TY - JOUR
AU - Koh, Hae-Young
AB - Abstract Nitric oxide serves as an orthograde synaptic cotransmitter between identified neurons in the cerebral ganglion of Aplysia. Nitric oxide synthase, the enzyme that produces nitric oxide, is localized in a few specific neurons in the ganglia, including neuron C2. Guanylyl cyclase the target enzyme of nitric oxide, is found in neurons C4 and MCC, which are synaptic followers of C2. Stimulation of C2 causes a vsEPSP in these neurons that is reduced to 50% of its amplitude by nitric oxide synthase inhibitors and guanylyl cyclase inhibitors. The remaining portion of the vsEPSP is mediated by histamine. Thus, nitric oxide and histamine act as orthograde cotransmitters in producing the vsEPSP. Both cotransmitters cause closure of a background potassium channel, which depolarizes the neuron and enhances its response to synaptic inputs. Exogenous nitric oxide (released by nitric oxide donor molecules) and histamine mimic the vsEPSP's depolarization and decreased membrane conductance. When neurons C4 or MCC are isolated in cell culture they respond just as they do in the ganglion, i.e., the nitric oxide response but not the histamine response is blocked by guanylyl cyclase inhibitors, and the membrane conductance is decreased by both histamine and nitric oxide. Aplysia hemolymph partially suppresses the response to nitric oxide, due to nitric oxide scavenging by hemocyanin, which contains copper and is the equivalent of hemoglobin. Neuron C2 followers that are hyperpolarized by histamine are insensitive to nitric oxide. Thus, only select follower neurons respond to both transmitters. INTRODUCTION Evidence that nitric oxide (NO) is a neurotransmitter and neuromodulator continues to accumulate at a rapid pace. Its role as an orthograde transmitter is established in the mammalian PNS (review, Rand and Li, 1995) and its role as a retrograde transmitter at hippocampal synapses, in relation to long term potentiation is well known (review, Schuman and Madison, 1994). NO serves as both an orthograde and retrograde transmitter in the retina and central projection of the vertebrate visual system (review, Cudeiro and Rivadulla, 1999). Much of the direct evidence for its orthograde role in the CNS has come from studies on identified neurons of invertebrate nervous systems (review, Jacklet, 1997). NO's use as an orthograde cotransmitter with histamine (HA) was first shown for slow depolarizing synaptic potentials between identified neuron C2 (cerebral neuron 2) and its synaptic follower neurons C4 (cerebral neuron 4) and MCC (metacerebral cell) in the CNS of Aplysia (Jacklet, 1995). NO also mediates a slow depolarizing synaptic potential between identified neurons B2 and B7nor in the snail Lymnaea (Park et al., 1998). When those neurons were placed together in cell culture the synaptic interaction was reestablished, but before synaptic contact was made between the neurons direct depolarization of B2 released NO, which depolarized neuron B7nor. Depolarization was greater for short distances but was detected when the neurons were up to 50 μm apart. This demonstrates NO ability to perform both synaptic transmitter functions and non-synaptic modulatory actions at a distance. Neuron C2 in Aplysia contains HA and makes synaptic connections with many E-cluster neurons in the cerebral ganglion (Chiel et al., 1986; McCaman and Weinreich, 1982, 1985; Weinreich, 1977) and MCC (Weiss et al., 1986) Stimulation of C2 evokes a variety of synaptic response types in follower neurons, including: fast, slow and very slow EPSPs and fast and slow IPSPs. They were thought to be mediated by HA alone, but now there is evidence (Jacklet, 1995) that the very slow EPSPs (vsEPSP) in neurons C4 and MCC are mediated in part by NO. C2 contains nitric oxide synthase (NOS), the enzyme that produces NO, and NOS inhibitors and NO scavengers block the vsEPSP. NO-donors mimic the membrane depolarization and decreased membrane conductance