Effects of alpha-adrenoceptor agonists and antagonists on pre- and postsynaptically located alpha-adrenoceptors.

The effects of alpha adrenoceptor agonists and antagonists have been examined at pre- and post-synaptically located alpha-adrenoceptors in the pithed rat. The presynaptic receptors were those located at the cardiac sympathetic nerve terminals and the postsynaptic receptors were those present in vascular smooth muscle. Clonidine was approximately equipotent at pre- and post-synaptic alpha-adrenoceptors, whilst LSD and BAY-1470 were more active at the pre- than at post-synaptic sites. Oxymetazoline, naphazoline, methoxamine and phenylephrine were all much more active at the postsynaptic alpha-adrenoceptors. Phentolamine was the most potent antagonist at both pre- and post-synaptic alpha-adrenoceptors. Piperoxan, yohimbine and tolazoline were about 3-7X less potent than phentolamine at both sites. Thymoxamine was about 10X less potent than phentolamine at postsynaptic alpha-adrenoceptors but about 1000X less active at the presynaptic receptors. The differential actions of both agonsists and antagonists at pre- and post-synaptic alpha-adrenoceptors suggest that the receptors may be of different types.     

The effect of age on the sensitivity of pre- and postsynaptic alpha-adrenoceptors to agonists and antagonists in the rat

Summary Age-related changes in presynaptic alpha-2 and postsynaptic alpha-1 adrenoceptors have been determined using the rat isolated vas deferens and the thoracic aorta, respectively. The IC₅₀ values of clonidine, B-HT 933 and UK 14,304 for inhibition of the electrically evoked contractions of the vas deferens were significantly higher in 50 week old rats when compared with rats of 5 weeks. Similarly, EC₅₀ values for the contraction of the thoracic aorta by noradenaline, methoxamine and phenylephrine were significantly increased in 50 week old rats compared with 5 week old rats. No age-related changes in the potency of the selective alpha-2 adrenoceptor antagonists yohimbine and Wy 26392 were detected in the vas deferens. Similarly, there were no age-related changes in the alpha-1 adrenoceptor antagonist potency of indoramin or prazosin on the aorta.The results of the present study suggest that the potency of both alpha-1 and alpha-2 adrenoceptor agonists, as measured by their respective EC and IC₅₀ values decreases with increasing age.     

The modulatory role of vascular endothelium in the interaction of agonists and antagonists with alpha-adrenoceptors in the rat aorta.

1. We have examined the effect of endothelium on the antagonistic action of prazosin, doxazosin, yohimbine and phentolamine against phenylephrine, clonidine and noradrenaline. 2. The action of prazosin against phenylephrine was similar to that earlier reported against noradrenaline, acting as a non-competitive antagonist in the presence of endothelium and as a competitive antagonist in the absence of endothelium. Prazosin also acted as a non-competitive antagonist against clonidine in the absence of endothelium. 3. Doxazosin behaved in a similar way to prazosin against noradrenaline, phenylephrine and clonidine acting as a non-competitive antagonist in the presence of endothelium and as competitive antagonist after removal of endothelium. In contrast, yohimbine and phentolamine acted as competitive antagonists both in the presence and in the absence of endothelium. 4. Analysis of the concentration-response curves for noradrenaline, phenylephrine and clonidine in the presence and in the absence of endothelium showed that the affinity for all three agonists was the same but not the efficacy and the receptor reserve, both of which were lower in the presence than in the absence of endothelium. 5. The rank order of agonist potency in the absence of endothelium was noradrenaline greater than phenylephrine greater than clonidine. The rank order of antagonist potency was prazosin greater than or equal to doxazosin greater than phentolamine greater than yohimbine. 6. The results show that vascular endothelium modulates the contractile response to alpha-adrenoceptor agonists and also modifies the action of the antagonists prazosin and doxazosin but not that of yohimbine and phentolamine. This effect of endothelium was related to a change in agonist efficacy and receptor reserve. These results also suggest that the alpha-adrenoceptors of the isolated aorta of the rat are predominantly, if not exclusively of the alpha 1-subtype.     

Effects of alpha-adrenoceptor agonists and antagonists and of antidepressant drugs on pre- and postsynaptic alpha-adrenoceptors

The effects of alpha-adrenoceptor agonists and antagonists and of antidepressant drugs were studied on pre- and postsynaptic alpha-adrenoceptors. The rat vas deferens, stimulated at low frequency (0.1 Hz) was used for presynaptic studies. The rat anococcygeus muscle was used in postsynaptic studies. In the agonist studies clonidine and guanfacine were selective for presynaptic alpha-adrenoceptors, methoxamine and phenylephrine were selective for postsynaptic alpha-adrenoceptors and noradrenaline and alpha-methylnoradrenaline were equipotent at pre- and post-synaptic alpha-adrenoceptors. In the antagonist studies piperoxane and yohimbine were selective for presynaptic alpha-adrenoceptors, phentolamine was equipotent at pre- and postsynaptic alpha-adrenoceptors and prazosin was a selective postsynaptic alpha-adrenoceptor antagonist. In the series of antidepressants studied, mianserin was the most potent antagonist at presynaptic alpha-adrenoceptors, followed by trazodone, amitriptyline and nortriptyline in descending order of potency. Mianserin was approximately two hundred times less potent than piperoxane as a presynaptic alpha-adrenoceptor antagonist. Viloxazine and desipramine were inactive. With the exception of viloxazine all of the antidepressants examined possessed postsynaptic alpha-adrenoceptor antagonist properties.     

