Regional dissociations within the hippocampus--memory and anxiety

The amnestic effects of hippocampal lesions are well documented, leading to numerous memory-based theories of hippocampal function. It is debatable, however, whether any one of these theories can satisfactorily account for all the consequences of hippocampal damage: Hippocampal lesions also result in behavioural disinhibition and reduced anxiety. A growing number of studies now suggest that these diverse behavioural effects may be associated with different hippocampal subregions. There is evidence for at least two distinct functional domains, although recent neuroanatomical studies suggest this may be an underestimate. Selective lesion studies show that the hippocampus is functionally subdivided along the septotemporal axis into dorsal and ventral regions, each associated with a distinct set of behaviours. Dorsal hippocampus has a preferential role in certain forms of learning and memory, notably spatial learning, but ventral hippocampus may have a preferential role in brain processes associated with anxiety-related behaviours. The latter's role in emotional processing is also distinct from that of the amygdala, which is associated specifically with fear. Gray and McNaughton's theory can in principle incorporate these apparently distinct hippocampal functions, and provides a plausible unitary account for the multiple facets of hippocampal function.

Action sets and decisions in the medial frontal cortex

Activations in human dorsomedial frontal and cingulate cortices are often present in neuroimaging studies of decision making and action selection. Interpretations have emphasized executive control, movement sequencing, error detection and conflict monitoring. Recently, however, experimental approaches, using lesions, inactivation, and cell recording, have suggested that these are just components of the areas' functions. Here we review these results and integrate them with those from neuroimaging. A medial superior frontal gyrus (SFG) region centred on the pre-supplementary motor area (pre-SMA) is involved in the selection of action sets whereas the anterior cingulate cortex (ACC) has a fundamental role in relating actions to their consequences, both positive reinforcement outcomes and errors, and in guiding decisions about which actions are worth making.

Distinct components of spatial learning revealed by prior training and NMDA receptor blockade

Synaptic plasticity dependent on N-methyl-D-aspartate (NMDA) receptors is thought to underlie certain types of learning and memory. In support of this, both hippocampal long-term potentiation and spatial learning in a watermaze are impaired by blocking NMDA receptors with a selective antagonist D(-)-2-amino-5-phosphonovaleric acid (AP5) or by a mutation in one of the receptor subunits. Here we report, however, that the AP5-induced learning deficit can be almost completely prevented if rats are pretrained in a different watermaze before administration of the drug. This is not because of stimulus generalization, and occurs despite learning of the second task remaining hippocampus dependent. An AP5-induced learning deficit is, however, still seen if the animals are pretrained using a non-spatial task. Thus, despite its procedural simplicity, the watermaze may involve multiple cognitive processes with distinct pharmacological properties; although required for some component of spatial learning, NMDA receptors may not be required for encoding the spatial representation of a specific environment.

Ventral hippocampal lesions affect anxiety but not spatial learning

Rats with cytotoxic ventral hippocampal lesions which removed approximately 50% of the hippocampus (including dentate gyrus) starting from the temporal pole, displayed a reduction in freezing behaviour following the delivery of an unsignalled footshock in an operant chamber. This was more plausibly a result of reduced susceptibility to fear than a result of a lesion-induced increase in general motor activity. There was no consistent difference between sham and lesioned animals in spontaneous locomotor activity, or locomotion following acute or chronic treatment with amphetamine. In contrast, ventral hippocampal lesioned animals were quicker to pass from the black to the white box during a modified version of the light/dark exploration test, and were quicker to begin eating during tests of hyponeophagia. Furthermore, rats with ventral hippocampal lesions defecated less than their sham counterparts both during open field testing and in extinction sessions following contextual conditioning. In contrast to these clear lesion effects, there were no signs of any spatial learning impairment either in the watermaze or on the elevated T-maze. Taken together these results suggest that the ventral hippocampus may play a role in a brain system (or systems) associated with fear and/or anxiety, and provide further evidence for a distinct specialisation of function along the septotemporal axis of the hippocampus.

