Inhibition of acetylcholine-induced activation of extracellular regulated protein kinase prevents the encoding of an inhibitory avoidance response in the rat.

It has been demonstrated that the forebrain cholinergic system and the extracellular regulated kinase signal transduction pathway are involved in the mechanisms of learning, encoding, and storage of information. We investigated the involvement of the cholinergic and glutamatergic systems projecting to the medial prefrontal cortex and ventral hippocampus and of the extracellular regulated kinase signal transduction pathway in the acquisition and recall of the step-down inhibitory avoidance response in the rat, a relatively simple behavioral test acquired in a one-trial session. To this aim we studied by microdialysis the release of acetylcholine and glutamate, and by immunohistochemistry the activation of extracellular regulated kinase during acquisition, encoding and recall of the behavior. Cholinergic, but not glutamatergic, neurons projecting to the medial prefrontal cortex and ventral hippocampus were activated during acquisition of the task, as shown by increase in cortical and hippocampal acetylcholine release. Released acetylcholine in turn activated extracellular regulated kinase in neurons located in the target structures, since the muscarinic receptor antagonist scopolamine blocked extracellular regulated kinase activation. Both increased acetylcholine release and extracellular regulated kinase activation were necessary for memory formation, as administration of scopolamine and of extracellular regulated kinase inhibitors was followed by blockade of extracellular regulated kinase activation and amnesia. Our data indicate that a critical function of the learning-associated increase in acetylcholine release is to promote the activation of the extracellular regulated kinase signal transduction pathway and help understanding the role of these systems in the encoding of an inhibitory avoidance memory.

Effects of novelty and habituation on acetylcholine, GABA, and glutamate release from the frontal cortex and hippocampus of freely moving rats.

The involvement of the forebrain cholinergic system in arousal, learning and memory has been well established. Other neurotransmitters such as GABA and glutamate may be involved in the mechanisms of memory by modulating the forebrain cholinergic pathways. We studied the activity of cortical and hippocampal cholinergic, GABAergic and glutamatergic systems during novelty and habituation in the rat using microdialysis. After establishing basal release of the neurotransmitters, the animals were transferred to a novel environment and allowed to explore it twice consecutively for 30 min (60 min apart; exploration I and II). The motor activity was monitored. Samples were collected throughout the experiment and the release of acetylcholine (ACh), GABA and glutamate was measured. During the two consecutive explorations of the arena, cortical and hippocampal, ACh release showed a significant tetrodotoxin-dependent increase which was higher during exploration I than II. The effect was more pronounced and longer-lasting in the hippocampus than in the cortex. Cortical GABA release increased significantly only during exploration II, while hippocampal GABA release did not increase during either exploration. Motor activity was higher during the first 10 min of exploration I and II and then gradually decreased during the further 20 min. Both cortical and hippocampal ACh release were positively correlated with motor activity during exploration II, but not during I. During exploration II, cortical GABA release was inversely correlated, while hippocampal GABA release was positively correlated to motor activity. No change in cortical and hippocampal glutamate release was observed. In summary, ACh released by the animal placed in a novel environment seems to have two components, one related to motor activity and one related to attention, anxiety and fear. This second component disappears in the familiar environment, where ACh release is directly related to motor activity. The negative relationship between cortical GABA levels and motor activity may indicate that cortical GABAergic activity is involved in habituation.

Neurotensin modulation of acetylcholine and GABA release from the rat hippocampus: an in vivo microdialysis study.

