Blocking of cortical inhibitory synapses by intravenous lidocaine

Lidocaine-Associated CNS Toxicity at Therapeutic Dosage: A Case Report and Literature Review

Lidocaine is a commonly used anesthetic. High doses or intravenous administration of lidocaine, as well as other local anesthetics, may result in systemic effects involving the cardiovascular and neurologic systems. Typically, effects are dependent on the serum concentrations of the offending agent. This is a case where a patient presented with symptoms of systemic lidocaine toxicity despite therapeutic dosage and an undetectable serum lidocaine concentration. A 47-year-old Caucasian male received a lumbar medial branch injection of lidocaine and presented with symptoms of perioral numbness, seizure-like activity, and confusion. The patient had a workup, including a CT head without acute findings and an undetectable serum lidocaine level. Due to symptoms consistent with systemic local anesthetic toxicity, intravenous lipid emulsion (ILE) was administered with resolution and without recurrence of symptoms. There should be a low threshold of suspicion for local anesthetic systemic toxicity when patients have neurologic or cardiovascular symptoms following exposure. Toxicity may be present despite therapeutic dosages and low serum concentrations. ILE may be beneficial and should be considered.

Mechanisms and Efficacy of Intravenous Lipid Emulsion Treatment for Systemic Toxicity From Local Anesthetics

Local anesthetics are widely used clinically for perioperative analgesia to achieve comfort in medical treatment. However, when the concentration of local anesthetics in the blood exceeds the tolerance of the body, local anesthetic systemic toxicity (LAST) will occur. With the development and popularization of positioning technology under direct ultrasound, the risks and cases of LAST associated with direct entry of the anesthetic into the blood vessel have been reduced. Clinical occurrence of LAST usually presents as a series of severe toxic reactions such as myocardial depression, which is life-threatening. In addition to basic life support (airway management, advanced cardiac life support, etc.), intravenous lipid emulsion (ILE) has been introduced as a treatment option in recent years and has gradually become the first-line treatment for LAST. This review introduces the mechanisms of LAST and identifies the clinical symptoms displayed by the central nervous system and cardiovascular system. The paper features the multimodal mechanism of LAST reversal by ILE, describes research progress in the field, and identifies other anesthetics involved in the resuscitation process of LAST. Finally, the review presents key issues in lipid therapy. Although ILE has achieved notable success in the treatment of LAST, adverse reactions and contraindications also exist; therefore, ILE requires a high degree of attention during use. More in-depth research on the treatment mechanism of ILE, the resuscitation dosage and method of ILE, and the combined use with other resuscitation measures is needed to improve the efficacy and safety of clinical resuscitation after LAST in the future.

Local Anesthetic-Induced Central Nervous System Toxicity during Interscalene Brachial Plexus Block: A Case Series Study of Three Patients

Local anesthetics are commonly administered by nuchal infiltration to provide a temporary interscalene brachial plexus block (ISB) in a surgical setting. Although less commonly reported, local anesthetics can induce central nervous system toxicity. In this case study, we present three patients with acute central nervous system toxicity induced by local anesthetics applied during ISB with emphasis on neurological symptoms, key neuroradiological findings and functional outcome. Medical history, clinical and imaging findings, and outcome of three patients with local anesthetic-induced toxic left hemisphere syndrome during left ISB were analyzed. All patients were admitted to our neurological intensive care unit between November 2016 and September 2019. All three patients presented in poor clinical condition with impaired consciousness and left hemisphere syndrome. Electroencephalography revealed slow wave activity in the affected hemisphere of all patients. Seizure activity with progression to status epilepticus was observed in one patient. In two out of three patients, cortical FLAIR hyperintensities and restricted diffusion in the territory of the left internal carotid artery were observed in magnetic resonance imaging. Assessment of neurological severity scores revealed spontaneous partial reversibility of neurological symptoms. Local anesthetic-induced CNS toxicity during ISB can lead to severe neurological impairment and anatomically variable cerebral lesions.

Brainstem Anaesthesia after Retrobulbar Block

Regional anaesthesia techniques in ophthalmology are usually utilized for day case surgery. During various procedures, profound akinesia of the eye and anaesthesia of the surgical site are required, both of which are achieved with retrobulbar block. Due to the anatomy of the eye, life-threatening complications are possible. An 82-year-old female with secondary post-herpetic uveitic glaucoma of the right eye presented at the Department of Ophthalmology for an elective trans-scleral laser cyclophotocoagulation. She was given a retrobulbar block to the right eye with 2 mL of 0.5% levobupivacaine and 2 mL of 2% lidocaine. The procedure was technically performed without any issues. 2-3 minutes after the injection she became lethargic and 5 minutes later she lost consciousness and developed severe hypotension with bradycardia and respiratory arrest. She was successfully intubated and resuscitated, using mechanical ventilation, vasoactive medications, fluid therapy and intravenous lipid emulsion. There are three mechanisms for local anaesthetic (LA) to reach the central nervous system after a retrobulbar block: systemic absorption of LA, direct intra-arterial injection and retrograde flow into the cerebral circulation, and injecting LA into the subdural space via puncturing the dural optic nerve sheath, the latter being most common. The clinical picture of our patient was very consistent with subdural anaesthesia after exposure of the pons, midbrain and cranial nerves to LA, i.e. brainstem anaesthesia. Following appropriate life support measures taken in our case, there was a successful outcome. To minimize the chance for brainstem anaesthesia after retrobulbar block, we recommend low volume with low concentration of LA and block performance by an experienced ophthalmologist or anaesthesiologist with proper technique. Patients receiving retrobulbar anaesthesia should be carefully monitored at least 20 minutes after the block. Life support equipment should be available before performing retrobulbar block.

Pre- and perioperative aspects of dermatosurgery

Dermatosurgery occupies an exceptional position among all surgical disciplines. Above all, this includes the fact that, with very few exceptions, the vast majority of surgical interventions can be performed under local or regional anesthesia, usually in smaller procedure rooms that are spatially separated from larger operating suites. Thus, peri- and postinterventional patient monitoring is the responsibility of the dermatosurgeon and his team. Though inherently smaller, this team still has to observe numerous perioperative requirements that - in larger surgical specialties - would be attended to by a host of various specialists working in concert. Said requirements include hygienic aspects, knowledge concerning pre- and intraoperative patient monitoring, managing surgical site infections, adequate postsurgical pain management, as well as detailed pharmacological knowledge with respect to common local anesthetics and the toxic and allergic reactions associated therewith. Not only does this require interdisciplinary collaboration and shared responsibility for the patient. It also necessitates the development and implementation of quality-oriented and evidence-based guidelines that, in the dermatosurgical setting, usually extend far beyond the scope of the specialty per se. The objective of the present CME article is the condensed presentation of interdisciplinary aspects relating to the most important perioperative issues.

