Does the spinal cord generate functionally significant sympathetic activity in the awake rat?

Area of Science:

• Physiology
• Neuroscience
• Cardiovascular Research

Background:

• Conflicting conclusions exist on sympathetic nerve activity (SNA) in rats with cervical spinal cord transection (CST).
• Adrenergic blockade in CST rats causes natriuresis and decreased heart rate, but the underlying mechanism is debated.

Purpose of the Study:

• To determine if spinally generated SNA or nonspecific antagonist effects cause natriuresis and bradycardia in CST rats.
• To investigate the specific mechanisms behind adrenergic blockade-induced physiological changes in CST rats.

Main Methods:

• Administered adrenergic antagonists (phentolamine, propranolol) to CST rats with renal denervation, adrenalectomy, or ganglionic blockade (hexamethonium).
• Tested for nonspecific drug effects by administering combinations of adrenergic antagonists (idazoxan, prazosin, yohimbine).

Main Results:

• Renal denervation, adrenalectomy, and ganglionic blockade did not prevent the significant increase in sodium excretion or decrease in heart rate.
• Idazoxan plus propranolol reproduced the natriuresis, but prazosin, yohimbine, and propranolol did not.
• No evidence for functionally significant spinally generated SNA was found.

Conclusions:

• There is no evidence for significant spinally generated sympathetic nerve activity in conscious CST rats.
• The natriuresis observed with phentolamine administration in CST rats is attributed to imidazoline binding, not SNA.