The growth cones of Aplysia sensory neurons: Modulation by serotonin of action potential duration and single potassium channel currents

Serotonin (5-HT) closes a specific K channel ("S") in the cell body of Aplysia sensory neurons, resulting in a slow excitatory postsynaptic potential and spike broadening. To determine whether the S channel is present and can be modulated in processes of the neuron other than the cell body, we studied the effects of 5-HT on growth cones of sensory neurons in culture by using the patch-clamp technique. Simultaneous application of 5-HT to the cell body and to the growth cones of sensory neurons produced, in both, a slow depolarization of approximately 5 mV. Also, 5-HT produced a lengthening of the duration of action potential in the growth cone and cell body by 20-30%. Similar effects were observed in isolated growth cones that had been severed from the rest of the neuron, implying that the growth cones contain all the molecular components (i.e., receptors, channels, cAMP cascade) necessary for 5-HT action. Cell-attached patch-clamp recordings demonstrated the presence of S channels in sensory neuron growth cones. Application of serotonin to the bath produced long-lasting all-or-none closures of these channels in a manner identical to the previously characterized action of 5-HT in the cell body. Thus, channel modulation is not restricted to the cell body and probably occurs throughout the sensory neuron. This strengthens the view that S-channel modulation may also occur at the sensory neuron presynaptic terminal, where it could play a role in the presynaptic facilitation produced by 5-HT.

Dual MAP kinase pathways mediate opposing forms of long-term plasticity at CA3-CA1 synapses

Although the function of the p42/p44 mitogen-activated protein (MAP) kinase pathway in long-term potentiation at hippocampal CA3-CA1 synapses has been well described, relatively little is known about the importance of the p38 MAP kinase pathway in synaptic plasticity. Here we show that the p38 MAP kinase pathway, a parallel signaling cascade activated by distinct upstream kinases, mediates the induction of metabotropic glutamate receptor-dependent long-term depression at CA3-CA1 synapses. Thus, two parallel MAP kinase pathways contribute to opposing forms of long-term plasticity at a central synapse.

Potent antihyperalgesic activity without tolerance produced by glycine site antagonist of N-methyl-D-aspartate receptor GV196771A

Central sensitization is a condition of enhanced excitability of spinal cord neurons that contributes to the exaggerated pain sensation associated with chronic tissue or nerve injury. N-methyl-D-aspartate (NMDA) receptors are thought to play a key role in central sensitization. We have tested this hypothesis by characterizing in vitro and in vivo a novel antagonist of the NMDA receptor acting on its glycine site, GV196771A. GV196771A exhibited an elevated affinity for the NMDA glycine binding site in rat cerebral cortex membranes (pKi = 7.56). Moreover, GV196771A competitively and potently antagonized the activation of NMDA receptors produced by glycine in the presence of NMDA in primary cultures of cortical, spinal, and hippocampal neurons (pKB = 7.46, 8. 04, and 7.86, respectively). In isolated baby rat spinal cords, 10 microM GV196771A depressed wind-up, an electrical correlate of central sensitization. The antihyperalgesic properties of GV196771A were studied in a model of chronic constriction injury (CCI) of the rat sciatic nerve and in the mice formalin test. In the CCI model GV196771A (3 mg/kg twice a day p.o.), administered before and then for 10 days after nerve ligature, blocked the development of thermal hyperalgesia. Moreover, GV196771A (1-10 mg/kg p.o.) reversed the hyperalgesia when tested after the establishment of the CCI-induced hyperalgesia. In the formalin test GV196771A (0.1-10 mg/kg p.o.) dose-dependently reduced the duration of the licking time of the late phase. These antihyperalgesic properties were not accompanied by development of tolerance. These observations strengthen the view that NMDA receptors play a key role in the events underlying plastic phenomena, including hyperalgesia. Moreover, antagonists of the NMDA glycine site receptor could represent a new analgesic class, effective in conditions not sensitive to classical opioids.

