Neocortical, hippocampal and septal parvalbumin- and somatostatin-containing neurons in young and aged rats: correlation with passive avoidance and water maze performance

Robust differences in cortical cell type proportions across healthy human aging inferred through cross-dataset transcriptome analyses

Age-related declines in cognitive function are driven by cell type-specific changes in the brain. However, it remains challenging to study cellular differences associated with healthy aging as traditional approaches scale poorly to the sample sizes needed to capture aging and cellular heterogeneity. Here, we employed cellular deconvolution to estimate relative cell type proportions using frontal cortex bulk gene expression from individuals without psychiatric conditions or brain pathologies. Our analyses comprised 8 datasets and 6 cohorts (1142 subjects and 1429 samples) with ages of death spanning 15-90 years. We found aging associated with profound differences in cellular proportions, with the largest changes reflecting fewer somatostatin- and vasoactive intestinal peptide-expressing interneurons, more astrocytes and other non-neuronal cells, and a suggestive "U-shaped" quadratic relationship for microglia. Cell type associations with age were markedly robust across bulk-and single nucleus datasets. Altogether, we present a comprehensive account of proportional differences in cortical cell types associated with healthy aging.

Effects of Aging on the Structure and Expression of NMDA Receptors of Somatostatin Expressing Neurons in the Mouse Hippocampus

Changes in the physiology, neurochemistry and structure of neurons, particularly of their dendritic spines, are thought to be crucial players in age-related cognitive decline. One of the most studied brain structures affected by aging is the hippocampus, known to be involved in different essential cognitive processes. While the aging-associated quantitative changes in dendritic spines of hippocampal pyramidal cells have already been studied, the relationship between aging and the structural dynamics of hippocampal interneurons remains relatively unknown. Spines are not a frequent feature in cortical inhibitory neurons, but these postsynaptic structures are abundant in a subpopulation of somatostatin expressing interneurons, particularly in oriens-lacunosum moleculare (O-LM) cells in the hippocampal CA1. Previous studies from our laboratory have shown that the spines of these interneurons are highly plastic and influenced by NMDA receptor manipulation. Thus, in the present study, we have investigated the impact of aging on this interneuronal subpopulation. The analyses were performed in 3−, 9−, and 16-month-old GIN mice, a strain in which somatostatin positive interneurons express GFP. We studied the changes in the density of dendritic spines, en passant boutons, and the expression of NMDA receptors (GluN1 and GluN2B) using confocal microscopy and image analysis. We observed a significant decrease in dendritic spine density in 9-month-old animals when compared with 3-month-old animals. We also observed a decrease in the expression of the GluN2B subunit in O-LM cells, but not of that of GluN1, during aging. These results will constitute the basis for more advanced studies of the structure and connectivity of interneurons during aging and their contribution to cognitive decline.

GABA levels in ventral visual cortex decline with age and are associated with neural distinctiveness

Age-related neural dedifferentiation-a decline in the distinctiveness of neural representations in the aging brain-has been associated with age-related declines in cognitive abilities. But why does neural distinctiveness decline with age? Based on prior work in nonhuman primates and more recent work in humans, we hypothesized that the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) declines with age and is associated with neural dedifferentiation in older adults. To test this hypothesis, we used magnetic resonance spectroscopy (MRS) to measure GABA and functional MRI (fMRI) to measure neural distinctiveness in the ventral visual cortex in a set of older and younger participants. Relative to younger adults, older adults exhibited lower GABA levels and less distinct activation patterns for faces and houses in the ventral visual cortex. Furthermore, individual differences in GABA within older adults positively predicted individual differences in neural distinctiveness. These results provide novel support for the view that age-related reductions of GABA contribute to age-related reductions in neural distinctiveness (i.e., neural dedifferentiation) in the human ventral visual cortex.

Septohippocampal transmission from parvalbumin-positive neurons features rapid recovery from synaptic depression

Parvalbumin-containing projection neurons of the medial-septum-diagonal band of Broca ([Formula: see text]) are essential for hippocampal rhythms and learning operations yet are poorly understood at cellular and synaptic levels. We combined electrophysiological, optogenetic, and modeling approaches to investigate [Formula: see text] neuronal properties. [Formula: see text] neurons had intrinsic membrane properties distinct from acetylcholine- and somatostatin-containing MS-DBB subtypes. Viral expression of the fast-kinetic channelrhodopsin ChETA-YFP elicited action potentials to brief (1-2 ms) 470 nm light pulses. To investigate [Formula: see text] transmission, light pulses at 5-50 Hz frequencies generated trains of inhibitory postsynaptic currents (IPSCs) in CA1 stratum oriens interneurons. Using a similar approach, optogenetic activation of local hippocampal PV ([Formula: see text]) neurons generated trains of [Formula: see text]-mediated IPSCs in CA1 pyramidal neurons. Both synapse types exhibited short-term depression (STD) of IPSCs. However, relative to [Formula: see text] synapses, [Formula: see text] synapses possessed lower initial release probability, transiently resisted STD at gamma (20-50 Hz) frequencies, and recovered more rapidly from synaptic depression. Experimentally-constrained mathematical synapse models explored mechanistic differences. Relative to the [Formula: see text] model, the [Formula: see text] model exhibited higher sensitivity to calcium accumulation, permitting a faster rate of calcium-dependent recovery from STD. In conclusion, resistance of [Formula: see text] synapses to STD during short gamma bursts enables robust long-range GABAergic transmission from MS-DBB to hippocampus.

Cognitive Reserve in Model Systems for Mechanistic Discovery: The Importance of Longitudinal Studies

The goal of this review article is to provide a resource for longitudinal studies, using animal models, directed at understanding and modifying the relationship between cognition and brain structure and function throughout life. We propose that forthcoming longitudinal studies will build upon a wealth of knowledge gleaned from prior cross-sectional designs to identify early predictors of variability in cognitive function during aging, and characterize fundamental neurobiological mechanisms that underlie the vulnerability to, and the trajectory of, cognitive decline. Finally, we present examples of biological measures that may differentiate mechanisms of the cognitive reserve at the molecular, cellular, and network level.

nNOS-expressing interneurons control basal and behaviorally evoked arterial dilation in somatosensory cortex of mice

Cortical neural activity is coupled to local arterial diameter and blood flow. However, which neurons control the dynamics of cerebral arteries is not well understood. We dissected the cellular mechanisms controlling the basal diameter and evoked dilation in cortical arteries in awake, head-fixed mice. Locomotion drove robust arterial dilation, increases in gamma band power in the local field potential (LFP), and increases calcium signals in pyramidal and neuronal nitric oxide synthase (nNOS)-expressing neurons. Chemogenetic or pharmocological modulation of overall neural activity up or down caused corresponding increases or decreases in basal arterial diameter. Modulation of pyramidal neuron activity alone had little effect on basal or evoked arterial dilation, despite pronounced changes in the LFP. Modulation of the activity of nNOS-expressing neurons drove changes in the basal and evoked arterial diameter without corresponding changes in population neural activity.

