1,2,5-Thiadiazole derivatives of arecoline stimulate M1 receptors coupled to phosphoinositide turnover

A series of alkoxy-1,2,5-thiadiazole derivatives of arecoline was synthesized in an effort to develop M1 muscarinic agonists. The 3-butenyloxy, 2-butynyloxy, cyclopropylmethyloxy, and hexyloxy derivatives stimulated phosphoinositide turnover through muscarinic receptors in the rat hippocampus. The dose-response curves of 2-butynyloxy, cyclopropylmethyloxy and hexyloxy compound together was the same as the response of each separately. Pirenzepine was somewhat more potent than AF-DX 116 for inhibiting the responses produced by low concentrations of thiadiazole derivatives. The data suggest that the cyclopropylmethyloxy-TZTP derivative is functionally a selective M1 agonist. Molecular mechanics calculations indicate that the anti form of the 1,2,5-thiadiazole derivatives of arecoline may be active at M1 receptors.

Regulation of the phosphoinositide cascade by polyamines in brain

The endogenous polyamines spermidine and spermine enhanced guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S)-stimulated phosphoinositide turnover with EC50 values of 100 +/- 30 and 50 +/- 15 microM, respectively, whereas the synthetic polyamines N,N'-bis(3-aminopropyl)-1,3-propanediamine and -ethylenediamine inhibited GTP-gamma-S-stimulated phosphoinositide turnover, with maximal inhibition at 1 mM. Kinetic analysis of GTP-gamma-S-stimulated phosphoinositide turnover in the absence and presence of spermidine showed that the Km for GTP-gamma-S was not changed (1,303 +/- 270 and 1,069 +/- 214 nM, respectively), whereas the Vmax was increased by 206% (1,566 +/- 141 and 4,792 +/- 84 cpm, respectively), indicating that spermidine and GTP-gamma-S acted at different sites. Spermidine also enhanced Ca(2+)-stimulated phosphoinositide turnover in the absence of GTP-gamma-S by decreasing the Ca2+ requirement of the phosphoinositide-specific phospholipase C. Arcaine and agmatine, polyamine antagonists at the NMDA receptor complex, did not block the effects of spermidine on GTP-gamma-S- and Ca(2+)-induced phosphoinositide turnover, suggesting that the spermidine effects are not mediated through these specific polyamine sites. Furthermore, spermidine increased the level of [3H]phosphatidylinositol 4-phosphate (EC50 = 120 +/- 10 microM), without affecting significantly the levels of [3H]-phosphatidylinositol and [3H]phosphatidylinositol 4,5-bis-phosphate. Collectively these data indicate that the enhanced phosphoinositide turnover induced by spermidine in the presence of GTP-gamma-S or Ca2+ is mediated through multiple levels of the phosphoinositide turnover cascade.

Steric and electronic requirements for muscarinic receptor-stimulated phosphoinositide turnover in the CNS in a series of arecoline bioisosteres

A series of arecoline derivatives was utilized to assess steric and electronic effects important for activating muscarinic receptors in the CNS. Arecoline derivatives in which the methyl ester moiety was replaced by hexyloxy-1,2,5-oxadiazole (2b), hexyloxythiophene (3b) or hexyloxypyrazine (4b) were compared with the hexyloxy-1,2,5-thiadiazole compound (1b) (Hexyloxy-TZTP), known from previous work to be active as an M1/M3 partial agonist. MNDO calculations showed that the N-S bonds of the alkoxythiadiazole ring were highly polarized with the ability to form H-bonds to the N's. On the other hand, the smaller oxadiazole had lower polarities in the N-O bonds and reduced ability to form H-bonds, the thiophene was of comparable size to the thiadiazole and had large C-S bond polarities without the H-bond capability and the pyrazine had limited ability to form H-bonds. The compounds were compared with respect to their abilities to stimulate phosphoinositide (Pl) turnover in the hippocampus of the rat brain. 1b was more active than 2b-4b for stimulating the Pl turnover response. The data suggest that the ability to form H-bonds is an important factor for the ability of 1 to stimulate M1 muscarinic receptors in the CNS.

Stereoselective binding and activity of oxotremorine analogs at muscarinic receptors in rat brain

The activities of the enantiomers of BM-5 were examined to measure muscarinic cholinergic selectivity in the central nervous system. Autoradiographic studies assessed the ability of each enantiomer to inhibit the binding of [3H]-(R)-quinuclidinyl benzilate ([3H]-(R)-QNB) to muscarinic receptors in the rat brain. (+)-(R)-BM-5 inhibited [3H]-(R)-QNB binding to rat brain sections at concentrations below 1.0 microM, while 100-fold higher concentrations of (-)-(S)-BM-5 were required for comparable levels of inhibition. Analysis of the autoradiograms indicated that both stereoisomers had a similar distribution of high affinity binding sites. Each enantiomer displayed higher affinity for muscarinic receptors in the superior colliculi and lower affinity for receptors in the cerebral cortex and hippocampus. (+)-(R)-BM-5 and oxotremorine inhibited adenylyl cyclase activity in the cerebral cortex with efficacies comparable to that for acetylcholine. (+)-(R)-BM-5 was 26-fold more potent than (-)-(S)-BM-5 in inhibiting adenylyl cyclase. Oxotremorine-M and carbamylcholine stimulated phosphoinositide turnover in the cerebral cortex. Oxotremorine had lower activity and (+)-(R)-BM-5 was essentially inactive at comparable concentrations. The difference in activity of the two enantiomers indicates a remarkable stereochemical selectivity for muscarinic receptors. The stereoselectivity index is comparable for both the autoradiographic assays (48) and measures of adenylyl cyclase activity (26) in the cerebral cortex.

