Fishes olfactory bulb

Central/Tertiary structures The fish olfactory bulb is a fourlayered structure much as in higher vertebrates. Within the 2nd layer, the first synapse for olfactory input is on the dendrites of the mitral cells (MC). About 1000 ORN axons converge on one MC, a ratio similar to mammals. The MC output, from cells at various levels, leads into several glomeruli and receives (inhibitory) input from granule cells. The latter also innervate a distinct cell type in the MC layer of teleosts — the ruffed cells (RC), with which they have reciprocal synapses [Fig. 2.18(a)] both relay cells send ascending fibres to forebrain centres (Kosaka and Hama, 1982). The RC are unlike the MC since they are not stimulated by the ORNs directly. Their interactions (Chap. 5) may contribute to the processing of pheromonal stimuli (Zippel, 2000). The main bulbar pathways project to several nuclei in the forebrain via two ipsilateral tracts, the lateral and medial [Fig. 2.18(b)], the latter mediates sexual behaviour and the former probably other behaviours (Hara,... 

Taken together, the fish olfactory bulb provides for the opportunity to study functionally segregated responses of all olfactory receptor neurons in a recepto-topic map. Due to the small size and semi-transparent nature of the zebrafish olfactory bulb, it is to be expected that odor responses of all three receptor neuron populations could be measured simultaneously and possibly identified by spatial position. Indeed, in the zebrafish olfactory bulb it has been possible to measure odor responses in lateral, medial, and ventral glomeruli (Friedrich and Korsching 1997, 1998). 

Andres K. (1970). Anatomy and ultrastructure of the olfactory bulb in fish, amphibia, reptiles, birds and mammals. In Taste and Smell in Vertebrates (Wolstenholme G. and Knight J., eds.). J A Churchill, London, pp. 177-193. 

Hofmann M. and Meyer D. (1995). The extrabulbar olfactory pathway — primary olfactory fibres by passing the olfactory bulb in bony fishes. Brain Behav Evol 46, 378-388. 

In fish, certain odorants activate specific glomeruli in the olfactory bulb. Amino acids stimulate glomeruli in the lateral region of the bulb, while glomeruli in the medial region of the bulb process information on bile acids (Yoshihara etah, 2001). 

Kyle AL, Sorensen PW, Stacey NE, Dulka JG (1987) Medial olfactory tract pathways controlling sexual reflexes and behavior in teleosts. Ann N Y Acad Sci 519 97-107 Laberge F, Hara TJ (2001) Neurobiology of fish olfaction a review. Brain Res Rev 36 41-59 Laberge F, Hara TJ (2003) Non-oscillatory discharges of an F-prostaglandin responsive neuron population in the olfactory bulb-telencephalin transition area in lake whitefish. Neuroscience 116 1089-1095... 

Within the AL about 300 non-spiking interneurons modulate the output of the PNs. The resulting activity has been experimentally characterized as patterned on two time scales. There are fast 20 Hz local field potential (LFP) oscillations to which spikes of PNs are locked (Laurent et al. 1996 Wehr and Laurent 1996). The PNs are active in synchronized groups and these groups evolve over time in a slower, odor-specific pattern. The slow switching dynamics has been hypothesized to improve odor discrimination for very similar odors (Laurent et al. 2001 Rabinovich et al. 2001) and some experimental evidence for a decorrelation, and therefore presumably disambiguation, of similar patterns in the olfactory bulb of zebra fish has been observed experimentally (Friedrich and Laurent 2001). 

The neural pathways mediating alarm responses were examined in the crucian carp (Carassius carassius L.). In these fish, two olfactory tracts convey information from the olfactory bulbs [adjacent to the olfactory organs (i.e., nostrils)] to other parts of the brain. One courses along the midline (the medial olfactory tract) and the other along the side (the lateral olfactory tract). The medial olfactory tract further divides into two bundles (the medial and the lateral bundles of the medial olfactory tract). Severing the medial bundle of the medial olfactory tract eliminated the alarm responses to skin extract, whereas severing the lateral bundle of the medial olfactory tract diminished the feeding behavior (Hamdani et al. 2000). 

Copper exposures at 20 pg/L or higher induce degenerating effects on the olfactory receptor cells in fish (Saucier and Astic 1995). Since it is a normal process that receptor cells are regenerating in the olfactory epithelium of fish and other vertebrates as long as basal cells are present, new functional olfactory cells will be continuously produced and the animal can recover its sense of smell (e.g. Zippel 1993). There will, however, be problems if the fish remains in contaminated water and the olfactory epithelium does not acclimate and protect the receptor cells from metal toxicity (e.g. by metal-lothioneins, mucus production). It has been shown that olfactory receptor neurons can be a transport route of metal ions and organic molecules to the olfactory bulbs and the brain in vertebrates, fish included, with severe disturbing effects on the function of the CNS (e.g. Tjalve and Henriksson 1999 Persson et al. 2002). 

Oka, Y., Ichikawa, M., and Ueda, K., 1982, Synaptic organization of the olfactory bulb and central projections of the olfactory tract, in Chemoreception in Fishes, T. J. Hara, ed., Elsevier, Amsterdam. 

During these tracking studies we collected water samples from different stratified layers within the water column. These samples were then used to stimulate the olfactory epithelium of hatchery-reared fish while we measured electrical activity from single olfactory bulb neurons. Ninety per cent of the responding neurons, presumably mitral cells, showed differential responses to the water samples (Figure 3) demonstrating the capacity of the olfactory system to discriminate among stratified water layers found in the ocean. 

In addition, in fish where the nasal epithelium was removed by surgery, and not in those where it was destroyed by cautery, enhanced ir-LHRH staining was also localized in tracts on the ventral region of the olfactory bulb between the NOR and the inception of the olfactory nerve. At autopsy, gonads of these fish appeared either normal or reduced in size. Gonads and pituitary glands have not yet been examined histologically. 

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