The mitogen-activated protein kinase p38-2 is necessary for the inhibition of N-type calcium current by bradykinin

Calcium influx via voltage-dependent calcium channels (ICa,V) links depolarization of excitable cells to critical cellular processes, such as secretion, contraction, and gene transcription. Fast regulation of ICa,V (<1 sec) by G-protein-coupled receptors is a relatively well-defined mechanism, whereas slow (30-60 sec) actions of transmitters and hormones on the same current remain poorly understood. In NG108-15 cells, the kinetically slow inhibition of N-type ICa,V by bradykinin (BK) requires the sequential activation of two G-proteins, heterotrimeric G13 and monomeric Rac1/Cdc42. We have now defined a role in this pathway for the relatively fast-acting p38 mitogen-activated protein kinase (MAPK). The slow inhibition of ICa,V by BK was suppressed specifically by SB203580, a compound that inhibits the p38 family of MAPKs. BK potently and selectively activated a newly discovered p38 family member, p38-2. These data provide the first evidence that a MAPK is involved in the regulation of ICa,V by a receptor-mediated process.

The monomeric G-proteins Rac1 and/or Cdc42 are required for the inhibition of voltage-dependent calcium current by bradykinin

Although regulation of voltage-dependent calcium current (ICa,V) by neurotransmitters is a ubiquitous mechanism among nerve cells, the signaling pathways involved are not well understood. We have determined previously that in a neuroblastoma-glioma hybrid cell line (NG108-15), the heterotrimeric G-protein G13 mediates the inhibition of ICa,V produced by bradykinin (BK) via an unknown mechanism. Various reports indicate that G13 can couple to RhoA, Rac1, and Cdc42, which are closely related members of the Rho family of monomeric G-proteins. We have investigated their role as signaling intermediates in the pathway used by BK to inhibit ICa,V. Using immunoblot analysis and the PCR, we found evidence that RhoA, Rac1, and Cdc42 all are expressed in NG108-15 cells. Intracellularly perfused recombinant Rho-GDI (an inhibitor of guanine nucleotide exchange specific for the Rho family) attenuated the inhibition of ICa,V by BK. These findings indicate that activation of RhoA, Rac1, or Cdc42 may be required for the response to BK. To determine whether any of these monomeric G-proteins mediate the response to BK, we have intracellularly applied blocking antibodies specific for each of the candidate proteins. Only the anti-Rac1 antibody blocked the response to BK. In parallel experiments, peptides corresponding to the C-terminal regions of Rac1 and Cdc42 blocked the same response. These data indicate a novel functional contribution of Rac1 and possibly also of Cdc42 to the inhibition of ICa,V by neurotransmitters.

Three distinct G protein pathways mediate inhibition of neuronal calcium current by bradykinin

1. In NG108-15 cells dialyzed with 10 mM ethylene glycolbis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) or bis (o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA), bradykinin (BK) selectively inhibited the N-type calcium current. This effect of BK was blocked by an antibody directed against the G protein G13. Thus under these conditions G13 mediates the inhibition of voltage-dependent calcium current (ICa, V) by BK. In contrast, activation of K+ currents by BK is mediated by Gq/11. BK also couples to Gi2. 2. We now examine the involvement of G proteins in the inhibition of ICa, V by BK when NG108-15 cells are dialyzed with 1 mM BAPTA. Under these conditions, BK inhibited both the N- and L-type, but not the T-type, calcium currents. Intracellular application of anti-G13 antibody did not suppress the response to BK. Applications of either anti-Gq/11 antibody or pertussis toxin (PTX, to block Gi2) were similarly ineffective. Even combined application of anti-Gq/11 and -G13 antibodies, or PTX together with either antibody, did not block inhibition of ICa, V by BK. However, the combination of both antibodies with PTX blocked the response to BK in low BAPTA. In conclusion, both Gq/11 and a PTX-sensitive G protein (presumably Gi2), together with G13, are involved in the inhibition of ICa, V by BK. 3. Gq/11 inhibited only the L-type calcium current, whereas the PTX-sensitive G protein inhibited both the N- and L-type calcium currents. 4. The BAPTA dependence of the Gq/11 and PTX-sensitive inhibitions may reflect a Ca2+ requirement of the pathway(s) acting on the L current and/or a direct suppressive effect of BAPTA.

The G protein G13 mediates inhibition of voltage-dependent calcium current by bradykinin

In neuroblastoma-glioma hybrid cells, bradykinin has dual modulatory effects on ion channels: it activates a K+ current as well as inhibits the voltage-dependent Ca2+ current (ICa,V). Both of these actions are mediated by pertussis toxin-insensitive G proteins. Antibodies raised against the homologous Gq and G11 proteins suppress only the activation of the K+ current; this suggested that at least two distinct G protein pathways transduce diverse effects of this transmitter. Here, we show that the inhibition of ICa,V by bradykinin is suppressed selectively by intracellular application of antibodies specific for G13. This novel G protein may play a general role in the inhibition of ICa,V by pathways resistant to pertussis toxin.

