Phencyclidine (angel dust)/sigma "opiate" receptor: visualization by tritium-sensitive film

Some distorted thoughts about ketamine as a psychedelic and a novel hypothesis based on NMDA receptor-mediated synaptic plasticity

Ketamine, a channel blocking NMDA receptor antagonist, is used off-label for its psychedelic effects, which may arise from a combination of several inter-related actions. Firstly, reductions of the contribution of NMDA receptors to afferent information from external and internal sensory inputs may distort sensations and their processing in higher brain centres. Secondly, reductions of NMDA receptor-mediated excitation of GABAergic interneurons can result in glutamatergic overactivity. Thirdly, limbic cortical disinhibition may indirectly enhance dopaminergic and serotonergic activity. Fourthly, inhibition of NMDA receptor mediated synaptic plasticity, such as short-term potentiation (STP) and long-term potentiation (LTP), could lead to distorted memories. Here, for the first time, we compared quantitatively the effects of ketamine on STP and LTP. We report that ketamine inhibits STP in a double sigmoidal fashion with low (40 nM) and high (5.6 μM) IC50 values. In contrast, ketamine inhibits LTP in a single sigmoidal manner (IC50 value ∼ 15 μM). A GluN2D-subunit preferring NMDA receptor antagonist, UBP145, has a similar pharmacological profile. We propose that the psychedelic effects of ketamine may involve the inhibition of STP and, potentially, associated forms of working memory. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

Phencyclidine-Based New Psychoactive Substances

The serendipitous discovery of phencyclidine (PCP) in 1956 sets the stage for significant research efforts that resulted in a plethora of analogs and derivatives designed to explore the biological effects of this class. PCP soon became the prototypical dissociative agent that eventually sneaked through the doors of clinical laboratories and became an established street drug. Estimations suggest that around 14 PCP analogs were identified as "street drugs" in the period between the 1960s and 1990s. Fast forward to the 2000s, and largely facilitated by advancements in electronic forms of communication made possible through the Internet, a variety of new PCP analogs began to attract the attention of communities interested in the collaborative exploration of these substances. Traditionally, as was the case with the first-generation analogs identified in previous decades, the substances explored represented compounds already known in the scientific literature. As the decade of the noughties unfolded, a number of new PCP-derived substances appeared on the scene, which included some analogs that have not been previously recorded in the published literature. The aim of this chapter is to present a brief introductory overview of substances that have materialized as PCP-derived new psychoactive substances (NPS) in recent years and their known pharmacology. Since N-methyl-D-aspartate receptor (NMDAR) antagonism is implicated in mediating the subjective and mind-altering effects of many dissociative drugs, additional data are included from other analogs not presently identified as NPS.

1,2-Diarylethylamine- and Ketamine-Based New Psychoactive Substances

While phencyclidine (PCP) and ketamine remain the most well-studied and widely known dissociative drugs, a number of other agents have appeared since the late 1950s and early 1960s, when the pharmacological potential of this class was first realized. For example, hundreds of compounds have been pursued as part of legitimate research efforts to explore these agents. Some of these found their way out of the research labs and onto illicit markets of the 1960s and following decades as PCP analogs. Other "illicit analogs" apparently never appeared in the scientific literature prior to their existence on clandestine markets, thus originating as novel innovations in the minds of clandestine chemists and their colleagues. Like so much else in this world, new technologies changed this dynamic. In the 1990s individuals separated by vast geographical distances could now communicate nearly instantaneously with ease through the Internet. Some individuals used this newly found opportunity to discuss the chemistry and psychoactive effects of dissociative drugs as well as to collaborate on the design and development of novel dissociative compounds. Similar to modern pharmaceutical companies and academic researchers, these seekers tinkered with the structure of their leads pursuing goals such as improved duration of action, analgesic effects, and reduced toxicity. Whether all these goals were achieved for any individual compound remains to be seen, but their creations have been let out of the bag and are now materialized as defined compositions of matter. Moreover, these creations now exist not only in and of themselves but live on further as permutations into various novel analogs and derivatives. In some cases these compounds have made their way to academic labs where potential clinical applications have been identified. These compounds reached wider distribution when other individuals picked up on these discussions and began to market them as "research chemicals" or "legal highs". The result is a continuously evolving game that is being played between legislatures, law enforcement, and research chemical market players. Two structurally distinct classes that have appeared as dissociative-based new psychoactive substances (NPS) are the 1,2-diarylethylamines and β-keto-arylcyclohexylamines. Examples of the former include diphenidine and various analogs such as fluorolintane and N-ethyl-lanicemine, and examples of the latter are analogs of ketamine such as methoxetamine, deschloroketamine, and 2-fluoro-2-deschloroketamine. The subject of this chapter is the introduction to some of the dissociative NPS from these classes and their known pharmacology that have emerged on the market in recent years.

Ketamine and phencyclidine: the good, the bad and the unexpected

The history of ketamine and phencyclidine from their development as potential clinical anaesthetics through drugs of abuse and animal models of schizophrenia to potential rapidly acting antidepressants is reviewed. The discovery in 1983 of the NMDA receptor antagonist property of ketamine and phencyclidine was a key step to understanding their pharmacology, including their psychotomimetic effects in man. This review describes the historical context and the course of that discovery and its expansion into other hallucinatory drugs. The relevance of these findings to modern hypotheses of schizophrenia and the implications for drug discovery are reviewed. The findings of the rapidly acting antidepressant effects of ketamine in man are discussed in relation to other glutamatergic mechanisms.

Sigma-1 receptor: the novel intracellular target of neuropsychotherapeutic drugs

Sigma-1 receptor ligands have been long expected to serve as drugs for treatment of human diseases such as neurodegenerative disorders, depression, idiopathic pain, drug abuse, and cancer. Recent research exploring the molecular function of the sigma-1 receptor started unveiling underlying mechanisms of the therapeutic activity of those ligands. Via the molecular chaperone activity, the sigma-1 receptor regulates protein folding/degradation, ER/oxidative stress, and cell survival. The chaperone activity is activated or inhibited by synthetic sigma-1 receptor ligands in an agonist-antagonist manner. Sigma-1 receptors are localized at the endoplasmic reticulum (ER) membranes that are physically associated with the mitochondria (MAM: mitochondria-associated ER membrane). In specific types of neurons (e.g., those at the spinal cord), sigma-1 receptors are also clustered at ER membranes that juxtapose postsynaptic plasma membranes. Recent studies indicate that sigma-1 receptors, partly in sake of its unique subcellular localization, regulate the mitochondria function that involves bioenergetics and free radical generation. The sigma-1 receptor may thus provide an intracellular drug target that enables controlling ER stress and free radical generation under pathological conditions.

