Sensitization and habituation of command neurons during a defensive reflex in grape snails

All investigated neurones were classified into functional groups depending on their participation in unconditioned avoidance reflex of pneumostome closure. Habituation was produced by tactile stimuli applied with a frequency of 0.1 c/s. Spike responses of neurons participating in processing sensory information habituated gradually, while reactions of command neurones of avoidance behaviour became sensitized to the second or third stimulus of the series. Behavioral reaction was sensitized in parallel with command neurones in spite of habituation in other types of nerve cells. A conclusion is drawn that behavioral habituation is due to waning of responses in all the participating neurones, but the independnet process of sensitization is due to sensitization of command neurones. Probable mechanisms of neuronal sensitization are discussed.     

Duration of changes in electrical characteristics of command neurons after defensive conditioning in snails

Electrical characteristics of snail command neurons were studied during and after defensive conditioning. Tapping on the shell was used as a conditioned stimulus. A light air blow into the lung cavity orifice (pneumostome) was used as an unconditioned stimulus. The conditioned defensive reflex is known to be retained for 40 days. We have shown earlier the decrease in membrane and threshold potentials of command neurons after defensive conditioning (Gainutdinov et al., 1996). In these experiments it has been found that the decreased level of membrane and threshold potentials are maintained during 40 days after defensive reflex conditioning.     

Effects of FMRFamide on the activity of command neurons in defensive behavior of fed and fasting snails, Helix pomatia

FMRFamide (Phe-Met-Arg-Phe-NH2) micropneumophoresis changed bimodally the activity of LPa2, LPa3, PPa2 and PPa3 neurones in fasting and fed Helix pomatia. In fasting creatures peptide application elicited hyperpolarization and decreased the neuronal membrane excitability and responses to tactile stimulation. In fed snails peptide application caused depolarization, decreased membrane resistance and increased the neuronal membrane excitability and responses to tactile stimulation. Neurophysiological mechanisms underlying FMRFamide effects on feeding and defense behaviour are discussed.     

The active electrogenesis of the command neurons in the defensive behavior of the mollusk during conditioning]

In the paper changes of active electrogenesis of the command neurones of defensive closure of snail pneumostome at elaboration, extinction and repeated elaboration of classic conditioned defensive reflex to tactile stimulus was described; the tactile stimulation of other point of the body served as a differentiating stimulus. During the increase of biological significance of conditioned stimulus as a result of learning the stimulation of the command neurones in response to this stimulus was raised. At the same time the neurones showed decreased excitability in response to differentiating stimulus. Possible mechanisms of quick reconstruction of neurones excitability and functional value of PA generation threshold changes were discussed.     

Effect of selective protein kinase C inhibitor on synaptic plasticity of command neurons of defensive behaviour during sensitization in snails

Polymyxin B (proteinkinase C inhibitor) effects on nociceptive sensitization of semiintact preparation were investigated in LP11 and RP11 snail neurons. It was found that application of sensitizing stimuli to control snail head initiated neural membrane depolarization, increase its excitability as well as depression of neural responses evoked by sensory stimulation during short-term stage. Polymyxin B application suppressed neural responses evoked by sensitizing (nociceptive) stimuli. At the same time changes in neural membrane excitability as well as neural responses evoked by tactile stimulation of snail foot or chemical stimulation of snail head were similar with ones in control snails. Polymyxin Bdid does not change the depression of neural responses evoked by tactile stimulation of snail head during short-term stages of sensitization but significantly suppressed facilitation of neural responses evoked by tactile stimulation of snail head during long-term stage of sensitization. It was suggested that proteinkinase C is involved in regulation of nociceptive mechanisms as well as in plasticity selective induction mechanisms in command neuron synaptic inputs activated by tactile stimulation of snail head.     

Effect of cyclic AMP on the excitation and response of command neurons responsible for the snail defensive behavior caused by sensory stimulation]

Extraneuronal application of db-cAMP or intraneuronal injection of cAMP were found to increase the neural membrane excitability and synaptic facilitation in neural responses to sensory stimulation of Lpl1 and Rpl1 neurones. The db-cAMP exerted no effects on neural responses to tactile stimulation of the snail foot or mantle. The intraneuronal injection of cAMP produced synaptic facilitation only in neural responses to quinine application to the snail head. The findings suggest the cAMP selective involvement in postsynaptic mechanisms of inducing the long-term facilitation transient stage.     

