Inositol trisphosphate regulation of photoreceptor membrane currents

In vitro extinction learning in Hermissenda: involvement of conditioned inhibition molecules

Extinction of a conditioned association is typically viewed as the establishment of new learning rather than the erasure of the original memory. However, recent research in the nudibranch, Hermissenda crassicornis (H.c.) demonstrated that extinction training (using repeated light-alone presentations) given 15 min, but not 23 h, after memory acquisition reversed both the cellular correlates of learning (enhanced Type B cell excitability) and the behavioral changes (reduced phototaxis) produced by associative conditioning (pairings of light, CS, and rotation, US). Here, we investigated the putative molecular signaling pathways that underlie this extinction in H.c. by using a novel in vitro protocol combined with pharmacological manipulations. After intact H.c. received either light-rotation pairings (Paired), random presentations of light and rotation (Random), or no stimulation (Untrained), B cells from isolated CNSs were recorded from during exposure to extinction training consisting of two series of 15 consecutive light-steps (LSs). When in vitro extinction was administered shortly (2 h, but not 24 h) after paired training, B cells from Paired animals showed progressive and robust declines in spike frequency by the 30th LS, while control cells (Random and Untrained) did not. We found that several molecules implicated in H.c. conditioned inhibitory (CI) learning, protein phosphatase 1 (PP1) and arachidonic acid (AA)/12-lipoxygenase (12-LOX) metabolites, also contributed to the spike frequency decreases produced by in vitro extinction. Protein phosphatase 2B (PP2B) also appeared to play a role. Calyculin A (PP1 inhibitor), cyclosporin A (PP2B inhibitor), and baicalein (a 12-LOX inhibitor) all blocked the spike frequency declines in Paired B cells produced by 30 LSs. Conversely, injection of catalytically-active PP1 (caPP1) or PP2B (caPP2B) into Untrained B cells partially mimicked the spike frequency declines observed in Paired cells, as did bath-applied AA, and occluded additional LS-produced reductions in spiking in Paired cells.

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.

Pavlovian conditioning of Hermissenda: current cellular, molecular, and circuit perspectives

The less-complex central nervous system of many invertebrates make them attractive for not only the molecular analysis of the associative learning and memory, but also in determining how neural circuits are modified by learning to generate changes in behavior. The nudibranch mollusk Hermissenda crassicornis is a preparation that has contributed to an understanding of cellular and molecular mechanisms of Pavlovian conditioning. Identified neurons in the conditioned stimulus (CS) pathway have been studied in detail using biophysical, biochemical, and molecular techniques. These studies have resulted in the identification and characterization of specific membrane conductances contributing to enhanced excitability and synaptic facilitation in the CS pathway of conditioned animals. Second-messenger systems activated by the CS and US have been examined, and proteins that are regulated by one-trial and multi-trial Pavlovian conditioning have been identified in the CS pathway. The recent progress that has been made in the identification of the neural circuitry supporting the unconditioned response (UR) and conditioned response (CR) now provides for the opportunity to understand how Pavlovian conditioning is expressed in behavior.

Involvement of the Ryanodine Receptor in Morphologic Modification ofHermissendaType B Photoreceptors After In Vitro Conditioning

We examined whether Ca2+induced Ca2+release through ryanodine receptors is involved in the conditioning of specific morphologic changes at the axon terminals of type B photoreceptors in the isolated circumesophageal ganglion of Hermissenda. Calcium chelation by bis(2-aminophenoxy) ethane- N,N,N′, N′-tetraacetic acid prevented the conformational change at the terminals after five paired presentations of light and vibration, which produce terminal branch contraction of B photoreceptors. Two ryanodine receptor blockers, dantrolene and micromolar concentrations of ryanodine, depressed the increase in excitability due to in vitro conditioning and the increase in intracellular Ca2+in response to membrane depolarization. Although the ability to increase intracellular Ca2+was depressed, synaptic transmission was preserved in the normal state from hair cells under dantrolene and ryanodine incubation. Ryanodine receptor blockers also prevented contraction at the B photoreceptor axon terminals. These results suggest that the ryanodine receptor has a crucial role in inducing the in vitro conditioning specific changes both physiologically and morphologically, including “focusing” at the B photoreceptor axon terminal.