Displacement by alpha-adrenergic agonists and antagonists of 3H-prazosin bound to the alpha-adrenoceptors of the dog aorta and the rat brain

To characterize the alpha 1-adrenoceptors in the dog aorta and the rat brain and to assess the antagonistic potencies of various chemicals inclusive of newly synthesized ones, radioligand binding assays were performed, and the potencies thus obtained were compared with those obtained from pharmacological observations. Reproducible binding to and displacement from the dog aorta of 3H-prazosin were observed. The rank orders of the inhibition of 3H-prazosin binding to the membrane preparations of the aorta expressed as IC50 (the concentration of drugs inhibiting 50% of maximal specific binding of 3H-prazosin) were: prazosin greater than YM09538 greater than phentolamine greater than yohimbine greater than phenoxybenzamine greater than labetalol greater than S-596 greater than dibenamine greater than K-351 greater than propranolol greater than hydralazine greater than N-696 for alpha-blockers and I-epinephrine greater than clonidine greater than I-norepinephrine greater than phenylephrine greater than I-isoproterenol for agonists. IC50 values of phenoxybenzamine, labetalol, dibenamine and K-351 obtained in the brain preparations were higher than those obtained in the preparation of the aorta. There was a good correlation (r = 0.90) between the IC50 values obtained in the dog aorta and in the rat brain, suggesting that the alpha 1-adrenergic receptors in the dog aorta and the rat brain resemble each other. Good correlations (aorta, r=0.97 and brain, r=0.94) were also observed between IC50 values derived from the binding assay in the aorta or the brain and the pA2 values obtained as regards to the contractile response of the rat aorta to phenylephrine. Thus, this method could be useful for the assessment of newly synthesized chemicals as alpha-adrenergic antagonists.     

Behavioural effects of serotonin agonists and antagonists in the rat and marmoset

The present study was conducted to investigate the effects of various 5-hydroxytryptamine (5-HT) agonists and antagonists on motor behaviour in rats and marmosets. Various motor-based responses were assessed after central or peripheral administration of 5-HT agents to rats and marmosets. Drugs acting as agonists at the 5-HT1A receptor (8-OHDPAT, gepirone, BMY-7378, NAN-190, PAPP (LY165163) and flesinoxan) and 5-HT2/1C receptors (DOI) were found to reverse neuroleptic-induced catalepsy in the rat whereas 5-HT2/1C antagonists (mianserin, ritanserin and ICI-170,809) and the 5-HT1 antagonist ((+/-)pindolol) increased catalepsy. Agonists acting at 5-HT3 receptors (phenylbiguanide and 2-methyl-5-HT) had no effect on catalepsy. The putative 5-HT1A antagonist, (+/-) pindolol, attenuated the reversal of catalepsy by 8-OHDPAT. Although both 8-OHDPAT and BMY-7378 were tested, only the latter was found to reduce apomorphine-induced stereotypy. Bilateral or unilateral infusions of 8-OHDPAT, BMY-7378 or pindolol into the substantia nigra of non-lesioned rats had no effect on spontaneous locomotor or rotational activity, respectively. However, 8-OHDPAT and BMY-7378 were found to increase or decrease motor activity, after injection into the median or dorsal raphe nuclei, respectively. Finally, 8-OHDPAT and BMY-7378 were found to be inactive against MPTP-induced bradykinesia in the marmoset. It is concluded that both 5-HT1A and 5-HT2/1C receptors are involved in the anti-cataleptic effects of 5-HT agents. The 5-HT1A receptors are probably situated within the raphe, whereas the location of the 5-HT2/1C receptors remains undetermined.     

Action of histamine receptor agonists and antagonists on the rat uterus.