Bradykinin modulates potassium and calcium currents in neuroblastoma hybrid cells via different pertussis toxin-insensitive pathways

In NG108-15 cells, bradykinin (BK) activates a potassium current (IK,BK) and inhibits the voltage-dependent calcium current (ICa,V). BK also stimulates a phosphatidylinositol-specific phospholipase C (PI-PLC). The subsequent release of inositol 1,4,5-trisphosphate and increase in intracellular calcium contribute to IK,BK, through activation of a calcium-dependent potassium current. In membranes from these cells, stimulation of PI-PLC by BK is mediated by Gq and/or G11, two homologous, pertussis toxin-insensitive G proteins. Here, we have investigated the role of Gq/11 in the electrical responses to BK. GTP gamma S mimicked and occluded both actions of BK, and both effects were insensitive to pertussis toxin. Perfusion of an anti-Gq/11 alpha antibody into the pipette suppressed IK,BK, but not the inhibition of ICa,V by BK. Thus, BK couples to IK,BK via Gq/11, but coupling to ICa,V is most likely via a different, pertussis toxin-insensitive G protein.

Inhibition of the ω-conotoxin-sensitive calcium current by distinct G proteins

Leu-enkephalin (Leu-Enk), norepinephrine (NE), somatostatin (SS), and bradykinin (BK) decrease the voltage-dependent calcium current in NG108-15 cells. Here we have investigated whether distinct G proteins, or a G protein common to all of the pathways, mediates this inhibition. We found that pertussis toxin (PTX) reduced all of these transmitter actions, except that of BK. To examine which of the PTX-sensitive pathways is transduced by GoA, we constructed an NG108-15 cell line that stably expresses a mutant, PTX-resistant alpha subunit of GoA. After treatment with PTX, the mutant GoA alpha rescued the Leu-Enk and NE pathways but not the SS pathway. At least three different G proteins can transduce receptor-mediated inhibition of calcium currents in nerve cells. The effects of these G proteins appear to converge on the omega-conotoxin GVIA-sensitive calcium current.

Serotonin inhibits the peptide FMRFamide response through a cyclic AMP-independent pathway in Aplysia

1. The S-K+ conductance was isolated by voltage-clamping near the resting potential pleural mechanosensory neurons of Aplysia in culture. This background conductance is modulated in opposite directions by two distinct, transmitter-controlled second-messenger cascades: it is enhanced by the peptide FMRFamide through the 12-lipoxygenase pathway of arachidonic acid, and it is decreased by serotonin (5-HT) through adenosine 3',5'-cyclic monophosphate (cAMP)-dependent phosphorylation. 2. The dose-dependent activating effect of FMRFamide (0.01-500 microM) on the S-K+ conductance was measured in the presence and the absence either of 1-100 microM 8-bromo-cAMP (8b-cAMP, a membrane-permeable and hydrolysis-resistant analogue of cAMP), or of 0.01-0.1 microM 5-HT. 3. When 8b-cAMP was applied, it produced a slow inward current response due to closure of the S-K+ conductance. This response was antagonized by FMRFamide in a dose-dependent mode. Application of 100 microM FMRFamide, in the presence of 1-10 microM 8b-cAMP, produced an outward current response larger than the control FMRFa response and equal to the sum of the responses to FMRFamide alone and to 8b-cAMP alone. Similarly, at 500 microM, FMRFamide completely antagonized the closing action of maximal 8b-cAMP levels (100 microM). This observation confirms previous work that indicated that FMRFamide can reopen S-K+ channels closed by FMRFamide. 4. In contrast, in the presence of moderate concentrations of 5-HT (0.01 microM), which produce a slow inward current due to the closing of the S-K+ conductance, FMRFamide elicited a response that only partially antagonized this 5-HT action. Under maximal 5-HT concentrations (0.1 microM), the 5-HT response was not antagonized by any FMRFamide concentration: instead, the FMRFamide response was smaller than the control response without 5-HT. This experiment suggests that 5-HT, with an action independent from cAMP, inhibits the effect of FMRFamide on the S-K+ channel. 5. The dose-dependent inhibitory effect of 5-HT (0.001-10 microM) on the S-K+ conductance was measured in the presence and the absence either of FMRFamide (1-50 microM), which stimulates the release and metabolism of arachidonic acid in Aplysia sensory neurons or of arachidonic acid (25 microM). 6. Under these conditions, supramaximal concentrations of 5-HT could not completely suppress the slow outward current evoked by FMRFamide or by arachidonic acid, indicating that a component of the arachidonic-mediated response to FMRFamide is resistant to actions that maximally increase the S-K+ channel phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)

