Localization and Characteristics of the Influence of some Neurotropic Drugs on the Bulbar Vasomotor Centre

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Localization and Characteristics of the Influence of some Neurotropic Drugs on the Bulbar Vasomotor Centre G. V. K O V A L Y d V

Although much can be found in the literature about the bulbar vasomotor centre (see reviews of McDowall, 1935; Folkow, 1955; Uvnas, 19611, its functional organization, afferent and efferent pathways and the participation of individual pontobulbar structures in circulatory regulation along with other important questions have not yet been clarified. The lack of exact physiological data is one of the causes of the unsatisfactory development of the pharmacology of central control of vascular mechanisms. The action of drugs on central vasomotor mechanisms is usually investigated with the aid of indirect parameters: e.g. alterations in arterial pressure, changes in reflex mechanisms, etc., without relating these parameters to exact morphological sites in the vasomotor centre. The indication that a drug inhibits or excites a particular centre is usually considered to be sufficient information, but it is clear that such brief statements about the action of pharmacological drugs is too schematic. It is well known that the vasomotor centre is located in the reticular formation of the brain stem and that it comprises several nervous structures. The morphological heterogeneity of this centre explains why certain drugs exert a specific influence upon different functional elements of the formation. The method of direct stimulation enables us to register certain specific reactions. The quantitative changes in these reactions after injection of various drugs could form a picture of the excitability of individual structural elements of the vasomotor centre. Only a small number of papers deals with this kind of investigation (Minami, 1958; Domino, 1958; Kitayev, 1960), and the results are limited by the methods used (bipolar stimulation, large electrode diameters, failure to check the experiments histologically). The lack of experimental data in this field made us investigate more thoroughly the influence of certain neurotropic drugs on vascular responses in order to determine the localization and characterization of drugs that act on different structural and functional elements controlling vascular tonus. Some results of our research have been published earlier (Kovaly6v, 1957, 1960, 1961a-c, 1962). In this paper we present new observations on the pharmacology of the bulbar vasomotor centre. On the basis of the results obtained we have attempted to elaborate certain physiological concepts. The experiments were performed in decerebrated and undecerebrated cats. We used local stimulation of different sites of the pons and the medulla oblongata. All experiments were matched by histological verification.iThe evoked vascular reactions were registered in the usual way. A detailed description of the method can be found in

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earlier publications (Kovaly6v, 1958a, b, 196la-c). The method adopted for recording the blood pressure and the methods of statistical evaluation can be found in the paper of Bondary6v (seep. 171). The figures in this paper show: (a) the site of stimulation and (b) the dynamics of the blood pressure response after injection of the drug into a certain brain structure. Furthermore the statistically treated data on quantitative shifts in blood pressure are correlated with the doses of a drug. 1.

NEMBUTAL AND URETHANE

The majority of investigators who have studied the influence of narcotic drugs on the vascular apparatus point out that barbiturates even in small doses (urethane only in narcotic doses) reduce the blood pressure. The cause of this hypotension has not yet been explained. Some authors explain the depressor response after injection of a narcotic drug by the inhibition of vasomotor centres in the cortex and the hypothalamic area (Schmiedeberg, 1885; Leiter and Grinker, 1934; Massermann, 1937; Schoettensack and Hahn, 1951; Gultyayev, 1952), others consider inhibition of the peripheral autonomic ganglia as the primary cause (Fedotov et al., 1947; Exley, 1954; Zakusov et al., 1955; Laporte and Bessau, 1956; and others). A third group of investigators look for a mechanism in the pontine vasomotor centre (Batrak, 1946; Chudnova, 1957; Linenko, 1959) as a result of direct stimulation (Minami, 1958; Kitayev, 1960). They conclude that barbiturates, along with urethane to a lesser degree (Nisikimi, 1956), inhibit the central mechanism of vascular control on this level. The question to what extent narcotic drugs exert their influence in controlling vascular tonus and where the dominating influence is localized, remains unsolved. The experiments discussed in this paper have proved that urethane and nembutal influence the vascular reactions evoked by stimulation of different structures of the pons and the medulla oblongata in more than one way. The site of stimulation and the reactions are shown schematically in Fig. 1. When studying the graphs which reflect the dynamics of the changes in vascular reaction under the influence of different doses of urethane (Fig. 2) and nembutal (Fig. 3) one gets the impression that the responses from the vestibular nuclei (type 111) are initially reduced by urethane in doses as low as 50-100 mg/kg, and by nembutal in doses of 1-5 mg/kg. The vascular reactions evoked by stimulation of the medial reticular nuclei (caudal pontine nucleus, magnocellular and ventral nuclei) are abolished by nembutal in doses of 20-30 mg/kg, whereas urethane does not show this suppressing action, even when its dose surpasses the narcotic level (1500-2000 mg/kg) (type 11). The pressor reactions arising from the lateral reticular structures (parvocellular nucleus) appeared to be more resistant to both drugs (type I), as they could not be completely suppressed even by narcotic doses of nembutal and urethane. The depressor reactions evoked by stimulation of the nucleus of the vagal nerve and some other structures were highly resistant to narcotic drugs. To suppress the depressor response, about half the narcotic doses of both nembutal and urethane were needed. Fig. 4 allows the comparison of the inhibiting influence of urethane and nembutal

