Hospital Medical School, W. 1 (Received 12 August 1946) mg. Knee jerk. intravenous. intravenous of semitendinosus muscle

Transcription

1 80 J. Physiol. (I947) io6, :6i ACTION OF INTRATHECALLY INJECTED ESERINE ON THE SPINAL CORD OF THE CAT BY I. CALMA AND SAMSON WRIGHT From the Department of Physiology, Middlesex Hospital Medical School, W. 1 (Received 12 August 1946) TABLE 1. Kremer 1942 Man Calma & 1944 Cat Wright Summary of literature on effects of eserine on the central nervous system General agreement on the central action of eserine has not yet been reached, as the results of different investigators have not been concordant. Table 1, which summarizes the literature, suggests that the animal species, the anaesthetic, the type of preparation, the route of administration, the dose of the drug and the level of the neuraxis affected by it, may modify, or even reverse, the action of eserine on the central nervous system. Author Year Specie. Preparation Dose and Route Reaction studied Schweitzer & 1937b Cat Chloralose mg. Knee jerk Wright intravenous Bonnet & 1937 Toad, Bremer Spinal Tog ug. Homolateral reflex twitch After-discharge + Bremer, Bonnet 1942 frog intravenous of semitendinosus muscle & Moldaver and after-discharges Merlis & 1939 Dog Na barbital or mg. Knee jerk + or - Lawson chloralose anaesthesia or intrathecal intravenous Flexor reflex + Chute, Feldberg 1940 Cat Perfused head, 1: 300,000- General reflex excitability + & Smyth almost completely isolated perfusion 1: 400,000 in Blinking reflex + fluid McKail, Obrador 1941 Cat mg. Flexor reflex + & Wilson intracarotid Bulbring & 1941 Dog mg. Knee jerk Burn intra-arterial Flexor reflex + Dial or urethane anaesthesia Chloralose anaesthesia. Isolated, perfused spinal cord Spinal block Decerebrate mg. intrathecal 1-4 mg. intravenous Results + potentiation - depression + (occasionally -) Reflex activity + (initial -) Muscle tone + (initial -) Deep reflexes + (initial -) Muscle tone of extensor + muscles

2 ESERINE ON SPINAL CORD 81 We have investigated the action of eserine injected intrathecally in cats. This method of administration was chosen to eliminate as far as possible the action of the drug on systems other than the central nervous system, and to obtain a high concentration of the drug on the structures under investigation, i.e. the whole of the neuraxis in the anaesthetized cat, the brain stem and spinal cord in the decerebrate cat, and the caudal part of the cord in the spinal animal. We have found that, under these conditions, eserine regularly increases the reflex (excitatory) responses of the extensor motoneurones of the spinal cord, but that its action is more variable on flexor reflexes. Some evidence of a central inhibitory action of eserine was also obtained. METHODS Experiments were performed on the animal. cat under chloralose anaesthesia and on the decerebrate Chioralosed cat The animals were anaesthetized with chloralose (0-08 g./kg. intravenously). The nerves to the muscles antagonistic to those reflexly stimulated were severed: in the case of the knee jerk and crossed-extensor reflex, the sciatic nerve high up in the thigh and, in many cases, the nerve to the sartorius muscle and the saphenous nerve were cut; in the case of the flexor reflex, the femoral nerve, the nerves to the hamstring muscles and often the tendon of the ileopsoas were divided. The knee jerk was elicited at regular intervals (every 9 or 10 sec.) by means of an electromagnetically operated tapper (Schweitzer & Wright, 1937a); the flexor and the crossed-extensor reflexes by single break-shocks applied at 9 or 10 sec. intervals to the central end of the cut tibial nerve through fluid electrodes of the Collison type. When both the knee jerk and the flexor reflex were elicited, the knee jerk was elicited first, and the stimulus for the flexor reflex was applied after an interval of sec. The knee jerk was recorded by connecting the ankle to a torsion lever myograph by means of a thread running over a pulley; the crossed-extensor reflex was recorded in the same way or else by connecting the patellar tendon directly to the myograph. The flexor reflex was recorded directly from the tendon of the tibialis anticus. Drills were passed into the upper and lower ends of the femur and tibia to fix the preparation. Eserine was injected into the theca through a blunt metal cannula inserted into the subarachnoid space at the level of the 7th-8th post-thoracic vertebra, i.e cm. below the lumbar enlargement. The spinal cord was exposed, care being taken not to damage the dura and the nerve roots. The dura was pierced with a sharp, large French needle. This was withdrawn and the blunt cannula inserted, tied in position with a thread passed externally round the dura and finally fixed with ligatures passed through the muscles of the back. Some animals were secondarily made spinal. Simple transection of the spinal cord at the required level was not employed, as that would have opened up the subarachnoid space and allowed the rapid escape of intrathecally injected eserine; instead, the spinal cord was tightly compressed to produce a physiological block df its long tracts. The spinal cord at the level of the proposed block was exposed, a thread was passed round it enveloping the meninges, and very firmly tied around the dura and spinal cord. We satisfied ourselves by the following criteria that the block produced in this way was complete: the cord inside the ligature appears crushed; in the decerebrate animal the hindlimbs become flaccid, while the forelimbs remain rigid and extended; the effects of injected eserine are limited entirely to the lower limbs. It may, therefore, be assumed that this type of block prevents impulses from passing up or down the spinal cord at the level of the ligature, and limits the local action of intrathecally injected eserine to the spinal cord caudal to the level of the ligature. PH. CVI. 6

