Tonic descending inhibition of the spinal somato-sympathetic reflex from the lower brain stem

Journal o f the Autonomic Nervous 8yBtem, 2 (1980) 157--182

157

~) EhwvierfNorth-Holland Biomedical Prt~

TONIC DESCENDING INHIBITION OF THE SPINAL SOMATO-SYMPATHETIC R E F L E X FROM THE LOWER BRAIN STEM

KLAUS DEMBOWSKY, JURGEN CZACHURSKI, KLAUS AMENDT and HORST SELLER Institute o f Physiology I, University o f Heidelberg, Im Neuenheimer Feld 326, D-6900 Heidelberg I (G.F.R.)

(R~.ceived February 11th, 1980) (Accepted April 28th. 1980)

K e y w o r d s : somato-sympathetic

reflexes -- descending inhibition -- dorsolateral funiculus -- catecholaminergic txansmission

ABSTRACT

In chloralose-anaesthetizedcats the spinal and supraspinalcomponents of the somato-sympathetic reflex were evoked in the white ramus at T3 and/or I.a by stimulation of intercostaland spinal nerves. A reversibleblockade of all ascending and descending spinal pathways was performed by cooling the spinal cord between the second and third cervicalsegment. Total blockade of conduction was produced at temperatures below 8.5°C (281.5 K). The spinal blockade produced the following reversibleeffects. (1) Mean arterial pressure fell to 30--50 mm Hg ( 4 . 0 - 6 . 7 kp,) and the tonic background activity in the white ramus was reduced to 0--24% of control (mean 12.1 ± 10.1%). (2) The amplitude of the early spinal reflex was increased from 100% to 111--316% (mean 200.9 e 49.5%, n = 49) at the thoracic level and to 125-34.2% (mean 181.4 ± 74.4%, n = 7) at the lumbar level. The onset latency of the spinal reflex at T3 (range 8--21 msec) was shortened by 0.5--3.0 msec (mean 1.7 ± 0.9 msec). (3) $upraspinal components were completely abolished. (4) Neithel baroreceptor denervation nor midcollicular decerebration ~tered these effects. (5) The cold block induced increase of the amplitude of the spinal reflex was reduced by the alpha-'~drenoceptor agonist clonidine; this effect was reversed b y the alpha-adrenocep+~or ~untagonist yohimbine. Selective cooling L . ,

L Parts of this study were presented ~t the 49th Meeting of the Deutsche Physiologische Gesellschaft, March 7--10, 1978 in G~tLingen, G.F.R. |24].

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of the dorsolateral funiculus caused the same effects on the spinal and supraspinal reflexes as cold block of the whole spinal cord. F~om these findings it is concluded that in the anaesthetized cat the spinal component of the somato-sympathetic reflex is modulated by a descending tonic inhibition. This inhibition acts at both the thoracic and the lumbar level and its origin is in the medulla oblongata. This inhibition is, however, independent of baroreceptor inputs. The pathway descends in the dorsolateral funiculus. It is suggested that noradrenaline or adrenaline might be invol~ed iv the transmission of this inhibitory influence.

INTRODUCTION It is now well established that both excitatory [3,15,32,36,43,46] and inhibitory [3,15,17,37,43,45,47,48,57] pathways of supraspinal origin as well as path'Nays originating from spinal afferents [51,66,78] converge onto sympathetic preganglionic neurones. Regarding the inhibitory control of sympathetic activity the most obvious aspect is the well-known inhibition exerted by activation of baroreceptor afferents. This reflex inhibition is transferred to the preganglionic neurones via the dorsolateral funiculus, although it is still unkaown if this inhibition takes place at the brain stem or at the spinal cord (for discussion see refs. 32, 36, 45). Several studies have revealed two b~oreceptor-i~dependent descending inhibitory systems: one in the dorsolsteral, the other in the ventrolateral funiculus. Stimulation of these pathway~ causes a reduction in blood pressure and in tonic background activity in pre- and postganglionic sympathetic nerves [ 15,43,45,57]. Since the work of Coote et al. [14], Kirchner et al. [47] and Rlert [42] it is known that descending pathways also e x e ~ an inhibitory action on the transmission of the spinal component of the somato-sympathetic reflex. Several reports in recent years concerning descending inhibition of the early spinal reflex have provided controversial results. It was concluded by some authors that baroreceptor afferents are involved in this inhibition at the soinal level [3,16,18] whereas others did not find any evidence for this hypothesis [ 14,25,47,52]. Furthermore, it has been reported that the descending inhibition of the spinal reflex is mediated by .. pathway running through the ventrolateral fun~culus [ 2 0 ] On the other hand, it could be shown that the amplitude of the spinal reflex can be reduced by excitation of inhibitory pathways both in the dorsolateral and in the ventrolateral funicu~us [15,17,42,44]. Barman and Wurster [3] assumed that the baroreceptor-inhibition of the spinal reflex descends in the dorsolateral funiculus [16,18] and the non-baroreceptor inhibition of this reflex descends in the ventrolat~ral funiculus. In addition, it has been sugge.,;ted by Kirchner et al. [48] that there are also propriospinal neuronal systems which exert an inhibitory effect on the spinal reflex and which are activated '~y stimulation in the lateral funiculus.

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There are also contradictory reports regarding the neuro-chemical transmission of the supraspinal inhibitory effect on the spinal reflex. The eerly spinal reflex could be depressed by serotonin (5-HT) or its precursor DL-5HTP [15,40] as well as by the precursor of noradrenaline, L-DOPA, and by alpha-adrenoceptor agonists [6,7,15,70]. An excitatory effect of the noradrenaline precursor L-DOPA on the spinal reflex was shown by Hare et al. [40]. In a more recent paper, Coote et ~d. [19] found no evidence for an influence of 5-HT on the spinal reflex. It was the aim of the present investigation to obtain more information about the descending inhibition of the ,-Arly ~pinal reflex. The following questions were asked. Is the descending inhibition tonically active? Where is the origin of the descending inhibition? Does baroreceptor denervation alter the effects of the descending inhibition? Which tracts in the spinM cord mediate the descending inhibition of the spinal reflex? Does the descending inhibition exert its effects by catecholaminergic transmitters? To resolve these questions experiments were undertaken using a reversible blockade of the spinal cord. METHODS

The experinlents were performed on 44 adult cats of either sex (2.0--2.8 kg body weight). After induction with ether, anaesthesia was continued with alpha-cl-loralose (60--70 mg/kg i.v.). Both ~emoral veins were cannulated for adr~ inistration of drugs and one femoral artery was cannulated for recording blood pressure. The trachea was cannulated and the cats were then paralyzed with hexacarbacholine (Imbretil: 0.5 mg/kg i.v.) and artificially ventilated. The ventilation was adjusted to maintain an end-tidal CO: of 3.5--4.0 vol.%. The rectal temperature was kept between 37 and 38°C (310-311 K) by means of a heai,ing pad or an infrared lamp. The wh~,te rami (WR) of the third thoracic (T3) and/or second lumbar (L2) segment were ~repared, dissected free from surrounding tissue and cut peripherally at thr junction with the stellate ganglion or the sympathetic trunk, respectively. *.n unsheathed white ramus was placed on a bipolar platinum electrode for l ecording nerve activity. The" activity was amplified (Tektronix AMS02, 2.0 [iz--3 kHz) and displayed on an oscilloscope (Tektronix 7603). The output of the amplifier was also connected to an averager (Nicolet 1072) and 16 consecutive reflex responses were averaged every 1 or 2 rain. The third and fourth thoracic intercostal nerves (IC) and/or the second and third lumbar spinal nerves (SpN) were prepared on the left side, cut peripherally and placed on bipolar silver electrodes for electrical stimulation. All exposed nerves "were kept in a pool of warm paraffin oil. The following stimulation parameters were used: a single pulse or two pulses st an interval of 2--4 msec, stimulation strength of 0.2--3.0 V for submaximal stimulation and greater than 3.0 V for supramaximal stimulation, pulse duration of 0.5 msec. The stimulation was repeated every 3--4 sec. In some experiments carotid sinus nerves and vagal nerves were bilaterally