1 Histamine and a series of compounds acting selectively on H1- and H2-receptors were tested on the isolated oestrous uterus of the rat. 2 Histamine had a dose-dependent inhibitory effect on the contractions elicited by acetylcholine. This action was unaffected by H1-blockers but was competitively inhibited by H2-blockers. The H1-selective agonist, 2-(2-aminoethyl)thiazole was ineffective at doses 100 times greater than those of histamine. Conversely, all the H2-agonists showed activity in the order of potency: N'-methylhistamine greater than histamine greater than N'-N'-dimethylhistamine greater than 5-methylhistamine greater than 5-methyl-N'-methylhistamine. Among the non-imidazole compounds, dimaprit had an activity identical to that of histamine, but all the dimaprit-like compounds showed negligible activity. 3 The data obtained suggest that in the rat uterus, (a) the activation of H2-receptors is responsible for the inhibitory effect of histamine and its analogues; (b) the integrity of the histamine molecule seems to be less crucial than that of the dimaprit molecule for the maintenance of the H2-activity, since changes in its structure modify but do not abolish the biological activity as they do in the case of dimaprit; (c) the order of activity of the various H2-receptor agonists is different from that observed in other tissues.     

The effects of dopaminergic agonists and antagonists on the frequency-response function for hypothalamic self-stimulation in the rat.

The function of dopaminergic synapses in generating the reinforcing effect of brain stimulation was examined in 8 rats. The animals were implanted with bipolar electrodes and trained to press a bar for lateral hypothalamic stimulation. The frequency of stimulation pulses was systematically changed, and a frequency-response curve was plotted for each rat after intraperitoneal injection of a test agent. Dopamine agonists and antagonists selective to either D1 or D2 subtypes of receptors were used. The curve was shifted to a high-frequency range by either SCH 23390 (D1 antagonist) or raclopride (D2 antagonist). SKF 38393 (D1 agonist) failed to shift the curve, and quinpirole and CV 205-502 (D2 agonists) shifted the lower part of the curve to a low-frequency range. The results suggest that an activation of D2 receptors generates a reinforcing effect, and that the effect is expressed only if D1 receptors are activated to an optimal level.     

Central effects of histamine H2-receptor agonists and antagonists on nociception in the rat

The effects of intracerebroventricular injection of histamine H2-receptor agonists (4-methylhistamine, 4-MeH; dimaprit, DIM), H2-antagonists (cimetidine, CIM; ranitidine, RAN; famotidine, FAM) and of the DIM chemical analogue SK&F 91487 on hot-plate latency in rats were examined. Both DIM (0.4-0.8 mumol/rat) and 4-MeH (0.4-0.8 mumol/rat) significantly enhanced the pain threshold, whereas SF&F 91487 (0.8 mumol/rat) had no effect, indicating that DIM antinociception is specifically due to its activity on histamine (HA) receptors. The H2-antagonists CIM (0.8 mumol/rat) and RAN (0.6 mumol/rat) also enhanced the pain threshold, while FAM (0.03 mumol/rat) did not modify pain latency. When injected before 4-MeH, FAM reduced the antinociceptive effect of 4-MeH. These findings suggest that the antinociceptive activity of CIM and RAN is not related to specific blockade of H2-receptors and that the activation of HA-H2-receptors is inhibitory to nociception.     

Muscarinic agonists and antagonists: effects on the urinary bladder

Voiding of the bladder is the result of a parasympathetic muscarinic receptor activation of the detrusor smooth muscle. However, the maintenance of continence and a normal bladder micturition cycle involves a complex interaction of cholinergic, adrenergic, nitrergic and peptidergic systems that is currently little understood. The cholinergic component of bladder control involves two systems, acetylcholine (ACh) released from parasympathetic nerves and ACh from non-neuronal cells within the urothelium. The actions of ACh on the bladder depend on the presence of muscarinic receptors that are located on the detrusor smooth muscle, where they cause direct (M?) and indirect (M?) contraction; pre-junctional nerve terminals where they increase (M?) or decrease (M?) the release of ACh and noradrenaline (NA); sensory nerves where they influence afferent nerve activity; umbrella cells in the urothelium where they stimulate the release of ATP and NO; suburothelial interstitial cells with unknown function; and finally, other unidentified sites in the urothelium from where prostaglandins and inhibitory/relaxatory factors are released. Thus, the actions of muscarinic receptor agonists and antagonists on the bladder may be very complex even when considering only local muscarinic actions. Clinically, muscarinic antagonists remain the mainstay of treatment for the overactive bladder (OAB), while muscarinic agonists have been used to treat hypoactive bladder. The antagonists are effective in treating OAB, but their precise mechanisms and sites of action (detrusor, urothelium, and nerves) have yet to be established. Potentially more selective agents may be developed when the cholinergic systems within the bladder are more fully understood.     

Effect of alpha-adrenergic agonists and antagonists on neurotransmission in the rat anococcygeus muscle.

