Xylazine-induced sedation in chicks is inhibited by opiate receptor antagonists

Central depressant effects and toxicity of propofol in chicks

Propofol is an ultra-short acting anesthetic agent. The information on the pharmacological and toxicological effects of propofol in the chicken is rather limited. This study examines the toxicity and pharmaco-behavioral effects of propofol given intraperitoneally in 7–10 day-old chicks. The median effective doses of propofol for the induction of sedation, analgesia to electric stimulation and sleep in the chicks were 1.82, 2.21 and 5.71 mg/kg, respectively. The 24-h median lethal dose of propofol in chicks was 57.22 mg/kg. The therapeutic indices of propofol for sedation, analgesia and sleep were 31.4, 25.9 and 10, respectively. Propofol at 0.5 and 1 mg/kg reduced the locomotor activity and increased the duration of tonic immobility in chicks. Propofol at 2 and 4 mg/kg caused analgesia to electric stimulation as well as analgesia and anti-inflammatory responses against formalin test in chicks. Propofol at 5, 10 and 20 mg/kg induced sleep in chicks for 8.4 to 25 min. Physostigmine shortened the sleep duration of propofol. Data suggest that propofol induces anti-inflammatory action and central nervous system depression in chicks resulting in sedation, analgesia and anesthesia with wide safety margin. These effects could form the basis of further pharmacological and toxicological studies on propofol in the young chick model, and the drug could be safely applied clinically in the chicken.

Sedative and hypnotic effects of combined administration of metoclopramide and ketamine in chickens

Metoclopramide is a dopamine receptor antagonist used in animals as both an antiemetic and a gastroprokinetic agent. In chickens, the drug causes central nervous system depression. The authors examined the potential sedative and hypnotic effects of metoclopramide when administered in combination with the anesthetic agent ketamine in 1-3-week-old chicks. Concomitant administration of metoclopramide and ketamine markedly reduced the median effective doses (ED50s) of both drugs for the induction of sedation and sleep in the chicks. The results suggest potential therapeutic applications of the metoclopramide-ketamine combination as a restraining agent in avian species not intended for human consumption.

Metoclopramide-induced central nervous system depression in the chicken

Background

Metoclopramide is a dopamine D2-receptor antagonist used as an antiemetic and gastroprokinetic agent in man and animals. The drug causes sedation as a side effect in man. Such a sedative action of metoclopramide has not been documented in the chicken as the drug is not used clinically in this species. The present study examines the central nervous system depressant effects of metoclopramide in 7–14 days old broiler chicks.

Results

Injection of metoclopramide at 50, 100 and 200 mg/kg, subcutaneously (s.c.) induced sedation in the chicks in a dose dependent manner. The chicks manifested, within 3.6–19 minutes of metoclopramide injection, signs of sedation characterized by drooping of the head and wings, closed eyelids, reduced motility and decreased distress calls. The duration of sedation ranged between 37.2 to 163.4 minutes. Metoclopramide at 100 and 200 mg/kg induced, within 12.2 and 6.2 minutes, sleep (loss of righting reflex) for 43.8 and 158.6 minutes, respectively. The median effective doses of metoclopramide for induction of sedation and sleep in the chicks were 11 and 53 mg/kg, s.c., respectively. Lower doses of metoclopramide (5 and 10 mg/kg, s.c.) significantly decreased the open-field activity of the chicks and increased the durations of their tonic immobility. All treated-chicks recovered from the central nervous system depressant effect of metoclopramide without any observable adverse effects.

Conclusion

The data suggest that metoclopramide induces central nervous system depression in chicks, and the drug could have potential clinical applications as a sedative-hypnotic agent in avian species not intended for human consumptions.

Intravenous anesthesia

Anticholinergics, tranquilizers, and sedative-hypnotics are the usual agents used for preanesthetic sedation of the horse. Of these drugs, the anticholinergics are of little importance in the horse. Acepromazine is the most useful and widely used tranquilizer, whereas xylazine is a safe and popular sedative. A newer sedative recently made available to the veterinarian for clinical use in horses is detomidine. Thiobarbiturates are seldom used alone any longer but are still useful when combined with guaifenesin for induction and maintenance of anesthesia. Other, more contemporary drug combinations that have largely replaced thiobarbiturates and chloral hydrate include xylazine with ketamine, xylazine with Telazol, detomidine with Telazol, and guaifenesin with ketamine and xylazine.

