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Thomas J Carew

Publications and source records attributed to Thomas J Carew.

At least 19 recordsLinked to original sources
PubMed2006

Regulation of behavioral and synaptic plasticity by serotonin release within local modulatory fields in the CNS of Aplysia.

In Aplysia, serotonergic neurons are widely activated during sensitization training, but the effects of exogenous serotonin (5-HT) on reflex circuits vary, inducing short- or long-term synaptic facilitation or synaptic inhibition, depending on the site of application. During learning, it is possible that specific spatial patterns of 5-HT release evoked by training may produce different phases of sensitization or behavioral inhibition. To test this hypothesis, we examined the modulation of the tail-induced siphon withdrawal reflex by repeated noxious stimuli applied to one of three sites: the (1) ipsilateral or (2) contralateral sides of the tail or (3) the head. Ipsilateral tail shock produced long-term sensitization, whereas contralateral tail shock induced only short-term sensitization, and head shock produced inhibition. In parallel cellular experiments, tail-nerve shock evoked large 5-HT release localized around the ipsilateral tail sensory neurons (SNs) and motor neurons (MNs) but only modest 5-HT release in the contralateral pleural-pedal ganglia and in the abdominal ganglion, in which the siphon MNs are located. Head-nerve shock, in contrast, produced only modest 5-HT release in the pleural, pedal, and abdominal ganglia. Thus, each training protocol evoked a specific pattern of 5-HT release within the CNS. In addition, we found that 5-HT released in the pleural ganglia was correlated with facilitation of SN-MN synapses; however, in the abdominal ganglion, it was associated with inhibition of the synapses between identified interneurons (L29s) and siphon MNs (LFSs). Because 5-HT differentially modulates synaptic efficacy at different synaptic sites, our data can explain how specific spatial patterns of 5-HT release in local modulatory fields can contribute to the induction of short- or long-term sensitization or to behavioral inhibition.

Action Potentials↗
PubMed2006

Intermediate-term processes in memory formation.

Neuroscientists have invested considerable effort in attempting to elucidate the molecular mechanisms that mediate short-term and long-term forms of learning and memory. For instance, the discovery of long-term potentiation inspired a field that has produced hundreds of studies examining both early and late forms of long-term potentiation. And at the behavioral level, most neuroscientists investigate either short- or long-term forms of memory or some combination of the two. The general belief that plasticity was restricted to short- and long-term temporal domains lasted for many years because of the apparent continuity of memory and its molecular characterization from one domain to the other. In cellular studies of plasticity, the short-term stage typically lasts in the range of minutes, and requires modification of pre-existing proteins, whereas long-term changes, such as synaptic growth, last for hours to days and require transcription and translation. As both behavioral and cellular studies covered a wider range of temporal domains, from the initiation of brief memory to the expression of long-lasting memory, it was at least tacitly assumed that these studies also captured any intervening domains as well. However, between these two temporal extremes lies a unique form of intermediate-term synaptic plasticity and memory, which mechanistically is a blend of the early and late forms.

Animals↗
PubMed2006

A tropomyosin-related kinase B ligand is required for ERK activation, long-term synaptic facilitation, and long-term memory in aplysia.

BDNF, which acts through tropomyosin-related kinase B (TrkB) receptors during mammalian development, also enhances long-term synaptic facilitation (LTF) in adult Aplysia. Because LTF is a substrate for long-term memory (LTM) in Aplysia, we examined the requirement of a secreted TrkB ligand in LTM formation at molecular, synaptic, and behavioral levels. Using an extracellular fusion protein that sequesters secreted TrkB ligands, we show that TrkB function is required for serotonin-induced activation of extracellular signal-regulated kinase, tail nerve shock-induced LTF in the CNS, and tail shock-induced LTM but is not necessary for short-term synaptic facilitation or short-term memory. These results show that a secreted growth factor, acting through a TrkB signaling cascade, is critical for the induction of long-lasting plasticity and memory formation in Aplysia.

Animals↗
PubMed2005

Differential role of inhibition in habituation of two independent afferent pathways to a common motor output.