I60

prepa_,~ before the beginning of the recording period for the later denervation of all tmroreceptors during the courie of a recordh~g period. The completeness of baroreceptor denervation was shown by an unc hsnged back. ground activity of the white ramus during a normirenalin~induced increase in blood pressure. A reversible blockade of all ascending and descendh~g spinal pathways was perf~_-'med by cooling the spinal cord i~:tween the second and third cervical segment. A laminectomy from C= to Cs was performed and the ventr~ and dorsal roots of C2 and C3 were cut. A hollow metal ring was placed around the spin,~l cord. For the purpose of coohng, the metal ring was perfused with cold ethanol. The temperature of the meted ring w~s measured at the surface of the spinal cord by a small thermistor probe (NTC-resistor with linear characteristics in the temperature range from 0 to 40°C (273--3] 3 K)). This temperature w ~ continuously recorded. In control experiments the dorsal funiculus cranial to the cold block was stim~flated and the evoked activity in the dorsal funiculus was recorded at the lew;1 of T3 (Fig. 1). Cooling the spinal cord resulted in a continuous reduction of the evoked potentials (Fig. 1A--I). At temperatu:es below 8.5°C (281.5 K) no evoked potentials could be recorded (Fig. 1J--K). Therefore in all experiments the temperature of the metal ring at the surface of the spinal cord wes kept at 4--6°C (277--279 K) during blockade by adjustments of the perfusion rate. Lowering the temperature to 4 - 6 ° C (277--279 K) required 90--150 sec. This blockade was fully reversible (Fig. 1L--T) when the spinal cord was rewarmed ~o body temperature by changing the perfusion from cold ethanol to warm water. Rewarming the spinal cord was achieved in 60--180 .~ec. the duration of the cold block was in most c~es 8--10 rain, but even after a cooling period of 24 rain complete recovery of WR activity was seen. In control experiments the temperature inside ~he spinal cord was measured by a needle thermistor. There was a maximal difference between the telaperature at the surface and the middle of the spinal cord of 3°C (3 K). A furtner indication for the completeness of the cold block was obtained by making a transection of the spinal cord cranial to the metal ring during a cooling p~riod. The sympathetic background activity in the white ramus and the blood pressure were not altered by this procedure. In 6 experiments the tips of both halves of the metal ring were placed bilaterally on the surface of the spinal cord just laterally to the entry zone of the dorsal rootlets for the purpose of sel~-cLwe cooling of the dorsolateral funiculus. In control experiments the temperature within the spinal cord was also measured by a needle thermistor during this selective cold blockade. Below the tips of the metal ring, i.e. in the dorsolateral fmdculus, the temperature was 6--7°C (279--280 K), at the most lateral smface of the spinai cord it was 2 0 - 2 1 ° C (293--294 K), and in the ventrolateral funiculus the temperature was 22.5--23.5°C (295.5--296.5 K). The average reflex responses were photographed and their amplitudes were measured from projected enlargements. The mean value of the control ~eflex responses was set at 100% and changes in reflex amplitudes were expressed in percentage of ~.hese control values. The mean values of control

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162

reflex amplitudes before and after the cold block as well as the mean value of reflex amplitudes during the cold blockade were calculated for each cooling series. Means were always expre~,'~i as means ± S.D. The Student's t-test was used to evaluate statistical significance. Oniy series in which there was a signific~t,~t increase of the reflex amplitude during cold blockade (P 0.05) are reported here. In some diagrams, however, the refle), amplitrde was given in pV. For pharmacological studies the folio,~ing drugs were injected intravenously: clonidine (10--30 pg/kg) end ychimbine (0.::,0---1.00 mg/kg) (supplied by C.H. Boehringer Sohn, Ingelheim, G.F.R.). RESULTS Effects of spinal blockade on blood pre~'ure and background activity in ~f~e white minus

A reversible blockade of all ~cending m,d descending ~pinal pathways by cooling the spinal cord between C2 and C3 to temperatures of 4--6°C (277-279 K) resulted in a decrease of mean arterial pressure to 30--50 mm Hg (4.0--6.7 kp,). This reversible f~l in blood pressure: was taken as an indication that the spinal cord was not injured by the surgical procedures. This test was routinely ca.,xied out belore the recordin;, pe,iod. Data were not evaluated ~om experiments in which this decrease in blood pressure could not be produced by the cold block. In order to exclude any influence of the low blood pressure on the transmission of the somato-sympathetic reflexes the blood pressure w:3s then kept at control levels during the cold block by a continuous infusion of angiotensin (2--25 pg/kg/min) or noradrenaline ( 1 - 1 5 pg/kg/min) (Fig. 2A). The background activity in the white ramus (Wit) at T3 and I~ was markedly reduced during ~;he spinal cord blockade. A quantitative analysis of the background activity was performed by means of counting the number of impulses in the WR-T3 mass activity and in WR multifibre activity (between 5 and 10 units/strand). By this method it was shown that the back~'ound activity was reduced to 0--24% of control values (mean 12.1 ± 10.1~, n = 14) during the cold block. These results were confirmed in experiments with sing]e fibre recordings (n -- 8). Most of the single units (n = 5) stopped their spontaneous firing during the period of cooling (Fig. 2B), the rest showed only a reduction, no change or even an increase in their activity. No quantitati~ ~ analysis of the background activity was made at the lumbar level. Some ~imultaneous recordings of WR-L2 and WR-T~, however, revealed that the backglound activity in WR-T3 was reduced to a greater extent than activity in WR-L2 during the cold block.

Effects of spinal blockade on somato-sympathetic reflexes The spinal and supraspinal component of the somato-sympathetic reflex was evoked by sub- or supramaximal stimulation oi somatic group II and III

