Intercellular calcium signaling and gap junctional communication in astrocytes

P2Y receptors and pain transmission

It is widely accepted that the most important ATP receptors involved in pain transmission belong to the P2X₃ and P2X_2/3 subtypes, selectively expressed in small diameter dorsal root ganglion (DRG) neurons. However, several types of the metabotropic ATP (P2Y) receptors have also been found in primary afferent neurons; P2Y₁ and P2Y₂ receptors are typically expressed in small, nociceptive cells. Here we review the results available on the involvement of P2Y receptors in the modulation of pain transmission.

The role of gap junctions in the brain in health and disease

Gap junctions connect the cytosolic compartments of adjacent cells for direct electrotonic and metabolic cell-to-cell communication. Gap junctions between glial cells or neurons are ubiquitously expressed in the brain and play a role in brain development including cell differentiation, cell migration and survival, tissue homeostasis, as well as in human diseases including hearing loss, skin disease, neuropathies, epilepsy, brain trauma, and cardiovascular disease. Furthermore, gap junctions are involved in the synchronization and rhythmic oscillation of hippocampal and neocotical neuronal ensembles which might be important for memory formation and consolidation. In this review the accumulated evidence from mouse mutant and pharmacological studies using gap junction blockers is summarized and the progress made in dissecting the physiological, pathophysiological and behavioral roles of gap junction mediated intercellular communication in the brain is discussed.

A co-operative regulation of neuronal excitability by UNC-7 innexin and NCA/NALCN leak channel

Gap junctions mediate the electrical coupling and intercellular communication between neighboring cells. Some gap junction proteins, namely connexins and pannexins in vertebrates, and innexins in invertebrates, may also function as hemichannels. A conserved NCA/Dmα1U/NALCN family cation leak channel regulates the excitability and activity of vertebrate and invertebrate neurons. In the present study, we describe a genetic and functional interaction between the innexin UNC-7 and the cation leak channel NCA in Caenorhabditis elegans neurons. While the loss of the neuronal NCA channel function leads to a reduced evoked postsynaptic current at neuromuscular junctions, a simultaneous loss of the UNC-7 function restores the evoked response. The expression of UNC-7 in neurons reverts the effect of the unc-7 mutation; moreover, the expression of UNC-7 mutant proteins that are predicted to be unable to form gap junctions also reverts this effect, suggesting that UNC-7 innexin regulates neuronal activity, in part, through gap junction-independent functions. We propose that, in addition to gap junction-mediated functions, UNC-7 innexin may also form hemichannels to regulate C. elegans' neuronal activity cooperatively with the NCA family leak channels.

Temporal and spatial profile of phosphorylated connexin43 after traumatic brain injury in rats

Gap junctions are conductive channels formed by membrane proteins termed connexins (Cx), which permit the intercellular exchange of metabolites, ions, and small molecules. Junctional permeability is regulated by pH, membrane potential, and intracellular secondary messengers. The purpose of this study was to elucidate the expression and distribution of astrocytic gap junctions in the hippocampus and the cortex after traumatic brain injury (TBI) in vivo. Adult male Sprague-Dawley rats (300-400 g) were subjected to lateral fluid percussion injury (FPI) at moderate severity (2.6-2.8 atm, 12 msec) using a Dragonfly device model. Phosphorylated gap junction protein levels were quantified using Western blot analysis. Spatial distribution of immunoreactivity for phosphorylated Cx43 (p-Cx43) was analyzed by immunohistochemistry. Our findings showed that p-Cx43 expression in the ipsilateral hippocampus was significantly induced at 1 h after TBI, and remained at a high level until 24 h after injury. The p-Cx43 protein content reached a maximum level at 6 h after injury. In addition, the immunoreactivity for p-Cx43 was localized in the astrocytes surrounding ipsilateral CA3 pyramidal neurons. On the other hand, the protein level in the ipsilateral cortex was not significantly different at any time point after TBI. Double immunostaining using phosphorylated ERK (p-ERK) showed that p-Cx43 and p-ERK immunoreactivities were enhanced in the same astrocytes at 6 h after injury. These findings suggest that astrocytic gap junctions participate in pathophysiological processes in the hippocampus after TBI.

Intercellular calcium waves in glial cells with bistable dynamics

A two-dimensional model is proposed for intercellular calcium (Ca(2 +)) waves with Ca(2 +)-induced IP(3) regeneration and the diffusion of IP(3) through gap junctions. Many experimental observations in glial cells, i.e. responding to local mechanical stimulation, glutamate application, mechanical stimulation followed by ACh application, and glutamate followed by mechanical stimulation, are reproduced and classified by the model. We show that a glial cell model with bistable dynamics, i.e. a Ca(2 +) oscillation state coexisting with a fixed point, can cause a prolonged plateau of Ca(2 +) signals in the cells nearby the stimulated cell when the cell network responds to the local mechanical stimulation.

Ca(2+) sources for the exocytotic release of glutamate from astrocytes

Astrocytes can exocytotically release the gliotransmitter glutamate from vesicular compartments. Increased cytosolic Ca(2+) concentration is necessary and sufficient for this process. The predominant source of Ca(2+) for exocytosis in astrocytes resides within the endoplasmic reticulum (ER). Inositol 1,4,5-trisphosphate and ryanodine receptors of the ER provide a conduit for the release of Ca(2+) to the cytosol. The ER store is (re)filled by the store-specific Ca(2+)-ATPase. Ultimately, the depleted ER is replenished by Ca(2+) which enters from the extracellular space to the cytosol via store-operated Ca(2+) entry; the TRPC1 protein has been implicated in this part of the astrocytic exocytotic process. Voltage-gated Ca(2+) channels and plasma membrane Na(+)/Ca(2+) exchangers are additional means for cytosolic Ca(2+) entry. Cytosolic Ca(2+) levels can be modulated by mitochondria, which can take up cytosolic Ca(2+) via the Ca(2+) uniporter and release Ca(2+) into cytosol via the mitochondrial Na(+)/Ca(2+) exchanger, as well as by the formation of the mitochondrial permeability transition pore. The interplay between various Ca(2+) sources generates cytosolic Ca(2+) dynamics that can drive Ca(2+)-dependent exocytotic release of glutamate from astrocytes. An understanding of this process in vivo will reveal some of the astrocytic functions in health and disease of the brain. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Intercellular calcium waves in the fire-diffuse-fire framework: Green's function for gap-junctional coupling

Calcium is a crucial component in a plethora of cellular processes involved in cell birth, life, and death. Intercellular calcium waves that can spread through multiple cells provide one form of cellular communication mechanism between various parts of cell tissues. Here we introduce a simple, yet biophysically realistic model for the propagation of intercellular calcium waves based on the fire-diffuse-fire type model for calcium dynamics. Calcium release sites are considered to be discretely distributed along individual linear cells that are connected by gap junctions and a solution of this model can be found in terms of the Green's function for this system. We develop the "sum-over-trips" formalism that takes into account the boundary conditions at gap junctions providing a generalization of the original sum-over-trips approach for constructing the response function for branched neural dendrites. We obtain the exact solution of the Green's function in the Laplace (frequency) domain for an infinite array of cells and show that this Green's function can be well approximated by its truncated version. This allows us to obtain an analytical traveling wave solution for an intercellular calcium wave and analyze the speed of solitary wave propagation as a function of physiologically important system parameters. Periodic and irregular traveling waves can be also sustained by the proposed model.

