Spontaneous and neurally evoked release of labelled noradrenaline from rabbit olfactory bulbs in vivo.

1. Rabbit olfactory bulbs were labelled with either [(3)H]noradrenaline (NA) and [(14)C]urea, or [(14)C]NA and [(3)H]inulin. The labelled substances released were collected by a modified cortical cup technique and estimated by liquid scintillation spectrometry.2. In many experiments only total radioactivity originating from labelled NA was measured. In such cases the term NA radioactivity is used.3. The spontaneous release of both NA radioactivity and marker radioactivity followed a multiphasic course. After 140 min the rate of efflux of NA radioactivity was significantly slower than that of the labelled marker.4. Stimulation of one medial olfactory tract, one lateral olfactory tract or the surface of one bulb resulted in a selective increase in release of NA radioactivity. The size of the increase was dependent on the intensity, frequency and duration of stimulation.5. There was a characteristic delay in the increased release of NA radioactivity following electrical stimulation. Acute lesioning of the olfactory tracts caudal to the stimulating electrode not only abolished this delay, but also resulted in a larger increase in release following stimulation.6. The radioactivity attributable to unchanged NA was initially 91%, but decreased to 66% after 4(1/2) hr of sample collection. Stimulation did not affect significantly the relative amounts of NA and metabolites released.7. Replacement of calcium in the Krebs solution by magnesium did not affect measurably the spontaneous release of NA radioactivity. However, after 1 hr; stimulation failed to increase the efflux of NA radioactivity.8. It is suggested that the results provide further evidence for a role for NA in synaptic transmission in the olfactory bulbs.