Absence of transient receptor potential vanilloid-1 accelerates stress-induced axonopathy in the optic projection.

How neurons respond to stress in degenerative disease is of fundamental importance for identifying mechanisms of progression and new therapeutic targets. Members of the transient receptor potential (TRP) family of cation-selective ion channels are candidates for mediating stress signals, since different subunits transduce a variety of stimuli relevant in both normal and pathogenic physiology. We addressed this possibility for the TRP vanilloid-1 (TRPV1) subunit by comparing how the optic projection of Trpv1(-/-) mice and age-matched C57 controls responds to stress from elevated ocular pressure, the critical stressor in the most common optic neuropathy, glaucoma. Over a 5 week period of elevated pressure induced by microbead occlusion of ocular fluid, Trpv1(-/-) accelerated both degradation of axonal transport from retinal ganglion cells to the superior colliculus and degeneration of the axons themselves in the optic nerve. Ganglion cell body loss, which is normally later in progression, occurred in nasal sectors of Trpv1(-/-) but not C57 retina. Pharmacological antagonism of TRPV1 in rats similarly accelerated ganglion cell axonopathy. Elevated ocular pressure resulted in differences in spontaneous firing rate and action potential threshold current in Trpv1(-/-) ganglion cells compared with C57. In the absence of elevated pressure, ganglion cells in the two strains had similar firing patterns. Based on these data, we propose that TRPV1 may help neurons respond to disease-relevant stressors by enhancing activity necessary for axonal signaling.

Short-term increases in transient receptor potential vanilloid-1 mediate stress-induced enhancement of neuronal excitation.

Progression of neurodegeneration in disease and injury is influenced by the response of individual neurons to stressful stimuli and whether this response includes mechanisms to counter declining function. Transient receptor potential (TRP) cation channels transduce a variety of disease-relevant stimuli and can mediate diverse stress-dependent changes in physiology, both presynaptic and postsynaptic. Recently, we demonstrated that knock-out or pharmacological inhibition of the TRP vanilloid-1 (TRPV1) capsaicin-sensitive subunit accelerates degeneration of retinal ganglion cell neurons and their axons with elevated ocular pressure, the critical stressor in the most common optic neuropathy, glaucoma. Here we probed the mechanism of the influence of TRPV1 on ganglion cell survival in mouse models of glaucoma. We found that induced elevations of ocular pressure increased TRPV1 in ganglion cells and its colocalization at excitatory synapses to their dendrites, whereas chronic elevation progressively increased ganglion cell Trpv1 mRNA. Enhanced TRPV1 expression in ganglion cells was transient and supported a reversal of the effect of TRPV1 on ganglion cells from hyperpolarizing to depolarizing, which was also transient. Short-term enhancement of TRPV1-mediated activity led to a delayed increase in axonal spontaneous excitation that was absent in ganglion cells from Trpv1(-/-) retina. In isolated ganglion cells, pharmacologically activated TRPV1 mobilized to discrete nodes along ganglion cell dendrites that corresponded to sites of elevated Ca(2+). These results suggest that TRPV1 may promote retinal ganglion cell survival through transient enhancement of local excitation and axonal activity in response to ocular stress.

GluN2B subunit deletion reveals key role in acute and chronic ethanol sensitivity of glutamate synapses in bed nucleus of the stria terminalis.

The bed nucleus of the stria terminalis (BNST) is a critical region for alcohol/drug-induced negative affect and stress-induced reinstatement. NMDA receptor (NMDAR)-dependent plasticity, such as long-term potentiation (LTP), has been postulated to play key roles in alcohol and drug addiction; yet, to date, little is understood regarding the mechanisms underlying LTP of the BNST, or its regulation by ethanol. Acute and chronic exposure to ethanol modulates glutamate transmission via actions on NMDARs. Despite intense investigation, tests of subunit specificity of ethanol actions on NMDARs using pharmacological approaches have produced mixed results. Thus, we use a conditional GluN2B KO mouse line to assess both basal and ethanol-dependent function of this subunit at glutamate synapses in the BNST. Deletion of GluN2B eliminated LTP, as well as actions of ethanol on NMDAR function. Further, we show that chronic ethanol exposure enhances LTP formation in the BNST. Using KO-validated pharmacological approaches with Ro25-6981 and memantine, we provide evidence suggesting that chronic ethanol exposure enhances LTP in the BNST via paradoxical extrasynaptic NMDAR involvement. These findings demonstrate that GluN2B is a key point of regulation for ethanol's actions and suggest a unique role of extrasynaptic GluN2B-containing receptors in facilitating LTP.

