Coding of pheromonal information in the goldfish olfactory bulb

Olfactory processing of pheromonal information is not well understood, largely due to a lack of identified pheromones in vertebrates. One of the best models of pheromone function in a vertebrate is the goldfish for which two hormonally-derived pheromonal cues have been identified. I conducted two experiments using local field potential (LFP) and single-unit recording from the goldfish olfactory bulb to test how components of these identified sex pheromones are processed by olfactory bulb neurons.;In the first experiment, LFP recording demonstrated that identified pheromonal odorants are encoded by spatial patterns of glomerular activation in the goldfish olfactory bulb. Pheromones evoked activity in localized areas of the bulb associated with output to the medial olfactory tract. In contrast, non-pheromonal cues evoked activity in broad glomerular regions spanning the entire bulb. Higher concentrations of odorants evoked activity in similar regions of the olfactory bulb as lower concentrations, but there was a spread in the initial peak of activity or additional peaks of activity.;In the second experiment, single unit recording provided evidence for both population and labeled-line coding of pheromonal information in the goldfish olfactory bulb. The molecular receptive ranges of 56 projection neurons were characterized. Five classes of biologically relevant odorants, discriminated by independent receptor mechanisms, were tested including sex steroids (pre-ovulatory pheromone), prostaglandins (post-ovulatory pheromone), bile acids (putative aggregation pheromone), nucleotides (possibly feeding), and amino acids (feeding). The majority of projection neurons (51 of 56) responded to more than one odorant class with excitation or suppression. Nearly all neurons (47 of 56) responded to least one sex pheromone with excitation or suppression, suggestive of population coding. Eight pheromone-sensitive neurons were excited by at least one additional odorant class, suggesting information from more than one receptor is being integrated in the olfactory bulb. Five neurons responded to only one pheromonal mixture and had no response to any other mixtures, suggesting labeled-line coding. Twenty-three neurons responded distinctly to one odorant class and those specifically sensitive sex steroids were spatially segregated in the medial olfactory bulb. In each experiment, fundamental differences in olfactory bulb processing of pheromonal and non-pheromonal stimuli were identified.