Mayumi Takasaki

Sympathetic and cardiovascular actions of orexins in conscious rats

The novel hypothalamic peptides orexin-A and orexin-B are known to induce feeding behavior when administered intracerebroventricularly, but little is known about other physiological functions. The renal sympathetic nerves play important roles in the homeostasis of body fluids and the circulatory system. We examined the effects of intracerebroventricularly administered orexins on mean arterial pressure (MAP), heart rate (HR), renal sympathetic nerve activity (RSNA), and plasma catecholamine in conscious rats. Orexin-A (0.3, 3.0 nmol) provoked an increase in MAP (94.3 ± 0.7 to 101.9 ± 0.7 mmHg and 93.1 ± 1.1 to 108.3 ± 0.8 mmHg, respectively) and RSNA (28.0 ± 7.0 and 57.9 ± 12.3%, respectively). Similarly, orexin-B (0.3, 3.0 nmol) increased MAP (93.9 ± 0.9 to 97.9 ± 0.9 mmHg and 94.5 ± 1.1 to 105.3 ± 1.7 mmHg, respectively). Orexin-A and -B at 3.0 nmol also increased HR. In other conscious rats, a high dose of orexin-A and -B increased plasma norepinephrine. Plasma epinephrine only increased with a high dose of orexin-A. These results indicate that central orexins regulate sympathetic nerve activity and affect cardiovascular functions.The novel hypothalamic peptides orexin-A and orexin-B are known to induce feeding behavior when administered intracerebroventricularly, but little is known about other physiological functions. The renal sympathetic nerves play important roles in the homeostasis of body fluids and the circulatory system. We examined the effects of intracerebroventricularly administered orexins on mean arterial pressure (MAP), heart rate (HR), renal sympathetic nerve activity (RSNA), and plasma catecholamine in conscious rats. Orexin-A (0.3, 3. 0 nmol) provoked an increase in MAP (94.3 +/- 0.7 to 101.9 +/- 0.7 mmHg and 93.1 +/- 1.1 to 108.3 +/- 0.8 mmHg, respectively) and RSNA (28.0 +/- 7.0 and 57.9 +/- 12.3%, respectively). Similarly, orexin-B (0.3, 3.0 nmol) increased MAP (93.9 +/- 0.9 to 97.9 +/- 0.9 mmHg and 94.5 +/- 1.1 to 105.3 +/- 1.7 mmHg, respectively). Orexin-A and -B at 3.0 nmol also increased HR. In other conscious rats, a high dose of orexin-A and -B increased plasma norepinephrine. Plasma epinephrine only increased with a high dose of orexin-A. These results indicate that central orexins regulate sympathetic nerve activity and affect cardiovascular functions.

Epidural anesthesia enhances sympathetic nerve activity in the unanesthetized segments in cats.

To evaluate compensatory sympathetic excitation during epidural anesthesia, we measured cardiac and renal sympathetic nerve activity during thoracic or lumbar epidural anesthesia in cats.Thirteen cats were divided into three groups: five cats received thoracic epidural anesthesia, five received lumbar epidural anesthesia, and three received lumbar epidural anesthesia after the carotid sinus and vagoaortic nerves were severed (denervated lumbar group). Heart rate (HR), mean arterial pressure (MAP), and cardiac and renal sympathetic nerve activity were measured repeatedly after administration of a single dose of 0.1 mL/kg of 1% lidocaine via the epidural catheter. Epidural solution spread from a median of C-8 to T-6 in the thoracic epidural group, T-8 to L-3 in the lumbar epidural group, and T-7 to L-3 in the denervated lumbar group. During thoracic epidural anesthesia, HR, MAP, and cardiac sympathetic nerve activity decreased, while renal nerve activity increased. Similarly, HR, MAP, and renal sympathetic nerve activity decreased during lumbar epidural anesthesia, and cardiac activity increased. In the denervated lumbar group, HR, MAP, and renal sympathetic nerve activity decreased but cardiac activity remained unchanged. Sympathetic nerve activity in corresponding unanesthetized segments increased during thoracic or lumbar epidural anesthesia in association with significant decreases in MAP and HR. After severance of the carotid sinus and vagoaortic nerves, the absence of sympathetic excitation in the unanesthetized segments during lumbar epidural anesthesia suggests that the compensatory response is produced by the baroreceptor reflex response to anesthesia-induced hypotension. (Anesth Analg 1997;84:391-7)