3 82 I. CALMA AND S. WRIGHT The eserine sulphate or salicylate was dissolved in acid (ph 4-4-5) or in neutral saline. The volume of fluid injected varied, the range being 01-2 c.c. Atropine was not used. Control injections of similar volumes of acid and neutral saline were always carried out before and after administration of eserine, until the control effects were well established. Decerebrate cat Under ether-chloroform anaesthesia, the limbs were suitably prepared and, when desired, spinal block was produced as in the chloralosed preparation. The animal was then decerebrated at the level of the anterior colliculi, or occasionally more cranially. The cannula was inserted into the theca and the electrodes applied to the nerves to be stimulated. The preparation was rested for j-1 hr., so that the anaesthetic might be blown off and a steady state reached. Control experiments were carried out to see whether eserine, in the dose and by the route employed, modified muscular contractions by a peripheral action, i.e. whether the amount of eserine which might be absorbed was sufficient to influence the peripheral muscular responses. Two procedures were used: (a) simultaneous records of the reflexes and of the contractions of the gastrocnemius muscle stimulated through its motor nerve (Fig. 1 A-G, upper records); (b) ischaemic preparations in which access of the drug to the muscles of the limbs was prevented by depriving the limbs of their blood supply, according to the technique devised by Schweitzer & Wright (1937c). We obtained no evidence that any significant peripheral actions were produced. RESULTS Control experiments were carried out by intrathecally injecting either acid or neutral saline (0x2-2 c.c.), slowly or quickly, at room temperature or at 370 C. The effects thus produced on the activity of the spinal cord were irregular and transient; if less than 0 5 c.c. of fluid was injected slowly no significant changes occurred (Fig. 1 A). Chloralosed cat In such animals, especially when the spinal cord has been tied (and thus transected) at Th. 9, intrathecal injections of physiological saline may produce an immediate, small, and brief contraction of the innervated muscles of the hindlimbs; a small temporary increase of tone and of the reflex contraction under examination often also occurs. After about 30 sec., however, the reflexes return to their initial level or show a secondary depression. In some cases, after the intrathecal injection of large volumes of saline, e.g c.c. (and especially when the injection was made rapidly), we observed an immediate and very marked rise of blood pressure, which was sometimes 100 mm. Hg in extent. This increase in blood pressure may be accompanied by an increase in the heart rate and development of extra-systoles. Similar vascular changes occurred with similar frequency when eserine was injected. The characteristic effects of eserine on the reflexes were independent of the occurrence or absence of these vascular changes. The responses obtained are independent of the reaction of the fluid employed, and are therefore mechanically produced. These effects are considered fully in another paper (Calma & Wright, 1947).