The effects of neurotensin (NT) on the release of acetylcholine (ACh), aspartate (Asp), glutamate (Glu) and gamma-aminobutyric acid (GABA) from the hippocampus of freely moving rats were studied by transversal microdialysis. ACh was detected by High Performance Liquid Chromatography (HPLC) with electrochemical detection while GABA, glutamate and aspartate were measured using HPLC with fluorometric detection. Neurotensin (0.2 and 0.5 microM) administered locally through the microdialysis probe to the hippocampus produced a long-lasting and concentration-dependent increase in the basal extracellular levels of GABA and ACh but not of glutamate and aspartate. The increase in the extracellular levels of GABA and ACh produced by 0.5 microM neurotensin in the hippocampus reached a maximum of about 310% for GABA and 250% for ACh. This stimulant effect of NT was antagonized by the NT receptor antagonist SR 48692 (100 microg/kg, i.p.). Local infusion of tetrodotoxin (1 microM) decreased the basal release of ACh, GABA, Asp, Glu and prevented the 0.2 microM NT-induced increase in GABA and ACh release. The effect of NT on the release of ACh was blocked by the GABA(A) receptor antagonist bicuculline (2-10 microM). Our findings indicate for the first time that neurotensin plays a neuromodulatory role in the regulation of GABAergic and cholinergic neuronal activity in the hippocampus of awake and freely moving rats. The potentiating effects of neurotensin on GABA and ACh release in the hippocampus are probably mediated by (i) NT receptors located on GABAergic cell bodies and (ii) through GABA(A) receptors located on cholinergic nerve terminals.

Neuronal receptors for caerulein in gallbladder.

Release of [3H]-acetylcholine ([3H]ACh) and muscle contractions in response to caerulein was measured and recorded simultaneously from isolated guinea-pig gallbladder. Caerulein (5 x 10(-9) M) enhanced the release of [3H]ACh and the contractions. Tetrodotoxin (10(-6) M) inhibited the caerulein-induced release of [3H]ACh by only 30%. Proglumide and loxiglumide (CR 1505) antagonized the effect of caerulein on both ACh-release and contractions. Neither proglumide nor loxiglumide affected the DMPP-induced contractions and the ACh release. The results suggest not only muscular but also neuronal receptors for caerulein in guinea-pig gallbladder.

Cholecystokinin-GABA interactions in rat striatum.

The release of [3H]gamma-aminobutyric acid ([3H]GABA) from rat striatal slices before and during electrical field stimulation (EFS) was measured. Electrical stimulation (10 Hz) induced an increase of Ca(++)- and tetrodotoxin-sensitive [3H]GABA release from the striatal slices. In the presence of sulphated octapeptide of cholecystokinin, CCK-8S (10(-9) M, 10(-8) M and 10(-7) M) both the basal and the electrically (10 Hz)-evoked release of [3H]GABA were dose-dependently increased. These effects of CCK-8S were abolished by tetrodotoxin (10(-6) M) and were not influenced by the CCK-A receptor antagonist loxiglumide (CR1505) (10(-7) M and 10(-6) M). The stimulant effect of CCK-8S was antagonized by the newly synthesized CCK-B selective receptor antagonist PD134308 (10(-7) M and 10(-6) M). These findings suggest that CCK-8 plays a neuromodulatory role in the regulation of GABAergic neuronal activity in the striatum. The activation of CCK-B receptors located on GABAergic neurons is involved in the GABA release-potentiating effect of CCK-8S in rat striatum.

Effect of somatostatin on the canine gallbladder motility.

Somatostatin (SOM) at doses up to 1 microgram was not effective on the motility of canine and guinea pig gallbladder smooth muscle preparations in vitro. When the preparations were contracted by field electrical stimulation (0.7 ms, 40 Hz) the cholecystokinin octapeptide (CCK OP) enhanced these contractions while SOM inhibited them. These effects were accompanied, respectively, by an increase or a decrease in [3H] acetylcholine (ACh) release in the intrinsic cholinergic nerve terminals. SOM (0.5 to 2 micrograms/kg i.v.) inhibited the spontaneous and the CCK OP-activated gallbladder pressure in conscious dogs. The effect of atropine (10-50 micrograms/kg) was similar to that of SOM when injected intravenously in conscious dogs. It is suggested that the inhibitory effect of SOM on gallbladder pressure in conscious dogs is probably mediated by a decrease in ACh release by cholinergic neurons.