DNA Methyltransferase 1 (DNMT1) Function Is Implicated in the Age-Related Loss of Cortical Interneurons

Increased life expectancy in modern society comes at the cost of age-associated disabilities and diseases. Aged brains not only show reduced excitability and plasticity, but also a decline in inhibition. Age-associated defects in inhibitory circuits likely contribute to cognitive decline and age-related disorders. Molecular mechanisms that exert epigenetic control of gene expression contribute to age-associated neuronal impairments. Both DNA methylation, mediated by DNA methyltransferases (DNMTs), and histone modifications maintain neuronal function throughout lifespan. Here we provide evidence that DNMT1 function is implicated in the age-related loss of cortical inhibitory interneurons. Dnmt1 deletion in parvalbumin-positive interneurons attenuates their age-related decline in the cerebral cortex. Moreover, conditional Dnmt1-deficient mice show improved somatomotor performance and reduced aging-associated transcriptional changes. A decline in the proteostasis network, responsible for the proper degradation and removal of defective proteins, is implicated in age- and disease-related neurodegeneration. Our data suggest that DNMT1 acts indirectly on interneuron survival in aged mice by modulating the proteostasis network during life-time.

Aging But Not Age-Related Hearing Loss Dominates the Decrease of Parvalbumin Immunoreactivity in the Primary Auditory Cortex of Mice

Alterations in inhibitory circuits of the primary auditory cortex (pAC) have been shown to be an aspect of aging and age-related hearing loss (AHL). Several studies reported a decline in parvalbumin (PV) immunoreactivity in aged rodent pAC of animals displaying AHL and conclude a relationship between reduced sensitivity and declined PV immunoreactivity. However, it remains elusive whether AHL or a general molecular aging is causative for decreased PV immunoreactivity. In this study, we aimed to disentangle the effects of AHL and general aging on PV immunoreactivity patterns in inhibitory interneurons of mouse pAC. We compared young and old animals of a mouse line with AHL (C57BL/6) and a mutant (C57B6.CAST-Cdh23Ahl+ ) that is not vulnerable to AHL according to their hearing status by measuring auditory brainstem responses (ABRs) and by an immunohistochemical evaluation of the PV immunoreactivity patterns in two dimensions (rostro-caudal and layer) in the pAC. Although AHL could be confirmed by ABR measurements for the C57BL/6 mice, both aged strains showed a similar reduction of PV+ positive interneurons in both, number and density. The pattern of reduction across the rostro-caudal axis and across cortical layers was similar for both aged lines. Our results demonstrate that a reduced PV immunoreactivity is a sign of general, molecular aging and not related to AHL.

Neuronal chemo-architecture of the entorhinal cortex: A comparative review

The identification of neuronal markers, that is, molecules selectively present in subsets of neurons, contributes to our understanding of brain areas and the networks within them. Specifically, recognizing the distribution of different neuronal markers facilitates the identification of borders between functionally distinct brain areas. Detailed knowledge about the localization and physiological significance of neuronal markers may also provide clues to generate new hypotheses concerning aspects of normal and abnormal brain functioning. Here, we provide a comprehensive review on the distribution within the entorhinal cortex of neuronal markers and the morphology of the neurons they reveal. Emphasis is on the comparative distribution of several markers, with a focus on, but not restricted to rodent, monkey and human data, allowing to infer connectional features, across species, associated with these markers, based on what is revealed by mainly rodent data. The overall conclusion from this review is that there is an emerging pattern in the distribution of neuronal markers in the entorhinal cortex when aligning data along a comparable coordinate system in various species.

Ontogeny of calcium-binding proteins in the cingulate cortex of the guinea pig: The same onset but different developmental patterns

This paper compared the density of calbindin D28k (CB), calretinin (CR) and parvalbumin (PV) containing neurons in prenatal, newborn and postnatal periods in the cingulate cortex (CC) of the guinea pig as an animal model. The distribution and co-distribution among calcium-binding proteins (CaBPs) was also investigated during the entire ontogeny. The study found that CB-positive neurons exhibited the highest density in the developing CC. The CC development in the prenatal period took place with a high level of CB and CR immunoreactivity and both of these proteins reached peak density during fetal life. The density of PV-positive neurons, in contrast to CB and CR-positive neurons, reached high levels postnatally. The observed changes of the CaBPs-positive neuron density in the developing CC coincide with developmental events in the guinea pig. E.g. the eyes opening moment may be preceded by elevated levels of CB and CR at E50, whereas high immunoreactivity of PV from P10 to P40 with a peak at P20 may indicate the participation of PV in enhancement of the inhibitory cortical pathway maturation.

The space where aging acts: focus on the GABAergic synapse

As it was established that aging is not associated with massive neuronal loss, as was believed in the mid‐20th Century, scientific interest has addressed the influence of aging on particular neuronal subpopulations and their synaptic contacts, which constitute the substrate for neural plasticity. Inhibitory neurons represent the most complex and diverse group of neurons, showing distinct molecular and physiological characteristics and possessing a compelling ability to control the physiology of neural circuits. This review focuses on the aging of GABAergic neurons and synapses. Understanding how aging affects synapses of particular neuronal subpopulations may help explain the heterogeneity of aging‐related effects. We reviewed the literature concerning the effects of aging on the numbers of GABAergic neurons and synapses as well as aging‐related alterations in their presynaptic and postsynaptic components. Finally, we discussed the influence of those changes on the plasticity of the GABAergic system, highlighting our results concerning aging in mouse somatosensory cortex and linking them to plasticity impairments and brain disorders. We posit that aging‐induced impairments of the GABAergic system lead to an inhibitory/excitatory imbalance, thereby decreasing neuron's ability to respond with plastic changes to environmental and cellular challenges, leaving the brain more vulnerable to cognitive decline and damage by synaptopathic diseases.

Age-related plasticity of the axon initial segment of cortical pyramidal cells in marmoset monkeys

Structural plasticity of the axon initial segment (AIS), the site of action potential initiation, is observed as part of the normal early development of the cortex, as well as in association with injury and disease. Here, we show that structural AIS plasticity also occurs with normal aging in adult marmosets. Immunohistochemical techniques were used to reveal the extent of the AIS of layer 2/3A pyramidal cells in 8 neocortical areas. We found that the AIS length varied significantly between areas in young adult (2-3 years old) marmosets, with neurons in frontal area 14C having the longest AIS, and those in the primary visual cortex the shortest. Similar interareal differences were observed in aged (12-14 year old) monkeys, but the AIS was significantly shortened in many areas, relative to the corresponding length in young adults. Shortening of the AIS is likely to represent a compensatory response to changes in the excitation-inhibition balance, associated with the loss of GABAergic interneurons in the aged cortex.

