Kiril Milenov

Antinociceptive effect of somatostatin microinjected into caudate putamen.

The effects of somatostatin microinjected bilaterally and unilaterally (left or right) at a dose of 10, 50 and 100 ng into the caudate putamen of male Wistar rats on nociception (analgesy-meter test) were studied. Somatostatin injected into caudate putamen resulted in analgesia. Bilateral microinjections of somatostatin significantly increased the pain threshold in a dose-dependent manner, i.e. somatostatin exerted antinociceptive effect. The pain threshold after left-side microinjections was significantly higher than that after injections into right-side. These findings suggest antinociceptive and asymmetric effects of somatostatin on pain in the caudate putamen.

Cholinergic-nitrergic interactions in the guinea-pig gastric fundus

The influence of nitric oxide (NO) on the spontaneous tone and on the contractile responses to electrical field stimulation or to exogenous acetylcholine (ACh) was studied. Circular strips from the guinea-pig gastric fundus were used. The NO-releasing compound sodium nitroprusside reduced the spontaneous tone while the NO-synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) increased it. The L-NAME-induced increase of the tone was antagonized by atropine or indomethacin, suggesting the involvement of cholinergic and prostaglandinergic pathways in this effect. L-NAME significantly potentiated the ACh (10(-8) to 10(-5) M)-induced contractions. L-NAME concentration-dependently potentiated the cholinergic contractions evoked by electrical field stimulation without affecting [3H]ACh overflow from [3H]choline-treated tissues. It is concluded that electrical field stimulation of gastric fundus muscle induces the release of endogenous nitrate which, in turn, functionally antagonizes cholinergic neurotransmission.

Somatostatin stimulates striatal acetylcholine release by glutamatergic receptors: an in vivo microdialysis study.

The modulation of striatal cholinergic neurons by somatostatin (SOM) was studied by measuring the release of acetylcholine (ACh) in the striatum of freely moving rats. The samples were collected via a transversal microdialysis probe. ACh level in the dialysate was measured by the high performance liquid chromatography method with an electrochemical detector. Local administration of SOM (0.1, 0.5 and 1 microM) produced a long-lasting and concentration-dependent increase in the basal striatal ACh output. The stimulant effect of SOM was antagonized by the SOM receptor antagonist cyclo(7-aminopentanoyl-Phe-D-Trp-Lys-Thr[BZL]) (1 microM). In a series of experiments, we studied the effect of 6,7-dinitroquinoxaline-2, 3-dione (DNQX), a selective non-NMDA (N-methyl-D-aspartate) glutamatergic antagonist, on the basal and SOM-induced ACh release from the striatum. DNQX, 2 microM, perfused through the striatum had no effect on the basal ACh output but inhibited the SOM (1 microM)-induced ACh release. The non-NMDA glutamatergic receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylendioxy-5H-2,3- benzodiazepine (GYKI-52466), 10 microM, antagonized the SOM (1 microM)-induced release of ACh in the striatum. Local administration of the NMDA glutamatergic receptor antagonist, 2-amino-5-phosphonopentanoic acid (APV), 100 microM, blocked SOM (1 microM)-evoked ACh release. Local infusion of tetrodotoxin (1 microM) decreased the basal release of ACh and abolished the 1 microM SOM-induced increase in ACh output suggesting that the stimulated release of ACh depends on neuronal firing. The present results are the first to demonstrate a neuromodulatory role of SOM in the regulation of cholinergic neuronal activity of the striatum of freely moving rats. The potentiating effect of SOM on ACh release in the striatum is mediated (i) by SOM receptor located on glutamatergic nerve terminals, and (ii) by NMDA and non-NMDA glutamatergic receptors located on dendrites of cholinergic interneurones of the striatum.

The effect of vasoactive intestinal polypeptide (VIP) on the canine gallbladder motility

1. VIP at doses of 10(-9) to 10(-8) M was ineffective and at doses of 5 x 10(-8) to 10(-7) M exerted a slight inhibitory effect on the tone of the canine gallbladder muscle strip. However, VIP (0.1-1 micrograms/kg) injected intravenously (i.v.) in conscious dogs dose-dependently decreased the gallbladder pressure. 2. VIP did not influence significantly the acetylcholine (ACh)- or carbachol- induced contractions of canine gallbladder under in vitro or in vivo conditions, but it decreased the electrically-induced, atropine-sensitive contractions of gallbladder muscle strips. 3. VIP (5 x 10(-9) to 5 x 10(-8) M) did not influence significantly the dose-response curve for cholecystokinin octapeptide (CCK OP) of canine and guinea-pig gallbladder muscle strips. VIP injected i.v. (0.1-0.5 micrograms/kg) in conscious dogs greatly decreased the CCK OP-induced gallbladder pressure.