Catechol-o-methyltransferase inhibition improves set-shifting performance and elevates stimulated dopamine release in the rat prefrontal cortex

The Val158Met polymorphism of the human catechol-O-methyltransferase (COMT) gene affects activity of the enzyme and influences performance and efficiency of the prefrontal cortex (PFC); however, although catecholaminergic neurotransmission is implicated, the underlying mechanisms remain elusive because studies of the role of COMT in PFC function are sparse. This study investigated the effect of tolcapone, a brain-penetrant COMT inhibitor, on a rat model of attentional set shifting, which is dependent on catecholamines and the medial PFC (mPFC). Additionally, we investigated the effect of tolcapone on extracellular catecholamines in the mPFC using microdialysis in awake rats. Tolcapone significantly and specifically improved extradimensional (ED) set shifting. Tolcapone did not affect basal extracellular catecholamines, but significantly potentiated the increase in extracellular dopamine (DA) elicited by either local administration of the depolarizing agent potassium chloride or systemic administration of the antipsychotic agent clozapine. Although extracellular norepinephrine (NE) was also elevated by local depolarization and clozapine, the increase was not enhanced by tolcapone. We conclude that COMT activity specifically affects ED set shifting and is a significant modulator of mPFC DA but not NE under conditions of increased catecholaminergic transmission. These data suggest that the links between COMT activity and PFC function can be modeled in rats and may be specifically mediated by DA. The interaction between clozapine and tolcapone may have implications for the treatment of schizophrenia.

Amygdala and Ventral Hippocampus Contribute Differentially to Mechanisms of Fear and Anxiety

Cytotoxic ventral hippocampal lesions produced anxiolytic effects on 4 ethologically based, unconditioned tests of anxiety in the rat (hyponeophagia, black/white 2-compartment box test, a successive alleys test that represents a modified version of the elevated plus-maze, and a social interaction test). Dorsal hippocampal lesions did not produce anxiolytic effects on these tests, suggesting a distinct specialization of function within the hippocampus. Furthermore, the effects of ventral hippocampal lesions were also distinct from those of amygdala lesions. This suggests that the effects of ventral hippocampal lesions are not simply due to direct or indirect effects on the amygdala, and that these 2 brain areas contribute differentially to a brain system (or systems) associated with the processing of fearful and/or anxiogenic stimuli.

Double dissociation of function within the hippocampus: a comparison of dorsal, ventral, and complete hippocampal cytotoxic lesions

Rats with complete cytotoxic hippocampal lesions exhibited spatial memory impairments in both the water maze and elevated T maze. They were hyperactive in photocell cages; swam faster in the water maze; and were less efficient on a nonspatial, differential reinforcement of low rates (DRL) task. Performance on both spatial tasks was also impaired by selective dorsal but not ventral lesions; swim speed was increased by ventral but not dorsal lesions. Both partial lesions caused a comparable reduction in DRL efficiency, although these effects were smaller than those of complete lesions. Neither partial lesion induced hyperactivity when rats were tested in photocell cages, although both complete and ventral lesion groups showed increased activity after footshock in other studies (Richmond et al., 1999). These results demonstrate possible functional dissociations along the septotemporal axis of the hippocampus.

Double dissociation of function within the hippocampus: spatial memory and hyponeophagia

Complete and dorsal hippocampal lesions impaired spatial performance on 2 working memory tasks: rewarded alternation on the T maze and matching to position in the water maze. In contrast, ventral hippocampal lesions had no effect on these tasks, even when task difficulty was increased by the introduction of delays. Ventral lesions did resemble complete lesions in reducing anxiety in 3 commonly used tests of anxiety (social interaction, plus-maze, and hyponeophagia). Dorsal lesions also appeared to be anxiolytic in the social interaction and plus-maze tests, but they did not affect hyponeophagia. Complete- and dorsal-lesioned rats displayed hyperactivity, whereas ventral-lesioned rats did not. These results show a double dissociation between dorsal and ventral hippocampal lesions (hyponeophagia vs. spatial memory), suggesting differentiation of function along the septotemporal axis of this structure.

