Dopaminergic mechanisms were Anatyzed in an isolated preparation of the turtle olfactory bulb. Field potentials were evoked by antidromic or orthodromic stimulation, and the effects determined of pharmacological manipulations of the bathing medium. In the presence of dopamine or a dopamine agonist, apomorphine, there was a reduction of amplitude and delay of onset of the component of the field potentials due to granule cell responses; fluphenazine, a dopamine antagonist, had generally opposite effects. Using paired volleys, it was found that the suppression of the response to the second test volley was reduced in the presence of dopamine or apomorphine, but enhanced in the presence of fluphenazine. The most likely explanation of these results is that exogenous dopamine depresses the response of mitral cells, which in turn decreases the dendrodendritic synaptic excitation of granule cells and reduces their inhibitory feedback onto mitral cells. This suggests that the dopaminergic cells in the bulb may suppress mitral cells by modulating excitable mechanisms in the mitral dendritic membrane, or modulating long-lasting synaptic potentials.
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