GABA(B) receptor-mediated selective peripheral analgesia by the non-proteinogenic amino acid, isovaline.

Peripherally restricted analgesics are desirable to avoid central nervous system (CNS) side effects of opioids. Nonsteroidal anti-inflammatory drugs produce peripheral analgesia but have significant toxicity. GABA(B) receptors represent peripheral targets for analgesia but selective GABA(B) agonists like baclofen cross the blood-brain barrier. Recently, we found that the CNS-impermeant amino acid, isovaline, produces analgesia without apparent CNS effects. On observing that isovaline has GABA(B) activity in brain slices, we examined the hypothesis that isovaline produces peripheral analgesia mediated by GABA(B) receptors. We compared the peripheral analgesic and CNS effect profiles of isovaline, baclofen, and GABA (a CNS-impermeant, unselective GABA(B) agonist). All three amino acids attenuated allodynia induced by prostaglandin E2 injection into the mouse hindpaw and tested with von Frey filaments. The antiallodynic actions of isovaline, baclofen, and GABA were blocked by the GABA(B) antagonist, CGP52432, and potentiated by the GABA(B) modulator, CGP7930. We measured Behavioural Hyperactivity Scores and temperature change as indicators of GABAergic action in the CNS. ED(95) doses of isovaline and GABA produced no CNS effects while baclofen produced substantial sedation and hypothermia. In a mouse model of osteoarthritis, isovaline restored performance during forced exercise to baseline values. Immunohistochemical staining of cutaneous layers of the analgesic test site demonstrated co-localization of GABA(B1) and GABA(B2) receptor subunits on fine nerve endings and keratinocytes. Isovaline represents a new class of peripherally restricted analgesics without CNS effects, mediated by cutaneous GABA(B) receptors.

The effect of deep inspiration breath-hold on tumour oxygenation.

AIM: To investigate the influence of deep inspiration breath-hold on the oxygen tension of in-vivo tumours measured using an Eppendorf pO2 histograph. MATERIALS AND METHODS: Patients with accessible primary or metastatic tumours > or = 2 cm diameter were entered into a protocol measuring tumour oxygenation with an Eppendorf pO2 histograph during normal breathing (NB) and deep inspiration breath-hold (DIBH). Change in oxygen tension was assessed using the Wilcoxon Signed Ranks test. RESULTS: Thirty patients were entered in to this protocol. The median maximum tumour dimension was 4 cm. The median of the median pO2 of these tumours was 18 mmHg. Tumours were assessed during NB and DIBH. Oxygen tension measurements along 1-3 pairs of tracks per tumour (median of 2) were obtained. The median number of measurements per track was 30 for NB and 29 for DIBH (range 17-59). In six tumours, the values during NB were significantly higher than during DIBH, whereas, for six other tumours, the relationship was the opposite; for the remaining 18 patients, no significant difference was observed. CONCLUSION: These data show heterogeneity of tumour oxygenation seen with in-situ tumours both at baseline and as a result of DIBH. No systematic change in the Eppendorf pO2 measurements was seen as a result of DIBH; however, the individual tumour responses to DIBH varied dramatically.

Combined pre- and post-surgical bupivacaine wound infiltrations decrease opioid requirements after knee ligament reconstruction.

PURPOSE: To test the efficacy of a combination of selective pre- and post-surgical local anesthetic infiltrations of the knee, compared with standard intra-articular injection at the end of surgery alone, to reduce postoperative opioid requirements following arthroscopic cruciate ligament reconstruction (ACLR). METHODS: In a double-blind, randomized, controlled trial, we studied 23 patients (ASA I or II) scheduled for elective ACLR under general anesthesia. The treatment group (n = 12) received infiltrations with bupivacaine 0.25% with epinephrine 1:200,000 presurgically (10 ml into the portals, 10 ml at the medial tibial incision site, 10 ml at the lateral femoral incision site, and 10 ml intra-articularly) and postsurgically (5 ml at the medial tibial incision and 10 ml at the lateral femoral incision). The control group (n = 11) received infiltrations with saline 0.9% in the same manner. All patients received a standard intra-articular local anesthetic instillation of the knee (25 ml of bupivacaine 0.25% with epinephrine 1:200,000) at the completion of surgery. RESULTS: Postoperative opioid requirements were lower in the treatment group (5.8 +/- 2.9 mg morphine equivalent) than in the control group ( 13.7 +/- 5.8 mg; P = 0.008). Treatment patients were ready for discharge approximately 30 min earlier than control patients (P = 0.046). There were no adverse events in the treatment group. In the control group, 2/11 patients vomited and a third experienced transient postoperative diaphoresis, dizziness and pallor. CONCLUSION: We conclude that a combination of selective pre- and post-surgical wound infiltration with bupivacaine 0.25% provides superior analgesia compared with a standard post-surgical intra-articular injection alone.

