Dopamine activates two different receptors to produce variability in sign at an identified synapse

The role of ligand-gated chloride channels in behavioural alterations at elevated CO2 in a cephalopod

Projected future carbon dioxide (CO2) levels in the ocean can alter marine animal behaviours. Disrupted functioning of γ-aminobutyric acid type A (GABAA) receptors (ligand-gated chloride channels) is suggested to underlie CO2-induced behavioural changes in fish. However, the mechanisms underlying behavioural changes in marine invertebrates are poorly understood. We pharmacologically tested the role of GABA-, glutamate-, acetylcholine- and dopamine-gated chloride channels in CO2-induced behavioural changes in a cephalopod, the two-toned pygmy squid (Idiosepius pygmaeus). We exposed squid to ambient (∼450 µatm) or elevated (∼1000 µatm) CO2 for 7 days. Squid were treated with sham, the GABAA receptor antagonist gabazine or the non-specific GABAA receptor antagonist picrotoxin, before measurement of conspecific-directed behaviours and activity levels upon mirror exposure. Elevated CO2 increased conspecific-directed attraction and aggression, as well as activity levels. For some CO2-affected behaviours, both gabazine and picrotoxin had a different effect at elevated compared with ambient CO2, providing robust support for the GABA hypothesis within cephalopods. In another behavioural trait, picrotoxin but not gabazine had a different effect in elevated compared with ambient CO2, providing the first pharmacological evidence, in fish and marine invertebrates, for altered functioning of ligand-gated chloride channels, other than the GABAAR, underlying CO2-induced behavioural changes. For some other behaviours, both gabazine and picrotoxin had a similar effect in elevated and ambient CO2, suggesting altered function of ligand-gated chloride channels was not responsible for these CO2-induced changes. Multiple mechanisms may be involved, which could explain the variability in the CO2 and drug treatment effects across behaviours.

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.

Dopamine-mediated calcium channel regulation in synaptic suppression in L. stagnalis interneurons

D2 dopamine receptor-mediated suppression of synaptic transmission from interneurons plays a key role in neurobiological functions across species, ranging from respiration to memory formation. In this study, we investigated the mechanisms of D2 receptor-dependent suppression using soma-soma synapse between respiratory interneuron VD4 and LPeD1 in the mollusk Lymnaea stagnalis (L. stagnalis). We studied the effects of dopamine on voltage-dependent Ca²⁺ current and synaptic vesicle release from the VD4. We report that dopamine inhibits voltage-dependent Ca²⁺ current in the VD4 by both voltage-dependent and -independent mechanisms. Dopamine also suppresses synaptic vesicle release downstream of activity-dependent Ca²⁺ influx. Our study demonstrated that dopamine acts through D2 receptors to inhibit interneuron synaptic transmission through both voltage-dependent Ca²⁺ channel-dependent and -independent pathways. Taken together, these findings expand our understanding of dopamine function and fundamental mechanisms that shape the dynamics of neural circuit.

Phylogenetic and individual variation in gastropod central pattern generators

Gastropod molluscs provide a unique opportunity to explore the neural basis of rhythmic behaviors because of the accessibility of their nervous systems and the number of species that have been examined. Detailed comparisons of the central pattern generators (CPGs) underlying rhythmic feeding and swimming behaviors highlight the presence and effects of variation in neural circuits both across and within species. The feeding motor pattern of the snail, Lymnaea, is stereotyped, whereas the feeding motor pattern in the sea hare, Aplysia, is variable. However, the Aplysia motor pattern is regularized with operant conditioning or by mimicking learning using the dynamic clamp to change properties of CPG neurons. Swimming evolved repeatedly in marine gastropods. Distinct neural mechanisms underlie dissimilar forms of swimming, with homologous neurons playing different roles. However, even similar swimming behaviors in different species can be produced by distinct neural mechanisms, resulting from different synaptic connectivity of homologous neurons. Within a species, there can be variation in the strength and even valence of synapses, which does not have functional relevance under normal conditions, but can cause some individuals to be more susceptible to lesion of the circuit. This inter- and intra-species variation provides novel insights into CPG function and plasticity.

