Autoradiographic localization and characterization of [125I]lysergic acid diethylamide binding to serotonin receptors in Aplysia

A 5-HT1A-like receptor is involved in the regulation of the embryonic rotation of Lymnaea stagnalis L

Cilia driven rotation of the pond snail Lymnaea stagnalis embryos is regulated by serotonin (5-HT). In the present study, physiological and biochemical assays were used to identify the 5-HT receptor type involved in rotation. The 5-HTergic agonists applied stimulated the rotation by 180-400% and their rank order potency was as follows: LSD>5-HT>8-OH-DPAT>WB4101>>5-CT. The applied antagonists, spiperone, propranalol and mianserin inhibited the 5-HT or 8-OH-DPAT stimulated rotation of the embryos by 50-70%. (3)H-5-HT was bound specifically to the washed pellet of the embryo homogenates. The specific binding of (3)H-5-HT was saturable and showed a single, high affinity binding site with K(d) 7.36 nM and B(max) 221 fmol/mg pellet values. This is the first report demonstrating the high affinity binding of (3)H-5-HT to the native receptor in molluscs. All of the pharmacons that stimulated the rotation or inhibited the 5-HT or 8-OH-DPAT evoked stimulation displaced effectively the binding of (3)H-5-HT. 5-HT resulted in the inhibition of forskolin stimulated cAMP accumulation, showing that 5-HT is negatively coupled to adenylate cyclase. Our results suggest that in the 5-HTergic regulation of the embryonic rotation in L. stagnalis a 5-HT(1A)-like receptor of the vertebrate type is involved.

Identification, molecular structure and expression of two cloned serotonin receptors from the pond snail, Helisoma trivolvis

Helisoma trivolvis has served as a model system to study the functions of serotonin (5-HT) from cellular, developmental, physiological and behavioural perspectives. To further explore the serotonin system at the molecular level, and to provide experimental knockout tools for future studies, in this study we identified serotonin receptor genes from the H. trivolvis genome, and characterized the molecular structure and expression profile of the serotonin receptor gene products. Degenerate oligonucleotide primers, based on conserved regions of the Lymnaea stagnalis 5-HT(1Lym) receptor, were used to amplify G protein-coupled biogenic amine receptor sequences from H. trivolvis genomic cDNA, resulting in the cloning of two putative serotonin receptors. The deduced gene products both appear to be G protein-coupled serotonin receptors, with well-conserved structure in the functional domains and high variability in the vestibule entrance of the receptor protein. Phylogenetic analysis placed these receptors in the 5-HT(1) and 5-HT(7) families of serotonin receptors. They are thus named the 5-HT(1Hel) and 5-HT(7Hel) receptors, respectively. In situ hybridization and immunofluorescence studies revealed that these genes and gene products are expressed most heavily in the ciliated pedal and mantle epithelia of H. trivolvis embryos. In adults, widespread expression occurred in all ganglia and connectives of the central nervous system. Expression of both receptor proteins was localized exclusively to neurites when examined in situ. In contrast, when isolated neurons were grown in culture, 5-HT(1Hel) and 5-HT(7Hel) immunoreactivity were located primarily in the cell body. This is the first study to reveal a 5-HT(7) receptor in a molluscan species.

Serotonergic enhancement of a 4-AP-sensitive current mediates the synaptic depression phase of spike timing-dependent neuromodulation

The mechanism underlying spike timing-dependent neuromodulation (STDN) was investigated in the opisthobranch mollusc Tritonia diomedea. The serotonergic dorsal swim interneurons (DSIs) dynamically modulated the synaptic output of ventral swim interneuron B (VSI); immediately after DSI stimulation, there was a potentiation of VSI synaptic strength followed by a longer-lasting synaptic depression. The potentiation phase of STDN was unaffected by spike broadening produced by the potassium channel blocker 4-aminopyridine (4-AP). In contrast, the depression phase was eliminated by 4-AP. Bath-applied serotonin (5-HT) decreased VSI spike duration and increased the magnitude of the A-current (IA), a voltage-dependent, transient, outward current. 4-AP preferentially blocked IA and prevented the spike narrowing caused by 5-HT, uncovering the full extent of 5-HT-induced synaptic potentiation. A consistent correlation was observed between IA and spike duration, but the correlation between synaptic strength and spike duration differed between preparations. Conductance-based simulations showed that the magnitude of A-current conductance could affect spike duration and gave an estimation of the change needed to produce spike narrowing. An artificial IA introduced into the VSI in the presence of 4-AP by means of the dynamic-clamp technique restored spike duration and gave a further approximation of the magnitude of modulation needed for spike narrowing. Together, these results suggest a mechanism for STDN: the DSIs release 5-HT, which causes a spike duration-independent enhancement of synaptic strength and a longer-lasting enhancement of IA that narrows the VSI spike and hence decreases VSI synaptic strength. Thus, STDN arises from the dynamics of independent intracellular signaling events.

