Action of serotonin on the hyperpolarization-activated cation current (Ih) in rat CA1 hippocampal neurons

Glucocorticoids Selectively Inhibit Hippocampal CA1 Pyramidal Neurons Activity Through HCN Channels

Glucocorticoids are known to influence hippocampal function, but their rapid non-genomic effects on specific neurons in the hippocampal trisynaptic circuit remain underexplored. This study investigated the immediate effects of glucocorticoids on CA1 and CA3 pyramidal neurons, and dentate gyrus (DG) granule neurons in rats using the patch-clamp technique. We found that a 5 min extracellular application of corticosterone significantly reduced action potential firing frequency in CA1 pyramidal neurons, while no effects were observed in CA3 or DG neurons. The corticosterone-induced inhibition in CA1 was blocked by the glucocorticoid receptor antagonist CORT125281, but remained unaffected by the mineralocorticoid receptor antagonist spironolactone. Notably, membrane-impermeable bovine serum albumin-conjugated dexamethasone mimicked corticosterone's effects on CA1 neurons, which exhibited prominent hyperpolarization-activated cyclic nucleotide-gated (HCN) channel currents. Pyramidal neurons in CA3 and granular neurons in the DG showed little HCN channel currents. Corticosterone enhanced HCN channel activity in CA1 neurons via glucocorticoid receptors, and the HCN channel inhibitor ZD7288 abolished corticosterone's suppressive effects on action potentials. These findings suggest that glucocorticoids selectively inhibit CA1 pyramidal neuron activity through HCN channels, providing new insight into the mechanisms of glucocorticoid action in hippocampal circuits.

Emergence of stochastic resonance in a two-compartment hippocampal pyramidal neuron model

In vitro studies have shown that hippocampal pyramidal neurons employ a mechanism similar to stochastic resonance (SR) to enhance the detection and transmission of weak stimuli generated at distal synapses. To support the experimental findings from the perspective of multicompartment model analysis, this paper aimed to elucidate the phenomenon of SR in a noisy two-compartment hippocampal pyramidal neuron model, which was a variant of the Pinsky-Rinzel neuron model with smooth activation functions and a hyperpolarization-activated cation current. With a bifurcation analysis of the model, we demonstrated the underlying dynamical structure responsible for the occurrence of SR. Furthermore, using a stochastically generated biphasic pulse train and broadband noise generated by the Orenstein-Uhlenbeck process as noise perturbation, both SR and suprathreshold SR were observed and quantified. Spectral analysis revealed that the distribution of spectral power under noise perturbations, in addition to inherent neurodynamics, is the main factor affecting SR behavior. The research results suggested that noise enhances the transmission of weak stimuli associated with elongated dendritic structures of hippocampal pyramidal neurons, thereby providing support for related laboratory findings.

Hippocampal cAMP regulates HCN channel function on two time scales with differential effects on animal behavior

[Figure: see text].

Ih from synapses to networks: HCN channel functions and modulation in neurons

Hyperpolarization-activated cyclic nucleotide gated (HCN) channels and the current they carry, I_h, are widely and diversely distributed in the central nervous system (CNS). The distribution of the four subunits of HCN channels is variable within the CNS, within brain regions, and often within subcellular compartments. The precise function of I_h can depend heavily on what other channels are co-expressed. In this review, we give an overview of HCN channel structure, distribution, and modulation by cyclic adenosine monophosphate (cAMP). We then discuss HCN channel and I_h functions, where we have parsed the roles into two main effects: a steady effect on maintaining the resting membrane potential at relatively depolarized values, and slow channel dynamics. Within this framework, we discuss I_h involvement in resonance, synaptic integration, transmitter release, plasticity, and point out a special case, where the effects of I_h on the membrane potential and its slow channel dynamics have dual roles in thalamic neurons.

Modulation of intrinsic excitability as a function of learning within the fear conditioning circuit

Experience-dependent neuronal plasticity is a fundamental substrate of learning and memory. Intrinsic excitability is a form of neuronal plasticity that can be altered by learning and indicates the pattern of neuronal responding to external stimuli (e.g. a learning or synaptic event). Associative fear conditioning is one form of learning that alters intrinsic excitability, reflecting an experience-dependent change in neuronal function. After fear conditioning, intrinsic excitability changes are evident in brain regions that are a critical part of the fear circuit, including the amygdala, hippocampus, retrosplenial cortex, and prefrontal cortex. Some of these changes are transient and/or reversed by extinction as well as learning-specific (i.e. they are not observed in neurons from control animals). This review will explore how intrinsic neuronal excitability changes within brain structures that are critical for fear learning, and it will also discuss evidence promoting intrinsic excitability as a vital mechanism of associative fear memories. This work has raised interesting questions regarding the role of fear learning in changes of intrinsic excitability within specific subpopulations of neurons, including those that express immediate early genes and thus demonstrate experience-dependent activity, as well as in neurons classified as having a specific firing type (e.g. burst-spiking vs. regular-spiking). These findings have interesting implications for how intrinsic excitability can serve as a neural substrate of learning and memory, and suggest that intrinsic plasticity within specific subpopulations of neurons may promote consolidation of the memory trace in a flexible and efficient manner.

Sleep Is for Forgetting

It is possible that one of the essential functions of sleep is to take out the garbage, as it were, erasing and "forgetting" information built up throughout the day that would clutter the synaptic network that defines us. It may also be that this cleanup function of sleep is a general principle of neuroscience, applicable to every creature with a nervous system.

Third Trimester Equivalent Alcohol Exposure Reduces Modulation of Glutamatergic Synaptic Transmission by 5-HT1A Receptors in the Rat Hippocampal CA3 Region

Fetal alcohol exposure has been associated with many neuropsychiatric disorders that have been linked to altered serotonin (5-hydroxytryptamine; 5-HT) signaling, including depression and anxiety. During the first 2 weeks of postnatal life in rodents (equivalent to the third trimester of human pregnancy) 5-HT neurons undergo significant functional maturation and their axons reach target regions in the forebrain (e.g., cortex and hippocampus). The objective of this study was to identify the effects of third trimester ethanol (EtOH) exposure on hippocampal 5-HT signaling. Using EtOH vapor inhalation chambers, we exposed rat pups to EtOH for 4 h/day from postnatal day (P) 2 to P12. The average serum EtOH concentration in the pups was 0.13 ± 0.04 g/dl (legal intoxication limit in humans = 0.08 g/dl). We used brain slices to assess the modulatory actions of 5-HT on field excitatory postsynaptic potentials in the hippocampal CA3 region at P13-P15. Application of the GABA_A/glycine receptor antagonist, picrotoxin, caused broadening of field excitatory postsynaptic potentials (fEPSPs), an effect that was reversed by application of 5-HT in slices from air exposed rats. However, this effect of 5-HT was absent in EtOH exposed animals. In slices from naïve animals, application of a 5-HT_1A receptor antagonist blocked the effect of 5-HT on the fEPSPs recorded in presence of picrotoxin, suggesting that third trimester ethanol exposure acts by inhibiting the function of these receptors. Studies indicate that 5-HT_1A receptors play a critical role in the development of hippocampal circuits. Therefore, inhibition of these receptors by third trimester ethanol exposure could contribute to the pathophysiology of fetal alcohol spectrum disorders.