characteristic of the vsEPSP (Jacklet, 1995). In addition, MCC shows NO induced, but not HA induced, cGMP immunoreactivity (IR) and 8-Br-cGMP mimics the vsEPSP membrane depolarization and decreased conductance (Koh and Jacklet, 1999). The cGMP-IR and the effects of the application of NO-donors, but not HA, are blocked by soluble guanylyl cyclase (sGC) inhibitors (Koh and Jacklet, 1999). Thus, NO released by C2 stimulates sGC in follower neurons. The cGMP decreases the membrane conductance and the neurons depolarize. This paper reviews this evidence, and shows that isolated neurons C4 and MCC in culture respond to NO and HA as they do in the ganglion. MATERIALS AND METHODS Aplysia californica (100–150g) were supplied by the Aplysia Resource Center (Miami, FL) and kept in an aquarium at 16–18°C. Cerebral ganglia were dissected from the animal and incubated in 1% protease (type IX, Sigma, P-6141) in 2 ml ASW (artificial sea water: 460 mM NaCl, 10 mM KCl, 10 mM CaCl2, 48 mM MgCl2, 10 mM HEPES Sigma, H-3375, pH 7.8) for 1.7 hr at 35°C before desheathing at room temperature. Neurons (MCC and E-cluster neurons C2, C4, C6 and others) were pulled from the ganglion using firepolished pipettes and plated in polylysine coated culture dishes containing 50% Aplysia culture medium (L15 solution made up in ASW) previously described (Jacklet et al., 1996) and 50% filtered (0.22 μm) hemolymph. Recording were made from cultured neurons from more than 50 cerebral ganglia and during recording cultured neurons were superfused with ASW containing SNC (S-nitroscysteine) or HA (flow rate 2.5–3 ml/min), or drugs were microfocally applied by pressure ejection (General Valve, Picospritzer) from a pipette. SNC stock solution (100 mM) was made (Lei et al., 1992) by dissolving 100 mM L-cysteine (Sigma, C-7755) and 100 mM sodium nitrite (Aldrich, 23721-3) in 959 μl distilled water on ice, and adding 41 μl HCl (12.1 N, Fisher), which makes the solution turn red. The stock solution was kept on ice and used within 2 hr. Histamine (hydrochloric form, Sigma, H-7250) in artificial sea water (ASW) was freshly made for each experiment and kept at 4°C. ODQ (lH-[1,2,4] oxadiazolo [4,3-a] quinoxaline-l-one) was purchased from Tocris Cookson. Micropipettes (10–20 MΩ) were filled with 3 M KAc/l M KCl, and connected to an NPI electronics, model SEC-10, amplifier, used in the bridge, current clamp or single electrode voltage clamp mode. Input resistance was measured as the voltage deflection induced by a −0.5 or −1.0 nA current pulse. Voltage clamping was performed at 30 kHz switching, duty cycle ½. Data were sampled at 1 kHz by computer. RESULTS The location of NOS-containing neurons and neurons showing NOinduced cGMP-IR in the cerebral ganglion are shown in Figure 1. NOS containing neurons in the central ganglia of Aplysia have been identified using NADPH-diaphorase histochemistry (Jacklet and Gruhn, 1994a, b). About 30 bilaterally represented pairs of stained neurons were found in the cerebral ganglion as well as distinctive staining of specific fiber tracts and synaptic glomeruli. Four pairs of the most prominent NADPH-diaphorase stained neurons are shown in Figure 1, including neuron C2, a small neighbor C2n, and an L cluster neuron C3n, that is either C3 or a neighbor. C3, like C2, contains a high level of HA (Ono and McCaman, 1980). Figure 1 also shows MCC and clusters of smaller, unnamed cerebral neurons that respond to NO by induced cGMP-IR (Koh and Jacklet, 1999). These neurons contain sGC, the enzyme that produces cGMP and the classical NO effector molecule (Garthwaite, 1991). The vsEPSP evoked in C4 or MCC by stimulation of C2 has two components, an initial rapid small depolarization and a very slow longer-lasting depolarization, described in detail earlier (Chiel et al., 1986; McCaman and Weinreich, 1985). These synaptic interactions between neuron C2 and its E-cluster followers occurs within the neuropil of the E-cluster. Synaptic interaction