Release of endogenous glutamate from rat cerebellar synaptosomes: interactions with adenosine and ethanol

Ethanol-Induced Cerebellar Ataxia: Cellular and Molecular Mechanisms

The cerebellum is an important target of ethanol toxicity given that cerebellar ataxia is the most consistent physical manifestation of acute ethanol consumption. Despite the significance of the cerebellum in ethanol-induced cerebellar ataxia (EICA), the cellular and molecular mechanisms underlying EICA are incompletely understood. However, two important findings have shed greater light on this phenomenon. First, ethanol-induced blockade of cerebellar adenosine uptake in rodent models points to a role for adenosinergic A1 modulation of EICA. Second, the consistent observation that intracerebellar administration of nicotine in mice leads to antagonism of EICA provides evidence for a critical role of cerebellar nitric oxide (NO) in EICA reversal. Based on these two important findings, this review discusses the potential molecular events at two key synaptic sites (mossy fiber-granule cell-Golgi cell (MGG synaptic site) and granule cell parallel fiber-Purkinje cell (GPP synaptic site) that lead to EICA. Specifically, ethanol-induced neuronal NOS inhibition at the MGG synaptic site acts as a critical trigger for Golgi cell activation which leads to granule cell deafferentation. Concurrently, ethanol-induced inhibition of adenosine uptake at the GPP synaptic site produces adenosine accumulation which decreases glutamate release and leads to the profound activation of Purkinje cells (PCs). These molecular events at the MGG and GPP synaptic sites are mutually reinforcing and lead to cerebellar dysfunction, decreased excitatory output of deep cerebellar nuclei, and EICA. The critical importance of PCs as the sole output of the cerebellar cortex suggests normalization of PC function could have important therapeutic implications.

A long Stokes shift red fluorescent Ca2+ indicator protein for two-photon and ratiometric imaging

The introduction of calcium ion (Ca(2+)) indicators based on red fluorescent proteins (RFPs) has created new opportunities for multicolour visualization of intracellular Ca(2+) dynamics. However, one drawback of these indicators is that they have optimal two-photon excitation outside the near-infrared window (650-1,000 nm) where tissue is most transparent to light. To address this shortcoming, we developed a long Stokes shift RFP-based Ca(2+) indicator, REX-GECO1, with optimal two-photon excitation at <1,000 nm. REX-GECO1 fluoresces at 585 nm when excited at 480 nm or 910 nm by a one- or two-photon process, respectively. We demonstrate that REX-GECO1 can be used as either a ratiometric or intensiometric Ca(2+) indicator in organotypic hippocampal slice cultures (one- and two-photon) and the visual system of albino tadpoles (two-photon). Furthermore, we demonstrate single excitation wavelength two-colour Ca(2+) and glutamate imaging in organotypic cultures.

Implication of the purinergic system in alcohol use disorders

In the central nervous system, adenosine and adenosine 5'-triphosphate (ATP) play an important role in regulating neuronal activity as well as controlling other neurotransmitter systems, such as, GABA, glutamate, and dopamine. Ethanol increases extracellular adenosine levels that regulate the ataxic and hypnotic/sedative effects of ethanol. Interestingly, ethanol is known to increase adenosine levels by inhibiting an ethanol-sensitive adenosine transporter, equilibrative nucleoside transporter type 1 (ENT1). Ethanol is also known to inhibit ATP-specific P2X receptors, which might result in such similar effects as those caused by an increase in adenosine. Adenosine and ATP exert their functions through P1 (metabotropic) and P2 (P2X-ionotropic and P2Y-metabotropic) receptors, respectively. Purinergic signaling in cortex-striatum-ventral tegmental area (VTA) has been implicated in regulating cortical glutamate signaling as well as VTA dopaminergic signaling, which regulates the motivational effect of ethanol. Moreover, several nucleoside transporters and receptors have been identified in astrocytes, which regulate not only adenosine-ATP neurotransmission, but also homeostasis of major inhibitory-excitatory neurotransmission (i.e., GABA or glutamate) through neuron-glial interactions. This review will present novel findings on the implications of adenosine and ATP neurotransmission in alcohol use disorders.

The role of nicotinic acetylcholine receptor (nAChR) α7 subtype in the functional interaction between nicotine and ethanol in mouse cerebellum

BACKGROUND

Many epidemiological studies report that alcoholics overwhelmingly smoke tobacco and vice versa, which suggests a possible functional interaction between ethanol and nicotine. Although nicotine-ethanol interaction is well documented within the central nervous system, the mechanism is not well understood. Therefore, it is important from a public health standpoint to understand the mechanisms involved in nicotine and ethanol functional interaction. The intracerebellar (ICB) administration of nicotine significantly attenuates ethanol ataxia through nicotinic acetylcholine receptor (nAChR) α(4)β(2) subtype. This study, an extension of earlier work, was intended to investigate the possible role of nAChR subtype α(7) in mitigating ethanol ataxia.

METHODS

The effect of ICB injection of PNU-282987 (α(7) agonist; 25 ng to 2.5 μg) and the antagonist methyllycaconitine was evaluated on ethanol (2 g/kg; i.p.)-induced ataxia with a Rotorod. Cerebellar nitric oxide was determined fluorometrically in the presence of ethanol and/or PNU-282987.

RESULTS

Attenuation of ethanol-induced ataxia following PNU-282987 microinfusion was dose-dependent suggesting the participation of α(7) subtype in nicotine and ethanol interaction. Intracerebellar pretreatment with methyllycaconitine (α(7) -selective antagonist; 6 ng) virtually abolished the attenuating effect of PNU-282987 as well as the effect of nicotine, but not of RJR-2403 (α(4)β(2) -selective agonist; 125 ng) on ethanol-induced ataxia. Finally, ethanol administration significantly decreased cerebellar NO(x), whereas ICB PNU-282987 significantly increased and/or opposed ethanol-induced decrease in NO(x). These results were functionally in agreement with our Rotorod data.

CONCLUSIONS

These observations confirmed the following: (i) α(7) participation in nicotine-ethanol interaction and (ii) α(7) selectivity of methyllycaconitine. Overall, the results demonstrate the role of cerebellar nAChR α(7) subtype in nicotine-induced attenuation of ethanol-induced ataxia in cerebellar NO(x)-sensitive manner.

