title: Presynaptic GABA(B) receptors regulate retinohypothalamic tract synaptic transmission by inhibiting voltage-gated Ca2+ channels.
abstract: Presynaptic GABA(B) receptor activation inhibits glutamate release from retinohypothalamic tract (RHT) terminals in the suprachiasmatic nucleus (SCN). Voltage-clamp whole cell recordings from rat SCN neurons and optical recordings of Ca2+-sensitive fluorescent probes within RHT terminals were used to examine GABA(B)-receptor modulation of RHT transmission. Baclofen inhibited evoked excitatory postsynaptic currents (EPSCs) in a concentration-dependent manner equally during the day and night. Blockers of N-, P/Q-, T-, and R-type voltage-dependent Ca2+ channels, but not L-type, reduced the EPSC amplitude by 66, 36, 32, and 18% of control, respectively. Joint application of multiple Ca2+ channel blockers inhibited the EPSCs less than that predicted, consistent with a model in which multiple Ca2+ channels overlap in the regulation of transmitter release. Presynaptic inhibition of EPSCs by baclofen was occluded by omega-conotoxin GVIA (< or = 72%), mibefradil (< or = 52%), and omega-agatoxin TK (< or = 15%), but not by SNX-482 or nimodipine. Baclofen reduced both evoked presynaptic Ca2+ influx and resting Ca2+ concentration in RHT terminals. Tertiapin did not alter the evoked EPSC and baclofen-induced inhibition, indicating that baclofen does not inhibit glutamate release by activation of Kir3 channels. Neither Ba2+ nor high extracellular K+ modified the baclofen-induced inhibition. 4-Aminopyridine (4-AP) significantly increased the EPSC amplitude and the charge transfer, and dramatically reduced the baclofen effect. These data indicate that baclofen inhibits glutamate release from RHT terminals by blocking N-, T-, and P/Q-type Ca2+ channels, and possibly by activation of 4-AP-sensitive K+ channels, but not by inhibition of R- and L-type Ca2+ channels or by Kir3 channel activation.
authors.affiliation: Center for Research on Occupational and Environmental Toxicology, Oregon Health and Science University, Portland 97239-30, USA.
authors.forename: Mykhaylo G
authors.initials: MG
authors.lastname: Moldavan
authors.forename: Robert P
authors.initials: RP
authors.lastname: Irwin
authors.forename: Charles N
authors.initials: CN
authors.lastname: Allen
mesh_terms: D000344:Afferent Pathways / Q000502:physiology; D000818:Animals; D015220:Calcium Channels / Q000378:metabolism*; D002478:Cells, Cultured; D007031:Hypothalamus / Q000502:physiology*; D015640:Ion Channel Gating / Q000502:physiology; D008297:Male; D009433:Neural Inhibition / Q000502:physiology*; D009474:Neurons / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D018080:Receptors, GABA-B / Q000378:metabolism*; D017661:Receptors, Presynaptic / Q000378:metabolism*; D012160:Retina / Q000502:physiology*; D009435:Synaptic Transmission / Q000502:physiology*
chemical_list: D015220:Calcium Channels; D018080:Receptors, GABA-B; D017661:Receptors, Presynaptic