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Mechanism of the CNS-mediated pressor response to intracerebroventricular injection of noradrenaline in unanaesthetized rats
NeuropharmacologyVol. 24, No. 9, pp. 831-837, 1985 Printed in Great Britain. All rights reserved
0028-3908/85 $3.00 + 0.00 Copyright 0 1985 Pergamon Press Ltd
MECHANISM OF THE CNS-MEDIATED PRESSOR RESPONSE TO INTRACEREBROVENTRICULAR INJECTION OF NORADRENALINE IN UNANAESTHETIZED RATS F. M. A. CORREA’, IDALINA A. S. MAGRO’, VERA L. PERES-P• LON~ and J. ANTUNES-RODRIGUES’ ‘Department of Pharmacology, 2Department of Physiology, School of Medicine of RibeirIo Preto-USP, 14100 RibeirIo Preto and “Department of Physiological Science, School of Dentistry of RibeirHo Preto-USP, 14100 RibeirIo Preto, SP, Brazil (Accepted
16 January 1985)
Summary-Catecholamines administered intracerebroventricularly (i.c.v.) have cardiovascular effects mediated by the CNS. Although hypotension constitutes the more prominent response, an increase in blood pressure has also been reported after the intracerebroventricular injection of these amines. Anaesthesia interferes with pressor responses mediated by the CNS to a larger extent than with depressor mechanisms and constitutes one of the major factors influencing the pattern of response to the amines. The depressor response observed after the intracerebroventricular administration of noradrenaline is reversed into increases in blood pressure in awake animals. In the present experiment, the action of intracerebroventricularly injected noradrenaline was compared in anaesthetized and conscious rats. The results indicated that the pressor response in awake rats was not mediated by the sympathetic nervous system and involved the release of a pituitary humoral-factor, most probably vasopressin, whereas the depressor response observed in anaesthetized animals was not dependent on pituitary mediation. The involvement of histaminergic mechanisms in the CNS in the control of the pressor response to intracerebroventricularly administered noradrenaline in the rat is proposed. Key words: blood pressure, CNS, noradrenaline vasopressin antagonist, rats.
administered intracerebroventricularly,
The intracerebroventricular administration of catecholamines, notably of noradrenaline, has hypotensive effects, mostly by activating a central cr-adrenergic mechanism (Day and Roach, 1974). The existence of such a mechanism provides the basis for understanding the mode of action of antihypertensive drugs such as a-methyldopa and clonidine (Van Zwieten and Timmermans, 1979). However, some discrepancies remain concerning the effects of intracerebroventricularly administered catecholamines, among them the fact that noradrenaline administered intracerebroventricularly has been reported to produce either an elevation or a fall in blood pressure (Forsyth and Pesouat, 1978; Schmidt and FCnard, 1971). Possible explanations for these discrepancies were previously discussed (Corr&a, Magro and Peres-Polon, 1982). The hypothesis of a predominant action of intracerebroventricularly administered noradrenaline on central a,-receptors is based on experiments using cats and dogs as subjects. However, the evidence suggests that, in the rat, there are other mechanisms being simultaneously activated. In awake rats, noradrenaline administered intracerebroventricularly causes increases in blood pressure which are blocked by local pretreatment with phentolamine (CorrCa and Graeff, 1974). Leakage into the peripheral circulation was proposed as an
vasopressin,
explanation for the pressor effects of intracerebroventricularly administered noradrenaline because ganglionic blockers failed to prevent or even potentiate such effects in the rat (Buccafusco and Brezenoff, 1977). Pressor responses were reported in awake rats, whereas only depressor responses were observed after intracerebroventricular administration of noradrenaline in anaesthetized animals (CorrCa and Graeff, 1974). If the idea of leakage to the periphery is accepted, as an explanation for the pressor effects of intracerebroventricularly administered noradrenaline, it must be assumed that leakage only occurs in awake animals. However, the lack of evidence for such a carrier or gradient mechanism, active only in awake animals, points to the necessity of an alternative explanation. In addition, the marked effects of anaesthesia on the central cardiovascular response to drugs or electrical stimulation (Bergmann and Gutman, 1966; Corrka et al., 1982; Kristt, 1975) may contribute to these discrepancies and should also be considered. The present study aimed to evaluate the extent of the involvement of the central nervous system (CNS) in the pressor response to intracerebroventricularly administered noradrenaline. Experiments were conducted using anaesthetized and awake rats. Intracerebroventricular administration of noradrenaline 831
F. M. A. CORRI?Aet al.
832
caused increases in blood pressure in awake rats and decreases in blood pressure in anaesthetized rats. Hypophysectomy or intravenous pretreatment with a specific vasopressin blocker, but not ganglionic blockade, were found effective in blocking the pressor response to intracerebroventricularly administered noradrenaline, suggesting the involvement of vasopressin in this response. Intracerebroventricular pretreatment with histamine antagonists was performed to provide evidence of the involvement of central histaminergic mechanisms in the pressor response to noradrenaline in the rat. METHODS
Wistar rats, 200-230 g were used. Anaesthetized animals were kept under urethane, 1.2 g/kg (i.v.), and intracerebroventricular cannulas were implanted according to the method of CorrZa and Graeff (1974). Blood pressure was recorded acutely through an intracarotid polyethylene catheter (PE-50). Awake animals had intracerebroventricular cannulas implanted under pentobarbital anaesthesia, 40 mg/kg (i.p.), and chronic intra-arterial catheters implanted according to Krieger (1964), for recording blood pressure. Intra-arterial catheters were implanted under ether anaesthesia, the day after the surgery. Experiments were carried out two days afterwards to allow for recovery. Whenever the intravenous route was used, a similar catheter was implanted simultaneously into the femoral vein for administration of drugs. Hypophysectomy was performed surgically under light anaesthesia with ether (Zarrow, Yochim and McCarthy, 1964). Animals were supplemented daily with deoxycorticosterone (Merck, Sharp & Dohme), 0.25 mg/kg (i.m.), alone or associated with vasopressin thanate in oil (Pitressin-CIBA; 5 U/kg, s.c.), and used 5 days later. Experiments with anaesthetized or awake hypophysectomized rats were carried out following the procedure previously mentioned. Drugs were dissolved in 0.9% NaCl and injected into the cerebral ventricles in a volume of 10 ~1 over a period of 30 set, using a hand-driven 705-N Hamilton 50 ~1 syringe and an odontologic injection needle (Mizzi, 0.2 mm o.d.). Dose-effect curves were obtained for intracerebroventricularly administered noradrenaline hydrochloride (Sigma) and for different histamine antagonists on the effect of intraventricular administration of noradrenaline. The H,-antagonists used were diphenhydramine HCl (Sigma) or pyrilamine maleate (Sigma) and H,-antagonists were metiamide (SKF) or cimetidine (SKF). Ganglioplegic efficacy was evaluated against the pressor effect evoked by electrical stimulation of the posterior hypothalamus. Rats were kept under urethane anaesthesia, the skull fixed to a stereotaxic apparatus (David Kopf 900) and a monopolar electrode implanted in the posterior hypothalamus at the
following coordinates: A + 4.2 mm, L + 0.5 mm, H + 2.3 mm, of the De Groot’s (1959) brain atlas of the rat brain. Blood pressure was recorded acutely through a intracarotid catheter. Square wave stimuli of 60 Hz and variable voltage were applied and voltage/response curves obtained prior to and after the administration of pentolinium tartrate (Sigma), (3 mg/kg, i.v.), alone or associated with scopolamine methylbromide (Boheringer; 4 mg/kg, i.v.). Further evaluation of the involvement of vasopressin in the pressor response to intracerebroventricularly administered noradrenaline was accomplished using the specific vasopressin antagonist d(CH,), Tyr(Me) AVP, (50 p g/kg, i.v.) in saline. The drug was kindly provided by Dr Maurice Manning from the Medical College of Ohio. RESULTS
Effect of ganglionic blockade on the blood pressure response to intracerebroventricularly administered noradrenaline and the electrical stimulation of the posterior hypothalamus Intracerebroventricular administration of noradrenaline caused different patterns of dosedependent effects in anaesthetized and awake rats. In anaesthetized animals a decrease in blood pressure was observed, whereas an increase in blood pressure was found in awake rats (Fig. 1). The administration
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Fig. 1. Dose-effect
curves for the pressor and depress0 responses observed after the intracerebroventricular admin istration of noradrenaline in freely moving and urethane anaesthetized rats, respectively. Points in the Figure repre sent the mean and the bars the SEM. Figures in parenthew represent the number of animals.
