B. Raymond Fink

Dynamics of colchicine effects on rapid axonal transport and axonal morphology

Abstract Concentration dependence and reversibility of inhibition of rapid axonal transport by colchicine were correlated with morphological changes in rabbit vagus nerve. Influx of [ 14 C]colchicine into the nerve was delayed by the presence of the nerve sheath, and inhibition of rapid transport took over twice as long as in desheathed nerves. Graded effects on transport of labeled protein were seen with 0.1–30 mM colchicine, associated with a progressive loss of microtubules. With 7.5 and 30 mM the number of neurofilaments increased. Inhibition of rapid transport was not reversed by washing for 3–20 h nor were the morphological effects. Conduction of the compound action potential was essentially unimpaired in all experiments. This contrasts with the effect of lidocaine, which blocks conduction as well as rapid transport reversibly and blocks rapid transport prior to affecting microtubule structure.

Lidocaine Inhibition of Rapid Axonal Transport

The effect of lidocaine hydrochloride on rapid axonal transport of protein in the rabbit vagus nerve has been studied in vitro. The study shows that 1) the rabbit vagus nerve, after labeling of its efferent nuclei by intracerebral injection of tritiated leucine, behaves in vitro as it does in vitro with regard to rapid axoplasmic transport; 2) 0.1–0.6 per cent lidocaine significantly depresses the rate of rapid transport of protein in these nerves; 3) the amount of inhibition is directly related to both concentration and length of treatment; 4) the inhibition of axoplasmic transport by 0.6 per cent lidocaine can be reversed by washing the nerves, if the treatment times are less than 60 minutes; 5) nerves treated with 0.6 per cent lidocaine for 90 minutes are almost completely lacking both axonal and Schwann cell microtubules.

Mechanisms of Differential Axial Blockade in Epidural and Subarachnoid Anesthesia

The mechanisms of persistent differential blocks that accompany subarachnoid and epidural anesthesia are clarified here with the aid of two principles derived from in vitro study of individual myelinated axons: 1) conduction can leap two consecutive blocked nodes but not three, and 2) a fiber length with more than three consecutive nodes bathed by weak anesthetic may block by decremental conduction, the requisite concentration varying inversely with the number of nodes bathed by anesthetic. Principle 1 applies in epidural blockade, where anesthetic bathes only a few millimeters of segmental nerve extradurally in the intervertebral foramen. Here, three-node block will be rare in large, long-internode fibers but likely in small, short internode fibers, thus explaining the differential retention of motor power in the presence of block of pain, which is achieved in epidural anesthesia when relatively weak solutions are used, as in obstetrics. Principle 2 may intervene in subarachnoid blockade where, cephalad to the site of puncture, increasingly concentrated anesthetic bathes increasing lengths of fibers in the craniocaudal succession of spinal nerve roots. This will produce decremental conduction block in increasingly long internode fibers in successive roots, reflected in a corresponding craniocaudal segmental sequence of blocked physiological functions: vasoconstriction, cutaneous temperature discrimination, pinprick pain sensibility, and skeletal motor activity. The segmental spatial differential sequence migrates with time but resembles the temporal differential sequence of loss seen at the onset of peripheral nerve blocks. Several other previously disparate clinical observations follow logically from the new interpretation.

The Teratogenicity of Halothane in the Rat

Rats were exposed to 0.8 per cent halothane for 12-hour periods at different stages of pregnancy. Fetuses near term were suitably prepared and the skeletons examined. Lumbar ribs and separation of normally single ossification centers into paired lateral centers in the lower thoracic vertebral bodies were common. Incidences of these anomalies were compared with those of a control series and found significantly higher following exposure to halothane on day 8 or day 91/2 (or day 10 for ribs only). Diurnal variation in the incidence of resorptions following halothane was noted, more fetal deaths occurring after exposure during the day than during the night. The anomalies found are similar to those seen following exposure to nitrous oxide.

Laryngeal Injury in a Dog Model of Prolonged Endotracheal Intubation

Using a dog model of prolonged translaryngeal intubation, the authors studied laryngeal injury. Segments of 10.7 mm diameter endotracheal tube were sutured in place in the larynges of anesthetized animals, and the animals were allowed to awaken and the tubes left in place for periods of 24 h to 84 days in a total of 13 dogs. Each animals larynx was studied endoscopically at weekly intervals and at the time of death. Both endoscopic and postmortem examination revealed erythematous laryngeal mucosa at 24 h and severe mucosal ulceration by 1 week. Microscopic examination revealed mucosal inflammation at 24 h with loss of mucosal architecture by 1 week. In several animals intubated 1 week or longer, inflammatory infiltrates were present in the arytenoid cartilage. While damage was generally severe by 1 week, it did not tend to become more severe after that time. Between week 1 and week 12, there was no significant correlation of the severity of laryngeal injury with the duration of endotracheal intubation. The results suggest that duration alone may not be a factor in laryngeal injury after the first week of intubation.