Inhibition of carbachol-stimulated phosphoinositide turnover by U-50,488H in rat hippocampus--involvement of GTP-binding protein

The effect of U-50,488H, a selective kappa-opioid agonist, on carbachol-stimulated phosphoinositide (PI) turnover response in rat hippocampal slices was examined. U-50,488H which stimulates PI turnover response in this preparation (Periyasamy and Hoss, 1990, Life Sci. 47, 219), inhibited carbachol-stimulated PI turnover in a concentration-dependent manner with an IC50 value of 33 +/- 9.0 microM. The inhibitory effect of U-50,488H was not blocked by the kappa-selective antagonists, e.g., nor-binaltorphimine (10 microM), and MR2266 (10 microM), or tetrodotoxin (1 microM) suggesting that the effect of U-50,488H was mediated neither through the kappa-receptors nor through the release of an endogenous neurotransmitter(s). A Lineweaver-Burke plot of the stimulation of PI turnover by carbachol in the presence and absence of U-50,488H showed that the Km was not changed (11.4 +/- 3.4 and 11.5 +/- 2.6 microM) whereas the Vmax was reduced from 3849 +/- 460 to 1534 +/- 31 cpm indicating that the inhibition was non-competitive. U-50,488H also inhibited guanosine 5'-[beta, gamma-imido]triphosphate (Gpp[NH]p)-stimulated PI turnover in rat hippocampal membranes in a concentration-dependent manner with an IC50 value of 33 +/- 12 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical and behavioral responses of pilocarpine at muscarinic receptor subtypes in the CNS. Comparison with receptor binding and low-energy conformations

Pilocarpine was tested biochemically in vitro for its ability to stimulate phosphoinositide (PI) turnover in the hippocampus (M1/M3 responses) where it displayed 35% of the maximal carbachol response with an EC50 value of 18 microM, and low-Km GTPase in the cortex (M2 response), where it had 50% of the maximal carbachol response with an EC50 value of 4.5 microM. Behaviorally, pilocarpine was able to restore deficits in a representational memory task (sensitive to M1 antagonists) produced by intrahippocampal injections of AF64A. Twenty-three low-energy conformations of protonated pilocarpine were generated using the program MacroModel. The data indicate that pilocarpine is a partial agonist at both M1 and M2 muscarinic receptors in the CNS. Behaviorally, with respect to the memory task, M1 effects of pilocarpine apparently predominate. It also is conceivable that different conformations of pilocarpine are active as agonists at different muscarinic receptor subtypes.

Biochemical and behavioral evidence for muscarinic autoreceptors in the CNS

Muscarinic autoreceptors of the M2 subclass were examined in rat forebrain using a number of different methodologies, including receptor autoradiography and image analysis, regulation of acetylcholine release, phosphoinositide turnover, low-Km GTP hydrolysis, and behavioral analysis. The relatively minor population of M2 receptors in coronal sections was visualized by autoradiography and image analysis using [3H]quinuclidinyl benzilate in the presence of a concentration of pirenzepine that blocked most of M1 (and M4) receptors. The highest densities of M2 receptors in forebrain regions were found in the outer layers of the cortex, CA1 region of the hippocampus and striatum. The M2-, but not M1-selective antagonists were able to block the oxotremorine-induced attenuation of acetylcholine release in forebrain synaptosomes. Low concentrations of the M2-selective antagonist gallamine increased phosphoinositide turnover, which is thought to be an M1 postsynaptic response in the forebrain, in brain slices by a Ca2(+)-dependent mechanism. The M2-selective agonist oxotremorine produced a substantial stimulation of low-Km GTPase in cortical membranes, suggesting that M2 forebrain receptors are efficiently coupled to G-proteins in the cortex. Behavioral signs of cholinergic stimulation were observed after intracerebroventricular injections of M2-, but not M1-selective antagonists. It is suggested that a minor population of forebrain M2 receptors regulates acetylcholine release by a mechanism that includes coupling through G-proteins presynaptically at synapses for which the postsynaptic response involves phosphoinositide turnover. Selective blockade of these receptors produces both biochemical and behavioral signs of acetylcholine release.

Differential coupling between muscarinic receptors and G-proteins in regions of the rat brain

The coupling of muscarinic receptors to G-proteins in various regions of the rat brain was assessed by measuring carbachol-stimulated, low-Km GTPase. The inhibition of carbachol-stimulated GTPase by the M1-selective antagonist pirenzepine was compared to the affinity of pirenzepine for various nuclei within the regions as measured autoradiographically. The rank order of potency of carbachol for stimulating GTPase in various brain regions was similar to that for binding to receptors in those areas. The maximal specific activity (efficacy) of carbachol-stimulated GTPase varied independently of the distribution of total receptors or receptor subtypes. The overall potency of pirenzepine for inhibiting carbachol-stimulated GTPase was not correlated with the overall affinity of pirenzepine for muscarinic receptors in the regions. Comparing results in various brain regions, the data suggest that there are differences in the efficiency of coupling between muscarinic receptors and G-proteins. For example, the pons-medulla appeared to have a small population of pirenzepine-sensitive (M1 or M4) receptors that were coupled very efficiently to G-proteins, whereas in the hippocampus all muscarinic receptors, most of which are pirenzepine-sensitive, appeared to be weakly coupled to G-proteins. It is suggested that variable interactions between receptors and G-proteins may be an important factor in the overall coupling between receptor occupancy and cellular responses to acetylcholine as well as other hormones and transmitters.

Subtype specificity of the muscarinic receptor-stimulated GTPase response in the rat cortex

Subtype selective agonists and antagonists were used to examine the pharmacology of the low-Km GTPase response in the rat cortex. The M2-selective agonist oxotremorine, which is a weak partial agonist for M1 receptors in the brain, stimulated low Km GTPase activity with an EC50 value of 1.0 microM. Oxotremorine was nearly a full agonist and demonstrated no partial agonist activity in the presence of optimal concentrations of the agonists carbachol or oxotremorine-M, which stimulate all muscarinic responses. These data suggest that the GTPase response is associated with M2 receptors. Pirenzepine (M1-selective) and AF-DX 116 (M2-selective) inhibited oxotremorine-stimulated GTPase activities in cortex with IC50 values of 4.0 and 2.2 microM, respectively. Since pirenzepine is substantially more potent than AF-DX 116 for binding muscarinic receptors in the cortex (Messer et al., J. Med. Chem., 32 (1989) 1164), M2 receptors contribute more to the GTPase response than M1 receptors.