Cost comparison of sevoflurane with isoflurane anesthesia in arthroscopic menisectomy surgery.

PURPOSE: To determine the "real world" cost of sevoflurane compared with isoflurane in balanced general anesthesia for daycare arthroscopic menisectomy, we prospectively investigated perioperative drug requirement and expense as well as recovery time. METHODS: Following intravenous induction, 40 consenting adult patients randomly received either sevoflurane- or isoflurane-based anesthesia with a standardized gas inflow rate of 3 l x min. Recovery was assessed in the postanesthetic recovery room (PARR) in a double-blind manner at 15 min intervals using the Aldrete scoring system until patients met discharge criteria. RESULTS: Patient demographics, anesthetic duration, volatile potency and adjunct drug requirements were similar in the two groups. Total perioperative drug cost per patient was CAN$38.10+/-10.13 (mean +/- SD) for the sevoflurane group and $23.87+/-6.59 for the isoflurane group (P<0.01). Although the nonvolatile drug cost was comparable between the two groups, the volatile drug cost per patient was $19.40+/-8.80 for sevoflurane and $4.50+/-1.90 for isoflurane (P<0.01). This four-fold sevoflurane-to-isoflurane cost difference was the product of two ratios, both based on the volume of liquid anesthetic: the ratio of consumption, 2.1; and the ratio of institutional price, 2.1. Intraoperative hemodynamic response, time until discharge from the PARR and incidences of postoperative nausea and vomiting did not significantly differ between the two groups. CONCLUSIONS: When used to maintain equipotent balanced general anesthesia for daycare arthroscopic menisectomy, volatile consumption and cost were greater for sevoflurane compared with isoflurane. Nonvolatile perioperative drug cost and recovery times were similar, however, in the two groups.

Addition of femoral 3-in-1 blockade to intra-articular ropivacaine 0.2% does not reduce analgesic requirements following arthroscopic knee surgery.

PURPOSE: To test the hypothesis that the addition of a preincisional femoral 3-in-1 block to intra-articular instillation with ropivacaine 0.2% at the end of surgery improves postoperative pain control in patients undergoing arthroscopic anterior cruciate ligament reconstruction (ACLR) under general anesthesia. METHODS: In a prospective, randomized, placebo-controlled, double-blind trial, we studied 44 patients scheduled for inpatient ACLR. Prior to incision, the treatment group (n = 22) received a femoral 3-in-1 block with 40 ml ropivacaine 0.2%, augmented by infiltrations of the lateral and anteromedial incisions with 20 ml ropivacaine 0.2% at the end of the procedure. The control group (n = 22) received saline 0.9% instead of ropivacaine. All patients received an intra-articular instillation with 30 ml ropivacaine 0.2% at the end of surgery. The primary efficacy variable was 24 hr morphine consumption postoperatively standardized by weight, administered intravenously via a patient-controlled analgesia (PCA) pump. RESULTS: There was no difference between both groups in 24 hr PCA morphine consumption postoperatively (control, 0.45 +/- 0.44 [mean +/- SD] mg x kg(-1); treatment, 0.37 +/- 0.50 mg x kg(-1); p = 0.55). No difference was found in postoperative visual analog scale pain scores, adverse events, or vital signs. In the treatment group, R = 10/22 patients did not require postoperative morphine compared with R = 6/22 in the control group (P = 0.35). CONCLUSION: We found no effect of a femoral 3-in-1 block with ropivacaine 0.2% on postoperative analgesic consumption, compared to intra-articular instillation with ropivacaine 0.2% alone, in patients undergoing ACLR under general anesthesia.

