Modulation of excitability in Aplysia tail sensory neurons by tyrosine kinases

A hypothesis regarding the molecular mechanism underlying dietary soy-induced effects on seizure propensity

Numerous neurological disorders including fragile X syndrome, Down syndrome, autism, and Alzheimer’s disease are co-morbid with epilepsy. We have observed elevated seizure propensity in mouse models of these disorders dependent on diet. Specifically, soy-based diets exacerbate audiogenic-induced seizures in juvenile mice. We have also found potential associations between the consumption of soy-based infant formula and seizure incidence, epilepsy comorbidity, and autism diagnostic scores in autistic children by retrospective analyses of medical record data. In total, these data suggest that consumption of high levels of soy protein during postnatal development may affect neuronal excitability. Herein, we present our theory regarding the molecular mechanism underlying soy-induced effects on seizure propensity. We hypothesize that soy phytoestrogens interfere with metabotropic glutamate receptor signaling through an estrogen receptor-dependent mechanism, which results in elevated production of key synaptic proteins and decreased seizure threshold.

The tail-elicited tail withdrawal reflex of Aplysia is mediated centrally at tail sensory-motor synapses and exhibits sensitization across multiple temporal domains

The defensive withdrawal reflexes of Aplysia californica have provided powerful behavioral systems for studying the cellular and molecular basis of memory formation. Among these reflexes the tail-elicited tail withdrawal reflex (T-TWR) has been especially useful. In vitro studies examining the monosynaptic circuit for the T-TWR, the tail sensory-motor (SN-MN) synapses, have identified the induction requirements and molecular basis of different temporal phases of synaptic facilitation that underlie sensitization in this system. They have also permitted more recent studies elucidating the role of synaptic and nuclear signaling during synaptic facilitation. Here we report the development of a novel, compartmentalized semi-intact T-TWR preparation that allows examination of the unique contributions of processing in the SN somatic compartment (the pleural ganglion) and the SN-MN synaptic compartment (the pedal ganglion) during the induction of sensitization. Using this preparation we find that the T-TWR is mediated entirely by central connections in the synaptic compartment. Moreover, the reflex is stably expressed for at least 24 h, and can be modified by tail shocks that induce sensitization across multiple temporal domains, as well as direct application of the modulatory neurotransmitter serotonin. This preparation now provides an experimentally powerful system in which to directly examine the unique and combined roles of synaptic and nuclear signaling in different temporal domains of memory formation.

The protective effect of capsaicin receptor-mediated genistein postconditioning on gastric ischemia-reperfusion injury in rats

BACKGROUND

No published study has addressed the effect of genistein postconditioning on gastric ischemia-reperfusion (GI-R) injury in rats.

AIM

To examine whether capsaicin receptor-mediated genistein postconditioning protects against gastric ischemia-reperfusion injury via the PI3K/Akt signal pathway.

METHODS AND RESULTS

Chloraldurat-anesthetized rats underwent occlusion of the celiac artery for 30 min, followed by 60 min of reperfusion. Based on this animal model of gastric ischemia-reperfusion injury, genistein at doses of 100, 500 or 1,000 μg/kg was administered via peripheral vein 5 min before reperfusion. The dose of 500 μg/kg was optimal for postconditioning, at which the severity of I-R-induced gastric injury significantly decreased. Immunohistochemistry also showed that gastric mucosal cell apoptosis decreased. Capsazepine (CPZ), a specific antagonist for the capsaicin receptor, was administered (1,000 μg/kg, i.v.) just before ischemia. Capsaicin (50 mg/kg, s.c.) once a day for 4 days reversed the protective effects of genistein. Reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting showed increased calcitonin gene-related peptide (CGRP) expression in genistein group but not in capsazepine or capsaicin group. CGRP inhibitor CGRP8-37 also prevented the effects of genistein in decreasing gastric mucosal injury index. In addition, PI3K inhibitor LY294002 (1.5 mg/kg) reversed the protective effect of genistein. Compared with genistein group, Western blots also demonstrated decreased Akt phosphorylation in LY294002 group.

CONCLUSION

Our data suggest that capsaicin receptors mediated the protective effects of genistein postconditioning. CGRP secreted by activated capsaicin-sensitive neurons played an important role in the protective effects of genistein. PI3K/Akt pathway was also involved in the protective effects of genistein.

