Fulfenamic acid sensitive, Ca(2+)-dependent inward current induced by nicotinic acetylcholine receptors in dopamine neurons

Nonlinear relationship between CAN current and C a 2 + influx underpins synergistic action of muscarinic and NMDA receptors on bursts induction in midbrain dopaminergic neurons

Bursting of midbrain dopamine (DA) neurons is believed to represent an important reward signal that instructs and reinforces goal-directed behaviors. In DA neurons, many afferents, including cholinergic and glutamatergic inputs, induce bursting, and it is suggested that a synergy exists between these afferents in bursting induction. However, the underlying mechanisms of the role and the synergy of muscarinic receptors (mAChRs) and NMDA receptors (NMDARs) in bursting induction remain unclear. Present work bestowed analysis using a mathematical model of DA neurons to demonstrate the underlying mechanisms. Activation of mAChRs, leading to rapid translocation of TRPC channels to cell surface, recruited C a 2 + -activated nonspecific (CAN) current ( I CAN ), meanwhile NMDARs excitation triggered C a 2 + influx, which induced the positive feedback loop of C a 2 + and I CAN , respectively, yielded a robust ramping depolarization with a superimposed high-frequency spiking. In some DA cells, neither NMDARs nor mAChRs induced positive feedback loop unless they were activated simultaneously to induce bursting. Our experimental results verified those theoretical findings. These together unveil the underlying mechanisms of the role and synergy of mAChRs and NMDARs in bursting induction emerge from the nonlinear relationship between C a 2 + influx and I CAN . Given the diverse and complex nature of neural circuitry and the DA neuron heterogeneity, our work provides new insights to understand specific afferents, the synergy between those afferents, and the differences in intrinsic excitability to be integrated by the bursting to accurately characterize the dopamine signals in the valances of reward and reinforcement, and a broad spectrum of neuropsychiatric disorders.

Cell-Specific Cholinergic Modulation of Excitability of Layer 5B Principal Neurons in Mouse Auditory Cortex

The neuromodulator acetylcholine (ACh) is crucial for several cognitive functions, such as perception, attention, and learning and memory. Whereas, in most cases, the cellular circuits or the specific neurons via which ACh exerts its cognitive effects remain unknown, it is known that auditory cortex (AC) neurons projecting from layer 5B (L5B) to the inferior colliculus, corticocollicular neurons, are required for cholinergic-mediated relearning of sound localization after occlusion of one ear. Therefore, elucidation of the effects of ACh on the excitability of corticocollicular neurons will bridge the cell-specific and cognitive properties of ACh. Because AC L5B contains another class of neurons that project to the contralateral cortex, corticocallosal neurons, to identify the cell-specific mechanisms that enable corticocollicular neurons to participate in sound localization relearning, we investigated the effects of ACh release on both L5B corticocallosal and corticocollicular neurons. Using in vitro electrophysiology and optogenetics in mouse brain slices, we found that ACh generated nicotinic ACh receptor (nAChR)-mediated depolarizing potentials and muscarinic ACh receptor (mAChR)-mediated hyperpolarizing potentials in AC L5B corticocallosal neurons. In corticocollicular neurons, ACh release also generated nAChR-mediated depolarizing potentials. However, in contrast to the mAChR-mediated hyperpolarizing potentials in corticocallosal neurons, ACh generated prolonged mAChR-mediated depolarizing potentials in corticocollicular neurons. These prolonged depolarizing potentials generated persistent firing in corticocollicular neurons, whereas corticocallosal neurons lacking mAChR-mediated depolarizing potentials did not show persistent firing. We propose that ACh-mediated persistent firing in corticocollicular neurons may represent a critical mechanism required for learning-induced plasticity in AC.