Mechanism of anesthesia revealed by shunting actions of isoflurane on thalamocortical neurons.

By using thalamic brain slices from juvenile rats and the whole cell recording technique, we determined the effects of aqueous applications of the anesthetic isoflurane (IFL) on tonic and burst firing activities of ventrobasal relay neurons. At concentrations equivalent to those used for in vivo anesthesia, IFL induced a hyperpolarization and increased membrane conductance in a reversible and concentration-dependent manner (ionic mechanism detailed in companion paper). The increased conductance short-circuited the effectiveness of depolarizing pulses and was the main cause for inhibition of tonic firing of action potentials. Despite the IFL-induced hyperpolarization, which theoretically should have promoted bursting, the shunt blocked the low-threshold Ca2+ spike (LTS) and associated burst firing of action potentials as well as the high-threshold Ca2+ spike (HTS). Increasing the amplitude of either the depolarizing test pulse or hyperpolarizing prepulse or increasing the duration of the hyperpolarizing prepulse partially reversed the blockade of the LTS burst. In voltage-clamp experiments on the T-type Ca2+ current, which produces the LTS, IFL decreased the spatial distribution of imposed voltages and hence impaired the activation of spatially distant T channels. Although IFL may have increased a dendritic leak conductance or decreased dendritic Ca2+ currents, the somatic shunt appeared to block initiation of the LTS and HTS as well as their electrotonic propogation to the axon hillock. In summary, IFL hyperpolarized thalamocortical neurons and shunted voltage-dependent Na+ and Ca2+ currents. Considering the importance of the thalamus in relaying different sensory modalities (i.e., somatosensation, audition, and vision) and motor information as well as the corticothalamocortical loops in mediating consciousness, the shunted firing activities of thalamocortical neurons would be instrumental for the production of anesthesia in vivo.

Ionic mechanism of isoflurane's actions on thalamocortical neurons.

We studied the actions of isoflurane (IFL) applied in aqueous solutions on ventrobasal neurons from thalamic brain slices of juvenile rats. By using the whole cell, patch-clamp method with current- and voltage-clamp recording techniques, we found that IFL increased a noninactivating membrane conductance in a concentration-dependent reversible manner. In an eightfold concentration range that extended into equivalent in vivo lethal concentrations, IFL did not produce a maximal effect on the conductance; this is consistent with a nonreceptor-mediated mechanism of action. TTX eliminated action potential activity but did not alter IFL effects. The effects on the membrane potential and current induced by IFL were voltage independent but depended on the external [K+], reversing near the equilibrium potential for K+. External Ba2+ or internal Cs+ applications, which block K+ channels, suppressed the conductance increase caused by IFL. External applications of the Ca2+ channel blockers Co2+ or Cd2+ or internal application of the Ca2+ chelator 1,2-bis-(2-aminophenoxy)-ethane-N,N, N',N'-tetraacetic acid did not prevent the effects of IFL, implying little involvement of Ca2+-dependent K+ currents. A contribution of inwardly rectifying K+ channels to the increased steady-state conductance seemed unlikely because IFL decreased inward rectification. An involvement of ATP-mediated K+ channels also was unlikely because application of the ATP-mediated K+ channel blocker glibenclamide (1-80 microM) did not prevent IFL's actions. In contrast to spiking cells, IFL depolarized presumed glial cells, consistent with an efflux of K+ from thalamocortical neurons. The results imply that a leak K+ channel mediated the IFL-induced increase in postsynaptic membrane conductance in thalamic relay neurons. Thus a single nonreceptor-mediated mechanism of IFL action was responsible for the hyperpolarization and conductance shunt of voltage-dependent Na+ and Ca2+ spikes, as reported in the preceding paper. Although anesthetics influence various neurological systems, an enhanced K+ leak generalized in thalamocortical neurons alone could account for anesthesia in vivo.

Isoflurane attenuates resonant responses of auditory thalamic neurons.

