Protein kinase C inhibitors block generation of anoxia-induced long-term potentiation

[EFFECT OF HYPOXIA ON SYNAPTIC TRANSMISSION BETWEEN RETINAL GANGLION CELLS AND SUPERIOR COLLICULUS NEURONS IN COCULTURE]

In this study we conducted a series of experiments to characterize the effect and define the mechanisms of hypoxia on synaptic transmission between retinal ganglion cells and superior colliculus (SC) neurons. Application of hypoxic solution leads to a long lasting potentiation (LTP) NMDA-mediated synaptic transmission. Analysis of the oxygen deficiency effect on the spontaneous and miniature postsynaptic currents (sPSC and mPSC respectively) revealed an increase in the frequency of their occurrence and the appearance of the second peak in the mPSC histogram distribution. The assessment of quantum and binomial parameters reflects the complex pre- and postsynaptic changes during the potentiation, independent of the release probability. Most likely this LTP can be caused by an increase in the total number of active synapses. Glutamatergic synaptic transmission mediated by non-NMDA activation receptor-channel complexes, responded to application of deoxygenated solution by the brief depression, which is the result of presynaptic dysfunction and associates with decrease in release probability and number of active zones. GABAergic synaptic transmission mediated by activation GABA(A)-receptor-channel complexes, responded to hypoxic action by long term depression (LTD). Analysis of sPSC and mPSC showed a significant decrease in the frequency of their occurrence and significant (P = 0.05) decrease in the quantum over a period of oxygen deficiency. In general, the effect of hypoxia-induced LTD of GABAergic synaptic transmission is based on complex changes of presynaptic (independent on the release probability) and postsynaptic (reduction sensitivity of receptors in postsynaptic membrane) mechanisms.

Ischemic cerebral damage: an appraisal of synaptic failure

In the human brain, ≈30% of the energy is spent on synaptic transmission. Disappearance of synaptic activity is the earliest consequence of cerebral ischemia. The changes of synaptic function are generally assumed to be reversible and persistent damage is associated with membrane failure and neuronal death. However, there is overwhelming experimental evidence of isolated, but persistent, synaptic failure resulting from mild or moderate cerebral ischemia. Early failure results from presynaptic damage with impaired transmitter release. Proposed mechanisms include dysfunction of adenosine triphosphate-dependent calcium channels and a disturbed docking of glutamate-containing vesicles resulting from impaired phosphorylation. We review energy distribution among neuronal functions, focusing on energy usage of synaptic transmission. We summarize the effect of ischemia on neurotransmission and the evidence of long-lasting synaptic failure as a cause of persistent symptoms in patients with cerebral ischemia. Finally, we discuss the implications of synaptic failure in the diagnosis of cerebral ischemia, including the limited sensitivity of diffusion-weighted MRI in those cases in which damage is presumably limited to the synapses.

A critical role of NO/cGMP/PKG dependent pathway in hippocampal post-ischemic LTP: modulation by zonisamide

Nitric oxide (NO) is an intercellular retrograde messenger involved in several physiological processes such as synaptic plasticity, hippocampal long-term potentiation (LTP), and learning and memory. Moreover NO signaling is implicated in the pathophysiology of brain ischemia. In this study, we have characterized the role of NO/cGMP signaling cascade in the induction and maintenance of post-ischemic LTP (iLTP) in rat brain slices. Moreover, we have investigated the possible inhibitory action of zonisamide (ZNS) on this pathological form of synaptic plasticity as well as the effects of this antiepileptic drug (AED) on physiological activity-dependent LTP. Finally, we have characterized the possible interaction between ZNS and the NO/cGMP/PKG-dependent pathway involved in iLTP. Here, we provided the first evidence that an oxygen and glucose deprivation episode can induce, in CA1 hippocampal slices, iLTP by modulation of the NO/cGMP/PKG pathway. Additionally, we found that while ZNS application did not affect short-term synaptic plasticity and LTP induced by high-frequency stimulation, it significantly reduced iLTP. This reduction was mimicked by bath application of NO synthase inhibitors and a soluble guanyl cyclase inhibitor. The effect of ZNS was prevented by either the application of a NO donor or drugs increasing intracellular levels of cGMP and activating PKG. These findings are in line with the possible use of AEDs, such as ZNS, as a possible neuroprotective strategy in brain ischemia. Moreover, these findings strongly suggest that NO/cGMP/PKG intracellular cascade might represent a physiological target for neuroprotection in pathological forms of synaptic plasticity such as hippocampal iLTP.

