Immunohistochemical localization of serotonin and choline acetyltransferase in sensory neurones of the locust

Temporal differences between load and movement signal integration in the sensorimotor network of an insect leg

Nervous systems face a torrent of sensory inputs, including proprioceptive feedback. Signal integration depends on spatially and temporally coinciding signals. It is unclear how relative time delays affect multimodal signal integration from spatially distant sense organs. We measured transmission times and latencies along all processing stages of sensorimotor pathways in the stick insect leg muscle control system, using intra- and extracellular recordings. Transmission times of signals from load-sensing tibial and trochanterofemoral campaniform sensilla (tiCS, tr/fCS) to the premotor network were longer than from the movement-sensing femoral chordotonal organ (fCO). We characterized connectivity patterns from tiCS, tr/fCS, and fCO afferents to identified premotor nonspiking interneurons (NSIs) and motor neurons (MNs) by distinguishing short- and long-latency responses to sensory stimuli. Functional NSI connectivity depended on sensory context. The timeline of multisensory integration in the NSI network showed an early phase of movement signal processing and a delayed phase of load signal integration. The temporal delay of load signals relative to movement feedback persisted into MN activity and muscle force development. We demonstrate differential delays in the processing of two distinct sensory modalities generated by the sensorimotor network and affecting motor output. The reported temporal differences in sensory processing and signal integration improve our understanding of sensory network computation and function in motor control.NEW & NOTEWORTHY Networks integrating multisensory input face the challenge of not only spatial but also temporal integration. In the local network controlling insect leg movements, proprioceptive signal delays differ between sensory modalities. Specifically, signal transmission times to and neuronal connectivity within the sensorimotor network lead to delayed information about leg loading relative to movement signals. Temporal delays persist up to the level of the motor output, demonstrating its relevance for motor control.

Descending octopaminergic neurons modulate sensory-evoked activity of thoracic motor neurons in stick insects

Neuromodulatory neurons located in the brain can influence activity in locomotor networks residing in the spinal cord or ventral nerve cords of invertebrates. How inputs to and outputs of neuromodulatory descending neurons affect walking activity is largely unknown. With the use of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and immunohistochemistry, we show that a population of dorsal unpaired median (DUM) neurons descending from the gnathal ganglion to thoracic ganglia of the stick insect Carausius morosus contains the neuromodulatory amine octopamine. These neurons receive excitatory input coupled to the legs’ stance phases during treadmill walking. Inputs did not result from connections with thoracic central pattern-generating networks, but, instead, most are derived from leg load sensors. In excitatory and inhibitory retractor coxae motor neurons, spike activity in the descending DUM (desDUM) neurons increased depolarizing reflexlike responses to stimulation of leg load sensors. In these motor neurons, descending octopaminergic neurons apparently functioned as components of a positive feedback network mainly driven by load-detecting sense organs. Reflexlike responses in excitatory extensor tibiae motor neurons evoked by stimulations of a femur-tibia movement sensor either are increased or decreased or were not affected by the activity of the descending neurons, indicating different functions of desDUM neurons. The increase in motor neuron activity is often accompanied by a reflex reversal, which is characteristic for actively moving animals. Our findings indicate that some descending octopaminergic neurons can facilitate motor activity during walking and support a sensory-motor state necessary for active leg movements.

NEW & NOTEWORTHY We investigated the role of descending octopaminergic neurons in the gnathal ganglion of stick insects. The neurons become active during walking, mainly triggered by input from load sensors in the legs rather than pattern-generating networks. This report provides novel evidence that octopamine released by descending neurons on stimulation of leg sense organs contributes to the modulation of leg sensory-evoked activity in a leg motor control system.