A Role for Sigma Receptors in Stimulant Self Administration and Addiction

Sigma₁ receptors (σ₁Rs) represent a structurally unique class of intracellular proteins that function as chaperones. σ₁Rs translocate from the mitochondria-associated membrane to the cell nucleus or cell membrane, and through protein-protein interactions influence several targets, including ion channels, G-protein-coupled receptors, lipids, and other signaling proteins. Several studies have demonstrated that σR antagonists block stimulant-induced behavioral effects, including ambulatory activity, sensitization, and acute toxicities. Curiously, the effects of stimulants have been blocked by σR antagonists tested under place-conditioning but not self-administration procedures, indicating fundamental differences in the mechanisms underlying these two effects. The self administration of σR agonists has been found in subjects previously trained to self administer cocaine. The reinforcing effects of the σR agonists were blocked by σR antagonists. Additionally, σR agonists were found to increase dopamine concentrations in the nucleus accumbens shell, a brain region considered important for the reinforcing effects of abused drugs. Although the effects of the σR agonist, DTG, on dopamine were obtained at doses that approximated those that maintained self administration behavior those of another agonist, PRE-084 required higher doses. The effects of DTG were antagonized by non-selective or a preferential σ₂R antagonist but not by a preferential σ₁R antagonist. The effects of PRE-084 on dopamine were insensitive to σR antagonists. The data suggest that the self administration of σR agonists is independent of dopamine and the findings are discussed in light of a hypothesis that cocaine has both intracellular actions mediated by σRs, as well as extracellular actions mediated through conventionally studied mechanisms. The co-activation and potential interactions among these mechanisms, in particular those involving the intracellular chaperone σRs, may lead to the pernicious addictive effects of stimulant drugs.

The novel sigma-2 receptor ligand SW43 stabilizes pancreas cancer progression in combination with gemcitabine

Background

Sigma-2 receptors are over-expressed in proliferating cancer cells, making an attractive target for the targeted treatment of pancreatic cancer. In this study, we investigated the role of the novel sigma-2 receptor ligand SW43 to induce apoptosis and augment standard chemotherapy.

Results

The binding affinity for sigma-2 ligands is high in pancreas cancer, and they induce apoptosis with a rank order of SV119 < SW43 < SRM in vitro. Combining these compounds with gemcitabine further increased apoptosis and decreased viability. Our in vivo model showed that sigma-2 ligand treatment decreased tumor volume to the same extent as gemcitabine. However, SW43 combination treatment with gemcitabine was superior to the other compounds and resulted in stabilization of tumor volume during treatment, with minimal toxicities.

Conclusions

This study shows that the sigma-2 ligand SW43 has the greatest capacity to augment gemcitabine in a pre-clinical model of pancreas cancer and has provided us with the rationale to move this compound forward with clinical investigations for patients with pancreatic cancer.

kappa-Opioid receptor signaling and brain reward function

The dynorphin-like peptides have profound effects on the state of the brain reward system and human and animal behavior. The dynorphin-like peptides affect locomotor activity, food intake, sexual behavior, anxiety-like behavior, and drug intake. Stimulation of kappa-opioid receptors, the endogenous receptor for the dynorphin-like peptides, inhibits dopamine release in the striatum (nucleus accumbens and caudate putamen) and induces a negative mood state in humans and animals. The administration of drugs of abuse increases the release of dopamine in the striatum and mediates the concomitant release of dynorphin-like peptides in this brain region. The reviewed studies suggest that chronic drug intake leads to an upregulation of the brain dynorphin system in the striatum and in particular in the dorsal part of the striatum/caudate putamen. This might inhibit drug-induced dopamine release and provide protection against the neurotoxic effects of high dopamine levels. After the discontinuation of chronic drug intake these neuroadaptations remain unopposed which has been suggested to contribute to the negative emotional state associated with drug withdrawal and increased drug intake. kappa-Opioid receptor agonists have also been shown to inhibit calcium channels. Calcium channel inhibitors have antidepressant-like effects and inhibit the release of norepinephrine. This might explain that in some studies kappa-opioid receptor agonists attenuate nicotine and opioid withdrawal symptomatology. A better understanding of the role of dynorphins in the regulation of brain reward function might contribute to the development of novel treatments for mood disorders and other disorders that stem from a dysregulation of the brain reward system.

Selective reduction of alcohol drinking in Sardinian alcohol-preferring rats by a sigma-1 receptor antagonist

RATIONALE AND OBJECTIVES

Sigma receptors have been implicated in appetitive effects of psychostimulants and in high levels of ethanol intake. This study tested the hypothesis that the sigma-1 receptor subtype (Sig-1R) may modulate ethanol intake.

MATERIAL AND METHODS

The effects of acute and repeated treatment with the potent, selective Sig-1R antagonist NE-100 on ethanol intake (10%) were studied in adult, male Sardinian alcohol-preferring (sP) rats, a model of genetic predisposition to high ethanol drinking. To assess the specificity of action, the acute effects of NE-100 on intake of an equally preferred sucrose solution and of a higher concentration of ethanol that sP rats did not prefer over water (28%), were determined. Finally, the ability of NE-100 administration to prevent the increased ethanol intake that occurs after deprivation was evaluated.

RESULTS

Acute treatment with NE-100 dose-dependently (10-30 mg/kg) reduced 1- and 3-h intake of 10% ethanol solution in sP rats, while increasing concurrent water intake and not affecting food intake. NE-100 (17.8-30 mg/kg) comparably reduced intake of the 28% ethanol solution, while not suppressing 1.25% sucrose solution intake, suggesting selectivity of action against ethanol intake. Acute NE-100 (30 mg/kg) also prevented an increase in ethanol intake after a 7-day deprivation period. Repeated, daily NE-100 (30 mg/kg) treatment continued to reduce 24-h ethanol intake across 7 days of administration, with some, but incomplete, tolerance, evident by day 6.

CONCLUSIONS

The results implicate the Sig-1R system in alcohol drinking, identifying a potential therapeutic target for the treatment of alcohol use disorders.

The sigma-receptor antagonist BD-1063 decreases ethanol intake and reinforcement in animal models of excessive drinking

Sigma-Receptors (SigRs) have been implicated in behavioral and appetitive effects of psychostimulants and may also modulate the motivating properties of ethanol. This study tested the hypothesis that SigRs modulate ethanol reinforcement and contribute to excessive ethanol intake. The effects of subcutaneous treatment with the potent, selective Sig-1R antagonist BD-1063 on operant ethanol self-administration were studied in two models of excessive drinking-Sardinian alcohol-preferring (sP) rats and acutely withdrawn ethanol-dependent Wistar rats-and compared to ethanol self-administration in nondependent Wistar controls. To assess the specificity of action, the effects of BD-1063 on self-administration of an equally reinforcing saccharin solution were determined in Wistar and sP rats. Gene expression of Sig-1R in reward-related brain areas implicated in ethanol reinforcement was compared between ethanol-naive sP and Wistar rats and withdrawn ethanol-dependent Wistar rats. BD-1063 dose dependently reduced ethanol self-administration in sP rats (3.3-11 mg/kg) and withdrawn, dependent Wistar rats (4-11 mg/kg) at doses that did not modify mean ethanol self-administration in nondependent Wistar controls. BD-1063 did not reduce concurrent water self-administration and did not comparably suppress saccharin self-administration, suggesting selectivity of action. BD-1063 also reduced the breakpoints of sP rats to work for ethanol under a progressive-ratio reinforcement schedule. Ethanol-naive sP rats and 24-h withdrawn, dependent Wistar rats showed reduced Sig-1R mRNA expression in the nucleus accumbens. The results suggest that SigR systems may contribute to innate or ethanol-induced increases in susceptibility to self-administer high ethanol levels, identifying a potential neuroadaptive mechanism contributing to excessive drinking and a therapeutic target for alcohol abuse and dependence.