Two interconnected kernels of reciprocally inhibitory interneurons underlie alternating left-right swim motor pattern generation in the mollusk Melibe leonina

The central pattern generator (CPG) underlying the rhythmic swimming behavior of the nudibranch Melibe leonina (Mollusca, Gastropoda, Heterobranchia) has been described as a simple half-center oscillator consisting of two reciprocally inhibitory pairs of interneurons called swim interneuron 1 (Si1) and swim interneuron 2 (Si2). In this study, we identified two additional pairs of interneurons that are part of the swim CPG: swim interneuron 3 (Si3) and swim interneuron 4 (Si4). The somata of Si3 and Si4 were both located in the pedal ganglion, near that of Si2, and both had axons that projected through the pedal commissure to the contralateral pedal ganglion. These neurons fulfilled the criteria for inclusion as members of the swim CPG: 1) they fired at a fixed phase in relation to Si1 and Si2, 2) brief changes in their activity reset the motor pattern, 3) prolonged changes in their activity altered the periodicity of the motor pattern, 4) they had monosynaptic connections with each other and with Si1 and Si2, and 5) their synaptic actions helped explain the phasing of the motor pattern. The results of this study show that the motor pattern has more complex internal dynamics than a simple left/right alternation of firing; the CPG circuit appears to be composed of two kernels of reciprocally inhibitory neurons, one consisting of Si1, Si2, and the contralateral Si4 and the other consisting of Si3. These two kernels interact with each other to produce a stable rhythmic motor pattern.

Hidden synaptic differences in a neural circuit underlie differential behavioral susceptibility to a neural injury

Individuals vary in their responses to stroke and trauma, hampering predictions of outcomes. One reason might be that neural circuits contain hidden variability that becomes relevant only when those individuals are challenged by injury. We found that in the mollusc, Tritonia diomedea, subtle differences between animals within the neural circuit underlying swimming behavior had no behavioral relevance under normal conditions but caused differential vulnerability of the behavior to a particular brain lesion. The extent of motor impairment correlated with the site of spike initiation in a specific neuron in the neural circuit, which was determined by the strength of an inhibitory synapse onto this neuron. Artificially increasing or decreasing this inhibitory synaptic conductance with dynamic clamp correspondingly altered the extent of motor impairment by the lesion without affecting normal operation. The results suggest that neural circuit differences could serve as hidden phenotypes for predicting the behavioral outcome of neural damage.

Excitatory neurotransmitters in the tentacle flexor muscles responsible for space positioning of the snail olfactory organ

Recently, three novel flexor muscles (M1, M2 and M3) in the posterior tentacles of the snail have been described, which are responsible for the patterned movements of the tentacles of the snail, Helix pomatia. In this study, we have demonstrated that the muscles received a complex innervation pattern via the peritentacular and olfactory nerves originating from different clusters of motoneurons of the cerebral ganglia. The innervating axons displayed a number of varicosities and established neuromuscular contacts of different ultrastructural forms. Contractions evoked by nerve stimulation could be mimicked by external acetylcholine (ACh) and glutamate (Glu), suggesting that ACh and Glu are excitatory transmitters at the neuromuscular contacts. Choline acetyltransferase and vesicular glutamate transporter immunolabeled axons innervating flexor muscles were demonstrated by immunohistochemistry and in Western blot experiments. Nerve- and transmitter-evoked contractions were similarly attenuated by cholinergic and glutamatergic antagonists supporting the dual excitatory innervation. Dopamine (DA, 10⁻⁵ M) oppositely modulated thin (M1/M2) and thick (M3) muscle responses evoked by stimulation of the olfactory nerve, decreasing the contractions of the M1/M2 and increasing those of M3. In both cases, the modulation site was presynaptic. Serotonin (5-HT) at high concentration (10⁻⁵ M) increased the amplitude of both the nerve- and the ACh-evoked contractions in all muscles. The relaxation rate was facilitated suggesting pre- and postsynaptic site of action. Our data provided evidence for a DAergic and 5-HTergic modulation of cholinergic nerves innervating flexor muscles of the tentacles as well as the muscles itself. These effects of DA and 5-HT may contribute to the regulation of sophisticated movements of tentacle muscles lacking inhibitory innervation.

Immunological and pharmacological identification of the dopamine D1 receptor in the CNS of the pond snail, Lymnaea stagnalis

We investigated the presence and distribution of the D1 dopamine receptor in the CNS of Lymnaea stagnalis applying immunobloting and immunocytochemistry. We also investigated the effect of dopamine as well as the specific D1 receptor blocker, SCH23390, on the firing activity of the feeding modulator serotonergic neuron, CGC, which displayed D1 immunoreactivity. Immunoblot experiments showed one specifically labeled band with 62 kDa mw which is close to that of the mammalian D1 receptor. Neurons displaying D1-like immunoreactivity can be observed in each ganglion of the CNS but particularly in the pedal ganglia which are the center for locomotion. Dopamine regularly evokes burst activity in the serotonergic CGC at 1 mM and this effect could be antagonized by SCH23390. These observations suggest that a D1-like receptor molecule is present in the CNS of Lymnaea.