Protein kinase M zeta regulation of Na/K ATPase: a persistent neuroprotective mechanism of ischemic preconditioning in hippocampal slice cultures

In ischemic preconditioning, a sublethal ischemic insult protects neurons from subsequent ischemia. In organotypic hippocampal slice cultures a sublethal 5-minute hypoxia-hypoglycemia treatment prevented neuronal loss after a 10-minute experimental ischemic (EI) treatment of hypoxia-hypoglycemia. Whereas preconditioning protected against EI given 24 h later, it did not protect when EI was given 2 h later, suggesting a slow development of neuroprotection. This model identified two regulators of ischemic preconditioning: the atypical protein kinase M zeta (PKMzeta), and the Na/K ATPase. Two hours following preconditioning, when there was no neuroprotection, Na/K ATPase activity was unchanged. In contrast, Na/K ATPase activity significantly increased 24 h after the preconditioning treatment. Elevated Na/K ATPase activity was accompanied by increased surface expression of the alpha1 and alpha2 isoforms of the Na/K ATPase. Similarly, active PKMzeta levels were increased at 24 h, but not 2 h, after preconditioning. PKMzeta overexpression by sindbis virus vectors also increased Na/K ATPase activity. To examine PKMzeta regulation of Na/K ATPase, occlusion experiments were performed using marinobufagenin to inhibit alpha1, dihydroouabain to inhibit alpha2/3 and a zeta-pseudosubstrate peptide to inhibit PKMzeta. These experiments showed that PKMzeta regulated both the activity and surface expression of the alpha1 isoform of the Na/K ATPase. Marinobufagenin, dihydroouabain, and zeta-pseudosubstrate peptide were used to determine if PKMzeta or the alpha1 and alpha2 Na/K ATPase isoforms protected neurons. All three compounds blocked neuroprotection following ischemic preconditioning. PKMzeta levels were elevated 3 days after ischemic preconditioning. These data indicate key roles of PKMzeta and Na/K ATPase in ischemic preconditioning.

Plasticity and repair in the post-ischemic brain

Stroke is the second commonest cause of death and the principal cause of adult disability in the world. In most cases ischemic injuries have been reported to induce mild to severe permanent deficits. Nevertheless, recovery is often dynamic, reflecting the ability of the injured neuronal networks to adapt. Plastic phenomena occurring in the cerebral cortex and in subcortical structures after ischemic injuries have been documented at the synaptic, cellular, and network level and several findings suggest that they may play a key role during neurorehabilitation in human stroke survivors. In particular, in vitro studies have demonstrated that oxygen and glucose deprivation (in vitro ischemia) exerts long-term effects on the efficacy of synaptic transmission via the induction of a post-ischemic long-term potentiation (i-LTP). i-LTP may deeply influence the plastic reorganization of cortical representational maps occurring after cerebral ischemia, inducing a functional connection of previously non-interacting neurons. On the other hand, there is evidence that i-LTP may exert a detrimental effect in the peri-infarct area, facilitating excitotoxic processes via the sustained, long-term enhancement of glutamate mediated neurotransmission. In the present work we will review the molecular and synaptic mechanisms underlying ischemia-induced synaptic plastic changes taking into account their potential adaptive and/or detrimental effects on the neuronal network in which they occur. Thereafter, we will consider the implications of brain plastic phenomena in the post-stroke recovery phase as well as during the rehabilitative and therapeutic intervention in human subjects.

Late-onset epileptogenesis and seizure genesis: lessons from models of cerebral ischemia

Patients surviving ischemic stroke often express delayed epileptic syndromes. Late poststroke seizures occur after a latency period lasting from several months to years after the insult. These seizures might result from ischemia-induced neuronal death and associated morphological and physiological changes that are only partly elucidated. This review summarizes the long-term morphofunctional alterations observed in animal models of both focal and global ischemia that could explain late-onset seizures and epileptogenesis. In particular, this review emphasizes the change in GABAergic and glutamatergic signaling leading to hyperexcitability and seizure genesis.

