The role of active zone protein Rab3 interacting molecule 1 alpha in the regulation of norepinephrine release, response to novelty, and sleep

Sleep mechanisms and synaptic plasticity are thought to interact to regulate homeostasis and memory formation. However, the influences of molecules that mediate synaptic plasticity on sleep are not well understood. In this study we demonstrate that mice lacking Rab3 interacting molecule 1 alpha (RIM1 alpha) (Rim1 alpha KO), a protein of the synaptic active zone required for certain types of synaptic plasticity and learning, had 53+/-5% less baseline rapid eye movement (REM) sleep compared with their wild type littermates. Also, compared with wild type littermates, exposure of the mice to an open field or to a novel object induced more robust and longer lasting locomotion suggesting altered habituation. This difference in exploratory behavior correlated with genotype specific changes in REM and deregulated release of norepinephrine in the cortex and basal amygdala of the Rim1 alpha KO mice. Also, moderate sleep deprivation (4 h), a test of the homeostatic sleep response, induced REM sleep rebound with different time course in Rim1 alpha KO and their wild type littermates. As norepinephrine plays an important role in regulating arousal and REM sleep, our data suggest that noradrenergic deficiency in Rim1 alpha KO animals impacts exploratory behavior and sleep regulation and contributes to impairments in learning.

Characterization of rabphilin phosphorylation using phospho-specific antibodies

Rab3A is a GTP-binding protein of synaptic vesicles that regulates neurotransmitter release and cycles on and off synaptic vesicles as a function of exocytosis. Rab3A presumably functions via GTP-dependent interactions with effectors. Two putative rab3A effectors have been described in neurons, rabphilin which is a soluble protein that moves onto and off synaptic vesicles in concert with rab3A, and RIM which is an active zone protein that only binds to rab3A on docked vesicles. Rabphilin is an abundant, evolutionarily conserved protein whose function has remained enigmatic since a knockout of rabphilin does not display the functional deficiencies observed in the rab3A knockout. However, previous studies have shown that rabphilin is phosphorylated by protein kinase A and CaM Kinase II, suggesting that it may have a regulatory role. In the present study, we have examined the site and regulation of rabphilin phosphorylation in living nerve terminals using phospho-specific antibodies raised against phospho-serine234 of rabphilin. With these antibodies, we demonstrate that rabphilin is physiologically phosphorylated on serine234, and that soluble rabphilin which is not bound to rab3A on synaptic vesicles is the primary target. However, different from synapsins which are induced to dissociate from synaptic vesicles by PKA phosphorylation, phosphorylation of rabphilin is not instrumental for dissociating rabphilin from synaptic vesicles. Our data support the notion that dissociated rabphilin is a synaptic phosphoprotein in vivo that may play a role in the regulation of nerve terminal protein-protein interactions.

Role of calcium in neurotransmitter release evoked by alpha-latrotoxin or hypertonic sucrose

At the synapse, neurotransmitter release is triggered physiologically by Ca(2+) influx through voltage-gated Ca(2+) channels. Non-physiologically, release can be evoked by a potent neurotoxin, alpha-latrotoxin, and by hypertonic sucrose. Controversy has arisen on whether release evoked by alpha-latrotoxin and hypertonic sucrose requires extracellular Ca(2+) or Ca(2+) from intracellular stores. Using synaptosomes, we have studied the Ca(2+) dependence of alpha-latrotoxin and sucrose action in different neurotransmitter systems. In agreement with previous data, no requirement for extracellular Ca(2+) in sucrose-induced secretion of norepinephrine, dopamine, glutamate or GABA was detected. Unexpectedly, we observed large differences between these neurotransmitters in the Ca(2+) dependence of alpha-latrotoxin-stimulated release: norepinephrine release required Ca(2+), dopamine release was only partially Ca(2+) dependent, and glutamate and GABA release did not require Ca(2+). To test if Ca(2+) derived from intracellular Ca(2+) stores participates in neurotransmitter release triggered by alpha-latrotoxin or hypertonic sucrose, we employed thapsigargin, a Ca(2+)-ATPase inhibitor that empties Ca(2+) stores. Thapsigargin did not induce neurotransmitter release, nor did it inhibit subsequent release stimulated by KCl depolarization, hypertonic sucrose or alpha-latrotoxin. However, intracellular Ca(2+) performs an important regulatory function, since thapsigargin increased the size of the readily releasable pool as measured by stimulation with hypertonic sucrose. This effect required extracellular Ca(2+) and protein kinase C, suggesting that depletion of internal Ca(2+) stores leads to store-operated Ca(2+) entry. The resulting Ca(2+) influx does not trigger release by itself, but activates protein kinase C that increases the readily releasable pool of neurotransmitters. Our data show that internal and external Ca(2+) is not acutely involved in hypertonic sucrose-evoked neurotransmitter release, while alpha-latrotoxin-triggered release requires external Ca(2+) for a subset of neurotransmitters. Although internal Ca(2+) is not essential for release, it modulates its extent, implying that the emptying of intracellular stores by activation of presynaptic receptors plays an important regulatory role in neurotransmitter release.

