Expression of a truncated receptor protein tyrosine phosphatase kappa in the brain of an adult transgenic mouse

Receptor protein tyrosine phosphatases (RPTPs) comprise a family of proteins that feature intracellular phosphatase domains and an ectodomain with putative ligand-binding motifs. Several RPTPs are expressed in the brain, including RPTP-kappa which participates in homophilic cell-cell interactions in vitro [Y.-P. Jiang, H. Wang, P. D'Eustachio, J.M. Musacchio, J. Schlessinger, J. Sap, Cloning and characterization of R-PTP-kappa, a new member of the receptor protein tyrosine phosphatase family with a proteolytically cleaved cellular adhesion molecule-like extracellular region, Mol. Cell. Biol. 13 (1993) 2942-2951; J. Sap, Y.-P. Jiang, D. Friedlander, M. Grumet, J. Schlessinger, Receptor tyrosine phosphatase R-PTP-kappa mediates homophilic binding, Mol. Cell. Biol. 14 (1994) 1-9]. The homology of RPTP-kappa's ectodomain to neural cell adhesion molecules indicates potential roles in developmental processes such as axonal growth and target recognition, as has been demonstrated for certain Drosophila RPTPs. The brain distribution of RPTP-kappa-expressing cells has not been determined, however. In a gene-trap mouse model with a beta-gal+neo (beta-geo) insertion in the endogenous RPTP-kappa gene, the consequent loss of RPTP-kappa's enzymatic activity does not produce any obvious phenotypic defects [W.C. Skarnes, J.E. Moss, S.M. Hurtley, R.S.P. Beddington, Capturing genes encoding membrane and secreted proteins important for mouse development, Proc. Natl. Acad. Sci. U.S.A. 92 (1995) 6592-6596]. Nevertheless, since the transgene's expression is driven by the endogenous RPTP-kappa promoter, distribution of the truncated RPTP-kappa/beta-geo fusion protein should reflect the regional and cellular expression of wild-type RPTP-kappa, and thus may identify sites where RPTP-kappa is important. Towards that goal, we have used this mouse model to map the distribution of the truncated RPTP-kappa/beta-geo fusion protein in the adult mouse brain using beta-galactosidase as a marker enzyme. Visualization of the beta-galactosidase activity revealed a non-random pattern of expression, and identified cells throughout the CNS that display RPTP-kappa promoter activity. Several neural systems highly expressed the transgene-most notably cortical, olfactory, hippocampal, hypothalamic, amygdaloid and visual structures. These well-characterized brain regions may provide a basis for future studies of RPTP-kappa function.

GGF/neuregulin is a neuronal signal that promotes the proliferation and survival and inhibits the differentiation of oligodendrocyte progenitors

We show that GGF/neuregulin is a mitogen for prooligodendrocytes (O4+/O1- cells), oligodendrocytes (O4+/O1+ cells), and type-2 astrocytes. Heregulin beta 1, another neuregulin isoform, is also mitogenic. The proliferative effect of glial growth factor (GGF) does not require, but is greatly potentiated by, serum factors. GGF also promotes the survival of pro-oligodendrocytes under serum-free conditions. High levels of GGF reversibly inhibit the differentiation and lineage commitment of oligodendrocyte progenitors and, in differentiated cultures, result in loss of O1 and myelin basic protein expression. All three erbB receptors are expressed by progenitors and are activated by GGF; the relative abundance of these receptors changes during differentiation. Finally, cortical neurons release a soluble mitogen for pro-oligodendrocytes that is specifically blocked by antibodies to GGF. These results implicate the neuregulins in the neuronal regulation of oligodendrocyte progenitor proliferation, survival, and differentiation.

Three forms of RPTP-beta are differentially expressed during gliogenesis in the developing rat brain and during glial cell differentiation in culture

In situ hybridization and Northern analysis demonstrate that the three splicing variants of RPTP-beta have different spatial and temporal patterns of expression in the developing brain. The 9.5-kb and 6.4-kb transcripts, which encode transmembrane protein tyrosine phosphatases with different extracellular domains, are predominantly expressed in glial progenitors located in the subventricular zone (SVZ). The 8.4-kb transcript, which encodes a secreted chondroitin sulfate proteoglycan (phosphacan), is expressed at high levels by more mature glia that have migrated out of the SVZ. The three transcripts are also differentially expressed in glial cell cultures; O2A progenitors express high levels of the 9.5- and 8.4-kb transcript, whereas type 1 astrocyte progenitors predominantly express the 6.4-kb transcript. C6 gliomas also express high levels of the 6.4-kb transcript. Treating C6 cells with the differentiating agent dibutyryl cyclic-AMP (DBcAMP), induces a decrease in the 6.4-kb transcript and a corresponding increase in the 8.4-kb transcript. O2A cells grown in the presence of basic fibroblast growth factor (bFGF) and platelet-derived growth factor (PDGF) remain highly proliferative and undifferentiated, and continue to express high levels of RPTP-beta. However, when O2A cells are grown in conditions that induce oligodendrocyte differentiation, there is a marked decrease in the expression of the transmembrane forms of RPTP-beta, as determined by immunofluorescence. These results demonstrate that RPTP-beta expression is regulated during glial cell differentiation and suggest that the different forms of RPTP-beta perform distinct functions during brain development.

Protein tyrosine kinase PYK2 involved in Ca(2+)-induced regulation of ion channel and MAP kinase functions

The protein tyrosine kinase PYK2, which is highly expressed in the central nervous system, is rapidly phosphorylated on tyrosine residues in response to various stimuli that elevate the intracellular calcium concentration, as well as by protein kinase C activation. Activation of PYK2 leads to modulation of ion channel function and activation of the MAP kinase signalling pathway. PYK2 activation may provide a mechanism for a variety of short- and long-term calcium-dependent signalling events in the nervous system.

Expression of receptor protein tyrosine phosphatase-sigma (RPTP-sigma) in the nervous system of the developing and adult rat

The expression of receptor protein tyrosine phosphatase-sigma (RPTP-sigma) mRNA during rat development was examined by Northern blot and in situ hybridization analyses. Northern blot analysis revealed that the two transcripts (5.7 kb and 6.9 kb) had different spatial and temporal patterns of expression. The 6.9-kb transcript was more abundant during embryonic development, whereas the 5.7-kb transcript was more abundant during postnatal development and in the adult. In situ hybridization revealed that RPTP-sigma mRNA was widely expressed throughout the central and peripheral nervous system during embryonic development. Very high levels were seen in the ventricular zone, subventricular zone, cortex, dorsal root ganglia, cranial nerve ganglia, olfactory epithelium, and retina. During postnatal development the level of expression decreased in most brain regions. However, high levels continued to be seen in the hippocampus. Emulsion autoradiography revealed that the majority of RPTP-sigma mRNA is expressed in neurons. Northern analysis showed that cultured glial cells expressed the 6.9-kb transcript, but not the 5.7-kb. RPTP-sigma mRNA expression profiles were clearly distinct from those of leukocyte antigen-related protein (LAR), a closely related RPTP. The spatiotemporal pattern of RPTP-sigma mRNA expression indicates that RPTP-sigma may play a role in the development of the nervous system.

