Jean-Marie Billard

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.

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.

Impaired long-term spatial and recognition memory and enhanced CA1 hippocampal LTP in the dystrophin-deficient Dmdmdx mouse

Duchenne muscular dystrophy (DMD) is associated with cognitive deficits that may result from dystrophin deficiency in neurons. However, in the dystrophin-deficient Dmd(mdx) mouse model of DMD, the nature of the memory impairment is not well characterised and its biological substrate is uncertain. Here, we demonstrate that dystrophin deficiency in Dmd(mdx) mice impairs long-term, but not short-term, object recognition memory and impairs long-term spatial memory, but not acquisition, following massed training in the water maze. Furthermore, we show that the abnormal enhancement of CA1 hippocampal LTP in Dmd(mdx) mice is not restricted to short-lasting mechanisms, but also affects the maintenance phase of LTP of both synaptic efficacy and neuronal excitability. We conclude that dystrophin loss alters memory consolidation in both spatial and nonspatial learning tasks, at least in part due to altered synaptic plasticity mechanisms, and suggest that the severity of the deficits may depend on the nature of the training procedure.

Neuronal d-Serine and Glycine Release Via the Asc-1 Transporter Regulates NMDA Receptor-Dependent Synaptic Activity

d-Serine and glycine are coagonists of NMDA receptors (NMDARs), but their relative contributions for several NMDAR-dependent processes are unclear. We now report that the alanine–serine–cysteine transporter-1 (Asc-1) mediates release of both d-serine and glycine from neurons, and, in turn, this modulates NMDAR synaptic activity. Asc-1 antiporter activity is enhanced by d-isoleucine (d-Ile), which releases d-serine and glycine from Asc-1-transfected cells, primary neuronal cultures, and hippocampal slices. d-Ile has no effect on astrocytes, which do not express Asc-1. We show that d-Ile enhances the long-term potentiation (LTP) in rat and mouse hippocampal CA1 by stimulating Asc-1-mediated endogenous d-serine release. d-Ile effects on synaptic plasticity are abolished by enzymatically depleting d-serine or by using serine racemase knock-out (SR-KO) mice, confirming its specificity and supporting the notion that LTP depends mostly on d-serine release. Conversely, our data also disclose a role of glycine in activating synaptic NMDARs. Although acute enzymatic depletion of d-serine also drastically decreases the isolated NMDAR synaptic potentials, these responses are still enhanced by d-Ile. Furthermore, NMDAR synaptic potentials are preserved in SR-KO mice and are also enhanced by d-Ile, indicating that glycine overlaps with d-serine binding at synaptic NMDARs. Altogether, our results disclose a novel role of Asc-1 in regulating NMDAR-dependent synaptic activity by mediating concurrent non-vesicular release of d-serine and glycine. Our data also highlight an important role of neuron-derived d-serine and glycine, indicating that astrocytic d-serine is not solely responsible for activating synaptic NMDARs.

Involvement of sst2 somatostatin receptor in locomotor, exploratory activity and emotional reactivity in mice.

Somatostatin (SRIF) controls many physiological and pathological processes in the central nervous system but the respective roles of the five receptor isotypes (sst1–5) that mediate its effects are yet to be defined. In the present study, we attempted to identify functions of the sst2 receptor using mice with no functional copy of this gene (sst2 KO mice). In contrast with control 129Sv/C57Bl6 mice, sst2 mRNA was no longer detectable in the brain of sst2 KO mice; 125I‐labeled Tyr0DTrp8‐SRIF14 binding was also greatly reduced in almost all brain structures except for the hippocampal CA1 area, demonstrating that sst2 accounts for most SRIF binding in mouse brain. Invalidation of this subtype generated an increased anxiety‐related behaviour in a number of behavioural paradigms, while locomotor and exploratory activity was decreased in stress‐inducing situations. No major motor defects could be detected. sst2 KO mice also displayed increased release of pituitary ACTH, a main regulator of the stress response. Thus, somatostatin, via sst2 receptor isotype pathways, appears involved in the modulation of locomotor, exploratory and emotional reactivity in mice.