between C2 and MCC occur near the E-cluster where the axon of MCC enters the cerebral-buccal connective. We have shown that the vsEPSP is mediated by both HA and NO (Koh and Jacklet, 1999) in about equal amounts. An example of this is seen in Figure 2, where the sGC inhibitor ODQ has partially suppressed the vsEPSP evoked by stimulation of C2. Note the characteristic very slow rate of EPSP rise and the prolonged depolarization following the cessation of C2 activity. Cerebral ganglion neurons isolated in culture To determine if neuron C2 followers respond directly to NO and HA they were isolated in cell culture. MCC and neurons of the E-cluster including C4, C6 and other C2 synaptic followers were identified by size, position and characteristic membrane activity were isolated (Fig. 3) and tested for responses to the NO-donor SNC and HA. Culture medium is composed of 50% Aplysia hemolymph, which is added to induce neurite growth, as well as L15 medium made up in ASW. In many cases about 1 mm of MCC's axon attached to the soma was obtained during isolation, and sometimes the bifurcation of the MCC axon was visible in culture (Fig. 3). Typically the axon retracts to one half or less of its length during the first day, before it stabilizes as shown in Figure 3 and then neurites begin to grow from the cut axon stump or other parts of the neuron. The soma of MCC is usually heavily invested with glial cells about 10 μm in diameter, while E-cluster neurons such as C4 are relatively free of glia. All neurons readily sprouted neuritic processes if hemolymph was added to the culture medium. Membrane responses of isolated neurons to NO and HA Isolated neurons responded to the NO-donor SNC and HA in essentially the same way as they did when they were in the ganglion. They have large stable resting potentials and can produce large (100 mV) overshooting action potentials. MCC typically had a −70 to −77 mV resting potential and 8–10 MΩ input resistance. The sGC inhibitor, ODQ, blocked the NO mediated depolarization in isolated neurons (Fig. 4), just as it did in the ganglion. Prior ODQ treatment for 1 hr was sufficient to block the response to bath applied SNC. Thus the isolated neurons have retained their normal sGC-cGMP second messenger pathway and NO responsiveness. ODQ did not block the HA induced depolarization and increased resistance (data not shown). A brief puff of SNC delivered by the picospritzer pipette depolarized MCCs membrane potential and increased the membrane resistance, as shown in Figure 5. The membrane potential shifted to a plateau potential near −55 mV and the neuron produced a continuous train of action potentials. Activity decreased with time and the membrane potential shifted back to the more negative, low resistance, membrane state as the NO diffused away or was scavenged. The membrane appears to have 2 quasi-stable states. This was most obvious when the resting potential was −75 mV or more negative and the SNC concentration was high enough to produce a rapid shift to the upper level. Note in Figure 5 that the membrane potential shifts to the upper level and remains there for about 10 msec before the first action potential is evoked. HA treatment evoked the same shift in Vm (data not shown). Standard puffs of either SNC or HA applied by the Picospritzer at 1 mM and 30 psi induced neuronal response if the pipette tip was less than 200 μm from the neuron, and puffs at 10 μm gave brisk responses, as expected. If the NO response is caused by closure of a potassium channel it should be null at the potassium equilibrium potential. To test this a puff of SNC was applied to the neuron at successively more negative membrane potentials. The SNC response diminished as the membrane was hyperpolarized as shown in Figure 6, but it did not reverse. A similar result was obtained by others for the HA response (Weiss et al., 1986). One expects the response to be null at the potassium equilibrium potential, near −85 mV, and to