Histochemistry of acetylcholinesterase and immunocytochemistry of an acetylcholine receptor-like antigen in the brain of the honeybee

Involvement of NO-synthase and nicotinic receptors in learning in the honey bee

Restrained worker honey bees (Apis mellifera) are one of the main models for the comparative study of learning and memory processes. Bees easily learn to associate a sucrose reward to antennal tactile scanning of a small metal plate (associative learning). Their proboscis extension response can also be habituated through repeated sucrose stimulations (non-associative learning). We studied the role of nitric oxide synthase and nicotinic acetylcholine receptors in these two forms of learning. The nicotinic antagonist MLA or the nitric oxide synthase inhibitor l-NAME impaired the formation of tactile associative long-term memory that specifically occurs during multiple-trial training; however these drugs had no effect on single-trial training. None of the drugs affected retrieval processes. These pharmacological results are consistent with data previously obtained with olfactory conditioning and indicate that MLA-sensitive nicotinic receptors and NO-synthase are specifically involved in long-term memory. MLA and l-NAME both reduced the number of trials required for habituation to occur. This result suggests that a reduction of cholinergic nicotinic neurotransmission promotes PER habituation in the honey bee.

Pilocarpine improves recognition of nestmates in young honey bees

Honey bees can distinguish nestmates from non-nestmates, directing aggressive responses toward non-nestmates and rarely attacking nestmates. Here we provide evidence that treatment with pilocarpine, a muscarinic agonist, significantly reduced the number of aggressive responses directed toward nestmates. By contrast, treatment with scopolamine, a muscarinic antagonist, significantly increased attacks on nestmates. Locomotor activity was not altered by these pharmacological treatments. When interpreted in light of known cholinergic pathways in the insect brain, our results provide the first evidence that cholinergic signaling via muscarinic receptors plays a role in olfaction-based social behavior in honey bees.

Effects of sublethal doses of acetamiprid and thiamethoxam on the behavior of the honeybee (Apis mellifera)

Acetamiprid and thiamethoxam are insecticides introduced for pest control, but they can also affect non-target insects such as honeybees. In insects, these neonicotinoid insecticides are known to act on acetylcholine nicotinic receptors but the behavioral effects of low doses are not yet fully understood. The effects of acetamiprid and thiamethoxam were studied after acute sublethal treatment on the behavior of the honeybee (Apis mellifera) under controlled laboratory conditions. The drugs were either administered orally or applied topically on the thorax. After oral consumption acetamiprid increased sensitivity to antennal stimulation by sucrose solutions at doses of 1 microg/bee and impaired long-term retention of olfactory learning at the dose of 0.1 microg/bee. Acetamiprid thoracic application induced no effect in these behavioral assays but increased locomotor activity (0.1 and 0.5 microg/bee) and water-induced proboscis extension reflex (0.1, 0.5, and 1 microg/bee). Unlike acetamiprid, thiamethoxam had no effect on bees' behavior under the conditions used. Our results suggest a particular vulnerability of honeybee behavior to sublethal doses of acetamiprid.

Widespread brain distribution of the Drosophila metabotropic glutamate receptor

Glutamate is the predominant excitatory neurotransmitter in the vertebrate brain, whereas acetylcholine has been considered to play the same role in insects. Recent studies have, however, questioned the latter view by showing a rather general distribution of glutamate transporters. Here, we describe the expression pattern of the receptor DmGlu-A (DmGluRA), the unique homolog of vertebrate metabotropic glutamate receptors. Metabotropic glutamate receptors play important roles in the regulation of glutamatergic neurotransmission. Using a specific antibody, we report DmGluRA expression in most neuropile areas in both larvae and adults, but not in the lobes of the mushroom bodies. These observations suggest a key role for glutamate in the insect brain.

Muscarinic excitation in grasshopper song control circuits is limited by acetylcholinesterase activity

The species- and situation-specific sound production of grasshoppers can be stimulated by focal application of both nicotinic and muscarinic receptor agonists into the central body complex of the protocerebrum. Pressure injection of the intrinsic transmitter acetylcholine only elicits fast and short-lived responses related to nicotinic receptor-mediated excitation. Prolonged sound production that includes complex song patterns requires muscarinic receptor-mediated excitation. In addition, basal muscarinic excitation in the central body neuropil seems to determine the general motivation of a grasshopper to stridulate. To demonstrate that endogenous acetylcholinesterase limits the activation of muscarinic receptors by synaptically released acetylcholine in the central body of Chorthippus biguttulus, we investigated both its presence in the brain and effects on sound production resulting from inhibition of esterase activity. Acetylcholinesterase activity was detected in the upper and lower division of the central body. Both these neuropils known to be involved in the cephalic control of stridulation were also shown to contain muscarinic acetylcholine receptors expressed by columnar neurons suggested to serve as output neurons of the central complex. Pressure injection of the acetylcholinesterase inhibitor eserine into protocerebral control circuits of restrained male grasshoppers stimulated long-lasting stridulation that depended on scopolamine-sensitive muscarinic receptors. In restrained males, eserine released the typical response song by potentiating the stimulatory effect of the conspecific female song. Eserine-mediated inhibition of acetylcholinesterase in the central body prolongs the presence of synaptically released acetylcholine at its postsynaptic receptors and increases its potency to activate muscarinic receptor-initiated signaling pathways acting to promote grasshopper sound production.

Study of nicotinic acetylcholine receptors on cultured antennal lobe neurones from adult honeybee brains

In insects, acetylcholine (ACh) is the main neurotransmitter, and nicotinic acetylcholine receptors (nAChRs) mediate fast cholinergic synaptic transmission. In the honeybee, nAChRs are expressed in diverse structures including the primary olfactory centres of the brain, the antennal lobes (AL) and the mushroom bodies. Whole-cell, voltage-clamp recordings were used to characterize the nAChRs present on cultured AL cells from adult honeybee, Apis mellifera. In 90% of the cells, applications of ACh induced fast inward currents that desensitized slowly. The classical nicotinic agonists nicotine and imidacloprid elicited respectively 45 and 43% of the maximum ACh-induced currents. The ACh-elicited currents were blocked by nicotinic antagonists methyllycaconitine, dihydroxy-beta-erythroidine and alpha-bungarotoxin. The nAChRs on adult AL cells are cation permeable channels. Our data indicate the existence of functional nAChRs on adult AL cells that differ from nAChRs on pupal Kenyon cells from mushroom bodies by their pharmacological profile and ionic permeability, suggesting that these receptors could be implicated in different functions.

Dual olfactory pathway in the honeybee, Apis mellifera

The antennal lobes (ALs) are the primary olfactory centers in the insect brain. In the AL of the honeybee, olfactory glomeruli receive input via four antennal sensory tracts (T1-4). Axons of projection neurons (PNs) leave the AL via several antenno-cerebral tracts (ACTs). To assign the input-output connectivity of all glomeruli, we investigated the spatial relationship of the antennal tracts and two prominent AL output tracts (medial and lateral ACT) mainly formed by uniglomerular (u) PNs using fluorescent tracing, confocal microscopy, and 3D analyses. Furthermore, we investigated the projections of all ACTs in higher olfactory centers, the mushroom-bodies (MB) and lateral horn (LH). The results revealed a clear segregation of glomeruli into two AL hemispheres specifically supplied by PNs of the medial and lateral ACT. PNs of the lateral ACT innervate glomeruli in the ventral-rostral AL and primarily receive input from T1 (plus a few glomeruli from T2 and T3). PNs of the medial ACT innervate glomeruli in the dorsal-caudal hemisphere, and mainly receive input from T3 (plus a few glomeruli from T2 and T4). The PNs of the m- and l-ACT terminate in different areas of the MB calyx and LH and remain largely segregated. Tracing of three mediolateral (ml) ACTs mainly formed by multiglomerular PNs revealed terminals in distinct compartments of the LH and in three olfactory foci within the lateral protocerebrum. The results indicate that olfactory input in the honeybee is processed via two separate, mainly uPN pathways to the MB calyx and LH and several pathways to the lateral protocerebrum.