The effects of yohimbine and four other antagonists on amitraz-induced depression of shuttle avoidance responses in dogs

The ability of five antagonists to prevent the central nervous system's depressant effects of amitraz in dogs was evaluated using a shuttle avoidance paradigm. All drugs were injected iv into six male dogs trained to avoid a 1-mA shock by jumping over a hurdle within 10 sec of the start of a tone. Dogs were given an antagonist or saline followed in 5 min by 3 mg/kg amitraz dissolved in dimethyl sulfoxide (DMSO) or DMSO alone. After pretreatment with saline, amitraz decreased significantly the means number of avoidance responses and increased significantly the means latencies of avoidance responses. After pretreatment with the alpha 2-adrenoreceptor antagonist yohimbine (0.1 mg/kg), amitraz no longer decreased the means number of avoidance responses or lengthened the means latencies of avoidance responses. The nonselective alpha-adrenoreceptor antagonist tolazoline (3.3 mg/kg) prevented amitraz-induced increases in means latencies, but did not prevent the decreases in the means number of avoidances. The alpha 1-adrenoreceptor antagonist prazosin (1 mg/kg), the muscarinic receptor antagonist atropine (0.04 mg/kg), and the opioid receptor antagonist naloxone (1 mg/kg) did not prevent either of amitraz's effects. The data suggest that the amitraz-induced suppression of avoidance responding is mediated by alpha 2-adrenoreceptors rather than by alpha 1-adrenergic, muscarinic, or opioid receptors.

Possible role of alpha-2 and alpha-1 adrenoceptors in the experimentally-induced depression of the central nervous system

The alpha-2 adrenoceptor agonists clonidine and xylazine were employed in chicks and rats to induce a loss of the righting reflex, a sign for depression of the central nervous system. These effects of clonidine and xylazine were antagonized by yohimbine, idazoxan and CH-38083 (7,8-(methylenedioxi)-14-alpha-hydroxyalloberbane HCl), compounds having alpha-2 adrenoceptor antagonist properties. Prazosin, an antagonist for alpha-1 adrenoceptors, enforced the alpha-2 adrenoceptor agonist-induced depression in both species. 6-Hydroxydopamine treatment, which reduced the norepinephrine concentrations in the rat cerebral cortex by 76%, increased the duration of the loss of righting reflex induced by xylazine indicating that central postsynaptic alpha-2 adrenoceptors might also be involved in this behavioral alteration. The electrically-stimulated tritium release was also determined from the isolated rat cerebral cortex slices which had been preloaded with 3H-norepinephrine. Clonidine and xylazine inhibited the stimulation-induced tritium release and this inhibition was counteracted by yohimbine, idazoxan or CH-38083, but not by prazosin. We have concluded from the present data that stimulation of alpha-2 adrenoceptors with pre- and postsynaptic locations or inhibition of alpha-1 adrenoceptors in the central nervous system may shift the depression/vigilance balance to the direction of depression which might be accompanied by a decreased activity of cortical noradrenergic neural transmission.

Xylazine-evoked depression of rat cerebral cortical neurons: a pharmacological study

1. Iontophoretic application of the veterinary tranquilizer xylazine suppressed the spontaneous firing of 134/137 neurons tested, an effect not antagonized by adenosine receptor and alpha-adrenergic receptor blocking agents. 2. Depressions in neuronal activity evoked by norepinephrine, gamma-aminobutyric acid or adenosine failed to be potentiated by xylazine. 3. However, the excitatory effects of acetylcholine and glutamate were both inhibited by xylazine. 4. Thus, the depressant effect of locally applied xylazine may involve a decrease in the excitability of the neuronal membrane.

