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D M Lovinger

Publications and source records attributed to D M Lovinger.

At least 37 records · Page 2Linked to original sources
PubMed1997

Relative contributions of G protein, channel, and receptor to voltage-dependent inhibition of neuronal N-type and P/Q-type calcium channels in HEK 293 cell lines.

The voltage-dependent modulation of neuronal voltage-gated calcium channels by heterotrimeric G protein-coupled receptors potentially provides a means for activity-dependent modulation of synaptic efficacy. Recent attention has focused upon the molecular mechanisms by which such G proteins influence the biophysical properties of calcium channels. We have used an HEK 293-based heterologous system which stably expresses human neuronal calcium channels to address the relative contributions of receptor, G protein, and channel to voltage-dependent inhibition. We find that the receptor and channel subtype only insignificantly influence the time it takes to re-establish modulation following voltage-dependent relief of inhibition. In contrast, the G protein subtype mediating inhibition appears to play a significant part in this process. These results emphasize the importance of G protein subtype in the modulation of neuronal calcium channels.

Calcium Channels↗
PubMed1997

Decreased frequency but not amplitude of quantal synaptic responses associated with expression of corticostriatal long-term depression.

We have investigated the site of expression of striatal long-term synaptic depression (LTD) using analysis of Sr2+-induced asynchronous release of quanta from stimulated synapses. The cumulative amplitude distribution of Sr2+-induced asynchronous synaptic responses overlaps with that of miniature EPSCs (mEPSCs), suggesting that Sr2+-induced asynchronous responses are quantal. Quantal amplitude at stimulated synapses is not significantly altered after LTD induction, whereas quantal frequency decreases after LTD induction. The decrease in quantal frequency is prevented when LTD expression is blocked by dialyzing 10 mM EGTA into the postsynaptic neuron. Our findings are most consistent with the idea that expression of striatal LTD involves decreased neurotransmitter release with no change in quantal amplitude, despite the fact that induction of striatal LTD involves postsynaptic mechanisms.

Animals↗
PubMed1997

Substitution of a mutant alpha2a-adrenergic receptor via "hit and run" gene targeting reveals the role of this subtype in sedative, analgesic, and anesthetic-sparing responses in vivo.

Norepinephrine contributes to antinociceptive, sedative, and sympatholytic responses in vivo, and alpha2 adrenergic receptor (alpha2AR) agonists are used clinically to mimic these effects. Lack of subtype-specific agonists has prevented elucidation of the role that each alpha2AR subtype (alpha2A, alpha2B, and alpha2C) plays in these central effects. Here we demonstrate that alpha2AR agonist-elicited sedative, anesthetic-sparing, and analgesic responses are lost in a mouse line expressing a subtly mutated alpha2AAR, D79N alpha2AAR, created by two-step homologous recombination. These functional changes are accompanied by failure of the D79N alpha2AAR to inhibit voltage-gated Ca2+ currents and spontaneous neuronal firing, a measure of K+ current activation. These results provide definitive evidence that the alpha2AAR subtype is the primary mediator of clinically important central actions of alpha2AR agonists and suggest that the D79N alpha2AAR mouse may serve as a model for exploring other possible alpha2AAR functions in vivo.

Analgesics↗
PubMed1997

Translocation of autophosphorylated calcium/calmodulin-dependent protein kinase II to the postsynaptic density.

Calcium/calmodulin-dependent protein kinase II (CaMKII) undergoes calcium-dependent autophosphorylation, generating a calcium-independent form that may serve as a molecular substrate for memory. Here we show that calcium-independent CaMKII specifically binds to isolated postsynaptic densities (PSDs), leading to enhanced phosphorylation of many PSD proteins including the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)-type glutamate receptor. Furthermore, binding to PSDs changes CaMKII from a substrate for protein phosphatase 2A to a protein phosphatase 1 substrate. Translocation of CaMKII to PSDs occurs in hippocampal slices following treatments that induce CaMKII autophosphorylation and a form of long term potentiation. Thus, synaptic activation leads to accumulation of autophosphorylated, activated CaMKII in the PSD. This increases substrate phosphorylation and affects regulation of the kinase by protein phosphatases, which may contribute to enhancement of synaptic strength.

