Chronic nicotine alters cannabinoid-mediated locomotor activity and receptor density in periadolescent but not adult male rats

A significant number of youths use cigarettes, and more than half of the youths who smoke daily also use illicit drugs. The focus of these studies is on how exposure to nicotine affects subsequent responses to both nicotine and cannabinoids in adolescents compared with adults. We have shown previously that chronic treatment with nicotine produces sensitization to its locomotor-activating effects in female and adult rats but not male adolescent rats. To better understand the effects of nicotine on adolescent and adult rats, rats were injected with nicotine or saline for 7 days and, on day 8, either challenged with delta-9-tetrahydrocannabinol (Delta 9-THC) or the cannabinoid agonist CP 55,940 and tested for locomotor activity, or the brains were removed for quantitative autoradiography studies of the cannabinoid(1) receptor. A separate group of rats was treated with nicotine plus the cannabinoid antagonist AM 251 and then challenged with CP 55,940. In adolescent male rats, nicotine administration led to sensitization to the locomotor-decreasing effects of both Delta 9-THC and CP 55,940, but in adult male rats, the response to either drug was unchanged compared to controls. The effect of nicotine on CP 55,940-mediated locomotor activity was blocked by co-administration of AM 251 with the nicotine. Further, cannabinoid receptor density was increased in the prelimbic prefrontal cortex, ventral tegmental area, and select regions of the hippocampus in adolescent male rats pretreated with nicotine compared to vehicle-treated controls. There were no significant changes in cannabinoid receptor binding, however, in any of the brain regions examined in adult males pretreated with nicotine. The prelimbic prefrontal cortex and the hippocampus have been shown previously to be involved in stimulant reinforcement; thus it is possible that these changes contribute to the unique behavioral effects of chronic nicotine and subsequent drug administration in adolescents compared with adults.

A comparison of the binding profiles of dextromethorphan, memantine, fluoxetine and amitriptyline: Treatment of involuntary emotional expression disorder

We compared the binding profiles of medications potentially useful in the treatment of involuntary emotional expression disorder at twenty-six binding sites in rat brain tissue membranes. Sites were chosen based on likelihood of being target sites for the mechanism of action of the agents in treating the disorder or their likelihood in producing side effects experienced by patients treated with psychoactive agents. We used radioligand binding assays employing the most selective labeled ligands available for sites of interest. Concentrations of labeled ligand were used at or below the K(i) value of the ligand for the target site. Compounds were initially screened at 1 muM. For compounds that competed for greater than 20-30% of specific binding at target sites of interest, full concentration curves were constructed. Dextromethorphan, amitriptyline and fluoxetine competed for binding to sigma(1) receptors and to serotonin transporters with high to moderate affinity. Of the target sites tested, these are the most likely to contribute to the therapeutic benefit of the various agents. In addition, all three drugs showed some activity at alpha(2) and 5-HT(1B/D) sites. Of the drugs tested, dextromethorphan bound to the fewest sites unlikely to be target sites. Although the mechanism of action of dextromethorphan or any drug that has been used in the treatment of involuntary emotional expression disorder is currently unknown, our data support that the affinity of the drug for sigma(1) receptors is consistent with its possible action through this receptor type in controlling symptoms of the disorder.

Dextromethorphan as a Potential Neuroprotective Agent With Unique Mechanisms of Action

BACKGROUND

Dextromethorphan (DM) is a widely-used antitussive. DM's complex central nervous system (CNS) pharmacology became of interest when it was discovered to be neuroprotective due to its low-affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonism.

REVIEW SUMMARY

Mounting preclinical evidence has proven that DM has important neuroprotective properties in various CNS injury models, including focal and global ischemia, seizure, and traumatic brain injury paradigms. Many of these protective actions seem functionally related to its inhibitory effects on glutamate-induced neurotoxicity via NMDA receptor antagonist, sigma-1 receptor agonist, and voltage-gated calcium channel antagonist actions. DM's protection of dopamine neurons in parkinsonian models may be due to inhibition of neurodegenerative inflammatory responses. Clinical findings are limited, with preliminary evidence indicating that DM protects against neuronal damage. Negative findings seem to relate to attainment of inadequate DM brain concentrations. Small studies have shown some promise for treatment of perioperative brain injury, amyotrophic lateral sclerosis, and symptoms of methotrexate neurotoxicity. DM safety/tolerability trials in stroke, neurosurgery, and amyotrophic lateral sclerosis patients demonstrated a favorable safety profile. DM's limited clinical benefit is proposed to be associated with its rapid metabolism to dextrorphan, which restricts its central bioavailability and therapeutic utility. Systemic concentrations of DM can be increased via coadministration of low-dose quinidine (Q), which reversibly inhibits its first-pass elimination. Potential drug interactions with DM/Q are discussed.

CONCLUSIONS

Given the compelling preclinical evidence for neuroprotective properties of DM, initial clinical neuroprotective findings, and clinical demonstrations that the DM/Q combination is well tolerated, this strategy may hold promise for the treatment of various acute and degenerative neurologic disorders.

In vitro regulation of serotonin transporter activity by protein kinase A and nicotinic acetylcholine receptors in the prefrontal cortex of rats

We investigated the effect of in vitro exposure to nicotinic acetylcholine receptors (nAChRs), agonists, antagonists, and protein kinase A (PKA) modulators on the activity of the serotonin transporter (SERT) in prefrontocortical (PFC) synaptosomes. The plasma membrane SERT is an active transport mechanism specific for serotonin. Receptors and second messengers capable of altering transporter activity would be expected to have profound effects on serotonergic neurotransmission and on functions involving serotonergic input, such as cognition, anxiety, and mood. Our data suggest that activation of nAChRs, quite likely via PKA, increase the activity of the SERT in the PFC and, thereby, can alter 5-HT levels in a region important in the behavioral effects of nicotine and 5-HT. Nicotine at 4 microM increased [(3)H]5-HT uptake by 75%. Because the nAChR antagonists mecamylamine and dihydro-beta-erythrodine (DHbetaE) both decreased [(3)H]5-HT uptake into synaptosomes, it appeared that the SERT might be tonically activated by acetylcholine present within our synaptosomal preparations. Blocking PKA significantly decreased [(3)H]5-HT, while stimulation of PKA activity significantly increased the uptake. A 66% decrease compared with control was produced by 100 microM Rp-cAMP, and a 41% increase in 5-HT uptake over control was observed with 30 microM Sp-cAMPs. Furthermore, the enhancement in uptake produced by 4 microM nicotine was inhibited in a time-dependent fashion by preincubation with 10 microM Rp-cAMP. A better understanding of the influence of the cholinergic system and the receptors involved in the trafficking of SERT would help clarify the important relationship between the cholinergic and serotonergic systems and the role these systems play in behavior.

Sigma2 (sigma2) receptors as a target for cocaine action in the rat striatum

Studies from our laboratory have shown that agonists at sigma1 and sigma2 receptors inhibit N-methyl-D-aspartate (NMDA)-stimulated dopamine release from motor and limbic areas of rat brain. In the current study, we examined the effects of cocaine on N-methyl-D-aspartate (NMDA)-stimulated [3H]dopamine release in rat striatal slices. Cocaine inhibited N-methyl-D-aspartate-stimulated [3H]dopamine release in a concentration-dependent manner with a Ki of approximately 10 microM, under conditions in which the dopamine transporter (DAT) was blocked by 10 microM nomifensine. The inhibition seen by cocaine was reversed by the selective sigma2 antagonist 1'-[4-[1-(4-fluorophenyl)-1H-indol-3-yl]-1-butyl]-spiro[isobenzofuran-1(3H), 4'piperidine] (Lu28-179). Inhibition of release by cocaine and (+)pentazocine, under conditions in which sigma1 receptors were blocked, was also reversed by the conventional PKC inhibitor 3-[1-[3-(dimethylamino)propyl-1H-indole-3-yl]-1-H-pyrpole-2-5'-dione. These results suggest that cocaine or other agonists, acting through the sigma2 receptor, require an intact conventional PKC (cPKC), most likely PKCalpha or PKCgamma in order to inhibit dopamine release.

Modulation of bradykinin-induced calcium changes in SH-SY5Y cells by neurosteroids and sigma receptor ligands via a shared mechanism

In this study we investigated the effects of sigma receptor ligands and neurosteroids on bradykinin-induced intracellular calcium concentration ([Ca2+]i) changes in SH-SY5Y neuroblastoma cells. [Ca2+]i levels in cells loaded with fura-2 were monitored with dual-wavelength ratiometric fluorescence measurement. Submicromolar concentrations of bradykinin elicited [Ca2+]i responses with a fast rise followed by a slow decline in these cells. Preincubation of low micromolar concentrations of the neurosteroids pregnenolone, dehydroepiandrosterone (DHEA), or the prototypic sigma (sigma) receptor agonist (+)pentazocine potentiated bradykinin-induced [Ca2+]i changes in SH-SY5Y cells. The sigma receptor antagonist haloperidol blocked the enhancing effects on [Ca2+]i by (+)pentazocine or pregnenolone. Progesterone did not significantly affect the basal [Ca2+]i level or bradykinin-induced [Ca2+]i changes in these cells. However, coincubation of progesterone with (+)pentazocine, pregnenolone, or DHEA reversed their potentiating effects. The antagonistic effects of haloperidol and progesterone on the potentiating effects of (+)pentazocine and pregnenolone suggested that these ligands might act through a common mechanism. We further showed that progesterone, pregnenolone, and DHEA competed for [3H]+pentazocine binding in SH-SY5Y cells with Ki values of 0.13 +/- 0.03 microM, 0.98 +/- 0.34 microM, and 5.2 +/- 1.4 microM, respectively. Thus, the modulation of bradykinin-induced [Ca2+]i changes by neurosteroids in these cells is likely due to their actions on sigma receptors.