Intraventricular
NA and blood pressure
^a x+5.0 E
833
animals after significantly lower in anaesthetized to controls hypophysectomy when compared respectively), (108.6 + 4.0 and 77.5 + 2.8 mmHg, whereas unanaesthetized animals had basal levels of of controls blood pressure similar to those (116.8 f 4.0 and 112.0 + 3.5 mmHg, respectively). Hypophysectomy was effective in reducing the intrain blood pressure to increase cerebroventricularly administered noradrenaline in the awake rat to the extent of 9&95% in animals supplemented with either deoxycorticosterone alone or in association with vasopressin, to maintain the hydro-ionic balance subsequent to the removal of the gland (Fig. 3A). The hypotensive response to intracerebroventricularly administered noradrenaline ob-
1 (A)
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PENTOLINIUM
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Fig. 2. (A) Effect of ganglionic blockade with 3 mg/kg (iv.) pentolium, alone (0-O) or associated to 4mg/kg (i.v.) scopolamine methylbromide (A-A), on the voltagedependent pressor response-curve to electrical stimulation of the posterior hypothalamus in the anaesthetized rat (@--0). Points in the Figure represent the mean and the bars the SEM. Figures in parentheses represent the number of animals. *Statistically significant at P < 0.05, Student’s t-test. (B) Absence of effects of ganglionic blockade with pentolinium on the pressor response to the intracerebroventricular administration of 60 nmol of noradrenaline (NA) in unanaesthetized rats. Dashed and open columns represent the mean of basal blood pressure and maximal response to NA, respectively. Bars represent the SEM. The percentage increase in mean arterial blood pres-
sure against control is presented at the top of columns. Figures in parentheses represent the number of animals. *Statistically significant at P < 0.05, Student’s t-test.
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of vehicle caused no effect on blood pressure. The pressor response to intracerebroventricularly administered noradrenaline did not produce tachyphylaxis after three successive injections in unanaesthetized, freely-moving rats. The response to intracerebroventricularly administered noradrenaline in the awake rat was not affected by intravenous pretreatment with pentolinium in a dose which was effective in blocking the increase in blood pressure induced by electrical stimulation of the posterior hypothalamus (Fig. 2A). At the dose used, pentolinium did shift the voltage/response curve to the right. The concomitant administration of scopolamine methylbromide, a muscarinic antagonist that does not cross the blood-brain barrier, did not increase the efficacy of ganglioplegic blockade. Effect of hypophysectomy on the blood pressure response to intracerebroventricularly administered noradrenaline The
basal
levels
of
blood
pressure
were
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HYPHD
Fig. 3. (A) Inhibitory effect of hypophysectomy on the pressor response to the intracerebroventricular administration of noradrenaline in unanaesthetized rats. Animals were treated with 0.25 mg/kg (i.m.) deoxycorticosterone, alone (CORT) or with 5 U/kg (s.c.) vasopressin (AVP) for 4 days after surgery. Columns represent the means of pressor responses to 60 nmol intracerebroventricularly administered noradrenaline in sham (dashed) and hypophysectomized animals (open). Bars represent the SEM. Figures the number of animals. in parentheses represent *Statistically significant at P < 0.05, Student’s t-test. (B) Absence of effect of hypophysectomy (HYPHD) on the depressor response to the intracerebroventricular administration of 60 nmol noradrenaline in urethane-anaesthetized rats. Animals were treated with deoxycorticosterone and vasopressin for 4 days after surgery. Columns represent the mean of basal blood pressure (dashed) and the maximal fall in blood pressure after intracerebroventricularly administered noradrenaline (open). Bars represent the SEM. The percentage fall in mean arterial pressure is presented at the top of the column. Figures in parentheses represent the number of animals. *Statistically significant at P < 0.05, Student’s t-test.
F. M. A. CORRBA et al.
834
Table 1. Effect of intravenous (i.v.) pretreatment with an a-adrenergic blocker on the pressor response to intravenous or intracerebroventricular (i.c.v.) administration of noradrenaline in the awake rat
A mmHe
Reduction of control (%)
MAP (mmHg + SEM) Control
Treatment* Noradrenaline 0.6 &kg (iv.) Noradrenaline 60 nmol (i.c.v.) Phenoxybenzamine 2 mg/kg (Lv.) Noradrenaline (i.v.)f phenoxybenzamine Noradrenaline (i.c.v.) phenoxybenzamine
after (i.v.) after (i.v.)
Treatmentt
103.5 + 2.9
159.9 f 4.0
+56.4 f 2.3
-
98.0 k 1.6
137.0 k 6.7
f39.0 f 6.2
-
96.1 f 2.1
84.0 f 4.0
- 12.1 * 3.7
-
86.1 * 3.5
124.9 f 4.9
+ 38.8 f 3.0$
30
98.0 + 4.2
100.3 * 4.4
+2.3 f 6.2§
94
‘Group of 6 animals. tMaximal response. MAP = mean arterial pressure. fNoradrenaline was injected 20 min (intravenously) and 10 min (intracerebroventricularly) after phenoxybenzamine. $Statistically different from control intravenously or intracerebroventricularly administered noradrenaline, P < 0.05, Student’s f-test.
