Fast cholinergic efferent inhibition in guinea pig outer hair cells

Apamin Improves Prefrontal Nicotinic Impairment in Mouse Model of Alzheimer's Disease

Disruption of attention is an early and disabling symptom of Alzheimer's disease (AD). The underlying cellular mechanisms are poorly understood and treatment options for patients are limited. These early attention deficits are evident in the TgCRND8 mouse, a well-established murine model of AD that recapitulates several features of the disease. Here, we report severe impairment of the nicotinic receptor-mediated excitation of prefrontal attentional circuitry in TgCRND8 mice relative to wild-type littermate controls. We demonstrate that this impairment can be remedied by apamin, a bee venom neurotoxin peptide that acts as a selective antagonist to the SK family of calcium-sensitive potassium channels. We probe this seeming upregulation of calcium-sensitive inhibition and find that the attenuated nicotinic firing rates in TgCRND8 attention circuits are mediated neither by greater cellular calcium signals nor by elevated SK channel expression. Instead, we find that TgCRND8 mice show enhanced functional coupling of nicotinic calcium signals to inhibition. This SK-mediated inhibition exerts a powerful negative feedback on nicotinic excitation, dampening attention-relevant signaling in the TgCRND8 brain. These mechanistic findings identify a new cellular target involved in the modulation of attention and a novel therapeutic target for early attention deficits in AD.

Existence of nicotinic receptors in a subset of type I vestibular hair cells of guinea pigs

In mammals, vestibular hair cells (VHCs) are classified as type I and II according to morphological criteria. Acetylcholine (ACh) is identified as the primary efferent neurotransmitter. To date, cholinergic activities have been reported in mammalian type II VHCs, but similar activities in type I VHCs have not been pursued presumably because the body of type I VHCs were suggested to be totally surrounded by afferent nerve calyces. A few reports showed that part of type I VHCs were incompletely surrounded by calyces and received contact from the efferent nerve endings in the mammals studied. The possibility of the expression of cholinergic receptors, their subunit composition, and their function in mammals' type I VHCs are still unclear. In this study, nicotinic responses were investigated by the whole-cell patch clamp technique in isolated type I VHCs of guinea pigs. Of the cells, 7.3% were sensitive to cholinergic agonists and showed an excitatory current at -40 mV which was not sensitive to nifedipine, iberiotoxin (IBTX), and apamin. The main carriers of this current were Na⁺ and K⁺. The rank order of activation potency was nicotine > 1,1-dimethyl-4-phenyl-piperazinium (DMPP) > ACh. These nicotinic ACh receptors (nAChRs) were not blocked by strychnine and α-bungarotoxin (α-BTX), but sensitive to d-tubocurarine (dTC) and mecamylamine (Mec). The findings provide physiological evidence that some subtypes of nAChRs may be located in a subset of type I VHCs, which were different from α9α10 nAChRs.

Two distinct channels mediated by m2mAChR and α9nAChR co-exist in type II vestibular hair cells of guinea pig

Acetylcholine (ACh) is the principal vestibular efferent neurotransmitter among mammalians. Pharmacologic studies prove that ACh activates a small conductance Ca2+-activated K+ channels (KCa) current (SK2), mediated by α9-containing nicotinic ACh receptor (α9nAChR) in mammalian type II vestibular hair cells (VHCs II). However, our studies demonstrate that the m2 muscarinic ACh receptor (m2mAChR) mediates a big conductance KCa current (BK) in VHCs II. To better elucidate the correlation between these two distinct channels in VHCs II of guinea pig, this study was designed to verify whether these two channels and their corresponding AChR subtypes co-exist in the same VHCs II by whole-cell patch clamp recordings. We found that m2mAChR sensitive BK currents were activated in VHCs II isolated by collagenase IA, while α9nAChR sensitive SK2 currents were activated in VHCs II isolated by trypsin. Interestingly, after exposing the patched cells isolated by trypsin to collagenase IA for 3 min, the α9nAChR sensitive SK2 current was abolished, while m2mAChR-sensitive BK current was activated. Therefore, our findings provide evidence that the two distinct channels and their corresponding AChR subtypes may co-exist in the same VHCs II, and the alternative presence of these two ACh receptors-sensitive currents depended on isolating preparation with different enzymes.

