Evaluation of Memantine in AAV-AD Rat: A Model of Late-Onset Alzheimer's Disease Predementia.

BACKGROUND: Though our understanding of Alzheimer's disease (AD) remains elusive, it is well known that the disease starts long before the first signs of dementia. This is supported by the large number of symptomatic drug failures in clinical trials and the increased trend to enroll patients at predementia stages with either mild or no cognitive symptoms. However, the design of pre-clinical studies does not follow this attitude, in particular regarding the choice of animal models, often irrelevant to mimic predementia Late Onset Alzheimer's Disease (LOAD). OBJECTIVES: We aimed to pharmacologically validate the AAV-AD rat model to evaluate preventive treatment of AD. METHODS: We evaluated an N-methyl-D-aspartate receptor antagonist, named memantine, in AAV-AD rats, an age-dependent amyloid rat model which closely mimics Alzheimer's pathology including asymptomatic and prodromal stages. Memantine was used at a clinically relevant dose (20 mg daily oral administration) from 4 (asymptomatic phase) to 10 (mild cognitive impairment phase) months of age. RESULTS: A 6-month treatment with memantine promoted a non-amyloidogenic cleavage of APP followed by a decrease in soluble Aß42. Consequently, both long-term potentiation and cognitive impairments were prevented. By contrast, the levels of hyperphosphorylated endogenous tau remained unchanged, indicating that a long-term memantine treatment is ineffective to restrain the APP processing-induced tauopathy. CONCLUSIONS: Together, our data confirm that relevant models to LOAD, such as the AAV-AD rat, can provide a framework for a better understanding of the disease and accurate assessment of preventive treatments.

Circuit-specific changes in D-serine-dependent activation of the N-methyl-D-aspartate receptor in the aging hippocampus.

Age-related memory deficits have recently been associated with the impaired expression of D-serine-dependent synaptic plasticity in neuronal networks of the hippocampal CA1 area. However, whether such functional alterations are common to the entire hippocampus during aging remains unknown. Here, we found that D-serine was also required for the induction of N-methyl-D-aspartate receptor (NMDA-R)-dependent long-term potentiation (LTP) at perforant path-granule cell synapses of the dentate gyrus. LTP as well as isolated NMDA-R synaptic potentials were impaired in slices from aged rats, but in contrast to the CA1, this defect was not reversed by exogenous D-serine. The lower activation of the glycine-binding site by the endogenous co-agonist does not therefore appear to be a critical mechanism underlying age-related deficits in NMDA-R activation in the dentate gyrus. Instead, our data highlight the role of changes in presynaptic inputs as illustrated by the weaker responsiveness of afferent glutamatergic fibers, as well as changes in postsynaptic NMDA-R density. Thus, our study indicates that although NMDA-R-dependent mechanisms driving synaptic plasticity are quite similar between hippocampal circuits, they show regional differences in their susceptibility to aging, which could hamper the development of effective therapeutic strategies aimed at reducing cognitive aging.

Unveiling an exceptional zymogen: the single-chain form of tPA is a selective activator of NMDA receptor-dependent signaling and neurotoxicity.

Unlike other serine proteases that are zymogens, the single-chain form of tissue plasminogen activator (sc-tPA) exhibits an intrinsic activity similar to that of its cleaved two-chain form (tc-tPA), especially in the presence of fibrin. In the central nervous system tPA controls brain functions and dysfunctions through its proteolytic activity. We demonstrated here, both in vitro and in vivo, that the intrinsic activity of sc-tPA selectively modulates N-methyl-D-aspartate receptor (NMDAR) signaling as compared with tc-tPA. Thus, sc-tPA enhances NMDAR-mediated calcium influx, Erk(½) activation and neurotoxicity in cultured cortical neurons, excitotoxicity in the striatum and NMDAR-dependent long-term potentiation in the hippocampal CA-1 network. As the first demonstration of a differential function for sc-tPA and tc-tPA, this finding opens a new area of investigations on tPA functions in the absence of its allosteric regulator, fibrin.

Rescue of cognitive aging by long-lasting environmental enrichment exposure initiated before median lifespan.

