The morphology and ultrastructure of antennal lobe cells from pupal honeybees (Apis mellifera) growing in culture

Cell Lines for Honey Bee Virus Research

With ongoing colony losses driven in part by the Varroa mite and the associated exacerbation of the virus load, there is an urgent need to protect honey bees (Apis mellifera) from fatal levels of virus infection and from the non-target effects of insecticides used in agricultural settings. A continuously replicating cell line derived from the honey bee would provide a valuable tool for the study of molecular mechanisms of virus-host interaction, for the screening of antiviral agents for potential use within the hive, and for the assessment of the risk of current and candidate insecticides to the honey bee. However, the establishment of a continuously replicating honey bee cell line has proved challenging. Here, we provide an overview of attempts to establish primary and continuously replicating hymenopteran cell lines, methods (including recent results) of establishing honey bee cell lines, challenges associated with the presence of latent viruses (especially Deformed wing virus) in established cell lines and methods to establish virus-free cell lines. We also describe the potential use of honey bee cell lines in conjunction with infectious clones of honey bee viruses for examination of fundamental virology.

Pharmacological properties of nicotinic acetylcholine receptors in isolated Locusta migratoria neurones

Mechanically dissociated neuronal cell bodies from the thoracic ganglia of Locusta migratoria were viable in culture conditions for up to 2 days and were voltage-clamped to record the effects of GABAergic drugs and physostigmine on the membrane conductance and ACh responses of the dissociated cells. Bicuculline, hydrastine, and gabazine inhibited the EC50 ACh responses of the cells. Both bicuculline and hydrastine were full inhibitors of the ACh responses but gabazine behaved as a partial inhibitor. Bicuculline, hydrastine, and gabazine inhibited the ACh responses in a non-competitive and voltage-independent fashion, suggesting that they are allosteric inhibitors of locust nicotinic ACh receptors. Physostigmine activated currents when applied onto isolated locust neurones. The responses activated by physostigmine were inhibited competitively by tubocurarine, which indicates that physostigmine interacts with the ACh site of locust nicotinic ACh receptors. However, maximal concentrations of physostigmine elicited currents of smaller amplitudes to those evoked by maximal ACh concentrations. Single-channel recordings suggest that the partial efficacy of physostigmine may reflect the low frequency of opening of physostigmine-induced single currents relative to that of ACh-single currents.

Distribution of dopamine receptors and dopamine receptor homologs in the brain of the honey bee, Apis mellifera L

In the brain of the honey bee, Apis mellifera, the radioligands [3H]-SCH23390 and [3H]-spiperone recognise D1- and D2-like receptors, respectively. In addition to being pharmacologically distinct and exhibiting significantly different expression profiles during the lifetime of the bee, [3H]-SCH23390- and [3H]-spiperone-binding sites differ markedly in their distribution within the brain. Estimates of [3H]-SCH23390-binding site density are highest in the somatal rind, whereas [3H]-spiperone-binding sites are most concentrated in the beta lobe neuropil of the mushroom bodies. Molecular cloning techniques have been used to identify two honey bee genes encoding dopamine receptor homologs. The first is the honey bee counterpart of a Drosophila D1-like dopamine receptor and is expressed in the mushroom bodies of both workers and drones. The second is related to D2-like dopamine receptors from vertebrates and is expressed in the brain of the bee, but the precise distribution of expression is not yet known.

Antennal lobe neurons of the honey bee, Apis mellifera, express a D2-like dopamine receptor in vitro

We have used the D2-specific dopamine receptor ligand spiperone [N-(p-aminophenethyl) spiperone; NAPS] coupled to the fluorophore 7-nitrobenz-2-oxa-1,3-diazole-4-yl (NBD) to visualize dopamine receptors expressed in vitro by neurons of the primary antennosensory centers (antennal lobes) of the brain of the honey bee, Apis mellifera. Changes in the percentage of antennal lobe neurons exhibiting spiperone binding sites over time in culture and at different stages of metamorphic adult development have been investigated. Neurons obtained from animals at all stages of development exhibited spiperone binding sites, but only after 2 days or more in vitro. The percentage of antennal lobe neurons in vitro expressing spiperone binding sites increased significantly with the development of the antennal lobe neuropil. Fluorescently labelled spiperone (120 nM) could be displaced effectively by 1 mM dopamine but not by the same concentration of tyramine, octopamine, or serotonin. In addition, the D2 antagonist spiperone and the D2/D1 antagonist fluphenazine were more effective at displacing the fluorescent ligand than the D1-specific antagonist SCH23390. Our results indicate that Apis antennal lobe neurons in culture express a dopamine receptor and that this receptor is more likely to be D2-like than D1-like in nature. The receptor is expressed early in the metamorphic adult development of the antennal lobe neuropil of the brain.

