Distributed neurodegenerative changes 2-28 days after ventral hippocampal excitotoxic lesions in rats

The 3-hit animal models of schizophrenia: Improving strategy to decipher and treat the disease?

Schizophrenia is a complex disease related to combination and interactions between genetic and environmental factors, with an epigenetic influence. After the development of the first mono-factorial animal models of schizophrenia (1-hit), that reproduced patterns of either positive, negative and/or cognitive symptoms, more complex models combining two factors (2-hit) have been developed to better fit with the multifactorial etiology of the disease. In the two past decades, a new way to design animal models of schizophrenia have emerged by adding a third hit (3-hit). This review aims to discuss the relevance of the risk factors chosen for the tuning of the 3-hit animal models, as well as the validities measurements and their contribution to schizophrenia understanding. We intended to establish a comprehensive overview to help in the choice of factors for the design of multiple-hit animal models of schizophrenia.

Molecular Factors Mediating Neural Cell Plasticity Changes in Dementia Brain Diseases

Neural plasticity-the ability to alter a neuronal response to environmental stimuli-is an important factor in learning and memory. Short-term synaptic plasticity and long-term synaptic plasticity, including long-term potentiation and long-term depression, are the most-characterized models of learning and memory at the molecular and cellular level. These processes are often disrupted by neurodegeneration-induced dementias. Alzheimer's disease (AD) accounts for 50% of cases of dementia. Vascular dementia (VaD), Parkinson's disease dementia (PDD), dementia with Lewy bodies (DLB), and frontotemporal dementia (FTD) constitute much of the remaining cases. While vascular lesions are the principal cause of VaD, neurodegenerative processes have been established as etiological agents of many dementia diseases. Chief among such processes is the deposition of pathological protein aggregates in vivo including β-amyloid deposition in AD, the formation of neurofibrillary tangles in AD and FTD, and the accumulation of Lewy bodies composed of α-synuclein aggregates in DLB and PDD. The main symptoms of dementia are cognitive decline and memory and learning impairment. Nonetheless, accurate diagnoses of neurodegenerative diseases can be difficult due to overlapping clinical symptoms and the diverse locations of cortical lesions. Still, new neuroimaging and molecular biomarkers have improved clinicians' diagnostic capabilities in the context of dementia and may lead to the development of more effective treatments. Both genetic and environmental factors may lead to the aggregation of pathological proteins and altered levels of cytokines, such that can trigger the formation of proinflammatory immunological phenotypes. This cascade of pathological changes provides fertile ground for the development of neural plasticity disorders and dementias. Available pharmacotherapy and disease-modifying therapies currently in clinical trials may modulate synaptic plasticity to mitigate the effects neuropathological changes have on cognitive function, memory, and learning. In this article, we review the neural plasticity changes seen in common neurodegenerative diseases from pathophysiological and clinical points of view and highlight potential molecular targets of disease-modifying therapies.

Glucocorticoid-sensitive ventral hippocampal-orbitofrontal cortical connections support goal-directed action - Curt Richter Award Paper 2019

In an ever-changing and often ambiguous environment, organisms must use previously learned associations between antecedents and outcomes to predict future associations and make optimal choices. Chronic stress can impair one's ability to flexibly adjust behaviors when environmental contingencies change, particularly in cases of early-life stress. In mice, exposure to elevated levels of the primary stress hormone, corticosterone (CORT), during early adolescence is sufficient to impair response-outcome decision making later in life, biasing response strategies towards inflexible habits. Nevertheless, neurobiological mechanisms are still being defined. Here, we report that exposure to excess CORT in adolescence causes a loss of dendritic spines on excitatory pyramidal neurons in the lateral, but not medial, orbital prefrontal cortex (loPFC) of mice, and spine loss correlates with the severity of habit biases in adulthood. Excess CORT also reduces the presence of ventral hippocampal (vHC) axon terminals in the loPFC. To identify functional consequences, we inactivated vHC→loPFC projections in typical healthy mice during a period when mice must update response-outcome expectations to optimally acquire food reinforcers. Inactivation impaired the animals' subsequent ability to sustainably choose actions based on likely outcomes, causing them to defer to habit-based response strategies. Thus, vHC→loPFC projections are necessary for response-outcome expectancy updating and a target of excess glucocorticoids during early-life development. Their degradation is likely involved in long-term biases towards habit-based behaviors following glucocorticoid excess in adolescence.