Products of heme-catalyzed transformation of the arachidonate derivative 12-HPETE open S-type K+ channels in Aplysia

In Aplysia mechanosensory neurons, the neuropeptide FMRFamide increases the opening of the background S-K+ channel. This action is mediated by activation of arachidonic acid metabolism. Arachidonic acid in Aplysia nervous tissue is transformed through the 12-lipoxygenase pathway to 12-HPETE, which undergoes further metabolism. In intact sensory cells, 12-HPETE simulates the FMRFamide response, raising the question of whether 12-HPETE is the messenger molecule ultimately acting on the S-K+ channel. Here we show that in cell-free (inside-out) patches from sensory cells, 12-HPETE fails to modulate the S-K+ channel, but in the presence of hematin (which catalyzes 12-HPETE metabolism), it triggers sharp increases in the channel opening probability. We also found that SKF-525A, an inhibitor of the cytochrome P450, reduces the response to FMRFamide, arachidonic acid, and 12-HPETE in intact cells. We conclude that a heme-catalyzed transformation of 12-HPETE is necessary and sufficient to promote the opening of the S-K+ channel and a heme-containing enzyme such as cytochrome P450 might play this key role.

Lipoxygenase metabolites of arachidonic acid as second messengers for presynaptic inhibition of Aplysia sensory cells

Biochemical and biophysical studies on Aplysia sensory neurons indicate that inhibitory responses to the molluscan peptide FMRFamide are mediated by lipoxygenase metabolites of arachidonic acid. These compounds are a new class of second messengers in neurons.

Neuronal inhibition by the peptide FMRFamide involves opening of S K+ channels

Neurotransmitters modulate the activity of ion channels through a variety of second messengers, including cyclic AMP, cyclic GMP and the products of phosphatidylinositol breakdown. Little is known about how different transmitters acting through different second-messenger systems interact within a cell to regulate single ion channels. We here describe the reciprocal actions of serotonin and the molluscan neuropeptide, FMRFamide, on individual K+ channels in Aplysia sensory neurons. In these cells, serotonin causes prolonged all-or-none closure of a class of background conductance K+ channels (the S channels) through cAMP-dependent protein phosphorylation. Using single-channel recording, we have found that FMRFamide produces two actions on the S channels; it increases the probability of opening of the S channels via a cAMP-independent second-messenger system and it reverses the closures of S channels produced by serotonin or cAMP.

Modulation of the serotonin-sensitive potassium channel in Aplysia sensory neurone cell body and growth cone

Using single-channel recording, we have been able to obtain some insight into the molecular mechanism of a modulatory transmitter action in Aplysia sensory neurones. Our results show that serotonin produces a slow EPSP and increases action potential duration in the sensory neurones by producing prolonged closures of the S potassium channel. Such closures appear to be mediated by cyclic AMP-dependent phosphorylation of a membrane protein which may be the channel. Modulation of S channels by serotonin also occurs in sensory neurone growth cones. This provides the first direct evidence that channel modulation occurs in nerve processes and increases the likelihood of channel modulation at the nerve terminal.

Action potentials, macroscopic and single channel currents recorded from growth cones of Aplysia neurones in culture

Action potentials, macroscopic ionic currents and single channel currents were recorded from growth cones of Aplysia right upper quadrant (r.u.q.) cells in culture, using the patch-clamp technique. Recordings were obtained from both intact growth cones and from growth cones that had been mechanically isolated from the rest of the neurone. In current-clamp mode, greater than half of the isolated growth cones display an all-or-none action potential when depolarized above 0 mV with outward current pulses. The remaining growth cones display only a graded depolarization that is unaffected by tetrodotoxin (TTX). In whole-cell voltage clamp almost all isolated growth cones display a rapidly activating and inactivating inward current followed by a delayed outward current in response to depolarizations positive to -20 mV. The rapid inward current reverses direction at around +70 to +80 mV and is completely suppressed by 100 microM-TTX, which suggests that this current is carried by the fast Hodgkin-Huxley sodium current channels. The delayed outward current appears to result from the activation of both the delayed rectifier potassium current, IK, and the calcium-activated potassium current, IC. The growth cones do not display any prominent early transient outward current, IA. The sodium current, INA, was studied in isolation by substituting caesium for potassium ions in the pipette solution. INa is half-inactivated at a holding potential of -36 mV, reaches half-maximal activation with a depolarization to 0 mV, and has a mean peak current density of 13 microA/cm2. The time course of inactivation is well described by a single exponential (tau = 3 ms at 0 mV). In cell-attached patches, a rapidly activating and inactivating inward current channel was recorded with an average unit conductance of 6.9 pS. The activation and inactivation parameters of the ensemble averaged current closely match the measured values from the macroscopic sodium current. At very positive potentials we recorded a voltage-dependent outward current channel with a conductance of around 35 pS. No significant inward calcium current was observed in whole-cell measurements and few single calcium channel currents were measured in cell-attached patches, suggesting a sparse distribution of calcium channels in the r.u.q. growth cones.