The rescue of cognitive function through environmental enrichment (EE) during aging has been extensively documented. However, the age at onset, the duration of EE, and the cerebral mechanisms required to obtain the greatest benefits still remain to be determined. We have recently shown that EE applied for 3 mo after the median lifespan, i.e., the age at which 50% of the population is still alive (from 17 to 20 mo in NMRI mice), failed to prevent cognitive deficits in senescent animals. In the present study, mice were exposed to EE prior to the median lifespan, and for a longer total duration (from 14 to 20 mo), before the assessment of memory performance and the electrophysiological properties of hippocampal neuronal networks. The EE prevented memory deficits and reduced anxiety as the animal aged. Moreover, EE attenuated the age-related impairment of basal glutamatergic neurotransmission in CA1 hippocampal slices, and reversed the decrease in isolated N-methyl-D-Aspartate receptor (NMDA-R)-dependent synaptic potentials. Surprisingly, EE did not prevent the age-related alteration of theta-burst-induced long-term potentiation (LTP). This study therefore suggests that EE needs to be initiated before the age corresponding to the median lifespan and/or required long duration (> 3 mo) to have an effect on cognitive aging. In addition, we show that EE probably acts through theta-burst-independent mechanisms of synaptic plasticity.

Reduced serine racemase expression contributes to age-related deficits in hippocampal cognitive function.

To gain insight into the contribution of d-serine to impaired cognitive aging, we compared the metabolic pathway and content of the amino acid as well as d-serine-dependent synaptic transmission and plasticity in the hippocampus of young and old rats of the Wistar and Lou/C/Jall strains. Wistar rats display cognitive impairments with aging that are not found in the latter strain, which is therefore considered a model of healthy aging. Both mRNA and protein levels of serine racemase, the d-serine synthesizing enzyme, were decreased in the hippocampus but not in the cerebral cortex or cerebellum of aged Wistar rats, whereas the expression of d-amino acid oxidase, which degrades the amino acid, was not affected. Consequently, hippocampal levels of endogenous d-serine were significantly lower. In contrast, serine racemase expression and d-serine levels were not altered in the hippocampus of aged Lou/C/Jall rats. Ex vivo electrophysiological recordings in hippocampal slices showed a marked reduction in N-methyl-d-aspartate-receptor (NMDA-R)-mediated synaptic potentials and theta-burst-induced long-term potentiation (LTP) in the CA1 area of aged Wistar rats, which were restored by exogenous d-serine. In contrast, NMDA-R activation, LTP induction and responses to d-serine were not altered in aged Lou/C/Jall rats. These results further strengthen the notion that the serine racemase-dependent pathway is a prime target of hippocampus-dependent cognitive deficits with aging. Understanding the processes that specifically affect serine racemase during aging could thus provide key insights into the treatment of memory deficits in the elderly.

Contribution of the d-Serine-Dependent Pathway to the Cellular Mechanisms Underlying Cognitive Aging.

An association between age-related memory impairments and changes in functional plasticity in the aging brain has been under intense study within the last decade. In this article, we show that an impaired activation of the strychnine-insensitive glycine site of N-methyl-d-aspartate receptors (NMDA-R) by its agonist d-serine contributes to deficits of synaptic plasticity in the hippocampus of memory-impaired aged rats. Supplementation with exogenous d-serine prevents the age-related deficits of isolated NMDA-R-dependent synaptic potentials as well as those of theta-burst-induced long-term potentiation and synaptic depotentiation. Endogenous levels of d-serine are reduced in the hippocampus with aging, that correlates with a weaker expression of serine racemase synthesizing the amino acid. On the contrary, the affinity of d-serine binding to NMDA-R is not affected by aging. These results point to a critical role for the d-serine-dependent pathway in the functional alterations of the brain underlying memory impairment and provide key information in the search for new therapeutic strategies for the treatment of memory deficits in the elderly.

Preserved memory capacities in aged Lou/C/Jall rats.