Rescue of Cyclic AMP Mediated Long Term Potentiation Impairment in the Hippocampus of Mecp2 Knockout (Mecp2(-/y) ) Mice by Rolipram

Rett syndrome (RTT) patients experience learning difficulties and memory loss. Analogous deficits of hippocampal plasticity are reported in mouse models of RTT. To elucidate the underlying pathophysiology, we studied long term potentiation (LTP) at the CA3 to CA1 synapses in the hippocampus in acute brain slices from WT and Mecp2(-/y) mice, by either activating cAMP dependent pathway or using high frequency stimulation, by means of patch clamp. We have observed that, the NMDA channel current characteristics remain unchanged in the Mecp2(-/y) mice. The adenylyl cyclase (AC) agonist forskolin evoked a long lasting potentiation of evoked EPSCs in WT CA1 neurons, but only minimally enhanced the EPSCs in the Mecp2(-/y) mice. This weaker potentiation in Mecp2 (-/) (y) mice was ameliorated by application of phosphodiesterase 4 inhibitor rolipram. The hyperpolarization activated cyclic nucleotide gated channel current (I h) was potentiated to similar extent by forskolin in both phenotypes. Multiple tetanus induced cAMP-dependent plasticity was also impaired in the Mecp2 (-/) (y) mice, and was also partially rescued by rolipram. Western blot analysis of CA region of Mecp2 (-/) (y) mice hippocampus revealed more than twofold up-regulation of protein kinase A (PKA) regulatory subunits, while the expression of the catalytic subunit remained unchanged. We hypothesize that the overexpressed PKA regulatory subunits buffer cAMP and restrict the PKA mediated phosphorylation of target proteins necessary for LTP. Blocking the degradation of cAMP, thereby saturating the regulatory subunits alleviated this defect.

Effects of polarization induced by non-weak electric fields on the excitability of elongated neurons with active dendrites

An externally-applied electric field can polarize a neuron, especially a neuron with elongated dendrites, and thus modify its excitability. Here we use a computational model to examine, predict, and explain these effects. We use a two-compartment Pinsky-Rinzel model neuron polarized by an electric potential difference imposed between its compartments, and we apply an injected ramp current. We vary three model parameters: the magnitude of the applied potential difference, the extracellular potassium concentration, and the rate of current injection. A study of the Time-To-First-Spike (TTFS) as a function of polarization leads to the identification of three regions of polarization strength that have different effects. In the weak region, the TTFS increases linearly with polarization. In the intermediate region, the TTFS increases either sub- or super-linearly, depending on the current injection rate and the extracellular potassium concentration. In the strong region, the TTFS decreases. Our results in the weak and strong region are consistent with experimental observations, and in the intermediate region, we predict novel effects that depend on experimentally-accessible parameters. We find that active channels in the dendrite play a key role in these effects. Our qualitative results were found to be robust over a wide range of inter-compartment conductances and the ratio of somatic to dendritic membrane areas. In addition, we discuss preliminary results where synaptic inputs replace the ramp injection protocol. The insights and conclusions were found to extend from our polarized PR model to a polarized PR model with I h dendritic currents. Finally, we discuss the degree to which our results may be generalized.

Afterhyperpolarization (AHP) regulates the frequency and timing of action potentials in the mitral cells of the olfactory bulb: role of olfactory experience

Afterhyperpolarization (AHP) is a principal feedback mechanism in the control of the frequency and patterning of neuronal firing. In principal projection neurons of the olfactory bulb, the mitral cells (MCs), the AHP is produced by three separate components: classical potassium-mediated hyperpolarization, and the excitatory and inhibitory components, which are generated by the recurrent dendrodendritic synaptic transmission. Precise spike timing is involved in olfactory coding and learning, as well as in the appearance of population oscillatory activity. However, the contribution of the AHP and its components to these processes remains unknown. In this study, we demonstrate that the AHP is developed with the MC firing frequency and is dominated by the potassium component. We also show that recurrent synaptic transmission significantly modifies MC AHP and that the strength of the hyperpolarization produced by the AHP in the few milliseconds preceding the action potential (AP) emission determines MC firing frequency and AP timing. Moreover, we show that the AHP area is larger in younger animals, possibly owing to increased Ca²⁺ influx during MC firing. Finally, we show that olfactory experience selectively reduces the early component of the MC AHP (under 25 msec), thus producing a modification of the AP timing limited to the higher firing frequency. On the basis of these results, we propose that the AHP, and its susceptibility to be selectively modulated by the recurrent synaptic transmission and olfactory experience, participate in odor coding and learning by modifying the frequency and pattern of MC firing.

Structural basis for the mutual antagonism of cAMP and TRIP8b in regulating HCN channel function

cAMP signaling in the brain mediates several higher order neural processes. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels directly bind cAMP through their cytoplasmic cyclic nucleotide binding domain (CNBD), thus playing a unique role in brain function. Neuronal HCN channels are also regulated by tetratricopeptide repeat-containing Rab8b interacting protein (TRIP8b), an auxiliary subunit that antagonizes the effects of cAMP by interacting with the channel CNBD. To unravel the molecular mechanisms underlying the dual regulation of HCN channel activity by cAMP/TRIP8b, we determined the NMR solution structure of the HCN2 channel CNBD in the cAMP-free form and mapped on it the TRIP8b interaction site. We reconstruct here the full conformational changes induced by cAMP binding to the HCN channel CNBD. Our results show that TRIP8b does not compete with cAMP for the same binding region; rather, it exerts its inhibitory action through an allosteric mechanism, preventing the cAMP-induced conformational changes in the HCN channel CNBD.

Binding of the auxiliary subunit TRIP8b to HCN channels shifts the mode of action of cAMP

Hyperpolarization-activated cyclic nucleotide–regulated cation (HCN) channels generate the hyperpolarization-activated cation current Ih present in many neurons. These channels are directly regulated by the binding of cAMP, which both shifts the voltage dependence of HCN channel opening to more positive potentials and increases maximal Ih at extreme negative voltages where voltage gating is complete. Here we report that the HCN channel brain-specific auxiliary subunit TRIP8b produces opposing actions on these two effects of cAMP. In the first action, TRIP8b inhibits the effect of cAMP to shift voltage gating, decreasing both the sensitivity of the channel to cAMP (K_1/2) and the efficacy of cAMP (maximal voltage shift); conversely, cAMP binding inhibits these actions of TRIP8b. These mutually antagonistic actions are well described by a cyclic allosteric mechanism in which TRIP8b binding reduces the affinity of the channel for cAMP, with the affinity of the open state for cAMP being reduced to a greater extent than the cAMP affinity of the closed state. In a second apparently independent action, TRIP8b enhances the action of cAMP to increase maximal Ih. This latter effect cannot be explained by the cyclic allosteric model but results from a previously uncharacterized action of TRIP8b to reduce maximal current through the channel in the absence of cAMP. Because the binding of cAMP also antagonizes this second effect of TRIP8b, application of cAMP produces a larger increase in maximal Ih in the presence of TRIP8b than in its absence. These findings may provide a mechanistic explanation for the wide variability in the effects of modulatory transmitters on the voltage gating and maximal amplitude of Ih reported for different neurons in the brain.

HCN1 subunits contribute to the kinetics of I(h) in neonatal cortical plate neurons

The distribution of ion channels in neurons regulates neuronal activity and proper formation of neuronal networks during neuronal development. One of the channels is the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel constituting the molecular substrate of hyperpolarization-activated current (I(h)). Our previous study implied a role for the fastest activating subunit HCN1 in the generation of Ih in rat neonatal cortical plate neurons. To better understand the impact of HCN1 in early neocortical development, we here performed biochemical analysis and whole-cell recordings in neonatal cortical plate and juvenile layer 5 somatosensory neurons of HCN1(-/-) and control HCN1(+/+) mice. Western Blot analysis revealed that HCN1 protein expression in neonatal cortical plate tissue of HCN(+/+) mice amounted to only 3% of the HCN1 in young adult cortex and suggested that in HCN1(-/-) mice other isoforms (particularly HCN4) might be compensatory up-regulated. At the first day after birth, functional ablation of the HCN1 subunit did not affect the proportion of Ih expressing pyramidal cortical plate neurons. Although the contribution of individual subunit proteins remains open, the lack of HCN1 markedly slowed the current activation and deactivation in individual I(h) expressing neurons. However, it did not impair maximal amplitude/density, voltage dependence of activation, and cAMP sensitivity. In conclusion, our data imply that, although expression is relatively low, HCN1 contributes substantially to I(h) properties in individual cortical plate neurons. These properties are significantly changed in HCN1(-/-), either due to the lack of HCN1 itself or due to compensatory mechanisms.