The evolution of pentameric ligand-gated ion channels

Fast, ionotropic neurotransmission mediated by ligand-gated ion channels is essential for timely behavioral responses in multicellular organisms. Metazoa employ more ionotropic neurotransmitters in more types of synapses, inhibitory or excitatory, than is generally appreciated. It is becoming increasingly clear that the adaptability of a single neurotransmitter receptor superfamily, the pentameric ligand-gated ion channels (pLGICs), makes the diversity in ionotropic neurotransmission possible. Modification ofa common pLGIC structure generates channels that are gated by ligands as different as protons, histamine or zinc and that pair common neurotransmitters with both cation and anion permeability. A phylogeny of the pLGIC gene family from representative metazoa suggests that pLGIC diversity is ancient and evolution of contemporary phyla was characterized by a surprising loss of pLGIC diversity. The pLGIC superfamily reveals aspects of early metazoan evolution, may help us identify novel neurotransmitters and can inform our exploration of structure/function relationships.

Ligand-gated chloride channels are receptors for biogenic amines in C. elegans

Biogenic amines such as serotonin and dopamine are intercellular signaling molecules that function widely as neurotransmitters and neuromodulators. We have identified in the nematode Caenorhabditis elegans three ligand-gated chloride channels that are receptors for biogenic amines: LGC-53 is a high-affinity dopamine receptor, LGC-55 is a high-affinity tyramine receptor, and LGC-40 is a low-affinity serotonin receptor that is also gated by choline and acetylcholine. lgc-55 mutants are defective in a behavior that requires endogenous tyramine, which indicates that this ionotropic tyramine receptor functions in tyramine signaling in vivo. Our studies suggest that direct activation of membrane chloride conductances is a general mechanism of action for biogenic amines in the modulation of C. elegans behavior.

Picrotoxin accelerates relaxation of GABAC receptors

Picrotoxin is a plant alkaloid that is often used to block the activity of neuronal GABA and glycine receptors. However, the mechanism by which picrotoxin inhibits these receptors is still in debate. In this study, we investigated the picrotoxin inhibition on perch-rho subunits expressed heterologously in Xenopus laevis oocytes, and on native GABA(C) receptors of perch bipolar cells. Both competitive and noncompetitive mechanisms were observed for picrotoxin inhibition of the GABA(C) receptor. In oocytes expressing the rho1A subunit, terminating simultaneously the coapplication of GABA and picrotoxin induced a large rebound of membrane current. In addition, picrotoxin significantly accelerated the kinetics of GABA responses, particularly in the relaxation (offset) phase of GABA currents. Both current rebound and the large acceleration of GABA relaxation were unique to picrotoxin inhibition and were not observed with the competitive antagonist (1,2,5,6-tetrahydropyridin-4-yl)-methylphosphinic acid or the allosteric modulator zinc. The change in kinetics induced by picrotoxin was also observed on receptors formed by other GABA rho subunits, as well as on the GABA(C) receptors of retinal bipolar cells. Based on these observations, we proposed a model in which picrotoxin binds to the GABA(C) receptor in both channel open and closed states. Overall, this model provides a remarkably good approximation of the experimental findings we observed for picrotoxin inhibition of GABA(C) receptors. These results support an allosteric mechanism of picrotoxin inhibition of ligand-gated chloride channels.

Regeneration of cerebral-buccal interneurons and recovery of ingestion buccal motor programs in Aplysia after CNS lesions

In the sea slug Aplysia, rhythmic biting is eliminated after bilateral cerebral-buccal connective (CBC) crushes and recovers within 14 days postlesion (dpl). The ability of cerebral-buccal interneuron-2 (CBI-2) to elicit ingestion buccal motor programs (iBMPs; i.e., fictive rhythmic ingestion) and to regenerate synaptic connections with target buccal neurons was assessed with intracellular recordings and dye injections. Isolated central ganglia were obtained from control animals and from lesioned animals at selected times after bilateral CBC crushes. Within 3 wk postlesion, transected CBI-2 axons sprouted at least 10 fine neurites confined to the core of the CBC that projected across the crush site toward the buccal ganglia. When fired with depolarizing current steps, CBI-2 was not observed to elicit iBMPs in preparations until 14 dpl. Thereafter a progressive enhancement in CBI-2's ability to elicit iBMPs was observed with time postlesion. By 40 dpl, CBI-2-elicited iBMPs were indistinguishable from those of controls. CBI-2 regenerated monosynaptic connections with appropriate buccal premotor- and motorneurons by 14 dpl, and the strength of these connections increased with time postlesion. Dramatic frequency facilitation was exhibited by the regenerating CBI-2 buccal synapses; for instance, at early postlesion times, no observable excitatory postsynaptic potentials (EPSPs) were obtained with 1- Hz stimulation of CBI-2, while at 7 Hz, a dramatic increase in EPSP amplitude was obtained with successive spikes. The present study shows that the time course of axonal and synaptic regeneration by command-like interneuron CBI-2 is correlated with the recovery of ingestion buccal motor programs elicited by CBI-2. These results parallel our previous findings of functional neural regeneration in the feeding system and suggest that functional neural regeneration is at least in part mediated by regeneration of specific synaptic pathways.