Assembly of SNARE core complexes prior to neurotransmitter release sets the readily releasable pool of synaptic vesicles

Precise regulation of neurotransmitter release is essential for the normal function of neural networks, but the mechanisms involved are largely unclear. Using superfused synaptosomes, we have studied the readily releasable pool of synaptic vesicles, measured as the amount of release triggered by hypertonic sucrose. We show that activation of presynaptic metabotropic glutamate receptors by dihydroxyphenylglycine and stimulation of protein kinase C by phorbol esters enhance the readily releasable pool of glutamate. Although the molecular nature of the readily releasable pool is unknown, one possibility is that during its generation, SNARE proteins form full core complexes, and that core complex formation occurs prior to neurotransmitter release. To test this possibility, we employed N-ethylmaleimide (NEM), an inhibitor of the ATPase N-ethylmaleimide-sensitive factor that dissociates core complexes, to study the relation of the readily releasable pool to core complex assembly in synaptosomes. NEM induced a dose-dependent increase in the readily releasable pool of neurotransmitters but by itself did not trigger release. Direct measurements of core complexes confirmed that NEM caused an increase in the levels of SNARE core complexes under these conditions. Our data suggest that in the readily releasable pool of synaptic vesicles, SNARE proteins are fully assembled into core complexes, and that SNARE complex assembly is a target of presynaptic regulation.

Mechanism of action of rab3A in mossy fiber LTP

In mossy fiber synapses of the hippocampal CA3 region, LTP is induced by cAMP and requires the synaptic vesicle protein rab3A. In contrast, CA1-region synapses do not exhibit this type of LTP. We now show that cAMP enhances glutamate release from CA3 but not CA1 synaptosomes by (1) increasing the readily releasable pool as tested by hypertonic sucrose; (2) potentiating release evoked by KCl depolarization, which opens voltage-gated Ca2+ channels; and (3) by enhancing Ca2+ action on the secretory apparatus as monitored by the Ca2+-ionophore ionomycin. In rab3A-deficient synaptosomes, forskolin still enhances KCl- and sucrose-induced glutamate release but not ionomycin-induced release. Our results show that cAMP has multiple actions in mossy fiber synapses, of which only the direct activation of the secretory apparatus requires rab3A and functions in mfLTP.

Region-specific phosphorylation of rabphilin in mossy fiber nerve terminals of the hippocampus

In mossy fiber synapses of the CA3 region of the hippocampus, long-term potentiation (LTP) is induced presynaptically by activation of cAMP-dependent protein kinase A (PKA). Rab3A is a synaptic vesicle protein that regulates vesicle fusion and is essential for mossy fiber LTP. Rab3A probably acts via two effector proteins, rabphilin and RIM, of which rabphilin is an in vitro substrate for PKA. To test if rabphilin is phosphorylated in nerve terminals and if its PKA-dependent phosphorylation correlates with the PKA-dependent induction of LTP in mossy fiber terminals, we have studied the phosphorylation of rabphilin in synaptosomes isolated from the CA1 and CA3 regions of the hippocampus. Rabphilin was phosphorylated in both CA1 and CA3 synaptosomes. However, when we treated the CA1 and CA3 synaptosomes with forskolin (an agent that enhances PKA activity) or induced Ca2+ influx into synaptosomes with high K+, rabphilin phosphorylation was increased selectively in mossy fiber CA3 synaptosomes, but not in CA1 synaptosomes. In contrast, the phosphorylation of synapsin, studied as a control for the specificity of the region-specific phosphorylation of rabphilin, was augmented similarly by both treatments in CA1 and CA3 synaptosomes. These results reveal that the phosphorylation states of two synaptic substrates for PKA and CaM KII, rabphilin and synapsin, are regulated differentially in a region-specific manner, an unexpected finding because rabphilin and synapsin are similarly present in CA1 and CA3 synaptosomes and are colocalized on the same synaptic vesicles. The region-specific phosphorylation of rabphilin agrees well with the restricted induction of LTP by presynaptic PKA activation in mossy fiber, but not CA1, nerve terminals.