Nicotinic receptor-mediated regulation of the dopamine transporter in rat prefrontocortical slices following chronic in vivo administration of nicotine

Low levels of dopaminergic activity in prefrontal cortex are thought to contribute to negative symptoms of schizophrenia. Negative symptoms are associated with the prefrontocortical area of the brain. Schizophrenic patients have a high rate of smoking, which by subjective as well as objective measures produces a cognitive benefit. We have previously shown that agonists at nicotinic receptors containing alpha4 and beta2 subunits can enhance amphetamine-stimulated [3H]dopamine ([3H]DA) release via the dopamine transporter (DAT) from slices of rat prefrontal cortex. This effect is selective for prefrontal cortex; the enhancement does not occur in striatum or nucleus accumbens. The enhancement is dependent upon activation of protein kinase C (PKC). In the current study, we show that the enhancement of amphetamine-stimulated [3H]DA release is maintained after 10 days of chronic nicotine treatment, delivered subcutaneously twice daily. There are no significant changes in the ability of prefrontocortical brain slices to take up [3H]DA in tissue prepared from nicotine-treated vs. saline-treated rats. Nicotinic receptors mediating enhancement of amphetamine-stimulated [3H]DA release are at least partially localized to nerve terminals, as an enhancement in release is also observed in synaptosomal preparations. Finally, the sensitivity of the nicotine enhancement in release to the PKC inhibitor chelerythrine is also seen in synaptosomal preparations, suggesting that the signaling mechanism activated through alpha4beta2 receptors is intact.

Acute and chronic effects of nicotine on serotonin uptake in prefrontal cortex and hippocampus of rats

We sought to investigate the effect of nicotine exposure (chronic and acute) on serotonin transporter (SERT) activity in two regions of the brain important for behavioral effects of nicotine. We first looked at the effects of chronic nicotine exposure (0.7 mg/kg nicotine, twice a day for 10 days) on [(3)H]5-HT uptake in prefrontal cortex (PFC) and hippocampus of rats. A significant increase in [(3)H]5-HT uptake was observed in synaptosomes prepared from both regions. To rule out the possibility that the increases were due to the last injection given, in a separate set of experiments a single injection of nicotine was administered the evening before sacrifice. No change in uptake occurred in either region, suggesting that the increases in uptake caused by nicotine was an effect of chronic exposure and not to an acute treatment. SERT binding studies, using prefrontocortical or hippocampal membrane preparations, revealed that chronic nicotine exposure significantly increased B(max) which correlated to an increase in SERT density. Lastly, we looked at the short-term effect of nicotine on [(3)H]5-HT uptake. Rats received a single nicotine injection 15-75 min before sacrifice. PFC synaptosomes displayed a time-dependent increase in uptake, whereas hippocampal synaptosomes showed an increase at only one time point.

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.

Sigma1 receptor agonist-mediated regulation of N-methyl-D-aspartate-stimulated [3H]dopamine release is dependent upon protein kinase C

We have previously shown that sigma1 receptor agonists inhibit N-methyl-D-aspartate (NMDA)-stimulated [3H]dopamine from slices of rat striatum in a concentration-related manner and that the inhibition is reversed by sigma1 receptor-selective and nonsubtype-selective sigma receptor antagonists. Based on previous evidence from our laboratory as well as other laboratories, we hypothesized that sigma1 receptors might use a protein kinase C (PKC) signaling pathway to modulate stimulated dopamine release. We tested several inhibitors of PKC isozymes, as well as a phospholipase C inhibitor for their effects on sigma1 receptor agonist-mediated regulation of [3H]dopamine release. Although none of the inhibitors tested affected the ability of NMDA to stimulate [3H]dopamine release, they all abolished regulation by the sigma1 receptor agonist (+)-pentazocine in a concentration-related manner. We also found that prior exposure to 1 microM phorbol 2-myristate 13-acetate for 30 min abolished regulation by (+)-pentazocine. We concluded that an intact PKC system was required for sigma1 agonist-mediated regulation of NMDA-stimulated [3H]dopamine release from rat striatal slices. Based on the pharmacological profile of the PKC inhibitors tested, as well as reports in the literature on PKC

Sigma(2)-receptor regulation of dopamine transporter via activation of protein kinase C

The elucidation of the mechanisms underlying sigma(2)-receptor activation and signal transduction is crucial to the understanding of sigma(2)-receptor function. Previous studies in our laboratory have demonstrated sigma(2)-receptor-mediated regulation of the dopamine transporter (DAT) as measured by amphetamine-stimulated release of [(3)H]dopamine (DA) from both rat striatal slices and PC12 cells. The regulation of the DAT in the PC12 cell model was dependent upon activation of Ca(2+)/calmodulin-dependent kinase II. We have now studied the second messenger systems involved in sigma(2)-receptor-mediated regulation of amphetamine-stimulated [(3)H]DA release in rat striatal slices, including Ca(2+)/calmodulin-dependent kinase II, protein kinase C, and sources of calcium required for the enhancement of release produced by sigma(2)-receptor activation. The Ca(2+)/calmodulin-dependent kinase II inhibitors 1-[N,O-bis-(5-isoquionolinesulfonyl)]-N-methyl-L-tyrosyl-4-phenylpiperazine and N-[2-[[[3-(4'-chlorophenyl)-2-propenyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4'-methoxy-benzenesulfonamide phosphate did not significantly affect the (+)-pentazocine-mediated enhancement of amphetamine-stimulated [(3)H]DA release. However, we found that an inhibitor of protein kinase C, 3-[1-[3-(dimethylamino)propyl]-1H-indol-3-yl)-1H-pyrrole-2,5-dione, blocks the (+)-pentazocine-mediated enhancement in rat striatal slices. The protein kinase C activator phorbol 12-myristate 13-acetate, but not the inactive isophorbol 4 alpha,9 alpha,12 alpha,13 alpha,20-pentahydroxytiglia-1,6-dien-3-one, enhanced the amphetamine-stimulated [(3)H]DA release comparable to the enhancement seen by (+)-pentazocine alone. Additionally, the L-type voltage-dependent calcium channel inhibitor nitrendipine or prior treatment with thapsigargin, but not the N-type voltage-dependent calcium channel omega-conotoxin MVIIA, attenuated the (+)-pentazocine-mediated enhancement. Together, these data suggest that activation of sigma(2)-receptors results in the regulation of DAT activity via a calcium- and protein kinase C-dependent signaling mechanism.

Binding of sigma receptor ligands and their effects on muscarine-induced Ca(2+) changes in SH-SY5Y cells

In human neuroblastoma SH-SY5Y cell preparations, sigma(1) receptor agonists (+)-pentazocine and 1S,2R-(-)-cis-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)cyclohexylamine (BD737) competed for [3H]haloperidol binding with K(i) values of 67+/-10 and 14+/-10 nM, respectively. (+)-Pentazocine or BD737 up to 10 microM did not affect basal levels of intracellular Ca(2+) concentration ([Ca(2+)](i)) in these cells, but they significantly reduced muscarine-induced [Ca(2+)](i) changes in a dose-related manner. However, the reduction by (+)-pentazocine was not reversed by the sigma(1) receptor antagonist haloperidol. Further studies showed (+)-pentazocine, BD737 and haloperidol could compete for [3H]quinuclidinyl benzilate binding in SH-SY5Y cells with K(i) values of 0.51+/-0.06, 0.32+/-0.07 and 4.4+/-2.3 microM, respectively. Thus, the inhibitory effects on muscarine-induced [Ca(2+)](i) changes by (+)-pentazocine and BD737 in SH-SY5Y cells were likely due to blockade of muscarinic receptors, not due to sigma(1) receptor activation by these ligands.

