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Studies of the mechanism of the cardiovascular action of central injections of histamine
Neuropharmacology Vol. 22. No. 9, pp. 1109-l 115, 1983 Printed in Great Britain
0028-3908/83%3.00 + 0.00 Pergamon Press Ltd
STUDIES OF THE MECHANISM OF THE CARDIOVASCULAR ACTION OF CENTRAL INJECTIONS OF HISTAMINE M. C. KLEIN* and S. B. GERTNER Department of Pharmacology, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, 100 Bergen Street, Newark, NJ 07103, U.S.A. (Accepted
24 February
1983)
Summary-The role of the sympathetic nervous system in the mediation of cardiovascular function, following administration of histamine centrally was investigated. Intracerebroventricular (i.c.v.) injections of histamine into the lateral ventricle of conscious rats caused a dose-dependent increase in blood pressure as well as a dose-dependent fall in heart rate. The pressure and heart rate responses were both attenuated by pretreatment with chlorpheniramine. Peripheral blockade of autonomic ganglia with hexamethonium failed to block the centrally-mediated pressor response to histamine. Peripheral chemical sympathectomy with 6-hydroxydopamine also did not inhibit the pressor responses to histamine. In this latter group of animals the pressor response to histamine was potentiated. It is concluded that in the conscious rat, increases in blood pressure resulting from central stimulation with histamine are not mediated solely through the activation of the sympathetic nervous system and are, at least in part, due to the release of a vasopressor substance.
Key words: intracerebroventricular, conscious rats.
histamine, blood pressure, hexamethonium,
6-hydroxydopamine,
in heart rate (Hoffman and Schmid, 1978; Klein and Gertner, 1981). In the conscious cat, central injections of histamine have been reported to increase blood pressure (Finch and Hicks, 1976b) without changing heart rate. In the conscious goat, centrally administered histamine increased blood pressure, but had variable effects on heart rate (Tuomisto and Eriksson, 1980). The purpose of this present study was to determine in the conscious rat, whether the sympathetic nervous system mediates the cardiovascular changes that occur following intracerebroventricular injections of histamine. This has been studied by blocking the output from the sympathetic nervous system by two different methods, i.e. ganglionic blockade and chemical sympathectomy.
The central administration of histamine in anesthetized rats (Brezenoff and Jenden, 1969; Finch and Hicks, 1976a, 1977) and cats (Trendelenburg, 1957; White, 1961) has been shown to produce a rise in both mean arterial blood pressure as well as heart rate. These changes in both blood pressure and heart rate were reported to occur through stimulation of central sympathetic centers, which resulted in an increase in sympathetic outflow to the peripheral vasculature and heart (Trendelenburg, 1957; White, 1965; Finch and Hicks, 1976a; Carroll and Clark, 1979). Additional pharmacological data by Finch and Hicks (1976a), were the basis for the suggestion that the increase in blood pressure and heart rate were mediated by two distinct central pathways, i.e. the increase in blood pressure was caused by the activation of central a-adrenergic pathways, whereas the increases in heart rate involved central muscarinic pathways. In conscious animals, central injections of histamine result in cardiovascular responses which are qualitatively different from those reported in anesthetized animals, and appear to be species-related. Thus, in unanesthetized rats intracerebroventricular (i.c.v.) injection of histamine, increases blood pressure, but produces opposite changes in heart rate, i.e. decreases
METHODS
*Present address: Department of Physiology and Pharmacology, Bowman Gray School of Medicine, 300 Hawthorne Road, Winston-Salem, NC 27103, U.S.A.
Thirty-one male Sprague-Dawley rats weighing 20&300 g (Taconic Farms, Gemantown, N.Y.) were used. Animals were housed in individual plastic cages, maintained on a 12 hr light-dark cycle and provided with food and water ad libitum. Experiments were designed to assess the requirement of an intact peripheral sympathetic nervous system in mediating the cardiovascular effects following central injections of histamine. To test whether ganglionic blockade could alter these cardiovascular responses, 12 rats were randomly divided into four groups. Two of the groups were pretreated with
1109
1110
M. C. KLEIN and S. B. GER~NER
hexamethonium and then challenged with either a small or a large dose of histamine. Two control groups received either the small or large dose of histamine (See Results, section 3.1.). In some cases, the same rat was used more than once. However, at least 48 hr was allowed for an animal to recover from a previous treatment. To determine the efficacy of ganglionic blockade, 4 rats were injected with the ganglionic stimulant l,l-dimethyl-4-phenyl-piperazinium (DMPP) (250 pg/kg, iv.) 15 min before and 5 min after injection of hexamethonium (20 mg/kg, i.v.). The first injection of DMPP increased mean arterial blood pressure by 56 + 6.7 mmHg. However, following ganglionic blockade, the rise in blood pressure after the second of DMPP was dose reduced by 66% (+ 19 f 3.1 mmHg). No reduction in blood pressure was observed following repeated injections of DMPP in the absence of ganglionic blockade. To test whether chemical sympathectomy could alter the cardiovascular responses to central injection of histamine, another group of rats was treated with the neurotoxin, 6_hydroxydopamine, to selectively destroy adrenergic nerve terminals. In this study, rats were injected with 6-hydroxydopamine (100 mg/kg, i.p.) daily for 3 days. On day 4, the animals were injected with histamine. On day 5, the pressor response to an intravenous injection of tyramine (100 fig/kg) was compared in control and 6-hydroxydopamine-treated rats. In 4 rats pretreated with 6_hydroxydopamine, tyramine had no effect on blood pressure (1.5 f 2.4 mmHg). In control rats this same dose of tyramine increased the mean arterial blood pressure by 12.7 + 1.8 mmHg. Surgical procedures