The sublethal effects of pesticides on beneficial arthropods

Traditionally, measurement of the acute toxicity of pesticides to beneficial arthropods has relied largely on the determination of an acute median lethal dose or concentration. However, the estimated lethal dose during acute toxicity tests may only be a partial measure of the deleterious effects. In addition to direct mortality induced by pesticides, their sublethal effects on arthropod physiology and behavior must be considered for a complete analysis of their impact. An increasing number of studies and methods related to the identification and characterization of these effects have been published in the past 15 years. Review of sublethal effects reported in published literature, taking into account recent data, has revealed new insights into the sublethal effects of pesticides including effects on learning performance, behavior, and neurophysiology. We characterize the different types of sublethal effects on beneficial arthropods, focusing mainly on honey bees and natural enemies, and we describe the methods used in these studies. Finally, we discuss the potential for developing experimental approaches that take into account these sublethal effects in integrated pest management and the possibility of integrating their evaluation in pesticide registration procedures.

Involvement of α-bungarotoxin-sensitive nicotinic receptors in long-term memory formation in the honeybee (Apis mellifera)

In the honeybee Apis mellifera, multiple-trial olfactory conditioning of the proboscis extension response specifically leads to long-term memory (LTM) which can be retrieved more than 24 h after learning. We studied the involvement of nicotinic acetylcholine receptors in the establishment of LTM by injecting the nicotinic antagonists mecamylamine (1 mM), alpha-bungarotoxin (alpha-BGT, 0.1 mM) or methyllycaconitine (MLA, 0.1 mM) into the brain through the median ocellus 20 min before or 20 min after multiple-trial learning. The retention tests were performed 1, 3, and 24 h after learning. Pre-training injections of mecamylamine induced a lower performance during conditioning but had no effect on LTM formation. Post-training injections of mecamylamine did not affect honeybees' performances. Pre-training injections of MLA or post-training injection of alpha-BGT specifically induced LTM impairment whereas acquisition as well as memory retrieval tested 1 or 3 h after learning was normal. This indicates that brain injections of alpha-BGT and MLA did not interfere with learning or medium-term memory. Rather, these blockers affect the LTM. To explain these results, we advance the hypothesis that honeybee alpha-BGT-sensitive acetylcholine receptors are also sensitive to MLA. These receptors could be essential for triggering intracellular mechanisms involved in LTM. By contrast, medium-term memory is not dependent upon these receptors but is affected by mecamylamine.

Acetylcholine, GABA and glutamate induce ionic currents in cultured antennal lobe neurons of the honeybee, Apis mellifera

The honeybee, Apis mellifera, is a valuable model system for the study of olfactory coding and its learning and memory capabilities. In order to understand the synaptic organisation of olfactory information processing, the transmitter receptors of the antennal lobe need to be characterized. Using whole-cell patch-clamp recordings, we analysed the ligand-gated ionic currents of antennal lobe neurons in primary cell culture. Pressure applications of acetylcholine (ACh), gamma-amino butyric acid (GABA) or glutamate induced rapidly activating ionic currents. The ACh-induced current flows through a cation-selective ionotropic receptor with a nicotinic profile. The ACh-induced current is partially blocked by alpha-bungarotoxin. Epibatidine and imidacloprid are partial agonists. Our data indicate the existence of an ionotropic GABA receptor which is permeable to chloride ions and sensitive to picrotoxin (PTX) and the insecticide fipronil. We also identified the existence of a chloride current activated by pressure applications of glutamate. The glutamate-induced current is sensitive to PTX. Thus, within the honeybee antennal lobe, an excitatory cholinergic transmitter system and two inhibitory networks that use GABA or glutamate as their neurotransmitter were identified.

Antennal tactile learning in the honeybee: Effect of nicotinic antagonists on memory dynamics

Restrained worker honeybees (Apis mellifera L.) are able to learn to associate antennal-scanning of a metal plate with a sucrose reinforcement delivered to the mouthparts. Learning occurs reliably in a single association of the two sensory stimuli. The involvement of nicotinic pathways in memory formation and retrieval processes was tested by injecting, into the whole brain through the median ocellus, either mecamylamine (0.6 microg per bee) or alpha-bungarotoxin (2.4 ng per bee). Saline served as a control. Mecamylamine injected 10 min before the retrieval test impairs the retention level tested 3 h and 24 h after single- or multi-trial learning. Retrieval tests performed at various times after the injection show that the blocking effect of mecamylamine lasts about 1 h. The drug has no effect on the reconsolidation or extinction processes. Mecamylamine injected 10 min before conditioning impairs single-trial learning but has no effect on five-trial learning and on the consolidation process. By contrast, alpha-bungarotoxin only impairs the formation of long-term memory (24 h) induced by the five-trial learning and has no effect on medium-term memory (3 h), on single-trial learning or on the retrieval process. Hence, owing to previous data, at least two kinds of nicotinic receptors seem to be involved in honeybee memory, an alpha-bungarotoxin-sensitive and an alpha-bungarotoxin-insensitive receptor. Our results extend to antennal mechanosensory conditioning the role of the cholinergic system that we had previously described for olfactory conditioning in the honeybee. Moreover, we describe here in this insect a pharmacological dissociation between alpha-bungarotoxin sensitive long-term memory and alpha-bungarotoxin insensitive medium-term memory, the last one being affected by mecamylamine.

Apisα2, Apisα7-1 and Apisα7-2: three new neuronal nicotinic acetylcholine receptor α-subunits in the honeybee brain

Acetylcholine is the principal excitatory neurotransmitter in the central nervous system of insects. Nicotinic acetylcholine receptors, which belong to the ligand-gated ion channel family, constitute important targets for insecticides. In the honeybee Apis mellifera, pharmacological evidence supports the existence of several nicotinic acetylcholine receptors. In this paper, we report the identification of three new genes that encode nicotinic acetylcholine receptor alpha-subunits in the honeybee. Phylogenetic comparisons with other ligand-gated ion channel subunit sequences support their classification as Apisalpha2, Apisalpha7-1 and Apisalpha7-2 subunits. Based on in situ hybridization experiments, we determined their expression patterns in the different brain regions of pupae and adult honeybees. Our results show that these nicotinic acetylcholine receptor subunits are differently expressed among the brain regions and that they appear at different stages of honeybee development.

Pharmacology of the neuronal nicotinic acetylcholine receptor of cultured Kenyon cells of the honeybee, Apis mellifera

We investigated the pharmacology of the nicotinic acetylcholine receptor of honeybee Kenyon cells, a subset of olfactory interneurons, which are crucial for olfactory learning and memory. Whole-cell currents were recorded using patch-clamp techniques. Pressure application of agonists induced inward currents in cultured Kenyon cells at holding potentials of -110 mV. Acetylcholine or carbamylcholine were full agonists, nicotine, epibatidine and cytisine were only partial agonists. Coapplications of these partial agonists with acetylcholine reduced the current amplitude. The most efficient antagonists were dihydroxy-beta-erythroidine (EC(50)=0.5 pmol x l(-1)) and methyllycaconitine (EC(50)=24 pmol x l(-1)). The open channel blocker mecamylamine, d-tubocurarine and hexamethonium were rather weak blockers of the honeybee nicotinic response. Bath applications of the muscarinic antagonist atropine inhibited nicotinic currents dependent on concentration (EC(50)=24.3 micromol x l(-1)). Muscarine, pilocarpine or oxotremorine (1 mmol x l(-1)) did not induce any measurable currents. The non-cholinergic drugs strychnine, bicuculline and picrotoxin partially and reversibly blocked the acetylcholine-induced currents. Our results indicate the expression of only one nicotinic acetylcholine receptor subtype in cultured Kenyon cells. Muscarinic as well as non-cholinergic antagonists also inhibit the receptor function, distinguishing the honeybee nicotinic receptor from the "typical" nicotinic receptor of vertebrates and from many described insects receptors.

