Activation of the alpha(2A)-adrenoceptor mediates deceleration of the deaggregation component of the response to ADP or 5-HT in human platelets in vitro.

Platelet aggregation requires the concomitant activation of at least one G(i)- and one G(q)-coupled receptor. Epinephrine (EPI) amplifies the response elicited by a number of agonists for platelet aggregation. This study tested the hypothesis that platelet alpha(2A)-adrenoceptor activation causes deceleration of the deaggregation component of the platelet aggregation response when activated concomitantly with the G(q)-coupled adenosine diphosphate (ADP) P2Y(1) or 5-hydroxytryptamine(2A) receptor. The time course of the aggregation response of human platelet-rich plasma following activation of combinations of two or three receptors was assessed by turbidometry using lepirudin anticoagulation. Simultaneous activation of specific two- and three-receptor combinations was achieved using selective antagonists for the P2Y(12) and P2Y(1) receptor subtypes. Steady-state and kinetic parameters, obtained using a four-compartment kinetic model, were used to assess the effects on the net aggregation response. Graded alpha(2A)-adrenoceptor activation was associated with a concentration-dependent decrease of the rate constant of deaggregation. Activation of both ADP receptor subtypes and the alpha(2A)-adrenoceptor produced a concentration-dependent, mutual amplification of the aggregation response. In addition, when three receptors were simultaneously activated, mutual amplification of the aggregation response was observed at physiologically relevant concentrations of epinephrine or norepinephrine (NE) and ADP. We propose that similar to the P2Y(12) receptor, activation of the alpha(2A)-adrenoceptor decelerates the deaggregation component shifting the balance toward increased net aggregation. The effects of EPI and NE on the aggregation response may contribute to the mechanism of increased thrombotic risk present in certain pathophysiological and disease states.

Differences in rapid desensitization of 5-hydroxytryptamine2A and 5-hydroxytryptamine2C receptor-mediated phospholipase C activation.

The serotonin (5-HT)2A and 5-HT2C receptors share a high degree of sequence homology and have very similar pharmacological profiles. Although it is generally believed that the cellular signal transduction mechanisms activated by these receptors are indistinguishable, recent data suggest significant differences in their signaling cascades. In this study we explored differences in the characteristics and mechanisms of rapid desensitization between the 5-HT2A and 5-HT2C receptor systems. For both receptor systems, pretreatment with 5-HT reduced the ability of a maximal concentration of 5-HT to stimulate phospholipase C-mediated inositol phosphate accumulation by about 65%, although the 5-HT2C receptor system was more sensitive to the desensitizing stimulus. Differences in the concentration dependence of the rate constant for desensitization (k(des)) suggested different mechanisms of desensitization for the 5-HT2A and 5-HT2C receptor systems. At very high receptor occupancy (>99%), the responsiveness of the 5-HT2A, but not the 5-HT2C, receptor system returned to control levels despite the continued presence of the agonist. This resensitization was dependent upon the activity of protein kinase C (PKC). Agonist-induced desensitization of the 5-HT2A, but not the 5-HT2C, receptor system was reduced by the PKC inhibitors staurosporine and bisindolylmaleimide, and by down-regulation of PKC. In addition, inhibitors of calmodulin (W-7) or of calmodulin-dependent protein kinase II, reduced 5-HT2A, but not 5-HT2C, desensitization. Desensitization of the 5-HT2C, but not the 5-HT2A, receptor system was dependent on G protein receptor kinase activity. These data further emphasize the major differences in the signaling systems coupled to 5-HT2A/2C receptors.

Deaggregation is an integral component of the response of platelets to ADP in vitro: kinetic studies of literature and original data.

