The inactin anaesthetized rat shews cardiovascular reflex
responses. Heat rate is under sympathetic but not vagal control.
Clonidine reduces the heart rate and blood pressure in the
inactin anaesthetized rat. The reduction in heart rate involves
reducing sympathetic cradiac drive. The fall in blood pressure
includes a reduction in peripheral resistance.
Using a newly developed "delayed" hindlimb perfusion the
reduction in peripheral resistance was seen to be neurally mediated.
A peripheral vasodilator action was not seen with clonidine.
Clonidine was administered by four different routes which were
expected to provide access to selected areas in the brain.
Intravenous, intracarotid artery, intraventricular and
intravertebral artery. Administration into the ventricular system of
the brain was slighty more potent in reducing arterial pressure than
intravenous injection. Intracarotid and intravenous were equipotent.
Intravertbral was by far the most effective, requiring 5% of the
intravenous dose to cause an equivalent cardiovascular response.
Autoradiography with H-clonidine was used to locate the
injected clonidine. The new CEA Verken tritium sensitive film was
used and proved able to detect very low levels of tritium. Each
route of administration resulted in a different pattern of
Clonidine administered intravenously distributed evenly
throughout the CN3.
Intacarotid administration selectively reached rostral areas.
Intraventricular administration had the spread limited to
Intravertebral clonidine reached the medulla, pons, areas of
the cerebellum and upper areas of the spinal cord.
Comparison with the different hypotensive affects led to the
conclusion that the site of action was within the medulla but not in
the periventricular areas.