PKC activators enhance GABAergic neurotransmission and paired-pulse facilitation in hippocampal CA1 pyramidal neurons.

Bryostatin-1, a potent agonist of protein kinase C (PKC), has recently been found to enhance spatial learning and long-term memory in rats, mice, rabbits and the nudibranch Hermissenda, and to exert profound neuroprotective effects on Alzheimer's disease (AD) in transgenic mice. However, details of the mechanistic effects of bryostatin on learning and memory remain unclear. To address this issue, whole-cell recording, a dual-recording approach and extracellular recording techniques were performed on young (2-4months) Brown-Norway rats. We found that bath-applied bryostatin-1 significantly increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs). The firing rate of GABAergic interneurons significantly was also increased as recorded with a loosely-attached extracellular recording configuration. Simultaneous recordings from communicating cell pairs of interneuron and pyramidal neuron revealed unique activity-dependent properties of GABAergic synapses. Furthermore, the bryostatin-induced increase of the frequency and amplitude of IPSCs was blocked by methionine enkephalin which selectively suppressed the excitability of interneurons. Pretreatment with RO-32-0432, a relatively specific PKCα antagonist, blocked the effect of bryostatin on sIPSCs. Finally, bryostatin increased paired-pulse ratio of GABAergic synapses that lasted for at least 20min while pretreatment with RO-32-0432 significantly reduced the ratio. In addition, 8-[2-(2-pentyl-cyclopropylmethl)-cyclopropyl]-octanoic acid (DCP-LA), a selective PKCε activator, also increased the frequency and amplitude of sIPSCs. Taken together, these results suggest that bryostatin enhances GABAergic neurotransmission in pyramidal neurons by activating the PKCα & ε-dependent pathway and by a presynaptic mechanism with excitation of GABAergic interneurons. These effects of bryostatin on GABAergic transmissions and modifiability may contribute to the improvement of learning and memory previously observed to be induced by bryostatin.

PKC activator therapeutic for mild traumatic brain injury in mice.

Traumatic brain injury (TBI) is a frequent consequence of vehicle, sport and war related injuries. More than 90% of TBI patients suffer mild injury (mTBI). However, the pathologies underlying the disease are poorly understood and treatment modalities are limited. We report here that in mice, the potent PKC activator bryostatin1 protects against mTBI induced learning and memory deficits and reduction in pre-synaptic synaptophysin and post-synaptic spinophylin immunostaining. An effective treatment has to start within the first 8h after injury, and includes 5 × i.p. injections over a period of 14 days. The treatment is dose dependent. Exploring the effects of the repeated bryostatin1 treatment on the processing of the amyloid precursor protein, we found that the treatment induced an increase in the putative α-secretase ADAM10 and a reduction in ß-secretase activities. Both these effects could contribute towards a reduction in ß-amyloid production. These results suggest that bryostatin1 protects against mTBI cognitive and synaptic sequela by rescuing synapses, which is possibly mediated by an increase in ADAM10 and a decrease in BACE1 activity. Since bryostatin1 has already been extensively used in clinical trials as an anti-cancer drug, its potential as a remedy for the short- and long-term TBI sequelae is quite promising.

Neuroprotective versus tumorigenic protein kinase C activators.

Protein kinase C (PKC) activators possess potent neurotrophic and neuroprotective activity, thus indicating potential applications in treating neurodegenerative diseases, stroke and traumatic brain injury. Although some activators, such as bryostatin and gnidimacrin, have been tested as antitumor agents, others, such as phorbol esters, are potent tumor promoters. All PKC activators downregulate PKC at high concentrations and long application times. However, tumorigenic activators downregulate certain PKC isozymes, especially PKCdelta, more strongly. Tumorigenic activators possess unique structural features that could account for this difference. At concentrations that minimize PKC downregulation, PKC activators can improve long-term memory, reduce beta-amyloid levels, induce synaptogenesis, promote neuronal repair and inhibit cell proliferation. Intermittent, low concentrations of structurally specific, non-tumorigenic PKC activators, therefore, could offer therapeutic benefit for a variety of neurologic disorders.

Bryostatin enhancement of memory in Hermissenda.

