examples for functional enhancements or facilitation after brain lesions are rather rare and usually not well studied. Here, the mammalian hippocampus seems to provide an exception, since substantial evidence shows that its damage can have facilitatory behavioral effects under certain conditions. This review will address these effects and their possible mechanisms. It will show that facilitatory effects of hippocampal lesions, although mostly studied in rats, can be found in many mammalian species, that is, they are apparently not species-specific. Furthermore, they can be found with various lesion techniques, from tissue ablation, to neurotoxic damage, and from damage of hippocampal structure itself to damage of fiber systems innervating it. The major emphasis of this review, however, lies on the behavioral effects and their interpretations. Thus, facilitatory effects can be found in several learning paradigms, especially active avoidance, and some forms of Pavlovian and instrumental conditioning. These will be discussed in light of pertinent theories of hippocampal function, such as inhibition, spatial cognition, and multiple memory systems theories, which state that facilitatory effects of hippocampal lesions may reflect the loss of interference between hippocampal spatial and striatal procedural cognition. Using the example of the rat sequential reaction time task, it will also be discussed how such lesions can have direct and indirect consequences on certain behavioral readouts. A final note will advocate considering possible functional facilitation also in neurologic patients, especially those with hippocampal damage, since such a strategy might provide new avenues for therapeutic treatments.

Hippocampus and two-way active avoidance conditioning: Contrasting effects of cytotoxic lesion and temporary inactivation

Hippocampal lesions tend to facilitate two-way active avoidance (2WAA) conditioning, where rats learn to cross to the opposite side of a conditioning chamber to avoid a tone-signaled footshock. This classical finding has been suggested to reflect that hippocampus-dependent place/context memory inhibits 2WAA (a crossing response to the opposite side is inhibited by the memory that this is the place where a shock was received on the previous trial). However, more recent research suggests other aspects of hippocampal function that may support 2WAA learning. More specifically, the ventral hippocampus has been shown to contribute to behavioral responses to aversive stimuli and to positively modulate the meso-accumbens dopamine system, whose activation has been implicated in 2WAA learning. Permanent hippocampal lesions may not reveal these contributions because, following complete and permanent loss of hippocampal output, other brain regions may mediate these processes or because deficits could be masked by lesion-induced extra-hippocampal changes, including an upregulation of accumbal dopamine transmission. Here, we re-examined the hippocampal role in 2WAA learning in Wistar rats, using permanent NMDA-induced neurotoxic lesions and temporary functional inhibition by muscimol or tetrodotoxin (TTX) infusion. Complete hippocampal lesions tended to facilitate 2WAA learning, whereas ventral (VH) or dorsal hippocampal (DH) lesions had no effect. In contrast, VH or DH muscimol or TTX infusions impaired 2WAA learning. Ventral infusions caused an immediate impairment, whereas after dorsal infusions rats showed intact 2WAA learning for 40-50 min, before a marked deficit emerged. These data show that functional inhibition of ventral hippocampus disrupts 2WAA learning, while the delayed impairment following dorsal infusions may reflect the time required for drug diffusion to ventral hippocampus. Overall, using temporary functional inhibition, our study shows that the ventral hippocampus contributes to 2WAA learning. Permanent lesions may not reveal these contributions due to functional compensation and extra-hippocampal lesion effects.

The marginal division of the striatum and hippocampus has different role and mechanism in learning and memory