Although memory impairments are a hallmark of aging, the degree of deficit varies across animal models, and is likely to reflect different states of deterioration in metabolic and endocrinological properties. This study investigated memory-related processes in young (3-4 months) and old (24 months) Sprague-Dawley rats (SD), which develop age-linked pathologies such as obesity or insulin-resistance and Lou/C/Jall rats, which do not develop such impairments. In short- and long-term memory recognition tasks, old Lou/C/Jall rats were never impaired whereas old SD rats were deficient at 1 and 24h latencies. The expression of N-methyl-d-aspartate receptors (NMDAR)-mediated synaptic plasticity in CA1 hippocampal networks shifted towards lower activity values in old Lou/C/Jall rats whereas long-term potentiation was impaired in age-matched SD rats. Age-related decrease in NR2A subunits occurred in both strains, extended to NR2B, NR1 and GluR1 subunits in older animals (28 months) but only in SD rats. Therefore, the Lou/C/Jall rats can be considered as a model of healthy aging, not only in terms of its preserved metabolism, but also in terms of cognition and synaptic plasticity.

The magnitude of hippocampal long term depression depends on the synaptic location of activated NR2-containing N-methyl-D-aspartate receptors.

Activation of N-methyl-D-aspartate receptors (NMDARs) is the first step in the induction of certain forms of synaptic plasticity in the hippocampus. In the adult rat hippocampus, NMDARs are composed almost exclusively of NR1 and NR2 subunits with NR1 subunits being mainly associated with either NR2A and/or NR2B subunits. The role played by the different subunits in synaptic plasticity is still controversial. In the present study, we used two different long term depression (LTD) -inducing protocols (electrical and chemical stimulation) to show that activation of NR2A-containing NMDAR subunits leads to the induction of LTD. We also demonstrated that extrasynaptic NR2B-containing NMDARs regulate the magnitude of LTD by exerting a control over the function of synaptic NR2A-containing NMDARs while having no effect on plasticity in the absence of synaptic receptor activation. Taken as a whole, these experiments demonstrate that NMDAR subunits play different roles according to their nature (NR2A or NR2B) and location (synaptic versus extrasynaptic). This sheds new light on the functional role of extrasynaptic NR2B containing-NMDARs. These results are particularly important for a better understanding of certain pathological disorders associated with glutamatergic overactivity.

Age-related alterations of GABAergic input to CA1 pyramidal neurons and its control by nicotinic acetylcholine receptors in rat hippocampus.

The aim of this study was to determine whether age-associated alterations in the GABAergic input to pyramidal neurons in the hippocampus are due to a dysfunction of GABAergic interneurons, and/or a decrease in their cholinergic control via nicotinic receptors (nAChRs). Electrophysiological recordings were obtained from pyramidal cells in the CA1 area of hippocampal slices from young (3-4 months old) and aged (25-30 months old) Sprague-Dawley rats. Synaptic GABA(A) receptor-mediated inhibitory postsynaptic currents and inhibitory postsynaptic potentials induced by stimulation of the stratum oriens were significantly smaller in aged rats. The frequency (but not amplitude) of spontaneous and miniature GABA inhibitory postsynaptic currents (IPSCs) was reduced in aged rats, suggesting a presynaptic alteration. Tetanic stimulation of cholinergic afferents to release endogenous acetylcholine, or an exogenous application of the nAChR agonist cytisine, increased the frequency of spontaneous IPSCs in young rats; however these effects were not evident in aged rats, indicating that the nicotinic control of GABA release is lowered during aging. None of these age-related alterations were reversed by a chronic treatment with donepezil, a cholinesterase inhibitor. Immunofluorescent labeling of GABA interneurons with somatostatin (SOM), parvalbumin (PV) or calbindin (CB), together with the vesicular acetylcholine transporter VAChT, revealed a selective loss of subpopulations of SOM and CB positive interneurons. This loss was associated with a general decrease in density of the cholinergic network in aged rats. Thus, the lower GABAergic inhibition observed in the aged rat hippocampus is due to a selective loss/dysfunction of subpopulations of GABAergic interneurons, associated with a widespread cholinergic deficit.

Motherhood-induced memory improvement persists across lifespan in rats but is abolished by a gestational stress.