Time domains of neuronal Ca2+ signaling and associative memory: steps through a calexcitin, ryanodine receptor, K+ channel cascade

Synaptic changes that underlie associative learning and memory begin with temporally related activity of two or more independent synaptic inputs to common postsynaptic targets. In turn, temporally related molecular events regulate cytosolic Ca2+ during progressively longer-lasting time domains. Associative learning behaviors of living animals have been correlated with changes of neuronal voltage-dependent K+ currents, protein kinase C-mediated phosphorylation and synthesis of the Ca2+ and GTP-binding protein, calexcitin (CE),and increased expression of the Ca2+-releasing ryanodine receptor (type II). These molecular events, some of which have been found to be dysfunctional in Alzheimer's disease, provide means of altering dendritic excitability and thus synaptic efficacy during induction, consolidation and storage of associative memory. Apparently, such stages of behavioral learning correspond to sequential differences of Ca2+ signaling that could occur in spatially segregated dendritic compartments distributed across brain structures, such as the hippocampus.

Phospholipases and arachidonic acid contribute independently to sensory transduction and associative neuronal facilitation in Hermissenda type B photoreceptors

During contiguous pairings of light and rotation, B photoreceptors in the Hermissenda eye undergo an increase in excitability that contributes to a modification of several light-elicited behaviors. This excitability increase requires a light-induced rise in intracellular Ca2+ in the photoreceptor concomitant with transmitter binding to G protein-coupled receptors as a result of presynaptic vestibular hair cell stimulation. Phospholipases and arachidonic acid (ArA) are here reported to be involved in independent signal transduction pathways that underlie both receptor function and activity-dependent facilitation of the B photoreceptor. 4-Bromophenacyl bromide (BPB), an inhibitor of phospholipases A2 (PLA2) and C (PLC), blocked the generation of light-induced depolarizing generator potentials, but had no affect on the inhibitory postsynaptic potential (IPSP) in the B cell that results from hair cell stimulation. Quinacrine, which predominantly blocks the activity of PLA2 in neurons, had no affect on either the light response or the IPSP, but did block increases in excitability (i.e. increased input resistance and elicited spike rate) of the B cell that results from pairings of light and presynaptic vestibular stimulation (i.e., in vitro associative conditioning). Neither nordihydroquararetic acid (NDGA), which inhibits metabolism of ArA by cyclooxygenase, nor indomethacin, which inhibits lipoxygenase metabolism of ArA, affected the light response or IPSP, but both blocked the increases in excitability in the B cell that accompanied in vitro conditioning. In combination with earlier results, these data suggest that ArA activates PKC in a synergistic fashion with Ca2+ and diacylglycerol in the B cell, and suggest that PLA2-induced ArA release, though not necessary for transduction of light or the hair cell-induced IPSP in the B cell, is a critical component of the convergence of signals that precipitates associative facilitation in this system.

Soluble beta-amyloid induces Alzheimer's disease features in human fibroblasts and in neuronal tissues

It has been shown that K+ channels, Cp20 (a 20kD GTP-binding protein), and intracellular calcium release, play a key role in associative memory storage. These same elements have been shown to be altered in fibroblasts from Alzheimer's Disease (AD) patients. In addition, it has been shown that PKC, also implicated in memory storage and closely related to the above mentioned components, is also altered in AD fibroblasts. Moreover, beta-amyloid was capable of inducing an AD-like phenotype for K+ channels and Cp20 in otherwise normal fibroblasts, providing additional evidence for the potential involvement of these components in AD and suggesting a possible pathological consequence of soluble beta-amyloid elevation in AD. Preliminary evidence shows that comparable changes in potassium channel function are also present in human olfactory neuroblasts from AD patients. These results indicate that the observed changes not only occur in peripheral tissues such as fibroblasts, but also in neural tissue, the primary site of AD pathology.

