339s
Clinical Science (1981) 61,339s-342s
P-Endorphin, an endogenous depressor agent in the rat?
M. A. P E T T Y , J . M. A . S I T S E N
AND
W. D E J O N G
Rudolf Magnus Institute f o r Pharmacology, Medical Faculty, University of Utrecht. Utrecht, The Netherlands
Summary
1. The role of opiates in cardiovascular
regulation has been investigated.
2. In urethane-anaesthetized renal hypertensive rats (two-kidney, one-clip Goldblatt
model), intracerebroventricular P-endorphin ( 10
pg) caused a greater fall in mean arterial pressure
than in sham-operated controls.
3. Unilateral injection of P-endorphin into the
nucleus tractus solitarii of the urethane-anaesthetized rat resulted in a U-shaped dose-response
relationship, with a fall in mean arterial pressure
and heart rate occurring at low doses. Doses
above 10 ng caused a rise in pressure, accompanied by a variable effect on heart rate.
4. The fall in blood pressure and heart rate
was prevented by prior subcutaneous administration of naloxone. Naloxone caused an
increase in blood pressure when administered
alone.
5. These results suggest a depressor role of an
endogenous brain opiate, possibly P-endorphin; a
site of action is probably the nucleus tractus
solitarii.
Key words: P-endorphin, nucleus tractus solitarii,
opiates.
Abbreviations: HR, heart rate; MAP, mean
arterial pressure; NTS, nucleus tractus solitarii.
Introduction
Anatomical and pharmacological evidence suggests a role for endogenous opiates in central
cardiovascular regulation. Two different opiate
systems exist, the enkephalins, which are distributed throughout the brain with numerous cell
Correspondence: Dr M. A. Petty, Rudolf Magnus
Institute for Pharmacology, Medical Faculty, University of Utrecht, Vondellaan 6, 3521 GD Utrecht, The
Netherlands.
groups and short neuronal connections, and,
secondly, the endorphin system. The latter is
located in a single set of hypothalamic neurons,
with axons projecting throughout the brain [ 11.
More specifically, enkephalinergic neurons and
their terminals, together with binding sites for
opiates, have been localized in brain-stem structures involved in cardiovascular control [2,31.
The intracisternal administration of opioid
peptides results in a decrease in baroreflex
sensitivity [41 and variable effects on blood
pressure and heart rate. Leu-enkephalin, Metenkephalin and the stable analogue D-Alaenkephalin cause a rise in blood pressure on
central administration [5, 61. Conversely,
morphine and P-endorphin produce an initial rise
in pressure followed by a prolonged hypotensive
phase [6, 71. Other authors observed only a
depressor response 181. All these effects are
readily reversed by the opiate antagonist
naloxone.
Intravenous and central naloxone causes a rise
in blood pressure in different shock states,
implicating the involvement of endogenous opiates in central cardiovascular control [9, 10, 111,
and in particular a depressor role of endorphins.
Furthermore Farsang and co-workers reported
that naloxone reverses the hypotensive effect of
clonidine and cc-methyldopa in spontaneously
hypertensive rats [12]. In addition, clonidine has
been reported to induce the release of a Pendorphin-like peptide from brain-stem slices
[ 131. These observations point to a depressor role
of P-endorphin.
Recently, Zamir et al. reported an altered pain
sensitivity in hypertensive rats and in essential
hypertensive patients. Administration of naloxone
to the hypertensive rats reversed the increased
pain threshold (hot plate latency) to control
values [14, 15, 161. These data suggest that under
these circumstances there is a change in endorphin systems.
In this study the cardiovascular effects of
P-endorphin injected into the lateral cerebral
M . A . Petty, J . M . A . Sitsen and W. de Jong
340s
ventricle in renovascular hypertensive rats and
sham-operated controls have been examined.
Freye & Arndt [ 171 have indicated that opiate
binding sites bordering the fourth cerebral ventricle mediate the hypotension and bradycardia
observed after fentanyl perfusion. The possibility
that the nucleus tractus solitarii (NTS) is a site of
action in mediating the cardiovascular effects of
opiates and in particular P-endorphin has been
investigated.
Methods
Male Wistar rats (Cpb, TNO, Zeist, The Netherlands) weighing 200-250 g were used.
Renal hypertension was induced by the application of a solid silver clip (0.20 mm internal
diameter) to the left renal artery of rats weighing
125-135 g (two-kidney, one-clip Goldblatt
model). The control group was subjected to the
same operative procedure, but no clip was
applied. The rats were used 3 - 4 weeks after
operation.
/3-Endorphin (10 pg) or an equivalent volume
of artificial cerebrospinal fluid vehicle was injected into the right lateral brain ventricle of
urethane-anaesthetized (1 a 5 g/kg) renovascular
hypertensive rats and sham-operated controls by
means of a cannula implated 1 week earlier under
Hypnorm anaesthesia (Duphar Nederland BV,
Amsterdam, The Netherlands).