Learning channels. Cellular physiology of odor processing neurons within the honeybee brain

To understand the cellular mechanisms of olfactory learning in the honeybee brain we study the physiology of identified neurons within the olfactory pathway. Here, we review data on the voltage-sensitive and ligand-gated ionic currents of mushroom body Kenyon cells and antennal lobe neurons in vitro and in situ. Both cell types generate action potentials in vitro, but have different voltage-sensitive K+ currents. They express nicotinic acetylcholine receptors and ionotropic GABA receptors, representing the major transmitter systems in the insect olfactory system. Our data are interpreted with respect to learning-dependent plasticity in the honeybee brain.

Physiological and morphological characterization of honeybee olfactory neurons combining electrophysiology, calcium imaging and confocal microscopy

The insect antennal lobe is the first brain structure to process olfactory information. Like the vertebrate olfactory bulb the antennal lobe is substructured in olfactory glomeruli. In insects, glomeruli can be morphologically identified, and have characteristic olfactory response profiles. Local neurons interconnect glomeruli, and output (projection) neurons project to higher-order brain centres. The relationship between their elaborate morphology and their physiology is not understood. We recorded electrophysiologically from antennal lobe neurons, and iontophoretically injected a calcium-sensitive dye. We then measured their spatio-temporal calcium responses to a variety of odours. Finally, we confocally reconstructed the neurons, and identified the innervated glomeruli. An increase or decrease in spiking frequency corresponded to an intracellular calcium increase or decrease in the cell. While intracellular recordings generally lasted between 10 and 30 min, calcium imaging was stable for up to 2 h, allowing a more detailed physiological analysis. The responses indicate that heterogeneous local neurons get input in the glomerulus in which they branch most strongly. In many cases, the physiological response properties of the cells corresponded to the known response profile of the innervated glomerulus. In other words, the large variety of response profiles generally found when comparing antennal lobe neurons is reduced to a more predictable response profile when the innervated glomerulus is known.

Fast synaptic currents in Drosophila mushroom body Kenyon cells are mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors and picrotoxin-sensitive GABA receptors

The mushroom bodies, bilaterally symmetric regions in the insect brain, play a critical role in olfactory associative learning. Genetic studies in Drosophila suggest that plasticity underlying acquisition and storage of memory occurs at synapses on the dendrites of mushroom body Kenyon cells (Dubnau et al., 2001). Additional exploration of the mechanisms governing synaptic plasticity contributing to these aspects of olfactory associative learning requires identification of the receptors that mediate fast synaptic transmission in Kenyon cells. To this end, we developed a culture system that supports the formation of excitatory and inhibitory synaptic connections between neurons harvested from the central brain region of late-stage Drosophila pupae. Mushroom body Kenyon cells are identified as small-diameter, green fluorescent protein-positive (GFP+) neurons in cultures from OK107-GAL4;UAS-GFP pupae. In GFP+ Kenyon cells, fast EPSCs are mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors (nAChRs). The miniature EPSCs have rapid rise and decay kinetics and a broad, positively skewed amplitude distribution. Fast IPSCs are mediated by picrotoxin-sensitive chloride conducting GABA receptors. The miniature IPSCs also have a rapid rate of rise and decay and a broad amplitude distribution. The vast majority of spontaneous synaptic currents in the cultured Kenyon cells are mediated byalpha-bungarotoxin-sensitive nAChRs or picrotoxin-sensitive GABA receptors. Therefore, these receptors are also likely to mediate synaptic transmission in Kenyon cells in vivo and to contribute to plasticity during olfactory associative learning.

A new ascending sensory tract to the calyces of the honeybee mushroom body, the subesophageal-calycal tract

The mushroom bodies of the honeybee are important neuropils for learning and memory. Therefore, knowledge about their input and output connections is essential to understanding how these neuropils function. A newly described input tract to the mushroom body is presented here, which is called the subesophageal-calycal tract (SCT) and connects the subesophageal ganglion with the calyces of the mushroom bodies. The neuronal somata of the SCT neurons lie in one cluster between the lobula of the optic lobe and a neuropil area that is formed from the fusion of the tritocerebrum and the subesophageal ganglion. Within the subesophageal ganglion, the dendritic fibers of SCT neurons overlap with terminals of sensory neurons from the proboscis. Therefore, we conclude that the SCT neurons might process gustatory and mechanosensory information from the proboscis. Individual SCT neurons receive unilateral input within the subesophageal ganglion and may connect to either the ipsilateral or the contralateral mushroom body. On their way to the mushroom bodies, the SCT neuron axons meet the roots of the antennocerebralis tracts (ACTs) and from this point follow the same path as the median ACT neurons for a short distance. Within the calyces, the SCT neurons innervate two separate areas, a small area within the dorsal collar just below the lip and a part of the basal ring. Double-labeling experiments show that the projections of the SCT neurons do not overlap with the projections of the olfactory projection neurons and visual projection neurons from the dorsal medulla. The possible function of the SCT neurons and the relation of the SCT to known input tracts of the mushroom bodies in other insects are discussed.

Organization and neural connections of the anterior optic tubercle in the brain of the locust, Schistocerca gregaria

The anterior optic tubercle is a small neuropil in the insect brain and a major target of visual interneurons from the optic lobe. The functional role of the tubercle is poorly understood, but recent evidence from locusts points to a possible involvement in polarization vision. The present study examines the organization of the anterior optic tubercle in the locust Schistocerca gregaria and its connections with other brain areas. The tubercle of the locust consists of an upper and a lower subunit. Both units are connected in parallel with the medulla and lobula of the optic lobe, with the contralateral tubercle, and with the lateral accessory lobe in the median protocerebrum. Wide-field transmedullary neurons provide input from the medulla. Neurons with processes in the dorsal rim of the medulla, a relay station in the polarization vision pathway, project exclusively to the lower unit of the tubercle. Visual input from the lobula to the upper and lower unit originates from topographically distinct strata. The most prominent output target of the tubercle is the lateral accessory lobe in the median protocerebrum. Neurons from the upper unit project widely in the lateral accessory lobe, whereas neurons from the lower unit have focused projections confined to the median olive and to the lateral triangle. The two subunits of the anterior optic tubercle are, therefore, processing stages in two parallel visual pathways from the optic lobe to the median protocerebrum. Pathways via the lower unit of the tubercle appear to be involved in polarization vision.

Identification and localization of the nicotinic acetylcholine receptor alpha3 mRNA in the brain of the honeybee, Apis mellifera

The nicotinic acetylcholine receptors are ligand-gated ion channels responsible for rapid neurotransmission and are target sites for pesticides in insects. In the honeybee Apis mellifera, pharmacological and electrophysiological studies have shown that different nicotinic acetylcholine receptor subtypes may exist in the brain. Here, we have identified a honeybee cDNA that encodes a 537 amino acid protein with features typical of nicotinic acetylcholine receptor alpha subunit, and sequence homology to human alpha3. In situ hybridization on cryosections shows that the Apisalpha3 mRNA is differently expressed in larvae and adult. In larvae, Apisalpha3 mRNA expression is restricted to the suboesophageal ganglia. In adult, it is further expressed in the optic lobes, the dorsal lobes, the antennal lobes and the calyces of mushroom bodies. Together our results suggest that Apisalpha3 shows a controlled expression pattern during development.