Chronic nicotine improves cognitive performance in a test of attention but does not attenuate cognitive disruption induced by repeated phencyclidine administration

RATIONALE

Nicotine-induced cognitive enhancement may be a factor maintaining tobacco smoking, particularly in psychiatric populations suffering from cognitive deficits. Schizophrenia patients exhibit higher smoking rates compared with the general population, suggesting that attempts to self-medicate cognitive schizophrenia deficits may underlie these high smoking levels.

OBJECTIVES

The present study explored pro-cognitive effects of nicotine in a model of schizophrenia-like cognitive dysfunction to test this self-medication hypothesis.

MATERIALS AND METHODS

We investigated whether chronic nicotine (3.16 mg/kg/day, base) would attenuate the performance disruption in the five-choice serial reaction time task (5-CSRTT, a task assessing various cognitive modalities, including attention) induced by repeated administration of phencyclidine (PCP), an N-methyl-D-aspartate receptor antagonist that induces cognitive deficits relevant to schizophrenia.

RESULTS

Chronic nicotine administration shortened 5-CSRTT response latencies under baseline conditions. Nicotine-treated rats also made more correct responses and fewer omissions than vehicle-treated rats. Replicating previous studies, repeated PCP administration (2 mg/kg, 30 min before behavioral testing for two consecutive days followed 2 weeks later by five consecutive days of PCP administration) decreased accuracy and increased response latencies, premature responding, and timeout responding. Chronic nicotine did not attenuate these PCP-induced disruptions.

CONCLUSIONS

Chronic nicotine had pro-cognitive effects by itself, supporting the hypothesis that cognitive enhancement may contribute to tobacco smoking. At the doses of nicotine and PCP used, however, no support was found for the hypothesis that the beneficial effects of nicotine on cognitive deficits induced by repeated PCP administration, assessed in the 5-CSRTT, are larger than nicotine effects in the absence of PCP.

Alterations in behavioral responses to a cholinergic agonist in post-pubertal rats with neonatal ventral hippocampal lesions: relationship to changes in muscarinic receptor levels

Excitotoxic neonatal ventral hippocampal (NVH) lesion in rats is considered as a putative animal model of schizophrenia as lesioned animals show characteristic post-pubertal emergence of neurochemical and behavioral abnormalities analogous to some of those seen in this disease. Converging evidence points to the involvement of central cholinergic system in this neuropsychiatric disorder, and our previous studies have suggested that cholinergic neurotransmission may be altered in post-pubertal NVH lesioned rats. We investigated here muscarinic receptor reactivity in NVH lesioned animals by measuring the effects of the muscarinic receptor agonist oxotremorine on physiological responses known to be modulated by these receptors such as body temperature, salivation, tremor, pain, and prepulse inhibition of the acoustic startle (PPI). Quantitative receptor autoradiography revealed that post-pubertal NVH lesioned animals display increased levels of [3H]pirenzepine/M1-like and [3H]AFDX-384/M2-like receptor binding sites in the striatum, nucleus accumbens, and in subareas of the dorsal hippocampus. Moreover, in response to the systemic administration of oxotremorine (0.25 mg/kg), post-pubertal NVH lesioned rats exhibited increases in salivation and tremor, and a greater reduction in body temperature compared to sham control animals. Increases in the hot-plate latency were also observed suggesting enhanced antinociceptive effects of oxotremorine in post-pubertal NVH lesioned animals. Finally, oxotremorine (0.1 and 0.25 mg/kg) disrupted PPI in post-pubertal sham control rats while the muscarinic receptor antagonist biperiden (0.5 and 1.0 mg/kg) normalized this behavior in NVH lesioned rats. Taken together, these findings reveal that post-pubertal NVH lesioned rats display enhanced muscarinic receptor responsiveness, which may relate to some behavioral abnormalities reported in this animal model.

Reduction in acetylcholine release in the hippocampus of dopamine D5 receptor-deficient mice

Activation of the dopamine D1-like receptor stimulates acetylcholine (ACh) release in the hippocampus, apparently through the molecularly defined d5 receptor. In the present study, we used a transgenic mouse completely deprived of functional d5 receptor (d5-/-) to confirm the role and elucidate the possible function of the d5 receptor subtype on hippocampal cholinergic neurotransmission. ACh release was measured using in vivo microdialysis in the mouse dorsal hippocampus of 4 months old homozygous (d5-/-), heterozygous (d5+/-), and the wild-type (d5+/+) littermates. Using the no net flux technique, a significant reduction in basal hippocampal ACh level was found in the d5-/- compared to d5+/- and d5+/+ mice. Moreover, the administration of SKF 38393, a D1-like receptor agonist, systemically (2.0 and 10.0 mg/kg ip), or locally through the dialysis probe (10 and 50 microM), produced a dose-dependent enhancement of ACh release in the d5+/+, a moderate stimulation in the d5+/- but had no effect in the d5-/- mice. Quantitative receptor autoradiography revealed significant increases in M1-like but not in M2-like muscarinic receptor binding sites in the hippocampal formation. These results confirm and extend the role of the d5 receptor in the modulation of hippocampal ACh release and provide evidence for long-term alteration of hippocampal cholinergic neurotransmission resulting from the absence of the d5 receptors including chronically reduced ACh release and change in M1-like receptor levels.

In rats, acute morphine dependence results in antagonist-induced response suppression of intracranial self-stimulation

RATIONALE

Lower (0.001-1.0 mg/kg) doses of the opioid antagonist naltrexone produce few behavioral effects in otherwise drug-free rats responding for ICSS, but reduce response rates by up to 75% after a single dose of morphine.

OBJECTIVES

The present study represents an effort to verify that other opioid antagonists produce this acute opioid dependence effect, and to characterize their relative pharmacological profiles.

METHODS

We implanted bipolar electrodes in the lateral hypothalamus of adult male rats, and then trained them to lever-press on an "autotitration" ICSS schedule, where responding on a "reset" lever allows the rat to control the frequency of stimulation; performance stabilized at approximately 1.5 responses/s.

RESULTS

During twice-weekly test sessions, cumulative doses of five of seven opioid antagonists produced significant response rate decreases (30-80%) in saline-pretreated rats; nalorphine (ED25=15.6 mg/kg)> naltrexone (ED25=13.1 mg/kg)>naloxone (ED25=7.3 mg/kg)>levallorphan (ED25=13.96 mg/kg)>(-)cyclazocine (ED25=0.028 mg/kg). A single MOR pretreatment (10 mg/kg, 4 h) significantly enhanced the rate-decreasing effects of six of the seven agonists tested; by 10-fold (-) cyclazocine>13-fold (nalorphine)>93-fold (levallorphan)>972-fold (naloxone)>2190-fold (naltrexone). The pure non-selective antagonist diprenorphine potently decreased rates after MOR pretreatment (ED25= 0.01 mg/kg), but did not after saline pretreatment. The mixed opioid agonist-antagonist drug nalbuphine (1.0-30 mg/kg) did not affect responding after either saline or MOR.

CONCLUSIONS

Antagonists with a high affinity for, and a lack of intrinsic activity at, the micro-opioid receptor precipitate the greatest behavioral changes in rats acutely dependent on MOR.

