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.     

The role of cAMP in generating excitatory serotonin responses of neurons in the edible snail

The neurons of the dorsal surface of snail Helix subesophageal ganglia respond similarly to the application of serotonin and the intracellular cAMP injection. These responses represent membrane depolarization. They increase in amplitude with membrane hyperpolarization and have a reverse potential between +10 and -30 mV. Presumably, these responses are associated with increased conductance for several ions. The values of the reverse potentials of serotonin and cAMP responses coincide in 7 out of 17 cells. Phosphodiesterase inhibitor theophylline caused a reversible increase in the amplitude and duration of both serotonin and cAMP responses and, used at a concentration of 1 mM, simulated them. The results obtained meet 2 out of 4 criteria demonstrating that cyclic nucleotides mediate a neurotransmitter response. It is suggested that cAMP may act as a second messenger in excitatory serotonin responses of snail Helix neurons.     

The role of brain serotonin in the expression of genetically determined defensive behavior

The review summarizes the results of long-term studies on the role of the brain mediator serotonin and genetic predisposition to various types of defensive behavior. The involvement of the serotonergic brain system in the mechanisms of genetic control of both active and passive defensive responses has been established using silver foxes, Norway rats of S40 selection for low and high aggressiveness to humans, aggressive mice with genetic knockout of monoaminoxidase A, and S40 rats selected for predisposition to passive defensive response of freezing (catalepsy). The changes in the serotonergic 5-HT1A-brain receptors of rats genetically predisposed to different strategies of defensive behavior were similar. However, the activity of the key enzyme of serotonin biosynthesis and the brain structures, in which serotonin metabolism was altered, significantly differed with regard to the preferred strategy. The conclusion was drawn that the 5-HT1A-receptors and enzymes of serotonin metabolism in the brain are involved in implementing genetic control of defensive behavior. Expression of the 5-HT1A-brain receptors was suggested to determine the levels of fear and anxiety and, consequently, the predisposition to defensive behavior, whereas the preferred strategy of defensive response (active or passive defensive) depends on genetically determined features of serotonin metabolism in the brain structures.     

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.     

The dynamics of the defensive and feeding conditioned reactions of the edible snail during long-term sensitization]

During the long-term sensitization in a snail there occurs an enhancement of a defensive reaction and a significant decrease of food intake. Defensive conditioning is facilitated and feeding conditioning deteriorated. The results give evidence that a formation of the state of the long-term sensitization is accompanied by a generation of a dominant defense focus.     

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.     

Behavioral changes during the acquisition of long-term sensitization of the defensive reflex in the edible snail

Reaction of long-term sensitization (LTS) of defensive reflex was elaborated in the snail. It was accompanied by a considerable increase of the time of closing of pneumostome. The results show that LTS is preserved for more than two week and may be a model of long-term memory.     

The mono- and polysynaptic connections between identified neurons in the system of the passive defensive reflex in the edible snail]

A structure of synaptic connections between the identified sensory and giant command neurons of Helix lucorum was studied. It was found that EPSPs arising in the giant neuron as responses to single action potentials generation in sensory neuron consist of several monosynaptic and several polysynaptic components having different magnitude, latencies, and plasticity. The latencies of monosynaptic components are determined by different presynaptic terminals' lengths.     

The role of cyclic adenosine monophosphate in simple forms of plasticity in the edible snail

cAMP-dependence of synaptic depression and facilitation was investigated in functionally identified synaptic connection in the snail. It was found that 5 mM imidazole (phosphodiesterase activator) as well as 2 mM tolbutamide (inhibitor of cAMP-dependent protein kinase) do not change the rate of EPSPs depression during rhythmic (0.1 Hz) nerve stimulation, and do not affect facilitation. But treatment with both these drugs decreases EPSPs amplitude. Possibility of cAMP-dependent modulation of synaptic effectiveness is discussed.     

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.     

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.     