Sex differences in caffeine neurotoxicity following chronic ethanol exposure and withdrawal

AIMS

Caffeine is a central nervous system stimulant that produces its primary effects via antagonism of the A(1) and A(2A) adenosine receptor subtypes. Previous work demonstrated a sex difference in neurotoxicity produced by specific adenosine A(1) receptor antagonism during ethanol withdrawal (EWD) in vitro that was attributable to effects downstream of A(1) receptors at NMDA receptors. The current studies were designed to examine the effect of non-specific adenosine receptor antagonism with caffeine during ethanol withdrawal on hippocampal toxicity in cultures derived from male and female rats.

METHODS

At 5 days in vitro (DIV), half of the male and female organotypic hippocampal slice cultures were exposed to 50 mM ethanol (EtOH) in culture media for 10 days before exposure to caffeine (5, 20 and 100 microM) for the duration of a 24 h EWD period. In keeping with this timeline, the remaining ethanol-naïve cultures were given media changes at 10 and 15 DIV and exposed to caffeine (5, 20 and 100 microM) for 24 h at 15 DIV. Cytotoxicity was assessed by fluorescent microscopy and quantification of propidium iodide (PI) uptake in the pyramidal cell layers of the CA1 and CA3 regions and the granule cell layer of the dentate gyrus (DG). A two-way (sex x treatment) ANOVA was conducted within each hippocampal region.

RESULTS

Twenty-four-hour withdrawal from 10-day exposure to 50 mM ethanol did not produce increased PI uptake in any hippocampal region. Caffeine exposure (5, 20 and 100 microM) in ethanol-naïve cultures did not produce toxicity in the DG or CA1 region, but 20 microM caffeine produced modest toxicity in the CA3 region. Exposure to 20 microM caffeine during EWD produced cytotoxicity in all hippocampal regions, though toxicity was sex-dependent in the DG and CA1 region. In the DG, both 5 and 20 microM caffeine produced significantly greater PI uptake in ethanol-exposed female cultures compared to ethanol-naïve female cultures and all male cultures. Similarly, 20 microM caffeine caused markedly greater toxicity in female cultures as compared to male cultures in the CA1 region.

CONCLUSIONS

Twenty-four-hour exposure to caffeine during EWD produced significant toxicity in the pyramidal cell layer of the CA3 region in male and female cultures, though toxicity in the granule cell layer of the DG and pyramidal cell layer of the CA1 region was observed only in female cultures. Greater sensitivity of the female slice cultures to toxicity upon caffeine exposure after prolonged ethanol exposure is consistent with previous studies of effects of a specific A(1) receptor antagonism during EWD on toxicity and indicate that this effect is independent of the hormonal milieu. Together, these data suggest that the A(1) receptor subtype is predominant in mediating caffeine's neurotoxic effects during EWD. These findings demonstrate the importance of considering gender/sex when examining neuroadaptive changes in response to ethanol exposure and withdrawal.

Sex differences in the neurotoxic effects of adenosine A1 receptor antagonism during ethanol withdrawal: reversal with an A1 receptor agonist or an NMDA receptor antagonist

BACKGROUND

Neuronal adaptations that occur during chronic ethanol (EtOH) exposure have been observed to sensitize the brain to excitotoxic insult during withdrawal. The adenosine receptor system warrants further examination in this regard, as recent evidence has implicated adenosine receptor involvement in the behavioral effects of both EtOH exposure and withdrawal.

METHODS

The current studies examined effects of adenosine A(1) receptor manipulation on neuronal injury in EtOH-naive and EtOH-withdrawn male and female rat hippocampal slice cultures. EtOH-naive and EtOH pretreated (43.1 to 26.9 mM from days 5 to 15 DIV) cultures were exposed to the A(1) receptor agonist 2-Chloro-N(6)-cyclopentyladenosine (CCPA; 10 nM), the A(1) receptor antagonist 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX;10 nM), or the N-methyl-D-aspartate (NMDA) receptor antagonist D,L,-2-amino-5-phosphovalerate (APV; 20 microM) at 15 days in vitro (DIV). Cytotoxicity was measured in the primary neuronal layers of the dentate gyrus, CA3 and CA1 hippocampal regions by quantification of propidium iodide (PI) fluorescence after 24 hours. Immunohistochemical analysis of A(1) receptor abundance was conducted in EtOH-naive and EtOH pretreated slice cultures at 15 DIV.

RESULTS

Twenty-four hour exposure to DPCPX in EtOH-naive slice cultures did not produced neurotoxicity in any region of slice cultures. Though withdrawal from 10 day EtOH exposure produced no toxicity in either male or female slice cultures, exposure to DPCPX during 24 hours of EtOH withdrawal produced a marked increase in PI uptake in all hippocampal culture subregions in female cultures (to approximately 160% of control values). A significant effect for sex was observed in the CA1 region such that toxicity in females cultures exposed to the A(1) antagonist during withdrawal was greater than that observed in male cultures. These effects of DPCPX in EtOH withdrawn female and male slices were prevented by co-exposure to either the A(1) agonist CCPA or the NMDA receptor antagonist APV for 24 hours. No differences in the abundance of A(1) receptors were observed in male and female EtOH-naive or EtOH pretreated cultures.

CONCLUSIONS

The current findings suggest that the female hippocampus possesses an innate sensitivity to effects of EtOH exposure and withdrawal on neuronal excitability that is independent of hormonal influences. Further, this sex difference is not related to effects of EtOH exposure on A(1) receptor abundance, but likely reflects increased NMDA receptor-mediated signaling downstream of A(1) inhibition in females.

