EFFECT OF LOW AND HIGH CONCENTRATIONS OF ALFENTANIL ADMINISTERED INTRATHECALLY ON Aδ AND C FIBRE MEDIATED SOMATOSYMPATHETIC REFLEXES

British Journal of Anaesthesia 1992; 68: 503-507

EFFECT OF LOW AND HIGH CONCENTRATIONS OF ALFENTANIL ADMINISTERED INTRATHECALLY ON A5 AND C FIBRE MEDIATED SOMATOSYMPATHETIC REFLEXES C. WANG, M. K. CHAKRABARTI AND J. G. WHITWAM

We have studied the effects of alfentanil administered intrathecally on somatosympathetic reflexes evoked by stimulation of radial and tibia/ nerves in 10 anaesthetized and paralysed dogs. In five animals, alfentanil was administered in doses of 100, WO, 200 and 400 fig in 0.8 ml and 800 fig in 1.6 ml prepared from the formulation of 500 fig mt'. Five others received alfentanil (high concentration formulation, 5 mg mf~'') in doses of 500 and 2000 fig in 0.5 ml and 5000 fig in 1 ml. C fibre tibia/ nerve reflexes were depressed by 100 fig and abolished with doses of 200-400 fig, but the AS response remained unaffected. AS responses were depressed also and abolished, respectively, by doses of alfentanil 2000 fig and 5000 fig. At the larger, but not the smaller, concentration there was evidence of baroreflex sensitization probably caused by systemic absorption of the drug. KEY WORDS Anaesthetic techniques: spinal. Analgesics: alfentanil. Nerve: axon. conduction, transmission. Reflexes: spinal, baroreceptor.

It is well known that nociceptive stimulation of afferent somatic sensory nerves induces changes in arterial pressure mediated by autonomic reflexes [1]. The sympathetic nervous system response to a single supramaximal somatic stimulus consists of two bursts of activity which may be recorded in sympathetic nerves and are a result of stimulation of afferent A5 and C fibre pathways, respectively; differences in reflex times depend on the different conduction velocities along the afferent pathway [1,2]. Previous studies have demonstrated that, in many reflex systems, the C fibre evoked response shows a greater sensitivity to depression by opioids than the AS response [3-8]. However, in most of these studies the opioids were administered systemically, which may result in antinociceptive activity through both supraspinal and spinal mechanisms. Typically localized pain (for example, pinprick) is affected less by opioids than more diffuse pain from less localized stimuli (for example, a heat stimulus), which are sometimes referred to "first" or "fast" pain and "second" or "slow" pain and which are regarded traditionally as being mediated by afferent A5 and C fibre pathways [9-11]. The dose of alfentanil, administered systemically,

required to abolish the A6 responses is 10-12 times that for C reflexes [12], which is the widest margin for any of the opioids tested to date. The present study was undertaken to determine if the A8 reflexes also show a similar reduction in sensitivity to the intrathecally administered drug compared with the C reflexes. MATERIALS AND METHODS

Ten greyhound dogs of both sexes, weighing 2230 kg, were studied (Home Office Licence No PPL 70/01654). Anaesthesia was induced with a single dose of methohexitone 10-15 mg kg"1 i.v. and maintained with alpha-chloralose (1 % solution) in an initial bolus dose of 30 mg kg"1 followed by a continuous infusion of 17-20 mg kg"1 h"1. The lungs were ventilated mechanically (Harvard pump) via a tracheal tube with oxygen enriched air. Muscle paralysis was maintained with suxamethonium 10 mg i.v. which was effective for approximately 30 min. The depth of anaesthesia of the animal was observed before each dose of suxamethonium (immobility, floppy ears and absent responses to glabellar tap and cutaneous stimulation). The left femoral artery and vein were catheterized for arterial pressure recording, blood sampling, drug administration and fluid infusion. Arterial pH, PaCOt, PaOt and oesophageal temperature were maintained in the ranges 7.30-7.35, 4.5-5.5 kPa, 20-27 kPa, and 37-39 °C, respectively. Laminectomy was performed at L2-3 and the dura mater exposed and cannulated (22-gauge YCan, Wallace) for intrathecal injection of drug. A lateral superficial branch of the radial and of the tibia] nerve were exposed in the left foreleg and right hindleg, and single fascicles of the renal sympathetic nerves were exposed retroperitoneally close to the renal artery. All nerves were desheathed at the stimulation or recording sites, cut distally and mounted on silver-silver chloride electrodes in a pool of mineral oil. Throughout these studies, the animal was maintained in a head-up tilt of 15° to reduce ascending spread of intrathecal drugs. C.WANG,

M.B.; M. K. CHAJCRABARTI,

B.SC.,

M.PHIL. ;

J. G.

WHITWAM, M.B., CH.B., PH.D., F.R.c.p., F.C.ANAES. ; Department of

Anaesthesia, Hammersmith Hospital, Royal Postgraduate Medical School, Du Cane Road, London W12 0NN. Accepted for Publication: December 12, 1991. Correspondence to C. W.

Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 17, 2015

SUMMARY

BRITISH JOURNAL OF ANAESTHESIA

504

RESULTS

The effects of alfentanil in small and large concentrations on tibial and radial evoked somatosympathetic reflexes from one animal in each group are illustrated

Radial

Tibial

FIG. 1. Effects of the smaller concentration of alfentanil i.t. and naloxone 2 mg i.v. on the radial and tibial evoked reflexes in renal sympathetic nerves in one animal. Upper traces: rectified integral (RI) of the averaged signal of the responses; lower traces: averaged transients (AT) of 16 responses. 1 = Control; 2 = 5 min after alfentanil total dose 100 ug (in 0.8 ml); 3 = 5 min after alfentanil total dose 200 ug (in 1.6 ml); 4 = 5 min after alfentanil total dose 1600 (ig (in 4.8 ml); 5 = 5 min after naloxone 2 mg i.v.

in figures 1 and 2. The mean processed data of responses evoked by tibial nerve stimulation from five animals in each group are shown in figures 3 and 4. Small concentration of alfentanil (500 fig ml'1)

It may be seen in figure 1 that the C fibre response to tibiarnerve stimulation was virtually abolished by alfentanil 100 ug i.t. However, at this dose the mean of the C fibre responses was reduced to 40 % in the five animals (fig. 3). Doses of alfentanil between 200 and 400 ug completely abolished these responses. In contrast, the A8 response was not affected by a dose of 800 ug in 1.6 ml of the undiluted formulation (a cumulative dose of 1600 ug and a total volume of alfentanil solution of 4.8 ml) (fig. 3). There were no significant changes in A8 and C reflexes from radial nerve stimulation throughout (fig. 1), indicating absence of a significant supraspinal component in the effect of the drug on the response to tibial nerve stimulation. Large concentration of alfentanil (5000 fig ml'1)

A single dose of alfentanil 500 ug in 0.5 ml of solution (1000 ug ml"1 concentration) eliminated the C reflexes (figs 2, 4), with only a small reduction in the A5 responses to tibial nerve stimulation. In the preparation illustrated in figure 2, an additional 2000 ug in 0.5 ml of solution (cumulative dose of 2500 ug in 1 ml of solution) caused a major reduction in the A8 response, with a mean reduction of 30 % in

Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 17, 2015

Supramaximal electrical stimuli (duration 0.5 ms; intensity 30 V; frequency 0.33 Hz) were applied to the peripheral nerves using a Grass S88 Stimulator with a matched, directly coupled isolation unit (Grass 478A). Efferent activity in renal sympathetic nerves was processed via a preamplifier (Tektronix type 122) and displayed on a dual beam oscilloscope (Tektronix type 565). Sixteen evoked responses from the renal sympathetic nerves were averaged using a Neurolog system (NL 750 Digitimer) and the averages rectified and integrated (Neurolog NL90). The rectified and integrated signals were displayed on a Devices MX2 pen recorder. A Gould system (Gould 1602) was used to measure the total electrical activity of this rectified, integrated signal, which was recorded in arbitrary units and changes expressed as a percentage of the control value. The arterial pressure was measured using calibrated strain gauges and was displayed together with beat-by-beat heart rate using a heated stylus recording system (Devices Ml9). After control data were set up and recorded, during each experimental sequence no adjustments were made to either the preparation or the instrumentation. After the surgical procedures, the preparation was allowed to stabilize for at least 30 min. Control averaged evoked responses in the renal sympathetic nerve were obtained in response to tibial and radial nerve stimulation. Resting mean arterial pressure (MAP) and heart rate (HR) were recorded also. Five animals were given alfentanil intrathecally (i.t.) in incremental doses of 100, 100, 200 and 400 ug in 0.8 ml and, finally, 800 ug in 1.6 ml (total volume 4.8 ml) at intervals of 10 min. These doses were made up from the standard formulation of alfentanil (500 ug ml"1) diluted with physiological saline when necessary. Five other animals were given greater concentrations of alfentanil, prepared from the formulation containing alfentanil 5 mg ml"1, at intervals of 15 min: doses of 500 and 2000 ug were administered in volumes of 0.5 ml by diluting the formulation with physiological saline; the final dose of 5000 ug was administered in 1 ml of the undiluted formulation. In all preparations 20 min after the final dose of alfentanil, naloxone 2 mg was administered i.v. After all measurements and recordings were completed, 1 % lignocaine 1 ml was administered i.t. to demonstrate abolition of peripheral evoked reflexes in renal sympathetic nerve and hence the correct placement of the intrathecal cannula. Averaged evoked responses in renal sympathetic nerves were recorded, starting 5 min after each drug injection. From four to eight stimuli were required for full activation of the C reflex, after which 16 responses were averaged. Resting MAP and HR were recorded throughout each experiment. Analysis of variance and paired t tests were used for statistical comparison.