Adenosine diphosphate (ADP) is recognized as an important mediator of platelet aggregation. Transient aggregation at low (< or =1 microM), and sustained aggregation at higher ADP concentrations are consistently observed. Dissociation of platelet aggregates has been described and may explain the reversible component of the aggregation response. We hypothesized that the net aggregation response to ADP in vitro results from the concurrent activation of two opposing processes, aggregation and deaggregation. Different purinergic receptor subtypes may mediate these effects. To test this hypothesis and its generalizability, we performed a kinetic analysis of representative published ADP-induced aggregation responses supplemented with original data from our laboratory. A four-compartment kinetic model was used to estimate k(3), a rate constant of deaggregation. Two model-independent parameters, the magnitude of the aggregation response (DeltaOD) and the time to reach maximal aggregation (t(peak)) were also assessed. Greater sustained aggregation at higher ADP concentrations was consistently associated with increased DeltaOD and t(peak) but decreased k(3) values. These relationships were independent of type of platelet preparation or experimental conditions and not due to ADP receptor desensitization. Conversely, blockade of the P2Y(12) receptor subtype (ticlopidine, clopidogrel or 2-MeS-AMP) decreased DeltaOD and t(peak) but increased k(3) values. This supports the presence of active deaggregation which is decelerated by activation of the P2Y(12) receptor subtype.

The balance of concurrent aggregation and deaggregation processes in platelets is linked to differential occupancy of ADP receptor subtypes.

Deaggregation, the partial reversal of the initial aggregation of platelets is observed following low, but not higher, micromolar ADP concentrations. This study tested the hypothesis that deaggregation results from a balance between concurrent, opposing, aggregation and deaggregation processes which are ADP (adenosine 5'-diphosphate) receptor occupancy-dependent. Aggregation of human platelet-rich plasma (PRP) prepared in r-hirudin was assayed in a 96-well plate reader over 20 min by measurement of the optical density (OD) at 580 nm. Aggregation and the time to reach peak aggregation were directly proportional to ADP receptor occupancy. The magnitude and time course of the response to ADP were comparable to those previously reported with standard aggregometry. The rate constant of platelet deaggregation, as assessed by a four-compartment kinetic model, was inversely proportional to agonist concentration. The ratio of the rate constants of aggregation and deaggregation was receptor occupancy-dependent and directly proportional to aggregation. Consequently, platelet aggregation was proportional, and deaggregation inversely proportional, to ADP receptor occupancy. We propose that the response of PRP to ADP and to 2-MeS-ADP (2-methylthioadenosine-diphosphate), in vitro, consists of at least two active, concurrent processes, aggregation and deaggregation. Incremental occupancy of the P2T ADP receptor subtype attenuates deaggregation and governs the balance between these two processes.

Activation of Gi-coupled receptors releases a tonic state of inhibited platelet aggregation.

Single-receptor pharmacology does not satisfactorily explain the physiology of the ADP-induced platelet aggregation response. It has been shown that, in addition to Gq-coupled receptor activation, one Gi-coupled receptor, either the ADP P2T or the alpha2-adrenoceptor, is required for elicitation of aggregation. The underlying mechanism of this action, however, has not been elucidated. By systematically assaying the entire time course of the aggregation and its fade using two methods of aggregometry, we have investigated the role of graded activation of these two Gi-coupled receptors. We demonstrate that constant activation of either of two Gq-coupled receptors, the ADP P2Y1 or the 5-HT2A, and incremental activation of either of the two Gi-coupled receptors, tightly regulates the aggregation response in vitro, through the apparent release of a tonic inhibition of platelet aggregation. This tightly regulated release of inhibition, which appears analogous to the phenomena of disinhibition observed in the central nervous system, may be instrumental for the continuous adaptation of the aggregation response to variable physiological conditions.

Novel actions of inverse agonists on 5-HT2C receptor systems.