Animals↗
PubMed1997

Decreased probability of neurotransmitter release underlies striatal long-term depression and postnatal development of corticostriatal synapses.

Changes in synaptic efficacy are crucial for the development of appropriate neural circuits and brain information storage. We have investigated mechanisms underlying long-term depression (LTD) at glutamatergic synapses in the striatum, a brain region important in motor performance and cognition, and a target for Huntington and Parkinson diseases. Induction of striatal LTD is dependent on postsynaptic depolarization and calcium influx through L-type channels. Surprisingly, LTD maintenance appears to involve a decrease in the probability of neurotransmitter release from presynaptic terminals as evidenced by increases in paired-pulse facilitation and the coefficient of variation of synaptic responses that are tightly associated with LTD expression. Furthermore, both the apparent probability of neurotransmitter release and the magnitude of LTD decrease concomitantly during postnatal development, consistent with the idea that striatal LTD is involved in a developmental decrease in the probability of neurotransmitter release at corticostriatal synapses. The presynaptic changes that underlie striatal LTD may also be important for motor performance and certain forms of learning and memory.

Aging↗
PubMed1997

Alcohols and neurotransmitter gated ion channels: past, present and future.

The neurotransmitter-gated ion channels form a superfamily of neurotransmitter receptors specialized for recognizing transmitters and rapidly gating ion channels that are contained within the same holoprotein complex. A large body of research indicates that alcohols alter the function of this class of receptor at concentrations relevant to the intoxicating and anesthetic effects of the alcohols. In addition, studies have implicated several types of neurotransmitter-gated channels in intoxicating and anesthetic alcohol actions. All of the neurotransmitter-gated ion channels contain conserved features such as N-terminal ligand binding domains, hydrophobic membrane spanning domains and charged pore-lining domains. However, at least two, and possibly three families of receptors have been identified within the superfamily, including the nicotinic ACh-like receptors, the ionotropic glutamate receptors and the ATP-gated ion channels. This review will begin with a brief overview of the structural features of the different receptors, with an emphasis on comparing and contrasting features of the different families of neurotransmitter-gated channels. The emphasis will be mostly on the nicotinic-like receptors and the iGluRs, since more is known about these receptors than about other types of ligand-activated cation channels. The remainder of the review focuses on the latest studies aimed at determining the mechanism of alcohol actions on this superfamily of receptor-channels as well as the relationship between the molecular structure of these channels and the effects of alcohols on channel function. In addition, emerging directions for future study of these effects of alcohols and possible regions of the protein that may be altered during alcohol exposure are discussed.

Animals↗
PubMed1997

Ethanol inhibition of N-methyl-D-aspartate-activated current in mouse hippocampal neurones: whole-cell patch-clamp analysis.

1. The action of ethanol on N-methyl-D-aspartate (NMDA)-activated ion current was studied in mouse hippocampal neurones in culture using whole-cell patch-clamp recording. 2. Ethanol inhibited NMDA-activated current in a voltage-independent manner, and did not alter the reversal potential of NMDA-activated current. 3. Concentration-response analysis of NMDA- and glycine-activated current revealed that ethanol decreased the maximal response to both agonists without affecting their EC50 values. 4. The polyamine spermine (1 microM) increased amplitude of NMDA-activated current but did not alter the percentage inhibition of ethanol. 5. Compared to an extracellular pH of 7.0, pH 6.0 decreased and pH 8.0 increased the amplitude of NMDA-activated current, but these changes in pH did not significantly alter the percentage inhibition by ethanol. 6. The sulphydryl reducing agent dithiothreitol (2 mM) increased the amplitude of NMDA-activated current, but did not affect the percentage inhibition by ethanol. 7. Mg2+ (10, 100, 500 microM), (5, 20 microM) or ketamine (2, 10 microM) decreased the amplitude of NMDA-activated current, but did not affect the percentage inhibition by ethanol. 8. The observations are consistent with ethanol inhibiting the function of NMDA receptors by a non-competitive mechanism that does not involve several modulatory sites on the NMDA receptor-ionophore complex.

