Perineuronal net degradation rescues CA2 plasticity in a mouse model of Rett syndrome.

Perineuronal nets (PNNs), a specialized form of extracellular matrix, are abnormal in the brains of people with Rett syndrome (RTT). We previously reported that PNNs function to restrict synaptic plasticity in hippocampal area CA2, which is unusually resistant to long-term potentiation (LTP) and has been linked to social learning in mice. Here we report that PNNs appear elevated in area CA2 of the hippocampus of an individual with RTT and that PNNs develop precociously and remain elevated in area CA2 of a mouse model of RTT (Mecp2-null). Further, we provide evidence that LTP could be induced at CA2 synapses prior to PNN maturation (postnatal day 8-11) in wild-type mice and that this window of plasticity was prematurely restricted at CA2 synapses in Mecp2-null mice. Degrading PNNs in Mecp2-null hippocampus was sufficient to rescue the premature disruption of CA2 plasticity. We identified several molecular targets that were altered in the developing Mecp2-null hippocampus that may explain aberrant PNNs and CA2 plasticity, and we discovered that CA2 PNNs are negatively regulated by neuronal activity. Collectively, our findings demonstrate that CA2 PNN development is regulated by Mecp2 and identify a window of hippocampal plasticity that is disrupted in a mouse model of RTT.

An Effect of Chronic Stress on Prospective Memory via Alteration of Resting-State Hippocampal Subregion Functional Connectivity.

The alteration of hippocampal function by chronic stress impairs higher order cognitive functions such as prospective memory (PM). However, how chronic stress affects hippocampal subregions related to PM remains largely unknown. In this study, the altered functional network of hippocampal subregions related to PM in chronic stress was explored. College students (N = 21) completed PM tasks and resting-state functional magnetic resonance imaging scans one month prior to (baseline) and during the final examination week (chronic stress). Hippocampal subregions' seed-based functional connectivity (FC) and PM were compared between baseline and chronic stress. PM performance declined in chronic stress. The FC of the cornu ammonis 2, 3 and dentate gyrus (CA23DG) with the bilateral caudate and precuneus was increased in chronic stress, while the FC of the subicular complex (SUBC) with the left middle frontal gyrus, the left inferior parietal gyrus and the right supramarginal gyrus was decreased. There was a negative correlation between PM performance and the FC of hippocampal subregions. We found chronic stress impairs PM by decreasing the FC of SUBC and increasing the FC of CA23DG. These findings suggest functional changes in hippocampal subregion networks as a mechanism underlying the impairment of PM in chronic stress.

Different patterns of epileptiform-like activity are generated in the sclerotic hippocampus from patients with drug-resistant temporal lobe epilepsy.

Human hippocampal slice preparations from patients with temporal lobe epilepsy (TLE) associated with hippocampal sclerosis (HS) are excellent material for the characterization of epileptiform-like activity. However, it is still unknown if hippocampal regions as cornu Ammonis (CA) 1, CA3 and CA4, generate population epileptiform-like activity. Here, we investigated epileptiform activities of the subiculum, CA1, CA2, CA3, CA4 (induced by elevation of extracellular potassium concentration) and the dentate gyrus (induced with hilar stimulation and elevation of potassium concentration) from sclerotic hippocampi of patients with drug-resistant TLE. Five types of epileptiform-like activity were observed: interictal-like events; periodic ictal spiking; seizure-like events; spreading depression-like events; tonic seizure-like events and no activity. Different susceptibilities to generate epileptiform activity among hippocampal regions were observed; the dentate gyrus was the most susceptible region followed by the subiculum, CA4, CA1, CA2 and CA3. The incidence of epileptiform activity pattern was associated with specific regions of the hippocampal formation. Moreover, it was observed that each region of the hippocampal formation exhibits frequency-specific ranges in each subfield of the sclerotic human tissue. In conclusion, this study demonstrates that epileptiform-like activity may be induced in different regions of the hippocampal formation, including regions that are severely affected by neuronal loss.

Regional vulnerability of the hippocampus to repeated motor activity deprivation.

Spontaneous vertical and horizontal exploratory movements are integral components of rodent behavior. Little is known, however, about the structural and functional consequences of restricted spontaneous exploration. Here, we report two experiments to probe whether restriction in vertical activity (rearing) in rats could induce neuro-hormonal and behavioral disturbances. Rearing movements in rats were deprived for 3h/day for 30 consecutive days by placing the animal into a circular tunnel task. Rats temporarily deprived of rearing behavior showed elevated plasma corticosterone levels but no detectable psychological distress and/or anxiety-related behavior within an elevated plus maze. However, rats emitted a greater number of 22-kHz ultrasonic vocalizations and spent significantly more time vocalizing than controls when deprived of their rearing behavior. Despite intact spatial performance within wet- and dry-land spatial tasks, rearing-deprived rats also exhibited a significant alteration in search strategies within both spatial tasks along with reduced volume and neuron number in the hippocampal subregion CA2. These data suggest a new approach to test the importance of free exploratory behavior in endocrine and structural manifestations. The results support a central role of the CA2 in spontaneous exploratory behavior and vulnerability to psychological stress.