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Fig. 1. Localization of stimulus points and changes on pressor response in experiments with narcotics. Left, urethane. Right, nembutal. Symbols represent site of stimulus application. The shape of the symbols indicates the type of change of the pressor response: type I, pressor responses elicited in lateral reticular formations; type 11, the same, in medial reticular formation; type 111, the same, in vestibular nuclei. Ciphers below drawings represent : (right) key number referring to frontal sections after Monnier, 1949; (left) key number referring to Brodal (1960). Abby. :Rgc= magnocellular nucleus of reticular formation; Rv= ventral reticular nucleus; R.p.c. = caudal reticular nucleus of the pons; Nrp = paramedian nucleus of the reticular formation; R.pc. = parvocellular nucleus; Vestib. = vestibular complex (superior, descending, medial, lateral nuclei); X = central nuclear complex of the vagus nerve.

(in equivalent doses, i.e. of 1/3-1/5 of the narcotic dose) upon the pressor effects evoked by stimulation of different structures in the bottom of the IVth ventricle. The height of the bars in the histogram reflects (according to the structures) the mean value of the pressor reaction after injection of the drug in the indicated doses. The pressor reactions produced by the nuclei of the vestibular complex (more than half suppressed) appeared to be particularly sensitive to both narcotics. The pressor reactions from the medial reticular nuclei (caudal pontine nucleus, magnocellular, ventral nuclei) were, on the whoie, more resistant to urethane than to nembutal. However, the pressor reactions from the medial structures were not equally sensitive to nembutal. Nembutal in doses

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Fig. 2. Changes in pressor responses evoked by stimulation of different structures of the pons and the

medulla oblongata, in relation to doses of urethane. The zero level equals the actual value of the pressor reaction just before injection. Vertical axis, degree of suppression of the pressor response in % of the initial value. Horizontal axis, doses in mg/kg. Continuous lines represent effects noted in individual experiments and interrupted lines mean values of all experiments. 1,II, 111: types of pressure reactions as described in text (see Fig. I). Lower left diagram shows projection of the nuclei upon the floor of the fourth ventricle.

of 5-10 mg/kg increased the suppression of pressor responses from the magnocellular nucleus, while the suppression of the responses from the ventral reticular nucleus was not so pronounced (75 versus 42 %). The pressor effects from the caudal nucleus of the pons ranged between these two. Analogously to nembutal, urethane unequally suppressed the pressor responses from each medial reticular nucleus. The pressor responses from the laterally located parvocellular nucleus were almost equally suppressed by both narcotics. The depressing influence of the narcotic drugs in this experiment are distinguished by a high degree of reliability (P< 0.05). Not only did urethane and nembutal unequally suppress the vasomotor reactions: they also exerted different influences upon the latency period of the vascular response. In no experiment did urethane alter the interval between the onset of the stimulus and

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Fig. 3. Changes in pressor responses evoked by stimulation of different structures of the pons and the medulla oblongata, in relation to doses of nembutal.

the beginning of the vascular reaction. Under the influence of nembutal the latency period was markedly prolonged when the pressor reactions arose as the result of stimulation of the vestibular and some reticular nuclei (the caudal pontine, magnocellular and ventral nuclei). Moreover, the prolongation of the latency period began during stimulation of the vestibular nuclei, when the dose of nembutal equalled 2-5 mg/kg. When the reticular nuclei were stimulated, the dose had to be increased to 10-1 5 mg/kg. When the reticular magnocellular nucleus and the lateral reticular tract were being stimulated, nembutal did not clearly influence the latency period even in narcotic doses (25-35 mg/kg). I n a number of experiments, urethane and nembutal evoked a reverse of the vascular reaction: we often noticed the transformation of the pressor reaction into a depressor reaction. In experiments with nembutal this phenomenon was observed less frequently than in those with urethane. The pressor reactions evoked by stimulation of the magnocellular nucleus were highly resistant to urethane. Without changing the value of the