Animals↗
PubMed1997

Serotonin's role in alcohol's effects on the brain.

Serotonin is an important brain chemical that acts as a neurotransmitter to communicate information among nerve cells. Serotonin's actions have been linked to alcohol's effects on the brain and to alcohol abuse. Alcoholics and experimental animals that consume large quantities of alcohol show evidence of differences in brain serotonin levels compared with nonalcoholics. Both short- and long-term alcohol exposure also affect the serotonin receptors that convert the chemical signal produced by serotonin into functional changes in the signal-receiving cell. Drugs that act on these receptors alter alcohol consumption in both humans and animals. Serotonin, along with other neurotransmitters, also may contribute to alcohol's intoxicating and re. warding effects, and abnormalities in the brain's serotonin system appear to play an important role in the brain processes underlying alcohol abuse.

Alcohol Drinking↗
PubMed1997

Chronic ethanol exposure leads to a selective enhancement of N-methyl-D-aspartate receptor function in cultured hippocampal neurons.

Effects of chronic ethanol exposure on N-methyl-D-aspartate (NMDA) receptor function were examined in hippocampal neurons. Rat hippocampal neurons grown in culture were chronically exposed to 100 mM ethanol to examine mechanisms that could underlie ethanol-induced changes in receptor function and excitotoxicity. NMDA-stimulated, but not kainic acid-stimulated, increases in intracellular calcium were enhanced after 1-, 2- and 7-day exposures to 100 mM ethanol. Chronic exposure to ethanol for 7 days duration increased the magnitude of cell death mediated by NMDA application, but not that mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate or kainic acid exposure. In addition, NMDA-induced excitotoxicity after chronic ethanol exposure (CEE) was not altered in the presence of nifedipine. The enhancement of NMDA-induced neuronal cell death was evident after 2 days of CEE, but not significantly different after a 1-day exposure to 100 mM ethanol. The enhancement of NMDA-induced calcium responses and excitotoxicity could be mimicked by a chronic 7-day exposure to aminophosphonovaleric acid. However, a concomitant chronic exposure of ethanol/aminophosphonovaleric acid did not enhance NMDA-induced calcium responses or excitotoxicity. Chronic exposure paradigms did not consistently alter basal intracellular calcium levels nor total cell number in the absence of exposure to glutamate receptor agonist. These findings support the hypothesis that NMDA receptor function is enhanced after CEE, and this predisposes hippocampal neurons to excitotoxicity.

Animals↗
PubMed1996

Synaptic transmission and modulation in the neostriatum.

The neostriatum is the entryway into the basal ganglia and is the site of many of the neurological defects involving basal ganglia function. Thus, it is important to understand the regulation of synaptic transmission at afferent synapses innervating the neostriatum. Cortical glutamatergic and nigral dopaminergic afferent input impinge on neurons in the neostriatum, providing the most significant afferent inputs to this structure. Our understanding of the mechanisms involved in transmission and modulation of transmission at these synapses has greatly increased. It is now apparent that the corticostriatal glutamatergic inputs produce rapid depolarization of striatal neurons via activation of ionotropic AMPA-type glutamate receptors. In addition, transmission is modulated by a number of presynaptic, G-protein-coupled receptors but, surprisingly, relatively little evidence of postsynaptic modulation has been observed. Corticostriatal synapses also express certain forms of plasticity, most notably short- and long- term synaptic depression (STI) and LTD, respectively). It appears that LTD may involve convergent actions of glutamate and dopamine. Striatal LTD may have important roles in information storage and motor set selection in the striatum. However, some aspects of synaptic transmission in the striatum remain unclear. In particular, the exact physiological roles of dopaminergic nigrostriatal input and the role of NMDA-type glutamate receptors are not well understood. In addition, intrastriatal synaptic connections have received relatively little attention as compared with extrinsic input to the neostriatum. Future studies will need to focus on elucidating these aspects of neostriatal function.