Dissociating context and space within the hippocampus: effects of complete, dorsal, and ventral excitotoxic hippocampal lesions on conditioned freezing and spatial learning

Rats with complete excitotoxic hippocampal lesions or selective damage to the dorsal or ventral hippocampus were compared with controls on measures of contextually conditioned freezing in a signaled shock procedure and on a spatial water-maze task. Complete and ventral lesions produced equivalent, significant anterograde deficits in conditioned freezing relative to both dorsal lesions and controls. Complete hippocampal lesions impaired water-maze performance; in contrast, ventral lesions improved performance relative to the dorsal group, which was itself unexpectedly unimpaired relative to controls. Thus, the partial lesion effects seen in the 2 tasks never resembled each other. Anterograde impairments in contextual freezing and spatial learning do not share a common underlying neural basis; complete and ventral lesions may induce anterograde contextual freezing impairments by enhancing locomotor activity under conditions of mild stress.

Weighing up the benefits of work: behavioral and neural analyses of effort-related decision making

How we decide whether a course of action is worth undertaking is largely unknown. Recently, neuroscientists have been turning to ecological approaches to address this issue, examining how animals evaluate the costs and benefits of different options. We present here evidence from rodents and monkeys that demonstrate the degree to which they take into account work and energetic requirements when deciding what responses to make. These calculations appear to be critically mediated by the anterior cingulate cortex (ACC) and mesolimbic dopamine (DA) pathways, with damage to either causing a bias towards options that are easily obtained but yield relatively smaller reward rather than alternatives that require more work but result in greater reward. The evaluation of such decisions appears to be carried out in systems independent of those involved in delay-discounting. We suggest that top-down signals from ACC to nucleus accumbens (NAc) and/or midbrain DA cells may be vital for overcoming effort-related response costs.

Differential involvement of serotonin and dopamine systems in cost-benefit decisions about delay or effort

RATIONALE

Although tasks assessing the role of dopamine in effort-reward decisions are similar to those concerned with the role of serotonin in impulsive choice in that both require analysis of the costs and benefits of possible actions, they have never been directly compared.

OBJECTIVES

This study investigated the involvement of serotonin and dopamine in two cost-benefit paradigms, one in which the cost was delay and the other in which it was physical effort.

METHODS

Sixteen rats were trained on a T-maze task in which they chose between high and low reward arms. In one version, the high reward arm was obstructed by a barrier, in the other, delivery of the high reward was delayed by 15 s. Serotonin and dopamine function were manipulated using systemic pCPA and haloperidol injections, respectively.

RESULTS

Haloperidol-treated rats were less inclined either to exert more effort or to countenance a delay for a higher reward. pCPA had no effect on the performance of the rats on the effortful task, but significantly increased the rats' preference for an immediate but smaller reward. All animals (drug treated and controls) chose the high reward arm on the majority of trials when the delay or effort costs were matched in both high and low reward arms.

CONCLUSION

A dissociation was found between the neurotransmitter systems involved in different types of cost-benefit decision making. While dopaminergic systems were required for decisions about both effort and delay, serotonergic systems were only needed for the latter.

Impaired spatial working memory but spared spatial reference memory following functional loss of NMDA receptors in the dentate gyrus