Expression and contribution of satellite glial cells purinoceptors to pain transmission in sensory ganglia: an update

The role of adenosine-5'-triphosphate (ATP) and of the ligand-gated P2X3 receptor in neuronal dorsal root ganglia (DRG) pain transmission is relatively well established. Much less is known about the purinergic system in trigeminal ganglia (TG), which are involved in certain types of untreatable neuropathic and inflammatory pain, as well as in migraine. Emerging data suggest that purinergic metabotropic P2Y receptors on both neurons and satellite glial cells (SGCs) may also participate in both physiological and pathological pain development. Here, we provide an updated literature review on the role of purinergic signaling in sensory ganglia, with special emphasis on P2Y receptors on SGCs. We also provide new original data showing a time-dependent downregulation of P2Y2 and P2Y4 receptor expression and function in purified SGCs cultures from TG, in comparison with primary mixed neuron-SGCs cultures. These data highlight the importance of the neuron-glia cross-talk in determining the SGCs phenotype. Finally, we show that, in mixed TG cultures, both adenine and guanosine induce intracellular calcium transients in neurons but not in SGCs, suggesting that also these purinergic-related molecules can participate in pain signaling. These findings may have relevant implications for the development of new therapeutic strategies for chronic pain treatment.

Astrocytes in Alzheimer's disease

The circuitry of the human brain is formed by neuronal networks embedded into astroglial syncytia. The astrocytes perform numerous functions, providing for the overall brain homeostasis, assisting in neurogenesis, determining the micro-architecture of the grey matter, and defending the brain through evolutionary conserved astrogliosis programs. Astroglial cells are engaged in neurological diseases by determining the progression and outcome of neuropathological process. Astrocytes are specifically involved in various neurodegenerative diseases, including Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, and various forms of dementia. Recent evidence suggest that early stages of neurodegenerative processes are associated with atrophy of astroglia, which causes disruptions in synaptic connectivity, disbalance in neurotransmitter homeostasis, and neuronal death through increased excitotoxicity. At the later stages, astrocytes become activated and contribute to the neuroinflammatory component of neurodegeneration.

Recent advances in (patho)physiology of astroglia

Our view of astrocytes in the operation of the brain is changing dramatically over the last 3 decades. Astroglial calcium excitability controls the release of gliotransmitters, which can occur at the tripartite synapse. Astrocytes not only modulate synaptic transmission by releasing and taking up transmitters, but also receiving neuronal signals that act upon astrocytic plasma membrane receptors. This process represents the bidirectional neurone-glia communication. Additionally, astrocytes play role in the regulation of blood flow as well as ion and water homeostasis. Many of the brain dysfunctions are primary astropathies, including hepatic encephalopathy and Alexander disease, while other brain malfunctions, such as epilepsy and Alzheimer disease, may have substantial astrocytic contribution. Thus, these star-shaped cells by their roles in (patho)physiology of the brain seem to live up to the expectation one can have from their given name - astrocyte.

Nonlinear gap junctions enable long-distance propagation of pulsating calcium waves in astrocyte networks

A new paradigm has recently emerged in brain science whereby communications between glial cells and neuron-glia interactions should be considered together with neurons and their networks to understand higher brain functions. In particular, astrocytes, the main type of glial cells in the cortex, have been shown to communicate with neurons and with each other. They are thought to form a gap-junction-coupled syncytium supporting cell-cell communication via propagating Ca²⁺ waves. An identified mode of propagation is based on cytoplasm-to-cytoplasm transport of inositol trisphosphate (IP₃) through gap junctions that locally trigger Ca²⁺ pulses via IP₃-dependent Ca²⁺-induced Ca²⁺ release. It is, however, currently unknown whether this intracellular route is able to support the propagation of long-distance regenerative Ca²⁺ waves or is restricted to short-distance signaling. Furthermore, the influence of the intracellular signaling dynamics on intercellular propagation remains to be understood. In this work, we propose a model of the gap-junctional route for intercellular Ca²⁺ wave propagation in astrocytes. Our model yields two major predictions. First, we show that long-distance regenerative signaling requires nonlinear coupling in the gap junctions. Second, we show that even with nonlinear gap junctions, long-distance regenerative signaling is favored when the internal Ca²⁺ dynamics implements frequency modulation-encoding oscillations with pulsating dynamics, while amplitude modulation-encoding dynamics tends to restrict the propagation range. As a result, spatially heterogeneous molecular properties and/or weak couplings are shown to give rise to rich spatiotemporal dynamics that support complex propagation behaviors. These results shed new light on the mechanisms implicated in the propagation of Ca²⁺ waves across astrocytes and the precise conditions under which glial cells may participate in information processing in the brain.

In recent years, the focus of Cellular Neuroscience has progressively stopped only being on neurons but started to include glial cells as well. Indeed, astrocytes, the main type of glial cells in the cortex, dynamically modulate neuron excitability and control the flow of information across synapses. Moreover, astrocytes have been shown to communicate with each other over long distances using calcium waves. These waves spread from cell to cell via molecular gates called gap junctions, which connect neighboring astrocytes. In this work, we used a computer model to question what biophysical mechanisms could support long-distance propagation of Ca²⁺ wave signaling. The model shows that the coupling function of the gap junction must be non-linear and include a threshold. This prediction is largely unexpected, as gap junctions are classically considered to implement linear functions. Recent experimental observations, however, suggest their operation could actually be more complex, in agreement with our prediction. The model also shows that the distance traveled by waves depends on characteristics of the internal astrocyte dynamics. In particular, long-distance propagation is facilitated when internal calcium oscillations are in their frequency-modulation encoding mode and are pulsating. Hence, this work provides testable experimental predictions to decipher long-distance communication between astrocytes.