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164

IC-T3 and it was 13.1 +- 2.5 msec (range 10--21 msec) by stimulation of IC-T4. In 5 experimer~ts IC-T3 and IC-T4 were alternatively stimulated. The onset latency of the spinal reflex by stimulation of IC-T3 was 2--5 msec (mean 3.6 ± 1.1 msec) shorter than by stimulation of IC-T4. At the lumbar level (WR-L2) the corr,:sponding data for the spinal reflex evoked by stimulation of SpN-L2 or Spb!-L3 were as follows: the amplitude ranged from 6 to 22 ~V (mean 13.3 ± 5.7 ~V, 7 series in 3 cats); the onset latency ranged from 6 to 9 msec. In all but one experiment a supraspinal component of the somato-sympathetic reflex was observed. The amplitude of the supraspinal reflex varied greatly but mostly it was as large or larger than the amplitude of the spinal reflex. The onset latency of the supraspinal reflex it Ts ranged from 30 to 73 msec (mean 42.3 ± 12.3 msec), the peak of the r~.~flex amplitude occurred at 47 to 103 msec (mean 56.3 *- 13.7 reset). The data for the supraspina] reflex at L., were: onset latency 56 to 76 msec (mean 67.3 +- 10.3 msec) and peak amplitude at 68 to 110 msec (mean 89.0-+ 29.7 msec). In 2 experiments a third reflex component, the so-called suprapontine reflex [65], vas observed (onset latency 178 and 215 msec). During the cooling period the following effects on the somato-sympathetic reflexes w e r e observed: the amplitude and duration of the spinal component were significantly increased, its latency was decreased, and the supraspinal and sugrapontine components were completely abolished. The amplitude of the spinal reflex at the thoracic level was increased from 100% to 111--316% during the cold block (mean value of 49 series 200.9 ± 49.5%). O~e representative series is shown in Fig. 3A; Fig. 4A illustrates the summary of all 49 series at the thoracic level. The largest amplitude was usually observed 2--4 rain after beginning of the cooling. Thereafter in most cases a stable plateau of the reflex amplitude was achieved during the mairttamed cold block. There was no difference in the increase of the reflex amplitudes during the co~d block between stimulation of IC-T3 or IC-T4. In all these series stimulation strength was submaximal for the spinal reflex. In 4 exlm.~riments stimulus strength was increased from sub- to supramaximal values. This resulted in an increase of the spinal reflex amplitude of 45--30i~9~ (mean 104 ± 85%). During supramaximal stimulation the spinal cord blockade produced an increase of the reflex amplitude which was 3 7 - 1 0 1 % (mean 71 ± 21%; n = 8) smaller as compared to the cooling series of submaximal stimulation in the same animal. In 13 experiments the latency of the spinal reflex was shortened by 0.5--3.0 msec (mean 1.7 + 0.9 msec) during the spinal cord blockade; in 7 exp(:riments no shortening of the latency could be observed. The reflex amplitud~.~ declined to control level within 1--4 rain after each cooling period. The mean values of the post-cooling amplitudes ranged from 55 to 154% (mean 110.6 ± 19.5%). Ir each experiment several (up to 5) consecutive cooling periods showed no considerable variations in the effects of the cold block on the spinal reflex. The effects of the cold block on somato-sympathetic reflexes at the lumbar level (WR-L2) were studied in 3 additional experiments. In 7 cooling

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167 At the thoracic as well as at the lumbar level the supraspinal components of the tomato-sympathetic reflex were completely abolished during the entire length of the cold block. Averaging during the decline of temperature usually revealed an abolition of the suprupinal reflexes before the amplitude of the spinal reflex was increased and before the background activity was reduced. At the end of each cooling period the supraspinaJ reflexes were re-established in full size.

Effects of selective blockade of the dorsolateral funiculus on background and reflex activity In an attempt to localize the pathway in the spinal cord by which the inhibitory control of the spinal reflex is transmitted the dorsolateral funiculus (DLF) was selectively blocked by cold in 3 experiments. The data were compared with results obtained by cold block of the whole spired cord in the same cat. Cooling the DLF bilaterally resulted in an increase of the spinal reflex amplitude (Fig. 5B). There was no significant difference in the increase of the spinal reflex amplitude whether cooling the whole spinal cord (n = 10) or only the DLF bilaterally (n = 7). In one cat the spin-,d cord blockade produced increases or" the reflex amplitude to mean values in the range of 197--261% and bilate.ral DLF blockade showed mean values of 200-226% (Fig. 5). Cooling of only the left DLF (n = 2, ipsilateral ~.o the side of recording and stimulating) resulted in an enhancement of the reflex amplitude to half of the increase observed by cold block of the whole spinal card or of both dotsolateral funiculi. In all cases no supraspinal reflex component could be recorded during each kind of cooling. In one experiment bilateral selective cold block of the DLF resulted in a reduction of the background activity to 28--30% of control values, whereas comparable blo~'kade of the whole spinal cord showed a reduction to 9 - 2 4 % of control values.

Effects of spinal blockade on tomato-sympathetic reflexes belt)re and after a ecerebration The increase of the amplitude of the spinal reflex during cold block was tested before and after midcollicular decerebration in two ather experiments. Before decerebration the mean values of the reflex amplitudes during the cold block ranged from 211 to 335% (4 series in 2 cats). During the process of suctionivg the brain rostral to the tentorium cerebelli, the spinal as well as the supr.tspinal reflex was absent for 4--6 min. After this period, in one cat both reflexes were fully re-established, whereas in the other cat the amplitude of I>)th reflexes was smaller than before decerebration. In this case a partial reco, rery of both reflexes to pre-decerebration amplitudes was observed within one hour after decerebration (Fig. 6A). The mean arterial blood pressure was. kept at the same level as before decerebration by infusion of a 6% dextran solution. After reaching a stable blood pressure further coolings of the spinal cord were performed (n = 3 series). During spinal

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170 blockade the stune effects as before decerebra~ion were seen (Pig. 6A). The spinal reflex an~plitudes were increased to the ,mine level as before decerebration and the sul)raspinal reflex c o m p o n e n t was reversibly abolished.

Effects of spiw.wl blockade on somato.sympatiwtic reflexes before and after baroreceptor d'enerva tion In a series ,~[ experiments (4 cats) the effc~t of complete baroreceptor denervation WzL~tested on the spinal reflex component and on the increase in reflex amp?~mde :luring cold blocksde. When the baroreceptors were denervated thez.~ wa~ an increase in arterial blood pressure within a few m!~nutes accoml~anied by an enhanced WR background activity. The amplitl:de of the spinal ~.~flex was not changed by this procedure (Fig. 6B) whereas the amplitud.~ of the supraspinal component was clearly increast~l aJ~ter baroreceptor der~ervation. The increase of :.he spinal reflex during the spinal cord blockade was not altered after the denervation. The series of cocling experi~cnts before and after denervation showed an enhancement of the spinal rel~iex amplitudes during the cold bl,)ck to mean values which lay within the saxae ow,raU range for each cat (Fig. 6B).

Effects of clonidi,w ant! yohimbine

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After the pre~ious e:.'perimentshad revealed the existence of a descending pathway modulating the spinal component of the somato-sympathetic reflex an atbempt was wade to answer the question of whether a catecholaminergic transmitter is inv,)Ived in the transmission of this effect. In 16 cats the influences of alph~dreaoceptor agonistic or antagonistic drugs were tested on the spinal reflex component. Clonidine as an alpha-r.-'ceptoragonistic and yo~imbine 8s an "~Ipha-receptor antagonistic substanc., were used in these experiments. In (; expeliments clonidine was given i.v. in a dose of 10--30 pg/kg. In ~I cases it was injected during spinal cord blockade. Clonidine then cav~ed the enhanced aml)litude of the spin&W reflex to decrease significantly by 4 2 - 1 9 8 % (me~tn 95.~ ± 57.8%) within £ - 4 rain after injection (Fig. 7A and C). These levei',s of reflex amplitude lay within the same range as precooling values (Fig. 7A). The subsequent rewarming of the spinal cord always ;esultvd in a further decrease of reflex amplitudes. Subsequent cooling periods revealed that the effect of clonidine lasted for more than one hottr (Fig. 7A). In addition, the amplitud,~, of the spinal reflex w~s decreased to the same exten|; when clonidine was given without spinal bl,~ckade. The described effects c.f clonidine were seen at both ~he thoracic and lumbar lew:l. In another 4 experiments yohimbine was inj~t.~cl i.v. in a dose of 0.4-1.0 mg/kg. In these experiments yohimbine was always given prior to spinal co~J blockade. An injection of yohimbine resulted ia a significant ;ncrease in the amplitude of the spin,~ reflex from 100% to 128--155% (mean 139.9 .+11..3%) (Fig. 7B). This increase, however, was alwa:1s less than the enhance-