Co-modulation of acute ethanol-induced motor impairment by mouse cerebellar adenosinergic A1 and GABAA receptor systems

We have previously demonstrated that cerebellar adenosine modulates ethanol ataxia. Using Rotorod method, we investigated the role of cerebellar GABA(A) receptors in the adenosinergic modulation of ethanol ataxia in mice. Direct cerebellar microinfusion of GABA(A) agonist, muscimol (2.5, 5 and 10 ng) and antagonist, bicuculline (50, 100 and 200 ng), via permanently implanted guide cannulas, produced a marked and dose-dependent accentuation and attenuation, respectively, of ethanol (2g/kg; IP) ataxia. The accentuation of ethanol ataxia by intracerebellar muscimol was through GABA(A) receptor because intracerebellar pretreatment with bicuculline virtually abolished muscimol effect. Intracerebellar microinfusion of adenosine A(1) agonist, N(6)-cyclohexyladenosine (CHA: 4 ng), and antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX: 100 ng) markedly accentuated and attenuated, respectively, ethanol ataxia consistent with our previously published data. Intracerebellar microinfusion of CHA (4 ng) or DPCPX (100 ng) markedly enhanced and reduced, respectively, muscimol (10 ng)-induced accentuation of ethanol ataxia suggesting co-modulation of ethanol ataxia by cerebellar adenosinergic A(1) and GABA(A) receptors. Similarly, intracerebellar bicuculline (200 ng) pretreatment not only prevented CHA-induced accentuation of ethanol ataxia, but caused further decrease in ethanol ataxia. No change in the normal coordination was observed when microinfusion of the highest dose of muscimol, bicuculline, DPCPX or CHA alone or in combination was followed by saline injection instead of ethanol. The results of the present study suggest a functional similarity between GABA(A) and adenosine A(1) receptors even though both receptor types are known to couple to different signaling system and their location is on the opposite ends of the cerebellar granule cells, axons and axonal terminals (i.e., GABA(A) at the granule cells and adenosine A(1) on axons and axonal terminals of the granule cells) and act as co-modulators of ethanol ataxia.

Behavioral interaction between nicotine and ethanol: possible modulation by mouse cerebellar glutamate

BACKGROUND

Epidemiological studies show that people who drink alcoholic beverages also smoke cigarettes and vice versa. Furthermore, animal studies provide circumstantial evidence for ethanol and nicotine interaction. Previously, we demonstrated that intracerebellar nicotine attenuates ethanol ataxia. This study investigated the possible role of glutamate in modulating the interaction of nicotine and ethanol.

METHODS

Glutamate drugs N-methyl-d-aspartate (NMDA) and (+)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrate (AMPA) as well as their antagonists were directly microinfused into the cerebellum of CD-1 male mice to evaluate their effect on ethanol (2 g/kg i.p.) ataxia. Drug microinfusions were made via stereotaxically implanted stainless-steel guide cannulas. Rotorod was used to evaluate the ataxic response of ethanol.

RESULTS

Microinfusion of nicotine (0.3125, 1.25, 5 ng) significantly attenuated ethanol ataxia dose-dependently, confirming the functional interaction between nicotine and ethanol as reported earlier. Intracerebellar pretreatment with hexamethonium, a nicotinic receptor (nAChR) antagonist, significantly blocked nicotine-induced attenuation of ethanol ataxia suggesting participation of nAChRs. When ethanol was injected before nicotine microinfusion, nicotine failed to attenuate ethanol ataxia, indicating the critical importance of initial activation of nAChRs by nicotine. Intracerebellar microinfusion of NMDA (30, 60, 125 ng) and its antagonist, (+)-MK-801 (50, 100, 200 ng), significantly increased and decreased, respectively, the nicotine-induced attenuation of ethanol ataxia in a dose-related manner, suggesting participation of the NMDA receptor. Similarly, intracerebellar microinfusion of AMPA (7.5, 15, 30 ng) and its antagonist, nitro -2, 3-dioxobenzoquinoxaline-sulfonamide (NBQX; 25, 50, 100 ng), significantly increased and decreased, respectively, the nicotine-induced attenuation of ethanol ataxia in a dose-dependent manner. This suggests participation of the AMPA receptor and further supports involvement of the glutamate system in the ethanol-nicotine interaction. Intracerebellar nicotine failed to attenuate sodium-pentobarbital (25 mg/kg i.p.) ataxia, suggesting the relative specificity of the nicotine-ethanol interaction.

CONCLUSIONS

The results suggested that glutamate modulates the functional interaction between nicotine and ethanol because NMDA and AMPA enhanced the nicotine-induced attenuation of ethanol ataxia, whereas (+)-MK-801 and NBQX reduced the attenuation.

Antagonism of ethanol ataxia by intracerebellar nicotine: possible modulation by mouse cerebellar nitric oxide and cGMP

We have reported previously that intracerebellar nicotine attenuates ethanol ataxia via nicotinic-cholinergic receptors. We report now that attenuation of ethanol ataxia by intracerebellar nicotine is modulated by cerebellar nitric oxide-guanylyl cyclase (GC) messenger system. Intracerebellar microinfusion of SNP (sodium nitroprusside, a nitric oxide donor; 15, 30, and 60 pg) and SMT (S-methylisothiourea; 70, 140, and 280 fg; an inhibitor of inducible nitric oxide synthase), significantly enhanced and reduced, respectively, intracerebellar nicotine-induced attenuation of ethanol ataxia in a dose-related manner. Similarly, intracerebellar isoliquiritigenin (an activator of GC; 1, 2, and 4 pg) and ODQ (1H [1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one, an inhibitor of GC; 375, 750, and 1500 fg), significantly enhanced and reduced, respectively, intracerebellar nicotine-induced attenuation of ethanol ataxia in a dose-related fashion. These results suggest that the functional interaction between nicotine and ethanol may involve modulation by cerebellar nitric oxide and cGMP. Intracerebellar microinfusion of isoliquiritigenin (4, 8, and 16 pg) in the absence of nicotine significantly attenuated ethanol ataxia dose-dependently indicating a tonic involvement of cGMP in ethanol ataxia. Finally, intracerebellar nicotine (5 ng) significantly increased and ethanol 2 g/kg i.p. decreased levels of total cerebellar nitrite+nitrate (NOx) which were functionally correlated with ethanol ataxia and its attenuation by intracerebellar nicotine. The ethanol-induced decrease in NOx was significantly antagonized by intracerebellar nicotine. The NOx data further supported an involvement of nitric oxide in the behavioral interaction between nicotine and ethanol. Overall, the results of the present investigation demonstrate a functional correlation between cerebellar nitric oxide messenger system and the behavioral interaction between nicotine and ethanol.

