The Parahippocampal Cortex and its Functional Connection with the Hippocampus are Critical for Nonnavigational Spatial Memory in Macaques

The Hamilton Search Task (HST) is a test of nonnavigational spatial memory that is dependent on the hippocampus. The parahippocampal cortex (PHC) is a major route for spatial information to reach the hippocampus, but the extent to which the PHC and hippocampus function independently of one another in the context of nonnavigational spatial memory is unclear. Here, we tested the hypotheses that (1) bilateral pharmacological inactivation of the PHC would impair HST performance, and (2) that functional disconnection of the PHC and hippocampus by contralateral (crossed) inactivation would likewise impair performance. Transient inactivation of the PHC impaired HST performance most robustly with 30 s intertrial delays, but not when color cues were introduced. Functional disconnection of the PHC and hippocampus, but not separate unilateral inactivation of either region, also selectively impaired long-term spatial memory. These findings indicate a critical role for the PHC and its interactions with the hippocampus in nonnavigational spatial memory.

Effect of Cannabidiol on Medial Temporal, Midbrain, and Striatal Dysfunction in People at Clinical High Risk of Psychosis: A Randomized Clinical Trial

Importance

Cannabidiol (CBD) has antipsychotic effects in humans, but how these are mediated in the brain remains unclear.

Objective

To investigate the neurocognitive mechanisms that underlie the therapeutic effects of CBD in psychosis.

Design, Setting, and Participants

In this parallel-group, double-blind, placebo-controlled randomized clinical trial conducted at the South London and Maudsley NHS Foundation Trust in London, United Kingdom, 33 antipsychotic medication-naive participants at clinical high risk (CHR) of psychosis and 19 healthy control participants were studied. Data were collected from July 2013 to October 2016 and analyzed from November 2016 to October 2017.

Interventions

A total of 16 participants at CHR of psychosis received a single oral dose of 600 mg of CBD, and 17 participants at CHR received a placebo. Control participants were not given any drug. All participants were then studied using functional magnetic resonance imaging (fMRI) while performing a verbal learning task.

Main Outcomes and Measures

Brain activation during verbal encoding and recall, indexed using the blood oxygen level-dependent hemodynamic response fMRI signal.

Results

Of the 16 participants in the CBD group, 6 (38%) were female, and the mean (SD) age was 22.43 (4.95) years; of 17 in the placebo group, 10 (59%) were female, and the mean (SD) age was 25.35 (5.24) years; and of 19 in the control group, 8 (42%) were female, and the mean (SD) age was 23.89 (4.14) years. Brain activation (indexed using the median sum of squares ratio of the blood oxygen level-dependent hemodynamic response effects model component to the residual sum of squares) was analyzed in 15 participants in the CBD group, 16 in the placebo group, and 19 in the control group. Participants receiving placebo had reduced activation relative to controls in the right caudate during encoding (placebo: median, -0.027; interquartile range [IQR], -0.041 to -0.016; control: median, 0.020; IQR, -0.022 to 0.056; P < .001) and in the parahippocampal gyrus and midbrain during recall (placebo: median, 0.002; IQR, -0.016 to 0.010; control: median, 0.035; IQR, 0.015 to 0.039; P < .001). Within these 3 regions, activation in the CBD group was greater than in the placebo group but lower than in the control group (parahippocampal gyrus/midbrain: CBD: median, -0.013; IQR, -0.027 to 0.002; placebo: median, -0.007; IQR, -0.019 to 0.008; control: median, 0.034; IQR, 0.005 to 0.059); the level of activation in the CBD group was thus intermediate to that in the other 2 groups. There were no significant group differences in task performance.

Conclusions and Relevance

Cannabidiol may partially normalize alterations in parahippocampal, striatal, and midbrain function associated with the CHR state. As these regions are critical to the pathophysiology of psychosis, the influence of CBD at these sites could underlie its therapeutic effects on psychotic symptoms.

Trial Registration

isrctn.org Identifier: ISRCTN46322781.

Differences in regional cerebral blood flow response to a 5HT3 antagonist in early- and late-onset cocaine-dependent subjects

5-hydroxytryptamine 3 (5HT3) receptors are important modulators of mesostriatal dopaminergic transmission and have been implicated in the pathophysiology of cocaine reward, withdrawal and self-administration. In addition, the 5HT3 antagonist ondansetron is effective in treating early-onset, but not late-onset, alcohol-dependent subjects. To explore the role of 5HT3 receptor systems in cocaine addiction using functioning imaging, we administered ondansetron to 23 abstinent, treatment-seeking cocaine-addicted and 22 sex-, age- and race-matched healthy control participants. Differences between early- (first use before 20 years, n = 10) and late-onset (first use after 20 years, n = 10) cocaine-addicted subjects were also assessed. On two separate days, subjects were administered ondansetron (0.15 mg/kg intravenously over 15 minutes) or saline. Regional cerebral blood flow (rCBF) was measured following each infusion with single photon emission computed tomography. No significant rCBF differences between the cocaine-addicted and control participants were observed following ondansetron relative to saline. Early-onset subjects, however, showed increased (P < 0.001) right posterior parahippocampal rCBF following ondansetron. In contrast, late-onset subjects showed decreased rCBF following ondansetron in an overlapping region of the right parahippocampal/hippocampal gyrus. Early-onset subjects also displayed increased rCBF in the left anterior insula and subthalamic nucleus following ondansetron; late-onset subjects showed decreased rCBF in the right anterior insula. These findings suggest that the age of drug use onset is associated with serotonergic biosignatures in cocaine-addicted subjects. Further clarification of these alterations may guide targeted treatment with serotonergic medications similar to those successfully used in alcohol-dependent patients.