In cell systems where ligand-independent receptor activity is optimized (such as when receptors are overexpressed or mutated), acute treatment with inverse agonists reduces basal effector activity whereas prolonged exposure leads to sensitization of receptor systems and receptor up-regulation. Few studies, however, have reported effects of inverse agonists in systems where nonmutated receptors are expressed at relatively low density. Here, we investigated the effects of inverse agonists at human serotonin (5-HT)2C receptors expressed stably in Chinese hamster ovary cells ( approximately 250 fmol/mg protein). In these cells, there is no receptor reserve for 5-HT and 5-HT2C inverse agonists did not reduce basal inositol phosphate (IP) accumulation nor arachidonic acid (AA) release but behaved as simple competitive antagonists, suggesting that these receptors are not overexpressed. Prolonged treatment (24 h) with inverse agonists enhanced selectively 5-HT2C-mediated IP accumulation but not AA release. The enhancing effect occurred within 4 h of treatment, reversed within 3 to 4 h (after 24-h treatment), and could be blocked with neutral antagonists or weak positive agonists. The enhanced responsiveness was not due to receptor up-regulation but may involve changes in the expression of the G protein, Galphaq/11 and possibly Galpha12 and Galpha13. Interestingly, 24-h exposure to inverse agonists acting at 5-HT2C receptors also selectively enhanced IP accumulation, but not AA release, elicited by activation of endogenous purinergic receptors. These data suggest that actions of inverse agonists may be mediated through effects on receptor systems that are not direct targets for these drugs.

Modification of sexual behavior of Long-Evans male rats by drugs acting on the 5-HT1A receptor.

Modulation of the sexual behavior of male rats by the anxiolytic buspirone (S-20499) and its analog gepirone were compared to the effects of 8-OH-DPAT (or DPAT, a selective 5-HT1A reference agonist), and BMY-7378 (a selective 5-HT1A partial agonist). Long-Evans rats were used; modulation of copulatory behavior and alteration of penile reflexes were examined. Modulation of copulatory behavior was assessed by three indices: frequency and length of intromission, and latency of ejaculation. DPAT, at doses of 1-8 mg/kg, reduced these three indices in a time dependent manner such that the effects peaked at 45 min and normalized at 90 min. The dose-effect relationship (assessed 45 min after DPAT injection) is bell-shaped with an ED50 approximately 1 mg/kg on the ascending limb of the curve. The effects of buspirone (2 mg/kg) and gepirone (2 mg/kg) on copulatory behavior were indistinguishable from control. BMY-7378 alone and in combination with these other 5-HT1A agonists reduced copulatory behavior, though not statistically significant. Penile reflexes, including number of erections, cups and flips, were inhibited by these agents: DPAT>buspirone>gepirone (inactive at 2 mg/kg). Furthermore, the latency period to erection was at least doubled by DPAT (2 mg/kg). Buspirone and gepirone, however, reduced the latency period to erection. BMY-7378 inhibited penile reflexes when administered alone and even more in combination with DPAT or buspirone. Two butyrophenone analogs, spiperone (a 5-HT1A and dopamine D2 antagonist) and haloperidol (a D2 antagonist), were also tested for their interaction with DPAT. Both of these drugs (at 0.25 mg/kg, 60 min after administration) reduced all indices of penile reflexes and copulation. Furthermore, in combination with DPAT (2 mg/kg, 45 min), the effects were synergistic such that sexual activity came nearly to a standstill. These opposing effects on putatively brain originated copulatory behavior and spinal mediated penile reflexes indicate that the effects of buspirone and DPAT on sexual behavior in the male rat may be possible at different parts of the central nervous system. If a tentative shared target site by DPAT and buspirone is the 5-HT1A receptor, than the same 5-HT receptor sub-type at different locations (brain, raphe nuclei, spinal cord and autonomic ganglia) may modulate rat sexual behavior in opposing ways.

Pharmacologic characterization of latex extracts by isolated guinea pig tracheal tissue.