Rats were anesthetized with ketamine (150 mg/kg, i.p.) and placed in a stereotaxic apparatus (David Kopf Instruments, Tujunga, California). A permanent cannula made of 23-gauge stainless-steel hypodermic tubing was placed in the right lateral ventricle according to a modification of the method of Feldberg and Sherwood (1953), for intracerebroventricular injections. Briefly, a small burr hole was made 1.5 mm lateral to the bregma. The cannula was positioned directly over the burr hole and inserted 3.0 mm below the surface of the skull as measured from bregma. After surgery, animals were injected with 30,000 U of procaine penicillin G (i.m.) and then allowed to recover for approx. 1 week. At least 24 hr before cardiovascular recordings, rats were anesthetized with ether and polyethylene catheters (PE 50), filled with heparinized saline (200 U/ml), were inserted into the left common carotid artery, to record blood pressure, and into the right jugular vein for the intravenous injection of drugs. The catheters were then passed under the skin and exteriorized to the back of the neck via a small incision. They were cut to approx. 4 cm in length and sealed with a 23-gauge stilette. The animals were
allowed to recover from anesthesia and were used the following day. Evans blue dye was injected into the ventricular cannula to verify that the cannula had been properly placed just prior to sacrifice. If the dye did not penetrate the ventricular system, those animals were excluded from the study. Cardiovascular recordings and injection of drugs
Within 24 hr after surgery, individual unanesthetized freely moving rats were placed in a lucite cage (28 x 28 x 32 cm). The exteriorized carotid catheter was fitted to a swivel cannula which was attached to a Statham (P23 AC) pressure transducer for blood pressure measurements. Heart rate was obtained from the pulse pressure using a Beckman cardiotachometer (model 9857B). Blood pressure and heart rate were recorded on a Beckman polygraph recorder (model R5 1lA). Animals were allowed to acclimatize in the lucite cage for at least 15 min before cardiovascular recordings were initiated. After the blood pressure and heart rate had stabilized, control values were recorded for at least another 15 min and the mean blood pressure and heart rate were averaged over the last 5 min of this recording period. Drugs were injected intravenously through the exteriorized venous cannula. The cannula was connected to a 3-way stop cock via a 15 cm length of PE 50 tubing, filled with saline. Drugs were dissolved in saline and injected in volumes of 0.1 ml/100 g body weight. The injections were made quickly and then immediately flushed with 0.2ml of saline. Drugs were injected intracerebroventricularly through a 30 gauge needle (11 mm in length) which was made from stainlesssteel hypodermic tubing and then attached to a 50 ~1 micro-syringe via polyethylene tubing (PE 10). The vehicle for all intracerebroventricular injections was artificial cerebrospinal fluid (Merlis, 1940) and the pH was adjusted to 7.4 with dilute HCl or NaOH. The volume of solution injected into the cerebral ventricles was 5 ~1 over a 15 set time period. Drugs
All drug solutions were freshly prepared on the day of each experiment. The following drugs were used in this study and were obtained from the indicated sources: histamine dihydrochloride, Fisher Chemical Company; hexamethonium bromide, DMPP and 6-hydroxydopamine hydrobromide, Sigma Chemical Company; chlorpheniramine maleate, Schering Corporation; and tyramine HCl, Nutritional Biochemicals Corporation. The doses of histamine are expressed as the base. The doses of all other drugs are expressed as the salt. Statistical analysis
All results are expressed as mean + standard error @EM) with P = 0.05 considered as the level of significance. Changes in blood pressure and heart rate
Cardiovascular action of central histamine
Ill1
Table 1. Blood pressure (BP) and heart rate (HR) response to histamine (i.c.v.) after pretreatment with chlorpheniramine (i.c.v.) (mean f SEM). Histamine was injected 15 min after injection of chlorpheniramine
(4 Control BP (mm Hg)
15 min after pretreatment (mm hg)
Maximum change after histamine (mm Hg)
Treatment
N
Vehicle + Histamine j.Opg (i.c.v.) Chlorpheniramine 5Opg + Histamine l.O/Jg (i.c.v.)
4
11524.7
115 + 4.8
+29 k 5.4
4
124f4.1
124 + 4.3
+ 12 f 2.9*
(B) Treatment
N
Control HR (beats/min)
15 min after pretreatment (beatsimin)
Maximum change after histamine (beats/min)
Vehicle + Histamine l.Opg (i.c.v.) Chlorpheniramine 5Ong + Histamine l.Opg (i.c.v.)
4
432 k 17.7
428 + 22.1
-47 k 7.9
4
413 + 10.2
402 &-11.5
- 15 + s.7*
*Significantly smaller change than vehicle plus histamine group by unpaired t-test, two-tailed P < 0.05.
were compared “t”-test.
by a paired or unpaired
Students
RESULTS Eflects of histamine receptor blockade on the cardiovascular response to intracerebroventricular injection of histamine. Table 1 shows that pretreatment of rats with chlorpheniramine (i.c.v.) significantly attenuated the pressor response to histamine compared to animals pretreated with vehicle. The Table also shows that the mean blood pressure did not differ from the control blood pressure 15 min after pretreatment. Heart rate changes following central histamine receptor blockade were also significantly reduced compared to vehicle-treated controls (Table 1). Eflects of ganglionic blockade on the cardiovascular response to intracerebroventricular injection of histamine
In rats pretreated with hexamethonium (20 mg/kg, i.v.), an immediate and precipitous fall in blood pressure of 56 &-4 mm Hg occurred in 8 animals. Five minutes later the blood pressure returned towards normal still but remained depressed by
30 f 3.7 mmHg, compared to control values. Heart rate changes following injections of hexamethonium were variable in that 3 animals responded with tachycardia, 4 animals responded with bradycardia and in 1 animal, the heart rate did not change. Figure 1 shows a typical response to hexamethonium followed by intracerebroventricular injection of histamine, 5 min later. In the hexamethonium-pretreated group, the magnitude of the pressor responses and bradycardia were not attenuated following injections of histamine (1 or 5 pg, i.c.v.), compared to animals receiving histamine alone (Tables 2 and 3). Effects of chemical sympathectomy with 6-hydroxydopamine on the cardiovascular response to intracerebroventricular injection of histamine
Control mean arterial blood pressure in 4 rats treated with 6-hydroxydopamine was 77 +_4.0 mm Hg. In 6 control rats, the mean arterial blood pressure was 116 + mm Hg. No significant difference was noted in control heart rate values between these 2 groups. In the 6-hydroxydopamine-treated group, injection of 5 pg (i.c.v.) of histamine evoked a 52 f 10 mm Hg rise in mean
arterial
blood
pressure.
