The action of serotonin on excitatory nerve terminals in lobster nerve-muscle preparations

Extrasynaptic Communication

Streams of action potentials or long depolarizations evoke a massive exocytosis of transmitters and peptides from the surface of dendrites, axons and cell bodies of different neuron types. Such mode of exocytosis is known as extrasynaptic for occurring without utilization of synaptic structures. Most transmitters and all peptides can be released extrasynaptically. Neurons may discharge their contents with relative independence from the axon, soma and dendrites. Extrasynaptic exocytosis takes fractions of a second in varicosities or minutes in the soma or dendrites, but its effects last from seconds to hours. Unlike synaptic exocytosis, which is well localized, extrasynaptic exocytosis is diffuse and affects neuronal circuits, glia and blood vessels. Molecules that are liberated may reach extrasynaptic receptors microns away. The coupling between excitation and exocytosis follows a multistep mechanism, different from that at synapses, but similar to that for the release of hormones. The steps from excitation to exocytosis have been studied step by step for the vital transmitter serotonin in leech Retzius neurons. The events leading to serotonin exocytosis occur similarly for the release of other transmitters and peptides in central and peripheral neurons. Extrasynaptic exocytosis occurs commonly onto glial cells, which react by releasing the same or other transmitters. In the last section, we discuss how illumination of the retina evokes extrasynaptic release of dopamine and ATP. Dopamine contributes to light-adaptation; ATP activates glia, which mediates an increase in blood flow and oxygenation. A proper understanding of the workings of the nervous system requires the understanding of extrasynaptic communication.

Regional Phenotypic Differences of the Opener Muscle in Procambarus clarkii: Sarcomere Length, Fiber Diameter, and Force Development

The opener muscle in the walking legs of the crayfish (Procambarus clarkii) has three distinct phenotypic regions although innervated by only one excitatory motor neuron. These regions (distal, central, and proximal) have varied biochemistry and physiology, including synaptic structure, troponin-T levels, fiber diameter, input resistance, sarcomere length, and force generation. The force generated by the central fibers when the excitatory neuron was stimulated at 40 Hz was more than the force generated by the other regions. This increase in force was correlated with the central fibers having longer sarcomeres when measured in a relaxed claw. These data support the idea that the central fibers are tonic-like and that the proximal fibers are phasic-like. The addition of serotonin directly on the fibers was hypothesized to increase the force generated by the central fibers more than in the other regions, but this did not occur at 40-Hz stimulation. We hypothesized that the central distal fibers would generate the most force due to the arrangement on the apodeme. This study demonstrates how malleable the motor unit is with modulation and frequency of stimulation.

Monoamines, Insulin and the Roles They Play in Associative Learning in Pond Snails

Molluscan gastropods have long been used for studying the cellular and molecular mechanisms underlying learning and memory. One such gastropod, the pond snail Lymnaea stagnalis, exhibits long-term memory (LTM) following both classical and operant conditioning. Using Lymnaea, we have successfully elucidated cellular mechanisms of learning and memory utilizing an aversive classical conditioning procedure, conditioned taste aversion (CTA). Here, we present the behavioral changes following CTA training and show that the memory score depends on the duration of food deprivation. Then, we describe the relationship between the memory scores and the monoamine contents of the central nervous system (CNS). A comparison of learning capability in two different strains of Lymnaea, as well as the filial 1 (F1) cross from the two strains, presents how the memory scores are correlated in these populations with monoamine contents. Overall, when the memory scores are better, the monoamine contents of the CNS are lower. We also found that as the insulin content of the CNS decreases so does the monoamine contents which are correlated with higher memory scores. The present review deepens the relationship between monoamine and insulin contents with the memory score.

Identification and developmental expression of the enzymes responsible for dopamine, histamine, octopamine and serotonin biosynthesis in the copepod crustacean Calanus finmarchicus

Neurochemicals are likely to play key roles in physiological/behavioral control in the copepod crustacean Calanus finmarchicus, the biomass dominant zooplankton for much of the North Atlantic Ocean. Previously, a de novo assembled transcriptome consisting of 206,041 unique sequences was used to characterize the peptidergic signaling systems of Calanus. Here, this assembly was mined for transcripts encoding enzymes involved in amine biosynthesis. Using known Drosophila melanogaster proteins as templates, transcripts encoding putative Calanus homologs of tryptophan-phenylalanine hydroxylase (dopamine, octopamine and serotonin biosynthesis), tyrosine hydroxylase (dopamine biosynthesis), DOPA decarboxylase (dopamine and serotonin biosynthesis), histidine decarboxylase (histamine biosynthesis), tyrosine decarboxylase (octopamine biosynthesis), tyramine β-hydroxylase (octopamine biosynthesis) and tryptophan hydroxylase (serotonin biosynthesis) were identified. Reverse BLAST and domain analyses show that the proteins deduced from these transcripts possess sequence homology to and the structural hallmarks of their respective enzyme families. Developmental profiling revealed a remarkably consistent pattern of expression for all transcripts, with the highest levels of expression typically seen in the early nauplius and early copepodite. These expression patterns suggest roles for amines during development, particularly in the metamorphic transitions from embryo to nauplius and from nauplius to copepodite. Taken collectively, the data presented here lay a strong foundation for future gene-based studies of aminergic signaling in this and other copepod species, in particular assessment of the roles they may play in developmental control.

Neuromodulation of neuronal circuits: back to the future

All nervous systems are subject to neuromodulation. Neuromodulators can be delivered as local hormones, as cotransmitters in projection neurons, and through the general circulation. Because neuromodulators can transform the intrinsic firing properties of circuit neurons and alter effective synaptic strength, neuromodulatory substances reconfigure neuronal circuits, often massively altering their output. Thus, the anatomical connectome provides a minimal structure and the neuromodulatory environment constructs and specifies the functional circuits that give rise to behavior.

Serotonin and synaptic transmission at invertebrate neuromuscular junctions

The serotonergic system in vertebrates and invertebrates has been a focus for over 50 years and will likely continue in the future. Recently, genomic analysis and discovery of alternative splicing and differential expression in tissues have increased the knowledge of serotonin (5-HT) receptor types. Comparative studies can provide useful insights to the wide variety of mechanistic actions of 5-HT responsible for behaviors regulated or modified by 5-HT. To determine cellular responses and influences on neural systems as well as the efferent control of behaviors by the motor units, preparations amenable to detailed studies of synapses are beneficial as working models. The invertebrate neuromuscular junctions (NMJs) offer some unique advantages for such investigations; action of 5-HT at crustacean NMJs has been widely studied, and leech and Aplysia continue to be key organisms. However, there are few studies in insects likely due to the focus in modulation within the CNS and lack of evidence of substantial action of 5-HT at the Drosophila NMJs. There are only a few reports in gastropods and annelids as well as other invertebrates. In this review we highlight some of the key findings of 5-HT actions and receptor types associated at NMJs in a variety of invertebrate preparations in hopes that future studies will build on this knowledge base.