For direct application of P-endorphin into the
NTS the rat was placed in a stereotactic
apparatus and the head fixed to an angle of 45O
downward. After exposure of the lower brain
stem micro-injections into the NTS on the right
side of the medulla were given through a glass
cannula (outer diameter 60 pm), in a volume of
0.4 p l , which was delivered in 10 s 1181. After
experimentation the correct position of the injection site was confirmed by histology.
The following drugs were used: ,&endorphin
(Organon International BV, Oss, The Netherlands) dissolved in artificial cerebrospinal fluid for
intracerebroventricular injection and in 0.9%
sodium chloride solution (saline) for direct
application into the NTS; naloxone/HCl (Endo
Laboratories, New York, U.S.A.) in saline
vehicle.
Results
The renal hypertensive rats had a significantly
(P < 0.001) higher mean arterial pressure (MAP)
(162 SD 8 mmHg) than the sham-operated
controls (100-120
mmHg). Intracerebroventricular administration of P-endorphin (10 pg)
in the hypertensive group resulted in a significantly (P < 0.05) greater fall in MAP than in
controls. In both groups the fall was delayed,
occurring 15 min after injection, when there was a
gradual decrease in MAP, which reached a
maximum at 60 min, 37 f 34 mmHg and 9 f 20
mmHg respectively. No significant changes in
heart rate (HR) or respiratory frequency were
observed,
The unilateral injection of /3-endorphin into the
NTS of normotensive animals resulted in a
U-shaped dose-response relationship with a fall in
MAP and HR occurring at low doses (1-1000
pg). The fall in both MAP and HR occurred
gradually over the following 60 min, the changes
50
I
0
1
5
10
15
30
45
0
60
Time after injection (rnin)
FIG. 1. Specimen traces (a) of the hypotension and bradycardia resulting from /3-endorphin
(100 pg) injected unilaterally into the nucleus tractus solitarii of the urethane-anaesthetized rat, (b)
after injection of saline control. Injection volume was 0.4 p l .
Cardiovascular effects of P-endorphin
being -20 f 5 mmHg and -41 f 17 beats/min
respectively (Fig. 1). No change in respiratory
frequency was observed. Both the hypotension
and bradycardia were prevented ( P < 0.001) by
the administration of naloxone (1 mg/kg, subcutaneously) given 15 min before the peptide.
Naloxone caused an increase in MAP of 8 f 3
mmHg, when administered subcutaneously, followed 10 min later by a saline injection into the
NTS. The maximum effect occurred 40 min after
naloxone administration.
Doses of P-endorphin greater than 10 ng
caused a rise in MAP accompanied by variable
effects on HR when administered into the NTS.
The maximum rise in pressure was observed after
30 min. Baseline values were reached 40-60 min
after injection. No decrease in MAP was seen.
Discussion
In this study the role of P-endorphin in central
cardiovascular regulation has been investigated.
The intracerebroventricular administration of the
opiate peptide caused a greater fall in blood
pressure in renal hypertensive rats than in
sham-operated controls. This observation indicates the alteration of an endogenous opiate
system in hypertension. Zamir et al. [141 have
shown a 45% higher level of ‘opioid activity’ in
the spinal cord of renal hypertensive animals than
in controls.
It has been postulated that opiate receptors
bordering the fourth cerebral ventricle mediate
the hypotensive and bradycardiac effects of some
opiates [ 171. The nucleus tractus solitarii (NTS) is
the site of the first synapse in the baroreflex arc
and it has been shown that opiate peptides affect
baroreceptor reflex function [4].Therefore, does
P-endorphin lower blood pressure through an
action in the NTS? The local application of the
peptide into this region resulted in a U-shaped
dose-response relationship with hypotension
accompanied by bradycardia occurring at low
doses. (AS seen in Fig. 1, the decrease in blood
pressure occurs gradually during a period of 1 h.)
At doses above 10 ng a rise in pressure was
observed, perhaps due to the simultaneous activation of other opiate receptors with a lower
affinity for P-endorphin or by diffusion to opiate
receptors inaccessible to lower doses. The
depressor effect appears to be due to the
activation of opiate receptors since it was
antagonized by naloxone. Naloxone treatment in
the absence of P-endorphin caused a significant
rise in blood pressure, as also shown by
Kumazawa et al. [ 191. Preliminary studies involving the local application of antiserum to P-
341s
endorphin into the NTS show a delayed but
significant rise in pressure.
These findings suggest a depressor role of
P-endorphin in the central nervous system, very
low doses causing a fall in blood pressure when
administered into the NTS. It is possible that the
hypotension results from an action within the
central connections of the baroreflex arc. These
findings and data from the literature give evidence
that P-endorphin may be involved in central blood
pressure regulation, in both hypertension and
shock states, as well as in normotension.
Acknowledgments
We thank Mr H. de Lang for his excellent
technical assistance. M.A.P. is supported by a
grant from the British and Dutch Heart
Foundations.
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