Effects of asymmetric vertebral blood flow upon the vestibulo-ocular reflex of the rabbit

Role of peripheral vestibular receptors in the control of blood pressure following hypotension

Hypotension is one of the potential causes of dizziness. In this review, we summarize the studies published in recent years about the electrophysiological and pharmacological mechanisms of hypotension-induced dizziness and the role of the vestibular system in the control of blood pressure in response to hypotension. It is postulated that ischemic excitation of the peripheral vestibular hair cells as a result of a reduction in blood flow to the inner ear following hypotension leads to excitation of the central vestibular nuclei, which in turn may produce dizziness after hypotension. In addition, excitation of the vestibular nuclei following hypotension elicits the vestibulosympathetic reflex, and the reflex then regulates blood pressure by a dual-control (neurogenic and humoral control) mechanism. In fact, recent studies have shown that peripheral vestibular receptors play a role in the control of blood pressure through neural reflex pathways. This review illustrates the dual-control mechanism of peripheral vestibular receptors in the regulation of blood pressure following hypotension.

The Effect of Acupuncture on Visual Function in Patients with Congenital and Acquired Nystagmus

Background: The aim of this study is to examine the short-term effect of visual function following acupuncture treatment in patients with congenital idiopathic nystagmus and acquired nystagmus (CIN and AN). Methods: An observational pilot study on six patients with confirmed diagnosis of nystagmus (three CIN and three AN patients (2♀, 4♂; mean age 42.67; SD ± 20.57 y)), was performed. Acupuncture treatment was done following a standardized protocol applying needle-acupuncture on the body and the ears. The treatment was scheduled with 10 sessions of 30 min duration over five weeks. To assess the effect of the treatment, we performed before, between, and after acupuncture objective measurement of the BCVA (EDTRS charts), contrast vision (CSV-1000, Vector Vision), nystagmography (Compact Integrated Pupillograph), complemented by evaluation questionnaires. A placebo non-acupuncture control group (Nr: 11, 22 eyes; 8♀, 3♂; mean age: 33.34 y (SD ± 7.33 y)) was taken for comparison. Results: The results showed that, following acupuncture treatment, CIN and AN patients showed improvement (SD± mean) in their binocular BCVA (baseline: 0.45 ± 0.36; between: 0.53 ± 0.34 and post-treatment: 0.51 ± 0.28), and in their monocular contrast sensitivity (baseline: 11.29 ± 12.35; between: 11.43 ± 11.45 and post-treatment: 14.0 ± 12.22). The post-/baseline-difference showed a significant improvement in contrast vision and in BCVA for CIN and AN patients, but not for controls (p = 0.029 and p = 0.007, respectively). The effect of the eye showed also, within CIN and AN, significant values for the examined parameters in the post-/baseline difference (p = 0.004 and p ≤ 0.001). Evaluated only binocularly, the respective between-/baseline and post-/baseline difference in the CIN and AN group showed significant values (p < 0.045). Two AN patients reported reduction of oscillations. Among general subjective symptoms, our patients reported reduction of tiredness and headache attacks, improvement of vision, and shorter sleep onset time. Conclusion: The applied acupuncture protocol showed improvement in the visual function of nystagmus patients and thus, in their quality of life. Further studies are mandatory to differentiate which group of nystagmus patients would benefit more from acupuncture.

Immunohistochemical detection of phosphorylated form of extracellular signal-regulated kinase 1/2 in rat vestibular nuclei following hemorrhagic hypotension

The purpose of this study was to evaluate the expression of the phosphorylated form of extracellular-regulated protein kinase 1/2 (pERK1/2), which is one of the major regulatory factors for transcription of the c-fos oncogene in neurons, within the vestibular nuclei (VN) of rats following acute arterial hypotension. Following acute arterial hypotension induced by rapid hemorrhage, a significant number of pERK1/2-immunoreactive neurons appeared bilaterally in the caudal aspect of the medial and inferior VN. No labeling of pERK1/2 was observed in the lateral VN. The peak expression of pERK1/2-immunoreactive neurons in these nuclei occurred within 5 min after hemorrhage. In bilaterally labyrinthectomized rats, the appearance of pERK1/2-immunoreactive neurons was eliminated in the VN. These results suggest that, following acute hypotension, afferent signals from the peripheral vestibular receptors are required for activation of extracellular signal-regulated kinase 1/2 in the VN.