Crustacean neuroendocrine systems and their signaling agents

Decapod crustaceans have long served as important models for the study of neuroendocrine signaling. For example, the process of neurosecretion was first formally demonstrated by using a member of this order. In this review, the major decapod neuroendocrine organs are described, as are their phylogenetic conservation and neurochemistry. In addition, recent advances in crustacean neurohormone discovery and tissue mapping are discussed, as are several recent advances in our understanding of hormonal control in this group of animals.

Increased Ca2+ influx through Na+/Ca2+ exchanger during long-term facilitation at crayfish neuromuscular junctions

Intense motor neuron activity induces a long-term facilitation (LTF) of synaptic transmission at crayfish neuromuscular junctions (NMJs) that is accompanied by an increase in the accumulation of presynaptic Ca2+ ions during a test train of action potentials. It is natural to assume that the increased Ca2+ influx during action potentials is directly responsible for the increased transmitter release in LTF, especially as the magnitudes of LTF and increased Ca2+ influx are positively correlated. However, our results indicate that the elevated Ca2+ entry occurs through the reverse mode operation of presynaptic Na+/Ca2+ exchangers that are activated by an LTF-inducing tetanus. Inhibition of Na+/Ca2+ exchange blocks this additional Ca2+ influx without affecting LTF, showing that LTF is not a consequence of the regulation of these transporters and is not directly related to the increase in [Ca2+]i reached during a train of action potentials. Their correlation is probably due to both being induced independently by the strong [Ca2+]i elevation accompanying LTF-inducing stimuli. Our results reveal a new form of regulation of neuronal Na+/Ca2+ exchange that does not directly alter the strength of synaptic transmission.

Temperature dependent modulation of lobster neuromuscular properties by serotonin

In cold-blooded species the efficacy of neuromuscular function depends both on the thermal environmental of the animal's habitat and on the concentrations of modulatory hormones circulating within the animal's body. The goal of this study is to examine how temperature variation within an ecologically relevant range affects neuromuscular function and its modulation by the neurohormone serotonin (5-HT) in Homarus americanus, a lobster species that inhabits a broad thermal range in the wild. The synaptic strength of the excitatory and inhibitory motoneurons innervating the lobster dactyl opener muscle depends on temperature, with the strongest neurally evoked muscle movements being elicited at cold (<5 degrees C) temperatures. However, whereas neurally evoked contractions can be elicited over the entire temperature range from 2 to >20 degrees C, neurally evoked relaxations of resting muscle tension are effective only at colder temperatures at which the inhibitory junction potentials are hyperpolarizing in polarity. 5-HT has two effects on inhibitory synaptic signals: it potentiates their amplitude and also shifts the temperature at which they reverse polarity by approximately +7 degrees C. Thus 5-HT both potentiates neurally evoked relaxations of the muscle and increases the temperature range over which neurally evoked muscle relaxations can be elicited. Neurally evoked contractions are maximally potentiated by 5-HT at warm (18 degrees C) temperatures; however, 5-HT enhances excitatory junction potentials in a temperature-independent manner. Finally, 5-HT strongly increases resting muscle tension at the coldest extent of the temperature range tested (2 degrees C) but is ineffective at 22 degrees C. These data demonstrate that 5-HT elicits several temperature-dependent physiological changes in the passive and active responses of muscle to neural input. The overall effect of 5-HT is to increase the temperature range over which neurally evoked motor movements can be elicited in this neuromuscular system.

cAMP acts on exchange protein activated by cAMP/cAMP-regulated guanine nucleotide exchange protein to regulate transmitter release at the crayfish neuromuscular junction

Glutamatergic synapses are highly modifiable, suiting them for key roles in processes such as learning and memory. At crayfish glutamatergic neuromuscular junctions, hyperpolarization and cyclic nucleotide-activated (HCN) ion channels mediate hormonal modulation of glutamatergic synapses and a form activity-dependent long-term facilitation (LTF) of synaptic transmission. Here, we show that a new target for cAMP, exchange protein activated by cAMP (Epac) or cAMP-regulated guanine nucleotide exchange protein, is involved in the hormonal enhancement of synaptic transmission by serotonin. Induction of LTF "tags" synapses, rendering them responsive to cAMP in an HCN-independent manner. Epac also mediates the enhancement of tagged synapses. Thus, the cAMP-dependent enhancement of transmission is mediated by two separate pathways, neither of which involves protein kinase A.

Modulation of available vesicles and release kinetics at the inhibitor of the crayfish neuromuscular junction

We have investigated the effect of serotonin (5-HT) and okadaic acid (OA) on presynaptic processes at the crayfish inhibitory neuromuscular junction. Two different physiological parameters of transmitter release were examined: release kinetics and the size of the readily releasable pool of vesicles (RRP). Using a paired pulse stimulus and high frequency trains, we established that a single broad action potential, recorded in 20 mM tetraethylammonium and 1 mM 4-amino-pyridine, released the RRP in its entirety. Thus, by measuring the amplitude of inhibitory postsynaptic potential (IPSC) we were able to directly assess the effects of 5-HT and OA on the RRP. Serotonin at 200 nM and OA at 2.5 microM each significantly increased IPSC above control levels and the effects of these two modulators were comparable. Both modulators also induced a leftward shift in the rising phase of IPSC, i.e. an apparent acceleration in release kinetics. The shift caused by OA was significantly more pronounced than that induced by 5-HT. This apparent acceleration in release was not associated with a corresponding change in the presynaptic Ca2+ transient measured at a 2 kHz resolution, suggesting that modulation was not due to an acceleration in Ca2+ channel kinetics. In view of the comparable increase in the size of the RRP by the modulators, the differential modulation of release kinetics suggests that these two parameters may be modulated by separate biochemical processes.

Regulation of serotonin levels by multiple light-entrainable endogenous rhythms

This study examined whether serotonin levels in the brain of the American lobster, Homarus americanus, are under circadian control. Using high-performance liquid chromatography and semi-quantitative immunocytochemical methods, we measured serotonin levels in the brains of lobsters at six time points during a 24-h period. Lobsters were maintained for 2 weeks on a 12 h:12 h light:dark cycle followed by 3 days of constant darkness. Under these conditions, brain serotonin levels varied rhythmically,with a peak before subjective dusk and a trough before subjective dawn. This persistent circadian rhythm in constant darkness indicates that serotonin levels are controlled by an endogenous clock. Animals exposed to a shifted light cycle for &gt;10 days, followed by 3 days in constant darkness,demonstrate that this rhythm is light entrainable. Separate analyses of two pairs of large deutocerebral neuropils, the accessory and olfactory lobes,show that serotonin levels in these functionally distinct areas also exhibit circadian rhythms but that these rhythms are out of phase with one another. The olfactory and accessory lobe rhythms are also endogenous and light entrainable, suggesting the presence of multiple clock mechanisms regulating serotonin levels in different brain regions.