Auditory steady-state responses in primary and non-primary regions of the auditory cortex in neonatal ventral hippocampal lesion rats

Auditory steady-state responses (ASSRs) represent the electrophysiological activity of the auditory nervous system in response to a periodic acoustic stimulus. Spectrogram analysis can reveal the frequency and phase information entrained in ASSRs. Clinically, the ASSR is used to detect abnormalities in electroencephalographs obtained from schizophrenia patients, who show reduced power and phase locking of ASSRs. The neonatal ventral hippocampal lesion (NVHL) rat is a widely used model to investigate the neurodevelopmental mechanisms of schizophrenia. It has been established that NVHL rats exhibit several schizophrenia-like behavioral and molecular abnormalities. However, no clear abnormalities in ASSRs have been reported to date. The present study compared ASSRs of adult NVHL and sham-operated rats. We inserted microelectrodes into the primary auditory cortex (A1) or posterior auditory field (PAF) and recorded the local field potential (LFP) in response to 40- and 80-Hz click train stimuli. Spectrogram analysis was performed to obtain the mean trial power (MTP) and phase-locking factor (PLF) of the click train-evoked LFPs. We found that in the control animals, A1 showed a stronger MTP and PLF of ASSR than PAF, and NVHL operation mainly impaired the ASSR in PAF. Analysis of spike activity also indicated that NVHL operation extended the duration of tone-evoked responses in PAF neurons. Our results reveal, for the first time, that NVHL may distinctly influence the neural activities of primary and non-primary fields of the auditory cortex.

Histological correlates of N40 auditory evoked potentials in adult rats after neonatal ventral hippocampal lesion: animal model of schizophrenia

The neonatal ventral hippocampal lesion (NVHL) is an established neurodevelopmental rat model of schizophrenia. Rats with NVHL exhibit several behavioral, molecular and physiological abnormalities that are similar to those found in schizophrenics. Schizophrenia is a severe psychiatric illness characterized by profound disturbances of mental functions including neurophysiological deficits in brain information processing. These deficits can be assessed by auditory evoked potentials (AEPs), where schizophrenics exhibit abnormalities in amplitude, duration and latency of such AEPs. The aim of the present study was to compare the density of cells in the temporal cerebral cortex and the N40-AEP of adult NVHL rats versus adult sham rats. We found that rats with NVHL exhibit significant lower amplitude of the N40-AEP and a significant lower number of cells in bilateral regions of the temporal cerebral cortex compared to sham rats. Because the AEP recordings were obtained from anesthetized rats, we suggest that NVHL leads to inappropriate innervation in thalamic-cortical pathways in the adult rat, leading to altered function of cortical networks involved in processing of primary auditory information.

Effects of caffeine or RX821002 in rats with a neonatal ventral hippocampal lesion

Rats with a neonatal ventral hippocampal lesion (NVHL) are used to model schizophrenia. They show enhanced locomotion and difficulties in learning after puberty. Such behavioral modifications are strengthened by dopaminergic psychostimulant drugs, which is also relevant for schizophrenia because illustrating its dopaminergic facet. But it remains questionable that only dopaminergic drugs elicit such effects. The behavioral effects could simply represent a non specific arousal, in which case NVHL rats should also be hyper-responsive to other vigilance enhancing drugs. We administered an adenosine (caffeine) or an adrenaline receptor antagonist, (RX821002) at doses documented to modify alertness of rats, respectively 5 mg/kg and 1 mg/kg. Rats were selected prior to the experiments using magnetic resonance imaging (MRI). Each group contained typical and similar NVHL lesions. They were compared to sham lesioned rats. We evaluated locomotion in a new environment and the capacity to remember a visual or acoustic cue that announced the occurrence of food. Both caffeine and RX82100 enhanced locomotion in the novel environment, particularly in NVHL rats. But, RX82100 had a biphasic effect on locomotion, consisting of an initial reduction preceding the enhancement. It was independent of the lesion. Caffeine did not modify the learning performance of NVHL rats. But, RX821002 was found to facilitate learning. Patients tend to intake much more caffeine than healthy people, which has been interpreted as a means to counter some cognitive deficits. This idea was not validated with the present results. But adrenergic drugs could be helpful for attenuating some of their cognitive deficits.

