Modification of the effects of glutamate by nitric oxide (NO) in a pattern-generating network.

Previous studies have shown that nitric oxide (NO) transforms the responses of various neurons to glutamate, though it remained unclear whether this mechanism is involved in the formation of behavior. We therefore studied the buccal generator of the feeding rhythm of the mollusk Lymnaea stagnalis (pond snail). In this organism, glutamate and NO are synthesized by defined neurons; glutamate is the neurotransmitter for the second phase of the standard triphasic feeding rhythm. Motoneuron B4 was used for monitoring. Studies using isolated CNS preparations showed that in some cases glutamate evoked hyperpolarization of B4 and terminated rhythmic network activity (n=17; group 1), while in other cases glutamate evoked depolarization of B4 and activation of a non-standard biphasic rhythm (n=12; group 2). In group 1, the NO donor nitroprusside lifted the inhibitory effect of glutamate (n=13), with transformation into an excitatory effect in nine cases. In group 2, the NO acceptor PTIO transformed the excitatory effect of glutamate into an inhibitory effect (n=7). These results provide evidence that: 1) the responses of the central generator of the buccal motor rhythm to glutamate depend on the NO level, and 2) this regulatory mechanism can modify feeding behavior.

Interaction between serotonin and nitric oxide (NO) in the activation of the serotoninergic system in the common snail.

The effects of serotonin and NO donors on serotoninergic neurons (more than 60) in the brain of the common snail Helix lucorum were studied. Serotonin and NO donors induced depolarization in all neurons. and increased spike activity and activated the synchronous synaptic input, including train-like input. resulting in the onset of synchronous train activity in all these neurons. The excitatory effect of serotonin was significantly decreased by 5,7-dihydroxytryptamine (5.7-DOT) and monomethylarginine--a blocker of endogenous NO synthesis. Both these substances blocked the serotonin activation of the synchronous train input. 5.7-DOT also blocked the activation of this input by NO donors. but had no effect on their excitatory actions. The effects of 5,7-DOT developed quickly, were reversible, and were comparable to the effects of serotonin receptor antagonists. The data obtained here provide evidence that serotonin and NO have similar regulatory effects on the serotoninergic system in the snail brain. Not only do they excite serotoninergic neurons, but they also coordinate their functioning by activating common synaptic inputs. which are apparently also serotoninergic. It is suggested that NO has the role of a second messenger during serotoninergic excitation and functions as a co-transmitter for the presynaptic input.

NO-producing compounds transform neuron responses to glutamate.

We have previously shown that NO increases the excitatory effects of glutamate and blocks the desensitization of neurons to glutamate in the brain of the common snail. The aim of the present work was to identify the possible effect of NO on inhibitory responses to glutamate in the neurons of this mollusk. Electrophysiological investigations were performed on three identified neurons. The results showed that glutamate (0.05-0.1 mM) initially induced hyperpolarization and blocked the spike activity of these neurons. Simultaneous exposure to glutamate and the NO donor nitroprusside or preincubation with an NO donor had the effect that cells again responded to glutamate with depolarization and excitation. The transformed excitatory response lasted several minutes and could be reproduced even after 24 h of washing. The NO synthase blocker monomethylarginine blocked the excitatory response to glutamate. Another agonist of glutamate receptors, N-methyl-D-aspartate (NMDA, 0.1-1 mM), initially had excitatory effects on these neurons; this effect was significantly enhanced after transformation of the response to glutamate by NO donors. The results obtained here show that NO is involved in transforming the inhibitory responses to glutamate to excitatory responses, and that this effect may be mediated by NMDA-type receptors.

The effects of nitrite on the excitability of brain neurons in the common snail.

Electrophysiological studies were performed on the effects of sodium nitrite (0.01-1 mM) on identified command neurons in the brain of the common snail. These studies showed that short periods of exposure to nitrite (from a few minutes up to 30 min) had little effect, producing a low level of depolarization and increases in neuron spike activity in only a few cases. Incubation of isolated brains with nitrite (1 mM) for periods ranging from 2 h to several days reduced excitation thresholds, significantly increased spike activity, increases responses to stimulation, and increases the levels of synaptic activity of the neurons studied. These effects increased with time and were stable, but were reversible on washing and were accompanied by depolarization and increased input resistance. The action of nitrite was imitated by known NO donors, and blockers of NO synthesis had the opposite effect. The possible mediation of the effects of nitrite by NO synthesis and the development of hypoxia is discussed.

The monosynaptic connection: modulating influence of opioid peptides on the plasticity of presynaptic neurons and identified synapses.

An investigation of the effect of opioid peptides (leucine-enkephalin and methionine-enkephalin) on the plastic properties of the system of monosynaptically connected LPa7-LPa3 and RPa3, and LPa8-LPa3 and RPa3, neurons in the brain of the edible snail was carried out. It was demonstrated that all three elements in the system under investigation (the presynaptic neuron, the postsynaptic neuron, and the synapse) display the same type of plasticity, i.e., habituation to rhythmic stimulation. The enkephalins exert a modulating effect on the plastic properties of the presynaptic neuron and synapse: they slow down the habituation of the presynaptic neuron in response to intracellular stimulation and the development of habituation at a level of the synapse as well. However, the changes in the character of the postsynaptic response in the presence of enkephalins are not the direct consequence of their influence on the plastic properties of the presynaptic neuron. In addition, the enkephalins decrease the efficiency of synaptic transmission in this system: they decrease the duration of the EPSP in the postsynaptic neuron.

Variation of the input resistance and membrane potential of a neuron in trace formation.

Trace changes in electrical activity, input resistance (Rinp), and membrane potential (MP) of brain neurons are studied in the mollusk Limnaea stagnalis for intracellular stimulation with a sinusoidal threshold current with a frequency of 0.1 Hz during 20 min. Some neurons are shown to exhibit an effect of facilitation, a rise in the level of activity being attending by an increase in Rinp and depolarization. Other neurons displayed lowered activity with a decrease of Rinp and hyperpolarization. The selectivity of the Rinp variations relative to the parameters of the stimuli (maximum changes at the frequency of the current used) suggests that it is precisely trace changes of Rinp which lie at the basis of the neuronal plasticity in "learning." Some neurons in this series of experiments did not alter their electrical response, Rinp, or MP for stimulation. The possible reason for the non-uniform reaction of different neurons to identical stimulation is discussed.