5-HT offsets homeostasis of synaptic transmission during short-term facilitation

In this study, we approach the topic of vesicle recruitment and recycling by perturbing neurotransmission at the crayfish neuromuscular junction with altered electrical activity and the presence of the neuromodulator serotonin (5-HT). After induction of short-term facilitation (STF) with stimulus pulse trains (40 Hz, 20 pulses), the amount of synaptic transmission can be maintained at a relatively constant level, producing a plateau in the amplitude of the excitatory postsynaptic potentials (EPSPs) throughout the remaining stimuli within a train of a few hundred milliseconds. With an increase in the frequency of the stimuli within a train (60 Hz, 20 pulses), an altered plateau of larger EPSP amplitudes occurs. This suggests that differential rates of vesicle recruitment can be rapidly reached and maintained. Exposure of nerve terminals to 5-HT further enhances the EPSP amplitudes to yet a higher plateau level. The effect of 5-HT is more pronounced for 40-Hz pulse trains than for 60-Hz trains. This suggests that 5-HT can recruit vesicles into the readily releasable pool (RRP) and that the recruitment is limited at higher stimulation frequencies. The attainment of a larger amplitude in the plateaus of the EPSPs at 60 Hz compared with 40 Hz also suggests that the rapid induction of STF enhances the entry of vesicles into the RRP. By direct quantal counts, mean quantal content increases linearly during STF, and 5-HT offsets the linear release. We propose that 5-HT and electrically induced recruitment of vesicles from a reserve pool to the RRP may share similar recruitment mechanisms.

Calcium influx through HCN channels does not contribute to cAMP-enhanced transmission

Serotonin is a native neuromodulator of synaptic transmission at glutamatergic neuromuscular junctions of crayfish limb muscles. During times of stress, serotonin binds to presynaptic receptors, which activate adenylyl cyclase to elevate presynaptic levels of cAMP. cAMP binds to two presynaptic target proteins, hyperpolarization and cyclic nucleotide-activated (HCN) ion channels and an exchange protein activated by cAMP (Epac), and activation of these effectors results in enhancement of transmitter release to action potentials. cAMP elevation also results in a small preterminal rise in [Ca(2+)](i), which we show here to result from Ca(2+) influx through the presynaptic HCN channels opened by cAMP. Little or no Ca(2+) influx occurs through voltage-dependent Ca(2+) channels, despite the small presynaptic depolarization caused by current through the HCN channels. Loading terminals with BAPTA delays the rise in preterminal [Ca(2+)](i) without affecting the enhancement of transmission to cAMP elevation. This dissociation of the dynamics of the [Ca(2+)](i) rise and synaptic enhancement, plus the small magnitude and location of [Ca(2+)](i) elevation distant from release sites, seems to preclude any direct role for this [Ca(2+)](i) elevation in cAMP-dependent enhancement of transmission.

Serotonin enhances the resistance reflex of the locomotor network of the crayfish through multiple modulatory effects that act cooperatively

Serotonin (5HT) is an endogenous amine that modifies posture in crustacea. Here, we examined the mechanisms of action of 5HT on the resistance reflex in crayfish legs. This reflex, which counteracts movements imposed on a limb, is based on a negative feedback system formed by proprioceptors that sense joint angle movements and activate opposing motoneurons. We performed intracellular recordings from depressor motoneurons while repetitively stretching and releasing a leg joint proprioceptor in a resting in vitro preparation (i.e., a preparation that lacks spontaneous rhythmic activity). 5HT increased the amplitude of the depolarization during the release phase of the proprioceptor (corresponding to an upward movement of the leg) and the discharge frequency of the motoneurons. The 5HT-induced increase in the resistance reflex is caused, to a large extent, by polysynaptic pathways because it was very attenuated in the presence of high divalent cation solution. In addition to this activation of the polysynaptic pathways, 5HT also has postsynaptic effects that enhance the resistance reflex. 5HT causes a tonic depolarization, as well as an increase in the time constant and input resistance of motoneurons. We developed a simple mathematical model to describe the integrative properties of the motoneurons. The conclusion of this study is that the input frequency and the decay time constant of the EPSPs interact in such a way that small simultaneous changes in these parameters can cause a large effect on summation. Therefore, the conjunction of presynaptic and postsynaptic changes produces a strong cooperative effect on the resistance reflex response.

Constant amplitude of postsynaptic responses for single presynaptic action potentials but not bursting input during growth of an identified neuromuscular junction in the lobster,Homarus americanus

As lobsters grow from early juveniles to adults their body size increases more than 20-fold, raising the question of how function is maintained during these ongoing changes in size. To address this question we studied the pyloric 1 (p1) muscle of the stomach of the lobster, Homarus americanus. The p1 muscle receives multiterminal innervation from one motor neuron, the lateral pyloric neuron of the stomatogastric ganglion. Staining with antibodies raised against synaptotagmin showed that as the muscle fibers increased in length, the spacing between the terminal innervation increased proportionally, so the number of synaptic contact regions/muscle fiber did not change. Muscle fibers were electrically coupled in both juveniles and adults. The amplitude of single intracellularly recorded excitatory junctional potentials evoked by motor nerve stimulation was the same in both juveniles and adults. Nonetheless, the peak depolarizations reached in response to ongoing pyloric rhythm activity or in response to high-frequency trains of stimuli similar to those produced during the pyloric rhythm were approximately twofold larger in juveniles than in adults. This suggests that homeostatic regulation of synaptic connections may operate at the level of the amplitude of the single synaptic potential rather than on the summed depolarization evoked during strong rhythmic activity.

The neural basis of dominance hierarchy formation in crayfish

Fifty years of study of the nervous system and behavior of crayfish have revealed neural circuits for movements that are similar to those seen during formation of a dominance hierarchy. Given this background, it is of interest to ask what is understood about the neural substrates of dominance hierarchy formation. Here we will consider the social behavior that crayfish display in the wild and in the laboratory, and its relationship to movements released by activation of specific neural circuits. We will consider how these movements might be knit together to produce the behavior patterns that are characteristic of dominant and subordinate animals.

