The spinal cord processing of input from the superior sagittal sinus: pathway and modulation by ergot alkaloids

Brain Research, 597 (1992) 321-330 © 1992 Elsevier Science Publishers B.V. All rights reserved 0006-8993/92/.~i05.00

321

BRES 18313

The spinal cord processing of input from the superior sagittal sinus: pathway and modulation by ergot alkaloids G.A. Lambert, A.J. Lowy, P.M. Boers, H. A n g u s - L e p p a n a n d A.S. Z a g a m i Institute of Neurological Sciences, The Prince Henry and Prince of Wales Hospitals and School of Medicine, Unit'ersity of New South Wales, Little Bay, N.S.W. (Australia) (Accepted 30 June 1992)

Key words: Craniovascular; Ergot alkaloid; lontophoresis; Migraine; Superior sagittal sinus; Trigeminal

The effects of ergot alkaloids on field potentials and unit responses produced in the upper cervical spinal cord by stimulation of the superior sagittal sinus (SSS) were examined in 57 anesthetized cats. Electrical stimulation of the SSS produced field potentials and single-unit responses at latencies of 5-20 ms. Field potentials were abolished by section of the first division of the trigeminal nerve but were unaffected or increased by section of the upper cervical nerves. Field potentials were reduced or abolished by intravenous injection of ergotamine or dihydroergotamine (DHE). The evoked response of 41 units (34.4%) were suppressed by either i.v. or iontophoretic administration of ergotamine, DHE or ergometrine. The results suggest that ergot alkaloids exert an effect at a spinal cord relay centre which receives trigeminally mediated input from cranial blood vessels.

INTRODUCTION The superior sagittal sinus (SSS) is innervated by A-8 and C-fibres ~, which play an important role in conveying the sensation of pain. Electrical stimulation of the SSS elicits pain resembling migraine headache in human subjects 31. The nerves supplying this and some other cranial blood vessels constitute a part of the trigeminal system25, with larger diameter fibres terminating in the chief sensory nucleus of the trigeminal nerve and trigeminal nucleus caudalis, and C-fibres coursing caudally in the spinal tract of the trigeminal nerve as far as the cervical cord 5. Fibres from the SSS, middle meningeal and middle cerebral arteries project to the trigeminal ganglion; those from the SSS project almost exclusively to the ophthalmic division of the ganglion24. Neurones in both the trigeminal nucleus caudalis and upper cervical spinal cord respond to electrical and chemical stimulation of cranial vessels. These responses are mediated by trigeminal pathways9't7"2°'39'43. A significant number of responsive units are found in

the dorsolateral area (DLA) of the cord, an area which contains the lateral cervical nucleus (LCN) 2°. The LCN receives trigeminal afferents7`s and is thought to play a role in the central processing of nociceptive information, including that from the head. Kajander and Giesler I" found that almost 60% of LCN neurones respond to noxious stimuli from the trunk and limbs. Goadsby et al. 12 demonstrated that stimulation of the SSS causes a significant increase in local glucose utilization in the DLA. The responses of trigeminal nucleus neurones or cervical cord elements to stimulation of structures such as the SSS may serve as useful models for testing hypotheses about the mechanisms of migraine and other vascular headaches 9"1°'3~. In recently published material we have reported on the properties of elements in the cervical cord in this regard 2°. The present experiments were designed to assess the peripheral pathway taken by impulses reaching the spinal cord from the superior sagittal sinus and whether the antimigraine drugs ergotamine, DHE and ergometrine affect the processing of craniovascular sensory infor-

Correspondence: G.A. Lambert, Room 147, CSB, Prince Henry Hospital, Little Bay 2036, N.S.W., Australia. Fax: (61) (2) 311-3483.

322 marion in the dorsolateral area of the upper cervical cord. The results have previously been presented in abstract form4't~'22.