Transcriptome analysis of frontal cortex in alcohol-preferring and nonpreferring rats

Although it is widely accepted that alcohol abuse and alcoholism have a significant genetic component of risk, the identities of the genes themselves remain obscure. To illuminate such potential genetic contributions, DNA macroarrays were used to probe for differences in normative cortical gene expression between rat strains genetically selected for alcohol self-administration preference, AA (Alko, alcohol) and P (Indiana, preferring), or avoidance, ANA (Alko, nonalcohol) and NP (Indiana, nonpreferring). Among 1,176 genes studied, six demonstrated confirmable, differential expression following comparison of ethanol-naive AA and ANA rats. Specifically, the mRNA level for metabotropic glutamate receptor 3 (mGluR3) was down-regulated in the AA vs. ANA lines. In contrast, calcium channel subunit alpha2delta1 (cacna2d1), vesicle-associated membrane protein 2 (VAMP2), syntaxin 1 (both syntaxin 1a and 1b; STX1a and STX1b), and syntaxin binding protein (MUNC-18) mRNAs were found to be increased in frontal cortex following comparison of AA with ANA animals. Bioinformatic analysis of these molecular targets showed that mGluR3 and cacna2d1 fall within chromosomal locations reported to be alcohol-related by the Collaborative Study on the Genetics of Alcoholism (COGA) as well as quantitative trait loci (QTL) studies. To determine further whether these differences were strain specific, the above-mentioned genes were compared in ethanol-preferring (P) and -nonpreferring (NP) selected lines. VAMP2 was the only gene that displayed statistically different mRNA levels in a comparison of P and NP rats. In conclusion, the altered cortical gene expression illuminated here would have the effect of altering neurotransmitter release in AA rats (compared with ANA rats). Such alterations, however, might not be a universal characteristic of all animal models of alcohol abuse and will also require further investigation in post-mortem human samples.

The tipsy terminal: presynaptic effects of ethanol

Considerable evidence suggests that the synapse is the most sensitive CNS element for ethanol effects. Although most alcohol research has focussed on the postsynaptic sites of ethanol action, especially regarding interactions with the glutamatergic and GABAergic receptors, few such studies have directly addressed the possible presynaptic loci of ethanol action, and even fewer describe effects on synaptic terminals. Nonetheless, there is burgeoning evidence that presynaptic terminals play a major role in ethanol effects. The methods used to verify such ethanol actions range from electrophysiological analysis of paired-pulse facilitation (PPF) and spontaneous and miniature synaptic potentials to direct recording of ion channel activity and transmitter/messenger release from acutely isolated synaptic terminals, and microscopic observation of vesicular release, with a focus predominantly on GABAergic, glutamatergic, and peptidergic synapses. The combined data suggest that acute ethanol administration can both increase and decrease the release of these transmitters from synaptic terminals, and more recent results suggest that prolonged or chronic ethanol treatment (CET) can also alter the function of presynaptic terminals. These new findings suggest that future analyses of synaptic effects of ethanol should attempt to ascertain the role of presynaptic terminals and their involvement in alcohol's behavioral actions. Other future directions should include an assessment of ethanol's effects on presynaptic signal transduction linkages and on the molecular machinery of transmitter release and exocytosis in general. Such studies could lead to the formulation of new treatment strategies for alcohol intoxication, alcohol abuse, and alcoholism.

Ethanol selectively inhibits enhanced vesicular release at excitatory synapses: real-time visualization in intact hippocampal slices

BACKGROUND

Conflicting information exists concerning the actions of ethanol on vesicular release at excitatory synapses. Because long-term alterations in synaptic transmission are thought to underlie neuroadaptive responses to ethanol, we have directly measured the actions of ethanol on release dynamics at an intact central synapse.

METHODS

Here we investigated the effects of ethanol on release dynamics in hippocampal slices using confocal microscopy with the lipophilic dye, FM1-43, complemented by a patch clamp analysis of AMPA miniature excitatory postsynaptic currents (mEPSCs). After a pretreatment/loading paradigm with sulforhodamine (S-Rhd) and FM1-43, stable, dense punctate FM1-43 staining in the CA1 stratum radiatum was evident.

RESULTS

FM1-43 fluorescence destaining was dose-dependently induced by perfusion with elevated K+ (20-60 mM). Cadmium inhibited K+-induced destaining, whereas nifedipine had no significant effect. Ethanol (25-75 mM) inhibited K+-induced destaining with high efficacy and had no effect on basal destaining. Both omega-Conotoxin GVIA and omega-Agatoxin IVA inhibited K+-induced destaining with high efficacy. The combination of omega-Conotoxin GVIA and omega-Agatoxin IVA occluded the inhibitory effect of ethanol, indicating that ethanol inhibition of release was dependent on inhibition of N/P/Q-voltage-gated calcium channels (VGCCs). Patch clamp studies of AMPA mEPSCs revealed similar findings in that vesicular release was enhanced with K+ depolarization in an ethanol-sensitive manner.

CONCLUSIONS

These findings indicate that the FM1-43/S-Rhd method is a stable and powerful approach for direct real-time measurement of vesicular release kinetics in intact brain slice preparations and that ethanol inhibits vesicular release induced by depolarization via inhibition of N/P/Q-VGCCs.

Adenosine A1 receptor activation selectively impairs the acquisition of contextual fear conditioning in rats

Three experiments were conducted to examine the importance of adenosine A1 receptors for the acquisition and expression of hippocampal-dependent and hippocampal-independent forms of conditioned fear. In Experiment 1, the selective adenosine A1 receptor agonist, N6-cyclopentyladenosine (CPA), or saline was administered intraperitoneally to male rats 30 min prior to Pavlovian fear conditioning, which consisted of 7 tone-shock pairings. Adenosine A1 receptor activation dose-dependently and selectively disrupted the acquisition of contextual fear conditioning while sparing tone-shock associations. Experiments 2 and 3 demonstrated that CPA's selective disruption of contextual learning could not be attributed to context being weaker than tone conditioning or to state-dependent learning. Adenosine A1 receptor activation also impaired the expression of both context- and tone-elicited fear. These results suggest that endogenous adenosine modulates the acquisition and expression of emotional (fear) memories by acting on A1 receptors in brain regions underlying fear conditioning.