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Nernbutal 5-10 mg/kg

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Fig. 4. Relative effect of narcotic drugs on pressor responses evoked by local stimulation of different pontobulbar structures. The 100% level equals the initial value of the pressor reaction in mm Hg, elicited by stimulation of the given area (mean values). Blocks represent the mean values of pressor reactions (for standard errors etc. seep. 172) after stimulation of the structures drawn in diagram of the IVth ventricle (centre) and after injection of urethane and nembutal.

vascular reaction urethane, in doses as small as 150-300 mg/kg, changed the shape of the graph -the steepness of rise and fall of the peak, alterations in shape, the occurrence of horizontal stretches of various durations and lengths. Evidently the vascular reactions evoked by stimulation of different compounds of the pontine vasomotor centre possessed unequal resistance to the same drug. The vascular responses arising in one and the same formation were unequally suppressed by urethane and nembutal. Control experiments in spinal animals (section at Cd showed that urethane and nembutal in doses suppressing the vascular reaction to stimulation of pontine structures, weakly depressed the pressor reactions evoked by stimulation of the lateral horns of the 6th and 7th thoracic segments of the spinal cord. The vascular responses upon stimulation of the peripheral segments of the splanchnic nerves also changed very little (Kovalyh, 1961a-c). Not excluding the possibility of a peripheral action of

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the narcotic drugs (especially with regard to nembutal) on the control of vascular tonus, it is safe to assume that they d o not dominate the response mechanism. In evaluating the observations one has to bear in mind that the pressor and depressor reactions of the vascular apparatus evoked by stimulation of a given brain structure not necessarily arise from structures that are locally stimulated. Morphological investigations have proved the existence of very widespread synaptic connections of the reticular neurons (the Scheibels, 1958) by which functionally different regions of the reticular formation are united into a whole. In our experiments (diameter of the electrodes, 30-50 p), certainly not all stimulated structures had a direct influence upon vascular regulation. Obviously, the unequal degree of suppression of the evoked vascular responses under the influence of narcotic drugs has to be explained by the morphological properties of the stimulated structures themselves and their connections with other nervous elements inside the brain stem and the spinal cord. The pressor reactions elicited via the magnocellular nucleus were almost three to four times as resistant to urethane as to nembutal. It is known that this nucleus contains large, medium and small cells (Brodal, 1960). One could imagine that the giant cells (or a certain part of them) are less sensitive to urethane than to nembutal. This notion is partially confirmed by the observation that the pressor reactions from the adjacent ventral reticular nucleus, which is almost free of giant cells, were more strongly suppressed by urethane than the responses from the magnocellular nucleus (Fig. 4). In the experiments on electrical activity of neurons in both regions of the reticular formation (Schlag et al., 1956; Tishchenko and Shapovalov, 1961; Yamamoto and Schaeppi, 1961) and the spinal cord (Lebedev, 1962; Shapovalov, 1963a-c) it has been noticed that, notwithstanding the clearly depressing influence of nembutal or urethane, part of the cells belonging to one of the formations retain a background activity and the capacity to respond to afferent stimulation. Such experiments prove the individual variations in sensitivity of different neurons in the medulla oblongata and the spinal cord to neurotropic drugs. 2.

MORPHINE A N D PROMEDOL

The influence of analgetic drugs on central vasomotor mechanisms, including the bulbar vasomotor centres, has seldom been investigated. In an earlier paper (Kovalybv, 1958a, b) we pointed out that analgetic drugs reinforce the pressor reactions of certain pontobulbar structures. A more detailed research made it clear that the influence of morphine and promedol on the pressor reactions is more variable and depends on the specific properties of the individual structures. As we have explained before, morphine and promedol exert a varying influence on the vascular reactions dependent on the site of stimulation; it was therefore possible to distinguish three types of changes in vascular tonus. The site of stimulation and the direction ofthechangesin vascular reaction dependent on the area of stimulation are reproduced in Fig. 5. The vascular reactions evoked by stimulation of the magnocellular nucleus were invariably reinforced; only after administration of large doses of the analgesics

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Fig. 5. Localization of stimulus points and changes in pressor response in experiments with analgesics. Left, morphine. Right, promedol. Shape of symbols at sites of stimulation represents type of pressor response according to morphological structure: type I, magnocellular and paramedian reticular nuclei; type 11, vestibular and ventral reticular nuclei; type 111, caudal reticular and parvocellular nuclei. For explanations see Fig. 1.