Effect of loxiglumide (CR 1505) on CCK-induced contractions and 3H-acetylcholine release from guinea-pig gallbladder.

Release of [3H]-acetylcholine (3H-ACh) and muscle contractions in response to cholecystokinin (CCK) were measured and recorded simultaneously from isolated guinea-pig gallbladder. Cholecystokinin octapeptide (CCK8) (10(-10)-10(-7) M) enhanced the release of [3H]ACh and the contractions of the muscle. TTX (10(-6) M) inhibited the CCK-induced release of 3H-ACh by only 30%. In Ca(2+)-free medium CCK8 had no effect. Loxiglumide, (CR 1505), a newly synthesized nonpeptide CCK-A-receptor antagonist, D.L-(3,4-dichlorbenzoilamino)-5-/N-(3-methoxypropyl)-pentylamin o-5-oxo-pentanoi c acid, antagonized both the ACh-releasing effect of CCK and the contractions in a dose-dependent manner. The affinity (pA2) of CR 1505 to CCK-receptors, determined by the shift of the concentration-response curves for CCK8 was 8.36. It was 5 logarithmic orders higher than the pA2 of proglumide. The IC50 value of CR 1505 calculated by the CCK-induced release of 3H-ACh was 10 nM. The results suggest the existence not only of muscular CCK receptors but also neuronal receptors for CCK probably located on cholinergic nerves.

Effect of neurotensin and immunneutralization with anti-neurotensin-serum on dopaminergic-cholinergic interaction in the striatum.

The effect of neurotensin (NT) on the release of acetylcholine (ACh) and dopamine (DA) from striatal slices of the rat brain was studied. Neurotensin, 1-150 nM, was able to release ACh from cholinergic interneurons of the striatum. Like the response to electrical stimulation, the ACh-releasing effect of NT was completely inhibited by tetrodotoxin indicating that neuronal firing is involved in its effect. Immunneutralization reduced the stimulation-evoked release of ACh, an effect that was much marked when the inhibitory dopaminergic input was suspended by sulpiride-selective antagonists of D2 receptors. Sulpiride, 0.1 mM, induced a 2-fold increase in the NT- and electrically-induced release of ACh. A quantitatively similar increase was also observed after degeneration of the nigrostriatal DA pathway with 6-hydroxydopamine (6-OHDA) (2 x 250 micrograms/animal, i.c.v.). However, the D2 receptor agonist quinpirole, 0.01 mM, significantly reduced the NT-induced release of ACh by 77%. Neurotensin enhanced the stimulation-evoked release of [3H]DA. These findings indicate that, using field stimulation when dopaminergic, cholinergic and NT-containing neurons are stimulated in concert, NT is capable of releasing both ACh and DA in the striatum, but its effect on ACh release is masked unless the D2 receptor-mediated tonic inhibitory effect of DA released from the nigro-striatal pathway is attenuated. Thus, in Parkinson's disease where the dopaminergic input is impaired, NT may be involved in producing cholinergic dominance.

Cholecystokinin [corrected] octapeptide modulates dopamine release in rat striatum.

The effect of cholecystokinin octapeptide sulfated (CCK8S) on the basal and electrically evoked release of [3H]dopamine ([3H]DA) in striatal slices from the rat brain was studied. Cholecystokinin octapeptide did not influence the basal release of [3H]DA. Field electrical stimulation (FES) (2 Hz) induced an increase of dopamine release from striatal slices, which was Ca2+ dependent and was abolished by tetrodotoxin, 10(-6) M. Cholecystokinin octapeptide (10(-9) M, 10(-8) M and 10(-7) M) dose dependently reduced the electrically evoked release of [3H]DA. This effect was antagonized by the CCK-A receptor antagonists loxiglumide (10(-7) M, 10(-6) M and 10(-5) M) or proglumide (10(-5) M, 10(-4) M and 10(-3) M). The results suggest that CCK receptors type A are involved in this effect of CCK8S in the striatum.