Effects of haloperidol and reduced haloperidol on binding to sigma sites

The s.c. administration of a single dose of 0.1 mg/kg of reduced haloperidol to guinea pigs produced a marked inhibition of the binding of [3H]dextromethorphan and [3H]3-(3-hydroxyphenyl)-N-(n-propyl)piperidine ([3H](+)-3-PPP) to brain. The inhibition was still evident 10 days later, and it was accompanied by residual brain levels of reduced haloperidol, and much lower levels of haloperidol. Scatchard and computer-assisted analysis demonstrated that the inhibition was due to a reduction in the number of binding sites without changes in the affinity. In the rat, haloperidol and reduced haloperidol also produced a rapid inhibition of binding to sigma sites. Interestingly, the brain of the reduced haloperidol-treated rats contained both haloperidol and reduced haloperidol, but the levels of reduced haloperidol in the haloperidol-treated rats were undetectable. However, the inhibition observed was of comparable magnitude, indicating that the haloperidol remaining in the brain is also inhibitory. In vitro experiments showed that the inhibition produced by haloperidol and reduced haloperidol was apparently competitive, but when brain membranes were preincubated with either drug, the inhibition was noncompetitive. By contrast, the inhibition produced by dextromethorphan was always competitive. Moreover, the inhibition produced by haloperidol and reduced haloperidol could not be reversed by washing. This investigation strongly suggests that the inhibition observed after the administration of haloperidol or reduced haloperidol is not a classic agonist-induced receptor down-regulation. The results indicated that the inhibition produced is a complex phenomenon, and suggest the formation of a slowly reversible or irreversible complex with reduced haloperidol or haloperidol.

A novel receptor tyrosine phosphatase-sigma that is highly expressed in the nervous system

A novel transmembrane receptor protein tyrosine phosphatase-sigma (RPTP-sigma) was cloned from a rat brain stem cDNA library. The extracellular segment of one form of RPTP-sigma contains 824 amino acids and is composed of three immunoglobulin-like and five fibronectin type III (FNIII)-like repeats. The 627-amino acid cytoplasmic region of RPTP-sigma consists of two catalytic domains oriented in tandem. Northern blot analyses indicate that RPTP-sigma is highly expressed in the brain as two major transcripts of 5.7 and 6.9 kilobases (kb). The 5.7-kb transcript is expressed exclusively in the brain while the 6.9-kb species can be detected in the lung and heart, but at significantly lower levels. In situ hybridization studies confirm that RPTP-sigma is localized predominantly in the nervous system and can be detected in the rat as early as embryonic day 12. During embryonic development, RPTP-sigma is expressed extensively in the central and peripheral nervous systems, including the trigeminal and dorsal root ganglia as well as the retina. In adult rat brain, expression is restricted primarily to the olfactory tubercule, cerebellum, and hippocampus. Within the latter structure, RPTP-sigma is present in the pyramidal cell layer and granular layer of the dentate gyrus. Transfection of RPTP-sigma cDNA into human embryonic kidney 293 cells results in the synthesis of a protein with an apparent molecular mass of 200 kDa as detected by immunoprecipitation and immunoblot analyses using polyclonal antibodies against the FNIII-like repeats present in the extracellular domain of RPTP-sigma. The gene for RPTP-sigma has been mapped to distal chromosome 17 in the mouse.

The expression of a novel receptor-type tyrosine phosphatase suggests a role in morphogenesis and plasticity of the nervous system

Analysis of the localization of receptor-type protein tyrosine phosphatase-beta (RPTP-beta) by in situ hybridization and immunocytochemistry indicates that it is predominantly expressed in the developing central nervous system (CNS). RPTP-beta is highly expressed in radial glia and other forms of glial cells that play an important role during development. The immunoreactivity localizes to the radial processes of these cells, which act as guides during neuronal migration and axonal elongation. The pattern of RPTP-beta expression changes with the progression of glial cell differentiation. In the adult, high levels of RPTP-beta are seen in regions of the brain where there is continued neurogenesis and neurite outgrowth. The spatial and temporal patterns of RPTP-beta expression suggest that this receptor phosphatase plays a role in morphogenesis and plasticity of the nervous system.

Cloning and characterization of R-PTP-kappa, a new member of the receptor protein tyrosine phosphatase family with a proteolytically cleaved cellular adhesion molecule-like extracellular region

We describe a new member of the receptor protein tyrosine phosphatase family, R-PTP-kappa, cDNA cloning predicts that R-PTP-kappa is synthesized from a precursor protein of 1,457 amino acids. Its intracellular domain displays the classical tandemly repeated protein tyrosine phosphatase homology, separated from the transmembrane segment by an uncharacteristically large juxta-membrane region. The extracellular domain of the R-PTP-kappa precursor protein contains an immunoglobulin-like domain and four fibronectin type III-like repeats, preceded by a signal peptide and a region of about 150 amino acids with similarity to the Xenopus A5 antigen, a putative neuronal recognition molecule (S. Takagi, T. Hsrata, K. Agata, M. Mochii, G. Eguchi, and H. Fujisawa, Neuron 7:295-307, 1991). Antibodies directed against the intra- and extracellular domains reveal that the R-PTP-kappa precursor protein undergoes proteolytic processing, following which both cleavage products remain associated. By site-directed mutagenesis, the likely cleavage site was shown to be a consensus sequence for cleavage by the processing endopeptidase furin, located in the fourth fibronectin type III-like repeat. In situ hybridization analysis indicates that expression of R-PTP-kappa in the central nervous system is developmentally regulated, with highest expression seen in actively developing areas and, in the adult, in areas capable of developmental plasticity such as the hippocampal formation and cerebral cortex. The mouse R-PTP-kappa gene maps to chromosome 10, at approximately 21 centimorgans from the centromere.

High-affinity dextromethorphan and (+)-3-(-3-hydroxyphenyl)-N-(1-propyl)piperidine binding sites in rat brain. Allosteric effects of ropizine

Dextromethorphan (DM) binds to high- and low-affinity sites in the rat brain. The high-affinity DM binding is inhibited by nonnarcotic antitussives, opipramol and sigma ligands with nanomolar affinities. Computer-assisted modeling of homologous and heterologous competition studies between DM and (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)-3-PPP] were performed at pH 8.4. These experiments confirmed the existence of the common high-affinity DM1/sigma 1 site (R1) for which DM and (+)-3-PPP have Kd values of 20 and 10 nM, respectively. DM also binds to a second high-affinity site (R2, Kd, 20 nM) for which (+)-3-PPP has only micromolar affinity. Similarly, (+)-3-PPP binds to another high-affinity site (R3, Kd, 60 nM) for which DM has micromolar affinity. The common high-affinity DM1/sigma 1 site is allosterically modulated by the anticonvulsant ropizine, and is (+)-pentazocine sensitive, as is the homologous site in the guinea pig. However, in the rat the common DM1/sigma 1 site is 10 times smaller than in the guinea pig. This explains the apparently different effects of the allosteric modifiers in both species. The multiplicity of binding sites for DM and (+)-3-PPP resolved in this investigation will help to establish the physiological role and the pharmacological potential of the different sites. Meanwhile, the pharmacological effects of DM and sigma ligands cannot be summarily attributed to any particular binding site or receptor. This investigation also demonstrates that the use of multiple labeled and unlabeled ligands, combined with computer-assisted modeling, is essential to resolve multiple binding sites with similar affinities and to characterize the complex effects of allosteric modifiers.