Motherhood modifies the biology and behavior of the female, a process which prepares the mother's cognitive systems that are needed for nurturance. It has recently been shown that motherhood enhances hippocampal-mediated spatial learning and synaptic plasticity. Deleterious and long-term effects of a stress experienced during gestation have been demonstrated on progeny. Surprisingly little is known about the effect of such stress on mothers. Here, we investigated the effect of gestational stress on the adaptive changes due to motherhood. Female rats were mated and stressed during the last week of gestation. Two weeks after weaning, they were submitted to behavioral tests or electrophysiological study. A group of females were then kept for 16 months after motherhood experience to study the long-term effect of gestational stress and motherhood on memory when they were 22 months old. We confirm that a single motherhood experience selectively increases hippocampal-mediated spatial memory during the entire lifespan of female rats and protects them from age-associated memory impairments. However, we demonstrate that a stressful experience during gestation totally abolishes the positive effects of motherhood both on spatial memory and on hippocampal synaptic plasticity (long-term potentiation). Environmental factors that induce biological vulnerability have negative effects even for fundamental biological behaviors.

A critical role for the glial-derived neuromodulator D-serine in the age-related deficits of cellular mechanisms of learning and memory.

Age-associated deficits in learning and memory are closely correlated with impairments of synaptic plasticity. Analysis of N-methyl-D-aspartate receptor (NMDAr)-dependent long-term potentiation (LTP) in CA1 hippocampal slices indicates that the glial-derived neuromodulator D-serine is required for the induction of synaptic plasticity. During aging, the content of D-serine and the expression of its synthesizing enzyme serine racemase are significantly decreased in the hippocampus. Impaired LTP and NMDAr-mediated synaptic potentials in old rats are rescued by exogenous D-serine. These results highlight the critical role of glial cells and presumably astrocytes, through the availability of D-serine, in the deficits of synaptic mechanisms of learning and memory that occur in the course of aging.

Somatostatin receptor subtypes 2 and 4 affect seizure susceptibility and hippocampal excitatory neurotransmission in mice.

We have investigated the role of somatostatin receptor subtypes sst2 and sst4 in limbic seizures and glutamate-mediated neurotransmission in mouse hippocampus. As compared to wild-type littermates, homozygous mice lacking sst2 receptors showed a 52% reduction in EEG ictal activity induced by intrahippocampal injection of 30 ng kainic acid (P < 0.05). The number of behavioural tonic-clonic seizures was reduced by 50% (P < 0.01) and the time to onset of seizures was doubled on average (P < 0.05). Seizure-associated neurodegeneration was found in the injected hippocampus (CA1, CA3 and hilar interneurons) and sporadically in the ipsilateral latero-dorsal thalamus. This occurred to a similar extent in wild-type and sst2 knock-out mice. Intrahippocampal injection of three selective sst2 receptor agonists in wild-type mice (Octreotide, BIM 23120 and L-779976, 1.5-6.0 nmol) did not affect kainate seizures while the same compounds significantly reduced seizures in rats. L-803087 (5 nmol), a selective sst4 receptor agonist, doubled seizure activity in wild-type mice on average. Interestingly, this effect was blocked by 3 nmol octreotide. It was determined, in both radioligand binding and cAMP accumulation, that octreotide had no direct agonist or antagonist action at mouse sst4 receptors expressed in CCl39 cells, up to micromolar concentrations. In hippocampal slices from wild-type mice, octreotide (2 micro m) did not modify AMPA-mediated synaptic responses while facilitation occurred with L-803087 (2 micro m). Similarly to what was observed in seizures, the effect of L-803087 was reduced by octreotide. In hippocampal slices from sst2 knock-out mice, both octreotide and L-803087 were ineffective on synaptic responses. Our findings show that, unlike in rats, sst2 receptors in mice do not mediate anticonvulsant effects. Moreover, stimulation of sst4 receptors in the hippocampus of wild-type mice induced excitatory effects which appeared to depend on the presence of sst2 subtypes, suggesting these receptors are functionally coupled.