Learning to learn - intrinsic plasticity as a metaplasticity mechanism for memory formation

"Use it or lose it" is a popular adage often associated with use-dependent enhancement of cognitive abilities. Much research has focused on understanding exactly how the brain changes as a function of experience. Such experience-dependent plasticity involves both structural and functional alterations that contribute to adaptive behaviors, such as learning and memory, as well as maladaptive behaviors, including anxiety disorders, phobias, and posttraumatic stress disorder. With the advancing age of our population, understanding how use-dependent plasticity changes across the lifespan may also help to promote healthy brain aging. A common misconception is that such experience-dependent plasticity (e.g., associative learning) is synonymous with synaptic plasticity. Other forms of plasticity also play a critical role in shaping adaptive changes within the nervous system, including intrinsic plasticity - a change in the intrinsic excitability of a neuron. Intrinsic plasticity can result from a change in the number, distribution or activity of various ion channels located throughout the neuron. Here, we review evidence that intrinsic plasticity is an important and evolutionarily conserved neural correlate of learning. Intrinsic plasticity acts as a metaplasticity mechanism by lowering the threshold for synaptic changes. Thus, learning-related intrinsic changes can facilitate future synaptic plasticity and learning. Such intrinsic changes can impact the allocation of a memory trace within a brain structure, and when compromised, can contribute to cognitive decline during the aging process. This unique role of intrinsic excitability can provide insight into how memories are formed and, more interestingly, how neurons that participate in a memory trace are selected. Most importantly, modulation of intrinsic excitability can allow for regulation of learning ability - this can prevent or provide treatment for cognitive decline not only in patients with clinical disorders but also in the aging population.

Shared cognitive and behavioral impairments in epilepsy and Alzheimer's disease and potential underlying mechanisms

Seizures in patients with Alzheimer's disease (AD) have been examined by many investigators over the last several decades, and there are diverse opinions about their potential relevance to AD pathophysiology. Some studies suggest that seizures appear to be a fairly uncommon co-morbidity, whereas other studies report a higher incidence of seizures in patients with AD. It was previously thought that seizures play a minor role in AD pathophysiology because of their low frequency, and also because they may only be noticed during late stages of AD, suggesting that seizures are likely to be a consequence of neurodegeneration rather than a contributing factor. However, clinical reports indicate that seizures can occur early in the emergence of AD symptoms, particularly in familial AD. In this case, seizures may be an integral part of the emerging pathophysiology. This view has been supported by evidence of recurrent spontaneous seizures in transgenic mouse models of AD in which familial AD is simulated. Additional data from transgenic animals suggest that there may be a much closer relationship between seizures and AD than previously considered. There is also evidence that seizures facilitate production of amyloid β (Aβ) and can cause impairments in cognition and behavior in both animals and humans. However, whether seizures play a role in the early stages of AD pathogenesis is still debated. Therefore, it is timely to review the similarities and differences between AD and epilepsy, as well as data suggesting that seizures may contribute to cognitive and behavioral dysfunction in AD. Here we focus on AD and temporal lobe epilepsy (TLE), a particular type of epilepsy that involves the temporal lobe, a region that influences behavior and is critical to memory. We also consider potential neurobiological mechanisms that support the view that the causes of seizures in TLE may be related to the causes of cognitive dysfunction in AD. We suggest that similar underlying mechanisms may exist for at least some of the aspects of AD that are also found in TLE.

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.

Serotonin regulates voltage-dependent currents in type I(e(A)) and I(i) interneurons of Hermissenda

Serotonin (5-HT) has both direct and modulatory actions on central neurons contributing to behavioral arousal and cellular-synaptic plasticity in diverse species. In Hermissenda, 5-HT produces changes in intrinsic excitability of different types of identified interneurons in the circumesophageal nervous system. Using whole cell patch-clamp techniques we have examined membrane conductance changes produced by 5-HT that contribute to intrinsic excitability in two identified classes of interneurons, types I(i) and I(eA). Whole cell currents were examined before and after 5-HT application to the isolated nervous system. A 4-aminopyridine-sensitive transient outward K(+) current [I(K(A))], a tetraethylammonium-sensitive delayed rectifier K(+) current [I(K(V))], an inward rectifier K(+) current [I(K(IR))], and a hyperpolarization-activated current (I(h)) were characterized. 5-HT decreased the amplitude of I(K(A)) and I(K(V)) in both type I(i) and I(eA) interneurons. However, differences in 5-HT's effects on the activation-inactivation kinetics were observed in different types of interneurons. 5-HT produced a depolarizing shift in the activation curve of I(K(V)) and a hyperpolarizing shift in the inactivation curve of I(K(A)) in type I(i) interneurons. In contrast, 5-HT produced a depolarizing shift in the activation curve and a hyperpolarizing shift in the inactivation curve of both I(K(V)) and I(K(A)) in type I(eA) interneurons. In addition, 5-HT decreased the amplitude of I(K(IR)) in type I(i) interneurons and increased the amplitude of I(h) in type I(eA) interneurons. These results indicate that 5-HT-dependent changes in I(K(A)), I(K(V)), I(K(IR)), and I(h) contribute to multiple mechanisms that synergistically support modulation of increased intrinsic excitability associated with different functional classes of identified type I interneurons.

The fast and slow ups and downs of HCN channel regulation

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (h channels) form the molecular basis for the hyperpolarization-activated current, I(h), and modulation of h channels contributes to changes in cellular properties critical for normal functions in the mammalian brain and heart. Numerous mechanisms underlie h channel modulation during both physiological and pathological conditions, leading to distinct changes in gating, kinetics, surface expression, channel conductance or subunit composition of h channels. Here we provide a focused review examining mechanisms of h channel regulation, with an emphasis on recent findings regarding interacting proteins such as TRIP8b. This review is intended to serve as a comprehensive resource for physiologists to provide potential molecular mechanisms underlying functionally important changes in I(h) in different biological models, as well as for molecular biologists to delineate the predicted h channel changes associated with complex regulatory mechanisms in both normal function and in disease states.

Nicotine blocks the hyperpolarization-activated current Ih and severely impairs the oscillatory behavior of oriens-lacunosum moleculare interneurons

In the brain, high cognitive functions are encoded by coherent network oscillations. Key players are inhibitory interneurons that, by releasing GABA into principal cells, pace targeted cells. Among these, oriens-lacunosum moleculare (O-LM) interneurons that provide a theta frequency patterned output to distal dendrites of pyramidal cells are endowed with HCN channels responsible for the slowly activating inwardly rectifying Ih current and their pacemaking activity. Here we show that, in transgenic mice expressing EGFP (enhanced green fluorescent protein) in a subset of stratum oriens somatostatin-containing interneurons that mostly comprise O-LM cells, nicotine, the active component of tobacco, reduced Ih and the oscillatory behavior of O-LM interneurons. In cells hyperpolarized at -90 mV, nicotine suppressed the theta resonance in the same way as ZD 7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride), a selective blocker of Ih. Nicotine blocked Ih in a concentration-dependent way with an EC50 of 62 nm. Similar effects were produced by epibatidine, a structural analog of nicotine. The effects of nicotine and epibatidine were independent on nicotinic ACh receptor (nAChR) activation because they persisted in the presence of nAChR antagonists. Furthermore, nicotine slowed down the interspike depolarizing slope and the firing rate, thus severely disrupting the oscillatory behavior of O-LM cells. Molecular modeling suggests that, similarly to ZD 7288, nicotine and epibatidine directly bind to the inner pore of the HCN channels. It is therefore likely that nicotine severely influences rhythmogenesis and high cognitive functions in smokers.