Computer-assisted modeling of multiple dextromethorphan and sigma binding sites in guinea pig brain

Computer-assisted, simultaneous analysis of self- and cross-displacement experiments demonstrated the existence of several binding sites in guinea pig brain for dextromethorphan, (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP), and 1,3-di-o-tolyl guanidine (DTG). Dextromethorphan binds with high affinity to two sites (R1 Kd 50-83 and R2 Kd 8-19 nM) and with low affinity to two additional sites (R3 and R4). (+)-3-PPP binds to one high-affinity (R1 Kd 24-36 nM), to one intermediate-affinity (R3 Kd 210-320 nM), and to two (R2 and R4) low-affinity sites. DTG binds with almost identical high affinity to two different sites (R1 Kd 22-24 and R3 Kd 13-16 nM). These results confirm that dextromethorphan, (+)-3-PPP, and DTG bind to the common DM1/sigma 1 site (R1). The binding of DTG to two different sites with identical affinities precludes the use of this compound as a specific marker for sigma receptors. Besides, haloperidol displaces labeled ligands from both high-affinity DTG sites (R1 and R3) with high affinity. Thus, haloperidol sensitivity should not be used as the single criterion to identify a putative receptor. The resolution of these novel sites also may provide new insights into the multiple effects of antipsychotic drugs. In addition, this investigation has important implications regarding the methods that must be applied to characterize multiple binding sites and their relations with putative receptors.

SKF 525-A and cytochrome P-450 ligands inhibit with high affinity the binding of [3H]dextromethorphan and sigma ligands to guinea pig brain

The DM1/sigma 1 site binds dextromethorphan (DM) and sigma receptor ligands. The broad binding specificity of this site and its peculiar subcellular distribution prompted us to explore the possibility that this site is a member of the cytochrome P-450 superfamily of enzymes. We tested the effects of the liver microsomal monooxygenase inhibitor SKF 525-A (Proadifen), and other P-450 substrates on the binding of [3H]dextromethorphan, [3H]3-(-3-Hydroxyphenyl)-N-(1-propyl)piperidine and (+)-[3H]1,3-Di-o-tolyl-guanidine ([3H]DTG) to the guinea pig brain. SKF 525-A, l-lobeline and GBR-12909 inhibited the binding of the three labeled ligands with nM affinity. Each drug has identical nM Ki values for the high-affinity site labeled by the three ligands. This indicated that they displaced the labeled ligands from the common DM1/sigma 1 site. Debrisoquine and sparteine, prototypical substrates for liver debrisoquine 4-hydroxylase, displayed Ki values of 9-13 and 3-4 microM respectively against the three labeled ligands. These results, the broad specificity of the DM1/sigma 1 binding site, and its peculiar subcellular distribution, raises the possibility that this binding site is a member of the cytochrome P-450 superfamily of isozymes, rather than a neurotransmitter receptor. These findings may have important implications for the understanding of the therapeutic, side effects and toxicity of several neurotropic drugs.

The psychotomimetic effects of opiates and the sigma receptor

A critical review of the literature shows that the dysphoric and psychotomimetic side effects of sigma opiates reside in the levorotatory and not in the dextrorotatory or (+)-isomer, as currently believed. Nalorphine, levallorphan, (-)-pentazocine, (-)-3-hydroxy-N-propargylmorphinan, and MR 2034, all levorotatory opiates, produce dysphoria and psychotomimetic effects, whereas the dextrorotatory isomers of pentazocine and MR 2034 do not. Moreover, the dysphoria and psychotomimetic effects produced by racemic cyclazocine and MR 2034 are antagonized dramatically by naloxone, which is levorotatory and has no affinity for the sigma receptor as currently defined. The findings reviewed demonstrate that the psychotomimetic effects of sigma opiates are mediated by opiate receptors, the type of which has not been determined. The haloperidol sensitive sigma receptor, with higher affinity for the dextrorotatory isomers than for the sigma opiates, cannot mediate the psychotomimetic effects produced by levorotatory opiates. The conclusions derived from this review have profound implications, because a putative psychotomimetic receptor with the wrong stereospecificity will mislead future research, frustrate investigators, and confound the granting agencies.

Ropizine concurrently enhances and inhibits [3H]dextromethorphan binding to different structures of the guinea pig brain: autoradiographic evidence for multiple binding sites

Ropizine (10 microM) produces a simultaneous enhancement and inhibition of [3H]dextromethorphan (DM) high-affinity binding to different areas of the guinea pig brain. These results imply that there are two distinct types of high-affinity [3H]DM binding sites, which are present in variable proportions in different brain structures. The ropizine-enhanced [3H]DM binding type was preferentially inhibited by (+)-pentazocine. This is consistent with the presumption that the (+)-pentazocine-sensitive site is identical with the common site for DM and 3-(-3-Hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP). The second binding type, which is inhibited by ropizine and is not so sensitive to (+)-pentazocine, has not been fully characterized. This study demonstrates that the biphasic effects of ropizine are due, at least in part, to the effects of ropizine on two different types of [3H]DM binding sites. However, this study does not rule out that common DM/(+)-3-PPP site also might be inhibited by higher concentrations of ropizine.

Computer-assisted analysis of dextromethorphan and (+)-3-(-3-hydroxyphenyl)-N-(1-propyl)piperidine binding sites in rat brain. Allosteric effects of ropizine

Computer-assisted analysis of self- and cross-displacement studies between dextromethorphan (DM) and (+)-3-(3-hydroxyphenyl)-N-(1-propyl) piperidine ((+)-3-PPP) demonstrated in the rat brain the existence of two high-affinity and one low-affinity binding site for each ligand. One high-affinity site is the common DM1/sigma 1 site, the affinity of which is allosterically increased 4 to 5-fold by 10 microM ropizine. The Kd values of the DM1/sigma 1 for DM and (+)-3-PPP are 17 and 11 nM respectively. DM binds to the second high-affinity site (R2) with a Kd of 15 nM; this site has low affinity for (+)-3-PPP. Conversely, (+)-3-PPP binds with high affinity (Kd 53 nM) to another site (R3), that has low-affinity for DM. The Bmax of the common DM1/sigma 1 site in the rat is about ten times smaller than that in the guinea pig. Thus, extreme caution should be exercised in extrapolating from one species to another. Since DM and most sigma ligands bind to more than one site, not all of which are shared, it is important not to attribute the complex pharmacological effects of these ligands to a single hypothetical receptor.

