The pharmacology of RS‐15385‐197, a potent and selective α 2 ‐adrenoceptor antagonist

1 The acute behavioural effects of the a2-adrenoceptor antagonists, yohimbine, idazoxan and delequamine (RS-15385-197) were compared in two tests of exploratory behaviour in the rat, operated in tandem. These were the elevated X-maze test (5 min) and a modified holeboard test (12 min), which comprised a holeboard arena with a small roof in one corner as a 'refuge'. Rats were first placed into this corner, thus enabling measurements of initial emergence latency and the number of forays. The experiments were always done with a concomitant vehicle control group, with 10-12 rats per group, and with the treatment blinded. 2 In order to validate the tests, the effects of representatives of four classes of psychoactive agents were examined, viz. picrotoxin (anxiogenic), chlordiazepoxide (anxiolytic), (+)-amphetamine (stimulant) and diphenhydramine (sedative). The modified holeboard tended to be more sensitive than the measurement of total arm entries in the elevated X-maze at detecting drug effects on exploratory behaviour, but unlike the X-maze it could not clearly identify each class of agent. Thus, picrotoxin (5 mg kg-', i.p.) reduced total arm entries and open arm exploration in the X-maze (P<0.02) and suppressed most measures of activity in the holeboard (P<0.05); chlordiazepoxide (7.5 mg kg-1, i.p) increased total arm entries and open arm exploration (P<0.02) in the X-maze, without clear-cut effects in the holeboard; (+)-amphetamine (1 mg kg-, i.p.) had no significant effects in the X-maze, but increased most holeboard activities (P<0.05), and diphenhydramine (30 mg kg-', i.p.) reduced total arm entries in the X-maze (P<0.002) and hole exploration in the holeboard (P<0.05). 3 The actions of yohimbine most closely resembled those of picrotoxin. In the elevated X-maze, yohimbine (3 mg kg , i.p.) decreased the total number of arm entries (P<0.02); a larger dose (10 mg kg-', i.p.) also reduced time spent on the open arms (P<0.02). In contrast, delequamine (3 mg kg-, i.p.) and idazoxan (3 mg kg-', i.p.) had no effect. 4 In the partially-shaded holeboard, yohimbine (3 mg kg-', i.p.) suppressed hole exploration (P<0.05); a higher dose (10 mg kg-', i.p.) increased emergence latency (P<0.002) and virtually abolished all activity. Delequamine (3 mg kg-, i.p.) and idazoxan (3 mg kg-', i.p.) did not influence emergence latency or holeboard activities.5 The extent of the blockade of central a2-adrenoceptors achieved during the tests was assessed by the ability of the doses used to reverse mydriasis induced by clonidine (300 Ag kg-', s.c.) in anaesthetized rats. At a dose of 3 mg kg-, i.p., delequamine and idazoxan produced a rapid, sustained reversal of the clonidine response (by 87 ±2 and 86 ± 2% respectively, 30 min after injection) whereas yohimbine produced a partial reversal of only 43 ± 13%. The higher dose of yohimbine used in the exploratory tests (10 mg kg'-, i.p.) was required in order to achieve 77 ± 4% reversal of clonidine-induced mydriasis.6 We therefore conclude that blockade of central a2-adrenoceptors per se...

Section: Introductionmentioning

confidence: 99%

A re‐evaluation of the role of α₂‐adrenoceptors in the anxiogenic effects of yohimbine, using the selective antagonist delequamine in the rat

Redfern

Williams

1995

“…Rat kidney membranes (300-500 pg protein) were incubated with 1.0 nM ['11-ida-zoxan (Amersham, U.K., 40-50 Ci mmol-') for 90 min at 25°C in the presence of the selective cx2-adrenoceptor antagonist, delequamine (RS-15385-197 ; 0.1 pM; Brown et al, 1993), and various concentrations of drugs in a final assay volume of 0.5 ml assay buffer (50 mM Tris HCl, pH 7.4 containing 0.5 mM EDTA). Non-specific binding was determined in the presence of the selective al-adrenoceptor antagonist, cirazoline (1 pM).…”

Section: Methodsmentioning

confidence: 99%

“…Previous studies have shown that inclusion of 0.1 gM delequamine (RS-15385-197;Brown et al, 1993) 50 yig i.c.v. (i.e.…”

