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.

Impairment of context memory by beta-amyloid peptide in terrestrial snail.

We examined influence of the beta-amyloid peptide (betaAP) (25-35) neurotoxic fragment on Helix lucorum food-aversion learning. Testing with aversively conditioned carrot showed that 2, 5 and 14 days after training the betaAP-injected group responded in a significantly larger number of cases and with a significantly smaller latency than the sham-injected control group. The results demonstrate that the AP partially impairs the learning process.

Effects of beta-amyloid (25-35) on learning in the common snail.

The effects of neurotoxic beta-amyloid fragment (25-35) on the formation of behavioral sensitization and a conditioned defensive reflex to food were studied. Administration of beta-amyloid (25-35) to common snails before the start of training led to a significant reduction in sensitization of the defensive reaction, weakening of the formation of the conditioned defensive reflex to food, and impairment of memory. These impairments to behavioral plasticity may be mediated by changes in synaptic plasticity previously observed in the presence of beta-amyloid.

Cannabinoid regulation in identified synapse of terrestrial snail.

In the terrestrial snail a direct monosynaptic glutamatergic connection between the primary sensory neuron and a premotor interneuron involved in withdrawal behaviour can be functionally identified using electrophysiological techniques. We investigated the involvement of cannabinoids in regulation of this synaptic contact. The results demonstrate that the specific binding sites for agonists to mammalian type 1 cannabinoid receptors (CB1Rs) exist in the snail's nervous system.

[Beta-amyloid peptide influences behavioral plasticity in terrestrial snail].

Influence of neurotoxic fragment of beta-amyloid peptide (25-35) on Helix lucorum behavioral plasticity (sensitization and food-aversion learning) was investigated. After beta-amyloid peptide (25-35) injection a significant reduction of behavioral long-term sensitization was observed. It was found, that beta-amyloid peptide (25-35) may interfere with associative learning and memory.

Caspase-like activity is essential for long-term synaptic plasticity in the terrestrial snail Helix.

Although caspase activity in the nervous system of mollusks has not been described before, we suggested that these cysteine proteases might be involved in the phenomena of neuroplasticity in mollusks. We directly measured caspase-3 (DEVDase) activity in the Helix lucorum central nervous system (CNS) using a fluorometrical approach and showed that the caspase-3-like immunoreactivity is present in the central neurons of Helix. Western blots revealed the presence of caspase-3-immunoreactive proteins with a molecular mass of 29 kDa.

Effect of beta-amyloid peptide on behavior and synaptic plasticity in terrestrial snail.

A large body of evidence implicates beta-amyloid peptide (betaAP) and other derivatives of the evolutionarily highly conserved amyloid precursor protein (APP) in the pathogenesis of Alzheimer's disease. However, the functional relationship of APP and its proteolytic derivatives to synaptic plasticity is not well known. We demonstrate that 30 min exposure to the 25-35 fragment of betaAP do not markedly change the dynamics of synaptic responses in identified neurons of terrestrial snail while a significant decrease of long-term sensitization was observed after 180 min betaAP bath application.

Ephaptic feedback in identified synapses in mollusk neurons.

The possible existence of intrasynaptic ephaptic feedback in the invertebrate CNS was studied. Intracellular recordings were made of excitatory postsynaptic potentials and currents arising on activation of the recently described monosynaptic connection between identified neurons in the snail CNS. In the presence of ephaptic feedback, tetanization of the postsynaptic neuron with hyperpolarizing impulses should activate presynaptic calcium channels, thus increasing the amplitude of excitatory postsynaptic potential, while sufficiently strong postsynaptic hyperpolarization applied during generation of the excitatory postsynaptic current should induce "supralinear" increases in its amplitude, as has been observed previously in rat hippocampal neurons.

[Ephaptic feedback in identified synapses of terrestrial snails].

A hypothesis for the existence of the intrasynaptic ephaptic feedback (EFB) in the invertebrate central nervous sytem was tested. Excitatory postsynaptic potentials (EPSPs) and currents (EPSCs) evoked by the activation of the recently described monosynaptic connection between the identified snail neurons were recorded intracellularly. In case of the EFB presence, the postsynaptic tetanization with hyperpolarization pulses could activate presynaptic Ca2+ channels and enhance the EPSP amplitude, whereas a steady postsynaptic hyperpolarization should induce a "supralinear" increase in EPSC amplitudes as it has been found in the rat hippocampus.

