Ionic mechanisms of cholinergic excitation in molluscan neurons

A Latin American Perspective on Ion Channels

Ion channels, both ligand- and voltage-gated, play fundamental roles in many physiologic processes. Alteration in ion channel function underlies numerous pathologies, including hypertension, diabetes, chronic pain, epilepsy, certain cancers, and neuromuscular diseases. In addition, an increasing number of inherited and de novo ion channel mutations have been shown to contribute to disease states. Ion channels are thus a major class of pharmacotherapeutic targets.

Ultrasound Modulates the Splenic Neuroimmune Axis in Attenuating AKI

We showed previously that prior exposure to a modified ultrasound regimen prevents kidney ischemia-reperfusion injury (IRI) likely via the splenic cholinergic anti-inflammatory pathway (CAP) and α7 nicotinic acetylcholine receptors (α7nAChR). However, it is unclear how ultrasound stimulates the splenic CAP. Further investigating the role of the spleen in ischemic injury, we found that prior splenectomy (-7d) or chemical sympathectomy of the spleen with 6-hydroxydopamine (6OHDA; -14d) exacerbated injury after subthreshold (24-minute ischemia) IRI. 6-OHDA-induced splenic denervation also prevented ultrasound-induced protection of kidneys from moderate (26-minute ischemia) IRI. Ultrasound-induced protection required hematopoietic but not parenchymal α7nAChRs, as shown by experiments in bone marrow chimeras generated with wild-type and α7nAChR(-/-) mice. Ultrasound protection was associated with reduced expression of circulating and kidney-derived cytokines. However, splenocytes isolated from mice 24 hours after ultrasound treatment released more IL-6 ex vivo in response to LPS than splenocytes from sham mice. Adoptive transfer of splenocytes from ultrasound-treated (but not sham) mice to naïve mice was sufficient to protect kidneys of recipient mice from IRI. Ultrasound treatment 24 hours before cecal ligation puncture-induced sepsis was effective in reducing plasma creatinine in this model of AKI. Thus, splenocytes of ultrasound-treated mice are capable of modulating IRI in vivo, supporting our ongoing hypothesis that a modified ultrasound regimen has therapeutic potential for AKI and other inflammatory conditions.

Identification and functional expression of a family of nicotinic acetylcholine receptor subunits in the central nervous system of the mollusc Lymnaea stagnalis

We described a family of nicotinic acetylcholine receptor (nAChR) subunits underlying cholinergic transmission in the central nervous system (CNS) of the mollusc Lymnaea stagnalis. By using degenerate PCR cloning, we identified 12 subunits that display a high sequence similarity to nAChR subunits, of which 10 are of the alpha-type, 1 is of the beta-type, and 1 was not classified because of insufficient sequence information. Heterologous expression of identified subunits confirms their capacity to form functional receptors responding to acetylcholine. The alpha-type subunits can be divided into groups that appear to underlie cation-conducting (excitatory) and anion-conducting (inhibitory) channels involved in synaptic cholinergic transmission. The expression of the Lymnaea nAChR subunits, assessed by real time quantitative PCR and in situ hybridization, indicates that it is localized to neurons and widespread in the CNS, with the number and localization of expressing neurons differing considerably between subunit types. At least 10% of the CNS neurons showed detectable nAChR subunit expression. In addition, cholinergic neurons, as indicated by the expression of the vesicular ACh transporter, comprise approximately 10% of the neurons in all ganglia. Together, our data suggested a prominent role for fast cholinergic transmission in the Lymnaea CNS by using a number of neuronal nAChR subtypes comparable with vertebrate species but with a functional complexity that may be much higher.