SIGNIFICANCE STATEMENT

Acetylcholine (ACh) is crucial for cognitive functions. Whereas in most cases the cellular circuits or the specific neurons via which ACh exerts its cognitive effects remain unknown, it is known that auditory cortex (AC) corticocollicular neurons projecting from layer 5B to the inferior colliculus are required for cholinergic-mediated relearning of sound localization after occlusion of one ear. Therefore, elucidation of the effects of ACh on the excitability of corticocollicular neurons will bridge the cell-specific and cognitive properties of ACh. Our results suggest that cell-specific ACh-mediated persistent firing in corticocollicular neurons may represent a critical mechanism required for learning-induced plasticity in AC. Moreover, our results provide synaptic mechanisms via which ACh may mediate its effects on AC receptive fields.

Dopaminergic and cholinergic modulation of striatal tyrosine hydroxylase interneurons

The recent electrophysiological characterization of TH-expressing GABAergic interneurons (THINs) in the neostriatum revealed an unexpected degree of diversity of interneurons in this brain area (Ibáñez-Sandoval et al., 2010, Unal et al., 2011, 2015). Despite being relatively few in number, THINs may play a significant role in transmitting and distributing extra- and intrastriatal neuromodulatory signals in the striatal circuitry. Here we investigated the dopaminergic and cholinergic regulation of THINs in vitro. We found that the dominant effect of dopamine was a dramatic enhancement of the ability of THINs to generate long-lasting depolarizing plateau potentials (PPs). Interestingly, the same effect could also be elicited by amphetamine-induced release of endogenous dopamine suggesting that THINs may exhibit similar responses to changes in extracellular dopamine concentration in vivo. The enhancement of PPs in THINs is perhaps the most pronounced effect of dopamine on the intrinsic excitability of neostriatal neurons described to date. Further, we demonstrate that all subtypes of THINSs tested also express nicotinic cholinergic receptors. All THIS responded, albeit differentially, with depolarization, PPs and spiking to brief application of nicotinic agonists. Powerful modulation of the nonlinear integrative properties of THINs by dopamine and the direct depolarization of these neurons by acetylcholine may play important roles in mediating the effects of these neuromodulators in the neostriatum with potentially important implications for understanding the mechanisms of neuropsychiatric disorders affecting the basal ganglia.

Acetylcholine excites neocortical pyramidal neurons via nicotinic receptors

The neuromodulator acetylcholine (ACh) shapes neocortical function during sensory perception, motor control, arousal, attention, learning, and memory. Here we investigate the mechanisms by which ACh affects neocortical pyramidal neurons in adult mice. Stimulation of cholinergic axons activated muscarinic and nicotinic ACh receptors on pyramidal neurons in all cortical layers and in multiple cortical areas. Nicotinic receptor activation evoked short-latency, depolarizing postsynaptic potentials (PSPs) in many pyramidal neurons. Nicotinic receptor-mediated PSPs promoted spiking of pyramidal neurons. The duration of the increase in spiking was membrane potential dependent, with nicotinic receptor activation triggering persistent spiking lasting many seconds in neurons close to threshold. Persistent spiking was blocked by intracellular BAPTA, indicating that nicotinic ACh receptor activation evoked persistent spiking via a long-lasting calcium-activated depolarizing current. We compared nicotinic PSPs in primary motor cortex (M1), prefrontal cortex (PFC), and visual cortex. The laminar pattern of nicotinic excitation was not uniform but was broadly similar across areas, with stronger modulation in deep than superficial layers. Superimposed on this broad pattern were local differences, with nicotinic PSPs being particularly large and common in layer 5 of M1 but not layer 5 of PFC or primary visual cortex (V1). Hence, in addition to modulating the excitability of pyramidal neurons in all layers via muscarinic receptors, synaptically released ACh preferentially increases the activity of deep-layer neocortical pyramidal neurons via nicotinic receptors, thereby adding laminar selectivity to the widespread enhancement of excitability mediated by muscarinic ACh receptors.