MATERIALS AND METHODS Fifty-seven cats (mean weight 2.8 kg) were anaesthetized with intraperitoneal injections of a mixture of 25% urethane (Sigma) 500 mg/kg and a-chloralose (Sigma) 20 mg/kg, or a-chloralose 60 mg/kg alone. The femoral artery and vein were cannulated to measure blood pressure and heart rate and to administer intravenous (i.v.) drugs and fluids, respectively. Animals were intubated and ventilated with air and end-expiratory CO, was kept in the range 3.5-4.0%. Throughout the experiment, the animal was paralysed with i.v. gallamine triethiodide, 20 mg/kg (May and Baker). Rectal temperature was monitored with a thermistor, and maintained at 37-38°C by means of a heating blanket. The depth of anaesthesia was assessed periodically during the experiment by testing for sympathetic responses to noxious stimulation (pupillary dilatation, tachycardia, raised blood pressure) during paralysis, or by allowing the paralysis to wear off so that withdrawal reflexes could be tested. Supplementary doses of either chloralose or gallamine were given when necessary.

Animals were mounted in a stereotaxic frame and prepared for electrophysiological recorOmg b? C I and C, laminectomies. A circular craniotomy (25 mm diameter) wits performed to expose the SSS, which was isolated in the following manner: two parallel incisions, running anteroposteriorly were made in the dura on either side of the SSS, the underlying cortex was depressed and the flaps of dura thus created were suspended clear of the cortex; a third longitudinal incision was made in the falx cerebri; the SSS, along with its remaining dural and falcical attachments, was draped over a pair of platinum hook stimulating electrodes; to prevent the SSS from becoming dry. it was bathed in paraffin oil constrained by a circular wall of dental acrylic', the SSS was isolated electrically from the underlying cortex by the paraffin oil, and by a circular piece of polyethylene sheeting threaded between the SSS and cortex. Artefacts due to respiration, vascular pulsation and other move. mcnls were reduced by the following procedures: the cat was suspended by a clamp on one thoracic spinous process; bilateral pneumothoraces were made and kept patent with a polythene spreader; both Ct lateral spinous processes were clamped to the auxiliary ear bar holders of the stereotaxic frame; and the remaining caudal portion of the dorsal C 2 spinous process was clamped from behind. The SSS was stimulated with bipolar platinum hook electrodes which delivered stimulus-isolated, supramaximal single shocks (40150 V, 0.4-1.5 mA) of 250 p,s, every 1-5 s, from a Grass $88 stimulator. The electrodes were insulated, except for the area in contact with the SSS. Drugs were applied to the exposed sinus by application of cotton pellets soaked in the drug of interest, Saline-soaked pellets were used as controls. The response of some cells to mechanical stimulation was tested by gently tugging on the SSS with a pair of fine forceps.

Field potential recordings Areas of responsiveness were detected in preliminary experiments by the use of a surface mapping technique. A silver ball electrode was used to record potentials from the dura overlying the cord at 1 mm intervals between an area just rostral to the obex and the C~ root entry zone. Recordings from deeper areas were made with tungsten semi-micro electrodes (exposed tip length 20-100/z). Field potentials were recorded with a FET amplifier (bandwidth 3 Hz-30 kHz), on a 1 mm interval, 3-dimensional grid and averaged for 25 sweeps by the use of microcomputer software developed in our laboratory. Measurements of latency and peak-to-peak amplitude of average potentials were performed with an interactive graphics program.

in some cats, the trigeminal ganglion, near the origin of the first division, was lesioned electrolytically by lowering a stainless steel electrode (Rhodes NEX-200) into the ganglion ( A / P = + 9.5, L = 6.0, H ~ - 7.8) 35 and passing a current of 1-2 mA for 5 mid between the two poles of the electrode. The brain was removed and ganglia were examined after the experiment to check for completeness of the lesions.