Cannabis affects people differently: inter-subject variation in the psychotogenic effects of Δ9-tetrahydrocannabinol: a functional magnetic resonance imaging study with healthy volunteers

BACKGROUND

Cannabis can induce transient psychotic symptoms, but not all users experience these adverse effects. We compared the neural response to Δ9-tetrahydrocannabinol (THC) in healthy volunteers in whom the drug did or did not induce acute psychotic symptoms. Method In a double-blind, placebo-controlled, pseudorandomized design, 21 healthy men with minimal experience of cannabis were given either 10 mg THC or placebo, orally. Behavioural and functional magnetic resonance imaging measures were then recorded whilst they performed a go/no-go task.

RESULTS

The sample was subdivided on the basis of the Positive and Negative Syndrome Scale positive score following administration of THC into transiently psychotic (TP; n = 11) and non-psychotic (NP; n = 10) groups. During the THC condition, TP subjects made more frequent inhibition errors than the NP group and showed differential activation relative to the NP group in the left parahippocampal gyrus, the left and right middle temporal gyri and in the right cerebellum. In these regions, THC had opposite effects on activation relative to placebo in the two groups. The TP group also showed less activation than the NP group in the right middle temporal gyrus and cerebellum, independent of the effects of THC.

CONCLUSIONS

In this first demonstration of inter-subject variability in sensitivity to the psychotogenic effects of THC, we found that the presence of acute psychotic symptoms was associated with a differential effect of THC on activation in the ventral and medial temporal cortex and cerebellum, suggesting that these regions mediate the effects of the drug on psychotic symptoms.

[Disorders of neurogenesis of cortical and subcortical structures in rat brain limbic system during fetal alcohol syndrome formation]

Disorders of neurogenesis of cortical and subcortical structures in rat brain limbic system were studied in the offspring of rats that received ethanol during pregnancy. The methods used included the staining of histological sections with cresyl violet, in vitro culture, and electron paramagnetic resonance. Prenatal alcohol intoxication was shown to induce the disturbances in proliferative activity of granular layer cells in the hippocampal dentate gyrus, neuron- and glioblast migration, enhancement of free NO and lipoperoxide production and cell death. This resulted in the changes in the number of neurons in cortical and subcortical structures of rat brain limbic system and in fetal alcohol syndrome formation.

In vitro ictogenesis and parahippocampal networks in a rodent model of temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is a chronic epileptic disorder involving the hippocampal formation. Details on the interactions between the hippocampus proper and parahippocampal networks during ictogenesis remain, however, unclear. In addition, recent findings have shown that epileptic limbic networks maintained in vitro are paradoxically less responsive than non-epileptic control (NEC) tissue to application of the convulsant drug 4-aminopyridine (4AP). Field potential recordings allowed us to establish here the effects of 4AP in brain slices obtained from NEC and pilocarpine-treated epileptic rats; these slices included the hippocampus and parahippocampal areas such as entorhinal and perirhinal cortices and the amygdala. First, we found that both types of tissue generate epileptiform discharges with similar electrographic characteristics. Further investigation showed that generation of robust ictal-like discharges in the epileptic rat tissue is (i) favored by decreased hippocampal output (ii) reinforced by EC-subiculum interactions and (iii) predominantly driven by amygdala networks. We propose that a functional switch to alternative synaptic routes may promote network hyperexcitability in the epileptic limbic system.

Differential activation of face memory encoding tasks in alcohol-dependent patients compared to healthy subjects: an fMRI study

It has been hypothesized that the right hemisphere of the brain is more sensitive to alcohol-related damage than the left hemisphere. The present study tested this hypothesis, using functional MRI to determine whether the pattern for right hemispheric activity is different for alcohol-dependent patients, compared to normal healthy individuals. Two different types of memory encoding tasks were performed separately: word and face encoding for both alcohol-dependent patients and normal healthy volunteers. The data for the normal volunteers indicate that the left prefrontal region is more active during word encoding, whereas the right parahippocampal region is more active during face encoding. The results for the patient data, however, demonstrated left lateralization in the prefrontal area during word encoding, while right lateralization in the parahippocampal region during face encoding was not observed. Therefore, alcoholism appears to have no influence on left hemispheric activity, since the activation pattern was similar to that observed for normal healthy persons. However, the absence of right hemispheric lateralization in alcohol-dependent patients is consistent with the hypothesis that the right hemisphere is more vulnerable to alcohol-related damage than the left hemisphere.

