A small-conductance Ca2+-dependent K+ current regulates dopamine neuron activity: a combined approach of dynamic current clamping and intracellular imaging of calcium signals

A piezoelectric micromachined ultrasound transducer combined with recording electrodes for acute brain preparations in vitro

BACKGROUND

Ultrasound stimulation is used to noninvasively stimulate the local and deep areas of the brain. However, the detailed cellular mechanisms of neural activation are still unclear because studies on micro-stimulation at the cellular level are lacking.

NEW METHOD

To modulate neural activity at the cellular level, we developed a piezoelectric micromachined ultrasound transducer (PMUT), having circular diaphragms for application on acute brain slice preparations. To monitor neural activities, additionally, we fabricated recording microelectrodes onto the same PMUT device for closed-loop application.

RESULTS

To examine the PMUT-driven cellular responses of a brain slice, intracellular calcium signals in individual cells were measured using two calcium indicators. We successfully observed the intracellular responses triggered by the ultrasound of our novel PMUT. In addition, we performed recordings of local field potentials in a brain slice, demonstrating its usefulness as a simultaneous recording interface.

COMPARISON WITH EXISTING METHOD(S)

Conventional ultrasound stimulators are open-loop systems that risk inducing excessive neural activity because of the absence of neural activity monitoring. In contrast, our PMUT is packaged in a single device with both stimulation and sensor interface for neuromodulation. Further, there are no published reports on in vitro microdevices that can be used for ultrasound stimulation in rodent cortical slices that are several hundred micrometers thick, which maintain the cortical laminar structure and intrinsic neural networks.

CONCLUSIONS

Our findings suggest that this novel PMUT device has the potential for being a powerful tool for in vitro brain slice applications and effective closed loop ultrasound stimulation.

Calcium-activated SK channels control firing regularity by modulating sodium channel availability in midbrain dopamine neurons

Dopamine neurons in the substantia nigra pars compacta and ventral tegmental area regulate behaviours such as reward-related learning, and motor control. Dysfunction of these neurons is implicated in Schizophrenia, addiction to drugs, and Parkinson’s disease. While some dopamine neurons fire single spikes at regular intervals, others fire irregular single spikes interspersed with bursts. Pharmacological inhibition of calcium-activated potassium (SK) channels increases the variability in their firing pattern, sometimes also increasing the number of spikes fired in bursts, indicating that SK channels play an important role in maintaining dopamine neuron firing regularity and burst firing. However, the exact mechanisms underlying these effects are still unclear. Here, we develop a biophysical model of a dopamine neuron incorporating ion channel stochasticity that enabled the analysis of availability of ion channels in multiple states during spiking. We find that decreased firing regularity is primarily due to a significant decrease in the AHP that in turn resulted in a reduction in the fraction of available voltage-gated sodium channels due to insufficient recovery from inactivation. Our model further predicts that inhibition of SK channels results in a depolarisation of action potential threshold along with an increase in its variability.

Transient Activation of GABAB Receptors Suppresses SK Channel Currents in Substantia Nigra Pars Compacta Dopaminergic Neurons

Dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) are richly innervated by GABAergic neurons. The postsynaptic effects of GABA on SNc DA neurons are mediated by a mixture of GABAA and GABAB receptors. Although activation of GABAA receptors inhibits spike generation, the consequences of GABAB receptor activation are less well characterized. To help fill this gap, perforated patch recordings were made from young adult mouse SNc DA neurons. Sustained stimulation of GABAB receptors hyperpolarized SNc DA neurons, as previously described. However, transient stimulation of GABAB receptors by optical uncaging of GABA did not; rather, it reduced the opening of small-conductance, calcium-activated K+ (SK) channels and increased the irregularity of spiking. This modulation was attributable to inhibition of adenylyl cyclase and protein kinase A. Thus, because suppression of SK channel activity increases the probability of burst spiking, transient co-activation of GABAA and GABAB receptors could promote a pause-burst pattern of spiking.

The mechanism of ethanol action on midbrain dopaminergic neuron firing: a dynamic-clamp study of the role of I(h) and GABAergic synaptic integration

Hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels are expressed in dopaminergic (DA) neurons of the ventral tegmental area (VTA) as well as in DA and GABAergic neurons of the substantia nigra (SN). The excitation of DA neurons induced by ethanol has been proposed to result from its enhancing HCN channel current, I(h). Using perforated patch-clamp recordings in rat midbrain slices, we isolated I(h) in these neurons by voltage clamp. We showed that ethanol reversibly increased the amplitude and accelerated the activation kinetics of I(h) and caused a depolarizing shift in its voltage dependence. Using dynamic-clamp conductance injection, we injected artificial I(h) and fluctuating GABAergic synaptic conductance inputs into neurons following block of intrinsic I(h). This demonstrated directly a major role of I(h) in promoting rebound spiking following phasic inhibition, which was enhanced as the kinetics and amplitude of I(h) were changed in the manner induced by ethanol. Similar effects of ethanol were observed on I(h) and firing rate in non-DA, putatively GABAergic interneurons, indicating that in addition to its direct effects on firing, ethanol will produce large changes in the inhibition and disinhibition (via GABAergic interneurons) converging on DA neurons. Thus the overall effects of ethanol on firing of DA cells of the VTA and SN in vivo, and hence on phasic dopamine release in the striatum, appear to be determined substantially by its action on I(h) in both DA cells and GABAergic interneurons.