Static and dynamic membrane properties of lateral vestibular nucleus neurons in guinea pig brain stem slices

In vitro intracellular recordings of central vestibular neurons have been restricted so far to the medial vestibular nucleus (MVN). We performed intracellular recordings of large Deiters' neurons in the lateral vestibular nucleus (LVN) to determine their static and dynamic membrane properties, and compare them with those of type A and type B neurons identified in the MVN. Unlike MVN neurons (MVNn), the giant-size LVN neurons (LVNn) form a homogeneous population of cells characterized by sharp spikes, a low-amplitude, biphasic after-hyperpolarization like type B MVNn, but also an A-like rectification like type A MVNn. In accordance with their lower membrane resistance, the sensitivity of LVNn to current injection was lower than that of MVNn over a large range of frequencies. The main difference between LVNn and MVNn was that the Bode plots showing the sensitivity of LVNn as a function of stimulation frequency were flatter than those of MVNn, and displayed a weaker resonance. Furthermore, most LVNn did not show a gradual decrease of their firing rate modulation in the frequency range where it was observed in MVNn. LVNn synchronized their firing with the depolarizing phase of high-frequency sinusoidal current injections. In vivo studies have shown that the MVN would be mainly involved in gaze control, whereas the giant LVNn that project to the spinal cord are involved in the control of posture. We suggest that the difference in the membrane properties of LVNn and MVNn may reflect their specific physiological roles.     

Self-stimulation and circling: Differentiation of the neural substrata by behavioral measurement with the use of the double pulse technique

Using a trade-off paradigm, the frequency of pairs of stimulating pulses (each pair consisting of a conditioning or C pulse and a test or T pulse) necessary to maintain a constant behavior was determined for different C-T intervals. Behaviors studied were self-stimulation (SS) in the lateral hypothalamus (5 rats) and circling in the raphe region (4 rats). It was postulated that the required changes in frequency of pairs reflect changes in the excitatory effectiveness of T pulses due to the excitability cycle of the stimulated neurons. In a first experiment, C and T pulses of the same intensity were used. The observed changes of frequency of pairs at different C-T intervals in both behaviors were found to be consistent with neural properties such as local potential summation (LPS) and refractory periods (RP). In an additional experiment using C bigger than T, the LPS was found to be decreased while a supernormal period was evidenced. It was concluded that the behavioral use of the double pulse technique reveals the same neuronal properties as in the classical use of this technique in neurophysiology. In addition, a difference of 0.3 msec was found between the absolute RPs of SS and circling. It was therefore concluded that this technique is able to differentiate two neural systems on the basis of their excitability cycles.     

Is acute compartment syndrome avoidable?

Complete absence of acute compartment syndrome was observed in a consecutive series of 966 tibial fractures in African patients of diverse ethnic groups, reviewed retrospectively at our institution. Considering the incidences reported in the literature, we should have experienced between 22 and 86 cases of acute compartment syndrome. The purpose of this prospective study was to confirm these findings and at the same time to look for possible explanations for this unexpected observation. During a period of one year and four months, 257 tibial fractures were prospectively analyzed for clinical signs and late sequelae of acute compartment syndrome. In 156 of these patients, presenting 158 fractures of the tibia, the pressure in the anterior compartment was systematically measured. No single case of compartment syndrome was diagnosed in this series, and no late sequelae of acute compartment syndrome were noted. The hypothesis we forward for total absence of acute compartment syndrome is the favourable effect of the continuously high surrounding temperatures on safeguarding the arteriovenous pressure gradient and lowering the vascular resistance. We suggest that further investigation should be carried out to study this hypothesis.     