Bryostatin, a potent agonist of protein kinase C (PKC), when administered to Hermissenda was found to affect acquisition of an associative learning paradigm. Low bryostatin concentrations (0.1 to 0.5 ng/ml) enhanced memory acquisition, while concentrations higher than 1.0 ng/ml down-regulated the pathway and no recall of the associative training was exhibited. The extent of enhancement depended upon the conditioning regime used and the memory stage normally fostered by that regime. The effects of two training events (TEs) with paired conditioned and unconditioned stimuli, which standardly evoked only short-term memory (STM) lasting 7 min, were--when bryostatin was added concurrently--enhanced to a long-term memory (LTM) that lasted about 20 h. The effects of both 4- and 6-paired TEs (which by themselves did not generate LTM), were also enhanced by bryostatin to induce a consolidated memory (CM) that lasted at least 5 days. The standard positive 9-TE regime typically produced a CM lasting at least 6 days. Low concentrations of bryostatin (<0.5 ng/ml) elicited no demonstrable enhancement of CM from 9-TEs. However, animals exposed to bryostatin concentrations higher than 1.0 ng/ml exhibited no behavioral learning. Sharp-electrode intracellular recordings of type-B photoreceptors in the eyes from animals conditioned in vivo with bryostatin revealed changes in input resistance and an enhanced long-lasting depolarization (LLD) in response to light. Likewise, quantitative immunocytochemical measurements using an antibody specific for the PKC-activated Ca2+/GTP-binding protein calexcitin showed enhanced antibody labeling with bryostatin. Animals exposed to the PKC inhibitor bisindolylmaleimide-XI (Ro-32-0432) administered by immersion prior to 9-TE conditioning showed no training-induced changes with or without bryostatin exposure. However, if animals received bryostatin before Ro-32, the enhanced acquisition and demonstrated recall still occurred. Therefore, pathways responsible for the enhancement effects induced by bryostatin were putatively mediated by PKC. Overall, the data indicated that PKC activation occurred and calexcitin levels were raised during the acquisition phases of associative conditioning and memory initiation, and subsequently returned to baseline levels within 24 and 48 h, respectively. Therefore, the protracted recall measured by the testing regime used was probably due to bryostatin-induced changes during the acquisition and facilitated storage of memory, and not necessarily to enhanced recall of the stored memory when tested many days after training.

Insulin and cholesterol pathways in neuronal function, memory and neurodegeneration.

Insulin and cholesterol play important roles in basic metabolic processes in peripheral tissues. Both insulin and cholesterol can also act as signalling molecules in the central nervous system that participate in neuronal function, memory and neurodegenerative diseases. A high-cholesterol diet improves spatial memory in experimental animals. beta-Amyloid, the toxic peptide in neurons of AD (Alzheimer's disease) patients, binds cholesterol and catalyses its oxidation to 7beta-hydroxycholesterol, a highly toxic oxysterol that is a potent inhibitor of alpha-PKC (alpha-protein kinase C), an enzyme critical in memory consolidation and synaptic plasticity and implicated in AD. Oxidized cholesterol also can act as a second messenger for insulin. Oxidized low-density lipoprotein inhibits insulin-dependent phosphorylation of the signalling kinases ERK (extracellular-signal-regulated kinase) and PKB/Akt. In sporadic AD patients, insulin levels are decreased, suggesting links between AD and diabetes. Insulin signalling is also important in synaptic plasticity. Insulin receptors are up-regulated and undergo translocation after spatial learning. Insulin modulates the activity of excitatory and inhibitory receptors including the glutamate and gamma-aminobutyric acid receptors and activates two biochemical pathways: the shc-ras-mitogen-activated protein kinase pathway and the PI3K (phosphoinositide 3-kinase)/PKC pathway, both of which are involved in memory processing. These findings point to a convergence at the biochemical level between pathways involved in AD and those important for normal memory.

Age-dependent differential expression of BACE splice variants in brain regions of tg2576 mice.

Plaques found in the brains of patients suffering from Alzheimer's disease (AD) mainly consist of beta-amyloid (Abeta), which is produced by sequential cleaving of amyloid precursor protein (APP) by two proteolytic enzymes, beta- and gamma-secretases. Any change in the fine balance between these enzymes and their substrate may contribute to the etio-pathogenesis of AD. Indeed, the protein level and enzymatic activity of beta-secretase (BACE), but not its mRNA level, were found elevated in brain areas of AD patients who suffer a high load of Abeta plaque formation. Similarly, increased BACE activity but no mRNA change was observed in a transgenic mouse model of AD, tg2576, in which over expression of the Swedish mutated human APP leads to Abeta plaque formation and learning deficits. Based on the recent demonstration of four BACE splice variants with different enzymatic activity, the discrepancy between BACE activity and mRNA expression may be explained by the altered BACE alternative splicing. To test this hypothesis, we studied the expression of all BACE splice variants in different brain areas of tg2576 mice at age of 4 months and 1 year old. We found developmental and regional differences between wild-type and tg2576 mice. Our results indicate that over expression of APP in tg2576 mice leads to the altered alternative splicing of BACE and the increase of its enzymatically more active splice variant (I-501).

