Potassium currents in isolated statocyst neurons and RPeD1 in the pond snail, Lymnaea stagnalis

Activity-dependent modulation of neuronal KV channels by retinoic acid enhances CaV channel activity

The metabolite of vitamin A, retinoic acid (RA), is known to affect synaptic plasticity in the nervous system and to play an important role in learning and memory. A ubiquitous mechanism by which neuronal plasticity develops in the nervous system is through modulation of voltage-gated Ca²⁺ (Ca_V) and voltage-gated K⁺ channels. However, how retinoids might regulate the activity of these channels has not been determined. Here, we show that RA modulates neuronal firing by inducing spike broadening and complex spiking in a dose-dependent manner in peptidergic and dopaminergic cell types. Using patch-clamp electrophysiology, we show that RA-induced complex spiking is activity dependent and involves enhanced inactivation of delayed rectifier voltage-gated K⁺ channels. The prolonged depolarizations observed during RA-modulated spiking lead to an increase in Ca²⁺ influx through Ca_V channels, though we also show an opposing effect of RA on the same neurons to inhibit Ca²⁺ influx. At physiological levels of Ca²⁺, this inhibition is specific to Ca_V2 (not Ca_V1) channels. Examining the interaction between the spike-modulating effects of RA and its inhibition of Ca_V channels, we found that inhibition of Ca_V2 channels limits the Ca²⁺ influx resulting from spike modulation. Our data thus provide novel evidence to suggest that retinoid signaling affects both delayed rectifier K⁺ channels and Ca_V channels to fine-tune Ca²⁺ influx through Ca_V2 channels. As these channels play important roles in synaptic function, we propose that these modulatory effects of retinoids likely contribute to synaptic plasticity in the nervous system.

Tale of tail current

The largest biomass of channel proteins is located in unicellular organisms and bacteria that have no organs. However, orchestrated bidirectional ionic currents across the cell membrane via the channels are important for the functioning of organs of organisms, and equally concern both fauna or flora. Several ion channels are activated in the course of action potentials. One of the hallmarks of voltage-dependent channels is a 'tail current' - deactivation as observed after prior and sufficient activation predominantly at more depolarized potentials e.g. for Kv while upon hyperpolarization for HCN α subunits. Tail current also reflects the timing of channel closure that is initiated upon termination of stimuli. Finally, deactivation of currents during repolarization could be a selective estimate for given channel as in case of HERG, if dedicated long and more depolarized 'tail pulse' is used. Since from a holding potential of e.g. -70 mV are often a family of outward K⁺ currents comprising I_A and I_K are simultaneously activated in native cells.

High sensitivity of spontaneous spike frequency to sodium leak current in a Lymnaea pacemaker neuron

The spontaneous rhythmic firing of action potentials in pacemaker neurons depends on the biophysical properties of voltage-gated ion channels and background leak currents. The background leak current includes a large K⁺ and a small Na⁺ component. We previously reported that a Na⁺ -leak current via U-type channels is required to generate spontaneous action potential firing in the identified respiratory pacemaker neuron, RPeD1, in the freshwater pond snail Lymnaea stagnalis. We further investigated the functional significance of the background Na⁺ current in rhythmic spiking of RPeD1 neurons. Whole-cell patch-clamp recording and computational modeling approaches were carried out in isolated RPeD1 neurons. The whole-cell current of the major ion channel components in RPeD1 neurons were characterized, and a conductance-based computational model of the rhythmic pacemaker activity was simulated with the experimental measurements. We found that the spiking rate is more sensitive to changes in the Na⁺ leak current as compared to the K⁺ leak current, suggesting a robust function of Na⁺ leak current in regulating spontaneous neuronal firing activity. Our study provides new insight into our current understanding of the role of Na⁺ leak current in intrinsic properties of pacemaker neurons.