Objective assessment of detomidine-induced analgesia and sedation in the horse

The effects of detomidine, a veterinary sedative analgesic, were studied in the horse. Novel objective techniques were employed to assess the analgesic and sedative potency of this compound. Intravenous doses of 0.010, 0.020 and 0.040 mg/kg were administered to eight adult Thoroughbred racehorses according to a double-blind crossover design. Analgesia was measured by determining the latency to onset of the skin twitch and hoof withdrawal reflexes following noxious thermal stimulation of the withers and fetlock, respectively. Sedation was assessed by quantifying spontaneous locomotor activity in horses confined to an activity chamber, and by electronic measurement of head ptosis. Linear, dose-related increases in the intensity and duration of analgesia were observed. However, dose-related increases in duration, rather than intensity of sedation were observed. In conclusion, detomidine is an effective analgesic in the horse, but it is more potent in producing sedation than analgesia.

Injectable anesthetics and anesthetic adjuncts

The purpose of this article is to review the use of selected anesthetics and anesthetic adjuncts in horses. Emphasis is placed on the pharmacologic bases of their use.

The ocular effects of xylazine in rabbits, cats, and monkeys

Xylazine is an agent frequently used in combination with ketamine to anesthetize rabbits. Xylazine is also related pharmacologically to clonidine, a relatively selective alpha 2-agonist. In the present studies, xylazine was examined for its effects on intraocular pressure (IOP) and pupil diameter (PD) in rabbits, cats and monkeys and on noradrenergic function in the cat nictitating membrane (CNM) preparation. Topical and unilateral administration of xylazine (1.0 mg) lowered IOP bilaterally in normal, unanesthetized rabbits, cats and monkeys and caused unilateral miosis in rabbits and cats. These ocular effects of xylazine were attenuated in superior cervical ganglionectomized (SX) cats and rabbits. In addition, intra-arterially administered xylazine (10, 33 and 100 micrograms) produced dose-related inhibition of contractions of the CNM elicited by electrically stimulating the pre- and postganglionic sympathetic trunks without altering the response to i.a. norepinephrine (10 micrograms). These data suggest that ocular effects of xylazine are mediated, in part, by alteration of sympathetic neuron function. Xylazine suppressed ocular hypertension induced by water loading and IOP recovery rate following hypertonic saline infusion in rabbits suggesting that aqueous flow was inhibited. Topical pretreatment with 0.05 mg of timolol caused potentiation of the ocular hypotensive response to 0.05 mg of xylazine in rabbits. These results indicate that xylazine lowers IOP, in part, by suppressing sympathetic neuronal function which causes a reduction in aqueous flow. The augmented response to timolol and xylazine, as compared with either agent alone, suggests a rational basis for combining a prejunctionally active agent with a postjunctionally active one.

Cardiovascular action of detomidine, a sedative and analgesic imidazole derivative with alpha-agonistic properties

The cardiovascular effects of detomidine, a new veterinary sedative and analgesic imidazole derivative were studied in rats and cats using as reference compound xylazine, a widely employed veterinary antinociceptive and sedative drug with alpha-agonistic potency. Detomidine (1-30 micrograms/kg i.v.) and xylazine (10-1000 micrograms/kg i.v.) had both dose-dependent hypotensive and bradycardiac effects in anaesthetized rats. After i.v. administration of 3-100 micrograms/kg detomidine and 0.1-3 mg/kg xylazine to conscious rats, detomidine was more active in reducing the heart rate than in lowering the blood pressure. In anaesthetized cats, detomidine (1-30 micrograms/kg i.v.) was hypotensive and bradycardiac in a dose-dependent manner. A low dose of detomidine into the vertebral artery was more effective than i.v. application in reducing blood pressure. Idazoxan (0.3 mg/kg i.v. and 0.03 mg/kg into the vertebral artery) antagonized the hypotensive and bradycardiac effects of detomidine injected into the femoral vein or vertebral artery, respectively. In pithed rats, detomidine and xylazine stimulated presynaptic and postsynaptic alpha 2-adrenoceptors, and to a lesser extent postsynaptic alpha 1-adrenoceptors. The results indicate that detomidine is an agonist of central and peripheral alpha 2-adrenoceptors which exerts its hypotensive and bradycardiac effects via activation of the central alpha 2-adrenoceptors.