The memory function of the hippocampal formation (Hip) and the marginal division (MrD) of neostriatum was compared. Rats with bilateral lesions of the MrD either immediate or 24 h after training in Y-maze were found to have decrease in correct runs in both groups. However, animals with transected afferent and efferent nerve bundles to isolate the Hip immediately or 24 h after training in Y-maze were found to show a decrease in correct runs only in the group injured immediately after Y-maze training but not in the 24 h group suggesting that MrD is likely involved in the entire process of long-term memory consolidation whereas the Hip only contributes to memory in the early stage. In addition, animals treated with a NMDA receptor (NMDAR) blocker, e.g. MK-801, showed decreased correct runs in Y-maze test and in expression level of phosphorylated CREB (pCREB) in neurons of the MrD but not in the Hip. Furthermore, animals treated with okadaic acid (OA), a potent protein phosphatase 1 inhibitor, showed increased correct runs in the Y-maze test. The expression level of pCREB and c-Fos and c-Jun was found increased in neurons of the MrD and the Hip in response to OA treatment. In conclusion, NMDAR and pCREB are involved in memory functions of both the Hip and the MrD. NMDAR might regulate pCREB level in neurons of the MrD but not in the Hip. Hence, the processes and mechanism of learning and memory involved in the MrD and the Hip may be different.

Identification of central projections from amylin-activated neurons to the lateral hypothalamus

The ability of the pancreatic hormone amylin to inhibit food intake relies on a direct activation of the area postrema (AP). This activation is synaptically transmitted to the nucleus of the solitary tract (NTS), the lateral parabrachial nucleus (LPB), the central amygdaloid nucleus (Ce) and the lateral bed nucleus of stria terminalis (BSTL). Interestingly, neurons of the rostro-dorsal lateral hypothalamic area (dLHA), which are activated during fasting, are inhibited by peripheral amylin, although they lack amylin receptors. Using the retrograde tracer cholera toxin-B (Ctb) we analyzed whether the dLHA receives neuronal projections from amylin-activated brain areas. The anterograde tracer biotinylated dextran-amine (BDA) was used to confirm the projections and to identify further neuronal pathways potentially involved in amylin signaling. We identified dense projections from the amylin activated neurons in the LPB and sparse projections from the NTS to the dLHA. LPB fiber efferents were found in close proximity to dLHA nuclei activated by 24h of fasting. The AP and the Ce showed no projections to the dLHA. Dense efferents were also observed from the LPB to other hypothalamic areas, namely to the ventromedial, dorsomedial, paraventricular and arcuate nuclei. This study provides neuroanatomical evidence that among the amylin activated areas, the LPB provides the strongest input to the dLHA, thus it may mediate the amylin-induced inhibition of the dLHA.

Using the MATRICS to guide development of a preclinical cognitive test battery for research in schizophrenia

Cognitive deficits in schizophrenia are among the core symptoms of the disease, correlate with functional outcome, and are not well treated with current antipsychotic therapies. In order to bring together academic, industrial, and governmental bodies to address this great 'unmet therapeutic need', the NIMH sponsored the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative. Through careful factor analysis and consensus of expert opinion, MATRICS identified seven domains of cognition that are deficient in schizophrenia (attention/vigilance, working memory, reasoning and problem solving, processing speed, visual learning and memory, verbal learning and memory, and social cognition) and recommended a specific neuropsychological test battery to probe these domains. In order to move the field forward and outline an approach for translational research, there is a need for a "preclinical MATRICS" to develop a rodent test battery that is appropriate for drug development. In this review, we outline such an approach and review current rodent tasks that target these seven domains of cognition. The rodent tasks are discussed in terms of their validity for probing each cognitive domain as well as a brief overview of the pharmacology and manipulations relevant to schizophrenia for each task.

Learning the morphine conditioned cue preference: Cue configuration determines effects of lesions