Developmental neuroplasticity and the origin of neurodegenerative diseases

OBJECTIVES

Neurodegenerative diseases like Alzheimer's and Parkinson's Disease, marked by characteristic protein aggregations, are more and more accepted to be synaptic disorders and to arise from a combination of genetic and environmental factors. In this review we propose our concept that neuroplasticity might constitute a link between early life challenges and neurodegeneration.

METHODS

After introducing the general principles of neuroplasticity, we show how adverse environmental stimuli during development impact adult neuroplasticity and might lead to neurodegenerative processes.

RESULTS

There are significant overlaps between neurodevelopmental and neurodegenerative processes. Proteins that represent hallmarks of neurodegeneration are involved in plastic processes under physiological conditions. Brain regions - particularly the hippocampus - that retain life-long plastic capacities are the key targets of neurodegeneration. Neuroplasticity is highest in young age making the brain more susceptible to external influences than later in life. Impacts during critical periods have life-long consequences on neuroplasticity and structural self-organization and are known to be common risk factors for neurodegenerative diseases.

CONCLUSIONS

Several lines of evidence support a link between developmental neuroplasticity and neurodegenerative processes later in life. A deeper insight into these processes is necessary to design strategies to mitigate or even prevent neurodegenerative pathologies.

Inhibitory dysfunction in amyotrophic lateral sclerosis: future therapeutic opportunities

In amyotrophic lateral sclerosis, motor neuron hyperexcitability and inhibitory dysfunction is emerging as a potential causative link in the dysfunction and degeneration of the motoneuronal circuitry that characterizes the disease. Interneurons, as key regulators of excitability, may mediate much of this imbalance, yet we know little about the way in which inhibitory deficits perturb excitability. In this review, we explore inhibitory control of excitability and the potential contribution of altered inhibition to amyotrophic lateral sclerosis disease processes and vulnerabilities, identifying important windows of therapeutic opportunity and potential interventions, specifically targeting inhibitory control at key disease stages.

Distinct behavioral consequences of short-term and prolonged GABAergic depletion in prefrontal cortex and dorsal hippocampus

GABAergic interneurons are essential for a functional equilibrium between excitatory and inhibitory impulses throughout the CNS. Disruption of this equilibrium can lead to various neurological or neuropsychiatric disorders such as epilepsy or schizophrenia. Schizophrenia itself is clinically defined by negative (e.g., depression) and positive (e.g., hallucinations) symptoms as well as cognitive dysfunction. GABAergic interneurons are proposed to play a central role in the etiology and progression of schizophrenia; however, the specific mechanisms and the time-line of symptom development as well as the distinct involvement of cortical and hippocampal GABAergic interneurons in the etiology of schizophrenia-related symptoms are still not conclusively resolved. Previous work demonstrated that GABAergic interneurons can be selectively depleted in adult mice by means of saporin-conjugated anti-vesicular GABA transporter antibodies (SAVAs) in vitro and in vivo. Given their involvement in schizophrenia-related disease etiology, we ablated GABAergic interneurons in the medial prefrontal cortex (mPFC) and dorsal hippocampus (dHPC) in adult male C57BL/6N mice. Subsequently we assessed alterations in anxiety, sensory processing, hyperactivity and cognition after long-term (>14 days) and short-term (<14 days) GABAergic depletion. Long-term GABAergic depletion in the mPFC resulted in a decrease in sensorimotor-gating and impairments in cognitive flexibility. Notably, the same treatment at the level of the dHPC completely abolished spatial learning capabilities. Short-term GABAergic depletion in the dHPC revealed a transient hyperactive phenotype as well as marked impairments regarding the acquisition of a spatial memory. In contrast, recall of a spatial memory was not affected by the same intervention. These findings emphasize the importance of functional local GABAergic networks for the encoding but not the recall of hippocampus-dependent spatial memories.

Simulating vertical and horizontal inhibition with short-term dynamics in a multi-column multi-layer model of neocortex

The paper introduces a multi-layer multi-column model of the cortex that uses four different neuron types and short-term plasticity dynamics. It was designed with details of neuronal connectivity available in the literature and meets these conditions: (1) biologically accurate laminar and columnar flows of activity, (2) normal function of low-threshold spiking and fast spiking neurons, and (3) ability to generate different stages of epileptiform activity. With these characteristics the model allows for modeling lesioned or malformed cortex, i.e. examine properties of developmentally malformed cortex in which the balance between inhibitory neuron subtypes is disturbed.

Trajectory of the main GABAergic interneuron populations from early development to old age in the rat primary auditory cortex

In both humans and rodents, decline in cognitive function is a hallmark of the aging process; the basis for this decrease has yet to be fully characterized. However, using aged rodent models, deficits in auditory processing have been associated with significant decreases in inhibitory signaling attributed to a loss of GABAergic interneurons. Not only are these interneurons crucial for pattern detection and other large-scale population dynamics, but they have also been linked to mechanisms mediating plasticity and learning, making them a prime candidate for study and modeling of modifications to cortical communication pathways in neurodegenerative diseases. Using the rat primary auditory cortex (A1) as a model, we probed the known markers of GABAergic interneurons with immunohistological methods, using antibodies against gamma aminobutyric acid (GABA), parvalbumin (PV), somatostatin (SOM), calretinin (CR), vasoactive intestinal peptide (VIP), choline acetyltransferase (ChAT), neuropeptide Y (NPY), and cholecystokinin (CCK) to document the changes observed in interneuron populations across the rat's lifespan. This analysis provided strong evidence that several but not all GABAergic neurons were affected by the aging process, showing most dramatic changes in expression of parvalbumin (PV) and somatostatin (SOM) expression. With this evidence, we show how understanding these trajectories of cell counts may be factored into a simple model to quantify changes in inhibitory signaling across the course of life, which may be applied as a framework for creating more advanced simulations of interneuronal implication in normal cerebral processing, normal aging, or pathological processes.

Age-dependent decline in learning and memory performances of WAG/Rij rat model of absence epilepsy

Recent clinical studies revealed emotional and cognitive impairments associated with absence epilepsy. Preclinical research with genetic models of absence epilepsy however have primarily focused on dysfunctional emotional processes and paid relatively less attention to cognitive impairment. In order to bridge this gap, we investigated age-dependent changes in learning and memory performance, anxiety-like behavior, and locomotor activity of WAG/Rij rats (a valid model of generalized absence epilepsy) using passive avoidance, Morris water maze, elevated plus maze, and locomotor activity cage. We tested 5 month-old and 13 month-old WAG/Rij rats and compared their performance to age-matched Wistar rats. Results revealed a decline in emotional and spatial memory of WAG/Rij rats compared to age-matched Wistar rats only at 13 months of age. Importantly, there were no significant differences between WAG/Rij and Wistar rats in terms of anxiety-like behavior and locomotor activity at either age. Results pointed at age-dependent learning and memory deficits in the WAG/Rij rat model of absence epilepsy.

