Decreased Alpha 1-adrenergic Receptors After Experimental Brain Injury

The magnitude of neuronal damage in central nervous system (CNS) injury may be related, in part, to alterations in the balance between excitatory and inhibitory neurotransmitters. Previous studies have implicated a role of central inhibitory noradrenergic mechanisms in the pathophysiologic sequelae of traumatic brain injury. In the present study, we examined alpha 1-adrenergic receptor binding after parasaggital lateral fluid percussion (FP) brain injury of moderate severity (2.3 atm) in the rat. At 30 min following injury, the specific binding of [3H]prazosin to membranes isolated from left cortex (injury site) was reduced by 37% in brain-injured animals when compared to sham-operated noninjured animals (p < 0.05). However, there were no significant differences in [3H]prazosin binding to membranes of either contralateral (right) cortex or left and right hippocampi between brain-injured and sham-operated animals. Conversely, at 24 h posttrauma, specific binding to membranes of left cortex, cortex adjacent to injury site, contralateral (right) cortex, and left hippocampus was reduced by 25%, 16%, 27%, and 24%, respectively (all p < 0.05). Scatchard analysis revealed that a reduction of [3H]prazosin binding to membranes of injured animals resulted from a decrease in alpha 1-receptor binding density (B-max) but not from changes in ligand affinity. Histopathologic assessment of neuronal damage at 24 h postinjury revealed neuronal loss within injury site cortex and left hippocampus but no clearly discernible cell loss in contralateral right cortex, suggesting that the decrease in B-max might be a consequence of early pathophysiology of trauma rather than of neuronal cell loss. We suggest that alterations in alpha 1-adrenergic receptors after brain injury may result in decreased inhibitory neurotransmitter action of norepinephrine and may thus contribute to the pathophysiology of traumatic brain injury.