Autoradiographic localization of [3H]dextromethorphan in guinea pig brain: allosteric enhancement by ropizine

Dextromethorphan (DM) is an antitussive with anticonvulsant activity that binds to high- and low-affinity sites in guinea pig brain homogenates. We examined the autoradiographic localization of [3H]DM using the anticonvulsant ropizine, an allosteric modifier that decreases the dissociation rate of [3H]DM. Competition studies demonstrated that the binding to brain sections was identical to that of brain homogenates [Craviso and Musacchio: Mol Pharmacol 23:629-640, 1983b]. Computer-assisted quantitative analysis of the autoradiographic images demonstrated that [3H]DM binds to discrete structures throughout the brain, but with higher density in the midbrain, pons, and medulla oblongata. The most intense labeling was observed in the rhabdoid, dorsal raphe, median raphe, caudal linear raphe nuclei, and cranial motor nerve nuclei. The central gray showed moderate to high-density labeling throughout its entire rostro-caudal extent, with very high binding in the dorsal tegmental nucleus and the locus coeruleus. Moderate and high binding was also seen in several hypothalamic structures. Distinct bands of moderate binding were seen in the pyramidal cell layer of the piriform cortex, the retrosplenial cortex, the granular cell layer of the dentate gyrus, the pyramidal cell layer of the hippocampus, and the Purkinje cell layer of the cerebellum. The striking similarity between the binding distribution of [3H]DM and sigma ligands, plus competition studies in brain homogenate, support the hypothesis that DM and sigma ligands share a common high-affinity binding site [Musacchio et al: Mol Pharmacol 35:1-5, 1989]. The distribution of [3H]DM binding provides possible anatomical substrates for both the antitussive and anticonvulsant actions of DM.

High affinity dextromethorphan binding sites in guinea pig brain. Effect of sigma ligands and other agents

Dextromethorphan (DM), a non-narcotic antitussive, binds in the guinea pig brain to specific high- and low-affinity sites with Kd values of 57 nM and 24 microM, respectively (Musacchio et al., 1988). The antitussives carbetapentane, caramiphen, butamirate and dimethoxanate competed with the high-affinity binding of [3H]DM at pH 7.4 with nanomolar Ki values. Sigma site ligands showed high affinity for [3H]DM binding sites. The rank order of potency was: haloperidol greater than (+)-pentazocine greater than (+)-cyclazocine greater than 3-(-3-hydroxyphenyl)-N-(1-propyl)piperidine greater than (+)-N-allylnormetazocine greater than (-)-butaclamol much greater than (+)-butaclamol (-)-N-allylnormetazocine. The antipsychotic perphenazine competed with low nanomolar Ki values, whereas rimcazole was weaker. The antidepressant opipramol and the benzomorphan (+)2'methoxyphenazocine were the most effective drugs tested, with Ki values of 0.4 nM. By contrast, MK-801 and phencyclidine hydrochloride were very weak competitors for [3H]DM binding. The diphenylalkylamines were the most effective competitors of the calcium channel blocking agents: prenylamine and cinnarizine had Ki values of 17 and 22 nM, respectively. Lidoflazine and hydroxyzine were slightly less potent, but nifedipine and the benzothiazepine diltiazem were much weaker. Potassium channel blockers inhibited DM binding in pharmacologically relevant concentrations: primaquine was the most effective with a Ki of 0.5 microM. Other antimalarial potassium channel blockers tested inhibited binding in the micromolar range. 4-Aminopyridine and tetraethylammonium had Ki values of 0.76 and 1.40 mM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of prototypic sigma ligands on the binding of [3H]dextromethorphan to guinea pig brain

We studied the effects of several prototypic sigma site ligands on the binding of [3H]dextromethorphan ([3H]DM) to guinea pig brain. Haloperidol, 3-(-3-Hydroxyphenyl)-N-(1-propyl)piperidine [+)-3-PPP) and (+)-N-allyl-N-normetazocine [+)-NANM or (+)-SKF10,047), which are potent sigma site ligands, showed high affinity for [3H]DM binding sites. The rank order of potency of sigma ligands, as indicated by the Ki values for the high-affinity sites is: haloperidol greater than (+)-pentazocine greater than (+)-cyclazocine greater than (+)-SKF10,047 greater than (-)-butaclamol much greater than (+)-butaclamol greater than (-)-SKF10,047. This rank order of potency is similar to that for the sites labeled with [3H](+)-3-PPP and [3H](+)-SKF10,047. The (+)-isomers of several benzomorphans displayed higher affinity than the (-)-isomers. (-)-Butaclamol competed against [3H]DM binding more effectively than the (+)-isomer, displaying the same stereospecificity shown for sigma sites. The findings reported here demonstrate that there are previously unrecognized similarities between DM and sigma sites. It is evident that further exploration of the DM, sigma and phencyclidine (PCP) sites will be necessary to establish the physiological role and therapeutic potential of these sites.

Effects of dextromethorphan site ligands and allosteric modifiers on the binding of (+)-[3H]3-(-3-hydroxyphenyl)-N-(1-propyl)piperidine

Equilibrium binding analysis demonstrated that (+)-[3H]3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)-[3H]3-PPP] binds in guinea pig brain homogenates to high and low affinity sites with Kd values of 25 nM and 0.9 microM, respectively. Competition studies with dextromethorphan (DM) site ligands and other drugs against (+)-[3H]3-PPP demonstrated that their Ki values and rank order of potency are identical to those found previously against [3H] DM. Most significant, ropizine produced a concentration-dependent increase in the binding of (+)-[3H]3-PPP, with an inhibitory component at high concentrations, as described previously for [3H]DM. Similarly, phenytoin increased the binding of (+)-[3H]3-PPP in the same fashion as that of [3H]DM. Computer-assisted analysis of equilibrium binding of (+)-[3H]3-PPP in the presence of 10 microM ropizine demonstrated that the binding increase produced is due to a 3-fold increase in the affinity for (+)-[3H]3-PPP. These results, and our previous finding that sigma ligands inhibit [3H] DM binding with a rank order of potency similar to that for sites labeled with (+)-[3H]3-PPP or (+)-[3H]SKF10,047 strongly suggest that sigma ligands bind to the high affinity DM site. These findings, and the inability of DM and other antitussives to produce psychotomimetic side effects, suggest that the high affinity DM sites can mediate only the nonpsychotomimetic effects of sigma ligands. However, further studies are necessary to determine the physiological role and therapeutic potential of the DM high affinity sites.

Dextromethorphan and sigma ligands: common sites but diverse effects

There is increasing evidence that sigma ligands and dextromethorphan (DM) bind to at least one common high-affinity site. DM and other antitussives do not produce psychotomimetic effects. This suggested that sigma ligands may produce their characteristic effects through another site, and prompted us to review critically the literature on the side effects of sigma opiates. Contrary to what is generally accepted, the dysphoric and psychotomimetic side effects of sigma opiates are mediated by the levo-and not by the dextrorotatory isomers. Moreover, these effects are unequivocally naloxone-reversible. Therefore, the current version of the "sigma receptor", with high affinity for the dextrorotatory sigma opiates, cannot explain the psychotomimetic effects of the levorotatory enantiomers. Thus, neither the "sigma ligands" nor its newly defined "receptor" are involved in the psychotomimetic effects of sigma opiates. Further experimentation with more selective drugs and with a combination of different methods will be necessary to identify the different binding sites, and to establish their physiological role and therapeutic potential.

