Changes in noradrenergic sensitivity to tumor necrosis factor-alpha in brains of rats administered clonidine

Tumor necrosis factor-alpha (TNF alpha) and the imidazoline clonidine modulate norepinephrine (NE) release from noradrenergic nerve terminals in the central nervous system. The present study demonstrates an intrinsic association between presynaptic alpha 2-adrenergic receptor sensitivity and TNF alpha responsiveness in governing this NE release. Superfusion and electrical field stimulation were applied to a series of rat hippocampal brain slices in order to study the regulation of [3H]-NE release. The alpha 2-adrenergic agonist clonidine and the cytokine TNF alpha concentration-dependently inhibit [3H]-NE release; whereas, the alpha 2-adrenergic antagonist idazoxan potentiates [3H]-NE release. The fractional release of [3H]-NE during field stimulation of control hippocampal slices was decreased by the addition of TNF alpha in a concentration-dependent manner, an effect which was potentiated by the alpha 2-adrenergic antagonist idazoxan; whereas, TNF alpha attenuated the concentration-dependent potentiating effect of idazoxan. Furthermore, constitutive TNF alpha, demonstrated to be present in several brain areas, was significantly decreased following administration of the alpha 2-adrenergic agonist clonidine (0.6 mg/kg, i.p., twice daily) to rats for either 1 or 14 days, without a change in TNF alpha mRNA accumulation. We next investigated whether the presynaptic sensitivity to TNF alpha was changed after clonidine administration to rats. TNF alpha enhanced, rather than inhibited, [3H]-NE release after 1 day of clonidine administration, while a suppressed sensitivity to TNF alpha was observed in the hippocampus after 14 days of clonidine administration. In addition, in the presence of idazoxan, TNF alpha potentiation of [3H]-NE release after 1 day clonidine administration was reversed to a decreased inhibition as compared to control slices exposed to idazoxan. Therefore, the temporary reversal in the presynaptic TNF alpha response after 1 day of clonidine administration illustrates a mechanism of action for its persistent antihypertensive effect, its transient sedative and antihyperpathic effects, and its acute ability to promote antidepressants. These results demonstrate a novel role for an immune mediator in the central nervous system, and demonstrates that presynaptic TNF alpha responsiveness is intimately associated with adrenergic receptor sensitivity.