Table 2. Effect of intravenous (iv.) pretreatment with a vasopressin antagonist on the pressor response to intracerebraventricularly administered (i.c.v.) noradrenaline in the awake rat Reduction of control I%)
MAP (mmHg + SEM) Treatment*
Control
Noradrenaline 60 mnol (i.c.v.) d(CH,),Tyr(Me)AVP 50 pg/kg (Lv.) Noradrenaline (i.c.v.)$ after d(CH,),Tyr(Me)AVP (i.c.v.)
Treatmentt
A mmHe
104.6 f 3.8
152.5 f 5.7
i-47.9 + 3.1
103.1 f 2.3
105.6 f 3.7
f2.5 f 3.6
105.0 f 3.5
109.4 f 4.4
f4.4 k 6.35
91
‘Group of 8 animals. tMaxima1 response. MAP = mean arterial pressure. fNoradrenaline was injected I5 min after d(CH,), Tyr AVP. &Statistically different from control intracerebroventricularly administered noradrenaline, P < 0.05, Student’s f-test.
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460 i.c.v.fnmol)
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Fig. 4. Dose-dependent blockade of the pressor response to the intracerebroventricular (i.c.v.) administration of 60 nmol noradrenaline in unanaesthetized rats by intracerebroventricular pretreatment with H, or H, histamine antagonists. Columns represent the mean of pressor responses to intracerebroventricularly administered noradrenaline prior (open) and after pretreatments (dashed). Bars represent the SEM. Figures in parentheses represent the number of animals. *Statistically significant at P < 0.05, Student’s f-test.
Intraventricular
NA and blood pressure
served in anaesthetized animals was not affected by hypophysectomy (Fig. 3B). Effect of the intravenous injection of noradrenaline awake, control or hypophysectomized rats
in
The intravenous administration of 0.6 pg/kg of noradrenaline caused increases in blood pressure of a similar magnitude in a group of 7 control (+43.8 + 2.0 mmHg) and 7 hypophysectomized (+ 39.7 + 2.2 mmHg) rats. Effect of intravenous pretreatment with phenoxybenzamine on the pressor response to intracerebroventricularly and intravenously administered noradrenaline in the awake rat
The intravenous pretreatment with 2mg/kg of phenoxybenzamine caused a slight fall in blood pressure and decreased significantly the pressor responses observed after the intravenous or intracerebroventricular administration of noradrenaline, to the extent of 30 and 94% respectively (Table 1). Noradrenaline was injected intravenously 20 min and intracerebroventricularly 40 min after pretreatment with the a-blocker. EfSect of intravenous pretreatment with d(CHJ, Tyr(Me) AVP on the pressor response to intracerebroventricularly administered noradrenaline in the awake rat
The intravenous pretreatment with 5Opg/kg of d(CH,), Tyr(Me) AVP, a specific vasopressin blocker, caused no change in blood pressure but inhibited the pressor response to intracerebroventricularly administered noradrenaline to the extent of 91% (Table 2). The vasopressin antagonist was injected 15 min prior to intracerebroventricularly administered noradrenaline. Effect of intracerebroventricular pretreatment with anti-histaminic agents on the blood pressure response norto intracerebroventricularly administered adrenaline
The intracerebroventricular injection of either H, or H, histamine antagonists caused transient increases in blood pressure (up to 20 mmHg) in the awake rat. Noradrenaline was injected intracerebroventricularly 20 min after the pretreatment, when the blood pressure was already at basal levels. The intracerebroventricular pretreatment with diphenhydramine or pyrilamine, H, antagonists, caused a dose-dependent blockade of the pressor response observed after the intracerebroventricular administration of 60 nmol of noradrenaline in unanaesthetized rats, of up to 100% (Fig. 4). The intracerebroventricular pretreatment with metiamide or cimetidine, H, antagonists, was less effective, causing a partial blockade of the pressor response to intracerebroventricularly administered noradrenaline, up to 50”/, (Fig. 4). The levels of blood pressure after histamine antagonists and immediately
835
before intracerebroventricularly administered noradrenaline (ranging from 105.9 f 3.7 to 116.4 f 8.0mmHg) were not statistically different from the control levels prior to administration of the histamine to from 107.5 + 5.4 antagonists (ranging 119.4 f 8.0 mmHg). DISCUSSION
Anaesthesia is known to interfere preferentially with central pressor pathways whereas depressor pathways are extremely resistent (Bergman and Gutman, 1966). The opposite dose-dependent patterns observed after the intracerebroventricular administration of noradrenaline to conscious or anaesthetized rats points to the existence of more than one mechanism mediating the response to the catecholamine in this species and contradicts the idea of leakage of noradrenaline into the peripheral circulation, because such leakage would occur only in awake animals. The present observation that ganglionic blockade was ineffective on the pressor response to intracerebroventricularly administered noradrenaline in the conscious rat agrees with a previous report by Buccafusco and Brezenoff (1977). In addition, the present results indicate that ganglionic muscarinic receptors are not important in this response, although a 30% participation of such receptors has been proposed in the response to ganglionic stimulation in the rat (Yoshizaki, 1973). Although the lack of effects of ganglioplegics points to the absence of sympathetic involvement in the response, it does not rule out the idea of leakage of intracerebroventricularly administered noradrenaline into the peripheral circulation. Excluding a major involvement of the sympathetic nervous system in the response, an alternative to the idea of leakage of intracerebroventricularly injected noradrenaline into the peripheral circulation would be the involvement of humoral factors in the CNS. To evaluate this possibility the effect of intracerebroventricularly administered noradrenaline was studied in hypophysectomized rats. The absence of pressor activity after hypophysectomy suggests the involvement of a hypophyseal factor, most probably vasopressin, in the response to the intracerebroventricular administration of noradrenaline. The idea of an involvement of vasopressin in this response is greatly reinforced by the present findings which showed that d(CH,), Tyr(Me) AVP, a potent and specific vasopressin antagonist (Sawyer, Pang, Seto, McEnroe, Lammek and Manning, 1981), blocked the pressor response to noradrenaline to a similar extent as did hypophysectomy. These results, in addition to the different pattern of response observed in anaesthetized and awake animals, constitute important evidence against the idea of leakage of the catecholamine into the periphery. In addition, differences in the vascular sensitivity of hypophysectomized rats to noradrenaline can also be ruled