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Fig. 7. Effects of clonidine and yohimbine on the spired sc matolympathetic refh,x. A: clonidine was injected intravenously during spinal cord blockade at the marking arrow Subsequent cooling periods show the diminishing effect of (lonidine. Stimulation parameters for IC-T4: one impulse every 3 sec, 0.6 msec, 0.8 V, B: yohimbine was injected intravenously between two cooling periods. Spinal somato-s} mpathetic reflex was evoked by stimulation of IC-T4 (one impulse every 3 sec, 0.6 m~:c, 0.8 V). C: clonidine and yohimbine were successively injected durin8 one cooling pel iod. After a second injection of clonidine which had no effect on the spinal reflex, yohir~bine "sis able to reverse the effect of clonidine. Stimulation parameters for IC-T4: tw,, impulses at 260 Hz every 3 sec, 0.5 nu~ec, 2.0 V. Results in A, B and C are from 3 different cats. Upper trace in each diagram shows the temperature at the surface of the spilLal cord at C~/Cs.

172 ment induced by cold block (Fig. 7B). Yohimbine exerted its effect within 1--2 min of injection, but the increased ampli~'ude was only maintained for 3--9 min. Thereafter it dec.~ined to control levels. An antagonism between these two drugs wa,~ also demonstrated with respect to their effect on the spinal reflex (Fig. 7(;). In one group of experiments (n ~ 2) yohimbine was injected 75--85 rain prior t~) a clonidine injection. Yohimbine caused the u~'ual increase in reflex amplitude whereas c l o n i d i n e - given during the cold block -- produced no further significant decrease (P ~ 0.05). In another 5 experiments both drtlgs were successively injected during one cooling period. The enhancement of the amplitude of the spinal reflex during cold blockade was again significantly decr.~ased by clonidine (20--25 /Jg/kg, i.v.). This decrease in reflex ar~plitude was reversed by yohimbin ~. (0.3--1.0 mg]kg, i.v.) given 8--20 rain after clcmidine (Rig. 7C). The incre~ se after yohimbine was always significant (P < 0.01). At the end of the cold block there was always a significant decrease of the amplitude though vaLles within the control range were not ~dways observed. In additional experiments yohimbine was injected during spizml co::d blockade without a prior administration of clonidine. During cold blockade the increased amplitude of the spinal reflex was not significantly altered ~Lfter an injection of yohim|)ine (0.3---0.4 mg/ kg, i.v.). A subsequent, h~jectic, n of clonidine (20---25/~g/kg, i.v.) also did not affect the amplitude ~,f l he spinal reflex. At the end of the cold block, however, there was always a signifi~'ant decrease of the reflex amplitude.

Effec;s of spinal block,zde and :lonidine on somato-somatic reflexe~ In 4 additional experir~lents an attempt was made to test the possibility of whether the inhibitory ~;y~tem contxolling the spinal somato-sympathetic reflex also acts on spinal somato-somatic refl~:o:es. Therefore either IC-T3 or IC-T4 was stimulated (stimulation parameters: 0.2--2.0 V: 0.5 msec; one impulse every 3--4 sec) and the somato-somatic reflex was recorded from the adjacent intercostal nerve. The amplitude c,f the somato-somatic r e f e x ranged from 4 to 36 ~tV ( m e ~ 13.3 ± 10.8/JV). The onset latency of this reflex ranged from 4 to 7 msec, thus being cor,~iderebly shorter than for the spinal somato-sympatheti~' reflex. During spinal cord blockade (n -- 8 cooling series) the same effects a~ described for the so ~lato-sympathetic reflex were seen: the latency was shortene(l, the duration was increased and the amplitude was significantly enhanced from 100% to 141--476% (mean of 8 series: 243.8 ± 17.4.8%) (Fig. 13A). In J of these experime,.~ts (n = 6 cooling series) it could be demonstrated I;hat bilateral cold block t,f the DI,F also ca~Jsed an increase of the reflex Ea-a~)litud.~ (Fig. 8B). As for the spinal soma*~)-sympathetic reflex there was no signi,'icant difference in the increase of the reflex amplitude whether the wJlole spinal cord or only the DLF ~vas cooled (Fig. 8A and B). In 3 e:~p,~:riments tests were performc-:! to see whether the somato-somatic reflex ,~-ar~ also be depressed by clonidine (Fig. 8C). The increase in re~ex ~mpli~u.:le during cold blockade was readily reversed to contl:ol level after i.v. in~vctiort of cionidine (25 /~g/kg). The subsequent

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174 re-warming of the spinal cord resulted in a further decrease of the reflex aml:litude. DISCUSSION These results indicat,~: that the spinal component of the somato-sympathetic reflex is contr,~lled by a tonic bulbospinal inhibition which acts independently of a bazoreceptor input. The transmission of this inhibition to the spinal cord occurs ,d:~ the dorsolat.~ral funiculus and might be mediated, at least in p~rt, by nor~.drenaline or adrenaline. Firstly, the method of reversibly inl:errupting pathways in the spinal cord by me:ins of cold block shall be disetLssed. Previous studies concerning the effect of temperature oL~ the condu,.'tion of nerve impulses in peripheral nerves revealed that cond Jction is corr pletely blocked at temperatures in the range of 4 C,°C (277--279 K) [56,61,62]. These data are also relevant for the spinal cord as was ~h,)wn in contlol experiments (Fig. 1). Therefore the tt:mpe]-ature at the su~:fvce of the si)inal cord was kept at 4 6°C (277-279 K) during any cooling period in ;hese experiments. The temperatme in the middle of the spinal c,~rd was 2--3°C (2--3 K) above the surface temperature. :['his temperature, howew~r, wa.~ still sufficiently low to produce complete blockade (Fig. 11,. The following observations during any cooling period support the completeness of the cold block: (i) a fall in m-terial blood pressure similar to that seen after spinal cord transection; (ii) disappearance of the supraspinal coml~,onent vf t h . somato-sympathetic reflex; (iii) reduction of the background activity in t? e white rami (WR); and (iv) no further reduction of background ~ctivity when the spinal cord was transected during cooling period cranial to the cold block. Another question that vzises is whether the effects of the cold block may })e at least partially produced by some central actions of the intravenously ~dministered drugs noradrenal~e or angiotensin which were used to comI)ensate the fall in blood pressure [47]. This possibility can be ruled out firstly by the observatic,n that all the described effects could also be seen ,#hen the fall in blood pt'e.,;sure was not compensated. Secondly, when some ncradrenaline had ,.~nteled the spinal cord [27] it would counteract the ,.'f~'~t of the cold block. The background activity in the white ramus was reduced to about the :;arne ..~xtent during cold block of the whole spinal cord as during bilateral cold block of the dorsol~tteral luniculus alone. Thus it was confirmed that a tonic excitatory input to sympathetic preganglionic neurones is transmitted t~hrough the dorsolateral ftD,~iculus (DLF) [32,43,45,46]. At the same time the supraspinal component of the somato-sympathetic reflex was abolished durm~: both kinds of cold t:lock {Fig. 5) as could be expected from findings ~)n th,.~ descending pathway of t~is reflex component [33,72]. Since it was showr however, that the ;ffferent ascending pathway of the supraspinal :.~eflex also runs ~hrough th~ dora)lateral sulcus at'ea and t~e DLF [11--13] ~t cannot be decided on th~ basi,~ of the present experiments whether only

175 one or both of these pathways were i,ltermpted by th e cold block.