Latex manufacturing workers are exposed to a heterogeneous aerosol of organic compounds. Previous studies of latex workers involved in glove production indicate that these individuals are at risk of developing respiratory symptoms and impaired lung function. The effect of latex extracts on isolated guinea pig tracheal smooth muscles was studied using latex water-soluble extracts obtained at different stages of the industrial process. Latex extracts were prepared as a 1:10 (w/v) solution. Dose-related contractions of nonsensitized guinea pig trachea were demonstrated using two latex extracts (latex 1 and latex 2). Latex 1 was prepared from the native latex and latex 2 from a processed form of latex which was relatively free of soluble proteins. Pharmacologic studies were performed by pretreating guinea pig tracheal tissue with drugs known to modulate smooth muscle contraction: atropine, indomethacin, pyrilamine, nordihydroguaiacetic acid, acivicin, trimethobenzoic acid and capsaicin. Constrictor effects of the dust extracts were inhibited by a wide variety of these agents. Atropine consistently and strikingly reduced the contractile effects of these extracts. This observation may suggest an interaction of the extracts with parasympathetic nerves or more directly with muscarinic receptors. Inhibition of contraction by blocking other mediators was less effective and varied with the dust extract. Pretreatment with capsaicin did not change the constrictor effects of latex 1 but enhanced the effects of latex 2. Depletion of neuropeptides, however, did not reduce the constrictor effect. We suggest that latex extracts cause dose-related airway smooth muscle constriction by nonimmunological mechanisms involving a variety of airway mediators and possibly cholinergic receptors. This effect is not dependent on the presensitization of guinea pigs.

Effector pathway-dependent relative efficacy at serotonin type 2A and 2C receptors: evidence for agonist-directed trafficking of receptor stimulus.

There are many examples of a single receptor coupling directly to more than one cellular signal transduction pathway. Although traditional receptor theory allows for activation of multiple cellular effectors by agonists, it predicts that the relative degree of activation of each effector pathway by an agonist (relative efficacy) must be the same. In the current experiments, we demonstrate that agonists at the human serotonin2A (5-HT2A) and 5-HT2C receptors activate differentially two signal transduction pathways independently coupled to the receptors [phospholipase C (PLC)-mediated inositol phosphate (IP) accumulation and phospholipase A2 (PLA2)-mediated arachidonic acid (AA) release]. The relative efficacies of agonists differed depending on which signal transduction pathway was measured. Moreover, relative to 5-HT, some 5-HT2C agonists (e.g., 3-trifluoromethylphenyl-piperazine) preferentially activated the PLC-IP pathway, whereas others (e.g., lysergic acid diethylamide) favored the PLA2-AA pathway. In contrast, when two dependent responses were measured (IP accumulation and calcium mobilization), agonist relative efficacies were not different. These data strongly support the hypothesis termed "agonist-directed trafficking of receptor stimulus" recently proposed by Kenakin [Trends Pharmacol Sci 16:232-238 (1995)]. Concentration-response curves to 5-HT2C agonists were fit well by a three-state model of receptor activation, suggesting that two active receptor states may be sufficient to explain pathway-dependent agonist efficacy. Rational drug design that optimizes preferential effector activity within a group of receptor-selective drugs holds the promise of increased selectivity in clinically useful agents.

Effects of recycled paper dust extracts on isolated guinea pig trachea.

The effect of paper dust collected at two different locations in a paper recycling plant (PD1 and PD2) on isolated nonsensitized guinea pig tracheal smooth muscle was studied in vitro. Dust extracts were prepared as a 1:10 w/v aqueous solution. Dose-related contractions of guinea pig tracheal rings were elicited with both PD1 and PD2. Pharmacologic studies were performed with atropine (10(-6) M), indometacin (10(-6) M), pyrilamine (10(-6) M), LY171883 (10(-5) M), nordihydroguaiaretic acid (10(-5) M), and TMB8 (10(-5) M). The possible role of endogenous neuropeptides in this constrictor process was studied by depleting neural mediators with capsaicin (5 x 10(-6) M) before challenge with dust extracts. Constrictor effects were partially inhibited by a wide variety of the mediator blocking agents. The effects of both extracts were almost totally inhibited by the anticholinergic agent atropine, suggesting that a principal pathway mediating this response may involve the parasympathetic nervous system. The intracellular calcium-blocking agent TMB8 also induced a reduction of the contractile responses to PD1 and PD2 consistent with the well established role of intracellular calcium in smooth muscle constriction. Pretreatment with capsaicin significantly increased the contractile activity of paper dust extracts but only at the higher doses of these extracts. This suggests that the effect of paper dust is not initiated by the release of mediators stored in sensory nerves but that the prerelease of these mediators may enhance the constrictor effects of these dusts. We suggest that paper dust extracts cause dose-related airway smooth muscle constriction possibly associated with the release of cholinergic as well as other mediators. The constrictor effect does not require tissue presensitization or the release of neuropeptides from sensory nerves.