The ionic mechanism associated with the action of putative transmitters on identified neurons of the snail, Helix aspersa

Intracellular recordings were made from identified neurons in the suboesophageal ganglionic mass of the snail, Helix aspersa. The ionic mechanisms associated with acetylcholine excitation and inhibition, dopamine excitation and inhibition, gamma-aminobutyric acid (GABA) excitation and inhibition and serotonin excitation were investigated. Acetylcholine excitation was found to involve an initial increase in sodium conductance while acetylcholine inhibition was a pure chloride event which reversed at membrane potentials more negative than the chloride equilibrium potential. Dopamine excitation appeared to involve only an increase in sodium conductance while serotonin excitation involved an increase in conductance to both sodium and calcium ions. Dopamine inhibition was associated with an increase in potassium conductance but failed to reverse at membrane potentials more negative than the potassium equilibrium potential. GABA excitation involved conductance increases to both sodium and chloride ions while GABA inhibition was a pure chloride event. An attempt was made to estimate the degree of co-operativity of the putative transmitters with their receptors using log-log and Hill plots. The slopes of the line for the log-log plots for acetylcholine excitation and inhibition were 0.88 and 1.1, respectively, suggesting the interaction of one molecule of acetylcholine with the receptor. The slope of the log-log plot for dopamine inhibition was 0.46 while that for serotonin excitation was 0.75. The Hill plots for GABA excitation and inhibition were 1.64 and 1.42, respectively, suggesting that two molecules of GABA are required for receptor activation.

Actions of acetylcholine on the salivary gland cells of the pond snail, Planorbis corneus

Intracellular recordings have been made from salivary gland cells of the pond snail Planorbis corneus. Gland cells produced a dose-dependent biphasic response to the bath application of acetylcholine (ACh), an initial depolarization being followed by a hyperpolarization. Nicotine and the nicotinic agonist tetramethylammonium had an excitatory action on the gland cells. The muscarinic agonists acetyl-beta-methyl choline and arecoline were also stimulants, but muscarine, bethanechol and pilocarpine produced no response from gland cells at 10(-3) M. A number of cholinergic antagonists, including atropine, hexamethonium and curare, effectively blocked the response to ACh. The depolarizing phase of the ACh response resulted from an increased membrane permeability to Na+ ions, though the participation of other ionic species cannot be ruled out. The hyperpolarizing phase of the ACh response was produced by the activity of an electrogenic Na+/K+ pump.

Acetylcholine responses of identified neurons in Helix pomatia--I. Interactions between acetylcholine-induced and potential-dependent membrane conductances

The influence of potential-dependent membrane conductances on amplitude and time course of acetylcholine (ACh) responses was studied. The investigations were performed on the identified neurons B1 and B3 of the buccal ganglion of Helix pomatia. The neurons B1 and B3 were depolarized by ACh. The depolarization was accompanied by a decrease of membrane resistance. An inward rectification occurring negative to the resting membrane potential (RMP) reduced the amplitude of the ACh depolarizations. An outward rectification occurring positive to the RMP consisted of two parts and ceiled the ACh responses. The early outward current reduced the amplitude and modified the time course of ACh responses. Local responses or axonal action potentials increased the amplitude of the ACh depolarizations.

Ionic mechanisms of the rapid (nicotinic) phase of acetylcholine response in identified Planobarius corneus neurones

Current responses to acetylcholine (ACh) and to suberyldicholine (D-6) applied from the double-barrelled ionophoretic micropipette were studied in two identified neurones (LPed-2 and LPed-3) isolated from the left ganglion of pulmonate mollusc, Planorbarius corneus. Experiments made with K2SO4-filled microelectrodes show that in LPed-2 neurone two kinds of cholinoreceptors are involved in the rapid phase of ACh response one of which induces chloride conductance and the other, sodium conductance. The Cl-dependent component can be separated from the cationic one by C-6 whereas the cationic component can be separated from the Cl--dependent one by furosemide. Cl- conductance can be induced selectively by D-6. In the LPed-3 neurone only Cl- conductance increases during rapid phase of ACh response. The reversal potential of Cl--dependent responses was found to be more negative than the resting potential in experiments made with K2SO4-filled microelectrodes but less negative than the resting potential in the case of KCl-filled microelectrodes. This difference seems to be due to the artificial increase of intracellular chloride concentration.