Prä- und perioperative Aspekte der Versorgung dermatochirurgischer Patienten

Die Dermatochirurgie nimmt hinsichtlich vieler Punkte eine Sonderstellung unter den operativen Fächern ein. Hierzu gehört in erster Linie die Tatsache, dass bis auf wenige Ausnahmen fast alle Eingriffe traditionell in Lokal- bzw. Regionalanästhesie und oft auch in räumlich-infrastruktureller Trennung von den großen Zentral-Operationssälen stattfinden können. Die peri- und postoperative Überwachung obliegt dabei dem dermatochirurgischen Operationsteam. Das sui generis kleinere OP-Team hat somit eine ganze Reihe perioperativer Notwendigkeiten zu beachten, um die sich in den "großen" chirurgischen Fächern eine Vielzahl verschiedener beteiligter Fachgruppen gemeinsam kümmern. Hierzu gehören neben Hygieneaspekten, Kenntnissen in der Überwachung der Patienten sowie dem Aspekt der surgical site infections auch Fragen zur postoperativen Schmerztherapie sowie detailliertes pharmakologisches Wissen über die zur Anwendung kommenden Lokalanästhetika und das Handling der damit assoziierten toxischen und allergischen Reaktionen. Eine interdisziplinäre Zusammenarbeit und Verantwortung für den Patienten ist notwendig und erfordert die Erarbeitung und Umsetzung qualitätsorientierter und evidenzbasierter Handlungsanweisungen, die im dermatochirurgischen OP-Setting meist weit über das eigentliche Fach hinausgehen. Ziel dieses Weiterbildungsartikels soll die komprimierte Darstellung der genannten fachübergreifenden Standpunkte bezüglich der wichtigsten perioperativen Aspekte sein.

Effectiveness of retigabine against levobupivacaine-induced central nervous system toxicity: a prospective, randomized animal study

PURPOSE

KCNQ2/3 channels play an important role in controlling neuronal excitability. Agents that decrease KCNQ2/3 current amplitudes are proconvulsant, whereas KCNQ2/3 current enhancers are anticonvulsant. Levobupivacaine is able to block the KCNQ2/3 channels and enhance neuronal excitation, whereas retigabine is able to reopen the channels and thus reduce overexcitation of neurons. In this study, we aimed to determine if retigabine is able to abolish local-anesthetic-induced seizures.

METHODS

Twenty New Zealand rabbits were randomly divided into two groups of ten. Levobupivacaine (0.5 %) was infused into conscious rabbits via the marginal ear vein at 8 ml/kg/h until the rabbits seized, and 5 mg/kg of retigabine were injected intravenously to terminate the seizure. The corresponding volume of saline was used as a control. The behavior of and the electroencephalogram (EEG) for each rabbit were continually monitored. Before levobupivacaine infusion, the rabbits were placed in a prostrate position calmly on the experimental platform, and the EEG pattern exhibited β waves. Intravenous levobupivacaine induced a typical EEG seizure characterized by multiple spike and slow wave complexes. The EEG changes were accompanied by behavioral convulsions which were characterized by clonic activity and opisthotonus.

RESULTS

Retigabine effectively terminated the electrographic and behavioral seizures. After receiving 5 mg/kg of retigabine, the animals became drowsy, and the EEG changed to δ waves.

CONCLUSIONS

We propose that KCNQ2/3 channels play an important role in levobupivacaine-induced central nervous system toxicity, and a KCNQ2/3 channel activator may be used to treat levobupivacaine-induced convulsions.

Implementation of Intraoperative Neurophysiological Monitoring during Endovascular Procedures in the Central Nervous System

BACKGROUND AND OBJECTIVE

Intraoperative monitoring (IOM) has been used in different surgical disciplines since the 1980s. Nonetheless, regular routine use of IOM in interventional neuroradiology units has only been reported in a few centers. The aim of this study is to report our experience, 1 year after deciding to implement standardized IOM during endovascular treatment of vascular abnormalities of the central nervous system.

METHODS

Basic recordings included somatosensory-evoked potentials (SEPs) and motor-evoked potentials (MEPs). Corticobulbar motor-evoked potentials and flash-visual-evoked potentials were also recorded depending on the topography of the lesion. Intra-arterial provocative tests (PTs) with amobarbital and lidocaine were also performed. All patients except 1 were under total intravenous anesthesia. Clinical outcome was assessed prospectively and correlated with IOM events.

RESULTS

Twelve patients and 15 procedures were monitored during the inclusion period. Significant IOM events were detected during 3 of the 15 procedures (20%). We observed temporary MEP changes in 2 cases which resolved after interruption of the embolization or application of corrective measures, leaving no postoperative neurological deficits. In 1 case, persistent SEP and MEP deterioration was detected secondary to a frontal hematoma, resulting in mild sensory-motor deficit in the right upper extremity after the procedure. Overall, 12 PTs (4 spinal cord and 8 brain abnormalities) were performed using lidocaine and sodium amytal injections. One positive result occurred after the injection of lidocaine. No false negatives were detected.

CONCLUSIONS

IOM may provide continuous real-time data about the functional status of eloquent areas and pathways of the central nervous system in patients under general anesthesia. It therefore allows us to detect early neurological damage in time to perform specific actions that may prevent irreversible neurological deficits.

[Update on the pharmacology and effects of local anesthetics]

Local anesthetics (LA) are broadly used in all disciplines and it could be considered that relatively little is reflected on the mechanisms of action of this old substance group. However, several molecular mechanisms of LAs mediating wanted and unwanted effects remain to be explored. Furthermore, the number of indications for application of LAs seems to be expanding. The local anesthetic effect of LAs is primarily mediated by a potent inhibition of voltage-gated sodium channels. However, this effect is due to much more than the interaction of LAs with one single molecule. Most recent studies indicated that the development of selective local anesthetics might be possible and LAs also interact with several other membrane molecules. Although the relevance of these effects is still unclear, they might play a role in systemic analgesia, tissue protection and anti-inflammatory effects of LA. The therapeutic index of systemically applied LA is very narrow. Systemic application is formally not permitted because the impending systemic toxicity is still a life-threatening complication. Although the cardiac and central nervous toxicity at least partly result from an unselective block of neuronal and cardiac sodium channels, preclinical studies suggest the involvement of several mechanisms. A local LA toxicity is less clinically impressive; however, all LAs induce a significant tissue toxicity for which the underlying mechanisms have been partly identified. This review reports on recent findings on mechanisms and on the clinical relevance of some LA-induced effects which are of relevance for anesthesiological activities.