Anatomical and electrophysiological changes in striatal TH interneurons after loss of the nigrostriatal dopaminergic pathway

Using transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of the tyrosine hydroxylase (TH) promoter, we have previously shown that there are approximately 3,000 striatal EGFP-TH interneurons per hemisphere in mice. Here, we report that striatal TH-EGFP interneurons exhibit a small, transient but significant increase in number after unilateral destruction of the nigrostriatal dopaminergic pathway. The increase in cell number is accompanied by electrophysiological and morphological changes. The intrinsic electrophysiological properties of EGFP-TH interneurons ipsilateral to 6-OHDA lesion were similar to those originally reported in intact mice except for a significant reduction in the duration of a characteristic depolarization induced plateau potential. There was a significant change in the distribution of the four previously described electrophysiologically distinct subtypes of striatal TH interneurons. There was a concomitant increase in the frequency of both spontaneous excitatory and inhibitory post-synaptic currents, while their amplitudes did not change. Nigrostriatal lesions did not affect somatic size or dendritic length or branching, but resulted in an increase in the density of proximal dendritic spines and spine-like appendages in EGFP-TH interneurons. The changes indicate that electrophysiology properties and morphology of striatal EGFP-TH interneurons depend on endogenous levels of dopamine arising from the nigrostriatal pathway. Furthermore, these changes may serve to help compensate for the changes in activity of spiny projection neurons that occur following loss of the nigrostriatal innervation in experimental or in early idiopathic Parkinson's disease by increasing feedforward GABAergic inhibition exerted by these interneurons.

Pharmacological comparison of novel synthetic fenamate analogues with econazole and 2-APB on the inhibition of TRPM2 channels

BACKGROUND AND PURPOSE

Fenamate analogues, econazole and 2-aminoethoxydiphenyl borate (2-APB) are inhibitors of transient receptor potential melastatin 2 (TRPM2) channels and are used as research tools. However, these compounds have different chemical structures and therapeutic applications. Here we have investigated the pharmacological profile of TRPM2 channels by application of newly synthesized fenamate analogues and the existing channel blockers.

EXPERIMENTAL APPROACH

Human TRPM2 channels in tetracycline-regulated pcDNA4/TO vectors were transfected into HEK293 T-REx cells and the expression was induced by tetracycline. Whole cell currents were recorded by patch-clamp techniques. Ca(2+) influx or release was monitored by fluorometry.

KEY RESULTS

Flufenamic acid (FFA), mefenamic acid (MFA) and niflumic acid (NFA) concentration-dependently inhibited TRPM2 current with potency order FFA > MFA = NFA. Modification of the 2-phenylamino ring by substitution of the trifluoromethyl group in FFA with -CH(3), -F, -CF(3), -OCH(3), -OCH(2)CH(3), -COOH, and -NO(2) at various positions, reduced channel blocking potency. The conservative substitution of 3-CF(3) in FFA by -CH(3) (3-MFA), however, gave the most potent fenamate analogue with an IC(50) of 76 µM, comparable to that of FFA, but unlike FFA, had no effect on Ca(2+) release. 3-MFA and FFA inhibited the channel intracellularly. Econazole and 2-APB showed non-selectivity by altering cytosolic Ca(2+) movement. Econazole also evoked a non-selective current.

CONCLUSION AND IMPLICATIONS

The fenamate analogue 3-MFA was more selective than other TRPM2 channel blockers. FFA, 2-APB and econazole should be used with caution as TRPM2 channel blockers, as these compounds can interfere with intracellular Ca(2+) movement.

Effect of non-steroidal anti-inflammatory drugs and new fenamate analogues on TRPC4 and TRPC5 channels