Plasticity of cholinoreceptors of neurons of the common snail after effects on inositol-1,4,5-triphosphate- and CA(2+)-dependant mobilization of stored CA2+ and the level of phosphatidic acid

The influences on the depth of extinction of the inward current induced by acetylcholine (the ACh-current) of a number of compounds affecting the mobilization of stored Ca2+ and the intracellular level of Ca(2+)-mobilizing second messengers, namely, inositol-1,4,5-trisphosphate (IP3), inositol hexakisphosphate, TMB-8 (an inhibitor of (IP3)-dependent Ca2+ mobilization), tetracaine (an inhibitor of Ca(2+)-dependant mobilization of Ca2+), as well as phospholipase D, which leads to the formation of phosphatidic acid through the hydrolysis of phosphatidylcholine, were investigated in identified RPa3 and LPa3 neurons of the common snail using the two-electrode voltage clamp technique for the recording of the potential on the membrane. The participation of IP3, of IP3-dependant, and Ca(2+)-dependant mobilized intracellular Ca2+, as well as phosphatidic acid in the regulation of the plasticity of the cholinoreceptors of the neurons was demonstrated.

Reconstruction of ionic currents in a molluscan photoreceptor

Two-microelectrode voltage-clamp measurements were made to determine the kinetics and voltage dependence of ionic currents across the soma membrane of the Hermissenda type B photoreceptor. The voltage-dependent outward potassium currents, IA and ICa(2+)-K+, the inward voltage-dependent calcium current, ICa2+ and the light-induced current, IIgt, were then described with Hodgkin-Huxley-type equations. The fast-activating and inactivating potassium current, IA, was described by the equation; IA(t) = gA(max)(ma infinity[1-exp(-t/tau ma)])3 x (ha infinity [1-exp(-t/tau ha)] + exp(-t/tau ha)) (Vm-EK), where the parameters ma infinity, ha infinity, tau ma, and tau ha are functions of membrane potential, Vm, and ma infinity and ha infinity are steady-state activation and inactivation parameters. Similarly, the calcium-dependent outward potassium current, ICa(2+)-K+, was described by the equation, ICa(2+)-K+ (t) = gc(max)(mc infinity(VC)(1-exp[-t/tau mc (VC)]))pc (hc infinity(VC) [1-exp(-t/tau hc)] + exp(-t/tau hc(VC)])pc(VC-EK). In high external potassium, ICa(2+)-K+ could be measured in approximate isolation from other currents as a voltage-dependent inward tail current following a depolarizing command pulse from a holding potential of -60 mV. A voltage-dependent inward calcium current across the type B soma membrane, ICa2+, activated rapidly, showed little inactivation, and was described by the equation: ICa2+ = gCa(max) [1 + exp](-Vm-5)/7]-1 (Vm-ECa), where gCa(max) was 0.5 microS. The light-induced current with both fast and slow phases was described by: IIgt(t) = IIgt1 + IIgt2 + IIgt3, IIgti = gIgti [1-exp(- ton/tau mi)] exp(-ton/tau hi)(Vm-EIgti) (i = 1, 2). For i = 3, /Igt(t) = gigt3m33h3(Vm - Eigt3)exp(-ton/Ton) x exp(-tfoff/t Off). Based on these reconstructions of ionic currents, learning-induced enhancement of the long lasting depolarization (LLD) of the photoreceptor'slight response was shown to arise from progressive inactivation of /A, lca2+ -K+, and lCa2+.

Different ionic conductances are modulated during the late receptor potential and the prolonged depolarizing afterpotential in Hermissenda type A photoreceptors

Wavelength-dependent, bistable phenomena were found in the receptor potential of Hermissenda crassicornis type A photoreceptors. Short exposure to blue light induced a prolonged depolarizing afterpotential (PDA) following the cessation of the light stimulus. Stronger adaptation to blue light, as caused by prolonged exposure and/or high intensity stimulation, effected a reduction in the early depolarizing transient of the late receptor potential (LRP) as elicited by subsequent stimuli. Vast separation of LRP emergence and PDA emergence could be obtained in photoreceptors in which a strong cancellation of the LRP was accomplished but a PDA still emerged after cessation of the light stimulus. Short exposure to yellow light cancelled the PDA, and stronger adaptation restored the LRP (opposite effect to blue light). The initial depolarizing part of the LRP had earlier been demonstrated to be mediated by the light-dependent increase of an inward conductance. In contrast, in this study the PDA was found to be accompanied by the reduction of an outward conductance, most likely a K+ conductance. A bistable photopigment system is thought to control the bistable receptor potential phenomenology by regulating the different membrane conductances during the LRP and the PDA.