Pharmacological analysis of tonic activity in motoneurons during stick insect walking

Stick insect middle leg (mesothoracic) motoneurons receive tonic excitatory input during front leg stepping on a treadmill. We studied the pharmacology of this excitatory input to the motoneurons during single-legged treadmill walking (in situ). During bath application of drugs restricted to the mesothoracic ganglion, activity in motoneurons contralateral to the stepping front leg was recorded from neuropilar processes. Application of the cholinergic antagonist atropine reduced the tonic depolarization amplitude. These results were compared with findings in acutely dissociated motoneuron cell bodies (in vitro) under whole cell voltage-clamp conditions. The presence of an acetylcholine-induced current in situ was supported by the finding of an acetylcholine evoked biphasic inward current with a sustained component that could be blocked by atropine. In situ the tonic depolarization was generally increased by application of the neuro-modulator octopamine and decreased by its antagonist mianserin. In vitro, however, octopamine reduced the inward current evoked by acetylcholine application to motoneurons. Intracellular application of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) into motoneurons in situ revealed a dependence of the tonic depolarization on Ca(2+) and application of the membrane-permeable cAMP analogue 8-bromo-cAMP increased the tonic depolarization. In contrast, 8-bromo-cAMP reduced the inward current evoked by acetylcholine application to motoneurons in vitro. We conclude that during walking, acetylcholine contributes to mediating the tonic depolarization possibly by acting on atropine-sensitive receptors on motoneurons. Octopamine that is released during walking increases the tonic depolarization. This increase, however, is not based on modulation of cholinergic action on motoneurons but rather on effects on premotor neurons. Both, Ca(2+) and cAMP are likely second messengers involved in mediating the tonic depolarization, but whereas Ca(2+) acts in motoneurons, cAMP does not appear to mediate a cholinergic depolarization in motoneurons.

Evidence that the swim afferent neurons of tritonia diomedea are glutamatergic

The escape swim response of the marine mollusc Tritonia diomedea is a well-established model system for studies of the neural basis of behavior. Although the swim neural network is reasonably well understood, little is known about the transmitters used by its constituent neurons. In the present study, we provide immunocytochemical and electrophysiological evidence that the S-cells, the afferent neurons that detect aversive skin stimuli and in turn trigger Tritonia's escape swim response, use glutamate as their transmitter. First, immunolabeling revealed that S-cell somata contain elevated levels of glutamate compared to most other neurons in the Tritonia brain, consistent with findings from glutamatergic neurons in many species. Second, pressure-applied puffs of glutamate produced the same excitatory response in the target neurons of the S-cells as the naturally released S-cell transmitter itself. Third, the glutamate receptor antagonist CNQX completely blocked S-cell synaptic connections. These findings support glutamate as a transmitter used by the S-cells, and will facilitate studies using this model system to explore a variety of issues related to the neural basis of behavior.

Serotonin: A coordinator of feeding-related physiological events in the blood-gorging bug, Rhodnius prolixus

Rhodnius prolixus is an obligatory blood-feeder that can ingest blood meals of up to 10 times its mass. Rapid production of urine commences within 2-3 min of the start of feeding in order to eliminate the load of water and salts, and so there is an increase of Malpighian tubule secretion greater than 1,000 fold in response to feeding. Feeding and post-prandial diuresis in Rhodnius are highly coordinated events, including for example, host recognition, probing, injection of saliva, cuticle plasticization, passage of blood through the digestive system, diuresis and excretion. This review illustrates that many of the known functions of serotonin in Rhodnius are feeding-related. Serotonin coordinates or 'orchestrates' feeding-related physiological events either as a neurotransmitter/neuromodulator, delivered to target tissues in the nerve supply, or as a neurohormone, delivered by the haemolymph. Thus, serotonin has physiological effects upon the salivary glands, cuticle, digestive tract, cardiac muscle, and Malpighian tubules. By discussing these aspects, the review illustrates that serotonin acts in a coordinated manner to prepare Rhodnius for this energy-demanding process of feeding and diuresis.

Substantial changes in central nervous system neurotransmitters and neuromodulators accompany phase change in the locust