A typical
re-
M. C. KLEINand S. B. GERTNER
Ill2
Hexomethonium 20 mg/kg
(i.v.1
Histamine (i.c.v.) 5.0 Irg
1
200
-
1
r -
OL
I min
600r
Fig. 1. Failure of hexamethonium (20 mg/kg) to attenuate the blood pressure (upper panel) and heart rate (lower panel) response following intraventricular injection of histamine (5.0 yg). Hexamethonium was injected intravenously 5 min prior to the injection of histamine.
sponse is illustrated in Fig. 2. In control animals 5 p g of histamine evoked a 24 f 1.6 mm Hg rise in mean arterial blood pressure. The data for these experiments are shown in Table 2, and demonstrate that chemical sympathectomy not only failed to attenuate the pressor response to histamine, but increased the response by 117%.
The
heart
rate
response
following
Table 2. Elevation of blood pressure (BP) in response to histamine (i.c.v.) after pretreatment with hexamethonium or 6-hydroxydopamine (mean + SEM). Histamine was injected 5 min after injection of hexamethonium 5 min after pretreatment (mm Hg)
Maximum change after histamine (mm Hg)
Treatment
N
Control BP (mm Hg)
Histamine l.Opg (i.c.v.) Histamine 5.Olln(icv.) Hexamethonium
4
120 + 6.8
+ 17 * 3.4*
6
116k4.4
+24 k 1.6*
4
103 * 4.9
73 f 5.6t
+ 18 f 4.5*
4
112 * 1.0
81 + 2.8t
+26 k 2.3*
4
77 + 4.0
20w/kg (iv.) + Histamine l.Ofln(i.c.v.) Hexamethonium
20w/k
(i.v.) + Histamine 5.Opg (i.c.v.) 6Hydroxydopamine + Histamine 5.0 pg (i.c.v.)
injection
of
histamine in the 6-hydroxydopamine-treated animals was variable. In one animal the heart rate decreased by 102 beats/min. However, in 3 other animals, the mean heart rate decreased by only 18 f 3.5 beats/min. In addition, 3 animals showed a biphasic response resulting in an initial decrease in heart rate
+ 52%
*Significantly change compared to blood pressure prior to histamine injection by paired r-test, two-tailed P < 0.05. tSignificantly different compared to control by paired t-test, two-tailed P < 0.05. fSignificantly greater change than histamine control group by unpaired f-test, two-tailed P i 0.05.
1113
Cardiovascular action of central histamine Table 3. Decrease of heart rate (HR) in response to histamine (i.c.v.) after pretreatment with hexamethonium (mean + SEM). Histamine was injected 5 min after injection of hexamethonium Maximum change after histamine (beatslmin)
Treatment
N
Control HR (beats/mm)
5 min after pretreatment (beatslmin)
Histamine
4
423 f 26
-
-63 + 10.2*
6
414 f 8
-
-70 + 6.5’
4
432 f 28
396 + 19
-45 * 15.0*
4
432 f 20
437 * 25
- 86 + 6.2’
l.Opg (i.c.v.) Histamine 5.0 fig_ (i.c.v.) Hexamethonium
20w/kg (i.v.) + Histamine l.Opg (icv.) Hexamethonium
20w/kg (Lv.) + Histamine 5.Opg (i.c.v.) *Significant change compared to heart rate prior to histamine injection by paired
r-test. two-tailed P < 0.05.
the blood pressure or heart rate response following of histamine. injections intracerebroventricular Moreover, chemical sympathectomy with repeated injections of 6-hydroxydopamine resulted in a potento intraresponse of the pressor tiation cerebroventricular injections of histamine rather than a blockade, while the bradycardia was diminished in 3 out of 4 animals. In contrast, the cardiovascular changes following intracerebroventricular injections of histamine in anesthetized animals have been reported to be mediated by the sympathetic nervous system. The evidence for this was based on the following experimental data; ganglionic blockade in the cat (Trendelenburg, 1957) and rat (Finch and Hicks, 1976a) or spinal cord transection in the cat
within the first l-2min after the injection of histamine followed by a tachycardia within the next 5-20min (Fig. 2). Injections of histamine in control rats resulted in a 70 f 6.5 beats/min decrease in heart rate. DISCUSSION
In this present study, intracerebroventricular injections of histamine in conscious rats resulted in dosedependent increase in blood pressure and heart rate. These cardiovascular effects were antagonized by pretreatment with the histamine receptor antagonist chlorpheniramine. Pretreatment of conscious rats with hexamethonium failed to modify the changes in
Histamine
-5pg
i.c.v.
IO min
20 min
250
2 &
600
460 o 360 % - 240
e
-
w
Fig. 2. Failure of pretreatment with 6-hydroxydopamine to attenuate the pressor response following intraventricular injection of histamine (5.0 pg). 6Hydroxydopamine (100 mg/kg, i.p.) was injected for 3 days. On day 4 the animal was tested with histamine.
