Acta pharmacol. et toxicol. 1976, 38,376-381.
From the Department of Pharmacology, University of Goteborg, Fack, S-400 33 Goteborg 33, Sweden
Circulatory Effects of Clonidine after Pre-hypothalamic Section in the Rat BY
Matts Henning, Giinter Stock* and Gustaf Trolin (Received September 1, 1975; Accepted September 22, 1975)
Abstract: Clonidine 15 pg/kg intravenously was administered to unanaesthetized rats during recording of blood pressure and heart rate. The rats were either sham operated or sectioned at a level rostral to the hypothalamus. The basal blood pressure and heart rate of the sectioned rats was significantly higher than that of the control rats. The blood pressure increase seen after clonidine in sham operated rats was absent in the rats with a pre-hypothalamic section and after 5-10 m h . hypotension was observed. The decrease in heart frequency was more pronounced in the rats with a diencephalic section than in the sham operated rats. The results suggest that the hypertensive effect of clonidine is dependent on intact structures rostral to the hypothalamus.
Key-words: Clonidine - blood pressure - heart rate - diencephalic section.
The clonidine induced circulatory changes, i. e. bradycardia and hypotension are generally thought to be mediated by central mechanisms located in the brain stem (Scmrrr & Scmrrr 1969; Smw et al. 1971). a-Adrenergic mechanisms in the higher parts of the brain tend to antagonize the hypotensive effect of clonidine (Smw et al. 1971) and noradrenaline (NA) nerve terminals in the hypothalamus (FUXE1965) seem to be of importance in blood pressure regulation as revealed by local administration of clonidine (PHILIPPUet al. 1974; STRUYKER BOUDIERet al. 1974). In a previous study (TROLIN1975) it is suggested that the initial hypertensive response after intravenous injection of clonidine is at least partly of central origin and located rostral to the medulla oblongata. The aim of the present series of experiments was to investigate more closely the localization of the hypertensive mechanism described above.
*
Present address: 1 Physiologisches Institut des Universitat Heidelberg, 68 Heidelberg, Im Neuenheimes-Feld 326, BRD.
CLONIDINE AFTER DIENCEPHALIC SECTION
377
Materials and Methods Male Sprague-Dawley rats (20Ck250 g) (Anticimex, Stockholm) were used. One week before the blood pressure experiments two holes in the skull were drilled on each side of the midline immediately rostra1 to the sutura coronaria under diethylether anaesthesia. 16-18 hrs before insertion of catheters (see below) sectioning of the brain was performed under pentobarbital sodium anaesthesia (nembutalm, 40 mg/kg intraperitoneally) at the level B as shown in fig. 1. Sectioning at the level B is called pre-hypothalamic section in the following text. The brains were always dissected after the blood pressure experiments in order to check the section. Only animals with a complete section were taken for statistical analysis. However, minor parts of the cortex were left caudal to the section. 0.9 % NaCl (10 ml subcutaneously) was administered after the sectioning. 16-18 hrs after the section of the brain was performed, a catheter was introduced under halothane (halothane@) anaesthesia into the aorta via the left carotid artery and connected to a Statham P23Dc transducer writing on a Grass Polygraph. Another catheter was introduced into the external jugular vein and exteriorized with the arterial catheter at the back of the neck. Control, sham operated rats were treated in a slightly different manner. Catheters were implanted 16-18 hrs before the blood pressure registrations under pentobarbital (nembutalm, 40 mg/kg Intraperitoneally) anaesthesia. One hour before the blood pressure registrations the rats were anaesthetized with halothane for 2&30 min. All rats were allowed to recover completely from the anaesthesia before the beginning of the blood pressure registrations. The heart rate was triggered on the pulse wave with a Grass Tachograph 7 P4C. All registrations were performed with the rats kept in their usual cages on a warm operation table (37"). Clonidine hydrochloride (Catapresanm, Boehringer Ingelheim AB, Stockholm) was dissolved in 0.9 % NaCl and doses were calculated as the salt. 0.9 % NaCl was always injected after the drug to a final volume of 0.5 ml. As the values could not be proved to be t-distributed, statistical analysis was performed by the non-parametric Mann-Whitney U-test (MA" & WHITNEY1947). P-values less than 0.05 were regarded as statistically significant.