Desert locusts (Schistocerca gregaria) can undergo a profound transformation between solitarious and gregarious forms, which involves widespread changes in behaviour, physiology and morphology. This phase change is triggered by the presence or absence of other locusts and occurs over a timescale ranging from hours, for some behaviours to change, to generations,for full morphological transformation. The neuro-hormonal mechanisms that drive and accompany phase change in either direction remain unknown. We have used high-performance liquid chromatography (HPLC) to compare amounts of 13 different potential neurotransmitters and/or neuromodulators in the central nervous systems of final instar locust nymphs undergoing phase transition and between long-term solitarious and gregarious adults. Long-term gregarious and solitarious locust nymphs differed in 11 of the 13 substances analysed: eight increased in both the brain and thoracic nerve cord (including glutamate,GABA, dopamine and serotonin), whereas three decreased (acetylcholine,tyramine and citrulline). Adult locusts of both extreme phases were similarly different. Isolating larval gregarious locusts led to rapid changes in seven chemicals equal to or even exceeding the differences seen between long-term solitarious and gregarious animals. Crowding larval solitarious locusts led to rapid changes in six chemicals towards gregarious values within the first 4 h(by which time gregarious behaviours are already being expressed), before returning to nearer long-term solitarious values 24 h later. Serotonin in the thoracic ganglia, however, did not follow this trend, but showed a ninefold increase after a 4 h period of crowding. After crowding solitarious nymphs for a whole larval stadium, the amounts of all chemicals, except octopamine, were similar to those of long-term gregarious locusts. Our data show that changes in levels of neuroactive substances are widespread in the central nervous system and reflect the time course of behavioural and physiological phase change.

A neurohormonal role for serotonin in the control of locust oviducts

Serotonin increases the frequency and amplitude of spontaneous contractions and leads to an increase in the basal tonus of the locust oviducts. These effects were dose-dependent and were seen on both the non-innnervated and innervated portion of the oviducts. Vertebrate type serotonin agonists and antagonists were used and the profile shows that the receptors on the non-innervated and innervated portion of the oviducts are more similar to 5-HT3 receptors than to either 5-HT1 or 5-HT2 receptors. No serotonin was found associated with the oviducts or the innervation to the oviducts using immunohistochemistry and HPLC coupled to electrochemical detection, suggesting a neurohormonal role for serotonin in the control of locust oviducts.

In vitro expression and pharmacology of the 5-HT7-like receptor present in the mosquito Aedes aegypti tracheolar cells and hindgut-associated nerves

We have previously reported the cloning of a 5-hydroxytryptamine receptor (Aedes 5-HT7-like receptor) from adult Aedes aegypti. For functional expression of the Aedes 5-HT7-like receptor, CHO-K1 cells were stably transfected with a receptor expression construct, pC5-HT7. The Aedes 5-HT7-like receptor positively coupled to Gs protein, increasing intracellular cAMP in response to 5-HT; adenylyl cyclase activity was induced in a concentration-dependent, saturable manner. Only 5-HT, and not octopamine, dopamine or tyramine, caused the induction of cAMP. At 10 nM 5-HT a weak synergism was observed between octopamine and 5-HT. Other known agonists of the mammalian 5-HT7 receptor were tested. Their order of potency was: 5-HT >> 5-CT = 8-OH-DPAT >> pimozide. This is the first report on the functional expression of a mosquito neurohormone receptor.

A contribution to the functional morphology of the femoral chordotonal organ in the green lacewing Chrysoperla carnea (Neuroptera)

The femoral chordotonal organ (FCO) and the subgenual organ (SGO) of the green lacewing Chrysoperla carnea were examined by conventional light and confocal laser scanning microscopy in order to search for neuroactive substances which are used for neurotransmission in sensory cells of these organs. Antibodies against serotonin, histamine and choline acetyltransferase were tested immunohistochemically. In the FCO, antiserum against serotonin strongly labelled cell bodies and axons of about 16 sensory cells. In the proximal scoloparium all 12 sensory cells showed immunoreaction with antiserotonin. In the distal scoloparium only four of 40 sensory cells were immunoreactive. These results suggest that different neuroactive substances are employed as neurotransmitters in the FCO of the green lacewing and that the proximal scoloparium and the distal scoloparium are functionally differentiated. Contrary to the FCO in the locust, acetylcholine was not found as a neurotransmitter in the FCO of the green lacewing. Additionally, histamine showed a negative result in the sensory cells of the FCO. Other neuroactive substances seem to be used as transmitters in the SGO because none of the tested antibodies showed positive reaction.