Ethanol selectively attenuates NMDAR-mediated synaptic transmission in the prefrontal cortex.

BACKGROUND: Brain imaging studies have revealed abnormal function in the prefrontal cortex (PFC) of alcoholics that may contribute to the impulsive behavior and lack of control over drinking that characterizes this disorder. Understanding how ethanol affects the physiology of PFC neurons may help explain this loss of control and lead to better treatments for alcohol addiction. In a previous study from this laboratory, we showed that ethanol inhibits complex patterns of persistent activity (known as "up-states") in medial PFC (mPFC) neurons in a reversible and concentration-dependent manner. METHODS: In the current study, whole-cell patch clamp recordings were used to directly examine the effects of ethanol on the glutamatergic and GABAergic components that underlie persistent activity. RESULTS: In deep-layer mPFC pyramidal neurons, ethanol reversibly attenuated electrically evoked N-methyl-D-aspartate-type glutamate receptor (NMDAR)-mediated EPSCs. Significant inhibition was observed at concentrations as low as 22 mM, equivalent to a blood ethanol concentration (0.1%) typically associated with legal limits for intoxication. In contrast to NMDA responses, neither evoked nor spontaneous EPSCs mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-type glutamate receptor were affected by ethanol at concentrations as high as 88 mM, a concentration that can be fatal to non-tolerant individuals. At similar concentrations, ethanol also had little effect on spontaneous or evoked IPSCs mediated by a-type gamma-aminobutyric acid receptor. Finally, mPFC neurons showed little evidence of GABAR-mediated tonic current and this was unaffected by ethanol. CONCLUSIONS: Together, these results suggest that NMDAR-mediated processes in the mPFC may be particularly susceptible to disruption following the acute ingestion of ethanol.

Ccl5 Mediates Proper Wiring of Feedforward and Lateral Inhibition Pathways in the Inner Retina.

The ß-chemokine Ccl5 and its receptors are constitutively expressed in neurons of the murine inner retina. Here, we examined the functional and structural significance of this constitutive Ccl5 signaling on retinal development. We compared outcomes of electrophysiology, ocular imaging and retinal morphology in wild-type mice (WT) and mice with Ccl5 deficiency (Ccl5-/- ). Assessment of retinal structure by ocular coherence tomography and histology revealed slight thinning of the inner plexiform layer (IPL) and inner nuclear layer (INL) in Ccl5-/- mice, compared to WT (p < 0.01). Assessment of postnatal timepoints important for development of the INL (P7 and P10) revealed Ccl5-dependent alterations in the pattern and timing of apoptotic pruning. Morphological analyses of major inner retinal cell types in WT, Ccl5-/- , gustducingfp and gustducingfp/Ccl5-/- mice revealed Ccl5-dependent reduction in GNAT3 expression in rod bipolar cells as well as a displacement of their terminals from the IPL into the GCL. RGC dendritic organization and amacrine cell morphology in the IPL was similarly disorganized in Ccl5-/- mice. Examination of the intrinsic electrophysiological properties of RGCs revealed higher spontaneous activity in Ccl5-/- mice that was characterized by higher spiking frequency and a more depolarized resting potential. This hyperactive phenotype could be negated by current clamp and correlated with both membrane resistance and soma area. Overall, our findings identify Ccl5 signaling as a mediator of inner retinal circuitry during development of the murine retina. The apparent role of Ccl5 in retinal development further supports chemokines as trophic modulators of CNS development and function that extends far beyond the inflammatory contexts in which they were first characterized.

Activation of NR2A-containing NMDA receptors is not obligatory for NMDA receptor-dependent long-term potentiation.