Emulsified isoflurane increases convulsive thresholds of lidocaine and produces neural protection after convulsion in rats

BACKGROUND

Local anesthetic-induced convulsions remain a concern of anesthesiologists when performing regional anesthesia. Our previous study found that the lidocaine requirement for IV regional anesthesia was reduced with coadministration of emulsified isoflurane. We designed this study to examine whether emulsified isoflurane could increase the convulsive threshold of lidocaine and produce protection after a lidocaine-induced convulsion.

METHODS

In experiment 1, the median convulsive dose of lidocaine with or without the addition of emulsified isoflurane was determined using the up-and-down method. In experiment 2, emulsified isoflurane (0.032 mL/kg for isoflurane), midazolam (1.6 mg/kg), 30% Intralipid (solvent of emulsified isoflurane) or saline was infused to treat lidocaine-induced convulsions, respectively. Convulsive behavior was scored by the modified Racine scale. Cognitive function and the pathology of hippocampus cornu ammonis 3 pyramid neurons of rats were evaluated on days 1, 3, 5, and 7 after convulsions.

RESULTS

In experiment 1, the median convulsive dose of lidocaine alone producing convulsions was 18.7 ± 2.6 mg/kg, and it was increased to 22.7 ± 2.6 (P = 0.010) and 26.7 ± 2.6 mg/kg (P < 0.001) with coadministration of emulsified isoflurane at doses of 0.016 and 0.032 mL/kg isoflurane, respectively. In experiment 2, both emulsified isoflurane and midazolam significantly suppressed lidocaine-induced tonic-clonic seizures. Rats treated with emulsified isoflurane regained full consciousness (convulsive score = 0) significantly earlier than rats treated with midazolam (8.7 ± 2.4 vs 19.5 ± 3.9 minutes, P < 0.001). Cognitive impairment and hippocampus cornu ammonis 3 pyramid neuron abnormalities were found after convulsions and improved with the administration of both emulsified isoflurane and midazolam.

CONCLUSION

Emulsified isoflurane increased the convulsive threshold of lidocaine and preserved neurological function in rats experiencing lidocaine-induced convulsions.

Effects of neural synchrony on surface EEG

It has long been assumed that the surface electroencephalography (EEG) signal depends on both the amplitude and spatial synchronization of underlying neural activity, though isolating their respective contribution remains elusive. To address this, we made simultaneous surface EEG measurements along with intracortical recordings of local field potentials (LFPs) in the primary visual cortex of behaving nonhuman primates. We found that trial-by-trial fluctuations in EEG power could be explained by a linear combination of LFP power and interelectrode temporal synchrony. This effect was observed in both stimulus and stimulus-free conditions and was particularly strong in the gamma range (30-100 Hz). Subsequently, we used pharmacological manipulations to show that neural synchrony can produce a positively modulated EEG signal even when the LFP signal is negatively modulated. Taken together, our results demonstrate that neural synchrony can modulate EEG signals independently of amplitude changes in neural activity. This finding has strong implications for the interpretation of EEG in basic and clinical research, and helps reconcile EEG response discrepancies observed in different modalities (e.g., EEG vs. functional magnetic resonance imaging) and different spatial scales (e.g., EEG vs. intracranial EEG).

Effect of PaCO2 and PaO2 on lidocaine and articaine toxicity

Alterations in arterial PaCO₂ can influence local anesthetic toxicity. The objective of this study was to evaluate the effect of stress-induced changes in PaCO₂ and PaO₂ on the seizure threshold of lidocaine and articaine. Lidocaine (2% with 1 : 100,000 epinephrine) or articaine (4% with 1 : 100,000 epinephrine) was administered intravenously under rest or stress conditions to 36 rats separated into 4 groups. Propranolol and prazosin were administered preoperatively to minimize cardiovascular effects of epinephrine. Mean arterial pressure (MAP), heart rate (HR), and arterial pH, PaCO₂, and PaO₂ were measured. Results showed no differences in MAP, HR, or pH. Stress significantly increased the latency period for the first tonic-clonic seizure induced by a toxic dose of both lidocaine and articaine (P < .05). Seizures were brought on more rapidly by articaine. No significant difference between toxic doses of lidocaine and articaine was noted. Stress raised the seizure threshold dose for both drugs and significantly (P < .01) increased arterial PaO₂ from 94.0 ± 1.90 mm Hg to 113.0 ± 2.20 mm Hg, and reduced PaCO₂ from 36.0 ± 0.77 mm Hg to 27.0 ± 0.98 mm Hg. In conclusion, reduction in PaCO₂ and/or increase in PaO₂ raised the seizure threshold of lidocaine and articaine. This study also confirmed that lidocaine and articaine have equipotent central nervous system toxicity.

Depolarizing shift in the GABA-induced current reversal potential by lidocaine hydrochloride

Lidocaine hydrochloride (LC-HCl) is widely used as a local anesthetic, while various adverse effects of LC-HCl, such as seizures have also been reported. Lidocaine is reported to inhibit various channels and receptors including GABA(A) receptors. Although the GABA(A) receptor-mediated response depends on Cl(-) equilibrium potential (E(Cl)), little is known about the effect of LC-HCl on E(Cl). In the present study, we investigated the effect of LC-HCl on GABA-induced currents in cultured rat hippocampal neurons with gramicidin-perforated patch-clamp recording which is known to keep the intracellular Cl(-) concentration intact. LC-HCl inhibited outward GABA-induced currents with depolarizing shift of the GABA reversal potential (E(GABA)). The LC-HCl-induced positive E(GABA) shift was not observed with conventional whole-cell patch-clamp method which cannot retain intact intracellular Cl(-) concentration. The LC-HCl action on E(GABA) was inhibited by either furosemide, a blocker of both Na(+)-K(+)-Cl(-) cotransporter (NKCC) and K(+)-Cl(-) cotransporter (KCC), or an increase in extracellular K(+) concentrations. Neither bumetanide, a specific inhibitor of NKCC, nor Na(+)-free external solution had any effect on the LC-HCl-induced E(GABA) shift. QX-314, a membrane impermeable lidocaine derivative, failed to shift E(GABA) to positive potential. Furthermore, LC-HCl caused a depolarizing shift of E(GABA) in cultured GT1-7 cells expressing KCC2 but failed to change E(GABA) in GT1-7 cells without expression of KCC2. These results suggest that the LC-HCl-induced positive E(GABA) shift is due to a blockade of KCC2. Together with the direct LC-HCl action to GABA(A) receptors, the positive E(GABA) shift induced by LC-HCl reduces the GABAergic inhibition in the central nervous system.

