Katalin S.-Rózsa

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)

Fluorimetric determination of 5-hydroxytryptamine and catecholamines in the central nervous system and heart of Locusta migratoria migratorioides

Abstract 5-Hydroxytryptamine (5HT) was measured in the nervous and heart tissues of Locusta migratoria migratorioides in concentrations of 2·34 and 2·69 μg/g wet weight respectively. Among catecholamines, dopamine was found in the nervous tissue (1·31 μg/g wet weight) as well as in the heart (2·42 μg/g wet weight). Noradrenaline was present only in the brain, but in smaller amounts (0·24 μg/g wet weight) than dopamine. Adrenaline was found neither in the ganglia nor in the heart. On the basis of their occurrence in the CNS, 5HT, dopamine, and noradrenaline, and in the heart 5HT and dopamine can be regarded as transmitter or modulator substances.

Molluscs in biological monitoring of water quality

Molluscs living in Lake Balaton accumulate persistent toxic substances, namely heavy metals, to a greater extent, than other organisms, and can serve as excellent passive biomonitors. Especially gills are good accumulators. Regular sampling showed that the level of Cd and Hg concentrations increased, while Pb contamination decreased during the past 20 years in mussels, corresponding probably to changes in pollution of the Lake. In functional, active monitoring various behavioral patterns of molluscs were employed. In mussels the periodicity of activity and rest (pumping activity vs. valve closure) is a sensitive indicator of unfavorable conditions and so of toxic substances. Low concentrations of inorganic and organic toxicants (heavy metals, PCBs, PAH compounds) cause reduction of the active and increase of the rest periods in a concentration dependent manner in a few hours. A second, suitable test for evaluating toxicity of chemicals is the measurement of the water flow through the exhalant siphon. Under the effect of toxicants the siphon activity, both the strength and duration of water flow changes characteristically within a few minutes. For both behavioral tests special techniques have been developed suitable for long duration recording, supported by mechano-electrical transduction and computerized data evaluation. In case of the pond snail (Lymnaea stagnalis L.) the behavior (positive/negative geotaxis and orientation) is disturbed in the presence of the above mentioned toxic chemicals. The execution and evaluation of the changes in the movement of the snail is based on video-recording and measurement of the direction and distance the animal performs in uncontaminated water and in the presence of the pollutants during the same period of time (0.5-4 h).

Distribution of serotonin-containing neurons in the central nervous system of the snail Helix pomatia

SummaryThe distribution of serotonin (5HT)-containing neurons in the central nervous system of the snail Helix pomatia has been determined in whole-mount preparations by use of immunocytochemical and in vivo 5,6-dihydroxy-tryptamine labelling. 5HT-immunoreactive neuronal somata occur in all but the buccal and pleural ganglia. Immunoreactive fibres are present throughout the central nervous system. The 5HT-immunoreactive neuronal somata characteristically appear in groups, located mainly in the cerebral, pedal, visceral and right parietal ganglia. The majority of 5HT-immunoreactive neurons is located in the pedal ganglia. Additionally a dense network of 5HT-immunoreactive varicose fibres is found in the neural sheath of the central nervous system including all the nerves and ganglia. The number and distribution of 5HT-immunoreactive neurons correlates with that demonstrated by 5,6-dihydroxytryptamine labelling method.

Modulation of voltage-activated ion currents on identified neurons of Helix pomatia L. by interleukin-1

Summary1.The effect of interleukin-1 (IL-1) was studied on voltage-activated ion currents of the identified central neurons ofHelix pomatia L. using a twomicroelectrode voltage clamp. The voltage-activated inward current (ICa) was decreased, whereas the outward current (Inet K) was increased by IL-1.2.IL-1 affects both the transient and the delayed rectifying potassium currents. The IL-1 modulatory effect on the voltage-activated ion currents was voltage and dose dependent. The threshold concentration for IL-1 was 2 U/ml.3.The proposed modulatory effect of IL-1 appears to have more than one site of action on the neuron membrane ion channels.4.Rabbit anti-human IL-1 polyclonal antiserum eliminated the IL-1 effects on the voltage-activated inward and outward currents. This is the first report demonstrating a direct effect of IL-1 modulation of voltage-activated ion currents on neurons of mollusks.