CNS stem and progenitor cell differentiation into functional neuronal circuits in three-dimensional collagen gels.

The mammalian central nervous system (CNS) has little capacity for self-repair after injury, and neurons are not capable of proliferating. Therefore, neural tissue engineering that combines neural stem and progenitor cells and biologically derived polymer scaffolds may revolutionize the medical approach to the treatment of damaged CNS tissues. Neural stem and progenitor cells isolated from embryonic rat cortical or subcortical neuroepithelium were dispersed within type I collagen, and the cell-collagen constructs were cultured in serum-free medium containing basic fibroblast growth factor. The collagen-entrapped stem and progenitors actively expanded and efficiently generated neurons, which developed neuronal polarity, neurotransmitters, ion channels/receptors, and excitability. Ca2+ imaging showed that differentiation from BrdU+/TuJ1- to BrdU-/TuJ1+ cells was accompanied by a shift in expression of functional receptors for neurotransmitters from cholinergic and purinergic to predominantly GABAergic and glutamatergic. Spontaneous postsynaptic currents were recorded by patch-clamping from precursor cell-derived neurons and these currents were partially blocked by 10-microM bicuculline, and completely blocked by additional 10 microM of the kainate receptor antagonist CNQX, indicating an appearance of both GABAergic and glutamatergic synaptic activities. Staining with endocytotic marker FM1-43 demonstrated active synaptic vesicle recycling occurring among collagen-entrapped neurons. These results show that neural stem and progenitor cells cultured in 3D collagen gels recapitulate CNS stem cell development; this is the first demonstration of CNS stem and progenitor cell-derived functional synapse and neuronal network formation in a 3D matrix. The proliferative capacity and neuronal differentiating potential of neural progenitors in 3D collagen gels suggest their potential use in attempts to promote neuronal regeneration in vivo.

Induced depressive behavior impairs learning and memory in rats.

While it is generally accepted that cognitive processes such as learning and memory are affected by emotion, the impact of depression on learning and memory has rarely been directly studied in experimental animals. Effects of induced depressive behavior on learning and memory were determined in rats, using an open space swim test, a novel animal model of depressive behavior that is developed recently in our laboratory. The model indexes searching activity of the animals, with the induced depressive immobility behavior showing specific sensitivity to three major prototypic classes of antidepressants and a selective serotonin reuptake inhibitor. The induced depressive behavior in rats showed a delayed response to chronic antidepressant treatment and had a lasting effect on the ability of rats to learn and recall the learned experience. It impaired the subsequent ability of rats to learn and recall both a spatial water maze task and a multi-trial passive avoidance task. These impairments were all sensitive to antidepressant therapeutics, but not to buspirone, an anxiolytic. By way of contrast, the ability of the rats to sense and move to a visible platform and to escape from an unconditioned shock stimulus was neither impaired by inducing the depressive behavior nor altered by the drug treatment, suggesting that non-specific changes in sensorimotor ability were not involved. These impairments of learning and memory indicate that the depressive behavior-induced deficits show generalizability and are not context-limited. This animal model of depressive behavior shows promising potential as a screen for novel antidepressive therapeutics and as a disease model for revealing network/cellular/molecular mechanisms in the pathophysiology of depression and depression-induced cognitive deficits.

Imaging learning and memory: classical conditioning.

The search for the biological basis of learning and memory has, until recently, been constrained by the limits of technology to classic anatomic and electrophysiologic studies. With the advent of functional imaging, we have begun to delve into what, for many, was a "black box." We review several different types of imaging experiments, including steady state animal experiments that image the functional labeling of fixed tissues, and dynamic human studies based on functional imaging of the intact brain during learning. The data suggest that learning and memory involve a surprising conservation of mechanisms and the integrated networking of a number of structures and processes.

Posttranscriptional regulation of gene expression in learning by the neuronal ELAV-like mRNA-stabilizing proteins.