Pleiotropic behavior of 5-HT2A and 5-HT2C receptor agonists.

There is now considerable evidence that a single receptor subtype can couple to multiple effector pathways within a cell. Recently, Kenakin proposed a new concept, termed "agonist-directed trafficking of receptor stimulus", that suggests that agonists may be able to selectively activate a subset of multiple signaling pathways coupled to a single receptor subtype. 5-HT2A and 5-HT2C receptors couple to phospholipase C-(PLC) mediated inositol phosphate (IP) accumulation and PLA2-mediated arachidonic acid (AA) release. Relative efficacies of agonists (referenced to 5-HT) differed depending upon whether IP accumulation or AA release was measured. For the 5-HT2C receptor system, some agonists (e.g. TFMPP) preferentially activated the PLC-IP pathway, whereas others (e.g. LSD) favored PLA2-AA. As expected, EC50's of agonists did not differ between pathways. For the 5-HT2A receptor system, all agonists tested had greater relative efficacy for PLA2-AA than for PLC-IP. In contrast, relative efficacies were not different for 5-HT2A agonists when sequential effects in a pathway were measured (IP accumulation vs. calcium mobilization). These data strongly support the agonist-directed trafficking hypothesis.

Interactions between effectors linked to serotonin receptors.

In general, there are two types of interactions between effector signaling pathways. "Homologous" interactions are those that occur within a receptor system to alter its own responsiveness, for example the loss of responsiveness (desensitization) that can occur upon agonist occupancy of a receptor. "Heterologous" interactions are those that occur between different receptor systems where the responsiveness of one receptor system is regulated (positively or negatively) by activation of another receptor system (i.e., "cross-talk"). Many, if not all receptors, couple to multiple cellular effector pathways and alterations in the responsiveness of a receptor system can be effector pathway-dependent which underscores the importance of studying each effector coupled to a receptor. Regulation of receptor system responsiveness, and consequently the efficacy of drugs, is a highly dynamic process. Perhaps by exploiting these interactions, new targets for pharmacotherapy may be uncovered which will provide for increased efficacy and specificity of drug action.

A modified citrulline assay of NOS activity in rat brain homogenates does not detect direct effects of halothane on the kinetics of NOS activity.

An improved citrulline radioassay of nitric oxide synthase (NOS) activity was developed to study the direct effects of the volatile anesthetic (VA) halothane on the enzyme kinetics of neuronal NOS derived from different regions of the rat central nervous system (CNS). The Vmax of NOS in both soluble cytosolic and membrane bound particulate fractions varied across regions with greatest activity in the cerebellum and least in the spinal cord. In contrast, the Km was not different across regions or in the cytosolic and particulate fractions. Halothane at 0.5, 1, 2 or 3% delivered concentration had no effect on either kinetic parameter of NOS in any of the regions studied indicating that the VAs have no direct effects on NOS activity.

Synthesis, affinity at 5-HT2A, 5-HT2B and 5-HT2C serotonin receptors and structure-activity relationships of a series of cyproheptadine analogues.

