One-Year Study of Spatial Memory Performance, Brain Morphology, and Cholinergic Markers After Moderate Controlled Cortical Impact in Rats

Abstract

Persistent cognitive deficits are one of the most important sequelae of head injury in humans. In an effort to model some of the structural and neuropharmacological changes that occur in chronic postinjury brains, we examined the longitudinal effects of moderate vertical controlled cortical impact (CCI) on place learning and memory using the Morris water maze (MWM) test, morphology, and vesicular acetylcholine (ACh) transporter (VAChT) and muscarinic receptor subtype 2 (M₂) immunohistochemistry. Vertical CCI (left parietal cortex, 4 m/sec, 2.5 mm; n = 10) or craniotomy (sham) was produced in male Sprague-Dawley rats (n = 10). Place learning was tested at 2 weeks, 4 weeks, 3 months, 6 months, and 12 months postinjury with the escape platform in a different maze quadrant for each time point. At each interval, rats received 5 days of water maze acquisition (latency to find hidden platform), a probe trial to measure place memory, and 2 days of visible platform trials to control for nonspecific deficits. At 3 weeks, half the animals were sacrificed for histology. At these injury parameters, CCI produced no significant differences in place learning between injured and sham rats at 2 weeks, 4 weeks, or 6 months after injury. However, at 3 and 12 months, the injured rats took significantly longer to find the hidden platform than the sham rats. Probe trial performance differed only at 12 months postinjury between injured (25.73 ± 2.1%, standard error of the mean) and sham rats (44.09 ± 7.0%, p < 0.05). The maze deficits at 1 year were not due to a worsening of performance, but may have resulted from a reduced ability of injured rats to benefit from previous water maze experience. Hemispheric loss of 30.4 ± 5.5 mm³ was seen at 3 weeks after injury (versus respective sham). However, hemispheric loss almost doubled by 1 year after injury (51.5 ± 8.5 mm³, p < 0.05 versus all other groups). Progressive tissue loss was also reflected by a three- to fourfold increase in ipsilateral ventricular volume between 3 weeks and 1 year after injury. At 1 year after injury, immunostaining for VAChT was dramatically increased in all sectors of the hippocampus and cortex after injury. Muscarinic receptor subtype 2 (M₂) immunoreactivity was dramatically decreased in the ipsilateral hippocampus. This suggests a compensatory response of cholinergic neurons to increase the efficiency of ACh neurotransmission. Moderate CCI in rats produces subtle MWM performance deficits accompanied by persistent alteration in M₂ and VAChT immunohistochemistry and progressive tissue atrophy. The inability of injured rats to benefit from repeated exposures to the MWM may represent a deficit in procedural memory that is independent of changes in hippocampal cholinergic systems.