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Distribution and Function of GABA_B Receptors in Spider Peripheral Mechanosensilla

¹ Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada; ² Department of Biophysics, Institute of General and Molecular Biology, N. Copernicus University, 87-100 Torun, Poland

Submitted 2 April 2003; accepted in final form 9 June 2003

The mechanosensilla in spider exoskeleton are innervated by bipolar neurons with their cell bodies close to the cuticle and dendrites attached to it. Numerous efferent fibers synapse with peripheral parts of the mechanosensory neurons, with glial cells surrounding the neurons, and with each other. Most of these efferent fibers are immunoreactive to -aminobutyric acid (GABA), and the sensory neurons respond to agonists of ionotropic GABA receptors with a rapid and complete inhibition. In contrast, little is known about metabotropic GABA_B receptors that may mediate long-term effects. We investigated the distribution of GABA_B receptors on spider leg mechanosensilla using specific antibodies against 2 proteins needed to form functional receptors and an antibody that labels the synaptic vesicles on presynaptic sites. Both anti-GABA_B receptor antibodies labeled the distal parts of the sensory cell bodies and dendrites but anti-GABA_BR1 immunoreactivity was also found in the axons and proximal parts of the cell bodies and some glial cells. The fine efferent fibers that branch on top of the sensory neurons did not show GABA_B receptor immunoreactivity but were densely labeled with anti-synapsin and indicated synaptic vesicles on presynaptic locations to the GABA_B receptors. Intracellular recordings from sensory neurons innervating the slit sensilla of the spider legs revealed that application of GABA_B receptor agonists attenuated voltage-activated Ca²⁺ current and enhanced voltage-activated outward K⁺ current, providing 2 possible mechanisms for controlling the neurons' excitability. These findings support the hypothesis that GABA_B receptors are present in the spider mechanosensilla where their activation may modulate information transmission.

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