Differences in subthreshold resonance of hippocampal pyramidal cells and interneurons: the role of h-current and passive membrane characteristics

The intrinsic properties of distinct types of neuron play important roles in cortical network dynamics. One crucial determinant of neuronal behaviour is the cell's response to rhythmic subthreshold input, characterised by the input impedance, which can be determined by measuring the amplitude and phase of the membrane potential response to sinusoidal currents as a function of input frequency. In this study, we determined the impedance profiles of anatomically identified neurons in the CA1 region of the rat hippocampus (pyramidal cells as well as interneurons located in the stratum oriens, including OLM cells, fast-spiking perisomatic region-targeting interneurons and cells with axonal arbour in strata oriens and radiatum). The basic features of the impedance profiles, as well as the passive membrane characteristics and the properties of the sag in the voltage response to negative current steps, were cell-type specific. With the exception of fast-spiking interneurons, all cell types showed subthreshold resonance, albeit with distinct features. The HCN channel blocker ZD7288 (10 microM) eliminated the resonance and changed the shape of the impedance curves, indicating the involvement of the hyperpolarization-activated cation current I(h). Whole-cell voltage-clamp recordings uncovered differences in the voltage-dependent activation and kinetics of I(h) between different cell types. Biophysical modelling demonstrated that the cell-type specificity of the impedance profiles can be largely explained by the properties of I(h) in combination with the passive membrane characteristics. We conclude that differences in I(h) and passive membrane properties result in a cell-type-specific response to inputs at given frequencies, and may explain, at least in part, the differential involvement of distinct types of neuron in various network oscillations.

Understanding effects on excitability of simulated I (h) modulation in simple neuronal models

The hyperpolarization-activated, inward, mixed cation current, I (h), appears in a wide variety of cells in the nervous system, contributes to diverse neuronal properties, and is up-regulated by a number of important neurotransmitters. Up-regulation of I (h) is usually associated with an excitability-enhancing depolarization of resting membrane potential and an excitability-depressing shunting effect caused by a decrease in input resistance. In order to gain a better understanding of the interaction of these effects and their influence on excitability with I (h) modulation, we systematically analyze changes in neuronal properties associated with excitability during I (h) modulation in simplified, yet, biophysical neuron models based on a hippocampal pyramidal neuron. We simulate I (h) modulation by varying both its maximal conductance and its half-activation voltage, mimicking the effects of cAMP-linked neurotransmitters, through ranges of physiologically realistic parameter regimes. Of particular interest is the contribution of the different effects of I (h) up-regulation when membrane potentials are held at common levels and neuronal excitability is probed. Our modeling results suggest that, although holding potentials at common levels may compensate for changes in resting membrane potentials, this protocol may exaggerate the excitability-depressing influences of changes in input resistances with I (h) up-regulation.

TRIP8b splice variants form a family of auxiliary subunits that regulate gating and trafficking of HCN channels in the brain

Hyperpolarization-activated cyclic nucleotide-regulated (HCN) channels, which generate the I(h) current, mediate a number of important brain functions. The HCN1 isoform regulates dendritic integration in cortical pyramidal neurons and provides an inhibitory constraint on both working memory in prefrontal cortex and spatial learning and memory in the hippocampus. Altered expression of HCN1 following seizures may contribute to the development of temporal lobe epilepsy. Yet the regulatory networks and pathways governing HCN channel expression and function in the brain are largely unknown. Here, we report the presence of nine alternative N-terminal splice forms of the brain-specific cytoplasmic protein TRIP8b and demonstrate the differential effects of six isoforms to downregulate or upregulate HCN1 surface expression. Furthermore, we find that all TRIP8b isoforms inhibit channel opening by shifting activation to more negative potentials. TRIP8b thus functions as an auxiliary subunit that provides a mechanism for the dynamic regulation of HCN1 channel expression and function.

Raphé neurons stimulate respiratory circuit activity by multiple mechanisms via endogenously released serotonin and substance P

Brainstem serotonin (5-HT) neurons modulate activity of many neural circuits in the mammalian brain, but in many cases endogenous mechanisms have not been resolved. Here, we analyzed actions of raphé 5-HT neurons on respiratory network activity including at the level of the pre-Bötzinger complex (pre-BötC) in neonatal rat medullary slices in vitro, and in the more intact nervous system of juvenile rats in arterially perfused brainstem-spinal cord preparations in situ. At basal levels of activity, excitation of the respiratory network via simultaneous release of 5-HT and substance P (SP), acting at 5-HT(2A/2C), 5-HT(4), and/or neurokinin-1 receptors, was required to maintain inspiratory motor output in both the neonatal and juvenile systems. The midline raphé obscurus contained spontaneously active 5-HT neurons, some of which projected to the pre-BötC and hypoglossal motoneurons, colocalized 5-HT and SP, and received reciprocal excitatory connections from the pre-BötC. Experimentally augmenting raphé obscurus activity increased motor output by simultaneously exciting pre-BötC and motor neurons. Biophysical analyses in vitro demonstrated that 5-HT and SP modulated background cation conductances in pre-BötC and motor neurons, including a nonselective cation leak current that contributed to the resting potential, which explains the neuronal depolarization that augmented motor output. Furthermore, we found that 5-HT, but not SP, can transform the electrophysiological phenotype of some pre-BötC neurons to intrinsic bursters, providing 5-HT with an additional role in promoting rhythm generation. We conclude that raphé 5-HT neurons excite key circuit components required for generation of respiratory motor output.

Dendritic HCN2 channels constrain glutamate-driven excitability in reticular thalamic neurons

Hyperpolarization activated cyclic nucleotide (HCN) gated channels conduct a current, I(h); how I(h) influences excitability and spike firing depends primarily on channel distribution in subcellular compartments. For example, dendritic expression of HCN1 normalizes somatic voltage responses and spike output in hippocampal and cortical neurons. We reported previously that HCN2 is predominantly expressed in dendritic spines in reticular thalamic nucleus (RTN) neurons, but the functional impact of such nonsomatic HCN2 expression remains unknown. We examined the role of HCN2 expression in regulating RTN excitability and GABAergic output from RTN to thalamocortical relay neurons using wild-type and HCN2 knock-out mice. Pharmacological blockade of I(h) significantly increased spike firing in RTN neurons and large spontaneous IPSC frequency in relay neurons; conversely, pharmacological enhancement of HCN channel function decreased spontaneous IPSC frequency. HCN2 deletion abolished I(h) in RTN neurons and significantly decreased sensitivity to 8-bromo-cAMP and lamotrigine. Recapitulating the effects of I(h) block, HCN2 deletion increased both temporal summation of EPSPs in RTN neurons as well as GABAergic output to postsynaptic relay neurons. The enhanced excitability of RTN neurons after I(h) block required activation of ionotropic glutamate receptors; consistent with this was the colocalization of HCN2 and glutamate receptor 4 subunit immunoreactivities in dendritic spines of RTN neurons. The results indicate that, in mouse RTN neurons, HCN2 is the primary functional isoform underlying I(h) and expression of HCN2 constrains excitatory synaptic integration.

Neuronal models for sleep-wake regulation and synaptic reorganization in the sleeping hippocampus

In this article, we discuss mathematical models that address the control of sleep-wake behavior in the infant and adult rodent and a model that addresses changes in single-cell firing patterns in the hippocampus across wake and rapid eye movement (REM) sleep states. Each of the models describes the dynamics of experimentally identified neuronal components--either the firing activity of wake-and sleep-promoting neuronal populations or the spiking activity of hippocampal pyramidal neurons. Our discussion of each model illustrates how a mathematical model that describes the temporal dynamics of the modeled neuronal components can reveal specifics about proposed neuronal mechanisms that underlie sleep-wake regulation or sleep-specific firing patterns. For example, the dynamics of the models developed for sleep-wake regulation in the infant rodent lend insight into the involved brain-stem neuronal populations and the evolution of the network during maturation. The results of the model for sleep-wake regulation in the adult rodent suggest distinct properties of the involved neuronal populations and their interactions that account for long-lasting and brief waking bouts. The dynamics of the model for sleep-specific hippocampal neural activity proposes neural mechanisms to account for observed activity changes that can invoke synaptic reorganization associated with learning and memory consolidation.

