Effects of the bradycardic agent ZD 7288 on membrane voltage and pacemaker current in sheep cardiac Purkinje fibres

Intrahippocampal infusion of the Ih blocker ZD7288 slows evoked theta rhythm and produces anxiolytic-like effects in the elevated plus maze

Hippocampal theta rhythm has been associated with a number of behavioral processes, including learning and memory, spatial behavior, sensorimotor integration and affective responses. Suppression of hippocampal theta frequency has been shown to be a reliable neurophysiological signature of anxiolytic drug action in tests using known anxiolytic drugs (i.e., correlational evidence), but only one study to date (Yeung et al. (2012) Neuropharmacology 62:155-160) has shown that a drug with no known effect on either hippocampal theta or anxiety can in fact separately suppress hippocampal theta and anxiety in behavioral tests (i.e., prima facie evidence). Here, we attempt a further critical test of the hippocampal theta model by performing intrahippocampal administrations of the Ih blocker ZD7288, which is known to disrupt theta frequency subthreshold oscillations and resonance at the membrane level but is not known to have anxiolytic action. Intrahippocampal microinfusions of ZD7288 at high (15 µg), but not low (1 µg) doses slowed brainstem-evoked hippocampal theta responses in the urethane anesthetized rat, and more importantly, promoted anxiolytic action in freely behaving rats in the elevated plus maze. Taken together with our previous demonstration, these data provide converging, prima facie evidence of the validity of the theta suppression model.

The h-current in the substantia Nigra pars compacta neurons: a re-examination

The properties of the hyperpolarization-activated cation current (I_h) were investigated in rat substantia nigra - pars compacta (SNc) principal neurons using patch-clamp recordings in thin slices. A reliable identification of single dopaminergic neurons was made possible by the use of a transgenic line of mice expressing eGFP under the tyrosine hydroxylase promoter. The effects of temperature and different protocols on the I_h kinetics showed that, at 37°C and minimizing the disturbance of the intracellular milieu with perforated patch, this current actually activates at potentials more positive than what is generally indicated, with a half-activation potential of −77.05 mV and with a significant level of opening already at rest, thereby substantially contributing to the control of membrane potential, and ultimately playing a relevant function in the regulation of the cell excitability. The implications of the known influence of intracellular cAMP levels on I_h amplitude and kinetics were examined. The direct application of neurotransmitters (DA, 5-HT and noradrenaline) physiologically released onto SNc neurons and known to act on metabotropic receptors coupled to the cAMP pathway modify the I_h amplitude. Here, we show that direct activation of dopaminergic and of 5-HT receptors results in I_h inhibition of SNc DA cells, whereas noradrenaline has the opposite effect. Together, these data suggest that the modulation of I_h by endogenously released neurotransmitters acting on metabotropic receptors –mainly but not exclusively linked to the cAMP pathway- could contribute significantly to the control of SNc neuron excitability.

The pelvis-kidney junction contains HCN3, a hyperpolarization-activated cation channel that triggers ureter peristalsis

Peristaltic waves of the ureteric smooth muscles move urine down from the kidney, a process that is commonly defective in congenital diseases. To study the mechanisms that control the initiation and direction of contractions, we used video microscopy and optical mapping techniques and found that electrical and contractile waves began in a region where the renal pelvis joined the connective tissue core of the kidney. Separation of this pelvis-kidney junction from more distal urinary tract segments prevented downstream peristalsis, indicating that it housed the trigger for peristalsis. Moreover, cells in the pelvis-kidney junction were found to express isoform 3 of the hyperpolarization-activated cation on channel family known to be required for initiating electrical activity in the brain and heart. Immunocytochemical and real-time PCR analyses found that hyperpolarization-activated cation-3 is expressed at the pelvis-kidney junction where electrical excitation and contractile waves originate. Inhibition of this channel caused a loss of electrical activity at the pelvis-kidney junction and randomized the origin of electrical activity in the urinary tract, thus markedly perturbing contractions. Collectively, our study demonstrates that hyperpolarization-activated cation-3 channels play a fundamental role in coordinating proximal-to-distal peristalsis of the upper urinary tract. This provides insight into the genetic causes of common inherited urinary tract disorders such as reflux and obstruction.

Ih without Kir in adult rat retinal ganglion cells

Antisera directed against hyperpolarization-activated mixed-cation ("I(h)") and K(+) ("K(ir)") channels bind to some somata in the ganglion cell layer of rat and rabbit retina. Additionally, the termination of hyperpolarizing current injections can trigger spikes in some cat retinal ganglion cells, suggesting a rebound depolarization arising from activation of I(h). However, patch-clamp studies showed that rat ganglion cells lack inward rectification or present an inwardly rectifying K(+) current. We therefore tested whether hyperpolarization activates I(h) in dissociated, adult rat retinal ganglion cell somata. We report here that, although we found no inward rectification in some cells, and a K(ir)-like current in a few cells, hyperpolarization activated I(h) in roughly 75% of the cells we recorded from in voltage clamp. We show that this current is blocked by Cs(+) or ZD7288 and only slightly reduced by Ba(2+), that the current amplitude and reversal potential are sensitive to extracellular Na(+) and K(+), and that we found no evidence of K(ir) in cells presenting I(h). In current clamp, injecting hyperpolarizing current induced a slowly relaxing membrane hyperpolarization that rebounded to a few action potentials when the hyperpolarizing current was stopped; both the membrane potential relaxation and rebound spikes were blocked by ZD7288. These results provide the first measurement of I(h) in mammalian retinal ganglion cells and indicate that the ion channels of rat retinal ganglion cells may vary in ways not expected from previous voltage and current recordings.