(10-25 mg/kg) a weak depression of the pressor effects was observed (Figs. 6 and 7, type 1). Morphine and promedol appeared to have a two-phasic action on the pressor reactions from the vestibular nuclei and the reticular ventral nucleus: in doses from 0.25 to 0.5 mg/kg they reinforced the effects, in doses of 1-3 mg/kg they reduced them (type 11). The pressor reactions arising from stimulation of the caudal pontine nucleus and the parvocellular nucleus were suppressed by the analgetic drugs without pre-emptive reinforcement, starting from very low doses (type 111). The depressor reactions evoked by stimulation of the nuclei of the vagus nerve and some other formations were not influenced by morphine, even in large doses, while promedol from 3.5-4 mg/kg reduced them. The facilitatory action of low doses of morphine and promedol on the pressor reactions from the magnocellular and para-

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Morphine Fig. 6. Pressor response changes evoked by stimulation of different structures of the pons and the medulla oblongata, in relation to doses of morphine. Types I, I1 and 111 as in Fig. 5. Explanations as under Fig. 1 .

median areas of the reticular formation was considerably (100 %) increased. The pressor effects from the vestibular and ventral reticular nuclei were definitely more weakly facilitated (20 %). The inhibitory influence of analgetic drugs on the vascular reactions evoked by stimulation of the parvocellular nucleus was more pronounced than those of the caudal nucleus of the pons. Fig. 8 shows the different actions of promedol in a dose of 3.5 mg/kg on vascular reactions arising from different pontine structures. It is obvious that the pressor reaction from the ventral reticular nucleus under the influence of analgesics is reinforced (l), while the pressor effect from the parvocellular nucleus is reduced (3). The depressor reaction evoked by stimulation of the motor nucleus of the vagus nerve is also suppressed (2). The analgetic drugs also changed the latency period of the vascular reactions. The prolongation of this period upon stimulaton of the vestibular nuclei and the parvocelM a r nucleus was observed after injection of morphine (0.5-2 mg/kg), and for the

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Prornedol Fig. I . Pressor response changes evoked by stimulation of different structures of the pons and the medulla oblongata, in relation to certain doses of promedol.Types I, I1 and I11 as in Fig. 5. Explanations as under Fig. 1 .

responses from the paramedian and ventral reticular nuclei by a dose of 2.5-5 mg/kg. Stimulation of the magnocellular nucleus did not change the latency period at all even after injection of large doses of morphine (10-1 5 mg/kg). The action of promedol in this respect tended to be different. Initially, a prolongation of the latency period of the reactions from the vestibular nuclei occurred (dose, 1-2 mg/kg). Analogous shifts occurred during stimulation of the reticular nuclei by largedoses of promedol during magnocellular and paramedian nuclei by doses of 2.5-3.5 mg/kg, of the parvocellular stimulation of the nucleus by 4-7 mg/kg, and of the ventral nucleus by 12 mg/kg. The character of the vascular reaction did not change. Not only the excitatory, but also the inhibitory action of the analgesics was conditioned by the influence of the structures of the medulla oblongata and the pons. In experiments with stimulation of the lateral horns of the spinal cord and the peripheral segments of the visceral nerves it became clear that the analgetic drugs did not

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Fig. 8. Influence of promedol on vascular reactions evoked by stimulation of bulbar structures. a, normal; b, 10 min after injection of 3.5 mg/kg of promedol. From top to bottom: Record of blood pressure, duration of stimulus, time base (unit: 5 sec). Ciphers inserted in stimulus recording indicate number of electrodes in accordance with site of stimulation: 1 = ventral reticular nucleus; 2 = motor nucleus of the vagal nerve; 3 = parvocellular reticular nucleus.

essentially influence the caudal elements of vasomotor centres (Kovaly6v, 1961a-c). The reinforcement of the pressor reactions that we observed could hardly be explained by assuming the existence of a ‘stimulating compound’ in the action of the analgesics, because simultaneously and with the same doses, there occurred a depression of pressor effects from the caudal nucleus and the parvocellular nucleus. The most plausible explanation for the increase i n the pressor reactions is the abolition of inhibitory influences of the medial parts of the reticular formation. Concerning the lateral parts such ‘de-inhibitory’ influence of the drugs seemed not be to present. It must be stated here that the analgetic drugs also increase various reflectory vascular reactions (Marri and Hauss, 1939; Rovenstine and Cullen, 1939; Kruglikova-Lvova, 1953; Kovalybv, 1958a, b, 1959; Ivanova, 1958, 1960; Kaverina, 1960). The reduction of pressor reactions under the influence of large doses of analgetic drugs is determined by the obviously direct inhibition of the stimulated structures. By microelectrode recording of the activity of individual neurons in the magnocellular nucleus it was established that large doses of morphine and promedol(l0-22 mg/kg) suppress the spontaneous activity of the nerve cells (Grantyn, 1962). On the action of analgesics on the central nuclei of the vagus nerve thereis still an incongruity of opinion in the literature. Some authors (Anders, 1913; Suy Bin, 1955) suppose that morphine and promedol stimulate the vagal centre directly, evoking hypotension at the same time. Others (Tatum et al., 1929; Shpuga, 1945) believe that the stimulation occurs in an indirect way; while still others (Witkovsky, 1877; Ngai, 1960) assume that centralvagal mechanisms are inhibited (Kravkov, 1928).Evans et al., 1952, suggest that analgesics influence - weakly, if at all - the nulcei of the vagus nerve. Our experiments with direct stimulation of the nuclei of the vagal centre have shown that morphine, whatever the dose, did not alter the depressor responses, while