The increase in amplitude and duration and the decrease in latency of the spinal component of the somato-sympathetic reflex during spinal cord blockade is due to the removal of a descending tonic inhibition. It might also be argued that the transmission in the spinal reflex arc is improved by a reduced tonic bac]cground activity in the whole white ramus and therefore the spinal reflex is ~nhanced during cold block. But the following findings are contradictory to this assumption: (i) the firing ra~e of single preganglionic neurones lies in the range of 1--2 irnp/sec [64,68] and their refractory period is 11--15 msec [31,64]; (ii) only a small fraction of the population of the tonically active preganglionic neurones at the thoracic level shows a spinal reflex i68]; (iii)changes in background activity which occurred for example after baroreceptor denervation or during baroreceptor activation did not significan~,ly al*,erthe amplitude of the spinal reflex. Thus it is improbable that the amplitude of the spinal reflex in the whole white ramus is limited by the background activity of preganglionic neurones and their refractory period. From these experiments it is evident that baroreceptor afferents in the carotid sinus and vagal nerves do not support the tonic inhibition of the spinal somato-sympathetic reflex. Variation of the baroreceptor input by an increase in blood pressure or by denervation of all baroreceptors also does not affect the spinal reflex [14,25,47,52]. Electrical stimulation of the carotid sinus nerves, however, produces a depression of the early spin~l reflex [16,18]. As was shown in a previous report [25] this depression is most likely due to the phenomenon of post-excitatory depression. The results obtained by Barman and Wurster [3] which showed a slight but significant reduction of the spinal reflex by a noradrenaline-induced increase in blood pressure are not conclusive because of the lack of control experiments in baroreceptor-denervated cats. Thus, in the cat there is no evidence so far for an action of the baroreceptors on the transmission of the spinal somatosympathetic reflex. O n the other hand, evidence was put forward by Gebber et al. [35,36], McCall et al. [60], Snyder and Gebber [71] and Taylor and Gebber [73,74] and by Barman and Wurster [3] to show that there might be baroreceptor inhibition at the spinal level. It was shown that the response in the sympathetic activity which was evoked by electrical stimulation of descending spinal pathways could be depressed by baroreceptor activation. Since there is a differential action of baroreceptors on different sub-populations of the sympathetic preganglionic neurones [68] it is possible that the population responding to spinal input is not affected by baroreceptor afferents, whereas that responding to descending input is. In general, it is well accepted that clonidine acts as an alpha-adrenoceptor agonist [49,67]. There are studies i~ the literature which indicate that clonidine stimulates both pre- and postsynaptic alpha.adrenoceptors [9,39, 50,54]. In part, it seems to depend on the dose of clonidine whether it acts preferentially on pre- or postsynaptic alpha-adrenoceptors [2]. These different actions of clonidine can be antagonized by different alpha-adrenoceptor blocking agents. Yohimbine and piperoxane preferentially block pre-

176

synaptic receptors [54], whereas phenoxybenzamine primarily acts on postsynaptic alpha-adrenoceptors [2,54]. In the present experiments it is unlikely that the effect of clonidine is due to stimulation •f presynaptic receptors which might be located on the axons of the descending inhibitory pathway described here, since the regulation of transmitter release by presynaptic receptors depends on the presence of orthodromically conducted nerve impulses [38]. A I ~ i b l e presynaptic site of action of clonidine at primary afferent terminals, howe'aer, cannot be excluded. It was also propo"oed that low doses of clon:dine ~;timulate adrenaline receptors in the spinal cord, w|~a:eas higher doses of clq~nldine .~timulate noradrenergic receptors [10]. Tne effects of cloni,line on ~tdrenaline receptors can be antagonized by yohimbine and piperoxane [ 101. An anatomical basis for this suggestion was provided by H6kfelt et al. [.,11] who demonstrated the existence of adrenaline neurones in *,he central ~ervo'~s system of the rat. Furthermore, it was assum(~l that clonidine m igh,. also stimulate 5-HT receptors on sympathetic preganglionic neuron,.~s and thus l~ad to the depression of the spinal somatosympathetic reflex [34]. The pr~;ent results with clonidine cannot clarify these controversial points, bttt they support previous studies in which other catecholan~inergic agonists ~,ere used. In these reports is was shown that L-DOPA, alpha-methyIDC|PA as well as clonidine can depress the spinal .~omato-sympathetic refle~ [6,7,1!~,70]. Therefore, it seems reasonable to assume that at lea~t part of the inhibitory control of the spinal somato-sympathetic reflex is mediated b:f cat~cholaminergic pathways. The possibility, however, that there migh", b~.• an ~Idditional serotonergic pathway with an inhibitory action on the si~in~fl reflex [15,40] was not t ~ t e d in the present experiments. Yohimbine is widely accepted a~; an alpha-adrenergic blocking agent [49, ~}7] although there are report..~ which indicate an agonistic [63] or antagonisiic [69] action on serotonergic recG~ptors. As was shown by Sinha et al. [70] tile depression of the spinal reflex by clonidine was readily reversed by yohimbine. The present res~dts ¢,btained with yohimbine, however, are puzzling since yohimbine igiwn up to 85 rain prior to clonidine was able to pre'lent the reduction of the spin~d reflex amplitude by clonidine. On the other hand, the release c.f ti~e sp:nal reflex from the tonic inhibition by yohimbine lasted for only 3--9 rain. Koss and Bernthal [53] reported that in (~ts the electrodermal reflex as a part of the soma~o-sympathetic reflex is significantly increased by yohimbine in the same dose that wa.,~ used in the p r e ~ n t experiments. This lell to the conclusion by the authors that this reflex is released from a i;onc inhibition descending from the. lower brain stem. In contrast to the findings i11 this study, the effect of yohimbine on the electrodermal reflex lasted for ~t least 1--2 h. The short-lasting increase of the spinal reflex after an injection of yohumbine does not s11pport the suggestion that the inhibitory cc.mtrol of the spinal reflex is e~.erted by catecholaminergic p~th;vays. IL c~uld be shown, however, that yohimbine did not alter the increased arrtpli~ude ,~f the ~pinal reflex whe~ it was intraver, ou.o.ly injected ~iu~i~]g the spinal cord blockade in the ~bsence of clonidine.