Serotonin in the developing stomatogastric system of the lobster, Homarus americanus

We studied the development of the serotonergic modulation of the stomatogastric nervous system of the lobster, Homarus americanus. Although the stomatogastric ganglion (STG) is present early in embryonic development, serotonin immunoreactivity is not visible in the STG until the second larval stage. However, incubation of the STG with exogenous serotonin showed that a serotonin transporter is present in embryonic and early larval stages. Serotonin uptake was blocked by paroxetine and 0% Na(+) saline. The presence of a serotonin transporter in the embryonic STG suggests that hormonally liberated serotonin could be taken up by the STG, and potentially released as a "borrowed transmitter". Consistent with a potential hormonal role, serotonin is found in the pericardial organs, a major neurosecretory structure, by midembryonic development. The rhythmic motor patterns produced by embryonic and larval STGs were decreased in frequency by serotonin. Lateral Pyloric (LP) neuron-evoked excitatory junctional potentials (EJPs) in the embryos and the first larval stage (LI) were larger, slower, and more variable than those in the adult. The amplitude of adult LP neuron-evoked EJPs was increased more than twofold in serotonin, but in embryos and LI preparations this effect was negligible. In embryos and LI preparations, serotonin increased the occurrence of muscle fiber action potentials and altered the EJP wave-form. These data demonstrate that serotonin receptors are present in the stomatogastric nervous system early in development, and suggest that the role of serotonin changes from modulation of muscle fiber excitability early in development to enhancement of neurally evoked EJPs in the adult.

Neurohormonal and glutamatergic neuronal control of the cardioarterial valves in the isopod crustacean Bathynomus doederleini

The heart of Bathynomus doederleini gives rise to an anterior median artery (AMA), one pair of anterior lateral arteries (ALAs) and five pairs of lateral arteries (LAs). Cardioarterial valves are located at the junctions between the heart and arteries, each composed of a pair of muscular flaps. All valves of the AMA and the ALAs receive valve excitatory (constrictor) nerves (VEs). The valves of the ALAs receive dual innervation from both constrictor and inhibitor (dilator) nerves, while the valves of the AMA receive innervation from a constrictor nerve alone. The effects of candidate neurohormones on cardioarterial valves were examined by measuring the pressure in each artery at which haemolymph flows out of the heart through the valve. Serotonin, octopamine, norepinephrine, glutamate (Glu) and proctolin constricted the cardioarterial valves and thus decreased the arterial pressure in all the arteries. Dopamine also decreased the arterial pressure of arteries except for the ALAs, in which pressure was increased. Among the neurohormones exerting excitatory effects on the valves, only Glu depolarized the membrane potential of valve muscle cells. The glutamatergic agonists kainate and quisqualate also depolarized the valve muscle cells of the AMA. Excitatory junctional potentials produced in the valves of the AMA in response to the stimulation of a VE were blocked by the glutamatergic antagonists Joro spider toxin and MK-801. Glu is the likeliest candidate for a neurotransmitter for the VEs.

Neuromodulatory complement of the pericardial organs in the embryonic lobster, Homarus americanus

The pericardial organs (POs) are a pair of neurosecretory organs that surround the crustacean heart and release neuromodulators into the hemolymph. In adult crustaceans, the POs are known to contain a wide array of peptide and amine modulators. However, little is known about the modulatory content of POs early in development. We characterize the morphology and modulatory content of pericardial organs in the embryonic lobster, Homarus americanus. The POs are well developed by midway through embryonic (E50) life and contain a wide array of neuromodulatory substances. Immunoreactivities to orcokinin, extended FLRFamide peptides, tyrosine hydroxylase, proctolin, allatostatin, serotonin, Cancer borealis tachykinin-related peptide, cholecystokinin, and crustacean cardioactive peptide are present in the POs by approximately midway through embryonic life. There are two classes of projection patterns to the POs. Immunoreactivities to orcokinin, extended FLRFamide peptides, and tyrosine hydroxylase project solely from the subesophageal ganglion (SEG), whereas the remaining modulators project from the SEG as well as from the thoracic ganglia. Double-labeling experiments with a subset of modulators did not reveal any colocalized peptides in the POs. These results suggest that the POs could be a major source of neuromodulators early in development.

Cellular, synaptic and network effects of neuromodulation

All network dynamics emerge from the complex interaction between the intrinsic membrane properties of network neurons and their synaptic connections. Nervous systems contain numerous amines and neuropeptides that function to both modulate the strength of synaptic connections and the intrinsic properties of network neurons. Consequently network dynamics can be tuned and configured in different ways, as a function of the actions of neuromodulators. General principles of the organization of modulatory systems in nervous systems include: (a) many neurons and networks are multiply modulated, (b) there is extensive convergence and divergence in modulator action, and (c) some modulators may be released extrinsically to the modulated circuit, while others may be released by some of the circuit neurons themselves, and act intrinsically. Some of the computational consequences of these features of modulator action are discussed.

Mechanisms involved in persistent facilitation of neuromuscular synapses in aplysia

Synaptic plasticity can last from a fraction of a second to weeks depending on how it was induced. The mechanisms that underlie short-, intermediate-, and long-term plasticity have been intensively studied at central synapses of both vertebrates and invertebrates; however, peripheral plasticity has not received as much attention. In this study, we investigated the mechanisms that contribute to a persistent form of plasticity at neuromuscular synapses in buccal muscle I3a of Aplysia. These synapses are reversibly facilitated by the small cardioactive peptide (SCP), a peptide cotransmitter that is intrinsic to the motor neurons, and persistently facilitated by serotonin (5HT) released from modulatory neurons that are extrinsic to the motor circuit. Many of the short-term effects of 5HT and SCP are mediated by the cAMP pathway, but little is known about the mechanisms that underlie persistent modulation. We were able to eliminate several possible mechanisms. One of these was the possibility that the apparent reversal of SCP's effects was due to desensitization of the SCP receptor. Superfusion for longer periods or with higher concentrations of SCP indicate that the SCP receptors do not desensitize. We also determined that new protein synthesis is not required for the persistent facilitation of EJPs. Another possibility was that 5HT was taken up and slowly re-released. Our results suggest that this mechanism is also unlikely. Activation of the cAMP pathway does not appear to mediate persistent effects; however, 5HT as well as SCP does cause persistent increases in cAMP levels that can prime I3a synapses and increase the effectiveness of activators of the cAMP pathway. Instead, the persistent effects of 5HT are mimicked by phorbol, suggesting that protein kinase C or an Aplysia homologue of unc13 may mediate these effects. These results, in combination with results from experiments on the sensory neurons that contribute to withdrawal reflexes in Aplysia, suggest that the mechanisms for intermediate- and long-term facilitation may reside in all of the synapses involved in the sensory to motor response reflex.

Chronic alterations in serotonin function: dynamic neurochemical properties in agonistic behavior of the crayfish, Orconectes rusticus

The biogenic amine serotonin [5-hydroxytryptamine (5-HT)] has received considerable attention for its role in behavioral phenomena throughout a broad range of invertebrate and vertebrate taxa. Acute 5-HT infusion decreases the likelihood of crayfish to retreat from dominant opponents. The present study reports the biochemical and behavioral effects resulting from chronic treatment with 5-HT-modifying compounds delivered for up to 5 weeks via silastic tube implants. High performance liquid chromatography with electrochemical detection (HPLC-ED) confirmed that 5,7-dihydroxytryptamine (5,7-DHT) effectively reduced 5-HT in all central nervous system (CNS) areas, except brain, while a concurrent accumulation of the compound was observed in all tissues analyzed. Unexpectedly, two different rates of chronic 5-HT treatment did not increase levels of the amine in the CNS. Behaviorally, 5,7-DHT treated crayfish exhibited no significant differences in measures of aggression. Although treatment with 5-HT did not elevate 5-HT content in the CNS, infusion at a slow rate caused animals to escalate more quickly while 5-HT treatment at a faster rate resulted in slower escalation. 5,7-DHT is commonly used in behavioral pharmacology and the present findings suggest its biochemical properties should be more thoroughly examined. Moreover, the apparent presence of powerful compensatory mechanisms indicates our need to adopt an increasingly dynamic view of the serotonergic bases of behavior like crayfish aggression.