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promedol, even in considerable quantities, only induced a slight change. From our results it is obvious therefore that the bulbar structures of the vagus nerve are barely inhibited by analgesics.

3.

AMINAZINE AND DIHYDROERGOTOXINE

Different opinions exist as to the mechanism of action of aminazine on the central regulation of vascular tonus. The inhibitory influence of aminazine on vasomotor reactions is explained by a number of authors by inhibition of suprabulbar structures (Donne, Zwirn et al., 1955; Benetato et al., 1957; Kalkoff, 1957; Shinagawa, 1958; and others). Other authors connect the hypotensive action of aminazine with inhibition of the pontine vasomotor centre (Dasgupta and Werner, 1954; Okuma, 1956; Murayama, 1957; Bakuradze el al., 1959) and the segmental vasomotor mechanisms (Cicardo, 1956). According to the observations of some investigators (Krause, 1956;

Fig. 9. Localization of stimulus points and changes in pressor response in experiments with aminazine (left) and ergotoxine (right). Explanations as under Fig. 1.

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Aminazine

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Fig. 10. Changes in pressor response evoked by stimulation of different structures of the pons and the medulla oblongata, in relation to doses of aminazine.

Shinagawa, 1957; Popescu et al., 1957a, b) an essential r6le in the hypotensive mechanisms of aminazine is played by peripheral influences. Concerning the influence on the central vascular control of dihydroergotoxine (DHE) and other hydrated alkaloids of ergot, there is no agreement either (Rothlin, 1946; Von Euler and Hesser, 1947; Wickerson and Gump, 1949; Kotova, 1958). The problem of the localization of the action of aminazine and DHE on the vasomotor centre is therefore far from being definitely solved. In our experiments the vascular reactions were reduced under the influence of aminazine and D H E alike, dependent on the localization of the stimulated structure (Fig. 9). The experience gathered with these drugs proved that the sensitivity of the pressor reactions evoked via different structures of the pons and the medullaoblongata, is highly variable with regard to aminazine and ergotamine. Less resistant to aminazine are the responses from the vestibular complex and the lateral reticular regions. The pressor effects from the medial reticular nuclei were not suppressed by large doses of

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Fig. 11. Changes in pressor response evoked by stimulation of different structures of the pons and the medulla oblongata, in relation to doses of ergotoxine.

aminazine (Fig. 10). The same rule applied to the experiments performed with DHE (Fig. 11). As quantitative analysis of the experimental results showed, aminazine in doses of 0.1-0.5 mg/kg suppressed the pressor effects evoked by stimulation of different pontobulbar structures variously. Especially sensitive to the drug were the responses from the vestibular complex. After injection of aminazine in the indicated doses the vascular responses were diminished by about 60%. The pressor reactions from the reticular nuclei were unequally suppressed by aminazine in the same doses. Most resistant of all were pressor effects from the caudal nucleus of the pons which were only reduced by about 35 %; least resistant were those from the ventral nucleus, which were reduced about 56 % (Fig. 12). The responses from other reticular structures were reduced to the same level, almost 50 %. The results of statistical analyses show a degree of probability, P < 0.05.

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Ergotoxine 0.1-0.3 mg/kg Fig. 12. Relative effects of aminazine and ergotoxine on pressor responses evoked by local stimulation of different pontobulbar structures.