Single unit recordings After removal of the dura and pia mater from the ~orsal surface of the spinal cord, the single or multi-barrelled recording electrodes were lowered into the DLA (0-6 mm caudal to the midpoint of the C 2 rootlets, 0-500 It lateral to the dorsal root entry zone, 0-3 mm below the dorsal surface of the spinal cord). From this point, the electrode was advanced or retracted in the spinal cord with a hydraulic microdrive. Bacteriological agar (3%) was poured over the exposed cord after insertion of the electrode to reduce movement of the cord due to vascular pulsation. In some animals, fine scissors were used to section the Ct and C2 roots at the point of their passage through the dural sleeves. This procedure was carried out under observation from a stereo dissecting microscope. The central tungsten wire of a single-barrelled or 7-barrelled glass microelectrode (impedance < 200 kCl) was used to record neuronal activity in the spinal cord t4. Each of the 6 barrels used for iontophoresis had an impedance of 50 M t'] (measured at 1 kHz in 0.9% NaCI). A Pagan iontophoresis generator provided the current for ejection of drugs from the surrounding pipette barrels. Amplifier bandwidth was usually 300 Hz to 10 kHz. When attempting to discriminate between some,tic and axonal recordings, an amplifier bandwidth of DC to 30 kHz was used. In order to record the response of single units to stimulation, post-stimulus histograms were constructed in real time and saved on disk for later processing. Peristimulus histograms were used to record spontaneous firing rates of units in the absence of electrical stimulation. An averaging routine, which sampled a delayed version of the analogue signal and was triggered by me window discriminator, was used to construct averaged action potentials for later analysis. The software was written in Micm~ft FOP TRAN and Assembler, to run on a 10-MHz MS-DOS microcomputer system.

Receptit,e fields Some units were tested for the presence of a cutaneous receptive field The skin and hair of the face and limbs were examined systematically with a variety of stimuli (brush, light touch, heavy pressure and pinch), and classified according to response. Low threshold mechanoreceptive (LTM) units responded to light touch or brush, and did not increase firing rate with noxious stimuli. Nociceptire specific (NS) units responded only to heavy pressure or pinch, and wide dynamic range (WDR) units responded to non-noxious stimuli, but increased firing rate in response to noxious stimuli Is'2°.

Drugs The following drugs were used: ergotamine tartrate (Sandoz) (1 mg/ml); DHE mesylate (Sandoz) (1 mg/mi); ergometrlne maleate (Sigma) (l mg/ml); ot.-homocysteic acid (DLH) (Sigma) (1.0 M), as sodium homocysteiate, phosphate-buffered horseradish peroxidase (HRP, 20%, Type VI, Sigma P8375); saline (NaCI) (1.0 M). In the iontophoresis experiments, one barrel contained NaCI 10 M, used for current-balancing purposes. Retaining currents of 1-3 nA were applied continuously to all barrels to keep drug leakage to a minimum. The following solutions of drugs were applied to the SSS by means of cotton pellets: DHE (1 mg/ml); llgnocaine (2%) (also applied as a gel).

Histology Selected single-unit sites were marked for subsequent identification either with an HRP stain, ejected from the electrode by the use of a constant current unit (500 ms duration, 1 Hz in a 50% duty cycle, 4 p,A for 20 rain)29, or an electrolytic lesion made by passing a

323 DC current through the tungsten recording wire for a few seconds (approximately 2-10 p.A-min). At the conclusion of each experiment, the animal was given pentobarbitone sodium 60 mg (Abbott) and perfused through the aorta with normal saline (containing potassium ferrocyanide to reveal trigeminal ganglion lesion sites), followed by phosphate-buffered sucrose formalin. The spinal cord was removed and sectioned on a freezing microtome. Specimens in which HRP stains were made were developed by incubating the sections in tetramethylbenzidine or diaminobenzidine 26. The resulting reaction with HRP produced a stain visible by light microscopy. All sections were stained with thionine. Recording sites were determined from electrode tracks, HRP stains and readings of depth from the microdrive.

50'

uV O'

-50 J 100"

Statistical analysis of results All group data are presented as mean :1:standard deviation (S.D.). Where comparisons are made between groups, the data are presented as mean +standard error (S.E.M.). Paired and unpaired Student t-tests were used to compare firing rates and the X 2 test of independence was used as a test of association between different unit properties such as receptive field characteristics and response to drugs. Significance was accepted at a level of P < 0.05. For experiments in which the effects of iontophoretic ejection of drugs on basal single unit firing rates were studied, a sustained change of 25% in basal firing rate or of 25% in stimulus-induced response was taken to be a significant change.

RESULTS

A

i ................... B

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250 -

C

. v o.