Kappa opioid receptors stimulate phosphoinositide turnover in rat brain

The effects of various subtype-selective opioid agonists and antagonists on the phosphoinositide (PI) turnover response were investigated in the rat brain. The kappa-agonists U-50,488H and ketocyclazocine produced a concentration-dependent increase in the accumulation of IP's in hippocampal slices. The other kappa-agonists Dynorphin-A (1-13) amide, and its protected analog D[Ala]2-dynorphin-A (1-13) amide also produced a significant increase in the formation of [3H]-IP's, whereas the mu-selective agonists [D-Ala2-N-Me-Phe4-Gly5-ol]-enkephalin and morphine and the delta-selective agonist [D-Pen2,5]-enkephalin were ineffective. The increase in IP's formation elicited by U-50,488H was partially antagonized by naloxone and more completely antagonized by the kappa-selective antagonists nor-binaltorphimine and MR 2266. The formation of IP's induced by U-50,488H varies with the regions of the brain used, being highest in hippocampus and amygdala, and lowest in striatum and pons-medulla. The results indicate that brain kappa- but neither mu- nor delta-receptors are coupled to the PI turnover response.

Identification of four brain areas each enriched in a unique muscarinic receptor subtype

The affinities of muscarinic agonists and antagonists were determined by autoradiography and image analysis in selected areas of the rat brain. IC50 values and Hill coefficients for the inhibition of the binding of 0.2 nM [3H]-QNB to dentate gyrus, superior colliculus, rhomboid thalamus and substantia nigra were measured in coronal sections. Pirenzepine displayed a high affinity for receptors in the dentate gyrus and AF-DX 116, the superior colliculus. Both pirenzepine and AF-DX 116 had high affinities for the substantia nigra and low affinities for the rhomboid thalamus. Gallamine displayed a 50-fold preference for superior colliculus over dentate gyrus receptors. Amitriptyline was less selective, showing a modest preference for substantia nigra receptors and 4-DAMP was essentially nonselective. Carbachol was the most selective agonist with a 4000-fold preference for superior colliculus over dentate gyrus receptors. Other agonists except RS 86 were also selective for superior colliculus receptors in the order carbachol much greater than arecoline greater than bethanechol greater than McN A343 = oxotremorine = pilocarpine.

Regional differences in the binding of selective muscarinic receptor antagonists in rat brain: comparison with minimum-energy conformations

The binding of selective muscarinic receptor antagonists to regions of rat brain was examined through quantitative autoradiographic techniques. 5,11-Dihydro-11-[(4-methyl-1-piperazinyl)acetyl]-6H- pyrido[2,3-b][1,4]benzodiazepin-6-one [pirenzepine (compound I)] and 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro- 6H-pyrido[2,3-b][1,4]benzodiazepin-6-one [AF-DX 116 (compound II)] were chosen on the basis of their selectivity for M1 and M2 muscarinic receptors, respectively, and similarities in chemical structure. Pirenzepine displayed a higher potency than II for inhibition of [3H]-l-quinuclidinyl benzilate ([3H]-l-QNB) binding to rat brain sections. Scatchard analyses of binding to brain sections revealed heterogeneous binding profiles for both antagonists, suggesting the presence of multiple receptor binding sites. Quantitative autoradiographic techniques were utilized in regional analyses of antagonist binding. Pirenzepine displayed the highest affinity for hippocampal, striatal, and amygdaloid muscarinic receptors (IC50 values less than 0.4 microM), with a slightly lower affinity for cortical receptors (IC50 values between 0.4 and 0.8 microM). Pirenzepine displayed the lowest affinity for thalamic and brainstem regions with IC50 values generally greater than 1.0 microM. In contrast, II bound with higher affinity to muscarinic receptors in brainstem, cerebellar, and hypothalamic nuclei (IC50 values less than 0.5 microM) than to receptors in thalamic nuclei (IC50 values between 0.5 and 2.0 microM). Binding sites with the lowest affinity for II were found in cortical, striatal, and hippocampal regions (IC50 values greater than 2.0 microM). The binding profiles of the two selective muscarinic antagonists reveal the complexity and diversity of muscarinic receptor subtypes throughout the brain. The data provide a basis for identifying muscarinic receptor subtypes (as defined through cloning procedures) with selective ligands. Minimum-energy conformations of pirenzepine and II were calculated by using the program MacroModel (version 2.0). Pirenzepine displayed three energy minima, differing in the relative position of the piperazine ring with respect to the tricyclic system. In contrast, the (diethylamino)methyl substituent on the piperidine ring conferred a much larger set of minimum-energy conformations on II. It is suggested that the greater conformational flexibility of the side chain allows II to achieve a conformation inaccessible to pirenzepine, which allows it to bind preferentially to M2 receptors.