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used anti-inflammatory therapeutic agents, among which the fenamate analogues play important roles in regulating intracellular Ca²⁺ transient and ion channels. However, the effect of NSAIDs on TRPC4 and TRPC5 is still unknown. To understand the structure-activity of fenamate analogues on TRPC channels, we have synthesized a series of fenamate analogues and investigated their effects on TRPC4 and TRPC5 channels. Human TRPC4 and TRPC5 cDNAs in tetracycline-regulated vectors were transfected into HEK293 T-REx cells. The whole cell current and Ca²⁺ movement were recorded by patch clamp and calcium imaging, respectively. Flufenamic acid (FFA), mefenamic acid (MFA), niflumic acid (NFA) and diclofenac sodium (DFS) showed inhibition on TRPC4 and TRPC5 channels in a concentration-dependent manner. The potency was FFA>MFA>NFA>DFS. Modification of 2-phenylamino ring by substitution of the trifluoromethyl group in FFA with F, CH₃, OCH₃, OCH₂CH₃, COOH, and NO₂ led to the changes in their channel blocking activity. However, 2-(2'-methoxy-5'-methylphenyl)aminobenzoic acid stimulated TRPC4 and TRPC5 channels. Selective COX1-3 inhibitors (aspirin, celecoxib, acetaminophen, and indomethacin) had no effect on the channels. Longer perfusion (> 5 min) with FFA (100 μM) and MFA (100 μM) caused a potentiation of TRPC4 and TRPC5 currents after their initial blocking effects that appeared to be partially mediated by the mitochondrial Ca²⁺ release. Our results suggest that fenamate analogues are direct modulators of TRPC4 and TRPC5 channels. The substitution pattern and conformation of the 2-phenylamino ring could alter their blocking activity, which is important for understanding fenamate pharmacology and new drug development targeting the TRPC channels.

A neural correlate of predicted and actual reward-value information in monkey pedunculopontine tegmental and dorsal raphe nucleus during saccade tasks

Dopamine, acetylcholine, and serotonin, the main modulators of the central nervous system, have been proposed to play important roles in the execution of movement, control of several forms of attentional behavior, and reinforcement learning. While the response pattern of midbrain dopaminergic neurons and its specific role in reinforcement learning have been revealed, the role of the other neuromodulators remains rather elusive. Here, we review our recent studies using extracellular recording from neurons in the pedunculopontine tegmental nucleus, where many cholinergic neurons exist, and the dorsal raphe nucleus, where many serotonergic neurons exist, while monkeys performed eye movement tasks to obtain different reward values. The firing patterns of these neurons are often tonic throughout the task period, while dopaminergic neurons exhibited a phasic activity pattern to the task event. The different modulation patterns, together with the activity of dopaminergic neurons, reveal dynamic information processing between these different neuromodulator systems.

Calcium-activated non-selective cation currents are involved in generation of tonic and bursting activity in dopamine neurons of the substantia nigra pars compacta

Nigral dopamine neurons are transiently activated by high frequency glutamatergic inputs relaying reward-predicting sensory information. The tonic firing pattern of dopamine cells responds to these inputs with a transient burst of spikes that requires NMDA receptors. Here, we show that NMDA receptor activation further excites the cell by recruiting a calcium-activated non-selective cation current (ICAN) capable of generating a plateau potential. Burst firing in vitro is eliminated after blockade of ICAN with flufenamic acid, 9-phenanthrol, or intracellular BAPTA. ICAN is likely to be mediated by a transient receptor potential (TRP) channel, and RT-PCR was used to confirm expression of TRPM2 and TRPM4mRNA in substantia nigra pars compacta.We propose that ICAN is selectively activated during burst firing to boost NMDA currents and allow plateau potentials. This boost mechanism may render DA cells vulnerable to excitotoxicity.