Characterization of the synaptic actions of an interneuron in the central nervous system of Tritonia

The motor program that drives the swimming behavior of the marine mollusk Tritonia diomedea is generated by three interneuronal populations in the cerebral ganglia. One of these populations, the pair of C2 neurons, is shown to also exert powerful synaptic actions upon most cells in the contralateral pedal ganglion. Intracellular staining with Co2+ showed that the C2 neurons projected to the contralateral pedal ganglion as a single unbranched axon, and nearly all contralateral pedal neurons received monosynaptic input from C2. Orthodromic stimulation of most peripheral nerves caused monosynaptic excitation of C2 by afferent sensory cells and, in some cases, monosynaptic inhibition from an unidentified source. C2 neurons produced four types of postsynaptic potential (PSP) on pedal neurons: (1) a fast, Cl- -mediated inhibition (FIPSP); (2) a fast, Na+ -mediated excitation (FEPSP); (3) a slow, K+ -mediated inhibition (SIPSP); and (4) a slow, conductance-decrease excitation (SEPSP). All four could be recorded simultaneously in some pedal neurons. The C2 neurons appear to be high-order, multiaction neurons involved in both the generation of a complex motor program and the coordination of ancillary neuronal activity.

Receptors for gamma-aminobutyric acid (GABA) on Aplysia neurons

Aplysia neurons show 5 different types of response (three excitatory and two inhibitory) to iontophoretic application of gamma-aminobutyric acid (GABA). Four of these are associated with a membrane conductance increase, but one is associated with a conductance decrease. The most common response is a fast hyperpolarization which reverses at about--58 mV and is sensitive to manipulation of external Cl- concentration, and thus is due to a specific increase in Cl- conductance. There is an infrequent, slower hyperpolarizing response which does not reverse above about--80 mV and is insensitive to external Cl-. This response appears to result from a conductance increase to K+. Two types of depolarizing responses are associated with conductance increases. These responses differ in their latency, duration and sensitivity to curare. The more frequent is relatively rapid (peak at 1-2 sec) and is depressed by curare at high concentrations. In other neurons, GABA causes a slower response, peaking at 6-10 sec, which is not curare-sensitive. Usually for both types of response, the voltage and conductance changes are completely abolished by perfusion with Na+-free seawater, and the responses cannot be reversed with depolarization. In other neurons such as L11, the response can be reversed with depolarization, and appears to result from a conductance increase to both Na+ and Cl-. In neuron R15, GABA causes a slow depolarizing response (peak at about 9 sec) which is associated with a decreased membrane conductance, probably to K+. The classical GABA antagonists, picrotoxin and bicuculline, block Cl- responses but no others, while the fast Na+ and Cl- responses are depressed by curare. Strychnine does not affect any GABA response. The multiplicity of GABA responses, the specificity of their organization and the fact that only some neurons have receptors for GABA, argue that GABA may have a role as a neurotransmitter in Aplysia. Furthermore, the existence of several types of excitatory GABA response suggests that GABA may function both as an inhibitory and excitatory neurotransmitter.

Pharmacological study of two kinds of cholinoreceptors on the membrane of identified completely isolated neurones of Planorbarius corneus

Completely isolated identified neurones (LPed-2 and LPed-3) of the left pedal ganglion of Planorbarius corneus were shown to have two kinds of cholinoreceptors (ChR) on their membrane. One kind of ChR is a classical nicotinic receptor which is sensitive to nicotinomimetics and can be blocked by tubocurarine; the depolarization caused by activation of this ChR type is chloride-dependent. The other kind of ChR, which mediates a potassium-dependent hyperpolarization, has some common features with muscarinic (M) ChR of vertebrates, being sensitive to the muscarinomimetics, dioxolane F-2268, methylfurmethide, mecholyl, and arecoline, although insensitive to oxotremorine. The sensitivity of this receptor to muscarine, itself, was not tested. Like the muscarinic ChR of vertebrates, it can be blocked by benzilylcholine mustard, but, in contrast to vertebrate muscarinic receptors, it cannot be blocked by atropine and cannot distinguish between the optical isomers of F-2268. TEA, mytolon, and cooling prevent the hyperpolarization caused by ACh and muscarinomimetics. The two kinds of ChR in P. corneus neurones seem to be similar to those found by Kehoe24-26 on the medial cells of Aplysia pleural ganglia, both in their pharmacological characteristics and in the ionic permeability changes which they control.