Learning-related long-term synaptic facilitation in snails: Possible mechanisms of long-term memory formation

The long-term changes in the electrical activity of command neurons related to sensitization and elaboration of associative defensive behavioral habit (food rejection) were studied inHelix snail. The long-term effects consisted of facilitation of synaptic components in neuronal responses to the test stimulations. Variations were found in the dynamics of long-term synaptic facilitation of responses to the applied chemical and tactile stimuli in the course of sensitization, as well as dependence of the degree of long-term facilitation of responses to the test stimulation at the site of its application with respect to the site of the sensitizing stimulation (site-specific sensitization). After conditioning, the synaptic response of command neurons to the conditioning stimulation appeared approximately 30 min later than did the long-term sensitization in these cells. The minimum duration of long-term synaptic facilitation of responses to the test stimulation varied from 1 h (for tactile stimulation) to 3 h (for chemical stimulation). The maximum duration of effects exceeded 4 h. It is suggested that the observed features of the synaptic plasticity in command neurons during learning are based on the selective regulation of synaptic inputs by specific protein regulators, whose lifespan does not exceed 1 h to 3 h.Neirofiziologiya/Neurophysiology, Vol. 25, No. 5, pp. 383–389, September–October, 1993.     

Elementary and compound postsynaptic potentials in the defensive command neurons of Helix lucorum

The present communication concerns with the analysis of elementary and the compound excitatory postsynaptic potentials (eEPSPs and cEPSPs) recorded by intracellular microelectrode from an identified defensive command neuron of the snail Helix lucorum. The eEPSPs were evoked by single presynaptic action potentials (APs) elicited by cationic current injection into one of the identified sensory neurons synapsing on the respective command neuron. The cEPSPs were elicited by local brief tactile stimuli on the skin or internal organs. It was shown that the cEPSPs amplitudes depend mainly on the number of activated sensory neurons. Compound EPSPs depend also on frequency and the number of APs in the bursts occurring in a single neuron. Presynaptic APs having frequency 2-10 Hz evoke high frequency depression of that eEPSPs after an interval is followed by post-tetanic potentiation of single eEPSPs. Preceding stimulation of a pneumostom area facilitates the cEPSPs elicited by repeated stimulation of viscera. The eEPSPs from the same visceral area demonstrate no heterosynaptic facilitation in experiments with double parallel intracellular recording from responsive sensory and command neurons. The different types of the eEPSPs plasticity are discussed according to their contribution cEPSPs plastic changes.     

Presynaptic mechanisms for intensifying reactivity of command neurons in Helix pomatia defensive behavior under the effects of vasopressin analog

This study set out to investigate presynaptic mechanisms underlying reactivity of command neurons inHelix pomatia defensive behavior under the effects of desglycine-arginine vasopressin, a vasopressin analog. Peptide perfusion was found to intensify accumulated EPSP in command neurons during stimulation of the nerve without affecting the monosynaptic connection between primary sensory and command neurons. It was concluded that parallel pathways exist for the spread of excitation from the periphery to command neurons. Properties of such pathways are described and neurons with similar properties are identified. It is in these same cells that long-term changes in spontaneous EPSP induced by application of the peptide were noted.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 22, No. 6, pp. 723–730, November–December, 1990.     

Involvement of intracellular calcium in sensitization of command neurons of defensive behavior in the edible snail (Helix lucorum)

Examinations carried out on command neurons of defensive behavior in the edible snail using electrophysiological methods and a chlortetracycline fluorescent probe revealed that a single sensitizing action alters electrical neuronal activity and the amount of bound calcium in the cells. An initial increase in the amount of bound calcium (the first 15–20 min after the sensitizing action) coincides in time with depolarization, enhancement of plasma membrane excitability, and a decrease of amplitude and duration of the excitatory postsynaptic potentials (EPSP) induced by sensory stimulations. Repeated pronounced increase in the bound calcium level develops 50–60 min after the sensitizing action and correlates with facilitation of neuronal responses to sensory stimuli. Alterations in the bound calcium level in command neurons of defensive behavior in the course of sensitization development differed in dynamics and direction from the previously described bound calcium shifts in the same cells in the course of habituation development.P. K. Anokhin Institute of Normal Physiology, Academy of Medical Sciences of the USSR Moscow. I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR Leningrad. Translated from Neirofiziologiya, Vol. 23, No. 4, pp. 418–427, July–August, 1991.     

Interrelationship between grape snail alimentary and defense behavior command neurons]

Food stimulus (carrot juice), releasing feeding behaviour in intact snails, evoked spike discharges in giant meta-erebral cells (considered to be command neurones of feeding behaviour) and subthreshold EPSPs in giant parietal cells (command neurones of avoidance behaviour) of a half-intact preparation. Tactile stimulation, eliciting avoidance reactions in intact snails, evoked hyperpolarization in command neurones of feeding behaviour and a spike discharge in command neurones of avoidance behaviour. Spikes induced into either of command neurones produced no changes in activity of other command cells. Inhibition of command neurones of feeding behaviour is assumed to be the basis of behavioural choice when food and tactile stimulus are presented simultaneously.     