Dibucaine toxicosis in a dog

INTRODUCTION

Dibucaine is a potent, long-lasting local anesthetic (LA). Topical dibucaine ointments are marketed directly to consumers in the USA without prescription. Dibucaine ointment is intended to treat discomfort associated with sunburn, eczema, minor rashes, minor scratches, insect bites, and poison ivy and is used alone or in combination with other active ingredients to treat pain associated with hemorrhoids or other anorectal disorders. Oral dibucaine toxicosis has been reported in children and includes gastrointestinal upset and neurologic and cardiovascular dysfunction.

CASE REPORT

An 18-month-old, female, Parson Russell terrier ingested approximately 23 g of 1% dibucaine ointment (approximately 38 mg/kg dibucaine) recommended to the owner for the treatment of hemorrhoids. Onset and resolution of clinical signs were relatively rapid, 5 min and 60 min, respectively. Clinical signs included vomiting, ptyalism, whole-body muscle fasciculations, disorientation, and severe ataxia.

DISCUSSION

Oral dibucaine toxicosis in dogs is similar to oral dibucaine toxicosis in children. Dibucaine ointment poses a real and potentially serious toxicological risk to pets and thus should be stored in a safe location.

Modulation of major voltage- and ligand-gated ion channels in cultured neurons of the rat inferior colliculus by lidocaine

AIM

The purpose of the present study was to explore how lidocaine as a therapeutic drug for tinnitus targets voltage- and ligand-gated ion channels and changes the excitability of central auditory neurons.

METHODS

Membrane currents mediated by major voltage- and ligand-gated channels were recorded from primary cultured neurons of the inferior colliculus (IC) in rats with whole-cell patch-clamp techniques in the presence and absence of lidocaine. The effects of lidocaine on the current-evoked firing of action potentials were also examined.

RESULTS

Lidocaine at 100 micromol/L significantly suppressed voltage-gated sodium currents, transient outward potassium currents, and the glycine-induced chloride currents to 87.66%+/-2.12%, 96.33%+/-0.35%, and 91.46%+/-2.69% of that of the control level, respectively. At 1 mmol/L, lidocaine further suppressed the 3 currents to 70.26%+/-4.69%, 62.80%+/-2.61%, and 89.11%+/-3.17% of that of the control level, respectively. However, lidocaine at concentrations lower than 1 mmol/L did not significantly affect GABA- or aspartate-induced currents. At a higher concentration (3 mmol/L), lidocaine slightly depressed the GABA-induced current to 87.70%+/-1.87% of that of the control level. Finally, lidocaine at 100 mumol/L was shown to significantly suppress the current-evoked firing of IC neurons to 58.62%+/-11.22% of that of the control level, indicating that lidocaine decreases neuronal excitability.

CONCLUSION

Although the action of lidocaine on the ion channels and receptors is complex and non-specific, it has an overall inhibitory effect on IC neurons at a clinically-relevant concentration, suggesting a central mechanism for lidocaine to suppress tinnitus.

Hippocampus and amygdala neurotoxicity produced by systemic lidocaine in adult rats

There is evidence that using lidocaine-treated cellular culture produces cell damage. However, there are no studies in vivo demonstrating the potential injurious effect of lidocaine on the central nervous system. Therefore, the aim of our study was to investigate if lidocaine is involved in neuronal damage in the CA3 hippocampus and amygdala regions when using a single subconvulsive or a convulsive lidocaine dose. Two-month-old male Wistar rats (57) were used. The animals were randomly assigned to one of three groups. Group I received 0.9% saline ip (n=9), group II received a single lidocaine dose of 60 mg/kg (n=18), and group III received 90 mg/kg ip (n=12). At day 2, 7, and 10 after the dosing, three to six rats per group were sacrificed. The brains of the rats were removed and were embedded in paraffin. Coronal cuts of 7 microm were made. Each brain section was stained with cresyl-eosin. We evaluated the number of normal and abnormal neurons in the hippocampal CA3 (pyramidal) and basolateral amygdala (large and medium neurons) regions in a 10,000 microm2 section. To explore an association between lidocaine-induced seizure and neuronal damage, diazepam was used (10 mg/kg ig) as an anticonvulsant two hours before a 90 mg/kg dose of lidocaine. Lidocaine causes a morphological neuronal alteration in the CA3 hippocampal region and the basolateral amygdala and possibly an inhibition-excitation imbalance. Diazepam prevents lidocaine-induced seizures, but not neuronal damage in brain structures. Interaction of lidocaine with the membrane components produces disrupted Ca+2 homeostasis and causes neuronal damage. Moreover, it is possible that lidocaine or its metabolites could actively participate in the neuronal damage observed.

Inhibition of serotonin transporters by cocaine and meprylcaine through 5-TH2C receptor stimulation facilitates their seizure activities

The present study examined whether the inhibition of serotonin transporters (SERT) contributes to cocaine- and other local anesthetics-induced convulsions, and which subtypes of 5-HT receptor are involved in the convulsions. For this purpose, cocaine, meprylcaine and lidocaine, all of which have different effects on SERT, were used as convulsants and the effects of serotonin reuptake inhibitors (SSRIs), specific agonists and antagonists for 5-HT receptor subtypes were evaluated in mice. Administration of SSRI, zimelidine, citalopram and fluoxetine, 5-HT(2A,2C) receptor agonist, R(-)-DOI and the 5-HT2C receptor agonists, mCPP, and MK212 resulted in a marked increase in incidence of convulsions and a reduction in the threshold of lidocaine-induced convulsions, while the 5-HT2B receptor agonist, BW723C86, had little influence. On the other hand, SSRI did not affect the measured parameters in meprylcaine- and cocaine-induced convulsions. R(-)-DOI, mCPP, and MK212 reduced the threshold of meprylcaine or cocaine with less extent than the reduction of lidocaine threshold. Incidence of cocaine- and meprylcaine-induced convulsions was significantly reduced by 5-HT(2A,2B,2C) antagonist, LY-53857, and 5-HT2C antagonist, RS 102221. The threshold of cocaine and meprylcaine was significantly increased by both antagonists. 5-HT2A antagonists MDL 11,939 and ketanserin, and 5-HT2B antagonist SB 204741 except at high doses had little effect on cocaine- and meprylcaine-induced convulsions. None of these antagonists altered the parameters of lidocaine-induced convulsions. Pretreatment with fluoxetine but not citalopram increased the plasma concentration of lidocaine. These results suggest that the increase of serotonergic neuronal activity through 5-HT2C receptor stimulation was responsible for increased activity of local anesthetics-induced convulsions and support the involvement of this mechanism in cocaine- and meprylcaine- but not in lidocaine-induced convulsions through their direct inhibitory action on central SERT.