The morphine conditioned cue preference was investigated using two different apparatus configurations. In one configuration, with a clear Plexiglas partition separating the drug-paired and unpaired compartments, rats could see the cues in both compartments while in either one. In the other configuration, with an opaque wood partition separating the two compartments, rats could see the cues in only one compartment at a time. The experiment had three phases: a session of pre-exposure to the entire apparatus; four 2-day training trials during each of which rats received pairings of 5 mg/Kg morphine sulphate with one compartment and saline with the other (compartments and order counterbalanced), and a test session in which the undrugged rats moved freely between the compartments while the time spent in each was measured. Four groups of rats were trained using the opaque partition in all three phases. Normal rats and rats with amygdala or nucleus accumbens lesions exhibited preferences for their morphine-paired compartments; rats with fimbria-fornix lesions had no preferences. Four additional groups were trained using the clear partition during pre-exposure, the opaque partition during training and the clear partition during testing. Normal rats and rats with fimbria-fornix lesions exhibited preferences, rats with amygdala or nucleus accumbens lesions had no preferences. This interaction between lesioned structures and the apparatus configuration is accounted for by the idea that different types of learning produced the preference for morphine-paired cues in the two apparatus configurations. Each type was learned in a different memory system and so was impaired by different lesions. These findings contribute to understanding the nature of the learning processes that produce the morphine CCP.

The selective Na+-Ca2+ exchange inhibitor attenuates brain edema after radiofrequency lesion in rats

2-[4-[(2,5-Difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400), a specific inhibitor of the Na+-Ca2+ exchanger, exerts cytoprotective action and reduces brain infarct volume after cerebral ischemia. We examined the effect of SEA0400 on vasogenic brain edema in rats. Histological observations showed that radiofrequency current caused brain infarct and extravasation of endogenous albumin in the brain. SEA0400 (3 and 10 mg/kg, i.v.) significantly suppressed the increase in brain water content with attenuation of Evans blue dye and sodium fluorescein extravasation after radiofrequency lesion. The findings suggest that the Na+-Ca2+ exchanger plays a role in vasogenic edema formation after radiofrequency lesion.

Effects of fimbria-fornix, hippocampus, and amygdala lesions on discrimination between proximal locations

The conditioned cue preference (CCP) task was used to study the information required to discriminate between spatial locations defined by adjacent arms of an 8-arm radial maze. Normal rats learned the discrimination after 3 unreinforced preexposure (PE) sessions and 4 food paired-unpaired training trials. Fimbria-fornix lesions made before, but not after, PE, and hippocampus lesions made at either time, blocked the discrimination, suggesting that the 2 structures processed different information. Lateral amygdala lesions made before PE facilitated the discrimination. This amygdala-mediated interference with the discrimination was the result of a conditioned approach response that did not discriminate between the 2 arm locations. A hippocampus/fimbria-fornix system and an amygdala system process different information about the same learning situation simultaneously and in parallel.

The ventral hippocampal regulation of prepulse inhibition and its disruption by apomorphine in rats are not mediated via the fornix

Prepulse inhibition (PPI) of startle is a measure of sensorimotor gating that is impaired in schizophrenia. We have reported that PPI is regulated by the ventral hippocampus (VH) and that the PPI disruptive effects of the dopamine agonist apomorphine are enhanced 4 weeks after excitotoxic lesions of the VH. The mechanisms responsible for the VH influence on PPI are not understood, but have been ascribed to interactions between the VH and nucleus accumbens. In the present study, we examined whether the VH influence on PPI and its dopaminergic regulation is dependent on the integrity of the VH-accumbens projection via the fornix. First, the PPI-disruptive effects of intra-VH NMDA infusion were assessed after sham or electrolytic transection of the fornix. Second, the PPI-disruptive effects of apomorphine were assessed 1 month after excitotoxic or electrolytic lesions of the VH, or after fornix transection. Intra-VH N-methyl-D-aspartate infusion significantly disrupted PPI; this effect was unaffected by fornix lesions. The PPI-disruptive effects of apomorphine were significantly enhanced by excitotoxic or electrolytic lesions of the VH, but not by fornix transection. The influence of the VH on PPI and its dopaminergic regulation does not appear to be mediated via the fornix. The enhanced sensitivity to the PPI-disruptive effects of apomorphine after VH lesions is not dependent on excitotoxin-induced changes in the VH or its downstream projections.