Cyproheptadine is a drug that shows high affinity for type 2 (5-HT2) receptors. We studied a series of compounds obtained by modification of the tricyclic system of Cyp (dibenzocycloheptadiene): 2f (thioxanthene), 2g (xanthene), 2h (dihydrodibenzocycloheptadiene), 2j (diphenyl), 2i (fluorene), and 3b (phenylmethyl). Their activities at the rat cerebral cortex 5-HT2A receptor were (pKi +/- S.E.M.): 8.80 +/- 0.11 (Cyp), 8.60 +/- 0.07 (2f), 8.40 +/- 0.02 (2g), 8.05 +/- 0.03 (2h), 7.87 +/- 0.12 (2j), 6.70 +/- 0.02 (2i) and 6.45 +/- 0.02 (3b); those at the rat stomach fundus 5-HT2B receptor (pA2 +/- S.E.M.) were: 9.14 +/- 0.25 (Cyp), 8.49 +/- 0.07 (2f), 7.58 +/- 0.58 (2g), 7.02 +/- 0.14 (2h), 6.07 +/- 0.20 (2j), and undetectable (2i, 3b): and those at the pig choroidal plexus 5-HT2C receptor (pKi +/- S.E.M.) were: 8.71 +/- 0.08 (Cyp), 8.68 +/- 0.01 (2f), 8.58 +/- 0.20 (2g), 7.95 +/- 0.05 (2h), 7.57 +/- 0.04 (2j), 6.98 +/- 0.04 (2i) and 6.63 +/- 0.20 (3b). The slopes did not differ significantly from unity. The compounds exhibited the same order of activities at every type of receptor, and the most active molecules presented certain steric (butterfly conformation of the tricyclic system) and electrostatic (proton affinity on the top of the central rings) patterns. It is concluded that the activity of cyproheptadine derivatives at 5-HT2 receptors is related to these molecular features, which make feasible a common disposition to interact with all three 5-HT2 subtypes.

Kinetic studies of desensitization and resensitization of the relaxation response to beta-2 adrenoceptor agonists in isolated guinea pig trachea.

Activation of beta-2 adrenoceptors (BAR) in smooth muscle preparations is associated with a rapid, reversible and incomplete receptor desensitization, resulting in a steady-state relaxation response to BAR agonists. Based on results from cell culture studies, we hypothesize that, in the isolated guinea pig trachea, this steady state is a result of a concurrent resensitization of desensitizing BAR. In tracheal segments maintained at mechanical tone (4-6 g), isoproterenol (ISO) and the partial BAR agonist salbutamol (SALB) elicited a monotonic, rapid (1-3 min) and reproducible relaxation response that could be maintained for up to 45 min and was completely reversed by propranolol. Similarly, tissues preconstricted with 0.1 microM carbachol (CARB) responded with a sustained relaxation response to ISO. In contrast, in tissues preconstricted with 0.3 to 10 microM CARB or with 75 mM KCl, the relaxation elicited by ISO was followed by a slow (20-30 min) and partial restoration of muscle tone ("fade"). The relaxation and fade were observed when CARB-constricted tissues were relaxed with SALB (0.2 or 10 microM) or 10 microM salmeterol. No response to SALB was observed when tissues were preconstricted with KCl. The fade met criteria for its classification as a homologous desensitization of the relaxation response at the BAR level. In desensitized washed tissues, a complete recovery of the original relaxation response could be detected within 60 min of drug removal. A propranolol- and ICI 118-551-sensitive steady state was achieved 30 to 35 min after the addition of BAR agonists to the isolated tissues. A three-compartment phenomenological kinetic model accurately described the observed data, defining one steady-state and three rate constants, describing relaxation (k1), desensitization (k2) and resensitization (k3). The values of k2 and k3 for the response to SALB and to salmeterol were significantly larger than those observed for ISO. In the presence of KCl, the values of k2 and k3 for the response to ISO were indistinguishable from those measured in the presence of CARB. Given the parameters defined by our model, we propose that desensitization and resensitization of BAR in the isolated guinea pig trachea are distinct concurrent processes whose net result actively maintains a sustained partial relaxation response to ISO, SALB or salmeterol. The component of resensitization in the presence of agonist may account for the clinical efficacy of inhaled BAR agonists.

5-hydroxytryptamine2C receptor activation inhibits 5-hydroxytryptamine1B-like receptor function via arachidonic acid metabolism.

