Spontaneous and odour evoked activity in single avian olfactory bulb neurones.

Extracellular single unit recordings were made from various depths in the left olfactory bulb of ten anaesthetised, freely breathing adult hens (Gallus domesticus) using glass insulated tungsten microelectrodes. The 66 spontaneously active neurons recorded had widely variable firing rates (mean 4.9 spikes/s, range 0.1-32.4 spikes/s) and variable temporal firing patterns (regular, bursting and random discharge). Interspike interval histograms were constructed for each unit and tested for goodness of fit to theoretical distributions. The activity of 23 units fitted gamma distributions, six units fitted lognormal distributions, three units fitted Weibull distributions, one unit fitted an exponential distribution and 11 units had bimodal distributions. Responses of some units to odour stimuli (Clove oil, Geraniol, Limonene and Ammonia all at 10% vapour saturation) delivered directly to the olfactory epithelium were examined. Odour stimulation modified spontaneous activity in 29 of the 44 units tested, causing either inhibition (56%) or excitation (44%) of firing. Of the 35 units stimulated with more than one odour, 15 responded to multiple stimuli, showing excitation or inhibition only or both inhibition and excitation, depending on the odour applied. Ammonia most commonly elicited a response (70% of applications) and all the odours were capable of inducing both inhibition and excitation of spontaneous firing. Examination of transverse sections of adult hen olfactory bulb in the region corresponding to the recording sites allowed unit recording depth to be related to cellular layers. There was some evidence of different neurone properties in each bulb layer. The spontaneous activity and odour responses observed resemble those seen in other vertebrate groups and the firing rates observed are between those previously reported for mammals and reptiles. This study provides detailed physiological evidence for a well-developed functional olfactory system in an avian species.