Many studies of the neural mechanisms of learning have focused on habituation, a simple form of learning in which a response decrements with repeated stimulation. In the siphon-elicited siphon withdrawal reflex (S-SWR) of the marine mollusk Aplysia, the prevailing view is that homosynaptic depression of primary sensory afferents underlies short-term habituation. Here we examined whether this mechanism is also utilized in habituation of the tail-elicited siphon withdrawal reflex (T-SWR), which is triggered by an independent, polysynaptic afferent pathway that converges onto the same siphon motor neurons (MNs). By using semi-intact preparations in which tail and/or siphon input to siphon MNs could be measured, we found that repeated tail stimuli administered in the presence of a reversible conduction block of the nerves downstream of the tail sensory neurons (SNs) completely abolished the induction of habituation. Subsequent retraining revealed no evidence of savings, indicating that the tail SNs and their immediate interneuronal targets are not the locus of plasticity underlying T-SWR habituation. The networks closely associated with the siphon MNs are modulated by cholinergic inhibition. We next examined the effects of network disinhibition on S-SWR and T-SWR habituation using an Ach receptor antagonist d-tubocurarine. We found that the resulting network disinhibition disrupted T-SWR, but not S-SWR, habituation. Indeed, repeated tail stimulation in the presence of d-tubocurarine resulted in an initial enhancement in responding. Lastly, we tested whether habituation of T-SWR generalized to S-SWR and found that it did not. Collectively, these data indicate that (1) unlike S-SWR, habituation of T-SWR does not involve homosynaptic depression of SNs; and (2) the sensitivity of T-SWR habituation to network disinhibition is consistent with an interneuronal plasticity mechanism that is unique to the T-SWR circuit, since it does not alter S-SWR.

Afferent Pathways↗
PubMed2004

Serotonergic modulation in aplysia. I. Distributed serotonergic network persistently activated by sensitizing stimuli.

A common feature of arousing stimuli used as reinforcement in animal models of learning is that they promote memory formation through widespread effects in the CNS. In the marine mollusk Aplysia, sensitization is typically induced by tail-shock, an aversive reinforcer that triggers a state of defensive arousal characterized by escape locomotion and increased heart rate. Serotonin (5-HT) contributes importantly to sensitization of defensive reflexes as well as to the regulation of locomotion and heart rate. Although specific serotonergic neurons increase their firing after tail-shock, it remains unclear whether this effect is restricted to these neurons or whether tail-shock recruits a more global serotonergic system. In this study, we recorded from serotonergic neurons throughout the CNS, which were prelabeled with 5,7-dihydroxytryptamine, during an in vitro analog of sensitization training, tail-nerve shock. We found that most of the serotonergic neurons that we recorded from (80%) increased their firing rate for several minutes after nerve shock. Most serotonergic neurons in the pedal and abdominal ganglion were also excited by 5-HT and by intracellular activation of the two serotonergic neurons CB1/CC3. This interconnectivity between serotonergic neurons might contribute to spread excitation within a large proportion of the serotonergic system during sensitization training. It is also possible that serotonergic neurons could be activated by 5-HT present in the hemolymph via a neuro-humoral positive feedback mechanism. Overall, these data indicate that sensitization training activates a large proportion of Aplysia serotonergic neurons and that this form of learning occurs in a context of increased serotonergic tone.

5,7-Dihydroxytryptamine↗
PubMed2004

Serotonergic modulation in aplysia. II. Cellular and behavioral consequences of increased serotonergic tone.

Serotonin (5-HT) plays an important role in sensitization of defensive reflexes in Aplysia and is also involved in several aspects of arousal, such as the control of locomotion and of cardiovascular tone. In the preceding paper, we showed that tail-nerve shock, a noxious stimulus that readily induces sensitization, increases the firing rate of a large number of serotonergic neurons throughout the CNS. However, the functional consequences of such an increase in serotonergic tone are still poorly understood. In this study, we examined this question by using the 5-HT precursor 5-hydroxytryptophan (5-HTP) to specifically increase 5-HT release in the CNS. Increased tonic 5-HT release after 5-HTP treatment was manifested by facilitation of sensorimotor (SN-MN) synapses, increased firing rate of serotonergic neurons in the pedal and abdominal ganglia, and enhanced 5-HT release evoked by tail-nerve shock. When 5-HTP was administered to freely moving animals, it produced a strong arousal response characterized by increased locomotion and heart rate, which was reminiscent of the defensive arousal reaction triggered by noxious stimulation such as tail-shock. In contrast, 5-HTP actually inhibited the tail-induced siphon-withdrawal reflex. It is possible that 5-HT-induced facilitation of SN-MN synapses was counteracted by inhibition of polysynaptic reflex pathways between SNs and MNs, resulting in transient behavioral inhibition of the reflex, which could favor escape locomotion and/or respiration shortly after an aversive stimulus. We conclude that a major function associated with the activation of the Aplysia serotonergic system evoked by noxious stimuli is the triggering of a defensive arousal response. It is known that tail-shock-induced serotonergic activation contributes to memory encoding at least in part by facilitating SN-MN synapses. However, this effect in isolation might not be sufficient for the behavioral expression of sensitization.

