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T J Carew

Publications and source records attributed to T J Carew.

At least 19 recordsLinked to original sources
PubMed2006

Molecular nodes in memory processing: insights from Aplysia.

Recent research in a variety of systems indicates that memory formation can involve the activation of a wide range of molecular cascades. In assessing this recent work it is clear that no single cascade is uniquely important for all forms of memory, nor is a single form of memory uniquely dependent on a single cascade. Rather, it appears that molecular networks are differentially engaged in the induction of various forms of memory. Despite this highly interactive array of possible cascades, specific 'molecular nodes' have emerged as critical regulatory points in memory formation. Functionally, these nodes can operate in two sequential steps, beginning with a convergence of inputs which coordinately influence the activation state of the node, in which the nature of stimulation determines the dynamics of nodal activity, followed by a divergence of substrate selection, in which the node serves as a gateway that activates specific downstream effectors. Finally, specific nodes can be differentially engaged (i.e. have different 'weights') depending upon the nature and pattern of the activating stimulus. The marine mollusk Aplysia has proven useful for a molecular analysis of memory formation. We will use this system to highlight some of the molecular strategies employed by the nervous system in the formation of memory for sensitization, and we will focus on extracellular signal-related kinase as a candidate node integral to these processes.

Animals↗
PubMed2001

Combined effects of intrinsic facilitation and modulatory inhibition of identified interneurons in the siphon withdrawal circuitry of Aplysia.

Synaptic plasticity can be induced through mechanisms intrinsic to a synapse or through extrinsic modulatory mechanisms. In this study, we investigated the relationship between these two forms of plasticity at the excitatory synapse between L29 interneurons and siphon motor neurons (MNs) in Aplysia. Using isolated ganglia, we confirmed that the L29-MN synapses exhibit a form of intrinsic facilitation: post-tetanic potentiation (PTP). We also found that L29-MN synapses are modulated by exogenous application of 5-HT: they are depressed after 5-HT exposure. We next investigated the functional relationship between an intrinsic facilitatory process (PTP) and extrinsic inhibitory modulation (5-HT-induced depression). First, we found that application of 5-HT just before L29 activation results in a reduction of PTP. Second, using semi-intact preparations, we found that tail shock (TS) mimics the effect of 5-HT by both depressing L29 synaptic transmission and by reducing L29 PTP. Third, we observed a significant correlation between L29 activity during TS and subsequent synaptic change: low-responding L29s showed synaptic depression after TS, whereas high-responding L29s showed synaptic facilitation. Finally, we found that we could directly manipulate the sign and magnitude of TS-induced synaptic plasticity by controlling L29 activity during TS. Collectively, our results show that the L29-MN synapses exhibit intrinsic facilitation and extrinsic modulation and that the sign and magnitude of L29-MN plasticity induced by TS is governed by the combined effects of these two processes. This circuit architecture, which combines network inhibition with cell-specific facilitation, can enhance the signal value of a specific stimulus within a neural network.

Animals↗
PubMed2001

Molecular mechanisms underlying a unique intermediate phase of memory in aplysia.

Short- and long-term synaptic facilitation induced by serotonin at Aplysia sensory-motor (SN-MN) synapses has been widely used as a cellular model of short- and long-term memory for sensitization. In recent years, a distinct intermediate phase of synaptic facilitation (ITF) has been described at SN-MN synapses. Here, we identify a novel intermediate phase of behavioral memory (ITM) for sensitization in Aplysia and demonstrate that it shares the temporal and mechanistic features of ITF in the intact CNS: (1) it declines completely prior to the onset of LTM, (2) its induction requires protein but not RNA synthesis, and (3) its expression requires the persistent activation of protein kinase A. Thus, in Aplysia, the same temporal and molecular characteristics that distinguish ITF from other phases of synaptic plasticity distinguish ITM from other phases of behavioral memory.

Analysis of Variance↗
PubMed2001

Contribution of postsynaptic Ca2+ to the induction of post-tetanic potentiation in the neural circuit for siphon withdrawal in Aplysia.