Serotonin and Retzius cell depress the hyperpolarization following impulses of leech touch cell

Intracellular recordings from T mechanosensory cells of Hirudo medicinalis showed, as previously demonstrated, that repetitive firing is followed by a long-lasting hyperpolarization. Serotonin application at two concentrations (1 microM and 50 microM) depressed this hyperpolarization by up to 2/3; the effect was dose-dependent, long-lasting and reversible. Intracellular stimulation of giant serotonergic neurons (Retzius cells, Rz) mimicked serotonin perfusion: the effect was proportional to the number of spikes fired by Retzius cells. The combined use of intracellular iontophoretic injection of horseradish peroxidase and lucifer yellow indicated the possible sites of contact between Rz and T cells. The effect of serotonin, released by Rz cells, is discussed with respect to its possible physiological significance.

Heterosynaptic facilitation and behavioral sensitization are inhibited by lowering endogenous cAMP in Aplysia

An adenylate cyclase inhibitor, RMI 12330A, is able to depress cAMP synthesis stimulated by serotonin in the abdominal ganglion of Aplysia depilans and punctata. This substance reversibly blocked the heterosynaptic facilitation, induced by activation of serotonergic pathways, of the EPSP recorded from L7 motoneuron in abdominal ganglion after electrical stimulation of the siphon nerve. RMI 12330A, injected into whole unrestrained animals, inhibited the short-term dishabituation of the siphon withdrawal reflex. These findings demonstrate that the increase of endogenous cAMP in the sensory neurons mediating the gill and siphon withdrawal reflex is an essential step in the mechanism of potentiation of the transmitter output underlying heterosynaptic facilitation and short-term behavioral sensitization.

Role of serotonin and cyclic AMP on facilitation of the fast conducting system activity in the leech Hirudo medicinalis

In the nervous system of the leech Hirudo medicinalis it has been possible to study short-term plastic changes. Depression and facilitation have been demonstrated in the fast conducting system (FCS) activity; this pathway consists of a chain of electrically linked neurons present in each ganglion. In semi-intact animals or in preparation of nerve cord and segments of body wall, both electrical stimulation of peripheral roots and tactile stimulation of the skin induced, after repetitive stimulation (0.1/s) a prolonged decrement of FCS response. Strong nociceptive stimulation applied onto the head or the body wall produced a sustained facilitation of the waned response. The same potentiation has been observed by perfusing the isolated ganglion with serotonin (5 x 10(-5) M). Such a potentiation is abolished by preincubation with methysergide, an antagonist of serotonin, and with imidazole, a cAMP-phosphodiesterase activator. Such an effect is mimicked by an analog of cAMP, db-cAMP. Simultaneous recordings of both T neurons (intracellularly) and FCS firing discharge showed that, during FCS response decrement, the T cell activity remained unchanged and no modification of conductance occurred, excluding therefore a detectable involvement of sensory neurons in the depression. These results suggest that short-term plastic changes of the FCS of the leech are due to a prolonged potentiation of synaptic transmission as a result of serotonin-mediated increase in cAMP.

[Preliminary studies on the characterization of adenylate cyclase from segmental ganglia of the leech Hirudo medicinalis]

Adenylate cyclase activity has been measured in segmental ganglia of the leech Hirudo m. Basal enzymatic activity was stimulated in a dose dependent fashion by serotonin. GTP is required in order to evoke maximal stimulation. The effect was blocked by methysergide, an antagonist of serotonin at receptor level. The stimulation of the enzyme was prevented by alpha beta-methy-len-ATP. In addition RMI 12330A inhibited basal as well as serotonin-stimulated adenylate cyclase activity presumably by acting at the catalytic subunit level.