Fig. 1. Sagittal section through the brain of the rat with the NA terminals of hypothalamus and cortex indicated (shadowed). The letters indicate the levels of the sections. Decerebration (A) was performed in a previous experiment (TROLIN1975) and referred to in the text. Pre-hypothalamic section (B) was performed in the present experiments.
378
M. HENNING, G. STOCK AND G. TROLIN
0
1
1
10
1'"
Fig. 2. Median blood pressure changes from the basal values of sham operated, control rats (n = 11) (triangles) and rats with a pre-hypothalamic section (n = 12) (circles) after clonidine 15 p d k g intravenously (for level of the section, see fig. 1.)
Results Median changes from basal blood pressure levels are shown in fig. 2. Median decreases from basal heart frequency levels are shown in fig. 3. Clonidine 15 pg/kg intravenously to control (sham operated) rats (n = 11). The median basal blood pressure was 117 mmHg. After clonidine the blood pressure increased with a median of 25 mmHg during the first minute (P < 0.002). After 10 min. the pressure had returned to the basal level. No decrease was seen after 10 min. The heart rate showed a decrease with a return to almost basal level after 10 min. Maximal decrease was observed after 1 min. The median basal heart rate was 380 beatshin.
. . 0
1
5
nln
10
Fig. 3. Median heart rate decrease from the basal levels of sham operated, control rats (n = 11) (triangles) and rats with a pre-hypothalamic section (n = 12) (circles) after clonidine 15 p d k g intravenously (for level of the section, see fig. 1).
CLONIDINE AFTER DIENCEPHALIC SECTION
379
Clonidine 15 pg/kg intravenously in rats after pre-hypothalamic section (n = 12). The median basal blood pressure level was 128.5 mmHg which is significantly higher than the basal pressure of the controls (P < 0.05). After the injection of clonidine the pressure increase seen in the controls during the first minute was absent and at 5 min. a hypotension was apparent. The changes in blood pressure from the basal level was significantly different from the changes from the basal level of the sham operated rats at all intervals (P
From the Department of Pharmacology, University of Goteborg, Fack, S-400 33 Goteborg 33, Sweden
Circulatory Effects of Clonidine after Pre-hypothalamic Section in the Rat BY
Matts Henning, Giinter Stock* and Gustaf Trolin (Received September 1, 1975; Accepted September 22, 1975)
Abstract: Clonidine 15 pg/kg intravenously was administered to unanaesthetized rats during recording of blood pressure and heart rate. The rats were either sham operated or sectioned at a level rostral to the hypothalamus. The basal blood pressure and heart rate of the sectioned rats was significantly higher than that of the control rats. The blood pressure increase seen after clonidine in sham operated rats was absent in the rats with a pre-hypothalamic section and after 5-10 m h . hypotension was observed. The decrease in heart frequency was more pronounced in the rats with a diencephalic section than in the sham operated rats. The results suggest that the hypertensive effect of clonidine is dependent on intact structures rostral to the hypothalamus.
Key-words: Clonidine - blood pressure - heart rate - diencephalic section.
The clonidine induced circulatory changes, i. e. bradycardia and hypotension are generally thought to be mediated by central mechanisms located in the brain stem (Scmrrr & Scmrrr 1969; Smw et al. 1971). a-Adrenergic mechanisms in the higher parts of the brain tend to antagonize the hypotensive effect of clonidine (Smw et al. 1971) and noradrenaline (NA) nerve terminals in the hypothalamus (FUXE1965) seem to be of importance in blood pressure regulation as revealed by local administration of clonidine (PHILIPPUet al. 1974; STRUYKER BOUDIERet al. 1974). In a previous study (TROLIN1975) it is suggested that the initial hypertensive response after intravenous injection of clonidine is at least partly of central origin and located rostral to the medulla oblongata. The aim of the present series of experiments was to investigate more closely the localization of the hypertensive mechanism described above.
*
Present address: 1 Physiologisches Institut des Universitat Heidelberg, 68 Heidelberg, Im Neuenheimes-Feld 326, BRD.
