Evaluation of a combined therapeutic regimen of 8-OH-DPAT and environmental enrichment after experimental traumatic brain injury

Abstract

When provided individually, both the serotonin (5-HT(1A))-receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and environmental enrichment (EE) enhance behavioral outcome and reduce histopathology after experimental traumatic brain injury (TBI). The aim of this study was to determine whether combining these therapies would yield greater benefit than either used alone. Anesthetized adult male rats received a cortical impact or sham injury and then were randomly assigned to enriched or standard (STD) housing, where either 8-OH-DPAT (0.1 mg/kg) or vehicle (1.0 mL/kg) was administered intraperitoneally once daily for 3 weeks. Motor and cognitive assessments were conducted on post-injury days 1-5 and 14-19, respectively. CA1/CA3 neurons and choline acetyltransferase-positive (ChAT(+)) medial septal cells were quantified at 3 weeks. 8-OH-DPAT and EE attenuated CA3 and ChAT(+) cell loss. Both therapies also enhanced motor recovery, acquisition of spatial learning, and memory retention, as verified by reduced times to traverse the beam and to locate an escape platform in the water maze, and a greater percentage of time spent searching in the target quadrant during a probe trial in the TBI + STD + 8-OH-DPAT, TBI + EE + 8-OH-DPAT, and TBI + EE + vehicle groups versus the TBI + STD + vehicle group (p ≤ 0.0016). No statistical distinctions were revealed between the TBI + EE + 8-OH-DPAT and TBI + EE + vehicle groups in functional outcome or CA1/CA3 cell survival, but there were significantly more ChAT(+) cells in the former (p = 0.003). These data suggest that a combined therapeutic regimen of 8-OH-DPAT and EE reduces TBI-induced ChAT(+) cell loss, but does not enhance hippocampal cell survival or neurobehavioral performance beyond that of either treatment alone. The findings underscore the complexity of combinational therapies and of elucidating potential targets for TBI.

FIG. 1.

Mean time (seconds) balancing on an elevated narrow beam prior to, and after, TBI or sham injury. All TBI groups were significantly impaired relative to the Sham group. While no overall difference was revealed among the TBI groups, individual day analyses showed that on post-injury days 4 and 5 the TBI + STD + vehicle group, which did not return to baseline, was significantly different from the TBI + EE + 8-OH-DPAT, TBI + EE +vehicle, and TBI + STD + 8-OH-DPAT groups (*p < 0.005).

FIG. 2.

Mean time (seconds) to traverse an elevated narrow beam prior to, and after, TBI or sham injury (*p < 0.0008 versus TBI + STD + vehicle; **p < 0.0001 versus all TBI groups).

FIG. 3.

Mean distance traveled along an elevated narrow beam prior to, and after, TBI or sham injury (*p < 0.005 versus TBI + STD + vehicle animals; **p < 0.0003 versus all TBI groups).

FIG. 4.

Mean time (seconds) to locate either a hidden (submerged) or visible (raised) platform in a water maze (*p < 0.0001 versus TBI + STD + vehicle animals; **p < 0.0001 versus all TBI groups, except the TBI + EE + 8-OH-DPAT group [^p < 0.0001 and ^p = 0.0016 versus the TBI + STD + vehicle and TBI + STD + 8-OH-DPAT groups, respectively, but was not statistically different from the TBI + EE + vehicle group]). No significant differences in time to locate the visible platform were revealed among groups.

FIG. 5.

Mean percentage of time spent in the target quadrant (i.e., where the platform was previously located) following a single probe trial 19 days after cortical impact or sham injury (*p ≤ 0.0009 versus the TBI + STD + vehicle group; **p < 0.0001 versus the TBI + STD + vehicle group). There were no statistically significant differences among the TBI + EE + 8-OH-DPAT, TBI + EE + vehicle, TBI + STD + 8-OH-DPAT, and Sham groups. The dashed line represents performance at the chance level (25%), and the diagrams of the mazes inserted in the data bars depict the representative swim paths showing the percentage of time in each quadrant.

FIG. 6.

Mean percentages of morphologically intact CA1 and CA3 neurons (% of the contralateral hippocampus) at 3 weeks after TBI or sham injury. No statistically significant differences were revealed among the TBI groups (A = **p < 0.0001 versus all TBI groups; B = *p < 0.002 versus TBI + STD + vehicle; **p < 0.0001 versus all TBI groups).

FIG. 7.

Mean ChAT⁺ medial septal neurons quantified at 3 weeks after TBI. No significant difference in ChAT⁺ cell expression was observed between the naïve and TBI + EE + 8-OH + DPAT groups (p > 0.05 for A, B, and F), but both had significantly more than the TBI + EE + vehicle, TBI + STD + 8-OH-DPAT, and TBI + STD + vehicle groups (**p < 0.005 for C, D, and E, respectively, and F). Although not significantly different from each other (p > 0.05), both the TBI + EE + vehicle and TBI + STD + 8-OH-DPAT groups had significantly more ChAT⁺ cells than the TBI + STD + vehicle group (*p < 0.005 for C and D, respectively, versus E; scale bar = 250 μm).