Somatotopic mapping of chordotonal organ neurons in a primitive ensiferan, the New Zealand tree weta Hemideina femorata: I. femoral chordotonal organ

The femoral chordotonal organ (FCO) in orthopteran insects comprises several hundred sensory neurons, making it one of the most complex insect proprioceptors. The sensory neurons are suspended from the proximal femur, connecting distally to ligaments and to a needle-like apodeme extending from the proximal tibia. They monitor the position and movement of the tibia. To address how this complexity depends on evolutionary status and function, the morphology of the FCO neurons in the primitive orthopteran Hemideina femorata was investigated by staining small populations of identified afferents. As in crickets, the FCOs in all legs of the weta comprise partly fused ventral and dorsal scoloparia, with the former containing two groups of somata, the ventral group (VG) and the dorsal group (DG). However, the dendrites of the DG insert into thin connective tissue attached to the ventral side of the dorsal ligament, forming a "third scoloparium." The VG afferents terminate mainly in the motor association neuropils, whereas afferents from the dorsal scoloparium neurons terminate exclusively in the vibratory neuropil as do the afferents from the subgenual organ, a substrate vibration detector. Several afferents originating in the DG have extensive terminations in the motor association-, vibratory-, and auditory-processing neuropils, indicating lesser functional specialization than in the other groups. The evolutionary development of the FCO is discussed from a comparative viewpoint.

Neuromodulation of arthropod mechanosensory neurons

Arthropod mechanosensory afferents have long been known to receive efferent synaptic connections onto their centrally located axon terminals. These connections cause presynaptic inhibition by attenuating the action potentials arriving at the axon terminals, thus reducing the synaptic potentials in the postsynaptic neurons. This type of inhibition can specifically reduce the excitation of selected postsynaptic neurons while leaving others unaffected. However, recent research has demonstrated that sensory signals detected by arthropod mechanosensory neurons can also be synaptically modulated before they ever arrive at the axon terminals. In arachnids and crustaceans, wide and complex networks of synapses on all parts of the afferent neurons, including the somata and dendrites, provide mechanisms to inhibit or enhance the responses to mechanical stimuli as they are being detected. This modulation will affect the signal transmission to all axonal branches and postsynaptic cells of the affected receptor neuron. In addition to the increased complexity of mechanosensory information transmission produced by these synapses, a variety of circulating neuroactive substances also modulate these neurons by acting on their postsynaptic receptors.

Synaptic structure, distribution, and circuitry in the central nervous system of the locust and related insects

The Orthopteran central nervous system has proved a fertile substrate for combined morphological and physiological studies of identified neurons. Electron microscopy reveals two major types of synaptic contacts between nerve fibres: chemical synapses (which predominate) and electrotonic (gap) junctions. The chemical synapses are characterized by a structural asymmetry between the pre- and postsynaptic electron dense paramembranous structures. The postsynaptic paramembranous density defines the extent of a synaptic contact that varies according to synaptic type and location in single identified neurons. Synaptic bars are the most prominent presynaptic element at both monadic and dyadic (divergent) synapses. These are associated with small electron lucent synaptic vesicles in neurons that are cholinergic or glutamatergic (round vesicles) or GABAergic (pleomorphic vesicles). Dense core vesicles of different sizes are indicative of the presence of peptide or amine transmitters. Synapses are mostly found on small-diameter neuropilar branches and the number of synaptic contacts constituting a single physiological synapse ranges from a few tens to several thousand depending on the neurones involved. Some principles of synaptic circuitry can be deduced from the analysis of highly ordered brain neuropiles. With the light microscope, synaptic location can be inferred from the distribution of the presynaptic protein synapsin I. In the ventral nerve cord, identified neurons that are components of circuits subserving known behaviours, have been studied using electrophysiology in combination with light and electron microscopy and immunocytochemistry of neuroactive compounds. This has allowed the synaptic distribution of the major classes of neurone in the ventral nerve cord to be analysed within a functional context.

Specific binding of [3H]ketanserin to hypothalamus membranes of juvenile rainbow trout, Oncorhynchus mykiss?