Detection of calcium transients in Drosophila mushroom body neurons with camgaroo reporters

Camgaroos are yellow fluorescent protein derivatives that hold promise as transgenically encoded calcium sensors in behaving animals. We expressed two versions of camgaroo in Drosophila mushroom bodies using the galactosidase-4 (GAL4) system. Potassium depolarization of brains expressing the reporters produces a robust increase in fluorescence that is blocked by removing extracellular calcium or by antagonists of voltage-dependent calcium channels. The fluorescence increase is not attributable to cytoplasmic alkalization; depolarization induces a slight acidification of the cytoplasm of mushroom body neurons. Acetylcholine applied near the dendrites of the mushroom body neurons induces a rapid and ipsilateral-specific fluorescence increase in the mushroom body axons that is blocked by antagonists of calcium channels or nicotinic acetylcholine receptors. Fluorescence was observed to increase in all three classes of mushroom body neurons, indicating that all types respond to cholinergic innervation.

Chemical neuroanatomy of the fly's movement detection pathway

In Diptera, subsets of small retinotopic neurons provide a discrete channel from achromatic photoreceptors to large motion-sensitive neurons in the lobula complex. This pathway is distinguished by specific affinities of its neurons to antisera raised against glutamate, aspartate, gamma-aminobutyric acid (GABA), choline acetyltransferase (ChAT), and a N-methyl-D-aspartate type 1 receptor protein (NMDAR1). Large type 2 monopolar cells (L2) and type 1 amacrine cells, which in the external plexiform layer are postsynaptic to the achromatic photoreceptors R1-R6, express glutamate immunoreactivity as do directionally selective motion-sensitive tangential neurons of the lobula plate. L2 monopolar cells ending in the medulla are accompanied by terminals of a second efferent neuron T1, the dendrites of which match NMDAR1-immunoreactive profiles in the lamina. L2 and T1 endings visit ChAT and GABA-immunoreactive relays (transmedullary neurons) that terminate from the medulla in a special layer of the lobula containing the dendrites of directionally selective retinotopic T5 cells. T5 cells supply directionally selective wide-field neurons in the lobula plate. The present results suggest a circuit in which initial motion detection relies on interactions among amacrines and T1, and the subsequent convergence of T1 and L2 at transmedullary cell dendrites. Convergence of ChAT-immunoreactive and GABA-immunoreactive transmedullary neurons at T5 dendrites in the lobula, and the presence there of local GABA-immunoreactive interneurons, are suggested to provide excitatory and inhibitory elements for the computation of motion direction. A comparable immunocytological organization of aspartate- and glutamate-immunoreactive neurons in honeybees and cockroaches further suggests that neural arrangements providing directional motion vision in flies may have early evolutionary origins.

Immunoreactivity against choline acetyltransferase, gamma-aminobutyric acid, histamine, octopamine, and serotonin in the larval chemosensory system of Dosophila melanogaster

We have studied the distribution of choline acetyltransferase (ChAT), gamma-aminobutyric acid (GABA), histamine, octopamine and serotonin in the larval chemosensory system of Drosophila melanogaster. Colocalization at the confocal level with green fluorescent protein (GFP) or Tau-GFP reporters, expressed in selected P[GAL4] enhancer trap lines, was used to identify the cells making up these neurotransmitters. As in the adult fly, larval olfactory afferents project into the (larval) antennal lobe (LAL), where they synapse onto local interneurons and projection neurons, whereas gustatory afferents terminate essentially in the tritocerebral-subesophageal (TR-SOG) region. We demonstrate that the neuropils of the LAL and the TR-SOG are immunoreactive to ChAT and GABA. In addition, serotonin- and octopamine-immunoreactive fibers are present in the LAL. ChAT immunostaining is localized in subsets of olfactory and gustatory afferents and in many of the projection neurons. In contrast, GABA is expressed in most, and perhaps all, of the local interneurons. Serotonin immunoreactivity in the LAL derives from a single neuron that is situated close to the LAL and projects to additional neuropil regions. Taken together, these findings resemble the situation in the adult fly. Hence, given the highly reduced numbers of odorant receptor neurons in the larva, as shown in a previous study (Python and Stocker [2002] J. Comp. Neurol. 445:374-387), the larval system may become an attractive model system for studying the roles of neurotransmitters in olfactory processing.

Peripheral synaptic contacts at mechanoreceptors in arachnids and crustaceans: morphological and immunocytochemical characteristics

Two types of sensory organs in crustaceans and arachnids, the various mechanoreceptors of spiders and the crustacean muscle receptor organs (MRO), receive extensive efferent synaptic innervation in the periphery. Although the two sensory systems are quite different-the MRO is a muscle stretch receptor while most spider mechanoreceptors are cuticular sensilla-this innervation exhibits marked similarities. Detailed ultrastructural investigations of the synaptic contacts along the mechanosensitive neurons of a spider slit sense organ reveal four important features, all having remarkable resemblances to the synaptic innervation at the MRO: (1) The mechanosensory neurons are accompanied by several fine fibers of central origin, which are presynaptic upon the mechanoreceptors. Efferent control of sensory function has only recently been confirmed electrophysiologically for the peripheral innervation of spider slit sensilla. (2) Different microcircuit configuration types, identified on the basis of the structural organization of their synapses. (3) Synaptic contacts, not only upon the sensory neurons but also between the efferent fibers themselves. (4) Two identified neurotransmitter candidates, GABA and glutamate. Physiological evidence for GABAergic and glutamatergic transmission is incomplete at spider sensilla. Given that the sensory neurons are quite different in their location and origin, these parallels are most likely convergent. Although their significance is only partially understood, mostly from work on the MRO, the close similarities seem to reflect functional constraints on the organization of efferent pathways in the brain and in the periphery.

The insecticide imidacloprid is a partial agonist of the nicotinic receptor of honeybee Kenyon cells

The main targets of the insecticide imidacloprid are neuronal nicotinic acetylcholine receptors (nAChRs) within the insect brain. We tested the effects of imidacloprid on ligand-gated ion channels of cultured Kenyon cells of the honeybee, Apis mellifera. Kenyon cells build up the mushroom body neuropils, which are involved in higher order neuronal processes such as olfactory learning. We measured whole-cell currents through nicotinic and gamma-aminobutyric acid (GABA) receptors using patch-clamp techniques. Pressure applications of imidacloprid elicited inward currents, which were irreversibly blocked by alpha-bungarotoxin. Imidacloprid was a partial nicotinic agonist, since it elicited only 36% of ACh-induced currents and competitively blocked 64% of the peak ACh-induced currents. GABA-induced currents were partially blocked when imidacloprid was coapplied and this block was independent upon activation of nAChRs. Our results identify the honeybee nAChR as a target of imidacloprid and an imidacloprid-induced inhibition of the insect GABA receptor.

Contrasting effects of imidacloprid on habituation in 7- and 8-day-old honeybees (Apis mellifera)

We examined the effects of sublethal doses (0.1, 1, and 10 ng per animal) of a new neonicotinoid insecticide, Imidacloprid, on habituation of the proboscis extension reflex (PER) in honeybees (Apis mellifera) reared under laboratory conditions. In untreated honeybees, the habituation of the proboscis extension reflex is age-dependent and there is a significant increase in the number of trials required for habituation in older bees (8-10 days old) as compared to very young bees (4-7 days old). Imidacloprid alters the number of trials needed to habituate the honeybee response to multiple sucrose stimulation. In 7-day-old bees, treatment with Imidacloprid leads to an increase in the number of trials necessary to abolish the response, whereas in 8-day-old bees, it leads to a reduction in the number of trials for habituation (15 min and 1 h after treatment), and to an increase 4 h after treatment. The temporal effects of Imidacloprid in both 7- and 8-day-old bees suggest that 4h after treatment the observed effects are due to a metabolite of Imidacloprid, rather than to Imidacloprid itself. Our results suggest the existence of two distinct subtypes of nicotinic receptors in the honeybee that have different affinities to Imidacloprid and are differentially expressed in 7- and 8-day-old individuals.