Lidocaine Excites Both Pre- and Postsynaptic Neurons of Reconstructed Respiratory Pattern Generator in Lymnaea stagnalis

Lidocaine causes both inhibition and excitation in the central nervous system, including the respiratory pattern. The excitation induced by an excessive dose of local anesthetic is thought to be the result of an initial blockade of an inhibitory pathway in the cerebral cortex. To clarify the effect of lidocaine on the pre- and postsynaptic neurons of an inhibitory synapse, a cultured soma-soma respiratory pattern generator model consisting of two neurons from the snail Lymnaea stagnalis were reconstructed in vitro. First we investigated the effects of lidocaine on single presynaptic (RPeD1) or postsynaptic (VD4) neurons. While RPeD1 and VD4 were simultaneously recorded, the number of action potentials, the membrane potential, and the wavelength of the action potential were compared before and after lidocaine (0.01, 0.1, and 1 mM) administration. Lidocaine increased the number of action potentials and the wavelength of a single action potential, and it depolarized the resting membrane potential in both RPeD1 and VD4 neurons in a dose-dependent manner. Furthermore, lidocaine decreased outward potassium currents. In soma-soma pairs, RPeD1 excitation and VD4 suppression occurred in 0.01 mM lidocaine, whereas both RPeD1 and VD4 neurons were excited by 0.1 and 1 mM lidocaine. In conclusion, lidocaine causes a reduction in synaptic transmission and general neuronal excitation in both presynaptic and postsynaptic neurons.

Seizures After a Bier Block with Clonidine and Lidocaine

A 47-yr-old man with history of complex regional pain syndrome type 1 underwent an IV Bier block with a mixture of lidocaine and clonidine. The tourniquet was deflated after 60 min, and approximately 10 min later he presented with complex partial seizures. The possible mechanisms for this are discussed, and the effects of clonidine, lidocaine, and the mixture of both are reviewed, as are four additional published cases reporting seizures after the administration of clonidine.

Bupivacaine and ropivacaine suppress glycine- and glutamate-induced ion currents in acutely dissociated rat hippocampal neurons

Bupivacaine and ropivacaine are local surgical anesthetics with great efficacy in post-operative pain relief and labor analgesia. In the present study, the effects of bupivacaine and ropivacaine on ion currents induced by glycine and glutamate in acutely dissociated hippocampal CA1 neurons of rats were investigated via a nystatin-perforated patch clamping method at a clamped voltage. The magnitude of the glycine-induced ion currents was decreased reversibly and in a time-dependent manner by continuous application of 0.1 mg/ml of either bupivacaine or ropivacaine. The magnitude of the glutamate-induced ion currents was also suppressed time-dependently by continuous application of either bupivacaine or ropivacaine. The inhibitory action of bupivacaine and ropivacaine on currents induced by glycine and glutamate could be one of the mechanisms behind the actions of these anesthetics.

Chronic inhibition of the norepinephrine transporter in the brain participates in seizure sensitization to cocaine and local anesthetics

The involvement of chronic inhibition of monoamine transporters (MAT) in the brain with respect to sensitization to cocaine- and local anesthetic-induced seizures was studied in mice. Repeated administration of subconvulsive doses of meprylcaine as well as cocaine, both of which inhibit MAT, but not lidocaine, which does not inhibit MAT, increased seizure activity and produced sensitization to other local anesthetics. The effects of five daily treatments of monoamine transporter inhibitors on lidocaine-induced convulsions were examined 2 or 3 days after the last dose of the inhibitors. Daily treatments of GBR 12935, a specific inhibitor of dopamine uptake, significantly increased the incidence and the intensity of lidocaine-induced convulsions at 20 mg/kg and decreased the threshold of the convulsions. Daily treatments of desipramine and maprotiline, selective norepinephrine uptake inhibitors, markedly increased the incidence and intensity of lidocaine-induced convulsions, and decreased the threshold in a dose-dependent manner at between 5 and 20 mg/kg. Daily treatments of citalopram, a selective serotonin uptake inhibitor, at 10 and 20 mg/kg, produced no significant increase in the incidence or intensity of lidocaine-induced convulsions, but decreased the threshold of the convulsions. These results suggest that the chronic intermittent inhibition of monoamine uptake increases susceptibility to cocaine- and local anesthetic-induced seizures, and the norepinephrine transporter is an integral component of this sensitization.

Neuroprotective role of neurophysiological monitoring during endovascular procedures in the spinal cord

The endovascular treatment of spinal vascular malformations places the spinal cord at risk for ischemia. When these procedures are performed using general anesthesia, the neurophysiological monitoring methods currently available provide the only means by which to assess the functional integrity of sensory and motor pathways. Neurophysiological monitoring allows a warning for the neuroradiologist of impending irreversible neurological damage so that action may be taken for the prompt restoration of adequate spinal cord perfusion. Muscle motor evoked potentials (mMEPs) better reflect spinal cord perfusion in the anterior spinal artery territory than do somatosensory evoked potentials (SEPs), although their use during spinal endovascular procedures remains anecdotal in the literature. In the study reported here we assessed: (1) the feasibility of intraoperative neurophysiological monitoring, (2) the role of provocative tests with Amytal and Xylocaine, and (3) the specific but complementary role played by SEPs and mMEPs, during endovascular embolization of spinal vascular malformations and tumors. The results suggest that: (1) neurophysiological monitoring is feasible during most endovascular procedures in the spine and spinal cord under general anesthesia, (2) provocative tests enhance the safety of the procedure, (3) mMEPs are more feasible than SEPs and more sensitive than SEPs to provocative tests. We strongly suggest the use of multimodal neurophysiological monitoring and provocative tests during the endovascular treatment of spinal and spinal cord vascular lesions.

Prevention of Complications during Endovascular Surgery on the External Carotid Arteries, with Special Reference to Use of Nitropaste and the Lidocaine test

SUMMARY

PURPOSE

To examine all complications of endovascular procedures on the external carotid arteries and to ascertain how to reduce their number.

METHODS

In 93 consecutive patients, 137 endovascular procedures on the external carotid arteries were performed during the last 6 years.