Glutamate receptor activation induces carrier mediated release of endogenous GABA from rat striatal slices

The regulation of striatonigral and striatopallidal GABAergic neurons by glutamatergic afferents is thought to play a critical role in normal basal ganglia function. Here we report that in striatal slices about 17% of K(+)-induced endogenous GABA release was Ca(2+)-independent and this could be blocked by a GABA transport inhibitor. Activation of N-methyl-D-aspartate (NMDA)- and quisqualate-sensitive receptors induced endogenous GABA efflux only in the presence of a GABA transaminase inhibitor; this efflux was inhibited by 60-80% with a GABA transport inhibitor. NMDA-induced GABA release was blocked by phencyclidine, Mg2+ and CGS 19755. Quisqualate-induced GABA release was blocked completely by a combination of the metabotropic antagonist, L-AP3 and CNQX, a non-NMDA receptor antagonist. These data indicate that excitatory amino acid agonists-induced GABA release is distinct from that induced by high K+ depolarization.

Characterization of nitric oxide generator-induced hippocampal [3H]norepinephrine release. I. The role of glutamate

In this study we compared the effects of two nitrogen monoxide (NO) generators, hydroxylamine and S-nitroso-L-cysteine (NO-CYS), on hippocampal [3H]norepinephrine ([3H]NE) release. A 10-min incubation with hydroxylamine (3-3,000 microM) or NO-CYS (30-10,000 microM) induced a concentration-dependent increase in the basal [3H]NE efflux with EC50 values of approximately 100 microM and 1 mM, respectively. Reduced hemoglobin, a NO scavenger, blocked both hydroxylamine- and NO-CYS-evoked [3H]NE release. Long-term exposure (> or = 25 min) to 100 microM hydroxylamine, or to millimolar concentrations of NO-CYS, evoked a tetrodotoxin-insensitive [3H]NE release. However, a 10-min stimulation with either 100 microM hydroxylamine or 300 microM NO-CYS was sensitive to 0.5 microM tetrodotoxin, a voltage-sensitive sodium channel blocker. This suggested that under these conditions hydroxylamine and NO-CYS induce [3H]NE release indirectly in part, perhaps via releasing an excitatory neurotransmitter. Indeed, kynurenate, a nonselective ionotropic glutamate receptor antagonist, produced an 80% inhibition of the NO generator-evoked [3H]NE release. CGS 19755, a N-methyl-D-aspartate receptor antagonist, had no significant effect, whereas the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptor antagonists, CNQX and GYKI 52446, inhibited the hydroxylamine response by 50%. In synaptosomes, a preparation in which synaptic interactions are nonsignificant, NO-CYS induced a dose-dependent release of both [3H]NE and [3H]glutamate. These data suggest that, in hippocampal slices, NO generators evoke [3H]NE release both directly from noradrenergic terminals and indirectly via releasing glutamate.

Characterization of nitric oxide generator-induced hippocampal [3H]norepinephrine release. II. The role of calcium, reverse norepinephrine transport and cyclic 3',5'-guanosine monophosphate

The mechanisms by which two nitrogen monoxide (NO) generators, hydroxylamine and S-nitroso-L-cysteine (NO-CYS), induce hippocampal [3H]norepinephrine ([3H]NE) release was investigated. Neither hydroxylamine- nor NO-CYS-induced release was affected by the guanylate cyclase inhibitors, methylene blue or LY 83,583. The effect of hydroxylamine was completely dependent on extracellular Ca++ and reduced by 40% in the presence of omega-conotoxin GVIA, an N-type Ca(++)-channel antagonist; however it was unaffected by Ni++, nifedipine, caffeine or thapsigargin. The stimulatory effect of hydroxylamine on hippocampal cyclic GMP formation was not significantly affected by removal of extracellular Ca++, indicating that Ca(++)-dependent release is not due to inhibition of NO formation from hydroxylamine. However, the response to NO-CYS was reduced by 35 to 50% in either nominally Ca(++)-free or 10 mM MgSO4-containing buffer. Interestingly, buffer containing ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid dramatically enhanced the formation of NO from NO-CYS and potentiated the NO-CYS response. Both NO-CYS- and hydroxylamine-induced [3H]NE release was inhibited by NE transport blockers, indicating a prominent role for reverse transport. NO-CYS completely inhibited synaptosomal uptake of [3H]NE (IC50 approximately, 300 microM). NO generator-induced [3H]NE release has a glutamate-dependent component (see accompanying article). Inhibition of glutamate-evoked [3H]NE release by mazindol, an inhibitor of NE transport, suggests that the glutamate-dependent component also involves reversal of the NE transporter. These data suggest that NO produced from hydroxylamine or NO-CYS evoke both vesicular and nonvesicular release of hippocampal [3H]NE. Putative NO target molecules and the role of extracellular Ca++ are discussed.