Novel spatially restricted genetic manipulations can be used to assess contributions made by synaptic plasticity to learning and memory, not just selectively within the hippocampus, but even within specific hippocampal subfields. Here we generated genetically modified mice (NR1(deltaDG) mice) exhibiting complete loss of the NR1 subunit of the N-methyl-D-aspartate receptor specifically in the granule cells of the dentate gyrus. There was no evidence of any reduction in NR1 subunit levels in any of the other hippocampal subfields, or elsewhere in the brain. NR1(deltaDG) mice displayed severely impaired long-term potentiation (LTP) in both medial and lateral perforant path inputs to the dentate gyrus, whereas LTP was unchanged in CA3-to-CA1 cell synapses in hippocampal slices. Behavioural assessment of NR1(deltaDG) mice revealed a spatial working memory impairment on a three-from-six radial arm maze task despite normal hippocampus-dependent spatial reference memory acquisition and performance of the same task. This behavioural phenotype resembles that of NR1(deltaCA3) mice but differs from that of NR1(deltaCA1) mice which do show a spatial reference memory deficit, consistent with the idea of subfield-specific contributions to hippocampal information processing. Furthermore, this pattern of selective functional loss and sparing is the same as previously observed with the global GluR-A L-alpha-amino-3-hydroxy-5-methyl-4-isoxazelopropionate receptor subunit knockout, a mutation which blocks the expression of hippocampal LTP. The present results show that dissociations between spatial working memory and spatial reference memory can be induced by disrupting synaptic plasticity specifically and exclusively within the dentate gyrus subfield of the hippocampal formation.

The contribution of distinct subregions of the ventromedial frontal cortex to emotion, social behavior, and decision making

Damage to the ventromedial frontal cortex (VMFC) in humans is associated with deficits in decision making. Decision making, however, often happens while people are interacting with others, where it is important to take the social consequences of a course of action into account. It is well known that VMFC lesions also lead to marked alterations in patients' emotions and ability to interact socially; however, it has not been clear which parts of the VMFC are critical for these changes. Recently, there has been considerable interest in the role of the VMFC in choice behavior during interpersonal exchanges. Here, we highlight recent research that suggests that two areas within or adjacent to the VMFC, the orbitofrontal cortex (OFC) and the anterior cingulate cortex (ACC), may play distinct but complementary roles in mediating normal patterns of emotion and social behavior. Converging lines of evidence from human, macaque, and rat studies now suggest that the OFC may be more specialized for simple emotional responses, such as fear and aggression, through its role in representing primary reinforcement or punishment. By contrast, the ACC may play a distinct role in more complex aspects of emotion, such as social interaction, by virtue of its connections with the discrete parts of the temporal lobe and subcortical structures that control autonomic responses.

The role of the GluR-A (GluR1) AMPA receptor subunit in learning and memory

It is widely believed that synaptic plasticity may provide the neural mechanism that underlies certain kinds of learning and memory in the mammalian brain. The expression of long-term potentiation (LTP) in the hippocampus, an experimental model of synaptic plasticity, requires the GluR-A subunit of the AMPA subtype of glutamate receptor. Genetically modified mice lacking the GluR-A subunit show normal acquisition of the standard, fixed-location, hidden-platform watermaze task, a spatial reference memory task that requires the hippocampus. In contrast, these mice are dramatically impaired on hippocampus-dependent, spatial working memory tasks, in which the spatial response of the animal is dependent on information in short-term memory. Taken together, these results argue for two distinct and independent spatial information processing mechanisms: (i) a GluR-A-independent associative learning mechanism through which a particular spatial response is gradually or incrementally strengthened, and which presumably underlies the acquisition of the classic watermaze paradigm and (ii) a GluR-A-dependent, non-associative, short-term memory trace which determines performance on spatial working memory tasks. These results are discussed in terms of Wagner's SOP model (1981).

Restoration of spatial working memory by genetic rescue of GluR-A–deficient mice

Gene-targeted mice lacking the AMPA receptor subunit GluR-A (also called GluR1 encoded by the gene Gria1,) have deficits in hippocampal CA3-CA1 long-term potentiation (LTP) and have profoundly impaired hippocampus-dependent spatial working memory (SWM) tasks, although their spatial reference memory remains normal. Here we show that forebrain-localized expression of GFP-tagged GluR-A subunits in GluR-A-deficient mice rescues SWM, paralleling its rescue of CA3-CA1 LTP. This provides powerful new evidence linking hippocampal GluR-A-dependent synaptic plasticity to rapid, flexible memory processing.