1114
M. C.
KLEIN and
(Trendelenburg, 1957; Sinha, Gupta and Bhargava, 1969) blocked the centrally-mediated cardiovascular effects of histamine; central administration of histamine increased hindlimb perfusion pressure in the cat (White, 1965) and rat (Carroll and Clark, 1979); and intracerebroventricular injections of histamine increased sympathetic nerve activity in the inferior cardiac nerve but paradoxically did not alter the blood pressure or heart rate in the anesthetized cat (Tackett and Holl, 1980). In this present study, the results indicate that the sympathetic nervous system does not appear to mediate the central cardiovascular effects of histamine, since ganglionic blockade, as well as chemical sympathectomy, did not block the cardiovascular action of intracerebroventricularly administered histamine. The precise reasons for the conflicting data between the conscious and anesthetized animal are not clear. One possible explanation is that histamine may stimulate centers in the brain which result in both an increase in sympathetic outflow to the peripheral vasculature, as well as the release of a neurohumoral vasoactive compound into the systemic circulation. This dual stimulation has previously been demonstrated following central injections of angiotensin II (Severs, Summy-hong, Taylor and Connor, 1970). Supporting this hypothesis, previous studies have shown the central administration of histamine to be a potent releaser of the pressor hormone, vasopressin, in conscious rats and goats (Dogterom, Avan Wilmersima-Greidanus and De Weid, 1976; Tuomisto, Eriksson and Fyhrquist, 1980). In addition, Gatti and Gertner (198 1) were able to attenuate significantly the pressor response to intracerebroventricular injections of histamine in more than 50% of the rats that had been pretreated with a selective vasopressin antagonist. Under the influence of anesthesia however, it is conceivable that the release of the humoral agent is significantly compromised, resulting primarily in an increase in sympathetic nerve activity following central histamine stimulation. This increased sympathetic drive would tend to raise both blood pressure and heart rate. Moreover, reflex slowing of the heart could be blunted under anesthesia. Thus, in an anesthetized animal, the central cardiovascular effects of histamine would presumably be mediated through the sympathetic nervous system as previously suggested. It appears, that when conscious rats are treated with a ganglion blocker or chemical sympathectomy, there is no attentuation in the centrally-mediated cardiovascular response to histamine. The reasons for this may involve at least two factors: (1) rats that have been treated with a ganglion blocker or chemical sympathectomy have abnormally low blood pressure and impaired cardiovascular reflexes. Hoffman, Phillips, Schmid, Falcon and West (1977) have reported that the pressor responses to centrally administered angiotensin II in conscious rats was potenafter peripheral sympathectomy with tiated
S. B. GERTNER 6-hydroxydopamine. They further demonstrated that the enhanced pressor responses was due to enhanced sensitivity of the peripheral vasculature to angiotensin II-induced release of vasopressin. Thus, under these conditions, the magnitude of the pressor response produced by pressor agents (e.g. norepinephrine or vasopressin) will be potentiated; and (2) the release of a vasoactive humoral substance may normally show an inverse relationship to sympathetic activity so that it is reflexly inhibited, in part, by increased activity of the sympathetic nervous system. Thus, when the peripheral outflow of the sympathetic nervous system is blocked, enhanced release of the humoral factor would be expected upon appropriate stimulation, i.e. histamine. It is concluded that in the conscious rat, increases in blood pressure and decreases in heart rate resulting from central histamine stimulation are due to the release of a vasopressor substance or hormone. The possibility that vasopressin is involved in these cardiovascular effects remains to be determined. The effect that the sympathetic nervous system normally contributes to these cardiovascular actions in the absence of ganglionic blockade in the conscious animal also remains to be determined. REFERENCES
Brezenoff H. E. and Jenden D. J. (1969) Modification of arterial blood pressure in rats after injections of drugs into the posterior hypothalamus. Neuropharmacology 8: 593-600.
Carroll G. J. and Clark D. W. J. (1979) Peripheral cardiovascular effects in rats after central administration of histamine and antihistamines. C&z. exp. Phurmac. Physiol. 6: 393-402.
Dogterom J., Avan Wilmersima-Greidanus T. B. and De Wied D. (1976) Histamine as an extremely potent releaser of vasopressin in the rat. Experien~~a 32: 659-660. Feldberg W. and Sherwood S. 1. (1953) A permanent cannula for intraventricular injections in cats. J. Physiol. 120: 3-5.
Finch L. and Hicks P. E. (1976a) The cardiovascular effects of intraventricularly administered histamine in the anesthetized rat. Naunyn-Schmiedebergs Arch. Pharmac. 293: 151-157.
Finch L. and Hicks P. E. (1967b) Central hypertensive action of histamine in conscious normotensive cats. Eur. J. Pharmac. 36: 262-266.
Finch L. and Hicks P. E. (1977) Involvement of hypothalamic histamine-receptors in the central cardiovascular actions of histamine. Ne~~opha~uco~ogy 16: 21 l-21 8. Gatti P. and Gertner S. B. (1981) The effect of a new potent antagonist of vasopressin on the pressor response of intracerebroventricularly administered histamine in unanesthetized freely moving rats. Sot. Neurosci. 7: 263-9. Hoffman W. E., Phillips M. I., Schmid P. G., Falcon J. and West J. F. (1977) Antidiuretic hormone release and the pressor response to central angiotensin II and cholinergic stimulation. Ne~~opha~ma~oio~y 16:463-472. Hoffman W. E. and Schmid P. G. (1978) Cardiovascular and antidiuretic effects of central histamine. tife Sci. 22: 170991714. Klein M. C. and Gertner S. B. (1981) Evidence for a role of endogenous histamine in central cardiovascular regulation: Inhibition of histamine-N-methyltransferase by SKF 91488. J. Pharmac. exp. Ther. 216: 315-320.
Cardiovascular action of central histamine Merlis J. (1940) The effect of changes in the calcium content of the cerebrospinal fluid on spinal reflex activity in the dog. Am. J. Physiol. 131: 67-72. 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. Thu. 174: 27-34.
Sinha J. N., Gupta M. L. and Bhargava K. P. (1969) Effect of histamine and antihistamines on central vasomoter loci. Eur. J. Pharmac. 5: 235-238. Tackett R. L. and Ho11 J. C. (1980) Histamine@ mechanisms involved in the centrally mediated effects of ouabain. J. Pharmac. exp. Ther. 215: 552-556.
1115
Trendelenburg U. (1957) Stimulation of sympathetic centers by histamine. Circulation Res. 5: 105-110. Tuomisto L. and Eriksson L. (1980) Cardiovascular and behavioral changes after i.c.v. infusions of histamine and agonists in conscious goat. Ag. Act. 10: 165-166. Tuomisto L., Eriksson L. and Fyhrquist F. (1980) Vasopressin release by histamine in the conscious goat. Eur. J. Pharmac. 63: 15-24.
White T. (1961) Some effects of histamine and two histamine metabolites on the cats brain. J. Physiol., Lond. 159: 198-202. White T. (1965) Peripheral vascular effects of histamine administered into the cerebral ventricles of anesthesized cats. Experientia 21: 132-133.