High affinity dextromethorphan binding sites in guinea pig brain: further characterization and allosteric interactions

Dextromethorphan (DM), a non-narcotic antitussive with anticonvulsant activity, binds to high (Kd, 57 nM)- and low-affinity sites (Kd, 24 microM) in the guinea pig brain. This work, done at physiological pH, expands previous results obtained at pH 8.3. Phenytoin (PHT) produces an allosteric increase in the binding of [3H]DM, which is more marked than that reported previously: PHT (100 microM,) at pH 7.4, increased the binding affinity of [3H]DM to brain homogenates 4-fold, without changing the concentration of DM sites. Moreover, ropizine (SC-13504) an anticonvulsant benzhydryl piperazine, also produced a marked concentration-dependent increase in the binding of [3H]DM, which is mediated by a decrease in the dissociation rate of [3H]DM. Importantly, the effects of ropizine are fully apparent at 10 microM, 10-fold lower than those of PHT. The effects of PHT and ropizine show that the affinity of the DM sites can be increased by other ligands, suggesting that these sites are located on macromolecules that can exist in at least two conformational states. [3H]DM also binds to peripheral tissues, but the brain displays the highest affinity. Besides, the central and peripheral sites are different as determined by competition studies with caramiphen and carbetapentane, which are DM site ligands with antitussive and anticonvulsant activity. The results reported in this communication are consistent with the hypothesis that the high-affinity DM binding sites mediate antitussive and anticonvulsant activity when occupied by the appropriate ligand.

Caramiphen: a non-opioid antitussive with potent anticonvulsant properties in rats

The effect of the non-opioid antitussive caramiphen was studied in the rat maximal electroshock test. Caramiphen produced a dose- and time-dependent blockade of tonic hindlimb extension and was nearly twice as potent as the prototypical anticonvulsant drug diphenylhydantoin. Pretreatment with a subthreshold-effective dose of caramiphen potentiated the anticonvulsant effects of diphenylhydantoin, lowering its ED50 33-fold. The anticonvulsant effects of caramiphen were not associated with its cholinolytic activity since (a) its anticonvulsant effects were not antagonized by physostigmine and (b) the more potent cholinolytic atropine was only weakly effective against maximal electroshock convulsions when tested at doses 25 times the minimally effective dose of caramiphen. Anticonvulsant effects of caramiphen were associated with minimal behavioral impairment. The results demonstrate that caramiphen is a potent anticonvulsant against generalized convulsions and, like other non-opioid antitussives, will enhance the anticonvulsant properties of diphenylhydantoin. It is suggested that the anticonvulsant effects of caramiphen result from specific binding to brain receptors labelled by the non-opioid antitussive dextromethorphan, and that the interactions with diphenylhydantoin involve allosteric interactions between the different binding sites.

Dextromethorphan binding sites in the guinea pig brain

1. Dextromethorphan (DM), a dextrorotatory nonopioid antitussive, binds to specific high-affinity sites in the central nervous system. These sites are distinct from the opioid and other known neurotransmitter receptor sites. Antitussives such as carbetapentane and caramiphen also bind to DM sites with a nanomolar affinity. 2. The anticonvulsant drugs phenytoin and ropizine produce an allosteric enhancement of the binding of [3H]DM to guinea pig brain. DM, carbetapentane, and caramiphen also are efficacious anticonvulsant agents in the rat maximal electroshock seizures test, and DM enhances the anticonvulsant effects of phenytoin (PHT). 3. These results suggest that drugs that bind to the DM sites could be used alone as anticonvulsants or in combination with PHT to lower its effective dose and reduce its side effects. 4. The investigation of the DM binding sites may help to open new approaches for the treatment of convulsive disorders and to explain further some of the molecular mechanisms of neutronal excitability.

Cross-dependence to opioid and alpha 2-adrenergic receptor agonists in NG108-15 cells

Clonidine, a partial alpha 2-agonist, has been used empirically to alleviate opiate withdrawal symptoms, but the mechanism of its effects is not completely understood. We studied the interactions of opioid and adrenergic receptor agonists in the NG108-15 cells, which are a model of opiate dependence. We determined that in these cells the adenylate cyclase (AC) [EC 4.6.1.1; ATP pyrophosphate-lyase (cyclizing) overshoot response to opioid or alpha 2-agonist withdrawal can be significantly attenuated or suppressed by the other agonist. Subsequently, the AC overshoot response can be triggered with the antagonist to the second agonist to which the cells were not dependent. These results demonstrate that convergent dependence to morphine and alpha 2 agonists can occur in a homogeneous cell population without neuronal loops. Therefore, the basic mechanisms that can account for convergent dependence in this model take place at the level of intracellular regulatory pathways that do not require neuronal networks.

Naloxone elicits an excitatory response in the morphine-tolerant mouse vas deferens

The effects of naloxone on the electrically stimulated vas deferens from mice implanted with morphine (tolerant) or placebo (naive) pellets were studied. In tolerant vas deferens, naloxone produced an 86.6% increase of the twitch contractions when the preparations were stimulated with 15 V, while only a 12% increase was observed with supramaximal voltage (40 V). Naloxone had no effect at either voltage in preparations from naive animals. The effect of morphine in naive vas deferens stimulated with 15 V, was reversed by naloxone without further increase over baseline contractions. The results suggest that opioid receptors other than those located in the neuronal soma and/or coupled to adenylate cyclase may be involved in the development of dependence.

Allosteric modulation of dextromethorphan binding sites

The nonopioid antitussives dextromethorphan (DM), carbetapentane and caramiphen are efficacious anticonvulsant agents in the rat MES test. The findings presented strongly suggest the existence of a novel allosteric mechanism by which drugs acting at two different but interacting sites, exert their effects. This mechanism has marked similarities with the gamma-aminobutyric acid (GABA)-benzodiazepine interactions, even though their binding sites are different. The allosteric interactions of dextromethorphan and phenytoin in the binding assay and the potentiation of the anticonvulsant effects of phenytoin by dextromethorphan suggest that drugs that bind to the dextromethorphan sites could be used to reduce the effective dose of phenytoin and reduce its side effects, at least those which are not an extension of its specific pharmacological actions. It is evident that the investigation of the molecular mechanisms described may help to open new approaches to understand and treat convulsive disorders, to find novel anticonvulsant drugs and to further explain some of the molecular mechanisms of neuronal excitability.