Double dissociation of function within the hippocampus: a comparison of dorsal, ventral, and complete hippocampal cytotoxic lesions

Rats with complete cytotoxic hippocampal lesions exhibited spatial memory impairments in both the water maze and elevated T maze. They were hyperactive in photocell cages; swam faster in the water maze; and were less efficient on a nonspatial, differential reinforcement of low rates (DRL) task. Performance on both spatial tasks was also impaired by selective dorsal but not ventral lesions; swim speed was increased by ventral but not dorsal lesions. Both partial lesions caused a comparable reduction in DRL efficiency, although these effects were smaller than those of complete lesions. Neither partial lesion induced hyperactivity when rats were tested in photocell cages, although both complete and ventral lesion groups showed increased activity after footshock in other studies (Richmond et al., 1999). These results demonstrate possible functional dissociations along the septotemporal axis of the hippocampus.

Deletion of glutamate receptor-A (GluR-A) AMPA receptor subunits impairs one-trial spatial memory

Genetically modified mice lacking the glutamate receptor A (GluR-A) subunit of the AMPA receptor (GluR-A-/- mice) display normal spatial reference memory but impaired spatial working memory (SWM). This study tested whether the SWM impairment in these mice could be explained by a greater sensitivity to within-session proactive interference. The SWM performance of GluR-A-/- and wild-type mice was assessed during nonmatching-to-place testing under conditions in which potential proactive interference from previous trials was reduced or eliminated. SWM was impaired in GluR-A-/- mice, both during testing with pseudotrial-unique arm presentations on the radial maze and when conducting each trial on a different 3-arm maze, each in a novel testing room. Experimentally naive GluR-A-/- mice also exhibited chance performance during a single trial of spontaneous alternation. This 1-trial spatial memory deficit was present irrespective of the delay between the sample information and the response choice (0 or 45 min) and the length of the sample phase (0.5 or 5 min). These results imply that the SWM deficit in GluR-A-/- mice is not due to increased susceptibility to proactive interference.

Impulsive choice in hippocampal but not orbitofrontal cortex-lesioned rats on a nonspatial decision-making maze task

Orbitofrontal cortical (OFC) and hippocampal (HPC) lesions in primates and rodents have been associated with impulsive behaviour. We showed previously that OFC- or HPC-lesioned rats chose the immediate low-reward (LR) option in preference to the delayed high-reward (HR) option, where LR and HR were associated with different spatial responses in a uniform grey T-maze. We now report that on a novel nonspatial T-maze task in which the HR and LR options are associated with patterned goal arms (black-and-white stripes vs. gray), OFC-lesioned rats did not show impulsive behaviour, choosing the delayed HR option, and were indistinguishable from controls. In contrast, HPC-lesioned rats exhibited impulsive choice in the nonspatial decision-making task, although they chose the HR option on the majority of trials when there was a 10-s delay associated with both goal arms. The previously reported impairment in OFC-lesioned rats on the spatial version of the intertemporal choice task is unlikely to reflect a general problem with spatial learning, because OFC lesions were without effect on acquisition of the standard reference memory water-maze task and spatial working memory performance (nonmatching-to-place) on the T-maze. The differential effect of OFC lesions on the two versions of the intertemporal choice task may be explained instead in terms of the putative role of OFC in using associative information to represent expected outcomes and generate predictions. The impulsivity in HPC-lesioned rats may reflect impaired temporal information processing, and emphasizes a role for the hippocampus beyond the spatial domain.

