Spermidine reverses arcaine's inhibition of N-methyl-D-aspartate-induced hippocampal [3H]norepinephrine release

The inhibition of N-methyl-D-aspartate (NMDA)-induced [3H]norepinephrine ([3HNE) release by a putrescine analog was studied. We report that arcaine, diguanidinobutane, a putative competitive polyamine antagonist, completely and noncompetitively antagonized NMDA-induced [3H]NE release from rat hippocampal minces with an IC50 value of 102 microM. Arcaine did not alter kainate- or potassium-induced [3H]NE release suggesting a specific effect on NMDA-mediated responses. Spermidine did not alter NMDA-induced [3H]NE release, nor did it reverse the effect of arcaine when introduced in a normal physiologic superfusion buffer. However, spermidine reversed the effect of arcaine when superfusing with buffer that contained 5% (v/v) of the organic solvent dimethylsulfoxide. This finding suggests that the polyamine site may be located at the intracellular surface of the cell membrane. Our results provide the first evidence for polyamine modulation of the NMDA receptor ionophore complex in a functional physiologic system.     

Ethanol inhibits N-methyl-D-aspartate-stimulated [3H]norepinephrine release from rat cortical slices

The effects of ethanol on N-methyl-D-aspartate (NMDA)-stimulated [3H]norepinephrine (NE) release from rat cortical slices was studied. NMDA-stimulated [3H]NE release was inhibited by tetrodotoxin, Mg++ and 2-amino-5-phosphonopentanoic acid, indicating that NMDA receptors in the cortex have characteristics similar to those observed using electrophysiological studies. Ethanol (60-200 mM) decreased the release of [3H]NE evoked by 100 microM NMDA in a concentration-dependent manner (32-52% inhibition), but it did not significantly alter the basal release. The inhibitory effect of 100 mM ethanol was due to a reduction in the maximal response with no significant change in the EC50 for NMDA. Pretreatment of the slices with 100 mM ethanol up to 6 min did not alter the magnitude of inhibition. The inhibition of NMDA-stimulated [3H]NE release due to ethanol was reversible after a 13-min recovery period. The presence of ethanol did not significantly affect the IC50 for Mg++ inhibition of NMDA-stimulated [3H]NE release (23 +/- 3 microM). Glycine (10-300 microM) potentiated the release of [3H]NE stimulated by 250 microM NMDA, and 60 mM ethanol did not alter this effect of glycine. Ethanol (100 mM) inhibited the release of [3H]NE evoked by 18.9 mM KCl in the presence or absence of 2-amino-5-phosphonopentanoic acid, but had no effect on release induced by 49.1 mM KCl. Tetrodotoxin (0.3 mM) significantly decreased the release of [3H] NE evoked by 23.2 mM KCl, and 60 to 200 mM ethanol did not alter this release. These results suggest that NMDA receptors in rat cortical slices are located on nerve cell bodies.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma l-[3H]norepinephrine, d-[14C]norepinephrine, and d,l-[3H]isoproterenol kinetics in essential hypertension.

We infused tracer-labeled l-[3H]-norepinephrine, d-[14C]norepinephrine, and d,l-[3H]-isoproterenol simultaneously into patients with essential hypertension and into normotensive control subjects, in order to determine whether abnormalities in the disappearance kinetics of these substances characterized the hypertensive patients. The mean preinfusion venous plasma norepinephrine concentration was somewhat higher in the hypertensive group (260 vs. 194 pg/ml, P = 0.06), but the groups did not differ in the disappearance kinetics of l- or d-norepinephrine or of isoproterenol. Preinfusion plasma norepinephrine was significantly positively correlated with calculated spillover rates in both the hypertensive and normotensive groups, but not with norepinephrine clearances. The d/l ratio in plasma norepinephrine was the same as in the infusate during and after the infusion, even after pretreatment with the neuronal norepinephrine uptake blocker, desipramine. Because isoproterenol is not taken up by nerve endings, the ratio of [3H]isoproterenol to l-[3H]norepinephrine increased after the infusion ended. This increase was almost completely abolished by pretreatment with desipramine. These results indicate that (a) increased plasma norepinephrine levels seen in some patients with essential hypertension result from increased sympathetic neural activity and not from decreased clearance of norepinephrine, (b) changes in the isoproterenol/norepinephrine ratio after simultaneous infusion of both provide an index of neuronal norepinephrine uptake in man, and (c) neuronal norepinephrine uptake is not stereospecific.     