177

A subsequent injection of clonidine durhlg the same cooling period did not decrease the amplitude of the spinal reflex. Therefore, an unspecific mechanism of action ~f yohimbine can be excluded. The origin of the descending tonic inhibition is confined to the brain stem as it was still present after midcollicular decerebration. This is consistent with a previous study [14]. There have been several attempts io identify the sites of the origin of this tonic inhibition in the brain stem. Illthese experiments either electrical stimulation or rew:rsible cold blockade of medullary regions was employed [15,17,21,47]. In view of the limitations of these methods and the suggested c~techolaminergic nature of the tonic inhibition it is assumed that area AI and also to a lesser degree area A2 [22] are the possible sources for this inhibition. Neurones of both areas have been shown to send axons to the spinal cord [23]. [n addition, adrenaline-containing neurones were detected in both of these areas and were called C~ and C2 respectively [41 ]. In the present experiments it was shown that the tonic bulbospinal inhibition acting on the early spinal reflex is mediated to the spinal cord mainly via the dorsolateral funiculus, since there was no difference in the amount of increase of the spinal reflex either during spinal cord blockade or during D L F blockade. This localization of an inhibitory pathway was also found by lllert [42], Illertand Gabriel [441 and by Coote and MacLeod [15,1 i], but they showed that there is an additional inhibitc,rypathway in the ventrolateral funiculus. In a more recent study, Coote and Sato [201 showed th~.t section of the ventrolateral funiculus alone eliminates the inhibition of the spinal reflex. A n inhibitory pathway in the ventrolateral funiculus was also demonstrated by Barman and Wurster [31. From the present experiments it cannot be excluded that some faster conducting axons in the dorsal zone of the ventrolateral funiculus were also blocked during selective cold blockade of the D L F as it was revealed from temperature measurements during this kind of cooling (see Fig. 1C--E~. Anatomical observations that axons of AI neurones descend in the D L F [5,23,551 support the suggestion tha*~this area is involved in the bulbospinal inhibition acting on the spinal reflex. :n addition, the reduction of the spinal somato:sympathetic reflex which is ,~roduced by electrical stimulation within the ventrolateral medulla is abolished after section of the D L F [17]. Regarding the nature of this inhibitory system there are stillother open questions. D o short catecholaminergic fibres act on a propriospinal system which then inhibits the spinal reflex [481, or is this inhibition mediated by long descending fibers? Are the descending fibres excitatory to inhibitory interneurones in the spinal cord or are they inhibitory to excitatory neurones? Does the inhibition take place at the interneurones in the reflex pathway or directly at the sympathetic preganglionic neurones? Like the spinal component of the sornato-sympathetic reflex there are other spinal reflex pathways t, at are tonically inhibited from the brain stem: viscero-somatic reflexes [1,26], electrodermal reflexes or galvanic skin reflexes [53,75,771, and somato-sornatic reflexes [26,28,581. The inhibition

178

of these spinal reflexes shows some c o m m o n properties: (i) it is always pre~ent after decerebratio n [1,26,~:8,58,75,77 ]; (ii)the tonic inhibitory control is not supported by baroreceptor afferen*~ [1,53]; (iii)except for the electrodermal reflexes it has be~.n shown that the inhibitory pathway descends in the dorsolat~,ral funiculus of the spinal cord [26,28,58]; and (iv) except for the viscero-:~)ntaticreflexes ithas been proposed that the inhibitory control is exerted by descending catecholaminergic pathways [53,58] (Fig. 8C). The ventromedial reticular formation which seems to enclose ~lagoun and ~,hines' inhibitory centre [59] and the magnocellular tegmental field according to the nc,m,~:nclatuze of Berman [8] are supposed to be sources for maintaining the ~nic inhibition of the spinal reflexes [1,26,28, 76,77]. Axons of neurones in this area descend in the dorsolateral and ventrolateral funiculus in the ~pinal cord [4,5]. Due to the lack of more detailed anatomical description it remains unclear how far area A~ and/or C~ are also involved in sustaining the tonic inhibition. The dorsal reticulospimd ..~ystem which has an inhibitory action upon somato-somatic reflexes de~ends in the D L F and originates in Magoun's inhibitory centre in the brain stem [28,58]. It could be demonstrated that the dorsal reticulospinal system acts on the interneurones of the spinal somato-s~matic reflex pathway [2~I. Due to its conduction velocity (faster than 20 m/sec) this inhibitory system is suggested to be non-monomninezgic [28]. O n the other hand, it could also be shown that the somato-somatic reflexes are depressed by L-DOPA, DL-5-HTP, and by clonidine [30,58] (Fig. 8C), thus indicating also a monoaminergic inhibitory control of the somatoThe similarities in the tonic inhibition of these spinal reflexes lead to the suggestion that there is ore c o m m o n catecholaminergic inhibitory system probably acting on interneurones of polysynaptic spinal reflex pathways. As shown for some of these reflexes there seems to be an additional serotonergic descending inhibitory system [15,30,40] originating in the raphe nuclei. Furthermore, the somato-somat~c reflexes are also tonically inhibited by a non-monoaminergic system [28]. ACKNOWLEDGEMENTS "2his study was supported by the Cerman Research Foundation within the SFB 90 "Cardiovascul~r,~ System". The authors are greatly indebted tc Miss Helga Schmidt-Prestin for her invaluable technicaJ assistance and to Mrs. D. Fischer.Barnicol for typing the manuscript. Thank.,; are due to Dr. G. S~ock m~d Dr. M. Spyer for comments and criticism during the preparation of the manuscript. REFERENCES

1 Alderson, A.M.a.~d Downmm, C.B.B., Supraspinai inhibition of thoracic reflexes of somatic and visceral origin, A~ch. ital. Diol., t04 (1966) 309--327.