Autoradiographic analyses of agonist binding to muscarinic receptor subtypes

The binding of four muscarinic receptor agonists to regions of rat brain was examined through quantitative autoradiographic techniques. Oxotremorine, arecoline, pilocarpine and bethanechol were chosen based on their different potencies and efficacies in muscarinic second messenger systems. Overall, the order of potency for inhibition of [3H]-l-quinuclidinyl benzilate ([3H]-l-QNB) binding to rat brain slices was oxotremorine greater than pilocarpine = arecoline much greater than bethanechol. Regional assays of agonist potency indicated that all agonists were more selective for brainstem and thalamic regions than for hippocampal and cortical regions. The high selectivity of agonists for areas such as the paraventricular thalamus and the superior colliculus, which also display low affinity for pirenzepine, suggests that muscarinic agonists bind with higher affinity to M2 receptors. Of the four agonists examined, pilocarpine displayed the lowest selectivity for M2 receptors in that IC50 values for pilocarpine were only 3-fold higher in the hippocampal and striatal regions (e.g. CA3: 40.6 +/- 9.4 microM) than in thalamic and brainstem regions (e.g. paraventricular thalamus: 14.9 +/- 6.2 microM). Oxotremorine was 8-fold more potent in the brainstem and thalamus, while arecoline and bethanechol were, respectively, 19- and 100-fold more selective for brainstem and thalamic receptors. Scatchard analyses revealed heterogeneous binding profiles for some agonists within single brain regions, suggesting that multiple agonist sites exist even within regions of predominantly M1 or M2 receptors. For example, arecoline displayed curved Scatchard plots within the external layers of the cerebral cortex, layer CA1 of the hippocampus (predominantly M1 subtype), and the paraventricular thalamus (predominantly M2 subtype). The ability of agonists to recognize multiple sites within a single region may reflect the ability to recognize receptors coupled or uncoupled to second messenger systems through G-proteins.

Inhibition of phosphoinositide turnover by selective muscarinic antagonists in the rat striatum. Correlation with receptor occupancy

In the rat corpus striatum, receptor occupancy and the inhibition of phosphoinositide turnover by muscarinic antagonists have been examined under very similar conditions with respect to tissue preparation and buffer composition. The results suggest a good correlation between receptor occupancy and inhibition by muscarinic antagonists, of the carbachol-stimulated turnover of inositol phospholipids, measured by the accumulation of [3H]inositol phosphates in the presence of 5 mM LiCl. In the presence of 10 mM carbachol (CCh), the accumulation of labeled inositol phosphates was increased 8-fold above basal levels (EC50 = 95 microM). Inclusion of antagonists resulted in a dose-dependent inhibition of the 0.1 mM CCh-stimulated inositol phosphate accumulation, with a rank order of potency of atropine greater than trihexyphenidyl greater than pirenzepine greater than or equal to gallamine. Radioligand binding studies with [3H]-l-quinuclidinyl benzilate [( 3H]QNB) in a cell aggregate preparation revealed a single class of saturable, high affinity [3H]QNB binding sites exhibiting a Kd of 74 pM and a Bmax of 2.85 pmol/mg protein. The antagonists examined were able to inhibit the binding of [3H]QNB with the same rank order of potency as for the inhibition of carbachol-stimulated phosphoinositide turnover (atropine greater than trihexyphenidyl greater than pirenzepine greater than or equal to gallamine). Although the inhibition of phosphoinositide turnover and [3H]QNB binding by the nonselective antagonist atropine was best described by interaction at a single site, inhibition of phosphoinositide turnover and [3H]QNB binding by both pirenzepine, which is selective for M1 receptors, and gallamine, which is selective for M2 receptors, is complex. Pirenzepine was much more potent than gallamine for both binding to receptors and inhibiting phosphoinositide turnover. Nonlinear curve-fitting analysis indicated that slope factors for inhibition of phosphoinositide turnover (analogous to Hill coefficient for binding) by only subtype selective antagonists were significantly less than unity. The above-mentioned antagonist interactions together with the apparently multicomponent stimulation of phosphoinositide turnover by carbachol suggest that phosphoinositide turnover may be coupled to more than one muscarinic receptor subtype in the corpus striatum.

Characterization of low Km GTPase activity in rat brain: comparison of opioid and muscarinic receptor stimulation

Guanosine 5'-triphosphate (GTP)-binding proteins (G-proteins) have an essential role in mediating the actions of drugs on neurotransmitter receptors by coupling them to their effectors with the attendant hydrolysis of GTP. The resulting GTPase activity was characterized in rat brain with a view toward selecting conditions under which specific hormone-stimulated activity could be monitored. Kinetic analysis with washed membranes suggested the presence of two distinct GTPases, a low Km GTPase with an apparent Km value of 0.35 +/- 0.04 microM and apparent Vmax of 108 pmol min-1 mg protein-1, together with a much higher Km component. Low Km (but not high Km) GTPase activity is stimulated by muscarinic and opioid agonists and inhibited by a nonhydrolyzable analogue of GTP, providing further evidence that the low Km component is a distinct enzyme. The activity of the low Km component is a linear function of protein concentration (20-100 micrograms/mL), time (2-10 min), and temperature (25-37 degrees C). The specific activity of the low Km component is selectively increased by approximately 50% in purified synaptic membranes compared with the washed membrane preparation. Both carbamylcholine-stimulated and basal low Km GTPase activities, but not the high Km component, are inhibited by a nonhydrolyzable analogue of GTP but not by the comparable analogue of ATP, demonstrating the specificity of low Km GTPase for guanine nucleotides. Opioid- and muscarinic-stimulated GTPase activities are additive in brain, suggesting that the two receptor systems are associated with different domains of G-proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Selectivity of pirenzepine in the central nervous system. I. Direct autoradiographic comparison of the regional distribution of pirenzepine and carbamylcholine binding sites

The binding capacities of the novel antagonist pirenzepine and the agonist carbamylcholine were examined autoradiographically to compare their abilities to reduce the binding of 1-[3H]quinuclidinyl benzilate ([3H]-1-QNB). This technique, which is applicable to any muscarinic ligand, permits a direct comparison between the binding of carbamylcholine and pirenzepine in the same assay. Analysis of the binding curves generated by standard scintillation counting of whole-brain slices indicated that the ligands bound heterogeneously to muscarinic receptors in the brain. Following apposition of slides to tritium-sensitive film, the binding profile for each ligand was examined visually and by microdensitometry. Regional analyses indicated that the agonist carbamylcholine displayed highest potency for thalamic nuclei, lower potency for cortical regions, and the lowest affinity for layers of the hippocampus. The M1-selective ligand pirenzepine displayed the highest potency for the dentate gyrus of the hippocampus, with lower inhibition levels in the cortex, and the lowest levels of inhibition found in the thalamus. The distribution of high affinity agonist sites was found to be distinct from the distribution of high-affinity antagonist binding sites. In a separate assay, the regional inhibition of pirenzepine and scopolamine was compared for the hippocampus and the forebrain. While scopolamine did not distinguish between muscarinic receptor sites in the hippocampus and cortex, pirenzepine inhibited [3H]-1-QNB labeling in the hippocampus significantly greater than in the cerebral cortex, providing additional evidence for the hypothesis that pirenzepine is a selective antagonist.