ATP-induced morphological changes in supporting cells of the developing cochlea

The developing cochlea of mammals contains a large group of columnar-shaped cells, which together form a structure known as Kölliker's organ. Prior to the onset of hearing, these inner supporting cells periodically release adenosine 5'-triphosphate (ATP), which activates purinergic receptors in surrounding supporting cells, inner hair cells and the dendrites of primary auditory neurons. Recent studies indicate that purinergic signaling between inner supporting cells and inner hair cells initiates bursts of action potentials in auditory nerve fibers before the onset of hearing. ATP also induces prominent effects in inner supporting cells, including an increase in membrane conductance, a rise in intracellular Ca(2+), and dramatic changes in cell shape, although the importance of ATP signaling in non-sensory cells of the developing cochlea remains unknown. Here, we review current knowledge pertaining to purinergic signaling in supporting cells of Kölliker's organ and focus on the mechanisms by which ATP induces changes in their morphology. We show that these changes in cell shape are preceded by increases in cytoplasmic Ca(2+), and provide new evidence indicating that elevation of intracellular Ca(2+) and IP(3) are sufficient to initiate shape changes. In addition, we discuss the possibility that these ATP-mediated morphological changes reflect crenation following the activation of Ca(2+)-activated Cl(-) channels, and speculate about the possible functions of these changes in cell morphology for maturation of the cochlea.

Where the thoughts dwell: the physiology of neuronal-glial "diffuse neural net"

The mechanisms underlying the production of thoughts by exceedingly complex cellular networks that construct the human brain constitute the most challenging problem of natural sciences. Our understanding of the brain function is very much shaped by the neuronal doctrine that assumes that neuronal networks represent the only substrate for cognition. These neuronal networks however are embedded into much larger and probably more complex network formed by neuroglia. The latter, although being electrically silent, employ many different mechanisms for intercellular signalling. It appears that astrocytes can control synaptic networks and in such a capacity they may represent an integral component of the computational power of the brain rather than being just brain "connective tissue". The fundamental question of whether neuroglia is involved in cognition and information processing remains, however, open. Indeed, a remarkable increase in the number of glial cells that distinguishes the human brain can be simply a result of exceedingly high specialisation of the neuronal networks, which delegated all matters of survival and maintenance to the neuroglia. At the same time potential power of analogue processing offered by internally connected glial networks may represent the alternative mechanism involved in cognition.

Characteristics of calcium signaling in astrocytes induced by photostimulation with femtosecond laser

Astrocytes have been identified to actively contribute to brain functions through Ca(2+) signaling, serving as a bridge to communicate with neurons and other brain cells. However, conventional stimulation techniques are hard to apply to delicate investigations on astrocytes. Our group previously reported photostimulation with a femtosecond laser to evoke astrocytic calcium (Ca(2+)) waves, providing a noninvasive and efficient approach with highly precise targeting. In this work, detailed characteristics of astrocytic Ca(2+) signaling induced by photostimulation are presented. In a purified astrocytic culture, after the illumination of a femtosecond laser onto one cell, a Ca(2+) wave throughout the network with reduced speed is induced, and intracellular Ca(2+) oscillations are observed. The intercellular propagation is pharmacologically confirmed to be mainly mediated by ATP through P(2)Y receptors. Different patterns of Ca(2+) elevations with increased amplitude in the stimulated astrocyte are discovered by varying the femtosecond laser power, which is correspondingly followed by broader intercellular waves. These indicate that the strength of photogenerated Ca(2+) signaling in astrocytes has a positive relationship with the stimulating laser power. Therefore, distinct Ca(2+) signaling is feasibly available for specific studies on astrocytes by employing precisely controlled photostimulation.

Visualization of melatonin's multiple mitochondrial levels of protection against mitochondrial Ca(2+)-mediated permeability transition and beyond in rat brain astrocytes

Melatonin protects cells against various types of oxidative stress-induced apoptosis due primarily to its ability to effectively scavenge pathological and disease condition-augmented generation of mitochondrial reactive oxygen species (mROS). Once produced, mROS indiscriminately damage mitochondrial components and more importantly they crucially activate directly the mitochondrial permeability transition (MPT), one of the critical mechanisms for initiating post mitochondrial apoptotic signaling. Whether or not melatonin targets directly the MPT, however, remains inconclusive, particularly during oxidative stress. This study, thus, investigated this possibility of an 'oxidation free Ca(2+) stress' in the presence of vitamin E after ionomycin exposure as a sole Ca(2+)-mediated MPT in order to exclude melatonin's primary antioxidative effects as well as Ca(2+)-mediated oxidative stress. The studies were carried out using cultured rat brain astrocytes RBA-1. With the application of laser scanning multiple fluorescence imaging microscopy, we visualized for the first time multiple mitochondrial protective effects provided by melatonin during Ca(2+) stress. First, melatonin, due to its primary antioxidative actions, completely prevented mCa(2+)-induced mROS formation during ionomycin exposure. Secondly, when melatonin(')s antioxidative effects were prevented due to the addition of vitamin E, melatonin significantly prevented mCa(2+)-mediated MPT and apoptosis suggesting its direct targeting of the MPT. Surprisingly, in the presence of cyclosporin A, a MPT inhibitor, melatonin reduced further mCa(2+)-mediated apoptosis during ionomycin exposure also suggesting its targeting beyond the MPT. As astrocytes are actively involve in regulating synaptic transmission and neurovascular coupling in the CNS, these multiple mitochondrial layers of protection provided by melatonin against mCa(2+)-and/or mROS-mediated apoptosis in astrocytes may be crucial for future therapeutic prevention and treatment of astrocyte-mediated neurodegenerative diseases in the CNS.

The subependymal zone neurogenic niche: a beating heart in the centre of the brain: how plastic is adult neurogenesis? Opportunities for therapy and questions to be addressed

The mammalian brain is a remarkably complex organ comprising millions of neurons, glia and various other cell types. Its impressive cytoarchitecture led to the long standing belief that it is a structurally static organ and thus very sensitive to injury. However, an area of striking structural flexibility has been recently described at the centre of the brain. It is the subependymal zone of the lateral wall of the lateral ventricles. The subependymal zone--like a beating heart--continuously sends new cells to different areas of the brain: neurons to the olfactory bulbs and glial cells to the cortex and the corpus callosum. Interestingly, the generation and flow of cells changes in response to signals from anatomically remote areas of the brain or even from the external environment of the organism, therefore indicating that subependymal neurogenesis--as a system--is integrated in the overall homeostatic function of the brain. In this review, it will be attempted to describe the fundamental structural and functional characteristics of the subependymal neurogenic niche and to summarize the available evidence regarding its plasticity. Special focus is given on issues such as whether adult neural stem cells are activated after neurodegeneration, whether defects in neurogenesis contribute to neuropathological conditions and whether monitoring changes in neurogenic activity can have a diagnostic value.