Alterations in dopaminergic modulation of prefrontal cortical acetylcholine release in post-pubertal rats with neonatal ventral hippocampal lesions

Excitotoxic lesion of the ventral hippocampus in neonatal rats is a putative animal model of schizophrenia with characteristic developmental abnormalities in dopaminergic neurotransmission and prefrontal cortical functions. Converging evidence also points to the involvement of the central cholinergic system in neuropsychiatric disorders. These two neurotransmitter systems are interlinked in the prefrontal cortex (PFC) where dopamine stimulates acetylcholine (ACh) release. In the present study, we investigated the role of dopamine in the developmental regulation of prefrontal cortical ACh release and the expression of nicotinic and muscarinic receptors in pre- and post-pubertal rats with neonatal ibotenic acid-induced lesions of the ventral hippocampus (NVH). In vivo microdialysis in the PFC revealed that systemic injections of the D(1)-like receptor agonist (+/-)-6-chloro-7,8-dihydroxy-1-phenyl2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF 81297) (2.5 and 5.0 mg/kg i.p.) caused significantly higher ACh release in post-pubertal NVH-lesioned animals (250 and 300% baseline for 2.5 and 5.0 mg/kg, respectively) compared with post-pubertal shams (150 and 220% baseline for 2.5 and 5.0 mg/kg, respectively). Most interestingly, while prefrontal cortical perfusion of SKF 81297 (100 and 250 microM) had no significant effect on ACh release in post-pubertal sham-operated animals, it significantly stimulated ACh release to approximately 250% baseline at both doses in post-pubertal NVH-lesioned animals. Receptor autoradiography demonstrated a significant and selective increase in M(1)-like receptor binding sites in the infralimbic area of the PFC in the post-pubertal NVH-lesioned animals. For all experiments, significant differences between sham and NVH-lesioned animals were observed only in post-pubertal rats. These results suggest a developmentally specific reorganization of the prefrontal cortical cholinergic system involving D(1)-like receptors in the NVH model.

Dissociative mental state in a patient with an intrathecal drug administration system

IMPLICATIONS

We describe a patient with acute mental status changes, which resolved on removal of medication from the reservoir of a Synchromed intrathecal pump. This report highlights the potential adverse mental affects of chronic spinal infusions for pain therapy and discusses pitfalls in toxicology analysis using immunoassay.

Influence of 3-PPP, a sigma receptor ligand, on the anticonvulsive action of conventional antiepileptic drugs

(+)-3-(3-Hydroxyphenyl)-N-(1-propyl)-piperidine (3-PPP; a sigma receptor ligand), administered at 30 mg kg-1, 30 min before the test, significantly decreased the electroconvulsive threshold in mice, being ineffective in lower doses. 3-PPP (20 mg kg-1) diminished the protective activity of diphenylhydantoin, phenobarbital and valproate, but not that of carbamazepine against maximal electroshock. The effect of 3-PPP upon the electroconvulsive threshold and the 3-PPP-induced inhibition of the protective action of antiepileptics was reversed by haloperidol (0.5 mg kg-1). Moreover, 3-PPP did not alter the total and free plasma levels of antiepileptic drugs, so a pharmacokinetic interaction is not probable. The combined treatment of 3-PPP with antiepileptic drugs, providing a 50% protection against maximal electroshock, did not affect motor performance in mice, although resulted in significant long-term memory deficits. Our data indicate that sigma receptor-mediated events may play some role in seizure processes in the central nervous system and can modulate the protective activity of some conventional antiepileptic drugs.

Relationship between modulation of the cerebellorubrospinal system in the in vitro turtle brain and changes in motor behavior in rats: effects of novel sigma ligands

Saturation and competition binding studies showed that the turtle brain contains sigma sites labeled by both [3H]di-o-tolylguanidine (DTG) and [3H](+)-pentazocine. There was a significant correlation between the IC50 values of sigma ligands for [3H]DTG sites in the turtle vs. rat brain, suggesting that the sites are comparable in the two species. In contrast, [3H](+)-pentazocine, which primarily labels sigma1 sites in the rodent brain, labels a heterogeneity of sites in the turtle brain. In extracellular recordings from the in vitro turtle brainstem, some sigma ligands enhanced the burst responses of red nucleus (RN) neurons (DTG, haloperidol, BD1031, BD1052, BD1069) while other sigma ligands decreased the burst responses (BD1047, BD1063). Control compounds (turtle Ringer vehicle control, opiate antagonist naloxone, atypical neuroleptic sulpiride) had no significant effects on the RN burst responses recorded from the in vitro turtle brain. The ED50s of the ligands for altering the burst responses in RN neurons from the turtle brain were correlated with their IC50s for turtle brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine; this pattern is identical to that previously reported in rats, where there is a correlation between the potencies of sigma ligands for producing dystonic postures after microinjection into the rat RN and their binding to rat brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine. When the novel sigma ligands were microinjected into the rat RN, dystonic postures were produced by ligands that increased the burst duration of RN neurons in the turtle brain. Novel sigma ligands that reduced the burst responses in the in vitro turtle brain have previously been reported to have no effects on their own when microinjected into the rat RN, but to block the dystonic postures produced by other sigma ligands. Taken together, the data suggest that the opposite effects of the novel ligands in the turtle electrophysiological studies represent the actions of agonists vs. antagonists, and that the directionality of the effects has predictive value for the expected motor effects of the drugs.

Intrahippocampal injections of phencyclidine but not naloxone disrupt acquisition of a spatial continuous recognition memory task

Rats with 36 nM or 54 nM of phencyclidine (PCP), 36 nM of naloxone or saline injected into the dentate gyrus of the hippocampus were tested for acquisition of a spatial continuous recognition memory task. Results indicate that relative to controls and rats with 36 nM of PCP or 36 nM of naloxone injections, rats with 54 nM of PCP injections were impaired in acquisition of the task across all lags as measured by increased in latency for repeated items. Since it is assumed that successful learning of this continuous recognition memory task depends upon processes associated with consolidation of new learning into long term memory, it appears that high doses of PCP, but not naloxone are sufficient to impair this process.

Autoradiographical and immunohistochemical analysis of receptor localization in the central nervous system

Quantitative receptor autoradiographic methods have been widely used over the past two decades. Some of the advantages and limitations of these techniques are reviewed here. Comparison with immunohistochemical and in situ hybridization methods is also highlighted, as well as the use of these approaches to study receptor gene over-expression in cell lines. Together, data obtained using these various methodologies can provide unique information on the potential physiological roles of a given receptor protein and/or binding sites in various tissues.