Cingulate cortex projections to the parahippocampal region and hippocampal formation in the rat

In the present study we aimed to determine the topographical and laminar characteristics of cingulate projections to the parahippocampal region and hippocampal formation in the rat, using the anterograde tracers Phaseolus vulgaris-leucoagglutinin and biotinylated dextranamine. The results show that all areas of the cingulate cortex project extensively to the parahippocampal region but not to the hippocampal formation. Rostral cingulate areas (infralimbic-, prelimbic cortices, rostral 1/3 of the dorsal anterior cingulate cortex) primarily project to the perirhinal and lateral entorhinal cortices. Projections from the remaining cingulate areas preferentially target the postrhinal and medial entorhinal cortices as well as the presubiculum and parasubiculum. At a more detailed level the projections show differences in topographical specificities according to their site of origin within the cingulate cortex suggesting the functional contribution of cingulate areas may differ at an individual level. This organization of the cingulate-parahippocampal projections relates to the overall organization of postulated parallel parahippocampal-hippocampal processing streams mediated through the lateral and medial entorhinal cortex respectively. The mid-rostrocaudal part of the dorsal anterior cingulate cortex appears to be connected to both networks as well as to rostral and caudal parts of the cingulate cortex. This region may therefore responsible for integrating information across these specific networks.

Depressive symptoms in adolescents: associations with white matter volume and marijuana use

BACKGROUND

Depressed mood has been associated with decreased white matter and reduced hippocampal volumes. However, the relationship between brain structure and mood may be unique among adolescents who use marijuana heavily. The goal of this study was to examine the relationship between white matter and hippocampal volumes and depressive symptoms among adolescent marijuana users and controls.

METHODS

Data were collected from marijuana users (n = 16) and demographically similar controls (n = 16) aged 16-18. Extensive exclusionary criteria included psychiatric and neurologic disorders, including major depression. Substance use, mood, and anatomical measures were collected after 28 days of monitored abstinence.

RESULTS

Marijuana (MJ) users demonstrated more depressive symptoms than controls (p < .05). MJ use (beta = .42, p < .005) and smaller white matter volume (beta = -.34, p < .03) each predicted higher levels of depressive symptoms on the Hamilton Depression Rating Scale. MJ use interacted with white matter volume (beta = -.55, p < .03) in predicting depression scores on the Beck Depression Inventory: among MJ users, but not controls, white matter volume was negatively associated with depressive symptoms.

CONCLUSIONS

Marijuana use and white matter volume were additive and interactive in predicting depressive symptoms among adolescents. Subtle neurodevelopmental white matter abnormalities may disrupt the connections between areas involved in mood regulation.

Reversible inactivation of amygdala and cerebellum but not perirhinal cortex impairs reactivated fear memories

The cerebellum, amygdala and perirhinal cortex are involved in fear learning but the different roles that these three structures play in aversive learning are not well defined. Here we show that in adult rats amygdala or cerebellar vermis blockade causes amnesia when performed immediately, but not 1 h, after the recall of fear memories. Thus, the cerebellum, as well as the amygdala, influences long-term fear memories. These effects are long lasting, as they do not recover over time, even after a reminder shock administration. However, all of the subjects were able to form new fear memories in the absence of inactivation. By increasing the strength of conditioning, we observed that stronger fear memories are affected by the combined but not independent amygdala and cerebellar blockade. These results demonstrate that the cerebellum supports the memory processes even in the absence of a crucial site for emotions like the amygdala. Furthermore, they suggest that the amygdala is only one of the neural sites underlying long-term fear memories. Finally, the inactivation of the perirhinal cortex never alters retrieved fear traces, showing important differences between the amygdala, cerebellum and perirhinal cortex in emotional memories.

Regional gray matter changes in bipolar disorder: a voxel-based morphometric study

OBJECTIVE

To investigate structural abnormalities in bipolar disorder (BD) using optimized voxel-based morphometry (VBM) in closely matched patients and controls, and to examine the relationship of clinical features with regional gray matter (GM) volumes.

METHODS

Twenty-four patients (six male) aged 19-59 years (mean=38.21 years, SD=11.04 years) with DSM-IV bipolar I disorder were compared with 25 control subjects, matched on age, sex, and years of education. VBM analyses were conducted on high-resolution T1-weighted brain magnetic resonance imaging to detect regional GM volume differences between groups, ensuring statistical correlation for age, sex and total intracranial volumes. Within the patient groups, regional GM changes were also investigated.

RESULTS

Compared to controls, BD patients had increased GM volume in left parahippocampal gyrus and decreased GM volume in left middle temporal gyrus. Family history, psychotic symptoms and lithium status were associated with regional GM abnormalities in BD patients.

CONCLUSIONS

This study presents evidence of gray matter volume abnormalities in adults with bipolar I disorder. Regional variation in relation to clinical factors suggests a neurobiological basis for clinical heterogeneity and posits the possibility of trait deficits.

Paradoxical facilitation of object recognition memory after infusion of scopolamine into perirhinal cortex: implications for cholinergic system function

The cholinergic system has long been implicated in learning and memory, yet its specific function remains unclear. In the present study, we investigated the role of cortical acetylcholine in a rodent model of declarative memory by infusing the cholinergic muscarinic receptor antagonist scopolamine into the rat perirhinal cortex during different stages (encoding, storage/consolidation, and retrieval) of the spontaneous object recognition task. Presample infusions of scopolamine significantly impaired object recognition compared with performance of the same group of rats on saline trials; this result is consistent with previous reports supporting a role for perirhinal acetylcholine in object information acquisition. Scopolamine infusions directly before the retrieval stage had no discernible effect on object recognition. However, postsample infusions of scopolamine with sample-to-infusion delays of up to 20 h significantly facilitated performance relative to postsample saline infusion trials. Additional analysis suggested that the infusion episode could cause retroactive or proactive interference with the sample object trace and that scopolamine blocked the acquisition of this interfering information, thereby facilitating recognition memory. This is, to our knowledge, the first example of improved recognition memory after administration of scopolamine. The overall pattern of results is inconsistent with a direct role for cortical acetylcholine in declarative memory consolidation or retrieval. Rather, the cholinergic input to the perirhinal cortex may facilitate acquisition by enhancing the cortical processing of incoming stimulus information.