Field potential recordings Preliminary recordings from the dura overlying the spinal cord in 12 cats revealed that stimulation of the superior sagittal sinus (SSS) produced a large diphasic artefact/evoked potential complex upon which was superimposed a group of low-amplitude potentials (Fig. 1A). Although these potentials could be recorded from areas rostral to the obex to as far caudally as the beginning of the Ca roots, their amplitude was greatest between the C 2 and C3 dorsal root entry zones. There was no change in the latencies of these small potentials as the recording electrode was moved between the obex and C3. The area with greatest peak-to-peak amplitude of the principal field potential wave was further explored with penetrating tungsten microelectrodes. Stimulation of the SSS produced either a single component slowwave response or a multi-component fast wave response according to the recording location. Slow-wave responses (Fig. 1B) of mean latency 9.4:1:0.8 ms (mean :t: standard error, n = 10) were largest at 2-6 mm caudal to the level of the mid-point of the C 2 dorsal root entry zone near the termination of the descending spinal nucleus and tract of the trigeminal nerve, in the region of the lateral cervical nucleus (LCN) (Fig. 2, right). The mean location of the area in which maximum field potentials were developed in this dorsolateral area (DLA) in 7 cats was 3.1 + 0.8 mm caudal to the midpoint of the C2 rootlets, 2.5 _+0.2 mm lateral to the midline and 1.9 + 0.3 mm below the surface. The

Fig. 1. A: field potential recorded from the surface of the spinal cord at the obex with a silver ball electrode. Supramaximal stimulation of the superior sagittal sinus delivered at the start of the trace. The initial 1-1.5 ms of the trace is distorted by stimulus artefact in this and the following figures. B: potential recorded with a tungsten semi-microelectrode in the dorsolateral area (DLA. 2 mm caudal to the midpoint of the C 2 rootlets, 2ram lateral to the midline and I mm below the dorsal surface) after stimulation of SSS. C: potentials recorded in the same cat as in B in the medioventral area (MVA), 6 mm caudal to the C 2 rootlets, in the midline and 6 mm below the surface of the cord) after stimulation of SSS. Tungsten semi-microelectrode. Potentials are averages of 25 sweeps and in cases B and C represent bulk neuronal activity from about 4 cubic mm 34 Positive is upward in all cases. X-axis time base in milliseconds.

maxima in the fast wave response were usually located in the midline, 4-6 mm from the dorsal surface of the cord and 4 mm caudal to the area of maximum slow wave responsiveness (medioventral area, MVA). A three-dimensional representation of 107 recording points in a cat spinal cord is shown in Fig. 3, which represents potentials at each point by symbol size and, in the coronal plane, by an isopotential contour map. The diagram also shows a coronal section of the spinal cord at C 2 (adapted from the diagrams in Rexed (1952) 32 and it can be seen that the maximum slow-wave potentials were developed just ventral to the lateral cervical nucleus.

324 Both types of field potentials were stable over many hours of recording and were not influenced by level of anaesthesia, administration of gallamine, anaesthetics or fluids, nor by changes in ventilatory parameters. Ablation and lesion studies. Electrolytic lesions of the ipsilateral ophthalmic division, at its junction with the trigeminal ganglion, abolished the DLA response (Fig. 4). Bilateral section of the C t and C 2 dorsal rootlets did not reduce the magnitude of the field potentials (Fig. 4). In all 5 cats in which this procedure was carried out, there was an increase in amplitude of the slow wave (average percentage increase = 14.0 ± 3.9; P < 0.05; Student's t-test). Effect of ergotamine. Intravenous injection of ergotamine (4/~g/kg and 40/.~g/kg) respectively reduced or blocked the evoked potential response (Fig. 5). Inhibition w~s slow to develop and did not reach a maximum until 30 min after administration. Inhibition

(32 - 4

was greater at the higher dose (Table I). Responses did not return to control values even after 4 - 6 h..