Effects of serotonin on the intrinsic membrane properties of layer II medial entorhinal cortex neurons

Although serotonin (5-HT) is an important neuromodulator in the superficial layers of the medial entorhinal cortex (mEC), there is some disagreement concerning its influences upon the membrane properties of neurons within this region. We performed whole cell recordings of mEC Layer II projection neurons in rat brain slices in order to characterize the intrinsic influences of 5-HT. In current clamp, 5-HT evoked a biphasic response consisting of a moderately short latency and large amplitude hyperpolarization followed by a slowly developing, long lasting, and small amplitude depolarization. Correspondingly, in voltage clamp, 5-HT evoked a robust outward followed by a smaller inward shift of holding current. The outward current evoked by 5-HT showed a consistent current/voltage (I/V) relationship across cells with inward rectification, and demonstrating a reversal potential that was systematically dependent upon the extracellular concentration of K(+), suggesting that it was predominantly carried by potassium ions. However, the inward current showed a less consistent I/V relationship across different cells, suggesting multiple independent ionic mechanisms. The outward current was mediated through activation of 5-HT(1A) receptors via a G-protein dependent mechanism while inward currents were evoked in a 5-HT(1A)-independent fashion. A significant proportion of the inward current was blocked by the I(h) inhibitor ZD7288 and appeared to be due to 5-HT modulation of I(h) as 5-HT shifted the activation curve of I(h) in a depolarizing fashion. Serotonin is thus likely to influence, in a composite fashion, the information processing of Layer II neurons in the mEC and thus, the passage of neocortical information via the perforant pathway to the hippocampus.

5‐HT4 receptor activation induces long‐lasting EPSP‐spike potentiation in CA1 pyramidal neurons

Recent studies implicated involvement of the 5-hydroxytryptamine4 (5-HT4) receptor in cognitive and emotional processes. The highest 5-HT4 receptor densities in the brain are found in the limbic system including the hippocampus. Here we used the selective 5-HT4 receptor full agonist, N-pentyl-N'-aminoguanidine carbazimidamide (SDZ-216454) to characterize effects of 5-HT4 receptor activation in whole-cell and field recordings in the area CA1 in hippocampal slices prepared from 3 to 4- and 6 to 9-week-old rats, respectively. Extracellular recordings showed that transient 5-HT4 receptor activation by 10-20 min application of SDZ-216454 induces field excitatory postsynaptic potential (fEPSP)-population spike potentiation (ESP(5-HT4)), which persisted for as long as we held the recordings (> 2 h). ESP(5-HT4) displayed characteristics different from EPSP-spike potentiation that accompanies long-term potentiation; it developed without an associated increase in synaptic transmission, was independent on afferent input, activity of postsynaptic neurons and N-methyl-d-aspartate receptor activation; and was expressed in the presence of GABA receptor antagonists. ESP(5-HT4) was also induced by transient application of the natural neurotransmitter, 5-HT. The increase in the evoked population spike (PS) induced by SDZ-216454 was not prevented by blockers of hyperpolarization-activated cation current (Ih), Cs+ and ZD-7288, but was mimicked and occluded by 150 microm Ba2+. Whole-cell voltage-clamp recordings from pyramidal neurons demonstrated that SDZ-216454 application increases membrane resistance with a concomitant decrease in a Ba2+-sensitive inwardly rectifying K+ current and the Ba2+-insensitive K+ current underlying slow afterhyperpolarization (I(sAHP)). We conclude that 5-HT4 receptor activation may cause a long-lasting excitability increase in CA1 pyramidal neurons by inhibition of a Ba2+-sensitive inwardly rectifying K+ current.

Learning-induced reversal of the effect of noradrenalin on the postburst AHP

Pyramidal neurons in the piriform cortex from olfactory-discrimination-trained rats have reduced postburst afterhyperpolarization (AHP), for 3 days after learning, and are thus more excitable during this period. Such AHP reduction is caused by decreased conductance of one or more of the calcium-dependent potassium currents, I(AHP) and sI(AHP), that mediate the medium and slow AHPs. In this study, we examined which potassium current is reduced by learning and how the effect of noradrenalin (NE) on neuronal excitability is modified by such reduction. The small conductance (SK) channels inhibitor, apamin, that selectively blocks I(A)(HP), reduced the AHP in neurons from trained, naïve, and pseudotrained rats to a similar extent, thus maintaining the difference in AHP amplitude between neurons from trained rats and controls. In addition, the protein expression level of the SK1, SK2, and SK3 channels was also similar in all groups. NE, which was shown to enhance I(AHP) while suppressing (S)I(AHP), reduced the AHP in neurons from controls but enhanced the AHP in neurons from trained rats. Our data show that learning-induced enhancement of neuronal excitability is not the result of reduction in the I(AHP) current. Thus it is probably mediated by reduction in conductance of the other calcium-dependent potassium current, sI(AHP). Consequently, the effect of NE on neuronal excitability is reversed. We propose that the change in the effect of NE after learning may act to counterbalance learning-induced hyperexcitability and preserve the piriform cortex ability to subserve olfactory learning.

Input source and strength influences overall firing phase of model hippocampal CA1 pyramidal cells during theta: relevance to REM sleep reactivation and memory consolidation

In simulation studies using a realistic model CA1 pyramidal cell, we accounted for the shift in mean firing phase from theta cycle peaks to theta cycle troughs during rapid-eye movement (REM) sleep reactivation of hippocampal CA1 place cells over several days of growing familiarization with an environment (Brain Res 855:176-180). Changes in the theta drive phase and amplitude between proximal and distal dendritic regions of the cell modulated the theta phase of firing when stimuli were presented at proximal and distal dendritic locations. Stimuli at proximal dendritic sites (proximal to 100 microm from the soma) invoked firing with a significant phase preference at the depolarizing theta peaks, while distal stimuli (>290 microm from the soma) invoked firing at hyperpolarizing theta troughs. The input location-related phase preference depended on active dendritic conductances, a sufficient electrotonic separation between input sites and theta-induced subthreshold membrane potential oscillations in the cell. The simulation results predict that the shift in mean theta phase during REM sleep cellular reactivation could occur through potentiation of distal dendritic (temporo-ammonic) synapses and depotentiation of proximal dendritic (Schaffer collateral) synapses over the course of familiarization.

Dynamics of learning-induced cellular modifications in the cortex

This aim of this review is to describe the dynamics of learning-induced cellular modifications in the rat piriform (olfactory) cortex after olfactory discrimination learning and to describe their functional significance to long-term memory consolidation. The first change to occur is in the intrinsic properties of the neurons. One day after learning, pyramidal neurons show enhanced neuronal excitability. This enhancement results from reduction in calcium-dependent conductance that mediates the post burst after-hyperpolarization. Such enhanced excitability lasts for 3 days and is followed by a series of synaptic modifications. Several forms of long-term enhancement in synaptic connections between layer II pyramidal neurons in the piriform cortex accompany olfactory learning. Enhanced synaptic release is indicated by reduced paired-pulse facilitation. Post-synaptic enhancement of synaptic transmission is indicated by reduced rise time of post-synaptic potentials and formation of new synaptic connections is indicated by increased spine density along dendrites of these neurons. Such modifications last for up to 5 days. Thus, olfactory discrimination rule learning is accompanied by a series of cellular modifications which occur and then disappear at different times. These modifications overlap partially, allowing the maintenance of the cortical system in a 'learning mode' in which memories for specific odors can be acquired rapidly and efficiently.