The role of dopamine and serotonin in regulating bone mass and strength: studies on dopamine and serotonin transporter null mice

Neurotransmitter regulation of bone metabolism has been a subject of increasing interest and investigation. Dopamine (DA) has been reported to have effects on calcium and phosphorus metabolism. The dopamine transporter (DAT) is believed to control the temporal and spatial activity of released DA by rapid uptake of the neurotransmitter into presynaptic terminals. We have evaluated the histologic and biomechanical properties of the skeleton in mice homozygous for deletion of the DA transporter gene (DAT (-/-)) to help delineate the role of DA in bone biology. We have demonstrated that DAT (-/-) mice have reduced bone mass and strength. DAT (-/-) animals have shorter femur length and dry weight, and lower ash calcium content. Cancellous bone volume in the DAT (-/-) proximal tibial metaphysis is significantly decreased with reduced trabecular thickness. DAT (-/-) vertebrae have lower cancellous bone volume as a consequence of increased trabecular spacing and reduced trabecular number, and cortical thickness and bone area in the femoral diaphysis are reduced. The ultimate bending load (femoral strength) for the DAT (-/-) mice is 30% lower than the wild-type mice. Thus, deletion of the DAT gene results in deficiencies in skeletal structure and integrity. Since serotonin (5-HT) plays a role as a regulator of craniofacial morphogenesis, we explored the expression and function of 5-HT receptors and the 5-HT transporter (5-HTT) in bone. Primary cultures of rat osteoblasts (rOB) and a variety of clonal osteoblastic cell lines including ROS 17/2.8, UMR 106-H5 and Py1a show mRNA expression for the 5-HTT, and the 5-HT(1A), 5-HT(1D), 5-HT(2A) and 5-HT(2B) receptors by RT-PCR analysis and immunoblot. A relatively high density of nanomolar affinity 5-HTT binding sites is present in ROS 17/2.8 and UMR 106-H5 cells. The maximal [(3)H]5-HT uptake rate in ROS cells was 110 pmol/10 min/well, with a K(m) value of 1.13 microM. In normal differentiating rOB cultures, 5-HTT functional activity was observed initially at day 25, and activity increased by almost eight-fold at day 31. In mature rOB cultures, the estimated density of [(125)I]RTI-55 binding sites was 600 fmol/mg protein. PMA treatment caused a significant 40% reduction in the maximal uptake rate of [(3)H]5-HT, an effect prevented by pretreatment with staurosporine. 5-HT potentiates the PTH-induced increase in AP-1 activity in UMR 106-H5 cells. In 5-HTT (-/-) animals, cancellous bone volume (BV/TV) in the lumbar vertebrae is reduced, with a trend toward decreased trabecular thickness and trabecular number. These results demonstrate that osteoblastic cells express a functional serotonin system, with mechanisms for responding to and regulating uptake of 5-HT, and disruption of the 5-HTT gene may cause osteopenia.     

Neural control of olfaction and tentacle movements by serotonin and dopamine in terrestrial snail

We investigated the role of serotonin (5HT) and dopamine (DA) in the regulation of olfactory system function and odor-evoked tentacle movements in the snail Helix. Preparations of the posterior tentacle (including sensory pad, tentacular ganglion and olfactory nerve) or central ganglia with attached posterior tentacles were exposed to cineole odorant and the evoked responses were affected by prior application of 5HT or DA or their precursors 5-hydroxytryptophan (5HTP) and l-DOPA, respectively. 5HT applications decreased cineole-evoked responses recorded in the olfactory nerve and hyperpolarized the identified tentacle retractor muscle motoneuron MtC3, while DA applications led to the opposite changes. 5HTP and l-DOPA modified MtC3 activity comparable to 5HT and DA action. DA was also found to decrease the amplitude of spontaneous local field potential oscillations in the procerebrum, a central olfactory structure. In vivo studies demonstrated that injection of 5HTP in freely moving snails reduced the tentacle withdrawal response to aversive ethyl acetate odorant, whereas the injection of l-DOPA increased responses to “neutral” cineole and aversive ethyl acetate odorants. Our data suggest that 5HT and DA affect the peripheral (sensory epithelium and tentacular ganglion), the central (procerebrum), and the single motor neuron (withdrawal motoneuron MtC3) level of the snail’s nervous system.     

The role of cGMP in extinguishing the reactions of identified neurons in the edible snail to acetylcholine

Possible role of cGMP is studied in control of extinction of snail neurones RPa4, RPa3 and LPa3 reactions to acetylcholine (ACh), applied rhythmically to neurone soma by means of microiontophoresis. It is shown that guanylate cyclase activators which raise the cGMP level in the cell--Na nitroprusside and Na azide (5,10(-4)-10(-3) mol/l)--intensify at extracellular application the extinction of inward transmembrane current and membrane depolarization in response to ACh. Suggestion is made about participation of cGMP-dependent phosphorylation of membrane proteins in control of the development rate, depth and duration of neurone cholinoreceptors short-term plasticity.     

