Depolarization-induced effects of Ca2+-calmodulin-dependent protein kinase injection, in vivo, in single neurons of cat motor cortex

Enhanced cAMP response element-binding protein activity increases neuronal excitability, hippocampal long-term potentiation, and classical eyeblink conditioning in alert behaving mice

The activity-regulated transcription factor cAMP response element-binding protein (CREB) is an essential component of the molecular switch that controls the conversion of short-term into long-term forms of plasticity, including those underlying long-term memory. Previous research in acute brain slices of transgenic animals expressing constitutively active CREB variants has revealed that enhancing CREB activity increases the intrinsic excitability of neurons and facilitates the late phase of long-term potentiation (LTP) in the Schaffer collateral pathway. Here, we report similar changes in plasticity at the Schaffer collateral pathway in alert behaving mice. Forebrain expression of a strong constitutively active CREB variant, VP16-CREB, enhanced in vivo LTP evoked in the Schaffer collateral pathway and caused significant changes in the input/output curve and paired-pulse facilitation in CA3-CA1 synapses, which could be explained by the increased excitability of hippocampal pyramidal neurons. In addition, classical eyeblink conditioning in transgenic mice and control littermates showed larger conditioned responses in mutant mice that were associated to a transient increase in the acquisition rate and in the concomitant learning-dependent change in synaptic strength. The sustained chronic activation of CREB activity, however, impaired the performance in this task. Our experiments demonstrate that the sustained enhancement of CREB function alters the physiology and plasticity of hippocampal circuits in behaving animals and that these changes have important consequences in associative learning.

In search of memory traces

The key issue in analyzing brain substrates of memory is the nature of memory traces, how memories are formed, stored, and retrieved in the brain. In order to analyze mechanisms of memory formation it is first necessary to find the loci of memory storage, the classic problem of localization. Various approaches to this issue are reviewed. A particular strategy is proposed that involves a number of different techniques (electrophysiological recording, lesions, electrical stimulation, pathway tracing) to identify the essential memory trace circuit for a given form of learning and memory. The methods of reversible inactivation can be used to localize the memory traces within this circuit. Using classical conditioning of eye blink and other discrete responses as a model system, the essential memory trace circuit is identified, the basic memory trace is localized (to the cerebellum), and putative higher-order memory traces are characterized in the hippocampus.

Neural Substrates of Eyeblink Conditioning: Acquisition and Retention

Classical conditioning of the eyeblink reflex to a neutral stimulus that predicts an aversive stimulus is a basic form of associative learning. Acquisition and retention of this learned response require the cerebellum and associated sensory and motor pathways and engage several other brain regions including the hippocampus, neocortex, neostriatum, septum, and amygdala. The cerebellum and its associated circuitry form the essential neural system for delay eyeblink conditioning. Trace eyeblink conditioning, a learning paradigm in which the conditioned and unconditioned stimuli are noncontiguous, requires both the cerebellum and the hippocampus and exhibits striking parallels to declarative memory formation in humans. Identification of the neural structures critical to the development and maintenance of the conditioned eyeblink response is an essential precursor to the investigation of the mechanisms responsible for the formation of these associative memories. In this review, we describe the evidence used to identify the neural substrates of classical eyeblink conditioning and potential mechanisms of memory formation in critical regions of the hippocampus and cerebellum. Addressing a central goal of behavioral neuroscience, exploitation of this simple yet robust model of learning and memory has yielded one of the most comprehensive descriptions to date of the physical basis of a learned behavior in mammals.