Neuregulin regulates the formation of radial glial scaffold in hippocampal dentate gyrus of postnatal rats

In the rodent hippocampus, the radial glial scaffold consists of radial glial cells (RGCs) and plays important roles in neurogenesis in this area after birth. However, the mechanisms that maintain the radial glial scaffold in the postnatal dentate gyrus (DG) area remain elusive. In the present work, we studied the role of Neuregulin (NRG) in the formation and maintenance of the radial glial scaffold in the hippocampal DG of postnatal rats using slice culture. We found that ErbB4 receptors were expressed in vimentin-positive RGCs in DG of postnatal day 6 (P6) rats. Treatment with NRG and Ab-3, the inhibitor of ErbB4, revealed that in P6 rats exogenous NRG promoted the proliferation of Vimentin-positive RGCs in DG. On the other hand, endogenous NRG was found necessary for maintaining the characteristic morphological and immunohistochemical features of these cells. These results indicated that NRG plays a critical role in the formation and maintenance of the radial glial scaffold in the hippocampal DG of postnatal rats.

Propagation Dynamics of Epileptiform Activity Acutely Induced by Bicuculline in the Hippocampal-Parahippocampal Region of the Isolated Guinea Pig Brain

PURPOSE

Aim of the study is to investigate the involvement of parahippocampal subregions in the generation and in the propagation of focal epileptiform discharges in an acute model of seizure generation in the temporal lobe induced by arterial application of bicuculline in the in vitro isolated guinea pig brain preparation.

METHODS

Electrophysiological recordings were simultaneously performed with single electrodes and multichannel silicon probes in the entorhinal, perirhinal, and piriform cortices and in the area CA1 of the hippocampus of the in vitro isolated guinea pig brain. Interictal and ictal epileptiform discharges restricted to the temporal region were induced by a brief (3-5 min) arterial perfusion of the GABA(A) receptor antagonist, bicuculline methiodide (50 microM). Current source density analysis of laminar field profiles performed with the silicon probes was carried out at different sites to establish network interactions responsible for the generation of epileptiform potentials. Nonlinear regression analysis was conducted on extracellular recordings during ictal onset in order to quantify the degree of interaction between fast activities generated at different sites, as well as time delays.

RESULTS

Experiments were performed in 31 isolated guinea pig brains. Bicuculline-induced interictal and ictal epileptiform activities that showed variability of spatial propagation and time course in the olfactory-temporal region. The most commonly observed pattern (n = 23) was characterized by the initial appearance of interictal spikes (ISs) in the piriform cortex (PC), which propagated to the lateral entorhinal region. Independent and asynchronous preictal spikes originated in the entorhinal cortex (EC)/hippocampus and progressed into ictal fast discharges (around 25 Hz) restricted to the entorhinal/hippocampal region. The local generation of fast activity was verified and confirmed both by CSD and phase shift analysis performed on laminar profiles. Fast activity was followed by synchronous afterdischarges that propagated to the perirhinal cortex (PRC) (but not to the PC). Within 1-9 min, the ictal discharge ceased and a postictal period of depression occurred, after which periodic ISs in the PC resumed. Unlike preictal ISs, postictal ISs propagated to the PRC.

CONCLUSIONS

Several studies proposed that reciprocal connections between the entorhinal and the PRC are under a very efficient inhibitory control (1). We report that ISs determined by acute bicuculline treatment in the isolated guinea pig brain progress from the PC to the hippocampus/EC just before ictal onset. Ictal discharges are characterized by a peculiar pattern of fast activity that originates from the entorhinal/hippocampal region and only secondarily propagates to the PRC. Postictal propagation of ISs to the PRC occurred exclusively when an ictal discharge was generated in the hippocampal/entorhinal region. The results suggest that reiteration of ictal events may promote changes in propagation pattern of epileptiform discharges that could act as trigger elements in the development of temporal lobe epilepsy.

Scopolamine reduces persistent activity related to long-term encoding in the parahippocampal gyrus during delayed matching in humans

Recent computational modeling and slice physiology studies have suggested that long-term encoding may depend on sustained spiking during brief memory delays in parahippocampal neurons, and that this persistent spiking activity is modulated by effects of acetylcholine at muscarinic receptors. Our recent functional magnetic resonance imaging (fMRI) study has shown that sustained parahippocampal delay period activity during delayed match-to-sample performance in healthy young individuals predicted subsequent memory of visual stimuli on a recognition memory assessment 20 min later (Schon et al., 2004). The current study combined this fMRI paradigm with a pharmacological manipulation to test whether this long-term encoding-related delay activity is reduced in subjects who receive the muscarinic cholinergic antagonist scopolamine before fMRI scanning. Subsequent memory was predicted by sustained activity during brief memory delays bilaterally in the perirhinal/entorhinal cortex, in the right posterior parahippocampal and mid-fusiform gyri, and in the hippocampal body in healthy young individuals without a scopolamine challenge. This activity was reduced in subjects receiving scopolamine. The results are consistent with computational modeling data and behavioral pharmacological studies, suggesting that long-term encoding-related activity may be reduced if cholinergic receptors are blocked by scopolamine.