836
F. M. A. CORRBAet al.
out because the present results indicated no difference in the pressor response to the intravenous injection of noradrenaline in awake control or hypophysectomized animals. Although the evidence suggests that vasopressin is less important in the long term regulation of arterial pressure, there is evidence for it having a major role in short-term regulation (Cowley, 1982). The involvement of vasopressin in the cardiovascular response to centrally administered drugs is supported by previous reports showing that in awake rats 50% of the pressor response to intracerebroventricularly administered angiotensin was blocked by hypophysectomy, whereas total blockade was only observed after both hypophysectomy and ganglionic blockade (Severs, Summy-Long, Taylor and Connor, 1970). In addition the intracerebroventricular administration of noradrenaline was shown to cause the release of vasopressin which was blocked intraby pretreatment with cerebroventricularly administered phentolamine but not by ganglioplegic agents (Hoffman, 1979). The fact that a-adrenergic antagonists injected intravenously also blocked the pressor response to intracerebroventricularly administered noradrenaline in awake rats has been used as evidence of leakage into the peripheral circulation (Buccafusco and Brezenoff, 1977). In the present experiment, a complete blockade of the pressor response to intracerebroventricularly administered noradrenaline was observed following a selected dose of phenoxybenzamine which was effective in blocking only 30% of the response to an intravenous dose of 0.6 pg/kg of noradrenaline, thus showing a greater sensitivity of the response to intracerebroventricularly administered noradrenaline to the a-blocker. Both, phenoxybenzamine and phentolamine cross the blood-brain barrier. In a previous report (Correa and Graeff, 1974) it was shown that intracerebroventricularly administered phentolamine blocked the pressor response to intracerebroventricularly administered noradrenaline, in a dose insufficient to interfere with vascular receptors. These results, taking into account the fact that hypophysectomy or the treatment with d(CH,), Tyr(Me) AVP blocked the pressor response to noradrenaline, point to a central mediation of the pressor response to intracerebroventricularly administered noradrenaline through cc-adrenoceptors as against the idea of leakage into the peripheral circulation. As a consequence the blockade observed after the intravenous administration of a-blockers would not indicate an action at a systemic level but an action of a-blockers in the CNS, after crossing the blood-brain barrier. The depressor effect observed after the intracerebroventricular administration of noradrenaline in anaesthetized rats was found to be independent of humoral factors because a similar decrease in blood pressure was observed in normal and hypophysectomized animals. Hypophysectomy however renders the animals less capable of control-
ling blood pressure under anaesthesia and the basal blood pressure was significantly lower in hypophysectomized as compared to sham-operated rats, whereas no difference was observed in awake animals. The mediation of the depressor responses to catecholamines involves both CI~and b-receptors in the rat. In this species, the hypotensive response to intracerebroventricularly administered adrenaline is blocked by b-antagonists but not cr,-antagonists, whereas clonidine shows a reverse situation (Borkowski and Finch, 1979). In addition, the depressor effect of the intracerebroventricular administration of noradrenaline to anaesthetized rats is reversed into a massive increase in blood pressure after intracerebroventricular pretreatment with propranolol (Correa et al., 1982). In a previous report, it was observed that the pressor response to the intracerebroventricular administration of bradykinin was blocked by pretreatment with histamine antagonists (Correa and Graeff, 1974). Bradykinin is known to release vasopressin (Baertschi and Dreifuss, 1978; Rocha e Silva and Malnic, 1964). The possible link between the pressor response to the peptide and a histaminergic system that modulates the release of vasopressin is reinforced by the existence of strong evidence for a physiological role of histamine in brain in the regulation of water balance. Histamine produces antidiuresis after intracerebroventricular (Tuomisto and Erickson, 1979) or local injection into the nucleus supraopticus (Bennet and Pert, 1974) and a marked increase in plasma-vasopressin in the rat (Dogteron, Van Wimersma-Greidanus and De Wied, 1976). In addition the intracerebroventricular administration of histamine causes an increase in blood pressure in this species (Finch and Hicks, 1976). The intracerebroventricular pretreatment with H,-antagonists, diphenhydramine and pyrilamine caused a dose-dependent blockade of the pressor response to intracerebroventricularly administered noradrenaline of up to 100x, whereas the intracerebroventricular pretreatment with HZ-antagonists, metiamide and cimetidine caused a partial blockade of up to 50% of the response. The blockade observed after histamine antagonists favours the idea of a histaminergic mechanism mediating the pressor response to intracerebroventricularly administered noradrenaline. The possibility of a non-specific blockade is less probable since pyrilamine and H,-blockers are practically devoid of local anaesthetic properties and large doses of intracerebroventricularly injected procaine were reported to be ineffective against the pressor response to histamine (Finchs and Hicks, 1976). Otherwise propranolol, which has potent local anaesthetic properties, was found to reverse the hypotensive response to intracerebroventricularly administered noradrenaline in the anaesthetized rat into a clear pressor response (Correa et al., 1982) and to
Intraventricular
NA and blood pressure
potentiate the pressor response in awake rats (unpublished results) after intracerebroventricular administration. Since histamine antagonists are devoid of sympatholytic activity, the blockade observed on the pressor response to intracerebroventricularly administered noradrenaline also reinforces the idea of an action at the level of the CNS and not after leakage to the periphery. In conclusion, the present results indicate the existence of two major systems activated by the intracerebroventricular administration of noradrenaline in the rat: (1) A pressor pathway, sensitive to anaesthesia, involving the release of a hypophyseal factor but not the activation of the sympathetic nervous system. This pathway is mediated by a histaminergic mechanism with the involvement of both H, and H, receptors, with a major participation by the former. This pressor pathway has a preponderant role in awake animals. (2) A depressor pathway, not involving hypophyseal mediation, is apparent under anaesthesia and masked in the awake animal. Acknowledgements-The authors are indebted to Dr M. Manning for providing the vasopressin blocker d(CH,), Tyr(Me) AVP, to MS Idalia I. Bonani-Aguiar for technical and to MS Sonia Maria Stefanelli for editorial assistance. Dr I.A.S. Magro is a PhD student and recipient fellowship from CAPES. The research was supported grant from FINEP (4/3/84/0306/00-FMRP).
of a by a
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Corrda F. M. A., Magro I. A. S. and Peres-Polon V. L. (1982) CNS mediation of cardiovascular responses to the intracerebroventricular administration of catecholamines. Trends Pharmac. Sci. 3: 33G332. Cowley A. W. Jr (1982) In: International Review of Physiology (Guyton‘A. C. and Hall J. E., Eds), Vol. 26, DD. 189-241. Universitv Park Press. Baltimore. Day M. D. and Roach A. S. (1974) Central a- and p-adrenoceptors modifying arterial blood pressure and heart rate in conscious cats. Br. J. Pharmac. 51: 325-333. De Groot J. (1959) The rat forebrain in stereotaxic coordinates, Trans. R. Neth. Acad. Sci. 52: No. 4. Dogteron J., Van Wimersma-Greidanus T. and De Wied D. (1976) Histamine as a potent releaser of vasopressin in the rat. Experientia 32: 6599660. Finch L. and Hicks P. E. (1976) The cardiovascular effects of intraventricularly administered histamine in the anaesthetized rat. Naunyn-Schmiedebergs Arch. Pharmac. 293: 151-157. Forsyth R. P. and Pesouat J. J. (1978) Cardiovascular effects of intracerebroventricular norepinephrine in conscious and unanaesthetized monkeys. Neuropharmacology 17: 103-108. Hoffman W. E. (1979) Central cardiovascular and antidiuretic action of adrenergic drugs. Neuropharmacology 18: 7-12. Krieger E. M. (1964) Neurogenic hypertension in the rat. Circulation Res. 15: 511-521. Kristt D. A. (1975) Vasodepressor response evoked from rat diencephalon: effect of pre-stimulation blood pressure. Brain Res. 91: 156159. Rocha e Silva M. and Malnic G. (1964) Release of antidiuretic hormone by bradykinin. J. Pharmac. 146: 2432. Sawyer W. H., Pang P. K. T., Seto J., McEnroe M., Lammek B. and Manning M. (1981) Vasopressin analogs that antagonize antidiuretic responses by rats to the antidiuretic hormone. Science 212: 49-51. Schmidt H. and Fenard S. (1971) Effets des substances sympathomimttiques sur les centres vasomoteurs. Archs int. Pharmacodyn. Ther. 190: 229-240. Severs W. B., Summy-Long J., Taylor J. S. and Connor J. D. (1970) A central effect of angiotensin: release of pituitary pressor material. J. Pharmac. exp. Ther. 174: 27-34. Tuomisto L. and Erickson L. (1979) Antidiuresis induced by infusions of histamine into the brain ventricles of conscious hydrated goats. Eur. J. Pharmac. 54: 191~201.