The depressor reactions evoked by stimulation of the central formation belonging to the vagus nerve appeared to be highly resistant to aminazine. A half reduction was manifested only after 2-3 mg/kg of the drug had been injected. The specificity of the inhibitory action of DHE in a dose of 0.1-0.3 mg/kg was more pronounced. The pressor reactions from the magnocellular nucleus were suppressed four times as much as those from the caudal nucleus of the pons. DHE appeared to be maximally effective on the responses going out from the vestibular nuclei, which were reduced by almost 80% (Fig. 12). This conclusion is also distinguished by a probability P < 0.05. The latency period elapsing between the onset of stimulation and the onset of the pressor reaction was differently influenced by the investigated drugs. With aminazine at doses of 0.05-0.1 mg/kg there was no alteration in the latency period of a n y structure. When the dose was increased, the latency period was prolonged, first in those

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structures that do not possess a descending pathway; the parvocellular (0.5 mg/kg), the vestibular (0.5 mg/kg) and the caudal pontine nuclei (1 mg/kg). Only after injection of aminazine a t 2-3 mg/kg did the first signs occur of a prolongation of the latency period of the reactions of the magnocellular and ventral nuclei of the reticular formation. DHE did not essentially inftuence the latency period of the response. Prolongation of this period which was effectuated by stimulation of a reaction site, was conditioned by the peripheral action of DHE in a dose of 0.3 mg/kg. Control experiments showed that aminazine in doses of 0.5-1.5 mg/kg would suppress the vascular responses only insignificantly when they were evoked by local stimulation of the spinal cord in the region of the lateral horns of the 7th andh8th thoracic segments; nor did it alter the response when the peripheral segment of the spIanchnic nerve was being stimulated. The overall a u l t of the action of aminazine and DHE is obviously determined by the morphological and physiological properties of the structures involved. The high sensitivity to DHE of pressor reactions evoked by stimulation of the parvocellular nucleus, as compared with other structures, may give rise to the assumption that the ergotropic systems could have their chief localization in the lateral compartments of the reticular formation. Further evidence for this finding can be looked for in the increase in the latency period before the onset of the vascular reaction and the elongation of the pressure peak after minimally effective doses of DHE and, sometimes, of aminazine. The ergotropic systems are represented to a lesser degree in the medial areas of the medulla oblongata, for the pressor reactions from the medial reticular nuclei were resistant throughout against the inhibitory influence of the drugs under investigation. The vascular reactions from the vestibular nuclei took a halfway position on the scale of sensitivity. The existence of adrenergic (ergotropic) structures in the reticular formation of the brain stem today is no longer doubted. Lately, new attempts have been made to localize them in certain regions of the reticular substance, namely in the region of the pons and the medulla oblongata. Also, a quantitative determination of the concentration of sympathetic intermediate metabolites in different areas of the brain has been presented (Vogt, 1954). Chen, Lim et al. (1936, 1937, 1938) andChai and Wang, 1962, have found, within the boundaries of the pons and the medulla oblongata, a sympathetic system concerned not only with vascular control but also with other vegetative functions. It is therefore assumed that in this region a great number of ergotropic structures are present which participate in the regulation of descending influences of the reticular formation (Rothballer, 1956). Comparison of the influence exerted by both drugs (which belong to the group of central and peripheral adrenolytic drugs) may help us to specify the links of central vasomotor regulation wherein egotropic systems are represented. 4.

A M I Z I L AND SCOPOLAMINE

We know that except for adrenergic structures, cholinergic neurons are also present

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Fig. 13. Localization of stimulus points and changes in pressor response shifts in experiments with anticholinergic drugs (amizil, left; scopolamine, right).

in the reticular formation of the brain stem (Bradley and Elkes, 1957). However, we have direct data bearing on the participation of cholinergic systems in the control of vascular tonus. Experimental data (Suh et al., 1935; Briicke, 1956; Alexandrova, 1960), which leave open the possibility of alterations in arterial pressure by means of intrathecal injection of different cholinergic drugs, cannot be counted as evidence for the hypothesis that these effects are determined by the action of the administered drugs on the vasomotor centre of the medulla oblongata. During investigation of the influence of amizil and scopolamine on the vascular reactions evoked by stimulation of different pontobulbar structures, we found that the action of these drugs is variable and depends on the site of stimulation (Fig. 13). Both drugs in doses as low as 0.05-0.1 mg/kg suppressed considerably, and in doses of 3-5 mg/kg abolished completely, the responses from the vestibular nuclei. The pressor effects produced by the medial reticular nuclei were invariably suppressed by amizil, whereas scopolamine in doses of 0.5-1 mg/kg, on the contrary, reinforced them. The vasomotor reactions produced by stimulation of the lateral reticular structures were by

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Fig. 14. Changes in pressor responsesevoked by stimulation of different structures of the pons and the medulla oblongata, in relation to doses of amizil.

no means suppressed by the cholinolytic drugs even in large doses. A summary of the results of the experiments with amizil and scopolamine is reproduced in Figs. 14 and 15. Comparing the quantitative values of the action of cholinolytic drugs administered within a circumscribed range of doses (0.1-0.5 mg/kg), we found out that the pressor reactions from the parvocellular nucleus were the most sensitive, while the vasomotor effects from the group of medially located (magnocellular, caudal pontine, ventral) nuclei were depressed by amizil, and especially by scopolamine, to a far lesser degree. Concerning reduction of the pressor responses, those produced by the vestibular