Latent inhibition is spared by n-methyl-d-aspartate (nmda)-induced ventral hippocampal lesions, but is attenuated following local activation of the ventral hippocampus by intracerebral nmda infusion

Repeated non-reinforced exposures of a neutral stimulus retard the development of a conditioned response to that stimulus when it is subsequently paired with a significant event. This stimulus pre-exposure effect is known as latent inhibition (LI). Early lesion studies have initially suggested an important role for the hippocampus in the normal development and expression of LI. This view has since been modified with the emergence of data derived from selective cell body lesions of the hippocampus and of the entorhinal cortex, with an abolition of LI only seen after lesions of the latter. This suggests that the significance of the hippocampus might have been overestimated in the past, possibly due to interruption of fibres en passage. However, intact behavioural expression of LI following hippocampal damage does not preclude the suggestion that the hippocampus participates in the control and regulation of LI expression in intact animals. The present study demonstrated that whilst cell body lesions of the ventral hippocampus spared LI (as expected), chemical activation of the ventral hippocampus by local N-methyl-D-aspartate infusion disrupted LI. These results parallel our earlier observations on prepulse inhibition (PPI) with similar manipulations [Neuroreport 10 (1999) 2533]. Thus, although the ventral hippocampus is itself not responsible for the behavioural manifestation of LI and PPI, it exerts at least a modulatory control over the form and/or magnitude of their expression. Our results should prompt a re-evaluation of the relative roles of the hippocampus and retrohippocampus in the development and expression of LI.

GABA transmission in the ventral pallidum is not involved in the control of latent inhibition in the rat

Latent inhibition describes a process of learning to ignore stimuli of no consequence, and is disrupted in acute, positive-symptomatic schizophrenia. Understanding the neural basis of latent inhibition in animals may help to elucidate the neural dysfunction underlying positive schizophrenic symptoms in man. Evidence suggests a crucial role for dopamine transmission in the nucleus accumbens in the control of latent inhibition. The present studies investigated the role of the GABA-ergic efferent from the nucleus accumbens to the ventral pallidum in latent inhibition. The GABA(A) agonist muscimol (4.56 ng/microl), and antagonist picrotoxin (0.2 microg/microl), were infused into the ventral pallidum, and effects on latent inhibition were assessed using a conditioned suppression procedure. Neither drug produced specific effects on latent inhibition when given alone and, in the case of muscimol, failed to reverse the disruption of latent inhibition induced by systemic amphetamine. In addition to significant non-specific drug effects, a positive control experiment revealed that intra-pallidal picrotoxin significantly enhanced locomotion, suggesting that our manipulations of ventral pallidal GABA function were behaviourally effective. We conclude that modulating ventral pallidal GABA transmission does not affect latent inhibition. The implications of this finding for theories of the neural circuitry mediating latent inhibition and for understanding the functional role of ventral pallidal GABA transmission are discussed.

The "two-headed" latent inhibition model of schizophrenia: modeling positive and negative symptoms and their treatment

RATIONALE

Latent inhibition (LI), namely, poorer performance on a learning task involving a previously pre-exposed non-reinforced stimulus, is disrupted in the rat by the dopamine (DA) releaser amphetamine which produces and exacerbates psychotic (positive) symptoms, and this is reversed by treatment with typical and atypical antipsychotic drugs (APDs) which on their own potentiate LI. These phenomena are paralleled by disrupted LI in normal amphetamine-treated humans, in high schizotypal humans, and in schizophrenia patients in the acute stages of the disorder, as well as by potentiated LI in normal humans treated with APDs. Consequently, disrupted LI is considered to provide an animal model of positive symptoms of schizophrenia with face, construct and predictive validity.

OBJECTIVES

To review most of the rodent data on the neural substrates of LI as well as on the effects of APDs on this phenomenon with an attempt to interpret and integrate these data within the framework of the switching model of LI; to show that there are two distinct LI models, disrupted and abnormally persistent LI; to relate these findings to the clinical condition.