Van Zwieten P. A. and Timmermans P. B. M. W. M. (1979) The role of central a-adrenoceptors in the mode of action of hypotensive drugs. Trends Pharmac. Sci. 1: 3941. Yoshizaki T. (1973) Participation of muscarinic receptors on splanchnic-adrenal transmission in the rat. Jap. J. Pharmac. 23: 8 13-8 16. Zarrow M. X., Yochim J. M. and McCarthy J. L. (1964) Experimental Endocrinology. A Sourcebook of Basic Techniques. Academic Press, New York.
0028-3908/85 $3.00 + 0.00 Copyright 0 1985 Pergamon Press Ltd
MECHANISM OF THE CNS-MEDIATED PRESSOR RESPONSE TO INTRACEREBROVENTRICULAR INJECTION OF NORADRENALINE IN UNANAESTHETIZED RATS F. M. A. CORREA’, IDALINA A. S. MAGRO’, VERA L. PERES-P• LON~ and J. ANTUNES-RODRIGUES’ ‘Department of Pharmacology, 2Department of Physiology, School of Medicine of RibeirIo Preto-USP, 14100 RibeirIo Preto and “Department of Physiological Science, School of Dentistry of RibeirHo Preto-USP, 14100 RibeirIo Preto, SP, Brazil (Accepted
16 January 1985)
Summary-Catecholamines administered intracerebroventricularly (i.c.v.) have cardiovascular effects mediated by the CNS. Although hypotension constitutes the more prominent response, an increase in blood pressure has also been reported after the intracerebroventricular injection of these amines. Anaesthesia interferes with pressor responses mediated by the CNS to a larger extent than with depressor mechanisms and constitutes one of the major factors influencing the pattern of response to the amines. The depressor response observed after the intracerebroventricular administration of noradrenaline is reversed into increases in blood pressure in awake animals. In the present experiment, the action of intracerebroventricularly injected noradrenaline was compared in anaesthetized and conscious rats. The results indicated that the pressor response in awake rats was not mediated by the sympathetic nervous system and involved the release of a pituitary humoral-factor, most probably vasopressin, whereas the depressor response observed in anaesthetized animals was not dependent on pituitary mediation. The involvement of histaminergic mechanisms in the CNS in the control of the pressor response to intracerebroventricularly administered noradrenaline in the rat is proposed. Key words: blood pressure, CNS, noradrenaline vasopressin antagonist, rats.
administered intracerebroventricularly,
The intracerebroventricular administration of catecholamines, notably of noradrenaline, has hypotensive effects, mostly by activating a central cr-adrenergic mechanism (Day and Roach, 1974). The existence of such a mechanism provides the basis for understanding the mode of action of antihypertensive drugs such as a-methyldopa and clonidine (Van Zwieten and Timmermans, 1979). However, some discrepancies remain concerning the effects of intracerebroventricularly administered catecholamines, among them the fact that noradrenaline administered intracerebroventricularly has been reported to produce either an elevation or a fall in blood pressure (Forsyth and Pesouat, 1978; Schmidt and FCnard, 1971). Possible explanations for these discrepancies were previously discussed (Corr&a, Magro and Peres-Polon, 1982). The hypothesis of a predominant action of intracerebroventricularly administered noradrenaline on central a,-receptors is based on experiments using cats and dogs as subjects. However, the evidence suggests that, in the rat, there are other mechanisms being simultaneously activated. In awake rats, noradrenaline administered intracerebroventricularly causes increases in blood pressure which are blocked by local pretreatment with phentolamine (CorrCa and Graeff, 1974). Leakage into the peripheral circulation was proposed as an
vasopressin,
explanation for the pressor effects of intracerebroventricularly administered noradrenaline because ganglionic blockers failed to prevent or even potentiate such effects in the rat (Buccafusco and Brezenoff, 1977). Pressor responses were reported in awake rats, whereas only depressor responses were observed after intracerebroventricular administration of noradrenaline in anaesthetized animals (CorrCa and Graeff, 1974). If the idea of leakage to the periphery is accepted, as an explanation for the pressor effects of intracerebroventricularly administered noradrenaline, it must be assumed that leakage only occurs in awake animals. However, the lack of evidence for such a carrier or gradient mechanism, active only in awake animals, points to the necessity of an alternative explanation. In addition, the marked effects of anaesthesia on the central cardiovascular response to drugs or electrical stimulation (Bergmann and Gutman, 1966; Corrka et al., 1982; Kristt, 1975) may contribute to these discrepancies and should also be considered. The present study aimed to evaluate the extent of the involvement of the central nervous system (CNS) in the pressor response to intracerebroventricularly administered noradrenaline. Experiments were conducted using anaesthetized and awake rats. Intracerebroventricular administration of noradrenaline 831
