Enhancement of the defensive withdrawal reflex ofAplysia involves a prolongation of the action potentials of mechanosensory neurons, which contributes to facilitation of transmitter release from these cells. Recent reports have suggested that whereas cAMP-dependent modulation of K+ current increases sensory neuron excitability, a cAMP-independent decrease in K+ current may increase the action potential duration and, thus, facilitate transmitter release. We have tested this proposal using Walsh cAMP-dependent protein kinase inhibitor or activators of the cAMP cascade and found that cAMP plays a major role in the spike-broadening effects of facilitatory transmitter; however, broadening requires higher levels of activation of the cAMP-dependent kinase than does increasing excitability. A steeply voltage-dependent transient K+ current, termed IKv,ealy, and the slowly activating S-type K+ (S-K+) current are both reduced by activation of the cAMP cascade, although with different sensitivities to the second messenger, enabling excitability and spike duration to be regulated independently. Differences in cAMP sensitivity also suggested that the originally described S-K+ current actually consists of two independent components, a slowly activating component and a time-independent, "steady-state" current that is activated at rest.During both sensitization and classical conditioning of defensive withdrawal reflexes in Aplysia, increased transmitter release from sensory neurons (SNs) (1-4) contributes to the enhancement of withdrawal responses. One important mechanism for short-term facilitation of transmitter release is a prolongation of the presynaptic action potential (5-7), which increases the Ca2+ influx during each spike (8). This spike broadening results from a decrease in outward K+ current (5) triggered by facilitatory transmitters, primarily serotonin (5-HT) (9,19), that are released by sensitizing stimuli. Several K+ currents are modulated by 5-HT in the siphon and pleural SNs, which have similar properties (10)(11)(12)(13)(14). Klein et al. (11) first characterized the K+ current reduced by 5-HT and found it is partially active at rest and further activates slowly when the membrane is depolarized to potentials from -30 to 0 mV. The 5-HT-induced reduction in this S-type K+ (S-K+) current (IKS) is mediated by cAMP (10, 11). Because several "classical" K+ currents were unaffected by 5-HT (11), it was believed that the modulation of IKS probably accounted for spike broadening. However, more recently, Baxter and Byrne (14) discovered that 5-HT also modulates a distinct steeply voltage-dependent K+ current that activates rapidly with membrane depolarization above +20 mV and then inactivates; we have named this current IKv,,,wly to emphasize both its kinetics and steep voltage dependence (15). 5-HT slows both its activation and inactivation and reduces the peak current activated.It has been difficult to evaluate which of the K+ currents reduced by 5-HT is primarily responsible for slowing repolarization of the S...

Facilitation of the monosynaptic connection between siphon sensory neurons and gill and siphon motor neurons contributes to sensitization and dishabituation of the gill and siphon withdrawal reflex in Aplysia. The facilitatory transmitter serotonin (5-HT) initiates two mechanisms that act in parallel to increase transmitter release from siphon sensory neurons. 5-HT acts, at least partly through cAMP, to broaden the presynaptic action potential. 5-IT also initiates a second process that facilitates depressed sensory neuron synapses by a mechanism independent of changes in action potential duration. Recent experiments indicated that either of two protein kinases, cAMP-dependent protein kinase A and protein kinase C, are capable of effectively activating this second facilitatory mechanism, restoring synaptic transmission in depressed synapses. We have used the adenylyl cyclase inhibitor SQ 22,536 [9-(tetrahydro-2-furyl)adenine or THFA] to explore the contribution ofcAMP to the reversal ofsynaptic depression. TIFA effectively inhibited both adenylyl cyclase activity in vitro and known cyclase-mediated effects in intact sensory neurons. THFA also completely blocked facilitation of depressed synapses by 5-4T. These results suggest that adenylyl cyclase plays a critical role in the reversal of synaptic depression that contributes to dishabituation in this system.Since the work of Sharpless and Jasper (1) and Thompson and Spencer (2), neurobiologists, as well as psychologists, have been interested in the relationship between two forms of nonassociative behavioral plasticity: sensitization and dishabituation. The gill and siphon withdrawal reflex of the marine mollusc Aplysia californica undergoes both forms of plasticity. In naive animals, a noxious stimulus, such as a shock to the skin, produces sensitization of the defensive withdrawal reflex, enhancing the withdrawal response elicited by a subsequent siphon touch. The same noxious stimulus can also cause dishabituation, an enhancement of the withdrawal response after it has previously been depressed by repeated siphon stimuli. During both sensitization and dishabituation, there is a parallel enhancement of the synaptic connections between the siphon sensory neurons that initiate the reflex and the postsynaptic motoneurons that produce the reflex; this synaptic facilitation contributes to the behavioral change. Nevertheless, recent behavioral and cellular evidence suggests that these two forms of behavioral enhancement depend, at least partly, upon different cellular mechanisms of synaptic plasticity (3)(4)(5).At the synapse between the siphon sensory neuron and the motoneuron, repeated presynaptic activity results in synaptic depression, which contributes to habituation of the withdrawal response. Noxious stimuli result in facilitation of these same synapses by activating modulatory interneurons that release facilitatory transmitters, including serotonin (5-HT) (6, 7). Klein and Kandel (8) identified one facilitatory mechanism that contributes to the incre...

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Nerve growth factor (NGF) treatment of rat PC12 pheochromocytoma cells results in an increase in the tyrosine phosphorylation of the NGF receptor, TrkA, leading to differentiation to a neuronal phenotype. Dephosphorylation by protein tyrosine phosphatases (PTPases) is thought to play an important role in regulating this signaling pathway. To identify PTPases that are recruited to the activated TrkA receptor, we used an ingel PTPase assay to examine the presence of PTPases in TrkA immunoprecipitates. The Src homology 2 domain containing PTPase SHP‐2 was found to associate transiently with TrkA following receptor activation, reaching a peak after 1 min of NGF treatment and then decreasing rapidly. The association of SHP‐2 with TrkA was accompanied by the tyrosine phosphorylation of SHP‐2 and an association of SHP‐2 with multiple tyrosine‐phosphorylated proteins. In addition, the PTPase activity in SHP‐2 immunoprecipitates increased greater than twofold after 1 min of NGF treatment. This is the first demonstration that the association of SHP‐2 with TrkA is induced by NGF and that this association leads to SHP‐2 activation and tyrosine phosphorylation. We conclude that SHP‐2 plays a significant role in early biochemical events in TrkA‐mediated signal transduction.