Spatial learning and synaptic hippocampal plasticity in type 2 somatostatin receptor knock-out mice.

Somatostatin is implicated in a number of physiological functions in the CNS. These effects are elicited through the activation of at least five receptor subtypes. Among them, sst2 receptors appear the most widely expressed in the cortex and hippocampal region. However, the specific role of this somatostatin receptor subtype in these regions is largely undetermined. In this study, we investigated the role of the sst2 receptor in the hippocampus using mice invalidated for the sst2 gene (sst2 KO mice). Complementary experimental approaches were used. First, mice were tested in behavioral tests to explore the consequences of the gene deletion on learning and memory. Spatial discrimination learning in the radial maze was facilitated in sst2 KO mice, while operant learning of a bar-pressing task was slightly altered. Mice were then processed for electrophysiological study using the ex vivo hippocampal slice preparation. Extracellular recordings in the CA1 area showed an enhancement in glutamatergic (AMPA and NMDA) responses in sst2 KO mice which displayed an increase in the magnitude of the short-term potentiation and long-term depression. In contrast, long-term potentiation was not significantly altered. Taken together, these data demonstrate that somatostatin, acting via sst2 hippocampal receptors, may contribute to a global decrease in glutamate efficiency and consequently alter glutamate-dependent plasticity and spatial learning.

Decrease in calbindin content significantly alters LTP but not NMDA receptor and calcium channel properties.

The contribution of the cytosolic calcium binding protein calbindin D(28K) (CaBP) to the synaptic plasticity was investigated in hippocampal CA1 area of wild-type and antisense transgenic CaBP-deficient mice. We showed that long-term potentiation (LTP) induced by tetanic stimulation in CaBP-deficient mice was impaired. The fundamental biophysical properties of NMDA receptors and their number were not modified in CaBP-deficient mice. We also demonstrated that the physiological properties of calcium channels were identical between genotypes. An insufficient Ca(2+) entry through NMDA receptors or calcium channels, or a decrease in NMDA receptor density are unlikely to explain this impairment of LTP. Interestingly, we showed that the loss of LTP was not prevented by glycine but was restored in the presence of a low concentration of the NMDA receptor antagonist D-APV (5 microM) and of the calcium chelator BAPTA-AM (5 microM). Moreover, we observed a loss of LTP in the wild-type mice when the postsynaptic tetanic-induced [Ca(2+)](i) rise is excessively increased. Conversely, a weaker tetanus stimulation allowed LTP induction and maintenance in CaBP-deficient mice. These results suggest that a higher cytosol [Ca(2+)](i), due to the decrease of CaBP expression may impair LTP induction and maintenance mechanisms without affecting the mechanisms of calcium entry. Thus, CaBP plays a critical role in long term synaptic plasticity by limiting the elevation of calcium rise in the cytosol to some appropriate spatio-temporal pattern.

NMDA receptor activation in the aged rat hippocampus.

Age-related alterations of N-methyl-D-aspartate receptor (NMDAr) activation were investigated in the CA1 field of hippocampal slices from young (3-6 months old) and aged (25-33 months old) Sprague-Dawley rats by using ex vivo extracellular electrophysiological recording techniques. NMDAr-mediated field excitatory postsynaptic potentials (fEPSPs) were induced by electrical stimulation of glutamatergic fibers in a magnesium (Mg(2+))-free medium supplemented with the non-NMDAr antagonist CNQX. The fEPSPs were significantly smaller in aged rats, whereas the response of presynaptic afferent fibers remained unaffected. No significant age-related differences were found in the ability of Mg(2+) to depress the magnitude of NMDAr-mediated fEPSPs. The responsiveness of postsynaptic NMDAr to the agonist was assessed in both groups of animals. No age-related differences were recorded either in the depolarizing effect of bath-applied NMDA or in the magnitude of the depolarization after altering extracellular Mg(2+) concentration. Finally, short-term potentiation (STP) of excitatory transmission was studied in young and aged rats considering the pivotal role of NMDAr in synaptic plasticity. No age-related alterations of the magnitude and the time course of STP in response to 10 or 30Hz conditioning stimulation were found. Because of the decrease in the magnitude of NMDAr-mediated synaptic transmission in aged animals, the absence of obvious modifications of synaptic plasticity suggests the occurrence of compensatory mechanisms that are discussed.

