Hyperpolarization-activated current (Ih): A characterization of subicular neurons in brain slices from socially and individually housed rats

Chronic methylphenidate exposure during adolescence reduces striatal synaptic responses to ethanol

Dopamine (DA) plays an important role in integrative functions contributing to adaptive behaviors. In support of this essential function, DA modulates synaptic plasticity in different brain areas, including the striatum. Many drugs used for cognitive enhancement are psychostimulants, such as methylphenidate (MPH), which enhance DA levels. MPH treatment is of interest during adolescence, a period of enhanced neurodevelopment during which the DA system is in a state of flux. Recent epidemiological studies report the co-abuse of MPH and ethanol in adolescents and young adults. Although repeated MPH treatment produces enduring changes that affect subsequent behavioral responses to other psychostimulants, few studies have investigated the interactions between MPH and ethanol. Here we addressed whether chronic therapeutic exposure to MPH during adolescence predisposed mice to an altered response to ethanol and whether this was accompanied by altered DA release and striatal plasticity. C57BL/6J mice were administered MPH (3-6 mg/kg/day) via the drinking water between post-natal days 30 and 60. Voltammetry experiments showed that sufficient brain MPH concentrations were achieved during adolescence in mice to increase the DA clearance in adulthood. The treatment also increased long-term depression and reduced the effects of ethanol on striatal synaptic responses. Although the injection of 0.4 or 2 g/kg ethanol dose-dependently decreased locomotion in control mice, only the higher dose decreased locomotion in MPH-treated mice. These results suggested that the administration of MPH during development promoted long-term effects on synaptic plasticity in forebrain regions targeted by DA. These changes in plasticity might, in turn, underlie alterations in behaviors controlled by these brain regions into adulthood.

Cholinergic modulation of GABAergic and glutamatergic transmission in the dorsal subcoeruleus: mechanisms for REM sleep control

STUDY OBJECTIVES

Dorsal subcoeruleus (SubCD) neurons are thought to promote PGO waves and to be modulated by cholinergic afferents during REM sleep. We examined the differential effect of the cholinergic agonist carbachol (CAR) on excitatory and inhibitory postsynaptic currents (PSCs), and investigated the effects of CAR on SubCD neurons during the developmental decrease in REM sleep.

DESIGN

Whole-cell patch clamp recordings were conducted on brainstem slices of 7- to 20-day-old rats.

MEASUREMENTS AND RESULTS

CAR acted directly on 50% of SubCD neurons by inducing an inward current, via both nicotinic and muscarinic M1 receptors. CAR induced a potassium mediated outward current via activation of M2 muscarinic receptors in 43% of SubCD cells. Evoked stimulation established the presence of NMDA, AMPA, GABA, and glycinergic PSCs in the SubCD. CAR was found to decrease the amplitude of evoked EPSCs in 31 of 34 SubCD cells, but decreased the amplitude of evoked IPSCs in only 1 of 13 SubCD cells tested. Spontaneous EPSCs were decreased by CAR in 55% of cells recorded, while spontaneous IPSCs were increased in 27% of SubCD cells. These findings indicate that CAR exerts a predominantly inhibitory role on fast synaptic glutamatergic activity and a predominantly excitatory role on fast synaptic GABAergic/glycinergic activity in the SubCD.

CONCLUSION

We hypothesize that during REM sleep, cholinergic "REM-on" neurons that project to the SubCD induce an excitation of inhibitory interneurons and inhibition of excitatory events leading to the production of coordinated activity in SubCD projection neurons. The coordination of these projection neurons may be essential for the production of REM sleep signs such as PGO waves.

Differential Expression of Intrinsic Membrane Currents in Defined Cell Types of the Anterolateral Bed Nucleus of the Stria Terminalis

The anterolateral group of the bed nucleus of the stria terminalis (BNSTALG) plays a critical role in a diverse array of behaviors, although little is known of the physiological properties of neurons in this region. Using whole cell patch-clamp recordings from rat BNSTALG slices in vitro, we describe three distinct physiological cell types. Type I neurons were characterized by the presence of a depolarizing sag in response to hyperpolarizing current injection that resembled activation of the hyperpolarization-activated cation current Ih and a regular firing pattern in response to depolarizing current injection. Type II neurons exhibited the same depolarizing sag in response to hyperpolarizing current injection, but burst-fired in response to depolarizing current injection, which was indicative of the activation of the low-threshold calcium current IT. Type III neurons did not exhibit a depolarizing sag in response to hyperpolarizing current injection, but instead exhibited a fast time-independent rectification that became more pronounced with increased amplitude of hyperpolarizing current injection, and was indicative of activation of the inwardly rectifying potassium current IK(IR). Type III neurons also exhibited a regular firing pattern in response to depolarizing current. Using voltage-clamp analysis we further characterized the primary active currents that shaped the physiological properties of these distinct cell types, including Ih, IT, IK(IR), the voltage-dependent potassium current IA, and the persistent sodium current INaP. The functional relevance of each cell type is discussed in relation to prior anatomical studies, as well as how these currents may interact to modulate neuronal activity within the BNSTALG.