Activation of non-NMDA receptors stimulates acetylcholine and GABA release from dorsal hippocampus: a microdialysis study in the rat.

The effect of the non-N-methyl-D-aspartate (NMDA) agonists (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and quisqualate (QUIS) on the release of acetylcholine (ACh), gamma-amino butyric acid (GABA), aspartate (Asp) and glutamate (Glu) from the hippocampus of freely moving rats was studied by transversal microdialysis. Intracerebroventricular (i.c.v.) administration of the non-NMDA receptor agonist AMPA (0.5 nmol) enhanced (by about 200%) ACh release from the hippocampus. The effect of AMPA was completely antagonized by 6-nitro-7-sulphamoyl-benz(f)quinoxaline-2,3-dione (NBQX; 2 nmol, i.c.v). No effect was seen when AMPA was perfused through the septum. However, AMPA (200 microM) locally applied to the hippocampus, increased (by about 200%) ACh release. QUIS (200 microM) applied locally to the hippocampus produced a long-lasting increase in the release of ACh (by about 215%) and GABA (by about 460%). Local infusion of tetrodotoxin (1 microM) decreased ACh and GABA basal extracellular levels, and abolished the QUIS-induced increase in ACh and GABA. Our results demonstrate that non-NMDA glutamatergic receptors in the hippocampus regulate hippocampal release of GABA and ACh.

The role of prostaglandins in the spontaneous and cholinergic nerve-mediated motility of guinea-pig gastric muscle.

The role of PGE1, PGE2 and PGF2 alpha in the spontaneous and cholinergic nerve-mediated motility of guinea-pig gastric muscle strips was investigated. The prostaglandins (PGs) studied induce dose-dependent tonic activation in the longitudinal strips from fundus, corpus and antrum and inhibition of the phasic activity of circular strips from antrum and pyloric sphincter. These effects are not significantly changed in the presence of adrenergic and cholinergic blocking agents nor with tetrodotoxin, and are, therefore, direct effects on the smooth muscle. Indomethacin exerts effects opposite to those of PGs. The contractile responses of the longitudinal and circular smooth muscles to field stimulation (frequency of 10-20 Hz, duration of 0.5 ms and supramaximal voltage) are inhibited by atropine (1 X 10(-6) M), indicating that cholinergic transmission is involved. PGs potentiate the response of the longitudinal muscles but inhibit the response of the circular muscles to field stimulation. On the contrary, indomethacin (1 X 10(-6) to 1 X 10(-5) M) inhibits the response of the longitudinal and potentiates the response of the circular muscles to field stimulation. The data suggest that PGs may exert a negative feedback control over the excitatory transmission in the circular muscle of the guinea-pig stomach.

Effect of prostacyclin (PGI2) on the mechanical activity of isolated longitudinal and circular muscle strips of guinea-pig stomach.

Muscle strips isolated in longitudinal and circular directions from the fundus, corpus and antrum, and from the pyloric sphincter of the guinea-pig stomach were placed in organ baths for recording their spontaneous contractility. Concentrations of the order of 10(-9) to 10(-6) M of prostacyclin (PGI2) were tested and compared with the effect of PGE1, PGE2, PGF2 alpha and acetylcholine. Furthermore, a modification of PGI2 effect was studied in the presence of adrenergic and cholinergic blocking agents, tetrodotoxin, indomethacin and the PG antagonist SC-19220. Like PGE1, PGE2, PGF2 alpha and acetylcholine, PGI2 increased the tone of the longitudinal strips from fundus, corpus and antrum, its effect being less potent than the effects of PGE1 and PGE2 and more potent than the effects of PGF2 alpha and acetylcholine. All the PGs inhibited the phasic contractions of the circular muscle of antrum and pyloric sphincter in a dose-dependent manner. These effects were not significantly changed in the presence of adrenergic and cholinergic blocking agents, nor in the presence of tetrodotoxin, and could therefore be interpreted as being myogenic in nature. Indomethacin exerted effects opposite to those of PGI2. It shifted the log concentration-effect curve for PGI2 to the right suppressing the maximum response of PGI2 by about 50%. SC-19220 reversibly inhibited the spontaneous tone and the excitatory responses of the gastric muscle to PGI2. The concentration-effect curves for PGI2 were shifted to the right in the presence of SC-19220. Analysis of the data gave the pA2 value for PGI2 5.3, the slope of Schild plot being 1.23, which suggests that SC-19220 is a competitive antagonist.

