Abstract
The mammalian olfactory bulb (OB) has a vast population of dopamine (DA) neurons, whose function is to increase odor discrimination through mostly inhibitory synaptic mechanisms. However, it is not well understood whether there is more than one neuronal type of OB DA neuron, how these neurons respond to different stimuli, and the ionic mechanisms behind those responses. In this study, we used a transgenic rat line (hTH-GFP) to identify fluorescent OB DA neurons for recording via whole-cell electrophysiology. These neurons were grouped based on their localization in the glomerular layer (“Top” vs. “Bottom”) with these largest and smallest neurons grouped by neuronal area (“Large” vs. “Small,” in μm2). We found that some membrane properties could be distinguished based on a neuron’s area, but not by its glomerular localization. All OB DA neurons produced a single action potential when receiving a sufficiently depolarizing stimulus, while some could also spike multiple times when receiving weaker stimuli, an activity that was more likely in Large than Small neurons. This single spiking activity is likely driven by the Na+ current, which showed a sensitivity to inactivation by depolarization and a relatively long time constant for the removal of inactivation. These recordings showed that Small neurons were more sensitive to inactivation of Na+ current at membrane potentials of −70 and −60 mV than Large neurons. The hyperpolarization-activated H-current (identified by voltage sags) was more pronounced in Small than Large DA neurons across hyperpolarized membrane potentials. Lastly, to mimic a more physiological stimulus, these neurons received ramp stimuli of various durations and current amplitudes. When stimulated with weaker/shallow ramps, the neurons needed less current to begin and end firing and they produced more action potentials at a slower frequency. These spiking properties were further analyzed between the four groups of neurons, and these analyses support the difference in spiking induced with current step stimuli. Thus, there may be more than one type of OB DA neuron, and these neurons’ activities may support a possible role of being high-pass filters in the OB by allowing the transmission of stronger odor signals while inhibiting weaker ones.
Highlights
Olfaction is central to the perception of chemical environments and is a necessary sensory system for the survival of most animals
Neurons expressed in layers deep to the glomerular layer (GL) are likely the neonatal and adult-born DA neurons that are migrating from the subventricular zone and rostral migratory stream to their final destination within the GL (Betarbet et al, 1996; Baker et al, 2001; Pignatelli et al, 2009)
Some areas in the GL have DA neurons that are distributed around their respective glomeruli’s circumferences (Figures 1B,C). We determined whether these neurons express differences that may account for them being more than one olfactory bulb (OB) DA neuron subtype
Summary
Olfaction is central to the perception of chemical environments and is a necessary sensory system for the survival of most animals. Chemical odors are first transduced by the OSNs in the olfactory epithelium. OSNs form glutamatergic axodendritic synapses (Berkowicz et al, 1994; Ennis et al, 1996) with interneurons of the GL and the main output neurons of the OB, mitral and tufted cells (M/TCs) (Pinching and Powell, 1971; Bardoni et al, 1996a,b; Kosaka et al, 1997; Keller et al, 1998). As the odor signal is being transmitted to the M/TCs, the JGCs modify the signal by the release of neurotransmitters such as glutamate, GABA, and DA
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