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Structural and functional synapse reorganization is one of the key issues of learning and memory mechanisms. Specific proteases, called matrix metalloproteinases (MMPs), play a pivotal role during learning-related modification of neural circuits. Different types of MMPs modify the extracellular perisynaptic environment, leading to the plastic changes in the synapses. In recent years, there has been an increasing interest in the role played by matrix metalloproteinase-3 (MMP-3) in various processes occurring in the mammalian brain, both in physiological and pathological conditions. In this review, we discuss a crucial function of MMP-3 in synaptic plasticity, learning, neuronal development, as well as in neuroregeneration. We discuss the involvement of MMP-3 in synaptic long-term potentiation, which is likely to have a profound impact on experience-dependent learning. On the other hand, we also provide examples of deleterious actions of uncontrolled MMP-3 activity on the central nervous system (CNS) and its contribution to Alzheimer's and Parkinson's diseases (AD and PD). Since the molecular mechanisms controlled by MMP-3 have a profound and diverse impact on physiological and pathological brain functioning, their deep understanding may be crucial for the development of more specific methods for the treatment of neuropsychiatric diseases.

Astrocytic glycogen metabolism sustains neuronal activity but its impact on basal glutamatergic synaptic transmission is not clear. To address this issue, we have compared the effect of glycogen breakdown inhibition on miniature excitatory postsynaptic currents (mEPSCs) in rat hippocampal pure neuronal culture (PNC) and in astrocyte-neuronal co-cultures (ANCC). Amplitudes of mEPSC in ANCC were nearly twice as large as in PNC with no difference in current kinetics. Inhibition of glycogen phosphorylase reduced mEPSC amplitude by roughly 40% in ANCC being ineffective in PNC. Altogether, these data indicate that astrocyte-neuronal interaction enhances basal mEPSCs in ANCC mainly due to astrocytic glycogen metabolism.

Gamma aminobutric acid (GABA) is the major inhibitory neurotransmitter in the adult brain and mechanisms of GABAergic inhibition have been intensely investigated in the past decades. Recent studies provided evidence for an important role of astrocytes in shaping GABAergic currents. One of the most obvious, but yet poorly understood, mechanisms of the cross-talk between GABAergic currents and astrocytes is metabolism including neurotransmitter homeostasis. In particular, how modulation of GABAergic currents by astrocytes depends on key enzymes involved in cellular metabolism remains largely unknown. To address this issue, we have considered two simple models of neuronal culture (NC): nominally astrocyte-free NC and neuronal-astrocytic co-cultures (ANCC). Miniature Inhibitory Postsynaptic Currents (mIPSCs) were recorded in control conditions and in the presence of different enzyme blockers. We report that enrichment of NC with astrocytes results in a marked increase in mIPSC frequency. This enhancement of GABAergic activity was accompanied by increased number of GAD65 and vGAT puncta, indicating that at least a part of the frequency enhancement was due to increased number of synaptic contacts. Inhibition of glutamine synthetase (Glns) (with MSO) strongly reduced mIPSC frequency in ANCC but had no effect in NC. Moreover, treatment of ANCC with inhibitor of glycogen phosphorylase (Gys) (BAYU6751) or with selective inhibitor of astrocytic Krebs cycle, fluoroacetate, resulted in a marked reduction of mIPSC frequency in ANCC having no effect in NC. We conclude that GABAergic synaptic transmission strongly depends on neuron-astrocyte interaction in a manner dependent on key metabolic enzymes as well as on the Krebs cycle.

Astrocytes releasing glucose- and/or glycogen-derived lactate and glutamine play a crucial role in shaping neuronal function and plasticity. Little is known, however, how metabolic functions of astrocytes, e.g., their ability to degrade glucosyl units, are affected by the presence of neurons. To address this issue we carried out experiments which demonstrated that co-culturing of rat hippocampal astrocytes with neurons significantly elevates the level of mRNA and protein for crucial enzymes of glycolysis (phosphofructokinase, aldolase, and pyruvate kinase), glycogen metabolism (glycogen synthase and glycogen phosphorylase), and glutamine synthetase in astrocytes. Simultaneously, the decrease of the capability of neurons to metabolize glucose and glutamine is observed. We provide evidence that neurons alter the expression of astrocytic enzymes by secretion of as yet unknown molecule(s) into the extracellular fluid. Moreover, our data demonstrate that almost all studied enzymes may localize in astrocytic nuclei and this localization is affected by the co-culturing with neurons which also reduces proliferative activity of astrocytes. Our results provide the first experimental evidence that the astrocyte-neuron crosstalk substantially affects the expression of basal metabolic enzymes in the both types of cells and influences their subcellular localization in astrocytes.

The organomercurial, thimerosal, is at the center of medical controversy as a suspected factor contributing to neurodevelopmental disorders in children. Many neurotoxic effects of thimerosal have been described, but its interaction with principal excitatory and inhibitory neurotransmiter systems is not known. We examined, using electrophysiological recordings, thimerosal effects on GABA and NMDA-evoked currents in cultured hippocampal neurons. After brief (3 to 10 min) exposure to thimerosal at concentrations up to 100 μM, there was no significant effect on GABA or NMDA-evoked currents. However, following exposure for 60-90 min to 1 or 10 μM thimerosal, there was a significant decrease in NMDA-induced currents (p0.05) and GABAergic currents (p0.05). Thimerosal was also neurotoxic, damaging a significant proportion of neurons after 60-90 min exposure; recordings were always conducted in the healthiest looking neurons. Mercuric chloride, at concentrations 1 μM and above, was even more toxic, killing a large proportion of cells after just a few minutes of exposure. Recordings from a few sturdy cells revealed that micromolar mercuric chloride markedly potentiated the GABAergic currents (p0.05), but reduced NMDA-evoked currents (p0.05). The results reveal complex interactions of thimerosal and mercuric ions with the GABA(A) and NMDA receptors. Mercuric chloride act rapidly, decreasing electrophysiological responses to NMDA but enhancing responses to GABA, while thimerosal works slowly, reducing both NMDA and GABA responses. The neurotoxic effects of both mercurials are interwoven with their modulatory actions on GABA(A) and NMDA receptors, which most likely involve binding to these macromolecules.