Spatiotemporal characteristics of calcium dynamics in astrocytes

Although Ca(i)(2+) waves in networks of astrocytes in vivo are well documented, propagation in vivo is much more complex than in culture, and there is no consensus concerning the dominant roles of intercellular and extracellular messengers [inositol 1,4,5-trisphosphate (IP(3)) and adenosine-5'-triphosphate (ATP)] that mediate Ca(i)(2+) waves. Moreover, to date only simplified models that take very little account of the geometrical struture of the networks have been studied. Our aim in this paper is to develop a mathematical model based on realistic cellular morphology and network connectivity, and a computational framework for simulating the model, in order to address these issues. In the model, Ca(i) (2+) wave propagation through a network of astrocytes is driven by IP(3) diffusion between cells and ATP transport in the extracellular space. Numerical simulations of the model show that different kinetic and geometric assumptions give rise to differences in Ca(i)(2+) wave propagation patterns, as characterized by the velocity, propagation distance, time delay in propagation from one cell to another, and the evolution of Ca(2+) response patterns. The temporal Ca(i)(2+) response patterns in cells are different from one cell to another, and the Ca(i)(2+) response patterns evolve from one type to another as a Ca(i)(2+) wave propagates. In addition, the spatial patterns of Ca(i)(2+) wave propagation depend on whether IP(3), ATP, or both are mediating messengers. Finally, two different geometries that reflect the in vivo and in vitro configuration of astrocytic networks also yield distinct intracellular and extracellular kinetic patterns. The simulation results as well as the linear stability analysis of the model lead to the conclusion that Ca(i)(2+) waves in astrocyte networks are probably mediated by both intercellular IP(3) transport and nonregenerative (only the glutamate-stimulated cell releases ATP) or partially regenerative extracellular ATP signaling.

Infusion of gliotoxins or a gap junction blocker in the prelimbic cortex increases alcohol preference in Wistar rats

Postmortem research has revealed that there is a lower density of glial cells in regions of the prefrontal cortex (PFC) of uncomplicated alcoholics when compared with control subjects. Impairment of astrocyte function in the PFC may contribute to malfunction in circuits involved in emotion- and reward-related subcortical centers, heavily connected with the PFC and directly involved in the pathophysiology of addictive behaviours. The hypothesis was tested that infusion of gliotoxins known to injure astrocytes or of a gap junction blocker into the prelimbic area of the rat PFC results in increased preference for ethanol in rats exposed to free choice between water and 10% ethanol. Fluorocitric acid, L-alpha-aminoadipic acid (AAD) or the gap junction blocker 18-alpha-glycyrrhetinic acid (AGA) were bilaterally infused once into the rat prelimbic cortex and alcohol preference (ratio of 10% ethanol consumed to total liquid ingested) was measured before and after infusion. Infusion of AAD or AGA dissolved in their vehicles, but not of their vehicles alone, resulted in significant transient increase of preference for 10% ethanol. The present data suggest that impaired integrity of glial cells or the gap junctional communication between them in the rat PFC may contribute to changes in ethanol preference.

Astrocytic protection of spinal motor neurons but not cortical neurons against loss of Als2/alsin function

Three neurodegenerative diseases affecting upper and/or lower motor neurons have been associated with loss of ALS2/Alsin function: juvenile amyotrophic lateral sclerosis, primary lateral sclerosis and infantile-onset ascending hereditary spastic paralysis. The distinct neuronal vulnerability and the role of glia in these diseases remains, however, unclear. We here demonstrate that alsin-depleted spinal motor neurons can be rescued from defective survival and axon growth by co-cultured astrocytes. The astrocytic rescue is mediated by a soluble protective factor rather than by cellular contact. Cortical neurons are intrinsically as vulnerable to alsin depletion as spinal motor neurons but cannot be rescued by co-cultured astrocytes. To our knowledge, these data provide the first example of non-cell-autonomous glial effects in a recessive form of motor neuron disease and a potential rationale for the higher vulnerability of upper versus lower motor neurons in ALS2/Alsin-linked disorders.

Differential NMDA-dependent activation of glial cells in mouse hippocampus

In the hippocampus, the NMDA receptor is thought to be an important glutamate receptor involved in synaptic plasticity and in memory processes. Until recently, NMDA receptors have been considered solely as neuronal components, but some evidence suggests that glial cells in the hippocampus, and in particular astrocytes, also could be activated by NMDA applications. On the basis of their shape and electrophysiological properties (linear and rectified I/V curve), we describe two different populations of glial cells from GFAP-GFP transgenic mice that are activated differentially by NMDA. We found that linear glial cells were depolarized by NMDA that was not dependent on Ca2+ rise but partially involved a Ca2+ entry. Additionally, NMDA-induced depolarization of linear glial cells involved both a TTX-independent pathway likely through a direct activation, and a TTX-dependent pathway that required neuronal activity. The NMDA-induced depolarization in these cells was in part due to the activation of glutamate transporters and GABA B receptors. Furthermore, TTX-dependent NMDA-induced activation regulates the level of gap junction coupling between linear glial cells. In contrast, NMDA-induced depolarization in outward rectifying cells do not require a Ca2+ rise but are mediated directly by Ca2+ entry and are independent of glutamate transporters, GABA B and GABA A receptors. Our findings reveal that NMDA differentially activates hippocampal glial cells and the glial network through heterogeneous mechanisms in a cell-type specific manner.

Modulation of brain hemichannels and gap junction channels by pro-inflammatory agents and their possible role in neurodegeneration

In normal brain, neurons, astrocytes, and oligodendrocytes, the most abundant and active cells express pannexins and connexins, protein subunits of two families forming membrane channels. Most available evidence indicates that in mammals endogenously expressed pannexins form only hemichannels and connexins form both gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activity, hemichannels communicate the intra- and extracellular compartments and serve as a diffusional pathway for ions and small molecules. A subthreshold stimulation by acute pathological threatening conditions (e.g., global ischemia subthreshold for cell death) enhances neuronal Cx36 and glial Cx43 hemichannel activity, favoring ATP release and generation of preconditioning. If the stimulus is sufficiently deleterious, microglia become overactivated and release bioactive molecules that increase the activity of hemichannels and reduce gap junctional communication in astroglial networks, depriving neurons of astrocytic protective functions, and further reducing neuronal viability. Continuous glial activation triggered by low levels of anomalous proteins expressed in several neurodegenerative diseases induce glial hemichannel and gap junction channel disorders similar to those of acute inflammatory responses triggered by ischemia or infectious diseases. These changes are likely to occur in diverse cell types of the CNS and contribute to neurodegeneration during inflammatory process.