Analysis of hyperpolarizations induced by glutamate and acetylcholine on Onchidium neurones

1. Four giant neurones, designated G-H cells, in the right pleural ganglion of the marine pulmonate mollusc, Onchidium verruculatum, showed characteristic membrane hyperpolarization during applications of either acetylcholine (ACh) or L-glutamate. In the presence of ACh the membrane was hyperpolarized only transiently, while in the presence of glutamate the response was maintained. Significant increases in membrane conductance accompanied the changes in membrane potential.2. In excess potassium sea water, a slight hyperpolarization occurred when the normal concentration was increased between one- and twofold. However, depolarization usually occurred when the concentration was increased tenfold except on a few occasions when a slight but definite hyperpolarization occurred. These changes were all accompanied by a substantial increase in the membrane conductance. This hyperpolarization was in all probability the result of an increase in chloride ion permeability caused by the release of an ACh-like transmitter from depolarized presynaptic nerve terminals.3. The reversal levels for glutamate- and ACh-induced hyperpolarization respectively were approximately - 20 and - 17 mV with respect to the resting membrane potential.4. By changing the external ion composition, glutamate- and ACh-induced hyperpolarization were shown to be the result of an increased permeability of the subsynaptic membrane to potassium and chloride ions respectively. It appears therefore that inhibition in the same G-H cells can be activated by two different transmitter substances and that each of them activates a change in the membrane permeability to a different ion.5. The relationship between the concentration of glutamate and the membrane conductance change was suggestive of two glutamate molecules reacting with a single receptor site.

Intracellular chloride activity and the effects of acetylcholine in snail neurones

1. Cl(-)-sensitive micro-electrodes were used to measure intracellular Cl(-) in snail neurones. The electrodes consisted of a sharpened and chlorided silver wire mounted inside a glass micropipette.2. The electrodes appeared to record changes in internal Cl(-) accurately but in H cells the chloride equilibrium potential (E(Cl)) as measured by the Cl(-)-sensitive electrode was always less negative than E(ACh).3. In some H cells ACh caused a measurable increase in internal Cl(-) when the cell was at its resting potential. In voltage-clamped cells there was a close correlation between the change in internal Cl(-) and the extra clamp current caused by a brief application of ACh. This confirmed that ACh increases the cell's membrane permeability only to Cl(-) ions, and that E(ACh) was equal to E(Cl).4. There was good agreement between the measured change in internal Cl(-) and that calculated from the cell size and clamp charge only when it was assumed that a constant voltage offset was added to the potential of the Cl(-)-sensitive electrode while it was inside the nerve cell.5. Cl(-)-sensitive electrodes with AgCl as the sensitive material appear to be unsuitable for intracellular measurement of Cl(-), although they might be suitable for following changes in E(Cl).6. In certain D cells ACh also caused an increase in internal Cl(-) although it decreased the membrane potential. In the presence of hexamethonium, ACh caused a hyperpolarization and a smaller increase in internal chloride.7. It is concluded that the intracellular Cl(-) in both H and D cells is about 8.3 mM, giving an E(Cl) of about -58 mV.

Acetylcholine receptors

The idea that certain drugs and neurotransmitters produce their effects by combining with specific receptors was first clearly expressed by Langley (1905) on the basis of the selective and localized effect of nicotine on striated muscle fibres. In 1914, Langley published a paper in which the antagonism between ‘curari’ and nicotine was analysed and measured as the ratio by which the nicotine concentration had to be increased in order to produce a standard response in the presence of tubocurarine. It was clear that Langley had in mind the idea of competition between nicotine and curare for the receptor sites and it was surprising that he did not formulatethe theory quantitatively, particularly since Hill, working in Langley's laboratory in 1909, published a mathematical analysis of the action of nicotine on frog muscle giving kinetic and equilibrium equations based on the law of mass action, which could easily have been extended to give an account of competitive antagonism. Barger & Dale (1910) did not favour the idea of specific receptors for sympathomimetic amines.