Neural response in vestibular organ of Helix aspersa to centrifugation and re-adaptation to normal gravity

Gravity plays a key role in shaping the vestibular sensitivity (VS) of terrestrial organisms. We studied VS changes in the statocyst of the gastropod Helix aspersa immediately after 4-, 16-, and 32-day exposures to a 1.4G hypergravic field or following a 7-day recovery period. In the same animals we measured latencies of behavioral "negative gravitaxis" responses to a head-down pitch before and after centrifugation and found significant delays after 16- and 32-day runs. In an isolated neural preparation we recorded the electrophysiological responses of the statocyst nerve to static tilt (±19°) and sinusoids (±12°; 0.1 Hz). Spike sorting software was used to separate individual sensory cells' patterns out of a common trace. In correspondence with behavior we observed a VS decrease in animals after 16- (p < 0.05) and 32-day (p < 0.01) centrifugations. These findings reveal the capability of statoreceptors to adjust their sensitivity in response to a prolonged change in the force of gravity. Interestingly, background discharge rate increased after 16 and 32 days in hypergravity and continued to rise through the recovery period. This result indicates that adaptive mechanisms to novel gravity levels were long lasting, and re-adaptation from hypergravity is a more complex process than just "return to normal".

Intermediate and long-term memory are different at the neuronal level in Lymnaea stagnalis (L.)

Both intermediate-term memory (ITM) and long-term memory (LTM) require novel protein synthesis; however, LTM also requires gene transcription. This suggests that the behavioural output of the two processes may be produced differently at the neuronal level. The fresh-water snail, Lymnaea stagnalis, can be operantly conditioned to decrease its rate of aerial respiration and, depending on the training procedure, the memory can last 3h (ITM) or >24h (LTM). RPeD1, one of the 3 interneurons that form the respiratory central pattern generator (CPG) that drives aerial respiration, is necessary for memory formation. By comparing RPeD1's electrophysiological properties in naïve, 'ITM-trained', 'LTM-trained' and yoked control snails we discovered that while the behavioural phenotype of memory at 3 and 24h is identical, the situation at the neuronal level is different. When examined 3h after either the 'ITM' or 'LTM' training procedure RPeD1 activity is significantly depressed. That is, the firing rate, input resistance, excitability and the number of action potential bursts are all significantly decreased. In snails receiving the ITM-training, these changes return to normal 24h post-training. However, in snails receiving the 'LTM-training', measured RPeD1 properties (firing rate, excitability, membrane resistance, and the number of action potential bursts fired) are significantly different at 24h than they were at 3h. Additionally, 24h following LTM training RPeD1 appears to be functionally "uncoupled" from its control of the pneumostome as the link between RPeD1 excitation and pneumostome opening is weakened. These data suggest that the behavioural changes occurring during LTM are due to more widespread neuronal reorganization than similar behavioural changes occurring during ITM. Thus ITM and LTM are not just distinct in a chronological and transcriptional manner but are also distinct at the level of neuronal properties.

Functional changes in the snail statocyst system elicited by microgravity

BACKGROUND

The mollusk statocyst is a mechanosensing organ detecting the animal's orientation with respect to gravity. This system has clear similarities to its vertebrate counterparts: a weight-lending mass, an epithelial layer containing small supporting cells and the large sensory hair cells, and an output eliciting compensatory body reflexes to perturbations.