Differential expression of NMDA receptor subunits and splice variants among the CA1, CA3 and dentate gyrus of the adult rat

N-Methyl-d-aspartate (NMDA)-type glutamate receptors in the hippocampus are important mediators of both memory formation and excitotoxicity. It is thought that glutamatergic neurons of the CA1, CA3 and dentate gyrus regions of the hippocampus contribute differentially to memory formation and are differentially sensitive to excitotoxicity. The subunit and/or splice variant composition of the NMDA receptor controls many aspects of receptor function such as ligand affinity, calcium permeability and channel kinetics, as well as interactions with intracellular anchoring and regulatory proteins. Thus, one possible explanation of the differences in NMDA receptor-dependent processes, such as synaptic plasticity and excitotoxicity, among the hippocampal sub-regions is that they differ in subunit and/or splice variant expression. Here we report that the NMDA receptor subunits NR1 and NR2B, along with the four splice variant cassettes of the NR1 subunit are differentially expressed in the CA1, CA3 and dentate gyrus of the hippocampus. Expression of the AMPA receptor subunits GluR1 and GluR2 also differ. These differences may contribute to functional differences, such as with excitotoxicity and synaptic plasticity, that exist between the sub-regions of the hippocampus.

Protein kinase C and synaptic dysfunction after cardiac arrest

It is now understood that the mechanisms leading to neuronal cell death after cardiac arrest (CA) are highly complex. A well established fact in this field is that neurons continue to die over days and months after ischemia. It has been suggested that decreases in electrophysiological activities precede the morphologic deterioration in postischemic CA1 neurons and that this deterioration may be one cause for delayed cell death. The link between synaptic dysfunction and cardiac arrest is evident by the fact that about 50% of long-term survivors of cardiac arrest exhibit impaired mental abilities, manifested as learning impairment, memory disturbance. Since PKC is known to be a key player in synaptic function and has been implicated in promoting cell death after cerebral ischemia, it is a logical candidate as a modulator of synaptic derangements after CA. In this review, we provide an overview of synaptic dysfunction following CA and the putative role of PKC on this dysfunction.

Dual effects of bryostatin-1 on spatial memory and depression

Dementia and depression are clinical symptoms commonly associated in patients. Emerging evidence suggests that the two diseases share many profiles in their development and underlying neural/molecular mechanisms. Thus, interest is raised in developing new classes of antidepressant agents with activity of cognitive enhancement. Here, we show that bryostatin-1, a protein kinase C substrate activator, at bilateral intracerebroventricular doses of 0.64 or 2 pmol/site, significantly enhanced learning and memory of rats in a spatial water maze task. When applied at the doses at which it exhibits memory-enhancing activity, bryostatin-1 showed a significant antidepressant activity, as determined in an open space swim test. Both effects were not observed when a smaller dose was administered and were largely eliminated by co-administration of 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), a protein kinase C inhibitor. These results support the hypothesis that memory processing and mood regulation share common neural mechanisms. Restoring impaired mood regulation with antidepressant agents that also exhibit memory-enhancing activity may represent one of the new strategies in the fight against depression associated with memory impairments.

Ischemia induces short- and long-term remodeling of synaptic activity in the hippocampus

One of the most vulnerable areas to ischemia or hypoglycemia is CA1 hippocampal region due to pyramidal neurons death. Glutamate receptors are involved together with protein-kinase C and nitric oxide synthase. Long-term potentiation (LTP) is generated in anoxic or hypoglycemic conditions via activation of NMDA while inhibition of these receptors attenuates this response. Protein-kinase C and nitric oxide synthase are involved in anoxic LTP mechanism. Postischemic neurons are hyperexcitable in CA3 area while CA1 pyramidal neurons degenerate and disappear. Changes of glutamate receptors triggered by ischemia and hypoglycemia are discussed in this review.

Synaptic plasticity in the ischaemic brain

Activity-dependent long-term potentiation (LTP) of excitatory neurotransmission underlies specific forms of associative learning and memory. A brief period of energy deprivation induces LTP in specific subsets of neurons; this synaptic plasticity might contribute to the delayed effects of brain ischaemia. In this review, we discuss the similarities and differences between LTP induced by energy deprivation and "physiological" LTP. On the basis of recent studies, we propose that pathological plasticity induced by energy deprivation can play a part in delayed neuronal death in the hippocampus and the striatum after global ischaemia and in the conversion of ischaemic penumbra to infarct core after focal ischaemia. We discuss evidence that ischaemia could also induce protective and reparative forms of neuronal plasticity that may play a part in ischaemic tolerance and poststroke recovery.

