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Biomedical subjects

T Osaka

Publications and source records attributed to T Osaka.

At least 91 records · Page 5Linked to original sources
PubMed1987

gamma-Aminobutyric acid antagonist blocks baroreceptor-activated inhibition of neurosecretory cells in the hypothalamic paraventricular nucleus of rats.

The effects of gamma-aminobutyric acid (GABA) antagonists on baroreceptor-activated inhibition of neurosecretory cells in the paraventricular nucleus (PVN) were examined in urethane-chloralose anesthetized rats. In 11 neurosecretory cells which were inhibited by baroreceptor activation induced by intravenous application of phenylephrine, microiontophoretically applied bicuculline and/or picrotoxin blocked the inhibition (n = 9) completely or partially, whereas strychnine (n = 4) did not. The results suggest that GABA is, at least in part, involved in the baroreceptor-activated inhibition of PVN neurosecretory cells.

Action Potentials↗
PubMed1987

Thermosensitivity of neurons in the paraventricular nucleus of the rat slice preparation.

The thermosensitivity of 65 spontaneously active neurons in the paraventricular nucleus (PVN) was investigated by extracellular recording in the rat hypothalamic slice preparation. The firing rate of these cells was comparatively low, ranging from 0.03 to 10.0 (mean 2.46) impulses/s at 37 degrees C, and only a minority showed a phasic firing pattern. Of 65 neurons tested, 23 (35%) increased their firing rate when the slice was warmed (warm-sensitive neurons) and 9 (14%) showed the opposite response (cold-sensitive neurons). Thermosensitivity was also tested in solutions with reduced [Ca2+] and high [Mg2+]. Eight out of 10 warm-sensitive neurons and 5 of 7 cold-sensitive neurons retained thermosensitivity after synaptic blockade. Out of 6 phasic firing neurons tested, one showed warm-sensitivity and another one showed cold-sensitivity. The thermosensitive neurons were diffusely distributed throughout the PVN and were not located in particular areas of the nucleus. Thus a group of cells in the PVN, including probably both magno- and parvocellular neurons, showed an inherent thermosensitivity, which suggests an important role for the PVN in thermoregulation.

Animals↗
PubMed1987

Neurons in the paraventricular nucleus projecting to the median eminence: a study of their afferent connections from peripheral baroreceptors, and from the A1-catecholaminergic area in the ventrolateral medulla.

In male rats anesthetized with urethane-chloralose, extracellular recordings were made from tuberoinfundibular (TI) neurons in the paraventricular nucleus (PVN) identified by antidromic stimulation of the median eminence. Activation of baroreceptors induced by intravenous administration of phenylephrine inhibited approximately two-thirds of the 32 TI-neurons tested. Electrical stimulation of the A1-catecholaminergic area in the ventrolateral medulla produced an excitation in one half of the 28 neurons. Microinjection of L-glutamate (40 nl of 0.5 M solution of pH 7.4-7.6) to the same A1-area evoked an excitation in 7 out of 8 TI-neurons tested. The results show that some TI-neurons in the PVN receive inhibitory synaptic inputs from the baroreceptors and excitatory inputs from neurons in the A1-catecholaminergic area.

Adrenergic Fibers↗
PubMed1987

Decrease in blood pressure by stimulation of the rat hypothalamic paraventricular nucleus with L-glutamate or weak current.

Effects of electrical and chemical stimulation of the paraventricular nucleus (PVN) of the hypothalamus on blood pressure were examined in rats anesthetized with urethane-chloralose. Focal electrical stimulation (10-25 microA, 50 Hz, 0.5 ms) of sites within the PVN consistently decreased blood pressure, while the stimulation of regions adjacent to the PVN produced no change or weak depressor response. Microinjection of L-glutamate (0.5 M, 80 nl) to the PVN consistently led to decreased blood pressure, while the same amount of saline injection had no effect. The depressor response was not affected by cervical vagotomy but was markedly reduced after intravenous administration of hexamethonium. The results suggest that activation of PVN neurons in rats leads to a decrease in blood pressure.