Inhibitory effects of nitric oxide on the uptake of [3H]dopamine and [3H]glutamate by striatal synaptosomes

Ten-minute incubation with 1 mM S-nitroso-L-cysteine (NO-CYS), a nitric oxide (NO) generator, induced a 3.5-fold increase in the basal dopamine (DA) efflux from rat striatal slices. Reduced hemoglobin (100 microM), a NO scavenger, blocked this response. Nomifensine (10 microM), an inhibitor of high-affinity DA transport, reduced the NO-CYS effect by 39%. In a synaptosomal preparation, NO-CYS induced a concentration-dependent efflux of [3H]DA that was also significantly inhibited by 10 microM nomifensine. Because these findings indicated that NO could alter the activity of the striatal DA transporter, we tested the effect of NO and NO-CYS on the uptake of [3H]DA into crude striatal synaptosomes. Although both compounds inhibited [3H]DA uptake with similar dose-response characteristics (IC50 approximately 300 microM and approximately 400 microM, respectively), the effect of NO was quicker in onset. The presence of superoxide dismutase (30 U/ml) and catalase (50 U/ml) had a small impact on the NO-CYS inhibition of [3H]DA uptake (1.8-fold increase in IC50). NO (1 mM) inhibited striatal [3H]glutamate uptake by 23%, but lower concentrations had no significant effect. The duration of the effect of NO gas on [3H]DA uptake varied from < 5 min to > 10 min, depending on the concentration used. All concentrations of NO-CYS tested produced an inhibition of [3H]DA uptake that lasted at least 10 min. However, only concentrations < or = 1 mM NO-CYS (or NO) were washable. The effect of 3 mM NO-CYS lasted > 20 min and could not be reversed by washing. Reduced hemoglobin (300 microM) prevented the action of 3 mM NO-CYS, whereas methemoglobin had no effect. The action of 3 mM NO-CYS was temperature dependent and took about 2 min to fully develop. Scatchard analysis revealed that NO-CYS diminished the capacity of the DA transporter without having a significant effect on the affinity. NO-CYS had no effect on striatal [3H]-mazindol binding, suggesting that NO-CYS did not interact with the DA recognition site of the transporter. These data suggest that NO may play a role in the regulation of DA and, to a lesser extent, glutamate transport in the striatum.

Regulation of nitric oxide synthase activity in cortical slices by excitatory amino acids and calcium

Nitric oxide synthase (NOS) activity was determined in adult rat frontal cortex and hippocampus by measuring the conversion of L-[3H]arginine to L-[3H]citrulline. N-methyl-D-aspartate (NMDA), but not kainate or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), stimulated NOS activity. This effect was concentration dependent (EC50 approximately 30 microM) and was inhibited by tetrodotoxin, EGTA, N omega-nitro-L-arginine (NOARG), Mg2+, phencyclidine, and (cis)-4-phosphonomethyl-2-piperidine carboxylate (CGS 19755), but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). NOS activity was increased to an even greater extent by the calcium ionophores ionomycin and A23187 and by depolarization with 50 mM K+. Interestingly, neither caffeine nor 1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), drugs that would be expected to increase intracellular Ca2+ concentration by release of Ca2+ from intracellular ryanodine- and inositol-1,4,5-trisphosphate-sensitive stores, respectively, had any significant effect on NOS activity. It is concluded that NOS can be activated by NMDA binding to a classic NMDA glutamate receptor subtype as well as by depolarization or other agents that increase the influx of extracellular Ca2+. The paradoxical lack of effect of caffeine, as well as the inhibitory effect of tetrodotoxin, are discussed.