Selectivity of pirenzepine in the central nervous system. II. Differential effects of pirenzepine and scopolamine on performance of a representational memory task

The behavioral effects of the two muscarinic antagonists scopolamine and pirenzepine were examined using a representational memory task for rats in a T-maze. Rats were pretrained to a criterion of 100% correct responses for daily sessions of 10 paired-run trials. The training procedures eliminated all neophobic or wary responses, and response times were invariably short (less than 3 s). Following the initial training sessions, guide cannulae were surgically implanted bilaterally over the hippocampus of each animal. Following recovery from surgery, animals were injected with saline (0.5 microliter to each hippocampus), scopolamine hydrobromide (0.5 microliter of a 60 mg/ml solution (30 micrograms) to each side), or pirenzepine (0.5 microliter of a 69.1 mg/ml solution (34.6 micrograms) to each side) according to a fixed schedule. Saline injections aimed between the blades of the dorsal dentate gyrus failed to produce any change in the performance of the memory task. Initial doses of scopolamine, applied to the same area, produced a decrease in the percentage of correct responses as did the initial dose of pirenzepine. In contrast to pirenzepine, scopolamine also produced increases in response times even to the point of defaulted trials (response times greater than 90 s) in some animals following drug injections. Saline injections failed to produce significant impairments on the days following scopolamine injections, although animals receiving pirenzepine injections were still impaired on the two days immediately following the initial pirenzepine injection. Subsequent doses of pirenzepine were ineffective in producing an impairment of performance while scopolamine injections were less effective than the initial dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Selectivity of pirenzepine in the central nervous system. III. Differential effects of multiple pirenzepine and scopolamine administrations on muscarinic receptors as measured autoradiographically

The effects of intrahippocampal injections of scopolamine and pirenzepine on muscarinic receptor binding were examined by quantitative autoradiographic techniques. Brain slices from animals which had received 7 injections of either scopolamine (n = 5) or pirenzepine (n = 5) over a 22-day injection schedule were compared with slices from 5 saline-injected controls for receptor binding to the whole slice and within selected regions of the brain as measured autoradiographically. The total number of receptors was determined from direct binding assays with 1-[3H]quinuclidinyl-benzilate ([3H]-1-QNB), while the binding of the selective ligands pirenzepine, carbamylcholine, and scopolamine was examined through inhibition studies. The data from the whole slices indicated that pirenzepine-treated animals contained more receptors for [3H]-1-QNB than either saline- or scopolamine-injected controls. Slices from the same animals also displayed a lower affinity for pirenzepine. Slices from scopolamine-injected animals revealed neither an increase in receptor number nor a decrease in antagonist affinity, although the binding of the agonist carbamylcholine was increased. Quantitative analysis of the autoradiograms generated from the slices indicated that the increase in receptor number for pirenzepine-injected animals was predominantly within the cerebral and cingulate cortices. The inhibition by pirenzepine was also lower in these areas in the same group of animals. Agonist inhibition was altered in the central layers of the cerebral cortex and in the pretectal area in scopolamine-treated animals. The results suggest separate mechanisms of drug action and adaptation for pirenzepine and scopolamine.

Regional distribution of muscarinic receptors preferring gallamine in the rat brain

The regional distribution of muscarinic receptors recognized by the antagonist gallamine was determined autoradiographically by the ability of gallamine to reduce the binding of [3H]quinuclidinyl benzilate in rat brain slices. The inhibition data obtained from indirect binding assays on whole slices indicated that gallamine distinguished at least two sites with differing affinities. Analysis using a two-site model gave Kh = 0.6 microM, K1 = 10 microM. The regions of highest and lowest affinity for gallamine were apparent qualitatively by visual inspection of the autoradiograms. A number of regions in coronal sections at three different levels were compared by microdensitometry. Gallamine possessed greater overall affinity for the diencephalon and brainstem than for the forebrain. Within the forebrain, the septal nucleus was unique in that it displayed high affinity for gallamine. Within the brainstem, the superior colliculus had the greatest proportion of sites with high affinity for gallamine. In general, the binding profile of gallamine was opposite to that of the antagonist pirenzepine and similar to that of the agonist carbachol, suggesting that gallamine is selective for M2 muscarinic receptors.

Inactivation of depolarization-induced calcium uptake in rat brain synaptosomes

The inactivation of depolarization-induced Ca uptake into rat brain synaptosomes was demonstrated biochemically by comparing 45Ca fluxes after various intervals of predepolarization achieved by abruptly increasing [K+]o. The chemical composition of the medium was maintained throughout the predepolarization and Ca uptake steps. Under these conditions, inactivation was dependent on depolarization, i.e., basal unstimulated Ca uptake in the presence of 5 mM [K+]o did not inactivate. Inactivation of stimulated Ca uptake was dependent on the predepolarization interval, moderately dependent on [Ca]o and relatively independent of membrane potential, i.e., [K+]o and ions such as Ni2+ and Co2+ that blocked Ca uptake. Both cinnarizine and lidoflazine blocked stimulated Ca uptake in a concentration-dependent manner without affecting the % inactivation. Although the amount of stimulated uptake increased greatly between 10 and 30 degrees C, the % inactivation was unaffected by temperature. These findings suggest that inactivation of the presynaptic Ca uptake is an intrinsic property of the channel independent of calcium uptake.