A comparison of N-methyl-D-aspartate-evoked release of adenosine and [3H]norepinephrine from rat cortical slices

Tetrodotoxin reduced N-methyl-D-aspartate (NMDA)-evoked release of adenosine by 35% but virtually abolished [3H]norepinephrine release. Although [3H]norepinephrine release from rat cortical slices evoked by 500 microM NMDA was abolished by 1.2 mM Mg++, which produces a voltage-sensitive, uncompetitive block of NMDA-channels, adenosine release was increased in the presence of Mg++. Partial depolarization with 12 mM K+ relieved the Mg++ block of 500 microM NMDA-evoked [3H]norepinephrine release but did not affect adenosine release, indicating that a Mg++ requirement for the adenosine release process per se cannot account for this discrepancy. NMDA was 33 times more potent in releasing adenosine than [3H]norepinephrine. At submaximal concentrations of NMDA (10 and 20 microM), adenosine release was augmented in Mg+(+)-free medium. Although a high concentration of the uncompetitive NMDA antagonist MK-801 [(+)-5-methyl-10,11,dihydro-5H-dibenzo[a,d]cyclohepten-5-10-imine maleate] (3 microM) blocked NMDA-evoked release of [3H]norepinephrine and adenosine, a lower concentration (300 nM) decreased NMDA-evoked [3H]norepinephrine release by 66% without affecting adenosine release. These findings suggest that maximal adenosine release occurs when relatively few NMDA receptors are activated, raising the possibility that spare receptors exist for NMDA-evoked adenosine release. Rather than acting as a protectant against excessive NMDA excitation, released adenosine might provide an inhibitory threshold which must be overcome for NMDA-mediated neurotransmission to proceed.     

Bupropion inhibits nicotine-evoked [(3)H]overflow from rat striatal slices preloaded with [(3)H]dopamine and from rat hippocampal slices preloaded with [(3)H]norepinephrine

Bupropion, an efficacious antidepressant and smoking cessation agent, inhibits dopamine and norepinephrine transporters (DAT and NET, respectively). Recently, bupropion has been reported to noncompetitively inhibit alpha3beta2, alpha3beta4, and alpha4beta2 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes or established cell lines. The present study evaluated bupropion-induced inhibition of native alpha3beta2* and alpha3beta4* nAChRs using functional neurotransmitter release assays, nicotine-evoked [(3)H]overflow from superfused rat striatal slices preloaded with [(3)H]dopamine ([(3)H]DA), and nicotine-evoked [(3)H]overflow from hippocampal slices preloaded with [(3)H]norepinephrine ([(3)H]NE). The mechanism of inhibition was evaluated using Schild analysis. To eliminate the interaction of bupropion with DAT or NET, nomifensine or desipramine, respectively, was included in the superfusion buffer. A high bupropion concentration (100 microM) elicited intrinsic activity in the [(3)H]DA release assay. However, none of the concentrations (1 nM-100 microM) examined evoked [(3)H]NE overflow and, thus, were without intrinsic activity in this assay. Moreover, bupropion inhibited both nicotine-evoked [(3)H]DA overflow (IC(50) = 1.27 microM) and nicotine-evoked [(3)H]NE overflow (IC(50) = 323 nM) at bupropion concentrations well below those eliciting intrinsic activity. Results from Schild analyses suggest that bupropion competitively inhibits nicotine-evoked [(3)H]DA overflow, whereas evidence for receptor reserve was obtained upon assessment of bupropion inhibition of nicotine-evoked [(3)H]NE overflow. Thus, bupropion acts as an antagonist at alpha3beta2* and alpha3beta4* nAChRs in rat striatum and hippocampus, respectively, across the same concentration range that inhibits DAT and NET function. The combination of nAChR and transporter inhibition produced by bupropion may contribute to its clinical efficacy as a smoking cessation agent.     