Postsynaptic calcium contributes to reinforcement in a three-neuron network exhibiting associative plasticity.

We show that activation of a single serotonergic cell is sufficient to trigger long-term associative enhancement of synaptic input to the withdrawal interneuron in a simple network consisting of three interconnected identified cells in the nervous system of terrestrial snail Helix. 1,2-bis (2-aminophenoxy) Ethane-N,N,N',N'-tetraacetic acid (BAPTA) injection in the postsynaptic neuron abolishes the pairing-specific enhancement of synaptic input. Activation of a single modulatory cell that we used to reinforce the synaptic input induced an increase of the intracellular [Ca2+] in the ipsilateral withdrawal interneuron without any changes of its membrane potential or input resistance.

Synaptic contact between mechanosensory neuron and withdrawal interneuron in terrestrial snail is mediated by L-glutamate-like transmitter.

The properties of the monosynaptic input from mechanosensory neurons to withdrawal interneurons were examined in Helix lucorum. The instantaneous I-V relation of the excitatory postsynaptic current in withdrawal interneurons was nonlinear, having a plateau region between -40 and -60 mV. On application of the blocker of vertebrate N-methyl-D-aspartate (NMDA) receptors AP5, or reduction of the Mg(2+) concentration, the current-voltage relation became more linear, suggesting that Mg(2+) may partially block the ion channel underlying the EPSC at voltages ranging from -40 to around -60 mV and the involvement of NMDA-like receptors.

Phase-dependent coordination of two motor programs in the buccal ganglion of a pteropod mollusk.

Rhythmic activity in two independent structures of the digestive apparatus of Clione limacina--the radula and the hooks--is coordinated by neural networks in the buccal ganglion during feeding behavior. Optical recording of neuron activity in the buccal ganglion, which allows simultaneous recording of large numbers of neurons, showed that the activity of all neurons producing volley discharges can be assigned to only two phases of a single rhythm. Instead of the four theoretically possible phases of rhythmic neural activity, all experiments yielded recordings of biphasic activity, even in conditions of electrical stimululation of the cerebrobuccal connectives, which triggers rhythmic movements of this apparatus in preparations.

[Phase-dependent coordination of two motor programs in the buccal ganglion of a pteropod mollusk].

Rhythmic activities of two feeding structures of the pteropod mollusk Clione limacina, redula and hooks, controlled by the neural networks in the buccal ganglia must be coordinated in order to produce a meaningful feeding response. Optical recording from the buccal ganglia, which allows the simultaneous activities of numerous neurons to be traced, revealed that such coordination exists in a phase-dependent manner. Instead of recording four theoretically possible phases of neuronal rhythmic activity, we always recorded only two phases, even after the electrical stimulation of the cerebro-buccal connective, which triggers both radula and hook rhythmic movements in the preparation.

Participation of GABA in establishing behavioral hierarchies in the terrestrial snail.

GABA-immunoreactive fibers were observed in the neuropile of each ganglion of Helix lucorum, while GABA-immunoreactive neural somata were found only in the buccal, cerebral, and pedal ganglia. Bath application of 10(-5) M GABA to the preparation "buccal mass-buccal ganglia" elicited a sequence of radula movements characteristic of feeding behavior. Corresponding bursts of activity were recorded in the buccal nerves under GABA application and in the buccal neurons recorded optically.

A single serotonergic modulatory cell can mediate reinforcement in the withdrawal network of the terrestrial snail.

A cluster of 40 serotonergic cells in the rostral part of pedal ganglia of the terrestrial snail Helix lucorum was shown previously to participate in the modulation of withdrawal behavior and to be necessary during the acquisition of aversive withdrawal conditioning in intact snails. Local extracellular stimulation of the serotonergic cells paired with a test stimulus elicited a pairing-specific increase (the difference between paired and explicitly unpaired sessions was significant, p <.01) of synaptic responses to test stimulation in the premotor interneurons involved in withdrawal.

Two modulatory inputs exert reciprocal reinforcing effects on synaptic input of premotor interneurons for withdrawal in terrestrial snails.