GABA through the ages: regulation of cortical function and plasticity by inhibitory interneurons

Inhibitory interneurons comprise only about 20% of cortical neurons and thus constitute a clear minority compared to the vast number of excitatory projection neurons. They are, however, an influential minority with important roles in cortical maturation, function, and plasticity. In this paper, we will highlight the functional importance of cortical inhibition throughout brain development, starting with the embryonal formation of the cortex, proceeding by the regulation of sensory cortical plasticity in adulthood, and finishing with the GABA involvement in sensory information processing in old age.

Dissecting the age-related decline on spatial learning and memory tasks in rodent models: N-methyl-D-aspartate receptors and voltage-dependent Ca2+ channels in senescent synaptic plasticity

In humans, heterogeneity in the decline of hippocampal-dependent episodic memory is observed during aging. Rodents have been employed as models of age-related cognitive decline and the spatial water maze has been used to show variability in the emergence and extent of impaired hippocampal-dependent memory. Impairment in the consolidation of intermediate-term memory for rapidly acquired and flexible spatial information emerges early, in middle-age. As aging proceeds, deficits may broaden to include impaired incremental learning of a spatial reference memory. The extent and time course of impairment has been be linked to senescence of calcium (Ca²⁺) regulation and Ca²⁺-dependent synaptic plasticity mechanisms in region CA1. Specifically, aging is associated with altered function of N-methyl-D-aspartate receptors (NMDARs), voltage-dependent Ca²⁺ channels (VDCCs), and ryanodine receptors (RyRs) linked to intracellular Ca²⁺ stores (ICS). In young animals, NMDAR activation induces long-term potentiation of synaptic transmission (NMDAR-LTP), which is thought to mediate the rapid consolidation of intermediate-term memory. Oxidative stress, starting in middle-age, reduces NMDAR function. In addition, VDCCs and ICS can actively inhibit NMDAR-dependent LTP and oxidative stress enhances the role of VDCC and RyR-ICS in regulating synaptic plasticity. Blockade of L-type VDCCs promotes NMDAR-LTP and memory in older animals. Interestingly, pharmacological or genetic manipulations to reduce hippocampal NMDAR function readily impair memory consolidation or rapid learning, generally leaving incremental learning intact. Finally, evidence is mounting to indicate a role for VDCC-dependent synaptic plasticity in associative learning and the consolidation of remote memories. Thus, VDCC-dependent synaptic plasticity and extrahippocampal systems may contribute to incremental learning deficits observed with advanced aging.

Alcohol administration during adulthood induces alterations of parvalbumin and glial fibrillary acidic protein immunoreactivity in rat hippocampus and cingulate cortex

Alcohol induces impairment of cognition, learning and memory. Neurotoxic effects of alcohol on the pathology of the hippocampus and the cingulate cortex were investigated in experimental rats. Parvalbumin (PV), a calcium-binding protein, is a crucial component of GABAergic neurons and glial fibrillary acidic protein immunoreactive (GFAP-ir) astrocytes have been used as markers. We investigated the effects of ethanol exposure during adulthood on the PV-ir neurons and GFAP-ir astrocytes in the hippocampus and the cingulate cortex of 3-month-old male Wistar rats. The rats were divided into 2 groups: control (C) and alcohol-exposed groups. The control group received distilled water whereas the alcohol-exposed groups received either a low dose (20%w/v, LD) or high dose (40%w/v, HD) of ethanol for periods of 21 days, 3 or 6 months. The brains of the animals were processed for immunohistochemistry using anti-parvalbumin and anti-GFAP antibodies and the numbers of PV immunoreactive (PV-ir) neurons and GFAP-ir astrocytes were counted/unit area. For each period of administration, the number of PV-ir neurons was significantly reduced for groups exposed to both the low and the high doses of ethanol compared to those of control groups in both the hippocampus and the cingulate cortex (p<0.01). In addition, the number of PV-ir neurons was progressively reduced after prolonged ethanol exposure. In contrast, there was a significantly increased number of GFAP-ir astrocytes observed in the hippocampus and the cingulate cortex in all groups exposed to ethanol and this was a function of both the duration and the dose of ethanol exposure, indicating that PV-ir neurons are as sensitive as the GFAP-ir astrocytes to ethanol exposure. Our data indicate that alcohol exposure induced a reduction of PV-ir neurons and an increase of GFAP-ir astrocytes in the hippocampus and the cingulate cortex and this may be associated with the impairment of cognition, learning and memory after chronic alcohol administration.

Neurotoxic effects induced by gammahydroxybutyric acid (GHB) in male rats

Gammahydroxybutyric acid (GHB) is an endogenous constituent of the central nervous system that has acquired great social relevance for its use as a recreational 'club drug'. GHB, popularly known as 'liquid ecstasy', is addictive when used continuously. Although the symptoms associated with acute intoxication are well known, the effects of prolonged use remain uncertain. We examined in male rats the effect of repeated administration of GHB (10 and 100 mg/kg) on various parameters: neurological damage, working memory and spatial memory, using neurological tests, the Morris water maze and the hole-board test. The results showed that repeated administration of GHB, especially at doses of 10 mg/kg, causes neurological damage, affecting the 'grasping' reflex, as well as alteration in spatial and working memories. Stereological quantification showed that this drug produces a drastic neuronal loss in the CA1 hippocampal region and in the prefrontal cortex, two areas clearly involved in cognitive and neurological functions. No effects were noted after quantification in the periaqueductal grey matter (PAG), a region lacking GHB receptors. Moreover, NCS-382, a putative antagonist of GHB receptor, prevented both neurological damage and working- memory impairment induced by GHB. This suggests that the effects of administration of this compound may be mediated, at least partly, by specific receptors in the nervous system. The results show for the first time that the repeated administration of GHB, especially at very low doses, produces neurotoxic effects. This is very relevant because its abuse, especially by young persons, could produce considerable neurological alterations after prolonged abuse.

Effect of gestational ethanol exposure on parvalbumin and calretinin expressing hippocampal neurons in a chick model of fetal alcohol syndrome

Fetal alcohol syndrome (FAS), a condition occurring in some children of mothers who have consumed alcohol during pregnancy, is characterized by physical deformities and learning and memory deficits. The chick hippocampus, whose functions are controlled by interneurons expressing calcium-binding proteins parvalbumin (PV) and calretinin (CR), is involved in learning and memory mechanisms. Effects on growth and development and hippocampal morphology were studied in chick embryos exposed to 5% and 10% ethanol volume/volume (vol/vol) for 2 or 8 days of gestation. There was a significant dose-dependent reduction (P<.05) in body weight and mean number per section of PV and CR expressing hippocampal neurons in ethanol-exposed chicks, without alterations in neuronal nuclear size or hippocampal volume, compared appropriate controls. Moreover, when chicks exposed to 5% ethanol for 2 and 8 days of gestation were compared, no significant differences were found in body parameters or neuronal counts. Similarly, exposure to 10% ethanol did not induce any significant changes in chicks exposed for 2 or 8 gestational days. Thus, these results suggest that gestational ethanol exposure induces a reduction in the mean number per section of PV and CR expressing hippocampal neurons, and could be a possible mechanism responsible for learning and memory disorders in FAS.