Effects of selected muscarinic cholinergic antagonists on [3H]acetylcholine release from rat hippocampal slices

A number of cholinergic muscarinic (M) agonists and antagonists were studied for their ability to enhance tritiated acetylcholine ([3H]ACh) release from electrically field-stimulated rat hippocampal slices. A Ca++-free medium and carbachol, but not nicotine, inhibited [3H]ACh release. Atropine, methylatropine and dexetimide produced concentration-dependent increases in [3H]ACh release to a maximum of about 50% above control. Aprophen and benactyzine produced a maximal response 25 to 35% above control. The selective M1 antagonist pirenzepine had the least effect on [3H]ACh release. Of the nonspecific M1-M2 antagonists studied, benactyzine produced the least amount of [3H]ACh release. The order of potency of the M antagonists in promoting a 15% increase in [3H]ACh release was aprophen greater than benactyzine greater than methylatropine greater than dexetimide greater than pirenzepine greater than atropine. However, the order of promoting maximal release of [3H]ACh was atropine greater than dexetimide greater than methylatropine greater than aprophen greater than benactyzine greater than pirenzepine.     

Alpha-2 adrenergic inhibition of Ca(++)-evoked [3H]norepinephrine release from synaptosomes is blocked by depolarization

The Ca(++)-evoked release of [3H]norepinephrine was used in these studies to investigate presynaptic regulation of norepinephrine release. In hippocampal synaptosomes, previously unexposed to Ca++ during isolation and superfusion, 1.25 mM Ca++ evoked a modest (4 to 7% of total stores) release of [3H]norepinephrine with 4.5 mM [K+] present. The alpha-2 adrenergic agonist clonidine inhibited 60% of the Ca(++)-evoked [3H]norepinephrine release. The alpha-2 adrenergic antagonists idazoxan and yohimbine reversed clonidine inhibition of release whereas the alpha-1 antagonist prazosin did not. Increasing the [K+] before Ca++ exposure increased [3H]norepinephrine release, and at 20 [K+] the release increased to over 20% of total stores. However, at [K+] above 9 mM, inhibition of Ca(++)-evoked release by clonidine decreased, and by 20 mM [K+] clonidine no longer inhibited release. Release was unaffected by 5 microM idazoxan or the opiate antagonist naloxone at 15 or 20 mM [K+]. The K+ channel blockers tetraethylammonium (5 mM) and 4-aminopyridine (0.1 mM) increased Ca(++)-evoked release almost 4-fold above control (4.5 mM [K+] present). Neither clonidine nor idazoxan affected Ca(++)-evoked release with the K+ channel blockers present. Therefore, even though K+ channel blockers and 20 mM [K+] increase neurotransmitter release, it is not autoreceptor activation by released endogenous norepinephrine that is responsible for blocking alpha-2 inhibition, but the depolarization produced by these treatments. The 20 mM [K+] blockade of alpha-2 inhibition was decreased by lowering the [Ca++] in the superfusion buffer. Therefore, synaptosomal accumulation of Ca++ may partially explain the loss of alpha-2 inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nicotinic acetylcholine receptor-mediated [3H]dopamine release from hippocampus

The mechanism of nicotinic acetylcholine receptor (nAChR)-induced hippocampal dopamine (DA) release was investigated using rat hippocampal slices. nAChRs involved in hippocampal DA and norepinephrine (NE) release were investigated using prototypical agonists and antagonists and several relatively novel compounds: ABT-594 [(R)-5-(2-azetidinylmethoxy)-2-chloropyridine], (+/-)-UB-165 [(2-chloro-5-pyridyl)-9-azabicyclo [4.2.1]non2-ene], and MG 624 [N,N,N-triethyl-2-[4-(2 phenylethenyl)phenoxy]-ethanaminium iodine]. (+/-)-Epibatidine, (+/-)-UB-165, anatoxin-a, ABT-594, (-)-nicotine, 1,1-dimethyl-4-phenyl-piperazinium iodide, and (-)-cytisine (in decreasing order of potency) evoked [(3)H]DA release in a mecamylamine-sensitive manner. Aside from (+/-)-UB-165, all the agonists displayed full efficacy relative to 100 microM (-)-nicotine in [(3)H]DA release. In contrast, (+/-)-UB-165 was a partial agonist, evoking 58% of 100 microM (-)-nicotine response. Mecamylamine, MG 624, hexamethonium, d-tubocurare, and dihydro-beta-erythroidine (in decreasing order of potency), but not alpha-conotoxin-MII, methyllycaconitine, alpha-conotoxin-ImI, or alpha-bungarotoxin, attenuated 100 microM (-)-nicotine-evoked [(3)H]DA release in a concentration-dependent manner. (+/-)-UB-165, ABT-594, and MG 624 exhibited different pharmacologic profiles in the [(3)H]NE release assay when compared with their effect on [(3)H]DA release. ABT-594 was 4.5-fold more potent, and (+/-)-UB-165 was a full agonist in contrast to its partial agonism in [(3)H]DA release. MG 624 potently and completely blocked NE release evoked by 100 microM (-)-nicotine and 10 microM (+/-)-UB-165, whereas it only partially inhibited (-)-nicotine-evoked [(3)H]DA release. In conclusion, we provide evidence that [(3)H]DA can be evoked from the hippocampus and that the pharmacologic profile for nAChR-evoked hippocampal [(3)H]DA release suggests the involvement of alpha3beta4(*) and at least one other nAChR subtype, thus distinguishing it from that of nAChR-evoked hippocampal [(3)H]NE release.     