Unilateral labyrinthectomy modifies the membrane properties of contralesional vestibular neurons

Vestibular compensation after a unilateral labyrinthectomy leads to nearly complete disappearance of the static symptoms triggered by the lesion. However, the dynamic vestibular reflexes associated with head movements remain impaired. Because the contralesional labyrinth plays a prominent role in the generation of these dynamic responses, intracellular recordings of contralesional medial vestibular nucleus neurons (MVNn) were done after 1 mo of compensation. Their firing properties and cell type were characterized at rest, and their response dynamics investigated using step, ramp, and sinusoidal current stimulations. The sensitivity of the contralesional MVNn firing rates to applied current was increased, which, along with increased phase leads, suggests that significant changes in active conductances occurred. We found an increased proportion of the phasic type B neurons relative to the tonic type A neurons in the contralesional MVN. In addition, the remaining contralesional type A MVNn response dynamics tended to approach those of type B MVNn. Thus the contralesional MVNn in general showed more phasic response dynamics than those of control MVNn. Altogether, the firing properties of MVNn are differentially modified on the ipsilesional and contralesional sides of the brain stem 1 mo after unilateral labyrinthectomy. Ipsilesional MVNn acquire more "type A-like" tonic membrane properties, which would contribute to the stabilization of the spontaneous activity that recovers in the deafferented neurons during vestibular compensation. The bilateral increase in the sensitivity of MVNn and the acquisition of more "B-like" phasic membrane properties by contralesional MVNn should promote the restoration of the vestibular reflexes generated by the remaining, contralesional labyrinth.     

Intrinsic choroidal neurons in the human eye: projections,targets, and basic electrophysiological data

PURPOSE: The chemical coding of intrinsic choroidal neurons (ICNs) has features in common with extrinsic fibers (e.g., from the pterygopalatine ganglion) making it impossible to assess whether a neuronal nitric oxide synthase (nNOS)/vasoactive intestinal polypeptide (VIP)-immunoreactive nerve fiber is of intrinsic or extrinsic origin. Neurobiotin injections into single neurons allow the visualization of projections of these cells and the determination of the origin of target innervation. Thus, this technique was used in the present study to help characterize the organization of the ICN in the human eye. METHODS: ICNs were visualized with the fluorescent vital dye 4-Di-2-ASP. Electrophysiological properties were determined by means of intracellular recordings. The impaled neurons were iontophoretically filled with neurobiotin. After fixation, immunohistochemistry for neuronal nitric oxide synthase (nNOS), alpha-smooth muscle actin, and calcitonin gene-related peptide (CGRP) was conducted. RESULTS: ICN processes were traced over distances of up to 2.612 micro m. They were found in the immediate vicinity of other nNOS-positive or -negative ICNs and were also found apposed to smooth muscle fibers (vascular and stromal nonvascular). CGRP-positive fibers forming boutons were observed closely associated with ICNs. Electrophysiological recording showed phasic firing without slow afterhyperpolarization, no spontaneous activity, an input resistance of 136 +/-73 MOmega, and a membrane time constant of 7 +/- 1 ms. CONCLUSIONS: Apart from the first functional characterization of ICNs, this study provided more precise evidence of reciprocal ICN-to-ICN contacts and innervation of both choroidal nonvascular and vascular smooth muscle. The presented technique offers promising perspectives to further investigate the function of ICNs in ocular homeostasis.     

Alinidine modifies the pacemaker current in sheep Purkinje fibers

(1) The specific bradycardic agent alinidine reduces the slope of the diastolic depolarization in sinoatrial tissue and Purkinje fibers. In short Purkinje fibers of sheep, alinidine (28 M) decreased the pacemaker current by a dual action. The voltage dependence ofi _f activation was shifted in the hyperpolarizing direction by 7.8±0.6 mV (n=18,p<0.001) and the conductance of the fully activated current was reduced to 73±2% (n=18,p<0.001) of its control value. These effects were reversible and dose-dependent. (2) Ionophoretic injections of alinidine caused reversible reductions of the diastolic depolarization rate and simultaneous transient hyperpolarizing shifts of thei _f activation range. (3) Some prolongation of the action potential duration was observed at 28 M and more pronounced at higher concentration. This was presumably the consequence of a reduction by alinidine of outward repolarizing current carried by the background inward rectifier and plateau currenti _x. (4) The action of alinidine oni _f resulted in a slower activation of a reduced fraction of the pacemaker current at the maximal diastolic potential level. This explains the decrease of the diastolic depolarization rate observed in Purkinje fibers.     