0028-3908/83%3.00 + 0.00 Pergamon Press Ltd
STUDIES OF THE MECHANISM OF THE CARDIOVASCULAR ACTION OF CENTRAL INJECTIONS OF HISTAMINE M. C. KLEIN* and S. B. GERTNER Department of Pharmacology, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, 100 Bergen Street, Newark, NJ 07103, U.S.A. (Accepted
24 February
1983)
Summary-The role of the sympathetic nervous system in the mediation of cardiovascular function, following administration of histamine centrally was investigated. Intracerebroventricular (i.c.v.) injections of histamine into the lateral ventricle of conscious rats caused a dose-dependent increase in blood pressure as well as a dose-dependent fall in heart rate. The pressure and heart rate responses were both attenuated by pretreatment with chlorpheniramine. Peripheral blockade of autonomic ganglia with hexamethonium failed to block the centrally-mediated pressor response to histamine. Peripheral chemical sympathectomy with 6-hydroxydopamine also did not inhibit the pressor responses to histamine. In this latter group of animals the pressor response to histamine was potentiated. It is concluded that in the conscious rat, increases in blood pressure resulting from central stimulation with histamine are not mediated solely through the activation of the sympathetic nervous system and are, at least in part, due to the release of a vasopressor substance.
Key words: intracerebroventricular, conscious rats.
histamine, blood pressure, hexamethonium,
6-hydroxydopamine,
in heart rate (Hoffman and Schmid, 1978; Klein and Gertner, 1981). In the conscious cat, central injections of histamine have been reported to increase blood pressure (Finch and Hicks, 1976b) without changing heart rate. In the conscious goat, centrally administered histamine increased blood pressure, but had variable effects on heart rate (Tuomisto and Eriksson, 1980). The purpose of this present study was to determine in the conscious rat, whether the sympathetic nervous system mediates the cardiovascular changes that occur following intracerebroventricular injections of histamine. This has been studied by blocking the output from the sympathetic nervous system by two different methods, i.e. ganglionic blockade and chemical sympathectomy.
The central administration of histamine in anesthetized rats (Brezenoff and Jenden, 1969; Finch and Hicks, 1976a, 1977) and cats (Trendelenburg, 1957; White, 1961) has been shown to produce a rise in both mean arterial blood pressure as well as heart rate. These changes in both blood pressure and heart rate were reported to occur through stimulation of central sympathetic centers, which resulted in an increase in sympathetic outflow to the peripheral vasculature and heart (Trendelenburg, 1957; White, 1965; Finch and Hicks, 1976a; Carroll and Clark, 1979). Additional pharmacological data by Finch and Hicks (1976a), were the basis for the suggestion that the increase in blood pressure and heart rate were mediated by two distinct central pathways, i.e. the increase in blood pressure was caused by the activation of central a-adrenergic pathways, whereas the increases in heart rate involved central muscarinic pathways. In conscious animals, central injections of histamine result in cardiovascular responses which are qualitatively different from those reported in anesthetized animals, and appear to be species-related. Thus, in unanesthetized rats intracerebroventricular (i.c.v.) injection of histamine, increases blood pressure, but produces opposite changes in heart rate, i.e. decreases
METHODS
*Present address: Department of Physiology and Pharmacology, Bowman Gray School of Medicine, 300 Hawthorne Road, Winston-Salem, NC 27103, U.S.A.
Thirty-one male Sprague-Dawley rats weighing 20&300 g (Taconic Farms, Gemantown, N.Y.) were used. Animals were housed in individual plastic cages, maintained on a 12 hr light-dark cycle and provided with food and water ad libitum. Experiments were designed to assess the requirement of an intact peripheral sympathetic nervous system in mediating the cardiovascular effects following central injections of histamine. To test whether ganglionic blockade could alter these cardiovascular responses, 12 rats were randomly divided into four groups. Two of the groups were pretreated with
1109
1110
M. C. KLEIN and S. B. GER~NER
hexamethonium and then challenged with either a small or a large dose of histamine. Two control groups received either the small or large dose of histamine (See Results, section 3.1.). In some cases, the same rat was used more than once. However, at least 48 hr was allowed for an animal to recover from a previous treatment. To determine the efficacy of ganglionic blockade, 4 rats were injected with the ganglionic stimulant l,l-dimethyl-4-phenyl-piperazinium (DMPP) (250 pg/kg, iv.) 15 min before and 5 min after injection of hexamethonium (20 mg/kg, i.v.). The first injection of DMPP increased mean arterial blood pressure by 56 + 6.7 mmHg. However, following ganglionic blockade, the rise in blood pressure after the second of DMPP was dose reduced by 66% (+ 19 f 3.1 mmHg). No reduction in blood pressure was observed following repeated injections of DMPP in the absence of ganglionic blockade. To test whether chemical sympathectomy could alter the cardiovascular responses to central injection of histamine, another group of rats was treated with the neurotoxin, 6_hydroxydopamine, to selectively destroy adrenergic nerve terminals. In this study, rats were injected with 6-hydroxydopamine (100 mg/kg, i.p.) daily for 3 days. On day 4, the animals were injected with histamine. On day 5, the pressor response to an intravenous injection of tyramine (100 fig/kg) was compared in control and 6-hydroxydopamine-treated rats. In 4 rats pretreated with 6_hydroxydopamine, tyramine had no effect on blood pressure (1.5 f 2.4 mmHg). In control rats this same dose of tyramine increased the mean arterial blood pressure by 12.7 + 1.8 mmHg. Surgical procedures