4 ESERINE ON SPINAL CORD Decerebrate cat In the decerebrate cat with spinal cord intact, the mechanical effects of intrathecal injection of large volumes of saline are more regularly observed (Fig. 2). In decerebrate preparations made spinal, intrathecal saline injections of the order of c.c., given rapidly, cause larger changes in tone and reflexes, as might be expected if rise of intrathecal pressure were the causal factor. Thus, in one experiment, the tone of the tibiabls anticus was increased, the flexor reflex was almost doubled in extent, and the knee jerk after a short delay was temporarily abolished; after a longer delay the flexor reflex was depressed for some time and the small crossed-extensor reflex disappeared. When the fluid is injected slowly and its volume does not exceed 0 5 c.c., the mechanical effects are much reduced or are absent. With saline injections, howevei, the preparation, after 1 min. or so, settles down to a steady state, approximating to that present before the injection, or showing some lasting depression. In view of these results, changes occurring during the first minute after the intrathecal injection of eserine have not been regarded as significant. Following the injection of eserine, as will be explained below, after a variable further latent period, characteristic changes make their appearance, and they can be distinguished from the immediate mechanical effects by the intervening 'normal' period, and also because changes of the type noted do not occur after saline injections. Observations on the knee jerk Preparations with spinal cord intact Effects of eserine in the chloralosed cat. Eserine salicylate or sulphate injected intrathecally in doses of ,ug. has no effect on the size or shape of the knee jerk in most preparations and does not produce 'spontaneous' discharges. In a few instances, repeated injections of small doses of eserine ( pg.) at intervals of 3-5 min., prolonged the relaxation phase of the knee jerk without any change in the peak tension of the reflex. Doses of eserine of 0-2 mg. or more produced regularly, after the initial mechanical effects (if any) had passed away, the following effects on the knee jerk: increased tension of the reflex contraction, alterations in the shape and duration of the relaxation curve, increase of the initial level of tension in the quadriceps muscle, irradiation of the reflex response to neighbouring muscles. The characteristic effects set in after a latent period of about 2-21 min. from the onset of the injection, which was usually rapidly completed, i.e. in 2-5 sec. They develop gradually and reach their maximum intensity after 3-5 min. The total duration of the changes varies widely in different preparations: the maximum duration observed was 20 min., the shortest 5 min. The effects wear off gradually

5 84 I. CALMA AND S. WRIGHT The maximal increase in the tension of the knee jerk was as much as fourfold in some preparations; in others the increase was only 30 %. There seems to be Fig. 1. Cat. 2-4 kg. Chloralose. In each section of the tracing, upper record is contraction of left gastrocnemius stimulated through its motor nerve; lower record is right knee jerk elicited by taps on right patellar tendon. The parts of the tracing taken on a slow drum are underlined, and the intervals between the individual contractions represent 10 sec. Otherwise the fast drum was employed, the time scale for which is given in D. A. Control; at arrow inject 05 c.c. of saline intrathecally. B. At arrow inject 0*3 mg. of eserine sulphate in 0 5 c.c. saline intrathecally. C, D, E, F, G were recorded 5, 11, 12, 14 and 27 min. respectively after the injection of eserine. no relationship between the increase in tension and the increase of the aftereffects as depicted by the character of the relaxation curve; in some instances the predominant effect was on the peak tension, with only a modest increase in the after-effects, while in other cases the condition was reversed. Fig. 1 (lower