F. M. A. CORRI?Aet al.
832
caused increases in blood pressure in awake rats and decreases in blood pressure in anaesthetized rats. Hypophysectomy or intravenous pretreatment with a specific vasopressin blocker, but not ganglionic blockade, were found effective in blocking the pressor response to intracerebroventricularly administered noradrenaline, suggesting the involvement of vasopressin in this response. Intracerebroventricular pretreatment with histamine antagonists was performed to provide evidence of the involvement of central histaminergic mechanisms in the pressor response to noradrenaline in the rat. METHODS
Wistar rats, 200-230 g were used. Anaesthetized animals were kept under urethane, 1.2 g/kg (i.v.), and intracerebroventricular cannulas were implanted according to the method of CorrZa and Graeff (1974). Blood pressure was recorded acutely through an intracarotid polyethylene catheter (PE-50). Awake animals had intracerebroventricular cannulas implanted under pentobarbital anaesthesia, 40 mg/kg (i.p.), and chronic intra-arterial catheters implanted according to Krieger (1964), for recording blood pressure. Intra-arterial catheters were implanted under ether anaesthesia, the day after the surgery. Experiments were carried out two days afterwards to allow for recovery. Whenever the intravenous route was used, a similar catheter was implanted simultaneously into the femoral vein for administration of drugs. Hypophysectomy was performed surgically under light anaesthesia with ether (Zarrow, Yochim and McCarthy, 1964). Animals were supplemented daily with deoxycorticosterone (Merck, Sharp & Dohme), 0.25 mg/kg (i.m.), alone or associated with vasopressin thanate in oil (Pitressin-CIBA; 5 U/kg, s.c.), and used 5 days later. Experiments with anaesthetized or awake hypophysectomized rats were carried out following the procedure previously mentioned. Drugs were dissolved in 0.9% NaCl and injected into the cerebral ventricles in a volume of 10 ~1 over a period of 30 set, using a hand-driven 705-N Hamilton 50 ~1 syringe and an odontologic injection needle (Mizzi, 0.2 mm o.d.). Dose-effect curves were obtained for intracerebroventricularly administered noradrenaline hydrochloride (Sigma) and for different histamine antagonists on the effect of intraventricular administration of noradrenaline. The H,-antagonists used were diphenhydramine HCl (Sigma) or pyrilamine maleate (Sigma) and H,-antagonists were metiamide (SKF) or cimetidine (SKF). Ganglioplegic efficacy was evaluated against the pressor effect evoked by electrical stimulation of the posterior hypothalamus. Rats were kept under urethane anaesthesia, the skull fixed to a stereotaxic apparatus (David Kopf 900) and a monopolar electrode implanted in the posterior hypothalamus at the
following coordinates: A + 4.2 mm, L + 0.5 mm, H + 2.3 mm, of the De Groot’s (1959) brain atlas of the rat brain. Blood pressure was recorded acutely through a intracarotid catheter. Square wave stimuli of 60 Hz and variable voltage were applied and voltage/response curves obtained prior to and after the administration of pentolinium tartrate (Sigma), (3 mg/kg, i.v.), alone or associated with scopolamine methylbromide (Boheringer; 4 mg/kg, i.v.). Further evaluation of the involvement of vasopressin in the pressor response to intracerebroventricularly administered noradrenaline was accomplished using the specific vasopressin antagonist d(CH,), Tyr(Me) AVP, (50 p g/kg, i.v.) in saline. The drug was kindly provided by Dr Maurice Manning from the Medical College of Ohio. RESULTS
Effect of ganglionic blockade on the blood pressure response to intracerebroventricularly administered noradrenaline and the electrical stimulation of the posterior hypothalamus Intracerebroventricular administration of noradrenaline caused different patterns of dosedependent effects in anaesthetized and awake rats. In anaesthetized animals a decrease in blood pressure was observed, whereas an increase in blood pressure was found in awake rats (Fig. 1). The administration
f
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Fig. 1. Dose-effect
curves for the pressor and depress0 responses observed after the intracerebroventricular admin istration of noradrenaline in freely moving and urethane anaesthetized rats, respectively. Points in the Figure repre sent the mean and the bars the SEM. Figures in parenthew represent the number of animals.
Intraventricular
NA and blood pressure
^a x+5.0 E
833
animals after significantly lower in anaesthetized to controls hypophysectomy when compared respectively), (108.6 + 4.0 and 77.5 + 2.8 mmHg, whereas unanaesthetized animals had basal levels of of controls blood pressure similar to those (116.8 f 4.0 and 112.0 + 3.5 mmHg, respectively). Hypophysectomy was effective in reducing the intrain blood pressure to increase cerebroventricularly administered noradrenaline in the awake rat to the extent of 9&95% in animals supplemented with either deoxycorticosterone alone or in association with vasopressin, to maintain the hydro-ionic balance subsequent to the removal of the gland (Fig. 3A). The hypotensive response to intracerebroventricularly administered noradrenaline ob-
1 (A)
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UNANAESTHETIZED
1 (A)
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Fig. 2. (A) Effect of ganglionic blockade with 3 mg/kg (iv.) pentolium, alone (0-O) or associated to 4mg/kg (i.v.) scopolamine methylbromide (A-A), on the voltagedependent pressor response-curve to electrical stimulation of the posterior hypothalamus in the anaesthetized rat (@--0). Points in the Figure represent the mean and the bars the SEM. Figures in parentheses represent the number of animals. *Statistically significant at P < 0.05, Student’s t-test. (B) Absence of effects of ganglionic blockade with pentolinium on the pressor response to the intracerebroventricular administration of 60 nmol of noradrenaline (NA) in unanaesthetized rats. Dashed and open columns represent the mean of basal blood pressure and maximal response to NA, respectively. Bars represent the SEM. The percentage increase in mean arterial blood pres-
sure against control is presented at the top of columns. Figures in parentheses represent the number of animals. *Statistically significant at P < 0.05, Student’s t-test.
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of vehicle caused no effect on blood pressure. The pressor response to intracerebroventricularly administered noradrenaline did not produce tachyphylaxis after three successive injections in unanaesthetized, freely-moving rats. The response to intracerebroventricularly administered noradrenaline in the awake rat was not affected by intravenous pretreatment with pentolinium in a dose which was effective in blocking the increase in blood pressure induced by electrical stimulation of the posterior hypothalamus (Fig. 2A). At the dose used, pentolinium did shift the voltage/response curve to the right. The concomitant administration of scopolamine methylbromide, a muscarinic antagonist that does not cross the blood-brain barrier, did not increase the efficacy of ganglioplegic blockade. Effect of hypophysectomy on the blood pressure response to intracerebroventricularly administered noradrenaline The
basal
levels
of
blood
pressure
were
c
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Id SHAM
HYPHD
Fig. 3. (A) Inhibitory effect of hypophysectomy on the pressor response to the intracerebroventricular administration of noradrenaline in unanaesthetized rats. Animals were treated with 0.25 mg/kg (i.m.) deoxycorticosterone, alone (CORT) or with 5 U/kg (s.c.) vasopressin (AVP) for 4 days after surgery. Columns represent the means of pressor responses to 60 nmol intracerebroventricularly administered noradrenaline in sham (dashed) and hypophysectomized animals (open). Bars represent the SEM. Figures the number of animals. in parentheses represent *Statistically significant at P < 0.05, Student’s t-test. (B) Absence of effect of hypophysectomy (HYPHD) on the depressor response to the intracerebroventricular administration of 60 nmol noradrenaline in urethane-anaesthetized rats. Animals were treated with deoxycorticosterone and vasopressin for 4 days after surgery. Columns represent the mean of basal blood pressure (dashed) and the maximal fall in blood pressure after intracerebroventricularly administered noradrenaline (open). Bars represent the SEM. The percentage fall in mean arterial pressure is presented at the top of the column. Figures in parentheses represent the number of animals. *Statistically significant at P < 0.05, Student’s t-test.
