In the tail suspension test (), mice injected with cytisine showed significantly less immobility at the 1.5 mg/kg dose compared to saline-treated animals (F1,14=13.15, p=0.003), as did mice injected with mecamylamine (1 mg/kg; F1,14=19.28, p=0.0006). Treatment with other cytisine doses showed a trend to induce decreased immobility but did not reach significance compared to saline treatment.
Total time spent immobile in the tail suspension (A) and in the forced swim (B) tests by C57BL/6J male mice treated with saline, cytisine and mecamylamine. Error bars represent SEM. * P< 0.05, ** P< 0.01. N=8 for each treatment group.
Similarly, in the forced swim test (), mice treated with cytisine (1.5 mg/kg) spent significantly less time immobile than control animals treated with saline (F1,14=4.72, p=0.047), as was seen following mecamylamine treatment (F1,14=4.87, p=0.045). Again, the lower doses of cytisine (<1.5 mg/kg) did not show significant effects compared to saline treatment.
As has been reported for classical antidepressants previously, chronic (21 days, 1.5 mg/kg), but not acute (1.5 mg/kg), cytisine treatment decreased the latency to initiate feeding as compared to saline treatment (F1,13=15.63, p=0.0017 and F1,14=0.006, p=0.94, respectively; ). A similar effect was observed in mecamylamine-treated animals (F1,13=7.16, p=0.019), but the mecamylamine effect was significantly different from the cytisine response (F1,14=8.75, p=0.0104). Weight loss after fasting or food intake after the test were not different across treatment groups (F2,20=1.26, p=0.30 and F(2,20)=0.206, p=0.82, respectively; data not shown).
Figure 2 Latency to initiate a feeding episode in the novelty-suppressed feeding test in male C57BL/6 mice. Effects of acute cytisine treatment is represented on the right. Error bars represent SEM. ** P<0.01, *** P<0.001. N=8 for each treatment (more ...)
In the light/dark test, cytisine treatment increased time spent in the dark compartment compared to saline treatment (F(1,13)=7.17, p=0.019; ). Locomotor activity in the box was unchanged by cytisine treatment (number of crossings: F(1,13)=1.56, p=0.23, data not shown). No change in overall locomotor activity was detected between treatment-groups either after acute (F(1,14)=0.62, p=0.44, ) or chronic cytisine treatment (F(1,14)=0.004, p=0.95, data not shown).
Figure 3 Total time spent in the dark compartment of the light/dark box test after acute cytisine or saline treatment in male C57BL/6 mice (A). Total number of beam breaks in an open-field for 20 minutes after acute injection of cytisine of cytisine or saline (more ...)
An overall reduction of c-fos immunostaining was observed in mice treated with cytisine compared to the saline treated group (F(1,112)=3.45, p=0.03). A dramatic reduction of c-fos immunostaining was observed in the basolateral amygdala () after cytisine treatment (F1,14=18.51, p=0.001), similar to what was observed with mecamylamine (F(1,14=26.20, p=0.0003).
Figure 4 A. c-fos immunostaining in brain areas of C57BL/6J male mice, following 24-days of treatment with saline, cytisine or mecamylamine. BLA: basolateral amygdala; Hypoth: hypothalamus; Nacc: nucleus accumbens; PVA: paraventricular nucleus; Sch: suprachiasmatic (more ...)
No other cytisine-induced differences in c-fos immunoreactivity were significant, although there was a general trend toward a reduction of c-fos staining in each brain region studied. However, mecamylamine showed a more pronounced effect than cytisine, with a significant reduction of c-fos staining in the hypothalamus (F1,14=5.48, p=0.03), the nucleus accumbens (F1,14=10.14, p=0.0072), and the suprachiasmatic nucleus (F1,14=11.09, p=0.005).