We previously reported that in Chinese hamster ovary (CHO) cells, 5-hydroxytryptamine (5-HT)1B-like (CHO/5-HT1B) receptor-mediated inhibition of forskolin-stimulated cAMP accumulation is inhibited by activation of transfected human 5-HT2C receptors but not 5-HT2A receptors. In the current study, we investigated the mechanism involved in the regulation of receptor-mediated inhibition of adenylyl cyclase as a means to further elucidate differences between the signal transduction cascades of the 5-HT2A and 5-HT2C receptor subtypes. Activation of 5-HT2C receptors with 5-HT or (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane increased release of arachidonic acid via a phospholipase A2 (PLA2)-dependent mechanism. Incubation with (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (1 microM) abolished 5-carboxamidotryptamine (5 nM)-mediated inhibition of forskolin-stimulated cAMP accumulation, which was blocked by the PLA2 inhibitor mepacrine (100 microM) and the cyclooxygenase inhibitor indomethacin (2 microM). Furthermore, purinergic receptor-mediated PLA2 activation as well as direct activation of PLA2 with melittin reduced CHO/5-HT1B responsiveness. These data indicate that activation of the PLA2/arachidonic acid signaling cascade mediates 5-HT2C receptor regulation of the CHO/5-HT1B receptor pathway. Consistent with our previous report and in contrast to activation of 5-HT2C or purinergic receptors, activation of 5-HT2A receptors had no effect on CHO/5-HT1B receptor function, although 5-HT2A receptor-mediated activation of PLA2 was measured. Interestingly, purinergic receptor-mediated inhibition of CHO/5-HT1B receptor function was blocked when 5-HT2A receptors were activated simultaneously. These data suggest that the lack of 5-HT2A mediated regulation of CHO/5-HT1B receptors may be due to activation of a third pathway (in addition to PLC and PLA2 pathways), which results in the inhibition of the production or the actions of a cyclooxygenase-dependent arachidonic acid metabolite.

Lack of 5-hydroxytryptamine1A-mediated inhibition of adenylyl cyclase in dorsal raphe of male and female rats.

In the rat hippocampus, 5-hydroxytryptamine (5-HT)1A receptors couple to two independent effector mechanisms, the inhibition of adenylyl cyclase activity and the opening of a K+ channel. In the dorsal raphe, 5-HT1A receptors also open K+ channels; however, coupling to adenylyl cyclase has not been demonstrated. In this study, the selective 5-HT1A agonists (+/-)- 8-hydroxy-2-(di-n-propylamino)tetralin, (R+)-8-hydroxy-2-(di-n-propylamino)tetralin and dipropyl-5-carboxamidotryptamine, did not inhibit forskolin-stimulated adenylyl cyclase (FSAC) activity in raphe region homogenates, although these drugs were efficacious in hippocampal homogenates. Other 5-HT1A agonists, NAN-190, BMY-7378, buspirone and gepirone, were also ineffective in raphe region homogenates. Estrogen-treatment of ovariectomized female rats, which is known to enhance 5-HT1A-mediated inhibition of FSAC in the hippocampus, did not promote the action of 5-HT1A agonists. Nor did activation of 5-HT1A receptors stimulate basal adenylyl cyclase activity in raphe homogenates as it does in the hippocampus. FSAC activity was inhibited in raphe region homogenates by activation of adenosine A1 or gamma-aminobutyric acidB receptors or by direct activation of the inhibitor G-protein, Gi, with guanyl-5'-6'-imidodiphosphate, indicating that the raphe homogenates have the biochemical machinery for inhibition of FSAC. High affinity binding studies showed that, in raphe homogenates, 5-HT1A receptors were expressed at a density comparable to that of adenosine A1 receptors and that they were coupled to G-proteins. It should be noted that our failure to observe 5-HT1A-mediated inhibition of adenylyl cyclase in the raphe does not prove that such coupling does not exist. However, a lack of 5-HT1A receptor coupling to adenylyl cyclase in the raphe would support contentions that coupling of the 5-HT1A receptor to adenylyl cyclase may be independent of its coupling to the K+ channel and that there may be distinct differences between pre- and postsynaptic 5-HT1A receptor systems.

A locus of the gonadotropin-releasing hormone receptor that differentiates agonist and antagonist binding sites.