Comparative ontogenic profile of cholinergic markers, including nicotinic and muscarinic receptors, in the rat brain

The ontogenic profiles of several cholinergic markers were assessed in the rat brain by using quantitative in vitro receptor autoradiography. Brain sections from animals at different stages of development were processed with [3H]AH5183 (vesamicol; vesicular acetylcholine transport sites), [3H]N-methylcarbamylcholine (alpha(4)beta(2) nicotinic receptor sites), [3H]hemicholinium-3 (high-affinity choline uptake sites), [3H]3-quinuclidinyl benzilate (total population of muscarinic receptor sites), [3H]4-DAMP (muscarinic M1/M3 receptor sites), [3H]pirenzepine (muscarinic M1 receptor sites), and [3H]AF-DX 116 and [3H]AF-DX 384 (muscarinic M2 receptor sites) as radiolabeled probes. The results revealed that, by the end of the prenatal period (embryonic day 20), the densities of nicotinic receptor and vesicular acetylcholine transport sites already represented a considerable proportion of those observed in adulthood (postnatal day 60) in different laminae of the frontal, parietal, and occipital cortices, in the layers of Ammon's horn fields and the dentate gyrus of the hippocampal formation, as well as in the amygdaloid body, the olfactory tubercle, and the striatum. In contrast, at that stage, the densities of total muscarinic, M1/M3, M1, and possibly M2 receptor and high-affinity choline uptake sites represent only a small proportion of levels seen in the adult. Differences were also observed in the postnatal ontogenic profiles of nicotinic, muscarinic, vesamicol, and high-affinity choline uptake sites. For example, between postnatal weeks 3 and 5, the levels of M1/M3 and M1 sites were at least as high as in the adult, whereas M2 and high-affinity choline uptake site densities appeared to be delayed and to reach adult values only after postnatal week 5. With regard to cholinergic innervation in the developing rat brain, the present findings suggest a temporal establishment of several components of the cholinergic systems. The first components are the vesicular acetylcholine transporter and nicotinic sites; these are followed by M1/M3 and M1 sites and, finally, by M2 and high-affinity choline uptake sites.

Phencyclidine injections into the dorsal hippocampus disrupt long- but not short-term memory within a spatial learning task

Since the hippocampus is likely to be a major site of phencyclidine (PCP) action, the effects of various doses of PCP (1.8, 18 or 36 nM) as well as 3.6 nM MK-801 or saline injected directly into the dentate gyrus of the hippocampus was tested for acquisition of a spatial navigation task (dry land version of a water maze) using a paradigm that assesses short term memory based on learning within a day and long term memory based on learning between days. Results indicated that relative to saline or 1.8 nM PCP injected rats, rats with 18 or 36 nM PCP or 3.6 nM MK-801 injections were impaired in acquisition of the task as measured by increased distances traveled to find the food location between days but not within days. In additional experiments 36 nM PCP or 3.6 nM MK-801 did not produce any deficits in the acquisition of an object discrimination task. It is suggested that PCP through its blocking action of the NMDA receptor in the dentate gyrus or CA1 region of the dorsal hippocampus mediates the consolidation of new spatial location information.

Quantitative autoradiographic localization of [3H]3-OH-PCP (1-(1(3-hydroxyphenyl)cyclohexyl)piperidine) binding sites in rat brain

Binding of a novel radioligand, [3H]3-OH-PCP (1(1(3-hydroxyphenyl) cyclohexyl)piperidine), to N-methyl-D-aspartate (NMDA) receptor-coupled and -uncoupled PCP sites was investigated in the rat brain. The highest densities of [3H]3-OH-PCP binding were observed in the hippocampal formation, notably in the stratum radiatum and oriens of CA1 region, and dentate gyrus. There were relatively high levels of binding in the olfactory system, superficial layer of cortices, the amygdala and the thalamus. In contrast, lower levels of binding were found in the globus pallidus, cerebellum, and brain stem, except for the superior colliculus. These findings demonstrate that [3H]3-OH-PCP binds to discrete regions within the rat brain. Its distribution is consistent with autoradiographic localization of [3H]TCP and [3H]MK-801 binding sites in the rat brain, suggesting that [3H]3-OH-PCP binds to NMDA/PCP ion-channel complexes in preference to sigma sites.

An electrophilic affinity ligand based on (+)-MK801 distinguishes PCP site 1 from PCP site 2

The electrophilic affinity ligand, (+)-3-isothiocyanato-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cycl ohepten-5,10 - imine hydrochloride [(+)-MK801-NCS] was characterized for its ability to acrylate phencyclidine (PCP) and sigma binding sites in vivo. Initial studies, conducted with mouse brain membranes, characterized the binding sites labeled by [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP). The Kd values of [3H]TCP for PCP site 1 (MK801-sensitive) and PCP site 2 (MK801-insensitive) were 12 nM and 68 nM, with Bmax values of 1442 and 734 fmol/mg protein, respectively. Mice were sacrificed 18-24 hours following intracerebroventricular administration of the acylator. The administration of (+)-MK801-NCS increased [3H]TCP binding to site 2, but not to site 1. Although (+)-MK801-NCS decreased [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d; ccyclohepten-5,10-imine maleate ([3H](+)-MK801) binding to site 1, it had no effect on [3H]TCP binding to site 1. Viewed collectively with other published data, these data support the hypothesis that PCP sites 1 and 2 are distinct binding sites, and that [3H]TCP and [3H](+)-MK801 label different domains of the PCP binding site associated with the NMDA receptor.

Biological rhythms as organization and information

While it is generally acknowledged that modern science began with the quantification of time in the measurement of linear physical processes in space by Galileo and Newton, the biological sciences have only recently developed appropriate experimental and mathematical methods for the description of living systems in terms of processes of non-linear, recursive dynamics. We now recognize that living organisms have patterns of exquisitely timed processes that are as intricate as their spatial structure and organization. Self-similarities of life processes in time and space have evolved to generate an ensemble of oscillators within which analogous functions may be discerned on many different time scales. The increasing complexity of periodic relationships on and between the many levels of biological organization are uncovered by current research. Recent efforts to reformulate the foundation of physics from the quantum to the cosmological level by using the concept of information as the common denominator integrating time, structure and energy remind us of an apparently analogous suggestion in the chronobiological literature which also describes the periodic dynamics of living systems as information processing. In this paper we review the periodic processes of living systems on all levels from the molecular, genetic and cellular to the neuroendocrinological, behavioural and social domains. Biological rhythms may be conceptualized as the evolution of ever more complex dynamics of information transduction that optimize the temporal integrity, development, and survival of the organism.

Current hypotheses on sigma receptors and their physiological role: possible implications in psychiatry

During the last years, due to the availability of selective ligands, numerous investigations have been dedicated to sigma receptors. The existence of different subtypes of these receptors is now accepted; their endogenous ligand has not yet been identified, but some candidates have been proposed. Evidence suggests that one of their major roles might be to regulate the activity of the glutamatergic system via the N-methyl-D-aspartate receptor. The potential involvement of sigma receptors in psychiatry was suggested by the psychotomimetic effects of their earliest ligands and the fact that several neuroleptics have a high affinity for them. Recently, new arguments have strengthened this hypothesis: some molecules with high sigma affinity but low dopaminergic affinity display a "neuroleptic-like" pharmacological profile; post-mortem studies have shown a reduction of sigma binding sites in the brain of patients with schizophrenia; cocaine, which can induce psychotic episodes, has high affinity for sigma receptors. Hence, by modulating the glutamatergic inputs, by regulating directly the firing activity of dopaminergic neurons, or by both mechanisms, sigma receptors could be involved in the pathophysiology and/or in the treatment of schizophrenia.

