Efficacy of an adenosine A1 receptor agonist compared with atropine and pralidoxime in a rat model of organophosphate poisoning

The objective of this study was to evaluate the effects of an adenosine A1 agonist, phenylisopropyl adenosine (PIA), on metamidophos poisoning compared to specific antidotes. Rats were poisoned with metamidophos (30 mg/kg, oral) and observed for 24 hours. One group received sodium chloride (1 mL/kg) and four experimental groups received atropine (5 mg/kg), pralidoxime (PAM, 20 mg/kg), atropine/PAM (5/20 mg/kg) or PIA (1 mg/kg) intraperitoneally. Atropine reduced salivation and prevented respiratory distress when compared to sodium chloride-treated rats. Treatment with PAM did not cause any suppression of cholinergic signs. Atropine and PAM combination prevented salivation, convulsion and respiratory distress. PIA delayed initial time of the salivation, convulsion and time to death. However, PIA was found ineffective against the metamidophos-induced cholinergic symptoms and mortality. All treatments, except PIA, lead to survival of these animals. Acetylcholinesterase (AChE) activity was not normalized by PIA or PAM. PIA prevented metamidophos-induced diaphragmatic muscle necrosis as much as PAM. In conclusion, a single dose of PIA was unable to protect the rats from metamidophos toxicity. Further studies are needed involving a combination of PAM and/or atropine with repeated doses of PIA to clarify the efficacy of adenosine agonists in OP poisoning.

Morphine can enhance the antiallodynic effect of intrathecal R-PIA in rats with nerve ligation injury

Nerve ligation injury may produce a tactile allodynia. Intrathecal adenosine receptor agonists or morphine have an antiallodynic effect. In this study, we examined the effect of intrathecal morphine on the antiallodynic state induced by the adenosine A1 receptor agonist, N(6)-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA), in a rat model of nerve ligation injury. Rats were prepared with ligation of left L5-6 spinal nerves and intrathecal catheter implantation. Tactile allodynia was measured by applying von Frey filaments to the lesioned hindpaw. Thresholds for withdrawal response were assessed. Morphine and R-PIA were administered to obtain the dose-response curve and the 50% effective dose (ED(50)). Fractions of ED(50)s were administered concurrently to establish the ED(50) of the drug combination. The drug interaction was analyzed using the isobolographic method. Intrathecal 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an A1 receptor antagonist, and naloxone were administered to examine the reversal of the antiallodynic effect. Side effects were also observed. Intrathecal morphine and R-PIA and their combination produced a dose-dependent antagonism without severe side effects. Intrathecal morphine synergistically enhanced the antiallodynic effect of R-PIA when coadministered. Intrathecal naloxone and DPCPX reversed the maximal antiallodynic effect in the combination group. These results suggest that activation of mu-opioid and A1 receptors at the spinal level is required for the synergistic interaction on tactile allodynia.

Effects of adenosine agonist R-phenylisopropyl-adenosine on halothane anesthesia and antinociception in rats

AIM

To investigate the antinociceptive effect of adenosine agonist R-phenylisopropyl-adenosine (R-PIA) given to conscious rats by intracerebroventricular (ICV) and intrathecal (IT), and identify the effect of R-PIA on minimum alveolar concentration (MAC) of halothane with pretreatment of A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) or K+ channel blocker 4-aminopyridine (4-AP).

METHODS

Sprague-Dawley rats were implanted with 24-gauge stainless steel guide cannula using stereotaxic apparatus and ICV method, and an IT catheter (PE-10, 8.5 cm) was inserted into the lumbar subarachnoid space, while the rats were under pentobarbital anesthesia. After one week of recovery from surgery, rats were randomly assigned to one of the following protocols: MAC of halothane, or tail-flick latency. All measurements were performed after R-PIA (0.8-2.0 microg) microinjection into ICV and IT with or without pretreatment of DPCPX or 4-AP.

RESULTS

Microinjection of adenosine agonist R-PIA in doses of 0.8-2.0 microg into ICV and IT produced a significant dose- and time-dependent antinociceptive action as reflected by increasing latency times and ICV administration of adenosine agonist R-PIA (0.8 microg) reducing halothane anesthetic requirements (by 29%). The antinociception and reducing halothane requirements effected by adenosine agonist R-PIA was abolished by DPCPX and 4-AP.

CONCLUSION

ICV and IT administration of adenosine agonist R-PIA produced an antinociceptive effect in a dose-dependent manner and decreased halothane MAC with painful stimulation through activation of A1 receptor subtype, and the underlying mechanism involves K+ channel activation.