Slow synaptic inhibition mediated by metabotropic glutamate receptor activation of GIRK channels.

Glutamate is the predominant excitatory neurotransmitter in the vertebrate CNS. Ionotropic glutamate receptors mediate fast excitatory actions whereas metabotropic glutamate receptors (mGluRs) mediate a variety of slower effects. For example, mGluRs can mediate presynaptic inhibition, postsynaptic excitation, or, more rarely, postsynaptic inhibition. We previously described an unusually slow form of postsynaptic inhibition in one class of projection neuron in the song-control nucleus HVc of the songbird forebrain. These neurons, which participate in a circuit that is essential for vocal learning, exhibit an inhibitory postsynaptic potential (IPSP) that lasts several seconds. Only a portion of this slow IPSP is mediated by GABA(B) receptors. Since these cells are strongly hyperpolarized by agonists of mGluRs, we used intracellular recording from brain slices to investigate the mechanism of this hyperpolarization and to determine whether mGluRs contribute to the slow synaptic inhibition. We report that mGluRs hyperpolarize these HVc neurons by activating G protein-coupled, inwardly-rectifying potassium (GIRK) channels. MGluR antagonists blocked this response and the slow synaptic inhibition. Thus, glutamate can combine with GABA to mediate slow synaptic inhibition by activating GIRK channels in the CNS.

Glutamatergic synaptic responses and long-term potentiation are impaired in the CA1 hippocampal area of calbindin D(28k)-deficient mice.

The contribution of the cytosolic calcium binding protein calbindin D(28K) (CaBP) to glutamatergic neurotransmission and synaptic plasticity was investigated in hippocampal CA1 area of wild-type and antisense transgenic CaBP-deficient mice, with the use of extracellular recordings in the ex vivo slice preparation. The amplitude of non-N-methyl-D-aspartate receptor (non-NMDAr)-mediated extracellular field excitatory postsynaptic potentials (fEPSPs) recorded in control medium was significantly greater in CaBP-deficient mice, whereas the afferent fiber volley was not affected. In contrast, the amplitude of NMDAr-mediated fEPSPs isolated in a magnesium-free medium after blockade of non-NMDAr and GABAergic receptors was significantly depressed in these animals. No alteration in the magnitude of paired-pulse facilitation was found, indicating that the presynaptic calcium mechanisms controlling glutamate release were not altered in CaBP-deficient mice. The magnitude and time course of the short-term potentiation (STP) of fEPSPs induced by a 30 Hz conditioning stimulation, which was blocked by the NMDAr antagonist 2-amino-5-phosphonovalerate acid (2-APV), was not impaired in the transgenic mice, whereas long-term potentiation (LTP) induced by a 100 Hz tetanus was not maintained. The long-term depression (LTD) induced by low-frequency stimulation (1 Hz, 15 min) in the presence of the GABA antagonist bicuculline was not altered. These results argue for a contribution of CaBP to the mechanisms responsible for the maintenance of long-term synaptic potentiation, at least in part by modulating the activation of NMDA receptors.

Pharmacological characterization of an unusual mGluR-evoked neuronal hyperpolarization mediated by activation of GIRK channels.

The metabotropic glutamate receptor (mGluR) agonist ACPD exerts an unusual inhibitory effect on a population of neurons of the song-control nucleus HVc of the zebra finch via activation of the GIRK channel. We report in the present study the pharmacology of this response. ACPD directly hyperpolarized the neurons by a mechanism independent of GABA(B) receptors. The group I mGluR agonist DHPG had no effect on membrane properties and the group I mGluR antagonist 4-CPG did not affect the ACPD-induced hyperpolarization. In contrast, the ACPD response was mimicked by the group II mGluR agonist LY314593 and the group II and III agonist L-CCG-I. The group II mGluR antagonist LY307452 fully antagonized the ACPD response and reduced the response induced by L-CCG-I. The group III mGluR agonist L-AP4 induced a small hyperpolarization, which was antagonized by the group III mGluR antagonist MAP-4. These data indicate that group II and group III mGluRs are present and functional in the postsynaptic membrane of these HVc neurons, and mediate the hyperpolarizing action of mGluR agonists. In contrast, group I mGluRs are absent from these neurons, nonfunctional, or coupled to different effector systems that do not influence membrane potential or input resistance.