F. M. A. CORRBA et al.
834
Table 1. Effect of intravenous (i.v.) pretreatment with an a-adrenergic blocker on the pressor response to intravenous or intracerebroventricular (i.c.v.) administration of noradrenaline in the awake rat
A mmHe
Reduction of control (%)
MAP (mmHg + SEM) Control
Treatment* Noradrenaline 0.6 &kg (iv.) Noradrenaline 60 nmol (i.c.v.) Phenoxybenzamine 2 mg/kg (Lv.) Noradrenaline (i.v.)f phenoxybenzamine Noradrenaline (i.c.v.) phenoxybenzamine
after (i.v.) after (i.v.)
Treatmentt
103.5 + 2.9
159.9 f 4.0
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-
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137.0 k 6.7
f39.0 f 6.2
-
96.1 f 2.1
84.0 f 4.0
- 12.1 * 3.7
-
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124.9 f 4.9
+ 38.8 f 3.0$
30
98.0 + 4.2
100.3 * 4.4
+2.3 f 6.2§
94
‘Group of 6 animals. tMaximal response. MAP = mean arterial pressure. fNoradrenaline was injected 20 min (intravenously) and 10 min (intracerebroventricularly) after phenoxybenzamine. $Statistically different from control intravenously or intracerebroventricularly administered noradrenaline, P < 0.05, Student’s f-test.
Table 2. Effect of intravenous (iv.) pretreatment with a vasopressin antagonist on the pressor response to intracerebraventricularly administered (i.c.v.) noradrenaline in the awake rat Reduction of control I%)
MAP (mmHg + SEM) Treatment*
Control
Noradrenaline 60 mnol (i.c.v.) d(CH,),Tyr(Me)AVP 50 pg/kg (Lv.) Noradrenaline (i.c.v.)$ after d(CH,),Tyr(Me)AVP (i.c.v.)
Treatmentt
A mmHe
104.6 f 3.8
152.5 f 5.7
i-47.9 + 3.1
103.1 f 2.3
105.6 f 3.7
f2.5 f 3.6
105.0 f 3.5
109.4 f 4.4
f4.4 k 6.35
91
‘Group of 8 animals. tMaxima1 response. MAP = mean arterial pressure. fNoradrenaline was injected I5 min after d(CH,), Tyr AVP. &Statistically different from control intracerebroventricularly administered noradrenaline, P < 0.05, Student’s f-test.
-
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Fig. 4. Dose-dependent blockade of the pressor response to the intracerebroventricular (i.c.v.) administration of 60 nmol noradrenaline in unanaesthetized rats by intracerebroventricular pretreatment with H, or H, histamine antagonists. Columns represent the mean of pressor responses to intracerebroventricularly administered noradrenaline prior (open) and after pretreatments (dashed). Bars represent the SEM. Figures in parentheses represent the number of animals. *Statistically significant at P < 0.05, Student’s f-test.
Intraventricular
NA and blood pressure
served in anaesthetized animals was not affected by hypophysectomy (Fig. 3B). Effect of the intravenous injection of noradrenaline awake, control or hypophysectomized rats
in
The intravenous administration of 0.6 pg/kg of noradrenaline caused increases in blood pressure of a similar magnitude in a group of 7 control (+43.8 + 2.0 mmHg) and 7 hypophysectomized (+ 39.7 + 2.2 mmHg) rats. Effect of intravenous pretreatment with phenoxybenzamine on the pressor response to intracerebroventricularly and intravenously administered noradrenaline in the awake rat
The intravenous pretreatment with 2mg/kg of phenoxybenzamine caused a slight fall in blood pressure and decreased significantly the pressor responses observed after the intravenous or intracerebroventricular administration of noradrenaline, to the extent of 30 and 94% respectively (Table 1). Noradrenaline was injected intravenously 20 min and intracerebroventricularly 40 min after pretreatment with the a-blocker. EfSect of intravenous pretreatment with d(CHJ, Tyr(Me) AVP on the pressor response to intracerebroventricularly administered noradrenaline in the awake rat
The intravenous pretreatment with 5Opg/kg of d(CH,), Tyr(Me) AVP, a specific vasopressin blocker, caused no change in blood pressure but inhibited the pressor response to intracerebroventricularly administered noradrenaline to the extent of 91% (Table 2). The vasopressin antagonist was injected 15 min prior to intracerebroventricularly administered noradrenaline. Effect of intracerebroventricular pretreatment with anti-histaminic agents on the blood pressure response norto intracerebroventricularly administered adrenaline
The intracerebroventricular injection of either H, or H, histamine antagonists caused transient increases in blood pressure (up to 20 mmHg) in the awake rat. Noradrenaline was injected intracerebroventricularly 20 min after the pretreatment, when the blood pressure was already at basal levels. The intracerebroventricular pretreatment with diphenhydramine or pyrilamine, H, antagonists, caused a dose-dependent blockade of the pressor response observed after the intracerebroventricular administration of 60 nmol of noradrenaline in unanaesthetized rats, of up to 100% (Fig. 4). The intracerebroventricular pretreatment with metiamide or cimetidine, H, antagonists, was less effective, causing a partial blockade of the pressor response to intracerebroventricularly administered noradrenaline, up to 50”/, (Fig. 4). The levels of blood pressure after histamine antagonists and immediately
835
before intracerebroventricularly administered noradrenaline (ranging from 105.9 f 3.7 to 116.4 f 8.0mmHg) were not statistically different from the control levels prior to administration of the histamine to from 107.5 + 5.4 antagonists (ranging 119.4 f 8.0 mmHg). DISCUSSION
Anaesthesia is known to interfere preferentially with central pressor pathways whereas depressor pathways are extremely resistent (Bergman and Gutman, 1966). The opposite dose-dependent patterns observed after the intracerebroventricular administration of noradrenaline to conscious or anaesthetized rats points to the existence of more than one mechanism mediating the response to the catecholamine in this species and contradicts the idea of leakage of noradrenaline into the peripheral circulation, because such leakage would occur only in awake animals. The present observation that ganglionic blockade was ineffective on the pressor response to intracerebroventricularly administered noradrenaline in the conscious rat agrees with a previous report by Buccafusco and Brezenoff (1977). In addition, the present results indicate that ganglionic muscarinic receptors are not important in this response, although a 30% participation of such receptors has been proposed in the response to ganglionic stimulation in the rat (Yoshizaki, 1973). Although the lack of effects of ganglioplegics points to the absence of sympathetic involvement in the response, it does not rule out the idea of leakage of intracerebroventricularly administered noradrenaline into the peripheral circulation. Excluding a major involvement of the sympathetic nervous system in the response, an alternative to the idea of leakage of intracerebroventricularly injected noradrenaline into the peripheral circulation would be the involvement of humoral factors in the CNS. To evaluate this possibility the effect of intracerebroventricularly administered noradrenaline was studied in hypophysectomized rats. The absence of pressor activity after hypophysectomy suggests the involvement of a hypophyseal factor, most probably vasopressin, in the response to the intracerebroventricular administration of noradrenaline. The idea of an involvement of vasopressin in this response is greatly reinforced by the present findings which showed that d(CH,), Tyr(Me) AVP, a potent and specific vasopressin antagonist (Sawyer, Pang, Seto, McEnroe, Lammek and Manning, 1981), blocked the pressor response to noradrenaline to a similar extent as did hypophysectomy. These results, in addition to the different pattern of response observed in anaesthetized and awake animals, constitute important evidence against the idea of leakage of the catecholamine into the periphery. In addition, differences in the vascular sensitivity of hypophysectomized rats to noradrenaline can also be ruled