Single-unit recordings In 45 experiments, a total of 147 units with a synchronised response to SSS stimulation were recorded in the DLA of the upper cervical spinal cord, with a mean latency of 9.0 + 4.3 ms. Six units had an additional long latency response (mean 235.8 + 97.8 ms) to SSS stimulation. The spontaneous firing rate of 66 units was measured, and was found to be 13.9 + 20.1 Hz. Sixty-three units were tested with iontophoretic application of DLH ( - 10 to - 100 nA) and this immediately increased the spontaneous firing rate of 28 units (44.4%) by up to 10 times 2° indicating that they were cell bodies. A cutaneous receptive field was found in 33 of 50 units tested: 14 on the ipsilateral face and 19 on the ipsilateral fore and hind limbs. Based on the

C2

Fig. 2. Diagrammatic reconstruction of a transverse section of the cervical spinal cord of a cat showing recording sites and recorded field potentials. Both DLA and MVA wave components can be seen. Left: section at 4 mm csudal to the: ¢72 rootlets ('Ca-4') showing the series of fast waves at a maximum near the ventral limit of the ventromedian fissure and immediateb, adjacent to the midline. Right: section at the mid C a rootlets ('C2') showing a maximum in the slow wave response 1 mm from the spinal co:d surface and 2 mm lateral to the midline. In each case the location from which the recordings were made is shown by the dot at the start cf each trace. Each trace represents the average of 20-30 sweeps. The spinal cord gray matter, including the LCN, is shown stippled. Positive upward.

325

Control

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Fig. 3. Three-dimensional representation of the magnitude of the slow wave response in the dorsolateral quadrant of the spinal cord. The reference point for the coordinate system is on the surface of the cord (ventral = negative), in the mid-line at the A / P mid-point of the C 2 rootlets (caudal = negative). Size of the symbols is proportional to the peakqo-valley amplitude of the slow wave response, with the largest potential shown (at A / P -- C2-2, H = 2, L = 2) being 298 ~V. An isopotential contour map of field potentials in the coronal plane is shown superimposed on a cross-section of the cord at C,. Contour interval is 50 tzV.

characteristics of their receptive fields, 14 units were nociceptive in nature (either NS or WDR) and 12 units were non-nociceptive. The nature of 7 receptive fields was not determined. Effects of ergot alkaloids. The i.v. injection of ergotamine (40 /zg/kg) suppressed the stimulus-induced discharge of 5 out of 7 DLA units. As with evoked field potentials, suppression was slow to develop (about 20 min) and lasted for several hours. At 1 h after injection, the mean change in probability of firing for the 5 units was a reduction to 18 + 9% (n -- 5, P < 0.05). Local application to the SSS of a 1 mg/ml solution of DHE failed to block the responses of 4 out of 4 units tested. Application of lignocaine 2% to the SSS produced a reversible block of response in 3 of 4 units tested. This effect was reproducible for each unit. These were not the same 4 units tested with local DHE. One hundred and twenty four units were tested with iontophoretic application of one of the three ergot alkaloids (Table II). No unit was tested with more than one agent. Each agent suppressed the firing of a proportion of units with SSS input. In the following text and tables, the response to ergot alkaloids is calculated as the probability of firing during drug application, as a percentage of the mean pre- and postdrug probability of firing in response to SSS stimulation. The evoked activity of 46 (about one-third) of units was reversibly reduced to 18-36% of control values

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--

I

20msec

Trigeminal lesion

I

Fig. 4. Effects of lesions on the slow wave response in the DLA. Control responses (top) were not blocked by bilateral section of the first and second cervical roots (middle) but were abolished by bilateral destructive electrolytic lesions of the trigeminal ganglia (bottom). Vertical scale in tzV.

(mean values for each drug group; Table 2, Fig. 6). The effect of these drugs developed over 2-10 rain and took up to 30 rain for responses to return to normal. Equivalent saline currents did not affect the response of units to SSS stimulation. Another 14 units were either irreversibly suppressed or lost from recording, and 3 units were reversibly potentiated by drug application. The remaining 61 units were unaffected by drug application. Analysis of the latency, spontaneous firing rate, response to DLH and receptive field characteristics

TABLE I Inhibition of slow-wave field potentials evoked by SSS stimulation the DLA area of the spinal cord by 4 and 40 I~g / kg ergotamine administered intravenously

Amplitude (/~V) p *

Control response

Ergotamine (4 i~g / kg)

Ergotamine (40 Izg / kg)

233_+34

115_+45 < 0.02

45+ 15 < 0.01

* Significance level, Student's paired t-test, n = 9.