The role of P-type calcium channels in the depolarization-induced activation of nitric oxide synthase in frontal cortex

In this study we demonstrate that 50 mM K+ stimulates the conversion of L-[3H]arginine to L-[3H]citrulline and that this effect is blocked by 10 microM N omega-nitro-L-arginine, a nitric oxide synthase inhibitor, and Ca(2+)-free conditions. Amiloride (1 mM) and low Na+ conditions were used to test the possible involvement of the Na(+)-Ca2+ exchanger. These treatments were without effect. The calcium channel blockers 10 mM Mg2+, 100 microM Cd2+, and 10 mM Co2+ also blocked the K+ response, suggesting the involvement of voltage-dependent calcium channels (VDCCs). The specific VDCC involved seems to be the P type, as funnel-web spider toxin blocked the response whereas 200 microM Ni2+, 10 microM nifedipine, and 100 nM omega-conotoxin did not.

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) induces burst firing via an inositol-1,4,5-triphosphate-independent pathway at rat dorsolateral septal nucleus

We have previously reported that a L-2-amino-3-phosphonopropionate (L-AP3)-sensitive metabotropic glutamate receptor was required for the induction of long-term potentiation (LTP) in rat dorsolateral septal nucleus neurons. (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a selective agonist for metabotropic glutamate receptors, also causes burst firing of dorsolateral septal nucleus (DLSN) neurons. In this study, we investigated whether this response was mediated by a phospholipase C-(PLC) coupled metabotropic glutamate receptor. The threshold concentration of 1S,3R-ACPD for the induction of burst firing was about 5 microM, while 10 microM 1S,3R-ACPD produced a maximal effect. L-AP3 (50 microM) reduced the burst firing induced by 1S,3R-ACPD (5 microM). Although 1S,3R-ACPD stimulated the formation of inositol-1,4,5-triphosphate [Ins(1,4,5)P3] suggesting the presence of PLC-coupled metabotropic glutamate receptors, it was only effective in a higher (30-100 microM) concentration range. In addition, the 1S,3R-ACPD-stimulated formation of Ins(1,4,5)P3 level was not affected by L-AP3. These observations suggest that the 1S,3R-ACPD induced burst firing is not mediated by PLC-coupled metabotropic glutamate receptors.

Nitric oxide induces calcium-dependent [3H]dopamine release from striatal slices

Hydroxylamine (0.01-30 mM), a nitric oxide (NO) generator, produced a concentration-dependent release of [3H]dopamine ([3H]DA) from rat striatal slices. Hemoglobin (10 microM), a NO scavenger, reduced basal [3H]DA release and blocked hydroxylamine (100 microM)-stimulated [3H]DA efflux. Tetrodotoxin (0.5 microM) had no significant effect. Sodium cyanide was used as a model compound to test the possibility that NO acted through blockade of mitochondrial electron transport. Calcium-free experimental buffer (1 mM EGTA) reduced basal release and the hydroxylamine response, while sodium cyanide-induced release did not change under these experimental conditions. Cadmium (200 microM), a non-selective inhibitor of voltage-dependent calcium channels, reduced the hydroxylamine response by 69%. Methylene blue (10 microM), an inhibitor of guanylate cyclase, produced a 3-fold increase in the basal release but had no significant effect on the hydroxylamine response. These data suggest that NO induces calcium-dependent [3H]DA release from the striatum via a mechanism which is independent of blockade of electron transport or activation of guanylate cyclase.

(R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors mediate a calcium-dependent inhibition of the metabotropic glutamate receptor-stimulated formation of inositol 1,4,5-trisphosphate

L-glutamate (3-1,000 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 10-1,000 microM), a selective agonist for the metabotropic glutamate receptor, stimulated the formation of inositol 1,4,5-trisphosphate in a concentration-dependent manner. L-Glutamate was half as efficacious as 1S,3R-ACPD. N-methyl-D-aspartate (NMDA; 1 nM to 1 mM) did not significantly influence the response to a maximally effective concentration of 1S,3R-ACPD (100 microM). On the other hand, coapplication of (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 1-300 nM) produced a concentration- and time-dependent inhibition of the 1S,3R-ACPD effect, with a maximal inhibition (97%) at 100 nM. Ten micromolar 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of the AMPA receptor, blocked the inhibitory effect of AMPA. Reduced extracellular calcium concentration, as well as 10 microM nimodipine, an L-type calcium channel antagonist, inhibited the AMPA influence on the 1S,3R-ACPD response. W-7, a calcium/calmodulin antagonist, prevented the inhibition by AMPA, whereas H-7, an inhibitor of protein kinase C, had no effect. These data suggest that activation of AMPA receptors has an inhibitory influence on inositol 1,4,5-trisphosphate formation mediated by stimulation of the metabotropic glutamate receptor. The mechanism of action involves calcium influx through L-type type calcium channels and possible activation of calcium/calmodulin-dependent enzymes.