reverse at more negative potentials. Nevertheless, the decline in amplitude with hyperpolarization is evidence that the response is mediated by closure of a potassium channel. Voltage clamping the membrane with a single electrode switching amplifier (NPI) demonstrated the conductance changes associated with SNC (Fig. 7). In ASW, before SNC treatment successive hyperpolarizing voltage steps from the holding potential of −60 mV produced an increase in the membrane conductance. SNC treatment reduced the conductance for each voltage step, but the increase in conductance with each step was still evident. Complete recovery was observed after ASW wash. Cultured neurons were routinely tested for SNC sensitivity before the culture medium was replaced with ASW by superfusion. There was consistently greater SNC sensitivity in ASW than there was in culture medium, which is 50% hemolymph and 50% Aplysia L15 solution. Aplysia culture medium contains Ll5 amino acids (including cysteine), vitamins and gentamicin made up in ASW. Hemolymph was of particular interest because it bathes the tissues of the animal, including the nervous system, and it is rich in hemocyanin, a copper containing protein, that can act as a scavenger of NO (Solomon, 1981). Aplysia culture medium alone were observed to suppress the SNC effect. Fifth percent hemolymph-50% ASW was tested and it suppressed the SNC sensitivity substantially, as shown in Figure 8. To control for the sequence of testing, the neuron was first tested in 50% hemolymph-ASW, switched to ASW and tested, and then switched back to 50% hemolymph-ASW and tested again. Hemolymph consistently suppressed the response. In specific tests using the number of action potentials evoked by a standard SNC puff, the response in ASW was 2.2 times (n = 3) the response in hemolymph. Other E- cluster neurons such as C6 that receive an IPSP input from C2 and are known to be hyperpolarized by HA (Chiel et al., 1986; McCaman and Weinreich, 1982) were tested with HA and SNC. Figure 9 shows that a neuron whose membrane potential was strongly hyperpolarized by HA and accompanied by a decrease in membrane resistance was only weakly depolarized by SNC, even at ten times the puff pulse duration that would have induced a robust response in either neuron C4 or MCC. All 3 E-cluster neurons tested that responded to HA with hyperpolarization and a decrease in membrane resistance gave null or very weak depolarizing responses to SNC. DISCUSSION Cultured neurons C4 and MCC retained their sensitivity to HA and SNC, suggesting that their responses in the ganglion are largely direct. Since ODQ, the potent sCG inhibitor (Garthwaite et al., 1995) blocked the SNC response completely, it appears that the NO-sGC-cGMP pathway leading to potassium channel closure is active in these cultured neurons. MCC shows strong cGMP-IR in the ganglion and it reaches a peak in 2–5 min (Koh and Jacklet, 1999), but neuron C4 cGMP-IR is faint or undetectable under the same condition. However, the vsEPSPs in both C4 and MCC evoked by C2 stimulation are reduced to 50% by sGC inhibitors ODQ and LY83583, and the SNC response in both neurons is completely blocked by the sGC inhibitors (Koh and Jacklet, 1999; Koh, unpublished results). The reason for the weak cGMP-IR in C4 is unclear, but C4 may have a strong cGMP quenching mechanism involving phosphodiesterase activity or sGC desensitization. Bellamy et al. (2000) have shown that NO-stimulated sGC activity in cerebellar cells is degraded swiftly, likely by a combination of these mechanisms. The SNC and HA responses are membrane voltage dependent, decreasing in size with hyperpolarization. These responses are minimal but not null at −85 mV the potassium equilibrium potential in these neurons (McCaman and Weinreich, 1982). The vsEPSP does not reverse at this potential (Weiss et al., 1986). In voltage clamp studies of MCC in the ganglion Weiss et al. (1986) found that histamine induced current reached a null but