Regulation of the hyperpolarization-activated cationic current Ih in mouse hippocampal pyramidal neurones by vitronectin, a component of extracellular matrix

Because the hyperpolarization-activated cation-selective current I(h) makes important contributions to neural excitability, we examined its long-term regulation by vitronectin, an extracellular matrix component commonly elevated at injury sites and detected immunochemically in activated microglia. Focusing on mouse hippocampal pyramidal neurones in organotypic slice cultures established at postnatal day 0 or 1 and examined after 3-4 days in vitro, we observed differences in the amplitude and activation rate of I(h) between neurones in naive and vitronectin-exposed slices (10 microg ml(-1) added to serum-free medium), and between neurones in slices derived from wild-type and vitronectin-deficient mice. The potassium inward rectifier I(K(ir)), activated at similar voltages to I(h), was not affected by vitronectin. In CA1, differences in I(h) amplitude primarily reflected changes in maximum conductance (G(max)): a 23.3% increase to 3.18 +/- 0.64 nS from 2.58 +/- 0.96 nS (P < 0.05) in vitronectin-exposed neurones, and a 17.9% decrease to 2.24 +/- 0.26 nS from 2.73 +/- 0.64 nS (P < 0.05) in neurones from vitronectin-deficient slices. The voltage of one-half maximum activation (V(1/2)) was not significantly affected by vitronectin exposure (-78.1 +/- 2.3 mV versus -80.0 +/- 4.9 mV in naive neurones; P > 0.05) or vitronectin deficiency (-83.8 +/- 3.1 mV versus -82.0 +/- 2.9 mV in wild-type neurones; P > 0.05). In CA3 neurones, changes in I(h) reflected differences in both G(max) and V(1/2): in vitronectin-exposed neurones there was a 35.4% increase in G(max) to 1.30 +/- 0.49 nS from 0.96 +/- 0.26 nS (P < 0.01), and a +3.0 mV shift in V(1/2) to -89.8 mV from -92.8 mV (P < 0.05). The time course of I(h) activation could be fitted by the sum of two exponential functions, fast and slow. In both CA1 and CA3 neurones the fast component amplitude was preferentially sensitive to vitronectin, with its relatively larger contribution to total current in vitronectin-exposed cells contributing to the acceleration of I(h) activation. Further, HCN1 immunoreactivity appeared elevated in vitronectin-exposed slices, while HCN2 levels appeared unaltered. We suggest that vitronectin-stimulated increases in I(h) may potentially affect excitability under pathological conditions.

Immunohistochemical localization of Ih channel subunits, HCN1-4, in the rat brain

Hyperpolarization-activated cation currents (I(h)) contribute to various physiological properties and functions in the brain, including neuronal pacemaker activity, setting of resting membrane potential, and dendritic integration of synaptic input. Four subunits of the Hyperpolarization-activated and Cyclic-Nucleotide-gated nonselective cation channels (HCN1-4), which generate I(h), have been cloned recently. To better understand the functional diversity of I(h) in the brain, we examined precise immunohistochemical localization of four HCNs in the rat brain. Immunoreactivity for HCN1 showed predominantly cortical distribution, being intense in the neocortex, hippocampus, superior colliculus, and cerebellum, whereas those for HCN3 and HCN4 exhibited subcortical distribution mainly concentrated in the hypothalamus and thalamus, respectively. Immunoreactivity for HCN2 had a widespread distribution throughout the brain. Double immunofluorescence revealed colocalization of immunoreactivity for HCN1 and HCN2 in distal dendrites of pyramidal cells in the hippocampus and neocortex. At the electron microscopic level, immunogold particles for HCN1 and HCN2 had similar distribution patterns along plasma membrane of dendritic shafts in layer I of the neocortex and stratum lacunosum moleculare of the hippocampal CA1 area, suggesting that these subunits could form heteromeric channels. Our results further indicate that HCNs are localized not only in somato-dendritic compartments but also in axonal compartments of neurons. Immunoreactivity for HCNs often occurred in preterminal rather than terminal portions of axons and in specific populations of myelinated axons. We also found HCN2-immunopositive oligodendrocytes including perineuronal oligodendrocytes throughout the brain. These results support previous electrophysiological findings and further suggest unexpected roles of I(h) channels in the brain.

Effect of early life stress on serotonin responses in the hippocampus of young adult rats

In this study, we investigated the effects of early life stress on several aspects of serotonin (5-HT) transmission in hippocampus, later on in life. Three-day-old rats were subjected to 24-hour maternal deprivation or control treatment. Maternal deprivation is known to activate the hypothalamo-pituitary-adrenal axis, resulting in increased corticosterone levels at a time-point in life when the axis is particularly insensitive to most stressful stimuli. When these animals had matured to 3 months of age, functional responses to 5-HT as well as 5-HT1A-receptor mRNA expression were examined. Also, indices for hypothalamo-pituitary-adrenal function were studied in the adult state, including hippocampal mRNA expression for the mineralocorticoid and the glucocorticoid receptor. Resting membrane potential of CA1 pyramidal neurons was significantly depolarized in animals earlier subjected to maternal deprivation compared to the controls. Despite this depolarized resting potential, hyperpolarizing responses induced by 5-HT in CA1 pyramidal neurons from deprived compared to non-deprived rats were attenuated. This attenuation in 5-HT response was not accompanied by changes in mRNA expression of the 5-HT1A-receptor. Maternal deprivation was not found to change any of the neuroendocrine parameters investigated once animals had matured. We conclude that maternal deprivation can alter specific aspects of hippocampal 5-HT transmission later on in life, possibly by post-translational modification of the 5-HT1A-receptor or changes in the 5-HT1A-receptor signal transduction pathway.

Failure of Repeated Electroconvulsive Shock Treatment on 5-HT4-Receptor-Mediated Depolarization Due To Protein Kinase A System in Young Rat Hippocampal CA1 Neurons

We previously demonstrated that repeated electroconvulsive shock (ECS) treatment enhanced serotonin (5-HT)(1A)- and 5-HT(3)-receptor-mediated responses in hippocampal CA1 pyramidal neurons. The electrophysiological studies were performed to elucidate the effects of ECS treatment on depolarization, which was an additional response induced by 5-HT, and the second messenger system involved in this depolarization of hippocampal CA1 neurons. Both application of 5-HT (100 microM) induced depolarization of the membrane potential in the presence of 5-HT(1A)-receptor antagonists. This depolarization was mimicked by 5-HT(4)-receptor agonists, RS 67506 (1-30 microM) and RS 67333 (0.1-30 microM), in a concentration-dependent manner. 5-HT- and RS 67333-induced depolarization was attenuated by concomitant application of RS 39604, a 5-HT(4)-receptor antagonist. H-89, a protein kinase A (PKA) inhibitor, inhibited 5-HT-, RS 67506-, and RS 67333-induced depolarizations, while forskolin (10 microM), an activator of adenylate cyclase, induced depolarization. Furthermore, RS 67333-induced depolarization was not significantly different between hippocampal slices prepared from rats administered ECS once a day for 14 days and those from sham-treated rats. These findings suggest that 5-HT(4)-receptor-mediated depolarization is caused via the cAMP-PKA system. In addition, repeated ECS-treatment did not modify 5-HT(4)-receptor functions in contrast to 5-HT(1A)- and 5-HT(3)-receptor functions.