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Fig. 15. Changes in pressor responses evoked by stimulation of different structures of the pons and the the medulla oblongata, in relation to doses of scopolamine.

nuclei were comparable to those of the medial reticular nuclei when amizil was administered ;but under the influence of scopolamine the difference was more pronounced (Fig. 16). The inhibitory action of cholinolytic drugs in doses of 0.1-0.5 mg/kg shows statistically a high degree of reproducibility (P < 0.05). The depressor reactions were suppressed by amizil and scopolamine alike. The effect of amizil and scopolamine in equal doses cannot be compared without

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Fig. 16. Relative effects of anticholinergicdrugs on pressor reactions evoked by local stimulation of different pontobulbar structures.

stipulating that the central action of amiziI in otherwise equal circumstances will be more definitely manifested (Denisenko, 1961a-c). If this factor is taken into account, however, it may be said that the experiments showed great differences in sensitivity to each cholinolytic drug. The fact that pressor reactions from the lateral structures were suppressed by lesser doses of cholinolytic drugs than the analogous reactions elicited by stimulation of the medial reticular structures which have direct connection with the spinal cord, gives evidence that the cholinergic systems of associative connections between lateral and medial structures of the medulla oblongata are inhibited. Amizil clearly influenced the latency period before the onset of the vascular reactions. The increase in the latency period, when the central vagal structures (i.e. the solitary tract, the motor nucleus etc.) were stimulated, began after minimal doses of amizil (0.01-0.02 mg/kg). ,An analogous effect was produced by injecting somewhat larger doses (0.05-0.1 mg/kg) during stimulation of the vestibular and reticular

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(parvocellular and caudal pontine) nuclei. To prolong this period by stimulating the magnocellular and ventral reticular nuclei, relatively large doses of amizil were needed : 1.5-3 mg/kg. Scopolamine, unlike amizil, did not evoke changes in the latency period, when given in small doses (0.05-0.1 mg/kg), during stimulation of the vestibular and some reticular (magnocellular, paramedian) nuclei. An analogous influence of this drug was noticed when the lateral reticular tract was stimulated but then the dose had to be increased. When the doses of scopolamine were increased, prolongation of the latency period was seen, but different quantities of the injected cholinolytic drug were needed in different places : during stimulation of the vestibular and parvocellular nuclei, 0.1-0.5 mg/kg; of the vagal structures, 0.5 mg/kg; and of the ventral reticular nucleus, 1-1.5 mg/kg. The caudal pontine and magnocellular nuclei in this respect were resistant to scopolamine. Even if the injected dose reached 1.5-2 mg/kg, all attempts to prolong the latency period connected with the said structures failed. Control experiments with stimulation of lower structures which deal with vascular control-the neurons of the lateral horns ofthe spinal cord and thesplanchnic nerveshowed that neither cholinolytic drug altered the response reactions in doses which usually suppressed the pressor effects evoked by stimulation of the supraspinal structures (Kovaly6v, 1961a-c). CONCLUSIONS

The disagreement on the functional organization of the bulbar vasomotor centre considerably complicates a final anlaysis of the observations. Various authors have attempted to draft schematic pictures of the morphological organization of the vascular control centre. Alexander (1946), for instance, tried to determine the approximate boundaries of the pressor and depressor areas of the medulla oblongata. Some years later Bach (1952) doubted the reliability of making such a rigid border line between the two zones. Other authors (Bach, 1948, 1952; Thomson and Bach, 1950; Pitts et at., 1939a, b ; Amoroso et al., 1954; Domino, 1958) showed the existence of various neuronal vascular units belonging to the pons and medulla oblongata. Dependent on the circumstances these units function together or apart; therefore the cited authors do not support the concept of specific centres in the reticular formation of the medulla oblongata. According to our observations (Kovaly6v, 1961a-c; Kovaly6v and Bondarybv, 1962) the pressor and depressor zones are diffusely spread in the pons and the medulla oblongata. We observed response reactions by stimulating very divergent sites of the medulla oblongata. Does this mean that the whole medulla oblongata is in fact identical with the bulbar vasomotor centre ? Definitely not! The fact that local stimulation may give rise to diverging changes in vascular, respiratory and somatic functional responses (Bach, 1952; Minami, 1958: Kovaly6v, 1961a-c) points to a n extensive network of connections between the neurons in the pontobulbar reticular formation, which is supported by morphological investigations (Brodal, 1960). The