Synaptic plasticity: stairway to memory

Since the idea that memory is associated with alterations in synaptic strength was accepted, studies on the cellular and molecular mechanisms responsible for the plastic changes in neurons have attracted wide interest in the scientific community. Recent studies on memory processes have also pointed out some unifying themes emerging from a wide range of nervous systems, suggesting that regardless of the species or brain regions, a common denominator for memory may exist. Thus, the present review attempted to create a hypothetical and universal synaptic model valid for a variety of nervous systems, ranging from molluscs to mammals. The cellular and molecular events leading to short- and long-term modifications of memory have been described in a sequential order, from the triggering signals to the gene expression, synthesis of new proteins and neuronal growth. These events are thought to represent the late phases of memory consolidation leading to persistent modifications in synaptic plasticity, thereby facilitating the permanent storage of acquired information throughout the individual's life.

Calmodulin blockers decrease short-term plasticity of the cholinoreceptors of neurons of the edible snail

A reversible decrease in the rate and depth of the extinction of the reactions of the cholinoreceptive membrane to repeated iontophoretic applications of acetylcholine to the soma by a number of calmodulin blockers was demonstrated in identified RPa3 and LPa3 neurons in the edible snail using the method of recording transmembrane ionic currents: R 24571 (20-50 mumols/liter), trifluoperazine (50-200 mumols/liter), chlorpromazine (20-60 mumols/liter), and prenylamine lactate (30-400 mumols/liter). The results obtained attest to the positive regulation of short-term plasticity of the cholinoreceptors of the neurons in question by calmodulin.

Molecular mechanisms of neuronal plasticity during learning: the role of secondary messengers

We present published data along with our own results concerning the role of second messengers and their intracellular receptors in molecular mechanisms associated with the plasticity of neurons during learning. The participation of cyclic 3',5'-adenosine monophosphate, cyclic 3',5'-guanosine monophosphate, calcium, calmodulin, and also the metabolic products of inositol phospholipids, inositol-1,4,5-triphosphate, diacylglycerol and the protein kinase C activated by it, arachidonic acid, and the products of its lipoxygenase oxidation during the regulation of neuronal plasticity over the course of prolonged potentiation, sensitization, habituation, and classical associative training are discussed.

Polymyxin B, an inhibitor of protein kinase C, prevents the maintenance of synaptic long-term potentiation in hippocampal CA1 neurons

The involvement of protein kinase C (PKC)-mediated processes in mechanisms of long-term potentiation (LTP) was suggested by recent studies which have demonstrated a correlation between PKC activation and LTP. However, it was not possible to tell whether there is a causal relationship between the two events. Therefore, we have examined the induction and maintenance of LTP in rat hippocampal slices in the presence of a relatively selective PKC inhibitor, using extracellular electrophysiological techniques. Bath application of 0.1-100 microM polymyxin B did not influence the occurrence of post-tetanic and long-term potentiation usually seen in test responses 1 and 10 min after a 100-Hz/1 s tetanic stimulation of stratum radiatum fibers. However, 20 microM polymyxin B significantly depressed the increase in population spike amplitude and population excitatory postsynaptic potential (EPSP) slope from 30 to 120 min onwards, following repeated tetanization. Immediately after the drug application only weak and reversible effects were seen by the same parameters in test responses of a non-tetanized control input. A late (greater than 6 h) heterosynaptic potentiation of the population spike in the control input was blocked by polymyxin B treatment. Whereas the EPSP-LTP was fully blocked, some potentiation of the population spike still remained, suggesting the independence of PKC of the additional spike (E/S) potentiation for the first 6 h. These results provide direct evidence that the PKC activation is not essential for the initial phase of LTP, but is a necessary condition for a medium and a late, protein synthesis-dependent phase in this monosynaptic pathway, i.e. for the maintenance of synaptic LTP.

