The magnetic map sense and its use in fine-tuning the migration programme of birds.

The Earth's magnetic field is one of several natural cues, which migratory birds can use to derive directional ("compass") information for orientation on their biannual migratory journeys. Moreover, magnetic field effects on prominent aspects of the migratory programme of birds, such as migratory restlessness behaviour, fuel deposition and directional orientation, implicate that geomagnetic information can also be used to derive positional ("map") information. While the magnetic "compass" in migratory birds is likely to be based on radical pair-forming molecules embedded in their visual system, the sensory correlates underlying a magnetic "map" sense currently remain elusive. Behavioural, physiological and neurobiological findings indicate that the sensor is most likely innervated by the ophthalmic branch of the trigeminal nerve and based on magnetic iron particles. Information from this unknown sensor is neither necessary nor sufficient for a functional magnetic compass, but instead could contribute important components of a multifactorial "map" for global positioning. Positional information could allow migratory birds to make vitally important dynamic adaptations of their migratory programme at any relevant point during their journeys.

Magnetic activation in the brain of the migratory northern wheatear (Oenanthe oenanthe).

Behavioural and neurobiological evidence suggests the involvement of the visual and trigeminal sensory systems in avian magnetoreception. The constantly growing array of new genetic approaches becoming available to scientists would bear great potential to contribute to a generally accepted understanding of the mechanisms underlying this ability, but would require to breed migratory birds in captivity. Here we show that the transcontinental night-migratory Northern Wheatear (Oenanthe oenanthe), which is currently the only migratory songbird successfully being bred in reasonable numbers in captivity, shows magnetic-field-induced neuronal activation in the trigeminal brainstem areas receiving their input through the ophthalmic branch of the trigeminal nerve. In addition, preliminary data indicate night vision-triggered activation in the anterior visual forebrain. This brain area could represent the same brain region, which has previously been named "Cluster N" and shown to be involved in processing magnetic compass information in European Robins. Thus, based on brain activation data, both visually and trigeminally mediated magnetic senses known from other birds seem to exist in Northern Wheatears. This makes this species a potentially excellent model species for future genetic research on magnetoreception in migratory birds.

Selective synaptic cadherin expression by traced neurons of the chicken visual system.

The stable and specific locking-in of pre- and postsynaptic membranes in synaptogenesis may be mediated by integral membrane proteins, such as members of the cadherin family. Cadherins are ideal candidate molecules for mediating synaptic specificity because they are differentially expressed in functionally connected brain structures. We studied the expression of four classic cadherins (R-cadherin, N-cadherin, cadherin-6B and cadherin-7) at the synaptic level on the somata and the proximal neurites of identified neuron populations that were traced selectively in the developing chicken visual system. Three major findings were observed. (1) Synapses on somata of shepherd's crook cells of the optic tectum are associated preferentially with one cadherin subtype. (2) In an isthmic nucleus that contains a mixed population of cells expressing different cadherins, somatic synapses tend to express the same cadherin subtype as the rest of the cell. (3) In the oculomotor complex, two cadherin subtypes are expressed only by synapses on the axon hillock. However, another neuron type that projects from the tectum to the isthmic nucleus does not show such selective synaptic cadherin staining. Our findings support the idea that a cadherin-based adhesive mechanism can mediate synaptic specificity.