Environmental manipulations early in development alter seizure activity, Ih and HCN1 protein expression later in life

Although absence epilepsy has a genetic origin, evidence from an animal model (Wistar Albino Glaxo/Rijswijk; WAG/Rij) suggests that seizures are sensitive to environmental manipulations. Here, we show that manipulations of the early rearing environment (neonatal handling, maternal deprivation) of WAG/Rij rats leads to a pronounced decrease in seizure activity later in life. Recent observations link seizure activity in WAG/Rij rats to the hyperpolarization-activated cation current (Ih) in the somatosensory cortex, the site of seizure generation. Therefore, we investigated whether the alterations in seizure activity between rats reared differently might be correlated with changes in Ih and its channel subunits hyperpolarization-activated cation channel HCN1, 2 and 4. Whole-cell recordings from layer 5 pyramidal neurons, in situ hybridization and Western blot of the somatosensory cortex revealed an increase in Ih and HCN1 in neonatal handled and maternal deprived, compared to control rats. The increase was specific to HCN1 protein expression and did not involve HCN2/4 protein expression, or mRNA expression of any of the subunits (HCN1, 2, 4). Our findings provide the first evidence that relatively mild changes in the neonatal environment have a long-term impact of absence seizures, Ih and HCN1, and suggest that an increase of Ih and HCN1 is associated with absence seizure reduction. Our findings shed new light on the role of Ih and HCN in brain functioning and development and demonstrate that genetically determined absence seizures are quite sensitive for early interventions.

WITHDRAWN: Immunohistochemical localization of Ih channel HCN3 in the rat brain

Ahead of Print article withdrawn by publisher

Theta-frequency membrane resonance and its ionic mechanisms in rat subicular pyramidal neurons

The neuron population of the hippocampal formation exhibits oscillatory activity within the theta (theta) frequency band (4-10 Hz), and the intrinsic resonance properties of individual hippocampal neurons contribute to this network oscillation. The subiculum is the pivotal output region of the hippocampal formation and it is involved in many of the physiological and pathological functions of the limbic system. To study the characteristics and underlying mechanisms of resonance activity in subicular pyramidal neurons, we performed whole-cell patch-clamp recordings from these neurons in rat horizontal brain slices. We applied sinusoidal currents with constant amplitudes and linearly increasing frequencies to measure the resonance frequency of subicular pyramidal neurons. We found that the resonance frequency of subicular pyramidal neurons was about 2 Hz at room temperature and 4-6 Hz at 32-35 degrees C. The resonance frequency increased at hyperpolarized membrane potentials and decreased at depolarized membrane potentials. We also investigated three sub-threshold currents involved in the resonance: a slow hyperpolarization-activated cation current; an instantaneously activating, inwardly rectifying potassium current; and an inwardly persistent sodium current. The application of ZD7288 abolished the resonance hump, indicating that hyperpolarization-activated cation current generated resonance. The application of Ba(2+) enlarged the resonance hump at hyperpolarized potentials below -80 mV, indicating that inwardly rectifying potassium current attenuated resonance. The application of TTX suppressed the resonance at depolarized potentials, indicating that persistent sodium current amplified resonance when neurons were depolarized. Thus, there is a theta-frequency resonance mediated by hyperpolarization-activated cation current in subicular pyramidal neurons. This theta-frequency resonance of individual subicular pyramidal neurons may participate in the population's theta oscillation and contribute to the functions of the subiculum.

The subiculum comes of age

The subiculum has long been considered as a simple bidirectional relay region interposed between the hippocampus and the temporal cortex. Recent evidence, however, suggests that this region has specific roles in the cognitive functions and pathological deficits of the hippocampal formation. A group of 20 researchers participated in an ESF-sponsored meeting in Oxford in September, 2005 focusing on the neurobiology of the subiculum. Each brought a distinct expertise and approach to the anatomy, physiology, psychology, and pathologies of the subiculum. Here, we review the recent findings that were presented at the meeting.

Physiology of long pranayamic breathing: Neural respiratory elements may provide a mechanism that explains how slow deep breathing shifts the autonomic nervous system

Pranayamic breathing, defined as a manipulation of breath movement, has been shown to contribute to a physiologic response characterized by the presence of decreased oxygen consumption, decreased heart rate, and decreased blood pressure, as well as increased theta wave amplitude in EEG recordings, increased parasympathetic activity accompanied by the experience of alertness and reinvigoration. The mechanism of how pranayamic breathing interacts with the nervous system affecting metabolism and autonomic functions remains to be clearly understood. It is our hypothesis that voluntary slow deep breathing functionally resets the autonomic nervous system through stretch-induced inhibitory signals and hyperpolarization currents propagated through both neural and non-neural tissue which synchronizes neural elements in the heart, lungs, limbic system and cortex. During inspiration, stretching of lung tissue produces inhibitory signals by action of slowly adapting stretch receptors (SARs) and hyperpolarization current by action of fibroblasts. Both inhibitory impulses and hyperpolarization current are known to synchronize neural elements leading to the modulation of the nervous system and decreased metabolic activity indicative of the parasympathetic state. In this paper we propose pranayama's physiologic mechanism through a cellular and systems level perspective, involving both neural and non-neural elements. This theoretical description describes a common physiological mechanism underlying pranayama and elucidate the role of the respiratory and cardiovascular system on modulating the autonomic nervous system. Along with facilitating the design of clinical breathing techniques for the treatment of autonomic nervous system and other disorders, this model will also validate pranayama as a topic requiring more research.