METHODOLOGY/PRINCIPAL FINDINGS

In terrestrial gastropod snail we studied the impact of 16- (Foton M-2) and 12-day (Foton M-3) exposure to microgravity in unmanned orbital missions on: (i) the whole animal behavior (Helix lucorum L.), (ii) the statoreceptor responses to tilt in an isolated neural preparation (Helix lucorum L.), and (iii) the differential expression of the Helix pedal peptide (HPep) and the tetrapeptide FMRFamide genes in neural structures (Helix aspersa L.). Experiments were performed 13-42 hours after return to Earth. Latency of body re-orientation to sudden 90° head-down pitch was significantly reduced in postflight snails indicating an enhanced negative gravitaxis response. Statoreceptor responses to tilt in postflight snails were independent of motion direction, in contrast to a directional preference observed in control animals. Positive relation between tilt velocity and firing rate was observed in both control and postflight snails, but the response magnitude was significantly larger in postflight snails indicating an enhanced sensitivity to acceleration. A significant increase in mRNA expression of the gene encoding HPep, a peptide linked to ciliary beating, in statoreceptors was observed in postflight snails; no differential expression of the gene encoding FMRFamide, a possible neurotransmission modulator, was observed.

CONCLUSIONS/SIGNIFICANCE

Upregulation of statocyst function in snails following microgravity exposure parallels that observed in vertebrates suggesting fundamental principles underlie gravi-sensing and the organism's ability to adapt to gravity changes. This simple animal model offers the possibility to describe general subcellular mechanisms of nervous system's response to conditions on Earth and in space.

Effects of weak environmental magnetic fields on the spontaneous bioelectrical activity of snail neurons

We examined the effects of 50-Hz magnetic fields in the range of flux densities relevant to our current environmental exposures on action potential (AP), after-hyperpolarization potential (AHP) and neuronal excitability in neurons of land snails, Helix aspersa. It was shown that when the neurons were exposed to magnetic field at the various flux densities, marked changes in neuronal excitability, AP firing frequency and AHP amplitude were seen. These effects seemed to be related to the intensity, type (single and continuous or repeated and cumulative) and length of exposure (18 or 20 min). The extremely low-frequency (ELF) magnetic field exposures affect the excitability of F1 neuronal cells in a nonmonotonic manner, disrupting their normal characteristic and synchronized firing patterns by interfering with the cell membrane electrophysiological properties. Our results could explain one of the mechanisms and sites of action of ELF magnetic fields. A possible explanation of the inhibitory effects of magnetic fields could be a decrease in Ca(2+) influx through inhibition of voltage-gated Ca(2+) channels. The detailed mechanism of effect, however, needs to be further studied under voltage-clamp conditions.

Action potential bursts in central snail neurons elicited by paeonol: roles of ionic currents

AIM

to investigate the effects of 2'-hydroxy-4'-methoxyacetophenone (paeonol) on the electrophysiological behavior of a central neuron (right parietal 4; RP4) of the giant African snail (Achatina fulica Ferussac).

METHODS

intracellular recordings and the two-electrode voltage clamp method were used to study the effects of paeonol on the RP4 neuron.

RESULTS

the RP4 neuron generated spontaneous action potentials. Bath application of paeonol at a concentration of ≥ 500 micromol/L reversibly elicited action potential bursts in a concentration-dependent manner. Immersing the neurons in Co(2+)-substituted Ca(2+)-free solution did not block paeonol-elicited bursting. Pretreatment with the protein kinase A (PKA) inhibitor KT-5720 or the protein kinase C (PKC) inhibitor Ro 31-8220 did not affect the action potential bursts. Voltage-clamp studies revealed that paeonol at a concentration of 500 micromol/L had no remarkable effects on the total inward currents, whereas paeonol decreased the delayed rectifying K(+) current (I(KD)) and the fast-inactivating K(+) current (I(A)). Application of 4-aminopyridine (4-AP 5 mmol/L), an inhibitor of I(A), or charybdotoxin 250 nmol/L, an inhibitor of the Ca(2+)-activated K(+) current (I(K(Ca))), failed to elicit action potential bursts, whereas tetraethylammonium chloride (TEA 50 mmol/L), an I(KD) blocker, successfully elicited action potential bursts. At a lower concentration of 5 mmol/L, TEA facilitated the induction of action potential bursts elicited by paeonol.

CONCLUSION

paeonol elicited a bursting firing pattern of action potentials in the RP4 neuron and this activity relates closely to the inhibitory effects of paeonol on the I(KD).