A cluster of serotonergic cells in the rostral part of pedal ganglia of the terrestrial snail Helix lucorum was shown previously to participate in modulation of withdrawal behavior, and to be necessary for elaboration of aversive withdrawal conditioning in intact snails. In the present experiments local extracellular stimulation of the serotonergic cells elicited a pairing-specific increase (difference between paired and explicitly unpaired sessions was significant, P<0.01) of synaptic responses in the premotor interneurons involved in withdrawal to paired nerve stimulation.

Effects of serotonin levels on postsynaptically induced potentiation of snail neuron responses.

Studies on identified neurons in the common snail were performed to investigate potentiation of EPSP arising after intracellular tetanization of the post-synaptic neuron. These experiments showed that high-frequency intracellular tetanization of a command neuron leads to biphasic long-term increases in the amplitude of synaptic responses to test stimulation. The role of serotonin in forming potentiation was studied.

[The effect of the serotonin level on the postsynaptically induced potentiation of neuronal responses in the snail].

The EPSP potentiation induced by the intracellular tetanization of a postsynaptic neuron was studied in the identified neurons of a terrestrial Helix lucorum snail. The high-frequency intracellular tetanization of the command neuron induced a two-phase long-term increase in the amplitude of synaptic responses. The role of 5-HT was studied in potentiation formation.

Dependence of synaptic facilitation postsynaptically induced in snail neurones on season and serotonin level.

The problem of stability of long-lasting synaptic facilitation postsynaptically induced by intracellular current pulses without concomitant presynaptic activation was addressed. A short (15-20 min) phase of synaptic facilitation induced by intracellular tetanization in identified snail neurones was stable and present in all experiments, while a long-term phase (lasting > or = 50 min) was observed only some experiments. Data analysis revealed dependence of the long-term phase on season.

Postsynaptic induction of long-term synaptic facilitation in snail central neurones.

Long-term facilitation in molluscs is believed to be induced due to purely presynaptic activations. We recorded excitatory postsynaptic potentials (EPSPs) simultaneously from two identified neurones of snail parietal ganglia. We report a non-decrementing facilitation induced by intracellular tetanization with concomitant presynaptic activation.

Participation of cardioactive peptides in habituation and sensitization of the synaptic input of command neurons of snail defense behavior.

The influence of serotonin, a small cardioactive peptide (SCPb) and FMRFamide on the time course of the responses of defense behavior command neurons in response to rhythmic stimulation of the intestinal nerve was studied in a preparation of the isolated common snail nervous system. It was found that the application of serotonin and SCPb induces an increase in the absolute magnitude of the synaptic response of the neurons under investigation; however, testing of the time course of the responses against the background of the action of these substances reveals the absence of sensitization and an increase in the rate of habituation to the rhythmic stimulus. The effect of the action of FMRFamide was entirely opposite to the effect of serotonin and SCPb, in both the influence on the amplitude of the single response and on the rate of habituation of the responses of the neurons.

[The participation of cardioactive peptides in the processes of habituation and sensitization of the synaptic input to the command neurons in snail defensive behavior].

Small cardioactive peptide (SCPb) application in a concentration of 5 x 10(-8) M in the saline results in a significant increase of the amplitude of the summary excitatory postsynaptic potential elicited by the intestinal nerve stimulation in the command neurons (CN) for withdrawal reactions. Serotonin (5-HT) exerts comparable effects in a concentration of 10(-6) M. FMRFamide application (10(-6) M) significantly decreases the amplitude of the synaptic response to a single stimulus.

Long-term sensitization and environmental conditioning in terrestrial snails.

The hypothesis that a long-term increase of behavioural responses in snails (over a period of days) might be due to environmental conditioning was examined. Training consisted of delivering electric shocks non-contingently with test stimuli twice per day for 5 days to freely moving snails on a ball floating in water. After training, a significant difference in amplitude of a withdrawal reaction to tactile test stimulation appeared between shocked and control snails.

The neurochemical basis of backward inhibition in a defense reaction reflex arc.

An investigation was carried out on a preparation of the isolated nervous system of the edible snail of the mechanism of habituation in the network of defense behavior neurons. It was established that the intracellular activation of one of the systems of the defense behavior command neurons, as in the case of the application of the neuropeptide FMRF-amide, leads to a decrease in the amplitude of the postsynaptic potentials in the remaining neurons of this class. An inference is reached, when account is taken of the presence of endogenous FMRF-amide in the defense behavior command neurons, regarding the possibility of active inhibition of the behavioral response to an appropriate stimulus through this neurochemical mechanism of the backward connection.