Characterization of noncholinergic nicotine receptors on human granulocytes

The noncholinergic nicotine receptor on leukocytes identified earlier [Davies et al., Molec. Cell. Biochem. 44, 23 (1982)] was further characterized. Structure-activity relationships showed that a pyrrolidine ring containing a basic N atom is an important structural feature for ligands that bind to the receptor. Accordingly, the carcinogenic component of tobacco smoke, N-nitrosonornicotine, does not bind to the receptor. The stereoselectivity for the d-isomer, which was confirmed using [3H]d-nicotine as a ligand, together with the absolute configurational relationship between d-nicotine and L-proline, suggested that basic peptides containing proline as the N-terminal amino acid would bind to the receptor. The finding that Pro-Lys-Pro-Arg, which has been reported to inhibit granulocyte phagocytosis, bound to the receptor with an IC50 value of 3.5 microM is compatible with this idea. An increase in receptor binding, which was observed in the presence of plasma, could be ascribed to bicarbonate. The presence of bicarbonate in the binding assay, even when the pH of the buffer was carefully controlled, resulted in an increase (approximately 2-fold) in the apparent number of receptors without affecting the Kd value significantly. Increasing the pH of the buffer in the absence of bicarbonate also increased receptor binding, suggesting that bicarbonate may increase receptor binding by its known ability to increase intracellular pH at constant extracellular pH. Preincubation of cells with d-nicotine under certain conditions reduced the subsequent binding of [3H]d-nicotine to the receptor.

Mechanism of phospholipase A2-induced conduction block in bullfrog sciatic nerve. I. Electrophysiology and morphology

The effects of exogenously added phospholipase A2 (PLA2) and its hydrolytic products in isolated bullfrog sciatic nerve were investigated. Nerves were pretreated for 3 h with a dose of trypsin which did not affect conduction in order to enhance penetration of the added agents. Treatment of nerves with beta-glucosidase, neuraminidase or chymotrypsin had no effect on conduction. Whereas incubation of the nerves with normal Ringers for 2 h had no significant effect on conduction, incubation with PLA2 in Ringers caused decrements in the height of the compound action potential in a dose-related manner. In addition, incubation of the nerves with 10 mg/ml lysolecithin, arachidonic acid, or docosahexaenoic acid caused marked decrements in the height of the compound action potential. Electron microscopic analysis of nerves after each treatment which caused conduction block revealed varying levels of myelin damage. Although myelin was damaged at the paranodal and/or internodal region, depending on the agents used, the axonal membrane appeared to be intact at the ultrastructural level. It was concluded that the block in conduction resulting from PLA2 was due to the formation of lysolecithin and long chain polyunsaturated fatty acids.

Mechanism of phospholipase A2-induced conduction block in bullfrog sciatic nerve. II. Biochemistry

The biochemical changes associated with conduction block following exogenous application of purified phospholipase A2 to bullfrog sciatic nerves were investigated. Nerves were treated with concentrations of phospholipase A2 needed to produce at least a 50% decrease in the compound action potential after a 2-2 1/2 h incubation. This phospholipase A2-induced conduction block was associated with lipid hydrolysis, depletion of high energy phosphates and decreases in [3H]saxitoxin binding. Forty-two percent of the nerve phosphatidylcholine and 45% of the nerve phosphatidylethanolamine were hydrolyzed. [3H]Saxitoxin binding was decreased by 85% in association with conduction block attained with phospholipase A2 incubation. There were significant decrements in adenosine triphosphate levels (-58%) and in phosphocreatine levels (-78%), but no difference in the specific activities of these phosphate compounds. It is concluded that a number of mechanisms can account for the conduction failure resulting from phospholipase A2, including disruption of sodium channels needed for propagation of regenerative nerve impulses and the depletion of high energy phosphates needed to maintain ionic gradients.

Characterization of muscarinic cholinergic receptors in the brains of copper-deficient rats

In order to assess a possible role for copper as a regulator of muscarinic receptors in vitro, the receptor was characterized in rats made copper deficient by a dietary regimen. In forebrain regions there was a decrease in both the affinity of the receptors for [3H]-1-quinuclidinyl benzilate and the density of receptors in the copper-deficient animals compared with control animals. Copper treatment in vitro of homogenates from deficient animals did not reverse the in vivo effects on antagonist binding but, rather, decreased receptor occupancy and ligand affinity in a manner similar to copper treatment of control homogenates. Minimally deficient rats displayed very similar changes in receptor properties compared with the more severely deficient animals. Minimal copper deficiency produced robust effects on the binding of agonists, increasing ID50 and derived dissociation constants. The addition of copper to the assay medium caused an apparent reversal of the in vivo effect of copper deficiency on agonist binding, decreasing ID50 and derived dissociation constants to values near those observed with homogenates from normal animals in the presence of copper. Since copper deficiency has dramatic effects on both receptor number and the binding of agonists to muscarinic receptors in the central nervous system, it is suggested that copper, because of its ability to form complexes with some proteins, may have an endogenous role in the regulation of the receptor.

Nicotine-induced membrane perturbation of intact human granulocytes spin-labeled with 5-doxylstearic acid. Correlation with chemotaxis

The effects of nicotine on intact human granulocytes were examined, using 5-doxylstearic acid as a spin probe. At micromolar concentrations, (-)-nitocine produces a membrane perturbation in granulocytes not observable with oriented lipid bilayers. The effect, which is stereoselective for the (-)-isomer, occurs at concentrations of nicotine that bind to noncholinergic nicotine receptors on granulocytes and which are present in the blood after smoking. At comparable concentrations, (-)-nicotine modulates granulocyte chemotaxis towards a chemotactic peptide in a stereospecific and dose-dependent manner. Cotinine, the major metabolite of nicotine, does not bind to the receptor, does not produce the membrane perturbation observed with nicotine, and has no effect on chemotaxis. These results suggest that (-)-nicotine present in the blood after smoking binds to a receptor on granulocytes, perturbs granulocyte membranes and modulates chemotaxis.