Comparative localization of two serotonin receptors and sensorin in the central nervous system of Aplysia californica

Aplysia californica is a powerful model for understanding the cellular and molecular mechanisms underlying modulation of neuronal plasticity and learning. In the central nervous system of Aplysia, serotonin is associated with various behaviors. For example, it induces short-, intermediate-, and long-term synaptic changes in sensory neurons during learning and inhibits the afterdischarge of the bag cells that initiate egg-laying behavior. Little is known about the nature and contribution of serotonin receptors involved in the numerous serotonin-mediated physiological responses in Aplysia. Recently, two G(i)-coupled serotonin receptors (5-HT(ap1) and 5-HT(ap2)) were cloned. We now report that, by using in situ hybridization to express the profile of these receptors, we are able to gain critical insight into their roles in the behavior of Aplysia. We compared their distribution to that of sensorin-A, a peptide specifically found in sensory neurons. We wished to determine their involvement in some simple forms of behavioral modifications. 5-HT(ap1) and 5-HT(ap2) mRNAs are expressed in all ganglia of the Aplysia central nervous system. Stronger signal was observed with the 5-HT(ap2) antisense probe than with the 5-HT(ap1) antisense probe. Notably, mRNA coding for the receptors was found in several identified neurons, in the bag cells, in characterized serotonergic neurons, and in neurons of the mechanosensory clusters that expressed sensorin. We also observed heterogeneity of receptor expression between R2 and LPl1 and among neurons of a single cluster of sensory neurons. These results suggest that 5-HT(ap1) and 5-HT(ap2) receptors may regulate the response to serotonin and/or its release in several neurons.

Multiple serotonergic mechanisms contributing to sensitization in aplysia: evidence of diverse serotonin receptor subtypes

The neurotransmitter serotonin (5-HT) plays an important role in memory encoding in Aplysia. Early evidence showed that during sensitization, 5-HT activates a cyclic AMP-protein kinase A (cAMP-PKA)-dependent pathway within specific sensory neurons (SNs), which increases their excitability and facilitates synaptic transmission onto their follower motor neurons (MNs). However, recent data suggest that serotonergic modulation during sensitization is more complex and diverse. The neuronal circuits mediating defensive reflexes contain a number of interneurons that respond to 5-HT in ways opposite to those of the SNs, showing a decrease in excitability and/or synaptic depression. Moreover, in addition to acting through a cAMP-PKA pathway within SNs, 5-HT is also capable of activating a variety of other protein kinases such as protein kinase C, extracellular signal-regulated kinases, and tyrosine kinases. This diversity of 5-HT responses during sensitization suggests the presence of multiple 5-HT receptor subtypes within the Aplysia central nervous system. Four 5-HT receptors have been cloned and characterized to date. Although several others probably remain to be characterized in molecular terms, especially the Gs-coupled 5-HT receptor capable of activating cAMP-PKA pathways, the multiplicity of serotonergic mechanisms recruited into action during learning in Aplysia can now be addressed from a molecular point of view.

Structure and function of invertebrate 5-HT receptors: a review

Over the last decade, knowledge of invertebrate serotonin receptors has expanded greatly. The first 5-HT receptor from Drosophila was cloned 10 years ago, and subsequently, 11 additional receptor genes have been cloned from Drosophila, molluscs (Lymnaea and Aplysia) and nematodes (Caenorhabditis and Ascaris). Information has also accumulated from physiological and biochemical studies that have used vertebrate serotonergic ligands to characterize endogenous invertebrate receptors. Although the endogenous receptors are often classified according to mammalian-based categories, in many cases the pharmacological properties of vertebrate and invertebrate receptors differ significantly and the actual identity of the latter is questionable. By providing information on the gene structure and amino acid sequence, molecular cloning studies offer a more definitive way to identify and classify invertebrate 5-HT receptors. This review summarizes information on the pharmacological and transductional properties of cloned invertebrate 5-HT receptors, and considers recent studies of endogenous receptors in the light of this new data.