Implications of antiinflammatory properties of the anticonvulsant drug levetiracetam in astrocytes

There is accumulating evidence that epileptic activity is accompanied by inflammatory processes. In the present study, we evaluated the effect of levetiracetam (Keppra), an anticonvulsant drug with decisive antiepileptic features, with regard to its putative antiinflammatory potential. We previously established an in vitro cell culture model to mimic inflammatory conditions: Primary astrocytic cultures of newborn rats were cocultured with 30% (M30) microglial cells. Alternatively, cocultures containing 5% microglia (M5) were incubated with the proinflammatory mediator, the cytokine interleukin-1beta (IL-1beta), and lipopolysaccharide (LPS), a potent bacterial activator of the immune system. For the M30 cocultures, we observed reduced expression of connexin 43 (Cx43), the predominant gap junction protein. Impaired functional dye coupling and depolarized membrane resting potential (MRP) were monitored in M30 cocultures as well as in M5 cocultures treated with IL-1beta and LPS. We could show that the Cx43 expression, the coupling property, and the membrane resting potential on which we focused our inflammatory coculture model were normalized to noninflammatory level under treatment with levetiracetam (Keppra). Cumulatively, our results provide evidence for antiinflammatory properties of levetiracetam in seizure treatment.

Brain micro-ecologies: neural stem cell niches in the adult mammalian brain

Neurogenesis persists in two germinal regions in the adult mammalian brain, the subventricular zone of the lateral ventricles and the subgranular zone in the hippocampal formation. Within these two neurogenic niches, specialized astrocytes are neural stem cells, capable of self-renewing and generating neurons and glia. Cues within the niche, from cell-cell interactions to diffusible factors, are spatially and temporally coordinated to regulate proliferation and neurogenesis, ultimately affecting stem cell fate choices. Here, we review the components of adult neural stem cell niches and how they act to regulate neurogenesis in these regions.

Electrophysiological properties and gap junction coupling of striatal astrocytes

The striatum is the biggest nucleus of the basal ganglia and receives input from almost all cortical regions, substantia nigra and the thalamus. Striatal neuronal circuitry is well characterized, but less is known about glial physiology. To this end, we evaluated astrocyte electrophysiological properties using whole-cell patch-clamp recording in dorsal striatal brain slices from P15 to P21 rat. The majority of cells (95%) were passive astrocytes that do not express any detectable voltage-gated channels. Passive astrocytes were subcategorized into three groups based on time-dependent current properties. The observed proportion of the different astrocyte subtypes did not change within the age range evaluated here, but was modulated during reduction of specific conductances and gap junction coupling. Striatal astrocytes were extensively interconnected and closure of gap junctions with octanol (1mM), carbenoxolone (100 microM) or increased intracellular calcium (2mM), significantly altered intrinsic properties. When simultaneously blocking potassium channels and gap junction coupling almost no passive conductance was detected, implying that the major currents in striatal astrocytes derive from potassium and gap junction conductance. Uncoupling of the syncytium reduced currents activated in response to a hyperpolarizing pulse, suggesting that changes in gap junction coupling alters astrocyte electrophysiological responses. Our findings indicate that the prevalent gap junction coupling is vital for astrocyte function in the striatum, and that whole-cell recordings will be distorted by currents activated in neighboring cells.

Ultrastructural localization of extranuclear progestin receptors relative to C1 neurons in the rostral ventrolateral medulla

To better understand the role of progestins in the C1 area of the rostral ventrolateral medulla (RVLM), immunocytochemical localization of progestin receptors (PRs) was combined with tyrosine hydroxylase (TH) in single sections of RVLM from proestrus rat brains prepared for light and electron microscopy. By light microscopy, PR-immunoreactivity (-ir) was detected in a few nuclei that were interspersed between TH-labeled perikarya and dendrites. Electron microscopy revealed that PR-ir was in several extranuclear locations. The majority of PR-labeling was in non-TH immunoreactive axons (51+/-9%) near the plasma membrane. Additional dual labeling studies revealed that PR-immunoreactive axons could give rise to terminals containing the GABAergic marker GAD65. PR-ir also was found in non-neuronal processes (29+/-9%), some resembling astrocytes. Occasionally, PR-ir was in non-TH-labeled terminals (10+/-3%) affiliated with clusters of small synaptic vesicles, or in patches contained in the cytoplasm of dendrites (10+/-1%). These findings suggest that progestins can primarily modulate neurons in the C1 area of the RVLM by presynaptic mechanisms involving GABAergic transmission. Moreover, they suggest that PR activation may contribute to progestin's effects on arterial blood pressure during pregnancy as well as to sex differences in central cardiovascular regulation.

Purinergic junctional transmission and propagation of calcium waves in cultured spinal cord microglial networks

In order to elucidate the mechanisms of purinergic transmission of calcium (Ca(2 + )) waves between microglial cells, we have employed micro-photolithographic methods to form discrete patterns of microglia that allow quantitative measurements of Ca(2 + ) wave propagation. Microglia were confined to lanes 20-100 [Formula: see text] wide and Ca(2 + ) waves propagated from a point of mechanical stimulation, with a diminution in amplitude, for about 120 [Formula: see text]. The number of cells participating in propagation also decreased over this distance. Ca(2 + ) waves could propagate across a cell-free lane from one microglia lane to another if this distance of separation was less than about 60 [Formula: see text], indicating that propagation involved diffusion of a chemical transmitter. This transmitter was identified as ATP since all Ca(2 + ) wave propagation was blocked by the purinoceptor antagonist suramin, which blocks P2Y(2) and P2Y(12) at relatively low concentrations. Antibodies to P2Y(12) showed these at very high density compared with P2Y(2), indicating a role for P2Y(12) receptors. These observations were quantitatively accounted for by a model in which the main determinants are the diffusion of ATP released from a stimulated microglial cell and differences in the dissociation constant of the purinoceptors on the microglial cells.