Parvalbumin immunoreactivity and protein level are altered in the gerbil hippocampus during normal aging

Hippocampal interneurons are local circuit neurons which are responsible for inhibitory activity in the hippocampus. Parvalbumin (PV) is one of useful markers for GABAergic interneurons, not for principle cells, in the hippocampus. In the present study, we investigated age-related changes in PV immunoreactive neurons and protein levels in the gerbil hippocampus during normal aging. PV immunoreactive neurons were detected in all hippocampal subregions of all groups. PV immunoreactive neurons, which innervated principal neurons, were non-pyramidal neurons in the hippocampal CA1-3 regions, and were polymorphic neurons in the dentate gyrus. In the hippocampal CA1 region, the number of PV immunoreactive neurons was significantly reduced in the postnatal month 3 (PM 3) group, which was sustained by PM 18, and, at PM 24, the number of PV immunoreactive neurons was significantly decreased. In the CA2/3 region and dentate gyrus, the number of PV immunoreactive neurons was significantly decreased at PM 6: Thereafter, the number of PV immunoreactive neurons was sustained until PM 24. In addition, changes in PV protein levels in the gerbil hippocampus were similar to immunohistochemical changes during normal aging: PV protein levels were significantly decreased with age by PM 6: Thereafter, PV protein levels were sustained by PM 24. These results suggest that PV immunoreactive interneurons were decreased in the hippocampus with age in gerbils.

Maintenance of inhibitory interneurons and boutons in sensorimotor cortex between middle and old age in Fischer 344 X Brown Norway rats

Ultrastructurally identified inhibitory synapses in layer II of rat sensorimotor cortex decline between middle and old age [Poe, B.H., Linville, C., Brunso-Bechtold, J., 2001. Age-related decline of presumptive inhibitory synapses in the sensorimotor cortex as revealed by the physical disector. J. Comp. Neurol. 439, 65-72]. The current study investigated whether a loss or shrinkage of gamma-aminobutyric acid (GABA)ergic interneurons contribute to that decline. Coronal sections from middle-aged (15-17 months) and old (25-29 months) Fischer 344 X Brown Norway male rats were immunoreacted with antibodies to the GABA synthesizing enzyme glutamic acid decarboxylase (GAD); the calcium-binding protein parvalbumin (PV), or the neuronal marker NeuN. The number of GAD-immunoreactive (IR), PV-IR, and NeuN-IR cells were determined stereologically using the optical disector technique and the cross-sectional areas of GAD-IR cells were measured in layers II/III, IV, V and VI of sensorimotor cortex. Neither the number of GAD-IR or NeuN-IR cells, nor the size of GAD-IR cells, declined significantly between middle and old age. A modest decline in the PV-IR subset of inhibitory interneurons was observed, predominantly due to changes in layers V and VI. Stereological analysis of layer II/III GAD-IR boutons revealed a stability of immunocytochemically identified inhibitory terminals. Taken together, these results indicate a general maintenance of overall GABAergic neurons in sensorimotor cortex between middle and old age and the loss of ultrastructurally identified inhibitory synapses may be due to the decline of a subset of GABAergic terminals.

Localization of m2 muscarinic receptor protein in parvalbumin and calretinin containing cells of the adult rat entorhinal cortex using two complementary methods

We investigated parvalbumin (PV) and calretinin (CR) containing interneurons in the rat entorhinal cortex. RNA amplification following single cell dissection of immunohistochemically labeled cells from layers II to VI revealed that PV cells, in contrast to CR cells, express the m2 muscarinic receptor (M2AchR) protein. Double immunostaining to confirm the results of RNA amplification indicated that the majority of PV cells contain M2AchR protein, whereas only a small proportion of CR cells do. In contrast, a large number of layer I CR cells, which are mostly Cajal-Retzius cells, were positive for M2AchR. RNA amplification following dissection of these cells also revealed that they contain the M2AchR protein. These findings emphasize that there are significant differences in the expression of different proteins, even among similar neuronal types in the same brain region. This highlights the importance of accurately collecting single cells, and knowledge of anatomical details in molecular biological studies.

Stereological quantification of GAD-67-immunoreactive neurons and boutons in the hippocampus of middle-aged and old Fischer 344 x Brown Norway rats

The aging process in rodents is associated with learning and memory impairments that are correlated with changes in multiple neurotransmitter systems in the hippocampus. For example, the gamma-aminobutyric acid (GABA)ergic system is compromised in old compared with young rats (Shetty and Turner [1998] J. Comp. Neurol. 394:252-269; Vela et al. [2003] J. Neurochem. 85:368-377; Potier et al. [1992] Neuroscience 48:793-806; Potier et al. [1994] Brain Res. 661:181-188). The present study investigated the important issue of whether there is a decline of the GABAergic inhibitory system between middle and old age. Five middle-aged (15-17 months) and five old (25-29 months) Fischer 344 x Brown Norway male rats were perfused, and coronal sections through the dorsal hippocampus were immunoreacted with antibodies either to NeuN, a neuronal marker, or to the 67-kDa isoform of glutamic acid decarboxylase (GAD), the rate-limiting enzyme for GABA synthesis. Using the optical dissector technique, NeuN-immunoreactive (IR) cells, GAD-IR cells, and GAD-IR boutons were quantified stereologically in the dentate gyrus, CA3, and CA1. The resulting GAD-IR cell and GAD-IR bouton densities then were normalized to NeuN-IR cell density to exclude the possible confound of tissue shrinkage. The results revealed a significant decline in GAD-IR cells between middle and old age in CA1 but not in dentate gyrus or CA3. Interestingly, GAD-IR boutons did not show a decline in CA1, CA3, or dentate gyrus between middle and old age. It is possible that loss of CA1 inhibitory interneurons in the dorsal hippocampus contributes to the learning and memory impairments reported in old rats.

Effects of hippocampal cholinergic deafferentation on learning strategy selection in a visible platform version of the water maze

Recent evidence has suggested that the relative levels of acetylcholine (ACh) between brain structures may be an important factor in the choice of behavioral strategy in settings in which either hippocampal or dorsal striatal brain systems can be employed both effectively and independently (McIntyre and Gold. 1999. Soc Neurosci Abs 25:1388). The current investigation used the neurotoxin 192 IgG-saporin to deplete the hippocampus of ACh selectively, while leaving ACh in other brain regions, including dorsal striatum, intact. Rats were then trained on a version of the Morris water maze, in which behavioral strategies attributed to the hippocampus and dorsal striatum are placed in direct competition. It was predicted that rats with hippocampal ACh depletion would display a cue bias. Contrary to this prediction, depleting hippocampal ACh did not bias against and, in fact, promoted use of a hippocampal place strategy in this task, as indicated by choice in competition tests and performance on hidden platform training trials. These data add to a growing literature demonstrating that the septohippocampal cholinergic system is not required for accurate spatial learning and suggest a complex role for basal forebrain projections in processing information about the spatial environment.

