Neurons establish morphological polarity by specifying two different compartments: the axon and the somatodendritic domain. This polarity is also functional, because in most neurons dendrites receive the majority of inputs while axons generate output signals via neurotransmitter release. However, many exceptions to this dogma occur in the olfactory bulb (OB), where most GABAergic interneurons are anaxonic and can only release neurotransmitters from their dendrites. OB glutamatergic cells, however, have classic morphological polarity but release neurotransmitters from both axonal and somatodendritic domains. Dendritic release is, therefore, a common feature of OB neurons.
A subset of GABAergic interneurons in the OB also release dopamine. These dopaminergic (DA) cells comprise two groups – axonic and anaxonic – depending on the presence or absence of an axon. Here, we provide structural and functional evidence showing that, unlike their anaxonic counterparts, axon-bearing DA neurons rarely if ever release GABA from their dendrites. We injected a Cre-dependent AAV in embryonic VGAT-Cre mice to obtain sparse cell morphology (GFP) plus structural evidence for putative neurotransmitter release sites (synaptophysin-mRuby), finding dendritic mRuby puncta almost exclusively in anaxonic cells. We then obtained electrophysiological recordings in acute slices from DAT-tdT mice, using an auto-evoked inhibition (AEI) protocol to detect dendritic GABA release. All anaxonic neurons displayed an AEI response, while almost all axonic DA cells did not. Our results suggest that axon-bearing DA neurons are the only OB cell type to not effect dendritic neurotransmitter release, placing a key spatial constraint on their ability to shape olfactory sensory processing.