In thalamocortical neurons, sensory signals are transformed differently during various states of consciousness. We investigated the effects of a general anesthetic, isoflurane, on the frequency responses of neurons in the ventral medial geniculate body, the primary nucleus of the auditory thalamus. Using slice preparations, whole cell current-clamp recording techniques, and frequency-domain analyses with oscillatory inputs, we observed a resonance in the hyperpolarized voltage range, implying a frequency preference near 1 Hz in the subthreshold frequency responses of medial geniculate neurons. As in other thalamocortical neurons, an interaction of a T-type Ca2+ current with passive membrane properties generates the resonant responses. The frequency preference shapes the input-output signal transformation, coupling oscillatory inputs at preferred frequencies to firing. Thus resonance may contribute to the rhythmic synchronization of the output to the cortex. In a concentration range of 0.5-3%, isoflurane application reversibly decreased the resonant responses of medial geniculate neurons. Throughout the subthreshold voltage range, it reduced impedance at frequencies < 10 Hz. At depolarized potentials near -60 mV, isoflurane reduced the low-pass filter selectivity of the neuron membrane. At rest near -70 mV or at hyperpolarized potentials, isoflurane had a greater effect on resonance (centered at approximately 1 Hz), reducing the peak impedance more than the magnitudes at other frequencies. At concentrations of > or = 2%, isoflurane completely blocked the resonance peak, thereby imposing low-pass characteristics of poor quality throughout the subthreshold voltage range. Application of isoflurane reversibly increased membrane conductance and the current threshold for firing evoked by depolarizing pulses from potentials between -60 and -90 mV. The neurons discharged in a tonic pattern on depolarization from about -60 mV and in a phasic (burst) mode from potentials negative to about -70 mV. An increase in current amplitude compensated the suppression of tonic firing much more readily than that of the burst firing on a low-threshold Ca2+ spike. Although a reduction in T-type Ca2+ channel activation may occur during isoflurane application, the depression of resonance is consistent with an interaction of a greatly increased leak conductance with the low-threshold Ca2+ current and the membrane capacitance. In the intact animal, this would tend to disrupt synchronized neural oscillations and the transfer of auditory information.

Midazolam premedication delays recovery after propofol without modifying involuntary movements.

Midazolam has GABAergic effects in children that may modify propofol-induced involuntary movements, yet delay recovery. In a double-blind, randomized study, 24 children (2-7 yr of age, ASA physical status I or II) undergoing short surgical procedures received midazolam 0.5 mg/kg (Group M) or placebo (Group P) per os 20-30 min before propofol anesthesia (5 mg/kg intravenously followed by an infusion). Blind observers scored sedation and anxiety levels (scale 1-4) before premedication, at separation from parents, and at induction of anesthesia. Induction and emergence were videotaped, and body movements were recorded. During recovery, times to eye opening and maximum Steward (SS = 6) and Vancouver Sedative Recovery (VSRS = 22) scores were noted. Parents were questioned about side effects that may have occurred during the following week. Both groups were similar in age, sex, weight, timing of premedication, propofol dose, and duration of surgery. The incidence of involuntary movements did not differ between groups but was higher at induction (79%) than on emergence (25%) (P < 0.05). Anxiety and sedation scores were similar in Group P and Group M, but recovery took longer after midazolam, with eye opening (mean +/- SD) 24 +/- 7 vs 43 +/- 18 min, maximum SS (median and range) 27 (13-37) vs 55 (24-138) min, and maximum VSRS 51 (30-100) vs 80 (50-130) min. Children returned to normal activity in 1 (0-5) day, and none exhibited neurological complications. We conclude that an oral premedicant dose of midazolam prolongs recovery from anesthesia in children without affecting dystonic movements after propofol.

Effects of anipamil, a long acting analog of verapamil, in pigs subjected to myocardial ischemia.

