Differential Effects of Central and Peripheral Desipramine on Sympathoadrenal Function and Heart Rate in Conscious Rabbits

Abstract

The effects of the tricyclic antidepressant, desipramine, on the baroreflex regulation of renal sympathetic nerve activity (SNA) and heart rate (HR), the nasopharyngeal reflex, plasma epinephrine and blood pressure (BP) were studied in conscious rabbits. Renal SNA and HR were recorded during slow ramp changes in mean arterial pressure (MAP) and during inhalation of cigarette smoke. Intracisternal (i.c.) and intravenous (i.v.) drug administration were compared, using doses which produced similar total central nervous system (CNS) concentrations. After a brief sympathoexcitation, i.c. desipramine inhibited renal SNA and MAP and increased plasma adrenaline and HR. The renal sympathetic baroreflex was substantially attenuated, with reflex range and gain reduced by 46 and 31%, respectively, but the cardiac baroreflex and nasopharyngeal reflex were affected minimally. Sixty-four percent of the desipramine remaining in the brain was concentrated in the medulla oblongata and spinalis; levels in cortex, thalamus, midbrain, lower spinal cord, and peripheral tissues were minimal. Treatment with i.v. desipramine decreased renal SNA and increased HR without altering MAP or epinephrine release. There was a slight attenuation of the nasopharyngeal reflex, a slight baroreceptor-independent reduction in renal SNA at most MAP levels, and an augmentation of the cardiac baroreflex. The drug was uniformly distributed throughout the CNS; only 20% of the centrally accumulated dose was in the medulla. Thus, i.c. desipramine produces a differentiated pattern of sympathoadrenal effects, probably by increasing norepinephrine (NE) concentrations at several sites within the medulla. The effects of i.v. desipramine were different, owing to poorer access to the medulla and the consequences of peripheral neuronal uptake blockade, which may include a modest inhibition at the sympathetic ganglia and an excitation at cardiac and vasoconstrictor neuroeffector junctions.