Dopamine regulation of [3H]acetylcholine release from guinea-pig stomach

The involvement of dopamine receptors in cholinergic transmission of guinea-pig stomach was investigated by analyzing the effects of dopamine receptor agonists and antagonists on acetylcholine (ACh) release from this organ. Electrical stimulation (1-20 Hz) of strips of guinea-pig stomach preloaded with [3H] choline induced a [3H]ACh release that was calcium dependent and tetrodotoxin sensitive. Dopamine inhibited this transmural stimulation-induced [3H]ACh release in a concentration-dependent manner (10(-8)-10(-4) M). This effect of dopamine was not altered by 10(-5) M hexamethonium, thereby suggesting that the major dopamine receptors are located on the postganglionic cholinergic neurons. Concentration-response curves for dopamine on [3H]ACh release were inhibited by haloperidol, sulpiride and domperidone but not by prazosin, yohimbine, propranolol and ketanserin. LY 171555, an agonist for the D2 dopamine receptor, but not SKF 38-393, an agonist for the D1 dopamine receptor, to some extent decreased the release of [3H]ACh induced by transmural stimulation. In view of the results, the release of ACh from postganglionic cholinergic neurons is probably required through dopamine receptors antagonized by D2 antagonists but not by adrenergic or serotonin receptor antagonists.     

Long-lasting facilitation of 4-amino-n-[2,3-(3)H]butyric acid ([(3)H]GABA) release from rat hippocampal slices by nicotinic receptor activation

In this study we explored the effect of the stimulation of nicotinic acetylcholine receptors located on interneurons by measuring 4-amino-n-[2,3-(3)H]butyric acid ([(3)H]GABA) release and monitoring [Ca (2+)](i) in superfused hippocampal slices. In the presence of 6-cyano-7-nitroquinoxaline-2,3-dione, (+/-)-2-amino-5-phosphonopentanoic acid, and atropine, i.e., under the blockade of N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate and muscarinic receptors, nicotine did not alter the spontaneous outflow of [(3)H]GABA, but significantly increased the stimulation-evoked [(3)H]GABA efflux. This effect of nicotine depended on the time interval between nicotine treatment and electrical stimulus, the concentration of nicotine (1-100 microM), and the parameters of electrical depolarization. Acetylcholine (0.03-3 mM), and the alpha 7 subtype-selective agonist choline (0.1-10 mM), also potentiated stimulus-evoked release of [(3)H]GABA, whereas 1,1-dimethyl-4-phenilpiperazinium iodide failed to increase the tritium outflow significantly. The effect of nicotine treatment was prevented by tetrodotoxin (1 microM) and by the nicotinic acetylcholine receptor antagonist mecamylamine (10 microM), and the alpha 7 subtype-selective antagonists alpha-bungarotoxin (100 nM) and methyllycaconitine (10 nM), whereas dihidro-beta-erythroidine (20 nM) was without effect. Perfusion of 100 microM nicotine caused a [Ca(2+)](i) transient in about one-third of the tested interneurons; however, the response to subsequent electrical stimulation remained unchanged. Inhibition of the GABA transporter system by nipecotic acid (1 mM) or by decreasing the bath temperature to 12 degrees C abolished completely the effect of nicotine to potentiate the stimulation-evoked release of GABA. These findings indicate that the activation of alpha 7-type nicotinic receptors of hippocampal interneurons results in a long-lasting ability of these cells to respond to depolarization with an increased release of GABA mediated by the transporter system.     