Cyclic adenosine monophosphate in the nervous system of Aplysia californica. II. Effect of serotonin and dopamine

Serotonin and dopamine, both likely transmitter substances in Aplysia, stimulated formation of adenosine-3',5' monophosphate (cAMP) in ganglia, connectives, and identified nerve cell bodies. This widespread distribution suggests that receptors for the response are localized throughout the nervous system, as is adenyl cyclase. Both synthesis of cAMP-(3)H from precursor previously labeled in incubations with adenine-(3)H and total content of cAMP were stimulated up to 15-fold. The acetylcholine analogue carbachol, glutamate, norepinephrine, and histamine were inactive. Full stimulation occurred within 2-4 min of applying serotonin; the extent of the effect was half maximal at 6micro serotonin. Even in the continued presence of serotonin, the increased cAMP diminished with time. When serotonin was removed, tissue remained refractory for 15-20 min; sensitivity returned after 25 min. Serotonin stimulated cAMP after removal of extracellular Na, K, or Cl and in isotonic sucrose, with all extracellular ions removed. Elevating Mg, which blocked the stimulation of cAMP caused by synaptic activity, did not affect the response to serotonin. Thus the response appeared to be independent of transmitter release and of changes in synaptic potentials and current flow. The role of cAMP in neuronal functioning remains to be determined. Conditions which markedly increased cAMP in neurons, however, did not affect the rate of RNA synthesis, nor did they alter the distribution of phosphorylated adenine or uridine nucleotides.

Membrane conductances and spectral sensitivities of Pecten photoreceptors

The electrical and spectral properties of depolarizing (proximal) and hyperpolarizing (distal) photoreceptors in the eye of the scallop, Pecten irradians, were examined. Both depolarizing and hyperpolarizing responses are associated with an increase in membrane conductance; in addition, the depolarizing response is characterized by a secondary decrease in conductance at light intensities which inactivate the response. Both responses can be reversed in polarity by applied current across the cell membrane. The depolarizing response has a reversal potential of approximately +10 mv, whereas the estimated reversal potential for the hyperpolarizing response is near -70 mv. The two responses have the same spectral sensitivity function, which agrees with a Dartnall nomogram for a rhodospin with a lambda(max) at 500 nm. It is suggested that the photochemical reactions produce different end products which give responses of opposite polarity in proximal and distal cells, or alternatively, that the reactions of the respective cell membranes to the same end product are different.

A direct synaptic connection mediating both excitation and inhibition

Neurons have generally been thought to produce only one synaptic action on any particular cell which they innervate. An identified interneuron in the abdominal ganglion of Aplysia mediates both direct excitation and inhibition to an identified follower cell. At low firing rates the interneuron produces excitatory postsynaptic potentials; however at higher firing rates these gradually diminish in size and eventually invert to inhibitory postsynaptic potentials. Electrophysiological and pharmacological evidence indicates that the connection between these cells is monosynaptic, and that a single transmitter, acetylcholine, mediates both actions. These opposite synaptic responses appear to result from the transmitter's acting on two types of postsynaptic receptors having different thresholds for activation and different susceptibilities for desensitization.

Ionic mechanisms of cholinergic excitation in molluscan neurons

Acetylcholine appears to be an excitatory transmitter at synapses on two different types of molluscan nerve cells: the so-called D- and CILDA neurons. The action of this substance is different in the two cases. In D-neurons, this compound increases the permeability of the subsynaptic or somatic membrane to chloride ions, and through a net efflux of this anion, depolarizes the cell. In CILDA neurons, on the other hand, acetylcholine depolarzies the cell by increasing its permeability to sodium ions.