Properties and maturation of adult-generated dopaminergic neurons in the mammalian olfactory bulb

The olfactory bulb (OB) of mammals contains a large population of dopaminergic interneurons (DA) within the glomerular layer (GL). Dopamine has been shown both in vivo and in vitro to modulate several aspects of olfactory information processing, but the functional properties of DA neurons have rarely been described due to the inability to recognize these cells in living preparations. We succeeded in this by using a transgenic mouse strain harboring an eGFP reporter construct under the promoter of tyrosine hydroxylase (TH), the rate-limiting enzyme for cathecolamine synthesis. As a result, we could identify dopaminergic neurons (TH-GFP cells) in living preparations and study the functional properties of such neurons in the OB, in both slices and dissociated cells. The most prominent feature of DA neurons was the autorhythmicity. In these cells we identified five main voltage-dependent conductances: the two having largest amplitude were a fast transient Na+ current and a delayed rectifier K+ current. In addition, we observed three smaller inward currents, sustained by Na+ ions (persistent type) and by Ca2+ ions (low- and high-voltage activated). Using pharmacological tools and ion substitution methods we showed that the pacemaking process is supported by the interplay of the persistent Na+ current and of a T-type Ca2+current. A significant fraction of the interneurons added in adulthood to the GL of the OB are dopaminergic. Typically, bulbar DA neurons are restricted to the GL, but we also detected the presence of TH-GFP+ cells in the mitral and external plexiform layers. We hypothesized that these could be adult-generated neurons committed to become DA but not yet entirely differentiated. Accordingly, TH-GFP+ cells outside the GL exhibit functional properties (appearance of pacemaker currents, synaptic connection with the olfactory nerve, intracellular chloride concentration, and other), marking a gradient of maturity toward the dopaminergic phenotype along the mitral-glomerular axis. Finally, we propose that the establishment of a synaptic contact with the olfactory nerve is the key event allowing these cells to complete their differentiation toward the DA phenotype and to reach their final destination.