Immunohistochemical localization of calcium-binding proteins, parvalbumin and calbindin-D 28k, in the adult and developing visual cortex of cats: a light and electron microscopic study

In the cat primary visual cortex, we investigated with immunohistochemical techniques the developmental changes in the cellular and subcellular localization of the Ca2+-binding proteins parvalbumin (PV) and calbindin-D 28K (CBP), in order to determine whether there is a correlation between the expression of Ca2+-dependent processes and the time course of the critical period for use-dependent plasticity. On the 54th day of gestation and at 1 week postnatally, both calcium-binding proteins were present only in a subpopulation of neurons in layers V and VI. During subsequent maturation, the number of PV(+) and CBP(+) neurons increased significantly and labeled cells were detected in more superficial layers. Moreover, the homogeneous labeling of some CBP(+) neurons in layers IV to VI decreased and changed to a punctate pattern. In adult cats PV(+) neurons were evenly distributed throughout layers II to VI, whereas CBP(+) neurons were concentrated in layers II/III. Only a few immunoreactive cells had morphological features characteristic of pyramidal cells; the large majority were nonpyramidal. Electron microscopy confirmed the presence of PV- and CBP-reaction product within the perikarya, axons, and dendrites of labeled cells. It was associated preferentially with microtubules, postsynaptic densities, and intracellular membranes. Immunoreactive neurons received immunonegative asymmetric synapses on their dendritic shafts and made symmetric synaptic contacts with labeled and unlabeled somata and with unlabeled dendritic shafts. The large number and widespread distribution of immunoreactive neurons implies that PV and CBP play an important role in the regulation of calcium-dependent processes in the visual cortex. Furthermore, the developmental redistribution of PV and CBP points to changes in the organization of Ca2+-dependent processes during maturation.

Independent protein kinases associated with the rat cerebral synaptic junction: comparison with cyclic AMP-dependent and Ca2+/calmodulin-dependent protein kinases in the synaptic junction

Independent protein kinases in the synaptic junction (SJ) isolated from rat cerebrum were characterized. SJ showed a protein kinase activity, phosphorylating intrinsic proteins, even in the absence of cyclic AMP or Ca2+ plus calmodulin (CaM) exogenously added. The activity was affected neither by Ca2+ concentrations in the physiological fluctuation range nor by the addition of specific ligands such as glutamate, aspartate, acetylcholine, and concanavalin A. The activity was not due to cyclic AMP-dependent protein kinase in SJ, since the activity was not inhibited by an inhibitor protein for cyclic AMP-dependent protein kinase, and since synapsin I was not specifically phosphorylated whereas cyclic AMP-dependent kinase appeared to phosphorylate selectively the protein in SJ. Phosphorylation of SJ proteins by the independent kinases was about one-third of that of the Ca2+/CaM-dependent protein kinase intrinsic to SJ. The apparent Km for ATP was estimated to be 700 microM. Proteins of 16K Mr and 117K Mr were specifically phosphorylated under the basic condition (in the absence of the substances known to activate specifically protein kinases), as well as six other proteins both under the basic conditions and in the presence of Ca2+ and CaM. The phosphorylation of 150K Mr, 60K Mr, 51K Mr, and 16K Mr SJ proteins was enhanced after prephosphorylation of SJ proteins by intrinsic kinase in the presence of Ca2+ and CaM.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular injection of cGMP-dependent protein kinase results in increased input resistance in neurons of the mammalian motor cortex

Purified, cyclic GMP-dependent protein kinase (cGPK) was pressure-injected into neurons of the precruciate cortex of awake cats. Input resistances increased within seconds after injection and remained elevated for 2 min or longer. The increases were larger when cGPK was injected in a mixture with 10 microM cGMP than when injected alone. Injections of heat-inactivated cGPK, with or without 10 microM cGMP, failed to produce increases in input resistance. The present results indicate that injection of activated cGPK into neurons of the mammalian motor cortex can mimic actions of extracellularly applied acetylcholine and intracellularly applied cGMP, the latter in 100-fold higher concentrations than those used here, in neurons of the same cortical areas.