20-hydroxyecdysone causes increased aggressiveness in female American lobsters, Homarus americanus

Lobsters become transiently more aggressive before ecdysis. This aggressiveness accompanies an increase in hemolymph titers of 20-hydroxyecdysone (20-HE). Combats between intermolt female lobsters, injected with premolt levels of 20-HE, and larger, saline-injected opponents were videotaped. Aggressive, defensive, and avoidance behaviors were ranked according to aggressiveness in a Rank of Aggression hierarchy, which included opponent-directed and (nonopponent) redirected behaviors. Treated animals performed more and more highly aggressive behaviors than saline-injected controls. Opponents of treated animals performed fewer aggressive behaviors than saline-injected control opponents. Controls performed more defensive behaviors than treated animals, when redirected behaviors were considered. Differences in avoidance behaviors among the four types of combatants were not significant. The total aggressive content was the same in treated and control fights, but the interactions between combatants in the two fights were significantly different. Treated animals were equally as aggressive and defensive as their opponents; controls were relatively less aggressive and more defensive than their opponents. These results correlate with molt-cycle variations in behavior, 20-HE titers, and the effects of 20-HE and molt-differentiated hemolymph on the claw opener muscle. They suggest that 20-HE orchestrates intrinsic, cellular, and nuclear events that produce the molt-cycle transformations in agonistic behavior and aggressive state of lobsters.

The effects of 5-HT on sensory, central and motor neurons driving the abdominal superficial flexor muscles in the crayfish

Serotonin (5-HT) induces a variety of physiological and behavioral effects in crustaceans. However, the mechanisms employed by 5-HT to effect behavioral changes are not fully understood. Among the mechanisms by which these changes might occur are alterations in synaptic drive and efficacy of sensory, interneurons and motor neurons, as well as direct effects on muscles. We investigated these aspects with the use of a defined sensory-motor system, which is entirely contained within a single abdominal segment and consists of a 'cuticular sensory neurons segmental ganglia abdominal superficial flexor motor neurons-muscles' circuit. Our studies address the role of 5-HT in altering (1) the activity of motor neurons induced by sensory stimulation; (2) the inherent excitability of superficial flexor motor neurons; (3) transmitter release properties of the motor nerve terminal and (4) input resistance of the muscle. Using en passant recordings from the motor nerve, with and without sensory stimulation, and intracellular recordings from the muscle, we show that 5-HT enhances sensory drive and output from the ventral nerve cord resulting in an increase in the firing frequency of the motor neurons. Also, 5-HT increases transmitter release at the neuromuscular junction, and alters input resistance of the muscle fibers.

Sensitivity of transformed (phasic to tonic) motor neurons to the neuromodulator 5-HT

Long-term adaptation resulting in a 'tonic-like' state can be induced in phasic motor neurons of the crayfish, Procambarus clarkii, by daily low-frequency stimulation [Lnenicka, G.A., Atwood, H.L., 1985b. Long-term facilitation and long-term adaptation at synapses of a crayfish phasic motoneuron. J. Neurobiol. 16, 97-110]. To test the hypothesis that motor neurons undergoing adaptation show increased responses to the neuromodulator serotonin (5-HT), phasic motor neurons innervating the deep abdominal extensor muscles of crayfish were stimulated at 2.5 Hz, 2 h/day, for 7 days. One day after cessation of conditioning, contralateral control and conditioned motor neurons of the same segment were stimulated at 1 Hz and the induced excitatory post-synaptic potentials (EPSPs) were recorded from DEL(1) muscle fibers innervated by each motor neuron type. Recordings were made in saline without and with 100 nM 5-HT. EPSP amplitudes were increased by 5-HT exposure in all cases. Conditioned muscles exposed to 5-HT showed a 2-fold higher percentage of increase in EPSP amplitude than did control muscles. Thus, the conditioned motor neurons behaved like intrinsically tonic motoneurons in their response to 5-HT. While these results show that long-term adaptation (LTA) extends to 5-HT neuromodulation, no phenotype switch could be detected in the postsynaptic muscle. Protein isoform profiles, including the myosin heavy chains, do not change after 1 week of conditioning their innervating motor neurons.

Enhancement of synaptic transmission by cyclic AMP modulation of presynaptic Ih channels

Presynaptic activation of adenylyl cyclase and subsequent generation of cAMP represent an important mechanism in the modulation of synaptic transmission. In many cases, short- to medium-term modulation of synaptic strength by cAMP is due to activation of protein kinase A and subsequent covalent modification of presynaptic ion channels or synaptic proteins. Here we show that presynaptic cAMP generation via serotonin receptor activation directly modulated hyperpolarization-activated cation channels (Ih channels) in axons. This modulation of Ih produced an increase in synaptic strength that could not be explained solely by depolarization of the presynaptic membrane. These studies identify a mechanism by which cAMP and Ih regulate synaptic plasticity.

Heart rate within male crayfish: social interactions and effects of 5-HT

Behaviors, such as those that establish dominant and subordinate social status, are thought to be driven by various neuromodulators and hormones. In crustaceans, the level of serotonin (5-HT) in the hemolymph is correlated with degree of aggressiveness. The crustacean heart is neurogenic and is modulated by neural secretion of 5-HT in the hemolymph, which bathes the cardiac tissue. We discuss and present the results of measuring heart rate (HR) of crayfish during interactions, as an indication of their state of excitability. HR is the result of multiple influences: a cocktail of hormones and modulators. HR was monitored during the periods in which crayfish established aggressive and submissive social status, during sham injections, and following injections of various doses of 5-HT. Crayfish, during an interaction to establish social status, can increase HR. Both the aggressive and submissive crayfish can dampen their HR within seconds during a pause in the interaction, while still posturing in an aggressive or submissive state. Injections of 5-HT to obtain systemic levels of approximately 100 nM-10 microM increase HR substantially for hours. This suggests that aggressive interactions and the establishment of a dominant posture may not be related to large increases in the free concentrations of 5-HT within the circulating hemolymph, since a sustained HR is not observed in aggressive animals. Instead, the results may demonstrate that inhibitory cardiac regulation is present in the aggressors during interactions and that a regulator is possibly 5-HT.