This study examines the existence and pharmacological specificity of [3H]ketanserin binding in hypothalamus of juvenile rainbow trout. Hypothalamic membranes were incubated with [3H]ketanserin (selective 5HT2 antagonist) under several experimental conditions; reactions were terminated by filtration and bound radioactivity was counted by liquid scintillation spectroscopy. Tissue dilution experiments revealed that specific [3H]ketanserin binding (Bsp) was tissue dependent; 1 hypothalamus equivalent per tube (1100 ± 115 cpm/mg protein) was subsequently used throughout the rest of this study. In association experiments, Bsp increased progressively with time, achieved equilibrium binding levels (1192 ± 120 cpm/mg protein) within 80 min, and remained stable for at least 60 min thereafter; kobs, and k+1 were 0.032 and 0.048 min-1·nM-1, respectively. In dissociation experiments, Bsp completely dissociated within 20 min following addition of excess ketanserin; k-1 and t1/2 were 0.0803 min-1 and 8.7 min, respectively. Bsp was saturable (2500 ± 256 cpm/mg protein); Scatchard-calculated values for the equilibrium dissociation constant (KD) and capacity (Bmax) were 0.48 nM, and 125 fmol/mg protein, respectively. Bsp was differentially displaced by structurally related competitors, with a rank order of potency of ketanserin = mianserin > ritanserin > serotonin (5HT) = spiperone >> methiothepin mesylate > metergoline = DOI ((±)-2-5-dimethoxy-4-iodoamphetamine hyrobromide) > 2-methyl-5HT > alpha-methyl-5HT >>>> 5HIAA (5-hydroxyindole acetic acid) = reserpine. These findings provide pharmacological evidence for the presence of a 5HT2-like receptor subtype in the trout hypothalamus.Key words: hypothalamus, [3H]ketanserin, 5HT2 receptor subtype, rainbow trout.

Localization of choline acetyltransferase-expressing neurons in Drosophila nervous system

A variety of approaches have been developed to localize neurons and neural elements in nervous system tissues that make and use acetylcholine (ACh) as a neurotransmitter. Choline acetyltransferase (ChAT) is the enzyme catalyzing the biosynthesis of ACh and is considered to be an excellent phenotypic marker for cholinergic neurons. We have surveyed the distribution of choline acetyltransferase (ChAT)-expressing neurons in the Drosophila nervous system detected by three different but complementary techniques. Immunocytochemistry, using anti-ChAT monoclonal antibodies results in identification of neuronal processes and a few types of cell somata that contain ChAT protein. In situ hybridization using cRNA probes to ChAT messenger RNA results in identification of cell bodies transcribing the ChAT gene. X-gal staining and/or beta-galactosidase immunocytochemistry of transformed animals carrying a fusion gene composed of the regulatory DNA from the ChAT gene controlling expression of a lacZ reporter has also been useful in identifying cholinergic neurons and neural elements. The combination of these three techniques has revealed that cholinergic neurons are widespread in both the peripheral and central nervous system of this model genetic organism at all but the earliest developmental stages. Expression of ChAT is detected in a variety of peripheral sensory neurons, and in the brain neurons associated with the visual and olfactory system, as well as in neurons with unknown functions in the cortices of brain and ganglia.

Distribution of input and output synapses on the central branches of bushcricket and cricket auditory afferent neurones: immunocytochemical evidence for GABA and glutamate in different populations of presynaptic boutons

In order to investigate the synapses on the terminals of primary auditory afferents in the bushcricket and cricket, these were impaled with microelectrodes and after physiological characterisation, injected intracellularly with horseradish peroxidase. The tissue was prepared for electron microscopy, and immunocytochemistry for gamma-aminobutyric acid (GABA) and glutamate was carried out on ultrathin sections by using a post-embedding immunogold technique. The afferent terminals received many input synapses. Between 60-65% of these were made by processes immunoreactive for GABA and approximately 25% from processes immunoreactive for glutamate. The relative distribution of the different classes of input were analysed from serial section reconstruction of terminal afferent branches. Inputs from GABA and glutamate-immunoreactive processes appeared to be scattered at random over the terminal arborisation of the afferents both with respect to each other and to the architecture of the terminals. They were, however, always found close to the output synapses. The possible roles of presynaptic inhibition in the auditory afferents is discussed in the context of the auditory responses of the animals.

Neuropeptides in insect sensory neurones: tachykinin-, FMRFamide- and allatotropin-related peptides in terminals of locust thoracic sensory afferents