Parallel Loss of Hippocampal LTD and Cognitive Flexibility in a Genetic Model of Hyperdopaminergia.

Dopamine-mediated neurotransmission has been implicated in the modulation of synaptic plasticity and in the mechanisms underlying learning and memory. In the present study, we tested different forms of activity-dependent neuronal and behavioral plasticity in knockout mice for the dopamine transporter (DAT-KO), which constitute a unique genetic model of constitutive hyperdopaminergia. We report that DAT-KO mice exhibit slightly increased long-term potentiation and severely decreased long-term depression at hippocampal CA3–CA1 excitatory synapses. Mutant mice also show impaired adaptation to environmental changes in the Morris watermaze. Both the electrophysiological and behavioral phenotypes are reversed by the dopamine antagonist haloperidol, suggesting that hyperdopaminergia is involved in these deficits. These findings support the modulation by dopamine of synaptic plasticity and cognitive flexibility. The behavioral deficits seen in DAT-KO mice are reminiscent of the deficits in executive functions observed in dopamine-related neuropsychiatric disorders, suggesting that the study of DAT-KO mice can contribute to the understanding of the molecular basis of these disorders.

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 µm) did not modify AMPA‐mediated synaptic responses while facilitation occurred with L‐803087 (2 µm). 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.

Deficit of NMDA receptor activation in CA1 hippocampal area of aged rats is rescued by D-cycloserine

Activation of the glycine modulatory site of the N‐methyl‐d‐aspartate glutamate receptor (NMDAR) may reduce cognitive impairments associated with normal ageing. In order to test this hypothesis, we assessed the effects of the partial agonist d‐cycloserine (DCS) on cellular activities involved in memory formation. This was performed in CA1 cellular networks of adult and aged Sprague–Dawley rat hippocampal slices using extracellular field excitatory postsynaptic potential recordings. Synaptic potentials specifically mediated by NMDAR were significantly reduced in aged animals. DCS increased the magnitude of these responses in both adult and old rats but this effect was significantly higher in the latter, thus reversing the age‐related decrease in NMDAR synaptic potentials. NMDAR‐mediated theta burst long‐term potentiation (TBS‐LTP) as well as long‐term depression (LTD) of synaptic transmission, prominent models for the cellular basis of learning and memory, were also weakened in aged animals. Age‐related alterations of both forms of synaptic plasticity were rescued by DCS. In addition, the DCS‐induced decrease in basal fast glutamatergic neurotransmission involving the activation of inhibitory glycinergic receptors, previously reported in young rats ( Rouaud & Billard, 2003 ), was severely attenuated in aged animals. In summary, our results indicate that the facilitation of NMDAR activation through its glycine‐binding site rescues the age‐related deficit of cellular mechanisms of learning and memory. Such physiological evidences suggest that this modulation site of NMDAR represents an important target to alleviate cognitive deficits associated with normal ageing.

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 mothers 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.

Age-related effects of the neuromodulator D-serine on neurotransmission and synaptic potentiation in the CA1 hippocampal area of the rat.

The effects of the co‐agonist of the N‐methyl‐d‐aspartate receptor (NMDAr) d‐serine on glutamatergic neurotransmission and synaptic potentiation were studied in the CA1 hippocampal field of young (3–5 months old) and aged (25–27 months old) Sprague–Dawley rats using ex vivo extracellular electrophysiological recording techniques. Exogenous d‐serine depressed fast neurotransmission mediated by the α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid/kainate subtype of glutamate receptors in young but not in aged rats by acting on inhibitory glycinergic interneurons. In contrast, d‐serine dose‐dependently enhanced NMDAr‐mediated synaptic responses in both groups of animals, but with a larger magnitude in aged rats, thus preventing the age‐related decrease in NMDAr activation. d‐serine also increased the magnitude of long‐term potentiation in aged but not in young rats. Finally, d‐serine levels were dramatically reduced in hippocampal tissues of aged rats. Taken together, these results indicate a weaker activation of the NMDAr glycine modulatory site by endogenous d‐serine in aged animals, which accounts for a reduced NMDAr contribution to synaptic plasticity in ageing.