Gene transfer of constitutively active protein kinase C into striatal neurons accelerates onset of levodopa-induced motor response alterations in parkinsonian rats

Alterations in motor response that complicate levodopa treatment of Parkinson's disease appear to involve sensitization of striatal ionotropic glutamate receptors. Since protein kinase C (PKC)-mediated phosphorylation regulates glutamatergic receptors of the alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionic acid (AMPA) subtype and has been linked to several forms of behavioral plasticity, activation of PKC signaling in striatal spiny neurons may also contribute to the motor plasticity changes associated with chronic levodopa therapy. To evaluate this possibility, we sought to augment PKC signaling by using Herpes Simplex Virus type 1 vectors (pHSVpkcDelta) to directly transfer the catalytic domain of the PKCbetaII gene into striatal neurons of parkinsonian rats. Microinjection of pHSVpkcDelta vectors lead to the persistent expression of PkcDelta (35% loss over 21 days) in medium spiny neurons together with an increase in serine 831 phosphorylation on AMPA receptor GluR1 subunits and hastened the appearance of the shortened response duration produced by chronic levodopa treatment (P<0.05). In pHSVpkcDelta-infected animals, intrastriatal injection of the PKC inhibitor NPC-15437 (1.0 microg) attenuated both the increased GluR1 phosphorylation (P<0.01) and the accelerated onset of the levodopa-induced response modifications (P<0.01). However, in rats that received levodopa treatment for 21 days without the gene transfer, intrastriatal NPC-15437 had no effect on the response shortening or on GluR1 S831 phosphorylation. The results suggest that an increase in PKC-mediated signaling, including, in part, phosphorylation of AMPA receptors, on striatal spiny neurons may be sufficient to promote the initial appearance, but not necessary the ultimate expression, of the levodopa-induced motor response changes occurring in a rodent model of the human motor complication syndrome.

The adaptive effects of hypoxic preconditioning of brain neurons

Prophylactic transient hypoxia (preconditioning) increased neuron resistance to subsequent induction of severe hypoxia. Published data and results obtained by the authors on the molecular-cellular mechanisms of hypoxic preconditioning are presented. The roles of intracellular signal transduction, genome function, stress proteins, and neuromodulatory peptides in this process are discussed. The roles of glutamatergic as well as calcium and phosphoinositide regulatory systems and neuromodulatory factors as components of "volume" signal transmission are analyzed in hypoxic preconditioning-associated induction of functional tolerance mechanisms against the acute harmful effects of hypoxia on neurons in olfactory slices.

Hypoxia-induced down-regulation of CYP1A1/1A2 and up-regulation of CYP3A6 involves serum mediators

1. Acute moderate hypoxia modifies the catalytic activity and expression of certain isoenzymes of hepatic cytochrome P450 (P450). The aim of this study was to document whether hypoxia affects hepatic P450 directly or through the release of serum mediators. 2. Rabbits were subjected to a FiO(2) of 8% for 48 h, sacrificed, and serum and hepatocytes were isolated; hepatocytes from control and rabbits with hypoxia were incubated with serum from control and hypoxic rabbits for 4 and 24 h, and total P450 content, CYP1A1, 1A2 and 3A6 activities and expressions were assessed. Sera were fractionated by size exclusion chromatography and fractions tested for their ability to modify activity and amount of P450, and serum mediators were identified through neutralization experiments. 3. Total serum and fractions with proteins of 15-23 and 65-94 kDa of M(r) reduced P450 content and expression of CYP1A1, 1A2 and 3A6, as well as CYP1A1, 1A2 and 3A6 mRNA. Total serum and the fraction with 32-44 kDa proteins increased CYP3A6 activity and protein and mRNA. The serum mediators implicated in the decrease in activity and expression of CYP1A1, 1A2 and 3A6 were interferon-gamma (IFN-gamma), interleukin-1beta (IL-1beta) and IL-2. Erythropoietin (Epo) was partly responsible for the increase in P450 content and CYP3A6 expression. 4. In conclusion, acute moderate hypoxia diminishes the activity and expression of CYP1A1, 1A2 and CYP1A1, 1A2 mRNA, and increases CYP3A6 protein, activity and CYP3A6 mRNA. Several mechanisms contribute to these changes in P450, among them the release of cytokines acting as serum mediators.