Distribution and functional anatomy of amine-containing neurons in decapod crustaceans

One of the lessons learned from studying the nervous systems of phylogenetically distant species is that many features are conserved. Indeed, aminergic neurons in invertebrate and vertebrate systems share a multitude of common characteristics. In this review, the varied roles of serotonin, octopamine, dopamine, and histamine in decapod crustaceans are considered, and the distributions of the amine-containing cells are described. The anatomy of these systems reinforces the idea that amine neurons are involved in widespread modulation and coordination within the nervous system. Many aminergic neurons have long projections, linking multiple regions with a common input, and therefore are anatomically perfected as "gain setters." The developmental patterns of appearance of each amine in the crustacean nervous system are described and compared. The developmental picture suggests that transmitter acquisition is distinctive for each amine, and that the pace of acquisition may be co-regulated with target maturation. The distinctive roles that transmitters play during specific developmental periods may, ultimately, provide important clues to their functional contributions in the mature organism.

Temporal dynamics of convergent modulation at a crustacean neuromuscular junction

At least 10 different substances modulate the amplitude of nerve-evoked contractions of the gastric mill 4 (gm4) muscle of the crab, Cancer borealis. Serotonin, dopamine, octopamine, proctolin, red pigment concentrating hormone, crustacean cardioactive peptide, TNRNFLRFamide, and SDRNFLRFamide increased and -allatostatin-3 and histamine decreased the amplitude of nerve-evoked contractions. Modulator efficacy was frequency dependent; TNRNFLRFamide, proctolin, and allatostatin-3 were more effective when the motor neuron was stimulated at 10 Hz than at 40 Hz, whereas the reverse was true for dopamine and serotonin. The modulators that were most effective at high stimulus frequencies produced a significant decrease in muscle relaxation time; those that were most effective at low stimulus frequencies produced modest increases in relaxation time. Thus modulator actions that appear redundant when examined only at one stimulus frequency are differentiated when a range of stimulus dynamics is studied. The effects of TNRNFLRFamide, serotonin, proctolin, dopamine, and -allatostatin-3 on the amplitude and facilitation of nerve-evoked excitatory junctional potentials (EJPs) in the gm4 and gastric mill 6 (gm6) muscles were compared. The EJPs in gm4 have a large initial amplitude and show relatively little facilitation, whereas the EJPs in gm6 have a small initial amplitude and show considerable facilitation. Modulators that enhanced contractions also enhanced EJP amplitude; -allatostatin-3 reduced EJP amplitude. The effects of these modulators on EJP amplitude were modest and showed no significant frequency dependence. This suggests that the frequency dependence of modulator action on contraction results from effects on excitation-contraction coupling. The modulators affected facilitation at these junctions in a manner consistent with a change in release probability. They produced a change in facilitation that is inversely related to their action on EJP amplitude.

Serotonergic neurons differentially modulate the efficacy of two motor neurons innervating the same muscle fibers in Aplysia

Feeding behavior in Aplysia shows substantial plasticity. An important site for the generation of this plasticity is the modulation of synaptic transmission between motor neurons and the buccal muscles that generate feeding movements. We have been studying this modulation in the anterior portion of intrinsic buccal muscle 3 (I3a), which is innervated by two excitatory motor neurons and identified serotonergic modulatory neurons, the metacerebral cells (MCCs). We have shown previously that serotonin (5-HT) applied selectively to the muscle potently modulates excitatory junction potentials (EJPs) and contractions. All the effects of 5-HT were persistent, lasting many hours after wash out. We examined whether the release of endogenous 5-HT from the MCC could produce effects similar to the application of 5-HT. Stimulation of the MCCs did produce similar short-term effects to the application of 5-HT. MCC stimulation facilitates EJPs, potentiates contractions, and decreases the latency between the onset of a motor neuron burst and the onset of the evoked contraction. The effects of MCC stimulation reached a maximum at quite low firing frequencies, which were in the range of those previously recorded during feeding behavior. The maximal effects were similar to those produced by superfusion with approximately 0.1 microM 5-HT. Although the effects of MCC stimulation on EJPs were persistent, they were less persistent than the effects of 0.1 microM 5-HT. Mechanisms that may account for differences in the persistence between released and superfused 5-HT are discussed. Thus activity in the MCCs has dramatic short-term effects on the behavioral output of motor neurons, increasing the amplitude and relaxation rate of contractions evoked by both B3 and B38 and shifting the temporal relationship between B38 bursts and evoked contractions.

Neuromodulators enhance transmitter release by two separate mechanisms at the inhibitor of crayfish opener muscle

A presynaptic voltage control method has been used to investigate the modulatory effects of serotonin (5-HT) and okadaic acid (OA) on the inhibitory junction of the crayfish opener muscle. Instead of using action potentials, we used 20 msec pulses depolarized to 0 mV to activate transmitter release. This approach allowed us to monitor two separate physiological parameters related to the release process. The first parameter, transmitter release kinetics, is characterized as the delay when inhibitory postsynaptic conductance reaches its half-maximum (IPSG50). The second parameter, the total area of IPSG (IPSGarea), estimates total transmitter output. We have reported previously that the F2 component of synaptic facilitation is associated with a decrease in IPSG50 but without a change in IPSGarea. These results raised the possibility that IPSG50 and IPSGarea could be mediated by separate mechanisms that were modulated independently. To explore this possibility, we investigated the effects of 5-HT (100-200 nM) and OA (2.5 microM) on the two parameters. 5-HT and OA enhanced IPSG neither by changing the sensitivity of postsynaptic receptors, as tested by iontophoretically ejected GABA, nor by elevating resting and action potential-activated presynaptic free calcium, as monitored by fura-2 imaging. 5-HT and OA decreased IPSG50 by 3.0 +/- 1.4 and 3.6 +/- 1.1 msec, respectively, and increased IPSGarea by 50 +/- 21 and 37 +/- 6%, respectively. The ability of F2 facilitation to accelerate release kinetics was reduced in the presence of the modulators, suggesting that the mechanism underlying the accelerated release kinetics was shared by the two modes of synaptic enhancement. This report demonstrates that the acceleration in release kinetics and the increase in total release are two separate mechanisms for enhancing transmitter output and that these two mechanisms can be activated without changes in presynaptic calcium dynamics.