326 C

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-

0

/ -200.

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Fig. 5. Effects of i.v. ergotamine on the field potentials evoked by SSS stimulation. Each graph of 312 points represents the mean of the field potentials in 9 cats. The dotted lines represent the standard error envelope of the means.

indicated that there was no difference between those units suppressed and those unaffected by ergot alkaloids. The mean latencies of those units which were suppressed and those which were unaffected by ergot alkaloids were 8.9 + 0.6 ms and 7.8 + 0.4 ms Similarly, there was no significant difference between the mean spontaneous firing rate of units in either group (12.3 + 2.7 Hz (n = 27) and 16.7 + 4.6 Hz (n = 31). Responses to ergot alkaloids were not dependent on the type of receptive field. In 9 units with a nociceptive (NS or WDR) receptive field, 5 (56%) were suppressed by ergot alkaloids, compared with only 2 of 10 units (20%) with non-nociceptive receptive fields (a, 2 = 1.27, n.s.). Fifty-eight units were tested with both DLH and one of the ergot alkaloids. Of the 32 units suppressed by ergot alkaloids, the spontaneous firing rate of 17 (53%) was increased by DLH. Of the 26 units unresponsive to ergot alkaloids, 69% were also unresponsive to DLH. There was no statistical relationship between the response of units to DLH and one of the ergot derivatives.

Histology The location of 32 units was reconstructed in the ~pinal cord by correlating microdrive readings with lesions or stains made during an experiment. Nine of these units were suppressed by one of the ergot alka-

loids. Of the 32 units, 7 were placed in the dorsal horn, 19 in the dorsolateral quadrant of white matter (including 2 within the lateral cervical nucleus) and 6 lay outside the boundaries of these regions (Fig. 7). Of the 9 units suppressed by ergot alkaloids, 3 were placed in the dorsal horn, 5 were in the dorsolateral white matter and 1 was outside these regions (Fig. 7). Three of these 9 units were also accelerated by application of DLH. DISCUSSION

We have previously shown that both field potentials and single unit activity can be recorded in the dorsolateral region of the cervical spinal cord in response to craniovascular stimulation 17-2°. The electrodes used (semi-micro tungsten with an exposed tip of 50-100/~) have optimal characteristics for recording field potentials from small populations of electrically active elements 3s. We have found two such active areas, differing in their response properties and separated by about 4-5 mm, but their identity is uncertain. Localized areas of evoked responses probably represent anatomically circumscribed, radially symmetrical groupings of units 34. The rostro-caudal limitation of the areas over which both types of potential could be recorded argues against their being longitudinal

TABLE II

Effect of ergot alkaloids on single units Ergot alkaloids were administered intravenously (iv), or iontophoretically (ionto) onto DLA units. The number of units (percentage in brackets) showing reversible inhibition of responses to SSS stimulation are shown, Mean responses of all cells tested after drug administration are expressed as a percentage of the control response of these same cells before drug administration.

Drug

Route

Amount

Units tested

Units inhibited (%)

Response (% of control)

Ergoiamine DHE Ergotamine Ergometrine

i,v. ionto ionto ionto

40p, g.kg-t 0 - 30 nA 0 - 30 nA 20-200 nA

7 46 9 64

5 (71.0) 17(37.0) 5 (55.5) 19 (30.0)

18,0+ 9.0 29.9+20.5 30.9 + 24.6 36.1 + 23.5

327 tracts. The slow wave response was centred on the DLA, an observation which supports the concept that it is radially symmetrical, probably a collection of cell bodies 27. The latency, duration and negativity of the slow wave potential in the cervical cord was similar to those produced in tLe lumbar cord by electrical stimulation of visceral afferents (splanchnic nerve) 36 or by mechanical distension of limb veins40. In cats in which the cervical roots had been severed, field potentials ~vere significantly larger than those

go

O0

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o o o

0

o

o

6

l Fig. 7, Histologically verified recording sites of SSS units in DLA from I5 cats. <3, linked to SSS, not inhibited by ergot alkaloids; o, linked to SSS, responses blocked by iontophoretic application of ergot alkaloids, Although the units are mapped on only one side, they represent results obtained from both left and right sides of the cord.