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2 And~n, N.-E., Grabowska, M. and Str~mbom, U., Different alpha-adrenoceptors in the central nervous system mediating biochemiclU and functional effects of clonidine and receptor blocking qcnts, Naunyn-Schmiedt berg's Arch. exp. Path. Pharmak., 292 (1976) 43--52. 3 Barman, S.M. and Wurster, R.D., Interaction of descending spinal sympathetic pathways ana tLfferent nerves, A~.~er. J. Physiol., 2~,4 (1978) H223--H229. 4 B u b a u m , A.I., Clanton, C.F.. and Fields, H.L., Three bulbospinal pathways from the rostral medulla of the cat: an autoradiographic study of pain modulating systems. J. comp. Neurol., 178 (1978) 209--224. 5 Basbaum, A.I. and Fields, H.L., The origin of descending patt~ways in the dorsolateral funieulus of the spinal cord of the cat and rat: further studi~b on the anatomy of pain modulation, J. comp. Neurol., 187 (1979) 513-~32. 6 Baum, T. and Shropshire, A.T., Susceptibility of spontaneous sympathetic outflow and sympathetic reflexes to depression by clonidine, Europ. J. Pharmacol., 44 (1977) 121--129. 7 Baum, T. and Shropshire, A.T., Evidence f o in inhibitory action of methyldopa on spinal sympathetic reflexes, Europ. J. Pharmac ~1., 46 (1977) 259--253. 8 Berman, A.I..., The Bran.nstem of the Cat. A C ~,toarchitectonic Atlas with Stereotaxic Coordinates, Univ. of Wisconsin Press, Madison, Wisc., 1968. 9 Berthelsen, S. and Pettinger, W.A., A functional basis for classification of ~-adrenergic receptors, Life Sci., 21 (1977) 595---606. 10 Bolme, P., Corrodi, H., Fuxe, K., H~kfeit, T., l,idbrink, P. and Goldstein, M., Possible involvement of central adrenaline neurons ill vasomotor and respiratory control. Studies with clonidine and its interactions with piperoxane and yohimbine, Europ. J. Pharmacol., 28 (1974) 89--94. 11 Chung, J.M. Webber, C.L. and Wurster, R.D., Ascending spinal pathwa~,s for the somatosympathetic A and C reflexes, Amer. J. Physiol., 237 (1979) H342--H347. 12 Chung, J.M. and Wurster, R.D., Ascending pre~or and deprer~,or pathways in the cat spinal cord, Amer. J. Physiol., 231 (1976) 786--792. 13 Coote, J.H. and Downman, C.B.B., Central pathways of some autonomic reflex discharges, J. Physiol. (Loncl.), 183 (1966) 714--729. 14 Coote, J.H., Downman, C.B.B. and Weber, W V., Reflex dis~.harges into thoracic white rami elicited by somatic and visceral afferent excitation, J. Physiol. (Lond.), 202 (1969) 147--159. 15 Coote, J.H. and MacLeod, V.H., The influence of bulbospinal monoaminergic pathways on sympathetic nerve activity, J. Physiol. (Lond.), 241 (1974) 453--475. 16 Coote, J.H. and MacLeod, V.H., Evidence for the involvemen ~.in the baroreceptor reflex of a descending inhibitory pathway, J. Physiol. (Lond.), "~41 (1974) 477 -496. 17 Coote, J.H. and MacLeod, V.H., The spinal route of sympatho-inhibitory path'~ays descending from the medulla oblongata, Pfli~l;ers Arch. ges. ]>hysiol., 359 (1975) 335--347. 18 Coote, J.H. and MacLeod, V.H., The effect of i~raspinal microinjections of 6-hydroxydopamine on the inhibitory influence exerted on spinal sympathetic activity by the baroreceptors, Pfltigers Ar¢h. ges. Physiol., 371 (1977) 271--277. 19 Coote, J.H., MacLeod, V.H. and Martin, I.L., Bt~lbospinal tryptaminergic neurones. A search for the role of bttlbospinal tryptaminergic neurones ill the control of the sympathetic activity, Pfliigers Arch. ges. PhysioL. 377 (1978) 10~--116. 20 Coote, J.H. and Sato, A., Supraspinal regulation ~f spinal reflex Oischarge into cardiac sympathetic nerves, Brain Res., 142 (1978) 425--437. 21 Czachurski, J., Amendt, K., Dembowsky, K. and Seller, H., The effect of reversible cold block of dorsomedisl or ventrolateral regions in the brainste,n on blood pressure, sympathetic activity, and respiration, Pfliigers Arch. ges. Physiol., 379 (1979) R48. 22 Dahlstr~m, A. and Fuxe, K., Evidence for the existence of monoamine-containing neurons in the central nervous system. I. Demol~tration of monoamines in the cell bodies of brains/era neurons, Acts physiol, scand. 62 Suppl., 232 (1964) 1--55.

180

23 Dahl~tri~m, A. and Fuxe, K., Evidence for the existence of monoamine neurons in the central nervous system. H. Experimentally induced changes in the intraneuronal mnin,: levels of bulbospinal neuron systems, Acta physiol, stand., 64 Suppl., ',!47 ()965) 6--J6. 24 Demhowsky, K., Czachurski, ]., Amendt, K. and Seller, H., Tonic, supraspinal, mcnoamin..,~,ic inhibition on spinal somato-sympathetic reflexes, Pfliigers Arch. ges. Physiol., 373 (1978) R76. 25 Dem]sowskv, K., Czachurski, J., Amendt, K. and Seller, H., Bulbospina~ inhibitory influ,.nces on sympathetic pr1:ganglionic neurons. In C. McC. Brooks, K. K~izumi and A. S~Lto (Eds.), Integrative Functions o~ the Autonomic Nervous System, Elspvier, Amster,]am, 1979, pp. 376--~'.84. 26 Down,man, C.B.B. and Hussain, A., Spinal lracts and supraspinal centres influencing ~i~cetornotor and allied reflex,.,s in cats, J P~ysiol. (Lond.), 141 (1958) 48cJ--499. 27 ~]virsrv~n, L. and MacKenzie,, E.T., Amin,: mechanisms in the cerebral circulation, Pha~ta.*ol. Rev., 28 (1976) 2"5--384. 28 Engber~, I., Lundberg, A. am~ Ryall, R.W., Reticulospina] inhibition of Lran~mission in reflex padlways, J. Physiol. (Lond.), 194 ~1968)201--223. 29 Engb,~rg, I., Lundberg, A. and Ryall, R.W., ;~eticulospinal inhibition of interneurones, J. Ph:rsiol. (Lond.), 194 (1968) 225--236. 30 Engb~:rg, I.. Lundtfrg, A. a~td Ryall, R.W., Is the tonic decerebrate inhibition of reflex paths medlar d by morloaminergic l:athways?, Acta physiol, scand., 72 (1968) 123--133. 31 Fernandez de Molina, A., Ku~lo, M. and Perl, E.R., Antidrondcally evok,~ r~sponses from ~ympathetic preganglionic neurones, J. Physiol. (Lond.), 180 (1965) 32".--335. 32 Forer~an, R.D. and Wurster, R.D., Localization and functional characteristics of desce:iding syml:atEetic spinal pathways, Amer. J. Physiol., 225 (1973) 212-- ? 17. 33 Foren~an, R.D. and Wurster, R.D., Conduction in descending :~pinal pathways initic:ted by somatosympathetic re~exes, Amer. J. Physiol., 228 (1975) 905--908. 34 Franz, D.N., Hare, B.D. and l~leurnayr, FLJ., Depression of sympathetic preganglionic n~'Jrones by clonidine: evidence- for stimulation of 5-HT receptors, Clin. exp. Hyperten., ~ (1978) 115--140. 35 Gebb,:r, G.L and McCall. R.B., Identification and discharge patterns of spi,al sympathe'.ic int,:rneurones, Amer. J. Physiol., 231 (1976) 722--733. 3~ Gebbcr, G.I,., Taylor, D.G. and Weaver. L.C.. Electrophysiological studies on organizatior of c~ntral vasopressor p;~thways, Amer. J. Physiol., 224 (1973) 470--481. 37 Gootrnan, P.M. and Cohen, M.I., Evoked splaachnic potentials produced by electrical st~mui ation of medullary vason~otor regions, Exp. Brain Res., 13 ( 1971 ) 1--14. 38 G~bcws|:a, M and And~n, N.-E, Noradrenaline synthesis and utilization: Control by m;rve impulse flow under nornmd conditions and after treatment with alpha-adrenoceptor blocking agents, Nau,~yr~.Schmiedeberg's Arch. exp. Path. Pharmak., 292 (!.976) 53--38. 39 Haeusler, G , Clonidine-induc~.d inhibition of sympathet;.c nerve activity: no indication let a central presynapti,." or an indirect sympatho-mimetic .~ode ¢,f action, Naunyn-Schmiedeberg's Arch. exp. Path. Pharmak., 286 (1974) 97--111. 40 Flare, B.D, Neumayr, R.J. and Franz, D N., Opposite effects of L-DOPA and 5-HTP ~>nt;pi hal sympathetic reflexes, Nature (Lond.), 239 ( 1972) 336--337. 41 t'Skfelt, T., Fuxe, K., Goldstein, M. and Johansson, O., Immuno-histochemical e,,idence for tee existence of adrenaline neurones in the rat brain, Brain Res., G6 (197-1) 235--q51. 42 lllert. M., E~'fects upon the somato-sympathetic reflex transmission in spinalized cats, Pfliigers Arc~. ges. Physiol., 332 (1972) R65. 43 Illert, M. and Gabriel, M., De~:ending pathways in the cervical co~d of cats affecting blood pressure and sympathetic activity, Pfliigers Arch. ges., Physiol. 335 (1972) 109--] 24. 44 Illert, M. and Gabriel, M., De;cending pathways in the spinal cord affect~nt~ sympathetic activity in spinalized cat~, Acta physiol, pol., 24 (1973) 115--I 17.