Effects of axotomy on cultured sensory neurons of Aplysia: long-term injury-induced changes in excitability and morphology are mediated by different signaling pathways

To facilitate an understanding of injury-induced changes within the nervous system, we used a single-cell, in vitro model of axonal injury. Sensory neurons were individually dissociated from the CNS of Aplysia and placed into cell culture. The major neurite of some neurons was then transected (axotomized neurons). Axotomy in hemolymph-containing culture medium produced long-term hyperexcitability (LTH-E) and enhanced neuritic sprouting (long-term hypermorphogenesis [LTH-M]). Axotomy in the absence of hemolymph induced LTH-E, but not LTH-M. Hemolymph-derived growth factors may activate tyrosine receptor kinase (Trk) receptors in sensory neurons. To examine this possibility, we treated uninjured (control) and axotomized sensory neurons with K252a, an inhibitor of Trk receptor activity. K252a depressed the excitability of both axotomized and control neurons. K252a also produced a distinct pattern of arborizing outgrowth of neurites in both axotomized and control neurons. Protein kinase C (PKC) is an intracellular signal downstream of Trk; accordingly, we tested the effects of bisindolylmaleimide I (Bis-I), a specific inhibitor of PKC, on the axotomy-induced cellular changes. Bis-I blocked LTH-E, but did not disrupt LTH-M. Finally, because Trk activates the extracellular signal regulated kinase pathway in Aplysia sensory neurons, we examined whether this pathway mediates the injury-induced changes. Sensory neurons were axotomized in the presence of U0126, an inhibitor of mitogen-activated/extracellular receptor-regulated kinase. U0126 blocked the LTH-M due to axotomy, but did not impair LTH-E. Therefore distinct cellular signaling pathways mediate the induction of LTH-E and LTH-M in the sensory neurons.

TRP channels: Targets for the relief of pain

Patients with inflammatory or neuropathic pain experience hypersensitivity to mechanical, thermal and/or chemical stimuli. Given the diverse etiologies and molecular mechanisms of these pain syndromes, an approach to developing successful therapies may be to target ion channels that contribute to the detection of thermal, mechanical and chemical stimuli and promote the sensitization and activation of nociceptors. Transient Receptor Potential (TRP) channels have emerged as a family of evolutionarily conserved ligand-gated ion channels that contribute to the detection of physical stimuli. Six TRPs (TRPV1, TRPV2, TRPV3, TRPV4, TRPM8 and TRPA1) have been shown to be expressed in primary afferent nociceptors, pain sensing neurons, where they act as transducers for thermal, chemical and mechanical stimuli. This short review focuses on their contribution to pain hypersensitivity associated with peripheral inflammatory and neuropathic pain states.

A transient receptor potential vanilloid 4-dependent mechanism of hyperalgesia is engaged by concerted action of inflammatory mediators

The transient receptor potential vanilloid 4 (TRPV4) is a primary afferent transducer that plays a crucial role in neuropathic hyperalgesia for osmotic and mechanical stimuli, as well as in inflammatory mediator-induced hyperalgesia for osmotic stimuli. In view of the clinical importance of mechanical hyperalgesia in inflammatory states, the present study investigated the role of TRPV4 in mechanical hyperalgesia induced by inflammatory mediators and the second-messenger pathways involved. Intradermal injection of either the inflammogen carrageenan or a soup of inflammatory mediators enhanced the nocifensive paw-withdrawal reflex elicited by hypotonic or mechanical stimuli in rat. Spinal administration of TRPV4 antisense oligodeoxynucleotide blocked the enhancement without altering baseline nociceptive threshold. Similarly, in TRPV4(-/-) knock-out mice, inflammatory soup failed to induce any significant mechanical or osmotic hyperalgesia. In vitro investigation showed that inflammatory mediators engage the TRPV4-mediated mechanism of sensitization by direct action on dissociated primary afferent neurons. Additional behavioral observations suggested that multiple mediators are necessary to achieve sufficient activation of the cAMP pathway to engage the TRPV4-dependent mechanism of hyperalgesia. In addition, direct activation of protein kinase A or protein kinase C epsilon, two pathways that mediate inflammation-induced mechanical hyperalgesia, also induced hyperalgesia for both hypotonic and mechanical stimuli that was decreased by TRPV4 antisense and absent in TRPV4(-/-) mice. We conclude that TRPV4 plays a crucial role in the mechanical hyperalgesia that is generated by the concerted action of inflammatory mediators present in inflamed tissues.

Origin and evolution of the Trk family of neurotrophic receptors

Among the numerous tyrosine kinase receptors, those belonging to the Trk family are distinctively involved in the development of complex traits within the vertebrate nervous system. Until recently, the lack of a proper Nt/Trk system in invertebrates has lead to the belief that they were a vertebrate innovation. Recent data, however, have challenged the field, and proved that bona fide Trk receptors do exist in invertebrates. Here, we review and discuss the evolutionary history of the Trk receptor family, and draw a comprehensive scenario that situates the origin of the Nt/Trk signalling prior to the origin of vertebrates. Probably, a ProtoTrk receptor was invented by means of domain and exon shuffling from pieces of ancient genes, generating the unique combination of domains found in extant Trk receptors. It is suggestive to propose that subtle protein mutations, gene duplications, and co-options in particular territories of a primitive Nt/Trk system were instrumental to the development of a complex vertebrate nervous system.