6 ESERINE ON SPINAL CORD 85 records) illustrates some of the points to which reference has been made. After 5 min. (Fig. I C) the amplitude of the knee jerk has increased slightly; relaxation is followed by one secondary contraction. After 11 min. (Fig. 1 D), the tension of the contraction is doubled, and its duration is prolonged; relaxation is initially not complete, but is interrupted by a secondary contraction which is larger than in Fig. 1 C. After 12 min. (Fig. 1 E) this secondary contraction lasts still longer and the beginning of a further after-contraction is visible. In Fig. 1 F (14 min.) two very well-developed after-contractions are seen. In Fig. 1 G (27 min.) the effects have to a large extent passed away. The changes observed are not due to a peripheral action of absorbed eserine on the muscles. The gastrocnemius muscle stimulated through its motor nerve shows no changes whatever (upper records Fig. 1 A-G). Results similar to those described have been obtained in 'ischaemic' preparations in which the blood supply to the hindlimbs had been temporarily cut off. As eserine in these concentrations has no effect on peripheral nerves, the results can be confidently attributed to an action of eserine on elements in the spinal cord. The increase in the tension of the knee jerk must be due either to an increase in the frequency of the discharge of the affected motoneurones, or to recruitment of additional motoneurones, or to both changes occurring together. In either case they indicate increased excitability ofthe spinal cord, e.g., additional delay paths being opened up, and/or regions of subliminal stimulation becoming liminally excited. The after-effects (delayed relaxation, secondary contractions) must be ascribed to after-discharge, in the sense that they occur after the afferent impulses set up by the patellar tap and travelling along the direct route to the motoneurones have died down. We have not determined how much of the after-effects is due to the 'back-lash' from the contracting quadriceps itself and how much is due to a greater true central after-discharge, independent of such fresh afferent impulses. Whatever the intimate nature of the mechanism, the duration of the discharge of the motoneurones is greatly increased. Bonnet & Bremer (1937) showed that eserine in the frog and toad increased the afterdischarge of the semitendinosus muscle after ipsilateral stimulation, even when the initial contraction was unaffected or depressed, and after section of posterior nerve roots containing the proprioceptive afferents from the muscle. Irradiation of the reflex to adjacent muscles was not graphically recorded, but was frequently noted by us in the muscles of the anterior abdominal wall of one or both sides. General convulsions (as described by Schweitzer & Wright (1937b) following intravenous injection of eserine) were not observed. Effects of eserine in the decerebrate cat. In a previous paper (Calma & Wright, 1944) it was shown that intravenously injected eserine in doses of 1-4 mg. markedly increases the tone of the extensor muscles in the decerebrate cat. In this series of experiments, in which eserine was injected intrathecally,

7 86 I. CALMA AND S. WRIGHT similar results were obtained (Fig. 2). The latent period is similar to that of the chloralosed preparation, i.e. about 2 min. In Fig. 2, after the initial transient changes of mechanical origin, each successive individual knee jerk is followed by a well-marked shortening reaction. The resting tone of the quadriceps muscle between stimuli thus rises in a step-like manner, while the knee jerks become progressively smaller. The new base line between any two consecutive knee jerks runs absolutely level. After 130 sec. the shortening reaction develops at the peak of the knee jerk, so that the maximal tension obtained during the jerk is maintained until the next reflex is produced. Ultimately a Fig. 2. Cat. 4-8 kg. Decerebrate. Records from above downwards: knee jerk, arterial blood pressure. At arrow inject mg. of eserine sulphate intrathecally. The injection is followed by a small initial-transient increase of the knee jerk and quadriceps tone (which is an effect of the volume of fluid injected) and is followed by the specific eserine effect. plateau develops which shows only slight irregularities and is maintained at a fairly steady level for 14 min. and then begins to decline slowly. Intermission of one 'or several stimuli to the patellar tendon did not significantly alter the character of the curve at this stage. The changes described seem to indicate progressive irradiation of the reflex maintaining quadriceps.tone, together with increasing occlusion of the knee jerk. Ultimately, reflex tone alone activates a larger pool of extensor motoneurones than came originally under excitation from the combined effects of reflex muscle tone and knee jerk. Another type of reaction was observed in so-me preparations. Before the injection of eserine, the base line on whioh the knee jerk was inscribed, runs horizontally, without any change of level; 2-3 min. after administration of eserine the base line shows irregular and unpredictable waves on which smaller knee jerks are inscribed. In other words the muscular tone after eserine shows temporary increases which were absent before eserine, accompanied by some degree of occlusion of the knee jerk. In all the decerebrate preparations with