RESULTS

We encountered no neurologic complications in these patients. In eight (9%) of the 93 patients who underwent lidocaine tests of the external carotid branches, nine transient cranial nerve palsies occured, seven of 67 vessels (10%) involving the middle meningeal artery and two of 18 vessels (11%) the ascending pharyngeal artery. However, they soon disappeared and successful embolisations were achieved. Vasospasm that prevented endovascular procedures occurred in only two cases of this series as a result of the application of isosorbide dinitrate.

CONCLUSIONS

The lidocaine test effectively provoked cranial nerve palsies without false negatives. The complication rate of endovascular surgery will decrease as long as awareness of complications such as cranial nerve palsy is maintained.

Unintentional hypotension from lidocaine infiltration during orthognathic surgery and general anaesthesia

BACKGROUND

A probable causal relationship between submucosal infiltration of lidocaine, with adrenaline 12.5 microg x mL(-1), and untoward arterial hypotension during general anaesthesia for orthognathic surgery was investigated in two parts.

METHODS

The first part of the study was open and non-randomised. The patients served as their own controls, receiving 5.4 mL saline/adrenaline before mandibular or maxillary surgery on the first side of the jaw and 5.4 mL lidocaine (2%)/adrenaline on the other side a minimum of 40 min later. Nine patients received in total 11 pairs of injections of the study drugs. In Part Two, 50 mg or 100 mg of lidocaine with adrenaline was infiltrated in an observer-blinded and randomised order in either side of the jaw in another 13 patients.

RESULTS

A significant and dose-dependent blood pressure decrease was observed after infiltration of lidocaine/adrenaline, but not after saline/adrenaline. The heart rate increased significantly after both types of injections, most pronounced after saline/adrenaline. Mean arterial blood pressure decreased on average 11% and 23% with lidocaine 50 mg and 100 mg, respectively (P<0.05, ANOVA).

CONCLUSION

Lidocaine induced dose-dependent arterial hypotension when combined with a defined level of general anaesthesia. The causative mechanism is unclear.

Role of Multimodality Intraoperative Neurophysiological Monitoring during Embolisation of a Spinal Cord Arteriovenous Malformation. A Paradigmatic Case

SUMMARY

The decision whether or not to embolise during endovascular procedures for arteriovenous malformations (AVMs) of the spinal cord under general anesthesia, relies primarily on neurophysiological results of provocative tests with Lidocaine and short-acting barbiturates. Because of the complex haemodynamics of spinal AVMs, when either sensory (CSEPs) or muscle motor evoked potentials (mMEPs) are used independently, they can mislead the interpretation of provocative tests. This report illustrates the specific but complementary role played by provocative tests using CSEPs and mMEPs during embolisation of a low thoracic spinal cord AVM. We present the case of a 46 year old male with six year history of right lower extremity weakness. At that time, Magnetic Resonance (MR) imaging of the spine disclosed an intramedullary AVM at T11. He remained neurologically stable up to seven months before admission, when he developed sudden onset of low back pain, followed by progressive paraparesis, numbness in lower extremities, urinary retention and fecal incontinence. A new MR imaging study indicated venous thrombosis of the AVM. A two-stage embolisation was performed. During the first procedure, after provocative tests did not affect either CSEPs or mMEPs, an embolisation was performed through a sulcocommisure feeder from the anterior spinal artery (ASA) at T9. Conversely, provocative tests with Lidocaine performed from a right posterior spinal artery (PSA) feeder to the AVM nidus resulted in a significant (> 50%) decrease of CSEPs, while mMEPs remained unchanged. The repeatedly positive tests warranted further investigation of the vascular anatomy which disclosed a normal right PSA distal to the nidus; the distal normal PSA was protected with coils. A repeated Lidocaine test was negative and the posterior feeder was embolised with no subsequent changes in CSEPs or mMEPs. After the procedure, the patient experienced only a mild transitory increase in right leg numbness, but no additional motor deficits. Five days later, the embolisation through the ASA feeder at T9 was completed on the basis of negative provocative tests. No additional neurological deficits were observed. Favoring either CSEPs or MEPs during endovascular procedures in the spinal cord is not justified by a solid scientific background. This case report illustrates that monitoring both CSEPs and mMEPs combined with provocative tests allows the safest and most effective embolisation of spinal cord AVMs under general anesthesia.

The alpha and gamma subunit-dependent effects of local anesthetics on recombinant GABA(A) receptors

Although convulsions due to local anesthetic systemic toxicity are thought to be due to inhibition of GABA(A) receptor-linked currents in the central nervous system, the mechanism of action remains unclear. We therefore examined the effects of local anesthetics on gamma-aminobutyric acid (GABA)-induced currents using recombinant GABA(A) receptors with specific combinations of subunits. Murine GABA(A) receptors were expressed by injection of cRNAs encoding each subunit into Xenopus oocytes. The effects of local anesthetics (lidocaine, bupivacaine, procaine and tetracaine) on GABA-induced currents of receptors expressing different subunit combinations (alpha1beta2, alpha1beta2gamma2s, alpha4beta2gamma2s and beta2) were examined via the two electrode voltage clamp method. At alpha1beta2, alpha1beta2gamma2s and alpha4beta2gamma2s GABA(A) receptors, all local anesthetics inhibited GABA-induced currents in a dose-dependent manner. The presence of the gamma2s subunit resulted in a greater inhibition by all local anesthetics, but the presence of the alpha4 subunit resulted in less inhibition. At beta2 homomeric receptors, local anesthetics directly induced an outward current similar to that of picrotoxin. These data indicated that (1) the alpha and gamma subunits of GABA(A) receptors modulated the inhibitory effects of local anesthetics on GABA(A) function, and (2) local anesthetics can activate the beta2 subunit and may block the GABA(A) receptor channel pore.

Lidocaine and seizures

Lidocaine has a concentration-dependent effect on seizures. At lower concentrations it has anticonvulsant properties, whereas concentrations above 15 microg/mL frequently result in seizures in laboratory animals and man. Seizures induced by lidocaine in experimental conditions invariably start in the amygdala. Despite the clear focal onset in these experimental models, the seizures emerging in patients given intravenous (i.v.) lidocaine are almost invariably generalized and without any clear signs of focality. Given the prevalence of partial seizures and the frequent use of lidocaine, a higher incidence of partial seizures would be expected with its use. Yet this is clearly not the case. These facts suggest that a history of partial seizures is not a major risk factor for the precipitation of partial seizures in patients treated with intravenous lidocaine.

