Botanisches Institut, Auf der Morgenstelle 1, D-72076 Tübingen,
An accumulating body of evidence shows that the period-protein (PER)-expressing lateral neurons (LNs) are circadian pacemaker cells in the brain of Drosophila (1). Two clusters of LNs can be distinguished, a dorsally located cluster (LNd) and a ventral cluster (LNv). In addition to PER, the LNv contains a peptide hormone, the pigment-dispersing hormone (PDH) (2). With the aid of an antiserum against PDH the arborizations of these neurons were revealed in detail (3). The processes of the LNv extend into the central brain and into the optic lobe, suggesting a centripetal and centrifugal transfer of rhythmicity. LNv central brain processes terminate in the superior protocerebrum. Studies with different neuroanatomical mutants show that these terminals are crucial for the transfer of the circadian message to locomotor centers in the brain. disconnected (disco) mutants normally lack LNs (4) and are behaviorally arrhythmic (5). However, in rare cases, single LNs are present in individual disco mutants. Their presence is sufficient to generate rhythmic locomotor activity, but only if projections to the superior protocerebrum exist. LNv optic lobe processes terminate in the distal medulla and may control rhythmic changes in the complex eye (6). They are not important for rhythmic behavior because mutants with reduced optic lobes, and consequently a reduced LNv arborization field in the medulla (3), exhibit rather normal rhythms (7). Only when the optic lobes were reduced to less than 5% of their original size, as in the double mutant small optic lobes; sine oculis (sol; so), was an altered locomotor activity rhythm pattern observed. These flies frequently showed two freerunning components in their activity patterns (8). Additionally, LNv processes normally targeting the optic lobes were rerouted into the superior protocerebrum (3). This may provide an additional optic lobe circadian signal which is superimposed over the central brain signal. This conclusion was corroborated by studies of individual sol; so flies with residual eyes. When the optic lobes exceeded a critical size, a wild-type pattern of LNv arborizations in the central brain and a single locomotor activity rhythm were found. The pathway from the superior protocerebrum to the locomotor center in the brain is still unknown. However, I expect that diffusible factors participate because the location of LNv terminals in the superior protocerebrum is variable in different mutants which retain their rhythmic behavior. Therefore, the transfer of a circadian rhythm signal may not depend on physical contacts to specific target neurons.