Trishomocubanes: novel sigma-receptor ligands modulate amphetamine-stimulated [3H]dopamine release

Several trishomocubane analogues of the type 4-azahexacyclo [5.4.1.0(2,6).0(3,10).0(5,9).0(8,11)]dodecane exhibited moderate to high affinity at sigma-receptor subtypes and low or negligible affinity at dopamine and serotonin transporters (SERT). Selected compounds were examined for their effects on amphetamine-stimulated [3H]dopamine release from striatal slices in vitro. Compounds 1, 2, 3 and 4 significantly enhanced amphetamine-stimulated release in a concentration-dependent manner. Compound 4, with the highest affinity and selectivity for the sigma(2)-receptor subtype, displayed the greatest potency. The enhancement produced by 1 and 2 was fully reversed by the selective sigma(2) antagonists 1'-[4-[1-(4-fluorophenyl)-1-H-indol-3-yl]-1-butyl]spiro[iso-benzofuran-1(3H), 4'piperidine] (Lu28-179), endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-(1-methyl)ethyl-2-oxo-1-H-benzimidazole-1-carboxyamidehydrochloride (BIMU-8) and the non-subtype selective antagonist N-[2-(3,4-dichlorophenyl)-ethyl]-N-methyl-2-pyrrolidinyl)ethylamine (BD1008). These data suggested a potential role for compounds 1 through 4 as sigma(2)-receptor agonists in functional studies. In addition, a D(3)-trishomocubane compound 5 displayed low affinity at sigma receptors (K(i)=3 microM) and moderate affinity at dopamine transporters (K(i)=623 nM). Compound 5 significantly inhibited the potentiation mediated by compound 2, presumably through sigma(2)-receptor antagonism, or a direct action on dopamine transporters.

Protein kinase C regulation of dopamine transporter initiated by nicotinic receptor activation in slices of rat prefrontal cortex

We previously reported that activation of nicotinic receptors causes an enhancement in amphetamine-stimulated release of dopamine via its transporter from slices of prefrontal cortex, but no such enhancement of release from slices of nucleus accumbens or striatum. The nicotinic receptors mediating the enhancement most likely contain alpha4 and beta2 subunits based upon pharmacological characterization. In this study, we sought to characterize the second messenger systems associated with the nicotine-mediated response. Sodium channel involvement was confirmed by the observation that tetrodotoxin blocked nicotine-mediated enhancement, whereas veratridine or elevated K(+) mimicked the enhancement seen with nicotine. Inclusion of EGTA blocked nicotine-mediated enhancement, suggesting that, even though no exogenous Ca(2+) was added, endogenous stores were required for the enhancement. The enhancement by nicotine was also abolished by the L-type voltage-dependent calcium channel (VDCC) antagonist nitrendipine, but not by the N-type VDCC antagonist omega-conotoxin GVIA. Finally, inhibition of protein kinase C also abolished the nicotine-mediated enhancement of amphetamine-stimulated dopamine release, whereas inhibitors of Ca(2+)/calmodulin kinase II did not. These findings establish that nicotine can exert selective effects on dopamine transporter activity in prefrontal cortex, an area involved in cognition and learning.

Evidence that the sigma(1) receptor is not directly coupled to G proteins

Sigma (sigma) receptors have been implicated in psychosis, cognition, neuroprotection, and locomotion in the central nervous system. The signal transduction mechanisms for sigma receptors have not been fully elucidated. In this study, we examined the possible coupling between sigma(1) receptors and heterotrimeric guanine nucleotide-binding proteins (G proteins) in rodent brain. In sigma(1) receptor-rich cerebellar membrane preparations, the competitive binding curves of two sigma(1) agonists, (+)pentazocine and 1S,2R-(-)-cis-N-[2-(3, 4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)cyclohexylamine (BD737), were unaffected by the addition of 10 microM guanosine-5'-O-(gamma-thio)-triphosphate (GTPgammaS). Neither (+)pentazocine (1-100 microM) nor BD737 (0.01-10 microM) stimulated GTPase activities significantly above basal levels in agonist-stimulated GTPase activity assays in cerebellar membranes. Furthermore, when using the method of agonist-stimulated [35S]GTPgammaS binding as assessed by autoradiography, we did not observe significant stimulation of [35S]GTPgammaS binding in rat brain sections by either (+)pentazocine or BD737. The above results demonstrate that the sigma(1) receptor is not likely be directly coupled to G proteins.

Nicotinic receptor-mediated regulation of dopamine transporter activity in rat prefrontal cortex

The objective of this study was to determine whether nicotine could selectively influence dopamine levels in the prefrontal cortex as compared with other dopaminergic areas of brain. Using a superfusion system, we found that nicotine and other agonists at nicotinic acetylcholine receptors enhanced the release of radiolabeled dopamine that was stimulated by 10 microM amphetamine from slices prepared from rat prefrontal cortex. In contrast, nicotine had no effect on amphetamine-stimulated [(3)H]dopamine release from slices of nucleus accumbens nor striatum. Under the conditions used, which included no added calcium to exclude contribution by exocytotic release, nicotine had no effect on basal release of [(3)H]dopamine. The enhancement by nicotine was concentration-dependent, reaching a maximum at 5 microM, and producing less release at higher concentrations. Enhancement by nicotine was fully reversed by 30 microM dihydro-beta-erythroidine, and by 10 microM mecamylamine, but was not affected by alpha-bungarotoxin. The potencies of nicotine, epibatidine, cytisine, and A85380 to enhance amphetamine-stimulated dopamine release, as well as the sensitivity of nicotine enhanced release to antagonists, are consistent with mediation via a high-affinity nicotinic acetylcholine receptor containing alpha 4 and beta 2 subunits, the major species of nicotinic receptor in forebrain. Since low dopaminergic activity in prefrontal cortex is correlated with cognitive deficits in schizophrenia, our findings may help explain why these deficits are improved in schizophrenics by smoking or nicotine administration.

SH-SY5Y cells as a model for sigma receptor regulation of potassium-stimulated dopamine release

Previous studies in our laboratory using rat brain tissue have shown that neuropeptide Y (NPY) can enhance NMDA- and potassium-stimulated dopamine release from various brain regions and that this enhancement is reversed by sigma (sigma) receptor antagonists. In the current study, we sought to determine whether SH-SY5Y cells are suitable for investigating sigma receptor effects and whether any sigma receptors present are of the subtype responsive to NPY. We compare mechanisms by which the prototypical sigma receptor agonist (+)-pentazocine, and the proposed endogenous sigma receptor ligand NPY regulate potassium-stimulated [(3)H]dopamine release from SH-SY5Y cells. Both (+)-pentazocine and NPY inhibit potassium-stimulated [(3)H]dopamine release. Unlike our studies in rat brain tissue, the effect of NPY on [(3)H]dopamine release is not reversed by sigma receptor antagonists. SH-SY5Y cells appear to be an appropriate model to study the regulation of dopamine release by sigma receptors or by NPY receptors, but this population is not identical to that population identified in brain slices.

Phencyclidine and dizocilpine modulate dopamine release from rat nucleus accumbens via sigma receptors

Phencyclidine (PCP) binds to many sites in brain, including PCP receptors located within the N-methyl-D-aspartate (NMDA) receptor-operated cation channel and sigma (sigma) receptors. In this study, we compare mechanisms by which PCP, dizocilpine (MK-801), the prototypical sigma receptor agonist (+)-pentazocine, and the proposed endogenous sigma receptor ligand neuropeptide Y regulate potassium (K(+))-stimulated [3H]dopamine release from slices of rat nucleus accumbens. (+)-Pentazocine inhibits K(+)-stimulated [3H]dopamine release, and neuropeptide Y enhances it. Both effects are blocked by sigma(1) and neuropeptide Y receptor antagonists, suggesting possible inverse agonism at a subpopulation of sigma/neuropeptide Y receptors. In contrast, PCP and MK-801 both enhance K(+)-stimulated [3H]dopamine release via sigma(1) and sigma(2) receptor subtypes, as demonstrated by antagonist sensitivity. Regulation of release by both (+)-pentazocine and neuropeptide Y persists in the presence of tetrodotoxin suggests that the sigma/neuropeptide Y receptors mediating the modulation are located presynaptically on dopaminergic nerve terminals, but tetrodotoxin eliminates regulation by PCP and MK-801, suggesting that receptors mediating their effects are located upstream from dopaminergic nerve terminals.

Modulation of amphetamine-stimulated [3H]dopamine release from rat pheochromocytoma (PC12) cells by sigma type 2 receptors

An important regulatory mechanism of synaptic dopamine (DA) levels is activation of the dopamine transporter (DAT), which is a target for many drugs of abuse, including amphetamine (AMPH). sigma receptors are located in dopaminergic brain areas critical to reinforcement. We found previously that agonists at sigma2 receptors enhanced the AMPH-stimulated release of [3H]DA from slices of rat caudate-putamen. In the present study, we modeled this response in undifferentiated pheochromocytoma-12 (PC12) cells, which contain both the DAT and sigma2 receptors but not neural networks that can complicate investigation of individual neuronal mechanisms. We found that enhancement of AMPH-stimulated [3H]DA release by the sigma agonist (+)-pentazocine was blocked by sigma2 receptor antagonists. Additionally, the reduction in the effect of (+)-pentazocine by the inclusion of ethylene glycol bis(beta-aminoethyl ether)-N,N,N', N'-tetraacetic acid led us to hypothesize that sigma2 receptor activation initiated a Ca2+-dependent process that resulted in enhancing the outward flow of DA via the DAT. The source of Ca2+ required for the enhancement of reverse transport did not appear to be via N- or L-type voltage-dependent Ca2+ channels, because it was not affected by nitrendipine or omega-conotoxin. However, two inhibitors of Ca2+/calmodulin-dependent protein kinase II blocked enhancement in AMPH-stimulated release by (+)-pentazocine. Our findings suggest that sigma2 receptors are coupled to the DAT via a Ca2+/calmodulin-dependent protein kinase II transduction system in PC12 cells, and that sigma2 receptor antagonists might be useful in the treatment of drug abuse by blocking elevation of DA levels via reversal of the DAT.