Isolation of a G protein that is modified by learning and reduces potassium currents in Hermissenda

In Hermissenda crassicornis conditioned to associate light and rotation, type B photoreceptor neurons exhibit pairing-specific decreases in the potassium currents IA and IK-Ca, which account for many of the behavioral changes elicited by associative conditioning. To determine which proteins are involved in storage of this memory, high-performance liquid chromatography was used to examine proteins from Hermissenda eyes. Conditioning-specific changes in four phosphoproteins were observed 24 hours after conditioning. One of these proteins, cp20, was purified to apparent homogeneity and found to be a G protein. When injected back into Hermissenda type B cells, cp20 reduced IK and IK-Ca in a manner indistinguishable from the reduction caused by conditioning, suggesting that this protein may play a crucial role in memory acquisition or retention.

Isolation of a G protein that is modified by learning and reduces potassium currents in Hermissenda

In Hermissenda crassicornis conditioned to associate light and rotation, type B photoreceptor neurons exhibit pairing-specific decreases in the potassium currents IA and IK-Ca, which account for many of the behavioral changes elicited by associative conditioning. To determine which proteins are involved in storage of this memory, high-performance liquid chromatography was used to examine proteins from Hermissenda eyes. Conditioning-specific changes in four phosphoproteins were observed 24 hours after conditioning. One of these proteins, cp20, was purified to apparent homogeneity and found to be a G protein. When injected back into Hermissenda type B cells, cp20 reduced IK and IK-Ca in a manner indistinguishable from the reduction caused by conditioning, suggesting that this protein may play a crucial role in memory acquisition or retention.

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.

Sequential modification of membrane currents with classical conditioning

Pavlovian conditioning of the nudibranch mollusc Hermissenda crassicornis was previously shown to produce long-lasting reduction of two K+ currents measured across the Type B photoreceptor soma membrane (Alkon et al., 1982a; Alkon et al., 1985). Pavlovian conditioning of the rabbit was also shown to be followed by persistent K+ current reduction (Disterhoft et al., 1986). Here we report the first evidence that Ca2+ currents can also be modified by conditioning. The amplitude of the currents rather than their voltage-dependence remains reduced at least 1-2 d after conditioning (but not control procedures). Conditioning-induced changes of both K+ and Ca2+ currents increased as a function of training, the Ca2+ currents only changing substantially with greater than or equal to 250 trials. The later changes of the Ca2+ current may function to limit the magnitude of excitability increases due to associative learning.

Effects of inositol phosphates on the membrane activity of smooth muscle cells of the rabbit portal vein

The effects of intracellular perfusion of inositol 1,4,5-trisphosphate (InsP3) or inositol 1,3,4,5-tetrakisphosphate (InsP4) on electrical responses of smooth muscle cell membranes of the rabbit portal vein were studied using the whole cell voltage clamp technique. Depolarisation to 0 mV from a holding potential of -60 mV, evoked inward Ca (Ica), transient outward (ISO), oscillatory outward (IOO) and sustained outward (ISO) currents. Generation of IOO was dependent on the [Ca]o, but it was also generated in 0 mM Ca solution for over 10 min. From amplitude histograms, IOO was divided into two components. Reduction in [Ca]o inhibited the appearance of but not the amplitudes of both IOO components. However, the larger component of IOO was more resistant to a reduction in [Ca]o than the smaller one. InsP3 (10 microM) increased the frequency of both IOO components to a greater extent than their amplitude, but the larger component was more sensitive to InsP3 than the smaller one. The increase in the occurrence of IOO induced by InsP3 did not occur following pretreatment with 3 mM caffeine or 1 nM A23187. In normal PSS, InsP3 was evoked by a depolarising pulse positive to -40 mV, whereas following perfusion with InsP3 (10 microM), IOO was evoked at -60 mV. In normal PSS, intracellular perfusion with 10 microM InsP4 changed neither the frequency nor the amplitude of IOO, and the amplitudes of ICa, ITO and ISO were also unchanged. However, in 10 mM Ca solution, 10 microM InsP4 generated IOO at a membrane potential of -60 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of Hermissenda K+ channels by cytoplasmic and membrane-associated C-kinase