Structure and response patterns of olfactory interneurons in the honeybee, Apis mellifera

To analyze morphologic and physiological properties of olfactory interneurons in the honeybee, Apis mellifera, antennal lobe (AL) neurons were intracellularly recorded and subsequently labeled with Neurobiotin. Additional focal injections were carried out with cobalt hexamine chloride and dextran fluorescent markers. Olfactory interneurons (projection neurons, PNs) project by means of five tracts, the lateral, the median, and three mediolateral antennocerebral tracts (l-, m-, and ml-ACT, respectively) to the mushroom bodies (MBs) and the protocerebral lobe (PL) of the ipsilateral protocerebrum. Uniglomerular PNs of the m- and l-ACT receiving input from a single glomerulus of the AL also arborize in different regions of the AL. The vast majority of l-ACT innervate the T1 region, whereas m-ACT neurons arborize exclusively in the T2, T3, and T4 regions (T1-4 : AL projection area of sensory cells from the antennae). In the calyces of the MB, uniglomerular PNs form varicosities in the basal ring and the lip region. Individual neurons of both types exhibit unequal innervation within and between the two calyces. In addition, m-ACT fibers ramify more densely within the lip neuropil and show a higher incidence of spine-like processes than l-ACTs. In the PL, l-ACTs arborize exclusively within the lateral horn, whereas some m-ACT neurons innervate a broader region. Multiglomerular neurons of the ml-ACT leave the AL by means of three subtracts (ml-ACT 1-3). Two different types can be distinguished according to their protocerebral target areas: ml-ACTs projecting to the lateral PL (LPL) and to the neuropil around the alpha-lobe (tracts 2 and 3) and neurons projecting only to the LPL (tract 1). Intracellular recordings indicate that both l- and m-ACT neurons respond to general odors but with different response properties, indicating that odor information is processed in parallel pathways with different functional characteristics. Just like m-ACT neurons, ml-ACT neurons respond to odors with complex activity patterns. Bilateral interneurons, originating in the suboesophageal ganglion, connect glomeruli of both AL, and send an axon through the m-ACT in each hemisphere of the brain, terminating in the lip region of the calyces. These neurons respond to contact chemical stimuli.

Synchronized neural activity in the Drosophila memory centers and its modulation by amnesiac

The mushroom bodies are key features of the brain circuitry for insect associative learning, especially when evoked by olfactory cues. Mushroom bodies are also notable for the close-packed parallel architecture of their many intrinsic neuronal elements, known as Kenyon cells. Here, we report that Kenyon cells of adult Drosophila exhibit synchronous oscillation of intracellular calcium concentration, with a mean period of approximately 4 min. Robust oscillation within a dissected brain persists for hours in insect saline and is strongly modulated in amplitude by the product(s) of the memory consolidation gene, amnesiac. It is also sensitive to pharmacological agents specific for several classes of ion channel and for acetylcholine and GABA receptors. A role in memory consolidation involving transcriptionally mediated synaptic strengthening is proposed.

Nicotinic acetylcholine receptor ligands differently affect cytochrome oxidase in the Honeybee brain

The objective of this study was to determine if nicotinic receptor antagonists known for their ability to impair memory in the honeybee could induce changes in brain metabolism. We tested the effect of antagonists [hexamethonium, mecamylamine, alpha-bungarotoxin (alpha-BTX)] and agonist (nicotine) brain injections on cytochrome oxidase (CO) histochemistry. Within as little as 30 min following nicotine injection, an increase of the staining was observed in almost all the structures analyzed. The increase was limited to the alpha-lobe after alpha-BTX injection. In contrast, the antagonists hexamethonium and mecamylamine reduced CO staining in this structure that seems to be involved in information retrieval. These results suggest that the decrease of metabolism in the alpha-lobe obtained with hexamethonium and mecamylamine injections could be related to the impairment of retrieval processes previously observed with these drugs.

Functional cytochrome oxidase histochemistry in the honeybee brain

The variations of neural metabolism induced by surgical and chemical treatments were studied in the honeybee brain by the means of cytochrome oxidase (CO) histochemistry. CO staining is considerably reduced in the alpha-lobe by antennal input deprivation. Chemical stimulation (50 mM K(+)) was linked to an increase of CO staining in antennal lobes (AL) and to a decrease in the basal ring of calyces (Cal). Application of the nicotinic ligand imidacloprid (10(-4) M) resulted in increased CO labelling within 30 min in all the structures analysed. Treatment with a lower concentration (10(-8) M) resulted in reduced staining in Cal and central body (CB). We conclude that CO histochemistry can be used to identify the target structures of cholinergic ligands in the honeybee brain.

Neurochemical control of cricket stridulation revealed by pharmacological microinjections into the brain

Neuroactive substances were administered into the frontal protocerebrum of tethered male Gryllus bimaculatus by pressure injections from microcapillaries. All three types of species-specific song pattern (calling song, rivalry song and courtship song) could be elicited by injection of acetylcholine and cholinergic agonists. Injection of nicotine led to short bouts of calling song that occurred after a short latency. In contrast, muscarine elicited long-lasting stridulation that took longer to develop. The pharmacologically induced song patterns showed transitions from rivalry song to calling song and from calling song to courtship song, which also occur during natural behaviour. Stridulation induced by a cholinergic agonist could be immediately blocked by microinjection of (γ)-aminobutyric acid (GABA) into the same neuropile sites. Administration of picrotoxin in resting crickets led to enhanced motor activity that incorporated the three different song patterns. We propose that, in the brain of the cricket, acetylcholine and GABA are putative transmitters involved in the control of stridulation. Histological analysis located the stimulation sites to an area between the pedunculus and the (α)-lobe of the mushroom body in which the command neurons for calling song have dendritic arborizations.

Cholinergic synaptic transmission in insect mushroom bodies in vitro

The mushroom body of the bee brain is an important site for learning and memory. Here we investigate synaptic transmission in the mushroom body using extracellular recording techniques in a whole bee brain in vitro preparation. The postsynaptic response showed attenuation by cadmium and paired-pulse facilitation, similar to in vivo findings. This confirms the viability of the in vitro preparation and supports the isolated whole bee brain as a useful model of the in vivo preparation. Bath application of the acetylcholine receptor antagonists, D-tubocurarine and alpha-bungarotoxin attenuated the postsynaptic response by 61 and 62% of control, respectively. The glutamate receptor antagonists, (+)-2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitroquinoxaline-2,3-dione, had no effect. The invertebrate monoamine and neuromodulator, octopamine, transiently increased the postsynaptic response by 130% of control. These results suggest that synaptic transmission of the olfactory input pathway in the mushroom body is 1) mediated primarily by acetylcholine and 2) modulated by octopamine.

Histochemistry of classical neurotransmitters in antennal lobes and mushroom bodies of the honeybee

This paper summarizes histochemical and immunocytochemical investigations of cholinergic, GABAergic, and glutamatergic pathways in the central brain and suboesophageal ganglion of the honeybee. Acetylcholinesterase histochemistry, immunocytochemical staining for nicotinic acetylcholine receptors, and mapping for alpha-bungarotoxin binding sites indicate cholinergic synaptic interactions in the antennal lobe and a cholinergic pathway via a subset of olfactory projection neurons into the mushroom bodies. Calcium imaging experiments in cell cultures prepared from mushroom bodies demonstrate the expression of nicotinic cholinergic receptors on Kenyon cells. Neurons synthesizing GABA and glutamate are stained with well-defined polyclonal antisera against the amino acids. GABA-immunoreactivity is mainly localized in local interneurons of the antennal lobe and in extrinsic neurons innervating the mushroom bodies. High levels of glutamate-immunoreactivity are found in motoneurons of the suboesophageal ganglion, the dorsal lobe, and also in interneurons. A subgroup of the Kenyon cells shows distinct but weaker levels of glutamate-immunoreactivity. The detailed knowledge about the chemical neuroanatomy of the bee provides a framework for behavioral pharmacological approaches, which implicate the involvement of cholinergic mechanisms in olfactory learning and GABAergic mechanisms in odor discrimination.

