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Olfactory input increases visual sensitivity in zebrafish: a possible function for the terminal nerve and dopaminergic interplexiform cells

Hans Maaswinkel and Lei Li^*

Departments of Physiology, University of Kentucky College of Medicine, Lexington, KY 40536, USA

View larger version (15K): [in a new window]   Fig. 1. Olfactoretinal centrifugal pathway in zebrafish. (A) Schematic representation of the location and projections of the terminal nerve (TN). Red circles and red lines label TN cell bodies and their axons, respectively. The broken line drawn between OB and TE indicates the location of the olfactory bulb lesion. OB, olfactory bulb; TE, telencephalon; TC, tectum opticum; RE, retina. (B) A confocal image of a whole-mount preparation of the olfactory bulb showing TN cell bodies and their axons (labeled with an antibody against FMRFamide). Anterior is to the left. Scale bar, approximately 35 µm for B.

View larger version (17K): [in a new window]   Fig. 2. Diagram of the behavioral test apparatus. The transparent water container is placed in the center of a drum that rotates clockwise or counterclockwise. A black segment printed on white paper attached on the inside of the drum serves as a visual stimulus. When the black segment comes into view, the fish reacts with an `escape' response, i.e. by reversing its swimming direction. The column in the center of the container prevents the fish from swimming across the center. Above the apparatus is suspended a light-source, the intensity of which is controlled by neutral density filters. An infrared video camera was used to observe the behavior of the fish at low light levels.

View larger version (24K): [in a new window]   Fig. 3. Changes of behaviorally assessed visual sensitivity in zebrafish induced by different concentrations of amino acids. Values are means ±S.E.M.; n.s., not significant; *P<0.05; **P<0.01; *** P<0.005; ****P<0.001.

View larger version (12K): [in a new window]   Fig. 4. Effect of olfactory stimulation with methionine (10-3 mol l-1) on the ERG b-wave sensitivity. Changes of the b-wave threshold after application of either methionine or regular tank water were determined at early morning, 06.00–19.00 h and late morning, 09.00–12.00 h. Values are means ± S.E.M.; n.s., not significant; ***P<0.001.

View larger version (62K): [in a new window]   Fig. 5. Immunostaining of midbrain and retinal sections of control (A,C) and bulbectomized (B,D) zebrafish. (A,B) FMRFamide staining was seen in the midbrain of control (A, arrows) but not bulbectomized fish (B). (C,D) Double-labeled retinal sections of control (C) and bulbectomized (D) zebrafish with antibodies against FMRFamide and tyrosine hydroxylase identifying terminal nerve (TN) axons (red) and dopaminergic interplexiform cells (DA-IPCs) (green), respectively. Note the absence of FMRFamide staining in bulbectomized zebrafish. Scale bar, approximately 350 µm (A,B), 35 µm (C,D).

View larger version (12K): [in a new window]   Fig. 6 Behavioral visual threshold changes in response to olfactory stimulation with methionine (10-3 mol l-1). (A) Visual threshold changes in control, bulbectomized (OBX) and 6-OHDA-treated zebrafish. (B) Visual threshold changes in control, dopamine D1 (SCH23390) and D2 (sulpiride) receptor antagonist-injected zebrafish. Values are means ± S.E.M.; n.s., not significant; ***P<0.005; ****P<0.001.