ApTrkl, a Trk-like Receptor, Mediates Serotonin- Dependent ERK Activation and Long-Term Facilitation in Aplysia Sensory Neurons

The Trk family of receptor tyrosine kinases plays a role in synaptic plasticity and in behavioral memory in mammals. Here, we report the discovery of a Trk-like receptor, ApTrkl, in Aplysia. We show that it is expressed in the sensory neurons, the locus for synaptic facilitation, which is a cellular model for memory formation. Serotonin, the facilitatory neurotransmitter, activates ApTrkl, which, in turn, leads to activation of ERK. Finally, inhibiting the activation of ApTrkl with the Trk inhibitor K252a or using dsRNA to inhibit ApTrkl blocks the serotonin-mediated activation of ERK in the cell body, as well as the cell-wide long-term facilitation induced by 5-HT application to the cell body. Thus, transactivation of the receptor tyrosine kinase ApTrkl by serotonin is an essential step in the biochemical events leading to long-term facilitation in Aplysia.

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.

Presynaptic and postsynaptic mechanisms of a novel form of homosynaptic potentiation at aplysia sensory-motor neuron synapses

Previous studies have shown that homosynaptic potentiation produced by rather mild tetanic stimulation (20 Hz, 2 sec) at Aplysia sensory-motor neuron synapses in isolated cell culture involves both presynaptic and postsynaptic Ca2+ (Bao et al., 1997). We have now investigated the sources of Ca2+ and some of its downstream targets. Although the potentiation lasts >30 min, it does not require Ca2+ influx through either NMDA receptor channels or L-type Ca2+ channels. Rather, the potentiation involves metabotropic receptors and intracellular Ca2+ release from both postsynaptic IP3-sensitive and presynaptic ryanodine-sensitive stores. In addition, it involves protein kinases, including both presynaptic and postsynaptic CamKII and probably MAP kinase. Finally, it does not require transsynaptic signaling by nitric oxide but it may involve AMPA receptor insertion. The potentiation, thus, shares components of the mechanisms of post-tetanic potentiation, NMDA- and mGluR-dependent long-term potentiation, and even long-term depression, but is not identical to any of them. These results are consistent with the more general idea that there is a molecular alphabet of basic components that can be combined in various ways to create novel as well as known types of plasticity.

Antagonistic modulation of a hyperpolarization-activated Cl(-) current in Aplysia sensory neurons by SCP(B) and FMRFamide

Whole cell voltage-clamp recordings from Aplysia mechanosensory neurons obtained from the pleural ganglion were used to investigate the actions on membrane currents of the neuropeptides SCP(B) and FMRFamide. At the start of whole cell recording, SCP(B) typically evoked an inward current at a holding potential of -40 mV, due to the cAMP-mediated closure of the S-type K+ channel, whereas FMRFamide evoked an outward current, due to the opening of the S-type K+ channels mediated by 12-lipoxygenase metabolites of arachidonic acid. However, after several minutes of whole cell recording with a high concentration of chloride in the whole cell patch pipette solution, the responses to SCP(B) and FMRF-amide at -40 mV were inverted; SCP(B) evoked an outward current, whereas FMRFamide and YGGFMRFamide evoked inward currents. Ion substitution experiments and reversal potential measurements revealed that these responses were due to the opposing regulation of a Cl(-) current, whose magnitude was greatly enhanced by dialysis with the high Cl(-) - containing pipette solution. SCP(B) inhibited this Cl(-) current through production of cAMP and activation of PKA. YGGFMRFamide activated this Cl(-) current by stimulating a cGMP-activated phosphodiesterase that hydrolyzed cAMP. Thus a cAMP-dependent Cl(-) current undergoes antagonistic modulation by two neuropeptides in Aplysia sensory neurons.

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.

Learning in simple systems

Cellular processes that mediate learning and memory show a remarkable level of conservation between vertebrates and invertebrates. Recent studies have shown that learning and memory formation in invertebrates, so-called 'simple systems', involves a highly complex arrangement of cellular pathways. Some pathways contribute to a single stage of memory formation, whereas others impact on multiple stages of memory development. Distinct cellular pathways may also act in series or in parallel during various stages of memory formation.