Effect of serotonin and noradrenaline on the magnitude of the response of the command neurons for defensive behavior in Helix lucorum L

Changes in synaptic responses of identified command neurones of avoidance behaviour to the electric nerve stimulation were investigated in the isolated nervous system of the snail during bath application of serotonin or noradrenaline. Serotonin (10(-5) M) elicited an increase of summary EPSP amplitude in the cells without changes of input resistance and resting potential. Noradrenaline (10(-5) M) application evoked an increase of EPSP amplitude, accompanied by an increase of the input resistance. Mechanisms of serotonin and noradrenaline influence on synaptic responses are discussed.     

The effect of a vasopressin analog on the reaction of the command neurons in the defensive behavior of the edible snail during nerve stimulation

On the level of snail command neurones of the defensive behaviour an increase was revealed of the amplitude of summary EPSP recorded in command neurones in response to nerve stimulation, an increase of probability of appearance of action potential in the reaction and, under certain conditions, a significant decrease of habituation speed at rhythmic (0.1 Hz) nerve stimulation against the background of peptide. The latter effect was found during comparison of groups of neurones--control neurones and those to which the peptide was presented before the first series of stimulation in the test, i. e. without preliminary elaboration of habituation. The decrease of the speed of habituation concerned both the amplitude of summary EPSP and the probability of action potential appearance in the reaction. All these changes against the background of vasopressin analogue may be the basis of the increase of spike reactions of command neurones of the defensive behaviour and thus the basis of the initiation or the increase of behavioural defensive reactions. The obtained effects were not protracted and took place only in the presence of the peptide in the extracellular medium.     

The effect of the small cardioactive peptide (b) on synaptic transmission efficiency and on the excitability of command neurons in the defensive behavior of the edible snail]

Small cardioactive peptide (b) (SCPb) application in concentration 5 X 10(-8) mol/l into the saline surrounding the snail CNS leads to an increase of amplitude of summate excitatory postsynaptic potential in the command neurones (CN) elicited by the intestinal nerve stimulation. Besides, SCPb causes an increase of excitability of the CN. Described effects can change the threshold of the neuronal net underlying avoidance behaviour. The possibility of integrative effects of peptides in low concentration on the behaviour is discussed.     

Dynamic defensive and feeding reactions to the acquisition of sensitization in edible snails]

In snails changes of defensive and alimentary behaviour and of reactions of command and modulator neurones of these kinds of behaviour were studied during elaboration of sensitization. After a single action of 50% chinine solution on mollusc's head, a short-term (during 50-70 min) and long-term (hours or days) facilitation was found of animals defensive reactions and responses of command neurones of defensive behaviour to tactile and chemical sensory stimuli. Alimentary behaviour of the snails and reactions of modulator neurones of alimentary behaviour to carrot juice presentation were inhibited in sensitized animals. Dynamics differences of defensive responses to tactile and chemical stimuli in the same sensitized animals were observed. Short-term responses facilitation under sensitization correlated chiefly with depolarization of the membrane potential of defensive behaviour neurones and with an increase of plasmatic membrane excitability. Prolonged facilitation of responses was due mainly to a change of effectiveness of synaptic transmission. The described model of sensitization elaboration can be a base for studying of molecular-cellular mechanisms, underlying the learning.     

The defensive behavior of the edible snail]

Literary and experimental data are summarized on organization of defensive behaviour in Helix. Muscular and neuronal mechanisms participating in this form of behaviour are reviewed.     

Effect of desglycine-arginine vasopressin on the excitability of the command neurons of the defensive reflex in the edible snail

Perfusion of the snail (Helix lucorum L.) CNS with DG-AVP (concentration 10(-6) M) in the course of low frequency intracellular stimulation (2-4-minute interval) of the defensive reflex command neurons led to an increase in the excitability. It was expressed both in the reduction of the spike generation latency, in the increased number of spikes in response to fixed stimuli, and in the activation of pacemaker potentials. If DG-AVP was added to the medium during endoneuronal habituation, there was no increase in the excitability. It is supposed that modification of the neuronal excitability may be caused by the DG-AVP effect on the pacemaker mechanism.     

Sensitization inHelix snail: Morphofunctional correlates in command neurons of withdrawal behavior

Electrophysiological effects have been studied in command neurons of withdrawal behavior inHelix snail. In parallel, correlated changes in the content of bound calcium (Ca-b), as well as changes in DNA condensation, were investigated using a chlortetracycline fluorescent probe and the fluorescent dye bisbenzimide, respectively. Short-term electrophysiological changes (depolarization of the membrane and elevation of its excitability) in sensitized snails have been found to be accompanied by an increase in the Ca-b level in the cell nucleus and by partial DNA decondensation. Long-term effects were characterized by more pronounced synaptic components of the responses — slow EPSPs evoked by sensory stimuli, as well as by further DNA decondensation and considerable elevation of the Ca-b content in the nucleus and cytoplasm. The results obtained inin vitro conditions have shown that Ca-binding nonhistone proteins of chromatin are components of the cell nucleus whose content may be measured by chlortetracycline fluorescence.Translated from Neirofiziologiya, Vol. 25, No. 2, pp. 150–157, March–April, 1993.