Effect of neonatal dentate gyrus lesion on allothetic and idiothetic navigation in rats

Goal-directed navigation is believed to be the combined product of idiothetic and allothetic orientation. Although both navigation systems require the hippocampal formation, it is probable that different circuits implement them. Examination of Long-Evans rats with dentate gyrus lesions induced by neonatal X-ray irradiation may show the dissociation of these two components of navigation. Two recently developed place avoidance tasks on a rotating circular arena were used to test this hypothesis. In the first test, the position of the punished area is stable in the room frame but is permanently changing on the surface of the arena. This task requires the rat to use allothetic orientation and to disregard idiothetic orientation. In the second test, the prohibited area is fixed in the coordinate system of the arena and the experiment is conducted in complete darkness, forcing the rat to rely exclusively on idiothesis supported by substratal cues. The results suggest that the dentate gyrus lesion interferes less with idiothetic orientation than with allothetic orientation. In addition, an attempt was made to control the number of developing granule cells by exact timing of a single high dose of perinatal irradiation, and to measure the ensuing behavioral deficits. Rats irradiated at 6, 18, or 24 h after birth were tested as adults in the Morris water maze. Irradiated animals showed significant, but highly variable, learning deficit, but histological examination indicated that the granule cell loss did not correlate with the degree of behavioral impairment.

Behavioral training-induced c-Fos expression in the rat nucleus basalis of Meynert during aging

The present study investigated the behavioral training-induced c-Fos expression in the nucleus basalis of Meynert (nbM) in differently aged rats. This study demonstrated that the c-Fos expression in nbM was significantly increased and the peak occurred at 2 h after dark-avoidance training. Although the increase of c-Fos expression was also observed after pseudotraining, the number of Fos-like immunoreactive neurons in pseudotrained rats was significantly less than that in dark-avoidance trained rats at each time-point. This result suggested that c-Fos expression might be involved in learning and memory processes. In addition, all the pseudotraining-, training- and memory arousing-induced c-Fos expression was decreased with increasing age, and the decrease was more notable in trained and memory aroused rats. This suggested that the total number of nbM neurons and/or the sensitivity of nbM neurons to experimental manipulations, especially learning and memory performance, might reduce during aging.

Immunohistochemical localization of the somatostatin sst(4) receptor in rat brain

The biological actions of the neuromodulator somatostatin are mediated through a family of G-protein-coupled receptors, of which five members, sst(1-5), have been identified. Although the messenger RNA distribution of the sst(4) receptor has been reported, no information about the distribution of the receptor protein in the central nervous system is available. We have therefore raised a polyclonal peptide antibody against a rat carboxy-terminal sst(4) peptide. The selectivity of the affinity-purified antibody was demonstrated by western blotting of membrane proteins isolated from Chinese hamster ovary-K1 cells expressing the recombinant sst(4) receptor and from the rat hippocampus. This resulted in both cases in the identification of a single band of approximately 42,000 mol. wt. Furthermore, the sst(4) receptor antibody selectively labelled Chinese hamster ovary-K1 cells expressing the recombinant sst(4) receptor in immunocytochemistry. No cross-reactivity was observed with other recombinant somatostatin receptors. Immunohistochemistry on adult rat brain sections showed the sst(4) receptor to have a widespread distribution. This included labelling of cell bodies as well as processes in the cerebral cortex, hippocampus and several nuclei in the brainstem. All signals were absent following antibody preabsorption with the synthetic sst(4) peptide. This study provides the first detailed analysis of the distribution of sst(4) receptor protein in the rat brain.

Effect of neonatal ethanol exposure on parvalbumin-expressing GABAergic neurons of the rat medial septum and cingulate cortex

This study was performed to determine the long-term effects of ethanol exposure during the brain growth spurt (postnatal days 4-10) on the number of parvalbumin-immunoreactive (PA+) GABAergic neurons in the adult (P60) rat medial septum and anterior cingulate cortex. Significant loss of neurons within each of these populations has previously been demonstrated following prenatal ethanol exposure. In the present study, no significant differences in the number of PA+ neurons were found in either the medial septum or the cingulate cortex when control and ethanol-exposed animals were compared. The cellular densities and volumetric measures in both brain regions were also similar in the two groups. We speculate that compensatory up-regulative mechanisms may have accounted for the protection of the PA neuronal populations in these two areas following the early neonatal exposure.

Somatostatin and behaviour: the need for genetically engineered models

Somatostatin was originally characterised as a hypothalamic neurohormone responsible for the inhibition of pituitary Growth Hormone secretion. In mammals two genes encode for somatostatin-related peptides, somatostatin 14 and 28, and cortistatins, respectively. All peptides bind with similar affinities to the five cloned somatostatin receptors (sst), which belong to the GPCR family. Despite numerous studies, no clear behavioural function has yet been attributed to somatostatin-related peptides. This is due to the lack of good pharmacological tools (selective antagonists) and animal models. This review will focus on the recent development of such tools.

Peptide immunoreactivity in aged rat cortex and hippocampus as a function of memory and BDNF infusion

Brain-derived neurotrophic factor (BDNF) modulates neuropeptide levels in hippocampus and cortex of young adult rats. Neuropeptide levels are altered in some age-related disorders, such as Alzheimer's and Parkinson's Disease. BDNF may be able to rectify peptide abnormalities but, because plasticity decreases with age, BDNF may not alter peptide levels as readily in aged animals. To determine if BDNF would regulate peptide levels in aged rats, young, aged memory-impaired, and unimpaired rats were infused with BDNF or vehicle into hippocampus and cortex. Cell profile counts, cell profile areas, fiber counts, and/or fiber terminal densities were measured for sections immunostained for neuropeptide Y (NPY), somatostatin (SOM), cholecystokinin-8 (CCK), and dynorphin A(1-8) (DYN). Results showed that BDNF upregulated cortical NPY-immunoreactivity (ir) and SOM-ir, upregulated hippocampal NPY-ir, and downregulated hippocampal DYN-ir in both aged and young rats. In addition, BDNF significantly and selectively normalized the areas of atrophied deep cortical CCK-ir cell profiles in aged-impaired rats. Finally, decreased CCK-ir fiber density was found in the hippocampal formation of aged memory-impaired rats.

