3.1. Signal detection accuracy
The strains did not differ significantly [F(1,16)=2.607, p>.05] when compared in the saline treatment only, but there was a significant difference in cue interval [F(2,32)=89.437, p<.0005; ], reflecting more correct responses in longer intervals. There was also a significant interaction [F(2,32)=6.944, p=.01] reflecting stronger cue interval differences in SHR [F(2, 16)=53.951, p<.0005] than WKY [F(2,16)=36.314, p<.0005]. Within-session performance accuracy did not differ between the two strains ().
Fig. 2 (2.1)WKY and SHR performance in different light cue conditions during the vehicle treatment. The three light cues (100 ms, 300 ms, 1 s) are shown on the X axis and the percent correct responses are shown on the Y axis. The two line graphs represent the (more ...)
When all treatments were considered (), there were significant main effects of strain [F(1, 16) =60.183, p<.0005], reflecting more correct responses in SHR than WKY, stimulus duration [F(1.367, 21.872) =103.600, p<.0005], reflecting more correct responses in longer intervals, and treatment condition [F(2.636, 42.176)=40.011, p<.0005], reflecting fewer correct responses with higher MP doses. There were significant interactions for cue interval X strain [F(1.367, 21.872)=4.811, p<.034] and treatment X strain [F(2.636, 42.176)=26.780, p<.0005], reflecting stronger differences in both for WKY than SHR. There was a non-significant trend for the interaction of strain X cue interval X treatment [F(4.795, 76.728)=1.974, p=.095], reflecting a significant cue interval X treatment interaction for WKY [F(2.78, 22.26)=37.257, p<.0005] but not for SHR.
Fig. 3 WKY and SHR correct lever presses in different light cue conditions with different treatments. The four dose treatments (vehicle, 2 mg/kg, 5 mg/kg, 10 mg/kg) are shown on the X axis and the percent correct responses are shown on the Y axis. The line graphs (more ...)
3.2. Response omissions
There were significant main effects () of strain [F(1, 16)= 82.310, p<.0005], stimulus duration [F(1.709, 27.351)=4.937, p<.019] and treatment [F(2.076, 33.210)=19.379, p<.0005]. The two way interaction was significant for treatment X strain [F(2.076, 33.210)= 18.650, p<.0005]. No significant strain X cue interval, cue interval X treatment, or strain X cue interval X treatment interactions were detected. Separate analyses by strain also revealed a significant main effect of treatment, but only for WKY rats [F(2.194, 17.550)=19.339, p<.005]. There were no significant main effects of cue interval for either strain, and no significant cue interval by treatment interaction.
Fig. 4 Behavioral measures for WKY and SHR with different treatments. 4.1. Response omissions. The four dose treatments (vehicle, 2 mg/kg, 5 mg/kg, 10,g/kg) are shown on the X axis. The percent omissions are shown on the Y axis. MP treatments with 5 mg/kg and (more ...)
The significant findings from between-treatment comparisons are summarized in , and show that in all three stimulus durations the treatment effect for WKY was larger than for SHR (larger increase of number of omissions with higher dose treatments).
3.3. Intertrial interval responses
There was a significant main effect () for strain [F(1, 16)= 12.173, p<.003] but not treatment. There was also a significant interaction for strain X treatment [F(2.482, 39.718)=8.529, p<.005]. Within-strain analyses revealed that the effect of treatment was significant for WKY [F(3, 24)=13.600, p<.005] but not SHR rats. Similarly, pair-wise comparisons in SHR rats showed no significant difference in ITI by treatment condition. In contrast, in WKY the higher dose treatments (5 and 10 mg/kg) significantly decreased the intertrial interval responses compared to both vehicle and the 2 mg/kg treatment. However, significant differences in WKY performance should be interpreted with caution since high dose treatments also produced an overall decrease in absolute number of responses to all light cues (i.e. response omissions).
3.4. Locomotor activity during VSPDT
There were significant main effects () for both strain [F(1, 16) = 42.967, p<.0005] and treatment [F(2.593, 16)=14.727, p<.0005], but there were no significant strain X treatment interactions. Overall, SHR rats exhibited higher mean locomotor activity than WKY rats at baseline (t=6.238, df=16, p<.0001). Within-strain analyses revealed significant main effects of treatment for both SHR [F(2.182, 17.452)=8.446, p<.002] and WKY [F(2.481, 19.844)=8.769, p<.002] rats. Pair-wise comparisons within the SHR group found a significant increase in locomotor activity with the 2, 5 and 10 mg/kg doses compared to saline. For WKY, all treatments also increased locomotor activity compared to saline. The data also indicates that treatment condition produced similar patterns of change for both strains (see ). All three treatments increased locomotor activity for both strains compared to vehicle, but not in a dose-linear fashion; the 5.0 mg/kg dose produced a higher magnitude of increase than both the 2 and 10 mg/kg dose treatments.