Mutual modulating effects of serotonin and dopamine on neurons of rat spinal ganglia

It was established in experiments on isolated rat spinal ganglia that the introduction of dopoamine (0.01–1.0 µM) into a superfusate potentiates the depolarizing responses of the neurons evoked by the action of serotonin, which is delivered from a micropipette under pressure, while the addition of serotonin in the same concentrations potentiates the depolarizing responses of the neurons evoked by the action of dopamine. The mutual potentiation of the effects of dopamine and serotonin depends on the concentration of the monoamines and is eliminated by blockers of the D₁- (but not D₂-dopamine) and type 2 serotonin (but not IA) receptors. The mutual potentiation of the effects of monoamines is of a postsynaptic nature and is associated with a change in the intracellular concentration of second messengers (Ca²⁺ and cAMP).A. M. Gor'kii Donetsk Medical Institute, Ministry of Health of the Ukrainian SSR. Translated from Neirofiziologiya, Vol. 23, No. 2, pp. 168–173, March–April, 1991.     

The role of dopamine and serotonin in the prolongation of post-decapitation convulsions in mice.

L-DOPA (320 mg/kg, i.p.) increased the duration of the clonic phase of post-decapitation convulsions (PDC) by 60% in mice pretreated with the peripheral decarboxylase inhibitor, Ro 4-4602 (50 mg/kg, i.p.). Assays of brains at the time of decapitation showed a 300% increase in dopamine (DM), an 80% reduction in serotonin (5-HT) and no change in norepinephrine (NE) levels. The effect of L-DOPA on PDC was not blocked by haloperidol (0.5 – 5.0 mg/kg), a blocker of DM receptors, nor by diethyldithiocarbamate (400 mg/kg) an inhibitor of NE synthesis. Parachlorophenylalanine (300 mg/kg × 3 days) produced an 80% reduction in 5-HT and a prolongation of PDC similar to that observed after L-DOPA. Prolongation of PDC was also seen after the 5-HT antagonists methysergide (5 mg/kg) and cinanserin (10 mg/kg), but not after cyproheptadine (10 mg/kg). The 5-HT precursor, 5-hydroxytryptophan (100 mg/kg), produced no change in PDC when used alone but inhibited L-DOPA's prolongation of PDC. The results suggest that L-DOPA acts by depleting 5-HT in bulbospinal pathways and thus enhancing reflex activity in the spinal cord.     

Viscero-cardiac reflexes in the edible snail]

Only two inhibitory neurons in the visceral ganglia provide the viscero-cardial and cardio-cardial reflexes of Helix pomatia. These neurons are connected in parallel and do not interact with each other. The cells have extensive receptive fields in all visceral organs which are considerably overlapped. These inhibitory neurons can provide the afferent function because of a high sensitivity to tactile stimulation of their endings. The analysis of the data showed that morphological and functional characteristics of the neurons in question corresponded completely to previously identified multifunctional interneuron V21.     

Effects of temperature on respiration defensive behavior and locomotion of fresh water snail Lymnaea stagnalis

Temperature dependence of lung respiration, defensive behavior and locomotion of Lymnaea stagnalis snail was studied. At the temperature in the range of 4-6 degrees C the rates of locomotion and respiration were reduced (as compared to control temperature of 14-16 degrees C), whereas defensive reactions were much more intense. Vice versa, the temperature rise to 24-26 degrees C activates respiration and locomotion but inhibits defensive behavior. It is suggested that the observed changes in Lymnaea behavior result from temperature-dependent reactions of neurons underlying these activities.     

The role of dopamine and opioid systems in the regulation of maternal behavior

This review describes in detail the different components and neuroanatomical basis of maternal behavior and also methodological approaches to investigation of parental reactions. The contributions of some endocrinal and neuromediator brain systems (in the first place, opioid and dopaminergic) to the regulation of maternal behavior are reported. The influences of ligands of opioid and dopamine receptors on the expressions of paternal reactions are analyzed especially. In concluding part the reasons of maternal depression and possibilities of this malfunction pharmacological correstion are discussed.