Supersensitivity of the noradrenergic system in the spinal cord following intracisternal injection of 6-hydroxydopamine

The intracisternal administration of 6-hydroxydopamine (6-OHDA; 100 μg on two consecutive days) resulted in a 90-95% depletion of norepinephrine (NE) in the spinal cord 3-60 days following administration of the drug. At 90 days following injection of 6-OHDA, the depletion was only 60% suggesting the regrowth of NE-containing fibers. Consistent with the depletion of NE, 3-60 days after the injection of 6-OHDA a 50-80% decrease in the uptake NE occurred with only a 25% decrease 90 days after administration of 6-OHDA. Associated with decreases in NE and its uptake was a 100-200% increase in the accumulation of NE-stimulated cyclic AMP in slices of spinal cord tissue from 6-OHDA-treated rats. The increased response was evident at 3-60 days after injection of 6-OHDA decreasing on day 90 to 40% above control. No shift in the EC₅₀ for NE was evident and the specificity of agonists for this response was limited to agents with alpha and/or beta-adrenergic receptor activation properties. Readioligand receptor binding techniques, utilizing [³H]prazosin for alpha₁ receptors and [³H]dihydroalprenolol for beta receptors demonstrated that increases in the B_max of response of alpha₁ (30-50%) and beta (15-20%) receptors in spinal cord may mediate the enhanced response of NE-stimulated cyclic AMP following the administration of 6-OHDA. This is indicated by similar time-dependent patterns of onset and termination of increased receptor binding and the accumulation of NE-stimulated cyclic AMP. Such an adaptation of adrenergic receptor systems following the administration of 6-OHDA suggests a possible role for this system in mediating supersensitivity phenomena, such as motor hyperreflexia following denervating injuries to the spinal cord.