Animals↗
PubMed1987

Effects of activation sequence and anisotropic cellular geometry on the repolarization phase of action potential of dog ventricular muscles.

The influence of activation sequences on action potential configuration, especially in the repolarization phase, was examined in isolated canine ventricular muscles. Action potentials were recorded from the epicardial surface in the center of a preparation having nearly uniform fiber orientation (25 X 25 mm). Stimuli applied just adjacent to the recording site produced nearly centrifugal propagation. An activation sequence either parallel (longitudinal) or perpendicular (transverse) to the long axis of the muscle fibers was produced by peripheral stimulation. Action potential duration at -60 mV (APD-60 mV) during centrifugal propagation was significantly longer than that during longitudinal propagation. Further shortening of APD-60 mV was observed during transverse propagation. When a collision of longitudinal or transverse wavefronts (longitudinal or transverse collision) was produced at the action potential recording site, the shortest APD was recorded. During centrifugal propagation, action potential mapping around the stimulating electrodes revealed that APD-60 mV shortened gradually as the recording site was moved further from the stimulation site. The spatial gradient of APD was steeper in the transverse than in the longitudinal direction, causing a distortion in the repolarization sequence and the recovery of excitability near the center of the tissue. Premature stimuli applied to an area near the central stimulation site induced one-way block and circus movement of the wavefront, indicating reentry of excitation. We concluded that the activation sequence and anisotropic cellular geometry substantially affect APD, and that such a change contributes to the spatial inhomogeneity of refractoriness leading to reentrant arrhythmias.

Action Potentials↗
PubMed1987

Endocardial excitation and conduction during reperfusion arrhythmias.

In order to clarify the role of Purkinje fibers in the occurrence of reperfusion arrhythmias, endocardial mapping was performed on perfused canine hearts by attaching 42 close bipolar electrodes to the endocardial surface of the left ventricular septum. Reperfusion with oxygenated Krebs-Ringer solution following 30 min of coronary occulusion induced ventricular tachycardia (VT) in 14 out of 23 preparations. These VT degenerated into ventricular fibrillation (VF) within 1 min after the reperfusion in all but 3 cases. Endocardial mapping revealed that the excitations during VT were always initiated by the Purkinje activities and that myocardial excitations were expanded in a centrifugal manner through Purkinje-muscle junctional area. Furthermore, this excitation pattern was preserved, in the early phase of VT, even though the propagation pattern was distorted. VF was always induced by reperfusion following 30 min of ischemic condition, that is, coronary perfusion with a hyperkalemic (K = 10 mM), acidic (pH = 6.8) and hypoxic (PO2 = 20-40 mmHg) solution (4/4 cases). Elimination of hyperkalemia from the ischemic condition markedly prevented occurrence of VF (1/6 cases) during reperfusion but it did not affect occurrence of VT (4/6 cases); this implies that hyperkalemia causes the onset of VF but has less effect on the occurrence of VT. It has been separately confirmed by micro-electrode experiment, using the dissected papillary muscle of the canine right ventricle, that abnormal impulse formation during re-oxygenation was triggered in Purkinje fibers around Purkinje-muscle junction.

Animals↗
PubMed1986

Electrophysiological properties of neurons in the caudal ventrolateral medulla projecting to the paraventricular nucleus of the hypothalamus in rats.