The glycine/NMDA receptor antagonist, R-(+)-HA-966, blocks activation of the mesolimbic dopaminergic system induced by phencyclidine and dizocilpine (MK-801) in rodents

1. The effects of the glycine/N-methyl-D-aspartate (NMDA) receptor antagonist, R-(+)-HA-966 on the neurochemical and behavioural responses to phencyclidine (PCP) and dizocilpine (MK-801) have been determined in rodents. 2. In rats, pretreatment with PCP (5 and 10 mg kg-1) or MK-801 (0.25 and 0.5 mg kg-1) dose-dependently stimulated dopamine turnover in nucleus accumbens, amygdala and medial prefrontal cortex, but had no effect in striatum. In contrast, pretreatment with (+)-HA-966 (10 and 30 mg kg-1) did not affect dopamine turnover in any brain region investigated. 3. Pretreatment with (+)-HA-966 (10 and 30 mg kg-1) significantly antagonized the stimulation of dopamine turnover induced by both PCP (10 mg kg-1) and MK-801 (0.5 mg kg-1) in rat nucleus accumbens, amygdala and medial prefrontal cortex. 4. Intracerebral dialysis studies in conscious rats demonstrated that systemic injection of PCP (10 mg kg-1) markedly stimulated dopamine release from the nucleus accumbens, an effect that was abolished by pretreatment with (+)-HA-966 (30 mg kg-1). 5. Pretreatment with PCP (3-30 mg kg-1) or MK-801 (0.1-1.6 mg kg-1) significantly increased locomotor activity in mice. In contrast, subcutaneous injection of (+)-HA-966 (10-100 mg kg-1) failed to stimulate activity. 6. Pretreatment with (+)-HA-966 (10 and 30 mg kg-1) dose-dependently antagonized both PCP (10 mg kg-1) and MK-801 (0.4 mg kg-1) induced hyperactivity in mice. 7. Blockade of PCP-induced hyperactivity by (+)-HA-966 is unlikely to be explained by the induction or potentiation of sedation/ataxia since PCP-induced rotarod deficits were not significantly different in mice pretreated with (+)-HA-966 (30 mg kg-1) or saline.8. The results demonstrate that (+ )-HA-966 antagonizes both the neurochemical and behavioural effects of PCP and MK-801, possibly through interactions at the glycine/NMDA receptor.

Binding properties of 3-[125I]iodophencyclidine, a new radioligand for N-methyl-D-aspartate-gated ionic channels

The binding properties of the 125I-labeled phencyclidine derivative N-[1-(3-[125I]iodophenyl)cyclohexyl]piperidine (3-[125I]iodo-PCP), a new ligand of the N-methyl-D-aspartate (NMDA)-gated ionic channel, were investigated. Association and dissociation kinetic curves of 3-[125I]iodo-PCP with rat brain homogenates were well described by two components. About 32% of the binding was of fast association and fast dissociation, and the remaining binding was of slow association and slow dissociation. Saturation curves of 3-[125I]iodo-PCP also were well described using two binding sites: one of a high affinity (KDH = 15.8 +/- 2.3 nM) and the other of a low affinity (KDL = 250 +/- 40 nM). 3-Iodo-PCP inhibited the binding of 3-[125I]iodo-PCP with inhibition curves that were well fitted by a two-site model. The binding constants (KiH, BmaxH; KiL, BmaxL) so obtained were close to those obtained in saturation experiments. Ligands of NMDA-gated ionic channels also inhibited the binding of 3-[125I]iodo-PCP with two constants, KiH and KiL. There was a very good correlation (r = 0.987) between the affinities of these ligands to bind to NMDA-gated ionic channels and their potencies to inhibit the binding of 3-[125I]iodo-PCP with a high affinity. Moreover, the regional distribution of the high-affinity binding of 3-[125I]-iodo-PCP paralleled that of tritiated N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP). In contrast to that of [3H] TCP, the binding of 3-[125I]iodo-PCP to well-washed rat brain membranes was fast and insensitive to glutamate and glycine.(ABSTRACT TRUNCATED AT 250 WORDS)

Protection after transient focal cerebral ischemia by the N-methyl-D-aspartate antagonist dextrorphan is dependent upon plasma and brain levels

Dextrorphan is a dextrorotatory morphinan and a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist. We studied the dose response characteristics of dextrorphan's neuroprotective efficacy and side effects, correlating these beneficial and adverse responses with plasma and brain levels in a rabbit model of transient focal cerebral ischemia. Thirty-three rabbits, anesthetized with halothane, underwent occlusion of the left internal carotid and anterior cerebral arteries for 1 h, followed by 4.5 h of reperfusion. One hour after the onset of ischemia, they were treated with an i.v. infusion of varying dextrorphan doses or normal saline. After killing, the brains were analyzed for ischemic high signal intensity using magnetic resonance imaging (MRI) and for ischemic neuronal damage with histopathology. A separate group of 12 anesthetized ischemic rabbits received similar doses of dextrorphan, correlating plasma with brain dextrorphan levels. Twenty-six additional dextrorphan unanesthetized, nonischemic rabbits received infusions of dextrorphan to correlate behavioral side effects with dextrorphan dose and levels. Compared with controls, dextrorphan 15 mg/kg group had significantly less cortical ischemic neuronal damage (5.3 versus 33.2%, p = 0.01) and a reduction in cortical MRI high signal area (9.1 versus 41.2%, p = 0.02). The dextrorphan 10 mg/kg rabbits showed less cortical ischemic neuronal damage (27.2%) and less MRI high signal (34.8%) but this was not statistically significant (p = 0.6). Dextrorphan 5 mg/kg had no benefit on either neocortical ischemic neuronal damage (35.8%) or MRI high signal (42.9%). The protective effect of dextrorphan was correlated with plasma free dextrorphan levels (r = -0.50, p less than 0.02 for ischemic neuronal damage; r = -0.66, p less than 0.001 for ischemic MRI high signal). All the rabbits with plasma levels greater than 2,000 ng/ml had less than 12% cortical ischemic neuronal damage and less than 34% MRI high signal. All rabbits with plasma levels greater than 3,000 ng/ml showed less than 7% ischemic neuronal damage and less than 11% MRI high signal. Plasma levels of approximately 2,500 ng/ml correlated with brain dextrorphan levels of approximately 6,000 ng/g. Unanesthetized rabbits with plasma levels of approximately 2,500 ng/ml demonstrated loss of the righting reflex. These results demonstrate that systemic treatment with dextrorphan after 1 h focal ischemia can significantly protect against cerebral damage if adequate plasma and brain levels of dextrorphan are achieved. The brain levels necessary to obtain in vivo protection are similar to concentrations that prevent glutamate or NMDA-induced injury in neuronal culture.