Neuropeptide Y-mediated enhancement of NMDA-stimulated [3H]dopamine release from rat prefrontal cortex is reversed by sigma1 receptor antagonists

Sigma (sigma) receptors are located in limbic areas, including the prefrontal cortex, where decreased dopamine levels have been linked to negative symptoms. Although the endogenous ligands for sigma receptors are unknown, neuropeptide Y (NPY) has been named as the potential endogenous agonist at these receptors. NPY enhanced NMDA-stimulated [3H]dopamine release in rat prefrontal cortex. This was in contrast to the inhibition produced by the sigma agonists (+)pentazocine and BD737. However, four sigma antagonists, including one which is sigma1 selective, that reverse (+)pentazocine- or BD737-mediated inhibition all reversed the NPY-mediated enhancement. In addition, PYX-1, a Y receptor antagonist, reversed both the (+)pentazocine- and BD737-mediated inhibition and the NPY-mediated enhancement of release. Peptide YY (PYY), [Leu31,Pro34]NPY and NPY(13-36) did not mimic the effect of NPY. Our findings are consistent with NPY acting as an endogenous ligand for a subtype of sigma receptor with characteristics different from Y1, Y2 and Y3 receptors but sensitive to PYX-1. These findings suggest a role for NPY, via sigma receptors, as a modulator of dopamine levels in the prefrontal cortex.

Modulation of amphetamine-stimulated (transporter mediated) dopamine release in vitro by sigma2 receptor agonists and antagonists

Some sigma receptor ligands have been shown to bind with low affinity to the dopamine transporter and to inhibit [3H]dopamine uptake. It has not previously been shown whether any of these compounds influence release of dopamine via facilitated exchange diffusion. To further examine the nature of the interaction between sigma receptor ligands and the dopamine transporter, the effects of sigma receptor ligands on amphetamine-stimulated [3H]dopamine release were examined in slices prepared from rat caudate putamen. In the absence of exogenous Ca2+, both (+)-pentazocine and (-)-pentazocine potentiated amphetamine-stimulated [3H]dopamine release at concentrations consistent with their affinities for sigma2 receptors. In contrast, BD737 (1S.2R-(-)-cis-N-¿2-(3,4-dichlorophenyl)ethyl¿-N-methyl-2-(1-pyrrolidiny l)cyclohexylamine), a sigma1 receptor agonist, had no effect on amphetamine-stimulated release. Neither isomer of pentazocine alone had any effect on basal [3H]dopamine release under these conditions. Three antagonists at sigma receptors, one of which is non-selective for subtypes, and two of which are sigma2-selective, all blocked the enhancement of stimulated release produced by (+)-pentazocine. Enhancement of stimulated release by (-)-pentazocine was similarly blocked by sigma2 receptor antagonists. Our data support the contention that it is possible to regulate transporter-mediated events with drugs that act at a subpopulation of sigma receptors pharmacologically identified as the sigma2 subtype.

Modulation of [3H]Dopamine release from rat nucleus accumbens by neuropeptide Y may involve a sigma1-like receptor

Sigma receptors are located in limbic areas, including the nucleus accumbens, where increased dopamine levels have been linked to psychosis and reinforcement. Neuropeptide Y (NPY) has been named as a possible endogenous ligand for a subpopulation of sigma receptors on the basis of its ability to compete for sigma receptor binding. Using a superfusion system, we found that NPY enhanced N-methyl-D-asparate-stimulated [3H]dopamine release in rat nucleus accumbens, whereas the prototypical sigma agonist (+)pentazocine inhibited release. However, four sigma antagonists, one of which is sigma1 selective, as well as a Y receptor antagonist, all reversed the enhancement by NPY and the inhibition by (+)pentazocine. A sigma2-selective antagonist had no effect on either NPY-mediated enhancement or (+)pentazocine-mediated inhibition. [Leu31,Pro34]NPY and NPY13-36 also enhanced release, but the effects were not reversed by sigma antagonists. Peptide YY did not mimic the effect of NPY. Our findings are consistent with the potential role of NPY as an endogenous ligand for a subtype of sigma receptor with characteristics different from Y1, Y2 and Y3 receptors but sensitive to Ac-[3-(2,6-dichlorobenzyl)Tyr27,D-Thr32NPY-(27-36)amide. Our findings suggest a role for NPY, via sigma receptors, in the regulation of dopamine levels in areas of brain critical to psychosis and reinforcement.

Release of [3H]dopamine from guinea pig striatal slices is modulated by sigma1 receptor agonists

Sigma receptors are found in motor and limbic areas in the brains of humans, non-human primates, and rodents. The most extensive pharmacological studies of ligand binding to sigma receptors have utilized brain tissue from guinea pigs, where two subtypes of sigma receptor, designated sigma1 and sigma2, have been identified. Few functional roles for sigma receptors have been described. Their location in guinea pig striatum, a terminal field of dopaminergic projections arising from the substantia nigra, suggested that this tissue would be a logical choice in which to examine physiological properties of sigma receptor activation. We found that sigma1 receptor agonists inhibited N-methyl-D-aspartate-stimulated [3H]dopamine release from guinea pig striatal slices in a concentration-dependent manner. The inhibition by sigma1 receptor agonists was reversed by a selective sigma1 receptor antagonist, as well as by a non-subtype-selective sigma receptor antagonist. The ability of agonists working through sigma1 receptors, but not through sigma2 receptors, to inhibit the stimulated release of catecholamines appears to be a unique characteristic of guinea pig striatum. We have previously reported that in rat striatum and hippocampus, as well as in guinea pig nucleus accumbens, prefrontal cortex, and hippocampus, activation of either sigma receptor subtype inhibits such release. Stimulated release of [3H]dopamine from guinea pig striatum was also inhibited by the phencyclidine receptor agonist dizocilpine, but this inhibition was not reversed by the sigma receptor antagonists. Therefore, the inhibition produced by sigma receptor agonists was not mediated via the phencyclidine binding site within the N-methyl-D-aspartate-operated cation channel. Our findings support the hypothesis that sigma receptor activation provides a mechanism of modulating dopamine release from striatum, and that striatal tissue from guinea pigs appears to be an appropriate model for characterizing sigma1 receptor-mediated effects.

Differential modulation of NMDA-stimulated [3H]dopamine release from rat striatum by neuropeptide Y and sigma receptor ligands

Although the identity of the endogenous ligands for sigma (sigma) receptors is unknown, neuropeptide Y (NPY) has been named as a possible candidate for a natural transmitter at these receptors. Using a superfusion system, we compared the effect of NPY on NMDA-stimulated [3H]dopamine release in rat striatum to that of the sigma agonists (+)-pentazocine and BD737. In contrast to (+)-pentazocine- or BD737-mediated inhibition of release, NPY enhanced release. However, the same sigma antagonists (BD1008, DuP734, haloperidol and DTG) that reverse (+)-pentazocine- or BD737-mediated inhibition, as well as a Y receptor antagonist, PYX-1, all reversed the enhancement. PYX-1 also reversed the (+)-pentazocine- and BD737-mediated inhibition of release. Peptide YY (PYY) and [Leu31,Pro34]NPY did not mimic the effect of NPY. NPY13-36 enhanced release to the same extent as NPY but the effect was not reversed by sigma antagonists. Our findings are consistent with the potential role of NPY as an endogenous ligand for a subtype of sigma receptor with characteristics different from Y1, Y2 and Y3 receptors but sensitive to PYX-1.

Regulation of [3H]norepinephrine release from guinea pig hippocampus by sigma2 receptors

The binding profile of the sigma2 receptor ligand endo-N-(8-methyl-8-azabicyclo[3.2.1.]oct-3-yl)-2,3-dihydro-(1-methyl)eth yl-2-oxo-1H-benzimidazole-1-carboxamidehydrochloride (BIMU-8) had previously been determined, but its agonist/antagonist status at sigma2 receptors had not been identified. We therefore investigated the effects of BIMU-8 for its ability to regulate the stimulated release of [3H]norepinephrine from slices of guinea pig hippocampus. BIMU-8 alone, at a concentration chosen to occupy 50% of sigma2 receptors, had no significant effect on N-methyl-D-aspartate (NMDA)-stimulated release of [3H]norepinephrine. We have shown previously that the sigma receptor agonist (+)-pentazocine inhibits NMDA-stimulated release in a concentration-dependent manner, producing a biphasic inhibition curve. Similarly, the sigma receptor agonist 1S,2R-(-)-cis-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl )cyclohexylamine (BD737) produced a broad inhibition curve. The inhibition by low concentrations of (+)-pentazocine or BD737 that selectively activated sigma1 receptors was reversed by the sigma1-selective receptor antagonist (1-(cyclopropylmethyl)-4-2'-oxoethyl)piperidine HBr (DuP 734). In the current study, when the sigma1 component of inhibition by (+)-pentazocine was blocked by DuP 734, the remaining component of inhibition mediated by sigma2 receptors was reversed by BIMU-8. Our results suggest that (1) BIMU-8 is an antagonist at sigma2 receptors and that (2) sigma2 receptors contribute to regulation of norepinephrine release in guinea pig hippocampus.