In vivo labeling of serotonin-containing neurons by 5,7-dihydroxytryptamine inAplysia

Intrahemocoelial administration of 5,7-dihydroxytryptamine (5,7-DHT) to Aplysia californica induces a transient (less than 4 h) behavioral alteration. About 5 weeks after 5,7-DHT treatment, 5-hydroxytryptamine (5-HT)-containing neurons develop dark brown pigmentation. These labeled 5-HT neurons have normal physiological and pharmacological properties when investigated electrophysiologically. This contrasts with the long-term neurotoxic effect of 5,7-DHT on vertebrate neurons. This technique will greatly facilitate visual identification of 5-HT-containing neurons and study of their physiology and actions.

Physiological and histochemical evidence for neuroendocrine regulation of heart activity in the snail Lymnaea stagnalis L.

Abstract 1. 1. Lymnaea heart possesses a functionally double innervation (inhibitory and stimulatory fibres). 2. 2. On the isolated Lymnaea heart only ACh has an inhibitory effect; 5HT, adrenaline, noradrenaline, dopamine and glutamine are stimulating agents. The effect of ACh is blocked by mytolon, that of 5HT by BOL. 3. 3. From aqueous and acetone homogenates of Lymnaea heart four heart-active factors could be separated, three of them are excitatory, one is inhibitory. 4. 4. In Lymnaea heart there are neurosecretory neurons containing catecholamines (shown by Falcks method). 5HT is localized in the muscle cells. By stimulating the extracardial nerve the catecholamine content of these neurons can be depleted completely. 5. 5. The sequence is postulated: cardiac nerve→natural excitatory transmitter →catecholamine→5HT→heart muscle contracts.

Single neurone responses to tactile stimulation of the heart in the snail, Helix pomatia L.

SummaryThe electrical activity of the heart nerve and of single neurons in the suboesophageal ganglia were recorded during tactile stimulation of the heart. 15 neurons were identified which responded to heart stimulation by inhibiting or accelerating activity. Cells influenced by heart afferents are scattered in the visceral and in the right and left parietal ganglia.In most of the cases both decrease and increase of cell activity are caused by synaptic potentials, in some cases, however, the neuron is assumed to have a sensory character.The activity of three neurons influenced by heart stimulation was conducted into the heart nerve. These cells are central neurons of a heart-CNS-heart reflex.Some of the neurons located in the right parietal and visceral ganglia have no connection with the mechanoreceptors of the heart. Since their spikes propagate into the heart nerve, they probably take part in the extracardial regulation of heart activity.One of the neurons located in the visceral ganglion (cell V12) sends its axon into the heart nerve. The response of this neuron to heart stimulation was an increase in activity and an inhibition of the heart rate. This is an inhibitory neuron of the extracardial heart regulatory system.

Actions of methionine enkephalin and morphine on single neuronal activity in Helix pomatia L.

1. Methionine enkephalin and morphine accelerated the firing frequency of the cells in the right parietal (RPa2) and visceral (V) ganglia of Helix pomatia L. The effect of both agents was blocked by naloxone. 2. Naloxone blocked the inhibitory effect caused by dopamine application to the RPa2 cell. 3. Dopamine had a stimulatory effect on the neurosecretory cell (NS) which was not altered by naloxone pretreatments. 4. Naloxone specificity may be extended to a type of dopamine receptor. 5. In Helix pomatia L. an opiate receptor with similar pharmacological properties to that reported in mammals exists.

Heavy metals regulate physiological and behavioral events by modulating ion channels in neuronal membranes of molluscs

The interaction of heavy metals (HgCl2, CdCl2, CuCl2, PbCl2 and ZnCl2) and neurotransmitters (ACh, 5HT and DA) was studied on the excitable membrane of identified neurons of Lymnaea stagnalis and Helix pomatia. It was shown that,(1)The excitability and chemosensitivity of molluscan neurons were modified under the influence of the heavy metals Hg2+, Cd2+, Cu2+, Pb2+ and Zn2+.(2)Change in excitability to transmitters occurred as a potentiation or depression of the evoked response both in duration of membrane polarization and in frequency of spike activity.(3)The chemosensitivity changes in various ways, namely:excitatory effect was totally eliminated;one component of the effect was depressed.Different neurons may show different reactions to the same heavy metal.(4)There were differences in the effects of various heavy metals. Hg2+ has a more generalized effect than Cd2+; Cu2+, Pb2+ and Zn2+ were less effective in a number of neurons. The heavy metal effect was dose dependent, too.(5)Both inward and outward currents, which were evoked by neurotransmitters or voltage induced, were modified in most of the tested neurons. Both an increase and decrease of the membrane permeability occurred in different neurons in response to the same or different heavy metals.(6)The changes can be interpreted as a result ofdirect effect on specific ionic channels;modification of receptors binding ACh, 5HT, or DA;modification of intracellular processes responsible for the regulation of membrane permeability.