Sensory afferents in the thoracic ganglia of the locust Locusta migratoria were labelled with antisera to different neuropeptides: locustatachykinins, FMRFamide and allatotropin. The locustatachykinin-immunoreactive (LTKIR) sensory fibres were derived from the legs and entered the ventral sensory neuropil of each of the thoracic ganglia via nerve 5. In the thoracic neuropil, the LTKIR sensory fibres formed a distinct plexus of terminations ventrally in the ipsilateral hemisphere. The peripheral cell bodies of the sensory neurones could not be revealed, but lesion experiments indicated that origin of the LTKIR fibres was the tarsus of each leg. Possibly the thin fibres are from tarsal chemoreceptors. Double labelling immunocytochemistry revealed that all the LTKIR sensory fibres contained colocalized FMRFamide immunoreactivity. A larger population of sensory fibres reacted with antiserum to moth (Manduca sexta) allatotropin. By means of double labelling immunocytochemistry, we could show that the LTKIR fibres constituted a subpopulation of the larger set of allatotropin-like immunoreactive fibres. Thus some sensory fibres may contain colocalized peptides related to locustatachykinins, FMRFamide-related peptide(s) and allatotropin-like peptide. A separate non-overlapping small set of sensory fibres in nerve 5 reacted with an antiserum to serotonin. Sensory fibres of the other nerves of the ventral nerve cord, including the abdominal ganglia, did not react with the peptide antisera. Since acetylcholine is the likely primary neurotransmitter of insect sensory fibres, it is possible that the peptides and serotonin are colocalized with this transmitter and serve modulatory functions in a subset of the leg afferents.

Cytoarchitecture of histamine-, dopamine-, serotonin- and octopamine-containing neurons in the cricket ventral nerve cord

The present article provides a comparative neuroanatomical description of the cellular localization of the biogenic amines histamine, dopamine, serotonin and octopamine in the ventral nerve cord of an insect, namely the cricket, Gryllus bimaculatus. Generally, different immunocytochemical staining techniques reveal a small number of segmentally distributed immunoreactive (-IR) amine-containing neurons allowing single cell reconstruction of prominent elements. Aminergic neurons share common morphological features in that they innervate large portions of neurophil and often connect different neuromeres by intersegmental 'wide-field' projections of varicose appearance. In many cases aminergic terminals are also found on the surface of peripheral nerves suggesting additional neurohemal release sites. Despite such morphological similarities histological analysis demonstrates for any given amine functionally distinct neuron types with specific innervation patterns establishing discrete pathways. Histamine-IR interneurons are characterized by both ascending and descending projections forming central and peripheral terminals. The descending branches from dopamine-IR cells mainly converge within the terminal ganglion, whereas serotonin-IR interneurons with ascending projections often terminate within the brain. Serotonin is also present in sensory and motor neurons. In contrast to other aminergic neurons, most octopamine-IR cells represent unpaired neurons projecting through motor nerves of the soma-containing neuromere. Octopamine-IR cells with intersegmental branches are only rarely found. Based on these findings, a colocalization of different amines within the same neuron seems to be unlikely to occur in the cricket ventral nerve cord. With respect to the neuroanatomical description of amine-containing neurons known physiological effects of biogenic amines and their possible neuromodulatory functions in insects are discussed.

Insect neurotransmission: neurotransmitters and their receptors

The roles of acetylcholine, dopamine, octopamine, tyramine, 5-hydroxytryptamine, histamine, glutamate, 4-aminobutanoic acid (gamma-aminobutyric acid) and a range of peptides as insect neurotransmitters are evaluated in terms of the criteria used to identify transmitters. Of the biogenic amines considered, there is good evidence that acetylcholine, dopamine, octopamine, 5-hydroxytryptamine, and histamine should be considered to be neurotransmitters, but the case for tyramine is less convincing at the moment. The evidence supporting neurotransmitter roles for glutamate and gamma-aminobutyric acid at specific insect synapses is overwhelming, but much work remains to be undertaken before the full significance of these molecules in the insect nervous system is appreciated. Attempts to characterise biogenic amine and amino acid receptors using pharmacological and molecular biological techniques have revealed considerable differences between mammalian and insect receptors. The number of insect neuropeptides isolated and identified has increased spectacularly in recent years, but genuine physiological or biochemical functions can be assigned to very few of these molecules. Of these, only proctolin fulfills the criteria expected of a neurotransmitter, and the recent discovery of proctolin receptor antagonists should enable the biology of this pentapeptide to be explored fully.