Rats were anesthetized with ketamine (150 mg/kg, i.p.) and placed in a stereotaxic apparatus (David Kopf Instruments, Tujunga, California). A permanent cannula made of 23-gauge stainless-steel hypodermic tubing was placed in the right lateral ventricle according to a modification of the method of Feldberg and Sherwood (1953), for intracerebroventricular injections. Briefly, a small burr hole was made 1.5 mm lateral to the bregma. The cannula was positioned directly over the burr hole and inserted 3.0 mm below the surface of the skull as measured from bregma. After surgery, animals were injected with 30,000 U of procaine penicillin G (i.m.) and then allowed to recover for approx. 1 week. At least 24 hr before cardiovascular recordings, rats were anesthetized with ether and polyethylene catheters (PE 50), filled with heparinized saline (200 U/ml), were inserted into the left common carotid artery, to record blood pressure, and into the right jugular vein for the intravenous injection of drugs. The catheters were then passed under the skin and exteriorized to the back of the neck via a small incision. They were cut to approx. 4 cm in length and sealed with a 23-gauge stilette. The animals were
allowed to recover from anesthesia and were used the following day. Evans blue dye was injected into the ventricular cannula to verify that the cannula had been properly placed just prior to sacrifice. If the dye did not penetrate the ventricular system, those animals were excluded from the study. Cardiovascular recordings and injection of drugs
Within 24 hr after surgery, individual unanesthetized freely moving rats were placed in a lucite cage (28 x 28 x 32 cm). The exteriorized carotid catheter was fitted to a swivel cannula which was attached to a Statham (P23 AC) pressure transducer for blood pressure measurements. Heart rate was obtained from the pulse pressure using a Beckman cardiotachometer (model 9857B). Blood pressure and heart rate were recorded on a Beckman polygraph recorder (model R5 1lA). Animals were allowed to acclimatize in the lucite cage for at least 15 min before cardiovascular recordings were initiated. After the blood pressure and heart rate had stabilized, control values were recorded for at least another 15 min and the mean blood pressure and heart rate were averaged over the last 5 min of this recording period. Drugs were injected intravenously through the exteriorized venous cannula. The cannula was connected to a 3-way stop cock via a 15 cm length of PE 50 tubing, filled with saline. Drugs were dissolved in saline and injected in volumes of 0.1 ml/100 g body weight. The injections were made quickly and then immediately flushed with 0.2ml of saline. Drugs were injected intracerebroventricularly through a 30 gauge needle (11 mm in length) which was made from stainlesssteel hypodermic tubing and then attached to a 50 ~1 micro-syringe via polyethylene tubing (PE 10). The vehicle for all intracerebroventricular injections was artificial cerebrospinal fluid (Merlis, 1940) and the pH was adjusted to 7.4 with dilute HCl or NaOH. The volume of solution injected into the cerebral ventricles was 5 ~1 over a 15 set time period. Drugs
All drug solutions were freshly prepared on the day of each experiment. The following drugs were used in this study and were obtained from the indicated sources: histamine dihydrochloride, Fisher Chemical Company; hexamethonium bromide, DMPP and 6-hydroxydopamine hydrobromide, Sigma Chemical Company; chlorpheniramine maleate, Schering Corporation; and tyramine HCl, Nutritional Biochemicals Corporation. The doses of histamine are expressed as the base. The doses of all other drugs are expressed as the salt. Statistical analysis
All results are expressed as mean + standard error @EM) with P = 0.05 considered as the level of significance. Changes in blood pressure and heart rate
Cardiovascular action of central histamine
Ill1
Table 1. Blood pressure (BP) and heart rate (HR) response to histamine (i.c.v.) after pretreatment with chlorpheniramine (i.c.v.) (mean f SEM). Histamine was injected 15 min after injection of chlorpheniramine
(4 Control BP (mm Hg)
15 min after pretreatment (mm hg)
Maximum change after histamine (mm Hg)
Treatment
N
Vehicle + Histamine j.Opg (i.c.v.) Chlorpheniramine 5Opg + Histamine l.O/Jg (i.c.v.)
4
11524.7
115 + 4.8
+29 k 5.4
4
124f4.1
124 + 4.3
+ 12 f 2.9*
(B) Treatment
N
Control HR (beats/min)
15 min after pretreatment (beatsimin)
Maximum change after histamine (beats/min)
Vehicle + Histamine l.Opg (i.c.v.) Chlorpheniramine 5Ong + Histamine l.Opg (i.c.v.)
4
432 k 17.7
428 + 22.1
-47 k 7.9
4
413 + 10.2
402 &-11.5
- 15 + s.7*
*Significantly smaller change than vehicle plus histamine group by unpaired t-test, two-tailed P < 0.05.
were compared “t”-test.
by a paired or unpaired
Students
RESULTS Eflects of histamine receptor blockade on the cardiovascular response to intracerebroventricular injection of histamine. Table 1 shows that pretreatment of rats with chlorpheniramine (i.c.v.) significantly attenuated the pressor response to histamine compared to animals pretreated with vehicle. The Table also shows that the mean blood pressure did not differ from the control blood pressure 15 min after pretreatment. Heart rate changes following central histamine receptor blockade were also significantly reduced compared to vehicle-treated controls (Table 1). Eflects of ganglionic blockade on the cardiovascular response to intracerebroventricular injection of histamine
In rats pretreated with hexamethonium (20 mg/kg, i.v.), an immediate and precipitous fall in blood pressure of 56 &-4 mm Hg occurred in 8 animals. Five minutes later the blood pressure returned towards normal still but remained depressed by
30 f 3.7 mmHg, compared to control values. Heart rate changes following injections of hexamethonium were variable in that 3 animals responded with tachycardia, 4 animals responded with bradycardia and in 1 animal, the heart rate did not change. Figure 1 shows a typical response to hexamethonium followed by intracerebroventricular injection of histamine, 5 min later. In the hexamethonium-pretreated group, the magnitude of the pressor responses and bradycardia were not attenuated following injections of histamine (1 or 5 pg, i.c.v.), compared to animals receiving histamine alone (Tables 2 and 3). Effects of chemical sympathectomy with 6-hydroxydopamine on the cardiovascular response to intracerebroventricular injection of histamine
Control mean arterial blood pressure in 4 rats treated with 6-hydroxydopamine was 77 +_4.0 mm Hg. In 6 control rats, the mean arterial blood pressure was 116 + mm Hg. No significant difference was noted in control heart rate values between these 2 groups. In the 6-hydroxydopamine-treated group, injection of 5 pg (i.c.v.) of histamine evoked a 52 f 10 mm Hg rise in mean
arterial
blood
pressure.