5,7-Dihydroxytryptamine↗
PubMed2004

Parallel somatic and synaptic processing in the induction of intermediate-term and long-term synaptic facilitation in Aplysia.

The induction of different phases of memory depends on the amount and patterning of training, raising the question of whether specific training patterns engage different cellular mechanisms and whether these mechanisms operate in series or in parallel. We examined these questions by using a cellular model of memory formation: facilitation of the tail sensory neuron-motor neuron synapses by serotonin (5-hydroxytryptamine, 5-HT) in the CNS of Aplysia. We studied facilitation in two temporal domains: intermediate-term facilitation (1.5-3 h) and long-term facilitation (LTF, >24 h). Both forms can be induced by using several different temporal and spatial patterns of 5-HT, including (i) repeated, temporally spaced pulses of 5-HT to both the sensory neuron soma and the sensory neuron-motor neuron synapse, and (ii) temporally asymmetric exposure of 5-HT to the soma and synapse under conditions in which neither exposure alone induces LTF. We first examined the protein and RNA synthesis requirements for LTF induced by these two patterns and found that asymmetric (but not repeated) 5-HT application induced LTF that required postsynaptic protein and RNA synthesis. We next focused on the patterning and protein synthesis requirements for intermediate-term facilitation. We found that intermediate-term facilitation (i) is induced locally at the synapse, (ii) requires multiple pulses of 5-HT, and (iii) requires synaptic protein synthesis. Our findings show that different temporal and spatial patterns of 5-HT induce specific temporal phases of long-lasting facilitation in parallel by engaging different cellular and molecular mechanisms.

Animals↗
PubMed2004

Intermediate-term memory for site-specific sensitization in aplysia is maintained by persistent activation of protein kinase C.

Recent studies of long-term synaptic plasticity and long-term memory have demonstrated that the same functional endpoint, such as long-term potentiation, can be induced through distinct signaling pathways engaged by different patterns of stimulation. A critical question raised by these studies is whether different induction pathways either converge onto a common molecular mechanism or engage different molecular cascades for the maintenance of long-term plasticity. We directly examined this issue in the context of memory for sensitization in the marine mollusk Aplysia. In this system, training with a single tail shock normally induces short-term memory (<30 min) for sensitization of tail-elicited siphon withdrawal, whereas repeated spaced shocks induce both intermediate-term memory (ITM) (>90 min) and long-term memory (>24 hr). We now show that a single tail shock can also induce ITM that is expressed selectively at the trained site (site-specific ITM). Although phenotypically similar to the form of ITM induced by repeated trials, the mechanisms by which site-specific ITM is induced and maintained are distinct. Unlike repeated-trial ITM, site-specific ITM requires neither protein synthesis nor PKA activity for induction or maintenance. Rather, the induction of site-specific ITM requires calpain-dependent proteolysis of activated PKC, yielding a persistently active PKC catalytic fragment (PKM) that also serves to maintain the memory in the intermediateterm temporal domain. Thus, two unique forms of ITM that have different induction requirements also use distinct molecular mechanisms for their maintenance.

Animals↗
PubMed2003

Neural circuit of tail-elicited siphon withdrawal in Aplysia. I. Differential lateralization of sensitization and dishabituation.

The tail-elicited siphon withdrawal reflex (TSW) has been a useful preparation in which to study learning and memory in Aplysia. However, comparatively little is known about the neural circuitry that translates tail sensory input (via the P9 nerves to the pleural ganglion) to final reflex output by siphon motor neurons (MNs) in the abdominal ganglion. To address this question, we examined the functional architecture of the TSW circuit by selectively severing nerves of semi-intact preparations and recording either tail-evoked responses in the siphon MNs or measuring siphon withdrawal responses directly. We found that the neural circuit underlying TSW is functionally lateralized. We next tested whether the expression of learning in the TSW reflects the underlying circuit architecture and shows side-specificity. We tested behavioral and physiological correlates of three forms of learning: sensitization, habituation, and dishabituation. Consistent with the circuit architecture, we found that sensitization and habituation of TSW are expressed in a side-specific manner. Unexpectedly, we found that dishabituation was expressed bilaterally, suggesting that a modulatory pathway bridges the two (ipsilateral) input pathways of the circuit, but this path is only revealed for a specific form of learning, dishabituation. These results suggest that the effects of a descending modulatory signal are differentially "gated" during sensitization and dishabituation.