Cloning and functional expression of an Aplysia 5-HT receptor negatively coupled to adenylate cyclase

Serotonin (5-HT) is involved in the control of various behaviors in Aplysia californica, including reproduction, feeding, locomotion, circadian rhythm, synaptic plasticity, and synaptic growth. The large variety of functions of 5-HT is mediated by different receptor subtypes that are coupled to different second-messenger systems. Here, we report the cloning of a cDNA coding for an Aplysia G-protein-coupled 5-HT receptor (5-HTap1). Its deduced amino acid sequence resembles those of the 5-HT1 receptor subfamily. When expressed in stable cell lines, 5-HTap1 exhibits high-affinity binding for the serotonergic radioligand [N-methyl-3H]lysergic acid diethylamide. This binding is competed by several 5-HT agonists and antagonists, and the pharmacological profile of inhibition has some similarities with those of 5-HT1 and 5-HT7 receptors. Application of 5-HT or its agonists 5-carboxamidotryptamine maleate and (+/-)-8-hydroxy-2-(di-n-propyl-amino) tetralin hydrobromide on cells transformed with 5-HTap1 produced a dose-dependent inhibition of forskolin-stimulated cAMP accumulation. 5-HTap1 is thus negatively coupled to adenylate cyclase. The production of antiserum against the 5-HTap1 receptor allowed us to examine its expression in animal tissues. The receptor protein is detected in every tissue examined, although it seems only weakly expressed in some samples. The receptor is also found in every ganglia of the nervous system, both in the sheath and in the neurons. 5-HTap1 mRNA is absent from the sheath, indicating that the protein observed there is probably located on the nerve terminals.

Age dependent changes in serotonin and dopamine receptors in Aplysia californica

Age related changes in dopaminergic and serotonergic receptors were examined in Aplysia californica. In this study dopamine (DA) and serotonin (5-HT) receptor levels were examined for animals belonging to 4-, 5-, 6-, 8-, 9- and 12-month age groups. Receptors analysis was performed using radio-labeled d-[3H] lysergic acid diethylamide (LSD) as the specific ligand. Specific binding for 5-HT was found to be significantly greater than that for DA in the young (4-month post-hatch) animals. The total DA and 5-HT receptor levels changed significantly with age. Dopamine levels increased from 5.34 fmol/mg of protein at 4 months to 19.11 fmol/mg at 12 months. Serotonin receptor levels increased from 7.35 fmol/mg at 4 months to 20.45 at 12 months.

Identification of planarian serotonin receptor by ligand binding and PCR studies

Binding sites for [3H]lysergic acid diethylamide (LSD), a serotonin receptor agonist, were identified in planarian membranes by ligand binding studies. The Kd and Bmax values were approximately 4 microM and 0.2 pmol mg-1 protein, respectively. The serotonin (5-HT) receptor antagonists methiothepin and dihydroergocriptine were also efficient displacers of [3H]LSD binding. When planarians were decapitated in the presence of these antagonists, head regeneration was significantly retarded. These results suggest that planarians possess LSD binding sites which are involved in the process of regeneration. Furthermore, we found a putative (5-HT) receptor expressed in planarians by the degenerate primer polymerase chain reaction.

Pharmacological characterization of a serotonin receptor involved in an early embryonic behavior of Helisoma trivolvis

In contrast to the abundance of information on the many physiological and developmental actions of serotonin in molluscan nervous systems, comparatively little is known about the serotonin receptors involved in these responses. Embryos of the pulmonate gastropod, Helisoma trivolvis, display a cilia-driven rotational behavior that is regulated by endogenous serotonin. In the present study, two functional assays were used to determine some of the pharmacological properties of the receptors that mediate the cilio-excitatory action of serotonin. Time-lapse video microscopy was used to measure whole embryo rotation rate and cilia beat frequency in isolated cells. In dose-response experiments, serotonin was approximately 10 times more potent in stimulating cilia beat frequency over embryo rotation. In rotation experiments, 5-carboxyamidotryptamine and methysergide had effective agonist activity in dose ranges similar to that of serotonin (1 to 100 microM). In contrast, 8-hydroxydipropylaminotetralin HBr (8-OH-DPAT) displayed agonist activity of lower potency and effectiveness. Several compounds displayed antagonist activity in the 1 to 100 microM dose range, including mianserin, spiperone, ritanserin, 1-(1-naphthyl)piperazine, and propranolol. alpha-Methylserotonin had mixed agonist-antagonist activity, and metoclopramide, MDL-72222, and ketanserin were inactive. Experiments on isolated cells suggested that the extremely effective antagonism displayed by mianserin in the embryo rotation assay was due to its specific activity at ciliary serotonin receptors. These results implicate the presence of a novel serotonin receptor on embryonic ciliated cells that is pharmacologically distinct from those previously characterized in vertebrate or invertebrate systems.