Pontine and mesencephalic substrates of self-stimulation

Single or twin, moveable monopolar stimulating electrodes were implanted in male adult rats in order to map the medial pons and mesencephalon for self-stimulation behaviour. The electrodes were implanted 6 mm below the surface of the skull and subsequently moved down by steps of 0.13 or 0.16 mm. Each bar press in a Skinner box delivered a train (0.4 s in duration) of cathodal rectangular pulses of fixed intensity (200 microA) and width (0.1 ms). Self-stimulation was recorded from zero to the maximum performance by varying the number of pulses per stimulating train. The rewarding efficacy of the stimulation at each electrode location was inferred from determination of the pulse period corresponding to the threshold and half-maximal performance. Out of 361 mesencephalic and pontine sites sampled, 289 supported self-stimulation. Within the metencephalon, the study revealed a continuous band of positive sites, extending over a dorso-ventral distance of 4 mm, between the floor of the aqueduct and the pontine nuclei. Hence, all electrode locations in the central grey, dorsal raphe and median raphe supported self-stimulation. Within the mesencephalon, the positive band was restricted between the floor of the central grey and the middle part of the interpeduncular nucleus. At the rostral mesencephalon, it shifted laterally towards the substantia nigra. The overlap between the self-stimulation sites and some of the best known ascending and descending pathways is discussed.     

Evidence that the reward attenuating effect of the D1-like antagonist, SCH-23390, is not mediated by its agonist action at the 5-HT2c receptors

This study investigated the effect of the 5-HT2c receptor antagonist, SB-242,084, on the attenuation of brain stimulation reward by SCH-23390. An additional experiment determined the effectiveness of SB-242,084 at blocking the reward attenuating effect of 5-HT2c agonist, CP-809,101. Results show that SB-242,084 blocked the reward attenuating effect of CP-809,101 but failed to alter that of SCH-23390. These findings provide evidence that the agonist action of SCH-23390 at the 5-HT2c receptors does not contribute to its attenuating effect on reward.     

Vestibular evoked myogenic potentials in patients suffering from an unilateral acoustic neuroma: a study of 170 patients.

OBJECTIVE: To determine the value of investigating the vestibular evoked myogenic potentials (VEMP's) induced by clicks and 500 Hz short tone burst (STB) for the diagnosis of acoustic neuromas. METHODS: We studied the average responses to 100dB clicks and 500 Hz STB in the ipsilateral sternomastoid muscle. Ninety-five healthy subjects and 170 patients suffering from a unilateral acoustic neuroma were included in that study. Caloric and audiometric tests results were also analyzed. RESULTS: Thirty-six/170 patients (21.2%) exhibited normal responses to clicks and to STB whereas 134/170 (78.8%) gave abnormally low or no responses. 78/170 (45.9%) showed no responses to both clicks and STB. In 56/170 patients (32.9%), VEMP's induced by high level clicks and STB were discordant: STB VEMP's were either normal (n=32) or low (n=24) in patients with an abnormal response to clicks (no response n=40 or low response n=16). In contrast, STB-induced VEMP's were always normal in cases of normal responses to clicks. No strong, systematic correlation could be found between saccular nerve dysfunction and either the degree of 4-8 kHz hearing loss or the extent of horizontal canalar impairment. CONCLUSIONS: These data indicate that high level clicks and STB provide complementary information about the functionality of the saccular nerve. Clicks are useful to detect a minor saccular nerve dysfunction. In cases in which there is no response to clicks, STB gives valuable information about a potential residual function of the saccular nerve.