Effect of FMRFamide on the electrical and plastic properties of identified Helix pomatia neurons

Effect of the neuropeptide FMRFamide on two types of nerve cells differing in plastic properties: habituating and non-habituating to rhythmic intracellular stimulation, has been studied. FMRFamide causes a slow developing, continuously growing depolarisation and an increase of input resistance of the most part of habituating cells resulting in inhibition of their habituation to intracellular stimulation. No desensitisation of cells to the action of FMRFamide was observed. The data obtained by using Ca-ionophore, imidazole and caffeine show that the effect of the peptide may be caused by inhibition of Ca-dependent K-conductance and depends on cAMP metabolism. FMRFamide exerts a less pronounced action on non-habituating cells and does not change their plastic properties. Inhibition by FMRFamide of the habituation at the level of electroexcitable membrane may play a significant part in regulation of neuronal plasticity.     

Some effects of short-chain phospholipids and n-alkanes on a transient potassium current (I A ) in identified Helix neurons

Many effects of short-chain phospholipids and n-alkanes on the squid axon sodium current (I _Na) are consistent with mechanisms involving changes in membrane thickness. Here, we suggest that the actions of short-chain phospholipids on an A-type potassium current (I _A ) in two-microelectrode voltage clamped Helix D1 and F77 neurons are incompatible with such simple mechanisms. Diheptanoyl phosphatidylcholine (diC₇PC, 0.2 and 0.3 mm) caused substantial (58 and 79%), and in some cases partially reversible, increases in I _A amplitude. These were correlated with hyperpolarizing shifts of up to –7 mV in the voltage dependence of current activation. The voltage dependence of steady-state inactivation was also moved in the hyperpolarizing direction. These effects are the opposite of those described for squid I _Na. 0.5 Saturated n-pentane and saturated n-hexane caused significant (–3 and –6 mV) hyperpolarizing shifts in the voltage dependence of I _A inactivation, qualitatively consistent with their effects on squid I _Na, while the voltage dependence of activation was moved slightly to the left or unchanged. Hydrocarbons had variable effects on peak current amplitude, although saturated n-pentane produced a clear suppression. DiC₇PC caused a 25% increase in the time constant of macroscopic I _A inactivation ( _b ) but 0.5 saturated n-pentane and saturated n-hexane reduced _b by 40%. The effects of these agents on current-clamped cells were broadly consistent with their opposing actions on _b —phospholipids tended to reduce excitability and n-alkanes tended to increase it. Possible mechanisms of I _A perturbation are discussed.We gratefully acknowledge financial support from the Science and Engineering Research Council and the Wellcome Trust. We would also like to thank Prof. H. Meves, Dr. N. Franks and Dr. W. Lieb for helpful discussions.     

Structure-activity and possible mode of action of S-Iamide neuropeptides on identified central neurons of Helix aspersa

Intracellular recordings were made from identified neurons from the suboesophageal ganglia of Helix aspersa. The inhibitory action of nine S-Iamide peptides was investigated. Structure-activity studies suggest that all act through a common receptor, which normally requires FVRIamide at the C terminal, with a preferred length of seven amino acids. Substitution at the N-terminal with alanine (A), threonine (T), proline (P) or leucine (L) results in little change in potency, suggesting the N-terminal requirements are relatively flexible. Ion substitution experiments suggest that potassium is the main ion involved in the inhibitory response to S-Iamide application. Studies using a range of compounds, which modify second messenger systems, would suggest that S-Iamide peptides may interact with adenylate cyclase. No evidence was found for an interaction with either guanylate cyclase or nitric oxide synthase.     

Effect of intracellular injection of cyclic amp on electrical characteristics of identified neurons in Helix pomatia

The effect of intracellular iontophoretic injection of cyclic AMP on electrical activity of neurons RPa1, RPa3, LPa2, LPa3, and LPl1 in the corresponding ganglia ofHelix pomatia was investigated. Injection of cyclic AMP into neuron LPl1 was found to cause the appearance of rhythmic activity (if the neuron was originally "silent"), an increase in the frequency of spike generation (if the neuron had rhythmic activity), and a decrease in amplitude of waves of membrane potential, in the duration of the interval between bursts, and in the number of action potentials in the burst (if the neuron demonstrated bursting activity). In the remaining "silent" neurons injection of cyclic AMP led to membrane depolarization. Injection of cyclic AMP into neurons whose membrane potential was clamped at the resting potential level evoked the development of an inward transmembrane current (cyclic AMP current), the rate of rise and duration of which increased proportionally to the size and duration of the injection. Theophylline in a concentration of 1 mM led to an increase in the amplitude and duration of the cyclic AMP current by about 50%. It is concluded that a change in the cyclic AMP concentration within the nerve cell may modify the ionic permeability of its membrane and, correspondingly, its electrical activity.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 5, pp. 517–525, September–October, 1980.     

Study of effects of antibody to protein S100 on ionic channels of input and output currents of identified neurons of the snail Helix lucorum

In the course of electrophysiological experiments, two types of the neurons of the edible snail Helix lucorum were detected, which responded by different way to application of antibodies to the neuron-specific calcium-binding S 100 protein (AS1000). Under effect of AS100, frequency of the action potential (AP) generation in the spontaneously active V1, V3, V17, and RPa6 cells decreased, whereas in V4 and V6 cells increased. On addition of quinine solution the AP generation frequency of these neurons decreased more than twice, while the AP duration (t _S) rose 6 times. The combined action of AS100 and quinine did not change statistically significantly the AP generation frequency, membrane potential (MP) and AP generation threshold (AP_t), as compared with the effect of AS100 in saline. The value of the AP duration (t _S) increased 1.6 times, which was less pronounced as compared with the quinine action in saline. This means that AS100 prevents an increase of the AP duration after the quinine application (block of the Ca-depended K-channels). The main AS100 effect at the level of the ionic currents is shown to consist in a decrease of the maximal value of the input current, on average, by 20%, while of the output current, on average, by 12%.     

