title: Peripheral GABAergic inhibition of spider mechanosensory afferents.
abstract: Spider mechanosensory neurons receive an extensive network of efferent synapses onto their sensory dendrites, somata and distal axonal regions. The function of these synapses is unknown. Peripheral synapses are also found on crustacean stretch-receptor neurons but not on mechanosensory afferents of other species, although inhibitory GABAergic synapses are a common feature of centrally located axon terminals. Here we investigated the effects of GABA receptor agonists and antagonists on one group of spider mechanosensory neurons, the slit sense organ VS-3, which are accessible to current- and voltage-clamp recordings. Bath application of GABA activated an inward current that depolarized the membrane and increased the membrane conductance leading to impulse inhibition. VS-3 neuron GABA receptors were activated by muscimol and inhibited by picrotoxin but not bicuculline, and their dose-response relationship had an EC(50) of 103.4 microm, features typical for insect ionotropic GABA receptors. Voltage- and current-clamp analysis confirmed that, while the Na(+) channel inhibition resulting from depolarization can lead to impulse inhibition, the increase in membrane conductance (i.e. 'shunting') completely inhibited impulse propagation. This result argues against previous findings from other preparations that GABA-mediated inhibition is caused by a depolarization that inactivates Na(+) conductance, and it supports those findings that assign this role to membrane shunting. Our results show that GABA can rapidly and selectively inhibit specific mechanoreceptors in the periphery. This type of peripheral inhibition may provide spiders with a mechanism for distinguishing between signals from potential prey, predators or mates, and responding with appropriate behaviour to each signal.
authors.affiliation: Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia, B3H 4H7, Canada.
authors.forename: Izabela
authors.initials: I
authors.lastname: Panek
authors.forename: Andrew S
authors.initials: AS
authors.lastname: French
authors.forename: Ernst-August
authors.initials: EA
authors.lastname: Seyfarth
authors.forename: Shin-ichi
authors.initials: S
authors.lastname: Sekizawa
authors.forename: Päivi H
authors.initials: PH
authors.lastname: Torkkeli
mesh_terms: D000344:Afferent Pathways / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D018755:GABA Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D009118:Muscimol / Q000494:pharmacology; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D009475:Neurons, Afferent / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010852:Picrotoxin / Q000494:pharmacology; D015222:Sodium Channels / Q000378:metabolism; D013112:Spiders; D005680:gamma-Aminobutyric Acid / Q000502:physiology*
chemical_list: D001640:Bicuculline; D018755:GABA Agonists; D018756:GABA Antagonists; D009118:Muscimol; D010852:Picrotoxin; D015222:Sodium Channels; D005680:gamma-Aminobutyric Acid