The HDAC Inhibitor Phenylbutyrate Reverses Effects of Neonatal Ventral Hippocampal Lesion in Rats

Recent evidence suggests that epigenetic mechanisms play a role in psychiatric diseases. In this study, we considered rats with neonatal ventral hippocampal lesions (NVHL) that are currently used for modeling neurodevelopmental aspects of schizophrenia. Contribution of epigenetic regulation to the effects of the lesion was investigated, using a histone deacetylase (HDAC) inhibitor. Lesioned or sham-operated rats were treated with the general HDAC inhibitor phenylbutyrate, which was injected daily from the day after surgery until adulthood. Changes in the volume of the lesion were monitored by magnetic resonance imaging (MRI). Anxiety was analyzed in the Plus Maze Test. Hypersensitivity of the dopaminergic system was evaluated by measuring the locomotor response to apomorphine. An associative conditioning test rewarded with food was used to evaluate learning abilities. The volume of the lesions expanded long after surgery, independently of the treatment, as assessed by MRI. Removal of the ventral hippocampus reduced anxiety, and this remained unchanged when animals were treated with phenylbutyrate. In contrast, NVHL rats' hypersensitivity to apomorphine and deterioration of the associative learning were reduced by the treatment. Global HDAC activity, which was increased in the prefrontal cortex of lesioned non-treated rats, was found to be reversed by HDAC inhibition. The study provides evidence that chromatin remodeling may be useful for limiting behavioral consequences due to lesioning of the ventral hippocampus at an early age. This represents a novel approach for treating disorders resulting from insults occurring during brain development.

MRI and X-ray scanning images of the brain of 3-, 6- and 9-month-old rats with bilateral neonatal ventral hippocampus lesions

Rats with bilateral neonatal ventral hippocampus lesions (NVHL) are commonly used for modeling developmental aspects of the pathophysiology of schizophrenia. Given that functional changes become significant only after puberty, NVHL as well as sham-operated rats were analyzed at the ages of 21, 42 and 63days (i.e. as pups, adolescents and adults), using MRI to examine the damage caused by surgery over time. Morphometric evaluations were considered and lesions were classified as small, medium and large. The volume of lesions increased regularly with age, to a greater extent than increases in overall brain size. This was relatively linear, corresponding to a gradually shrinking forebrain, and these observations held true for each class of lesions considered. Following the observation that the lesion procedure elicited calcifications in the brain, the same rats were subjected to 3D X-ray scanning the day after each MRI session, allowing precise measurements of skull size to be carried out. The NVHL rats had smaller skulls; however, the dimensions of the calcifications did not grow more than the skull size over time. The mechanisms underlying the progressive anatomical changes following surgery are discussed, and we propose this in vivo follow-up method to investigate therapeutic strategies aimed at countering or limiting the post-lesion consequences of a neonatal brain damage.

Dorsal hippocampal N-methyl-D-aspartate receptors underlie spatial working memory performance during non-matching to place testing on the T-maze

Previous lesion studies have suggested a functional dissociation along the septotemporal axis of the hippocampus. Whereas the dorsal hippocampus has been implicated in spatial memory processes, the ventral hippocampus may play a role in anxiety. However, these lesion studies are potentially confounded by demyelination of fibres passing through the lesion site, and the possibility of secondary, downstream changes in associated brain structures as a consequence of their chronic denervation following the lesion. In the present study, we have used the microinfusion of muscimol to temporarily inactivate either the dorsal or ventral hippocampus in order to re-examine the contribution of the hippocampal sub-regions to spatial memory. Microinfusion studies spare fibres of passage and offer fewer opportunities for compensatory changes because the effects are transient and short-lasting. Rats were infused prior to spatial working memory testing on a non-matching to place T-maze alternation task. Spatial working memory was impaired by dorsal but not ventral hippocampal inactivation. In a second experiment, infusion of the NMDAR antagonist, D-AP5, into dorsal hippocampus also impaired spatial working memory performance, suggesting that NMDAR function within the dorsal hippocampus makes an essential contribution to this aspect of hippocampal information processing.