Rat striatal adenosinergic modulation of ethanol-induced motor impairment: possible role of striatal cyclic AMP

We have previously reported the involvement of the striatum in acute ethanol-induced motor incoordination and the striatal adenosinergic modulation of ethanol-induced motor incoordination through A1 receptor-mediated mechanism(s). The present study, a continuation of our previous work, was carried out to investigate the possible functional correlation between striatal cyclic AMP and ethanol-induced motor incoordination, and its modulation by striatal adenosine in Sprague-Dawley rats. Forskolin (0.1, 0.5 and 1.0 pmol), a known activator of adenylate cyclase, significantly attenuated ethanol-induced motor incoordination in a dose-dependent manner following its direct intrastriatal microinfusion. Forskolin also antagonized the accentuating effect of intrastriatal N6-cyclohexyladenosine on ethanol-induced motor incoordination. These results suggested that ethanol-induced motor incoordination might be functionally correlated to a decrease in the striatal cyclic AMP levels and that the striatal adenosine A1 receptors might modulate ethanol-induced motor incoordination through cyclic AMP signaling mechanism(s). Further support to this hypothesis was obtained by the actual measurement of the striatal cyclic AMP levels in the same experimental conditions as in motor coordination studies using high-performance liquid chromatography with fluoroscence detection. Regardless of the method (focused microwave irradiation, cervical dislocation or decapitation into a dry ice-ethanol mixture) used to kill the animals, a significant decrease in the striatal cyclic AMP levels was observed due to ethanol. Intrastriatal adenosine A1-selective agonist, N6-cyclohexyladenosine (24 ng), caused a further significant decrease in the striatal cyclic AMP levels in the ethanol- but not in the vehicle-treated animals. The further enhancement in the ethanol-induced decrease in the striatal cyclic AMP levels by intrastriatal N6-cyclohexyladenosine, therefore, functionally correlated with the observed potentiating effect of intrastriatal N6-cyclohexyladenosine on ethanol-induced motor incoordination. The effects of intrastriatal N6-cyclohexyladenosine+ethanol and of ethanol alone on the striatal cyclic AMP levels were blocked by intrastriatal pertussis toxin (500 ng) pretreatment, indicating the involvement of pertussis toxin-sensitive G-proteins (Gi, Go) and possibly of the adenosine A1 receptor coupled to the G-proteins in the striatum. Furthermore, ethanol alone significantly decreased the basal as well as the cyclic AMP-stimulated catalytic activities of the striatal cyclic AMP protein kinase, which were further reduced by intrastriatal N6-cyclohexyladenosine. The results of the present study therefore support an involvement of a cyclic AMP signaling pathway in the striatal adenosinergic modulation of ethanol-induced motor incoordination at the post-adenosine A1 receptor level.

Adenosinergic modulation of ethanol-induced motor incoordination in the rat motor cortex

1. On going work in our laboratory has shown that adenosine modulates ethanol-induced motor incoordination (EIMI) when given systemically as well as directly into the cerebral ventricles, cerebellum and corpus striatum of the rat and/or mouse. 2. The objective of this study was to determine what effect adenosine agonists and antagonists would have within the rat motor cortex on EIMI. 3. The participation of the motor cortex in EIMI was suggested when microinfusion of the anti-ethanol compound, Ro15-4513, an inverse agonist of the benzodiazepine binding site, directly into the motor cortex significantly attenuated EIMI. Further, the adenosine agonists N6-cyclohexyladenosine (CHA) and 2-p-(2-carboxyethyl)-phenethylamino-5'-N-carboxaminoadenosine++ + hydrochloride (CGS-21680) significantly accentuated EIMI in a dose-related manner. The adenosine A1 receptor-selective agonist, CHA, appeared most potent in this modulatory effect when compared to the A2-selective agonist, CGS-21680. 4. The extent of diffusion of the adenosine drugs within the cortical tissue after their microinfusion was also checked by measuring the dispersion of microinfused [3H]CHA. The [3H]CHA dispersion study indirectly confirmed that the results of the present investigation were based on the effect of adenosine drugs within the motor cortex only. 5. Accentuation by the A1- and A2-selective adenosine agonists was significantly attenuated by the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by the A2 receptor-selective antagonist 8-(3-chlorostyryl)caffeine (CSC) further suggesting modulation mainly by the A1-subtype. 6. Pretreatment of the motor cortex with pertussis toxin (PT) significantly reduced the capacity of both A1- and A2-selective adenosine agonists to accentuate EIMI suggesting the involvement of a PT-sensitive Gi/Go protein. 7. These data support earlier work which showed that adenosine modulates EIMI within the central nervous system (CNS), most likely via the A1 receptor, and moreover, extend that work by including the motor cortex as a brain area participating in the adenosinergic modulation of ethanol-induced motor impairment.

Discriminative stimulus effects of glutamate release inhibitors in rats trained to discriminate ethanol

In a drug discrimination paradigm with rats trained to discriminate ethanol (1 g/kg IP) from saline we studied two substances, lamotrigine and riluzole, which are regarded as glutamate release inhibitors concerning their ability to substitute for ethanol. Both substances have been shown to act primarily on voltage-gated sodium channels; however, Lamotrigine dose dependently generalized to the ethanol cue, whereas riluzole did not. These results reflect the different high-dose effects of both sustances at voltage-gated calcium channels, where lamotrigine has inhibitory effects, but not riluzole, and provide further evidence for a role of voltage-gated calcium channels in the mediation of the effects of ethanol.

The striatal adenosinergic modulation of ethanol-induced motor incoordination in rats: possible role of chloride flux

Previous studies from our laboratory have provided strong evidence that brain adenosine modulates acute ethanol (i.p.)-induced motor incoordination (MI) through receptor mediated mechanism(s). Recently, we have reported the involvement of the striatum in ethanol-induced MI as well as the striatal adenosinergic modulation of the ethanol-induced motor deficit. The present study was thus designed to further characterize the modulatory effect of striatal adenosine on ethanol-induced MI and to look for its functional correlation with chloride flux within the rat striatum. Intrastriatal microinfusion of adenosine A1 receptor agonist N6-cyclohexyladenosine (CHA) and antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), significantly accentuated and attenuated, respectively, the motor incoordinating effect of ethanol while having no effect on the normal motor coordination in saline-treated control animals. These data confirmed the role of striatal adenosine in ethanol-induced MI. The selectivity of interactions between adenosine A1 agonist and antagonist and ethanol was further confirmed by the study in which neither intrastriatal CHA nor DPCPX significantly altered the MI induced by sodium pentobarbital. Previously, we have shown that intrastriatal Ro15-4513 not only significantly attenuated ethanol-induced MI but also blocked its accentuation by intrastriatal CHA. It is well known that Ro15-4513 antagonizes many, but not all, CNS effects of ethanol by blocking the ethanol potentiation of GABA-stimulated uptake of chloride. Therefore, experiments using striatal microsac preparations were carried out to investigate the possible modulation of chloride conductance by CHA and its relationship to ethanol. High concentrations of CHA (10 and 100 nM) increased the total chloride uptake by the striatal microsacs. Corresponding to the ethanol-adenosine interaction observed behaviorally, a much lower concentration (1 nM) of CHA, being ineffective itself, significantly enhanced the stimulatory action of ethanol on chloride uptake. This effect was blocked by either Ro15-4513 (100 nM) or DPCPX (10 nM). The modulatory effect of GABA and/or ethanol on chloride influx was also evaluated, and the results supported the appropriateness to use striatal microsac preparations in the present study. Overall, the data suggested a functional interaction between ethanol and striatal adenosine and further supported the hypothesis that striatal adenosine might, in part, modulate ethanol-induced MI through its effect on chloride conductance through chloride channels coupled to GABA-benzodiazepine receptor complex.

