On the properties of identified dopaminergic neurons in the mouse substantia nigra and ventral tegmental area

Abstract

We studied the properties of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA) in mice expressing the enhanced green fluorescent protein (eGFP) under the control of the tyrosine hydroxylase promoter (TH-GFP). By using a practical map of cell positioning in distinct SNpc and VTA subregions in horizontal midbrain slices we saw that the spontaneous firing, membrane properties, cell body size and magnitude of the hyperpolarization-activated current (I h ) in TH-GFP-positive neurons (TH-GFP + ) vary significantly among subregions, following a mediolateral gradient. Block of I h with Zd7288 inhibited firing in the most lateral subregions, but had little effect in the intermediate/medial VTA. In addition, TH-GFP + cells were excited by Met 5 -Enkephalin. Extracellular recordings from a large neuron number showed that all TH-GFP + cells were inhibited by dopamine, suggesting that this is a reliable approach for identifying dopaminergic neurons in vitro. Simultaneous recordings from dopamine-sensitive and dopamine-insensitive neurons showed that dopamine-insensitive cells (putative nondopaminergic neurons) are unaffected by Zd7288 but inhibited by Met 5 -Enkephalin. Under patch-clamp, dopamine generated a quantitatively similar outward current in most TH-GFP + neurons, although medial VTA cells showed reduced dopamine sensitivity. Pargyline prolonged the dopamine current, whereas cocaine enhanced dopamine-mediated responses in both the SNpc and the VTA. Our work provides new insights into the variability in mouse midbrain dopaminergic neurons along the medial-lateral axis and points to the necessity of a combination of different electrophysiological and pharmacological approaches for reliably identifying these cells to distinguish them from non-dopaminergic neurons in the midbrain.

Reversal of dopamine‐mediated firing inhibition through activation of the dopamine transporter in substantia nigra pars compacta neurons

“…The spontaneous firing of SNc neurons in slices was detected with an MEA device as extracellular single‐unit spikes, as previously reported (Berretta et al ., ). The slice was exposed to 100 μM dopamine, and a total of 58 active neurons (six slices, from three animals) responded with inhibition of their firing rate (from 1.93 ± 0.20 to 0.24 ± 0.10 Hz) that reached a maximal effect within 30 s to 1 min, as expected from typical dopaminergic neurons, due to activation of their D 2 autoreceptors (Lacey et al ., ; Kim et al ., ; Krashia et al ., ). We thus focused our investigation on this presumed dopaminergic neuronal population.…”

“…Nigral dopaminergic neurons were identified as tightly packed cells located close to the medial terminal nucleus of the accessory optic tract. Moreover, at the beginning of each patch-clamp experimental session, the neurons were tested in current-clamp mode, in order to verify the presence of normal spontaneous firing (<5 Hz) and a depolarizing 'sag' in response to hyperpolarizing current steps, paralleled in voltage-clamp mode by the presence of a prominent I h (hyperpolarization-activated cation current) in response to hyperpolarizing voltage commands (Krashia et al, 2017).…”

“…In recent years, data have been put forward illustrating specific properties that differentiate these two subpopulations of dopaminergic neurons. However, there is a common consensus that, although to a variable extent, virtually all dopaminergic neurons of the ventral midbrain are inhibited by dopamine in response to stimulation of somato-dendritic D 2 receptors, also called D 2 autoreceptors, by interfering with G-protein-coupled inwardly rectifying potassium conductance (K ir 3.1, also known as GIRK) (Lacey et al, 1987;Lacey et al, 1988;Kim et al, 1995;Beckstead et al, 2004;Luscher and Slesinger, 2010;Krashia et al, 2017). Thus, through its somato-dendritic release, dopamine acts on D 2 receptors located on the dopaminergic neurons acting as a negative feedback system to modulate dopaminergic neurons' excitability (Beckstead et al, 2004;Ford, 2014;Evans et al, 2017).…”

Dopamine neuronal loss contributes to memory and reward dysfunction in a model of Alzheimer’s disease

“…Curiously, DA outflow to the NAc core (and DAT levels) was unchanged in Tg2576 mice, even though this region is heavily innervated by VTA DAergic neurons34. However, it should be noted that the NAc shell and core are innervated by anatomically distinct subpopulations of highly heterogeneous VTA neurons regarding their properties346263, with the majority of shell-projecting neurons located in the paranigral nucleus in the medial VTA and the majority of core-projecting neurons located in the parabrachial pigmented area of the lateral VTA3435, suggesting that medial VTA DAergic neurons may show more vulnerability in Tg2576 mice.…”

The Hyperpolarization-Activated Current Determines Synaptic Excitability, Calcium Activity and Specific Viability of Substantia Nigra Dopaminergic Neurons

“…The implications of Ih dysfunction in pathological states of the nervous system has prompted the quest for selective, subunit-specific modulators (Del Lungo et al, 2012; Novella Romanelli et al, 2016). In midbrain DAergic neurons, Ih has been the object of deep molecular and electrophysiological characterization (Mercuri et al, 1995; Seutin et al, 2001; Neuhoff et al, 2002; Lammel et al, 2008; Dufour et al, 2014; Krashia et al, 2017), but its implication in diseases of the DAergic system has remained largely unexplored. In the present work, we demonstrate that potentiation of synaptic excitability resulting from Ih inhibition leads to the amplification of somatic calcium responses (SCRs).…”