Neurotensin, N-acetyl-aspartylglutamate and beta-endorphin modulate [3H]dopamine release from guinea pig nucleus accumbens, prefrontal cortex and caudate-putamen

Dopaminergic hyperactivity in nucleus accumbens and dopaminergic hypoactivity in prefrontal cortex are thought to underlie positive and negative symptoms of schizophrenia, respectively. The caudate putamen is the neuroanatomical substrate for extrapyramidal side effects resulting from chronic antipsychotic treatment. We sought to identify potential endogenous regulators of dopamine release that might produce differential effects in these brain areas. We tested neurotensin, N-acetyl-aspartyl-glutamate and beta-endorphin for potential regulation of [3H]dopamine release in these regions of guinea pig brain. All three peptides stimulated dopamine release, above basal activity, at all concentrations tested in the three regions. Neurotensin significantly enhanced and N-acetyl-aspartyl-glutamate had no significant effect on N-methyl-D-aspartate-stimulated release from all three regions. In contrast, beta-endorphin significantly inhibited N-methyl-D-aspartate-stimulated release in nucleus accumbens and caudate putamen. These results suggest that these neuropeptides may regulate endogenous dopamine release and therefore may be potential therapeutic targets for antipsychotic drug development.

Regulation of [3H] dopamine release from mesolimbic and mesocortical areas of guinea pig brain by sigma receptors

The role of sigma (sigma) receptors in brain function is poorly defined. They are located in limbic areas, including nucleus accumbens (NAC) and prefrontal cortex (PFC), both of which are thought to be involved in schizophrenia. Many antipsychotics (APs), including haloperidol, bind with high affinity to sigma receptors. Dopaminergic hyperactivity in NAC is thought to underlie positive symptoms of schizophrenia, while dopaminergic hypoactivity in PFC is thought to underlie negative symptoms. Sigma receptors regulate N-methyl-D-aspartate (NMDA)-stimulated [3H] dopamine ([3H]DA) release in caudate-putamen (CP), the neuroanatomical substrate for extrapyramidal side effects resulting from chronic AP treatment. In the current study, we investigated whether sigma receptors could similarly regulate DA release in mesolimbic and mesocortical tissue, and the relative participation of different sigma receptor subtypes in this process. We found that, in NAC, regulation of DA release by the prototypical sigma agonist (+)pentazocine was mediated predominantly by the sigma 1 receptor, whereas in the PFC a portion of the (+)pentazocine effect was likely mediated by the sigma 2 receptor. We also observed, in both the NAC and PFC, that regulation of DA release by the sigma agonist BD737 was mediated primarily by the sigma 1 receptor. In addition, we determined that (+)pentazocine or BD737 effects on DA release were not mediated via opioid receptors, nor the phencyclidine (PCP) binding site within the NMDA receptor-operated cation channel, nor by sigma receptor effects upon [3H]DA accumulated by noradrenergic terminals in PFC.

sigma1 Receptors in rat striatum regulate NMDA-stimulated [3H]dopamine release via a presynaptic mechanism

The role of the sigma1 receptor in the regulation of N-methyl-D-aspartate (NMDA)-stimulated [3H]dopamine release from rat striatal slices was examined. The sigma receptor agonist 1S,2R-(--)-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)cy clohexylamine (BD737) inhibited stimulated release in a concentration-dependent manner. The sigma1 receptor antagonist, 1-(cyclopropylmethyl)-4-(2'-(4"-fluorophenyl)-2'-oxoethyl)piperidi ne HBr (DuP 734), reversed inhibition of release by BD737. Haloperidol, di-o-tolylguanidine (DTG) and N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine (BD1008) reversed the BD737-mediated inhibition of release. Haloperidol and DTG also antagonized inhibition of stimulated release by (+)-pentazocine. Furthermore, BD737 and (+)-pentazocine inhibited stimulated release in the presence of tetrodotoxin, suggesting that sigma1 receptors regulating dopamine release are located on dopaminergic nerve terminals. These data suggest that sigma1 receptors may be important in the regulation of glutamate-stimulated dopamine release.

Functional and binding properties of sigma receptors in rat cerebellum

Autoradiographic studies have shown that sigma receptors are enriched in the locus coeruleus, the origin of noradrenergic projections to the cerebellum, as well as in the Purkinje, molecular, and granular layers and the interpositus cerebellar nucleus of the cerebellum itself. In contrast, the cerebellum is relatively poor in phencyclidine (PCP) binding sites, which have been historically confused with sigma sites. The high ratio of sigma to PCP receptors in cerebellum is advantageous for discriminating sigma-mediated physiological effects. sigma agonists and antagonists have been shown to regulate N-methyl-D-aspartate (NMDA)-stimulated norepinephrine release in hippocampus, which is innervated by locus coeruleus projections. We now report that sigma drugs also regulate norepinephrine release from cerebellum. In contrast to findings in the hippocampus, where regulation is via sigma 1 and sigma 2 receptors, sigma-mediated regulation in cerebellum seems to be primarily via sigma 1 receptors. In radioligand binding studies, we find that sigma receptors primarily of the sigma 1 type are present in the cerebellum. We further report that binding to sigma receptors in cerebellum is not affected by the addition of NMDA or glycine or by the presence of NMDA antagonists, suggesting that sigma receptors are not located within the NMDA-operated cation channel in this brain region.

The role of L-type voltage dependent calcium channels in stimulated [3H]norepinephrine release from canine hippocampal slices following global cerebral ischemia and reperfusion

The hippocampus is among those brain regions which are selectively vulnerable to ischemic damage. Hippocampal damage due to transient cerebral ischemia is mainly of the delayed, non-necrotic type which may arise after disruption or activation of specific cellular systems, including transmitter release through excitatory amino acid receptors. We investigated the contribution of L-type voltage dependent calcium channels (VDCCs) to glycine (GLY) potentiated N-methyl-D-aspartate (NMDA) receptor- and potassium-stimulated [3H]norepinephrine (NE) release in a canine model of global cerebral ischemia and reperfusion. Tissue was collected from four experimental groups: non-arrested controls (NA), global cerebral ischemia induced by 10 minute cardiac arrest (CA), and CA followed by 30 min or 24 hours reperfusion after restoration of spontaneous circulation. Brain slices prepared from all groups accumulated approximately equivalent amounts of [3H]NE. The sensitivity of [3H]NE release to stimulation by NMDA/GLY or elevated potassium was unchanged after ischemia and reperfusion. About 30% of release stimulated by the addition of 20 mM potassium was inhibited by the NMDA receptor-operated channel antagonist MK801 in all groups except CA in which only 4% of release was inhibited by MK801. The ability of 1 microM nitrendipine (NTP) to block stimulated release indicated that the contribution of the L-type VDCC to potassium or NMDA/GLY-stimulated release was significant only in NA and 24 hour reperfused animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Sigma receptor regulation of norepinephrine release from rat hippocampal slices

Multiple sigma receptor subtypes have been identified in the hippocampus, yet their physiological role remains largely undefined. In the current study, we examined the role of sigma receptors in the regulation of N-methyl-D-aspartate (NMDA)-stimulated [3H]norepinephrine ([3H]NE) release from rat hippocampal slices. Both sigma agonists (+)pentazocine and BD737 inhibited stimulated norepinephrine release in a concentration-dependent manner. The sigma1 antagonist DuP 734 completely antagonized the inhibition of release by all concentrations of BD737 tested. However, DuP 734 only partially reversed inhibition of release by (+)pentazocine concentrations above 100 nM. 1,3 Di-o-tolylguanidine (DTG), but not haloperidol, antagonized BD737-mediated inhibition of release. DTG also completely antagonized inhibition of release by 100 nM (+)pentazocine yet haloperidol produced only a partial reversal. A combination of DuP 734 and haloperidol produced complete reversal of (+)pentazocine-mediated inhibition, suggesting potential involvement of multiple sigma receptor subtypes in the regulation of norepinephrine release. Both (+)pentazocine and BD737 failed to inhibit stimulated release in the presence of tetrodotoxin, suggesting that sigma receptors regulating NE release are not located on noradrenergic nerve terminals. These results suggest that sigma receptors may be a therapeutic target for disorders resulting from noradrenergic imbalance in hippocampus.