Intracellular application of TNF-alpha impairs cell to cell communication via gap junctions in glioma cells

Human gliomas are the most common class of brain neoplasm. In order to better characterize their response to inflammation, we evaluated the influence of tumor necrosis factor alpha (TNF-alpha) on the coupling behaviour and the membrane resting potential (MRP) of glioma cells (F98 glioma cell line) compared to primary astrocytes. In contrast to cultured primary astrocytes which exhibited a profound inhibition of gap junction mediated intercellular communication (GJIC), extracellular exposure of TNF-alpha to F98 glioma cells gained no effect on the functional coupling. Whereas, intracellular application of TNF-alpha into the glioma cells elicited similar effects as those found in primary astrocytes indicating a compromised accessibility of the TNF-alpha receptor in F98 cells. Western blotting, immunocytochemical staining and real time RT PCR analysis revealed a differential expression and distribution of TNF-alpha receptor 1 (TNFR1) in the glioma cells. Connexin 43 (Cx43) is the major astrocytic gap junction protein which when phosphorylated has been shown to reveal altered gating properties. Here we show that TNF-alpha increases the level of phosphorylated Cx43 in primary astrocytes but not in the F98 glioma cells. Our observations could account for the decreased regulatory effects of TNF-alpha on GJIC of F98 glioma cells.

Expression and functional characterization of transient receptor potential vanilloid-related channel 4 (TRPV4) in rat cortical astrocytes

Cell-cell communication in astroglial syncytia is mediated by intracellular Ca(2+) ([Ca(2+)](i)) responses elicited by extracellular signaling molecules as well as by diverse physical and chemical stimuli. Despite the evidence that astrocytic swelling promotes [Ca(2+)](i) elevation through Ca(2+) influx, the molecular identity of the channel protein underlying this response is still elusive. Here we report that primary cultured cortical astrocytes express the transient receptor potential vanilloid-related channel 4 (TRPV 4), a Ca(2+)-permeable cation channel gated by a variety of stimuli, including cell swelling. Immunoblot and confocal microscopy analyses confirmed the presence of the channel protein and its localization in the plasma membrane. TRPV4 was functional because the selective TRPV4 agonist 4-alpha-phorbol 12,13-didecanoate (4alphaPDD) activated an outwardly rectifying cation current with biophysical and pharmacological properties that overlapped those of recombinant human TRPV4 expressed in COS cells. Moreover, 4alphaPDD and hypotonic challenge promoted [Ca(2+)](i) elevation mediated by influx of extracellular Ca(2+). This effect was abolished by low micromolar concentration of the TRPV4 inhibitor Ruthenium Red. Immunofluorescence and immunogold electron microscopy of rat brain revealed that TRPV4 was enriched in astrocytic processes of the superficial layers of the neocortex and in astrocyte end feet facing pia and blood vessels. Collectively, these data indicate that cultured cortical astroglia express functional TRPV4 channels. They also demonstrate that TRPV4 is particularly abundant in astrocytic membranes at the interface between brain and extracerebral liquid spaces. Consistent with its roles in other tissues, these results support the view that TRPV4 might participate in astroglial osmosensation and thus play a key role in brain volume homeostasis.

Connexin channel permeability to cytoplasmic molecules

Connexin channels are known to be permeable to a variety of cytoplasmic molecules. The first observation of second messenger junctional permeability, made approximately 30 years ago, sparked broad interest in gap junction channels as mediators of intercellular molecular signaling. Since then, much has been learned about the diversity of connexin channels with regard to isoform diversity, tissue and developmental distribution, modes of channel regulation, assembly, expression, biochemical modification and permeability, all of which appear to be dynamically regulated. This information has expanded the potential roles of connexin channels in development, physiology and disease, and made their elucidation much more complex--30 years ago such an orchestra of junctional dynamics was unanticipated. Only recently, however, have investigators been able to directly address, in this more complex framework, the key issue: what specific biological molecules, second messengers and others, are able to permeate the various types of connexin channels, and how well? An important related issue, given the ever-growing list of connexin-related pathologies, is how these permeabilities are altered by disease-causing connexin mutations. Together, many studies show that a variety of cytoplasmic molecules can permeate the different types of connexin channels. A few studies reveal differences in permeation by different molecules through a particular type of connexin channel, and differences in permeation by a particular molecule through different types of connexin channels. This article describes and evaluates the various methods used to obtain these data, presents an annotated compilation of the results, and discusses the findings in the context of what can be inferred about mechanism of selectivity and potential relevance to signaling. The data strongly suggest that highly specific interactions take place between connexin pores and specific biological molecular permeants, and that those interactions determine which cytoplasmic molecules can permeate and how well. At this time, the nature of those interactions is unclear. One hopes that with more detailed permeability and structural information, the specific molecular mechanisms of the selectivity can be elucidated.

Where and how could intentional programs be generated in the brain?

Based on glial-neuronal interaction a formalism (negative language) for the generation of intentional programs is proposed. An intentional program generates a specific multirelational structure in an inner or outer appropriate environment according to the principle of feasibility. After description of the glial spatio-temporal boundary-setting function in its interaction with the neuronal system, it is hypothesized that intentional programs may be generated in glial networks (syncytia) in line with the formalism of negative language. Gap junctions are interpreted as multirelational negation operators, generating cycles in a permutation system. These cycles could represent intentional programs that can either be realized or not in neuronal networks embodying a permutation system. The feasibility of these intentional programs is essentially dependent on appropriate environmental information. Since the realization of intentional programs in neuronal networks allows high degrees of freedom, the problem of free will is tackled, as well. Free will is defined as the subjective freedom to choose between the inner determination of intentional programs and the overdetermination of their feasibility in an appropriate environment. Finally, the possible implementation of the proposed brain model in robot brains is briefly discussed.

Adenosine released by astrocytes contributes to hypoxia-induced modulation of synaptic transmission

Astrocytes play a critical role in brain homeostasis controlling the local environment in normal as well as in pathological conditions, such as during hypoxic/ischemic insult. Since astrocytes have recently been identified as a source for a wide variety of gliotransmitters that modulate synaptic activity, we investigated whether the hypoxia-induced excitatory synaptic depression might be mediated by adenosine release from astrocytes. We used electrophysiological and Ca2+ imaging techniques in hippocampal slices and transgenic mice, in which ATP released from astrocytes is specifically impaired, as well as chemiluminescent and fluorescence photometric Ca2+ techniques in purified cultured astrocytes. In hippocampal slices, hypoxia induced a transient depression of excitatory synaptic transmission mediated by activation of presynaptic A1 adenosine receptors. The glia-specific metabolic inhibitor fluorocitrate (FC) was as effective as the A1 adenosine receptor antagonist CPT in preventing the hypoxia-induced excitatory synaptic transmission reduction. Furthermore, FC abolished the extracellular adenosine concentration increase during hypoxia in astrocyte cultures. Several lines of evidence suggest that the increase of extracellular adenosine levels during hypoxia does not result from extracellular ATP or cAMP catabolism, and that astrocytes directly release adenosine in response to hypoxia. Adenosine release is negatively modulated by external or internal Ca2+ concentrations. Moreover, adenosine transport inhibitors did not modify the hypoxia-induced effects, suggesting that adenosine was not released by facilitated transport. We conclude that during hypoxia, astrocytes contribute to regulate the excitatory synaptic transmission through the release of adenosine, which acting on A1 adenosine receptors reduces presynaptic transmitter release. Therefore, adenosine release from astrocytes serves as a protective mechanism by down regulating the synaptic activity level during demanding conditions such as transient hypoxia.