Acute hippocampal slices and primary neuronal cultures are often used with a tacit assumption that basic characteristics of the two models closely resemble each other. The use of the cell cultures, however, may raise controversies because of non-physiological conditions resulting from e.g. glial cells deficit, random neuronal sprouting, lack of specificity in the synaptic connections, impaired homeostasis, etc. Importantly, alteration in neuronal environment, especially when occurring over a prolonged period of time, may give rise to a profound homeostatic modulation. In the present study we have compared the properties of GABAergic and glutamatergic (non-NMDA) currents in pyramidal neurons from hippocampal slices and neuronal cell culture. We show that, most strikingly, amplitude ratio of currents elicited by ultrafast applications of saturating GABA and glutamate was nearly one order of magnitude larger in cultured neurons than that in slices. Miniature IPSCs and EPSCs also showed substantial differences between these two models. In particular, mEPSC amplitudes were larger and more frequent in cultured neurons but their time duration was longer in slices. Miniature IPSCs did not show differences in amplitude when comparing slices and cultures but their time duration was faster and occurrence more frequent in slices. In conclusion, we provide evidence that expression pattern of GABA(A) and glutamate receptors as well as synaptic current properties in the neuronal cell culture show profound differences with respect to that in the physiological conditions.

We applied the whole-cell patch-clamp technique to study the influence of zinc ions (Zn(2+)) and extracellular protons at acidic pH (pH(o)) on voltage-gated potassium currents in cultured rat hippocampal neurons. The first goal of the study was to estimate whether Kv1.3 currents significantly contributed to voltage-gated potassium currents in examined cells. Then, the influence of both ions on the activity of other voltage-gated potassium currents in the neurons was examined. We examined both the total current and the delayed - rectifier component. Results obtained in both cases were not significantly different from each other. Available data argued against any significant contribution of Kv1.3 currents to the recorded currents. Nevertheless, application of Zn(2+) in the concentration range from 100 microM to 5 mM reversibly modulated the recorded currents. The activation midpoint was shifted by about 40 mV (total current) and 30 mV (delayed-rectifier current) towards positive membrane potentials and the activation kinetics were slowed significantly (2 - 3 fold) upon application of Zn(2+). The inactivation midpoint was also shifted towards positive membrane potentials, but less significantly (about 14 mV). The current amplitudes were reduced in a concentration-dependent manner to about 0.5 of the control value. The effects of Zn(2+) were saturated at the concentration of 1 mM. Raising extracellular proton concentration by lowering the pH(o) from 7.35 to 6.4 did not affect significantly the currents. Possible mechanisms underlying the observed phenomena and their possible physiological significance are discussed.

Brain-derived calcium-binding protein S100 induces apoptosis in a significant fraction of rat phaeochromocytoma (PC12) cells. We used single cell techniques (patch clamp, videomicroscopy and immunocytochemistry) to clarify some of the specific aspects of S100-induced apoptosis, the modality(ies) of early intracellular Ca2+ concentration increase and the expression of some classes of genes (c-fos, c-jun, bax, bcl-x, p-15, p-21) known to be implicated in apoptosis of different cells. The results show that S100: (1) causes an increase of [Ca2+]i due to an increased conductance of L-type Ca2+ channels; (2) induces a sustained increase of the Fos levels which is evident since the first time point tested (3 h) and remains elevated until to the last time point (72 h). All these data suggest that S100-derived apoptosis in PC12 cells may be the consequence of a system involving an increase in L-type Ca2+ channel conductance with consequent [Ca2+]i increase which up-regulates, directly or indirectly, the expression of Fos.

Metalloproteinases (MMPs) have been shown to play a crucial role in synaptic plasticity and cognitive processes. We recently reported that in the mossy fiber - CA3 hippocampal pathway, LTP maintenance required fine-tuned MMP-9 activity, as both MMP-9 excess and absence impaired LTP. Here we used acute brain slices from transgenic (TG) rats overexpressing MMP-9 to investigate the impact of excessive MMP-9 activity on the excitatory synaptic transmission in the CA3-CA1 projection. Using field potential recordings, we have demonstrated that MMP-9 overexpression increased the strength of basal synaptic transmission but had no effect on the short-term plasticity in comparison to the wild-type (WT) group. In attempt to shed light on mechanisms underlying this observation, miniature excitatory postsynaptic potentials (mEPSCs) were recorded from pyramidal CA1 neurons. We found that mEPSCs in the TG group had a significantly slower decaying phase than in WT but amplitudes and frequencies were similar. The lack of differences in mEPSC frequency and short-term plasticity between TG and WT groups suggests that MMP-9 overexpression effect on fEPSPs was mainly postsynaptic. Additionally, we have found that excess of MMP-9 in TG rats was associated with impaired late-phase of LTP in the considered pathway. It seems thus that augmented synaptic strength in TG rats occurred in expense of impaired long-term plasticity induced by tetanization. In conclusion, overexpression of MMP-9 leads to increase in the strength of basal excitatory synaptic transmission and impairs of LTP maintenance phase in the CA3-CA1 pathway in vitro.