A 5-month period of epilepsy impairs spatial memory, decreases anxiety, but spares object recognition in the lithium-pilocarpine model in adult rats

PURPOSE

In temporal lobe epilepsy (TLE), interictal behavioral disorders affect patients' quality of life. Therefore we studied long-term behavioral impairments in the lithium-pilocarpine (li-pilo) model of TLE.

METHODS

Eleven li-pilo adult rats exhibiting spontaneous recurrent seizures (SRSs) during 5 months were compared with 11 li-saline rats. Spatial working memory was tested in a radial arm maze (RAM), anxiety in an elevated plus-maze (EPM), and nonspatial working memory in an object-recognition paradigm. Neuronal loss was assessed on thionine brain sections after behavioral testing.

RESULTS

In the RAM, the time to complete each session and the number of errors per session decreased over a 5-day period in li-saline rats but remained constant and significantly higher in li-pilo rats. In the EPM, the number of entries in and time spent on open arms were significantly higher in li-pilo than li-saline rats. In the object-recognition task, the two groups exhibited a comparable novelty preference for the new object. Neuronal loss reached 47-90% in hilus, CA1, amygdala, and piriform and entorhinal cortex.

CONCLUSIONS

In li-pilo rats having experienced SRS for 5 months, performance in the object-recognition task is spared, which suggests that object discrimination remains relatively intact despite extensive damage. Neuronal loss in regions mediating memory and anxiety, such as hippocampus, entorhinal cortex, and amygdala, may relate to impaired spatial orientation and decreased anxiety.

Periodic oscillatory activity in parahippocampal slices maintained in vitro

Brain slices maintained in vitro have been extensively used for studying neuronal synchronization. However, the validity of this approach may be questioned since pharmacological procedures are usually required to elicit spontaneous events similar to the EEG activity recorded in vivo. Here, we report that when superfused with control medium, rat brain slices comprising the entorhinal and perirhinal cortices along with a portion of the basolateral/lateral nuclei of the amygdala can synchronously generate periodic oscillatory activity at 5-11 Hz every 5-30 s. The periodic events: (i) correspond intracellularly to synaptic depolarizations in regularly firing neurons analyzed in the three areas; (ii) have no fixed site of onset; (iii) spread with time lags of 8-20 ms; and (iv) continue to occur asynchronously after their surgical isolation. NMDA receptor antagonism reduced the duration of the oscillatory events, while glutamatergic non-NMDA receptor antagonism abolished them. Activation of mu-opioid receptors, a procedure that hyperpolarizes interneurons thus decreasing GABA release, reversibly decreased the rate of occurrence of periodic oscillatory activity (POA). However, periodic events continued to occur during application of GABA(A) or GABA(B) receptor antagonists as well as in the presence of the cholinergic agent carbachol. We also found that POA was abolished by baclofen and irreversibly reduced by the gap junction decoupler carbenoxolone. These findings demonstrate that parahippocampal networks in a brain slice preparation can generate periodic, synchronous activity under quasi-physiological conditions. These network oscillations (i) reflect the activation of ionotropic glutamatergic and GABAergic receptors, (ii) are contributed by gap-junction interactions, and (iii) are controlled by GABA(B) receptors that are presumably located presynaptically.

Interaction between perirhinal and medial prefrontal cortex is required for temporal order but not recognition memory for objects in rats

The present study investigated the roles of the perirhinal cortex, medial prefrontal cortex, and intrahemispheric interactions between them in recognition and temporal order memory for objects. Experiment 1 assessed the effects of bilateral microinfusions of the sodium channel blocker lidocaine into either the anterior perirhinal or medial prefrontal cortex immediately before memory testing in a familiarity discrimination task and a recency discrimination task, both of which involved spontaneous exploration of objects. Inactivation of the perirhinal cortex disrupted performance in both tasks, whereas inactivation of the medial prefrontal cortex disrupted performance in the recency, but not the familiarity, discrimination task. In a second experiment, the importance of intrahemispheric interactions between these structures in temporal order memory were assessed by comparing the effects of unilateral inactivation of either structure alone with those of crossed unilateral inactivation of both structures on the recency discrimination task. Crossed unilateral inactivation of both structures produced a significant impairment, whereas inactivation of either structure alone produced little or no impairment. Collectively, these findings suggest that the perirhinal cortex, but not the medial prefrontal cortex, contributes to retrieval of information necessary for long-term object recognition, whereas both structures, via intrahemispheric interactions between them, contribute to retrieval of information necessary for long-term object temporal order memory. These data are consistent with models in which attributed information is stored in posterior cortical sites and supports lower-order mnemonic functions (e.g., recognition memory) but can also be retrieved and further processed via interactions with the prefrontal cortex to support higher-order mnemonic functions (e.g., temporal order memory).