Neuromodulators enhance transmitter release by two separate mechanisms at the inhibitor of crayfish opener muscle

A presynaptic voltage control method has been used to investigate the modulatory effects of serotonin (5-HT) and okadaic acid (OA) on the inhibitory junction of the crayfish opener muscle. Instead of using action potentials, we used 20 msec pulses depolarized to 0 mV to activate transmitter release. This approach allowed us to monitor two separate physiological parameters related to the release process. The first parameter, transmitter release kinetics, is characterized as the delay when inhibitory postsynaptic conductance reaches its half-maximum (IPSG50). The second parameter, the total area of IPSG (IPSGarea), estimates total transmitter output. We have reported previously that the F2 component of synaptic facilitation is associated with a decrease in IPSG50 but without a change in IPSGarea. These results raised the possibility that IPSG50 and IPSGarea could be mediated by separate mechanisms that were modulated independently. To explore this possibility, we investigated the effects of 5-HT (100-200 nM) and OA (2.5 microM) on the two parameters. 5-HT and OA enhanced IPSG neither by changing the sensitivity of postsynaptic receptors, as tested by iontophoretically ejected GABA, nor by elevating resting and action potential-activated presynaptic free calcium, as monitored by fura-2 imaging. 5-HT and OA decreased IPSG50 by 3.0 +/- 1.4 and 3.6 +/- 1.1 msec, respectively, and increased IPSGarea by 50 +/- 21 and 37 +/- 6%, respectively. The ability of F2 facilitation to accelerate release kinetics was reduced in the presence of the modulators, suggesting that the mechanism underlying the accelerated release kinetics was shared by the two modes of synaptic enhancement. This report demonstrates that the acceleration in release kinetics and the increase in total release are two separate mechanisms for enhancing transmitter output and that these two mechanisms can be activated without changes in presynaptic calcium dynamics.

Regulation of synaptic vesicle recycling by calcium and serotonin

Serotonin, a neuromodulator at the crayfish neuromuscular junction, regulates neurotransmission without changing intracellular calcium levels. However, the mechanism of this regulation remains unclear. By analysis of synaptic depression using a depletion model and measurement of vesicle recycling using the styryl dye FM1-43, we show that serotonin increases the number of vesicles available for transmitter release (total synaptic vesicle pool size). This regulation is due either to an increase in the number of vesicles at each release site or to an activation of previously nonsecreting or silent synapses. We also observed that low calcium medium rendered part of the vesicle pool unavailable for release. These results suggest a new mechanism for regulating synaptic transmission.

Neuromodulation of activity-dependent synaptic enhancement at crayfish neuromuscular junction

Action potential-evoked transmitter release is enhanced for many seconds after moderate-frequency stimulation (e.g. 15 Hz for 30 s) at the excitor motorneuron synapse of the crayfish dactyl opener muscle. Beginning about 1.5 s after a train, activity-dependent synaptic enhancement (ADSE) is dominated by a process termed augmentation (G.D. Bittner, D.A. Baxter, Synaptic plasticity at crayfish neuromuscular junctions: facilitation and augmentation, Synapse 7 (1991) 235-243'[4]; K.L. Magleby, Short-term changes in synaptic efficacy, in: G.M. Edelman, L.E. Gall, C.W. Maxwell (Eds.), Synaptic Function, John Wiley and Sons, New York, 1987, pp. 21-56; K.L. Magleby; J.E. Zengel, Augmentation: a process that acts to increase transmitter release at the frog neuromuscular junction, J. Physiol. (Lond.) 257 (1976) 449-470) which decays approximately exponentially with a time constant of about 10 s at 16 degrees C, reflecting the removal of Ca2+ which accumulates during the train in presynaptic terminals (K.R. Delaney, D.W. Tank, R.S. Zucker, Serotonin-mediated enhancement of transmission at crayfish neuromuscular junction is independent of changes in calcium, J. Neurosci. 11 (1991) 2631-2643). Serotonin (5-HT, 1 microM) increases evoked and spontaneous transmitter release several-fold (D. Dixon, H.L. Atwood, Crayfish motor nerve terminal's response to serotonin examined by intracellular microelectrode, J. Neurobiol. 16 (1985) 409-424; J. Dudel, Modulation of quantal synaptic release by serotonin and forskolin in crayfish motor nerve terminals, in: Modulation of Synaptic Transmission and Plasticity in Nervous Systems, G. Hertting, H.-C. Spatz (Eds.), Springer-Verlag, Berlin, 1988; S. Glusman, E.A. Kravitz. The action of serotonin on excitatory nerve terminals in lobster nerve-muscle preparations, J. Physiol. (Lond.) 325 (1982) 223-241). We found that ADSE persists about 2-3 times longer after moderate-frequency presynaptic stimulation in the presence of 5-HT. This slowing of the decay of ADSE by 5-HT was not accompanied by significant changes in the initial amplitude of activity-dependent components of enhancement 1.5 s after the train. Measurements of presynaptic [Ca2+] indicated that the time course of Ca2+ removal from the presynaptic terminals after trains was not altered by 5-HT. Changes in presynaptic action potential shape, resting membrane potential or postsynaptic impedance after trains cannot account for slower recovery of ADSE. Axonal injection of EDTA slows the removal of residual Ca2+ and the decay of synaptic augmentation after trains of action potentials (K.R. Delaney, D.W. Tank, A quantitative measure of the dependence of short-term synaptic enhancement on presynaptic residual calcium, J. Neurosci. 14 (1994) 5885-5902), but has little or no effect on the 5-HT-induced persistence of ADSE. This also suggests that the time course of ADSE in the presence of 5-HT is not determined primarily by residual Ca2+ removal kinetics. The slowing of ADSE recovery after trains by 5-HT reverses with washing in 5-HT-free saline along with the 5-HT-mediated enhancement of release.

Serotonin and the small cardioactive peptides differentially modulate two motor neurons that innervate the same muscle fibers in Aplysia

The anterior portion of intrinsic buccal muscle 3 (I3a) is innervated by two motor neurons, B3 and B38, which appear to use glutamate as their fast excitatory transmitter. B3 and B38 express the neuropeptides FMRFamide and the small cardioactive peptides (SCPs), respectively. We have shown previously that stimulation of B38 causes release of the SCPs from terminals in the muscle. The I3a muscle also receives input from neurons that use 5HT as a modulatory transmitter. The SCPs and 5HT potently facilitated B38-evoked excitatory junction potentials (EJPs) but had only a small effect on B3-evoked EJPs; however, both the SCPs and 5HT strongly potentiated contractions evoked by both B3 and B38, indicating that the two substances must also act on excitation-contraction coupling. The selective facilitation of B38-evoked EJPs, however, did manifest itself in other parameters. Decreases in the firing frequencies and burst durations that were threshold to evoke contractions and decreases in the latency between the onset of a burst and the onset of the evoked contraction were all much larger for B38 than for B3. Indeed, B38 bursts recorded during feeding-like behavior would be subthreshold for evoking contractions in the absence of this modulation. All of the effects of the SCPs reversed during washout, whereas those of 5HT were persistent, lasting many hours after washout. Thus, the SCPs and 5HT dramatically change the behavioral output of these motor neurons, increasing the amplitude of contractions evoked by both B3 and B38, and shifting the temporal relationship between bursts in B38 and its evoked contractions.