Mode of action of cholecystokinin octapeptide on smooth muscles of stomach, ileum and gall bladder.

Mechanical activity of preparations isolated from canine and guinea-pig stomach, ileum and gall bladder was recorded. At least 2 strips were cut out from each organ and investigated simultaneously in thermostatically controlled organ baths. Responses to acetylcholine (ACH) were used for comparison. Octapeptide of cholecystokinin (CCK-OP) at concentrations of 5 X 10(-11) M to 10(-8) M produced dose-dependent tonic concentrations in all muscle strips and showed a higher affinity but lower efficacy as compared to ACH. Atropine (10(-7) M 10(-5) M) had no effect on the CCK-OP responses in stomach and gall bladder muscle strips but it significantly decreased the CCK-OP responses in ileum muscle strips. Dibutyryl cyclic GMP (dbc GMP) at concentrations of 10(-5) M-5 X 10(-4) M did not change the ACH dose-response curves but shifted to the right in parallel to the control the dose-response curves for CCK-OP in all muscle preparations with pA2 values 5, 5.3, 7.2, and 6 for canine stomach and guinea-pig stomach, ileum and gall bladder, respectively. Michaelis-Menten's analysis suggested a competitive type of interaction of dbc GMP on CCK-OP contractile responses of guinea-pig ileum and gall bladder; at a higher concentration (5 X 10(-4) M) the antagonistic effect of dbc GMP in canine and guinea-pig stomach appeared to be a mixed or uncompetitive type. The data suggest that the contractile effects of CCK-OP in the iliac smooth muscle are caused by cholinergic and direct mechanisms whereas on the gall bladder and gastric smooth muscles by direct myogenic mechanisms only.

Cholinergic mechanism in cholecystokinin action on gall bladder motility.

The participation of cholinergic mechanisms in cholecystokinin octapeptide (CCKOP) action on canine gall bladder was studied in vivo and in vitro, using three different experimental conditions. In vitro the responses of canine gall bladder smooth muscle to CCKOP (0.01 to 10 nm) were insensitive to atropine (1 to 10 microM) and tetrodotoxin (3 microM). When gall bladder muscle preparations were contracted by field electrical stimulation (0.7 ms, 40 Hz) CCKOP (0.001 to 0.1 nM) enhanced these contractions while atropine (1 microM) abolished them. This suggests that CCKOP is able to influence acetylcholine (ACH)-release from intrinsic cholinergic nerve terminals. In vivo the responses of canine gall bladder smooth muscle to CCKOP (1 to 10 ng/kg i.v.) were reduced and even abolished by atropine (10 to 50 micrograms/kg i.v.) and hexamethonium (0.5 to 3 mg/kg i.v.). The results suggest the participation of at least two mechanisms in CCKOP action on canine gall bladder motility: a direct action on smooth muscle cells, insensitive to atropine or tetrodotoxin, and an indirect action, which is dependent on pre- and post ganglionic cholinergic pathways.

Antagonistic effect of polyphloretin phosphate and di-4-phloretin phosphate on the contractile responses of guinea-pig gastric muscles to prostaglandin E2.