Alpha-2 adrenoceptors modulate [3H]dopamine release from rabbit retina

In the rabbit retina, preloaded in vitro with [3H]dopamine, calcium-dependent release of radioactivity was elicited by a 1-min period of field stimulation at 3 Hz (20 mA, 2 msec). In the presence of the catecholamine uptake inhibitor nomifensine (30 microM), unlabeled catecholamines (0.01-3 microM), namely, dopamine, norepinephrine and epinephrine, inhibited in a concentration-dependent manner the field stimulation-evoked release of [3H]dopamine from the retina. The concentrations of dopamine, norepinephrine or epinephrine which inhibited by 50% the release of [3H]dopamine (IC50) were 0.30, 0.25 and 0.25 microM, respectively. In the presence of 30 microM nomifensine, S-sulpiride (1 microM) significantly increased the calcium-dependent release of [3H]dopamine, suggesting that this dopamine antagonist blocks a receptor tonically activated by endogenous dopamine in the rabbit retina. In contrast, the alpha receptor antagonist phentolamine (1 microM) alone did not affect the release of [3H]dopamine from the retina. The inhibitory effect of norepinephrine and epinephrine on [3H]dopamine overflow was not modified by S-sulpiride which, on the contrary, selectively antagonized the effect of exogenous dopamine. Phentolamine (1 microM) competitively antagonized the inhibitory effect of norepinephrine and epinephrine on [3H]dopamine release, suggesting that these catecholamines activate alpha adrenoceptors in retina. In the absence of nomifensine, the selective alpha-2 agonist clonidine (IC50 = 0.056 microM) inhibited the stimulation-evoked release of [3H]dopamine from retina, whereas the alpha agonist methoxamine was without effect. The inhibitory effect of clonidine was antagonized by yohimbine (1 microM), but not prazosin, suggesting that the release modulating alpha receptors of the retina are of the alpha-2 subtype.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulatory role of alpha adrenoceptors on the release of [3H]norepinephrine elicited by preganglionic stimulation of the cat superior cervical ganglion

In the isolated cat superior cervical ganglion labeled in vitro with [3H]norepinephrine ([3H]NE), the overflow of radioactivity evoked by preganglionic stimulation at 10 Hz (80 V, 2 msec duration for 5 min) was reduced to 50% of control values by the alpha adrenoceptor agonists clonidine (0.001 microM) and methoxamine (12.0 microM). The alpha adrenoceptor antagonist phenoxybenzamine (2.9 microM) produced a 2-fold increase in the overflow of [3H]NE elicited by nerve stimulation. Preincubation with drugs that reduce the neuronal uptake of norepinephrine in the isolated ganglion (8.8 microM cocaine and 0.33 microM desipramine) did not modify the release of [3H]NE by preganglionic stimulation. However, a higher concentration of desipramine (3.3 microM) produced a 4-fold increase in the overflow of tritium evoked by stimulation. As this concentration of desipramine produced a shift to the right in the concentration-response curve to methoxamine in the isolated nictitating membrane of the cat, the conclusion is drawn that a feedback mechanism mediated through presynaptic alpha adrenoceptors regulates the release of [3H]NE induced by preganglionic stimulation of the cat superior cervical ganglion. In addition, it is suggested that regulatory mechanisms for norepinephrine release by nerve stimulation are not restricted to nerve terminals but are also present in dendrites of the postganglionic adrenergic neurons.     

Stimulus-dependent modulation of [(3)H]norepinephrine release from rat neocortical slices by gabapentin and pregabalin

Gabapentin (GBP; Neurontin) has proven efficacy in several neurological and psychiatric disorders yet its mechanism of action remains elusive. This drug, and the related compounds pregabalin [PGB; CI-1008, S-(+)-3-isobutylgaba] and its enantiomer R-(-)-3-isobutylgaba, were tested in an in vitro superfusion model of stimulation-evoked neurotransmitter release using rat neocortical slices prelabeled with [(3)H]norepinephrine ([(3)H]NE). The variables addressed were stimulus type (i.e., electrical, K(+), veratridine) and intensity, concentration dependence, onset and reversibility of action, and commonality of mechanism. Both GBP and PGB inhibited electrically and K(+)-evoked [(3)H]NE release, but not that induced by veratridine. Inhibition by these drugs was most pronounced with the K(+) stimulus, allowing determination of concentration-effect relationships (viz., 25 mM K(+) stimulus: GBP IC(50) = 8.9 microM, PGB IC(50) = 11.8 microM). R-(-)-3-Isobutylgaba was less effective than PGB to decrease stimulation-evoked [(3)H]NE release. Other experiments with GBP demonstrated the dependence of [(3)H]NE release inhibition on optimal stimulus intensity. The inhibitory effect of GBP increased with longer slice exposure time before stimulation, and reversed upon washout. Combination experiments with GBP and PGB indicated a similar mechanism of action to inhibit K(+)-evoked [(3)H]NE release. GBP and PGB are concluded to act in a comparable, if not identical, manner to preferentially attenuate [(3)H]NE release evoked by stimuli effecting mild and prolonged depolarizations. This type of modulation of neurotransmitter release may be integral to the clinical pharmacology of these drugs.     