Recent studies in Aplysia have revealed a novel postsynaptic Ca(2+) component to posttetanic potentiation (PTP) at the siphon sensory to motor neuron (SN-MN) synapse. Here we asked whether the postsynaptic Ca(2+) component of PTP was a special feature of the SN-MN synapse, and if so, whether it reflected a unique property of the SN or the MN. We examined whether postsynaptic injection of BAPTA reduced PTP at SN synapses onto different postsynaptic targets by comparing PTP at SN-MN and SN-interneuron (L29) synapses. We also examined PTP at L29-MN synapses. Postsynaptic BAPTA reduced PTP only at the SN-MN synapse; it did not affect PTP at either the SN-L29 or the L29-MN synapse, indicating that the SN and the MN do not require postsynaptic Ca(2+) for PTP with all other synaptic partners. The postsynaptic Ca(2+) component of PTP is present at other Aplysia SN-MN synapses; tail SN-MN synapses also showed reduced PTP when the MN was injected with BAPTA. Surprisingly, in both tail and siphon SN-MN synapses, there was an inverse relationship between the initial size of the EPSP and the postsynaptic component to PTP; only the initially weak SN-MN synapses showed a BAPTA-sensitive component. Homosynaptic depression of initially strong SN-MN synapses into the size range of initially weak synapses did not confer postsynaptic Ca(2+) sensitivity to PTP. Finally, the postsynaptic Ca(2+) component of PTP could be induced in the presence of APV, indicating that it is not mediated by NMDA receptors. These results suggest a dual model for PTP at the SN-MN synapse, in which a postsynaptic Ca(2+) contribution summates with the conventional presynaptic mechanisms to yield an enhanced form of PTP.

2-Amino-5-phosphonovalerate↗
PubMed2001

Aplysia ror forms clusters on the surface of identified neuroendocrine cells.

The ror receptors are a highly conserved family of receptor tyrosine kinases genetically implicated in the establishment of cellular polarity. We have cloned a ror receptor from the marine mollusk Aplysia californica. Aplysia ror (Apror) is expressed in most developing neurons and some adult neuronal populations, including the neuroendocrine bag-cell neurons. The Apror protein is present in peripheral neuronal processes and ganglionic neuropil, implicating the kinase in the regulation of growth and/or synaptic events. In cultured bag-cell neurons, the majority of the protein is stored in intracellular organelles, whereas only a small fraction is expressed on the surface. When expressed on the cell surface, the protein is clustered on neurites, suggesting that Apror is involved in the organization of functional domains within neurons. Apror immunoreactivity partially colocalizes with the P-type calcium channel BC-alpha1A at bag-cell neuron varicosities, suggesting a role for Apror in organizing neuropeptide release sites.

Age Factors↗
PubMed2001

Modulation of excitability in Aplysia tail sensory neurons by tyrosine kinases.

Tyrosine kinases have recently been shown to modulate synaptic plasticity and ion channel function. We show here that tyrosine kinases can also modulate both the baseline excitability state of Aplysia tail sensory neurons (SNs) as well as the excitability induced by the neuromodulator serotonin (5HT). First, we examined the effects of increasing and decreasing tyrosine kinase activity in the SNs. We found that tyrosine kinase inhibitors decrease baseline SN excitability in addition to attenuating the increase in excitability induced by 5HT. Conversely, functionally increasing cellular tyrosine kinase activity in the SNs by either inhibiting opposing tyrosine phosphatase activity or by direct injection of an active tyrosine kinase (Src) induces increases in SN excitability in the absence of 5HT. Second, we examined the interaction between protein kinase A (PKA), which is known to mediate 5HT-induced excitability changes in the SNs, and tyrosine kinases, in the enhancement of SN excitability. We found that the tyrosine kinases function downstream of PKA activation since tyrosine kinase inhibitors reduce excitability induced by activators of PKA. Finally, we examined the role of tyrosine kinases in other forms of 5HT-induced plasticity in the SNs. We found that while tyrosine kinase inhibitors attenuate excitability produced by 5HT, they have no effect on short-term facilitation (STF) of the SN-motor neuron (MN) synapse induced by 5HT. Thus tyrosine kinases modulate different forms of SN plasticity independently. Such differential modulation would have important consequences for activity-dependent plasticity in a variety of neural circuits.

4-(3-Butoxy-4-methoxybenzyl)-2-imidazolidinone↗
PubMed2000

Parallel molecular pathways mediate expression of distinct forms of intermediate-term facilitation at tail sensory-motor synapses in Aplysia.

Three distinct temporal phases of synaptic facilitation (short-, intermediate-, and long-term) are induced by serotonin (5-HT) at sensory (SN) to motor (MN) synapses in Aplysia. Here, we characterize two mechanistically distinct forms of intermediate-term facilitation (ITF) at tail SN-MN synapses. One form, activity-independent ITF, is produced by five spaced pulses of 5-HT in the absence of SN activity. Its induction requires protein synthesis, and its expression requires persistent activation of PKA but not PKC. The other form, activity-dependent ITF, is produced by a single pulse of 5-HT coincident with SN activation. Its induction does not require protein synthesis, and its expression requires persistent activation of PKC but not PKA. These results demonstrate that SN-MN synapses can exhibit two distinct forms of ITF that are mediated by parallel molecular pathways.