Neuronal effects of orexins: relevant to sympathetic and cardiovascular functions

Orexin A and B, also called hypocretin 1 and 2, were recently discovered in the hypothalamus. This organ, in which a number of neuropeptides have been demonstrated to stimulate or suppress food intake, is considered important for the regulation of appetite and energy homeostasis. Orexins were initially reported as a regulator of food intake. More recent reports suggest their possible important roles in the multiple functions of neuronal systems, such as narcolepsy, a sleep disorder. Orexins and their receptors are distributed in neural tissue and brain regions involved in the autonomic and neuroendocrine control. Functional studies have shown that these peptides evoke changes in cardiovascular and sympathetic responses. The data from our in vivo and in vitro studies suggest that the peptide acting on neurons in the hypothalamic paraventricular nucleus increases the cardiovascular responses. This review will focus on the neural effects of orexins and how these peptides may participate in the regulation of cardiovascular and sympathetic functions.

Dural puncture with a 26-gauge spinal needle affects spread of epidural anesthesia

Combined spinal and epidural anesthesia may increase the risk of epidurally administered drugs spreading into the subarachnoid space through the dural hole.We studied the effect of dural puncture with a 26-gauge needle on the spread of analgesia induced by epidural injection of local anesthetics. Forty patients were randomly assigned to control and dural puncture groups. In the dural puncture group, the dura was punctured with a 26-gauge Whitacre spinal needle at L2-3 but no drug was injected. In both groups, an 18-gauge epidural catheter was inserted 4 cm cephalad into the epidural space at L2-3 and 15 mL of 2% mepivacaine without epinephrine was injected. Analgesia was assessed by pinprick at 5, 10, 15, and 20 min after injection and at the end of surgery. The caudal spread of analgesia was significantly greater in the dural puncture group than in the control group 15 and 20 min after injection (P < 0.01), but the cranial spread of analgesia was not different between the two groups. We conclude that dural puncture (without drugs) using a 26-gauge Whitacre spinal needle before epidural injection increases caudal spread of analgesia induced by epidural local anesthetics. (Anesth Analg 1996;82:1040-2)

Procaine and mepivacaine have less toxicity in vitro than other clinically used local anesthetics.

The neurotoxicity of local anesthetics can be demonstrated in vitro by the collapse of growth cones and neurites in cultured neurons. We compared the neurotoxicity of procaine, mepivacaine, ropivacaine, bupivacaine, lidocaine, tetracaine, and dibucaine by using cultured neurons from the freshwater snail Lymnaea stagnalis. A solution of local anesthetics was added to the culture dish to make final concentrations ranging from 1 × 10−6 to 2 × 10−2 M. Morphological changes in the growth cones and neurites were observed and graded 1 (moderate) or 2 (severe). The median concentrations yielding a score of 1 were 5 × 10−4 M for procaine, 5 × 10−4 M for mepivacaine, 2 × 10−4 M for ropivacaine, 2 × 10−4 M for bupivacaine, 1 × 10−4 M for lidocaine, 5 × 10−5 M for tetracaine, and 2 × 10−5 M for dibucaine. Statistically significant differences (P < 0.05) were observed between mepivacaine and ropivacaine, bupivacaine and lidocaine, lidocaine and tetracaine, and tetracaine and dibucaine. The order of neurotoxicity was procaine = mepivacaine < ropivacaine = bupivacaine < lidocaine < tetracaine < dibucaine. Although lidocaine is more toxic than bupivacaine and ropivacaine, mepivacaine, which has a similar pharmacological effect to lidocaine, has the least-adverse effects on cone growth among clinically used local anesthetics.

