THE DISTRIBUTION AND FUNCTION OF METABOTROBIC GABA RECEPTORS IN THE SPIDER PERIPHERAL MECHANOSENSILLAB

A dense network of fine efferent fibers innervates the peripheral parts of the mechanosensilla of the tropical wandering spider (Cupiennius salei, Keys.). They form synapses with the axons, somata and dendrites of the mechanosensory neurons and they also synapse with each other and with glial cells surrounding the sensory neurons (Fabian-Fine et al. 1999a,b). The majority of efferent neurons are immunoreactive to an antibody against the inhibitory neurotransmitter GABA (Fabian-Fine et al. 1999b) and the sensory neurons are inhibited by agonists of ionotropic GABA receptors (Panek et al. 2002). In addition to relatively fast inhibition via ionotropic receptors, GABA can modulate neuronal activity by activating metabotropic G-protein coupled GABA receptors that B play critical roles in long-term modulation of synaptic transmission. Two different GABA B receptor proteins, GABA R1 and R2, have been cloned from mammalian and Drosophila B nervous systems, and both exist in several alternatively spliced forms. The functional receptor comprises a heterodimer with these subunits (Jones et al. 1998; Filippov et al. 2000; Mezler et al. 2001). Specific antibodies against both of these receptor subunits are commercially available and we tested them using Western blot analysis and immunocytochemistry in spider brain and peripheral nerve tissues. To learn how the GABA receptor immunoreactivity correlated with the synaptic sites we performed B double labeling experiments using a monoclonal antibody against synapsin that labels the synaptic vesicles on the fine efferent fibers on the spider sensilla (Fabian-Fine et al. 1999a,b). We also tested if specific GABA receptor agonists modulate the voltage-B activated currents and the excitability of identified peripheral mechanosensory neurons