Long-term increases in excitability of facial motoneurons and other neurons in and near the facial nuclei after presentations of stimuli leading to acquisition of a Pavlovian conditioned facial movement

Levels of neuronal excitability to injected current were measured intracellularly in facial motoneurons and other neurons in and near the facial nuclei of three groups of awake cats: a "Conditioned" group consisting of animals that had previously received sufficient numbers of paired presentations of click CSs and glabella tap USs to produce eyeblink CRs; a "US-only" group that had received presentations of the USs only; and a "Naive" group that had received neither of these stimuli. Thresholds of intracellularly applied, depolarizing pulse currents required to elicit repeatable spike activity were significantly lower in the "Conditioned" and "US-only" groups than in the "Naive" group. The increased levels of neuronal excitability were correlated with increases in neuronal input resistance. Levels of neuronal excitability remained elevated when measured more than a month after presentations of both CSs and USs, whereas the increases in neuronal excitability decayed within a few weeks in animals given USs only. The increases in neuronal excitability and input resistance following repetitive presentations of glabella tap USs alone appeared to support a latent facilitation of motor performance reflected by an absence of a blink CR to click CS after such presentations but an increased rate of acquisition of subsequent eyeblink conditioning using paired click CS and tap US. The rate of eyeblink conditioning was found to be accelerated in a group of cats given repetitive presentations of tap USs seven days prior to conditioning with paired CSs and USs, compared to a group that was not given USs or CSs before similar conditioning. These findings provide direct, in vivo evidence that increases in the excitability and input resistance of neurons in and near the facial nucleus can occur in cats following presentations of the stimuli used for conditioning.

The role of neurochemical modulation in learning

Tsukahara creatively exploited the advantages of a "simple system" approach in a vertebrate context to gain cellular insights into the learning process. The molluscs Aplysia and Hermissenda have provided useful invertebrate examples of this approach. For classical conditioning of Hermissenda a temporal sequence of cellular transformations has been found to correspond to and to substantially account for a learning-specific behavioral transformation. For at least days after the conditioning a biophysical record persists: two voltage-dependent K+ currents, IA and ICa2+-K+, remain reduced in amplitude and at least IA shows an increased rate of inactivation. More recently, a similar biophysical record of associative memory has been identified in the mammalian brain (Disterhoft et al., 1986). Other experiments suggest that a synergistic interaction of C-kinase activation with Ca2+/CaM-kinase activation enhances and prolongs Ca2+-mediated K+ current reduction. The effects of alpha-receptor agonists to enhance depolarization of type B cells (a site of visual-vestibular convergence) and in turn acquisition of classical conditioning are in contrast to the effects of serotonin which can hyperpolarize and thereby reduce depolarization during the acquisition process. For both LTP and LTD, application of a neurotransmitter itself is not sufficient to produce long-lasting neural modification. In this respect, both the LTP and LTD models are more similar to the biochemical sequence implicated in Hermissenda conditioning than to the mechanism initiated by serotonin-like substances proposed for Aplysia sensitization.

Modulation of calcium-mediated inactivation of ionic currents by Ca2+/calmodulin-dependent protein kinase II

Iontophoretic injection of Ca2+ causes reduction of I0A (an early rapidly activating and inactivating K+ current) and I0C (a late Ca2+-dependent K+ current) measured across the isolated type B soma membrane (Alkon et al., 1984, 1985; Alkon and Sakakibara, 1984, 1985). Similarly, voltage-clamp conditions which cause elevation of [Ca2+]i are followed by reduction of I0A and I0C lasting 1-3 min. Iontophoretic injection of highly purified Ca2+/CaM-dependent protein kinase II (CaM kinase II) isolated from brain tissue (Goldenring et al., 1983) enhanced and prolonged this Ca2+-mediated reduction of I0A and I0C. ICa2+, a voltage-dependent Ca2+ current, also showed some persistent reduction under these conditions. Iontophoretic injection of heat-inactivated enzyme had no effect. Agents that inhibit or block Ca2+/CaM-dependent phosphorylation produced increased I0A and I0C amplitudes and prevented the effects of CaM kinase II injection. The results reported here and in other studies implicate Ca2+-stimulated phosphorylation in the regulation of type B soma ionic currents.