tlml (ms)

Ergomeirine 200hA

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Fig. 6. Post-stimulus time histograms showing the effects of iontophoretic application of ergometrine (200 nA) on responses of a cervical cord unit to electrical stimulation of the superior sagittal sinus. Ergometrine produced a reversible inhibition of responses to electrical stimulation. There was no effect of ejection of the same current of saline.

recorded in intact cats. We have observed in other experiments that dorsal root fibres entering the cord at the C2 to C 3 level appear to have an inhibitory effect on the single unit response to SSS stimulation. The increased field potentials seen in the experiments described here may represent a release from a background inhibitory input of this type. The area in which fast wave evoked potentials could be recorded was much larger and it was possible to detect such potentials on the surface. We believe this indicates that these potentials originate in a spinal tract 27, possibly the cervico-thalamic tract. A preliminary report on observations on these potentials has been made n9 and will be expanded upon in a further report. In this study, we found that most responsive single units were recorded in the dorsolateral region of the upper cervical (C 2 to C3) spinal cord, lateral to the dorsal horn and in the dorsolateral white matter (Fig. 7). This is consistent with the location of single units in the earlier studies cited, and lends further weight to the argument that the craniovascular afferent pathway projects to this region. As in the previous study, we have shown that many (but not all) units recorded in the dorsolateral white matter appear to be cell bodies, since their spontaneous firing rates could be increased by iontophoretic application of homocysteic acid. The failure of homocyst¢ic acid to excite some units could

328 be due to either the remoteness of the unit from the iontophoresis barrel (thus reducing the effect concentration of DLH) or to the recorded units being fibres of passage rather than cell bodies. The results are also in agreement with those of Yezierski and Broton (1991) 43 who reported on units in the dorsal horn, LCN and spinal white matter, some of which responded to mechanical stimulation of the dura. The latency to activation of the cells reported in the present study are the same or slightly shorter than those reported by Davis and Dostrovsky 9 and Strassman et al. 39. for the trigeminal nucleus and by ourselves2° for the cervical cord, but in other respects their properties are similar. We were unable to assign units to a sensory fibre category because we did not distinguish between a direct input to the cervical cord or a relayed input such as that described by Craig (1978) ~ and Wall and Tauh (1962) 41. Arbab et al. (1988) 2 reported that HRP placed on the proximal portion of the middle cerebral artery produced labelling in the pars oralis of the trigeminal nucleus. Much more intense labelling was seen in the pars interpolaris of the trigeminal nucleus and a small amount of labelling was seen in the C 2 dorsal horn. The latter they considered to be of direct trigeminal origin and such a pathway could provide an explanation for the results we have reported here and elsewhere 2°. In the experiments we report here, we have used only electrical stimulation to activate neurones in the cervical spinal cord and it can be argued that this type of stimulation activates neurones which would not normally be activated under physiological conditions. However, as we report elsewhere, such units can also be activated by mechanical stimulation of the sagittal sinus or application of bradykinin to the sinus 2°. Some units in this area also receive convergent input from the teeth and can be activated by a cooling of the tooth pulp 2s. As we have previously reported 2°, we found cells with receptive fields on the face and the limbs. In our previous communication we reported that we found units with nociceptive specific receptive fields on the face but most units with facial receptive fields were found to be wide dynamic range units, while those responding to limb or paw stimulation usually responded to low threshold mechanoreceptor stimuli only. Our findings in this report are similar, although the proportion of units having receptive fields on the limbs is smaller. Convergence of input from f~ial receptive fields provides a basis for the referral of migraine pain, as pointed out by Davis and Dostrovsky ~ Strassman et al. 3~ and ourselves 2°. Referral of headache pain to the limbs in migraine and cluster