N-ethylmaleimide blocks the modulatory effects of divalent cations and guanine nucleotides on the brain substance P receptor

The binding of [3H]physalaemin ([3H]PHY) to rat brain substance P receptors is modulated by cations and guanine nucleotides. [3H]PHY binding in the presence of either monovalent or divalent cations (125 mM Na2SO4 or 2.5 mM MnCl2) shows a KD of 5.9 and 5.5 nM and a Bmax of 44.4 and 63.9 fmol/mg protein respectively. In the presence of both, there is a 2-fold increase in the affinity (KD 2.8 nM) and a 25-80% increase in the Bmax (81.6 fmol/mg protein). Addition of 100 microM GTP or Gpp(NH)p in either 125 mM Na2SO4 or 2.5 mM MnCl2 or both decreases the Bmax by 25-55%. However, the receptor affinity for [3H]PHY is not significantly altered by guanine nucleotides. N-Ethylmaleimide (NEM) irreversibly inhibits the receptor binding with an IC50 of 1.0 mM, demonstrating that SH groups play a critical role in the interaction of the ligand with the receptor. If the SP receptors are protected with 1 microM PHY, NEM irreversibly inhibits the effect of divalent cations and guanine nucleotides. Analysis of [3H]PHY binding in 125 mM Na2SO4, 2.5 mM MnCl2 on membranes that were protected with 1 microM PHY and then preincubated with NEM demonstrates a variable decline in receptor number and a 2-fold decrease in the affinity (KD, from 2.8 to 6.9 nM). These observations indicate the existence of a second class of SH groups that are essential for the interaction of divalent cations and guanine nucleotides with the receptor. The blockade of the modulatory effects of divalent cations and guanine nucleotides by NEM treatment further suggests that brain SP receptors are coupled to a guanine nucleotide binding regulatory protein.

Dextromethorphan and carbetapentane: centrally acting non-opioid antitussive agents with novel anticonvulsant properties

The non-opioid antitussives dextromethorphan and carbetapentane, the active ingredients of several over-the-counter cough suppressants, provide a dose-related protection against maximal electroshock seizures in rats. Both drugs, which bind with high affinity to the same site in the brain, potentiated the effects of the prototypic antiepileptic drug diphenylhydantoin. We propose that these novel anticonvulsant drugs may represent potentially useful therapeutic agents for the treatment of some forms of epilepsy, either alone or in combination with existing antiepileptic drugs.

The adenylate cyclase rebound response to naloxone in the NG108-15 cells. Effects of etorphine and other opiates

The adenylate cyclase (AC) of the neuroblastoma-glioma hybrid cells (NG108-15), is generally considered to be a model for the study of the biochemical correlates of opiate tolerance and dependence. However, the naloxone-induced rebound response of adenylate cyclase, described in some recent reports, is much smaller than that originally described by Sharma, Klee and Nirenberg (1975). Possible explanations for these discrepancies are: (1) a marked down-regulation of opioid receptors and tolerance produced by the use of delta agonists or (2) the use of etorphine, a relatively hydrophobic drug which has slower dissociation rates than morphine. To test these possibilities, neuroblastoma-glioma hybrid cells were treated cells with morphine, etorphine, [D-Ala2,D-Leu5]enkephalin (DADLE), [D-Ala2]Leu-enkephalinamide (DALAMID) or vehicle. In addition, some of the cells treated with etorphine were washed with DADLE to replace the etorphine without producing the rebound response of adenylate cyclase prior to the addition of naloxone. The cells treated with morphine, DADLE and DALAMID, and incubated with prostaglandin E1 (PGE1) and naloxone showed a significant rebound of adenylate cyclase when compared with control groups and opiate-treated cells, incubated only with PGE1. In contrast, naloxone did not induce any significant rebound response in cells treated with etorphine unless they were previously washed with DADLE. These results demonstrate that the lack of a rebound response in cells treated with etorphine was due to the slow dissociation rates of the opiate and not to tolerance or to down-regulation of opioid receptors produced by agonists of high intrinsic activity.

Specific binding of [3H]substance P to the rat submaxillary gland. The effects of ions and guanine nucleotides

[3H]Substance P ([3H]SP), in a high ionic strength incubation medium, binds to a single class of saturable, noninteracting binding sites on rat submaxillary gland membranes with a KD = 2.8 +/- 0.34 nM and maximum binding (Bmax) = 220 +/- 31 fmol/mg of protein. The rank order of potency of various tachykinins, SP fragments and analogs to compete against [3H]SP is correlated with their potency to induce salivation. These findings indicate that, under the conditions described, [3H]SP binds to a physiologically relevant tachykinin receptor of the SP-P subtype. [3H]SP binding increases by 35% in the presence of optimal concentrations of Mn++ and Mg++ whereas guanine nucleotides reduce [3H]SP binding. The effect produced by either divalent cations or guanine nucleotides is due to increasing or decreasing the Bmax, respectively, without changing the affinity of [3H]SP. Guanine nucleotides reduce the Bmax of [3H]SP to the same level in the presence or absence of divalent cations, indicating that divalent cations increase the population of SP receptors that are sensitive to guanine nucleotides. In low ionic strength media, and when the nonspecific binding is defined by 1 microM SP, [3H]SP binds to two sites: a high affinity site with a KD of 0.14 nM and a Bmax of 370 fmol/mg of protein and a low affinity high capacity site. When the nonspecific binding is defined by 1 microM physalaemin, the high affinity is the only detectable site. However, in low ionic strength media, physalaemin has about one-fiftieth the potency of SP in competing with [3H]SP. These results prove that increasing the ionic strength of the media reduces the affinity of SP and some of its fragments and allows the determination of physiologically relevant SP-P binding sites.

Specific labeling of rat brain substance P receptor with [3H]physalaemin

The binding of [3H]physalaemin [( 3H]PHY) to rat brain membranes is specific, saturable and reversible in the presence of monovalent cations and peptidase inhibitors. Monovalent cations increase the binding of [3H]PHY in an ionic strength (mu)-dependent manner with an optimal effect at mu higher than 0.3. Addition of 2.5 mM MnCl2 results in a 2-fold increase in the affinity (KD) and a 40% increase in the maximal receptor density (Bmax). Scatchard analysis under these conditions indicates the existence of a single population of noninteracting sites with KD of 3.6 nM and a Bmax of 76 fmol/mg of protein. Substance P (SP) and physalaemin are equipotent in inhibiting the binding of [3H]PHY, whereas the potency of SP(2-11), SP(3-11), and SP(4-11) decreased in inverse proportion to their length. The relative affinity of the different tachykinins, SP, and SP fragments in competing with [3H]PHY correlates with their potency to stimulate several bioassay systems, indicating that [3H]PHY labels a physiologically relevant binding site that correspond to the SP-P tachykinin receptor. Guanine nucleotides completely abolish the increase in the binding of [3H]PHY produced by 2.5 mM MnCl2, but in its absence, the nucleotides reduce binding only by 15%. Guanine nucleotides reduce binding to the same level regardless of the presence or absence of the divalent cation. Regional distribution studies confirm that the density of SP receptors is maximal in the olfactory bulb, followed by the hypothalamus, striatum, hippocampus, cortex, and cerebellum.