8 ESERINE ON SPINAL CORD spinal cord intact, the predominant features of the response, namely the increase in tone and the associated disappearance of the knee jerk, do not allow an analysis either of the shape of the knee jerk or of the after-discharges. Preparations with spinal block Effects of eserine in the chloralosed cat. Eserine, injected intrathecally into preparations with spinal block at Th. 9, has the same action as in preparations with the spinal cord intact. In order to localize more exactly the site of action of eserine, we performed a few experiments in cats with spinal block at Th. 13, L 1 and L 2. In four out of five experiments eserine produced no effect on the knee jerk (or on the blood pressure). The 'immediate' reflex centre must have been intact because the knee jerk could still be elicited. Spinal shock, which might have been produced by the manipulations and by the actual ligature used to produce the block, must be considered; it is well known, however, that spinal shock does not occur readily in cats and is, in any event, a quite temporary condition. In two decerebrate animals made spinal at the level of L1, when larger doses of eserine were used (0.6 mg.) some degree of potentiation did occur. The experiments suggest that some degree of 'long circuiting' is necessary for eserine to have its characteristic effects on the knee jerk, or at any rate for it to facilitate such responses (Fig. 3). Effects of eserine in the decerebrate cat. Eserine injected intrathecally in doses of 0-3-0O65 mg. after a latent period of 2-3 min. (and 1-2 min. after the initial mechanical effects of the injection have subsided) causes the knee jerk to be modified in the following way (Fig. 3): the tension of the reflex contraction of the quadriceps muscle is increased in relation to the pre-injection level, or to the level at which the knee jerk settles down when the mechanical effects have passed away. This effect of eserine develops gradually, reaches its maximum intensity 5-15 min. after the injection and may last for over 15 min. The extent of the increase varies: in some preparations the tension of the knee jerk is doubled, whereas in others there is a threefold or fourfold increase. The relaxation curve of the knee jerks shows striking changes: it may be followed by small after-contractions (Fig. 3 B), which increase in number and may summate to attain a considerable level of tension which declines very gradually (Fig. 3 C); the relaxation may be interrupted by further short bursts of motor activity (Fig. 3 D). On the other hand, the greatly enhanced knee jerk contraction may be followed by an abrupt fall in tension to well below the initial base line (Fig. 3 E). This after-fall lasts for periods of sec.; the curve then climbs to or above the base line (Fig. 3 E), and finally declines in the manner already described. Irregular quick or slow oscillations of the level of resting muscle tone may occur even when the taps on the patellar tendon are suspended. 87

9 88 I. CALMA AND S. WRIGHT These fluctuations in tone do not (for the reasons already given) represent fibrillation set up by a peripheral action of eserine which has been absorbed into the general circulation; they do not occur in the flexor muscles. Spontaneous movements, which might be called fibrillation, have never been seen in these experiments in muscles with -their motor nerves cut. Fig. 3. Cat. 2-6 kg. Decerebrate. Spinal block at Th. 9. Knee jerk. Time scale is shown in A (fast drum). A. Before eserine. B, C, D and E were recorded 5, 7, 9 and 15 min. respectively, after the injection of eserine. The abrupt after-fall which so commonly followed the main jerk must represent a period of central inhibition of the 'resting' reflex tone of the quadriceps muscle. Fulton & Pi-Sunier (1928) and Denny-Brown (1928) showed in action current studies that a temporary arrest of central discharge occurs after the production of the knee jerk in the decerebrate cat. Denny- Brown demonstrated that this 'silent period' was due to central inhibition reflexly set up by impulses originating in the quadriceps muscle during its contraction. Sherrington (1907) has shown that some of the afferent impulses set up by a contracting extensor muscle produce central inhibition of the motoneurone pool of that muscle. The after-fall in the experiments with eserine presumably represents a well-developed 'silent period', which might be due to a greater inhibitory volley set up by the muscle, because of its greater