Extracellular recordings were made from neurons in the caudal ventrolateral medulla in urethane-chloralose-anesthetized rats. Stimulation of the paraventricular nucleus (PVN) in the hypothalamus evoked antidromic action potentials in 71 neurons. On the basis of antidromic spike latencies, these neurons could be divided into fast- (24 neurons) and slow-conducting cell groups (47 neurons). Slow-conducting cells showed irregular and slow spontaneous discharges, while a majority of the fast-conducting cells did not show spontaneous discharges. The spontaneous activity of slow-conducting cells was suppressed by i.v. clonidine administration. The effects of clonidine could be consistently reversed by administration of the alpha 2-adrenergic antagonist, yohimbine. The responses by clonidine and yohimbine remained unimpaired in baroreceptor-denervated rats. Vagus nerve stimulation produced an excitation in 80% of slow-conducting cells tested. Baroreceptor activation induced by i.v. administration of phenylephrine inhibited about half of slow-conducting cells tested. Similar elevation of blood pressure in baroreceptor-denervated rats did not show any effect. These physiological and pharmacological properties of slow-conducting cells were similar to those previously reported for catecholaminergic cells in other parts of the brain. The results show the existence of two different populations among neurons in the caudal ventrolateral medulla which project directly to the PVN, and suggest that the presumed A1 catecholaminergic cells are involved in the afferent pathway from cardiovascular baroreceptors and the vagus nerve to the PVN.

Animals↗
PubMed1986

[Electrophysiological studies on the central control of feeding and drinking under hyperbaric environment].

Increased urinary output, decreased fluid intake and reduction in body weight have been reported in divers exposed to hyperbaric environments. Single unit activity of hypothalamic neurons in the lateral hypothalamic area and paraventricular nucleus which control energy and body fluid balance was recorded with chronically implanted electrodes in rats. Neurons in the prefrontal cortex and central medical thalamus were also recorded for comparison. Out of 33 hypothalamic neurons, 19 reversibly decreased their firing rate to 67%, in average, under high pressure (7 atmospheres absolute) of air compared with precompression control. On the other hand, the remaining 14 neurons and all prefrontal and thalamic neurons tested were unaffected. Decreased hypothalamic unit activity was also found under normobaric hyperoxic environment and normoxic hyperbaric environment. The results suggest that hyperbaric air specifically affected the hypothalamic neurons by means of increased partial pressure of both oxygen and nitrogen.

Animals↗
PubMed1986

The significance of myocardial bridge upon atherosclerosis in the left anterior descending coronary artery.

The relation between myocardial bridges (MB) and atherosclerosis in the left anterior descending coronary artery (LAD) was explored using morphometric methods in 642 hearts. The location of myocardial bridges in the LAD was classified according to distribution as proximal, middle and distal. Myocardial bridges were found in 48 per cent of males and 36 per cent of females. When proximal myocardial bridging was present intimal thickening and macroscopic raised lesion were increased just before the bridge as compared with the corresponding site in the other two categories. Underneath bridges eccentric plaques and raised lesions are absent although there is often concentric intimal thickening. The overall frequency of myocardial infarction was the same in patients with and without myocardial bridges. However, when infarction occurred in the patients having bridges, it was almost confined to those in the proximal group despite this being infrequent in the general distribution of myocardial bridges in the left anterior descending artery. It is postulated that hypertension may enhance infarction in the case of myocardial bridges in the very proximal left anterior descending artery. It is concluded that the location of myocardial bridges greatly alters the distribution of physical force against the arterial wall and influences the extent of atherosclerosis.

Aged↗
PubMed1986

Comparison of influences of stimulation of the Ammon's horn and subiculum to thermosensitive preoptic and septal neurons.

Thermosensitive preoptic and septal (PO/SP) neurons were studied for their responsiveness to electrical stimulation of the dorsal Ammon's horn (DAH) and the ventral subiculum (VSB) of the hippocampal formation in the rat. DAH stimulation affected only 14 (16%) of 90 PO/SP thermosensitive neurons whereas VSB stimulation affected 77 (86%) units. The results suggests that the DAH has no major role in thermoregulation in the rat.

Animals↗
PubMed1986

Endocardial mapping of ventricular tachycardia in one-day-old myocardial infarction.