Graded, irreversible changes in crayfish giant axon as manifestations of lidocaine neurotoxicity in vitro

UNLABELLED High concentrations of lidocaine induce irreversible conduction block with little effect on resting membrane potential (Em). We assumed the mechanism of persistent neurologic deficit caused by local anesthetics may result from neural death, as represented by the loss of Em. We investigated the effects of lidocaine on Em and action potential (AP) in single crayfish giant axons in vitro. Axons were perfused with two doses of lidocaine for either 15 or 30 min, and they were continuously washed. No axons exposed to 80 mM lidocaine for 30 min showed recovery of AP and Em. Those exposed to 40 mM for 30 min and 80 mM for 15 min showed a return to baseline for Em, but no recovery of AP. Those exposed to 40 mM lidocaine for 15 min showed full recovery of Em and AP immediately after washing. The membrane depolarization was significantly greater during exposure to 80 mM lidocaine for 30 min than in other groups. We conclude that lidocaine has a direct neurotoxic effect on crayfish giant axons and that the generation of AP is more vulnerable than the maintenance of Em. The irreversibility of AP and Em is dose- and time-dependent. IMPLICATIONS Highly concentrated lidocaine induced an irreversible conduction block and a complete loss of resting membrane potential in crayfish giant axons in vitro. Our results may represent a possible explanation for various grades of local anesthetic-induced neurotoxicity in clinical cases if the same toxicity occurs in mammalian nerves in vivo.

Lidocaine disrupts axonal membrane of rat sciatic nerve in vitro

Highly concentrated lidocaine has been reported to induce irreversible loss of membrane potential in crayfish nerve, which implies membrane disruption as one of the direct mechanisms of lidocaine-induced neurotoxicity. To confirm lidocaine-induced membrane disruption in mammalian nerve, a lactate dehydrogenase (LDH) leakage from rat sciatic nerve was measured in vitro. Before applying lidocaine, the desheathed nerve was incubated for 60 min in Krebs-Ringer solution at 37°C to examine basal LDH activity. It was then incubated in 80 mM lidocaine solution at pH 7.3 for 15, 30, 60, or 120 min. Other nerves were immersed in 800 mM choline solution for 120 min. Total LDH activity per wet weight of nerve tissue was assayed using spectrophotometry. It was also determined using nerves cut into 10 segments and incubated in distilled water for 60 min. The LDH activity in the lidocaine group showed a time-dependent increase. After the 60- and 120-min incubation with lidocaine, the amount of LDH activity was significantly increased compared with the choline group and was similar to that of the group incubated in distilled water. We conclude that 80 mM lidocaine may be sufficient to cause membrane damage and facilitate the leakage of enzymes from cytoplasm. Implications This study demonstrates that exposing the rat myelinated nerve to lidocaine at a clinically used concentration for more than 30 min causes enough membrane damage to allow enzyme leakage. In clinical practice, the smallest effective dose should be used.

Activation of a G protein-coupled inwardly rectifying K+ current and suppression of Ih contribute to dexmedetomidine-induced inhibition of rat hypothalamic paraventricular nucleus neurons.

Background:&agr;2-Adrenoceptor agonist has been reported to produce inhibition of arginine vasopressin release, diuresis, and sympatholytic effects. However, its mechanisms of central action remain incompletely understood. Hypothalamic paraventricular nucleus (PVN) neurons, which are in direct contact with noradrenergic synapses and are controlled by the hyperpolarization-activated currents, are called Ih (H current). The effect of dexmedetomidine, a highly selective and potent agonist, at &agr;2 adrenoceptors on Ih is unknown. The purpose of this study was to examine the effects of dexmedetomidine on the PVN neuron, which is involved in the arginine vasopressin release and autonomic regulation. Methods:The authors investigated the effects of dexmedetomidine on the membrane properties in PVN magnocellular neurons and an Ih in PVN parvocellular neurons with a whole cell patch clamp technique using a rat brain slice preparation. Results:Dexmedetomidine dose-dependently hyperpolarized PVN magnocellular neurons. In the voltage clamp mode, dexmedetomidine induced an outward current, with a reversal potential of −94 mV, and this was shown to depend on the external concentration of K+. Pretreatment with Ba2+ or peptide toxin tertiapin blocked hyperpolarization induced by dexmedetomidine. The effect of dexmedetomidine was blocked by an &agr;2-adrenoceptor antagonist, yohimbine. Ih was suppressed dose dependently by dexmedetomidine in PVN parvocellular neurons. Pretreatment with Cs+ occluded the Ih suppression by dexmedetomidine. Yohimbine blocked the Ih suppression by dexmedetomidine. The Ih sensitive to dexmedetomidine was weakly modulated by intracellular cyclic adenosine monophosphate. Conclusions:Dexmedetomidine inhibited PVN magnocellular neurons by activation of the G protein–coupled inwardly rectifying K+ current and inhibited PVN parvocellular neurons by suppression of Ih.