Animals↗
PubMed2003

Neural circuit of tail-elicited siphon withdrawal in Aplysia. II. Role of gated inhibition in differential lateralization of sensitization and dishabituation.

In the preceding report, we observed that tail-shock-induced sensitization of tail-elicited siphon withdrawal reflex (TSW) of Aplysia was expressed ipsilaterally but that dishabituation induced by an identical tail shock was expressed bilaterally. Here we examined the mechanisms of this differential lateralization. We first isolated the modulatory pathway responsible for the induction of contralateral dishabituation by making selective nerve cuts. We found that an intact pleural-abdominal connective, the descending pathway connecting the ring ganglia with the abdominal ganglion, ipsilateral to the shock was required for contralateral dishabituation. We examined whether network inhibition suppresses the contralateral effects of tail shock in nonhabituated preparations. We found that blockade of inhibitory transmission in the CNS by the nicotinic ACh inhibitor d-tubocurarine (d-TC) rendered tail shock capable of inducing bilateral sensitization. We next asked whether serotonin (5-HT), a neuromodulator released in the CNS in response to tail shock, was affected by d-TC. We found that d-TC does not alter 5-HT processes in the ring ganglia: it had no effect on the lateralized pattern of tail nerve shock-induced changes in tail sensory neuron excitability, a 5-HT-dependent process, and it did not alter tail nerve shock-evoked release of 5-HT. By contrast, d-TC enhanced 5-HT release in the abdominal ganglion. Consistent with this observation, restricting d-TC to the abdominal ganglion rendered tail nerve shock capable of producing bilateral sensitization. Together with the results of the preceding paper, our results suggest a model in which TSW sensitization and dishabituation can be dissociated both anatomically and mechanistically.

Animals↗
PubMed2003

Differential role of mitogen-activated protein kinase in three distinct phases of memory for sensitization in Aplysia.

The mitogen-activated protein kinase (MAPK) pathway has been implicated recently in synaptic plasticity and memory. Here we used tail shock-induced sensitization of the tail-elicited siphon withdrawal reflex in Aplysia to examine the role of MAPK in three different phases of memory. We show that a specific pattern of serotonin (5-HT) application that produces intermediate-term and long-term synaptic facilitation (ITF and LTF, respectively) of the sensory-motor (SN-MN) synapses in Aplysia leads to sustained activation of extracellular signal-regulated kinase in the ventrocaudal cluster sensory neurons (SNs), which include the tail SNs. Furthermore, repeated tail shocks that induce intermediate-term and long-term memory (ITM and LTM, respectively) for sensitization also lead to sustained MAPK activation in the SNs. Given these results, we next examined the requirement of MAPK activity in (1) SN-MN synaptic facilitation and (2) memory for sensitization in Aplysia, by inhibiting MEK, the upstream kinase that phosphorylates and activates MAPK. In cellular experiments, we show that MAPK activity is required for ITF of tail SN-tail MN synapses, and, in parallel behavioral experiments, we show that ITM requires MAPK activity for its induction but not its expression. In contrast, short-term memory for sensitization does not require MAPK activity. Finally, 5-HT-induced LTF has been shown previously to require MAPK activity. Here we show that LTM for sensitization also requires MAPK activity. These results provide evidence that MAPK plays important roles specifically in long-lasting phases of synaptic plasticity and memory.

Action Potentials↗
PubMed2003

Inhibition of calcineurin facilitates the induction of memory for sensitization in Aplysia: requirement of mitogen-activated protein kinase.

The induction of both synaptic plasticity and memory is thought to depend on the balance between opposing molecular regulatory factors, such as protein kinases and phosphatases. Here we show that inhibition of protein phosphatase 2B (calcineurin, CaN) facilitates the induction of intermediate-term memory (ITM) and long-term memory (LTM) for tail shock-induced sensitization in Aplysia without any effect on short-term memory. To identify the molecular cascade underlying the improvement of memory by inhibition of CaN, we examined the role of extracellular signal-regulated kinase 1/2/mitogen-activated protein kinase (MAPK). Molecular experiments revealed that one pulse of serotonin, which by itself does not activate MAPK, leads to significant MAPK activation in the sensory neurons of the pleural ganglia when CaN is inhibited. Extending these observations, behavioral experiments showed that the facilitated induction of ITM and LTM produced by CaN inhibition depends on MAPK activity. These results demonstrate: (i) that CaN acts as an inhibitory constraint in the formation of long-lasting phases of memory, and (ii) that facilitated induction of ITM and LTM by CaN inhibition requires MAPK activity.

