Lateralized activation of Cluster N in the brains of migratory songbirds

Abstract

Cluster N is a cluster of forebrain regions found in night-migratory songbirds that shows high activation of activity-dependent gene expression during night-time vision. We have suggested that Cluster N may function as a specialized night-vision area in night-migratory birds and that it may be involved in processing light-mediated magnetic compass information. Here, we investigated these ideas. We found a significant lateralized dominance of Cluster N activation in the right hemisphere of European robins (Erithacus rubecula). Activation predominantly originated from the contralateral (left) eye. Garden warblers (Sylvia borin) tested under different magnetic field conditions and under monochromatic red light did not show significant differences in Cluster N activation. In the fairly sedentary Sardinian warbler (Sylvia melanocephala), which belongs to the same phyolgenetic clade, Cluster N showed prominent activation levels, similar to that observed in garden warblers and European robins. Thus, it seems that Cluster N activation occurs at night in all species within predominantly migratory groups of birds, probably because such birds have the capability of switching between migratory and sedentary life styles. The activation studies suggest that although Cluster N is lateralized, as is the dependence on magnetic compass orientation, either Cluster N is not involved in magnetic processing or the magnetic modulations of the primary visual signal, forming the basis for the currently supported light-dependent magnetic compass mechanism, are relatively small such that activity-dependent gene expression changes are not sensitive enough to pick them up.

Fig. 1

ZENK expression in European robin brains under different eye-cover conditions. (A) Orientation of European robins during spring migration in an NMF in our behavioural apparatus. Each dot represents the mean orientation of one individual bird. The arrow indicates the mean orientation of the group. The length of the arrow represents the length (r-value) of the group mean vector. The dashed circles indicate the radius of the group mean vector needed for significance (P < 0.05 and P < 0.01) according to the Rayleigh Test of uniformity. (B) ZENK expression levels in Cluster N of the right (r) and left (l) hemisphere for four groups of birds with various eye open and covered conditions. (C) Direct within-bird subtracted comparisons of the interhemispheric differences in ZENK expression levels for the four experimental groups. One-way ANOVA comparison between all groups revealed highly significant differences for all group comparisons (P < 0.001), except for the comparison between the two control groups: birds with both eyes open (control) and birds with both eyes covered. (D) Darkfield image of ZENK expression in right and left hemisphere saggital sections showing Cluster N expression under different eye-cover conditions. White arrows mark the caudal and rostral boundaries of Cluster N for each brain section. Rostral is right, dorsal is up. (E) Anatomical profile of the right image (condition: both eyes covered) shown in (D). Abbreviations: A, arcopallium; Cn, Cluster N; E, entopallium; GP, globus pallidus; HF, hippocampal formation; H, hyperpallium; ICo, inferior colliculus; M, mesopallium including ventral (MV) and dorsal (MD) part; N, nidopallium; OT, optic tectum; St, striatum; v, ventricle; W, visual Wulst. Error bars are SEM. Scale bar, 1 mm.

Fig. 2

Relative ZENK expression levels in Cluster N (difference in expression between Cluster N and the brain subdivisions rostral to Cluster N; see Materials and methods) in garden warbler brains under different magnetic field conditions. (A and B) Orientation of garden warblers during autumn migration in (A) an NMF and (B) a ZMF (symbols are as in Fig. 1). Panels A and B reproduced (with permission from Elsevier) from Mouritsen et al. (2004b), as the results reported here originate from the same individual birds as used in that study. (C) Relative ZENK expression levels in Cluster N in birds exposed to NMF, CMF or ZMF compared with birds collected during day-time (day, NMF). Error bars are SEM.

Fig. 3

ZENK expression in garden warbler brains under dim white vs. dim red light. (A) Darkfield image of Cluster N ZENK expression under dim white light at sagittal section, night (left) and after 45 min exposure to dim-monochromatic red light (right). Rostral is right, dorsal is up. (B) Anatomical profile of the right image shown in A. (C) Quantification of ZENK expression levels in Cluster N, comparing exposure to standard white light with red light. IHA, intercalated hyperpallium apicale; for other abbreviations see Fig. l legend. Error bars are SEM. Scale bar, 1 mm.

Fig. 4

ZENK expression in Sardinian warblers. (A) Darkfield image of ZENK expression in a sagittal section showing Cluster N. Rostral is right, dorsal is up. (B) Anatomical profile of the image shown in (A). The red dashed line marks the boundary of Cluster N. (C) Quantification of ZENK expression levels in Cluster N, comparing day-time and night-time groups. For abbreviations see Fig. 1 legend. Error bars are SEM. Scale bar, 1 mm.