1 Compound Eyes and the World of Vision Research.- 2 Autrum's Impact on Compound Eye Research in Insects.- 3 Optics and Evolution of the Compound Eye.- 4 Photoreceptor Optics, Theory and Practice.- 5 Variations in the Structure and Design of Compound Eyes.- 6 Visual Pigments of Compound Eyes - Structure, Photochemistry, and Regeneration.- 7 Distribution of Insect Visual Chromophores: Functional and Phylogenetic Aspects.- 8 Pigments in Compound Eyes.- 9 Phototransduction in Limulus Ventral Photoreceptors: Roles of Calcium and Inositol Trisphosphate.- 10 The Retina-Lamina Pathway in Insects, Particularly Diptera, Viewed from an Evolutionary Perspective.- 11 Coding Efficiency and Design in Visual Processing.- 12 Neurotransmitters in Compound Eyes.- 13 Circadian Rhythms in the Invertebrate Retina.- 14 Color Vision in Honey Bees: Phenomena and Physiological Mechanisms.- 15 Polarization Sensitivity in Compound Eyes.- 16 Beneath the Compound Eye: Neuroanatomical Analysis and Physiological Correlates in the Study of Insect Vision.- 17 Directionally Selective Motion Detection by Insect Neurons.- 18 Neural Mechanisms of Visual Course Control in Insects.- 19 Size and Distance Perception in Compound Eyes.

Facets of Vision

Publisher Summary This chapter focuses on receptors for the neurotransmitter molecule GABA (γ-aminobutyric acid) with particular emphasis on insect GABA receptors. It discusses GABA receptors of the insect nervous system and insect muscle. GABAergic neurons are found in the brain, spinal cord, and ganglia of the mammalian nervous system. Ionophoretic application of GABA on to mammalian neurons normally results in an increased membrane conductance and an inhibitory (hyperpolarizing) response of the cell under test. GABA-sensitive cells have been detected in sympathetic ganglia cerebral cortex and Deiter's nucleus. Most responses are attributable to a GABA-operated chloride channel; they are blocked by bicuculline, the active form of which is the 1S,9R-(-)isomer and picrotoxin. The chapter also discusses GABA receptors of invertebrates other than insects, insect GABA receptors as targets for insecticides, and GABA receptors as members of a ligand-operated receptor super-family.

GABA receptors of insects

γ-Aminobutyrate (GABA)- and glutamate-gated chloride channels (GABACls and GluCls) are members of the Cys-loop receptor channel family. Both channels are widely distributed in the nervous system of invertebrates and function as neurotransmitter receptors to mediate inhibitory synaptic transmission. These receptor channels are important targets for widely used insecticides and parasiticides. Diverse noncompetitive antagonists, including the phenylpyrazole insecticide fipronil, inhibit inhibitory neurotransmission by binding to a site deep within the pentameric channel. Macrocyclic lactones, such as the insecticide/parasiticide avermectin, activate GluCls by irreversibly binding to the subunit interfaces. Recent genome sequencing and molecular biology studies have revealed a diversity of subunit isoforms arising from the duplication, alternative splicing, and RNA editing of the genes encoding these channels in various insect species, which may lead to physiological and pharmacological diversification of the receptor channels. Fipronil resistance due to target-site insensitivity in GABACls is a growing concern in several insect pest species. Nevertheless, identification of new chemistry and multiple potential binding sites for agonists, antagonists, and modulators in these channels suggests potential for developing novel pest control chemicals.

γ-Aminobutyrate- and Glutamate-gated Chloride Channels as Targets of Insecticides

Responses to GABA by isolated insect neuronal somata: pharmacology and modulation by a benzodiazepine and a barbiturate.

The pharmacology of a gamma-aminobutyric acid (GABA) receptor on the cell body of an identified motor neuron of the cockroach (Periplaneta americana) was investigated by current-clamp and voltage-clamp methods. Iontophoretic application of GABA increased membrane conductance to chloride ions, and prolonged application resulted in desensitization. Hill coefficients, determined from dose-response data, indicated that binding of at least two GABA molecules was required to activate the chloride channel. Differences between vertebrate GABAA receptors and insect neuronal GABA receptors were detected. For the GABA receptor of motor neuron Df, the following rank order of potency was observed: isoguvacine greater than muscimol greater than or equal to GABA greater than 3-aminopropanesulphonic acid. The GABAB receptor agonist baclofen was inactive. Of the potent vertebrate GABA receptor antagonists (bicuculline, pitrazepin, RU5135 and picrotoxin), only picrotoxin (10(-7) M) produced a potent, reversible block of the response to GABA of motor neuron Df. Both picrotoxinin and picrotin also blocked GABA-induced currents. Bicuculline hydrochloride (10(-4) M) and bicuculline methiodide (10(-4) M) were both ineffective when applied at resting membrane potential (-65 mV), although at hyperpolarized levels partial block of GABA-induced current was sometimes observed. Pitrazepin (10(-4) M) caused a partial, voltage-independent block of GABA-induced current. The steroid derivative RU5135 was inactive at 10(-5) M. In contrast to the potent competitive blockade of vertebrate GABAA receptors by bicuculline, pitrazepin and RU5135, none of the weak antagonism caused by these drugs on the insect GABA receptor was competitive. Flunitrazepam (10(-6) M) potentiated GABA responses, providing evidence for a benzodiazepine site on an insect GABA-receptor-chloride-channel complex.

GABA receptors on the cell-body membrane of an identified insect motor neuron.

Taurine resembles G ABA in its synaptic effects in the cockroach cereal nerve giantfiber synapse where it exerts a depressant action upon synaptic transmission. Both taurine and G A B A produce an increased conductance of pre- and postsynaptic membranes through changes in the permeability of chloride ions.

Pre- and postsynaptic effects of taurine and GABA in the cockroach central nervous system.

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Pre- and postsynaptic effects of taurine and GABA in the cockroach central nervous system.