Nitric oxide induces neurotransmitter release from hippocampal slices

Hydroxylamine (1-300 microM), a nitric oxide generator, stimulated the release of [3H]norepinephrine ([3H]NE) and [14C]acetylcholine ([14C]ACh) from rat hippocampal slices in a concentration-dependent manner (EC50 approximately 30 microM). A maximally effective concentration of hydroxylamine (300 microM) produced a 24-fold increase in the basal [3H]NE and 3.6-fold increase in the in the basal [14C]ACh efflux. Sodium nitroprusside (SNP), also stimulated the release of [3H]NE, but only at high concentrations (10-30 mM). Calcium-free experimental buffer (1 mM EGTA) abolished the response. Hemoglobin (0.3 microM) inhibited the effect of 100 microM hydroxylamine in a manner which was specific for nitric oxide. In addition, 100 microM hydroxylamine increased the efflux of endogenous GABA and glutamate by 3- and 6-fold, respectively.

Inhibition of the phospholipase C-linked metabotropic glutamate receptor by 2-amino-3-phosphonopropionate is dependent on extracellular calcium

D,L-2-Amino-3-phosphonopropionate (AP-3), a proposed metabotropic receptor antagonist, produced a concentration-dependent increase in the formation of inositol 1,4,5-trisphosphate in rat hippocampal slices. The response was maximal at 1 mM and completely due to the L-isomer. D,L-AP-3 was half as efficacious as (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a selective agonist of this receptor. The response produced by maximally effective concentrations of L-AP-3 and 1S,3R-ACPD together for 5 min was not significantly different from that produced by 1S,3R-ACPD alone. However, pretreatment for 40 min with either 1 mM L-AP-3 or D,L-AP-3 completely inhibited the response to 1S,3R-ACPD. This inhibition was long-lasting (wash-resistant) and was reversed by reduction of the extracellular Ca2+ concentration. Also, pretreatment for 40 min with 1S,3R-ACPD reduced, but did not completely block, the response to readdition of 1S,3R-ACPD. L-AP-3 (1 mM) also produced a stereoselective 2.3-fold increase in the efflux of glutamate from the hippocampal slices. These data suggest that incubation of hippocampal slices with AP-3 induces a time-dependent desensitization of the metabotropic response by a mechanism that is dependent on extracellular Ca2+. The possible roles of receptor occupancy and inhibition of glutamate uptake by AP-3 are also discussed.

Incubation of tissue slices in the absence of Ca2+ and Mg2+ can cause nonspecific damage

Superfusion of striatal slices with a medium deficient in Ca2+ and Mg2+ caused a large and sustained increase in release of lactate dehydrogenase, a finding indicative of the disruption of plasma membranes. This was associated with an efflux of dopamine (DA) and the depletion of DA from the tissue. In addition, whereas DA efflux was stimulated by either D-amphetamine (10 microM) or L-glutamate (10 mM) in the absence of Ca2+, these effects were greatly reduced when Mg2+ also was withdrawn from the buffer. These results suggest that (a) incubation in a Ca2+/Mg2(+)-free buffer disrupts plasma membranes, (b) this disruption affects dopaminergic neurons as well as those of other striatal elements, and (c) the failure of a treatment to stimulate DA release in a Ca2+/Mg2(+)-free buffer cannot be used as a test of Ca2+ dependence.