The protective effect of Allium sativum L. clove aqueous and methanolic extracts against hypoxia-induced lethality in mice

The antihypoxic activity of Allium sativum clove (garlic) aqueous and methanolic extracts was studied in mice. The extracts of garlic showed that the antihypoxic effect was dose-dependent. The minimum effective doses of aqueous and methanolic extracts were 0.2 g/kg and 5.12 g/kg, respectively. Phenytoin, 50 mg/kg, and R-phenylisopropyladenosine (R-PIA), 1.6 mg/kg (R-PIA) as positive controls increased survival time up to 52.5 +/- 2.9 min and 120.5 +/- 6 min, respectively, compared to normal saline (34.73 +/- 0.71 min). The high doses of aqueous (16.9 g/kg) and methanolic (12.8 g/kg) extracts increased survival time up to 73.17 +/- 4.9 and 68.41 +/- 3.7, respectively. These results indicated that the extracts of A. sativum cloves have a protective effect against hypoxia-induced lethality in mice.

Marked enhancement of anti-allodynic effect by combined intrathecal administration of the adenosine A1-receptor agonist R-phenylisopropyladenosine and morphine in a rat model of central pain

BACKGROUND

There is often no satisfactory treatment for chronic pain after spinal cord injury. We have previously reported that intrathecal (i.t.) administration of the adenosine A1-receptor agonist R-phenylisopropyl-adenosine (R-PIA) or the opioid morphine has anti-allodynic effects in a model of presumed chronic central pain after photochemically induced spinal cord injury in rats. In the present study, we set out to investigate the possible interaction between i.t. R-PIA and morphine in spinally injured rats.

METHODS

Sprague-Dawley rats displaying allodynia-like behaviors to mechanical and cold stimuli after photochemically induced spinal cord injury with minor motor deficits were used. R-PIA and morphine, either alone or in combination, were administered i.t. through an implanted catheter to lumbar spinal cord.

RESULTS

Cumulative doses of R-PIA or morphine dose-dependently reduced the mechanical allodynia-like behavior, with a threshold of 1 nmol and 1.5 nmol, respectively. When co-administrated, R-PIA and morphine produced marked suppression of mechanical allodynia at doses of 5 pmol and 7.5 pmol, respectively. The effect of i.t. co-administration of R-PIA and morphine on cold allodynia was comparable to i.t. R-PIA alone. The combination of R-PIA and morphine did not increase adverse effects such as motor deficits in comparison to either drug alone.

CONCLUSION

These results demonstrate a supra-additive interaction between the adenosine A1-receptor agonist R-PIA and morphine to reduce mechanical allodynia-like behavior in rats with chronic spinal cord injury. The combination of R-PIA and morphine administered spinally may be superior to R-PIA or morphine alone for treating such pain.

Influence of adenosinergic drugs on ethanol withdrawal syndrome in rats

The influence of adenosine, its analogs: (-)N6-(R-phenylisopropyl)-adenosine (R-PIA), N6-cyclohexyladenosine (CHA), 5-(N-ethylcarboxamido)-adenosine (NECA), adenosine uptake inhibitor-dipyridamole, and theophylline and caffeine (adenosine receptor antagonists) on ethanol withdrawal syndrome was investigated in rats. Adenosine (100 mg/kg ip), all adenosine analogs and dipyridamole (30 mg/kg ip) reduced the number of rats in which audiogenic convulsions appeared. Caffeine and theophylline (5-25 mg/kg ip) did not influence significantly the audiogenic seizures, but they antagonized the depressing effects of adenosine analogs on these withdrawal symptoms. The results suggest that adenosine mechanisms in the brain may be implicated in the development of ethanol withdrawal syndrome.

Adenosine analogues modulate the incidence of sleep apneas in rats

The effects of adenosine A1 and A2 agonists on spontaneous central sleep apneas in rats have been examined by simultaneously monitoring sleep and respiration in freely moving unanesthetized animals. Intraperitoneal administration of 1.0 mg/kg of the A1 receptor agonist R(-)N6-L-(2-phenyl-isopropyl)adenosine (L-PIA) and 150 and 300 micrograms/kg of 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido-adenosine hydrochloride (CGS 21680), a selective A2 agonist, decreased the apnea index (AI) during sleep. Within a sleep period, AI increased over time in both control and drug-treated animals. For both agonists, doses effective in reducing AI also significantly reduced sleep efficiency.

Effect of central adenosine on brainstem blood flow in fetal sheep

The effect of the adenosine analogue R-N6-(phenylisopropyl)adenosine (R-PIA) on blood flow to the medulla and pons was examined in unanaesthetized fetal sheep. Microspheres labelled with isotopes were used to determine blood flow before and after instillation of 0.2 or 0.5 microgram R-PIA into the cerebrospinal fluid of the fourth ventricle. Blood flow to the medulla, which had a mean value (+/- S.E.M.) of 285 +/- 41 ml min-1 (100 g)-1 during the control period, was not changed by the central instillation of R-PIA. Blood flow to the pons was also not affected. These data indicate that central adenosine, which depresses respiratory drive in fetal sheep, acts by mechanisms independent of removal of carbon dioxide from the brainstem.