The fruit essential oil of Pimpinella anisum L. (Umblliferae) induces neuronal hyperexcitability in snail partly through attenuation of after-hyperpolarization

AIM OF THE STUDY

Many biological actions of Pimpinella anisum L. (Ainse), including antiepileptic activity have been demonstrated; however, there is no data concerning its precise cellular mechanisms of action. We determined whether the fruit essential oil of anise affect the bioelectrical activity of snail neurons in control condition or after pentylenetetrazol (PTZ) induced epileptic activity.

MATERIALS AND METHODS

Intracellular recordings were made under the current clamp condition and the effects of anise oil (0.01% or 0.05%) alone or in combination with PTZ were assessed on the firing pattern, action potential configuration and postspike potentials.

RESULTS

Anise oil changed the firing pattern from regular tonic discharge to irregular and then to bursting in intact cells or resulted in the robustness of the burst firing and the steepness of the paroxysmal shift induced by PTZ treatment. It also significantly increased the firing frequency and decreased both the after-hyperpolarization potential (AHP) following single action potential and the post-pulse AHP.

CONCLUSIONS

Likely candidate cellular mechanisms underlying the hyperexcitability produced by anise oil include enhancement of Ca(2+) channels activity or inhibition of voltage and/or Ca(2+) dependent K(+) channels activity underlying AHPs. These finding indicates that a certain caution is needed when Pimpinella anisum is used for treating patients suffering from epilepsy.

The functional consequences of paraoxon exposure in central neurones of land snail, Caucasotachea atrolabiata, are partly mediated through modulation of Ca2+ and Ca2+-activated K+-channels

Toxicity of paraoxon has been attributed to inhibition of cholinesterase, but little is known about its direct action on ionic channels. The effects of paraoxon (0.3 microM-0.6 microM) were studied on the firing behaviour of snail neurones. Paraoxon significantly increased the frequency of spontaneously generated action potentials, shortened the afterhyperpolarization (AHP) and decreased the precision of firing. Short periods of high frequency-evoked trains of action potentials led to an accumulation in the depth and duration of post-train AHPs that was evidenced as an increase in time to resumption of autonomous activity. The delay time in autonomous activity initiation was linearly related to the frequency of spikes in the preceding train and the slope of the curve significantly decreased by paraoxon. The paraoxon induced hyperexcitability and its depressant effect on the AHP and the post-train AHP were not blocked by atropine and hexamethonium. Calcium spikes were elicited in a Na+ free Ringer containing voltage dependent potassium channel blockers. Paraoxon significantly decreased the duration of calcium spikes and following AHP and increased the frequency of spikes. These findings suggest that a reduction in calcium influx during action potential may decrease the activation of calcium dependent potassium channels that participate in AHP generation and act as a mechanism of paraoxon induced hyperexcitability.

Comparative study of visuo-vestibular conditioning in Lymnaea stagnalis

In this review, we compare the current understanding of visuo-vestibular conditioning in Hermissenda crassicornis and Lymnaea stagnalis on the basis of behavioral, electrophysiologic, and morphologic studies. Paired presentation of a photic conditioned stimulus (CS) and an orbital rotation unconditioned stimulus (US) results in conditioned escape behavior in both species. In Hermissenda, changes in excitability of type B photoreceptors and morphologic modifications at the axon terminals follow conditioning. Caudal hair cells, which detect mechanical turbulence, have reciprocal inhibition with type B photoreceptors. In Lymnaea, the interaction between photoreceptors and hair cells is dependent on statocyst location. Furthermore, the organization of the Lymnaea eye is complex, with more than 100 photoreceptors distributed in a uniquely folded retina. Although the optimal conditions to produce long-term memory (memory persistent for >1 week) are almost identical in Hermissenda and Lymnaea, physiologic and morphologic differences suggest that the neuronal mechanisms underlying learning and memory are distinct.