In this study we examined the cardiovascular and possible antiarrhythmic actions of anipamil, a long acting analog of verapamil. Initial dose-response studies for anipamil (0.25-6.0 mg/kg, i.v.) in pentobarbitone-anesthetized pigs (n = 4) were conducted to determine the effects of the drug on EKG and hemodynamic measures. In this initial study anipamil was found to produce a dose-dependent reduction in blood pressure, left-ventricular pressure and its derivative (dP/dtmax), cardiac output, and increase in heart rate. These results were used as a basis from which to choose doses for a second study to assess antiarrhythmic actions of anipamil against arrhythmias induced by regional myocardial ischemia. The antiarrhythmic effects of the two doses were compared with verapamil when the latter was given at a dose producing cardiovascular effects mid-way between those produced by the two doses of anipamil. Anesthetized pigs were randomly assigned to receive one of three drug treatments, or vehicle control, prior to occlusion of the left-anterior descending coronary artery. Antiarrhythmic effectiveness of low (1.0 mg/kg + 0.10 mg/kg/min infusion, n = 8) and high (5.0 mg/kg + 0.50 mg/kg/min infusion, n = 12) dose anipamil was compared to that of verapamil (0.5 mg/kg + 0.60 mg/kg/min infusion, n = 8) in a vehicle controlled study (n = 15). Arrhythmic events (VPB, VT and VF incidence) were monitored and grouped according to their time of occurrence after occlusion. Thus phase 1a arrhythmias occurred 0-5 min after initiation of occlusion, phase 1b, 5-30 min, and phase 2, 0.5-4 hr after occlusion. This study showed that during phase 1a there was a low incidence of arrhythmias in all groups except the one receiving 5 mg/kg anipamil where the group incidence of VT was 58% as compared to 20% in controls (n = 15). Most ventricular arrhythmias occurred in all groups during phase 1b. In this phase verapamil abolished VF and reduced VT, as compared with controls. Anipamil (high and low doses) tended to reduce VT but not VF. In the period 0.5 to 4 hours post occlusion (phase 2) all three drug treatments were associated with fewer arrhythmias but this only reached statistical significance with verapamil. Thus verapamil was more efficacious than anipamil at providing antiarrhythmic protection against both early and late onset arrhythmias. Anipamil may have been proarrhythmic in the early phase of arrhythmias and only moderately antiarrhythmic, if at all, in the later phase.(ABSTRACT TRUNCATED AT 400 WORDS)

Opisthotonos following propofol: a nonepileptic perspective and treatment strategy.

In this report of opisthotonos during recovery from propofol anaesthesia, we relate clinical observations with scientific considerations, and propose a strategy for treatment of this rare side effect. Following a brief operative procedure, a healthy 29-yr-old woman developed recurrent opisthotonos while recovering from anaesthesia with alfentanil, propofol, and nitrous oxide. In contrast to accumulating reports, the patient remained conscious during each episode of back extension and retrocollis. The preservation of consciousness and similarities to strychnine-induced opisthotonos suggest to us that the mechanism may have a brainstem and spinal origin. Recent investigations show that propofol potentiates the inhibitory transmitters glycine and gamma-aminobutyric acid (GABA) which would enhance spinal inhibition during anaesthesia. Postanaesthetic opisthotonos, however, may be due to a propofol-induced tolerance to inhibitory transmitters. This rebound phenomenon would lead to an acute, enduring refractoriness in inhibitory pathways of the brainstem and spinal cord, resulting in increased activity of extensor motoneurons. We recommend a therapeutic strategy that restores inhibition by glycine and GABA at multiple sites; the preferred therapeutic agents would be diazepam and physostigmine. The episodes are usually short-lived, but two of the reviewed 17 patients developed recurrent retrocollis for four and 23 days following antiepileptic drug therapy. Since high doses of phenytoin and carbamazepine can result in opisthotonos, we recommend that anticonvulsants be reserved for postanaesthetic patients with electroencephalographic evidence of seizure activity.

Isoflurane prevents transitions to tonic and burst firing modes in thalamic neurons.

We used patch-clamp whole-cell techniques to assess the effects of a volatile anaesthetic on thalamic firing modes in rat brain slices. Isoflurane application in clinical concentrations (0.5-2%) reversibly prevented voltage-dependent transitions to repetitive spike and burst firing modes in ventrobasal neurons. In voltage-clamp studies, isoflurane increased leak conductance, which shunted tonic and burst firing. Isoflurane also blocked the low-threshold Ca(2+)-current underlying the burst mode of firing, by increasing leak current and depressing membrane Ca(2+)-channel activity. We suggest that the mechanism of anaesthesia is distinct from sleep, although both states critically involve excitabilities of thalamic neurons.