Long-term pregabalin treatment protects basal cortices and delays the occurrence of spontaneous seizures in the lithium-pilocarpine model in the rat

PURPOSE

To determine whether a pharmacologic treatment could delay or prevent the epileptogenesis induced by status epilepticus (SE) through the protection of some brain areas, we studied the effects of the long-term exposure to pregabalin (PGB) on neuronal damage and epileptogenesis induced by lithium-pilocarpine SE.

METHODS

SE was induced in adult and 21-day-old (P21) rats. At 20 min after pilocarpine, rats received 50 mg/kg PGB (pilo-preg) or saline (pilo-saline). PGB treatment was given daily at the dose of 50 mg/kg for 7 days after SE and at 10 mg/kg from day 8 until killing. Neuronal damage was assessed in hippocampus and piriform and entorhinal cortices in brain sections stained with thionine and obtained from adult and P21 animals killed 6 days after SE. The number of glial fibrillary acidic protein (GFAP)-reactive astrocytes was tested by immunohistochemistry in sections adjacent to those used for cell counting. The latency to spontaneous seizures was controlled by visual observation and EEG recording.

RESULTS

PGB induced neuroprotection in layer II of piriform cortex and layers III-IV of ventral entorhinal cortex of adult rats, whereas no hippocampal region was protected. In P21 rats, damage was limited to the hilus and similar in pilo-preg and pilo-saline animals. The number of GFAP-positive astrocytes was higher in pilocarpine- than in saline-treated rats. It was decreased in pilo-preg compared with pilo-saline rats in layer II of the piriform cortex. Adult pilo-preg rats became epileptic after a longer latency (39 days) than did pilo-saline rats (22 days).

CONCLUSIONS

These data underline the antiepileptogenic consequences of long-term PGB treatment, possibly mediated by the protection of piriform and entorhinal cortices in the lithium-pilocarpine model of epilepsy.

Glucocorticoid-induced impairment of declarative memory retrieval is associated with reduced blood flow in the medial temporal lobe

Previous work indicates that stress levels of circulating glucocorticoids can impair retrieval of declarative memory in human subjects. Several studies have reported that declarative memory retrieval relies on the medial temporal lobe. The present study used H(2)(15)O-positron emission tomography to investigate whether acutely elevated glucocorticoid levels affect regional cerebral blood flow in the medial temporal lobe, as well as in other brain regions, during declarative memory retrieval in healthy male human subjects. When measured over four different declarative memory retrieval tasks, a single, stress-level dose of cortisone (25 mg) administered orally 1 h before retention testing, induced a large decrease in regional cerebral blood flow in the right posterior medial temporal lobe, the left visual cortex and the cerebellum. The decrease in the right posterior medial temporal lobe was maximal in the parahippocampal gyrus, a region associated with successful verbal memory retrieval. Cortisone administration also significantly impaired cued recall of word pairs learned 24 h earlier, while drug effects on performance in the other tasks (verbal recognition, semantic generation and categorization) were not significant. The present results provide further evidence that acutely elevated glucocorticoid levels can impair declarative memory retrieval processes and suggest that such impairments may be related to a disturbance of medial temporal lobe function.

Presurgical evaluation of epilepsy by brain diffusion: MR-detected effects of flumazenil on the epileptogenic focus

PURPOSE

After focal status epilepticus, focal alterations of the apparent diffusion coefficient (ADC) have been demonstrated in the epileptogenic zone by using diffusion-weighted magnetic resonance (MR)imaging (DWI). Effects of flumazenil on an epileptogenic focus have been demonstrated by EEG recordings, but not by functional MRI. We hypothesized that dynamic spatiotemporal alterations of brain diffusion of the epileptogenic focus after application of flumazenil will be detectable by DWI and correlate with the epileptogenic zone.

METHODS

Twelve adult patients considered for epilepsy surgery with medically intractable temporal lobe epilepsy (TLE; n = 7), extratemporal lobe epilepsy (ETE; n = 2), and TLE+ETE (n = 3) were prospectively examined with DWI interictally (serving as baseline) and 10 min after application of 1 mg flumazenil i.v.

RESULTS

The baseline interictal ADC was significantly elevated in the hippocampus on the ictogenic side in the patients with TLE (p = 0.002) as compared with healthy volunteers. The following changes of the mean ADC were seen in different regions of interest (ROIs) after injection of flumazenil: decreases in the hippocampus on the seizure-onset side by 14.8% (p = 0.005); decreases in the parahippocampal gyrus on both sides by 6.8% (epileptogenic side; p = 0.044) or 7.9% (nonepileptogenic side; NS), respectively; decreases in the cortex on the nonictogenic side by 7.9% (p = 0.047); and no significant changes of the ADC in the other ROIs.

CONCLUSIONS

ADC decreases measured after application of flumazenil were seen in the seizure-onset zone as revealed by EEG and structural MRI and are an indicator of focus localization in patients with TLE.