Role of adenosine in the ethanol-induced potentiation of the effects of general anesthetics in rats

Acetate, derived from ethanol metabolism in the liver, is released into the circulation and utilized in many tissues including the brain. The subsequent metabolism of acetate results in the production of adenosine that has a number of effects in the central nervous system. The purpose of the present studies, therefore, was to investigate the contribution of metabolically generated adenosine to the ethanol-induced potentiation of the inhalational agents isoflurane and sevoflurane. Changes in the anesthetic requirement for isoflurane and sevoflurane were determined in rats using the tail-clamp procedure. Both ethanol and sodium acetate reduced anesthetic requirement for isoflurane and sevoflurane in a dose-dependent fashion. The effect of acetate on anesthetic requirement was completely blocked by the administration of the adenosine receptor blocker, 8-phenyltheophylline. The ethanol-induced reduction in anesthetic requirement, however, was only partially blocked by 8-phenyltheophylline. Direct intracerebroventricular (i.c.v.) administration of the water-soluble adenosine receptor blocker, 8-sulfophenyltheophylline, also completely blocked the effect of acetate and partially blocked the effect of ethanol. This i.c.v. administration demonstrates that the actions of ethanol and acetate on anesthetic requirement are a central nervous system effect. The i.c.v. administration of the adenosine A1 receptor subtype agonist, R-phenylisopropyl adenosine, potentiated the anesthetic effects of isoflurane and suggests that the A receptor mediates the observed potentiation of anesthetic effect. This is further supported by the concomitant administration of 5-N-ethylcarboxamido adenosine, a non-selective adenosine agonist, with the selective A1 antagonist, 8-cyclopentyltheophylline, showing A1 receptor potentiation of anesthetic requirements. The studies show that (1) acetate potentiates the anesthetic effects of the inhalational anesthetics, sevoflurane and isoflurane; (2) acetate contributes in part to the effect of ethanol on anesthetic potency through metabolically generated adenosine; (3) these effects are likely mediated via adenosine A1 receptor subtypes.

Mouse cerebellar adenosinergic modulation of ethanol-induced motor incoordination: possible involvement of cAMP

As an extension of our previous work pertaining to brain adenosinergic modulation of ethanol-induced motor incoordination, the effect of direct intracerebellar administration of the A1-selective adenosine agonist, N6-cyclohexyladenosine (CHA) on ethanol-induced motor incoordination was evaluated. Marked accentuation of ethanol-induced motor impairment by CHA was observed. No change in the normal motor coordination was noted when CHA administration was followed by saline instead of ethanol. Intracerebellar cAMP or its analog, 8-(4-chlorophenylthio)-cAMP, significantly inhibited ethanol's motor impairment in a dose-related manner as well as abolished CHA's accentuating effect on ethanol-induced motor incoordination. These observations suggested a possible involvement of cAMP in the adenosinergic modulation and in the expression of ethanol-induced motor incoordination. Further support was provided by the observation of a marked accentuation and attenuation in a dose-related manner of ethanol-induced motor impairment as well as CHA's accentuation of ethanol's motor impairment by intracerebellar miconazole and forskolin, respectively. However, equimolar intracerebellar doses of miconazole and forskolin (inhibitor and stimulator of adenylyl cyclase, respectively) failed to significantly alter ethanol-induced motor incoordination probably due to their mutual functional antagonism. The expression of adenosinergic modulation and that of ethanol-induced motor impairment most likely involved Gi protein-coupled receptor(s) (such as adenosine receptors). The involvement of receptors linked to pertussis toxin-sensitive G-proteins was suggested because intracerebellar pertussis toxin pretreatment markedly inhibited ethanol-induced motor incoordination as well as CHA's accentuation of ethanol's motor impairment. Finally, cAMP, unlike its antagonism to CHA's accentuation, failed to antagonize the accentuation of ethanol-induced motor impairment by intracerebellar GABA(A) agonist (+)-muscimol. This indicated selectivity of cAMP participation in G protein coupled receptor (such as adenosine)-mediated response and not in ionic channel coupled receptor (such as GABA(A))-mediated mechanism. Overall, the data suggested a possible involvement of cerebellar adenylyl cyclase-cAMP signalling pathway in the adenosinergic modulation of ethanol's ataxia.

Anxiolytic activity of adenosine receptor activation in mice

1. Purine analogues have been examined for anxiolytic- and anxiogenic-like activity in mice, by use of the elevated plus-maze. 2. The selective A1 receptor agonist, N6-cyclopentyladenosine (CPA) had marked anxiolytic-like activity at 10 and 50 microg kg(-1), with no effect on locomotor performance at these doses. 3. The A1 selective adenosine receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (CPX) had no significant effect on anxiety-related measures or locomotor behaviour, but blocked the anxiolytic-like activity of CPA. The hydrophilic xanthine, 8-(p-sulphophenyl) theophylline did not prevent anxiolysis by CPA. 4. Caffeine had anxiogenic-like activity at 30 mg kg(-1) which was prevented by CPA at 50 micro kg(-1). 5. The A2 receptor agonist, N6-[2-(3,5-dimethoxyphenyl)-2(2-methylphenyl)-ethyl]adenosine (DPMA) had no effect on anxiety behaviour but depressed locomotor activity at the highest dose tested of 1 mg kg(-1). The A2 receptor antagonist, 1,3-dimethyl-l-propargylxanthine (DMPX) had no effect on anxiety-related measures or locomotion and did not modify the anxiolytic-like activity of CPA. 6. Administration of DPMA in combination with anxiolytic doses of CPA prevented the anxiolytic-like activity of the latter. 7. The results suggest that the selective activation of central A1 adenosine receptors induces anxiolytic-like behaviour, while the activation of A2 sites causes locomotor depression and reduces the effects of A1 receptor activation. The absence of any effect of CPX alone suggests that the receptors involved in modulating behaviour in the elevated plus-maze in mice are not activated tonically by endogenous adenosine.