Nicotinic acetylcholine currents of cultured Kkenyon cells from the mushroom bodies of the honey bee Aapis mellifera

1. Acetylcholine-induced currents of mushroom body Kenyon cells from the honey bee Apis mellifera were studied using the whole-cell configuration of the patch clamp technique. Pressure application of 1 mM acetylcholine (ACh) induced inward currents with amplitudes between -5 and -500 pA. 2. The cholinergic agonists ACh and carbamylcholine had almost equal potencies of current activation at concentrations between 0.01 and 1 mM; nicotine was less potent. The muscarinic agonist oxotremorine did not elicit any currents. 3. Approximately 80 % of the ACh-induced current was irreversibly blocked by 1 microM alpha-bungarotoxin. Atropine (1 mM) did not block the ACh-induced current. 4. Upon prolonged ACh application the current desensitized with a time course that could be approximated by the sum of two exponentials (tau1 = 276 +/- 45 ms (mean +/- s.e.m. ) for the fast component and tau2 = 2.4 +/- 0.7 s for the slow component). 5. Noise analyses of whole-cell currents yielded elementary conductances of 19.5 +/- 2.4 pS for ACh and 23.7 +/- 5.0 pS for nicotine. The channel lifetimes, calculated from the frequency spectra, were tauo = 1.8 ms for ACh and tauo = 2.5 ms for nicotine. 6. Raising the external calcium concentration from 5 to 50 mM shifted the reversal potential of the ACh-induced current from +4. 6 +/- 0.9 to +37.3 +/- 1.3 mV. The calcium-to-sodium permeability ratio (PCa : PNa) was 6.4. 7. In high external calcium solution (50 mM) the ACh-induced current rectified in an outward direction at positive membrane potentials. 8. We conclude that Kenyon cells express nicotinic ACh receptors with functional profiles reminiscent of the vertebrate neuronal nicotinic ACh receptor subtype.

Colocalisation of taurine- with transmitter-immunoreactivities in the nervous system of the migratory locust

The expression of taurine immunoreactivity (TAU-IR) by neurones immunoreactive for octopamine (OA-IR), gamma-aminobutyric acid (GABA-IR), and the C-terminal peptide sequence arginine-phenylalanine (RFamide-IR) was investigated in the migratory locust (Locusta migratoria). TAU-IR is colocalised with OA-IR in the dorsal unpaired median neurones, which are efferent neuroparacrine cells. TAU-IR is not, however, expressed by OA-IR interneurones in the thoracic ganglia and brain. The only other TAU-IR somata found with peripheral axons are the medial neurosecretory cells in abdominal ganglia that project to the neurohaemal organs. These cells exhibit RFamide-IR. The majority of TAU-IR somata in the thoracic abdominal nervous system exhibit GABA-IR. These cells correspond to populations of identified local and intersegmentally projecting inhibitory interneurones. TAU-IR is not, however, exhibited by the well-known GABAergic common inhibitor neurones, which have peripherally projecting axons. This differential distribution of TAU-IR in basically two, functionally different, neuronal subsets (efferent neurosecretory and neuroparacrine cells, inhibitory interneurones) conforms with the concept of taurine acing as a depressive agent to limit excitation during stressful conditions.

Cultured insect mushroom body neurons express functional receptors for acetylcholine, GABA, glutamate, octopamine, and dopamine

Fluorescence calcium imaging with fura-2 and whole cell, patch-clamp electrophysiology was applied to cultured Kenyon cells (interneurons) isolated from the mushroom bodies of adult crickets (Acheta domesticus) to demonstrate the presence of functional neurotransmitter receptors. In all cells investigated, 5 microM acetylcholine (ACh, n = 52) evoked an increase in intracellular free calcium ([Ca2+]i). Similar effects were observed in response to 10 microM nicotine. The ACh response was insensitive to atropine (50 microM) but was reduced by mecamylamine (50 microM) and alpha-bungarotoxin (alpha-bgt, 10 microM). ACh-induced inward ion currents (n = 28, EACh approximately 0 mV) were also blocked by 1 microM mecamylamine and by 1 microM alpha-bgt. Nicotine-induced inward currents desensitized more rapidly than ACh responses. Thus functional alpha-bgt-sensitive nicotinic ACh receptors are abundant on all Kenyon cells tested, and their activation leads to an increase in [Ca2+]i. gamma-Aminobutyric acid (GABA, 100 microM) triggered a sustained decrease in [Ca2+]i. Similar responses were seen with a GABAA agonist, muscimol (100 microM), and a GABAB agonist, 3-APPA (1 mM), suggesting that more than one type of GABA receptor can affect [Ca2+]i. This action of GABA was not observed when the extracellular KCl concentration was lowered. All cells tested (n = 26) with patch-clamp electrophysiology showed picrotoxinin (PTX)-sensitive, GABA-induced (30-100 microM) currents with a chloride-sensitive reversal potential. Thus, an ionotropic PTX-sensitive GABA receptor was found on all Kenyon cells tested. Most (61%) of the 54 cells studied responded to -glutamate (100 microM) application either with a biphasic increase in [Ca2+]i or with a single, delayed, sustained [Ca2+]i increase. Nearly all cells tested (95%, n = 19) responded to (100 microM) -glutamate with rapidly desensitizing, inward currents that reversed at approximately -30 mV. Dopamine (100 microM) elicited either a rapid or a delayed increase in [Ca2+]i in 63% of the 26 cells tested. The time course of these responses varied greatly among cells. Dopamine failed to elicit currents in patch-clamped cells (n = 4). A brief decrease in [Ca2+]i was induced by octopamine (100 microM) in approximately 54% of the cells tested (n = 35). However, when extracellular CaCl2 was lowered, octopamine triggered a substantial increase in [Ca2+]i in 35% of the cells tested (n = 26). No octopamine-elicited currents were detected in patched-clamped cells (n = 10).

Distribution of acetylcholinesterase in the central nervous system of harvestmen (Arachnida: Opilionida)

Enzyme histochemical technique (Gomori and Koelle methods, reaction product development according to Lewis) was applied on 10 microm frozen sections of formalin-fixed material to demonstrate the distribution of acetylcholinesterase in the central nervous system (CNS) of harvestmen (Opilionida: Phalangiidae). Distinctly positive reactions were confined to the neuropil, showing strong staining especially in protocerebral brain centres (optic lobes, cerebral ganglia), the cheliceral ganglia (stomodeal bridge, afferent tracts), and the connective ring systems of the subesophageal nerve mass. The results obtained are discussed with regard to central coordinative functions of the respective brain parts.

Effects of the muscarinic antagonists atropine and pirenzepine on olfactory conditioning in the honeybee

One-trial conditioning of the proboscis extension reflex (PER) in honeybees was used to examine the qualitative effects of two muscarinic antagonists, atropine and pirenzepine, on the acquisition and retrieval of memory following intracranial injection. The main result of this study is that atropine, at a relatively high concentration of 10(-2) M, impairs memory retrieval but not acquisition of memory after a single olfactory conditioning trial (at this concentration, there is no effect of atropine on the sensorimotor components of the PER). This result is in agreement with the effects of scopolamine, reported in a previously published article. Pirenzepine, at the same concentration as atropine, had no effect on either acquisition or retrieval of memory. These results suggest that blockade of muscarinic-like receptors, except those that bind to pirenzepine, induces solely an impairment of memory retrieval.