Excitotoxic septal lesions result in spatial memory deficits and altered flexibility of hippocampal single-unit representations

The septal nuclei are reciprocally connected with the hippocampal formation and contribute importantly to spatial and memory processing. Using excitotoxic lesions of the septal area, we investigated whether neurodegeneration in subcortical projections to hippocampus can compromise flexible information processing by hippocampal single units. In agreement with the mild effects of excitotoxic septal lesions on hippocampal physiology compared with fimbria-fornix lesions and septal inactivation, we observed limited lesion effects on single-unit activity. The location specificity of hippocampal complex spike cells remained unchanged, but a less reliable location-dependent discharge was observed in experimental animals with a pronounced postoperative working memory deficit. Testing in the absence of ambient illumination and in a new environment revealed that the spatial correlates of complex spike cells in lesioned animals may rely on a more limited set of sensory cues. Altered sensory cues resulted in a significantly different response pattern between the control and lesion group in the new environment, a situation that normally results in place field reorganization. Such a group difference was not observed during dark testing, a condition in which place field reorganization is less prominent. A contribution of hippocampal interneurons to the observed alterations in the spatial properties of the principal cells was suggested by decreased theta modulation in the lesioned group. Because excitotoxic lesions result in memory deficits that resemble age-related memory problems in the absence of age-related degenerative processes, we suggest that septal neurodegeneration could directly contribute to those behavioral changes with advanced age that correlate with functional alterations in the hippocampal formation.

Entorhinal cortex and fimbria-fornix role in rat's passive avoidance response memorization

The stereotaxic administration of tetrodotoxin (TTX) was employed to induce the fully reversible inactivation of the fimbria-fornix complex (FF) and of the entorhinal cortex (EC), in order to ascertain the role of these structures in the memorization of a passive avoidance response (PAR). On permanently cannulated rats TTX (5 ng in 0.5 microliter saline) or saline (0.5 microliter) was injected uni- or bilaterally, respectively, in the FF and in the EC, 60 min before PAR acquisition, immediately after PAR acquisition and 60 min before PAR retrieval, always performed 48 h after the acquisition trial. It was shown that EC unilateral or bilateral pre-acquisition inactivation was followed by amnesia, while TTX inactivation in post-acquisition and pre-retrieval had no effects. Identical results were obtained by TTX administration in FF. The experimental evidence indicates that both EC and FF play a role during acquisition of PAR engram. The results are discussed in comparison with previous ones concerning dorsal and ventral hippocampus TTX inactivation effects on rat's PAR, and in relation to hippocampal and medial septal area connectivity.

Increased calcium buffering in basal forebrain neurons during aging

Increased calcium buffering in basal forebrain neurons during aging. J. Neurophysiol. 80: 350-364, 1998. Alterations of neuronal calcium (Ca2+) homeostasis are thought to underlie many age-related changes in the nervous system. Basal forebrain neurons are susceptible to changes associated with aging and to related dysfunctions such as Alzheimer's disease. It recently was shown that neurons from the medial septum and nucleus of the diagonal band (MS/nDB) of aged (24-27 mo) F344 rats have an increased current influx through voltage-gated Ca2+ channels (VGCCs) relative to those of young (1-4. 5 mo) rats. Possible age-related changes in Ca2+ buffering in these neurons have been investigated using conventional whole cell and perforated-patch voltage clamp combined with fura-2 microfluorimetric techniques. Basal intracellular Ca2+ concentrations ([Ca2+]i), Ca2+ influx, Ca2+ transients (Delta[Ca2+]i), and time course of Delta[Ca2+]i were quantitated, and rapid Ca2+ buffering values were calculated in MS/nDB neurons from young and aged rats. The involvement of the smooth endoplasmic reticulum (SER) was examined with the SER Ca2+ uptake blocker, thapsigargin. An age-related increase in rapid Ca2+ buffering and Delta[Ca2+]i time course was observed, although basal [Ca2+]i was unchanged with age. The SER and endogenous diffusible buffering mechanisms were found to have roles in Ca2+ buffering, but they did not mediate the age-related changes. These findings suggest a model in which some aging central neurons could compensate for increased Ca2+ influx with greater Ca2+ buffering.

A comparison of the density of NADPH-diaphorase-reactive neurons in the fascia dentata and Ammon's horn between 6-month and 12-month old dark agouti rats

The present study aimed to assess the developmental progress of the hippocampal nitric oxide (NO) system within adulthood by comparing the density of NO-producing neurons in the fascia dentata and Ammon's horn in two groups of adult male rats using NADPH-diaphorase (NADPH-d) histochemistry. One group comprised 6-month-old rats (early adulthood), and the other 12-month-old rats (middle-adulthood). Areal density (number of neurons per unit area) of NADPH-d positive neurons along the three hippocampal axes (septo-temporal, transverse and radial axes) were subjected to quantitative analyses. There were significant variations in the density of NADPH-d-reactive neurons along the transverse and radial axes of the hippocampus, similar to what have been described previously. Comparison between 6-month and 12-month-old rats indicated a substantial reduction in the density of NADPH-d-reactive neurons in the fascia dentata (69%) and Ammon's horn (54%) of the latter group. This reduction was relatively uniform along the septotemporal and radial axes, but appeared to be more pronounced in the fascia dentata and in the proximal region of Ammon's horn. Our finding showed that the hippocampal NO system can undergo significant changes within adulthood. It further highlighted the possibility that an age-related reduction in the capacity to produce NO may not be directly responsible for the cognitive decline associated with senescence, but rather predisposes neuronal degeneration in later life.

The onset of parvalbumin-expression in interneurons of the rat parietal cortex depends upon extrinsic factor(s)

Parvalbumin (PV) belongs to the large family of EF-hand calcium-binding proteins and is an excellent marker for a subpopulation of GABAergic neocortical interneurons. During cortical development, PV first appears on postnatal day (P)8, in the infragranular layers; after P14, it also becomes apparent within the supragranular layers. However, nothing is known about the factors controlling its expression, which could involve functional activity, neuronal connectivity and/or neurotrophic factors. It being difficult to manipulate these parameters in vivo, their role may be more readily assessed in organotypic cultures, which are deprived of their subcortical afferents and efferents, and hence of subcortically derived neurotrophic factors and extrinsic functional activity. We prepared slices of the rat brain on P3, P5, P7 and P9, maintained them in culture for 2-5 weeks, and compared the temporal and spatial distribution pattern of PV-immunoreactivity within these slices with the in vivo situation. We found, first, that during late postnatal in vivo development and ageing, the number of PV-immunoreactive neurons in the parietal cortex decreases significantly, and second, that the expression of PV-immunoreactivity in the parietal cortex was markedly influenced by the phase of postnatal development at which slice cultures were explanted. In those removed on P7 and P9, the number of PV-immunoreactive cells, as well as the temporal and spatial distribution pattern of PV-immunoreactivity corresponded to the in vivo situation, but in explants obtained on P3 or P5, PV-immunoreactivity remained confined to layer V of the cortex, reminiscent of the expression profile manifested at the end of the second postnatal week in vivo. Also, the number of PV-immunoreactive cells in these cultures was significantly lower than in explants at the later stages. Our results indicate that the onset of PV-expression in the parietal cortex depends upon extrinsic cortical factors subsisting prior to P7. Once the production of this protein has been initiated, such influences are no longer required.