Regulation of [3H]dopamine release from rat striatal slices by sigma receptor ligands

Sigma receptors have been located in several areas of the brain that control motor function, including on the dopaminergic projections from substantia nigra to striatum. In the current study, the regulation of N-methyl-D-aspartate-stimulated [3H]dopamine release from slices of rat striatum by several sigma ligands has been tested. Both isomers of the benzomorphans SKF10,047 and pentazocine inhibited the stimulated release of dopamine in a concentration-related manner. All these compounds probably activate sigma and non-sigma receptors, including phencyclidine receptors, over the broad concentration ranges tested. However, concentrations of (+)pentazocine below about 100 nM appear to act solely through sigma receptors. This phase of inhibition was reversed by the sigma antagonist N-[-2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-[1- pyrimidinyl-1-piperazine butanol and by the sigma1-selective antagonist (1-(cyclopropylmethyl)-4-2'4"-fluorophenyl)-(2'-oxoethyl)piperi din e HBr. Neither of these antagonists affected stimulated release in the absence of (+)pentazocine. The synthetic sigma ligands 2-(4-morpholino)ethyl 1-phenylcyclohexane-1-carboxylate hydrochloride, 6-[6-(4-hydroxypiperidinyl)-hexoxy]-3-methylflavone hydrochloride and alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1- piperazine butanol enhanced NMDA-stimulated DA release significantly in the presence of (+)pentazocine. These drugs have affinity at non-sigma receptors as well, and their stimulatory effects may be mediated through these receptors along with nonreceptor mechanisms. Our findings on the regulation of dopamine support earlier assertions that sigma receptors may be important in the regulation of motor function.

Sigma-receptor regulation of [3H]arachidonic acid release from rat neonatal cerebellar granule cells in culture

Sigma receptors have been identified in many brain areas and are especially abundant in those regions known to be involved in control of movement. Sigma receptors have been located autoradiographically in the granule cell layer of cerebellum in adult rat brain. In the current study, we identified sigma receptors in rat neonatal granule cells in culture using radioligand binding. The tritium labeled form of the putative sigma antagonist haloperidol bound with high affinity to membranes prepared from these cells, and ligands selective for sigma receptors competed well against [3H]haloperidol binding. The excitatory amino acid N-methyl-D-aspartate and the direct phospholipase A2 activator melittin stimulated the release of [3H]arachidonic acid from cerebellar granule cells. The N-methyl-D-aspartate-stimulated, but not the melittin-stimulated, release was inhibited in a concentration-dependent manner by the sigma-selective agonist (+)-pentazocine. In addition, the novel sigma 1 agonist BD737 inhibited N-methyl-D-aspartate-stimulated release. Pentazocine inhibition was almost completely reversed by the sigma antagonists NPC-16377 and opipramol. A 1 microM concentration of the phencyclidine receptor-selective ligand MK-801 inhibited approximately 65% of N-methyl-D-aspartate-stimulated release. These results suggest that sigma receptors may play a role in modulating arachidonic acid release in cerebellar granule cells.

Increased activation of L-type voltage-dependent calcium channels is associated with glycine enhancement of N-methyl-D-aspartate-stimulated dopamine release in global cerebral ischemia/reperfusion

We investigated the relationships among N-methyl-D-aspartate, glycine, L-type voltage-dependent calcium channels, and [3H]dopamine release in a canine model of global cerebral ischemia/reperfusion. The binding of [3H]PN200-110 ([3H]isradipine) to L-type voltage-dependent calcium channels, that open as a consequence of N-methyl-D-aspartate-induced changes in membrane potential, was approximately doubled in striatal membranes prepared from ischemic animals relative to controls, and remained significantly elevated at 30 min and 2 h of reperfusion. These changes coincided temporally with changes in the ability of the voltage-sensitive calcium channel blocker nitrendipine to inhibit glycine enhancement of N-methyl-D-aspartate-stimulated [3H]dopamine release in striatal slices prepared from the same animals. Compared with nonischemic controls, N-methyl-D-aspartate-stimulated [3H]dopamine release was increased in ischemic animals and remained increased throughout reperfusion up to at least 24 h. Glycine enhanced N-methyl-D-aspartate-stimulated release in all treatment groups. The enhancement of N-methyl-D-aspartate-stimulated dopamine release by glycine was reduced by the inclusion of nitrendipine in striatal slices from ischemic and 30-min reperfused animals. These data suggest that glycine may facilitate opening of the voltage-dependent calcium channels activated by N-methyl-D-aspartate and that this facilitation is blocked by the antagonist nitrendipine.

N-acetyl-aspartylglutamate modulation of N-methyl-D-aspartate-stimulated [3H]norepinephrine release from rat hippocampal slices

The release of preloaded radiolabeled norepinephrine ([3H]NE) from slices of rat hippocampus can be stimulated by excitatory amino acids that interact with the N-methyl-D-aspartate (NMDA) receptor. The acidic dipeptide N-acetyl-L-aspartylglutamate (NAAG) is colocalized with NE in the cell bodies of locus coeruleus (the origin of the noradrenergic projections to the hippocampus) and the hippocampus itself. The function of NAAG in these neurons has not been demonstrated, although evidence exists that it may serve as a neuromodulator in other neuronal pathways. NAAG inhibited the release of [3H]NE stimulated by NMDA and L-glutamate in a concentration-related manner. The maximal inhibition produced by NAAG was about 25% of the control release stimulated by 25 microM NMDA. The effects observed were caused by the intact dipeptide and not the degradation artifacts produced by the enzyme N-acetylated-alpha-linked-acidic dipeptidase because N-acetyl-L-aspartate had no significant effect on the release and L-glutamate was stimulatory. The activity of this enzyme appears to be suppressed under the assay conditions used. Although the addition of glycine did not enhance NMDA-stimulated release, 7-chlorokynurenate and 1-hydroxy-3-amino-pyrrolidone-2 decreased the release in a concentration-dependent manner. Furthermore, the attenuation produced by NAAG plus 7-chlorokynurenate or 1-hydroxy-3-aminopyrrolidone-2 was greater than the inhibitory actions of either glycine antagonist alone. Similarly, NAAG produced additional inhibition over that produced by either of two different voltage-dependent calcium channel blockers.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine release from canine striatum following global cerebral ischemia/reperfusion

The elevation of extracellular dopamine (DA) levels in the striatum of experimental animals subjected to ischemic insult has been well documented. The contribution of excessive DA to neuronal damage can be inferred from the ability of DA antagonists, as well as selective destruction of dopaminergic tracts, to confer neuroprotection in models of ischemia. In the current study, we report an enhanced releasability of preloaded [3H]DA in response to either elevated potassium or N-methyl-D-aspartate (NMDA) from striatal slices of beagles that had experienced 10 min of ischemia induced by cardiac arrest. The elevation in sensitivity to potassium stimulation was transient, approaching control levels after 30 min of reperfusion. In contrast, release stimulated by NMDA was elevated immediately after cardiac arrest and remained elevated for as long as 24 h of reperfusion. Release stimulated by NMDA was enhanced by glycine (Gly) and inhibited by MK801, consistent with mediation through the NMDA receptor/channel complex. The increased sensitivity of DA release, coupled with the high levels of excitatory amino acids (EAAs), including glutamate (Glu), aspartate (Asp) and Gly in ischemic brain, probably contribute to the extensive neuronal cell damage.

Comparison of dopamine uptake and release in vitro in sheep and rat striatum

Cocaine inhibits tritium-labeled dopamine ([3H]DA) uptake in rat (IC50 approximately 400 nM) and sheep (IC50 approximately 1 microM) striatum. GBR 12909, a selective DA uptake inhibitor, potently inhibits [3H]DA uptake in rat (IC50 less than 10 nM), but is less effective (only 60% of the uptake is inhibited at a concentration of 10 microM) and less potent (IC50 approximately 300 nM) in sheep. [3H]DA release from slices of rat or sheep striatum is stimulated by potassium (15-50 mM). In the presence of nomifensine (10 microM), cocaine (10 microM) had no effect on potassium-stimulated [3H]DA release in either species. [3H]DA release is increased by N-methyl-D-aspartate (NMDA) (10-1000 microM) in rat striatum but NMDA did not stimulate [3H]DA release in sheep striatum. These findings suggest that NMDA receptors either are absent from or do not regulate release of preloaded [3H]DA in sheep striatum.

Characterization of binding of [3H]GBR 12935 (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)-piperazine) to membranes and to solubilized membrane extracts from terminal field regions of mesolimbic, mesocortical and nigrostriatal dopamine pathways

The binding characteristics of [3H]GBR 12935 (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine), a selective dopmaine uptake inhibitor, were examined in intact membrane preparations and solubilized extracts of terminal field regions of dopamine pathways in the brain of the rats. There were many similarities in the properties of binding sites for [3H]GBR 12935 in the striatum, nucleus accumbens and olfactory tubercle. The binding of [3H]GBR 12935 was saturable and the affinity constants were not significantly different between regions of the brain. The binding of [3H]GBR 12935 was inhibited by amfonelic acid, GBR 12909, mazindol, methylphenidate and cocaine, with comparable affinities in each region of the brain and with the same order of potency in both preparations. Furthermore, the rank order of potencies for inhibiting the binding of [3H]GBR 12935 was the same as for inhibiting the uptake of [3H]dopamine in these regions of the brain. There did appear to be some degree of heterogeneity of binding sites for [3H]GBR 12935 in each of these regions of the brain, as both amfonelic acid and mazindol were best fitted by two-site models. Whether this apparent heterogeneity was due to the existence of two distinct binding sites or to two components of a single site is unclear. It did not, however, appear to be due to binding to uptake sites for norepinephrine or serotonin, as neither nisoxetine nor fluoxetine, selective inhibitors of the uptake of norepinephrine and serotonin, respectively, inhibited the binding of [3H]GBR 12935, at concentrations which inhibit the uptake of norepinephrine or serotonin.