10 ESERINE ON SPINAL CORD tension, or to a potentiation (by the drug) of the central inhibitory effect. The observation that the after-fall may be well developed in experiments in which the enhancement of the jerk itself is small supports the latter view. Jar reflex In some decerebrate preparations made spinal, simultaneously with the knee jerk, a contraction of the'contralateral tibialis anticus muscle was elicited during the control period. Sherrington (1898) described this reaction in decerebrate animals made spinal and showed that it was reflex in character. The sequence of events that brings about this reflex is, according to Sherrington, as follows: the tapping of the patellar tendon (say right knee) which causes the knee jerk, sets up vibrations, which are transmitted through the bones to the left leg. These vibrations stimulate the afferent nerve fibres in the posterior nerve roots corresponding with the left tibialis anticus muscle near or at the ganglion and so set up a reflex contraction. Accordingly, this 'jar reflex', as Sherrington calls it, is abolished by section of the posterior roots of the left side but not by longitudinal section of the spinal cord along the midline. In our experience the jar reflex has been present only when the tap applied to the patellar tendon was unusually vigorous. If this reflex is initially present, it is characteristically potentiated by the intrathecal injection of eserine. Thus in one experiment the jar reflex in the control period was small and appeared irregularly. Six min. after injection of eserine, the reflex began to appear quite regularly, but was still unchanged in tension. Subsequently, the response was markedly enhanced or showed a double contraction, i.e. a small initial contraction followed'by the main contraction. The effect finally passed away. Observations on the crossed-extensor reflex In the chloralosed cat, with spinal cord intact or transected, break-shocks, even when maximal, did not, in our experience, elicit a crossed-extensor reflex. After eserine, the crossed-extensor reflex gradually made its appearance. Initially it tended to be small and pendular in character and later showed a larger initial contraction followed by smaller secondary contractions with a gradual return to base line. The potentiation of the crossed-extensor reflex by eserine is more strikingly shown in the decerebrate animal. The results depend on the method of recording employed, i.e. whether the tension is recorded directly from the patellar tendon or whether recording takes place from the ankle. Fig. 4 is from a decerebrate animal made spinal in which direct recording was employed. The crossed-extensor reflex (lower records) is initially small with a smooth, slightly delayed relaxation phase. Following the intrathecal injection of eserine, there were the usual transient mechanical disturbances. After 2 min. (Fig. 4 B), the reflex contraction was enhanced, and the relaxation phase showed changes 89

11 90 I. CALMA AND S. WRIGHT very similar to those already described for the knee jerk. Thus the initial relaxation was incomplete and was interrupted by secondary contractions. Fig. 4. Cat. 1 9 kg. Decerebrate. Spinal block at Th. 9. In each tracing the upper record is the flexor reflex recorded from left tibialis anticus; the lower record is the crossed-extensor reflex recorded from right quadriceps tendon. Time scale is shown in A (fast drum). A. Before eserine. Between A and B inject 0-3 mg. of eserine intrathecally. B, C, D and E were recorded 2, 4, 7 and 14 min. respectively after the injection of eserine. Subsequently the initial relaxation was very slight in extent, the tension curve maintained a plateau level and then declined slowly and irregularly (Fig. 4 D). In the intervals between reflexes 'spontaneous' contractions may occur. These phenomena of the relaxation phase are to be regarded as manifestations of

12 ESERINB ON SPINAL CORD 91 different patterns of after-discharge. In Fig. 4 E (after 14 min.) though the initial reflex contraction is less potentiated, the relaxation is still prolonged and irregular. When recording takes place from the ankle (Fig. 5), after the potentiated initial reflex contraction there is frequently (as in the case of the knee jerk) an abrupt relaxation of the quadriceps muscle with an after-fall below the base line. The tension subsequently mounts and is maintained steadily or with Fig. 5. Cat. 2 8 kg. Decerebrate. Spinal block at Th. 9. In each tracing the upper record is the flexor reflex recorded from left tibialis anticus; the lower record is the crossed-extensor reflex recorded from right ankle by thread running over pulley. Time scale is shown in B. A. Before eserine. Note conspicuous after-discharges of flexor reflex. B. 15 min. after injection of eserine. irregular oscillations until an approximate return to the base line occurs (Fig. 5 B). The after-fall presumably represents a 'silent period' and can be accounted for in the same way as the after-fall of the knee jerk. The main changes in the crossed-extensor reflex represent irradiation and/or increased discharge rates of the motoneurones and prolonged after-discharge, associated sometimes with apparent intensification of some central inhibitory effects. Observations on the flexor reflex We have had considerable experience of the effects of intrathecally injected eserine on the flexor reflex. The outstanding feature of the results is their