The decapeptide gonadotropin-releasing hormone controls reproductive function via interaction with a heptahelical G protein-coupled receptor. Because of molecular model of the receptor predicts that Lys121 in the third transmembrane helix contributes to the binding pocket, the function of this side chain was studied by site-directed mutagenesis. Substitution of Arg at this position preserved high affinity agonist binding, whereas Gln at this position reduced binding below the limits of detection. Leu and Asp at this locus abolished both binding and detectable signal transduction. The EC50 of concentration-response curves for coupling to phosphatidyl inositol hydrolysis obtained with the Gln121 receptor was more than 3 orders of magnitude higher than that obtained for the wild-type receptor. In order to determine whether the increased EC50 obtained with this mutant reflects an altered receptor affinity, the effect of decreases in wild-type receptor density on concentration-response curves was determined by irreversible antagonism. Progressively decreasing the concentration of the wild-type receptor increased the EC50 values obtained to a maximal level of 2.4 +/- 0.2 nM. Comparison of this value with the EC50 of 282 +/- 52 nM observed with the Gln121 receptor mutant indicates that the agonist affinity for this mutant is reduced more than 100-fold. In contrast, antagonist had comparable high affinities for the wild-type, Arg121, and Gln121 mutants. The results indicate that a charge-strengthened hydrogen bond donor is required at this locus for high affinity agonist binding but not for high affinity antagonist binding.

Respiratory function in poultry workers and pharmacologic characterization of poultry dust extract.

A group of 343 workers (252 males and 91 females) employed in four poultry farms in Croatia was studied for the prevalence of acute and chronic respiratory symptoms and lung function changes. There were significantly higher prevalences of chronic cough, chronic phlegm, chronic bronchitis, and chest tightness in poultry workers than in control workers. Male poultry workers who were smokers had significantly higher prevalences of chronic cough, chronic phlegm, and chronic bronchitis than poultry workers who were nonsmokers (P<0.01). Poultry workers exposed for more than 10 years had significantly higher symptoms prevalences than those workers with shorter exposures (except among female smokers). There was also a high prevalence in poultry workers of acute symptoms which developed during the work shift. The measured FVC, FEV1, and FEF25 in poultry workers were significantly lower than predicted normal values. Workers exposed for more than 10 years had lower ventilatory capacity tests (expressed as percentage of predicted) than those workers with shorter exposures. Changes in FEV1, FEF50, and FEF25 were less pronounced than FVC. Additionally we showed that a water-soluble poultry dust extract obtained from this workplace caused a dose-related contraction of nonsensitized guinea pig tracheal smooth muscle when studied in vitro. Pharmacologic studies of this response indicate that it may result from the release of multiple endogenous mediators. Our data suggest that work in poultry farms may, for some workers, cause the development of acute and chronic respiratory symptoms and lung function changes.

Pharmacologic characterization of wool dust extract in isolated guinea pig trachea.

Wool mill workers develop respiratory symptoms and lung function abnormalities associated with their work in the textile industry. As in other workplaces, which process organic materials, the dust generated in the manufacture of wool has been implicated as a cause of these respiratory problems. Pharmacologic studies of wool dust extract were performed in vitro on guinea pig tracheal (GPT) segments. A wool dust extract (WDE) was prepared from material collected from a mill previously surveyed. When the standardized WDE solution was added to an organ bath in increments of 10, 30, 100, 300, and 1000 microliters it caused a consistent, dose-dependent constriction of GPT. Pretreatment of guinea pig tracheas, prior to WDE challenge, with atropine (10(-6) M), pyrilamine (10(-6) M), indomethacin (10(-6) M), verapamil (10(-6) M), TMBS (10(-6) M), BW755C (10(-6) M) and LY171883 (10(-6) M) was studied in order to evaluate receptor-dependent and -independent characteristics of WDE-induced constriction. WDE-induced bronchoconstriction was partially inhibited by the antihistamine pyrilamine. Atropine and leukotriene inhibitors (LY171883 and BW755C) were not found to have a significant protective effect on WDE-induced constriction. Both TMBS and verapamil (intra- and extracellular calcium blocking agent) suppressed the effect of wool dust extract in the range tested. These findings suggest that in this model, WDE-induced airway constriction is only partly attributable to common mediators of bronchoconstriction (e.g., histamine). The airway effects of WDE may be modulated by calcium channel blocking agents.