Astrocytic-inducible nitric oxide synthase in the ischemic developing human brain

Variability in the expression of apoptotic and inflammatory mediators with time after an ischemic insult and their role in the expansion of cerebral infarcts are still controversial. This study examines DNA degradation and the expression of activated caspase-3 and iNOS, inducible nitric oxide (iNOS) in the human developing brain. Autopsy specimens from children with a neuropathologic diagnosis of focal ischemic infarct were included in the study. The specimens were classified based on the clinical history as acute (< 24 h, n = 5), subacute (24-72 h, n = 8), or old (> 72 h, n = 6) infarcts. Immunohistochemical staining for caspase-3, iNOS and TUNEL were then performed on all infarcts alongside age-matched controls. TUNEL staining was detected in regions of all infarcts. Expression of iNOS was significantly higher than that of caspase-3 in the penumbra of subacute infarcts (p = 0.02). Glial fibrillary acidic protein and iNOS staining co-localized in the penumbra of acute and subacute infarcts. These results suggest that cell death continues to occur for more than 3 d post ischemic insult. Cell death in the penumbra of subacute infarcts is partially caspase-3 independent and may be attributed to nitric oxide. Astrocytes are a source of iNOS and may play a role in the evolution of pediatric brain injury days after the initial insult.

Astrocyte calcium elevations: properties, propagation, and effects on brain signaling

The possibility that astrocytes are involved in brain signaling began to emerge in the late 1970s, when it was first shown that astroglia in vitro possess numerous receptors for neurotransmitters. It was later demonstrated that cultured astroglia and astrocytes in situ respond to neurotransmitters with increases in intracellular second messengers, including cyclic AMP and calcium. Astrocyte calcium responses have since been extensively studied both in culture and in intact tissue. We continue to gather information regarding the various compounds able to trigger astrocyte calcium increases, as well as the mechanisms involved in their initiation, propagation as a calcium wave within and between astrocytes, and effects on signaling within the brain. This review will focus on each of these aspects of astrocyte calcium regulation, and attempt to sort out which effects are more likely to occur in developmental, pathological, and physiological conditions. While we have come far in our understanding of the properties or potential of astrocytes' ability to signal to neurons using our array of pharmacological tools, we still understand very little regarding the level of involvement of astrocyte signaling in normal brain physiology.

Astrocyte calcium waves: what they are and what they do

Several lines of evidence indicate that the elaborated calcium signals and the occurrence of calcium waves in astrocytes provide these cells with a specific form of excitability. The identification of the cellular and molecular steps involved in the triggering and transmission of Ca(2+) waves between astrocytes resulted in the identification of two pathways mediating this form of intercellular communication. One of them involves the direct communication between the cytosols of two adjoining cells through gap junction channels, while the other depends upon the release of "gliotransmitters" that activates membrane receptors on neighboring cells. In this review we summarize evidence in favor of these two mechanisms of Ca(2+) wave transmission and we discuss that they may not be mutually exclusive, but are likely to work in conjunction to coordinate the activity of a group of cells. To address a key question regarding the functional consequences following the passage of a Ca(2+) wave, we list, in this review, some of the potential intracellular targets of these Ca(2+) transients in astrocytes, and discuss the functional consequences of the activation of these targets for the interactions that astrocytes maintain with themselves and with other cellular partners, including those at the glial/vasculature interface and at perisynaptic sites where astrocytic processes tightly interact with neurons.

Ethanol effects on electrophysiological properties of astrocytes in striatal brain slices

Ethanol (EtOH) is known to alter neuronal physiology, but much less is known about the actions of this drug on glial function. To this end, we examined acute effects of ethanol on resting and voltage-activated membrane currents in striatal astrocytes using rat brain slices. Ten minutes exposure to 50mM EtOH reduced slope conductance by 20%, increased input resistance by 25% and decreased capacitance by 38% but did not affect resting membrane potential. Current generated by a hyperpolarizing pulse was inhibited in a concentration dependent manner in passive astrocytes, while no significant EtOH effect was observed in complex astrocytes or neurons. The EtOH effect was blocked when intracellular KCl was replaced with CsCl, but not during chelation of intracellular calcium with BAPTA. During blockage of gap junction coupling with high intracellular CaCl(2) or extracellular carbenoxolone the EtOH effect persisted but was reduced. Interestingly, EtOH effects were largely irreversible when gap junctions were open, but were fully reversible when gap junctions were closed. Ethanol also reduced the spread to other cells of Lucifer Yellow dye from individual glia filled via the patch pipette. These data suggest that EtOH inhibits a calcium-insensitive potassium channel, most likely a passive potassium channel, but also affects gap junction coupling in a way that is sustained after ethanol withdrawal. Astrocytes play a critical role in brain potassium homeostasis, and therefore EtOH effects on astrocytic function could influence neuronal activity.

Purinergic junctional transmission and propagation of calcium waves in spinal cord astrocyte networks

Micro-photolithographic methods have been employed to form discrete patterns of spinal cord astrocytes that allow quantitative measurements of Ca(2+) wave propagation. Astrocytes were confined to lanes 20-100 microm wide and Ca(2+) waves propagated from a point of mechanical stimulation or of application of adenosine triphosphate; all Ca(2+) wave propagation was blocked by simultaneous application of purinergic P2Y(1) and P2Y(2) antagonists. Stimulation of an astrocyte at one end of a lane, followed by further stimulation of this astrocyte, gave rise to Ca(2+) transients in the same astrocytes; however, if the second stimulation was applied to an astrocyte at the other end of the lane, then this gave rise to a different but overlapping set of astrocytes generating a Ca(2+) signal. Both the amplitude and velocity of the Ca(2+) wave decreased over 270 microm from the point of initiation, and thereafter remained, on average, constant with random variations for at least a further 350 microm. Also, the percentage of astrocytes that gave a Ca(2+) transient decreased with distance along lanes. All the above observations were quantitatively predicted by our recent theoretical model of purinergic junctional transmission, as was the Ca(2+) wave propagation along and between parallel lanes of astrocytes different distances apart. These observations show that a model in which the main determinants are the diffusion of adenosine triphosphates regeneratively released from a stimulated astrocyte, together with differences in the properties and density of the purinergic P2Y receptors on astrocytes, is adequate to predict a wide range of Ca(2+) wave transmission and propagation phenomena.