Basal ganglia control of substantia nigra dopaminergic neurons

Although substantia nigra dopaminergic neurons are spontaneously active both in vivo and in vitro, this activity does not depend on afferent input as these neurons express an endogenous calcium-dependent oscillatory mechanism sufficient to drive action potential generation. However, afferents to these neurons, a large proportion of them GABAergic and arising from other nuclei in the basal ganglia, play a crucial role in modulating the activity of dopaminergic neurons. In the absence of afferent activity or when in brain slices, dopaminergic neurons fire in a very regular, pacemaker-like mode. Phasic activity in GABAergic, glutamatergic, and cholinergic inputs modulates the pacemaker activity into two other modes. The most common is a random firing pattern in which interspike intervals assume a Poisson-like distribution, and a less common pattern, often in response to a conditioned stimulus or a reward in which the neurons fire bursts of 2-8 spikes time-locked to the stimulus. Typically in vivo, all three firing patterns are observed, intermixed, in single nigrostriatal neurons varying over time. Although the precise mechanism(s) underlying the burst are currently the focus of intensive study, it is obvious that bursting must be triggered by afferent inputs. Most of the afferents to substantia nigra pars compacta dopaminergic neurons comprise monosynaptic inputs from GABAergic projection neurons in the ipsilateral neostriatum, the globus pallidus, and the substantia nigra pars reticulata. A smaller fraction of the basal ganglia inputs, something less than 30%, are glutamatergic and arise principally from the ipsilateral subthalamic nucleus and pedunculopontine nucleus. The pedunculopontine nucleus also sends a cholinergic input to nigral dopaminergic neurons. The GABAergic pars reticulata projection neurons also receive inputs from all of these sources, in some cases relaying them disynaptically to the dopaminergic neurons, thereby playing a particularly significant role in setting and/or modulating the firing pattern of the nigrostriatal neurons.

Flufenamic acid affects multiple currents and causes intracellular Ca2+ release in Aplysia bag cell neurons

Flufenamic acid (FFA) is a nonsteroidal antiinflammatory agent, commonly used to block nonselective cation channels. We previously reported that FFA potentiated, rather than inhibited, a cation current in Aplysia bag cell neurons. Prompted by this paradoxical result, the present study examined the effects of FFA on membrane currents and intracellular Ca2+ in cultured bag cell neurons. Under whole cell voltage clamp, FFA evoked either outward (I out) or inward (I in) currents. I out had a rapid onset, was inhibited by the K+ channel blocker, tetraethylammonium, and was associated with both an increase in membrane conductance and a negative shift in the whole cell current reversal potential. I in developed more slowly, was inhibited by the cation channel blocker, Gd3+, and was concomitant with both an increased conductance and positive shift in reversal potential. FFA also enhanced the use-dependent inactivation and caused a positive-shift in the activation curve of the voltage-dependent Ca2+ current. Furthermore, as measured by ratiometric imaging, FFA produced a rise in intracellular Ca2+ that persisted in the absence of extracellular Ca2+ and was reduced by depleting either the endoplasmic reticulum and/or mitochondrial stores. Ca2+ appeared to be involved in the activation of I in, as strong intracellular Ca2+ buffering effectively eliminated I in but did not alter I out. Finally, the effects of FFA were likely not due to block of cyclooxygenase given that the general cyclooxygenase inhibitor, indomethacin, failed to evoke either current. That FFA influences a number of neuronal properties needs to be taken into consideration when employing it as a cation channel antagonist.

A calcium-activated nonselective cation conductance underlies the plateau potential in rat substantia nigra GABAergic neurons

Plateau potentials can be elicited in nigral GABAergic neurons by injection of 500 ms depolarizing current pulses from hyperpolarized holding potentials in whole-cell recordings in vitro. In approximately one-third of these neurons, plateau potentials were observed under control conditions and could be elicited in the remaining neurons after blocking potassium conductances. Application of the L-type calcium channel agonist Bay K 8644 or activation of NMDA receptors enhanced plateau potentials observed under control conditions and caused a plateau to be elicited in neurons not exhibiting it previously. The plateau potential was abolished in calcium-free buffer, as well as by nickel or cadmium. The L-type calcium channel blockers nimodipine and nifedipine abolished the plateau potential observed under control conditions but did not affect plateaus unmasked by tetraethylammonium. Plateau potentials observed under control conditions as well as those observed in the presence of Bay K 8644, NMDA, or tetraethylammonium were abolished in low-sodium buffer and by the calcium-activated nonselective cation conductance blocker flufenamic acid. These data suggest that nigral plateau potentials are mediated by a calcium-activated nonselective cation conductance (I(CAN)) that is activated by calcium entry predominantly through L-type calcium channels. In many nigral neurons, I(CAN) is masked by tetraethylammonium-sensitive potassium conductances, but plateaus can be evoked after increasing calcium conductances. The I(CAN)-mediated plateau potential in nigral GABAergic neurons likely affects the way these neurons integrate input and may represent a mechanism contributing to the rhythmic firing of these neurons seen in pathological conditions such as Parkinson's disease.