M2 muscarinic ACh receptors sensitive BK channels mediate cholinergic inhibition of type II vestibular hair cells

There are two types of hair cells in the sensory epithelium of vestibular end organ. Type II vestibular hair cell (VHC II) is innervated by the efferent nerve endings, which employ a cholinergic inhibition mediated by SK channels through the activation of α9-containing nAChR. Our previous studies demonstrated that a BK-type cholinergic inhibition was present in guinea pig VHCs II, which may be mediated by an unknown mAChR. In this study, BK channel activities triggered by ACh were studied to determine the mAChR subtype and function. We found the BK channel was insensitive to α9-containing nAChR antagonists and m1, m3, m4 muscarinic antagonists, but potently inhibited by the m2 muscarinic antagonist. Muscarinic agonists could mimic the effect of ACh and be blocked by m2 antagonist. cAMP analog activated the BK current and adenyl cyclase (AC) inhibitor inhibited the ACh response. Inhibitor of Giα subunit failed to affect the BK current, but inhibitor of Giα and Giβγ subunits showed a potent inhibition to these currents. Our findings provide the physiological evidence that mAChRs may locate in guinea pig VHCs II, and m2 mAChRs may play a dominant role in BK-type cholinergic inhibition. The activation of m2 mAChRs may stimulate Giβγ-mediated excitation of AC/cAMP activities and lead to the phosphorylation of Ca(2+) channels, resulting in the influx of Ca(2+) and opening of the BK channel.

Responses of pigeon vestibular hair cells to cholinergic agonists and antagonists

Acetylcholine (ACh) is the major neurotransmitter released from vestibular efferent terminals onto hair cells and afferents. Previous studies indicate that the two classes of acetylcholine receptors, nicotinic (nAChRs) and muscarinic receptors (mAChRs), are expressed by vestibular hair cells (VHCs). To identify if both classes of receptors are present in VHCs, whole cell, voltage-clamp- and current-clamp-patch recordings were performed on isolated pigeon vestibular type I and type II HCs during the application of the cholinergic agonists, acetylcholine and carbachol, and the cholinergic antagonists, D-tubocurarine and atropine. By applying in different combinations, these compounds were used to selectively activate either nAChRs or mAChRs. The effects of nAChR and mAChR activation on HC currents and zero electrode current potential (V(z)) were monitored. It was found that presumed mAChR activation decreased both inward and outward currents in both type I and type II HCs, resulting in either a depolarization or hyperpolarization. Conversely, nAChR activation mainly increased both inward and outward currents in type II HCs, resulting in a hyperpolarization of their V(z). nAChR activation also increased outward currents in type I HCs resulting in either a depolarization or hyperpolarization of their V(z). The decrease of inward and outward currents and the depolarization of the V(z) in type I pigeon HCs by activation of mAChRs represents a new finding. Ion channel candidates in pigeon vestibular HCs that might underlie the modulation of the macroscopic ionic currents and V(z) by different AChR activation are discussed.

Functional characterization of alpha9-containing cholinergic nicotinic receptors in the rat adrenal medulla: implication in stress-induced functional plasticity

An increase in circulating adrenal catecholamine levels constitutes one of the mechanisms whereby organisms cope with stress. Accordingly, stimulus-secretion coupling within the stressed adrenal medullary tissue undergoes persistent remodeling. In particular, cholinergic synaptic neurotransmission between splanchnic nerve terminals and chromaffin cells is upregulated in stressed rats. Since synaptic transmission is mainly supported by activation of postsynaptic neuronal acetylcholine nicotinic receptors (nAChRs), we focused our study on the role of alpha9-containing nAChRs, which have been recently described in chromaffin cells. Taking advantage of their specific blockade by the alpha-conotoxin RgIA (alpha-RgIA), we unveil novel functional roles for these receptors in the stimulus-secretion coupling of the medulla. First, we show that in rat acute adrenal slices, alpha9-containing nAChRs codistribute with synaptophysin and significantly contribute to EPSCs. Second, we show that these receptors are involved in the tonic inhibitory control exerted by cholinergic activity on gap junctional coupling between chromaffin cells, as evidenced by an increased Lucifer yellow diffusion within the medulla in alpha-RgIA-treated slices. Third, we unexpectedly found that alpha9-containing nAChRs dominantly (>70%) contribute to acetylcholine-induced current in cold-stressed rats, whereas alpha3 nAChRs are the main contributing channels in unstressed animals. Consistently, expression levels of alpha9 nAChR transcript and protein are overexpressed in cold-stressed rats. As a functional relevance, we propose that upregulation of alpha9-containing nAChR channels and ensuing dominant contribution in cholinergic signaling may be one of the mechanisms whereby adrenal medullary tissue appropriately adapts to increased splanchnic nerve electrical discharges occurring in stressful situations.