Differences in GABAergic transmission between two inputs into the perirhinal cortex

We have investigated properties of GABAergic synaptic transmission in perirhinal cortex evoked by stimulation of temporal and entorhinal cortex sides. GABAA IPSCs were isolated by blockade of glutamatergic transmission in slices of adult perirhinal cortex; IPSC decay was best fitted with two exponentials. Interestingly, temporal IPSCs had a larger slow component of decay (P < 0.05) compared to entorhinal IPSCs. Depression of IPSCs by the GABAB receptor agonist baclofen was greater (P < 0.05) in the temporal input (79 +/- 4% depression) than the entorhinal input (65 +/- 3% depression). Furthermore, baclofen abolished the slow component of IPSC decay in both inputs. Activity-dependent depression of IPSCs at 5 Hz was greater (P < 0.05) in the temporal input [paired pulse ratio (PPR) 0.5 +/- 0.04] compared to that in the entorhinal input (PPR 0.67 +/- 0.02, n = 10). The differences in paired pulse depression between the inputs were removed by the GABAB receptor antagonist CGP55845A. This study demonstrates several differences in GABA transmission between temporal and entorhinal inputs including the differential activation of presynaptic GABAB receptors and differential regulation of inhibitory synaptic transmission. These properties may be important in the control of neuronal activity within perirhinal cortex.

Facilitation of dopamine-mediated locomotor activity in adult rats following cholinergic denervation

The dopamine hypothesis of schizophrenia postulates hyperactivity of dopaminergic neurotransmission in the mesolimbic system. However, the possible underlying causes for this dopaminergic overfunction are not well understood. Therefore, the main aim of this study was to examine the effect of central cholinergic denervation on dopamine-mediated functions. We also examined the effect of neonatal cholinergic denervation upon adult brain function. The immunotoxin 192 IgG-saporin causes severe lesions of the basal forebrain cholinergic system when infused into the lateral ventricles by targeting neurons expressing the p75 neurotrophin receptor. The toxin may also damage p75-expressing Purkinje neurons in the cerebellum. We have compared the behavioral effects of intracerebroventricular injections of 192 IgG-saporin to adult rats with that of injections to neonate rats. As expected, adult treated rats displayed an almost complete cholinergic denervation of forebrain corticohippocampal areas concomitant with a marked impairment in the Morris water maze. When tested as adults, neonatally treated animals had a less complete cholinergic denervation and showed lesser impairments in water maze behaviors. Interestingly, adult treated rats showed increased spontaneous horizontal activity and a remarkable increase in locomotor response to d-amphetamine as evidenced by increased horizontal and vertical activity. There were no marked changes of spontaneous or drug-induced locomotor activity in adult rats treated with 192 IgG-saporin as neonates. These results suggest that cholinergic denervation of the forebrain causes a marked enhancement of behavioral responses related to dopaminergic activity, probably mainly mediated presynaptically. However, it cannot be fully excluded that damage to noncholinergic systems, e.g., Purkinje cells, might contribute to the effects. The striking overreaction to dopaminergic stimuli, presumably caused by the cholinergic deficit, is discussed in relation to the suggested role of cholinergic malfunctioning in schizophrenia.

Long-term enhancement of excitatory synaptic inputs to layer V parahippocampal neurons by low frequency stimulation in rat brain slices

Excitatory inputs to layer V neurons of the parasubiculum and medial entorhinal cortex were examined in rat brain slices with intracellular and field potential recordings. Single extracellular stimuli to layer V evoked subthreshold excitatory postsynaptic potentials (EPSPs) or a long duration (>100 ms) depolarization that sustained high frequency firing. Repetitive stimulation at low frequencies (from 1/10 s to 1/min) induced stable long-lasting decreases in the threshold for firing in individual cells or population events, and also induced stable long-lasting increases in evoked intracellular or field response amplitudes. More stimuli were required to produce the equivalent changes in threshold and amplitude in the presence of MCPG (200 microM). Smaller changes in amplitude, but equivalent changes in threshold were elicited in the presence of CPP (10 microM), or CPPG (20 microM). No changes in threshold or amplitude were detected in the presence of CNQX (10 microM), even when used in combination with picrotoxin (100 microM). EPSP facilitation was enhanced greatly by firing in postsynaptic cells. It is suggested that stable changes in excitatory inputs to layer V parahippocampal neurons involve the activation of NMDA and metabotropic glutamate receptors, but requires AMPA receptor activation and postsynaptic cell firing.

NMDA and muscarinic blockade in the perirhinal cortex impairs object discrimination in rats

To determine the possible involvement of NMDA and muscarinic activation of the perirhinal cortex in object discrimination, an NMDA antagonist, D,L-2-amino-5-phosphonopentanoic acid (AP5), and a muscarinic antagonist, scopolamine (SCP) were injected into the perirhinal cortex of rats. Each drug at the higher dose (AP5 60 mM, SCP 80 mM) significantly decreased correct choices on the retention test of object discrimination. SCP, but not AP5, also significantly increased response latency, but this increase was not necessarily related to the time spent for a choice. These results suggest that activation of both NMDA and muscarinic receptors contributes to object discrimination.

Neurotoxic lesions of perirhinal cortex impair visual recognition memory in rhesus monkeys

Recent excitotoxic lesion studies in monkeys have shown that the recognition memory deficits originally attributed to amygdalo-hippocampal damage were due in whole or in part to the accompanying damage to surrounding tissue, including fibers of passage. Here we show that the same conclusion does not apply to the visual recognition impairment produced by aspiration lesions of perirhinal cortex inasmuch as equally severe impairment was found after excitotoxic lesions of this cortex. The finding demonstrates that damage limited to perirhinal neurons is sufficient to impair visual memory and that damage to fibers of passage neither caused nor exacerbated the effect described initially.