Behavioral effects of somatostatin microinjected into caudate putamen.

The present study examined the behavioral responses to bilateral microinjections of somatostatin (SRIF) into caudate putamen of male Wistar rats. SRIF locally administered at doses of 10, 50 and 100 ng/side dose-dependently affected locomotor activity, as reflected in both horizontal and vertical movements. SRIF modulated locomotor activity in a biphasic manner, exerting an inhibitory and a facilitatory effect. In the elevated plus-maze experiments, SRIF at doses of 50 and 100 ng/side microinjected bilaterally into caudate putamen decreased only the total number of entries in the open and closed maze arms, confirming the suppressing effect of SRIF on locomotion at the first 5 min.

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 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 cholecystokinin octapeptide and somatostatin on the mechanical and electrical activity of the colon of conscious dogs

Intraluminal pressure and electrical activity of the colon of conscious dogs were recorded using silver bipolar electrodes and a pressure transducer. Two phases in the mechanogram of the transverse colon were observed: a quiescent phase (lack of contractions) lasting 12.05 ± 0.54 min and a contractile phase lasting 14.46 ± 0.88 min. The electrical activity was characterized by a quiescent phase (only slow waves in the electrogram) with a duration of 13.55 ± 1.73 min. Groups of spike potentials bursting in the rhythm of the slow waves appeared during the activity phase lasting 15.60 ± 2.02 min. Cholecystokinin octapeptide (CCK8) (10–20 ng/kg i.v.) significantly increased the duration of 1–3 active phases and shortened the duration of the quiescent phases. The percentage of slow waves with spike potentials during the active phase in the electro‐gram increased. Colonic contractions also increased, i.e. CCK8 evoked an enhanced motility of the colon. Somatostatin (1–2 μg/kg i.v.) increased by two to three times the duration of the quiescent phases. Atropine (50–100 μg/kg i.v.) or somatostatin (1–2 μg/kg i.v.) inhibited both the spontaneous and the CCK8‐induced colonic motility. It is suggested that the inhibitory effect of somatostatin on spontaneous and CCAV induced colonic activity in conscious dogs is mediated by a decrease in cholinergic neurotransmission.

Effect of cholecystokinin octapeptide and somatostatin on the motility of guinea pig and canine gallbladder

Species differences have been observed in the effect of cholecystokinin octapeptide (CCK OP) on the canine and guinea pig gallbladder smooth muscle motility. 1. CCK OP was more potent stimulant in canine than in guinea pig gallbladder smooth muscles. Its pD2 values were 10 and 9.2, respectively. 2. The acetylcholine (10(-4) M)-induced maximum contractions in canine gallbladder muscle strips were by 50% lower as compared to the CCK OP (10(-8) M) maximum responses while in guinea pig gallbladder muscle strips the acetylcholine (ACh) maximum responses were by 20% lower than the CCK OP maximum responses. 3. CCK OP increased [3H]ACh release by 27% in canine gallbladder and by 40% in guinea pig gallbladder. 4. Somatostatin (SOM) had not any direct myogenic effect in guinea pig and canine gallbladder but it decreased [3H]ACh release from gallbladder intrinsic cholinergic neurons.

Effect of neurotensin on the canine gallbladder motility: in vivo and in vitro experiments

Neurotensin (NT) (10(-8)-10(-6)) exerted a dose-dependent increase in the tone and release of [3H]ACh in the guinea-pig gallbladder muscle strips but was inefficient in the canine gallbladder muscle strips. However, in conscious dogs NT (2.5-20 ng/kg intravenously (i.v.)) dose-dependently increased the gallbladder pressure. Similar was the effect of CCK8 (1-10 ng/kg i.v.) and carbachol (0.5-2 micrograms/kg i.v.). The NT- or CCK8-induced gallbladder pressure was inhibited by atropine (10-50 micrograms/kg i.v.) or hexamethonium (0.5-3 mg/kg i.v.). Somatostatin (1-2 micrograms/kg i.v.) or VIP (0.5-1 microgram/kg i.v.) also reduced or even abolished the NT- or CCK8-induced gallbladder pressure. The NT-induced increase of the tone of guinea-pig gallbladder preparations was accompanied by an increase of [3H]ACh release, suggesting the involvement of cholinergic innervation.