Spontaneous ryanodine-receptor-dependent Ca2+-activated K+ currents and hyperpolarizations in rat medial preoptic neurons

The aim of the present study was to clarify the identity of slow spontaneous currents, the underlying mechanism and possible role for impulse generation in neurons of the rat medial preoptic nucleus (MPN). Acutely dissociated neurons were studied with the perforated patch-clamp technique. Spontaneous outward currents, at a frequency of approximately 0.5 Hz and with a decay time constant of approximately 200 ms, were frequently detected in neurons when voltage-clamped between approximately -70 and -30 mV. The dependence on extracellular K(+) concentration was consistent with K(+) as the main charge carrier. We concluded that the main characteristics were similar to those of spontaneous miniature outward currents (SMOCs), previously reported mainly for muscle fibers and peripheral nerve. From the dependence on voltage and from a pharmacological analysis, we concluded that the currents were carried through small-conductance Ca(2+)-activated (SK) channels, of the SK3 subtype. From experiments with ryanodine, xestospongin C, and caffeine, we concluded that the spontaneous currents were triggered by Ca(2+) release from intracellular stores via ryanodine receptor channels. An apparent voltage dependence was explained by masking of the spontaneous currents as a consequence of steady SK-channel activation at membrane potentials > -30 mV. Under current-clamp conditions, corresponding transient hyperpolarizations occasionally exceeded 10 mV in amplitude and reduced the frequency of spontaneous impulses. In conclusion, MPN neurons display spontaneous hyperpolarizations triggered by Ca(2+) release via ryanodine receptors and SK3-channel activation. Thus such events may affect impulse firing of MPN neurons.

The coupling of acetylcholine-induced BK channel and calcium channel in guinea pig saccular type II vestibular hair cells

Molecular biological studies and electrophysiological data have demonstrated that acetylcholine (ACh) is the principal cochlear and vestibular efferent neurotransmitter among mammalians. However, the functional roles of ACh in type II vestibular hair cells (VHCs II) among mammalians are still unclear, with the exception of the well-known alpha9-containing nicotinic ACh receptor (alpha9-containing nAChR)-activated small conductance, calcium-dependent potassium current (SK) in cochlear hair cells and frog saccular hair cells. The activation of SK current was necessary for the calcium influx through the alpha9-containing nAChR. Recently, we have demonstrated that ACh-induced big conductance, calcium-dependent potassium current (BK) was present in VHCs II of the vestibular end-organ of guinea pig. In this study, the nature of calcium influx for the activation of ACh-induced BK current in saccular VHCs II of guinea pig was investigated. Following extracellular perfusion of ACh, saccular VHCs II displayed a sustained outward current, which was sensitive to iberiotoxin (IBTX). High concentration of apamin failed to inhibit the current amplitude of ACh-induced outward current. Intracellular application of Cs(+) completely abolished the current evoked by ACh. ACh-induced current was potently inhibited by nifedipine, nimodipine, Cd(2+) and Ni(2+), respectively. The inhibition potency of these four calcium channel antagonists was nimodipine>nifedipine>cadmium>nickel. The L-type Ca(2+) channels agonist, (-)-Bay-K 8644 mimicked the effect of ACh and activated an IBTX-sensitive current. In addition, partial VHCs II displayed a biphasic waveform. In conclusion, the present data showed that in the guinea pig saccular VHCs II, ACh-induced BK channel was coupled with the calcium channel, but not the receptor. The perfusion of ACh will drive the opening of calcium channels; the influx of calcium ions will then activate the BK current.