Differential effects of traumatic brain injury on vesicular acetylcholine transporter and M-2 muscarinic receptor mRNA and protein in rat

Experimental traumatic brain injury (TBI) produces cholinergic neurotransmi ssion deficits that may contribute to chronic spatial memory deficits. Chol inergic neurotransmission deficits may result from presynaptic alterations in the storage and release of acetylcholine (ACh) or from changes in the re ceptors for ACh. The vesicular ACh transporter (VAChT) mediates accumulatio n of ACh into secretory vesicles, and the M-2 muscarinic receptor subtype c an modulate cholinergic neurotransmission via a presynaptic inhibitory feed back mechanism. We examined the effects of controlled cortical impact (CCI) injury on hippocampal VAChT and M-2 muscarinic receptor subtype protein an d medial septal mRNA levels at 4 weeks following injury. Rats were anesthet ized and surgically prepared for CCI injury (4 m/sec, 2.5 to 2.9 mm in dept h) and sham surgery. Animals were sacrificed, and coronal sections (35 mu m thick) were cut through the dorsal hippocampus for VAChT and M-2 immunohis tochemistry. Semiquantitative measurements of VAChT and M-2 protein in hipp ocampal homogenates from injured and sham rats were assessed with Western b lot analysis. Changes in VAChT and M-2 mRNA levels were evaluated by revers e transcriptase polymerase chain reaction (RT-PCR). At 4 weeks after injury , both immunohistochemical and Western blot methods demonstrated an increas e in hippocampal VAChT protein. An increase in VAChT mRNA was also observed . Immunohistochemistry demonstrated a loss of M-2; however, there was no si gnificant change in M-2 mRNA levels in comparison with sham controls. These changes may represent a compensatory response of cholinergic neurons to in crease the efficiency of ACh neurotransmission chronically after TBI throug h differential transcriptional regulation.