[3H]1-[2-(2-thienyl)cyclohexyl]piperidine labels two high-affinity binding sites in human cortex: further evidence for phencyclidine binding sites associated with the biogenic amine reuptake complex

Previous work demonstrated two high-affinity PCP binding sites in guinea pig brain labeled by [3H]TCP (1-(1-[2-thienyl]cyclohexyl)piperidine): site 1 (N-methyl-D-aspartate [NMDA]-associated) and site 2 (dopamine-reuptake complex associated). The present study examined brain membranes prepared from various species, including human, for the presence of site 2, defined as binding in the presence of (+)-5-methyl-10,11-dihydro-5H-dibenzo [a, d]cyclohepten-5,10-imine maleate ((+)-MK801) minus binding in the presence of 10 microM TCP (nonspecific binding). Studies were conducted in absence of sodium which was found to be inhibitory to [3H]TCP binding. The results demonstrated detectable levels of site 2 in brain membranes of guinea pig, rabbit, pig, mouse, sheep, and human but not in the rat or chicken. Using human cortical membranes, site 2 was the predominant binding site. Detailed studies conducted with human cortical tissue showed that high-affinity dopamine (1-[2- [bis(4-fluorophenyl)-methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR12909)], [1,2]benzo(b)thiophenylcyclo-hexylpiperidine (BTCP), and serotonin (fluoxetine) uptake inhibitors produced a wash-resistant inhibition of [3H]TCP binding to site 2, but not site 1. Preincubation of guinea pig brain membranes with BTCP was shown to produce an increase in the dissociation rate of [3H]TCP from PCP site 2. Structure activity studies with various uptake inhibitors showed that GBR12909, benztropine, fluoxetine, and BTCP have higher affinity for site 2 than for site 1. (+)-MK801, ketamine, and tiletamine were very selective for site 1, whereas dexoxadrol and TCP were moderately selective for site 1. These results suggest that human cortex possesses high-affinity PCP binding sites associated with biogenic reuptake binding sites, and that guinea pig brain, but not rat brain, may be an appropriate animal model for studying PCP site 2 in human brain.

Cerebral metabolic effects of sigma ligands in the rat

The 2-deoxy-D-[1-14C]glucose method was used to study the effects of sigma-type drugs, BMY 14802, (+)-pentazocine and BW 234U (rimcazole), on cerebral metabolism in 44 male Fischer 344 rats. Drug effects were observed in epithalamic, methathalamic, hypothalamic and mesencephalic regions, as well as in cranial nerve nuclei, the cerebellum, and the medulla oblongata. BMY 14802 and (+)-pentazocine increased local cerebral glucose utilization (LCGU) in areas that generally did not overlap; BW 234U administered 30 min before the radiotracer decreased LCGU. Most of the areas that showed changes in LCGU are known to contain sigma receptor sites. Differences in pharmacological properties, including effects on neuronal electrical excitability, may have resulted in the different distributions and effects of the drugs on LCGU. The present findings did not discriminate the compounds as agonists or antagonists for sigma receptor sites. However, the 2-deoxy-D-[1-14C]glucose method appeared useful in delineating the distribution of CNS responses to sigma drugs in the rat.

Solubilization and characterization of sigma-receptors from guinea pig brain membranes

The sigma-receptor, a distinct binding site in brain tissue that may mediate some of the psychotomimetic properties of benzomorphan opiates and phencyclidine, has been solubilized using the ionic detergent sodium cholate. Binding assays were performed with the solubilized receptor using vacuum filtration over polyethyleneimine-treated glass fiber filters. The pharmacological specificity of the solubilized binding site for sigma-receptor ligands is nearly identical to the membrane-bound form of the receptor, with the order of potencies for displacement of the selective sigma-ligand [3H]di-o-tolylguanidine ([3H]DTG) closely correlated. The stereoselectivity for (+)-benzomorphan opiate enantiomers was retained by the solubilized receptor. The soluble receptor retained high affinity for binding of [3H]DTG (KD = 28 +/- 0.5 nM) and (+)-[3H]3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)-[3H]3-PPP] (KD = 36 +/- 2 nM). Photoaffinity labeling of the solubilized receptor by [3H]p-azido-DTG, a sigma-selective photoaffinity label, resulted in labeling of a 29-kilodalton polypeptide identical in size to that labeled in intact membranes. Estimation of the Stokes radius of the [3H]DTG binding site was obtained by Sepharose CL-6B chromatography in the presence of 20 mM cholate and calculated to be 8.7 nm. This value was identical to the molecular size found for the binding sites of the sigma-selective ligands (+)-[3H]3-PPP and (+)-[3H]SKF-10,047, supporting the hypothesis that all three ligands bind to the same macromolecular complex.

Regulation of sigma-receptors: high- and low-affinity agonist states, GTP shifts, and up-regulation by rimcazole and 1,3-Di(2-tolyl)guanidine

The regulation of the central sigma-binding site was investigated using both in vitro and in vivo manipulations in conjunction with radioligand binding. The displacement of the binding of R(+)-[3H]3-[3-hydroxyphenyl]-N-(1-propyl)piperidine [R(+)-[3H]3-PPP] to cortical homogenates by a range of drugs was consistent with the site labelled being a sigma-receptor. (+)-SKF 10,047, (-)-SKF 10,047, (+/-)-cyclazocine, phencyclidine, and dexoxadrol displaced R(+)-[3H]3-PPP with pseudo-Hill coefficients of less than 1. Further analysis employing nonlinear curve fitting techniques demonstrated that displacement data for these compounds were described better by a model whereby R(+)-[3H]3-PPP was displaced from two discrete sites; approximately 65% of the total sites were in the high-affinity state. In the presence of 10 mM Mg2+ and 0.3 mM GTP, displacement curves for (+)-SKF 10,047 and (+/-)-cyclazocine were shifted to the right. These findings were due to the shift of some 15% of the high-affinity binding sites to a low-affinity state. Saturation experiments revealed that 0.3 mM GTP acted competitively to decrease the affinity of R(+)-[3H]3-PPP for the sigma sites. The sigma-binding site was thus likely to be linked to a guanine nucleotide regulatory (G) protein. Thus sigma drugs could be subdivided on the basis of their GTP sensitivity and pseudo-Hill coefficients, and by analogy with other receptors R(+)-3-PPP, (+)-SKF 10,047, and (+/-)-cyclazocine, may be putative sigma-agonists. 1,3-Di(2-tolyl)guanidine (DTG), rimcazole, and haloperidol displaced R(+)-[3H]3-PPP with pseudo-Hill coefficients of approximately unity and thus may be sigma-antagonists. Subchronic treatment with rimcazole was characterized by slight sedation and a concomitant up-regulation, with a decrease in the affinity, of sigma-binding sites. The schedule of rimcazole also increased dopamine turnover in the nucleus accumbens; both the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) and the DOPAC/dopamine ratio were elevated. DTG produced similar alterations to the binding parameters of the sigma-binding site; however, changes were not observed in general behavior or accumbal dopamine turnover. sigma-Receptors are likely to be linked to a G protein and are functionally involved in the CNS.