RESULTS

The nucleus accumbens (NAC) and its DA innervation form a crucial component of the neural circuitry of LI, and are involved at the conditioning stage. There is a clear functional differentiation between the NAC shell and core subregions whereby damage to the shell disrupts LI and damage to the core renders LI abnormally persistent under conditions that disrupt LI in normal rats. The effects of shell and core lesions parallel those produced by lesions to the major sources of input to the NAC: entorhinal cortex lesion, like shell lesion, disrupts LI, whereas hippocampal lesion, like core lesion, produces persistent LI with changes in context, and basolateral amygdala (BLA) lesion, like core lesion, produces persistent LI with extended conditioning. Systemically induced blockade of glutamatergic as well as DA transmission produce persistent LI via effects exerted at the conditioning stage, whereas enhancement of DA transmission disrupts LI via effects at the conditioning stage. Serotonergic manipulations can disrupt or potentiate LI via effects at the pre-exposure stage. Both typical and atypical APDs potentiate LI via effects at conditioning whereas atypical APDs in addition disrupt LI via effects at pre-exposure. Schizophrenia patients can exhibit disrupted or normal LI as a function of the state of the disorder (acute versus chronic), as well as persistent LI.

CONCLUSIONS

Different drug and lesion manipulations produce two poles of abnormality in LI, namely, disrupted LI under conditions which lead to LI in normal rats, and abnormally persistent LI under conditions which disrupt it in normal rats. Disrupted and persistent LI are differentially responsive to APDs, with the former reversed by both typical and atypical APDs and the latter selectively reversed by atypical APDs. It is suggested that this "two-headed LI model" mimics two extremes of deficient cognitive switching seen in schizophrenia, excessive and retarded switching between associations, mediated by dysfunction of different brain circuitries, and can serve to model positive symptoms of schizophrenia and typical antipsychotic action, as well as negative symptoms of schizophrenia and atypical antipsychotic action.

Effect of excitotoxic lesions of rat medial prefrontal cortex on spatial memory

The involvement of medial prefrontal cortex (mPFC) in spatial learning was examined in two memory tasks using spatial components, the Morris water maze and the three-panel runway. Using the Morris water maze task, with an invisible platform, the effects of NMDA mPFC lesions were assessed in a procedure reflecting spatial learning and memory, including a spatial reversal. In the three-panel runway, a delayed matching-to-position procedure was used in which rats were required to find food at the end of the runway after passing through one of three panel gates set into four barriers spaced equally apart along the maze. In addition, mPFC lesions were assessed behaviorally in two behavioral tests known to be sensitive to mPFC dysfunction: the food hoarding paradigm and spontaneous locomotion in the open field. Consistent with the documented effects of mPFC damage, NMDA mPFC lesions impaired food hoarding behavior and increased spontaneous exploratory locomotion. In the Morris water maze and the three-panel runway, mPFC-lesioned rats showed relatively few effects, supporting the conclusion that the damage inflicted to the mPFC had no consequence for the processing of spatial information. However, mPFC lesioned animals showed slower acquisition during both the training trial in the three-panel runway and the reversal training in the Morris water maze. These results suggest that spatial memory did not depend on mPFC integrity in the Morris water maze and the three-panel runway experiments, and address the issue of deficits induced by mPFC lesions in memory tasks dependent on non-mnemonic processes such as attentional processes and/or a reduced behavioral flexibility to environmental changes.