The pallial basal ganglia pathway modulates the behaviorally driven gene expression of the motor pathway

The discrete neural network for songbird vocal communication provides an effective system to study neural mechanisms of learned motor behaviors in vertebrates. This system consists of two pathways--a vocal motor pathway used to produce learned vocalizations and a vocal pallial basal ganglia loop used to learn and modify the vocalizations. However, it is not clear how the loop exerts control over the motor pathway. To study the mechanism, we used expression of the neural activity-induced gene ZENK (or egr-1), which shows singing-regulated expression in a social context-dependent manner: high levels in both pathways when singing undirected and low levels in the lateral part of the loop and in the robust nucleus of the arcopallium (RA) of the motor pathway when singing directed to another animal. Here, we show that there are two parallel interactive parts within the pallial basal ganglia loop, lateral and medial, which modulate singing-driven ZENK expression of the motor pathway nuclei RA and HVC, respectively. Within the loop, the striatal and pallial nuclei appear to have opposing roles; the striatal vocal nucleus lateral AreaX is required for high ZENK expression in its downstream nuclei, particularly during undirected singing, while the pallial vocal lateral magnocellular nucleus of the anterior nidopallium is required for lower expression, particularly during directed singing. These results suggest a dynamic molecular interaction between the basal ganglia pathway and the motor pathway during production of a learned motor behavior.

The price of seizure control: dynorphins in interictal and postictal psychosis

Postictal and interictal psychoses are relatively common complicating factors in the clinical course of epilepsy, yet their neurobiological substrates are poorly understood. Recent evidence shows that kappa opioid receptor (KOR) activation elicits anticonvulsant and psychotomimetic effects. In view of this background, here we introduce the hypothesis that epilepsy-related psychoses may partially result from excessive hippocampal dynorphin release and kappa opioid receptor overstimulation aimed at seizure control.

Cerebral metabolic consequences in the adult brain after neonatal excitotoxic lesions of the amygdala in rats

In the present study the effects of neonatal excitotoxic lesions of the amygdala or ventral hippocampus on local cerebral glucose utilisation in the adult rat were studied by means of the [14C]2-deoxyglucose autoradiographic method. Our hypothesis was that damage to the brain during early development leads to long-term functional activity changes in brain regions outside the primary lesioned area which might underlie the behavioural deficits observed in animals with neonatal brain damage. Cerebral glucose utilisation in animals with a neonatal amygdala lesion was significantly decreased in the amygdala itself and in several other brain regions. The neonatal ventral hippocampal lesion did not cause significant changes in cerebral glucose utilisation, except for a decrease in the primary damaged region (i.e. caudal ventral hippocampus). Behaviourally, animals lesioned in the amygdala displayed increased ambulatory activity both before and after puberty when exposed to a novel open field, while neonatal ventral hippocampal lesions did not affect adult exploratory behaviour as compared to sham controls. These results support our hypothesis that neonatal brain damage leads to long-term functional activity changes in brain regions outside the primary lesioned area. Moreover, they suggest that this long-term effect depends on the primary area lesioned since only damage to the amygdala, and not to the ventral hippocampus, affects the functional organisation of the brain of the animals later in life. Additionally, the findings may suggest that the functional changes in the brain may underlie the behavioural deficits observed after neonatal amygdala lesion in the rat.