did not reverse. In high external potassium the current did reverse at the expected potassium equilibrium potential but it did not follow the expected ohmic relations. The possible reasons for non-reversal of the decreasing conductance vsEPSP have been discussed at length by others (Weiss et al., 1986), but no clear explanation has emerged. The increasing potassium conductance IPSP and membrane response induced by HA in other E-cluster neurons (e.g., C6) does reverse at −85 mV as expected (McCaman and Weinreich, 1982). Since NO produces much the same membrane effects as HA in MCC, it appears that the primary membrane conductance effect of NO is on a potassium channel. When an isolated MCC was hyperpolarized from the holding potential of −60 mV under voltage clamp the membrane conductance increased. This property of MCC has also been observed in the intact ganglion (Weiss et al., 1986). It is a reflection of the pronounced inward rectification of the membrane at potential more negative than −55 mV (Weiss et al., 1986). This produces a bistability of the membrane potential, which we observed in isolated neurons MCC and C4. The neurons had preferred stable levels of membrane potential at about −55 mV with a slow rate of spiking and a high conductance level at about −70 mV. The inward rectification produces an interesting positive feedback depolarizing effect. If the membrane is at rest at −70 mV and NO or HA induces a membrane conductance decrease, the membrane depolarizes and as the potential shifts to more depolarized levels the membrane conductance decreases further as the inward rectification current decreases, adding to the NO and HA induced decrease in conductance. Thus, the NO or HA induced conductance decrease and depolarization will be enhanced by the intrinsic properties of the membrane inward rectifier. Since NO acts to decrease the potassium conductance and thus increase the membrane resistance in neurons MCC and C4, it will enhance the effects of synaptic currents induced by other neurons. NO in this way may augment and prolong an excitatory state. Neuron B2 of Lymnaea, which also uses NO as a transmitter, appears to reduce the potassium conductance of its follower neuron, B7nor, and is capable of producing a similar enhanced and prolonged excitation (Park et al., 1998). Culture medium reduced the sensitivity to SNC. The medium is a mixture of L15 made up in ASW and hemolymph obtained from the animal. Both components alone reduced the SNC sensitivity compared to ASW alone and both contain potential NO scavengers. L15 contains a mixture of amino acids including cysteine. Hemolymph is a complex mixture of ions and biological molecules including hemocyanin, a copper-containing blood oxygen carrier. Analysis of Aplysia hemolymph shows that it has 4 large proteins: acetylcholinesterase, hemocyanin, hemagglutinin and erythrocurorin (Bevelaqua et al., 1975). It contains 2 mg/ml total protein and 2 μg/ml copper (Srivatsan et al., 1992). The copper in hemocyanin reacts with NO (Solomon, 1981) and is an NO scavenger. Like hemoglobin in vertebrate systems (Lancaster, 1997), hemocyanin may be a major factor in limiting the spread and effectiveness of NO released as a neurotransmitter and neuromodulator in invertebrate systems. Not all HA sensitive synaptic followers of C2 respond to NO. The followers we studied that hyperpolarize in response to HA and show increasing membrane conductance are insensitive to NO. These are the cerebral E-cluster neurons that receive slow inhibitory synaptic input from C2 (Chiel et al., 1986; McCaman and Weinreich, 1985). The IPSPs are produced by increased potassium conductance and have a reversal potential at the calculated potassium equilibrium potential (McCaman and Weinreich, 1982). There is strong evidence that NO serves as a cotransmitter along with HA in neuron C2. NO is likely to serve as a cotransmitter or neuromodulator in other systems as well because NOS is commonly found in