Neuronal cAMP-dependent protein kinase type II is concentrated in mushroom bodies of Drosophila melanogaster and the honeybee Apis mellifera

In both Drosophila melanogaster and the honeybee Apis mellifera, cyclic adenosine monophosphate (cAMP)-dependent processes have been implicated in mechanisms of learning. This study characterizes the type II cAMP-dependent protein kinase (PKAII), the major target of cAMP in adult animals. In both species, PKAII is restricted to neuronal tissue, in which it accounts for more than 90% of total PKA activity. Although the intensity of PKAII immunoreactivity differs between distinct brain regions, labeling is detectable in all neuropiles and most somata. While the visual neuropiles, the antennal lobes, and structures of the central brain exhibit intermediate immunostaining, the mushroom bodies show high labeling and contain a three- to fourfold higher PKA activity compared to other neuropiles. Since the mushroom bodies are central sites of olfactory learning mediated via cAMP-dependent signaling, the modulatory functions of transmitters on PKA activity in Kenyon cells from the honeybee were tested. Agents which elevate cytoplasmic Ca2+ levels have no effects on PKA activity in cultured Kenyon cells. Dopamine, serotonin, and octopamine, however, cause an increase in PKA activity in Kenyon cells. The modulation of PKA activity by octopamine, the putative transmitter of the unconditioned stimulus in associative olfactory learning in the honeybee, together with the findings on the central role of the cAMP cascade in Drosophila mushroom bodies, suggests a major implication of PKAII-mediated phosphorylation in learning and memory in both Drosophila and Apis.

Mecamylamine-induced impairment of acquisition and retrieval of olfactory conditioning in the honeybee

Mecamylamine, a nicotinic receptor antagonist, was injected into the honeybee brain haemolymph. The effects of the drug were investigated on Pavlovian conditioning of the proboscis extension reflex. The conditioned response was acquired after a one-trial learning session, consisting of an olfactory-conditioned stimulus combined with a gustatory antennal unconditioned stimulus. The drug was injected at different times before or after the learning session in order to dissociate its effects on acquisition, consolidation and retrieval processes. The performance was evaluated in short-delayed recall tasks. To control potential effects on sensory-motor activity, the effects of the drug were also investigated on sensory processes (through olfactory and gustatory functions) and on motor processes of proboscis extension. The results of conditioning experiments showed that pretrial injection induced a decrease of retention performance 1 h after the learning trial. Mecamylamine injected 20 min after the learning session induced a time-dependent impairment of retention performance, as has been shown by the performance level registered from 10 to 80 min after injection. A 5-min post-trial injection had no effect on retention performance. Control experiments did not reveal any effect of mecamylamine on the response reflex of proboscis extension and on responsiveness to olfactory stimuli (geraniol, lavender and vanillin). The absence of effects on sensory perception combined with the amnestic effect induced by pre- or late post-trial injections lead us to conclude that mecamylamine specially impaired acquisition and retrieval processes. The involvement of nicotinic-like receptors in these processes is discussed.

Transmitter-induced calcium signalling in cultured neurons of the insect brain

The insect brain offers a unique opportunity for cell biological investigation of calcium signalling in relatively homogeneous neuronal populations, such as the mushroom body neurons. This review summarizes fluorescence imaging experiments with the indicator dye fluo-3 to investigate the calcium responses to transmitter stimulation of honeybee mushroom body neurons in primary culture. Application of acetylcholine (ACh) or nicotine promoted a calcium influx into the cell body and neurites. The increase in intracellular calcium after ACh stimulation was blocked by alpha-bungarotoxin. These results support previous histochemical studies that suggested the expression of nicotinic cholinergic receptors on Kenyon cells. An increase in cytoplasmic calcium levels leads in specific neurons to the generation of nitric oxide (NO) by a Ca2+/calmodulin activated NO synthase. NO is thought to diffuse as a short-lived messenger molecule through the plasma membrane. Using a sensitive photometric assay it could be shown that dissociated cells from the locust brain release NO after stimulation by agents elevating cytoplasmic Ca2+ levels and by the excitatory neurotransmitter ACh. The experiments in dissociated culture therefore allow the demonstration of a Ca(2+)-dependent release during nerve cell depolarization, which is a basic requirement for identifying NO as a messenger molecule.

GABAergic synapses in the antennal lobe and mushroom body of the locust olfactory system

To help elucidate the role of inhibitory feedback in the genesis of odour-evoked synchronization of neural activity, we investigated the distribution of gamma-aminobutyric acid (GABA)ergic synaptic terminals in the antennal lobes (AL) and mushroom bodies (MB) of the locust olfactory system. Electron-microscopy, intracellular horseradish peroxidase labelling, and immunocytochemistry were combined to assess the distribution of GABAergic synapses, using established methods (Leitch and Laurent [1993] J. Comp. Neurol. 337:461-470). In the AL, GABA-immunoreactive presynaptic terminals contacted both immunoreactive and immunonegative profiles. Conversely, GABA-immunoreactive profiles received direct input from both reactive and negative terminals. The tract containing the axons of the projection neurons that run from the AL to the MB contained about 830 axons of fairly uniform size, none of which was immunoreactive for GABA. In the calyx of the MB, large immunoreactive terminals contacted very-small-diameter profiles thought to belong to the Kenyon cells (KCs). This was confirmed by combining immunocytochemistry with intracellular HRP-labelling of KCs. KCs were not immunoreactive for GABA. Although some GABAergic contacts were made onto the spiny profiles of KCs, others were made onto their dendritic shafts. Large GABA-immunoreactive profiles were also found to contact large negative profiles that were presynaptic to KC terminals. This suggests that KC dendrites can be both pre- and post-synaptically inhibited in the calyx. The MB pedunculus contained ca. 50,000 tightly packed KC axons, showing conspicuous en passant and often reciprocal synaptic contacts between neighbouring axons. KC axons were immunonegative, but received direct input from, and contacted directly, large immunoreactive profiles running across or along the KC axons. In the alpha- and beta-lobes of the MB, connections similar to those in the pedunculus were seen with two main differences: (1) The density of synaptic profiles was higher, giving on occasion numerous serially connected profiles in a single section; (2) large immunonegative profiles with dense-core vesicles were abundant and were frequently presynaptic to GABAergic processes and to very-small-diameter profiles which possibly belong to KCs. These results are discussed in the context of the known physiological data on olfactory processing in these complex circuits.

Morphology of neuroglia in the antennal lobes and mushroom bodies of the brain of the honeybee

We investigated the distribution and anatomical organization of glial cells in the antennal lobes and mushroom bodies of the honeybee. Reconstructions from serial sections, prepared according to the ethyl gallate method, revealed the entire morphology of glial cells in neuropiles, tracts, and the soma rind. The distribution of the glial cell bodies in the neuropiles was derived from the staining of cell nuclei with a fluorescent dye. There are glial cells of different shape in the soma rind which are wrapped around the neuronal cell bodies of the antennal lobes and the Kenyon cells of the mushroom bodies. Glial cells surround neuropilar areas such as the external and lateral sides of the glomeruli of the antennal lobes. Whereas we could not detect glia in the glomerular neuropile, glial cells with long processes are located in the core of the antennal lobe. Extensions of these glial cells also invade tracts containing the olfactory projection neurons. A layer of glial cells separates the mushroom body neuropile from the surrounding protocerebral neuropile. The neuropile of the mushroom bodies is clearly compartmented by glial cells. There is a high density of astrocyte-like glia in a column of the pedunculus which can be followed to the ventral part of the alpha-lobe. A network of mushroom body intrinsic glial cells separates the alpha-lobe from the beta-lobe and the pedunculus. This anatomical description of glial cell types in olfactory information processing pathways of an insect brain provides a framework for further physiological studies of neuroglia in dissociated cell culture.