A typical
re-
M. C. KLEINand S. B. GERTNER
Ill2
Hexomethonium 20 mg/kg
(i.v.1
Histamine (i.c.v.) 5.0 Irg
1
200
-
1
r -
OL
I min
600r
Fig. 1. Failure of hexamethonium (20 mg/kg) to attenuate the blood pressure (upper panel) and heart rate (lower panel) response following intraventricular injection of histamine (5.0 yg). Hexamethonium was injected intravenously 5 min prior to the injection of histamine.
sponse is illustrated in Fig. 2. In control animals 5 p g of histamine evoked a 24 f 1.6 mm Hg rise in mean arterial blood pressure. The data for these experiments are shown in Table 2, and demonstrate that chemical sympathectomy not only failed to attenuate the pressor response to histamine, but increased the response by 117%.
The
heart
rate
response
following
Table 2. Elevation of blood pressure (BP) in response to histamine (i.c.v.) after pretreatment with hexamethonium or 6-hydroxydopamine (mean + SEM). Histamine was injected 5 min after injection of hexamethonium 5 min after pretreatment (mm Hg)
Maximum change after histamine (mm Hg)
Treatment
N
Control BP (mm Hg)
Histamine l.Opg (i.c.v.) Histamine 5.Olln(icv.) Hexamethonium
4
120 + 6.8
+ 17 * 3.4*
6
116k4.4
+24 k 1.6*
4
103 * 4.9
73 f 5.6t
+ 18 f 4.5*
4
112 * 1.0
81 + 2.8t
+26 k 2.3*
4
77 + 4.0
20w/kg (iv.) + Histamine l.Ofln(i.c.v.) Hexamethonium
20w/k
(i.v.) + Histamine 5.Opg (i.c.v.) 6Hydroxydopamine + Histamine 5.0 pg (i.c.v.)
injection
of
histamine in the 6-hydroxydopamine-treated animals was variable. In one animal the heart rate decreased by 102 beats/min. However, in 3 other animals, the mean heart rate decreased by only 18 f 3.5 beats/min. In addition, 3 animals showed a biphasic response resulting in an initial decrease in heart rate
+ 52%
*Significantly change compared to blood pressure prior to histamine injection by paired r-test, two-tailed P < 0.05. tSignificantly different compared to control by paired t-test, two-tailed P < 0.05. fSignificantly greater change than histamine control group by unpaired f-test, two-tailed P i 0.05.
1113
Cardiovascular action of central histamine Table 3. Decrease of heart rate (HR) in response to histamine (i.c.v.) after pretreatment with hexamethonium (mean + SEM). Histamine was injected 5 min after injection of hexamethonium Maximum change after histamine (beatslmin)
Treatment
N
Control HR (beats/mm)
5 min after pretreatment (beatslmin)
Histamine
4
423 f 26
-
-63 + 10.2*
6
414 f 8
-
-70 + 6.5’
4
432 f 28
396 + 19
-45 * 15.0*
4
432 f 20
437 * 25
- 86 + 6.2’
l.Opg (i.c.v.) Histamine 5.0 fig_ (i.c.v.) Hexamethonium
20w/kg (i.v.) + Histamine l.Opg (icv.) Hexamethonium
20w/kg (Lv.) + Histamine 5.Opg (i.c.v.) *Significant change compared to heart rate prior to histamine injection by paired
r-test. two-tailed P < 0.05.
the blood pressure or heart rate response following of histamine. injections intracerebroventricular Moreover, chemical sympathectomy with repeated injections of 6-hydroxydopamine resulted in a potento intraresponse of the pressor tiation cerebroventricular injections of histamine rather than a blockade, while the bradycardia was diminished in 3 out of 4 animals. In contrast, the cardiovascular changes following intracerebroventricular injections of histamine in anesthetized animals have been reported to be mediated by the sympathetic nervous system. The evidence for this was based on the following experimental data; ganglionic blockade in the cat (Trendelenburg, 1957) and rat (Finch and Hicks, 1976a) or spinal cord transection in the cat
within the first l-2min after the injection of histamine followed by a tachycardia within the next 5-20min (Fig. 2). Injections of histamine in control rats resulted in a 70 f 6.5 beats/min decrease in heart rate. DISCUSSION
In this present study, intracerebroventricular injections of histamine in conscious rats resulted in dosedependent increase in blood pressure and heart rate. These cardiovascular effects were antagonized by pretreatment with the histamine receptor antagonist chlorpheniramine. Pretreatment of conscious rats with hexamethonium failed to modify the changes in
Histamine
-5pg
i.c.v.
IO min
20 min
250
2 &
600
460 o 360 % - 240
e
-
w
Fig. 2. Failure of pretreatment with 6-hydroxydopamine to attenuate the pressor response following intraventricular injection of histamine (5.0 pg). 6Hydroxydopamine (100 mg/kg, i.p.) was injected for 3 days. On day 4 the animal was tested with histamine.