AM841, a covalent cannabinoid ligand, powerfully slows gastrointestinal motility in normal and stressed mice in a peripherally restricted manner.

BACKGROUND AND PURPOSE: Cannabinoid (CB) ligands have been demonstrated to have utility as novel therapeutic agents for the treatment of pain, metabolic conditions and gastrointestinal (GI) disorders. However, many of these ligands are centrally active, which limits their usefulness. Here, we examine a unique novel covalent CB receptor ligand, AM841, to assess its potential for use in physiological and pathophysiological in vivo studies. EXPERIMENTAL APPROACH: The covalent nature of AM841 was determined in vitro using electrophysiological and receptor internalization studies on isolated cultured hippocampal neurons. Mouse models were used for behavioural analysis of analgesia, hypothermia and hypolocomotion. The motility of the small and large intestine was assessed in vivo under normal conditions and after acute stress. The brain penetration of AM841 was also determined. KEY RESULTS: AM841 behaved as an irreversible CB1 receptor agonist in vitro. AM841 potently reduced GI motility through an action on CB1 receptors in the small and large intestine under physiological conditions. AM841 was even more potent under conditions of acute stress and was shown to normalize accelerated GI motility under these conditions. This compound behaved as a peripherally restricted ligand, showing very little brain penetration and no characteristic centrally mediated CB1 receptor-mediated effects (analgesia, hypothermia or hypolocomotion). CONCLUSIONS AND IMPLICATIONS: AM841, a novel peripherally restricted covalent CB1 receptor ligand that was shown to be remarkably potent, represents a new class of potential therapeutic agents for the treatment of functional GI disorders.

Plasticity of hippocampal subfield volume cornu ammonis 2+3 over the course of withdrawal in patients with alcohol dependence.

IMPORTANCE: Research focusing on plasticity has shown adult neurogenesis in hippocampal subfields. Chronic alcoholism is associated with decreased plasticity and reduced whole hippocampal volume that could contribute to neuropsychiatric characteristics and outcome of the disease. OBJECTIVE: To investigate the effect of alcohol abstinence on neuronal plasticity measured as longitudinal volume change in distinct hippocampal subfields. DESIGN, SETTING, AND PARTICIPANTS: We acquired high-resolution structural images of 42 patients addicted to alcohol and 32 healthy control participants. Patients and control participants were both scanned twice, once after withdrawal and 2 weeks later. MAIN OUTCOMES AND MEASURES: Volumes of hippocampal subfields cornu ammonis (CA) 2+3, CA4+dentate gyrus, and subiculum were determined with a user-independent segmentation method. RESULTS: We found plasticity effects in bilateral CA2+3 in patients addicted to alcohol. Compared with healthy control participants, patients had lower CA2+3 volume at pretest (t31 = -0.73, P = .47) and showed a significant normalization of gray matter volume 2 weeks later. Pretest CA2+3 (t31 = -3.93, P < .001) volume was negatively associated with years of regular alcohol consumption (r42 = -0.32, P < .05) and more severe alcohol-withdrawal symptoms (r38 = -0.35, P < .05). Patients with stronger withdrawal symptoms displayed the largest volume increase of CA2+3 (r38 = 0.55, P < .001). CONCLUSIONS AND RELEVANCE: The observed normalization of the bilateral hippocampal CA2+3 volume deficit matches animal data, showing a strong increase of hippocampal neurogenesis after cessation of alcohol consumption, and fits the reported increase of patients' cognitive function within a few months of alcohol abstinence. The role of CA3 in pattern separation and completion is also critical for formation of hallucinations, which constitute a severe symptom of the withdrawal syndrome. The study adds further biological arguments from structural brain research to abstain from alcohol.

Association between glucocorticoid receptor methylation and hippocampal subfields in major depressive disorder.