Opiates stimulate low Km GTPase in brain

Low Km GTP hydrolysis in rat brain is stimulated in a concentration-dependent manner by the opiate alkaloid etorphine, and by the opioid peptide D-Ala2-leucine-enkephalinamide. The opiate antagonist naloxone inhibits the maximal D-Ala2-leucine-enkephalinamide stimulation of the GTPase, also with concentration dependency. The magnitude of maximally stimulated, opioid-sensitive, GTP hydrolysis is differentially distributed across brain regions. Opioid-stimulated GTPase may represent one means of identifying a specific type of opioid receptor.

Effects of copper on the binding of agonists and antagonists to muscarinic receptors in rat brain

Studies were performed to assess the effects of copper treatment in vitro on muscarinic binding parameters in rat brain homogenates. Brainstem, an area low in copper, was found to be insensitive to copper treatment as compared to forebrain, a region of relatively high copper content. Inclusion of 3 microM copper in forebrain homogenates decreased the number of sites seen by [3H]-l-quinuclidinyl benzilate (QNB) by 40-50%. Copper-enhanced displacement of bound QNB was noted for agonists and antagonists. Both ligands showed maximal effects at 6 microM copper, although quantitative differences could be determined at any copper level. At levels of maximal effect, the increase in QNB displacement was greater than or less than 50% for agonists and antagonists respectively. Two-site analyses of carbamylcholine (CCH) binding showed that the addition of 1 microM copper to forebrain homogenates increased the percentage of high affinity sites (alpha) from 42 to 70%. The IC50 decreased from 3.1 to 1.7 microM, but the dissociation constants for the high and low affinity sites were not changed. The effect of added copper on CCH binding to muscarinic receptors was reversible with the addition of the copper-chelating agent triethylene tetramine.

Effect of 2450 MHz microwave energy on the blood-brain barrier to hydrophilic molecules. A. Effect on the permeability to sodium fluorescein

Significantly elevated levels of sodium fluorescein (MW 376) were found only in the brains of conscious rats made considerably hyperthermic (colonic temperatures greater than 41.0 degrees C) by exposure to ambient heat (42 +/- 2 degrees C) for 90 min or 2450 MHz CW microwave energy at 65 mW/cm2 (SAR approximately equal to 13.0 W/kg) for 30 or 90 min. For microwave-exposed rats, fluorescein levels within the cortex and hypothalamus appeared to increase with increasing duration of exposure. This trend was not apparent in the cerebellum or medulla. Exposure to ambient heat resulted in increased fluorescein with the cortex, hypothalamus and medulla, but not the cerebellum, and, in general, ambient heat was not as effective as microwave energy in raising tracer concentrations within the brain. By far the greatest elevation of fluorescein dye in the brain occurred in those animals whose blood-brain barrier had been opened osmotically by intracarotid injection of 10 M urea. It is suggested that increased levels of sodium fluorescein found in the brain tissue of ambient heat and microwave-exposed rats most likely represent technically derived artifact and not a breakdown of the blood-brain barrier.

Calcium channel activity in rat brain synaptosomes: effects of neuroleptics and other factors regulating phosphorylation and transmitter release

Neuroleptic drugs inhibit depolarization-induced Ca uptake in nerve endings, having IC50 values in the micromolar range. Dopamine and a variety of other substances including opiates and PGE1 are inactive. The effect is probably not mediated by the interaction of the neuroleptics with calmodulin, which itself is a potent inhibitor of stimulated Ca uptake. Dibutyryl cyclic AMP, but not fluoride, increases K+-stimulated Ca uptake. Phosphatidic acid, which is an intermediate in transmitter-stimulated phosphatidylinositol turnover, acts as a Ca ionophore in nerve endings and enhances K+-stimulated Ca uptake at a relatively low concentration. Carbamyl choline, a known stimulator of phosphatidylinositol turnover, did not, however, cause a significant increase in K+-stimulated Ca uptake. Treatment of the nerve ending fraction with relatively small amounts of phospholipase A2 greatly inhibited depolarization-induced Ca uptake, demonstrating the importance of phospholipids for the functioning of the potential-dependent Ca channel in nerve endings. These studies suggest that the regulation of voltage-sensitive Ca channels in nerve endings may be one mechanism controlling transmitter release.

Depolarization-induced increase in synaptosomal membrane calcium monitored by chlorotetracycline fluorescence

Chlorotetracycline (CT) was used as a fluorescent probe for membrane calcium with intact synaptosomes. The net increase in fluorescence intensity at 520 nm, which is a measure of membrane-bound Ca, increases with increasing Ca, saturating in the millimolar range. Membrane Ca can also be detected in the absence of added external Ca. Potassium-induced depolarization of synaptosomes leads to an increase in membrane Ca, reaching a new steady-state value within 5 min. Neither opiates nor phenytoin affected synaptosomal membrane Ca. Relatively high concentrations of chlorotetracycline increased depolarization-induced uptake of 45Ca into synaptosomes. The data suggest that the Ca-CT complex binds to synaptic plasma membranes, and that depolarization-induced Ca influx results in increased Ca binding to the internal surface of the plasma membrane and/or other internal membranes.

Activation of fluoride-stimulated adenylate cyclase by phospholipase A2 in the caudate nucleus of the rat brain

Phospholipase A2 (PLA2) increases adenylate cyclase (AC) activity in the rat caudate nucleus in a dose-dependent manner. After maximal stimulation by fluoride, PLA2 treatment further increases AC activity 2.4 fold. Adenylate cyclase activity is maximal after 45% hydrolysis of the phospholipids. Of the products of PLA2 treatment only lysophosphatidylcholine (LPC) produces such an increase in AC activity. In contrast to PLA2 treatment, LPC solubilizes the enzyme, decreases the Km value for ATP, and requires much larger amounts of LPC than that produced by lipase treatment. After maximal stimulation with fluoride and PLA2, removal of most of the LPC does not reduce the activity of adenylate cyclase. These findings suggest that removal of membrane lipid rather than generation of LPC is responsible for the activation of brain adenylate cyclase by phospholipase A2.