Animals↗
PubMed1996

Metabotropic glutamate receptor modulation of voltage-gated Ca2+ channels involves multiple receptor subtypes in cortical neurons.

Metabotropic glutamate receptor (mGluR) modulation of voltage-gated Ca2+ channels was examined in isolated deep layer frontoparietal cortical neurons under conditions designed to isolate calcium-independent modulatory pathways. Trans-1-aminocyclopentane-1,3-dicarboxylate (t-ACPD), a nonspecific mGluR agonist, produced rapid and reversible inhibition of Ca2+ channels. This effect was mimicked by agonists for group I and group II, but not group III, mGluRs. Effects of group I and II agonists often were observed in the same neurons, but separate subgroups of neurons were unresponsive to the group I agonist quisqualate or the group II agonist 2-(2,3-dicarboxycyclopropyl) glycine (DCG-IV). Inhibition by quisqualate and DCG-IV was nonocclusive in neurons responding to both agonists. These agonists thus appear to act on different mGluRs. The mGluR antagonist alpha-methyl-4-carboxylphenylglycine attenuated inhibition by t-ACPD, quisqualate, and DCG-IV. Inhibition by quisqualate and DCG-IV was voltage-dependent. Although the effects of both agonists were greatly reduced by N-ethylmaleimide (NEM), inhibition by DCG-IV was more sensitive to NEM than inhibition by quisqualate. t-ACPD-induced inhibition was reduced by omega-conotoxin GVIA (omega-CgTx) and omega-agatoxin IVA (omega-AgTx) but was affected little by nifedipine. Inhibition by DCG-IV and quisqualate also was reduced by omega-CgTx. We conclude that multiple mGluR subtypes inhibit Ca2+ channels in cortical neurons and that N- and possibly P-type channels are inhibited. Modulation is via a rapid-onset, voltage-dependent mechanism that likely involves a pertussis toxin (PTX)-sensitive G-protein. Type I mGluRs may work via additional PTX-insensitive pathways.

Animals↗
PubMed1996

Genetic evidence for involvement of multiple effector systems in alpha 2A-adrenergic receptor inhibition of stimulus-secretion coupling.

The alpha 2A-adrenergic receptor (alpha 2AAR), via its interaction with the pertussis toxin-sensitive Gi/G(o) class of G proteins, modulates multiple effector systems, including inhibition of adenylyl cyclase and Ca2+ channels and activation of K+ channels. Mutation of a membrane-embedded aspartate residue, highly conserved among G protein-coupled receptors, in the alpha 2AAR to asparagine (D79N alpha 2AAR) results in selective uncoupling of the receptor to K+ currents but retention of inhibition of cAMP production and of voltage-sensitive Ca2+ currents when expressed in AtT20 anterior pituitary cells in culture. It is known that attenuation of cAMP synthesis alone cannot account for alpha 2AAR suppression of stimulus-secretion coupling; thus, the D79N alpha 2AAR provides a unique tool with which to assess the relative contribution of K+ current activation and Ca2+ current suppression in mediating the cellular responses of alpha 2AAR. The wild-type alpha 2AAR suppresses basal and secretagogue-evoked adrenocorticotropic hormone (ACTH) release in a manner indistinguishable from response to the endogenous somatostatin receptor. In contrast, the D79N alpha 2AAR does not attenuate basal ACTH release and is only partially effective in suppressing ACTH secretion evoked by the secretagogue isoproterenol. Regulation of ACTH release evoked by 8-bromo-cAMP, which bypasses receptor regulation of cAMP synthesis, suggests that attenuation of cAMP production, although not sufficient for inhibition of ACTH secretion, nevertheless participates in a functionally relevant manner. Taken together, the present findings indicate that alpha 2AAR-mediated suppression of neuropeptide secretion requires concomitant regulation of K+ and Ca2+ currents in parallel with attenuation of cAMP production.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1996

Pharmacologic characteristics of potentiation of 5-HT3 receptors by alcohols and diethyl ether in NCB-20 neuroblastoma cells.