neurons that contain a classical neurotransmitter (Vincent, 1994). For example, cerebellar granular cells contain NOS and glutamate, basket cells contain NOS and GABA. Neuron B2 in Lymnaea appears to use acetylcholine, neuropeptides and NO as transmitters (Park et al., 1998). Neuron C2 was shown to contain high concentrations of HA by single neuron chemical analysis (Weinreich et al., 1975). Later, a tandem physiological recording, chemical analysis approach (Ono and McCaman, 1980) was used to identify another histaminergic neuron, C3, and determine that C2 and C3 are weakly electrotonically connected and share synaptic follower neurons in the E-cluster and neuron MCC. They found that C2 contains 2.8 mM and C3 contains 2.4 mM of HA. These neurons and others in the cerebral ganglion were also identified by tritium-labeled HA uptake and HA immunocytochemistry (Elste et al., 1990). The NADPH diaphorase positive neuron labeled as C3n in Fig. 1 has not been positively identified as histaminergic neuron C3, but it may well be because it is in the correct anatomical location (Ono and McCaman, 1980) and it should share the characteristic of C2, as indicated above. The accumulated evidence suggests the scheme for the NO and HA pathways involved in the vsEPSPs shown in Figure 10. Depolarization of neuron C2 causes calcium influx that activates NOS and also releases the HA stored in vesicles. HA binds to post-synaptic receptors and activates an unidentified second messenger pathways that leads to closure of potassium channels. NO diffuses widely and activates sGC, which leads to cGMP synthesis and closure of the potassium channels. The copper in hemocyanin acts as a scavenger of NO, and limits its spread. NO may also modulate membrane properties by nitrosylation of membrane thiols (Stamler, 1994). It appears that HA activity would be restricted to specialized conventional synaptic sites, whereas NO diffuses widely in all directions, and perhaps more than 100 μm, if it is not restricted by NO scavenging. Within that action sphere its effects are dictated by appropriate chemical reactions with molecules that contain heme, metals or thiols. Open in new tabDownload slide Fig. 1. View of the cerebral ganglion showing the location of cGMP-IR positive and NADPH-diaphorase positive bilaterally paired neurons. The metacerebral cells (MCC) are the principal cGMP-IR neurons, but other small neurons are at the base of the O and PT nerves. Although the ganglion is bilaterally symmetrical, the names of the NADPH-diaphorase positive neurons (C2, C2n, C3n) are indicated on the right hemiganglion and the neuron clusters (A, B etc.) are indicated on the left hemiganglion. Nerves or interganglionic connectives are labeled on the right: AT, anterior tentacular; C-B, cerebral-buccal; C-P, cerebral pedal; C-PL, cerebral-pleural; LLAB, lower labial; O, optic; PT, posterior tentacular; and ULAB, upper labial Open in new tabDownload slide Fig. 1. View of the cerebral ganglion showing the location of cGMP-IR positive and NADPH-diaphorase positive bilaterally paired neurons. The metacerebral cells (MCC) are the principal cGMP-IR neurons, but other small neurons are at the base of the O and PT nerves. Although the ganglion is bilaterally symmetrical, the names of the NADPH-diaphorase positive neurons (C2, C2n, C3n) are indicated on the right hemiganglion and the neuron clusters (A, B etc.) are indicated on the left hemiganglion. Nerves or interganglionic connectives are labeled on the right: AT, anterior tentacular; C-B, cerebral-buccal; C-P, cerebral pedal; C-PL, cerebral-pleural; LLAB, lower labial; O, optic; PT, posterior tentacular; and ULAB, upper labial Open in new tabDownload slide Fig. 2. ODQ partial suppression of the vsEPSP in MCC induced by neuron C2 firing. Current injection (2 nA) into C2 produced 18 spikes/sec for 3 sec. The amplitude of the control vsEPSP in ASW was reduced to about half after treatment with 20 μM ODQ for 1 hr. Membrane potential (Vm) was −70 mV at beginning of each trace Open in new tabDownload slide Fig. 2. ODQ partial suppression of the vsEPSP in MCC induced by neuron C2 firing. Current injection (2 nA) into C2 produced 18 spikes/sec for 3 sec. The amplitude of the control vsEPSP in ASW was reduced to about half after treatment with 20 μM ODQ for 1 hr. Membrane potential (Vm) was −70 mV at beginning of each trace Open in new tabDownload slide Fig. 3. Neurons MCC and C4 isolated in cell culture (day 2). The soma of MCC was invested with many small glia, unlike the majority of other isolated neurons. The MCC axon retracted several hundred μm soon after isolation but it stabilized. Neurite outgrowth from the cut ends of the bifurcated MCC axon and the C4 axon occurred. Scale applies to C4 and MCC Open in new tabDownload slide Fig. 3. Neurons MCC and C4 isolated in cell culture (day 2). The soma of MCC was invested with many small glia, unlike the majority of other isolated neurons. The MCC axon retracted several hundred μm soon after isolation but it stabilized. Neurite outgrowth from the cut ends of the bifurcated MCC axon and the C4 axon occurred. Scale applies to C4 and MCC Open in new tabDownload slide Fig. 4. Response of MCC in culture to SNC is blocked by ODQ. A) MCC was tested with SNC (100 μM) to observe the depolarization and increased resistance. After 30 min wash, B) MCC was treated with 20 μM ODQ for 1 hr, prior to bath treatment with ODQ plus 100 μM SNC for 10 min. C) after 40 min wash, bath treatment with 100μM SNC alone for 10 min had its usual depolarizing effect accompanied by increased membrane input resistance (Rin). Current induced membrane resistance tests were limited by the recorder response Open in new tabDownload slide Fig. 4. Response of MCC in culture to SNC is blocked by ODQ. A) MCC was tested with SNC (100 μM) to observe the depolarization and increased resistance. After 30 min wash, B) MCC was treated with 20 μM ODQ for 1 hr, prior to bath treatment with ODQ plus 100 μM SNC for 10 min. C) after 40 min wash, bath treatment with 100μM SNC alone for 10 min had its usual depolarizing effect accompanied by increased membrane input resistance (Rin). Current induced membrane resistance tests were limited by the recorder response Open in new tabDownload slide Fig. 5. Response of MCC in culture to SNC puff. A) Depolarization and spike activity were induced by a 100 msec puff of 1 mM SNC at the arrow. B) Vm was still depolarized 1 min later but spike frequency declined. C) Vm shifted to lower state at 2 min. D) Recovery from SNC was nearly complete by 3 min. Rin tested by −1 nA pulses. Note the Vm −55 mV plateau just prior to the first spike in A. Top trace is current monitor for all Open in new tabDownload slide Fig. 5. Response of MCC in culture to SNC puff. A) Depolarization and spike activity were induced by a 100 msec puff of 1 mM SNC at the arrow. B) Vm was still depolarized 1 min later but spike frequency declined. C) Vm shifted to lower state at 2 min. D) Recovery from SNC was nearly complete by 3 min. Rin tested by −1 nA pulses. Note the Vm −55 mV plateau just prior to the first spike in A. Top trace is current monitor for all Open in new tabDownload slide Fig. 6. Voltage dependence of the SNC response in an isolated neuron C4. Depolarization was induced by 100 msec puff of 1 mM SNC, 100 uM from the soma in each trace. A) Response at Vm −65 mV. B) Response at −75 mV. Response at −85 mV. Top trace is current monitor for all Open in new tabDownload slide Fig. 6. Voltage dependence of the SNC response in an isolated neuron C4. Depolarization was induced by 100 msec puff of 1 mM SNC, 100 uM from the soma in each trace. A) Response at Vm −65 mV. B) Response at −75 mV. Response at −85 mV. Top trace is current monitor for all Open in new tabDownload slide Fig. 7. Decrease in conductance during treatment with 100 μM SNC in isolated neuron MCC. A) Pre-treatment. B) response to SNC bath treatment after 10 min. C) Recovery 