836
F. M. A. CORRBAet al.
out because the present results indicated no difference in the pressor response to the intravenous injection of noradrenaline in awake control or hypophysectomized animals. Although the evidence suggests that vasopressin is less important in the long term regulation of arterial pressure, there is evidence for it having a major role in short-term regulation (Cowley, 1982). The involvement of vasopressin in the cardiovascular response to centrally administered drugs is supported by previous reports showing that in awake rats 50% of the pressor response to intracerebroventricularly administered angiotensin was blocked by hypophysectomy, whereas total blockade was only observed after both hypophysectomy and ganglionic blockade (Severs, Summy-Long, Taylor and Connor, 1970). In addition the intracerebroventricular administration of noradrenaline was shown to cause the release of vasopressin which was blocked intraby pretreatment with cerebroventricularly administered phentolamine but not by ganglioplegic agents (Hoffman, 1979). The fact that a-adrenergic antagonists injected intravenously also blocked the pressor response to intracerebroventricularly administered noradrenaline in awake rats has been used as evidence of leakage into the peripheral circulation (Buccafusco and Brezenoff, 1977). In the present experiment, a complete blockade of the pressor response to intracerebroventricularly administered noradrenaline was observed following a selected dose of phenoxybenzamine which was effective in blocking only 30% of the response to an intravenous dose of 0.6 pg/kg of noradrenaline, thus showing a greater sensitivity of the response to intracerebroventricularly administered noradrenaline to the a-blocker. Both, phenoxybenzamine and phentolamine cross the blood-brain barrier. In a previous report (Correa and Graeff, 1974) it was shown that intracerebroventricularly administered phentolamine blocked the pressor response to intracerebroventricularly administered noradrenaline, in a dose insufficient to interfere with vascular receptors. These results, taking into account the fact that hypophysectomy or the treatment with d(CH,), Tyr(Me) AVP blocked the pressor response to noradrenaline, point to a central mediation of the pressor response to intracerebroventricularly administered noradrenaline through cc-adrenoceptors as against the idea of leakage into the peripheral circulation. As a consequence the blockade observed after the intravenous administration of a-blockers would not indicate an action at a systemic level but an action of a-blockers in the CNS, after crossing the blood-brain barrier. The depressor effect observed after the intracerebroventricular administration of noradrenaline in anaesthetized rats was found to be independent of humoral factors because a similar decrease in blood pressure was observed in normal and hypophysectomized animals. Hypophysectomy however renders the animals less capable of control-
ling blood pressure under anaesthesia and the basal blood pressure was significantly lower in hypophysectomized as compared to sham-operated rats, whereas no difference was observed in awake animals. The mediation of the depressor responses to catecholamines involves both CI~and b-receptors in the rat. In this species, the hypotensive response to intracerebroventricularly administered adrenaline is blocked by b-antagonists but not cr,-antagonists, whereas clonidine shows a reverse situation (Borkowski and Finch, 1979). In addition, the depressor effect of the intracerebroventricular administration of noradrenaline to anaesthetized rats is reversed into a massive increase in blood pressure after intracerebroventricular pretreatment with propranolol (Correa et al., 1982). In a previous report, it was observed that the pressor response to the intracerebroventricular administration of bradykinin was blocked by pretreatment with histamine antagonists (Correa and Graeff, 1974). Bradykinin is known to release vasopressin (Baertschi and Dreifuss, 1978; Rocha e Silva and Malnic, 1964). The possible link between the pressor response to the peptide and a histaminergic system that modulates the release of vasopressin is reinforced by the existence of strong evidence for a physiological role of histamine in brain in the regulation of water balance. Histamine produces antidiuresis after intracerebroventricular (Tuomisto and Erickson, 1979) or local injection into the nucleus supraopticus (Bennet and Pert, 1974) and a marked increase in plasma-vasopressin in the rat (Dogteron, Van Wimersma-Greidanus and De Wied, 1976). In addition the intracerebroventricular administration of histamine causes an increase in blood pressure in this species (Finch and Hicks, 1976). The intracerebroventricular pretreatment with H,-antagonists, diphenhydramine and pyrilamine caused a dose-dependent blockade of the pressor response to intracerebroventricularly administered noradrenaline of up to 100x, whereas the intracerebroventricular pretreatment with HZ-antagonists, metiamide and cimetidine caused a partial blockade of up to 50% of the response. The blockade observed after histamine antagonists favours the idea of a histaminergic mechanism mediating the pressor response to intracerebroventricularly administered noradrenaline. The possibility of a non-specific blockade is less probable since pyrilamine and H,-blockers are practically devoid of local anaesthetic properties and large doses of intracerebroventricularly injected procaine were reported to be ineffective against the pressor response to histamine (Finchs and Hicks, 1976). Otherwise propranolol, which has potent local anaesthetic properties, was found to reverse the hypotensive response to intracerebroventricularly administered noradrenaline in the anaesthetized rat into a clear pressor response (Correa et al., 1982) and to
Intraventricular
NA and blood pressure
potentiate the pressor response in awake rats (unpublished results) after intracerebroventricular administration. Since histamine antagonists are devoid of sympatholytic activity, the blockade observed on the pressor response to intracerebroventricularly administered noradrenaline also reinforces the idea of an action at the level of the CNS and not after leakage to the periphery. In conclusion, the present results indicate the existence of two major systems activated by the intracerebroventricular administration of noradrenaline in the rat: (1) A pressor pathway, sensitive to anaesthesia, involving the release of a hypophyseal factor but not the activation of the sympathetic nervous system. This pathway is mediated by a histaminergic mechanism with the involvement of both H, and H, receptors, with a major participation by the former. This pressor pathway has a preponderant role in awake animals. (2) A depressor pathway, not involving hypophyseal mediation, is apparent under anaesthesia and masked in the awake animal. Acknowledgements-The authors are indebted to Dr M. Manning for providing the vasopressin blocker d(CH,), Tyr(Me) AVP, to MS Idalia I. Bonani-Aguiar for technical and to MS Sonia Maria Stefanelli for editorial assistance. Dr I.A.S. Magro is a PhD student and recipient fellowship from CAPES. The research was supported grant from FINEP (4/3/84/0306/00-FMRP).
of a by a
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