Effect of Polygala tenuifolia root on behavioral disorders by lesioning nucleus basalis magnocellularis in rat

We investigated whether an aqueous extract of Polygala tenuifolia Willd (PTW) could improve the rats' memory and behavioral disorders produced by lesioning nucleus basalis magnocellularis (NBM) in rats. The animals were divided into four groups for surgery, and following that they were orally administered PTW extract for 7 and 21 days. Each group consisted of eight male Sprague-Dawley rats and were treated as follows:

CONTROL

no surgery (n = 8), PBS: 1M (mol/L) phosphate buffered saline (n = 8), IBO: 0.12 M (n = 8), QUIS: 0.12 M (n = 8). Two 0.5 microL injections were made in the vicinity of the bilateral side of the nucleus basalis magnocellularis (NBM). All rats were tested in the neurological tests and the step-through passive avoidance memory test during pre-surgery, surgery and post-surgery drug treatment. The results suggest that PTW extract has some repairing effects on the memory and behavioral disorders produced by lesioning of the NBM in rats.

Disruption of neurogenesis on gestational day 17 in the rat causes behavioral changes relevant to positive and negative schizophrenia symptoms and alters amphetamine-induced dopamine release in nucleus accumbens

Gestational disruption of neurodevelopment has been proposed to lead to pathophysiological changes similar to those underlying schizophrenia. We induced such disruption by treating pregnant rat dams with methylazoxymethanol acetate (MAM) on gestational day 17 (GD17). Total brain size and that of the prefrontal cortex and hippocampus were reduced in adult rats exposed prenatally to MAM. When locomotor activity was assessed in an open field, MAM-exposed rats were hyper-responsive to a mild stress and to amphetamine (2 mg/kg, s.c.). They also engaged in less social interaction than controls. We studied, by microdialysis, the effect of amphetamine on extracellular dopamine in the nucleus accumbens and the medial prefrontal cortex of freely moving control and MAM-exposed rats. Amphetamine (2 mg/kg, s.c.) induced an increase in dopamine release that was larger in the nucleus accumbens of MAM-exposed rats than in controls, whereas no difference was seen in the medial prefrontal cortex. In controls, amphetamine infused into the medial prefrontal cortex (50 microM) led to a slight decrease in extracellular dopamine in the nucleus accumbens. This effect was absent in MAM-exposed rats, where a transient increase in nucleus accumbens dopamine levels was seen after amphetamine infusion. These results show that the late gestational disruption of neurogenesis in the rat leads to behavioral changes that mimic positive and negative schizophrenia symptoms, and also to a dysregulation of subcortical dopamine neurotransmission. This study contributes to the evaluation of the validity of the prenatal MAM GD17 treatment in rats as an animal model for schizophrenia.

Anxiety is functionally segregated within the septo-hippocampal system

Previous lesion studies have suggested that the septal-hippocampal system is involved in fear and anxiety. In this study we examined the effects on anxiety of temporary neuronal inhibition of various aspects of the septo-hippocampal system in rats. Infusions of tetrodotoxin (TTX) were used to induce reversible lesions in the fimbria fornix, medial septum, dorsal hippocampus, and ventral hippocampus. To assess anxiety we used the elevated plus-maze and the shock-probe burying tests. A reduction in anxiety in the elevated plus-maze is indicated by increased open arm exploration, whereas a reduction in anxiety in the shock-probe burying test is indicated by decreased burying behavior or increased contacts with the shock-probe. The results suggested that inhibition of the septal-hippocampal system induced site-specific anxiolytic effects that vary in nature. Tetrodotoxin lesions of the fimbria fornix increased both open arm exploration and the number of shocks taken by the rats, while having no effect on burying behavior. Both septal and ventral hippocampal lesions increased open arm exploration and decreased burying behavior, but had no effect on the number of probe shocks. Finally, TTX lesions of the dorsal hippocampus increased the number of shocks taken by the rats, but did not affect open arm activity or burying behavior. Neuroanatomical studies indicated that the effect on the number of shocks induced by dorsal hippocampal TTX lesions was not likely mediated by the amygdala. Collectively, the data suggest that the control of specific anxiety reactions is functionally segregated within different aspects of the septo-hippocampal system.