Long-lasting potentiation of a direct central connection between identified motor neurons in the locust

The plasticity of the direct central connection between the fast extensor and the posterior fast flexor tibiae motor neurons in the locust (Schistocerca gregaria) metathoracic ganglion was studied. An action potential in the fast extensor results in a monosynaptic excitatory postsynaptic potential (EPSP) in the flexor motor neuron. Antidromic stimulation of the fast extensor at 100 Hz for 3.5 s resulted in a long-lasting potentiation of the EPSP amplitude. The potentiation was not dependent on feedback caused by movement of the tibia, and was associated with an increase in the input resistance of the flexor motor neuron. The potentiation was heterosynaptic, and was not affected by bath application of the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid. The potentiation was voltage dependent, as hyperpolarizing the flexor motor neuron during the stimulation blocked the development of the potentiation whereas depolarizing the flexor in the absence of presynaptic activity caused potentiation of subsequent fast extensor-evoked EPSPs. The depolarization-induced potentiation was calcium dependent. Antidromic stimulation of the fast extensor at 100 Hz for 3.5 s also caused modulation of the presynaptic action potential. The spike duration was increased and the amplitude of the afterhyperpolarization reduced. These effects were dependent on movement of the tibia. Bath application of the 5-hydroxytryptamine (5-HT) receptor antagonist ketanserin blocked the changes in the presynaptic spike. The modulation was probably due to the release of 5-HT from proprioceptive afferents that monitor movement of the tibia about the femur. The modulation of the presynaptic action potential increases transmitter release onto the flexor motor neurons, and this acts in synergy with the postsynaptic modulation to potentiate the connection.

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.

Distribution of acetylcholine receptors in the central nervous system of adult locusts

A polyclonal antibody raised against nicotinic acetylcholine receptor protein from purified locust neuronal membrane was used to analyse the distribution of antigenic sites within the central nervous system of adult Schistocerca gregaria. Light microscopic examination showed that all principal neuropiles in the thoracic ganglia label with the antibody but that the major tracts and commissures do not. Analysis of this pattern of staining in the electron microscope reveals that the receptor is present on specific synaptic and extrajunctional neuronal membranes in the neuropile. Antigenic sites are also evident on the plasma membranes and within the cytoplasm adjacent to Golgi complexes of some neuronal somata, suggesting that these neurones synthesise nicotinic acetylcholine receptors. In addition to neuronal labelling, there is evidence that the receptor is also present on the membranes of three types of glial cells. The implications of this pattern of receptor distribution are discussed.

Presynaptic modulation of sensory afferents in the invertebrate and vertebrate nervous system

1. Ultrastructural examination of the central terminals of sensory afferent neurons in both invertebrates and vertebrates demonstrates that the synapses that form the substrate for presynaptic inhibition and facilitation are almost universally present. 2. Presynaptic modulation of afferent input acts in many ways which tailor the inflow of sensory information to the behaviour of the animal, effectively providing a means of turning this on and off, or of combining information of the same or different modalities to refine responsiveness or clarify ambiguity. 3. Presynaptic modulation may act in several different roles on the same afferent. 4. A comparison of the mechanisms of presynaptic inhibition in different animals demonstrates the likelihood of a variety of common mechanisms, several of which may act simultaneously on the same terminal. These include changes in the conductance of the afferent membrane to Cl-, K+ and Ca2+ ions, in addition to less well understood mechanisms that directly affect transmitter release. 5. A single transmitter can produce several effects on a terminal through the same or different receptors. 6. Ultrastructural studies of afferent terminals reveal that only a proportion of boutons on a given afferent may receive presynaptic input and that this may depend on the region of the nervous system in which these are found or on the identity of the postsynaptic neurons contacted. 7. The synaptic relationships of afferent terminals can be complex. In invertebrates different types of presynaptic neuron may interact synaptically, as may postsynaptic dendrites in vertebrates. 8. Axons presynaptic to afferent terminals in vertebrates frequently synapse also with dendrites postsynaptic to the afferents. 9. In both invertebrates and vertebrates reciprocal interactions between afferents and postsynaptic neurons are seen. 10. Ultrastructural immunocytochemistry reveals the likely dominance of GABA as an agent of presynaptic inhibition but also demonstrates the possible presence of other transmitters some of whose roles are less completely understood.

