Anatomical and functional characterization of the orientation network in the central and lateral complex of the desert locust Schistocerca gregaria
Spatial orientation is an indispensable basis of many behaviors that ensure the survival of an individual or a species. Foraging, finding mating partners, avoiding predators or developing new habitats rely on this ability. Insects show sophisticated skills for spatial orientation and navigation....
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|Spatial orientation is an indispensable basis of many behaviors that ensure the survival of an
individual or a species. Foraging, finding mating partners, avoiding predators or developing
new habitats rely on this ability. Insects show sophisticated skills for spatial orientation and
navigation. These abilities require integration of external and internal stimuli that provide
information about the animal’s own position in space or relative to an object. Under the open
sky the sun is – even for humans – a prominent orientation cue that can be utilized for
orientation by many insect species. Additionally, other celestial cues - that are not perceptible
by humans - can be detected and used by insects. One of these is the polarization pattern of
the sky that results from scattering of unpolarized sunlight in the earth’s atmosphere. It is
characterized by the systematic arrangement of the prevailing plane of oscillation of polarized
light (angle of polarization, AoP) and depends on the position of the sun. The degree of
polarization (DoP) that indicates the percentage of polarized light within a light beam also
depends on the sun’s position. In the insect brain, the AoP is encoded by the activity of
polarization-sensitive neurons that transmit this information from the compound eyes into the
central brain, where it is used to generate an internal compass. The internal compass is
represented by the activity of neuronal populations of the central complex (CX), a navigation
center that processes orientation-relevant information and is involved in the generation of
appropriate locomotor responses. The CX comprises four midline spanning neuropils in the
center of the brain; the protocerebral bridge (PB), the lower division of the central body
(CBL; also known as ellipsoid body, EB, in flies) the upper division of the central body
(CBU; also known as fan-shaped body, FB, in flies) and the paired noduli (NO). The
neuropils are characterized by vertical columns (or slices) and horizontal layers that result
from the neuronal projections of the neuron systems that constitute the CX. Arborizations of
tangential neurons establish distinct layers and arborizations of columnar and pontine neurons
result in distinct columns. Beside polarization information the neuronal network of the CX
also integrates other information that underlies context- and experience dependent behavior.
Another brain region, the lateral complex (LX) plays a major role in mediating information
flow to and from the CX. The LX is located in both hemispheres laterally from the CX and
consists of the lateral accessory lobe (LAL) with the associated gall and the bulb. A variety of
neuron types connects the LX and the CX and provides connections between the LX and other
brain regions as well as the thoracic ganglia.
The present thesis investigates the physiology of polarization-sensitive neurons of the CX
and the anatomical organization of the CX and the LX of the desert locust Schistocerca
gregaria (Figure I). Electrophysiological experiments were performed to investigate the
influence of the DoP on the coding of the AoP by polarization-sensitive neurons of the CX
(Chapter I). They revealed that even low DoPs allow a reliable coding of AoPs in the locust
brain. However, DoPs under a certain threshold result in a strong modulation of the activity of
neurons at the input stage of the CX. Neuron types within the CX are characterized in addition
to their physiology and morphology by the expression of neurotransmitters and neuropeptides.
Immunocytochemical stainings revealed the expression pattern of myoinhibitory peptide
(MIP) in the CX (Chapter II). We identified and characterized five MIP-expressing neuronal
systems comprising cell types that have so far been identified mainly based on single-cell
labeling. The combination of single-cell labelings and immunocytochemical stainings
revealed potential feedback loops between cell types of the CX and the LX and were used to
identify and describe novel cell types of the LAL (Chapter III).