Effects of acute hypotension on expression of cFos-like protein in the vestibular nuclei of rats

The expression and regional distribution of cFos protein, which is an oncogene product and metabolic marker of neural excitation, were investigated in the vestibular nuclear complex following acute hypotension in adult Sprague-Dawley rats. Intravenous administration of nitroprusside elicited a 10-50% reduction in mean blood pressure for 10 min. Unilateral or bilateral chemical labyrinthectomies were performed 14 days before the start of the experiment to eliminate afferent signals from the peripheral vestibular receptors in the inner ear. All of the animals were sacrificed and the tissues were fixed 2 h after the onset of acute hypotension using the cardiac perfusion method for c-Fos immunohistochemical staining. The cFos-like immunoreactive (cFLI) neurons were expressed selectively in the central area of the medial vestibular nucleus following a 10% reduction in blood pressure. Once the blood pressure had fallen by 30%, bilateral expression of cFLI neurons was observed in the superior, medial, and spinal vestibular nuclei, but not in the lateral vestibular nucleus, of control rats with intact labyrinths. The expression of cFLI neurons increased proportionately with reductions in blood pressure. In unilaterally labyrinthectomized rats, acute hypotension induced the expression of cFLI neurons in vestibular nuclei contra lateral to the injured labyrinth, but not in the ipsilateral vestibular nuclei. However, cFLI neurons were not expressed in bilateral vestibular nuclei following acute hypotension in bilateral labyrinthectomized rats. These results suggest that afferent signals from the peripheral vestibular receptors are essential for cFos protein expression in the vestibular nuclei following acute hypotension.

Effects of acute hypotension on neuronal activity in the medial vestibular nuclei of rats

The role of peripheral vestibular receptors in acute hypotension was investigated in anesthetized rats. In animals with intact labyrinths, acute hypotension induced by either i.v. infusion of sodium nitroprusside or hemorrhage produced excitation of electrical activity in two-thirds of type I neurons and inhibition in two-thirds of type II neurons recorded in the medial vestibular nuclei. In unilaterally labyrinthectomized animals, two-thirds of type I neurons ipsilateral to the lesion showed an inhibitory response, and two-thirds of contralateral type I neurons showed an excitatory response after the induction of acute hypotension. The response patterns of type II neurons were opposite to those of type I neurons. These results suggest that blood flow changes are detected by peripheral vestibular receptors, and that this might suggest a mechanism for control of blood pressure.

Effects of blood pressure change upon brain-stem pO2 in rabbits with surgically removed baroreceptor nerve. Possible mechanisms for vertigo due to autonomic nerve dysfunction

The effect of denervation of carotid sinus nerve (CSD) on the brain-stem tissue pO2 tension was studied in the awake rabbit. Tissue pO2 around the vestibular nuclei was measured in two groups of rabbits, normal and CSD, under two experimental conditions, viz. blood pressure (BP) modulation and tilt stimulation. BP was either depressed by hemorrhage (up to 50 mmHg) or elevated by intravenous injection of phenylephrine (up to 160 mmHg). In the normal rabbit, tissue pO2 showed minimum change during BP modulation (ranging between 70 and 160 mmHg. In contrast, CSD surgery resulted in the failure to maintain pO2 at a constant level when BP was elevated. However, tissue pO2 did not change when BP decreased. Head-up tilt stimulation (up to 40 degrees) also induced a transient decrease in BP and pO2 in the CSD rabbit; however, these were maintained at a constant level in the normal rabbit. These conditions were assumed to be due to the decrease in autoregulation of cerebral blood flow (CBF), which resulted in CSD. In these experiments, tissue pO2 recovered first, even when BP still remained low. Therefore, it can be proposed that the carotid baroreceptor reflex works for CBF autoregulation of brain stem primarily during an early phase of BP change. Therefore a disturbed carotid sinus reflex, as shown in the present experiment, may be a possible explanation for the mechanism of stress-related vertigo and/or orthostatic vertigo.