BACKGROUND: DNA methylation in the promoter region of the glucocorticoid receptor gene (NR3C1) is closely associated with childhood adversity and suicide. However, few studies have examined NR3C1 methylation in relation to major depressive disorder (MDD) and hippocampal subfield volumes. We investigated the possible association between NR3C1 methylation and structural brain alterations in MDD in comparison with healthy controls. METHODS: We compared the degree of NR3C1 promoter methylation in the peripheral blood of non-psychotic outpatients with MDD and that of healthy controls. Correlations among NR3C1 promoter methylation, structural abnormalities in hippocampal subfield volumes and whole-brain cortical thickness, and clinical variables were also analyzed. RESULTS: In total, 117 participants (45 with MDD and 72 healthy controls) were recruited. Patients with MDD had significantly lower methylation than healthy controls at 2 CpG sites. In MDD, methylations had positive correlations with the bilateral cornu ammonis (CA) 2-3 and CA4-dentate gyrus (DG) subfields. However, in healthy controls, methylations had positive correlation with the subiculum and presubiculum. There were no differences in total and subfield volumes of the hippocampus between patients with MDD and healthy controls. Compared with healthy controls, patients with MDD had a significantly thinner cortex in the left rostromiddle frontal, right lateral orbitofrontal, and right pars triangularis areas. CONCLUSIONS: Lower methylation in the NR3C1 promoter, which might have compensatory effects relating to CA2-3 and CA4-DG, is a distinct epigenetic characteristic in non-psychotic outpatients with MDD. Future studies with a longitudinal design and a comprehensive neurobiological approach are warranted in order to elucidate the effects of NR3C1 methylation.

Interneuronal membrane contacts and syncytial perforations in CA2 hippocampal area after brain trauma.

Membranes of pyramid neuron bodies located in CA2 hippocampal area were studied by electron microscopy after gunshot craniocerebral injury. In control group, asynaptic contacts and interneuronal syncytial perforations forming from tight junctions were observed. Contacts and perforations increased in size after trauma. Their number was maximum after severe gunshot injury. They reached their maximal size on days 5-7 after the injury.

The epileptic human hippocampal cornu ammonis 2 region generates spontaneous interictal-like activity in vitro.

The dentate gyrus, the cornu ammonis 2 region and the subiculum of the human hippocampal formation are resistant to the cell loss associated with temporal lobe epilepsy. The subiculum, but not the dentate gyrus, generates interictal-like activity in tissue slices from epileptic patients. In this study, we asked whether a similar population activity is generated in the cornu ammonis 2 region and examined the electrophysiological and neuroanatomical characteristics of human epileptic cornu ammonis 2 neurons that may be involved. Hippocampal slices were prepared from postoperative temporal lobe tissue derived from epileptic patients. Field potentials and multi-unit activity were recorded in vitro using multiple extracellular microelectrodes. Pyramidal cells were characterized in intra-cellular records and were filled with biocytin for subsequent anatomy. Fluorescent immunostaining was made on fixed tissue against the chloride-cation cotransporters sodium-potassium-chloride cotransporter-1 and potassium-chloride cotransporter-2. Light and electron microscopy were used to examine the parvalbumin-positive perisomatic inhibitory network. In 15 of 20 slices, the hippocampal cornu ammonis 2 region generated a spontaneous interictal-like activity, independently of population events in the subiculum. Most cornu ammonis 2 pyramidal cells fired spontaneously. All cells fired single action potentials and burst firing was evoked in three cells. Spontaneous excitatory postsynaptic potentials were recorded in all cells, but hyperpolarizing inhibitory postsynaptic potentials were detected in only 27% of the cells. Two-thirds of cornu ammonis 2 neurons showed depolarizing responses during interictal-like events, while the others were inhibited, according to the current sink in the cell body layer. Two biocytin-filled cells both showed a pyramidal-like morphology with axons projecting to the cornu ammonis 2 and cornu ammonis 3 regions. Expression of sodium-potassium-chloride cotransporter-1 and potassium-chloride cotransporter-2 was reduced in some cells of the epileptic cornu ammonis 2 region, but not to an extent corresponding to the proportion of cells in which hyperpolarizing postsynaptic potentials were absent. Numbers of parvalbumin-positive inhibitory cells and axons were shown to be decreased in the epileptic tissue. Electron microscopy showed the preservation of somatic inhibitory input of cornu ammonis 2 cells, and confirmed the loss of parvalbumin from the interneurons rather than their death. An extra excitatory input (partly coming from sprouted mossy fibres) was demonstrated to innervate cornu ammonis 2 cell bodies. Our results show that the cornu ammonis 2 region of the sclerotic human hippocampus can generate an independent epileptiform activity. Inhibitory and excitatory signalling were functional but modified in epileptic cornu ammonis 2 pyramidal cells. Overexcitation and the altered functional properties of perisomatic inhibitory network, rather than a modified chloride homeostasis, may account for the perturbed gamma-aminobutyric acid-ergic signalling and the generation of interictal-like activity in the human epileptic cornu ammonis 2 region.