13 92 1. CALMA AND S. WRIGHT variability, both in different experiments and in the course of the same experiment. In Fig. 4 the flexor reflex was enhanced. At times, though more rarely, the reflex was diminished in amplitude with decrease in the after-discharge (Fig. 5 A, B) or tended to wax and wane above and below the initial value during the course of the eserine effect. This variability of the flexor reflex was in clear contrast with the regularity of the eserine effect on extensor tone, knee jerk, crossed-extensor reflex and jar reflex. DISCUSSION The effects of intrathecally injected eserine on spinal reflexes do not -develop more rapidly than after intravenous injection. The effective intravenous dose in the chloralosed cat is mg., the drug being diluted by the whole blood volume (about 200 c.c.) and to an unknown extent by the tissue fluids before it reaches the neuraxis. The effective intrathecal dose is mg. diluted by say 2 c.c. of cerebro-spinal fluid. If we assume that equal central effects represent the results of approximately equal concentrations of eserine in the neuraxis, then it would seem that the uptake of the drug by the neuraxis takes place much more readily from the blood than from the subarachnoid space. Difficulty in. penetration through the white matter of the spinal cord may be the major source of the delay following intrathecal injection, as presumably eserine has to reach the grey matter to produce its effects. The effects of intrathecal eserine are limited to the distal part of the neuraxis, indicating that diffusion cranially along the subarachnoid space takes place very gradually, if at all. Kremer (1942) found in man that intrathecally injected prostigmine takes hours to progress from the lumbar to the cervical region of the spinal cord. The evidence reported here demonstrates that eserine acts directly on the central nervous system enhancing excitatory reflexes; both the immediate reaction to afferent stimulation and the after-discharge are potentiated. The drug produces its effect on crossed reflexes (crossed-extensor) and on ipsilateral reflexes (extensor tone, knee jerk and jar reflex); on reflexes employing a minimum of two neurones (knee jerk) or of three neurones (flexor reflex (irregularly)) or on multi-neuronal reflexes (crossed-extensor); on flexor responses (flexor reflex (irregularly), and jar reflex) and on extensor responses (knee jerk, crossed-extensor reflex and extensor tone). After the completion of this work a recent paper by Wikler (1945) came into our hands. Wikler recorded the reflex efferent discharge in the ventral roots of L7, S1 in response to single shocks (supramaximal for A fibres) applied to various afferent nerves in cats made spinal at the first cervical segment and kept under artificial respiration and continuous ether anaesthesia. Under these conditions eserine injected intravenously (0.25 mg./kg.) enhanced the reflex discharge of two neurone arcs, while the multineurone arcs were little