Polyphloretin phosphate (PPP) and di-4-phloretin phosphate (DPP) were studied for their ability to antagonize the contractions produced by prostaglandin E2 (PGE2) on isolated guinea-pig gastric muscles. PPP (100, 200 and 300 micrograms/ml) and DPP (10, 30 60 and 100 micrograms/ml) inhibited reversibly the PGE2-produced excitatory effect in longitudinal fundus and corpus muscles but did not influence the PGE2-produced inhibitory effect in circular muscle strips. In the presence of PPP and DPP the concentration-effect curves for PGE2 were shifted to the right parallel to control curves. The antagonistic action of PPP and DPP appeared to be a specific one since the compounds had little, if any, effect on the responses to histamine, acetylcholine and BaCL2. Analysis of the data (1) showed that DPP is a more potent antagonist than PPP against PGE2 on guinea-pig gastric muscle with pA2 6,1 and 4,8, respectively.

Study on the prostaglandin antagonistic activity of aligeron and piracetam.

The interaction of aligeron and piracetam with the effects of prostaglandin F2 alpha (PGF2 alpha) E2 (PGE2) was studied using in vitro and in vivo tests for evaluation of PG antagonistic activity. Aligeron was found to be a non-selective PG antagonist in isolated guinea-pig stomach smooth muscle preparations. It antagonized the PGF2 alpha and PGE2-induced fall in blood pressure in cats prevented diarrhoea induced by PGF2 alpha in mice, inhibited rat paw oedema induced by PGE2 in rats, but did not modify the PGF2 alpha induced bronchoconstriction in guinea-pigs. Piracetam did not antagonize the smooth muscle contractile effects of PGF2 alpha and PGE2 and in the in vivo tests it inhibited only the rat paw oedema induced by PGE2. It is concluded that aligeron is active in vitro and in vivo as an antagonist of some of the actions of PGs studied. Its effect in vitro lacks selectivity and is probably due to interference with the action of Ca2+. The probable clinical implication of these results will be discussed. Piracetam can not be considered a PG antagonist.

Responses of guinea-pig gastric, ileal and gall bladder smooth muscle to desamino-cholecystokinin-octapeptide (CCK 7).

Cholecystokinin 7 (CCK 7), a synthetic analogue of cholecystokinin/pancreozymin (CCK 33), increased in a dose-dependent manner the tone of the guinea-pig ileal, gastric and gall bladder smooth muscle preparations. In all these preparations CCK 7 was more potent than CCK 8 and CCK 33 and all three cholecystokinins were more potent than acetylcholine (ACH). Atropine and tetrodotoxin (TTX) did not influence the CCK 7 action in fundus and gall bladder muscle strips but reduced non-competitively its effect in ileal muscle strips. Neither GE 410 nor dbcGMP affected the ACH and histamine (His) response of the muscle strips but both antagonists shifted the dose-response curve of CCK 7 to the right, GE 410 (cholecystokinin antagonist) being a much more potent antagonist of CCK 7 as compared to dbcGMP. In all muscle strips a competitive action on the CCK 7 responses was found for GE 410. In gastric muscle strips a competitive influence on the CCK 7 responses was found for dbcGMP at low concentration (1 x 10(-5)M) and a non-competitive influence at high concentration (5 x 10(-4)M). The results suggest that the contractile effects of CCK 7 in the isolated ileal smooth muscle are realized by cholinergic and direct myogenic mechanisms, whereas in the isolated gall bladder and gastric smooth muscles, by a direct myogenic mechanism only.

Suc-Tyr(SE)-Met-Gly-Trp-Met-Asp-beta-phenethylamide(410): a competitive antagonist of cholecystokinin-induced contractions in smooth muscles in vitro.