Regulation of [3H]dopamine release from guinea pig striatum by NMDA receptor/channel activators and inhibitors

Excitatory amino acids, that interact with the N-methyl-D-aspartate (NMDA) receptor stimulate release of [3H]dopamine [3H]DA) from the striatum of the guinea pig and rat in a concentration-dependent manner. DA release was measured in the presence of domperidone and nomifensine to avoid complications associated with autoreceptor alteration of and reuptake of released DA. This release is inhibited by magnesium. Therefore, all experiments were performed in the absence of this ion. The competitive NMDA antagonists D-(-)2-amino-5-phosphonopentanoic acid and 3-[(+-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid and the noncompetitive antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine and phencyclidine also inhibit NMDA-stimulated release. Glycine enhances NMDA-stimulated release and can release [3H]DA in the absence of added NMDA. Release stimulated by glycine alone is not affected by 3-[(+-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonic acid. Conversely, if the glycine antagonist 3-amino-1-hydroxy-2-pyrrolidone or 6-cyano-7-nitroquinoxaline-2,3-dione is included, NMDA elicits less release of [3H]DA. This inhibition can be overcome by increasing the concentration of glycine. The kappa-selective opioid agonist trans-(+-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzene-acetamide is also capable of inhibiting the NMDA-stimulated release of [3H]DA from guinea pig and rat striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the effects of cocaine and other inhibitors of dopamine uptake in rat striatum, nucleus accumbens, olfactory tubercle, and medial prefrontal cortex

It is thought that inhibition of dopamine reuptake into neurons may play a major role in the mechanisms by which cocaine produces its reinforcing effects. The striatum, while rich in dopamine terminals, is not implicated in drug reinforcement, whereas the mesolimbic dopamine pathway appears to play a primary role. It is therefore possible that the properties and drug sensitivities of the dopamine uptake systems in the nigrostriatal, mesolimbic, and mesocortical tracts differ. The effects of cocaine, GBR 12909, amfonelic acid, and methylphenidate on dopamine uptake in the striatum, nucleus accumbens, olfactory tubercle, and medial prefrontal cortex were examined. Over 80% of the dopamine uptake in each of the 4 regions was sodium-dependent and exhibited Km values of approximately 100 nM. Cocaine, GBR 12909, amfonelic acid, and methylphenidate each biphasically inhibited uptake in the striatum, nucleus accumbens and olfactory tubercle with GBR 12909 and amfonelic acid being approximately 50-fold more potent than cocaine or methylphenidate. In the medial prefrontal cortex, cocaine and GBR 12909 could inhibit only about 40% of the [3H]dopamine uptake. There are similarities in the properties and drug sensitivities of the dopamine uptake systems in brain areas which are implicated in drug reinforcement and those which are not.

Selective changes in mu opioid receptor properties induced by chronic morphine exposure

Chronic infusion of morphine to guinea pigs produced selective changes in mu agonist binding properties in cerebrocortical membrane preparations. Employing the mu-selective opioid agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO) in direct binding studies and in competition of labeled antagonist binding, we found that the major changes were a decrease in the number of sites with high affinity for agonist, a small reduction in total receptor number, and a loss in the ability of guanosine 5'-[gamma-thio]triphosphate to regulate binding. A fraction of high-affinity mu receptors appeared to retain their high affinity for agonist and their sensitivity to guanine nucleotide analogue after the induction of morphine tolerance, possibly because the morphine concentrations achieved in brain were insufficient to uncouple all mu receptors from associated guanine nucleotide-binding regulatory proteins. Some membrane preparations were treated with pertussis toxin, which has been shown to functionally uncouple mu opioid receptors from their effector systems. In these preparations, a single agonist-affinity state of the receptor was observed. The apparent dissociation constant for this affinity state in pertussis toxin-treated membranes was similar to the lower-affinity state observed in preparations from morphine-tolerant animals. In contrast to the changes observed at mu opioid binding sites, no significant changes in agonist affinity or binding density were observed for selective delta or kappa agonists, consistent with the development of selective tolerance at mu receptors.

Effects of pertussis toxin on opioid regulation of catecholamine release from rat and guinea pig brain slices

Opioid agonists selective for mu-, delta-, and kappa-receptors are all capable of regulating the stimulated release of noradrenaline from three terminal fields (cortex, hippocampus, and cerebellum) of the noradrenergic projections from locus coeruleus in the guinea pig brain. Intracerebroventricular injections of pertussis toxin abolished the ability of a mu-selective agonist and of a delta-selective agonist to inhibit stimulated noradrenaline release, but left unaffected the concentration-related inhibition of NE release by a kappa agonist. Thus, mu- and delta-receptors have been shown to be coupled to their effector system in these noradrenergic neurons via guanyl nucleotide binding proteins (G proteins) which are sensitive to pertussis toxin, while kappa-receptors in the same neurons appear to be coupled through a different mechanism which is significantly less sensitive to pertussis toxin. In contrast to opioid receptor regulation of noradrenaline release in guinea pig hippocampus, mu-, but not delta- or kappa-agonists are capable of regulation of stimulated noradrenaline release from rat hippocampus and cortex, and kappa-, but not mu- or delta-agonists are capable of inhibiting the stimulated release of dopamine from rat striatum and cortex. Pertussis toxin injections significantly attenuated mu-agonist inhibition of noradrenaline release, but had no effect on the ability of a kappa-selective agonist to regulated dopamine release, confirming the insensitivity of the kappa-receptor-effector coupling system to pertussis toxin.

Selective opioid antagonist effects on opioid-induced inhibition of release of norepinephrine in guinea pig cortex

Opioid agonists with selectivity for mu, delta and kappa-receptors have each been shown to inhibit the K+-stimulated release of [3H]norepinephrine (NE) from slices of guinea pig cortex maintained in vitro. In order to provide further evidence that each of these types of opioid receptor can regulate the release of NE in this tissue, experiments with receptor-type selective opioid antagonists have been conducted. In initial experiments, the selectivity of the antagonists for specific types of opioid receptors in the cortex of the guinea pig in an incubation medium of the same composition as that used for release studies was confirmed. The delta-receptor selective antagonist, ICI 174,864, prevented the inhibitory actions of the delta-selective agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE), but had little effect on the inhibitory actions of the mu-selective agonist, Tyr-D-Ala-Gly-MePhe-Gly-ol (DAMGO), or the kappa-selective agonist, U-50,488H. In contrast, the kappa-selective antagonist, nor-binaltorphimine (nor-BNI) prevented the inhibitory actions of U-50,488H, but had little effect on the inhibitory actions of DPDPE or DAMGO. The greater potency of the partially mu-selective antagonist, naloxone, in reversing the effects of DAMGO relative to those of DPDPE or U-50,488H was confirmed. These results support the conclusion that mu- delta- and kappa-opioid receptors each exert a negative regulatory effect on the stimulated release of NE in the cortex of the guinea pig.

Selective tolerance at mu and kappa opioid receptors modulating norepinephrine release in guinea pig cortex

The development of selective tolerance, that is, a loss in the ability of an agonist to exert an effect without concomitant loss in the ability of an agonist which acts through another receptor type to similarly lose its effectiveness, has provided supporting evidence for the existence of multiple opioid receptor types in brain and peripheral tissues. In brain, this phenomenon has generally been demonstrated for agonists which produce different physiological effects. In this study, we describe selective tolerance at two opioid receptor types which converge upon a single function. The effects of chronic treatment for 6 days with the mu agonist morphine (1.7 mg/kg/hr) and the kappa agonist U50, 488H (300 micrograms/kg/hr) on the ability of mu and kappa opioid agonists to inhibit the stimulated release of [3H]norepinephrine from slices of guinea pig cortex were investigated. Mu, delta and kappa selective agonists have been shown previously to be capable of regulating the stimulated release of norepinephrine. Chronic administration of morphine resulted in a diminution in the ability of the mu agonists Tyr-D-Ala-Gly-N(Me)Phe-Gly-ol, morphine and etorphine, but not of U50, 488H, to inhibit the stimulated release of [3H]norepinephrine. Conversely, chronic U50, 488H infusion decreased the ability of U50, 488H to inhibit release, with no change in the effectiveness of Tyr-D-Ala-Gly-N(Me)Phe-Gly-ol. The degree of tolerance observed for mu agonists in tissue from morphinized animals is discussed with regard to their efficacy and selectivity.