Multiple cell types distinguished by physiological, pharmacological, and anatomic properties in nucleus HVc of the adult zebra finch.

Nucleus HVc of the songbird is a distinct forebrain region that is essential for song production and shows selective responses to complex auditory stimuli. Two neuronal populations within HVc give rise to its efferent projections. One projection, to the robust nucleus of the archistriatum (RA), serves as the primary motor pathway for song production, and can also carry auditory information to RA. The other projection of HVc begins a pathway through the anterior forebrain, (area X --> medial portion of the dorsolateral nucleus of the thalamus (DLM) --> lateral portion of the magnocellular nucleus of the anterior neostriatum (L-MAN) --> RA) that is crucial for song learning but, although active during singing, is not essential for adult song production. To test whether these different projection neuron classes have different functional properties, we recorded intracellularly from neurons in nucleus HVc in brain slices. We observed at least three classes of neuron based on intrinsic physiological and pharmacological properties as well as on synaptic inputs. We also examined the morphological properties of the cells by filling recorded neurons with neurobiotin. The different physiological cell types correspond to separate populations based on their soma size, dendritic extent, and axonal projection. Thus HVc neurons projecting to area X have large somata, show little spike-frequency adaptation, a hyperpolarizing response to the metabotropic glutamate receptor (mGluR) agonist (1S,3R)-trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), and exhibit a slow inhibitory postsynaptic potential (IPSP) following tetanic stimulation. Those HVc neurons projecting to motor nucleus RA have smaller somata, show strong accommodation, are not consistently hyperpolarized by ACPD, and exhibit no slow IPSP. A third, rarely recorded class of neurons fire in a sustained fashion at very high-frequency and may be interneurons. Thus the neuronal classes within HVc have different functional properties, which may be important for carrying specific information to their postsynaptic targets.

Spatial discrimination learning and CA1 hippocampal synaptic plasticity in mdx and mdx3cv mice lacking dystrophin gene products.

Duchenne muscular dystrophy is frequently associated with a non-progressive cognitive deficit attributed to the absence of 427,000 mol. wt brain dystrophin, or to altered expression of other C-terminal products of this protein, Dp71 and/or Dp140. To further explore the role of these membrane cytoskeleton-associated proteins in brain function, we studied spatial learning and ex vivo synaptic plasticity in the mdx mouse, which lacks 427,000 mol. wt dystrophin, and in the mdx3cv mutant, which shows a dramatically reduced expression of all the dystrophin gene products known so far. We show that reference and working memories are largely unimpaired in the two mutant mice performing a spatial discrimination task in a radial maze. However, mdx3cv mice showed enhanced emotional reactivity and developed different strategies in learning the task, as compared to control mice. We also showed that both mutants display apparently normal levels of long-term potentiation and paired-pulse facilitation in the CA1 field of the hippocampus. On the other hand, an increased post-tetanic potentiation was shown by mdx, but not mdx3cv mice, which might be linked to calcium-regulatory defects. Otherwise, immunoblot analyses suggested an increased expression of a 400,000 mol. wt protein in brain extracts from both mdx and mdx3cv mice, but not in those from control mice. This protein might correspond to the dystrophin-homologue utrophin. The present results suggest that altered expression of dystrophin or C-terminal dystrophin proteins in brain did not markedly affect hippocampus-dependent spatial learning and CA1 hippocampal long-term potentiation in mdx and mdx3cv mice. The role of these membrane cytoskeleton-associated proteins in normal brain function and pathology remains to be elucidated. Furthermore, the possibility that redundant mechanisms could partially compensate for dystrophins' deficiency in the mdx and mdx3cv models should be further considered.