We have examined the actions of alkanols, halogenated ethanol derivatives and diethyl ether on ion current mediated by 5-HT3 receptors in NCB-20 neuroblastoma cells. The alcohols and diethyl ether potentiated 5-HT3 receptor-mediated ion current at concentrations that had no effect on membrane current when applied in the absence of agonist. The potency of alcohols increased with increasing hydrophobicity. However, the maximal efficacy of alcohols was unrelated to hydrophobicity. Interactions between different drugs applied simultaneously to cells were examined to determine whether these compounds compete for a distinct modulatory site associated with the 5-HT3 receptor. Analysis of interactions observed at different drug concentrations indicated a variety of interactions between different compounds, ranging from negative to positive allosteric interactions. Interactions between trichloroethanol (TCEt) and isopentanol exhibited characteristics that might indicate competition for a single site of action. However, further examination of interactions between these two drugs indicated that although isopentanol altered the efficacy of co-applied TCEt, TCEt did not have a similar effect with respect to isopentanol. Furthermore, isopentanol did not alter the potency of TCEt for potentiation of receptor function. The absence of competitive interactions among alcohols indicates that a single "alcohol receptor" cannot be defined using established pharmacologic approaches. Our findings are most consistent with the idea that alcohols interact with several hydrophobic sites associated with the 5-HT3 receptor.

Animals↗
PubMed1996

Functional coupling of rat group II metabotropic glutamate receptors to an omega-conotoxin GVIA-sensitive calcium channel in human embryonic kidney 293 cells.

Metabotropic glutamate receptors are G protein-coupled receptors that perform a variety of modulatory roles in the central and peripheral nervous systems. The development of receptor subtype-specific agonists/antagonists has lagged far behind the isolation and characterization of receptor cDNAs. Further more, the coupling of specific metabotropic receptors to the various neuronal-specific effector molecules, such as voltage gated Ca2+ channels, has not been well studied. It was recently demonstrated that a rat group II metabotropic receptor (rm-GluR2) is capable of coupling to endogenous N-type Ca2+ channels when heterologously expressed in adult rat sympathetic ganglia neurons. To eventually understand the molecular aspects of metabotropic receptor modulation of the N-type Ca2+ channel, we have transiently expressed both group II receptors in a human embryonic kidney 293 cell line (G1A1) that stably expresses the human alpha 1B-1, alpha 2b, and beta 1-3 Ca2+ channel subunits. rmGluR2 and rmGluR3 modulate the omega-conotoxin GVIA-sensitive Ba2+ currents in G1A1 cells using a voltage-dependent mechanism via an endogenous pertussis toxin-sensitive G protein. Cell-attached "macropatch" recordings demonstrate that modulation by rmGluR2 and rmGluR3 is membrane delimited. This is the first report of Ca2+ channel modulation mediated by rmGluR3. In addition, an extensive pharmacological comparison between rmGluR2 and rmGluR3 reveals that these group II receptors interact with agonists and antagonists in unique ways.

Animals↗
PubMed1995

Activation of adenosine A1 receptors initiates short-term synaptic depression in rat striatum.