Use-dependent blockade of cardiac pacemaker current (If) by cilobradine and zatebradine

The action of the bradycardiac agents, cilobradine (DK-AH269) and zatebradine (UL-FS49), on the cardiac pacemaker current (If) was investigated on short Purkinje fibres from sheep hearts, using the two-microelectrode voltage-clamp technique, and on isolated rabbit sino-atrial cells with the patch clamp technique. These drugs reduce dose dependently the amplitude of the If, without modifying either the voltage dependence or the kinetics of channel activation. When voltage-clamp pulse trains were applied, cilobradine induced a use-dependent blockade of If that was stronger and faster than that with zatebradine. Recovery from blockade during prolonged hyperpolarization was significantly faster with zatebradine. Presumably, both drugs block the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel by gaining access to a binding site within the open channel pore, and are removed from the blocking site by strong hyperpolarization with large inward If through the open channel. Cilobradine, compared to zatebradine blocks If more effectively and faster in both preparations. Consequently cilobradine strongly reduces the pacemaker diastolic depolarization rate and the cell's firing frequency.

Modulation of the hyperpolarisation-activated current, Ih, in rat facial motoneurones in vitro by ZD-7288

ZD-7288 [4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride] and Cs(+) have been used to distinguish the currents contributing to inward rectification in neonatal rat facial motoneurones (FMs). ZD-7288 (0.1-10 microM) inhibited a current that reversed at -43.7+/-3.7 mV in artificial cerebrospinal fluid (ACSF) containing 3 mM K(+) (n=9), and displayed the time and voltage dependence of the hyperpolarisation-activated current, I(h). Depolarisation-activated transient (I(K(A))) and sustained outward currents were unaffected by ZD-7288. The IC(50) for block of I(h) by ZD-7288 was around 0.2 microM. Onset of inhibition was slow and no recovery was seen after washing in ZD-7288-free ACSF for up to 4 h. In the presence of ZD-7288, Ba(2+) and Rb(+) blocked an inwardly rectifying potassium (K(+)) current, confirming both the presence of I(K(IR)) and its insensitivity to ZD-7288. Cs(+) rapidly and reversibly blocked both I(h) and I(K(IR)). Inhibition of I(h) by ZD-7288 showed no use dependence, internally applied ZD-7288 also blocked I(h), and tail current analysis indicated inhibition to be voltage-independent. In the presence of internal guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and after previous exposure to ZD-7288, 5-hydroxytryptamine (5-HT), but not noradrenaline, promoted a recovery of I(h) that was not observed if ZD-7288 was present throughout the recording period. This interaction between ZD-7288 and irreversible 5-HT-receptor activation may be related to the mechanism underlying ZD-7288-mediated block of these channels.

Inhibition of pacemaker current by the bradycardic agent ZD 7288 is lost use-dependently in sheep cardiac Purkinje fibres

The inhibition of the pacemaker current (if) in sheep cardiac Purkinje fibers by ZD 7288 [4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)pyrimidinium++ + chloride] is lost use-dependently. This disinhibition of if was investigated by using the two-microelectrode voltage-clamp technique. The pulse protocol consisted of a rest period (holding potential of about -50 mV, 1-10 micromol/l ZD 7288) followed by a train of test pulses (potential negative to -100 mV, stimulation frequency 0.05 Hz). At the beginning of the first test pulse there was an immediate reduction of if but inhibition was lost during continued stimulation. Activation of if is sigmoidal and the early delay in current activation was prolonged from 33 ms (no ZD 7288) to 424 ms (10 micromol/l ZD 7288). Therefore hardly any disinhibition occurred during short test pulses (0.5s). During longer test pulses (5 s, -120 mV, 10 micromol/l) disinhibition developed with a time constant of about 2 s. The inhibition of if by ZD 7288 was lost voltage-dependently. With 10 micro mol/l ZD 7288 the half-maximal disinhibition occurred at -92 mV and the slope factor of the disinhibition/voltage curve (Boltzmann relation) was 4.8 mV. The voltage-dependent disinhibition could be abolished largely by extracellular application of protease (0.5 mg/ml, 7 min). After prior disinhibition, reinhibition at the holding potential (about -50 mV) followed a bi-exponential time course indicating that inhibition may be produced by a fast (tau=0.7 min) and a slow component (tau=20-30 min). Increasing ZD 7288 concentration from 1 to 10 mu mol/l accelerated reinhibition, mainly by an increase of the amplitude (A) of the fast component. The ratio Afast/Aslow was 0.399 at 1 micromol/l and 2.65 at 10 micromol/l ZD 7288. The reinhibition of if was unchanged by shifting the holding potential from -50 mV to -20 mV. Trials to wash out the effects of 10 micromol/l ZD 7288 gave two results. The inhibition of if was slightly reversed after a wash-out of 1.5 h with drug-free solution. A second effect of the drug, the fast reinhibition, could be completely removed by wash-out. In summary if is inhibited by ZD 7288 at membrane potentials at which the virtual if gate is closed. Disinhibition occurs during long-lasting hyperpolarization but will hardly be operative in unclamped fibres under physiological conditions.