The Action Of Lanthanum(La～(3+))on Amino Acids Receptors In The Sacral Dorsal Commissural Neurons

Background : Rare earth elements (REEs) consist of lanthanide series, as well as Scandium and Yttrium. REEs comprise seventeen elements in all. REEs don't belong to essential element of human. Lanthanum is a most abundant trivalent rare earth metal cation. Greater use of lanthanum, especially in industry, agriculture, husbandry, medicine, increases the chances of accidental exposure. However, early studies have shown that La3+ have dose dependent analgesic effect on rat by microcannulated into the waist subarachnoid space of rats and naloxone-reversible site dependent opiate analgesia after microinjection of lanthanum in rat brain. The effect on central nerve system by La3+ has aroused much interest to neurologists.The sacral dorsal commissural nucleus (SDCN) is a cell column located in the dorsal gray matter of central canal in the lower lumbar and sacral spinal cord. It receives primary afferent inputs from both pelvic and pudendal nerves, which provide the major innervation of pelvic organ. Accordingly, the SDCN serves as a relay of sensory information from the pelvic viscera. An important role of the nucleus is in nociception, which makes SDCN an ideal place to study the transmission and modulation of the pelvic visceral nociceptive signals. However, Up to date, little attention has been directed towards affect of La3+ on the SDCN.Objective: To characterize the Pharmacology and effect mechanism of La3+ on SDCN by investigating the Pharmacology and effect of La3+ on amino acids receptor function in neurons acutely dissociated from the SDCN. It will provide theory evidence for neurotoxicity and utilization of La3+.Method: I. Two-week-old Sprague–Dawley rats lumbosacral (L6-S2) segment (approximately10–15 mm) was dissected out from the spinal cord and The spinal segment was sectioned to yield several transverse slices, then neurons were mechanically dissociated and allowing electrophysiological studies to be conducted. II. At room temperature, the effect of La3+ on IKA , GABAA receptor and Gly receptor in neurons acutely dissociated from the SDCN was investigated using the nystatin-perforated patch-recording configuration under voltage-clamp conditions and applying via a'Y-tube'.III. The data were fitted to a modified Michaelis–Menten Eq by the use of a least-square fitting for EC50 and the Hill coefficient. Statistical comparisons were performed by paired Student's t-test.Results:I. The application of 3μM GABA to all tested SDCN neurons induced typical inward currents at a holding potential (VH) of 40mV. The IGABA was completely inhibited by adding 10μM BIC. Application of La3+ alone induced no noticeable current at concentrations up to 30mM, and with La3+ less than 10μM, there was not any change of the IGABA detected in SDCN neurons. The amplitudes of IGABA activated by 3μMGABA were increased with an increase of La3+ concentration. The enhancement was rapidly and completely reversed upon washing the cell with La3+-free solution. 100μM La3+ increased the amplitude of current induced by 3μM GABA to 171% of control and the maximal effect of La3+ was 231% of control. The degree of augmentation of IGABA by 100μM La3+ was decreased with increasing concentrations of GABA, while at high agonist concentrations (above 100μM) the effect of La3+ was almost completely abolished. It is apparent that the dose–response curve of GABA was shifted leftwards in the presence of 100μM La3+, without significantly changing the maximal response amplitude. In the presence of La3+, the linear voltage dependence of IGABA persisted at VH over -60mV to +60mV, thus the effect of La3+ is voltage-independent. The reversal potential was 1.73mV with 100μM La 3+. The value did not significantly differ from that of GABA in control group (-0.3mV; P>0.1) and were close to the ECl. II. La3+ markedly increased the amplitude of current activated by 10μM glycine. The enhancement was rapidly and completely reversed upon washing the cell with La3+-free solution. On average, 100μM La3+ increased the amplitude of current induced by 10μM glycine to 176% of control and the maximal effect was 215% of control. The potentiation of La3+ on IGly was not use-dependent, but in a concentration-dependent manner between 1μM and 1mM, with the EC50 value of 52.3±10.9μM. La3+ did not change the reversal potential of IGly, which is close to the ECl calculated from the given extra- and intracellular cation concentrations.Conclusion:It seems that the IC50 values of La3+ determined here are within the limits that may be expected to occur in vivo by La exposure. Lanthanum potentiates the action of GABA and Gly in acutely isolated rat SDCN neurons. The enhancement was mediated by an increase in apparent affinity of GABAA receptors for GABA. And La3+ produced a similar augmenting effect on glycine-activated current mediated by Gly receptors compared with the effect of La 3+ on GABAA receptors. The properties of the La3+-induced potentiation are more consistent with an allosteric model of action, in which La3+ binds to a distinct site on the GABAA receptor or glycine receptor and increases the potency of GABA or glycine.The presence ofα1β2γ2 type GABAA receptors in SDCN is quite possible.In summary, These results suggest that in SDCN, the potentiation of rejection capability amino acids receptor-mediated responses and the inhibition of excitatory amino acids receptors as expected may significantly contribute to anesthesia of pelvic viscera.