The sleep-modulating peptide cortistatin augments the h-current in hippocampal neurons

Cortistatin (CST) is a sleep-modulating peptide found exclusively in the brain. Although CST is closely related to somatostatin (SST) and binds to SST receptors, CST has effects on sleep and neuronal activity in cortex and hippocampus that differ from SST. To uncover the cellular mechanisms affected by CST, we studied the electrophysiological postsynaptic effects of CST and assessed its interaction with SST on hippocampal CA1 pyramidal neurons. CST altered intrinsic membrane properties and occluded SST effects, indicating that both peptides similarly augment the sustained K+ M- and leak-currents (IM and IK(L)). In the presence of SST, however, CST elicited an additional inwardly rectifying component in the hyperpolarized range. This effect was unaffected by barium, used to block K+ currents, but was completely prevented by the selective h-current (Ih) blocker ZD7288. CST, but not SST, selectively increased Ih in a concentration-dependent manner by augmenting its maximum conductance. CST did not shift the Ih activation curve, and the peptide effect was unaffected by a membrane-permeable analog of cAMP. We conclude that CST and SST similarly increase K+ conductances in hippocampal neurons, most likely by activating SST receptors. However, CST additionally augments Ih, a voltage-dependent current that plays a key role in the modulation of synaptic integration and regulates oscillatory activity. Our results indicate that CST targets a specific conductance unaffected by SST to modulate cellular mechanisms implicated in sleep regulation.

Serotonin reduces the hyperpolarization-activated current (Ih) in ventral tegmental area dopamine neurons: involvement of 5-HT2 receptors and protein kinase C

Dopaminergic neurons of the ventral tegmental area (VTA) have been implicated in the rewarding properties of drugs of abuse and in the etiology of schizophrenia; serotonin modulation of these neurons may play a role in these phenomena. Whole cell patch-in-the-slice recording in rat brain slices was used to investigate modulation of the hyperpolarization-activated cationic current Ih by serotonin in these neurons. Serotonin (50-500 microM) reduced the amplitude of Ih in a concentration-dependent manner; this effect was reversible after prolonged washout of serotonin. This effect was mimicked by the 5-HT2 agonist alpha-methylserotonin (25 microM) and reversed by the 5-HT2 antagonist ketanserin (25 microM). Serotonin reduced the maximal Ih current and conductance (measured at -130 mV) and caused a negative shift in the voltage dependence of Ih activation. The serotonin-induced reduction in Ih amplitude was antagonized by intracellular administration of the nonspecific protein kinase inhibitor H-7 (75 microM) and the selective protein kinase C inhibitor chelerythrine (25 microM). The protein kinase C activator phorbol 12, 13 diacetate (PDA, 2 microM) reduced Ih amplitude; when PDA and serotonin were applied together, the effect on Ih was less than additive. These data support the conclusion that serotonin reduces Ih in dopaminergic VTA neurons by acting at serotonin 5-HT2 receptors, which activate protein kinase C. This reduction of Ih may be physiologically important, as the selective inhibitor of Ih, ZD7288, significantly increased dopamine inhibition of firing rate of dopaminergic VTA neurons, an effect that we previously demonstrated with serotonin.

Gabapentin actions on Kir3 currents and N-type Ca2+ channels via GABAB receptors in hippocampal pyramidal cells

Gabapentin is a clinically effective anticonvulsant with an unclear mechanism of action. It was described as a GABA(B(1a,2)) receptor subtype-selective agonist, activating postsynaptic K(+) currents and inhibiting postsynaptic Ca(2+) channels in CA1 pyramidal cells, but without presynaptic actions. These activities appeared controversial and we therefore sought to further clarify gabapentin actions in rat hippocampal slices by characterizing K(+) currents and Ca(2+) channels targeted by gabapentin using whole-cell recording and multiphoton Ca(2+) imaging. 1) We found that gabapentin and baclofen induced inwardly rectifying K(+) currents (K(Gbp) and K(Bac), respectively), sensitive to Ba(2+) and Cs(+). 2) A constitutively active K(IR) current, independent of GABA(B) receptor activation and sensitive to Ba(2+) and Cs(+) was also present. 3) K(Gbp), K(Bac), and K(IR) currents showed some differences in sensitivity to Ba(2+) and Cs(+), indicating the possible activation of distinct Kir3 currents, independent of K(IR), by gabapentin and baclofen. 4) Gabapentin inhibition of Ca(2+) channels was abolished by omega-conotoxin GVIA, but not by omega-agatoxin IVA and nimodipine, indicating a predominant action of gabapentin on N-type Ca(2+) channels. 5) Gabapentin actions were linked to activation of pertussis toxin-sensitive G-proteins since N-ethylmaleimide (NEM) blocked K(Gbp) activation and Ca(2+) channel inhibition by gabapentin. 6) Finally, gabapentin reduced epileptiform discharges in slices via GABA(B) receptor activation. The anticonvulsant actions of gabapentin in hippocampal cells may thus involve GABA(B) receptor coupling to G-proteins and modulation of Kir3 and N-type Ca(2+) channels. Moreover, gabapentin and baclofen activation of GABA(B) receptors may couple to distinct cellular targets.

Functional expression of the hyperpolarization-activated, non-selective cation current I(f) in immortalized HL-1 cardiomyocytes

HL-1 cells are adult mouse atrial myocytes induced to proliferate indefinitely by SV40 large T antigen. These cells beat spontaneously when confluent and express several adult cardiac cell markers including the outward delayed rectifier K(+) channel. Here, we examined the presence of a hyperpolarization-activated I(f) current in HL-1 cells using the whole-cell patch-clamp technique on isolated cells enzymatically dissociated from the culture at confluence. Cell membrane capacitance (C(m)) ranged from 5 to 53 pF. I(f) was detected in about 30% of the cells and its occurrence was independent of the stage of the culture. I(f) maximal slope conductance was 89.7 +/- 0.4 pS pF(-1) (n = 10). I(f) current in HL-1 cells showed typical characteristics of native cardiac I(f) current: activation threshold between -50 and -60 mV, half-maximal activation potential of -83.1 +/- 0.7 mV (n = 50), reversal potential at -20.8 +/- 1.5 mV (n = 10), time-dependent activation by hyperpolarization and blockade by 4 mM Cs(+). In half of the cells tested, activation of adenylyl cyclase by the forskolin analogue L858051 (20 microM) induced both an approximately 6 mV positive shift of the half-activation potential and an approximately 37 % increase in the fully activated I(f) current. RT-PCR analysis of the hyperpolarization-activated, cyclic nucleotide-gated channels (HCN) expressed in HL-1 cells demonstrated major contributions of HCN1 and HCN2 channel isoforms to I(f) current. Cytosolic Ca(2+) oscillations in spontaneously beating HL-1 cells were measured in Fluo-3 AM-loaded cells using a fast-scanning confocal microscope. The oscillation frequency ranged from 1.3 to 5 Hz and the spontaneous activity was stopped in the presence of 4 mM Cs(+). Action potentials from HL-1 cells had a triangular shape, with an overshoot at +15 mV and a maximal diastolic potential of -69 mV, i.e. more negative than the threshold potential for I(f) activation. In conclusion, HL-1 cells display a hyperpolarization-activated I(f) current which might contribute to the spontaneous contractile activity of these cells.

Serotonergic control of cerebellar mossy fiber activity by modulation of signal transfer by rat pontine nuclei neurons