Section: Binding Studiesmentioning

confidence: 99%

RS‐45041‐190: a selective, high‐affinity ligand for I₂ imidazoline receptors

Brown¹,

MacKinnon²,

Redfern³

et al. 1995

Self Cite

1 RS-45041-190 (4-chloro-2-(imidazolin-2-yl)isoindoline) showed high affinity for 12 imidazoline receptors labelled by [3H]-idazoxan in rat (pKi = 8.66 ± 0.09), rabbit (pKi = 9.37 ± 0.07), dog (pKi = 9.32 ± 0.18) and baboon kidney (pKi = 8.85 ± 0.12), but had very low affinity for a2-adrenoceptors in rat cerebral cortex (pKi = 5.7 ± 0.09).2 RS-45041-190 showed low affinity for other adrenoceptors, dopamine, 5-hydroxytryptamine, and muscarinic receptors and dihydropyridine binding sites (selectivity ratio>1000). 3 RS-45041-190 showed moderate potency for the inhibition of monoamine oxidase A in vitro (pIC50= 6.12), but had much lower potency for monoamine oxidase B (pICs = 4.47), neither of which equated with its affinity for I2 receptors. 4 RS-45041-190 (0.001 to 3 mg kg-', i.v. and 1 ng-50 yg i.c.v.) had only small, transient effects on blood pressure and heart rate in anaesthetized rats. In conscious rats, RS-45041-190 had no effect on body core temperature or tail skin temperature (1 mg kg-', s.c.) or on activity or rotarod performance (10 mg kg-1, i.p.). There were also no effects on barbiturate sleeping time in mice after doses of 1-10 mg kg-, i.p.5 RS-45041-190 (10 and 25 mg kg-', i.p.) significantly increased food consumption in rats for up to 4 h after dosing, but unlike idazoxan (10 mg kg-, i.p.) did not increase water consumption. 6 RS-45041-190 is therefore a selective, high-affinity ligand at I2 imidazoline receptors and its hyperphagic effect may suggest a role for I2 imidazoline receptors in the modulation of appetite. However, in the absence of a selective agonist it is unclear whether this ligand is an agonist or an antagonist at I2 receptors.

“…Few of these studies addressed the question of the local pharmacology of the rat tail vasculature. During in vivo testing of the potent, selective M2-adrenoceptor antagonist, delequamine (RS-15385-197;Brown et al, 1993;Redfern et al, 1993), we casually observed an apparent warming of the tail following its administration to conscious rats. We therefore set out to study this phenomenon more systematically, addressing such questions as to whether it was a general feature of x2-adrenoceptor antagonists and whether this was a local or central effect.…”

Section: Introductionmentioning

confidence: 99%

The role of α₂‐adrenoceptors in the vasculature of the rat tail

Redfern¹,

MacLean

Clague³

et al. 1995

1 The effects of x2-adrenoceptor agonists and antagonists on rat tail skin temperature (t,,), an indicator of local cutaneous blood flow, were studied in conscious and anaesthetized rats and in the isolated, Krebs perfused, vascular bed of the rat tail.2 In conscious rats, at an ambient temperature of 18.5-20'C, t,, was 21.0 ± 0.2'C and core (rectal) temperature (tJ) was 38.2 ± 0.04'C (n = 126). The M2-adrenoceptor antagonist, delequamine (RS-15385-197 per dose).3 The maximum increases in t,, in response to a dose of 1 mg kg', s.c. of z2-adrenoceptor antagonists were as follows (n = 6 per drug): delequamine (+ 9.6 ± 0.8'C)> yohimbine (+ 9.0 ± 1.0C) > WY-26703 (+ 7.9 ± 1.3C)> piperoxan (+ 5.6 ± 1.7C)> idazoxan (+ 4.6 ± 1.3C)> imiloxan (+ 4.1 ± 1.3C)> SKF 104078 (+ 2.0 ± 1.9°C) > BDF-6143 (+ 1.3 ± 0.8°C).4 Prazosin (0.3 mg kg-', s.c.), hydralazine (10 mg kg-', s.c.) and nifedipine (3 mg kg', s.c.) did not increase tt,, whereas propranolol (10 mg kg-', s.c.) evoked a small increase in tt, (+ 2.9 ± 1.0C). Pentolinium (2-10 mg kg-', s.c.) elicited a dose-related increase in t,,, which was elevated by 4.4 ± 1.3°C after a dose of 10 mg kg-'; tc was reduced in a dose-related manner. Drug vehicles (1 ml kg'-, s.c.) had no effect on t,, or tc.5 In anaesthetized rats, idazoxan (300 pg, i.v.) produced a rapid increase in t,, which was detectable 2 min after beginning the injection, reaching a peak after 7 min. When the same dose was administered i.c.v., tt, also rose, but more slowly. The peak response (+ 3.6 ± 0.70C, n = 5) was significantly smaller than when idazoxan was administered intravenously (+ 6.3 ± 1.2°C, n = 5), which suggests that the increase in tt, following systemic administration of M2-adrenoceptor antagonists is not due to a central effect. The change in tt, was not secondary to changes in blood pressure. 6 In the isolated, Krebs perfused, tail vascular bed of the rat, at an ambient temperature of 20-21C, under constant flow conditions (3.5-4.0 ml min-'; n = 4), baseline perfusion pressure was 57 ± 4 mmHg. 5-Hydroxytryptamine (5-HT; 70-150 nM) increased perfusion pressure by 56± 9 mmHg. The M2-adrenoceptor agonist, UK-14,304 (10 nmol), elicited a further increase in perfusion pressure by 27.5 ± 15 mmHg but had no effect in the absence of 5-HT; this response to UK-14,304 was abolished by rauwolscine (1 JAM). 7 Under constant pressure conditions (-I100 mmHg; n = 9), baseline mean perfusion flow was 2.1 ± 0.2 ml min-', and mean tail skin temperature was 31.6 ± 0.6C. 5-HT (119 ± 28 nM) decreased t. by 3.3 ± 2.0°C and reduced flow by 1.2 ± 0.3 ml min-'. UK-14,304 (10 nmol) further reduced t,, by 3.0 ± 0.3°C without significant effect on flow; this effect was also abolished by 1 JAM rauwolscine.8 We conclude that post-junctional M2-adrenoceptors in the vasculature of the rat tail have a major vasoconstrictor role, controlling both the flow and distribution of blood within the tail and thereby thermoregulatory heat loss from its surface.