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different changes in these functions under the influence of pharmacological drugs Domino, 1958; Kovaly6v, 1961a-c; Tsyrlin, 1962) is evidence for the hypothesis that different morphofunctional units participate in their manifestation. Obviously we are dealing with specialized neurons which participate in vascular control either directly or indirectly. The study of the potentials even of individual ‘cardiovascular’ neurons by means’of capillary microelectrodes (Salmoiraghi, 1962) does not make it possible, however, to establish whether the changes in their activity are thecause or theeffect of the shifts in arterial pressure. Upon the basis of the present opinions of the general structure ofthe centres inside the nervous system and the latest anatomical and functional investigations of the reticular formation quoted in the review of Brodai (1950), and backed up by our own observations, however, we prefer to assume that the bulbar vasomotor centre can be represented as a diffuse network which interconnects the reticular neurons. In the lateral compartments of the pontobulbar reticular formations we find structures which consist of small cells and which are evidently ‘sensory’, as they have extensive afferent connections with structures localized outside the medulla oblongata, while they themselves send out efferent links mainly to the medial part of the medulla oblongata. In the latter area are situated the large ‘motor’ neurons which have only descending connections with the spinal cord. We have reason to suppose that direct connections between the associative lateral and the effector medial zones exist along with polysynaptic pathways. The switching over from afferent pathways to efferent in order to guide instantaneous responses can therefore be executed immediately. When taking these considerations into account one may be inclined to think that the central vasomotor mechanisms at the level of the medulla oblongata medial and lateral reticular structures participate to the same extent in circulatory control. The possibility of evoking pressor reactions from other structures of the medulla oblongata (vestibular nuclei etc.) which do not belong to the reticular formation and maintain only an indirect relation to the regulation of blood pressure, can be explained by the presence of a number of polysynaptic pathways in this region of the brain. These hypotheses on the organization of the vasomotor centre are supported by our own investigations. The vascular reactions produced by the vestibular nuclei, which are not, as will be remembered, directly related to circulatory control, were least resistant to almost all investigated drugs (in minimal doses). On administering 50 mg/kg of urethane or 1 rng/kg of nembutal, or 0.01-0.05 mg/kg of aminazine, DHE, amizil or scopolamine, a reduction in the vascular response of 3040% was observed. Only the analgesics (0.25-0.5 mg/kg) reinforced the pressor reaction. The most marked effect was produced by a strong adrenolytic drug, DHE, and also by another drug with adrenolytic properties, aminazine. This suggests the possible participation of adrenergic systems in the transmission of stimuli between vestibular and other structures of the medulla oblongata which deal with circulation. Both drugs also considerably prolonged the latency period before the onset of the pressor reactions from the vestibular nuclei. On the other hand, amizil and scopolamine had strong inhibitory influences on the

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magnitude of the pressor reactions evoked by stimulation of the vestibular nuclei, and just as strongly reduced the latency period of the reaction. This points to a probable participation of cholinergic systems in the stimulus transmission from the vestibular nuclei to the vasomotor neurons. The pressor effects produced by the lateral reticular nuclei were more resistant to the drugs than the vestibular pressor reactions. The indicated drugs in the same doses suppressed the responses by almost 10-20 %, while amizil in some experiments even reinforced the pressor reactions of these structures. The action of analgesics was again different : without a sign of facilitation they invariably suppressed the pressor reactions in doses as low as 0.25-0.5 mg/kg. It is remarkable that in the given case DHE appeared to be the most effective of all drugs; to a lesser degree this applied to aminazine, while scopolamine and amizil were least effective. This is an indication towards the importance of ergotropic systems in the pathways between the lateral and medial reticular structures, which evidently surpasses that of the cholinergic systems. A further confirmation of this hypothesis is the strong inhibitory effect of aminazine on the latency period of the pressor reactions produced by the parvocellular nucleus as compared to the same parameter for the vestibular nuclei. The pressor reactions which could be elicited by stimulation of the medially localized reticular nuclei were most resistant to all drugs with the exception of nembutal. Considerable doses of the drugs were needed before the elicited responses from these structures were completely abolished. Particularly resistant were those responses which arose during stimulation of the rnagnocellular and ventral nuclei. Morphine and promedol had no inhibitory influence at all upon the vascular reactions from the medial structures. l t merits attention that, as a rule, nearly all drugs barely altered the latency period before the onset of the pressor reaction of the medial reticular nuclei. In conclusion the pressor reactions elicited by different structures of the medulla oblongata and the pons were unequally suppressed by neurotropic drugs; first, the responses from the vestibular nuclei were reduced ;next, the pressor effects of the lateral reticular structures were inhibited, and lastly, a decrease in the pressor reactions evoked by the medial reticular layers was observed.