Sensory neuron development revealed by taurine immunocytochemistry in the honeybee

The formation of ommatidia in the compound eyes and sensilla on the antennae of the honeybee was followed and the development of their sensory neurons was traced using an antiserum against taurine as a marker. Taurine-like immunoreactivity (Tau-IR) is expressed in sensory neurons of several modalities, namely visual, olfactory, gustatory, and mechanosensory. Staining intensity is very high in the larva and in the first half of the pupal stage and gradually decreases towards the end of metamorphosis. In the photoreceptor cells of the compound eyes, Tau-IR can be detected from the fifth larval instar onwards, prior to differentiation of other components of the ommatidium. Already in the midstage larvae, when the antennal primordia of the adult still lie within the peripodial cavity, a few presumably mechanosensory neurons are labelled in the pedicellus of the developing antenna. The majority of the antennal sensory neurons which are located on the flagellum start to exhibit Tau-IR upon pupation, long before any cuticular specializations such as sensory hairs or plates are detectable. All known types of antennal sensilla were identified and it could be shown that all of them are innervated by Tau-IR sensory neurons. Thus, taurine immunocytochemistry can be applied as a useful label for developing sensory neurons. Functional implications of taurine during development are discussed.

Distribution of acetylcholinesterase activity in the deutocerebrum of the sphinx moth Manduca sexta

We have used a cytochemical technique to investigate the distribution of acetylcholinesterase (AChE) activity in the deutocerebrum of the brain of the sphinx moth Manduca sexta. To distinguish between extra- and intracellular pools of the enzyme, some brains were treated prior to histochemical staining with echothiophate, an irreversible AChE inhibitor which penetrates cell membranes very slowly and, therefore, inhibits only extracellular AChE. In the antennal nerve, fascicles of presumably mechanosensory fibers show echothiophate-insensitive AChE activity. They bypass the antennal lobe and project to the antennal mechanosensory and motor center of the deutocerebrum. In the antennal lobe, fibers in the coarse neuropil, cell bodies in the lateral cell group, and all glomeruli exhibit AChE activity. In most ordinary glomeruli, echothiophate-sensitive AChE activity is concentrated in the outer cap regions, corresponding to the terminal arborizations of olfactory afferents. A previously unrecognized glomerulus in the ventro-median antennal lobe shows uniform and more intense AChE-specific staining that the other glomeruli. No AChE activity appeared to be associated with male-specific pheromone-sensitive afferents in the macroglomerular complex. About 67 interneurons with somata in the lateral cell group of the antennal lobe show echothiophate-insensitive AChE activity. These neurons seem to be members of two types of antennal-lobe projection neurons with fibers passing through the outer-antennocerebral tract to the protocerebrum. AChE-stained arborizations of these neurons appear to invade all glomeruli, including three distinguishable subunits of the male-specific macroglomerular complex. In echothiophate-treated animals, the projections of one of these types of fiber form large terminals in the lateral horn of protocerebrum, which partly protrude into the adjacent glial cell layer. The results suggest that extracellularly accessible AChE is associated with ordinary olfactory receptor terminals but apparently not with pheromone-sensitive afferents. Intracellular AChE appears to be present in antennal mechanosensory fibers and in two types of olfactory projection neurons of the antennal lobe. The study provides further evidence for cholinergic neurotransmission of most antennal afferents. The AChE-containing interneurons might be cholinergic as well or use the enzyme for functions unrelated to hydrolysis of acetylcholine.

Localization of a nicotinic acetylcholine receptor-like antigen in the thoracic nervous system of embryonic locusts, Schistocerca gregaria

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subsequent immunoblotting of neuronal membrane proteins derived from thoracic ganglia of adult Locusta and Schistocerca reveal that a polyclonal antiserum raised against the Locusta nicotinic acetylcholine receptor (nAChR), binds strongly to an identical polypeptide band corresponding to 65 kDa in both locust species. This polyclonal antiserum was used to analyze the distribution of antigenic sites within the developing thoracic central nervous system of Schistocerca embryos. Axonal outgrowths from the earliest differentiated neurons are first labeled between 30% and 35% development. By 40% to 45% development, labeled granules appear in the cytoplasm of neuronal cell bodies. When the developing neuropil is first enclosed at approximately 45% to 50% development, it appears uniformly labeled, but by 55% development, unlabeled areas appear that represent the sites of future tracts and commissures. By 75%, an adult pattern of neuropil immunogenicity is established in which synaptic regions are stained but tracts and commissures are not. This suggests that during the early development of the thoracic nervous system nAChR-like antigenic sites are evenly distributed, but later become concentrated in the developing synaptic areas.

Octopamine-like immunoreactivity in the brain and subesophageal ganglion of the honeybee

The organization of putative octopaminergic pathways in the brain and subesophageal ganglion of the honeybee was investigated with a well-defined polyclonal antiserum against octopamine. Five prominent groups of just over 100 immunoreactive (IR) somata were found in the cerebral ganglion: Neurosecretory cells in the pars intercerebralis innervating the corpora cardiaca via NCC I, one cluster mediodorsal to the antennal lobe, one scattered on both sides of the midline of the protocerebrum, one between the lateral protocerebral lobes and the dorsal lobes, and a single soma on either side of the central body. With the exception of the pedunculi and beta-lobes of the mushroom bodies, varicose immunoreactive fibers penetrate all parts of the cerebral ganglion. Strong labelling was found in the central complex and the protocerebral bridge. Fine networks of labelled processes invade the antennal lobes, the calyces and a small part of the alpha-lobes of the mushroom bodies, the protocerebrum, and all three optic ganglia. In the subesophageal ganglion, one labelled cell body was found in the lateral soma layer of the mandibular segment. Each of the three neuromeres contains a group of six to ten somata in the ventral median parts. Most of the ventral median cells send their neurites dorsally through the midline tracts, whereas the neurites of a few cells follow the ventral cell body neurite tracts. Octopamine-IR was demonstrated in all neuropils that contain pathways for proboscis extension learning in honeybees. Because octopaminergic mechanisms seem to be involved in the behavioral plasticity of the proboscis extension reflex, our study provides anatomical data on the neurochemical organization of an appetitive learning paradigm.

Histochemical localization of acetylcholinesterase in the lateral eye and brain of Limulus polyphemus: might acetylcholine be a neurotransmitter for lateral inhibition in the lateral eye?

The distribution of acetylcholinesterase (AChE) in the lateral eye and brain of the horseshoe crab was investigated with histochemical means using standard controls to eliminate butyrylcholinesterase and nonspecific staining. Intense staining was observed in the neural plexus of the lateral compound eye, in the lateral optic nerve, and in various neuropils of the brain. Nerve fibers with moderate to weak staining were widespread in the brain. No somata were stained in either the lateral eye or the brain. The distribution of acetylcholinesterase in the supraesophageal ganglia and nerves of the giant barnacle was also investigated for comparison. Although both the median optic nerve of the barnacle and the lateral optic nerve of the horseshoe crab appear to contain the fibers of histaminergic neurons, only the lateral optic nerve of the horseshoe crab shows AChE staining. Other parts of the barnacle nervous system, however, showed intense AChE staining. These results along with the histochemical controls eliminate the possibility that some molecule found in histaminergic neurons accounted for the AChE staining but support the possibility that acetylcholine might be involved as a neurotransmitter in lateral inhibition in the horseshoe crab retina. Two reasonable neurotransmitter candidates for lateral inhibition, histamine and acetylcholine, must now be investigated.