Animals↗
PubMed2000

Dynamic regulation of the siphon withdrawal reflex of Aplysia californica in response to changes in the ambient tactile environment.

The state of an animal's environment can be viewed as a source of information that can be used to regulate both ongoing and future behavior. The present work examined how the ambient environment can regulate the Aplysia siphon withdrawal reflex (SWR) by changing the environment between calm and turbulent. Results indicate that the SWR is dynamically regulated on the basis of variations in external conditions, so that responsiveness (measured as both reflex duration and threshold) is matched to the state of the environment. Prior exposure to a noxious stimulus (tailshock) has selective effects on this regulation, suggesting the existence of multiple regulatory mechanisms. Further, neurophysiological correlates to behavioral observations were measured in sensory and motor neurons. This will allow for a detailed cellular analysis of environmental information-processing in this system.

Animals↗
PubMed1999

Coincident induction of long-term facilitation in Aplysia: cooperativity between cell bodies and remote synapses.

Induction of long-term synaptic changes at one synapse can facilitate the induction of long-term plasticity at another synapse. Evidence is presented here that if Aplysia sensory neuron somata and their remote motor neuron synapses are simultaneously exposed to serotonin pulses insufficient to induce long-term facilitation (LTF) at either site alone, processes activated at these sites interact to induce LTF. This coincident induction of LTF requires that (i) the synaptic pulse occur within a brief temporal window of the somatic pulse, and (ii) local protein synthesis occur immediately at the synapse, followed by delayed protein synthesis at the soma.

Animals↗
PubMed1999

Developmental dissociation of serotonin-induced spike broadening and synaptic facilitation in Aplysia sensory neurons.

In sensory neurons (SNs) of adult Aplysia, serotonin (5-HT)-induced spike broadening has long been implicated as important for synaptic facilitation [spike duration-dependent (SDD) facilitation], particularly at nondepressed synapses. At depressed synapses, spike broadening has less impact on synaptic facilitation; under these conditions, 5-HT induces a spike duration-independent (SDI) form of facilitation (). It has been difficult to dissociate clearly the cellular mechanisms underlying these two forms of facilitation. However, the observation that a major form of spike broadening emerges late in juvenile development () provides a unique opportunity to examine the relationship between spike broadening and synaptic facilitation in juvenile Aplysia. We have identified three forms of synaptic plasticity in juvenile Aplysia: homosynaptic depression, SDD facilitation, and SDI facilitation. We show that homosynaptic depression is fully developed in the juvenile and that 5-HT reliably induces synaptic facilitation at depressed synapses. However, in nondepressed synapses, 5-HT-induced facilitation is not reliable. Further analysis revealed that the relationship between spike broadening and synaptic facilitation for nondepressed synapses is the inverse of that in adults. Surprisingly, in juveniles, minor spike broadening induced by 5-HT results in significant synaptic facilitation, whereas major spike broadening, when it occurs, does not. These results suggest a model in which juvenile synapses predominantly use SDI facilitation, and with the emergence of major spike broadening, a developmentally transient inhibitory process emerges. This inhibitory process seems to be independent of major spike broadening induced by 5-HT because directly broadening the spike with 4-aminopyridine induces adult-like SDD synaptic facilitation. Finally, in the adult, the inhibitory process is either lost or masked, and SDD facilitation predominates at nondepressed synapses.

Action Potentials↗
PubMed1998

Developmental emergence of different forms of neuromodulation in Aplysia sensory neurons.

The capacity for neuromodulation and biophysical plasticity is a defining feature of most mature neuronal cell types. In several cases, modulation at the level of the individual neuron has been causally linked to changes in the functional output of a neuronal circuit and subsequent adaptive changes in the organism's behavioral responses. Understanding how such capacity for neuromodulation develops therefore may provide insights into the mechanisms both of neuronal development and learning and memory. We have examined the development of multiple forms of neuromodulation triggered by a common neurotransmitter, serotonin, in the pleural sensory neurons of Aplysia californica. We have found that multiple signaling cascades within a single neuron develop sequentially, with some being expressed only very late in development. In addition, our data suggest a model in which, within a single neuromodulatory pathway, the elements of the signaling cascade are developmentally expressed in a "retrograde" manner with the ionic channel that is modulated appearing early in development, functional elements in the second messenger cascade appearing later, and finally, coupling of the second messenger cascade to the serotonin receptor appearing quite late. These studies provide the characterization of the development of neuromodulation at the level of an identified cell type and offer insights into the potential roles of neuromodulatory processes in development and adult plasticity.

Action Potentials↗
PubMed1998

Serotonin induces temporally and mechanistically distinct phases of persistent PKA activity in Aplysia sensory neurons.