Possible central adenosinergic modulation of ethanol-induced alterations in [14C]glucose utilization in mice

The possible role of brain adenosine in acute ethanol-induced alteration in glucose utilization in the whole brain, as well as in the specific brain areas (cerebellum and brain stem), was investigated. Mice were killed 20-min postethanol, and the fresh tissue slices (300 microns) of brain and/or specific brain areas were incubated for 100 min in a 5.5 mM glucose medium in Warburg flasks using [6-(14)C]glucose as a tracer. Trapped 14CO2 was counted to estimate glucose utilization. Ethanol (2 g/kg, i.p.) markedly increased the glucose utilization in whole brain and in both motor areas of brain. Theophylline (50 mg/kg, i.p.), an adenosine antagonist, significantly reduced ethanol-induced increase in glucose utilization in whole brain, as well as in brain areas. However, adenosine agonist N6-cyclohexyladenosine (CHA; 0.1 mg/kg, i.p.) on the contrary, significantly accentuated ethanol-induced increase in glucose utilization in these tissues that was nearly completely blocked by theophylline pretreatment. Theophylline alone did not produce any significant change in glucose utilization, whereas CHA alone (in vivo and in vitro) significantly increased glucose utilization, as well as ethanol-induced increase in glucose utilization in an additive manner. Relevant supportive data were obtained by experiments in which adenosine deaminase (ADA), p-sulfophenyltheophylline (8-SPT), and CHA were administered in vitro to the slice preparations. Both ADA and 8-SPT were effective in almost completely blocking the ethanol-induced increase in glucose utilization, whereas CHA further enhanced the ethanol-induced increase in glucose utilization in an additive manner.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of adenosine A1 receptor activation in ethanol-induced inhibition of stimulated glutamate release in the hippocampus of the fetal and adult guinea pig

The role of adenosine A1 receptor activation in ethanol-induced inhibition of stimulated L-glutamate (Glu) release was determined in transverse hippocampal slices of the near-term fetal guinea pig and the adult guinea pig. Exposure of the slices to 48 mM ethanol inhibited K(+)-stimulated Glu efflux. Pretreatment with 8-cyclopentyltheophylline (CPT), a selective adenosine A1 receptor antagonist, blocked the ethanol-induced inhibition of K(+)-stimulated Glu efflux in the near-term fetal and adult hippocampus. In the near-term fetus, 2-chloro-N6-cyclopentyladenosine (CCPA), a selective adenosine A1 agonist, and exogenous adenosine each blocked K(+)-stimulated Glu efflux similar to that produced by 48 mM ethanol. In the adult, although K+ increased Glu efflux in the presence of CCPA or adenosine, the magnitude of increase was less than that of the K(+)-stimulated Glu efflux for the control conditions. Exposure to ethanol alone or ethanol plus CPT produced a transient increase in endogenous adenosine efflux in the near-term fetal and adult hippocampus, which was not temporally related to the ethanol-induced inhibition of K(+)-stimulated Glu efflux. Overall, the data indicate that adenosine A1 receptor activation mediates ethanol-induced inhibition of stimulated Glu release in the hippocampus of the near-term fetal and adult guinea pig.

Ethanol elevates fetal serum glutamate levels in the rat

Glutamate is an important excitatory neurotransmitter. However, a sustained elevation of glutamate in the extracellular space may be toxic to neurons. Because the blood-brain barrier is incomplete in the developing fetus, an elevation of fetal serum glutamate could expose the immature, growing brain to potentially toxic levels of extracellular glutamate. Chronic ethanol consumption during pregnancy is associated with an increased risk for a complex array of congenital anomalies, including alterations in the CNS, a hallmark of the fetal alcohol syndrome. Some central nervous system changes appear to involve the glutamate receptor, including reduced number and altered function. One mechanism for receptor downregulation may be a sustained elevation in extracellular glutamate. We hypothesize that chronic ethanol exposure during pregnancy leads to an elevation in fetal serum glutamate. When rats were fed ethanol-containing liquid diet throughout pregnancy, growth retardation of fetuses was observed at sacrifice (gestation day 20). Within each group, ethanol-fed, pair-fed, and ad libitum chow-fed, serum glutamate levels were generally higher in the fetus than in the dam. Ethanol treatment had no effect on fetal or maternal serum glutamine, a reciprocal metabolite of glutamate. In contrast, ethanol treatment increased serum glutamate levels in the fetal serum by nearly 50%, compared with either of the control groups. Maternal serum glutamate was not affected. The finding of ethanol-induced elevation of fetal serum glutamate suggests that the developing brain might be concurrently exposed to elevated levels of extracellular glutamate. Chronic exposure to elevated glutamate during critical periods of brain development may contribute to the pathogenesis of the fetal alcohol syndrome.

Effect of adenosine on bicuculline-resistant paired-pulse inhibition in the rat hippocampal slice

This study extends previous investigations into the effect of adenosine on bicuculline-resistant paired-pulse inhibition between field potentials evoked 300 ms apart in the CA1 area of the rat hippocampal slice. A direct assessment of the effect of adenosine on paired-pulse inhibition is complicated by the facts that adenosine directly depresses evoked potentials and bicuculline-resistant paired-pulse inhibition is greater between pairs of small potentials than between pairs of larger potentials. Adenosine increased bicuculline-resistant paired-pulse inhibition when stimulus strength was constant between adenosine and control but paired-pulse inhibition of responses in adenosine was markedly less than paired-pulse inhibition of control responses of the same size. Furthermore, adenosine decreased the size of conditioned potentials to a significantly lesser extent than unpaired potentials of the same initial size. Taken together the results indicate that adenosine can decrease bicuculline-resistant paired-pulse inhibition in the hippocampus. A possible mechanism for this effect is that adenosine is suppressing transmission at excitatory terminals onto interneurones which would suggest that these receptors are more sensitive to adenosine than those on the Schaffer collateral/CA1 pyramidal cell synapses. In this case adenosine should reduce paired-pulse inhibition at lower concentrations than are required for depression of single evoked potentials. A comparison of the concentration-response relationships for the effects of adenosine on paired-pulse inhibition and on single evoked potentials ruled out greater sensitivity of adenosine receptors at excitatory terminals onto interneurones as an explanation for adenosine's action on bicuculline-resistant paired-pulse inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Seizures during ethanol withdrawal are blocked by focal microinjection of excitant amino acid antagonists into the inferior colliculus and pontine reticular formation