The view that memory is encoded by variations in the strength of synapses implies that long-term biochemical changes take place within subcellular microdomains of neurons. These changes are thought ultimately to be an effect of transcriptional regulation of specific genes. Localized changes, however, cannot be fully explained by a purely transcriptional control of gene expression. The neuron-specific ELAV-like HuB, HuC, and HuD RNA-binding proteins act posttranscriptionally by binding to adenine- and uridine-rich elements (AREs) in the 3' untranslated region of a set of target mRNAs, and by increasing mRNA cytoplasmic stability and/or rate of translation. Here we show that neuronal ELAV-like genes undergo a sustained up-regulation in hippocampal pyramidal cells only of mice and rats that have learned a spatial discrimination paradigm. This learning-specific increase of ELAV-like proteins was localized within cytoplasmic compartments of the somata and proximal dendrites and was associated with the cytoskeleton. This increase was also accompanied by enhanced expression of the GAP-43 gene, known to be regulated mainly posttranscriptionally and whose mRNA is demonstrated here to be an in vivo ELAV-like target. Antisense-mediated knockdown of HuC impaired spatial learning performance in mice and induced a concomitant down-regulation of GAP-43 expression. Neuronal ELAV-like proteins could exert learning-induced posttranscriptional control of an array of target genes uniquely suited to subserve substrates of memory storage.

Intracellular correlates of spatial memory acquisition in hippocampal slices: long-term disinhibition of CA1 pyramidal cells.

Despite many advances in our understanding of synaptic models of memory such as long-term potentiation and depression, cellular mechanisms that correlate with and may underlie behavioral learning and memory have not yet been conclusively determined. We used multiple intracellular recordings to study learning-specific modifications of intrinsic membrane and synaptic responses of the CA1 pyramidal cells (PCs) in slices of the rat dorsal hippocampus prepared at different stages of the Morris water maze (WM) task acquisition. Schaffer collateral stimulation evoked complex postsynaptic potentials (PSP) consisting of the excitatory and inhibitory postsynaptic potentials (EPSP and IPSP, respectively). After rats had learned the WM task, our major learning-specific findings included reduction of the mean peak amplitude of the IPSPs, delays in the mean peak latencies of the EPSPs and IPSPs, and correlation of the depolarizing-shifted IPSP reversal potentials and reduced IPSP-evoked membrane conductance. In addition, detailed isochronal analyses revealed that amplitudes of both early and late IPSP phases were reduced in a subset of the CA1 PCs after WM training was completed. These reduced IPSPs were significantly correlated with decreased IPSP conductance and with depolarizing-shifted IPSP reversal potentials. Input-output relations and initial rising slopes of the EPSP phase did not indicate learning-related facilitation as compared with the swim and naïve controls. Another subset of WM-trained CA1 PCs had enhanced amplitudes of action potentials but no learning-specific synaptic changes. There were no WM training-specific modifications of other intrinsic membrane properties. These data suggest that long-term disinhibition in a subset of CA1 PCs may facilitate cell discharges that represent and record the spatial location of a hidden platform in a Morris WM.

Gene expression profiles during long-term memory consolidation.

Changes in gene expression have been postulated to occur during long-term memory (LTM). We used high-density cDNA microarrays to assess changes in gene expression 24 h after rabbit eye blink conditioning. Paired animals were presented with a 400 ms, 1000 Hz, 82 dB tone conditioned stimulus that coterminated with a 100 ms, 60 Hz, 2 mA electrical pulse unconditioned stimulus. Unpaired animals received the same conditioned and unconditioned stimuli but presented in an explicitly unpaired manner. Differences in expression levels between paired and unpaired animals in the hippocampus and cerebellar lobule HVI, two regions activated during eye blink conditioning, indicated the involvement of novel genes as well as the participation of previously implicated genes. Patterns of gene expression were validated by in situ hybridization. Surprisingly, the data suggest that an underlying mechanism of LTM involves widespread decreased, rather than increased, gene expression. These results demonstrate the feasibility and utility of a cDNA microarray system as a tool for dissecting the molecular mechanisms of associative memory.

Pharmacological enhancement of synaptic efficacy, spatial learning, and memory through carbonic anhydrase activation in rats.

