Identification and quantitative measurements of biogenic monoamines in the central nervous tissue of some gastropods

Dopamine as a Multifunctional Neurotransmitter in Gastropod Molluscs: An Evolutionary Hypothesis

AbstractThe catecholamine 3,4-dihydroxyphenethylamine, or dopamine, acts as a neurotransmitter across a broad phylogenetic spectrum. Functions attributed to dopamine in the mammalian brain include regulation of motor circuits, valuation of sensory stimuli, and mediation of reward or reinforcement signals. Considerable evidence also supports a neurotransmitter role for dopamine in gastropod molluscs, and there is growing appreciation for its potential common functions across phylogeny. This article reviews evidence for dopamine's transmitter role in the nervous systems of gastropods. The functional properties of identified dopaminergic neurons in well-characterized neural circuits suggest a hypothetical incremental sequence by which dopamine accumulated its diverse roles. The successive acquisition of dopamine functions is proposed in the context of gastropod feeding behavior: (1) sensation of potential nutrients, (2) activation of motor circuits, (3) selection of motor patterns from multifunctional circuits, (4) valuation of sensory stimuli with reference to internal state, (5) association of motor programs with their outcomes, and (6) coincidence detection between sensory stimuli and their consequences. At each stage of this sequence, it is proposed that existing functions of dopaminergic neurons favored their recruitment to fulfill additional information processing demands. Common functions of dopamine in other intensively studied groups, ranging from mammals and insects to nematodes, suggest an ancient origin for this progression.

Identification and characterization of catecholaminergic neuron B65, which initiates and modifies patterned activity in the buccal ganglia of Aplysia

Catecholamines are believed to play an important role in regulating the properties and functional organization of the neural circuitry mediating consummatory feeding behaviors in Aplysia. In the present study, we morphologically and electrophysiologically identified a pair of catecholaminergic interneurons, referred to as B65, in the buccal ganglia. Their processes innervate both the ipsi- and contralateral neuropil, and separate branches of B65 appeared to innervate the somata of both ipsi- and contralateral B4/5 neurons. B65 exhibited patterned burst(s) of activity during spontaneous cycles of fictive feeding. Patterned activity in B65 also was elicited by stimulation of the radula nerve, by depolarization of the pattern initiating neurons B31/32 or B63, and by bath application of -3,4-dihydroxyphenylalanine (DOPA). B65 appeared to be a member of the protraction group of neurons. Action potentials in B65 elicited fast one-for-one excitatory postsynaptic potentials (EPSPs) in neurons B4/5, B8A/B, B31/32, B63, and B64. In turn, B31/32 and B63 excited B65 and B64 inhibited B65. Some of the synaptic connections of B65 were plastic. For example, the fast EPSPs elicited in B4/5 and B64 decremented, whereas those in B31/32 andB8A/B facilitated. In addition to fast EPSPs, B65 elicited slow postsynaptic potentials in some of its follower cells. Depolarization of B65 elicited cycles of patterned activity indicative of fictive feeding in buccal neurons, including B65 itself. During series of B65-induced patterns, the properties of the buccal motor programs appeared to change. In particular, the activity of radula closure motor neurons B8A/B, which initially coincided mainly with the protraction phase of a cycle, gradually extended to overlap mostly with the retraction phase. This observation suggests that prolonged activity in B65 may play a role in transitioning from rejection-like to ingestion-like fictive feeding. The phase shift of the activity of B8A/B appears due, at least in part, to a decrease in activity of B4/5, and thus a reduction in inhibition from B4/5 onto B8A/B, during the retraction phase. The functional properties and synaptic connections of B65 suggest that it may play an important role in determining features of patterned neural activity in the buccal ganglia.

Behavioural and biochemical changes in the feeding system of Lymnaea induced by the dopamine and serotonin neurotoxins 6-hydroxydopamine and 5,6-dihydroxytryptamine

The neurotoxins 5,6-dihydroxytryptamine (5,6-DHT) and 6-hydroxydopamine (6-OH-DA) were used to examine the role of monoamines in the feeding system of the snail Lymnaea stagnalis . Biting responses to sucrose were monitored up to 25 days after injection with drugs. Cerebral and buccal ganglia and cerebro-buccal connectives from the same groups of snails were examined for changes in serotonin and dopamine levels by high performance liquid chromatography and the glyoxylic acid histo-fluorescence technique. Twelve to eighteen days after injection with 5,6-DHT only 57% of the snails responded to sucrose with biting movements, compared with 98% of controls. Those that did respond had a longer latency to the first bite and the bites were of shorter duration and occurred at a lower rate when compared with controls. This was accompanied by a 39% drop in 5-HT levels in the cerebro-buccal commissure and nerves and a loss of fluorescence in the axons of the paired cerebral giant cells, the main serotonergic neurons involved in feeding. Earlier behavioural effects of injecting 5,6-DHT at 20 min and 3-4 h after injection could not be explained by specific changes in 5-HT levels. Recovery of both behavioural response and serotonin levels occurred between 22-25 days after injection. Injection of 6-OH-DA also inhibited feeding responses but the effects were quicker (1-3 days) and more dramatic, with only 40% of snails showing any biting response to food compared with a 98% response in controls. Reduction in dopamine (DA) levels of 40%, together with a loss of DA fluorescence in nerve fibres accompanied the reduction of behavioural responsiveness. An early (3-4 h) effect of 6-OH-DA injection could not be correlated with a specific reduction in DA levels. Behavioural responses and DA levels returned to normal by 4-7 days after injection. Both neurotoxins inhibited uptake of their target monoamines and this appeared to be the main mechanism for depleting 5-HT and DA. Early effects of neurotoxin injection probably directly inhibited monoamine uptake, whereas long-term inhibition was a secondary effect because of degeneration of nerve fibres. Neither neurotoxin caused release of monoamines. Results from both neurotoxins suggest that monoamines play an important role in the initiation and maintainance of the feeding response, the consummatory phase of feeding. Neither neurotoxin prevented the occurrence of the initial appetitive phase of feeding when snails orientate towards food in the experimental chamber.