The effect of oxytocin on the identified neurons of the brain in the edible snail Helix pomatia L.]

Responses induced by a perfusion by a solution with oxytocin were examined in identified Helix pomatia L. neurons. Depolarizing, hyperpolarizing, and modulatory neuronal responses were observed. The responses under study were supposed to be associated in most of the cases with the system of cyclic nucleotides.     

Acetylcholine responses of identified neurons in Helix pomatia--II. Pharmacological properties of acetylcholine responses

A pharmacological separation of depolarizing and hyperpolarizing mechanisms involved in the generation of acetylcholine (ACh) depolarizations was attempted in the identified neurons B1 and B3 of the buccal ganglia of Helix pomatia. The selectivity of the drugs employed was assayed in non-identified buccal neurons in which ACh increased a hyperpolarizing Cl- conductance. Voltage clamp techniques were used. Under control conditions the depolarizing ACh currents increased non-linearly with more negative membrane potentials. The hyperpolarizing ACh currents showed a linear potential dependence. The buffer substance Tris (5 mmol/l) depressed the depolarizing ACh currents. The effect was accentuated with more negative membrane potentials. Tris failed to affect hyperpolarizing ACh responses. HEPES (5 mmol/l) did not change depolarizing or hyperpolarizing ACh responses. d-Tubocurarine (0.02-0.2 mmol/l), hexamethonium (0.5-5.0 mmol/l) and atropine (0.1 mmol/l) blocked the depolarizing and hyperpolarizing ACh responses. Arecoline (0.1 mmol/l) had neither an agonistic nor an antagonistic effect on the identified and on the non-identified neurons. It displayed an anticholinesterase activity. Anthracene-9-carbonic acid (0.5 mmol/l) depressed selectively the hyperpolarizing ACh responses. In the neurons B1 and B3 no pharmacologically separable hyperpolarizing ACh responses were detected to be superimposed on the ACh depolarizations.     

Axonal pathways and synaptic inputs of three identified neurons in the buccal ganglion of Helix pomatia.

The axonal pathways and the synaptic inputs of the identified neurons B1 through B3 in the buccal ganglia of Helix pomatia were studied. The axons of neurons B1, B2 and B3 were found to run invariably within the ipsilateral posterior oesophageal nerve, ipsi- and contralateral salivary gland nerves, and ipsilateral cerebrobuccal connective, respectively. Synaptic responses could be elicited by stimulation of most of the nerves of the buccal ganglia. These consisted of an early depolarization which was most frequently followed by a longlasting de- or hyperpolarization. The shape of the synaptic response proved to be related to the different neurons.     

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.     

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.     

Pre- and postsynaptic effects of eugenol and related compounds on Helix pomatia L. neurons

We showed how eugenol blocks the synaptic transmission and gave a possible interpretation how it inhibits the excitation-contraction coupling that several authors described previously. Eugenol acts both in the pre- and postsynaptic side of the neurons. It blocks the Ca2+-currents, decreases the membrane potential of the neurons, increases the inward resistance and decreases the GABA, ACh and glutamate evoked excitatory responses in submillimolar concentration.     

Norepinephrine effects on identified neurons of the rat dorsal motor nucleus of the vagus

The dorsal motor nucleus of the vagus (DMV) receives more noradrenergic terminals than any other medullary nucleus; few studies, however, have examined the effects of norepinephrine (NE) on DMV neurons. Using whole cell recordings in thin slices, we determined the effects of NE on identified gastric-projecting DMV neurons. Twenty-five percent of DMV neurons were unresponsive to NE, whereas the remaining 75% responded to NE with either an excitation (49%), an inhibition (26%), or an inhibition followed by an excitation (4%). Antrum/pylorus- and corpus-projecting neurons responded to NE with a similar percentage of excitatory (49 and 59%, respectively) and inhibitory (20% for both groups) responses. A lower percentage of excitatory (37%) and a higher percentage of inhibitory (36%) responses were, however, observed in fundus-projecting neurons. In all groups, pretreatment with prazosin or phenylephrine antagonized or mimicked the NE-induced excitation, respectively. Pretreatment with yohimbine or UK-14304 antagonized or mimicked the NE-induced inhibition, respectively. These data suggest that NE depolarization is mediated by alpha(1)-adrenoceptors, whereas NE hyperpolarization is mediated by alpha(2)-adrenoceptors. In 16 neurons depolarized by NE, amplitude of the action potential afterhyperpolarization (AHP) and its kinetics of decay (tau) were significantly reduced vs. control. No differences were found on the amplitude and tau of AHP in neurons hyperpolarized by NE. Using immunohistochemical techniques, we found that the distribution of tyrosine hydroxylase fibers within the DMV was significantly different within the mediolateral extent of DMV; however, distribution of cells responding to NE did not show a specific pattern of localization.     

Reduction of calcium currents in identified neurons of Helix pomatia: intracellular injection of D890

The actions of intracellularly applied D890 on membrane currents of the identified neurons B1, B2 and B3 of Helix pomatia were investigated. The TTX-resistant component of the inward current, the inward currents in Na+-free sucrose solution and in Ca2+-free Ba2+ solution were reduced. In Ca2+-free Co2+ solution the inward current was not affected. The late outward currents were strongly reduced. In solutions containing 20 mmol/l NiCl2 the remaining parts of these currents were blocked only to a lesser extent. The early outward current remained unchanged. It is concluded that intracellularly applied D890 mainly exerts its effects on the calcium current.