Pharmacologic activation of endogenous protein kinase C (PKC) together with elevation of the intracellular Ca2+ level was previously shown to cause reduction of two voltage-dependent K+ currents (IA and ICa2+-K+) across the soma membrane of the type B photoreceptor within the eye of the mollusc Hermissenda crassicornis. Similar effects were also found to persist for days after acquisition of a classically conditioned response. Also, the state of phosphorylation of a low-molecular-weight protein was changed only within the eyes of conditioned Hermissenda. To examine the role of PKC in causing K+ current changes as well as changes of phosphorylation during conditioning (and possibly other physiologic contexts), we studied here the effects of endogenous PKC activation and exogenous PKC injection on phosphorylation and K+ channel function. Several phosphoproteins (20, 25, 56, and 165 kilodaltons) showed differences in phosphorylation in response to PKC activators applied to intact nervous systems or to isolated eyes. Specific differences were observed for membrane and cytosolic fractions in response to both the phorbol ester 12-deoxyphorbol 13-isobutyrate 20-acetate (DPBA) or exogenous PKC in the presence of Ca2+ and phosphatidylserine/diacylglycerol. Type B cells pretreated with DPBA responded to PKC injection with a persistent reduction of K+ currents. In the absence of DPBA, PKC injection also caused K+ current reduction only following Ca2+ loading conditions. However, the direct effect of PKC injection in the absence of DPBA was only to increase ICa2+-K+. According to a proposed model, the amplitude of the K+ currents would depend on the steady-state balance of effects mediated by PKC within the cytoplasm and membrane-associated PKC. The model further specifies that the effects on K+ currents of cytoplasmic PKC require an intervening proteolytic step. Such a model predicts that increasing the concentration of cytoplasmic protease, e.g., with trypsin, will increase K+ currents, whereas blocking endogenous protease, e.g., with leupeptin, will decrease K+ currents. These effects should be opposed by preexposure of the cells to DPBA. Furthermore, prior injection of leupeptin should block or reverse the effects of subsequent injection of PKC into the type B cell. All of these predictions were confirmed by results reported here. Taken together, the results of this and previous studies suggest that PKC regulation of membrane excitability critically depends on its cellular locus. The implications of such function for long-term physiologic transformations are discussed.

Inhibition of protein synthesis prolongs Ca2+-mediated reduction of K+ currents in molluscan neurons

Elevated intracellular Ca2+ concentration within the Hermissenda type B cell has previously been shown to cause transient reduction of both the early K+ current IA and the delayed, Ca2+-dependent K+ current ICa2+-K+, a reduction that is more permanent with classical conditioning. Other earlier experiments suggested that Ca2+-mediated reduction of K+ currents initially involves the dual activation of Ca2+/calmodulin-dependent and Ca2+/lipid-dependent protein kinases. In the present study, voltage-clamp conditions that cause substantial increases in intracellular Ca2+ concentration (i.e., a Ca2+ "load") were used to produce IA and ICa2+-K+ reduction with and without the protein synthesis inhibitor anisomycin or cycloheximide or the control substance deacetylanisomycin in the bathing medium. Anisomycin (100 microM) and cycloheximide (100 microM) caused no significant change of resting membrane potential, holding current, or the non-voltage-dependent "leak" current. However, inhibition of protein synthesis prevented recovery from Ca2+-mediated K+-current reduction. This effect resembled the effect of injecting purified Ca2+-dependent kinases and was blocked by the presence of trifluoperazine in the bathing medium. Activation of protein kinase C with a water-soluble phorbol ester caused marked reduction of protein synthesis in Hermissenda neurons as monitored by two-dimensional gel electrophoresis. Synthesis of new proteins therefore may be important for reversal of initial steps during memory storage, and Ca2+-activated phosphorylation pathways may initiate long-term changes by turning off (as well as by turning on) the synthesis of particular proteins.