Impaired dodging in food-conflict following fimbria-fornix transection in rats: a novel hippocampal formation deficit

It is well known that damage to the hippocampal formation (Ammon's horn, dentate gyrus, fimbria-fornix, and other pathways) produces impairments in spatial navigation and in certain forms of learning. Lesions within these structures have also been reported to produce some motor impairments, but the nature of these impairments is less understood. The present study examined the effects of fimbria-fornix lesions on food wrenching and dodging, social interactions that occur when one rat attempts to steal food from a conspecific, who in turn attempts to protect the food by an evasive movement. Lesion effectiveness was confirmed histologically and electrophysiologically, by the loss of hippocampal rhythmical slow-wave activity (RSA or theta), and by changes in open field behavior (increased open field behavior, less thigmotaxis and more defecation). Analysis of the social interaction indicated when an eating control rat was approached by a conspecific that was attempting to steal its food, it prevented the theft by dodging, a rapid lateral maneuver involving forequarter turning and stepping with the rear limbs. Rats with fimbria-fornix lesions were significantly impaired in dodging and so were more likely to lose their food to the robber. This novel deficit in motor behavior is discussed in relation to contemporary theories of hippocampal function and it is suggested that the deficit may be caused by an inability of the fimbria-fornix damaged animals to disengage attention from eating in order to initiate an evasive movement to protect food. The finding of this novel deficit underscores the importance of considering both loss as well as release phenomena in the analysis of hippocampal formation function.

The entorhinal cortex-nucleus accumbens pathway and latent inhibition: a behavioral and neurochemical study in rats

Latent inhibition (LI) refers to the decrease in conditioned response produced by the repeated nonrein-forced preexposure to the to-be-conditioned stimulus. Experiment I investigated the effects of electrolytic lesions of the entorhinal cortex on LI in a conditioned emotional response procedure. Entorhinal cortex lesions attenuated LI. Experiments 2 and 3 investigated whether this attenuation of LI could result from a modification in nucleus accumbens (NAcc) dopamine (DA) release. Rats with entorhinal cortex lesions displayed normal spontaneous and amphetamine-induced locomotor activity, as well as normal basal and amphetamine-induced release of DA within the NAcc (assessed by microdialysis). Taken together, these results show that entorhinal cortex lesions disrupt LI in a way that is unlikely to be due to an alteration of DA release within the NAcc.

Nucleus accumbens, entorhinal cortex and latent inhibition: a neural network model

A neural network model of classical conditioning (Schmajuk, Lam, and Gray, J. Exp. Psychol.: Anim. Behav. Process, 22, 1996, 321-349) is applied to the description of the neural substrates of latent inhibition. Experimental data suggest that latent inhibition might be controlled by a circuit that involves the hippocampus, the entorhinal cortex, the nucleus accumbens, and the mesolimbic dopaminergic projection from the ventral tegmental area to the accumbens. By mapping different nodes and connections in the model onto this brain circuit, computer simulations demonstrate that, in most cases, the model provides a good quantitative description of: (1) the impairment of latent inhibition by lesions of the shell of the nucleus accumbens; (2) the restoration of latent inhibition by haloperidol following lesions of the shell; (3) the preservation of latent inhibition by lesions of the core of the nucleus accumbens; (4) the facilitation of latent inhibition by combined shell core lesions and by core lesions with extended conditioning; (5) the impairment of latent inhibition following lesions of the entorhinal cortex or the hippocampus; and (6) the restoration of latent inhibition by haloperidol following lesions of the entorhinal cortex and ventral subiculum. In addition, the model is able to describe neural activity in the nucleus accumbens.

Haloperidol reinstates latent inhibition impaired by hippocampal lesions: data and theory

The effect of haloperidol administration on the impairment of latent inhibition produced by aspirative lesions of the hippocampus was examined in the rat eyeblink response preparation. During the preexposure phase, rats with hippocampal or control lesions were either exposed to a tone or allowed to sit in the training apparatus. During the conditioning phase, the tone was paired with an airpuff to the eye after the rats were injected with either saline or haloperidol. Although saline-injected rats with hippocampal lesions did not show latent inhibition, the phenomenon was reinstated in rats that received haloperidol injections. A possible locus of the interaction between hippocampal lesions and haloperidol is the nucleus accumbens. The reported data are well described by a neural network model of classical conditioning. This study contributes to the understanding of the neurophysiology of latent inhibition as well as the neuropsychological bases of schizophrenia.