High-frequency stimulation (HFS) of afferent fibers produced short-term depression (STD) and long-term depression (LTD) of corticostriatal synaptic transmission. Application of the non-selective adenosine receptor antagonist 1,3-dipropyl-8-p-sulfophenylxanthine (DPSPX) or the selective A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) blocked the induction of STD but not LTD. Application of adenosine or the selective A1 receptor agonist R(-)N6-(2-phenylisopropyl)adenosine (R-PIA) induced synaptic depression, while the A2a receptor agonist CGS 21680 did not consistently alter synaptic transmission. Depression induced by adenosine or R-PIA was not accompanied by changes in postsynaptic input resistance and appeared to involve a presynaptic depressant effect previously characterized at this synapse. These observations indicate that HFS leads to the production of endogenous adenosine that acts on presynaptic A1 receptors to initiate STD at corticostriatal synapses. Initiation and maintenance of LTD appear to be independent of A1 receptor activation.

Adenosine↗
PubMed1995

Ifenprodil inhibition of the 5-hydroxytryptamine3 receptor.

The anti-hypertensive drug ifenprodil is known to interact potently with the alpha 1-adrenergic receptor as well as a number of other second messenger-linked receptors. In addition to these properties, ifenprodil has been shown to prevent glutamate-mediated excitotoxicity via non-competitive antagonism of NMDA receptors [Legendre and Westbrook (1991) Molec. Pharmac. 40: 289-298; Shalaby et al. (1992) J. Pharmac. Exp. Ther. 260: 925-932]. With these things in mind, we have begun to examine the specificity of ifenprodil for various ligand-gated ion channels using electrophysiological methods. While ifenprodil effectively inhibits NMDA-mediated currents in cortical neurons in culture, it does not interact with either kainate or GABA receptors. Surprisingly, ifenprodil also acts as a relatively potent antagonist of the 5-hydroxytryptamine3 (5-HT3) receptor in the NG108-15 neuroblastoma x glioma cell line. Furthermore, several aspects of ifenprodil action on the 5-HT3 receptor resemble its interaction with the NMDA receptor. Namely, inhibition of 5-HT3-mediated cation currents is readily reversible, has relatively slow onset, is non-competitive, and is not voltage dependent. Since most of the known 5-HT3 antagonists are competitive, it is possible that ifenprodil may define a unique modulatory site(s) on this neurotransmitter receptor.

Animals↗
PubMed1995

Metabotropic glutamate receptor modulation of synaptic transmission in corticostriatal co-cultures: role of calcium influx.

Modulation of excitatory glutamatergic transmission at corticostriatal synapses by a metabotropic glutamate receptor (mGluR) was examined using a newly developed cell culture preparation in which small explants of cortical tissue are grown in co-culture with isolated striatal neurons. Electrical stimulation of cortical tissue evoked excitatory postsynaptic currents (eEPSCs) observed during tight-seal, whole-cell recordings from striatal neurons. Transmission was mediated by activation of AMPA/kainate-type glutamate receptors. The mGluR agonists, 1SR,3RS-ACPD and DCG-IV, reduced eEPSC amplitude. The effect of 1SR,3RS-ACPD increased in a concentration-dependent manner. Application of phorbol diacetate (PDAc) potentiated eEPSC amplitude and reduced the inhibitory effect of mGluR activation. Pretreatment with pertussis toxin (PTX) also reduced inhibition by 1SR,3RS-ACPD. Under conditions in which transmission was independent of the function of voltage-gated calcium channels, mGluR activation reduced the frequency of occurrence of miniature EPSCs (mEPSCs), but did not alter mEPSC amplitude. This effect of mGluR activation was reduced by PDAc treatment. mGluR activation modulates glutamatergic transmission via a presynaptic autoreceptor at corticostriatal synapses in this newly-developed corticostriatal co-culture preparation as in striatal slices. Modulation of transmission occurs whether or not transmission involves activation of voltage-gated calcium channels. Furthermore, many of the characteristics of mGluR modulation of eEPSCs are shared by mGluR modulation of mEPSCs. These findings indicate that mechanisms downstream from calcium entry may contribute to modulation of synaptic transmission by mGluR autoreceptors.

Animals↗