Expression of the surface antigen A2B7 in adult and developing honeybee olfactory pathway

In order to identify molecules involved in the development of the honeybee olfactory pathway, hybridoma technology has been used. Among different cell lines, A2B7 has been selected. It produces a specific antibody for a surface glycoprotein of 91 kDa. This protein is mainly expressed by both the antennal receptor cells and mushroom body neurons. Based on (i) the spatio-temporal pattern of expression during pupal development; (ii) the cell surface location of the antigen; and (iii) the partial molecular characterization of the antigen, a putative role for this protein in axonal fasciculation and guidance is discussed.

Preparation of primary cultures and acute slices of the nervous system of the moth Manduca sexta

Interactions among receptor neurons, glial cells and neurons intrinsic to the antennal lobe of the moth underlie the formation of olfactory glomeruli. To isolate these interactions, as well as to understand the effect of a variety of humoral agents on differentiation of the neurons and glia, we generate primary cultures of neurons or glia. These methods are described. In addition. we describe a protocol for producing slice preparations of the developing moth brain that we are using to study the biophysical and morphological development of glial cells. This technique allows us to examine a class of glial cells associated with the glomeruli that otherwise are nearly inaccessible using standard intracellular recording techniques. It also preserves the 3-dimensional arrangement of glia that may strongly influence the development of glomeruli.

Embryonic development of the antennal lobes of a hemimetabolous insect, the cockroach Periplaneta americana: light and electron microscopic observations

In the hemimetabolous insect Periplaneta americana, the adult-like organization of the primary olfactory centers, the antennal lobes, is established during the approximately 31 days of embryogenesis. This report describes the temporal sequence of developmental events as viewed in the light and electron microscope by means of histological stains and by DiI labeling of antennal receptor axons with subsequent photoconversion. Glomeruli, characteristic differentiations of the antennal lobe neuropil, are first observed on day 19; their development, which is not synchronous in the various parts of the antennal lobe, lasts until about day 22. From day 10 on, glial cells begin to form a narrow boundary layer between the soma cortex and the central neuropil. They exhibit a lengthening of their processes in parallel with the formation of glomeruli. Marked proliferation or migration of these glial cells into the neuropil between glomeruli has not been observed. Antennal receptor axons could be labeled from stage 15 on. They terminate in an elongated growth cone with numerous filopodia. From day 18 on, some of these become bent or show an initial bifurcation. From day 22 on, the first afferent axons develop an adult-like arborization pattern. Synaptic contacts between receptor axons and unidentified neurons were observed as early as stages 16 and 19, in which the axons still have a growth cone-like form. In stage 27, in which the fibers have adult-like arborizations, many output contacts and few input contacts were found.

Cockroach glial cell cultures: morphological development and voltage-gated potassium channels

Insect glial cells derived from the embryonic brain of Periplaneta americana cockroaches have been kept in primary culture conditions for up to 3 weeks. Under the culture conditions used, the glial cells differentiated and formed a complex cellular network on the floor of the culture vessels from which glial-glial and glial-neuronal contacts could be seen. Single-channel currents from cell-attached glial membrane patches were recorded using the gigaseal technique. Depolarisation of membrane patches activated outward currents, which were abolished in the presence of extracellular 50 mM tetraethylammonium or 5 mM 4-aminopyridine, and were insensitive to 1 microM tetrodotoxin, 10 microM picrotoxin and 2 mM Cd2+. The amplitude of the outward currents increased linearly with depolarisation, and amplitude histograms obtained at several pipette potentials could be reasonably fitted with a single Gaussian corresponding to a single channel type with a slope conductance of 37 +/- 11 pS. The extrapolated equilibrium potential of the outward current was about 5 +/- 10 mV positive to the resting potential and both the channel open time constant and relative open time probability were sensitive to membrane potential, increasing markedly with depolarisation. The results presented in this paper show the presence of a cadmium-insensitive, voltage-dependent outward potassium channel in insect glial cells in vitro.

Dissociated neurons of the pupal honeybee brain in cell culture

Primary cell cultures were prepared from specific regions of the pupal honeybee brain which are involved in proboscis extension learning. Defined areas could be dissociated purely by mechanical treatment. We show that cultured neurons regenerate new neurites and remain viable for up to three weeks in a serum-free, chemically-defined medium. Several labelling techniques were employed to identify subpopulations of cultured neurons. For example, acetylcholinesterase staining; fluorescent beads to distinguish identified cell populations of co-cultured brain areas; various markers for surface antigens such as a monoclonal antibody to olfactory projection neurons of the antennoglomerular tracts and monopolar cells of the optic lobes, as well as anti-HRP immunoreactivity and alpha-bungarotoxin binding; and various antisera for detecting transmitter phenotype. The appearance of transmitter-immunoreactive cells agreed closely with that expected from their known distribution in situ. Our results suggest that cultured cells retain surface properties and transmitter phenotype of their in vivo counterparts, despite differences in basic morphology. Thus our culture system provides the important initial step for future in vitro investigations of the cellular and electrophysiological properties of neurons mediating proboscis extension learning.