The neurotoxic compound N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4) depletes endogenous norepinephrine and enhances release of [3H]norepinephrine from rat cortical slices

The alkylating compound N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4) injected to rodents blocks norepinephrine (NE) uptake and reduces endogenous NE levels in the central nervous system and in the periphery. To investigate the processes leading to these alterations, rat cortical slices were incubated in the presence of DSP4. Cortical NE was depleted by 40% after incubation of slices in 10(-5) M DSP4 for 60 min and this was blocked by desipramine. The spontaneous outflow of radioactivity from cortical slices labeled previously with [3H]NE was enhanced markedly both during exposure to DSP4 and during the subsequent washings, suggesting that NE depletion could be due to this stimulation of NE release. The radioactivity released by DSP4 was accounted for mainly by NE and its deaminated metabolite 3,4-dihydroxyphenylglycol. The enhanced release, independent of external Ca++, apparently originated from the vesicular pool as it was absent after reserpine pretreatment. Activities of the enzymes related to NE synthesis were not altered by DSP4 in vitro and only monoamine oxidase activity was inhibited at high concentrations. Thus, the depletion of endogenous NE produced by DSP4 is probably due to a persistent enhancement of its release from the vesicular pool. Fixation of DSP4 to the NE transport system is necessary but not sufficient to produce the acute NE depletion and the characteristic long-term actions of the compound.     

Modulation of the release of [3H]norepinephrine from the base and body of the rat urinary bladder by endogenous adrenergic and cholinergic mechanisms

Modulation of [3H]NE release was studied in rat urinary bladder strips prelabeled with [3H]NE. [3H]NE uptake occurred in strips from the bladder base and body, but was very prominent in the base where the noradrenergic innervation is most dense. Electrical field stimulation markedly increased [3H]NE outflow from the superfused tissue. The quantity of [3H]NE release was approximately equal during three consecutive periods of stimulation. Activation of presynaptic muscarinic receptors by 1.0 microM oxotremorine reduced [3H]NE release to 46% of the control. Atropine (1 microM) blocked the effect of oxotremorine and increased the release to 147% of predrug control levels. Activation of presynaptic alpha-2 adrenoceptors by 1 microM clonidine reduced [3H]NE release to 55% of control. Yohimbine blocked the action of clonidine and increased the release to 148% of control. The release of [3H]NE from the bladder base and body was increased by both 1 microM atropine (to 167% and 174% of control, respectively) and 1 microM yohimbine (to 286% and 425% of control, respectively). Atropine and yohimbine administered in combination had similar facilitatory effects as when administered alone. We conclude that the release of [3H]NE from adrenergic nerve endings in electrically stimulated bladder strips is modulated via endogenous transmitters acting on both muscarinic and alpha-2 adrenergic presynaptic receptors and that the latter provide the most prominent control.     

Steroids modulate N-methyl-D-aspartate-stimulated [3H] dopamine release from rat striatum via sigma receptors

Steroids have been proposed as endogenous ligands at sigma receptors. In the current study, we examined the ability of steroids to regulate N-methyl-d-aspartate (NMDA)-stimulated [3H]dopamine release from slices of rat striatal tissue. We found that both progesterone and pregnenolone inhibit [3H]dopamine release in a concentration-dependent manner similarly to prototypical agonists, such as (+)-pentazocine. The inhibition seen by both progesterone and pregnenolone exhibits IC50 values consistent with reported Ki values for these steroids obtained in binding studies, and was fully reversed by both the sigma1 antagonist 1-(cyclopropylmethyl)-4-2'-4"flurophenyl)-2'oxoethyl)piperidine HBr (DuP734) and the sigma2 antagonist 1'-[4-[1-(4-fluorophenyl)-1-H-indol-3-yl]-1-butyl]spiro[iso-benzofuran-1(3H), 4'piperidine] (Lu28-179). Lastly, to determine whether a protein kinase C (PKC) signaling system might be involved in the inhibition of NMDA-stimulated [3H]dopamine release, we tested the PKCbeta-selective inhibitor 5,21:12,17-dimetheno-18H-dibenzo[i,o]pyrrolo[3,4 - 1][1,8]diacyclohexadecine-18,20(19H)-dione,8-[(dimethylamino)methyl]-6,7,8,9,10,11-hexahydro-monomethanesulfonate (9Cl) (LY379196) against both progesterone and pregnenolone. We found that LY379196 at 30 nM reversed the inhibition of release by both progesterone and pregnenolone. These findings support steroids as candidates for endogenous ligands at sigma receptors.     