Animals↗
PubMed2003

Identification and characterization of Aplysia adducin, an Aplysia cytoskeletal protein homologous to mammalian adducins: increased phosphorylation at a protein kinase C consensus site during long-term synaptic facilitation.

Structural changes at synapses are associated with long-term facilitation (LTF) of synaptic transmission between sensory and motor neurons in Aplysia. We have cloned a cDNA encoding Aplysia adducin (ApADD), the Aplysia homolog of mammalian adducins that are regulatory components of the membrane cytoskeleton. ApADD is recovered in the particulate fraction of nervous system extracts and is localized predominantly in the submembraneous region of Aplysia neurons. ApADD is phosphorylated in vitro by protein kinase C (PKC) at a site homologous to the in vivo PKC phosphorylation site in mammalian adducins. Phosphorylation of ApADD at this site is also detected in vivo in the intact Aplysia nervous system and is increased 18 hr after serotonin-induced LTF. In contrast, there is no change in phosphorylation during short-term facilitation or 1 hr after initial LTF induction. Thus, ApADD is modulated specifically with later phases of LTF and provides an attractive candidate protein that contributes to structural changes accompanying long-lasting synaptic alteration.

Amino Acid Sequence↗
PubMed2003

Activation of a tyrosine kinase-MAPK cascade enhances the induction of long-term synaptic facilitation and long-term memory in Aplysia.

Tyrosine kinases have been implicated in cellular processes thought to underlie learning and memory. Here we show that tyrosine kinases play a direct role in long-term synaptic facilitation (LTF) and long-term memory (LTM) for sensitization in Aplysia. Tyrosine kinase activity is required for serotonin-induced LTF of sensorimotor (SN-MN) synapses, and enhancement of endogenous tyrosine kinase activity facilitates the induction of LTF. These effects are mediated, at least in part, through mitogen-activated protein kinase (MAPK) activation and are blocked by transcriptional and translational inhibitors. Moreover, brain-derived neurotrophic factor (BDNF) also enhances the induction of LTF in a MAPK-dependent fashion. Finally, activation of endogenous tyrosine kinases enhances the induction of long-term memory for sensitization, and this enhancement also requires MAPK activation. Thus, tyrosine kinases, acting through MAPK, play a pivotal role in LTF and LTM formation.

Animals↗
PubMed2003

Tyrosine kinases, synaptic plasticity and memory: insights from vertebrates and invertebrates.

Tyrosine kinases were first characterized in terms of their function during development. Over the past decade, it has become clear that tyrosine phosphorylation also plays an important role in the adult mammalian nervous system. This article reviews three different families of tyrosine kinase signaling cascades: the Trk receptor tyrosine kinases, the Src family of non-receptor tyrosine kinases and the Eph receptor tyrosine kinases. Each of these cascades has been implicated in both adult synaptic plasticity and memory formation. Evidence from invertebrate systems also demonstrates a role for tyrosine kinase signaling in the induction of long-term memory, suggesting that molecular mechanisms of memory formation are conserved across species.

Animals↗
PubMed2002

Improved electrochemical detection of biogenic amines in Aplysia using base-hydrolyzed cellulose-coated carbon fiber microelectrodes.

A major challenge with electrochemical techniques in vivo, using carbon-fiber microelectrodes, is to achieve sufficient sensitivity to detect the low concentrations of transmitters released by neurons. In particular, when an electrode is inserted into living tissue, its sensitivity is usually substantially decreased as a result of the degradation of the carbon surface by proteins. Here, we show that this decrease can be significantly attenuated by coating the electrode with cellulose acetate. The cellulose film offers a steric barrier that prevents macromolecules from diffusing to the carbon surface and its porosity can be progressively increased by controlled hydrolysis. We compared different cellulose-coated electrodes, either non-hydrolysed or hydrolyzed, in 0.08 N KOH for 10-30 min. We found that dopamine and serotonin detection was blocked by non-hydrolysed cellulose films, but that hydrolysis restored optimal detection similar to uncoated electrodes. Moreover, cellulose films (hydrolyzed for 20 min) significantly diminished electrode degradation in vivo and allowed reliable detection of fast concentration changes with <0.5 s delay, compared to uncoated electrodes. Finally, the sensitivity to endogenous 5-HT release in Aplysia central nervous system was more than doubled with these electrodes. We conclude that the optimal hydrolysis time of cellulose-coated electrodes is approximately 20 min with our protocol and carbon fiber electrodes prepared with this method offer improved sensitivity for the detection of biogenic amines.

Animals↗