181 46 Illert, M. and Seller, H., A descending sympathoinhibitory tract in the ver~trolateral column of the cat, Pfliigers Arch. ges. Physiol., 313 (1969) 343--360. 46 Kerr, F.W.L. and Alexander, S., Descending autonomic pathways in the spinal cord, Arch. Neurol. Psychiat. (Chic.), 10 (1964) 249--261. 47 Kirchner, F., Sato, A. and Weidinger, H., Bulbar inhibition of spinal and supraspinal sympathetic reflex discharges, Pfliigers Arch. ges. Physiol., 326 (1971) 324--333. 48 Kirchner, F., Wyszogrodski, I. and Polosa, C., Some properties of sympathetic neuron inhibition by depressor area and intraspinal stimulation, Pfliigers Arch. ges. Physiol., 357 (1975) 349--360. 49 Kobinger, W., Central ~-adrenergic systems as targets for hypotensive drug,., Rev. Physiol. Biochem. Pharnmcol., 81 (1978) 39--100. 50 Kobinger, W. and Picbler, L., The central modulatory effect of clonidine .m the cardiodepressor reflex qfte.~ suppre~si~ of "-y~J~hesis and storage of noradretlaline, Europ. J. Pharmacoi., 30 (1975) 56--62. 51 Koizumi, K. and Brooks, C. McC., The integral.ion of autonomic system reactions: discussion of autonomic reflexes, their control and their association with somatic reactions, Ergebn. Physiol., 67 ( 1972) 1---68. 52 Koizumi, K., Seller, H., Kaufman, A. and Brooks, C., McC., Pattern of sympathetic discharges and their relation to baroreceptor and respiratory activities, Brain Res., 27 (1971) 281--294. 53 Koss, M.C. and Bernthal, P.J., Potentiation of two sympathetic reflexes by yohimbine hydrochloride, Neuropharmacology, 18 (1979) 295--300. 34 Langer, S.Z., Presynaptic receptors and their role in the regulation of transmitter release, Brit. J. Pharmacoi., 60 (1977) 481--497. 55 Leichnetz, G.R., Watkins, L., Griffin, G., Murfin, R. and Mayer, D.J., The projection from nucleus raphe magnus and other brainstem nuclei to the spinal cord in the rat: ~t,,dy ~_=ingth~ IIRP biue-reaction, Neurosci. Lett., 8 (1978) 119--124. 56 Li, C.-L., Mathews, G. and Bak, A.F., Effect of temperature on the response of cervical vagai nerve, Exp. Neurol., 55 (1977) 709--718. 57 Lira, R.K.S., Wang, S.C. and Yi, C.L., On the question of a myelencephalic sympathetic centre. VII. The depressor area a sympatho-inhibitory centre, Chin. J. Physiol., 13 (1938) 61--78. 58 Lundberg, A., Integration in the reflex pathway. In. R. Granit (Ed.), Nobel Symposium I: Muscular Afferents and Motor Control, Almqvist and Wiksell, S~.ockhol'~, 1966, pp. 275--305. 59 Magoun, H.W. and Rhines, R., An inhibitory mechanism in the bulbar reticular formation, J. Neurophysiol., 9 (1946) 165--171. 60 McCall, R.B., Gebber, G.L. and Barman, S.M., Spinal interneurons in the baroreceptor reflex arc, Amer. J. Physiol., 232 (1977) H657--H665. 61 Puintal, A.S., Block of conduction in mammalian myelinated nerve fibres by low temperatures, J. Physiol. (Lond.), 180 (1965) 1--19. (;2 Paintal, A.S., A comparison of the nerve imp~,lses of mammalian non-medullated nerve fibres with those of the smallest diameter medullated fibres, J. Physiol. (Lond.), 193 (~967) 523--533. t~3 Papeschi, R., Sourkes, T.L. and Youdim, M.B.H., The effect of yohimbine on brain serotonin metabolism, motor behavior and body temperature of the rat, Europ. J. Pharmacol., 15 (1971) 318--326. (;4 Polosa, C., Spontaneous activity of sympathetic preganglionic neurons, Canad. J. Physiol. Pha~macol., 46 (1968) 887--896. 65 Sato, A., Somato~ympathetic reflex discharges evoked through supramedullary pathways, Pfliigers Arch. ges. Physiol., 332 (1972) 117--126. E,6 Sato, A. and Schmidt, R.F., Somatosympathetic reflexes: afferent fibers, c~.ntral pathways, discharge characteristics, Physiol. Rev., 53 (1973) 916--947. 67 Schmitt, H., The pharmacology of cionidine and related products. In F. Gross (Ed.)~ Antihypertensive Agents, Handbook of Experimental Pharmacology, Vol. 39, Springer-Verlag, Berlin, 1977, pp. 299--396.

68 Seller, H., The discharge pattern of single units in thorseie and lumbar white rami in relation to cardiovascular ev,,nts, Pfliigers Arch. ges. Physiol., 343 (1973) 31"/--330. 69 Shaw, E. and Wooley, D.W, Yohimbine and ergot alkaloids as naturally occurring antimetabohtes of serotonin, ,}. biol. Chem., 203 (1963) 979--989. 70 S'mh:, J.,q., Atkimon, J.M. nnd Sehmitt, H., Effects of clonidine and L-dopa on spontaneouJ and evoked splanchr4c n ~ e diseharl~, Europ. J. Pharmacol., 24 (1973) 113--119. 71 S n y d ~ , D.W. and Gebber, G.L., Relatlonships between medullary depressor region and central vasopremor pllthways, A m e r J. Physiol., 225 (1973) 1129--1137. 72 Szulczyk, P., Descending spinal sympathetic pathway utilized by somato-sympatY.etic reflex and carotid chemoi~flex~ Brain Res, 112 (1976) 190--193. 73 Taylor, D.G. and Gebm.r, G.~,., Sympathetic unit re~p(~nae$ to stimulation of cat medulla, Amer. J. Physiol., 22,6 (1973) ] 138--1146. 74 Taylor, D.~. and Gebber, G.L., Baroceptor meehevlsm~ ¢ontr,~|ling sympathe~,c hervous rhythn~ of ~.entra] crilir ~nn~r. J. Physi4~l., o.2~, (] 97~) 100,°--101 ~. 75 Wang, G.H. and Brown, V.W., Cb~ng~ in ~*Ivar.,'c skln reflex after acute spinal transection in normal and decerebr~te cats, J. Neurophysiol., 19 (195~) 446--451. 76 Wan~, G.H. and Brown, V.W., Suprasegmentql inhibition of an a atonomic telex, J. Neurophysiol., 1.~ (1956~. 564--572. 77 W~ng, C-.H., Ste;.~, P. an-:! Browv: V.W., Brainstern reticular s vstel~ ai,d gf.~v~nieskin refI,,x in acute dee~_rebra~e cats, J. Neurophysiol., 19 (1956) 350--" 55. '18 Wyszogrodski, I. and Po|osa, C., The inhibitir.n of sympathetic pre,~anglionic neurons by somatic afl'ev{.n'.~,Cal~ad. J. Physiol. Pharmacol., 51 (1973) 29---3~