14 ESERINE ON SPINAL CORD 93 affected. Morphine in preparations under nembutal was found to enhance two neurone reflex arcs and depress multineurone arcs. Eserine administered after morphine potentiated the responses from both the two neurone and the multineurone arcs. These results are in general agreement with those reported above and again emphasize the extent to which the precise experimental conditions influence the responses of the central nervous system to the effects of eserine. The evidence reported in this paper also suggests that eserine may enhance certain central inhibitory reactions (after-fall of the knee jerk and of the crossed-extensor reflex). These last findings merit further careful consideration, in view of the previous evidence of central inhibition produced by eserine: occasional inhibition of the knee jerk in the chloralosed cat (Schweitzer & Wright, 1937 b, c), and in man (Kremer, 1942), occasional inhibition of knee jerk in the dog under barbital or chloralose (Merlis & Lawson, 1939), and regular inhibition of the knee jerk in the dog with isolated perfused spinal cord (Biilbring & Burn, 1941). Our own results with the flexor reflex were (as already indicated) quite irregular, both potentiation and depression being produced in different animals or at different times in the course of the same experiment. Schweitzer, Stedman & Wright (1939) pointed out that a tertiary ammonium compound like eserine sulphate would dissociate in water into water-soluble (lipid-insoluble) ions and free base (which is lipid-soluble). They suggested that the former might be unable to penetrate the cell membrane and enter the cell, while the free base being lipid-soluble might penetrate the cell. They demonstrated that the quaternary ammonium anti-cholinesterases which give rise on dissociation only to water-soluble, lipid-insoluble ions were purely central depressant. The tertiary ammonium compounds were predominantly convulsant but tended to have a mixed central effect. The experimental evidence that has been published in recent years (including that recorded in this paper) indicates that the central inhibitory effects of eserine are of fairly common occurrence. Eserine may thus be supposed to have a dual central action, the central excitatory action generally predominating, but the inhibitory effect sometimes notably modifying or annulling it. The study of the central action of eserine on 'pure' inhibitory reflexes may help to clarify the situation. SUMMARY 1. The action of intrathecally injected eserine on the knee jerk, flexor reflex, crossed-extensor reflex, jar reflex and on muscle tone has been studied in the chloralosed and decerebrate cat. 2. Eserine increases the following reflex responses: knee jerk, crossedextensor reflex, jar reflex. It increases and prolongs the after-discharges of all

15 94 I. CALMA AND S. WRIGHT these reflexes. Irradiation, facilitation and occlusion phenomena occur during the action of eserine. 3. Eserine has a variable effect on the flexor reflex, potentiating it, inhibiting it or not modifying it at all. 4. Evidence is presented of some inhibitory central effects produced or intensified by eserine, superimposed on and modifying the predominantly potentiating action of the drug. This work was first submitted for publication on 5 September 1945, but the MS. and tracings were destroyed in a train accident after being received at the Editorial Office. The essential experiments were repeated and the original results confirmed in all important particulars. The expenses of this research were partially defrayed from a grant from the Thomas Smythe Hughes Fund of the University of London made to one of us (S. W.). REFERENCES Bonnet, V. & Bremer, F. (1937). J. Physiol. 90, 451'. Bremer, F., Bonnet, V. & Moldaver, J. (1942). Arch. int. Physiol. 52, 215. Biilbring, E. & Burn, J. H. (1941). J. Physiol. 100, 337. Calma, I. & Wright, S. (1944). J. Phy8iol. 103, 93. Calma, I. & Wright, S. (1947). In the Press. Chute, A. L., Feldberg, W. & Smyth, D. H. (1940). Quart. J. exp. Physiol. 30, 65. Denny-Brown, D. (1928). Proc. Roy. Soc. B, 103, 320. Fulton, J. F. & Pi-Sufier, J. (1928). Amer. J. Phy8iol. 83, 548. Kremer, M. (1942). Quart. J. exp. Physiol. 31, 337. McKail, R. A., Obrador, S. & Wilson, W. C. (1941). J. Physiol. 99, 312. Merlis, J. K. & Lawson, M. (1939). J. Neurophysiol. 2, 566. Schweitzer, A., Stedman, E. & Wright, S. (1939). J: Physiol. 96, 302. Schweitzer, A. & Wright, S. (1937a). J. Phy8iol. 88, 459. Schweitzer, A. & Wright, S. (1937b). J. Physiol. 89, 165. Schweitzer, A. & Wright, S. (1937c). J. Physiol. 89, 384. Sherrington, C. s. (1898). Philos. Trans. B, 190, 45. Sherrington, C. s. (1907). Proc. Roy. Soc. B, 79, 337. Wikler, A. (1945). Proc. Soc. exp. Biol., N. Y., 58, 193.