Differential Effects of Traumatic Brain Injury on Vesicular Acetylcholine Transporter and M 2 Muscarinic Receptor mRNA and Protein in Rat

Abstract

Experimental traumatic brain injury (TBI) produces cholinergic neurotransmission deficits that may contribute to chronic spatial memory deficits. Cholinergic neurotransmission deficits may result from presynaptic alterations in the storage and release of acetylcholine (ACh) or from changes in the receptors for ACh. The vesicular ACh transporter (VAChT) mediates accumulation of ACh into secretory vesicles, and the M₂ muscarinic receptor subtype can modulate cholinergic neuro-transmission via a presynaptic inhibitory feedback mechanism. We examined the effects of controlled cortical impact (CCI) injury on hippocampal VAChT and M₂ muscarinic receptor subtype protein and medial septal mRNA levels at 4 weeks following injury. Rats were anesthetized and surgically prepared for CCI injury (4 m/sec, 2.5 to 2.9 mm in depth) and sham surgery. Animals were sacrificed, and coronal sections (35 μm thick) were cut through the dorsal hippocampus for VAChT and M₂ immunohistochemistry. Semiquantitative measurements of VAChT and M₂ protein in hippocampal homogenates from injured and sham rats were assessed with Western blot analysis. Changes in VAChT and M₂ mRNA levels were evaluated by reverse transcriptase polymerase chain reaction (RT-PCR). At 4 weeks after injury, both immunohistochemical and Western blot methods demonstrated an increase in hippocampal VAChT protein. An increase in VAChT mRNA was also observed. Immunohistochemistry demonstrated a loss of M₂; however, there was no significant change in M₂ mRNA levels in comparison with sham controls. These changes may represent a compensatory response of cholinergic neurons to increase the efficiency of ACh neurotransmission chronically after TBI through differential transcriptional regulation.