Suc-Tyr(SE)-Met-Gly-Trp-Met-Asp-beta-phenethylamide(410) has been studied for its ability to antagonize contractile responses of guinea pig gall bladder, ileum and stomach muscle strips to desamino-cholecystokinin-octapeptide (CCK-7) and cholecystokinin octapeptide (CCK-8). Both CCK-7 and CCK-8 at concentrations of 10(-11)M to 10(-7)M produced dose-dependent tonic contractions in all muscle strips. Suc-Tyr(SE)-Met-Gly-Trp-Met-Asp-beta-phenethylamide (10(-8)M-10(-5)M) inhibited reversibly in a dose-dependent manner the contractile responses to CCK-7 and CCK-8. At the same concentrations the antagonist shifted to the right in parallel to the dose-response curves for CCK-7 and CCK-8 without decreasing their maximum response. Analysis of the data after Schild gave pA2 values (410 potency as antagonist) of CCK-7 in gall bladder, ileum and stomach of 8.36; 8.0 and 7.56, respectively, and pA2 values of CCK-8 of 7.64; 8.94 and 8.52, respectively. The slope of the Schild plots for both CCKs did not differ significantly from the unity, which suggests that 410 is a competitive antagonist. The antagonistic action of 410 is reversible and appeared to be specific since at concentrations of 5 X 10(-6), it had no effect on contractile responses of the gall bladder, ileum and gastric muscle strips to acetylcholine or histamine.

Functional and neurochemical evidence that neurotensin-induced release of acetylcholine from Auerbach's plexus of guinea-pig ileum is presynaptically controlled via alpha 2-adrenoceptors.

The effect of neurotensin (NT) on [3H]acetylcholine release and contraction from isolated longitudinal muscle strip of guinea-pig ileum was examined. Neurotensin dose-dependently enhanced the release of [3H]-acetylcholine. This effect of neurotensin was inhibited by stimulation of alpha 2-adrenoceptors: noradrenaline, clonidine, xylazine or dexmedetomidine (alpha 2-adrenoceptor agonists) inhibited neurotensin-induced release of acetycholine (ACh) as well as the contractions, while CH-38083 or yohimbine (alpha 2-adrenoceptor antagonist) prevented this inhibitory effect. Our findings suggest that neurotensin may play a neuromodulatory role in the regulation of cholinergic neuronal activity in the gut and this modulatory effect is continuously controlled by the tonic activity of the sympathetic nervous system: endogenous noradrenaline release is capable of reducing the release of ACh and the consequent contraction of the gut enhanced by neurotensin.

Antagonistic effect of SC-19220 on the responses of guinea-pig gastric muscles to prostaglandins E1, E2 and F2 alpha.

SC-19220 has been studied for its ability to antagonize the contractions produced by prostaglandins E1, E2 and F2 alpha (PGE1, PGE2, PGF2 alpha) on isolated guinea-pig gastric muscle. At concentrations of 1 X 10(-6) M to 3 X 10(-4) M SC-19220 inhibited reversibly in a dose-dependent manner both the spontaneous tone and the phasic activity. At the same concentrations the compound had no effect on the inhibitory responses of circular muscle strips to PGE1, PGE2 and PGF2 alpha. In contrast, SC-19220 antagonized the excitatory responses of longitudinal muscle strips to PGE1, PGE2 and PGF2 alpha. The concentration-effect curves for PGE1, PGE2 and PGF2 alpha were shifted to the right. Analysis of the data ( Arunlakshana and Schild, 1959) gave the pA2 values of PGE2, PGE1 and PGF2 alpha 4.90, 6.28 and 4.16, the slopes of the Schild plots being 1.05, 1.18 and 1.11 respectively. This suggests that SC-19220 is a competitive antagonist. Moreover, the antagonistic action of SC-19220 appeared to be a specific one since at concentrations of 3 X 10(-6) M to 3 X 10(-4) M the compound had a little, if any, effect on the responses of the gastric muscle to acetylcholine and histamine. This favors the suggestion that the longitudinal layer of the guinea-pig stomach contains prostaglandins receptors which are the same for PGE1, PGE2 and PGF2 alpha. SC-19220 specifically blocks these prostaglandin receptors characterized by the order of agonist potency PGE2 greater than PGE1 greater than PGF2 alpha.