Kappa receptor regulation of dopamine release from striatum and cortex of rats and guinea pigs

The effects of opioid agonists with selectivity for kappa, mu and delta types of opioid receptors on the K+-stimulated release of [3H]dopamine (DA) from striatum and cortex of rat and guinea pig loaded previously with the monoamine have been studied. The kappa agonist U50488H did not affect base-line release of [3H]DA measured in 5 mM K+, but produced a dose-dependent inhibition of the release of [3H]DA stimulated by 20 mM K+ from slices of striatum in rat and guinea pig, with an IC50 of about 0.5 nM in each case. In contrast, the mu-selective agonist, Tyr-D-Ala-Gly-(Me)Phe-Gly-ol, and the delta-selective agonist, [D-Pen2-D-Pen5]enkephalin, did not inhibit stimulated release from the slice preparations at concentrations up to 1 microM. The inhibitory effects of U50488H were antagonized by naloxone, and the potent and selective kappa antagonist, nor-binaltorphimine (nor-BNI). Similar results were obtained when release of [3H]DA from rat and guinea pig cortex slices was examined. In guinea pig cortex, the maximum inhibition of DA release induced by U50488H was 80% of control-stimulated fractional release. In rat cortex and in striatum of both species the maximum release was about 40% of control fractional release. Thus, in the guinea pig, the mesocortical dopaminergic pathway appears more sensitive to the inhibitory effects of U50488H than the nigrostriatal system. The effects of the opioids on the K+ (12.5 mM)-stimulated release of [3H]DA from guinea pig striatal synaptosomes also were determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic morphine exposure on opioid inhibition of adenylyl cyclase in 7315c cell membranes: a useful model for the study of tolerance at mu opioid receptors

The effects of prolonged morphine exposure on the mu opioid receptor in 7315c pituitary tumor cell membranes have been examined. Since a low concentration of naloxone reversed the inhibition of forskolin-stimulated adenylyl cyclase induced by the mu-selective agonist, Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO), and by high concentrations of [D-Pen2-D-Pen5]enkephalin (DPDPE), we suggest that these cells contain a homogeneous population of mu opioid receptors coupled to adenylyl cyclase via a guanyl nucleotide-binding protein. Studies measuring the ability of [D-Ala2-D-Leu5]enkephalin (DADLE), an opioid agonist, to inhibit adenylyl cyclase in cells that had been exposed to 100 microM morphine for varying periods of time, indicated that the agonist no longer inhibited enzyme activity after 5 hr of morphine exposure. Measurements of 3H-antagonist binding in membranes from cells exposed to morphine demonstrated a decreased receptor density after 24 hr of 100 microM morphine exposure with no change in the antagonist affinity. Computer analysis indicated a 20% decrease in the number of mu receptors labeled after 24 hr of morphine exposure and a 60% decrease after 72 hr of exposure. Computer analysis of agonist competition against 3H-antagonist binding confirmed the existence of one binding site with an affinity intermediate between the high and low apparent affinity states observed in membranes from untreated cells. Addition of 10 microM GTP gamma S did not affect the agonist affinity or receptor density in membranes from morphine-treated cells, suggesting that the receptors were uncoupled from G proteins, as observed in 7315c cell membranes that have been treated with pertussis toxin. Thus chronic morphine treatment induced a rapid loss of opioid mu receptor-mediated inhibition of adenylyl cyclase (desensitization), and a more slowly developing reduction in receptor number. The desensitization was accompanied by a loss of guanyl nucleotide regulation of agonist affinity. These findings are comparable to results reported for the delta opioid receptor and the beta-adrenergic receptor upon prolonged agonist exposure.

Multiple agonist-affinity states of opioid receptors: regulation of binding by guanyl nucleotides in guinea pig cortical, NG108-15, and 7315c cell membranes

Multiple affinity states of opioid receptors of the mu and delta types have been identified in membranes prepared from cells which bear only one type of opioid receptor (mu receptors in 7315c cells, delta receptors in NG 108-15 cells), and in guinea pig cortical membranes where both types of receptors were present in the membrane preparations. States of mu and delta receptors which have agonist affinities too low to be identified by radiolabeled agonist have been measured indirectly by agonist competition for sites labeled by radioactive antagonist. Using analogues of guanyl nucleotides, we have examined the competition of the mu and delta agonists DAGO and DSLET against [3H]DIP or [3H]NAL binding to opioid receptors and identified several agonist affinity states. In the absence of added nucleotide, competition of DSLET for [3H]DIP binding to delta opioid receptors revealed the presence of two binding sites with differing apparent agonist affinities. Addition of GDP beta S produced a steep monophasic curve which was best fit by a one-site model. In contrast, in the presence of added GTP or GTP gamma S, two affinity states were again apparent for DSLET competition at the delta receptor. The competition curve with GTP was shifted to the right relative to that produced in the absence of added guanyl nucleotide, indicating the presence of a lower apparent affinity state than any observed under other treatment conditions. DAGO competed against [3H]DIP or [3H]NAL binding to mu receptors over a wide concentration range in the absence of added guanyl nucleotide, consistent with the occupation by this ligand of more than one agonist affinity state of the mu receptor. However, when GDP beta S was added to the incubation mixture, only a single binding site was identified. Two mu receptor affinity states were again observed in the presence of added GTP or GTP gamma S. One of these had significantly lower apparent affinity than those states detected in the absence of added nucleotide or with GDP beta S. Pertussis toxin treatment resulted in a monophasic agonist competition curve which was best fitted by a single-site model in both 7315c and NG108-15 cell membranes. Addition of 100 microM GTP did not affect the agonist Kapp or Bmax after pertussis toxin treatment, suggesting that sites labeled under these conditions were not functionally associated with a G protein. In general, the effects of guanyl nucleotides were qualitatively similar at mu and delta receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Opioid receptor regulation of the release of norepinephrine in brain

The ability of opioids to inhibit the release of norepinephrine (NE) from slice preparations of brain has been tested. Slices of brain were preincubated with [3H]NE allowing uptake of the [3H]NE into intraneuronal stores of NE. After rinsing, the tissues were incubated at 37 degrees C in Krebs buffer containing 5mM K+, for estimation of baseline release and then in 20 mM K+ to stimulate release. The [3H]NE released into the incubation medium was increased by blockade of neuronal re-uptake with desipramine and by blockade of alpha 2-adrenoceptors with yohimbine. These agents were used routinely in subsequent incubations. Release was also Ca2+ dependent. Stimulated release of [3H]NE from slices of cortex of the guinea pig and rat was inhibited by the mu opioid receptor agonist, Tyr-D-Ala2-Gly-NMePhe-Gly-ol (DAGO) in a naloxone-reversible manner, although naloxone itself produced a measurable inhibitory effect in the absence of opioid agonist. Stimulated release of [3H]NE from slices of guinea pig cortex was also inhibited by the delta receptor selective peptide, [D-Pen2, D-Pen5] enkephalin (DPDPE), and the kappa receptor selective agent, U50,488H. The inhibitory effect of both agents was reversed by naloxone. In rat cortex, DAGO induced a similar inhibition of release to that seen in guinea pig cortex, but DPDPE and U50,488H were much less effective, producing only weak inhibition even in large doses. Similar results were obtained when effects of opioids on [3H]NE release from hippocampus and cerebellum of the guinea pig and rat were compared. In guinea pig tissues, agonists acting preferentially through mu, delta and kappa receptors were all active in inhibiting stimulated release of [3H]NE, but in hippocampus and cerebellum of the rat, only DAGO inhibited release while DPDPE and U50,488H either had no effect or potentiated the stimulated release. These results suggest that in the rat only mu type opioid receptors mediate an inhibitory regulation of NE release from the cortex, hippocampus and cerebellum terminal projections of locus coeruleus noradrenergic neurons. In the guinea pig, stimulated release of [3H]NE was subject to inhibitory regulation by mu, delta and kappa opioid receptors.

Sodium regulation of agonist binding at opioid receptors. II. Effects of sodium replacement on opioid binding in guinea pig cortical membranes

We have examined the effects of sodium on the binding of opioid agonists to mu-, delta-, and kappa-receptors in guinea pig cortical membranes. Concentration curves for sodium indicated that maximal inhibition of mu binding by this cation was about 60% and maximal inhibition for delta binding was about 70%, whereas that for kappa binding was only about 20%. The concentration of sodium required for half-maximal inhibition of binding to all three sites was about 10-30 mM, corresponding to the intracellular sodium concentration. The nature of the sodium effect was further characterized by saturation analysis of binding to each of the three receptor types by comparing results obtained in the presence of 120 mM sodium with those obtained with equimolar replacement of sodium by another cation. Two radiolabeled agonists with different structural characteristics were tested for each binding site. In the presence of sodium, the affinity of the labeled agonists for mu sites was approximately 2-3-fold less than in its absence, but the density of binding sites was not changed. At kappa sites, sodium reduced agonist affinity slightly but, again, did not alter the number of binding sites. In contrast, sodium reduced the apparent density of delta-binding sites while leaving the agonist affinity unchanged. Competition against antagonist binding to delta sites indicated that, in the presence of sodium, a higher proportion of sites was in a lower affinity state, as reflected by the biphasic nature of the agonist displacement curve. In contrast, the effect of sodium on displacement of antagonist from mu sites was to of sodium on displacement of antagonist from mu sites was to lower the affinity of the agonist. Competition against antagonist binding to kappa sites also showed a reduction in agonist affinity by sodium, but no change in number of receptors. The results indicate that sodium may differentially regulate agonist binding to opioid receptor types and that this regulation may occur at an intracellular site. The kappa site appears to be less sensitive to sodium than the mu and delta sites.