Pharmacological characterization of a 5-HT receptor in locust nervous tissue

A 5-HT receptor in the nervous tissue of the desert locust (Schistocerca gregaria Forsk.) was investigated, using [3H]LSD (lysergic acid diethylamide) as the radioligand. [3H]LSD labels in addition a putative dopamine receptor whose specific [3H]LSD binding nevertheless could easily be diminished by co-incubation with 1 microM dopamine. The binding site was characterized by a KD of 1.64 nM, and a maximal concentration of binding sites of 79.8 fmol/mg protein. Pharmacological investigation revealed a relatively low affinity for the putative natural agonist, serotonin (KI = 0.209 microM). In contrast to the high affinity of classical serotonergic antagonists (e.g. dihydroergotamine or (+)-butaclamol) substances with subtype specificity such as 8-OH-DPAT (8-hydroxyl-1-(N,N-dipropyl)-aminotetralin) or ketanserin have only moderate affinities. Quantitative comparison of the pharmacological data demonstrated that there is obviously no pharmacological homology with vertebrate 5-HT receptors characterized so far. The only receptors with a close pharmacological relationship to the 5-HT receptor of locusts are the 5-HTdro1 receptor expressed in Drosophila nervous tissue and a 5-HT receptor in snail nervous tissue which might be homologous to that of locusts. The 5-HT receptor investigated, was shown to be G-protein-coupled, as addition of stable GTP analogues or depletion of Mg2+ ions from the incubation medium led to agonist-specific lowering of the affinity.

Correlation of 125I-LSD autoradiographic labeling with serotonin voltage clamp responses in Aplysia neurons

Autoradiographic receptor binding studies using 125I-LSD (2-[125I]lysergic acid diethyamide) revealed intense labelling on the soma of a symmetrically located pair of cells in the abdominal ganglion of Aplysia californica. This binding was blocked by micromolar concentrations of serotonin and lower concentrations of the serotonergic antagonists, cyproheptadine and mianserin (Kadan and Hartig, 1988). Electrophysiological investigation of responses to serotonin of neurons in the left upper quadrant, where one of the labeled neurons is located, revealed a range of serotonin responses. Cells L3 and L6 have a K+ conductance increase in response to serotonin that is not blocked by cyproheptadine or mianserin. Cells L2 and L4 have a biphasic response to serotonin: a Na+ conductance increase, which can be blocked by cyproheptadine and mianserin, followed by a voltage dependent Ca2+ conductance which is blocked by Co2+ but not the serotonergic antagonists. Cell L1, and its symmetrical pair, R1, have in addition to the Na+ and Ca2+ responses observed in L2 and L4, a Cl- conductance increase blocked by LSD, cyproheptadine and mianserin. LSD had little effect on the other responses. We conclude that the symmetrically located cells L1 and R1 have a Cl- channel linked to a cyproheptadine- and mianserin-sensitive serotonin receptor that is selectively labelled by 125I-LSD. This receptor has many properties in common with the mammalian serotonin 1C receptor.

Cloning and characterization of a Drosophila serotonin receptor that activates adenylate cyclase

Using a strategy based on nucleotide sequence homology between genes encoding receptors that interact with guanine nucleotide-binding proteins, we have isolated Drosophila genomic and cDNA clones encoding a functional serotonin receptor (5HT-dro receptor). This protein is expressed predominantly in Drosophila heads and exhibits highest homology with the human 5HT1A receptor. The predicted structure of the 5HT-dro receptor reveals two unusual features: (i) eight putative transmembrane domains instead of the expected seven and (ii) a Gly-Ser repeat that is a potential glycosaminoglycan attachment site. When stably introduced into mouse NIH 3T3 cells, the 5HT-dro receptor activates adenylate cyclase in response to serotonin and is inhibited by serotonin receptor antagonists such as dihydroergocryptine. The 5HT-dro receptor or closely related receptors might be responsible for the serotonin-sensitive cyclase that has been suggested to play a role in learning and modulation of circadian rhythm in a number of invertebrate systems.