| Objective:To look for an easy, feasible and accurate method to locate horizontal semicircular canal plane. To explore the characteristics of field potentials within vestibular nuclei evoked by electrical stimulus. To investigate the responsiveness of medial vestibular nucleus (MVN) neurons to baclofen during the early, partially compensated period after unilateral surgical labyrinthectomy.Methods: Based on former authors'study, we measured the coordinate of the bregma, and calculated the coordinate of bregma when the head pitched down 48 degrees. In addition, we analyzed the possible error. Double silver electrodes were placed on the surface of round window, vestibular nuclei field potential was recorded under the condition of electrical stimulus. MVNs were localized in vestibular nucleus complexes using field potentials evoked by electrical stimulation, along with a stereotaxic atlas of rat brain. Neuronal activity in MVNs was recorded and analyzed in rats that had undergone labyrinthectomy with and without administration of baclofen, a type Bγ-aminobutyric acid receptor (GABAB receptor) agonist.Results: The coordinate of the bregma is (8.70±0.51mm,14.30±0.37mm), which changed to (16.48±0.52mm,3.14±0.38mm) when pitched down 48 degrees. The error usually limited to less than 1.28 degrees. The field potential consists of a negative wave (N1), the threshold is 0.59±0.07mA or 0.90±0.50V, the latency of N1 peak is 1.20±0.13ms. When double stimuli were applied, no response followed the second stimulus or the response was small if the time interval is less than 4ms. Field potentials were recorded in vestibular nucleus complexes in most rats, and the amplitude of the N1 wave (the first negative wave) correlated well with the position of the glass microelectrode tip within the nucleus. After left labyrinthectomy, the mean discharge rate in ipsilesional MVNs decreased, but it was nearly restored by postoperative day 8. Baclofen (3 mg/kg) reversed the mean discharge rates between bilateral MVNs at days 4 and 8 after surgical labyrinthectomy. At day 4, the discharge rates of left and right MVN neurons were 10.5±5.7 and 7.3±5.5 spikes/s respectively after baclofen administration, compared with 13.5±3.2 and 22.9±7.1 spikes/s respectively after unilateral labyrinthectomy without baclofen. At Day 8, the discharge rates of left and right MVN neurons were 13.9±8.0 and 12.1±8.1 spikes/s respectively after baclofen administration, compared with 19.4±13.0 and 21.6±4.8 spikes/s respectively without baclofen. In addition, the reduction ratio of the right MVN neurons was higher than that of the left MVN neurons.Conclusion: Use of 3.14 mm as y axis coordinate to locate horizontal semicircular canal is a kind of easy, feasible and accurate method. Evoked vestibular nuclei potential can be obtained by peripheral labyrinth electrical stimulus, which is quite stable. Ipsi- and contralateral MVNs neurons respond differently to systemic injection of baclofen after unilateral labyrinthectomy, illustrating the plastic changes of the GABA receptor during vestibular compensation.
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