Serotonin and the small cardioactive peptides differentially modulate two motor neurons that innervate the same muscle fibers in Aplysia

The anterior portion of intrinsic buccal muscle 3 (I3a) is innervated by two motor neurons, B3 and B38, which appear to use glutamate as their fast excitatory transmitter. B3 and B38 express the neuropeptides FMRFamide and the small cardioactive peptides (SCPs), respectively. We have shown previously that stimulation of B38 causes release of the SCPs from terminals in the muscle. The I3a muscle also receives input from neurons that use 5HT as a modulatory transmitter. The SCPs and 5HT potently facilitated B38-evoked excitatory junction potentials (EJPs) but had only a small effect on B3-evoked EJPs; however, both the SCPs and 5HT strongly potentiated contractions evoked by both B3 and B38, indicating that the two substances must also act on excitation-contraction coupling. The selective facilitation of B38-evoked EJPs, however, did manifest itself in other parameters. Decreases in the firing frequencies and burst durations that were threshold to evoke contractions and decreases in the latency between the onset of a burst and the onset of the evoked contraction were all much larger for B38 than for B3. Indeed, B38 bursts recorded during feeding-like behavior would be subthreshold for evoking contractions in the absence of this modulation. All of the effects of the SCPs reversed during washout, whereas those of 5HT were persistent, lasting many hours after washout. Thus, the SCPs and 5HT dramatically change the behavioral output of these motor neurons, increasing the amplitude of contractions evoked by both B3 and B38, and shifting the temporal relationship between bursts in B38 and its evoked contractions.

Comparative anatomy of serotonin-like immunoreactive neurons in isopods: putative homologues in several species

It is now commonly accepted that the arthropod nervous system has evolved only once, and so homologies between crustacean and insect nervous systems can be meaningfully sought. To do this, we have examined the distribution of serotonin (5-hydroxytryptamine)-like immunoreactive neurons in the central nervous system (CNS) of four common British isopods. Two species of terrestrial woodlouse, Oniscus asellus and Armadillidium vulgare, the littoral sea slater, Ligia oceanica, and the aquatic water hoglouse, Asellus meridianus, all possess approximately 40 pairs of serotonin-like immunoreactive neurons, distributed throughout the CNS in a very similar pattern. Interspecific homology is clearly suggested. Serotonin-like immunoreactive neurons in the first (T1) and fourth (T4) thoracic ganglia are particularly prominent in each of the four species studied. Whole-mount immunohistochemistry shows that the pair of T1 neurons have large dorsolateral cell bodies and prominent neurites that project medially and then anteriorly, whereas the pair of T4 neurons have ventrolateral cell bodies and neurites that bifurcate to form a thin axon projecting anteriorly to terminate in T3 and a thick medial axon that projects posteriorly into the abdominal neuromeres of the terminal ganglion. Intracellular cobalt staining of these neurons reveals more of their arborizations: the T1 neurons send three processes anteriorly, which arborize in the brain and exist from the CNS via peripheral nerves, whereas the T4 neurons contribute considerably to the extensive pattern of serotonin-like immunoreactive fibres in T3-T6 ganglia. The overall pattern of serotonin-like immunoreactive neurons in the isopods is similar to that in decapod crustacea, and a number of putative homologies can be assigned. It is more difficult to homologize the isopod serotonin-like immunoreactive neurons with those in the insect CNS, but some stained brain and thoracic neurons share common cell body positions and axon trajectories in isopods, decapods, and insects and may therefore be homologous.

Enhancement by serotonin of GABA-mediated inhibitory synaptic currents in rat cerebellar Purkinje cells

The actions of serotonin (5-HT) on synaptic responses in Purkinje cells were examined using the whole-cell voltage-clamp recording from thin slices of the rat cerebellum. Focal stimulation within the molecular layer elicited synaptic responses: (1) inhibitory postsynaptic currents (IPSCs) that were completely blocked by bicuculline; (2) excitatory postsynaptic currents (EPSCs) that were antagonized by CNQX; and (3) combination of IPSC and EPSC. Application of 5-HT (3-30 microM) increased the amplitude of the stimulation-evoked IPSCs but did not significantly affect the EPSCs. The frequency and the mean amplitude of spontaneous IPSCs were increased in the presence of 5-HT. Neither the sensitivity to exogenously applied GABA nor the holding current of Purkinje cells were noticeably altered by 5-HT. The results indicate that 5-HT differentially enhances the GABA-mediated IPSCs in rat cerebellar Purkinje cells via a presynaptic mechanism. Serotonergic neurons thus appear to play a role in the motor control through selective presynaptic facilitation that modulates the output from the cerebellar cortex.

Peripheral proprioceptive modulation in crayfish walking leg by serotonin

In vitro serotoninergic modulation of intracellularly recorded sensory responses was examined in primary afferent terminals of a crayfish leg proprioceptor, the coxo-basal chordotonal organ (CB CO). The effects of different concentrations of serotonin (5-HT) on static and dynamic sensory responses were analysed following bath or pressure applications of the monoamine directly on the strand of the mechanoreceptor. Consequently, the reported effects result from the direct peripheral action of 5-HT on the sensory organ itself. Serotonin modulates the sensory activity by modifying the sensory discharge frequency. The firing discharge of the primary afferents is increased in a dose-dependent manner. The maximal effect is obtained with a concentration of 10(-6) M. Higher concentrations are less effective, and for 20% of the recorded cells, 10(-4) M 5-HT induces a decrease of the sensory discharge, i.e. has an inhibitory effect. Alteration in the pattern of sensory firing, resulting in bursting discharge, was observed in some units. All the recorded sensory units were responsive to the neuromodulator whatever their functional properties. The effects of 5-HT lasted as long as the amine was applied and were reversible after wash. The results suggest that 5-HT could exert a modulatory action on the proprioceptive feedback, by peripheral action on the sensory organ. The natural modalities of 5-HT action are discussed on the basis of immunohistochemistry data suggesting: (i) connections between CB CO and central serotoninergic cells, (ii) 5-HT content in sensory cells of the CB CO. Since the CB CO is involved in the control of leg movement and position, the modulation of its primary afferents might influence the organization of the locomotor pattern. The functional significance of this peripheral sensory neuromodulation was approached by the analysis of the motor reflex activity.

Presynaptic calcium in transmitter release and posttetanic potentiation

This review gives some indication of the progress that has been made in understanding synaptic transmission by use of new methods for measuring and controlling presynaptic [Ca2+]i. Many unsolved problems remain. We still do not have a clear idea of the exact relationship between [Ca2+]i and transmitter release and whether this relationship is the same under all circumstances. The apparently different [Ca2+]i-dependence of evoked transmitter release and of PTP suggest multiple molecular sites of calcium action that remain to be identified. A complete and comprehensive model of transmitter release has yet to be devised, and questions raised by our experiments may indicate that it is still too early to try to construct a precise model. We also do not know just how serotonin acts to modulate transmitter release, only that it does not appear to alter either resting or entering calcium. Some of these questions may be approachable with the techniques described here; others are not and require different methods for their resolution. The work continues.