Endocardial mapping of ventricular tachycardia (VT) was performed in one-day-old myocardial infarction using a matrix electrode (6 X 7 = 42) placed on the endocardial surface of the perfused isolated canine heart. Spontaneous focal VT was observed in all 7 dogs. It was neither affected nor terminated by electrical stimulation. Focal VT was induced in 5 dogs by constant pacing or premature programmed stimulation of the main left bundle. It was terminated by electrical stimulation and induction of the VT was suppressed by verapamil (5 X 10(-7)). Endocardial mapping of the spontaneous focal VT and induced focal VT showed initiation of activation from the surviving Purkinje fiber in the infarcted or border zone with subsequent circular expansion of excitation. Reentrant VT was induced in 3 dogs by bursts of rapid pacing and deteriorated into ventricular fibrillation. Endocardial mapping showed a continuous arc of functional conduction block and slowly circulating activation front around the block. It is suggested that in experimental one-day-old myocardial infarction VT resulted from abnormal automaticity and triggered activity in the surviving Purkinje fiber and reentry in the subendocardial muscle layer.

Animals↗
PubMed1984

Effects of electrical and chemical stimulation of the paraventricular nucleus on neurons in the subfornical organ of cats.

Single unit recordings were made from neurons of the subfornical organ (SFO) in hemispherectomized cats following electrical stimulation in the paraventricular nucleus (PVN) of the hypothalamus. Of 132 neurons tested, 48 (36%) were inhibited, 3 (2.3%) excited and two (1.5%) antidromically activated. In separate experiments the wall of the third ventricle near the PVN was locally perfused with glutamate solution using a concentric push-pull cannula. Of 35 SFO neurons tested, 6 were inhibited, 5 were facilitated and 24 unaffected. The results indicate that one-third of SFO neurons receive synaptic inputs from cells in or near the PVN.

Angiotensin II↗
PubMed1984

Paraventricular neurosecretory neurons: synaptic inputs from the ventrolateral medulla in rats.

Effects of electrical stimulation of the ventrolateral medulla on discharge activity of neurosecretory neurons in the paraventricular nucleus (PVN) were studied in male rats anesthetized with urethane-chloralose. Among 35 phasically firing neurosecretory neurons, stimulation of the lateral reticular nucleus and its vicinity produced excitation in 10 and inhibition in 2. The stimulation also enhanced the activity of 40% of the PVN neurosecretory neurons that fired continuously (n = 81); of these responsive neurons, half of the neurons tested (n = 12) were inhibited by i.v. administration of phenylephrine. The result suggests that both vasopressin- and oxytocin-secreting neurons in the PVN receive mainly excitatory synaptic inputs from the ventrolateral medulla.

Animals↗
PubMed1984

Changes in body temperature and thermosensitivity of preoptic and septal neurons during hippocampal stimulation.

The effects of electrical stimulation of the ventral subiculum (VSB) of hippocampal formation on the thermosensitivity of neurons in the preoptic area and the septum (PO/SP) were studied in the urethane-anesthetized rat. The local thermosensitivity of sixteen of 28 PO/SP thermosensitive neurons (25 warm-units and 3 cold-units) was reduced or lost during low frequency (4.8 Hz) stimulation of VSB. The extent of reduction was positively correlated to the magnitude of inhibition by the VSB stimulation. Thermosensitivity did not change in 11 of the remaining 12 warm-units and increased slightly in one warm-unit during VSB stimulation. In the conscious rat, bilateral stimulation (5 Hz) of VSB for 10 min caused a rise in rectal temperature of 0.28-0.87 degrees C in a warm environment (30-33 degrees C) and a fall of 0.18-0.57 degrees C in a cold (9.4-9.8 degrees C) environment. The results suggest that the predominantly inhibitory input from VSB decreases the thermosensitivity of PO/SP warm- and cold-sensitive neurons and thereby reduces the ability of thermoregulation.

Animals↗