High glutamate concentrations evoke Ca(++)-independent dopamine release from striatal slices: a possible role of reverse dopamine transport

Glutamate stimulated the efflux of dopamine from slices of rat striatum superfused with a Krebs' bicarbonate buffer containing a physiological concentration of Mg++ (1.2 mM). This effect was observed in the presence of high concentrations of glutamate (3-10 mM), but not at lower concentrations (0.01-1 mM). The response was not accompanied by increased lactate dehydrogenase activity, a measure of glutamate neurotoxicity. At 10 mM, glutamate increased dopamine efflux by more than 9-fold. This was reduced to about 34% of the control response by either the competitive N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid (100 microM) or the noncompetitive N-methyl-D-aspartate receptor antagonist MK 801 [(+)-5-methyl-10,11-dihydro-5H-dibenso[a,d]cyclohepten-5,10- imine hydrogen maleate] (1-10 microM), but was unaffected by a kainate/quisqualate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10-100 microM). Glutamate-stimulated dopamine efflux also was unaffected by tetrodotoxin (0.5 microM), withdrawal of extracellular Ca++ [and addition of 1 mM ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid] or systemic administration of reserpine (5 mg/kg, 24 hr before the experiment), an inhibitor of the vesicular storage of dopamine. In contrast, nomifensine (10 microM), an inhibitor of high-affinity dopamine transport, reduced glutamate-induced dopamine efflux to 15% of the control response. Moreover, the response to glutamate was blocked by deleting NaCl from the medium. Collectively, these results suggest that, at high concentrations and in the presence of Mg++, glutamate can stimulate the release of dopamine by a mechanism that does not use Ca(++)-dependent exocytosis but instead involves a reversal of the dopamine transport system.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of selective antagonists (CH-38083 and idazoxan) to distinguish between prejunctional and postjunctional alpha 2-adrenoreceptors

1. The prejunctional alpha 2-adrenoreceptor antagonist activity of CH-38083 (7,8-(methylenedioxi)-14-alpha-hydroxyalloberbane HCl), idazoxan and prazosin was determined against B-HT 920 on the tachycardia induced by cardiac nerve stimulation in pithed rats. Antagonism of cirazoline and B-HT 920 pressor responses was used to assess postjunctional alpha 1- and alpha 2-adrenoreceptor antagonist activities. 2. CH-38083 was more potent than idazoxan in blocking pre- and postjunctional alpha 2-adrenoreceptor sites. There was no difference between the activities of the two selective alpha 2-adrenoreceptor blocking agents on pre- and postjunctional alpha 2-adrenoreceptors. 3. These data classify CH-38083 as a potent and highly selective alpha 2-adrenoreceptor antagonist in vivo and further support evidence of the homogeneity of pre- and postjunctional alpha 2-adrenoreceptors.

Berbanes: search for novel alpha-2 adrenoceptor antagonists

We have previously described the alpha-2 adrenoceptor antagonist properties of berbanes and its stereoisomers. Of the compounds studied CH-38083 (2,3-methylenedioxy-11-betahydroxyalloberbane) has been selected for further analysis based upon its high affinity and selectivity for central and peripheral alpha-2 adrenoceptors. Structure activity relationship study revealed that the aromatic ring with its substituents at C-2 and C-3 positions, the nitrogen atom, the hydroxy group at C-11 position and the methoxycarbonyl group at C-12 position are important for the binding of the berbanes to the alpha-2 adrenoceptors. Using alloberbane derivatives for characterization of the alpha-2 adrenoceptors it was speculated that xylazine sensitive alpha-2 adrenoceptors in the rat vas deferens and in the guinea-pig ileum are similar, whereas xylazine sensitive and noradrenaline sensitive alpha-2 adrenoceptors of the guinea-pig ileum may belong to different subtypes. Correlation studies indicated that modification of the molecular structure of the alloberbanes can lead to either increased or decreased alpha-2 adrenoceptor antagonists activity without parallel changes in the alpha-1 adrenoceptor antagonist potency. The low affinity of CH-38083 for other receptor populations (muscarinic, histamine, dopamine receptors) makes this compound attractive for investigation of alpha-2 adrenoceptor-mediated neural processes in the central nervous system and periphery.

Evidence that dopaminergic innervation is not involved in the vasodilatation of cat superior mesenteric arterial bed: the role of beta-adrenoceptors and circulating catecholamines

In the presence of phentolamine, stimulation of the cat splanchnic nerve decreased the resistance of the superior mesenteric arterial bed. This effect was not significantly influenced by sulpiride. Sulpiride, in the presence of phentolamine, did not inhibit the decrease in resistance in animals pretreated with guanethidine but propranolol or ablation of the adrenal glands prevented the effect of stimulation. These results are not compatible with the assumption that dopamine released from dopaminergic nerves is involved in the vasodilatation but do provide evidence for the role of beta-adrenoceptors and circulating catecholamines in the vasodilatation of mesenteric vessels.