30 min after wash out. Holding potential was −60 mV and −10, −20 and −30 mV steps were applied. Single electrode VC, 30 Hz switching, ½ duty cycle. Note increasing conductance from −60 mV to −90 mV, and suppression at all potentials by SNC Open in new tabDownload slide Fig. 7. Decrease in conductance during treatment with 100 μM SNC in isolated neuron MCC. A) Pre-treatment. B) response to SNC bath treatment after 10 min. C) Recovery 30 min after wash out. Holding potential was −60 mV and −10, −20 and −30 mV steps were applied. Single electrode VC, 30 Hz switching, ½ duty cycle. Note increasing conductance from −60 mV to −90 mV, and suppression at all potentials by SNC Open in new tabDownload slide Fig. 8. NO scavenging by hemolymph. A) Isolated neuron C4 was treated with SNC (1 sec puffs at 100 μM) in ASW. B) same SNC treatment with C4 bathed in 50% hemolymph-50% ASW. Response was partially suppressed in hemolymph and completely recovered (not shown) when superfused again with ASW. Vm is −70 mV in both tests Open in new tabDownload slide Fig. 8. NO scavenging by hemolymph. A) Isolated neuron C4 was treated with SNC (1 sec puffs at 100 μM) in ASW. B) same SNC treatment with C4 bathed in 50% hemolymph-50% ASW. Response was partially suppressed in hemolymph and completely recovered (not shown) when superfused again with ASW. Vm is −70 mV in both tests Open in new tabDownload slide Fig. 9. Isolated E-cluster neuron that was hyperpolarized by HA was insensitive to SNC. A) shows neuron action potential in response to depolarizing current pulse and strong hyperpolarization induced by 100 msec, 1 mM puff of HA. Rin decreased and Vm reached −80 mV. B) shows very weak depolarization in response to even 1 000 msec puff of 1 mM SNC Open in new tabDownload slide Fig. 9. Isolated E-cluster neuron that was hyperpolarized by HA was insensitive to SNC. A) shows neuron action potential in response to depolarizing current pulse and strong hyperpolarization induced by 100 msec, 1 mM puff of HA. Rin decreased and Vm reached −80 mV. B) shows very weak depolarization in response to even 1 000 msec puff of 1 mM SNC Open in new tabDownload slide Fig. 10. Simplified scheme of NO and HA synaptic pathways involved in the vsEPSP in neurons C4 and MCC induced by C2 stimulation. NOS is activated and HA is released by Ca++ influx. NO activates sGC and cGMP is produced, which directly or indirectly closes the potassium (K+) channels. Hemocyanin acts as an NO scavenger via the interaction of copper and NO (CuNO). NO may possibly react with membrane thiol groups (RS). HA activates a separate pathway and induces closure of K+ channels Open in new tabDownload slide Fig. 10. Simplified scheme of NO and HA synaptic pathways involved in the vsEPSP in neurons C4 and MCC induced by C2 stimulation. NOS is activated and HA is released by Ca++ influx. NO activates sGC and cGMP is produced, which directly or indirectly closes the potassium (K+) channels. Hemocyanin acts as an NO scavenger via the interaction of copper and NO (CuNO). NO may possibly react with membrane thiol groups (RS). HA activates a separate pathway and induces closure of K+ channels 1 From the Symposium: Nitric Oxide in the Invertebrates: Comparative Physiology and Diverse Functions presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 4–8 January 2000, at Atlanta, Georgia. 2 E-mail: jwj74@albany.edu We thank Nancy Pultz and David Tieman for assistance. Supported by NIMH grant MH57746 to JWJ. References Bellamy , T. C. , J. Wood, D. A. Goodwin, and J. 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TI - Nitric Oxide as an Orthograde Cotransmitter at Central Synapses of Aplysia: Responses of Isolated Neurons in Culture
JF - Integrative and Comparative Biology
DO - 10.1093/icb/41.2.282
DA - 2001-04-01
UR - https://www.deepdyve.com/lp/oxford-university-press/nitric-oxide-as-an-orthograde-cotransmitter-at-central-synapses-of-4dWbWD0soa
SP - 282
EP - 291
VL - 41
IS - 2
DP - DeepDyve
ER -