Specific binding of [3H-Tyr8]physalaemin to rat submaxillary gland substance P receptor

[3H]Physalaemin [( 3H]PHY) binds to a single class of noninteracting sites on rat submaxillary gland membranes suspended in high ionic strength media with a KD of 2.7 nM, a Bmax of 240 fmol/mg of protein, and low nonspecific binding. The relative potencies of substance P (SP) and its fragments in competing with [3H]PHY correlate with their relative salivation potencies. This indicates that [3H]PHY interacts with a physiologically relevant SP receptor. In low ionic strength media, the KD of [3H]PHY does not change, but SP and some of its fragments are more potent than PHY in competing with [3H] PHY. Computer-assisted analysis of [3H]PHY and [3H]SP binding in high and low ionic strength media demonstrated that both peptides are equipotent in high ionic strength but that the affinity of SP increases by 70-fold in low ionic strength. The SP fragments that contain a basic residue in positions 1 and/or 3 also display an increased affinity in low ionic strength. These findings document that [3H]PHY binding in high ionic strength (mu = 0.6) accurately reflects the pharmacological potencies of agonists on the SP-P receptor. The binding of [3H]PHY, like that of [3H]SP, increases by the addition of divalent cations (Mg2+ greater than Ca2+ greater than Mn2+). Guanine nucleotides decrease [3H]PHY binding by decreasing the Bmax to the same level (160 fmol/mg of protein), in the presence or absence of Mg2+.

The effect of tolerance on opiate dependence as measured by the adenylate cyclase rebound response to naloxone in the NG108-15 model system

The naloxone-induced adenylate cyclase rebound response produced by opiates in the NG108-15 cells is quite apparent after 48 h treatment with M but it is undetectable after incubation with the high intrinsic activity agonist etorphine. Replacement of agonists by antagonists in receptors that have been rendered inactive by tolerance does not elicit the AC rebound response, which is considered a biochemical correlate of dependence. This indicates that marked tolerance and receptor down-regulation induced by prolonged treatment with high efficacy agonists may reduce the level of dependence.

Specific binding of a 125I-labeled substance P analog to rat submaxillary gland

The substance P (SP) analog [tyrosine1, norleucine11]-substance P [( Tyr1, Nle11]-SP) was iodinated by the chloramine-T method to yield the mono iodinated derivative; both peptides were biologically active on the guinea-pig ileum SP receptor. Saturable, reversible binding of [125l-Tyr1, Nle11]-SP to a single class of noninteracting sites on the rat submaxillary gland homogenate was demonstrated with an affinity of 9.26 +/- 0.8 nM and a maximum binding of 15.7 +/- 1.25 or 250 +/- 20 fmol/mg of protein. The relative potencies of SP and its fragments SP(2-11) and SP(4-11), as measured by their IC50, are in agreement with their rank order in the salivation assay, whereas physalaemin was found apparently weaker than expected. However, their rank order in displacing [125I-Tyr1, Nle11]-SP was identical to their rank order in displacing [125I-Bolton-Hunter]-SP bound to mouse mesencephalic cells in primary culture.

Failure of opiates to increase the hydrolysis of GTP in neuroblastoma-glioma 108-15 cells

It has been repeatedly demonstrated that the neuroblastoma-glioma (NG 108-15) cell line has opiate receptors that inhibit adenylate cyclase and it has been proposed that this inhibition is mediated by a naloxone reversible stimulation of a low Km GTPase (Koski and Klee, Proc. Natl. Acad. Sci. 78:4185, 1981). The guanine nucleotides of NG cells were labeled with [3H]guanine followed by incubation with 10(-6)M guanine. Etorphine (10(-6)M) or vehicle were added and the incubations continued for 1-4 min. The reaction was stopped with 5 percent TCA containing nucleotides as carriers and markers for the HPLC. Marker nucleotides were detected at 254 nm and the labeled nucleotides by liquid scintillation spectrometry. In several experiments, etorphine failed to produce any measurable change in the labeled nucleotides or in the GTP/GDP ratios. To verify that the opiate receptors were functional we measured its capacity to inhibit the formation of cAMP induced by PGE1. We also studied the effects of naloxone and PGE1 on the formation of cAMP in opiate tolerant cells. Tolerant cells responded to naloxone with a 50 percent increase in cAMP, indicating again that the opiate receptors were functional. Our results are consistent with the idea that in intact NG108-15 cells the opiate-mediated hydrolysis of GTP observed in cell membrane preparations is of very small magnitude.

High-affinity dextromethorphan binding sites in guinea pig brain. I. Initial characterization

Tritiated dextromethorphan ([3H]DM) binds to two distinct sites in guinea pig brain, a high-affinity site (Kd = 13-20 nM) and a low-affinity site (Kd greater than 200 nM). Binding of [3H] DM to the high-affinity site is rapid, reversible, saturable, proportional to tissue concentration, and pH-dependent. The sites have a protein-like component, since preincubating brain homogenate in the presence of proteolytic enzymes and protein-modifying reagents significantly reduces binding. There is also a progressive loss of binding when brain homogenate is heated to temperatures in excess of 37 degrees. Millimolar concentrations of lithium, calcium, magnesium, and manganese decrease DM binding while sodium, in concentrations as high as 100 mM, has little effect; calcium in micromolar concentrations slightly enhances binding. The pons-medulla and cerebellum contain the highest density of sites. Subcellular localization studies have shown that high-affinity sites are confined almost exclusively to the microsomal fraction. Binding of DM to brain microsomes does not appear to be related to drug-metabolizing enzymes. The characteristics of DM binding suggest that DM sites are not a subclass of opiate receptors. Studies using tritiated dextrorphan as radioligand failed to reveal a high-affinity binding site for in brain.

The determination of dissociation constants for substance P and substance P analogues in the guinea pig ileum by pharmacological procedures

The dissociation constants (Kd values) of substance P (SP), physalaemin, kassinin, and SP analogues acting on SP receptors in guinea pig ileal longitudinal muscle strips were determined by the pharmacological procedures of Furchgott [Adv. Drug Res. 3:21-55 (1966)]. This method involves analysis of the concentration-response data before and after fractional inactivation of receptors with phenoxybenzamine (2 X 10(-5) M). Estimations of the Kd values for SP were similar when phenoxybenzamine was incubated for 10, 13, or 15 min. Coincubation with high concentrations of SP protected against receptor inactivation with phenoxybenzamine, but bradykinin and serotonin did not cross-protect SP receptors. Kd values for SP were similar when trypsin was substituted for phenoxybenzamine [Kd = 8.1 +/- 4 nM (n = 9) versus 10 +/- 6 nM (n = 5)]. In atropinized preparations the Kd values obtained for physalaemin were similar to those obtained for untreated preparations [Kd = 8.0 +/- 3.6 nM (n = 5) and 12.6 +/- 3 nM (n = 4), respectively]. The effects of phenoxybenzamine on concentration-response curves for kassinin showed greater shifts to the right with phenoxybenzamine. This indicated that kassinin may interact with another population of receptors, in addition to the sites that SP and other analogues bind. A direct correlation was found between EC50 values and Kd values and Kd values for SP and SP analogues. It was estimated that, for SP, a 20% receptor occupancy is required to elicit a 50% response.