Cocaine and lidocaine have additive inhibitory effects on the GABAA current of acutely dissociated hippocampal pyramidal neurons

Inhibition mediated by gamma-aminobutyric acid (GABA) is a major target for the central actions of cocaine and lidocaine, which can result in seizures, especially when these drugs are abused in combination. In the present study, we investigated how cocaine and lidocaine interact to depress GABA current (IGABA), recorded by the whole-cell technique in freshly isolated rat hippocampal neurons. Cocaine depressed IGABA in a concentration dependent manner, such that cocaine was more potent against lower than higher GABA concentrations: the cocaine IC50 was 0.13, 0.62 and 1.2 mM for GABA at 2, 10 and 100 microM, respectively. Cocaine depressed IGABA to the same extent in the absence and presence of 1 microM tetrodotoxin, indicating that cocaine inhibition of IGABA is distinct from its Na+ channel blocking action. Lidocaine reversibly depressed IGABA evoked by 10 microM GABA, with an IC50 of 9.8 mM. In the presence of 3 mM lidocaine, 0.3 mM cocaine depressed IGABA (10 microM GABA) to 30+/-7%. The significantly greater depression by the combined agents (p<0.05) indicates additive effects on the GABA receptor/channel complex, which are likely to contribute to the additive convulsant effects noted when these drugs are abused in combination.

Modulation of synaptosomal plasma membrane-bound enzyme activity through the perturbation of plasma membrane lipid structure by bupivacaine

We investigated modulations of lipid dynamics and lipid-protein interactions of rat brain synaptosomal plasma membrane (SPM) as one of the possible mechanisms by which the local anesthetic bupivacaine (BPV) has an adverse effect on nerve cell function, with SPM-bound enzyme activity used as a functional probe. The kinetics of BPV impact on the activity of the endoenzymes Ca2+/Mg2+-stimulated ATPase and Na+/K+-stimulated ATPase and the active concentrations of the drug were relevant to those that produce biphasic systemic toxicity. Arrhenius plots of these enzymes showed a transition temperature of 26.6 +/- 1.8 degrees C and 24.5 +/- 1.2 degrees C (mean +/- SD), respectively, in control SPM, which shifted to 17.1 +/- 0.95 degrees C (P < 0.01) and 18.2 +/- 0.85 degrees C (P < 0.05) in SPM treated with 10(-5) M BPV. The Hill coefficients for the allosteric inhibition of Ca2+/Mg2+-stimulated ATPase by Na+ and Na+/K+-stimulated ATPase by fluoride decreased from 1.73 +/- 0.20 and 1.95 +/- 0.25, respectively, in controls to 0.92 +/- 0.09 (P < 0.001) and 1.09 +/- 0.11 (P < 0.001) in the presence of 10(-5) M BPV. The fluidity perturbation in the microenvironment of the ectoenzyme acetylcholinesterase was observed only at 5 x 10(-3) M BPV, as confirmed by the disparity in transition temperature between the controls (22.3 +/- 1.2 degrees C) and the BPV-treated SPM (17.5 +/- 0.8 degrees C, P < 0.01) and that in the Hill coefficient in the two groups: 2.15 +/- 0.24 and 0.97 +/- 0.12 (P < 0.001), respectively.

IMPLICATIONS

We propose that under physiological conditions, the neutral and protonated forms of local anesthetics can affect nerve cell function through the asymmetric perturbation of the membrane lipid structure, accompanied by synaptosomal plasma membrane-bound enzyme dysfunction.

Cocaine depresses GABAA current of hippocampal neurons

Although blockade of dopamine re-uptake and the resulting elevation of excitatory agonists is commonly thought the primary mechanism of cocaine-induced seizures, it is possible that other neurotransmitters such as gamma-aminobutyric acid (GABA) are involved. To examine this possibility, the effects of cocaine on the whole cell GABA current (IGABA) of freshly isolated rat hippocampal neurons were investigated with the patch-clamp technique. Preincubation or acute application of cocaine reversibly suppressed IGABA. The IC50 was 127 microM when cocaine was applied before the application of GABA. The concentration-response relations of cocaine in various GABA concentrations revealed that cocaine inhibited IGABA non-competitively. This effect of cocaine appeared to be independent of voltage. The present study suggests that the GABA receptor/channel complex is also a target for cocaine's action. The suppression of IGABA may contribute to cocaine-induced seizures.

Prior brain injury protects death from local anaesthetic-induced convulsion

Minor brain injury was inflicted with a small hypodermic needle at four sites from the scalp 7 days before the production of convulsion by i.p. injection of 100 mg/kg lidocaine in mice. The latency to convulsion and survival rate were significantly longer and higher, respectively, in the brain-injured group than in the sham-operated one. Thus, the results suggest that a protective mechanism develops in the injured brain against asphyxia caused by lidocaine convulsion.

Decreased cocaine- and lidocaine-induced seizure response by dextromethorphan and DNQX in rat

The present study investigated the effect of dextromethorphan and 6,7-dinitroquinoxaline-2,3-dione (DNQX) pre-treatment on the development of cocaine- and lidocaine-induced seizures. The dopaminergic action of cocaine was also studied. The NMDA antagonist dextromethorphan and the non-NMDA (AMPA/kainate) antagonist DNQX both significantly decreased the intensity of the seizure response to intravenous convulsant doses of cocaine and lidocaine individually (20 mg/kg) and in combination (5 mg/kg each). The incidence of seizures in rats receiving cocaine or lidocaine individually was significantly reduced by pre-treatment with dextromethorphan but not DNQX. Haloperidol did not have an effect on the incidence or intensity of seizures induced by cocaine or lidocaine, alone or in combination. The results suggest that local anesthetic-induced convulsive seizures are mediated by excitatory glutamate transmission through both NMDA and non-NMDA receptor systems.

Local anaesthetics potentiate GABA-mediated Cl- currents by inhibiting GABA uptake

A two-electrode voltage clamp was used to examine the effects of the local anaesthetics (LAs) lidocaine, tetracaine and bupivacaine, and the meta-isomer of lidocaine (LL33), on the gamma-aminobutyric acid (GABA)-gated Cl- channel in the crayfish stretch receptor neurone. The voltage-induced current changes were recorded while exposing the neurone to GABA in saline containing different LAs. All LAs enhanced the voltage-induced Cl- current equivalent to a shift of the conductance vs GABA curve towards lower concentrations. The mechanism of the facilitation of GABA-induced conductance was found to be mainly a block of the GABA uptake in the stretch receptor neurone. Tetracaine, and to a lesser extent LL33, also seem to affect directly the GABA/receptor Cl-/channel complex resulting in an increased conductance of the channel.