Pathogenetic role of the deafness-related M34T mutation of Cx26

Mutations in the GJB2 gene, which encodes the gap junction protein connexin26 (Cx26), are the major cause of genetic non-syndromic hearing loss. The role of the allelic variant M34T in causing hereditary deafness remains controversial. By combining genetic, clinical, biochemical, electrophysiological and structural modeling studies, we have re-assessed the pathogenetic role of the M34T mutation. Genetic and audiological data indicate that the majority of heterozygous carriers and all five compound heterozygotes exhibited an impaired auditory function. Functional expression in transiently transfected HeLa cells showed that, although M34T was correctly synthesized and targeted to the plasma membrane, it inefficiently formed intercellular channels that displayed an abnormal electrical behavior and retained only 11% of the unitary conductance of the wild-type protein (HCx26wt). Moreover, M34T channels failed to support the intercellular diffusion of Lucifer Yellow and the spreading of mechanically induced intercellular Ca2+ waves. When co-expressed together with HCx26wt, M34T exerted dominant-negative effects on cell-cell coupling. Our findings are consistent with a structural model, predicting that the mutation leads to a constriction of the channel pore. These data support the view that M34T is a pathological variant of Cx26 associated with hearing impairment.

Purinergic signalling and intercellular Ca2+ wave propagation in the organ of Corti

Extracellular ATP is a key neuromodulator of visual and auditory sensory epithelia. In the rat cochlea, pharmacological dissection indicates that ATP, acting through a highly sensitive purinergic/IP(3)-mediated signaling pathway with (little or) no involvement of ryanodine receptors, is the principal paracrine mediator implicated in the propagation of calcium waves through supporting and epithelial cells. Measurement of sensitivity to UTP and other purinergic agonists implicate P2Y(2) and P2Y(4) as the main P2Y receptor isoforms involved in these responses. Ca2+ waves, elicited under highly reproducible conditions by carefully controlling dose (1 microM) and timing of focal agonist application (0.2s), extended over radial distance greater than 160 microm from the source, identical to those activated by damaging single outer hair cells. Altogether, these results indicate that intercellular calcium waves are a robust phenomenon that confers a significant ability for cell-cell communication in the mammalian cochlea. Further ongoing research will reveal the roles that such Ca2+ waves play in the inner ear.

Purinergic signaling and microglia

Microglial cells are considered as the pathologic sensors of the brain. In this paper, we review mechanisms of purinergic signaling in microglia. As ATP is not only considered as a physiological signaling substance but is also elevated in pathology, it is not surprising that microglia express a variety of P2X, P2Y and adenosin receptors. As a rapid physiological event, ATP triggers a cationic conductance, increases the potassium conductance and also elicits a calcium response. As a long-term effect, purinergic receptor activation is linked to the movement of microglial processes and, in the context of pathology, to chemotaxis. The purinoreceptors also modulate the release of substances from microglia, such as cytokines, nitric oxide, or superoxide, which are important in the context of a pathologic response.

Purinergic transmission in the central nervous system

The adenosine 5'-triphosphate (ATP), discovered in 1929 by Karl Lohman, Cyrus Hartwell Fiske, and Yellagaprada SubbaRow, acts as an important extracellular signaling molecule. In the CNS, ATP can be released from synaptic terminals, either on its own or together with other neurotransmitters. After the release from the presynaptic terminals, ATP binds to a plethora of ionotropic and metabotropic receptors, which mediate its action as an excitatory neurotransmitter. Furthermore, ATP also acts as an important mediator in neuronal-glial communications because glial cells are endowed with numerous ATP receptors, which trigger Ca(2+) signaling events and membrane currents in both macro and microglia. In addition, ATP can be released from astroglial cells, thereby acting as a mediator of glial-glial and glial-neuronal signaling.

Role of gap junction in the expression of morphine-induced antinociception

The present study was undertaken to investigate whether gap junctional communication could be involved in morphine-induced antinociceptive response using blockers of the gap junctional channel, carbenoxolone and Gap27. Intrathecal pretreatment with either carbenoxolone or Gap27 caused a dose-dependent attenuation of morphine-induced antinociception. Furthermore, the dose-response line for morphine-induced antinociception was shifted to the right by 2.53-fold following intrathecal treatment with carbenoxolone. These findings suggest that gap-junctional-dependent communication in the mouse spinal cord may play, at least in part, a role in the expression of morphine-induced antinociception.

Ultrastructural localization of estrogen receptor beta immunoreactivity in the rat hippocampal formation

Several lines of evidence indicate that estrogen affects hippocampal synaptic plasticity through rapid nongenomic mechanisms, possibly by binding to plasma membrane estrogen receptors (ERs). We have previously shown that ERalpha immunoreactivity (ir) is in select interneuron nuclei and in several extranuclear locations, including dendritic spines and axon terminals, within the rat hippocampal formation (Milner et al., [2001] J Comp Neurol 429:355). The present study sought to determine the cellular and subcellular locations of ERbeta-ir. Coronal hippocampal sections from diestrus rats were immunolabeled with antibodies to ERbeta and examined by light and electron microscopy. By light microscopy, ERbeta-ir was primarily in the perikarya and proximal dendrites of pyramidal and granule cells. ERbeta-ir was also in a few nonprincipal cells and scattered nuclei in the ventral subiculum and CA3 region. Ultrastructural analysis revealed ERbeta-ir at several extranuclear sites in all hippocampal subregions. ERbeta-ir was affiliated with cytoplasmic organelles, especially endomembranes and mitochondria, and with plasma membranes primarily of principal cell perikarya and proximal dendrites. ERbeta-ir was in dendritic spines, many arising from pyramidal and granule cell dendrites. In both dendritic shafts and spines, ERbeta-ir was near the perisynaptic zone adjacent to synapses formed by unlabeled terminals. ERbeta-ir was in preterminal axons and axon terminals, associated with clusters of small, synaptic vesicles. ERbeta-labeled terminals formed both asymmetric and symmetric synapses with dendrites. ERbeta-ir also was detected in glial profiles. The cellular and subcellular localization of ERbeta-ir was generally similar to that of ERalpha, except that ERbeta was more extensively found at extranuclear sites. These results suggest that ERbeta may serve primarily as a nongenomic transducer of estrogen actions in the hippocampal formation.