A systems approach to the cellular analysis of associative learning in the pond snail Lymnaea

We show that appetitive and aversive conditioning can be analyzed at the cellular level in the well-described neural circuitries underlying rhythmic feeding and respiration in the pond snail, Lymnaea stagnalis. To relate electrical changes directly to behavior, the snails were first trained and the neural changes recorded at multiple sites in reduced preparations made from the same animals. Changes in neural activity following conditioning could be recorded at the level of motoneurons, central pattern generator interneurons and modulatory neurons. Of significant interest was recent work showing that neural correlates of long-term memory could be recorded in the feeding network following single-trial appetitive chemical conditioning. Available information on the synaptic connectivity and transmitter content of identified neurons within the Lymnaea circuits will allow further work on the synaptic and molecular mechanisms of learning and memory.

Chloride channels of glycine and GABA receptors with blockers: Monte Carlo minimization and structure-activity relationships

GABA and glycine receptors (GlyRs) are pentameric ligand-gated ion channels that respond to the inhibitory neurotransmitters by opening a chloride-selective central pore lined with five M2 segments homologous to those of alpha(1) GlyR/ ARVG(2')LGIT(6')TVLTMTTQSSGSR. The activity of cyanotriphenylborate (CTB) and picrotoxinin (PTX), the best-studied blockers of the Cl(-) pores, depends essentially on the subunit composition of the receptors, in particular, on residues in positions 2' and 6' that form the pore-facing rings R(2') and R(6'). Thus, CTB blocks alpha(1) and alpha(1)/beta, but not alpha(2) GlyRs (Rundström, N., V. Schmieden, H. Betz, J. Bormann, and D. Langosch. 1994. Proc. Natl. Acad. Sci. U.S.A. 91:8950-8954). PTX blocks homomeric receptors (alpha(1) GlyR and rat rho(1) GABAR), but weakly antagonizes heteromeric receptors (alpha(1)/beta GlyR and rho(1)/rho(2) GABAR) (Pribilla, I., T. Takagi, D. Langosch, J. Bormann, and H. Betz. 1992. EMBO J. 11:4305-4311; Zhang D., Z. H. Pan, X. Zhang, A. D. Brideau, and S. A. Lipton. 1995. Proc. Natl. Acad. Sci. U.S.A. 92:11756-11760). Using as a template the kinked-helices model of the nicotinic acetylcholine receptor in the open state (Tikhonov, D. B., and B. S. Zhorov. 1998. Biophys. J. 74:242-255), we have built homology models of GlyRs and GABARs and calculated Monte Carlo-minimized energy profiles for the blockers pulled through the pore. The profiles have shallow minima at the wide extracellular half of the pore, a barrier at ring R(6'), and a deep minimum between rings R(6') and R(2') where the blockers interact with five M2s simultaneously. The star-like CTB swings necessarily on its way through ring R(6') and its activity inversely correlates with the barrier at R(6'): Thr(6')s and Ala(2')s in alpha(2) GlyR confine the swinging by increasing the barrier, while Gly(2')s in alpha(1) GlyR and Phe(6')s in beta GlyR shrink the barrier. PTX has an egg-like shape with an isopropenyl group at the elongated end and the rounded end trimmed by ether and carbonyl oxygens. In the optimal binding mode to alpha(1) GlyR and rho(1) GABAR, the rounded end of PTX accepts several H-bonds from Thr(6')s, while the elongated end enters ring R(2'). The lack of H-bond donors on the side chains of Phe(6')s (beta GlyR) and Met(6')s (rho(2) GABAR) deteriorates the binding. The hydrophilic elongated end of picrotin does not fit the hydrophobic ring of Pro(2')s/Ala(2')s in GABARs, but fit a more hydrophilic ring with Gly(2')s in GlyRs. This analysis provides explanations for structure-activity relationships of noncompetitive agonists and predicts a narrow pore of LGICs in agreement with experimental data on the permeation of organic cations.