High-affinity dextromethorphan binding sites in guinea pig brain. II. Competition experiments

Binding of dextromethorphan (DM) to guinea pig brain is stereoselective, since levomethorphan is 20 times weaker than DM in competing for DM sites. In general, opiate agonists and antagonists as well as their corresponding dextrorotatory isomers are weak competitors for tritiated dextromethorphan ([3H]DM) binding sites and display IC50 values in the micromolar range. In contrast, several non-narcotic, centrally acting antitussives are inhibitory in the nanomolar range (IC50 values for caramiphen, carbetapentane, dimethoxanate, and pipazethate are 25 nM, 9 nM, 41 nM, and 190 nM, respectively). Other antitussives, such as levopropoxyphene, chlophedianol, and fominoben, have poor affinity for DM sites whereas the antitussive noscapine enhances DM binding by increasing the affinity of DM for its central binding sites. Additional competition studies indicate that there is no correlation of DM binding with any of the known or putative neurotransmitters in the central nervous system. DM binding is also not related to tricyclic antidepressant binding sites or biogenic amine uptake sites. However, certain phenothiazine neuroleptics and typical and atypical antidepressants inhibit binding with IC50 values in the nanomolar range. Moreover, the anticonvulsant drug diphenylhydantoin enhances DM binding in a manner similar to that of noscapine. Preliminary experiments utilizing acid extracts of brain have not demonstrated the presence of an endogenous ligand for DM sites. The binding characteristics of DM sites studied in rat and mouse brain indicate that the relative potencies of several antitussives to inhibit specific DM binding vary according to species. High-affinity, saturable, and stereoselective [3H]DM binding sites are present in liver homogenates, but several differences have been found for these peripheral binding sites and those described for brain. Although the nature of central DM binding sites is not known, the potent interaction of several classes of centrally acting antitussives with DM sites suggests that they may be related to the mechanism of action of this drug.

Processing of enkephalin precursors by chromaffin granule enzymes

Subcellular localization studies indicate that the enzyme activities which cleave enkephalins from larger polypeptides are located in both membranous and soluble components of the chromaffin granules and not in the lysosomes. Cleavage of endogenous precursors produced methionine enkephalin [Met-E], leucine enkephalin [Leu-E], and Met-E-Arg6. Cleavage of synthetic peptide E produced Leu-E, Met-E, and Met-E-Arg6. The pH optimum for enkephalin production is pH 5.7. Dithiothreitol prevents the inhibition of enkephalin conversion produced by p-chloromecurobenzoate. Studies with peptide E indicate that cleavage appears to occur at pairs of basic amino acid residues. The presence of enkephalin producing enzymes with the precursors and the products in the chromaffin granules could be important in the elucidation of the factors that regulate enkephalin biosynthesis.

Hydrolysis of substance P and bradykinin by black widow spider venom gland extract

Black widow spider venom gland extract was found to contain significant peptidase activity. Aliquots of the venom gland extract incubated at 37 degrees inactivated substance P (SP) and bradykinin but not angiotensin II or the enkephalins. The peptide inactivation was proportional to the duration of the incubation and the amount of extract used. Analysis of the peptides on high pressure liquid chromatography demonstrated that the loss in biological activity of SP and bradykinin in the longitudinal muscle of the guinea pig ileum was correlated with cleavage of the peptides into several fragments. Kinetic studies revealed that SP was initially split into two fragments but that these products underwent further degradation into smaller peptides. The optimal pH for the peptidase activity was 6.5. At 0 degree the enzymatic activity was undetectable, and it was irreversibly destroyed by incubation at 100 degrees for 5 min or by pretreatment of the extract with 100 microM diisopropyl fluorophosphate. In addition, the gland extract preparation hydrolyzed artificial substrates designed to detect trypsin or chymotrypsin-like activity.

Fading and tachyphylaxis to the contractile effects of substance P in the guinea-pig ileum

Substance P (SP) caused an immediate and vigorous contraction of the longitudinal smooth muscle layer of the guinea-pig ileum. The contractile response to SP, unlike that to acetylcholine or histamine was not maintained but faded to baseline levels in about 6 min. When 0.3-1.0 nM SP was added the fading time was shorter than 6 min and tachyphylaxis did not develop. Higher concentrations of SP produced fading times of about 6 min that could not be increased even by adding extremely high concentrations of the peptide, up to 1800 nM. Short fading times and the lack of development of tachyphylaxis are the result of the rapid adsorption and/or metabolism of SP. The addition of exogenous peptidases such as pronase, chymotrypsin and an extract of black widow spider venom gland dramatically increased the rate of degradation of SP, shortened the fading response and blocked the development of tachyphylaxis. Tetrodotoxin and atropine reduced the fading time by 25%, while eserine increased its duration several-fold; these findings are consistent with the existence of a cholinergic nerve component in the mediation of some of the effects of SP receptor and, in part, to adsorption and metabolism of the peptide. The magnitude of the tachyphylaxis to SP was proportional to the concentration of the desensitizing dose of the peptide and was specific to SP and to the related peptide physalaemin; no cross-tachyphylaxis towards other agents was found.

Naloxone reversal of insulin-induced hypotension in reserpine pretreated rats

The administration of insulin caused a gradual lowering of systolic and diastolic blood pressure in anaesthetized reserpinized rats. Injection of 1 mg/kg naloxone at the peak of the hypotension resulted in immediate restoration of blood pressure to pre-insulin control values. The recovery of the arterial blood pressure caused by naloxone lasted for 5 to 10 min and was entirely dependent on the reserpine pretreatment. The lowering of the blood pressure caused by insulin and the increase in systemic blood pressure after naloxone were of about the same magnitude in rats with bilateral denervation of the adrenal glands as in sham operated rats. It is concluded that in anaesthetized reserpinized rats, hypoglycemia causes the release of opiate-like material that mediates a hypotensive response. The origin, nature and site of action of this opioid activity is as yet not established, but does not appear to derive from the adrenal gland.

Electrically induced opiate-like inhibition of the guinea-pig ileum: cross-tolerance to morphine

The guinea-pig myenteric plexus-longitudinal muscle (MPLM) preparation electrically stimulated at 0.1 Hz is very sensitive to the inhibitory effects of opiates. We used this preparation to detect an inhibitory response (IR) which was produced by electrical stimulation at 5 to 20 Hz. The magnitude and duration of the IR are determined by the parameters of the stimulation, mainly by the frequency and duration of the period of stimulation. Maximal IR is obtained with symmetrical biphasic stimuli of 2 msec duration and supramaximal voltage at 20 Hz applied for a period of about 5 min. The IR is calcium-dependent, cannot be attenuated by washing and is mediated by several components. About 55 to 70% of the IR can be reversed by specific narcotic antagonists and therefore it is considered to be produced by the release of endorphins. There are at least two additional components, one small, adrenergic in nature, and a third one which has not been identified. The offset rate of the IR is measured in minutes, while enkephalin and human beta-endorphin have half-lives of 10 and 85 sec, respectively, after washing. This suggests that the endorphin that mediates the opiate component of the IR may be a different one with slower offset rates. Myenteric plexus-longitudinal muscle strips obtained from guinea pigs which were made tolerant to morphine by subcutaneous implantation of three pellets were cross-tolerant to the opiate component of the IR elicited by electrical stimulation.