505

ALFENTANIL AND SOMATOSYMPATHETIC REFLEXES Radial 160 -i 140-

l m

S-clOO-

zz

0 O

8

40200 Control 100 Tibial

1000 2000 Naloxone Alfentanil

160500 ms

FIG. 2. Effects of the greater concentration of alfentanil i.t. and naloxone 2 mg i.v. on the radial and tibial nerve evoked reflexes in renal sympathetic nerves in one animal. Upper traces: rectified integral (RI) of averaged signals; lower traces: averaged transients (AT) of 16 responses. 1 = Control; 2 = 5 min after alfentanil 500 ug in 0.5 ml; 3 = 5 min after a second dose of alfentanil 2000 jig in 0.5 ml (total dose of 2500 ug in 1 ml); 4 = 5 min after third dose of alfentanil 5000 ng in 1 ml (total dose 7500 |ig in 2 ml); 5 = 5 min after naloxone 2 mg i.v.

the five animals (fig. 4). A further increment of alfentanil 5000 ug in the undiluted formulation (1 ml) abolished the A8 reflexes (figs 2, 4)—a cumulative dose of 7500 ug in a total volume of 2 ml. For radial nerve stimulation, there were no significant changes in the A5 responses throughout. However, the mean C fibre response was reduced to 75 % of control after the second dose of alfentanil (a total dose of 2500 ug in 1 ml) and it was abolished totally after the third dose (cumulative dose of 7500 ug in 2 ml of solution) (figs 2, 4). Naloxone 2 mg i.v.

The effects of alfentanil were antagonized completely by naloxone 2 mg i.v., the responses returning to values not significantly different from control, indicating absence of damage to the spinal cord. Cardiovascular changes

There were only minimal changes in MAP and HR in those animals receiving the smaller concentration of alfentanil. In contrast, in those receiving the greater concentration, there was clear evidence of baroreflex sensitization, with reductions in both heart rate and systemic arterial pressure. It may be seen in figure 5 that, while the mean MAP in these animals decreased from a control value of 169 mm Hg to 144 mm Hg after 500 ug (P < 0.05), and to approximately 115 mm Hg after cumulative

Control 100

1000 Alfentanil

10000 Naloxone

FIG. 4. Effect of large concentrations of alfentanil i.t. on the responses in renal sympathetic nerve, evoked by stimulation of tibial nerve ( • = AS reflexes; • = C reflexes) and radial nerve (A = A8 reflexes; A = C reflexes). Relative changes in integrated electrical activity of the averaged transient of 16 responses in five animals (expressed as percent of control) (mean, SD). Doses of alfentanil 500, 2500 and 7500 |ig i.t. are shown on a logarithmic scale as the cumulative dose (total cumulative volume of solution 2 ml). Naloxone dose was 2 mg i.v. Significant differences: * P < 0.05, **P<0.01, ***/>< 0.001—AS and C fibre mediated responses compared with control; t t ^ 3 < 0.01—C reflexes compared with AS.

doses of 2500 ug and 7500 ug, the HR declined from a mean control value of 185 beat min"1 to 148 beat min"1 (P < 0.05), 74 beat min"1 and 68 beat min"1 (P < 0.01) after the first, second and third doses, respectively. DISCUSSION

This study has shown that relatively small doses of alfentanil, in the range 200-400 ug injected i.t. in the lumbar region, abolished C fibre but not A5 sympathetic reflex responses to stimulation of tibial nerve in the dog. This suggests that the analgesic properties of this drug injeaed in an accepted dose of approximately 100 ug i.t. in man in a low concentration may also be attributed principally to depression of C reflexes [13]. Increasing the concentration of alfentanil from 125ugml~l to 500 ug ml"1 and the injected volume to 4.8 ml (which is a large volume in the dog) did not abolish the tibial

Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 17, 2015

FIG. 3. Effect of small concentrations of alfentanil administered i.t. on the A5 ( # ) and C (O) reflexes evoked in renal sympathetic nerve by stimulation of the tibial nerve. Relative changes in integrated electrical activity of the averaged transient of 16 responses infiveanimals (expressed as percent of control) (mean, SD). Doses of alfentanil 100, 200, 400, 800 and 1600 ug i.t. are shown as cumulative dose (total volume of solution 4.8 ml) on a logarithmic scale. Naloxone dose was 2 mg i.v. Significant differences: * P < 0.05, *** P < 0.001—A5 and C fibre mediated responses compared with control; tt-P < 0.01—C reflexes compared with A8.

BRITISH JOURNAL OF ANAESTHESIA

506 250 n _ 200 -

?'c 150 -

r s 100 H

il 50 0

J

Control 100

1000 Alfentanil

10000 Naloxone

A8 reflex and there was no depression of the C or A8 radial nerve reflexes or any cardiovascular changes such as baroreflex sensitization [14]. This indicates first, that the drug had not spread rostrally in the CSF to affect the thoracic dorsal root area or the medulla; second, that the drug had not affected the renal sympathetic efferent pathway; third, that systemic absorption of the drug was insufficient to cause significant effect; and finally, therefore, that the effects of the drug were localized to the spinal cord. A progressive increase in concentration to 5000 ug ml"1 in a total dose up to 7500 ug in a total volume of only 2 ml also abolished the tibial A5 reflex (in addition to the C fibre reflex). The lack of effect on the radial A8 reflex implies that the efferent sympathetic pathway remained unaffected, but the abolition of the radial C reflex and the reduction in both heart rate and arterial pressure, indicating baroreflex sensitization, suggests intrathecal spread, effective systemic absorption, or both. However, in view of the lack of effect of alfentanil 4.8 ml in the smaller concentration preparation on the radial C reflex, and in view of the maximum injected volume of only 2 ml in the large concentration, the more probable view is that the observed cardiovascular effects and the depression of the radial C reflexes were caused by systemic absorption. It has been reported previously that, during systemic administration, the maximum depression of the cardiovascular system by fentanyl, alfentanil and sufentanil occurs at the point of maximum baroreflex sensitization, which also coincides with the abolition of somatic C fibre mediated reflexes [6-8]. Thereafter, further massive doses of these drugs causes no further cardiovascular depression [6-8, 15] although A8 reflexes are abolished. The results of the present study suggest similar effects after i.t. administration. This study also shows that alfentanil i.t. has a greater effect on C fibre mediated reflexes than on AS. The ratio of the concentration of alfentanil i.t. required for elimination of the A8 and C fibres evoked responses was more than 10:1, which is similar to the ratio of the total dose of 11:1 for

REFERENCES 1. Sato A, Schmidt RF. Somatosympathetic reflexes: afferent fibres, central pathways, discharge characteristics. Physiological Reviews 1973; S3: 916-947. 2. Whirwam JG, Kidd C, Fussey IV. Responses in sympathetic nerves of the dog evoked by stimulation of somatic nerves. Brain Research 1979; 16S: 219-233. 3. Duggan AW, North RA. Electrophysiology of opioid. Pharmacological Reviews 1984; 35: 219-281. 4. Blinn G, Heinz G, Jurna I. Effects of substantia nigra stimulation on suralis-evoked spinal reflex activity: comparison with the effects of morphine and stimulation in the peraqueductal gray matter. Neuropharmacology 1980; 19: 75-85. 5. Jurna I, Heinz G. Differential effects of morphine and opioid analgesics on A and C fibre-evoked activity in ascending axons of the rat spinal cord. Brain Research 1979; 171: 537-576. 6. Niv D, Whitwam JG. Selective effect of fentanyl on group III and group IV somatosympathetic reflexes. Neuropharmacology 1983; 22: 703-709. 7. Swenzen GO, Chakrabarti MK, Sapsed-Byme S, Whitwam JG. Selective depression of alfentanil on group III and group IV somatosympathetic reflexes in dog. British Journal of Anaesthesia 1988; 61: 441^145. 8. Swenzen GO, Chflkrabarti MK, Sapsed-Byrne S, Whitwam JG. Selective effect of sufentanil on group III (A delta) and group IV (C) somatosympathetic reflexes. Acta Anaesthesiologica Scandinavica 1986; 30: 545-548. 9. De Castro J, Meynadier J, Zenz M. Pharmacodynamics of opioid. In: De Castro J, Meynadier J, Zenz M, cds. Regional Opioid Analgesia. Dordrecht: Klumer Academic Publishers, 1991; 85-94. 10. TorebjSrk HE, Ochoa JL. Specific sensations evoked by activity in single identified sensory units in man. Acta Physiologica Scandinavica 1980; 110: 445-^47. 11. Yaksh TL. Neurologic mechanisms of pain. In: Cousins MJ, Bridenbaugh PO, eds. Neural Blockade in Clinical Anesthesia and Management of Pain. Philadelphia: Lippincott, 1988; 791-844.

Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 17, 2015

FIG. 5. Cardiovascular changes during administration of the large concentration of alfentanil i.t. followed by naloxone 2 mg i.v. Cumulative dose of drug is shown on a logarithmic scale. O = Mean arterial pressure (MAP); • = heart rate (HR). Significant differences: *P < 0.05, ** P < 0.01, compared with control.

alfentanil administered systemically to produce comparable effects [7]. The precise mechanism for this selectivity of opioid on A8 and C fibre reflexes is uncertain [16]. There is evidence that different opioid receptors may modulate the effects of different types of stimulation. For example the K opioid agonists have a distinctly greater analgesic potency on tests of nociception using mechanical or visceral stimuli, in contrast with the u agonists which have a greater effect on noxious heat stimuli [17-20]. At the cellular level, whereas u and 8 agonists act via potassium channels, the K agonists act via calcium channels [16], which could possibly be one factor contributing to these observed differences. An intriguing problem is the relationship between the lipophilicity and potency of the opioids administered i.t. McQuay and his associates have suggested an inverse relationship between lipophilicity and potency for a range of synthetic opioids injected i.t. in a rat model [21]. It has been shown that fentanyl administered i.t., in the same dog model as was used in the present study, abolished both A8 and C reflexes at a total dose of only 128 ug [22]. This is in marked contrast with the results of the present study, showing that alfentanil, which is less lipophilic than fentanyl, was also less potent when administered i.t., particularly in relation to its effect on A8 mediated reflexes, and hence would not support their hypothesis.

ALFENTANIL AND SOMATOSYMPATHETIC REFLEXES 12. Bovill JG, Sebel PS, Blackburn CL, Heykants J. The pharmacokinetics of alfentanil (R 39209): a new opioid analgesic. Anesthesiology 1982; 57: 439-443. 13. De Castro J, Meynadier J, Zenz M. Drugs. In: De Castro J, Meynadier J, Zenz M, eds. Regional Opioid Analgesia. Dordrecht: KJumer Academic Publishers, 1991; 131-209. 14. Laubie M, Schmitt H, Vincent M. Vagal bradycardia produced by microinjections of morphine-like drugs into the nucleus ambiguus in anaesthetized dogs. European Journal of Pharmacology 1979; 59: 287-291. 15. Askitopoulou H, Whirwam JG, Al-Khudhairi D, Chakrabarti MK, Bower S, Hull CJ. Acute tolerance to fentanyl during anesthesia in dogs. Anesthesiology 1985; 63: 255-261. 16. Dickenson AH. Mechanisms of the analgesic actions of opiates and opioids. British Medical Bulletin 1991; 47: 690-702. 17. Tyers MB. A classification of opiate receptors that mediate antinociception in animals. British Journal of Pharmacology 1980; 69: 503-512.

507

18. Upton N, Sewell RD, Spencer PS. Differentiation of potent u- and K-opiate agonists using heat and pressure antinociceptive profiles and combined potency analysis. European Journal of Pharmacology 1982; 78: 421-429. 19. Schmauss C, Yaksh TL. In vivo studies on spinal opiate receptor systems mediating antinociception. II. Pharmacological profiles suggesting a differential association of u, S and K receptors with visceral chemical and cutaneous thermal stimuli in the rat. Journal of Pharmacology and Experimental Therapeutics 1984; 228: 1-12. 20. Fields HL, Basbaum AI. Endogenous pain control mechanism. In: Wall PD, Melzack R, eds. Textbook of Pain. Edinburgh: Churchill Livingstone, 1989; 206-217. 21. McQuay HJ, Sullivan AF, Smallman K, Dickenson AH. Intrathecal opioids, potency and lipophilicity. Pain 1989; 36: 111-115. 22. Wang C, Chakrabarti MK, Whitwam JG. Relative effects of intrathecal midazolam and fentanyl on AS and C reflexes. British Journal of Anaesthesia 1990; 65: 590P. Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 17, 2015