CLONIDINE AFTER DIENCEPHALIC SECTION
377
Materials and Methods Male Sprague-Dawley rats (20Ck250 g) (Anticimex, Stockholm) were used. One week before the blood pressure experiments two holes in the skull were drilled on each side of the midline immediately rostra1 to the sutura coronaria under diethylether anaesthesia. 16-18 hrs before insertion of catheters (see below) sectioning of the brain was performed under pentobarbital sodium anaesthesia (nembutalm, 40 mg/kg intraperitoneally) at the level B as shown in fig. 1. Sectioning at the level B is called pre-hypothalamic section in the following text. The brains were always dissected after the blood pressure experiments in order to check the section. Only animals with a complete section were taken for statistical analysis. However, minor parts of the cortex were left caudal to the section. 0.9 % NaCl (10 ml subcutaneously) was administered after the sectioning. 16-18 hrs after the section of the brain was performed, a catheter was introduced under halothane (halothane@) anaesthesia into the aorta via the left carotid artery and connected to a Statham P23Dc transducer writing on a Grass Polygraph. Another catheter was introduced into the external jugular vein and exteriorized with the arterial catheter at the back of the neck. Control, sham operated rats were treated in a slightly different manner. Catheters were implanted 16-18 hrs before the blood pressure registrations under pentobarbital (nembutalm, 40 mg/kg Intraperitoneally) anaesthesia. One hour before the blood pressure registrations the rats were anaesthetized with halothane for 2&30 min. All rats were allowed to recover completely from the anaesthesia before the beginning of the blood pressure registrations. The heart rate was triggered on the pulse wave with a Grass Tachograph 7 P4C. All registrations were performed with the rats kept in their usual cages on a warm operation table (37"). Clonidine hydrochloride (Catapresanm, Boehringer Ingelheim AB, Stockholm) was dissolved in 0.9 % NaCl and doses were calculated as the salt. 0.9 % NaCl was always injected after the drug to a final volume of 0.5 ml. As the values could not be proved to be t-distributed, statistical analysis was performed by the non-parametric Mann-Whitney U-test (MA" & WHITNEY1947). P-values less than 0.05 were regarded as statistically significant.
Fig. 1. Sagittal section through the brain of the rat with the NA terminals of hypothalamus and cortex indicated (shadowed). The letters indicate the levels of the sections. Decerebration (A) was performed in a previous experiment (TROLIN1975) and referred to in the text. Pre-hypothalamic section (B) was performed in the present experiments.
378
M. HENNING, G. STOCK AND G. TROLIN
0
1
1
10
1'"
Fig. 2. Median blood pressure changes from the basal values of sham operated, control rats (n = 11) (triangles) and rats with a pre-hypothalamic section (n = 12) (circles) after clonidine 15 p d k g intravenously (for level of the section, see fig. 1.)
Results Median changes from basal blood pressure levels are shown in fig. 2. Median decreases from basal heart frequency levels are shown in fig. 3. Clonidine 15 pg/kg intravenously to control (sham operated) rats (n = 11). The median basal blood pressure was 117 mmHg. After clonidine the blood pressure increased with a median of 25 mmHg during the first minute (P < 0.002). After 10 min. the pressure had returned to the basal level. No decrease was seen after 10 min. The heart rate showed a decrease with a return to almost basal level after 10 min. Maximal decrease was observed after 1 min. The median basal heart rate was 380 beatshin.
. . 0
1
5
nln
10
Fig. 3. Median heart rate decrease from the basal levels of sham operated, control rats (n = 11) (triangles) and rats with a pre-hypothalamic section (n = 12) (circles) after clonidine 15 p d k g intravenously (for level of the section, see fig. 1).
CLONIDINE AFTER DIENCEPHALIC SECTION
379
Clonidine 15 pg/kg intravenously in rats after pre-hypothalamic section (n = 12). The median basal blood pressure level was 128.5 mmHg which is significantly higher than the basal pressure of the controls (P < 0.05). After the injection of clonidine the pressure increase seen in the controls during the first minute was absent and at 5 min. a hypotension was apparent. The changes in blood pressure from the basal level was significantly different from the changes from the basal level of the sham operated rats at all intervals (P