Pharmacological characterization of ionotropic excitatory amino acid receptors in young and aged rat basal forebrain

Ionotropic glutamate receptors were characterized in acutely dissociated medial septum/nucleus of diagonal band neurons from one- to four-month- and 24-26-month-old male Fischer 344 rats. Whole-cell patch-clamp recordings were used to study glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, kainate and N-methyl-D-aspartate-induced currents. Pharmacological properties of these ionotropic receptors were studied across different age groups by comparing concentration response curves and EC50 for agonist-induced currents, as well as dissociation constants (Kb) for competitive receptor antagonists. Our results suggest that non-N-methyl-D-aspartate receptors on medial septum/nucleus of diagonal band neurons were predominantly of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate type and display biophysical and pharmacological properties similar to other central neurons. However, peak alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-induced currents were enhanced in aged (35.0+/-4.4 pA/pF) compared to young cells (16.2+/-1.7 pA/pF, P<0.005), and the EC50 shifted to the right (4.4+/-0.6 in young compared to 8.8+/-1.3 microM in aged, P<0.05). The Kb for 6,7-dinitroquinoxaline-2,3-dione inhibition of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-induced currents likewise shifted to the right (0.16+/-0.02 in young and 0.29+/-0.04 microM in aged, P<0.05) suggesting an age-related decrease in affinity for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors. N-Methyl-D-aspartate-induced currents were generated in standard physiological solutions with the addition of 1 microM glycine and the removal of Mg2+. The N-methyl-D-aspartate responses were predictably modulated by magnesium and glycine, and were antagonized by the competitive antagonist 2-amino-5-phosphonovaleric acid. No age-related change in N-methyl-D-aspartate maximum, EC50, magnesium sensitivity, glycine sensitivity or Kb for 2-amino-5-phosphonovaleric acid was observed. Overall, our results suggest that ionotropic glutamate receptors in the medial septum/nucleus of diagonal band have a similar pharmacological profile compared to glutamate receptors in other brain regions. More importantly, these data suggest that while medial septum/nucleus of diagonal band cells maintain N-methyl-D-aspartate receptors during ageing, a significant increase in current density and decrease in receptor affinity for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, during this same time period, may provide a mechanism for age-related changes in neuronal plasticity and excitotoxicity in the basal forebrain.

The effect of aging on experience-dependent plasticity of hippocampal place cells

The firing characteristics of 1437 CA1 pyramidal neurons were studied in relation to both spatial location and the phase of the theta rhythm in healthy young and old rats performing a simple spatial task on a rectangular track. The old rats had previously been found to be deficient on the Morris spatial learning task. Age effects on the theta rhythm per se were minimal. Theta amplitude and frequency during rapid eye movement sleep were virtually identical. During behavior, theta frequency was slightly reduced with age. In both groups, cell firing occurred at progressively earlier phases of the theta rhythm as the rat traversed the place field of the cell (i. e., there was "phase precession," as reported by others). The net phase shift did not differ between age groups. The main finding of the study was a loss of experience-dependent plasticity in the place fields of old rats. During the first lap around the track on each day, the initial sizes of the place fields were the same between ages; however, place fields of young rats, but not old, expanded significantly during the first few laps around the track in a given recording session. As the place fields expanded, the rate of change of firing with phase slowed accordingly, so that the net phase change remained constant. Thus changes in field size and phase precession are coupled. A deficit in plasticity of place fields in old rats may lead to a less accurate population code for spatial location.

Comparison of NADPH diaphorase histochemistry, somatostatin immunohistochemistry, and silver impregnation in detecting structural and functional impairment in experimental status epilepticus

Nitric oxide has been postulated as a retrograde intercellular messenger for long-term potentiation, a form of synaptic plasticity that is associated with learning and memory processes. In the present study we investigated whether the loss or survival of nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase-containing neurons, which are known to synthesize nitric oxide, would be an useful indicator for evaluating the structural and functional state of the rat hippocampus after status epilepticus that is induced by intraperitoneal injection of kainic acid. Besides NADPH diaphorase histochemistry, two other histological parameters were studied: the grade of cell damage evaluated from silver-impregnated sections, and the number of somatostatin-containing neurons in different hippocampal subfields. We found that the number of NADPH diaphorase-containing neurons in the hilus and granule cell layer correlated well with spatial learning and memory performance as assessed by the Morris water-maze test. The extent of cell damage in the CA1 subfield analysed in silver-impregnated sections and the number of hilar somatostatin-containing neurons also significantly correlated with latencies in the water-maze test. Furthermore, linear regression analysis revealed that the number of somatostatin-containing neurons in the hilus explains about 50% of the variation in water-maze learning. These findings emphasize that although general structural preservation is of crucial importance for the function of the hippocampus also interneurons, such as somatostatin- and NADPH diaphorase-containing neurons, may play an important role during the acquisition phase and processing of information in hippocampal circuitry. Therefore, in addition to evaluating general cell damage, analysis of the cell loss that occurs in the interneuron subpopulations will be beneficial in verifying structural and functional deficits of the hippocampus after status epilepticus.

Effects of combined chronic nimodipine and acute metrifonate treatment on spatial and avoidance behavior

The present experiment was designed to elucidate whether chronic dietary treatment with nimodipine (3 months, 1000 ppm) enhances water maze spatial navigation, passive avoidance behavior and locomotor activity, and whether such a treatment with nimodipine would interact with the therapeutic effect of acute metrifonate treatment. In young medial septum-lesioned rats, nimodipine had no effect by its own on cognitive or motor behavior, and did not enhance the water maze and passive avoidance behavior improving action of metrifonate (3 and 10 mg/kg. p.o.). Nimodipine treatment of aged rats did not markedly affect the deficit in motor performance. Single and combined nimodipine and metrifonate (3 and 10 mg/kg, p.o.) treatment of aged rats resulted in shorter escape distance values to the hidden water maze escape platform compared to those of control aged rats. The passive avoidance performance of aged rats was more effectively facilitated by a combined nimodipine and metrifonate treatment than by either of the drugs on their own. Following a washout period of 2.5 months the rats that were treated previously with nimodipine no longer performed better than aged controls in the water maze test. Furthermore, after the washout period metrifonate 10 mg/kg was no longer effective in improving the water maze behavior of the now 26-month-old rats irrespective of their chronic pretreatment. Taken together, these findings indicate that chronic nimodipine and acute metrifonate treatment may more effectively stimulate cognitive functioning than either of the treatments on their own.