Activation of NMDA receptors (NMDARs) within the CNS represents a major signal for persistent alterations in glutamatergic signaling, such as long-term potentiation (LTP) and long-term depression. NMDARs are composed of a combination of NR1 and NR2 subunits, with distinct NR2 subunits imparting distinct characteristics on the receptor. One particular NR2 subunit, NR2A (NRepsilon1), has been proposed to play an integral role in LTP induction in the hippocampus and cortex. Here, we report studies investigating the role of NR2A in LTP induction in the dorsolateral bed nucleus of the stria terminalis (dlBNST). The putative NR2A-specific inhibitor NVP-AAM077 (AAM077) has been used previously to demonstrate the dependence of cortical and hippocampal LTP on NMDARs containing NR2A subunits. We report here the same sensitivity of LTP to pretreatment with AAM077 (0.4 microm) in the dlBNST. However, inconsistent with the conclusion that LTP in the dlBNST is NR2A dependent, we see intact LTP in the dlBNST of NR2A knock-out mice. Because we also see blockade of this dlBNST LTP in NR2A knock-out mice after pretreatment with AAM077, we conclude that the antagonist is targeting non-NR2A subunit-containing receptors. Using a variety of cultured cell types, we find that AAM077 (0.4 microm) can attenuate transmission of NR2B subunit-containing NMDARs when preapplied rather than coapplied with an agonist. Therefore, we conclude that NR2A is not obligatory for the induction of LTP in the dlBNST. Furthermore, our data demonstrate that care must be exercised in the interpretation of data generated with AAM077 when the compound is applied before an agonist.

High-frequency stimulation induces ethanol-sensitive long-term potentiation at glutamatergic synapses in the dorsolateral bed nucleus of the stria terminalis.

Anatomical and functional data support a critical role for the bed nucleus of the stria terminalis (BNST) in the interaction between stress and alcohol/substance abuse. We report here that neurons of the dorsal anterolateral BNST respond to glutamatergic synaptic input in a synchronized way, such that an interpretable extracellular synaptic field potential can be readily measured. High-frequency stimulation of these glutamatergic inputs evoked NMDA receptor (NMDAR)-dependent long-term potentiation (LTP). We found that an early portion of this LTP is reduced by acute exposure to ethanol in a GABA(A) receptor-dependent manner. This effect of ethanol is accompanied by a significant and reversible dose-dependent attenuation of isolated NMDAR signaling and is mimicked by incomplete NMDAR blockade.

Synthesis and cellular effects of an intracellularly activated analogue of 4-hydroxynonenal.

4-Hydroxy-2-nonenal (HNE) has been recognized as reactive product of lipid peroxidation and has been suggested to play a role in the pathogenesis in several common diseases as well as injuries caused by environmental toxicants. Although formed intracellularly in vivo, for practical reasons this molecule is applied extracellularly in order to analyze its biological effects. The focus of this study was to develop an approach that would enable intracellular HNE production in the absence of the many other products and processes that occur in cells experiencing generalized oxidative stress. To this end, we synthesized 1,1,4-tris(acetyloxy)-2(E)-nonene (HNE[Ac]3), a triester analogue of HNE that is itself unreactive but could be hydrolyzed intracellularly presumably by lipases and/or esterases into the highly reactive HNE. In vitro lipase rapidly converted HNE(Ac)(3) initially to 4-acetyloxy-2-nonenal (HNE[Ac]1) and then to HNE. Neuro 2A cell lysate also caused a rapid hydrolysis of HNE(Ac)3 into HNE(Ac)1 and HNE. Incubation of BSA with HNE(Ac)3 resulted in protein-adduct formation only in the presence of lipase. We demonstrated adduction of HNE to proteins in Neuro 2A cells exposed to HNE(Ac)3 by immunoblotting and immunocytochemistry using antibodies specific for HNE-Michael adducts on proteins. We have previously shown that microtubule organization is very sensitive to HNE. Analysis of Neuro 2A cell microtubules showed that this cytoplasmic organelle is similarly sensitive to HNE and HNE(Ac)3.