Role of the locus coeruleus in the prolactin secretion of female rats

Since locus coeruleus (LC) lesion blocks preovulatory prolactin surge, the aim of this study was to determine if this lesion would also block prolactin surges induced by steroids in ovariectomized rats and would modify basal prolactin secretion. To determine the time of the steroid-induced prolactin surges, ovariectomized rats treated with estradiol (OVE) or estradiol and progesterone (OVEP) were cannulated at 08:00 h and blood samples were collected hourly between 14:00 and 18:00 h. Ovariectomized rats treated with oil (OV-Oil) were used as control. Prolactin peaked at 16:00 h in OVE rats and at 15:00 h in OVEP. In a second experiment, male rats, cycling rats, OVE, OVEP, and OV-Oil groups were cannulated at 08:00 h, followed by LC lesion or sham-surgery. Blood samples were withdrawn at times of basal and peak prolactin levels. LC lesion blocked afternoon prolactin surges of OVE, OVEP and proestrus rats. However, the low levels observed at 16:00 h in OV-Oil, diestrus and male rats as well as at 11:00 h in OVE, OVEP, estrus, and proestrus rats were not modified by LC lesion. The high prolactin levels observed on estrus afternoon were dramatically reduced by LC lesion. Data suggest that LC neurons are important for steroid-induced prolactin surge genesis, but not for prolactin basal secretion.

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.

Hippocampal modulation of sensorimotor processes

While the hippocampus makes unique contributions to memory, it has also long been associated with sensorimotor processes, i.e. innate processes involving control of motor responses to sensory stimuli. Moreover, hippocampal dysfunction has been implicated in neuropsychiatric diseases, such as schizophrenia and anxiety disorders, primarily characterized by non-mnemonic deficits in the processing of and responding to sensory information. This review is concerned with the hippocampal modulation of three sensorimotor processes in rats-locomotor activity, prepulse inhibition (PPI) of the startle reflex, and the startle reflex itself-whose alterations are related to human psychosis or anxiety disorders. Its main purpose is to present and discuss the picture emerging from studies examining the effects of pharmacological manipulations of the dorsal and ventral hippocampus by local drug microinfusions. While a role of the hippocampus in regulating locomotor activity, PPI, and startle reactivity has also been suggested based on the effects of hippocampal lesions, the microinfusion studies have revealed additional important details of this role and suggest modifications of notions based on lesion studies. In summary, the microinfusion studies corroborate that hippocampal mechanisms can directly influence locomotor activity, PPI, and startle reactivity, and that aberrant hippocampal function may contribute to neuropsychiatric diseases, in particular psychosis. The relation between different sensorimotor processes and hippocampal neurotransmission, the role of ventral and dorsal hippocampus, and the extrahippocampal mechanisms mediating the hippocampal modulation of different sensorimotor processes can partly be dissociated. Thus, the hippocampal modulation of these sensorimotor processes appears to reflect multiple operations, rather than one unitary operation.

Prepulse inhibition in rats with temporary inhibition/inactivation of ventral or dorsal hippocampus

Prepulse inhibition (PPI) of the acoustic startle response is a measure of sensorimotor gating and is decreased in neuropsychiatric diseases, including schizophrenia. Hippocampal involvement in PPI has been the subject of several studies, in particular, as aberrant hippocampal activity has been associated with schizophrenia. In rats, chemical stimulation of the ventral hippocampus reduced PPI, while normal PPI was found following hippocampal lesions, suggesting that ventral hippocampal overactivity is detrimental for PPI, but that normal hippocampal activity does not contribute substantially to PPI. In the present study, we investigated the importance of hippocampal activity for PPI by examining PPI in Wistar rats with temporarily decreased hippocampal activity, aiming to avoid compensatory processes that may occur with permanent lesions. Bilateral ventral or dorsal hippocampal infusions of the gamma-aminobutyric acid A (GABA(A)) receptor agonist muscimol (1 microg/side) or the sodium-channel blocker tetrodotoxin (TTX, 10 ng/side) reduced PPI. This reduction is probably neuroleptic-resistant since haloperidol and clozapine did not antagonize the muscimol-induced decreases in PPI. PPI reduction by muscimol inhibition or TTX inactivation of the dorsal or ventral hippocampus indicates that hippocampal activity contributes to sensorimotor gating, suggesting intact PPI after permanent hippocampal lesions to reflect compensatory processes. The data are discussed with respect to hippocampal dysfunction in schizophrenia.