Serotonergic modulation of precerebellar nuclei may be crucial for the function of the entire cerebellar system. To study the effects of serotonin (5-HT) on neurons located within the pontine nuclei (PN), the main source of cerebellar mossy fibers, we performed standard intracellular recordings from PN neurons in a slice preparation of the rat pontine brain stem. Application of 5 microM 5-HT significantly altered several intrinsic membrane properties of PN neurons. First, it depolarized the somatic membrane potential by 6.5 +/- 3.5 mV and increased the apparent input resistance from 49.5 +/- 14.6 to 62.7 +/- 21.1 MOmega. Second, 5-HT altered the I-V relationship of PN neurons: it decreased the inward rectification in hyperpolarizing direction, but increased it when depolarizing currents were applied. Third, it decreased the rheobase from 0.32 +/- 0.14 to 0.24 +/- 0.14 nA without affecting the firing threshold. Finally, the amplitude of medium-duration after hyperpolarizations was reduced from -14.9 +/- 2.0 to -12.3 +/- 2.4 mV. Together, these 5-HT effects on the intrinsic membrane properties result in an increase in excitability and instantaneous firing rate. In addition, application of 5 microM 5-HT also modulated postsynaptic potentials (PSPs) evoked by electric stimulations within the cerebral peduncle. The amplitude, maximal slope, and integral of these PSPs were reduced to 46.2 +/- 23.4%, 45.7 +/- 23.7%, and 61.4 +/- 28.4% of the control value, respectively. In contrast, we found no change in the decay and voltage dependence of PSPs. To test modulatory effects on short-term synaptic facilitation, we applied pairs of electrical stimuli at intervals between 10 and 1,000 ms. 5-HT selectively enhanced the paired-pulse facilitation for interstimulus-intervals >20 ms. The alteration of paired-pulse facilitation points to a presynaptic site of action for 5-HT effects on synaptic transmission. Pharmacological experiments suggested that pre- and postsynaptic effects of 5-HT were mediated by two different kinds of 5-HT receptors: changes in intrinsic membrane properties were blocked by the 5-HT(2) receptor antagonist cinanserin while the reduction of PSPs was prevented by the 5-HT(1) receptor antagonist cyanopindolol. In conclusion, 5-HT increases the excitability of PN neurons but decreases the synaptic transmission on them. The selective enhancement of synaptic facilitation may, however, allow high-frequency inputs to effectively drive PN neurons, thus the PN may act as a high-pass filter during periods of 5-HT release.

Functional characterisation and subcellular localisation of HCN1 channels in rabbit retinal rod photoreceptors

Gating of voltage-dependent conductances in retinal photoreceptors is the first step of a process leading to the enhancement of the temporal performance of the visual system. The molecular components underlying voltage-dependent gating in rods are presently poorly defined. In the present work we have investigated the isoform composition and the functional characteristics of hyperpolarisation-activated cyclic nucleotide-gated channels (HCN) in rabbit rods. Using immunocytochemistry we show the expression in the inner segment and cell body of the isoform 1 (HCN1). Electrophysiological investigations show that hyperpolarisation-activated currents (I(h)) can be measured only from the cell regions where HCN1 is expressed. Half-activation voltage (-75.0 +/- 0.3 mV) and kinetics (t(1/2) of 101 +/- 8 ms at -110 mV and 20 degrees C) of the I(h) in rods are similar to those of the macroscopic current carried by homomeric rabbit HCN1 channels expressed in HEK 293 cells. The homomeric nature of HCN1 channels in rods is compatible with the observation that cAMP induces a small shift (2.3 +/- 0.8 mV) in the half-activation voltage of I(h). In addition, the observation that within the physiological range of membrane potentials, cAMP does not significantly affect the gain of the current-to-voltage conversion, may reflect the need to protect the first step in the processing of visual signals from changes in cAMP turnover.

Differential modulation of I(h) by 5-HT receptors in mouse CA1 hippocampal neurons

CA1 pyramidal neurons of the hippocampus express various types of serotonin (5-HT) receptors, such as 5-HT(1A), 5-HT(4) and 5-HT(7) receptors, which couple to Galpha(i) or Galpha(s) proteins and operate on different intracellular signalling pathways. In the present paper we verify such differential serotonergic modulation for the hyperpolarization-activated current I(h). Activation of 5-HT(1A) receptors induced an augmentation of current-induced hyperpolarization responses, while the responses declined after 5-HT(4) receptors were activated. The resting potential of neurons hyperpolarized (-2.3 +/- 0.7 mV) after 5-HT(1A) receptor activation, activation of 5-HT(4) receptors depolarized neurons (+3.3 +/- 1.4 mV). Direct activation of adenylyl cyclase (AC) by forskolin also produced a depolarization. In voltage clamp, the Ih current was identified by its characteristic voltage- and time-dependency and by blockade with CsCl or ZD7288. Activation of 5-HT(1A) receptors reduced I(h) and shifted the activation curve to a more negative voltage by -5 mV at half-maximal activation. Activation of 5-HT(4) and 5-HT(7) receptors increased I(h) and shifted the activation curve to the right by +5 mV. Specific activation of 5-HT(4) receptors by BIMU8 increased membrane conductance and showed an increase in I(h) in a subset of cells, but did not induce a significant alteration in the activation curve. In order to verify spatial differences, we applied BIMU8 selectively to the soma and to the dendrites. Only somatic application induced receptor activation. These data are confirmed by immunofluorescence stainings with an antibody against the 5-HT(4) receptor, revealing receptor expression at the somata of the CA1 region. A similar expression pattern was found with a new antibody against 5-HT(7) receptors which reveals immunofluorescence staining on the cell bodies of pyramidal neurons.

The 5-HT(4) receptor-induced depolarization in rat hippocampal neurons is mediated by cAMP but is independent of I(h)

Intracellular recordings were used to examine the mechanisms underlying the 5-hydroxytryptamine (5-HT)4 receptor-mediated depolarization seen in CA1 region pyramidal cells in in vitro hippocampal brain slices. This depolarization was mimicked and occluded by administration of the membrane permeable cyclic adenosine monophosphate (cAMP) analog 8-bromo-cAMP but was unaffected by blockade of protein kinase A (PKA). These results suggest that 5-HT4 receptors signal this depolarization through a cAMP-dependent but PKA-independent mechanism. In many cell types, 5-HT elicits a depolarization via cAMP by facilitating Ih, a hyperpolarization-activated cation current. In contrast, we find no evidence for the involvement of Ih in this response. Rather, this depolarization may involve a cyclic nucleotide gated channel.

Muscarinic induction of hippocampal gamma oscillations requires coupling of the M1 receptor to two mixed cation currents

Oscillatory network activity at gamma frequencies is assumed to be of major importance in cortical information processing. Whereas the synaptic mechanisms of gamma oscillations have been studied in detail, the ionic currents involved at the cellular level remain to be elucidated. Here we show that in vitro gamma oscillations induced by muscarine require activation of M1 receptors on hippocampal CA3 pyramidal neurons and are absent in M1 receptor-deficient mice. M1 receptor activation depolarizes pyramidal neurons by increasing the mixed Na(+)/K(+) current I(h) and the Ca(2+)-dependent nonspecific cation current I(cat), but not by modulation of I(M). Our data provide important insight into the molecular basis of gamma oscillations by unequivocally establishing a novel role for muscarinic modulation of I(h) and I(cat) in rhythmic network activity.

Extrinsic modulation and motor pattern generation in a feeding network: a cellular study

Systems level studies have shown that the paired serotonergic cerebral giant cells (CGCs) of gastropod mollusks have important extrinsic modulatory actions on the central pattern generator (CPG) underlying rhythmic ingestion movements. Here we present the first study that investigates the modulatory actions of the CGCs and their released transmitter 5-HT on the CPG at the cellular level. In the snail, Lymnaea, motoneurons such as the B4, B8, and B4CL cells are part of the feeding CPG and receive serotonergic synaptic inputs from CGCs. These motoneurons were used to investigate the effect of serotonergic modulation on endogenous cellular properties of CPG neurons. Cells were isolated from the intact nervous system, and their properties were examined by pharmacological methods in cell culture. Motoneurons were also grown in coculture with CGCs to compare 5-HT effects with CGC stimulation. Three distinct modulatory effects of exogenously applied 5-HT/CGC activity were seen: all three motoneuron types were depolarized by 5-HT for prolonged periods leading to firing. Conditional bursting accompanied this depolarization in the B4/B8 cells, but not in B4CL cells. The frequency of the bursting was increased with increased CGC tonic firing. An increase in the size of postinhibitory rebound (PIR) occurred with 5-HT application in all three cell types, because of an increase in a CsCl-sensitive, hyperpolarization-activated inward current. Similar modulatory effects on membrane potential, endogenous bursting, and PIR properties could be observed in the intact nervous system and were necessary for motoneuron activation during feeding. Part of the systems gating and frequency control functions of the CGCs appear to be caused by these modulatory effects on feeding motoneurons.