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The zebrafish, Danio rerio, is becoming an important model system for developmental studies. We have used a variety of histological techniques to characterize the adult structure of the olfactory system in this teleost to form a base for future developmental work. The olfactory epithelium in this fish contains ciliated and microvillar sensory neurons, microvillar supporting cells, secretory goblet cells, and basal cells, and the adjacent nonsensory epithelium contains ciliated supporting cells. The olfactory bulb is a diffusely organized structure with four laminae: olfactory nerve, glomerular, mixed mitral cell/plexiform, and granule cell layers. These structures and the synapses observed in the olfactory bulb are typical of what is found in other vertebrates. We also examined the distribution of several neurotransmitter markers (tyrosine hydroxylase, neuropeptide Y, dopamine-beta-hydroxylase, and serotonin) in the olfactory bulb. Antibodies to neuropeptide Y, dopamine-beta-hydroxylase, and serotonin labeled fibers in the olfactory bulb and cell bodies in caudal regions of the brain in distributions comparable to other species. Tyrosine hydroxylase immunoreactivity was observed in a set of intrinsic bulb neurons with extensive processes in the glomerular layer. In addition, the structural proteins glial fibrillary acidic protein and vimentin have distributions similar to those in the olfactory bulbs of other animals. Thus, the adult olfactory structures are analogous to the structures in other vertebrate animals in morphology and chemical neuroanatomy. This similarity, along with its numerous advantages for developmental studies, makes the zebrafish a good model for studies of olfaction and forebrain maturation.

In standardized tests of huddling behavior, 5-, 10-, 15-, and 20-day-old rat pups spent substantial and equivalent amounts of time with an immobile rat or a heated, fur-covered tube, which suggests that the conspecific and inanimate stimuli were equally attractive to the pups. Two-choice preferences tests, however, revealed dramatic developmental differences in attraction. Younger pups preferred to huddle with the warmer, inanimate target, whereas older pups preferred the conspecific. The emergent conspecific preference appears mediated by attraction to species odors. The 5-and 10-day-olds huddled equally with an immobile rat and an immobile gerbil, stimuli with similar thermal and tactile properties, but older pups preferred the conspecific. Intranasal zinc sulfate treatment eliminated preference for the conspecific in 15and 20-day-olds but did not disrupt huddling per se. Thermal cues were sufficient to elicit huddling at all ages, but olfactory cues became a more salient influence before weaning. An ontogenetic transition from "physiological" to "filial" huddling is discussed in terms of changes in sensory control of early behavior.