The isolated rat anococcygeus muscle was found to be a sensitive preparation for the study of the pre- and postsynaptic actions of α-adrenergic agonists and antagonists. The pre- and postsynaptic actions of α-agonists were characterized by comparing the order of potency (1) for directly contracting smooth muscle, (2) for inhibiting the field-stimulated release of tritium (³H) or ³H-norepinephrine, following labelling of adrenergic stores with ³H-NE and (3) for inhibiting twitch contractions elicited by field-stimulation. Clonidine, oxymetazoline tetrahydrozoline and naphazoline, listed in their order of potency, inhibited field-stimulated ³H-release and twitch contractions. With the exception of clonidine, a clear separation of pre- and postsynaptic α-mediated effects was not achieved with the other imidazolines tested. In contrast, methoxamine and phenylephrine did not inhibit twitch response or ³H-release, but did cause a direct contraction of the smooth muscle. The α-antagonists, phentolamine, yohimbine and phenoxybenzamine increased the stimulated release of ³H in a concentration dependent manner. At 10^?7 M, yohimbine preferentially blocked presynaptic α-receptors. Phentolamine was the most potent antagonist at pre- or postsynaptic α-receptors, but like phenoxybenzamine showed no selectivity for pre- or postsynaptic α-receptors. The percent inhibition of neurotransmitter released produced by the α-agonists was found to be inversely proportional to the train lenght, frequency of stimulation and extracellular calcium. Because of the differential effects of α-agonists at pre- and postsynaptic sites, α-receptors on nerve varicosities may have different structural requirements for blockade and activation than the α-receptors on smooth muscle.     

Muscarinic receptor agonists and antagonists: effects on cancer

Many epithelial and endothelial cells express a cholinergic autocrine loop in which acetylcholine acts as a growth factor to stimulate cell growth. Cancers derived from these tissues similarly express a cholinergic autocrine loop and ACh secreted by the cancer or neighboring cells interacts with M3 muscarinic receptors expressed on the cancer cells to stimulate tumor growth. Primary proliferative pathways involve MAPK and Akt activation. The ability of muscarinic agonists to stimulate, and M3 antagonists to inhibit tumor growth has clearly been demonstrated for lung and colon cancer. The ability of muscarinic agonists to stimulate growth has been shown for melanoma, pancreatic, breast, ovarian, prostate and brain cancers, suggesting that M3 antagonists will also inhibit growth of these tumors as well. As yet no clinical trials have proven the efficacy of M3 antagonists as cancer therapeutics, though the widespread clinical use and low toxicity of M3 antagonists support the potential role of these drugs as adjuvants to current cancer therapies.     

Muscarinic agonists and antagonists: effects on gastrointestinal function

Muscarinic agonists and antagonists are used to treat a handful of gastrointestinal (GI) conditions associated with impaired salivary secretion or altered motility of GI smooth muscle. With regard to exocrine secretion, the major muscarinic receptor expressed in salivary, gastric, and pancreatic glands is the M? with a small contribution of the M? receptor. In GI smooth muscle, the major muscarinic receptors expressed are the M? and M? with the M? outnumbering the M? by a ratio of at least four to one. The antagonism of both smooth muscle contraction and exocrine secretion is usually consistent with an M? receptor mechanism despite the major presence of the M? receptor in smooth muscle. These results are consistent with the conditional role of the M? receptor in smooth muscle. That is, the contractile role of the M? receptor depends on that of the M? so that antagonism of the M? receptor eliminates the response of the M?. The physiological roles of muscarinic receptors in the GI tract are consistent with their known signaling mechanisms. Some so-called tissue-selective M? antagonists may owe their selectivity to a highly potent interaction with a nonmuscarinic receptor target.     

Neuromuscular effects of some opioid agonists and antagonists

The effects of morphine, pethidine, (-) naloxone, N-methylnaloxone, naltrexone and levallorphan on acetylcholine-induced contraction of the toad rectus were studied. The drugs were shown to inhibit the contraction, and their inhibitory effect was suggested to be partly mediated via a peripheral opiate binding site. The depression of acetylcholine-induced contraction by levallorphan and dextrallorphan might indicate possible involvement of stereospecific binding sites, as the latter required a significantly higher concentration to produce the same magnitude of depression. Statistical analysis of the slope of the computer-plotted dose-response of acetylcholine in the presence and absence of each of the opioids indicates that these drugs can be classified into four categories. Morphine and naltrexone each formed a class of its own; (-) naloxone, N-methylnaloxone and pethidine formed another class; levallorphan and dextrallorphan formed the fourth class. The classification of the opioids into four categories reveals the possible existence of multiple opiate binding sites on the skeletal muscle. The significance of each of the sub-types of binding sites in the contraction of skeletal muscle and the mechanism by which it affects the contraction remains to be investigated.     

The effects of ions on the binding of agonists and antagonists to muscarinic receptors.

1 There are no selective effects of Na+, K+, Ca2+, Mg2+ or Cl- on the binding of antagonists or agonists to muscarinic receptors in rat brain. A decrease in affinity related to ionic strength is found for all these ions. 2 Larger effects were produced by T1+, La3+, and some transition metal ions.