Acute pulmonary edema after intravenous liquid halothane in dogs

Intravenous liquid halothane causes severe pulmonary edema when administered for suicide attempts. This study was carried out to elucidate the cardiopulmonary effects of intravenous liquid halothane in 14 dogs. Subjects were divided into three groups: group 1 (n = 4) was the control; group 2 (n = 5) received 7.5 mmol intravenous liquid halothane; and group 3 (n = 5) received pretreatment of continuous infusion of prostaglandin E, at a rate of 0.02, μg kg−1 min−1, followed by 7.5 mmol intravenous liquid halothane. Hemodynamic values, extravascular lung water, and arterial blood gas tensions were measured for 240 min. In group 2, thromboxane B2, β-glucuronidase, and lipid peroxides were measured in four of five dogs. In group 2, intravenous liquid halothane caused pulmonary edema associated with hypoxemia, pulmonary hypertension, and left ventricular dysfunction. In group 3, prostaglandin E1, given to reduce pulmonary vasoconstriction and left ventricular preload, aggravated hypoxemia and pulmonary hypertension and impaired left ventricular contractility, although end-diastolic left ventricular pressure was low. Thromboxane B2 increased, whereas β-glucuronidase and lipid peroxides did not change after administration of intravenous halothane. We conclude that pulmonary edema induced by intravenous liquid halothane was due to direct pulmonary vascular damage, and that pulmonary vasoconstriction and increased left ventricular preload were not contributory causes.

The Effect of Pulsed Radiofrequency Current on Mechanical Allodynia Induced with Resiniferatoxin in Rats

BACKGROUND: Pulsed radiofrequency (PRF) is a popular pain treatment modality. The effect of PRF current on neuropathic pain has not been examined in detail. We investigated the effect of PRF current on mechanical allodynia induced with resiniferatoxin (RTX) in rats, especially regarding the influence of the duration of allodynia before PRF procedures and that of exposure time to PRF. METHODS: Adult male Sprague-Dawley rats (weighing 250–400 g) received a single intraperitoneal injection of RTX (200 &mgr;g/kg) under 2 to 3% sevoflurane anesthesia. Rats in group S2 (n = 5) were assigned to receive PRF current to the right sciatic nerve for 2 minutes 1 week after RTX treatment; rats in group M2 (n = 6), PRF current for 2 minutes 3 weeks after RTX treatment; rats in group L2 (n = 7), PRF current for 2 minutes 5 weeks after RTX treatment; rats in group S4 (n = 5), PRF current for 4 minutes 1 week after RTX treatment; rats in group S6 (n = 5), PRF current for 6 minutes 1 week after RTX treatment; and rats in group S0 (n = 3), no PRF current was delivered. Instead, the needle and electrode were inserted at proper points for 6 minutes 1 week after RTX treatment. All rats were evaluated for sensitivity to mechanical stimulation with von Frey filaments and to thermal stimulation with a thermal testing apparatus and for motor function using placing and grasping reflexes before injection of RTX, every week after injection of RTX, and 1, 2, 3, 4, and 5 weeks after PRF treatment. RESULTS: The paw withdrawal thresholds of both hindpaws 1 week after RTX treatment were significantly lower than the pre-RTX baseline in all groups. In groups S2, S4, S6, and M2, after PRF procedures, the ipsilateral paw withdrawal thresholds significantly increased. A statistically significant difference was detected between the PRF-treated and PRF-untreated hindpaws. The ipsilateral–contralateral paw withdrawal thresholds after PRF procedures in group S2 were significantly higher than those in groups M2 and L2. Between groups M2 and L2, significant differences were found 1, 2, 4, and 5 weeks after PRF procedures. The ipsilateral–contralateral paw withdrawal thresholds in group S6 were significantly higher than those in groups S2 and S4 5 weeks after PRF procedures. No significant difference was found between groups S2 and S4 at any time. After PRF procedures, no difference in the withdrawal latency after heat stimulation and no motor disturbance were observed at any time in all groups. CONCLUSIONS: PRF treatment was more effective when applied in the early stages of mechanical allodynia (1 week) in rats. Increased exposure time to PRF current from 2 to 6 minutes showed a significant antiallodynic effect without motor impairment. We propose the application of PRF current for 6 minutes adjacent to the nerve as soon as possible when allodynia appears.