headache has often been reported (for reports and review, see Guiioff and Fruns, 1988) 13 and may be a consequence of the convergence we have found between limb and dural sensory input. Cervical spinal cord potentials produced by SSS stimulation were reduced or abolished by lesions of the first division of the trigeminal nerve but not by sectioning of the cervical rerves. We interpret this to mean that input to the upper cervical cord from the sinus arises almost exclusively from the trigeminal system and not from the cervical nerves. The observation that section of the cervical nerves increases field potential responses to SSS stimulation is evidence for a cervicotrigeminal interaction, an observation that may be relevant for cervicogenic headache. Field potential and single unit responses in the DLA could he reduced by i.v. injection of ergotamine in doses equivalent to those used to treat raigraine headache and abolished by higher doses 42. This suggests that ergotamine acts on the initial central stages of sensory processing and not only on the peripheral nemo-effector junction or the cranial vascular smooth muscle, sometimes considered to be the site of action of ergotamine in migraine headache 2~. Electrical stimulation of the cranial vasculature would be expected to stimulate sensory fibres directly, thus bypassing the sensory nerve endings and neuro-effector junction region. This supposition is supported by our observation that local vascular application of ergotamine in concentrations far higher than would be achieved in the SSS after i.v. administration failed to block the responses of DLA units to SSS stimulation. We can find no evidence in the literature that ergotamine acts as a local anaesthetic or that it in any other way blocks the conduction of impulses from the periphery through the ganglion to the first central synapse. iontophoretie application of ergot derivatives to units in the DLA also suppresses craniovascular responses. This supports our hypothesis that ergot alkaloids have a central action in migraine. These three ergot derivatives, which have been used successfully in the treatment of migraine (ergotamine, DHE and ergometrine)3° reversibly inhibited the responses of about one third of central neurones to electrical stimulation of the SSS. The reversible nature of this suppression, and the absence of suppression with saline application suggest that ergot alkaloids have a specific effect on neurones in this region. These results, taken with those of Saito et al. 23. suggest that ergot alkaloids affect the initial central synapses of a craniovascular afferent pathway, as well as the peripheral ends of these nerves. The suppression of responses to SSS stimulation by ergot alkaloids could be caused by presynaptic inhibi-

329

tion of neurotransmitter release or postsynaptic activation of endogenous serotonin receptors. Other neurochemicals such as glycine, GABA or the enkephalins, which have been shown to be present or have an action in the trigeminal and spinal systems, could also be involved in the action of ergotamine, but there is insufficient evidence in the literature to support such speculation. Although field potential responses and the responses of 5 out of 7 units were effectively suppressed by i.v. injection of ergot alkaloids, only one-third of units tested were reversibly suppressed by ergot alkaloids administered iontophoretically. Responsive units did not differ from non-responsive units with regard to latency of firing, spontaneous firing rate, response to DLH or receptive field type. Methodological difficulties associated with the technique of iontophoresis may have confounded measurement of the responses of some units to ergot alkaloids. Alternatively, field potential and single-unit measurements may reflect different, response properties of cell populations in the cervical cord. Although there are no reports that ergotamine is an analgesic in any other system, evidence has been presented that it can potentiate the analgesic effects of non-opioid compounds such as baclofen 3. Bartolini et al 3. postulated that ergotamine was acting in that instance on the descending serotonergic system, an action that could explain our results, given that such systems can affect processing in other trigeminal components •~7 and the LCN It. The report by Saito et al. "~3 indicates that ergotamine can block extravasation induced in the SSS by electrical stimulation of trigeminal ganglion in rats, presumably by blocking peptide release from sensory nerve endings. Such an action, occurring at the central rather than the peripheral ends of the trigeminal nerve, may underly the effect we have reported here. Acknowledgements. We gratefully acknowledge the discussion and criticism of this work offered by Professor James W. Lance, and the technical assistance of Mr. Mark HeWer, Mr. John Duckworth, Mr. Paul Charalambous and Mrs. Jane Peralta. This research was supported by a grant from the National Health and Medical Research Council of Australia, by the J.A. Perini Family Trust, the Basser Trust, the Australian Brain Foundation and Warren and Cheryl Anderson.

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