A Comparison of GluR-A-Deficient and Wild-Type Mice on a Test Battery Assessing Sensorimotor, Affective, and Cognitive Behaviors

Previous studies have demonstrated a spatial working memory deficit in glutamate receptor (GluR)-A (GluR1) AMPA receptor subunit knockout mice. The present study evaluated male and female wild-type and GluR-A-/- mice on a test battery that assessed sensorimotor, affective, and cognitive behaviors. Results revealed a behavioral phenotype more extensive than previously described. GluR-A-/- mice were hyperactive, displayed a subtle lack of motor coordination, and were generally more anxious than wild-type controls. In addition, they showed a deficit in spontaneous alternation, consistent with previous reports of a role for GluR-A-dependent plasticity in hippocampus-dependent, spatial working memory. Although changes in motor coordination or anxiety cannot explain the dissociations already reported within the spatial memory domain, it is clear that they could significantly affect interpretation of results obtained in other kinds of behavioral tasks.

The drugs don't work-or do they? Pharmacological and transgenic studies of the contribution of NMDA and GluR-A-containing AMPA receptors to hippocampal-dependent memory

OBJECTIVE

The aim of this article is to provide a review of studies using N-methyl-D-aspartate (NMDA) receptor antagonists to assess the hippocampal long-term potentiation (LTP)/learning hypothesis.

DISCUSSION

In particular, we will re-examine the validity of both (1) the original hippocampal LTP/spatial learning hypothesis of Morris and (2) the sensorimotor account put forward by Cain, among others, both from the point of view of the pharmacological studies on which they were based and with regard to recent studies with genetically modified mice. More specifically, we will review the pharmacological studies in the light of recent work on the glutamate receptor A (GluR-A or GluR1) L-alpha-amino-3-hydroxy-5-methyl-4-isoxazelopropionate (AMPA) receptor sub-unit knockout mouse. We will argue that neither the original hippocampal LTP/spatial learning hypothesis nor a sensorimotor account can adequately explain all of the available data. We argue instead that hippocampal synaptic plasticity, which requires NMDA receptors for its induction and GluR-A-containing AMPA receptors for its continued expression, contributes to a process whereby appropriate behavioural responses are selected rapidly on the basis of conditional information provided by the context. These contextual cues could include not only the spatial context (i.e. the 'where') and the temporal context (the 'when'), but also other aspects of context, such as internal state cues (hunger and fear state), and can be used to rapidly and flexibly alter valences of specific response options.

RECOMMENDATIONS

We also suggest that there is a separate, distinct, NMDA/GluR-A-independent mechanism through which the context can gradually (incrementally or decrementally) alter the valence of a particular response option.

What causes aberrant salience in schizophrenia? A role for impaired short-term habituation and the GRIA1 (GluA1) AMPA receptor subunit

The GRIA1 locus, encoding the GluA1 (also known as GluRA or GluR1) AMPA glutamate receptor subunit, shows genome-wide association to schizophrenia. As well as extending the evidence that glutamatergic abnormalities have a key role in the disorder, this finding draws attention to the behavioural phenotype of Gria1 knockout mice. These mice show deficits in short-term habituation. Importantly, under some conditions the attention being paid to a recently presented neutral stimulus can actually increase rather than decrease (sensitization). We propose that this mouse phenotype represents a cause of aberrant salience and, in turn, that aberrant salience (and the resulting positive symptoms) in schizophrenia may arise, at least in part, from a glutamatergic genetic predisposition and a deficit in short-term habituation. This proposal links an established risk gene with a psychological process central to psychosis and is supported by findings of comparable deficits in short-term habituation in mice lacking the NMDAR receptor subunit Grin2a (which also shows association to schizophrenia). As aberrant salience is primarily a dopaminergic phenomenon, the model supports the view that the dopaminergic abnormalities can be downstream of a glutamatergic aetiology. Finally, we suggest that, as illustrated here, the real value of genetically modified mice is not as 'models of schizophrenia' but as experimental tools that can link genomic discoveries with psychological processes and help elucidate the underlying neural mechanisms.