CA1 pyramidal cells were recorded in rat hippocampal slices. In the presence of carbonic anhydrase activators, comicrostimulation of cholinergic inputs from stratum oriens and gamma-aminobutyric acid (GABA)ergic inputs from stratum pyramidale at low intensities switched the hyperpolarizing GABA-mediated inhibitory postsynaptic potentials to depolarizing responses. In the absence of the activators, however, the same stimuli were insufficient to trigger the synaptic switch. This synaptic switch changed the function of the GABAergic synapses from excitation filter to amplifier and was prevented by carbonic anhydrase inhibitors, indicating a dependence on HCO. Intralateral ventricular administration of these same carbonic anhydrase activators caused the rats to exhibit superior learning of the Morris water maze task, suggesting that the GABAergic synaptic switch is critical for gating the synaptic plasticity that underlies spatial memory formation. Increased carbonic anhydrase activity might, therefore, also enhance perception, processing, and storing of temporally associated relevant signals and represents an important therapeutic target in learning and memory pharmacology.

Role of insulin and insulin receptor in learning and memory.

As one of the most extensively studied protein hormones, insulin and its receptor have been known to play key roles in a variety of important biological functions. Until recent years, the functions of insulin and insulin receptor (IR) in the central nervous system (CNS) have largely remained unclear. IR is abundantly expressed in several specific brain regions that govern fundamental behaviors such as food intake, reproduction and high cognition. The IR from the periphery and CNS exhibit differences in both structure and function. In addition to that from the peripheral system, locally synthesized insulin in the brain has also been identified. Accumulated evidence has demonstrated that insulin/IR plays important roles in associative learning, as suggested by results from both interventive and correlative studies. Interruption of insulin production and IR activity causes deficits in learning and memory formation. Abnormal insulin/IR levels and activities are seen in Alzheimer's dementia, whereas administration of insulin significantly improves the cognitive performance of these patients. The synaptic bases for the action of insulin/IR include modifying neurotransmitter release processes at various types of presynaptic terminals and modulating the activities of both excitatory and inhibitory postsynaptic receptors such as NMDA and GABA receptors, respectively. At the molecular level, insulin/IR participates in regulation of learning and memory via activation of specific signaling pathways, one of which is shown to be associated with the formation of long-term memory and is composed of intracellular molecules including the shc, Grb-r/SOS, Ras/Raf, and MEK/MAP kinases. Cross-talk with another IR pathway involving IRS1, PI3 kinase, and protein kinase C, as well as with the non-receptor tyrosine kinase pp60c-src, may also be associated with memory processing.

Mobilization of calcium from intracellular stores, potentiation of neurotransmitter-induced calcium transients, and capacitative calcium entry by 4-aminopyridine.

In this study we analyzed the effect of 4-aminopyridine (4-AP) on free cytosolic calcium concentration ([Ca(2+)](i)) in basal conditions, after stimulation with neurotransmitters, and during capacitative calcium entry. Using fura-2 ratiometric calcium imaging, we found that 4-AP increased [Ca(2+)](i) in type I astrocytes, neurons, and in skeletal muscle cells. The [Ca(2+)](i) elevation induced by 4-AP was concentration-dependent and consisted of two phases: the first was dependent on intracellular calcium mobilization, and the second was dependent on extracellular calcium influx. 4-AP also increased the second messenger inositol trisphosphate in both neurons and astrocytes. In astrocytes, 4-AP treatment potentiated the sustained phase of the [Ca(2+)](i) elevation induced by ATP and bradykinin. In addition, capacitative calcium entry was potentiated severalfold by 4-AP, in astrocytes and muscle cells but not in neurons. These effects of 4-AP were completely and promptly reversible. 4-AP blocked voltage-sensitive K(+) currents in astrocytes. However, voltage-sensitive K(+) channel blockers inhibiting these currents did not affect agonist-induced calcium transients or capacitative calcium entry, indicating that 4-AP effects on [Ca(2+)](i) were not caused by the blockade of voltage-gated K(+) channels. We conclude that 4-AP is able to affect calcium homeostasis at multiple levels, from increasing basal [Ca(2+)](i) to potentiating capacitative calcium entry. The potentiation of capacitative calcium entry in astrocytes or muscle cells may explain some of the therapeutic activities of 4-AP as a neurotransmission enhancer.

"Either-or" two-slit interference: stable coherent propagation of individual photons through separate slits.

