Chronic Effects of Traumatic Brain Injury on Hippocampal Vesicular Acetylcholine Transporter and M2Muscarinic Receptor Protein in Rats

Abstract

Experimental traumatic brain injury (TBI) produces cholinergic neurotransmission deficits that may contribute to chronic spatial memory deficits. Cholinergic neurotransmission deficits may be due to 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 M2receptors can modulate cholinergic neurotransmission via a presynaptic inhibitory feedback mechanism. We examined the effects of controlled cortical impact (CCI) injury on hippocampal VAChT and M2muscarinic subtype receptor protein levels at four time points: 1 day, 1 week, 2 weeks, and 4 weeks following injury. Rats were anesthetized and surgically prepared for controlled cortical impact injury (4 m/s, 2.5- to 2.9-mm depth) and sham surgery. Animals were sacrificed and coronal sections (35 μm thick) were cut through the dorsal hippocampus for VAChT and M2immunohistochemistry. Semiquantitative measurements of VAChT and M2protein in hippocampal homogenates from injured and sham rats were assessed using Western blot analysis. Immunohistochemistry showed no obvious changes in VAChT and M2immunoreactivity at 1 day and 1 week postinjury. At 2 and 4 weeks postinjury, an increase in hippocampal VAChT protein and a corresponding loss of hippocampal M2protein was observed compared to sham controls. Consistent with these results, Western blot analyses at 4 weeks postinjury demonstrated a 40–50% increase in VAChT and a 25–30% decrease in M2. These changes may represent a compensatory response of cholinergic neurons to increase the efficiency of ACh neurotransmission chronically after TBI, by upregulating the storage capacity and subsequent release of ACh and downregulating presynaptic inhibitory receptors.