The cAMP signaling cascade has been implicated in several stages of memory formation. We have examined activation of this cascade by serotonin (5-HT) in the sensory neurons of Aplysia. We find that different patterns of 5-HT exposure induce three distinct modes of PKA activation. First, a single 5 min pulse induces transient (5 min) PKA activation that requires neither transcription nor translation. Second, 4-5 pulses induce intermediate-term persistent activation (3 hr duration) that requires translation but not transcription. Third, 5 pulses of 5-HT, as well as continuous (90 min) exposure, induce long-term persistent activation 20 hr later, which requires both transcription and translation. Thus, in the sensory neurons, different patterns of 5-HT give rise to three independent phases of PKA activation that differ in their induction requirements, their temporal profiles, and their molecular mechanisms.

Animals↗
PubMed1998

Protein kinase C regulates a vesicular class of calcium channels in the bag cell neurons of aplysia.

Protein kinase C (PKC) acutely increases calcium currents in Aplysia bag cell neurons by recruiting calcium channels different from those constitutively active in the plasma membrane. To study the mechanism of PKC regulation we previously identified two calcium channel alpha1-subunits expressed in bag cell neurons. One of these, BC-alpha1A, is localized to vesicles concentrated primarily in somata and growth cones. We used antibodies to BC-alpha1A to analyze its expression in the bag cell neurons of juvenile Aplysia at a developmental stage at which PKC-sensitive calcium currents have previously been shown to be low. We find that vesicular BC-alpha1A staining is generally reduced in juvenile bag cell neurons but that its expression level can vary among juvenile animals. In 17 bag cell clusters examined, the percentage of neurons that displayed punctate alphaBC-alpha1A staining ranged from 0 to 85%. Sampling of calcium currents from cells of the same clusters by whole cell patch-clamp techniques revealed that the PKC-sensitive calcium current density is significantly correlated with the degree of vesicular staining. In contrast, no correlation of basal calcium current levels with aBC-alpha1A staining was found. These results strongly suggest that BC-alpha1A, a member of the ABE-subfamily of calcium channels, carries the PKC-sensitive calcium current in bag cell neurons. They are consistent with a model in which PKC recruits channels from the vesicular pool to the plasma membrane.

Animals↗
PubMed1997

Metaplasticity at identified inhibitory synapses in Aplysia.

Synaptic plasticity is an important feature of neural networks involved in the encoding of information. In the analysis of long-term potentiation and long-term depression, several examples have emerged in which this plasticity is itself modulated. This higher-order form of plasticity has been referred to as 'metaplasticity', a modification of synapses reflected as a change in the ability to induce or maintain plasticity. These observations raise the question of the possible advantage of regulating the intrinsic plastic properties of a synapse. The neural circuit mediating the siphon withdrawal reflex in Aplysia provides a useful network in which to examine this question directly. Inhibitory synapses in this circuit (from L30 neurons) exhibit a variety of forms of activity-dependent short-term synaptic enhancement which contribute to dynamic gain control in the siphon withdrawal reflex. Here we report that tail shock, an extrinsic modulatory input of known behavioural relevance, induces differential metaplasticity at this synapse, attenuating its ability to exhibit short-term synaptic enhancement after presynaptic activation (augmentation and post-tetanic potentiation), while leaving intact its capacity for enhancement during activation. This attenuation of inhibition at the synaptic level seems to mediate comparable attenuation of inhibitory modulation at both network and behavioural levels.

Animals↗
PubMed1997

Ontogeny of serotonergic neurons in Aplysia californica.

Although the identity, projection patterns, and functions of serotonergic neurons in juvenile and adult Aplysia are relatively well understood, little is known about the development of these cells. We have used light and electron microscopic immunocytochemistry to investigate the genesis, differentiation, identity, and fate of the serotonergic cells in the embryonic, larval, and metamorphic stages of the life cycle of Aplysia. The results indicate that the first serotonergic cells emerge at midembryogenesis and that a total of five cells makes up the entire serotonergic system by hatching. These cells are part of a newly discovered ganglion in Aplysia, called the apical ganglion. This serotonergic system of five cells remains essentially intact throughout larval development. The apical ganglion, together with its serotonergic cells, is resorbed at metamorphosis. A distinct set of serotonergic cells, which begins to emerge by the end of the larval period, is rapidly elaborated during the metamorphic and early juvenile periods to form the adult serotonergic system. These results support the view that the larval and adult forms of the Aplysia nervous system consist of entirely distinct sets of serotonergic cells, each adapted to the stage-specific morphological and behavioral characteristics of the animal.

Animals↗