Perforated patch clamp recordings reveal new facets of dopaminergic modulation of striatal neurons

Dopaminergic neurons in the substantia nigra projecting to the basal ganglia nucleus of the striatum control movement initiation and acceleration. This effect is thought to be mediated by altering the cell excitability of the two main types of long-projection neurons in the striatum, which differ for their expression of either D1 or D2 dopamine receptors. In previous studies, the effect of dopamine has been investigated using reduced preparations (i.e. brain slices) and by analyzing the biophysical properties of striatal medium spiny neurons using whole-cell patch clamp recordings and by mimicking dopamine release through exogenous applications. Whole-cell patch clamp recordings disrupt the physiological composition of the intracellular milieu, as the solution of the patch pipette dialyzes the intracellular cytoplasm. In addition, the exogenous application of dopamine may not recapitulate the physiological time course of dopamine release in vivo. As a result, there are conflicting results on how D1 dopamine receptor activation alters cell excitability in striatal medium spiny neurons. Here, Lahiri and Bevan combine perforated-patch recordings from D1 medium spiny neurons (MSNs) with optogenetic stimulation of dopamine release from nigro-striatal afferents to the dorsolateral striatum. They show that dopamine release increases the firing rate of D1-MSNs elicited by long domatic current injections, which mimic up-states. This effect persists for more than 10 min and is mediated by PKA activation. To mimic both up and down states, the authors apply 250 ms current steps once a second for 41 seconds, a protocol that they applied every 5 minutes for 3-4 trials. Based on the fact that the latency to action potential firing decreases and the firing frequency increases during optogenetic stimulation, they conclude that dopamine promotes the transitions from down to up states. Through the use of a wide range of pharmacological assays, they show that dopamine reduces the fast and medium after-hyperpolarization, consistent with an effect on slowly inactivating A-type potassium channels and calcium activated potassium channels. Together, this extensive array of heroic experiments shed light on previously unknown molecular mechanisms  through which the firing output of MSNs responds to changing levels of extracellular dopamine. Although the optogenetic stimulation of nigro-striatal afferents used by the authors may not capture aspects of asynchrony in dopamine release from nigro-striatal afferents, this is the closest we have got to understand the molecular machinery regulating the complex functional properties of striatal neurons.

Ian Tschang and Sam Barron

Reference
Lahiri AK, Bevan MD (2020). Dopaminergic transmission rapidly and persistently enhances excitability of D1 receptor-expressing striatal projection neurons. Neuron (xx), xxx–xx.

Example of three patch clamp configurations