Effects of riluzole on locomotor activity and in models of antidepressant efficacy
In the first series of experiments, mice were treated with riluzole for a total of 22 days to allow for multiple tests and recovery time between them (). During this period, locomotor activity was periodically monitored. Chronic treatment at these concentrations did not appear to be sedative; by contrast, the lowest 6 μg/ml concentration increased ambulatory activity [main effect of concentration F(3,33)=6.7, p<0.001; post-hoc ps≤0.01 relative to all other groups] (). Mice administered 30 or 60 μg/ml did not differ from control mice (ps>0.8).
Riluzole has antidepressant-like properties in mice
An initial cohort of mice administered the highest 60 μg/ml concentration showed no response in the tail suspension test of antidepressant efficacy at 10 days of treatment (t14=−0.02, p>0.9) (); because this concentration had no effect, lower concentrations were not tested. Despite this finding, 30 and 60 μg/ml reduced immobility in the forced swim test [F(3,32)=3, p=0.05; post-hoc ps<0.04 relative to control] when measured at 14 days of treatment ().
By the end of the 22-day treatment period, mice treated with the lowest concentration (6 μg/ml) were lighter than control animals [F(3,33)=12.4, p<0.001; post-hoc p=0.03], while 60 μg/ml increased body weights (ps≤0.004 relative to all other groups) ().
In view of differing effects in the tail suspension and forced swim tests of antidepressant efficacy, we adapted a model of depressive-like behavior termed “incentive disengagement.” We first confirmed that when expected reinforcement was withheld in drug-naïve instrumentally-trained mice, locomotor activity acutely declined, as in previous reports in rats: Indeed, in drug-naïve mice, photobeams broken in the 15 min after extinction training were reduced relative to counts generated in the first 15 min after reinforced test sessions [time bin x condition F(9,63)=8.1, p<0.001; post-hoc ps<0.003] (), and photobeam counts generated during these initial 15 minutes after extinction training were predicted by the number of nonreinforced instrumental responses the mice had performed (r2=0.56, p=0.03) (), pointing to a relationship between instrumental responding and subsequent locomotor activity. Importantly, reinstatement of food-reinforced responding also reinstated robust locomotor activity equivalent to that prior to extinction training (p=0.003 relative to post-extinction; ps>0.9 relative to reinforced sessions) (), indicating the decline in activity was not simply a consequence of testing order.
Validation and utility of the incentive disengagement model of acute depressive-like behavior
When mice were monitored after a reinforced operant conditioning session, locomotor activity in the first 15 min was increased to approximately 166% of the last 15 min the previous day. By contrast, if reinforcement was withheld, locomotor activity was only approximately 119% of the same baseline (); antidepressant treatments would be expected to occlude this decline. Consistent with this model, mice administered chronic fluoxetine generated more locomotor counts than control mice after extinction training (t21=−2.2, p=0.04) (). Mice treated with 60 μg/ml riluzole were also more active than control and lower-dose groups [F(3,28)=12.1, p≤0.001; post-hoc ps<0.006 relative to all other groups] () consistent with an antidepressant-like response. Unexpectedly, the lowest dose—6 μg/ml—decreased normalized locomotor scores after extinction training (p=0.009 relative to control). Statistical analyses here refer to normalized scores, but raw photobeam counts are also provided in .
Photobeam counts before and after instrumental extinction training
We collected several peripheral measures from mice chronically (22 days) treated with riluzole at an antidepressant-like concentration (60 μg/ml): Thymus gland weights did not significantly differ between groups (t14=1.6, p=0.13) (). Adrenal gland weights did not differ (t14=0.4, p=0.4) (); consistent with this finding, blood serum CORT also did not differ between groups (t9=1.1, p=0.3) ().
Chronic riluzole (60 μg/ml) has no effects on thymus and adrenal gland weights or blood serum CORT
Central effects of riluzole in a chronic depression model
We next analyzed the central actions of riluzole (60 μg/ml for 3 weeks) in a glucocorticoid-based model of depression. When we quantified BDNF in dentate gyrus-rich tissue samples, a CORT x riluzole interaction was identified [F(1,39)=8.6, p=0.005] (). Post-hoc tests indicated prior CORT exposure decreased BDNF as expected (p=0.02), while subsequent riluzole treatment restored BDNF levels in CORT-exposed mice (p=0.01 relative to CORT alone). Surprisingly, however, post-hoc tests also indicated mice treated with riluzole alone did not differ from CORT-exposed mice, and levels were reduced at a trend level of significance relative to control values (p=0.1).
Biochemical effects of riluzole (60 μg/ml) after corticosteriod exposure
We also evaluated the expression of GLT-1 in CA1/CA3-rich tissue samples. Again, a CORT x riluzole interaction was identified [F(1,39)=5.3, p=0.03] (), with post-hoc tests indicating prior CORT exposure decreased GLT-1 expression (p=0.02), and levels were restored by subsequent riluzole treatment (relative to CORT p=0.004). A main effect of riluzole was also identified (p=0.02).