Distribution and Function of GABA_BReceptors in Spider Peripheral Mechanosensilla

Abstract

. 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 longterm 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 B R1 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 voltageactivated Ca 2ϩ current and enhanced voltage-activated outward K ϩ 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.

Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons

“…Previously, activation of metabotropic GABA B receptors was shown to reduce the amplitude of LVA Ca 2ϩ current in VS-3 neurons (Fig. 11) (Panek et al 2003). This effect occurred slowly, within about 5 min after agonist application, and was long-lasting.…”

“…11. Results from a previous investigation into the effects of GABA B -receptor activation on voltage-gated channels in VS-3 neurons (Panek et al 2003) are also included in this model.…”

“…In addition to rapidly activating GABA A receptors, VS-3 neurons also have metabotropic GABA B receptors that have been shown to mediate longer-term effects (Panek et al 2003). To verify that the effect on fluorescence ratio was induced by activation of GABA A receptors, we performed similar experiments using muscimol, a specific agonist of these receptors (Fig.…”

“…The only Ca 2ϩ current that has been identified in VS-3 neurons is the LVA Ca 2ϩ current that was estimated to activate at Ϫ50 mV (Panek et al 2003;Sekizawa et al 2000). Both mibefradil and Ni 2ϩ block LVA Ca channels, but they are not completely specific and can block other voltage-gated Ca channels (Cattaert et al 2005;Tsien et al 1991).…”

“…There are no completely specific blockers of LVA channels, making it difficult to separate them from other types of Ca channels. However, if GABA-induced depolarization in VS-3 neurons activates Ca channels, the most likely candidates would be the LVA channels, whose activation range is closest to the depolarization that occurred in response to GABA application (Panek et al 2003;Sekizawa et al 2000). FIG. 9.…”