The hippocampus and motivation revisited: appetite and activity

After reviewing the available data regarding the various effects of manipulating (e.g. lesions, chemical or electrical stimulation) the hippocampal formation, Jarrard concluded that this structure likely played a role in motivated behaviors, specifically in general behavioral activation and incentive motivation. Since that time there have been technical advances in lesion techniques and conceptual advances in theories of motivation and learning. Here, we present more recent data that demonstrates the effects of hippocampal lesions on general activity, the utilization of interoceptive state cues, ingestive behaviors, and appetitive responding. We critically evaluate several theories of hippocampal function that have been proposed to explain these data, including the hippocampus as an inhibitor of general activation, as a processor of energy state signals and as a mediator of reward valuation. Finally, we propose that these findings may also be accounted for based on a role for the hippocampus in the learned inhibition of appetitive behaviors. We conclude that, while the specific mechanism of hippocampal involvement may not yet be determined, it is clear that this structure is involved in food-related behaviors and we caution researchers to consider this as a possible confound in studies of learning and memory processes.

Lesion size and amphetamine hyperlocomotion after neonatal ventral hippocampal lesions: more is less

Neonatal hippocampal lesions in rats produce behavioral and neurochemical abnormalities post-puberty that are used in animal models for developmentally linked pathology in schizophrenia. In one model, adult rats exhibit enhanced sensitivity to the locomotor-activating effects of amphetamine, if they had sustained excitotoxic lesions of the ventral hippocampus on post-natal day 7. The hippocampal elements responsible for these lesion-induced developmental changes have not been fully characterized. The present study assessed the locomotor-activating effects of amphetamine in adult rats that on day 7 had sustained either sham or ibotenic acid lesions of the ventral hippocampus alone ("standard lesions"), or the ventral hippocampus plus surrounding portions of entorhinal cortex and dorsal hippocampus ("large lesions"). "Standard lesions" produced the expected "supersensitive" locomotor response to amphetamine, while "large lesions" did not. No differences between these lesion groups were observed in baseline levels of locomotor activity or habituation. These data suggest that models of enhanced behavioral sensitivity to dopamine agonists after neonatal hippocampal lesions require functionality in the entorhinal cortex and/or dorsal hippocampus. It is possible that the behavioral abnormalities in the "neonatal hippocampal lesion model" reflect, at least in part, aberrant function within spared elements of the hippocampal complex.

Regulation of sensorimotor gating in rats by hippocampal NMDA: anatomical localization

Prepulse inhibition (PPI) of the startle reflex is a measure of sensorimotor gating that is reduced in humans with certain neuropsychiatric disorders, including schizophrenia, and in rats after manipulations of limbic cortico-striato-pallido-pontine circuitry. We have reported that PPI is reduced after specific manipulations of the hippocampal complex (HPC) in rats, but the mechanisms for these effects remain poorly understood. For example, dopaminergic substrates clearly regulate PPI, but the PPI-disruptive effects of intra-HPC carbachol or NMDA are not reversed by D2 receptor antagonists. This study examined the anatomical specificity within the hippocampal complex of the PPI-disruptive effects of NMDA infusion. Startle magnitude and PPI were assessed after acute bilateral infusion of NMDA (0, 0.4 or 0.8 microg) into the dorsal subiculum (DS), region CA1, the ventral subiculum (VS), the rostral entorhinal cortex (ECr) and the caudal entorhinal cortex (ECc). A dorsal-ventral gradient for NMDA effects was observed, with a dose-dependent disruption of PPI after NMDA infusion into the VS or EC, but not the DS, and with intermediate level effects observed after NMDA infusion into CA1. A second set of studies confirmed that the failure of NMDA effects in the DS did not reflect site-related differences in startle magnitude or baseline levels of PPI. These findings demonstrate the importance of the ventral, but not the dorsal HPC, in the glutamatergic regulation of PPI.