Reward prediction error computation in the pedunculopontine tegmental nucleus neurons

In this article, we address the role of neuronal activity in the pathways of the brainstem-midbrain circuit in reward and the basis for believing that this circuit provides advantages over previous reinforcement learning theory. Several lines of evidence support the reward-based learning theory proposing that midbrain dopamine (DA) neurons send a teaching signal (the reward prediction error signal) to control synaptic plasticity of the projection area. However, the underlying mechanism of where and how the reward prediction error signal is computed still remains unclear. Since the pedunculopontine tegmental nucleus (PPTN) in the brainstem is one of the strongest excitatory input sources to DA neurons, we hypothesized that the PPTN may play an important role in activating DA neurons and reinforcement learning by relaying necessary signals for reward prediction error computation to DA neurons. To investigate the involvement of the PPTN neurons in computation of reward prediction error, we used a visually guided saccade task (VGST) during recording of neuronal activity in monkeys. Here, we predict that PPTN neurons may relay the excitatory component of tonic reward prediction and phasic primary reward signals, and derive a new computational theory of the reward prediction error in DA neurons.

Enhancement of excitatory postsynaptic potentials by preceding application of acetylcholine in mesencephalic dopamine neurons

Previously, we reported that Ca(2+) influx through nicotinic acetylcholine (ACh) receptors (nAChRs) activates a fulfenamic acid (FFA)-sensitive inward current, presumably a Ca(2+)-activated nonselective cation current (I(CAN)), in mesencephalic dopamine (DA) neurons. This current exhibited a negative slope conductance in the voltage range between -80 and -40mV and its activation led to a dramatic change in the responses to a transient application of glutamate, from single spikes to burst discharges. In this study, to examine the effect of activation of the FFA-sensitive current on EPSPs, we applied ACh (1mM) by transient air pressure shortly before electrical stimulation to evoke EPSPs in DA neurons. Application of ACh enhanced the amplitude of EPSPs when it preceded the electrical stimulation by less than 2 s, but not when the interval was longer than 3 s. In addition, this enhancement was critically dependent on intracellular Ca(2+) and the membrane potentials of the postsynaptic cell. Furthermore, the enhancing effect of ACh on EPSPs was sensitive to FFA and phenytoin. These results suggest that Ca(2+) influx caused by cholinergic inputs enhances EPSPs via activation of the FFA- and phenytoin-sensitive current.

Ca2+-dependent inward current induced by nicotinic receptor activation depends on Ca2+/calmodulin–CaMKII pathway in dopamine neurons

It is well known that midbrain dopamine (DA) neurons receive massive projection from cholinergic neurons in the brainstem. In our preceding report, we showed that Ca(2+)-influx through nicotinic acetylcholine (ACh) receptors in the DA neurons subsequently activated an inward current that was sensitive to fulfenamic acid (FFA) and phenytoin, presumably a Ca(2+)-activated non-selective cation current. The FFA-sensitive current exhibited a negative slope conductance and predominantly enhanced the depolarizing responses of DA neurons. In this study, we showed that the inward FFA-sensitive current was eliminated by antagonists of Ca(2+)/calmodulin (Ca(2+)/CaM), N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide hydrochloride (W-7; 1 microM), trifluoperazine (TFP; 1.5 microM) and calmidazolium (100 nM). Application of W-7 and TFP reduced the ACh-induced inward current and the current component suppressed by these drugs exhibited negative slope conductance, as well as the FFA-sensitive current. Further, intracellular application of KN-93, an antagonist of Ca(2+)/CaM-dependent protein kinase II (CaMKII), but not KN-92 eliminated the FFA-sensitive current. All these results suggest that Ca(2+)/CaM-CaMKII pathway is involved in an activation of the FFA-sensitive current.