Local alpha-bungarotoxin-sensitive nicotinic receptors modulate hippocampal norepinephrine release by systemic nicotine

Previous studies have shown that nicotinic receptors (NAChRs) accessible from the cerebral aqueduct of the brainstem mediate the hippocampal norepinephrine (NE) release induced by i.v. nicotine. The present study was designed to investigate the role of hippocampal NAChRs in this process. Nicotinic antagonists were microinjected or microdialyzed into the hippocampus (HP) before administering nicotine (0.09 mg/kg over 60 s, i.v.) to freely moving rats. alpha-Bungarotoxin (0.3 nmol by microinjection) blocked nicotine-induced hippocampal NE release by 47% (p <.05) and abolished the effect of 0.065 mg/kg nicotine. Methyllycaconitine (1.4-5.6 mM in the dialysate) inhibited the stimulatory effect of nicotine 0.09 mg/kg by 48 to 75% (p <.05). In contrast, mecamylamine (2.9-5.8 mM) and dihydro-beta-erythroidine (7-14 mM) were completely ineffective. The role of hippocampal NAChRs was demonstrated further by selectively desensitizing these receptors before the systemic infusion of nicotine. To do so, the HP was pretreated with nicotine (0.1 mM) delivered through the microdialysis probe; this concentration was calculated to yield tissue concentrations similar to those produced by the systemic infusions of nicotine. Dialyzing this concentration of nicotine into the HP inhibited the NE response to i.v. nicotine by 34% (p <.05), and 1.0 mM nicotine reduced the response by 40%. These studies indicate that alpha-bungarotoxin-sensitive hippocampal NAChRs, probably containing alpha7 subunits, modulate hippocampal NE release because of systemic nicotine.     

Mechanism and metabolic disposition of verapamil-evoked overflow of radioactivity from isolated atria preloaded with [3H]norepinephrine

In this study the mechanism and metabolic profile of verapamil-evoked release of radioactivity was investigated in the rat isolated atria preloaded with [3H]norepinephrine [( 3H]NE). Verapamil (10(-7) to 10(-3) M) caused a dose-related increase in the outflow (or the fractional release) of 3H. The fractional 3H-release produced by verapamil was reduced markedly in tissues which had been preloaded with [3H]NE in the presence of cocaine (10 microM) or after pretreatment of animals with reserpine (5 mg/kg i.p., 24 hr before sacrifice). Verapamil-evoked fractional 3H-release was unchanged in the presence of tetrodotoxin (5 X 10(-6) M) or in Ca++-free Krebs' medium containing 2 mM ethylene glycol bis(beta-aminoethyl ether)N,N'-tetraacetic acid. Whereas greater than 90% of tissue 3H-content consisted of unchanged [3H]NE, 60 to 75% of the spontaneous outflow and the verapamil-evoked overflow consisted of [3H]-3,4-dihydroxyphenylglycol and 2 to 10% was unchanged [3H]NE. When both cocaine (10 microM) and hydrocortisone (28 microM) (uptake-1 and uptake-2 blockers, respectively) were present, although the spontaneous outflow, as well as verapamil-evoked overflow, of radioactivity was increased, the metabolic profiles remained essentially unchanged. The addition of pargyline (10 microM), a monoamine oxidase inhibitor, in addition to the uptake-1 and uptake-2 blockers to the Krebs' solution significantly depressed both the spontaneous outflow and verapamil-evoked overflow of 3H; the verapamil-evoked overflow under this condition, however, consisted of unchanged [3H]NE (greater than 90%).(ABSTRACT TRUNCATED AT 250 WORDS)