Differential effects of traumatic brain injury on vesicular acetylcholine transporter and M2 muscarinic receptor mRNA and protein in rat.

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 neurotransmission 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 microm 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.