Physical dependence on ethanol can result in seizure susceptibility during ethanol withdrawal. In rats, generalized tonic-clonic seizures are precipitated by auditory stimulation during the ethanol withdrawal syndrome. Excitant amino acids (EAAs) are implicated as neurotransmitters in the inferior colliculus and the brain stem reticular formation, which play important roles in the neuronal network for genetic models of audiogenic seizures (AGSs). Ethanol blocks the actions of EAAs in various brain regions, including the inferior colliculus. In this study, dependence was produced by intragastric administration of ethanol for 4 days. During ethanol withdrawal, AGSs were blocked by systemic administration of competitive or noncompetitive NMDA antagonists 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or dizocilpine (MK-801). Focal microinjections of NMDA or non-NMDA antagonists into the inferior colliculus or the pontine reticular formation also inhibited AGSs. MK-801 was the most potent anticonvulsant systemically. When injected into the inferior colliculus, CPP had a more potent anticonvulsant effect than either MK-801 or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. The inferior colliculus was more sensitive than the pontine reticular formation to the anticonvulsant effects of both competitive NMDA and non-NMDA antagonists. The results of the present support the idea that continued ethanol administration may lead to development of supersensitivity to the action of EAAs in inferior colliculus and pontine reticular formation neurons. This may be a critical mechanism subserving AGS susceptibility during ethanol withdrawal.

Electrophysiological actions of acetate, a metabolite of ethanol, on hippocampal dentate granule neurons: interactions with adenosine

Acetate is the primary breakdown product of ethanol metabolism in the liver and has been found in the brain following ethanol ingestion in rats. Systemically administered acetate has been shown to cause motor impairment, an effect which is blocked by the adenosine receptor blocker, 8-phenyltheophylline (8-PT). The effects of sodium acetate were investigated in this study using intracellular recording techniques in rat hippocampal dentate granule cells, and were compared to the actions of ethanol and adenosine individually and in conjunction with 8-PT. Acetate hyperpolarized the membrane at 0.4-0.8 mM. The amplitude and duration of the postspike train afterhyperpolarization (AHP) were increased by acetate when the cell was repolarized to the control resting membrane potential. Comparable results were seen in voltage clamp. Acetate also decreased spike frequency adaptation. The effects of acetate were mimicked by adenosine (50 microM) and ethanol (20 mM). The ethanol effects occluded those produced by acetate. All of the effects of acetate, adenosine and ethanol could be inhibited with prior perfusion of 8-PT (1-10 microM). These data suggest that the actions of the major metabolite of ethanol, acetate, may be mediated by adenosine receptor activation.

The involvement of NMDA receptors in acute and chronic effects of ethanol

Recent evidence indicates involvement of excitatory amino acid receptors sensitive to N-methyl-d-aspartate (NMDA) in the action of ethanol (EtOH). Pronounced inhibition of NMDA receptor function is seen in vitro with concentrations of EtOH corresponding to those present during alcohol intoxication in humans. The present study was devoted to investigate the role of NMDA receptors in the action of EtOH in rats. Acute experiments showed antagonism by EtOH of convulsions induced by intracerebroventricular injection of NMDA. A similar effect was seen with a high dose of diazepam. Convulsions induced by an agonist of another excitatory amino acid receptor subtype, kainate, were also inhibited by EtOH. An uncompetitive antagonist of NMDA receptors, 5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate (MK-801), potentiated EtOH-induced loss of righting, but attenuated the hypothermic action of EtOH. Moreover, MK-801 inhibited audiogenic convulsions in EtOH withdrawn rats. At the same time the effect of a proconvulsive dose of NMDA was not enhanced. Tolerance to the myorelaxant action of both EtOH and MK-801 upon repetitive administration was seen. Also some degree of cross-tolerance was observed. Moreover, MK-801 failed to modify EtOH preference in rats. The present results support involvement of NMDA receptors in expression of some acute and subchronic actions of EtOH and in expression of EtOH withdrawal.

Glutamate-evoked release of endogenous adenosine from rat cortical synaptosomes is mediated by glutamate uptake and not by receptors

L-Glutamate (10 microM-1 mM) released endogenous adenosine from rat cortical synaptosomes. Studies with excitatory amino acid antagonists, (+)-5-methyl-16,11,dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), 6,7-dinitroquinoxaline-2,3-dione (DNQX), Mg2+, and agonists N-methyl-D-aspartate (NMDA), kainate, and quisqualate, indicated that this release was not receptor mediated. D,L-2-Amino-4-phosphonobutanoic acid (APB) also did not affect glutamate-evoked adenosine release. Inhibition of glutamate uptake by dihydrokainate or replacement of extracellular Na+ blocked glutamate-evoked adenosine release. D-aspartate, which is a substrate for the glutamate transporter but is not metabolized, also released adenosine, suggesting that release was due to amino acid transport and not to its subsequent metabolism. D-Glutamate, a relatively poor substrate for the transporter, was correspondingly less potent than L-glutamate at releasing adenosine. Glutamate-evoked adenosine release was not Ca2+ dependent or tetrodotoxin sensitive and did not appear to occur on the bidirectional nucleoside transporter. Inhibition of ecto-5'-nucleotidase virtually abolished glutamate-evoked adenosine release, indicating that adenosine was derived from extracellular metabolism of released nucleotide(s). However, L-glutamate did not release ATP and did not appear to release cyclic AMP. Therefore, transport of glutamate into presynaptic terminals releases some other nucleotide which is converted extracellularly to adenosine. This adenosine could act at P1-purinoceptors to modulate glutamatergic neurotransmission.

The effect of chronic ethanol on glutamate binding in human and rat brain

Quantitative autoradiographic techniques demonstrate that chronic alcohol administration causes a decrease in [3H]-glutamate binding to hippocampal N-methyl-D-aspartate (NMDA) receptors. A 14% decrease in [3H]-glutamate binding in the hippocampal CA1 region is seen both in the rat after five days of ethanol administration and in postmortem hippocampal tissues from alcoholics. In the rat, 24 hr ethanol withdrawal values are intermediate between control and alcohol binding levels. There was no significant effect of ethanol on [3H]-glutamate binding in the cortex or caudate.