Effects of opiates on synaptosomal calmodulin and calcium uptake

Acute opiate administration in vivo increases the level of cytoplasmic calmodulin in isolated rat brain synaptosomes. These synaptosomes do not, however, display decreased K+-stimulated 45Ca uptake in vitro. Opiates affect neither cytoplasmic calmodulin nor Ca uptake after incubation of synaptosomes with the drugs in vitro. In contrast to the interpretation of electrophysiological data, these results suggest that the observed inhibition by opiates of the release of several transmitters may not be mediated by presynaptic opiate receptors that inhibit Ca uptake.

Evidence for a noncholinergic nicotine receptor on human phagocytic leukocytes

A noncholinergic nicotine receptor on human phagocytic leukocytes has been characterized using the binding of 3H-(d,1)-nicotine. The average affinity +/- standard deviation of (d,1)-nicotine for the receptor on neutrophils is 36 +/- 18 nM (n = 6). The binding is saturable with an average of 8.7 x 10(4) sites per neutrophil. Monocytes and to a lesser extent lymphocytes but not erythrocytes also display specific binding. Bound nicotine is dissociable from the receptor and is not metabolized. Only close structural analogs of nicotine bind to the receptor, which is stereoselective for the (d)-isomer. The receptor can be occupied by (1)-nicotine at concentrations present in the blood of smokers. It is suggested that some of the adverse effects of smoking on leukocyte functions may be mediated by a specific nicotine receptor.

Competitive interaction of gallamine with multiple muscarinic receptors

Gallamine, a cholinergic antagonist at the (nicotinic) neuromuscular junction possesses antimuscarinic potency in several systems. We report here that gallamine inhibited the binding of [3H] quinuclidinyl benzilate (QNB) in a competitive manner in the brainstem and forebrain of the rat. The occupancy curves derived from these studies suggest that gallamine has widely varying affinities for different subpopulations of muscarinic receptors, a finding which sets gallamine apart from classical muscarinic antagonists such as atropine and QNB. The greatest difference in affinities for gallamine occurred in the brainstem, where the data could be satisfactorily fitted to a two-site model, with 77% of the receptors having high affinity (Kd = 25 nM) and 23% low affinity (93 microM). Further, these affinities displayed rank order correlation with those of carbachol (an agonist), although gallamine has not, so far, displayed agonist (or partial agonist) activity. The finding that antagonists as well as agonists can display multiple affinities for muscarinic receptors suggests that there are fundamental differences among subpopulations of these receptors.

Enhancement of synaptic vesicle attachment to the plasma membrane fraction by copper

Synaptic vesicles from rat brain were labeled with 125I, and the association of the vesicles with other subcellular components of brain was examined using a centrifugation assay. Copper at micromolar concentrations enhances the binding of the vesicles to the synaptic membrane as well as other fractions. Magnesium, Ca2+, and calmodulin with Ca2+ are ineffective. There is virtually no binding of synaptic vesicles to the microtuble fraction and only a slight enhancement with Cu2+. These findings support the hypothesis that Cu may serve as a bridge between synaptic vesicles and the plasma membrane.

Conversion between configurational states of the muscarinic receptor in rat brain

Reductive alkylation of neural membranes by N-ethyl maleimide (NEM) converts muscarinic acetylcholine receptors from a state of low to high affinity for receptor agonists. Interactions of muscarinic antagonists with the receptor are unaffected by this treatment. Muscarinic receptors from the rat telencephalon in the high agonist affinity state are increased from 34.2 to 53.4% of the total receptor population and the Ki for carbamylcholine inhibition of 3-quinuclidinyl benzilate binding is decreased from 1.2 X 10(-4) to 6.9 X 10(-5) M by NEM treatment.

Binding and immobilization of catecholamines by liposomes

The polarization of the native fluorescence of dopamine and noradrenaline has been used to measure their binding and immobilization by liposomes suspended in aqueous buffers. Whereas both catecholamines are significantly immobilized by brain phosphatidyl serine and yeast phosphatidyl inositol, phosphatidyl ethanolamine and phosphatidyl inositol from brain are ineffective. Dopamine is immobilized to a greater degree than noradrenaline. The dissociation constants determined from modified Scatchard plots of the polarization data are 1.7 X 10(-4) and 9.6 X 10(-5)M for dopamine with yeast phosphatidyl inositol and brain phosphatidyl serine, respectively. Apomorphine binds to a hydrophobic region of phosphatidyl serine liposomes with a KD value of 69 micrometer. It is suggested that a fraction of dopamine is complexed with membranous phosphatidyl serine in nerve terminals.

Binding of apomorphine to neural membranes

The intrinsic fluorescence of apomorphine has been used to measure its binding to neural membranes. A large number of relatively weak binding sites are concentrated in myelin and synaptic membrane fractions. Butyrophenones have the highest affinities for these sites--KD = 43 micrometer for haloperidol--while dopamine and dopamine releasers and reuptake blockers, as well as a variety of other alkaloids, have much lower affinities. The sites are hydrophobic and undergo a phase transition to a highly fluid state near 26 degrees C. Calcium is a noncompetitive inhibitor of apomorphine binding. Some of the actions of neuroleptic drugs may result from binding to these hydrophobic membrane sites in vivo, blocking conduction in small catecholamine axons.

Kinetics of interfacial ATP adsorption and anionic exchange with a synaptic membrane protein

A study was conducted on the adsorption of 14C-ATP to a surface film of a hydrophobic protein derived from synaptic membranes isolated from bovine cerebellum. The adsorption of ATP to the protein film followed a rate law based on diffusion and an energy barrier to adsorption, the rate law being generally applicable to the adsorption of ions to a charged interface. Studies were also carried out on the displacement of ATP from the film by the injection of other nucleotides, inorganic phosphates, and other anions. The kinetics conformed to a rate law based on diffusion and displacement. The most important factor in the displacement of ATP was the magnitude of charge of the anion, while steric factors were relatively minor. However, from a consideration of the energy barrier to adsorption it appeared that steric factors play a greater role in the adsorption of ATP. The results are discussed in relationship to the configurational aspects of the surface film as well as their possible significance in synaptic function.