Activation of muscarinic receptors induces protein synthesis-dependent long-lasting depression in the perirhinal cortex

There is strong evidence that decrements in neuronal activation in perirhinal cortex when a novel stimulus is repeated provide a neural substrate of visual recognition memory. There is also strong evidence that muscarinic acetylcholine (ACh) receptors are involved in learning and memory. However, the mechanisms underlying neuronal decrements in the perirhinal cortex and the basis of ACh involvement in learning and memory are not understood. In an in vitro preparation of rat perirhinal cortex we now demonstrate that activation of ACh receptors by carbachol (CCh) produces long-lasting depression (LLD) of synaptic transmission that is dependent on muscarinic M1 receptor activation. Crucially, the induction of this form of LLD requires neither N-methyl-D-aspartate receptor activation nor synaptic stimulation. CCh-induced LLD was not blocked by the protein kinase C inhibitors staurosporine or BIM, or by the protein phosphatase inhibitor okadaic acid. However, each of cyclopiazonic acid (an agent that depletes intracellular calcium stores) and anisomycin (an inhibitor of protein synthesis) significantly reduced the magnitude of CCh-induced LLD. These mechanisms triggered by muscarinic receptor activation could play a role in the induction and/or expression of certain forms of activity-dependent long-term depression in perirhinal cortex. An understanding of CCh-induced LLD may thus provide clues to the mechanisms underlying lasting neuronal decrements that occur in the perirhinal cortex and hence for neural substrates of visual recognition memory.

Instability in the place field location of hippocampal place cells after lesions centered on the perirhinal cortex

The perirhinal cortex appears to play a key role in memory, and the neighboring hippocampus is critically involved in spatial processing. The possibility exists, therefore, that perirhinal-hippocampal interactions are important for spatial memory processes. The purpose of the present study was to investigate the contribution of the perirhinal cortex to the location-specific firing ("place field") of hippocampal complex-spike ("place") cells. The firing characteristics of dorsal CA1 place cells were examined in rats with bilateral ibotenic acid lesions centered on the perirhinal cortex (n = 4) or control surgeries (n = 5) as they foraged in a rectangular environment. The activity of individual place cells was also monitored after a delay period of either 2 min, or 1 or 24 hr, during which time the animal was removed from the environment. Although the perirhinal cortex lesion did not affect the place field size or place cell firing characteristics during a recording session, it was determined that the location of the place field shifted position across the delay period in 36% (10 of 28) of the cells recorded from lesioned animals. In contrast, none of the place cells (0 of 29) recorded from control animals were unstable by this measure. These data indicate that although the initial formation of place fields in the hippocampus is not dependent on perirhinal cortex, the maintenance of this stability over time is disrupted by perirhinal lesions. This instability may represent an erroneous "re-mapping" of the environment and suggests a role for the perirhinal cortex in spatial memory processing.

A peculiar pattern of temporal involvement of rat perirhinal cortex in memory processing

By means of the fully reversible tetrodotoxin inactivation technique, perirhinal cortex (PC) mnemonic function was investigated in rats trained to a passive avoidance response (PAR). It was shown that PC functional integrity is necessary during PAR acquisition, during late and very late consolidation (from 24 hr up to 192 hr after the training session), and during retrieval. An unexpected finding was that the PC was not involved in the early consolidation period. Thus the PC may play a relatively simple relay or connective role during acquisition, but its very late and very long consolidative involvement may indicate a peculiar function in consolidation and possibly in the storage of the PAR engram. The results are discussed in terms of the mnemonic characteristics of other neural sites (amygdala, hippocampus, and entorhinal cortex) involved in the same learning process.

Auditory thalamus, dorsal hippocampus, basolateral amygdala, and perirhinal cortex role in the consolidation of conditioned freezing to context and to acoustic conditioned stimulus in the rat

On the basis of previous experimental evidence, it is known that the auditory thalamus (AT), the dorsal hippocampus (DH), the basolateral amygdala (BLA), and the perirhinal cortex (PC) are involved in the mnemonic processing of conditioned freezing. In particular, BLA and PC appear to be involved both in conditioned stimulus (CS) and context conditioned freezing. Through AT, the auditory CS is sent to other sites, whereas DH is involved in context conditioning. Nevertheless, the existing evidence does not make it possible to assess AT, DH, BLA, and PC involvement during the consolidation phase of conditioned freezing. To address this question, fully reversible tetrodotoxin (TTX) inactivation was performed on adult male Wistar rats having undergone CS and context fear training. Anesthetized animals were injected stereotaxically with TTX (either 5 or 10 ng in 0.5 or 1.0 microliter of saline, according to site dimensions) at increasing post-acquisition delays. Context and CS freezing durations were measured during retention testing, always performed 48 and 72 hr after TTX administration. The results showed that AT inactivation does not disrupt consolidation of either contextual or auditory fear memories. In contrast, inactivation of the other three structures disrupted consolidation. For the DH, this disruption was specific to contextual cues and only occurred when inactivation was performed early (up to 1.5 hr) after training. The BLA and PC were shown to be involved in the consolidation of both contextual and auditory fear. Their involvement persisted for longer periods of time (2d for BLA and 8 d for PC). These findings provide information to build a temporal profile for the post-training processing of fear memories in structures known to be important for this form of learning. The results are discussed in relation to previous studies on conditioned freezing and other aversive conditioned response neural correlates.