Phencyclidine and sigma receptors in rat spinal cord: binding characterization and quantitative autoradiography

These experiments were designed to compare phencyclidine (PCP) and sigma (sigma) receptor binding sites in the rat spinal cord by using receptor binding and autoradiographic techniques. Binding sites for 3H-TCP (3H-1-[1-(2-thienyl)cyclohexy]piperidine), a PCP receptor agonist, and (+)3H-3-PPP (3H-(+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine), a sigma receptor agonist, in the rat spinal cord were shown to represent two populations of recognition sites. Inhibition studies revealed that ligands with high affinity for the PCP receptor (MK-801 and PCP) were potent competitors at 3H-TCP binding sites whereas the putative sigma receptor ligands (+/-)pentazocine and haloperidol were potent competitors at (+)3H-3-PPP binding sites. The autoradiographic distribution of 3H-TCP and (+)3H-3-PPP binding sites in adjacent sections of rat spinal cord demonstrated the occurrence of two distinct populations of binding sites. 3H-TCP binding sites were localized primarily in laminae I and II in cervical and thoracic spinal segments. Binding sites in lamina I decreased in density along a rostral to caudal gradient in the spinal cord. The highest density of (+)3H-3-PPP binding sites was found in the ventral horn (lamina VIII and IX) and over perikarya in dorsal root ganglia. Significantly elevated densities of (+)3H-3-PPP binding sites were also found in lamina X within thoracic and lumbar segments and in the intermediolateral cell column. The results of the present study show that PCP and sigma receptor binding sites are differentially localized in the rat spinal cord and suggest that separate binding sites exist for PCP and sigma agonists.

Dextromethorphan protects against cerebral injury following transient focal ischemia in rabbits

We investigated dextromethorphan, both a dextrorotatory opioid derivative and a clinically tested N-methyl-D-aspartate (NMDA) receptor antagonist, in a rabbit model of transient focal cerebral ischemia. Fourteen rabbits were randomly assigned to treatment with a 20 mg/kg i.v. loading dose followed by a 10 mg/kg/hr infusion of 0.4% dextromethorphan in normal saline or with an equivalent volume of normal saline alone. One hour after treatment, the rabbits underwent a 1-hour occlusion of the left internal carotid and anterior cerebral arteries followed by 4 hours of reperfusion. The seven dextromethorphan-treated rabbits showed a significant decrease in the area of neocortical severe ischemic neuronal damage (10.5%) compared with the seven normal saline-treated controls (49.6%, p less than 0.001). The dextromethorphan-treated rabbits also demonstrated significantly smaller areas of cortical edema (10.2%) on magnetic resonance imaging than the controls (38.6%, p less than 0.01). Analysis of somatosensory evoked potentials revealed recovery of the ipsilateral amplitude to contralateral values within 5 minutes of reperfusion in the dextromethorphan-treated rabbits but not in the controls (p less than 0.01). In our rabbit model of transient focal cerebral ischemia, dextromethorphan appears to protect the brain against ischemic neuronal damage and edema, as well as to promote neurophysiologic recovery. This clinically available drug should be further investigated as having potential therapeutic value in the treatment of stroke.

Characterization of the rat mas oncogene and its high-level expression in the hippocampus and cerebral cortex of rat brain

The human mas oncogene was originally detected by its ability to transform NIH 3T3 cells. We previously showed that the protein encoded by this gene is unique among cellular oncogene products in that it has seven hydrophobic potential transmembrane domains and shares strong sequence similarity with a family of hormone-receptor proteins. We have now cloned the rat homolog of the mas oncogene, determined its DNA sequence, and examined its expression in various rat tissues. A comparison of the predicted sequences of the rat and human mas proteins shows that they are highly conserved, except in their hydrophilic amino-terminal domains. Our examination of the expression of mas, determined by RNA-protection studies, indicates that high levels of mas RNA transcripts are present in the hippocampus and cerebral cortex of the brain, but not in other neural regions or in other tissues. This pattern of expression and the similarity of mas protein to known receptor proteins suggest that mas encodes a receptor that is involved in the normal neurophysiology and/or development of specific neural tissues.

Solubilization of rat brain phencyclidine receptors in an active binding form that is sensitive to N-methyl-D-aspartate receptor ligands

Phencyclidine (PCP) receptors were successfully solubilized from rat forebrain membranes with 1% sodium cholate. Approximately 58% of the initial protein and 20-30% of the high-affinity PCP binding sites were solubilized. The high affinity toward PCP-like drugs, the stereo-selectivity of the sites, and the sensitivity to N-methyl-D-aspartate (NMDA) receptor ligands were preserved. Binding of the potent PCP receptor ligand N-[3H][1-(2-thienyl)cyclohexyl] piperidine ([3H]TCP) to the soluble receptors was saturable (KD = 35 nM), and PCP-like drugs inhibited [3H]TCP binding in a rank order of potency close to that observed for the membrane-bound receptors; the most potent inhibitors were TCP (Ki = 31 nM) and the anticonvulsant MK-801 (Ki = 50 nM). The NMDA receptor antagonist 2-amino-5-phosphonovaleric acid inhibited binding of [3H]TCP to the soluble receptors; glutamate or NMDA diminished this inhibition in a dose-dependent manner. Taken together, the results indicate that the soluble PCP receptor preparation contains the glutamate recognition sites and may represent a single receptor complex for PCP and NMDA, as suggested by electrophysiological data. The successful solubilization of the PCP receptors in an active binding form should now facilitate their purification.

Identification of the binding subunit of the sigma-type opiate receptor by photoaffinity labeling with 1-(4-azido-2-methyl[6-3H]phenyl)-3-(2-methyl[4,6-3H]phenyl)guanidine

The sigma-type opiate receptor is a distinct binding site in the brain that may mediate some of the psychotomimetic effects caused by benzomorphan opiates and phencyclidine in humans. We have developed a synthetic, highly selective ligand for this receptor, 1,3-di-o-tolylguanidine (DTG). To identify the binding protein(s) of the sigma receptor, we have now synthesized a radiolabeled azide derivative of DTG, 1-(4-azido-2-methyl[6-3H]phenyl)-3-(2-methyl[4,6-3H]phenyl)-guanidine ([3H]N3DTG). In guinea pig brain membrane binding assays conducted in the dark, [3H]N3DTG bound reversibly, selectively, and with high affinity (Kd = 10 nM) to sigma receptors. The drug specificity profile of reversible [3H]-N3DTG binding was identical to that of [3H]DTG and 3H-labeled (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine binding indicating that [3H]N3DTG is a selective sigma receptor ligand. Guinea pig brain membranes were photoaffinity-labeled with [3H]N3DTG. NaDodSO4/PAGE of detergent-solubilized membrane extract identified a single 29-kDa radioactive band. Sepharose Cl-6B gel chromatography of photolabeled brain membranes solubilized with the nondenaturing detergent sodium cholate showed a radioactive complex with a Stoke's radius of 4.6 nm (Mr, 150,000) that may represent the intact sigma receptor complex. NaDodSO4/PAGE of this complex showed that the radiolabeled material was a 29-kDa polypeptide that may be the binding subunit of the sigma receptor. The specific sigma receptor photoaffinity ligand described here should be a useful tool for purifying and characterizing the sigma receptor.

Visualization of growth factor receptor sites in rat forebrain

It is now known that various growth factors may also act in the central nervous system. Among them, it has recently been shown that epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I) may possess trophic effects in the mammalian brain. We report here on the respective autoradiographic distribution of [125I]EGF and [125I]IGF-I receptor binding sites in the rat brain, both during ontogeny and in adulthood. It appears that [125I]EGF sites are mostly found in the rat forebrain during brain development. On the other hand, [125I]IGF-I sites are more widely distributed both during ontogeny and in adulthood. These results reveal the plasticity of the expression of EGF and IGF-I receptor sites in the mammalian brain. This could be relevant for the respective role of these two growth factors in the development and maintenance of neuronal function.