Cocaine and lidocaine in combination are synergistic convulsants

The abuse of cocaine has dramatically increased in the recent decade. Cocaine obtained on the illegal market is rarely found in pure form. Most often it is adulterated with various substances, especially other local anesthetics. Lidocaine is one of the most common local anesthetics employed for adulteration of illicit cocaine. Toxicity due to the simultaneous ingestion of cocaine and lidocaine has been reported. Acute toxicity to cocaine and other local anesthetics is manifested in central nervous system aberrations, such as seizures and convulsions. This study investigated the convulsant potency of cocaine and lidocaine alone and in combination. Rats were administered intravenous injections of 5 mg/kg or 20 mg/kg of cocaine or lidocaine alone and in combination in equal proportion. Seizure activity and intensity were evaluated. The plasma concentration and brain content of each agent was also determined at the time of toxicity. The administration of 5 mg/kg of each drug alone did not yield seizure activity. However, the concomitant administration of 5 mg/kg of both cocaine and lidocaine produced a seizure response nearly equal to that produced after administration of 20 mg/kg of cocaine alone. Diazepam pre-treatment successfully antagonized the seizures induced by cocaine and lidocaine and raised the seizure threshold dose for the combination treatment by approximately four fold. The results suggest that cocaine and lidocaine interact synergistically to increase seizure activity and that the nature of this response occurs in part through a depression of inhibitory neuronal transmission.

Pharmacological analysis of local anaesthetic tolycaine-induced convulsions by modification of monoamines in rat brain

The effects of a local anaesthetic, tolycaine, on brain monoamine levels were investigated during the convulsive process in rats. The influence of central monoamine modifications on tolycaine-induced convulsions was also examined. Tolycaine (140 mg/kg, intraperitoneally) produced a significant elevation of noradrenaline and 5-hydroxytryptamine levels in all brain regions in the convulsive state from the levels in the non-convulsive state. Their levels returned to normal during the postconvulsive state. Dopamine levels were depleted in the cerebral cortex, the striatum, and the ponsmedulla oblongata during the convulsive process and increased in the cerebellum. Pretreatment with alpha-methyl-p-tyrosine, which depletes brain catecholamine, suppresses the tolycaine-induced convulsions, as shown by a decrease in the incidence; L-3,4-dihydroxyphenylalanine and bis-(1-methyl-4-homopiperazinyl-thiocarbonyl)-disulfide, which increase brain catecholamine, intensified the convulsions, as shown by shortening of the latency and increase in the mortality. Antagonists of beta-adrenergic and dopamine receptors, such as propranolol, chlorpromazine and pimozide, markedly suppressed the convulsions, but an antagonist of alpha-adrenergic receptor, phenoxybenzamine, had no effect. Furthermore, 5-hydroxytryptophan, which increases brain 5-hydroxytryptamine, suppressed the convulsions, and DL-p-chlorophenylalanine, which depletes brain 5-hydroxytryptamine, intensified them. Antagonists of 5-hydroxytryptamine receptor, methysergide and methiothepin, suppressed the convulsions. These results suggest that brain noradrenaline and 5-hydroxytryptamine are major regulators in the tolycaine-induced convulsive process and that central catecholaminergic neurones act in a stimulatory way on the tolycaine-induced convulsions, while serotonergic neurones act suppressively.

Effects of intrathecal local anesthetics on spinal excitability and on the development of autotomy

The effect of 500 micrograms intrathecal (i.t.) lidocaine and tocainide injected prior to unilateral sciatic nerve section on the development of autotomy behavior was examined in rats. Neither drug inhibited autotomy compared with saline controls. The effect of lidocaine and tocainide on the flexor reflex in decerebrate, spinalized rats was also examined. Both drugs at 500 micrograms i.t. totally blocked the flexor reflex and hyperexcitability following acute nerve section, with a longer duration of action with tocainide. Recovery from the drugs was followed by a prolonged period of reflex hyperexcitability, even without axotomy. Lower dose lidocaine (100 micrograms) briefly blocked the reflex and recovery was followed by reflex hyperexcitability. Very low dose i.t. lidocaine (1 and 10 micrograms) did not block the flexor reflex, but only induced a prolonged increase in reflex magnitude. The possible mechanisms underlying these observations are discussed. The results suggest that i.t. local anesthetic pretreatment alone may not be beneficial for the prevention of the development of postoperative or neuropathic pain.

Local anesthetics reduce the inhibitory neurotransmitter-induced current in dissociated hippocampal neurons of the rat

The effects of local anesthetics on amino acid-induced currents were examined using the whole-cell configuration of the patch clamp technique in dissociated hippocampal pyramidal neurons of the rat. Lidocaine (3 mM) decreased the glycine-induced Cl- current (Gly-ICl) more potently (to 46% of the control value) than the gamma-aminobutyric acid-induced Cl- current (GABA-ICl; to 75%), whereas the agent had little effect on the excitatory glutamate response. The reduction in the Gly-ICl was dose-dependent, with a dissociation constant (KD) of 3 mM and a Hill coefficient of 0.96. A non-competitive inhibition was suggested by a double reciprocal plot of the effects of lidocaine on the concentration-response curve of the Gly-ICl. Benzocaine, a neutral local anesthetic at physiological pH, decreased the Gly-ICl more potently than lidocaine, while QX314, a permanently charged quaternary derivative of lidocaine, produced a much smaller inhibition, thereby indicating that the neutral form of local anesthetics is more effective in reducing the Gly-ICl. The depression of the Gly-ICl and GABA-ICl in central neurons may contribute to local anesthetic-induced convulsions.

Inhibitory influence of frontal cortex on locus coeruleus neurons

The functional influence of the frontal cortex (FC) on the noradrenergic nucleus locus coeruleus (LC) was studied in the rat under ketamine anesthesia. The FC was inactivated by local infusion of lidocaine or ice-cold Ringer's solution while recording neuronal activity simultaneously in FC and LC. Lidocaine produced a transient increase in activity in FC, accompanied by a decrease in LC unit and multiunit activity. This was followed by a total inactivation of FC and a sustained increase in firing rate of LC neurons. Subsequent experiments revealed antidromic responses in the FC when stimulation was applied to the LC region. The antidromic responses in FC were found in a population of neurons (about 8%) restricted to the dorsomedial area, FR2. The results indicate that there is a strong inhibitory influence of FC on the tonic activity of LC neurons. The antidromic responses in FC to stimulation of the LC region suggest that this influence is locally mediated, perhaps through interneurons within the nucleus or neighboring the LC.