Evolutionary aspects of transmitter molecules, their receptors and channels

Classical transmitters are present in all phyla that have been studied; however, our detailed understanding of the process of neurotransmission in these phyla is patchy and has centred on those neurotransmitter receptor mechanisms which are amenable to study with the tools available at the time, for example, high-affinity ligands, tissues with high density of receptor protein, suitable electrophysiological recording systems. Studies also clearly show that many neurones exhibit co-localization of classical transmitters and neuropeptides. However, the physiological implications of this co-localization have yet to be elucidated in the vast majority of examples. The application of molecular biological techniques to the study of neurotransmitter receptors (to date mainly in vertebrates) is contributing to our understanding of the evolution of these proteins. Striking similarities in the structure of ligand-gated receptors have been revealed. Thus, although ligand-gated receptors differ markedly in terms of the endogenous ligands they recognize and the ion channels that they gate, the structural similarities suggest a strong evolutionary relationship. Pharmacological differences also exist between receptors that recognize the same neurotransmitter but in different phyla, and this may also be exploited to further the understanding of structure-function relationships for receptors. Thus, for instance, some invertebrate GABA receptors are similar to mammalian GABAA receptors but lack a modulatory site operated by benzodiazepines. Knowledge of the structure and subunit composition of these receptors and comparison with those that have already been elucidated for the mammalian nervous system might indicate the functional importance of certain amino acid residues or receptor subunits. These differences could also be exploited in the development of new agents to control agrochemical pests and parasites of medical importance. The study of the pharmacology of receptor proteins for neurotransmitters in invertebrates, together with the application of biochemical and molecular biological techniques to elucidate the structure of these molecules, is now gathering momentum. For certain receptors, e.g. the nicotinic receptor, we can expect to have fundamental information on the function of this receptor at the molecular level in both invertebrates and vertebrates in the near future.

Activity-dependent induction of facilitation, depression, and post-tetanic potentiation at an insect central synapse

In Manduca sexta larvae, sensory neurons innervating planta hairs on the tips of the prolegs make monosynaptic excitatory connections with motoneurons innervating proleg retractor muscles. Tactile stimulation of the hairs evokes reflex retraction of the proleg. In this study we examined activity-dependent changes in the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked in a proleg motoneuron by stimulation of individual planta hair sensory neurons. Deflection of a planta hair caused a phasic-tonic response in the sensory neuron, with a mean peak instantaneous firing frequency of greater than 300 Hz, and a tonic firing rate of 10-20 Hz. Direct electrical stimulation was used to activate individual sensory neurons to fire at a range of frequencies including those observed during natural stimulation of the hair. At relatively low firing rates (e.g., 1 Hz), EPSP amplitude was stable indefinitely. At higher instantaneous firing frequencies (greater than 10 Hz), EPSPs were initially facilitated, but continuous stimulation led rapidly to synaptic depression. High-frequency activation of a sensory neuron could also produce post-tetanic potentiation, in which EPSP amplitude remained elevated for several min following a stimulus train. Facilitation, depression, and post-tetanic potentiation all appeared to be presynaptic phenomena. These activity-dependent changes in sensory transmission may contribute to the behavioral plasticity of the proleg withdrawal reflex observed in intact insects.

GABA-like immunoreactivity in a population of locust intersegmental interneurones and their inputs

Intracellular labelling of locust intersegmental interneurones with lucifer yellow or horseradish peroxidase was carried out in combination with light and electron microscope immunocytochemistry by using an antibody raised against gamma amino butyric acid (GABA). Fifteen percent (four out of 27) of intracellularly stained interneurones showed GABA-like immunoreactivity. This is in agreement with previous physiological observations that 20% of the interneurones in this population make inhibitory output connections in the metathoracic ganglion. GABA-like immunoreactivity was also found in processes presynaptic to the interneurones in the mesothoracic ganglion. The presence of such immunoreactive inputs onto the intersegmental interneurones correlates well with physiological evidence that their receptive fields are in part shaped by direct input from GABA-ergic spiking local interneurones.

Serotonin-immunoreactive neurons in the antennal lobes of the American cockroach Periplaneta americana: light- and electron-microscopic observations

A large deutocerebral serotonin-immunoreactive neuron arborizes profusely in the glomeruli of the antennal lobes, and also sends neurites into the lateral lobe and the calyces of the mushroom bodies in the ipsilateral protocerebrum. Electron micrographs of the glomerular neuropil show that the main synapses of the serotonin-immunoreactive arborizations are output contacts with unidentified neuron profiles. Only a few synaptic input contacts with serotonin-labeled fibers were observed.