| Abstract No.: | C-B3044 |
| Country: | Canada |
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| Title: | STRYCHNINE-MEDIATED EXCITATION OF MAGNOCELLULAR NEUROSECRETORY CELLS IS INVERSELY RELATED TO EXTRACELLULAR FLUID OSMOLALITY IN SUPERFUSED EXPLANTS OF RAT HYPOTHALAMUS |
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| Authors/Affiliations: | 1 Yuri Katrina Choe*; 1 Charles W. Bourque;
1 McGill University, Montreal, QC, Canada
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| Content: | Objectives: Application of strychnine has been shown to excite magnocellular neurosecretory cells (MNCs) in the rat supraoptic nucleus (SON) in vivo, where strychnine is believed to act as a competitive antagonist to taurine-mediated activation of glycine receptors (GlyRs). In this system, taurine has been shown to be released by astrocytes in the ventral glial limitans, which arborize within and around the nucleus. Hypoosmotic conditions have been shown to increase the release of taurine, thereby inhibiting the electrical activity of MNCs in order to suppress vasopressin and oxytocin release into the bloodstream. Conversely, hypertonic conditions suppress the basal release of taurine and lessen GlyR-mediated inhibition on MNCs, leading to an increase in electrical activity and VP release. In this study we investigated if this process could be studied in vitro, in superfused explants of rat hypothalamus by applying strychnine while extracellularly recording from MNCs in varied osmotic conditions. The role of glia in this process was further investigated by bath-applying fluorocitrate, a toxin previously demonstrated to be specific for glial cells.
Materials and Methods: We performed single-unit extracellular recordings on rat SON explants The specificity of bath-applying 1 ¥ìM strychnine for blockade of GlyRs was tested by bolus injections of 1 mM Taurine or 0.4 mM GABA) in control (ACSF; hypertonic 315 mOsm) and during 1 ¥ìM strychnine. In isotonic (300 mOsm) or hypotonic (275 mOsm) oxygenated artificial cerebrospinalfluid (ACSF) perfusion, 1 ¥ìM strychnine was bath-applied for 2-5 minutes after collecting baseline activity of the neuron. Fluorocitrate was added to hypotonic ACSF to a final concentration of 67 ¥ìM and perfused the brain explant for at least 4 hours.
Results: In single-unit extracellular recordings from MNCs, bath-application of 1 ¥ìM strychnine reversibly blocked the inhibitory effect of taurine, but not that of GABA. Indeed, application of strychnine significantly reduced the per cent inhibition induced by taurine from 58.9 ¡¾ 8.4 to 18.4 ¡¾ 5.8 per cent (n=4; P<0.01), but not those evoked by GABA (44.6 ¡¾ 2.6 to 30.7 ¡¾ 5.0 per cent (n=3; P=0.07)). The degree of excitation induced by bath-applying 1 ¥ìM strychnine showed an inverse relationship with respect to extracellular osmolality: The excitatory effect of strychnine was significantly greater in hypoosmotic (1.0 ¡¾ 0.3 Hz; n=18) than isoosmotic medium (0.04 ¡¾ 0.3 Hz ;n=14; P<0.05). Strychnine application induced a statistically significant increase in mean firing frequency in control group (1.9 ¡¾ 0.2 Hz to 3.4 ¡¾ 0.4 Hz) but not in FC (3.8 ¡¾ 0.8 Hz to 3.9 ¡¾ 0.8 Hz). Baseline firing rate in control was significantly lower than in FC.
Conclusions: These results indicate that GlyRs mediate an inhibitory effect on firing rate in MNCs and that the strength of this inhibition varies as an inverse function of osmolality in superfused explants of rat hypothalamus. Taurine release from astrocytes is responsible for this inhibitory effect.
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