1114
M. C.
KLEIN and
(Trendelenburg, 1957; Sinha, Gupta and Bhargava, 1969) blocked the centrally-mediated cardiovascular effects of histamine; central administration of histamine increased hindlimb perfusion pressure in the cat (White, 1965) and rat (Carroll and Clark, 1979); and intracerebroventricular injections of histamine increased sympathetic nerve activity in the inferior cardiac nerve but paradoxically did not alter the blood pressure or heart rate in the anesthetized cat (Tackett and Holl, 1980). In this present study, the results indicate that the sympathetic nervous system does not appear to mediate the central cardiovascular effects of histamine, since ganglionic blockade, as well as chemical sympathectomy, did not block the cardiovascular action of intracerebroventricularly administered histamine. The precise reasons for the conflicting data between the conscious and anesthetized animal are not clear. One possible explanation is that histamine may stimulate centers in the brain which result in both an increase in sympathetic outflow to the peripheral vasculature, as well as the release of a neurohumoral vasoactive compound into the systemic circulation. This dual stimulation has previously been demonstrated following central injections of angiotensin II (Severs, Summy-hong, Taylor and Connor, 1970). Supporting this hypothesis, previous studies have shown the central administration of histamine to be a potent releaser of the pressor hormone, vasopressin, in conscious rats and goats (Dogterom, Avan Wilmersima-Greidanus and De Weid, 1976; Tuomisto, Eriksson and Fyhrquist, 1980). In addition, Gatti and Gertner (198 1) were able to attenuate significantly the pressor response to intracerebroventricular injections of histamine in more than 50% of the rats that had been pretreated with a selective vasopressin antagonist. Under the influence of anesthesia however, it is conceivable that the release of the humoral agent is significantly compromised, resulting primarily in an increase in sympathetic nerve activity following central histamine stimulation. This increased sympathetic drive would tend to raise both blood pressure and heart rate. Moreover, reflex slowing of the heart could be blunted under anesthesia. Thus, in an anesthetized animal, the central cardiovascular effects of histamine would presumably be mediated through the sympathetic nervous system as previously suggested. It appears, that when conscious rats are treated with a ganglion blocker or chemical sympathectomy, there is no attentuation in the centrally-mediated cardiovascular response to histamine. The reasons for this may involve at least two factors: (1) rats that have been treated with a ganglion blocker or chemical sympathectomy have abnormally low blood pressure and impaired cardiovascular reflexes. Hoffman, Phillips, Schmid, Falcon and West (1977) have reported that the pressor responses to centrally administered angiotensin II in conscious rats was potenafter peripheral sympathectomy with tiated
S. B. GERTNER 6-hydroxydopamine. They further demonstrated that the enhanced pressor responses was due to enhanced sensitivity of the peripheral vasculature to angiotensin II-induced release of vasopressin. Thus, under these conditions, the magnitude of the pressor response produced by pressor agents (e.g. norepinephrine or vasopressin) will be potentiated; and (2) the release of a vasoactive humoral substance may normally show an inverse relationship to sympathetic activity so that it is reflexly inhibited, in part, by increased activity of the sympathetic nervous system. Thus, when the peripheral outflow of the sympathetic nervous system is blocked, enhanced release of the humoral factor would be expected upon appropriate stimulation, i.e. histamine. It is concluded that in the conscious rat, increases in blood pressure and decreases in heart rate resulting from central histamine stimulation are due to the release of a vasopressor substance or hormone. The possibility that vasopressin is involved in these cardiovascular effects remains to be determined. The effect that the sympathetic nervous system normally contributes to these cardiovascular actions in the absence of ganglionic blockade in the conscious animal also remains to be determined. REFERENCES
Brezenoff H. E. and Jenden D. J. (1969) Modification of arterial blood pressure in rats after injections of drugs into the posterior hypothalamus. Neuropharmacology 8: 593-600.
Carroll G. J. and Clark D. W. J. (1979) Peripheral cardiovascular effects in rats after central administration of histamine and antihistamines. C&z. exp. Phurmac. Physiol. 6: 393-402.
Dogterom J., Avan Wilmersima-Greidanus T. B. and De Wied D. (1976) Histamine as an extremely potent releaser of vasopressin in the rat. Experien~~a 32: 659-660. Feldberg W. and Sherwood S. 1. (1953) A permanent cannula for intraventricular injections in cats. J. Physiol. 120: 3-5.
Finch L. and Hicks P. E. (1976a) The cardiovascular effects of intraventricularly administered histamine in the anesthetized rat. Naunyn-Schmiedebergs Arch. Pharmac. 293: 151-157.
Finch L. and Hicks P. E. (1967b) Central hypertensive action of histamine in conscious normotensive cats. Eur. J. Pharmac. 36: 262-266.
Finch L. and Hicks P. E. (1977) Involvement of hypothalamic histamine-receptors in the central cardiovascular actions of histamine. Ne~~opha~uco~ogy 16: 21 l-21 8. Gatti P. and Gertner S. B. (1981) The effect of a new potent antagonist of vasopressin on the pressor response of intracerebroventricularly administered histamine in unanesthetized freely moving rats. Sot. Neurosci. 7: 263-9. Hoffman W. E., Phillips M. I., Schmid P. G., Falcon J. and West J. F. (1977) Antidiuretic hormone release and the pressor response to central angiotensin II and cholinergic stimulation. Ne~~opha~ma~oio~y 16:463-472. Hoffman W. E. and Schmid P. G. (1978) Cardiovascular and antidiuretic effects of central histamine. tife Sci. 22: 170991714. Klein M. C. and Gertner S. B. (1981) Evidence for a role of endogenous histamine in central cardiovascular regulation: Inhibition of histamine-N-methyltransferase by SKF 91488. J. Pharmac. exp. Ther. 216: 315-320.
Cardiovascular action of central histamine Merlis J. (1940) The effect of changes in the calcium content of the cerebrospinal fluid on spinal reflex activity in the dog. Am. J. Physiol. 131: 67-72. 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. Thu. 174: 27-34.
Sinha J. N., Gupta M. L. and Bhargava K. P. (1969) Effect of histamine and antihistamines on central vasomoter loci. Eur. J. Pharmac. 5: 235-238. Tackett R. L. and Ho11 J. C. (1980) Histamine@ mechanisms involved in the centrally mediated effects of ouabain. J. Pharmac. exp. Ther. 215: 552-556.
1115
Trendelenburg U. (1957) Stimulation of sympathetic centers by histamine. Circulation Res. 5: 105-110. Tuomisto L. and Eriksson L. (1980) Cardiovascular and behavioral changes after i.c.v. infusions of histamine and agonists in conscious goat. Ag. Act. 10: 165-166. Tuomisto L., Eriksson L. and Fyhrquist F. (1980) Vasopressin release by histamine in the conscious goat. Eur. J. Pharmac. 63: 15-24.
White T. (1961) Some effects of histamine and two histamine metabolites on the cats brain. J. Physiol., Lond. 159: 198-202. White T. (1965) Peripheral vascular effects of histamine administered into the cerebral ventricles of anesthesized cats. Experientia 21: 132-133.