In quantum theory, nothing that is observable, be it physical, chemical, or biological, is separable from the observer. Furthermore, ". all possible knowledge concerning that object is given by its wave function" (Wigner, E. 1967. Symmetries and Reflections. Indiana University Press, Bloomington, IN), which can only describe probabilities of future events. In physical systems, quantum mechanical probabilistic events that are microscopic must, in turn, account for macroscopic events that are associated with a greater degree of certainty. In biological systems, probabilistic statistical mechanical events, such as secretion of microscopic synaptic vesicles, must account for macroscopic postsynaptic potentials; probabilistic single-channel events sum to produce a macroscopic ionic current across a cell membrane; and bleaching of rhodopsin molecules (responsible for quantal potential "bumps") produces a photoreceptor generator potential. Among physical systems, a paradigmatic example of how quantum theory applies to the observation of events concerns the interactions of particles (e.g., photons, electrons) with the two-slit apparatus to generate an interference pattern from a single common light source. For two-slit systems that use two independent laser sources with brief (<1 ms) intervals of mutual coherence (Paul, H. 1986. Rev. Modern Phys. 58:209-231), each photon has been considered to arise from both beams and has a probability amplitude to pass through each of the two slits. Here, a single laser source two-slit interference system was constructed so that each photon has a probability amplitude to pass through only one or the other, but not both slits. Furthermore, all photons passing through one slit could be distinguished from all photons passing through the other slit before their passage. This "either-or" system produced a stable interference pattern indistinguishable from the interference produced when both slits were accessible to each photon. Because this system excludes the interaction of one photon with both slits, phase correlation of photon movements derives from the "entanglement" of all photon wave functions due to their dependence on a common laser source. Because a laser source (as well as Young's original point source) will have stable time-averaged spatial coherence even at low intensities, the "either-or" two-slit interference can result from distinct individual photons passing one at a time through one or the other slit-rather than wave-like behavior of individual photons. In this manner, single, successive photons passing through separate slits will assemble over time in phase-correlated wave distributions that converge in regions of low and high probability.

Heterosynaptic transformation of GABAergic gating in the hippocampus and effects of carbonic anhydrase inhibition.

Recordings from CA1 pyramidal cells were made in rat hippocampal slices (in vitro). Activation of cholinergic receptors associated with tetanization of GABAergic inputs from stratum pyramidale transformed the hyperpolarizing GABA-mediated inhibitory postsynaptic potentials into depolarizing responses of rat hippocampal CA1 pyramidal neurons. The synaptic transformation was characterized by a significant shift of reversal potential of postsynaptic responses toward positive membrane potentials. This effect lasted more than 1 h and changed the function of the GABAergic synapses from excitation filter to amplifier. This long-term synaptic transformation was prevented by carbonic anhydrase inhibitors or the presence of HEPES buffer, indicating a dependence on HCO(3-). The presence or absence of an associated activation of cholinergic with GABAergic inputs thus gates the information processing through the pyramidal cells and network, forming an amplified "center" of attention and a filtered "surround". Information flow through the neural circuit is thereby directed according to temporal association of the relevant signals.

Theta rhythm of hippocampal CA1 neuron activity: gating by GABAergic synaptic depolarization.

Information processing and memory consolidation during exploratory behavior require synchronized activity known as hippocampal theta (theta) rhythm. While it is well established that the theta activity depends on cholinergic inputs from the medial septum/vertical limb of the diagonal band nucleus (MS/DBv) and theta discharges of GABAergic interneurons, and can be induced with cholinergic receptor agonists, it is not clear how the increased excitation of pyramidal cells could occur with increased discharges of GABAergic interneurons during theta waves. Here, we show that the characteristic theta activity in adult rat hippocampal CA1 pyramidal cells is associated with GABAergic postsynaptic depolarization and a shift of the reversal potential from Cl(-) toward HCO(3)(-) (whose ionic gradient is regulated by carbonic anhydrase). The theta activity was abolished by GABA(A) receptor antagonists and carbonic anhydrase inhibitors, but largely unaffected by blocking glutamate receptors. Carbonic anhydrase inhibition also impaired spatial learning in a water maze without affecting other sensory/locomotor behaviors. Thus HCO(3)(-)-mediated signaling, as regulated by carbonic anhydrase, through reversed polarity of GABAergic postsynaptic responses is implicated in both theta and memory consolidation in rat spatial maze learning. We suggest that this mechanism may be important for the phase forward shift of the place cell discharges for each theta cycle during the animal's traversal of the place field for that cell.