Cytotoxic lesions of the hippocampus increase social investigation but do not impair social-recognition memory

A number of studies have implicated the hippocampal formation in social-recognition memory in the rat. The present study addressed this issue directly by assessing the effects of cytotoxic lesions confined to the hippocampus proper, encompassing the four CA subfields and the dentate gyrus, on this behavioural task. Ibotenate-induced hippocampal lesions led to locomotor hyperactivity and a marked spatial working-memory impairment on the elevated T-maze. In addition, they also led to increased social investigation. However, despite these clear effects, there was no effect of the lesions on social-recognition memory. These results suggest that the hippocampus proper does not subserve social-recognition memory; but does not, however, preclude the possibility that other areas of the hippocampal formation (e.g. entorhinal cortex or subiculum) may support this memory process.

The role of the entorhinal cortex in two forms of spatial learning and memory

It is generally acknowledged that the rodent hippocampus plays an important role in spatial learning and memory. The importance of the entorhinal cortex (ERC), an area that is closely interconnected anatomically with the hippocampus, in these forms of learning is less clear cut. Recent studies using selective, fibre-sparing cytotoxic lesions have generated conflicting results, with some studies showing that spatial learning can proceed normally without the ERC, suggesting that this area is not required for normal hippocampal function. The present study compared cytotoxic and aspiration ERC lesions with both fimbria fornix (FFX) lesions and sham-operated controls on two spatial learning tasks which have repeatedly been shown to depend on the hippocampus. Both groups of ERC lesions were impaired during non-matching-to-place testing (rewarded alternation) on the elevated T-maze. However, neither of these lesions subsequently had any effect on the acquisition of a standard spatial reference memory task in the water maze. FFX lesions produced a robust and reliable impairment on both of these tasks. A second experiment confirmed that cytotoxic ERC lesions spared water maze learning but disrupted rewarded alternation on the T-maze, when the order of behavioural testing was reversed. These results confirm previous reports that ERC-lesioned animals are capable of spatial navigation in the water maze, suggesting that the ERC is not a prerequisite for normal hippocampal function in this task. The present demonstration that ERC lesions disrupt non-matching-to-place performance may, however, be consistent with the possibility that ERC lesions affect attentional mechanisms, for example, by increasing the sensitivity to recent reward history.

Medial Septal Lesions Mimic Effects of Both Selective Dorsal and Ventral Hippocampal Lesions

Electrolytic medial septal (MS) lesions, which depleted acetylcholinesterase staining in both dorsal and ventral hippocampus, produced a constellation of behaviors, combining aspects of both selective dorsal and ventral hippocampal lesion effects. MS lesions impaired spatial working memory on the T maze, thus resembling the effects of dorsal hippocampal lesions. In addition, MS lesions reduced anxiety during successive alleys (a modified form of the elevated plus-maze), social interaction, and hyponeophagia tests. MS lesions also reduced postshock freezing. These effects more closely resemble those of ventral hippocampal lesions. Therefore, the effects of electrolytic MS lesions derive from the resulting combined deafferentation of dorsal and ventral hippocampal regions, suggesting that previously reported effects of cytotoxic dorsal hippocampal lesions are unlikely to be due to a demyelination of fibers of passage coursing through the septal pole.

A role for dorsal and ventral hippocampus in inter-temporal choice cost-benefit decision making

Previous studies suggest a preferential role for dorsal hippocampus (dHPC) in spatial memory tasks, whereas ventral hippocampus (vHPC) has been implicated in aspects of fear and/or anxiety. In this study, we tested the hypothesis that vHPC may be a critical subregion for performance on a delay-based, cost-benefit decision making task. Rats chose between the two goal arms of a T maze, one containing an immediately available small reward, the other containing a larger reward that was only accessible after a delay. dHPC, vHPC, and complete hippocampal (cHPC) lesions all reduced choice of the delayed high reward (HR) in favor of the immediately available low reward (LR). The deficits were not due to a complete inability to remember which reward size was associated with which arm of the maze. When an equivalent 10-s delay was introduced in both goal arms, all rats chose the HR arm on nearly all trials. The deficit was, however, reinstated when the inequality was reintroduced. Our results suggest an important role for both dHPC and vHPC in the extended neural circuitry that underlies intertemporal choice.