EFFECTS OF LIDOCAINE ON RABBIT BRAIN MICROTUBULAR PROTEIN

Microtubule repolymerization from a crude supernatant fraction prepared from rabbit brain was followed quantitatively by viscometry and electron microscopy. Lidocaine inhibits this repolymerization in a dose‐dependent fashion and the effect is completely reversible upon removal of the lidocaine by dialysis. Direct counting of microtubules by electron microscopy indicates that the local anesthetic decreases the number of tubules without significantly affecting their length. An association constant of lidocaine for tubulin is estimated at 7.5 mm. Procaine and etidocaine were also found to affect the polymerization of microtubules with results similar to those found with lidocaine.

The Position and the State of the Larynx during General Anesthesia and Muscle Paralysis

Based on a chance observation in two patients in whom the larynges could be visualized during direct laryngoscopy using topical anesthesia but not after general anesthesia and muscle paralysis, the authors postulated that there will be a shift in the position of the larynx with the onset of general anesthesia and muscle paralysis. To verify this the authors measured the position of larynx in lateral radiographs of necks taken in human volunteers when they were awake, and after induction of general anesthesia and muscle paralysis. The authors found that the hyoid bone and epiglottis were shifted anteriorly and the supraglottic region or the vestibule of the larynx was enlarged with the onset of general anesthesia and muscle paralysis. In addition, the larynx was also stretched longitudinally with wide separation of the vestibular and vocal folds. The authors conclude that consciousness is associated with tonic muscular activity that folds the larynx and partially closes it and that onset of general anesthesia and muscle paralysis opens the larynx wider and shifts it anteriorly, which might make visualization of the larynx during direct laryngoscopy difficult in some patients.

Differential slowing and block of conduction by lidocaine in individual afferent myelinated and unmyelinated axons.

The study of compound action potentials has not succeeded in determining exact limits to differential block of nerve fibers by local anesthetics. Further observations on individual neurons therefore were undertaken. Rabbit vagus nerve and ganglion were superfused in vitro at 37° C, pH 7.4. Activity evoked by stimulating the distal end of the nerve was monitored extracellularly from individual somata by a microelectrode in the ganglion. Lidocaine HCl in steps of 0.2–0.1 mmol/l (0.0005–0.0025 g/dl) was applied to the intervening nerve to identify two concentrations, respectively, just sparing and extinguishing conduction in the axon belonging to the soma, the average being regarded as the blocking concentration. The mean blocking concentrations (mM,± SD) for axons conducting at control velocities between 3 and 26 m/s (myelinated axons) was 0.43 ± 0.15, N = 18, for axons slower than 1.4 m/s (unmyelinated axons) 0.63 ± 0.14, N = 11, for intermediate velocity axons 0.19 ± 0.18, N = 7. The differences were significant by Scheffé‘s multiple comparisons test (P < 0.01). Although the myelinated axons were blocked by a lower average concentration of lidocaine than unmyelinated axons, they manifested significantly more slowing of conduction before block (P < 0.001). No relation between blocking concentration or latency increase and conduction velocity (fiber size) was evident within any fiber group.

Rapid axonal transport in trigeminal nerve of rat

The rat trigeminal nerve was investigated as a model system for axonal transport to identifiable sensory nerve endings. Following stereotaxic injection of [3-H]proline into the ganglion, the distribution of label in protein and other extracts of ligated or unligated infraorbital nerve was measured in replicates at various time intervals from 30 min to 3 days. In unligated nerves the average maximum velocity of somatofugal protein transport was 228 mm/day. Light microscope autoradiography demonstrated rapid intraaxonal transport of radioactive material to nerve endings in molar teeth and vibrissae. The system appears suitable for investigating the fate of axonally transported material at peripheral receptors.

Neurokinetics of lidocaine in the infraorbital nerve of the rat in vivo : relation to sensory block

The kinetics of neural uptake and efflux of lidocaine hydrochloride were studies by means of a standardized technique for blocking the intraorbital nerve of the rat, using a palatal jig. Following injection of 14-C-labeled local anesthetic, groups of ten animals were saccraficed at incipient recovery from sensory block or at othertimes. The nerves were weighed and assayed for radioactivity. The lengths of nerve containing high levels of lidocaine varied inversely with the times elapsed since onset of block. In experiments where a fixed quantity (2 mg) drug was injected, the incidence of block 2 hours later was concentrated-dependent, occuring in 80 per cent of animals after 2 per cent, in 40 per cent after 1 per cent, and in none after 0.5 per cent lidocaine. Epinephrine, 1:200,000, prolonged by 80 per cent the block effected with 0.2 ml of 1 per cent lidocaine. At the onset of recovery the neural contents of lidocaine at the sites of injection were 484 plus or minus 404 ng/mg of nerve in epinephrine-treated nerves, and 274 plus or minus 218 ng/mg in nonepinephrine-treated nerved (N.S., P greater than 0.05). Quantitative comparisons of in-vivo effectiveness of local anesthetic solutions can be made with this technique.