Regeneration of the optic tracts and circadian pacemaker activity in the cockroachLeucophaea maderae

Summary

Recovery of the circadian rhythm of locomotor activity after bilateral section of the optic tracts (OT) of the cockroachLeucophaea maderae was investigated. After OT section rhythmicity consistently reappeared in 3–5 weeks (29±6.2 days,n=22) (Fig. 1), while removal of the optic lobes caused permanent (> 100 days) arrythmicity (n= 13) (Fig. 2A). Recovery of rhythmicity after OT section was likely due to regeneration since:(1) Histological examination showed structural regeneration had occurred (Fig. 3A). (2) Insertion of a glass barrier between the OL and midbrain prevented (> 75 days,n=6) or slowed (46±14.9 days,n=3) rhythm recovery (Fig. 2B). (3) Extracellular recording after optic tract section showed recovery of light evoked activity in the cervical connectives (Fig. 4) whose time course paralleled the recovery of behavioral rhythmicity (Fig. 5).

The freerunning period (τ) of the rhythm after regeneration was strongly correlated with τ before surgery (r=0.87) but was slightly longer (Δτ=0.2±0.35 h) (Fig. 6). Also the phase of the rhythm, projected back to the day of surgery, was correlated with preoperative phase (r=0.61) (Fig. 7). Exposure to light cycles the first 10 days after OT section shifted the phase of the subsequent rhythm (Fig. 8). These results suggest that an entrainable circadian oscillation persists in the optic lobes after OT section.