The effects of centrally administered adenosine on fetal sheep heart rate accelerations

OBJECTIVE

We examined the effect of administering a long-acting adenosine analog, L-2-N6-(phenylisopropyl) adenosine, into the cerebrospinal fluid of the fourth ventricle on fetal sheep heart rate accelerations.

STUDY DESIGN

Pregnant ewes between 123 and 130 days' gestation were anesthetized, and the fetal head was exteriorized. Catheters were placed in the fourth cerebral ventricle through the foramen magnum and in the brachial artery to record fetal heart rate. Studies were performed in unanesthetized fetuses 4 to 7 days after surgery. Accelerations were defined as a 10 beats/min rise in heart rate for at least 5 seconds.

RESULTS

The mean number of accelerations before administration of L-2-N6- (phenylisopropyl) adenosine was 3.9 +/- 0.7 (mean +/- SE) per 10-minute epoch, decreasing to 2.0 +/- 0.7 and 1.4 +/- 0.8 after instillation of 0.2 microgram and 0.5 micrograms of L-2-N6-(phenylisopropyl) adenosine, respectively (p < 0.05). Increasing the L-2-N6-(phenylisopropyl) adenosine dose to 10.0 micrograms resulted in loss of heart rate accelerations. Accelerations were not reduced when theophylline, an adenosine receptor blocker, was given before L-2-N6-(phenylisopropyl) adenosine.

CONCLUSION

Increasing amounts of a centrally administered adenosine analog progressively decreased the number of fetal sheep heart rate accelerations, most probably by suppression of brainstem sympathetic outflow.

Spinal cord blood flow after intrathecal injection of a N-methyl-D-aspartate receptor antagonist or an adenosine receptor agonist in rats

Evaluation of spinal cord blood flow (SCBF) is important as a preclinical screening for potential neurotoxicologic side effects before introducing new therapeutic drugs for intrathecal (IT) administration. This study was undertaken to determine whether two drugs with possible antinociceptive effects, the N-methyl-D-aspartate receptor antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and the adenosine agonist R-phenylisopropyladenosine (R-PIA), would affect SCBF after IT administration in the rat. SCBF was measured 30 min after IT injection of either saline, 4 nmol of CPP or 10 nmol or R-PIA in anesthetized rats by quantitative autoradiography with [14C]iodantipyrine as a tracer. No differences in SCBF were found between groups treated with saline or CPP. In the group where R-PIA was administered IT, grey matter blood flow was 10% greater than in the group that received saline, whereas no differences were found in white matter blood flow or in the grey/white matter ratio between the groups. We concluded that neither CPP nor R-PIA, when administered IT to anesthetized rats in provocative doses within the upper pharmacologic dose range, reduces SCBF in rats 30 min after IT injection. This indicates that neither neurotoxicologic nor antinociceptive effects are likely to occur due to decrease in SCBF. Our results do not indicate that these drugs should be avoided for the treatment of pain.

R-phenylisopropyl-adenosine increases spinal cord blood flow after intrathecal injection in the rat

The A1-adenosine receptor agonist, R-phenylisopropyl-adenosine (R-PIA), demonstrated antinociceptive properties in animal studies after intrathecal administration. In the evaluation of a drug for possible spinal injection in humans, the effects of intrathecal R-PIA on spinal cord blood flow (SCBF) were investigated using the laser-Doppler flow-metry technique in anesthetized rats. In low doses (0.1-1 nmol), no change in SCBF was recorded, whereas larger doses (10-100 nmol) caused a significant increase in SCBF. No change in systemic arterial blood pressure could be seen, except for a decrease after administration of the largest dose of R-PIA (100 nmol). It is concluded that R-PIA in doses of 10 nmol and larger induces an increase in SCBF after intrathecal injection in anesthetized rats and that an increase in blood flow is seen before any effect on the systemic circulation is detected. It can also be deduced that the antinociceptive effects of R-PIA after intrathecal injection are not a consequence of spinal ischemia and that disturbances in local blood flow cannot be expected to constitute a neurotoxic factor.

Ocular hypotensive activity of the adenosine agonist (R)-phenylisopropyladenosine in rabbits

Adenosine A1 agonists have been shown to induce a variety of pharmacological effects. In New Zealand White rabbits, the topical administration of 500 micrograms of the relatively selective adenosine A1 receptor agonist R(-) phenylisopropyladenosine (R-PIA) produced a biphasic response in IOP in the ipsilateral eye: an initial ocular hypertension (3.5 +/- 1.4 mm of Hg) at 0.5 hour, followed by significant reduction in IOP (5 to 8 mm of Hg) from 2 to 6 hours postadministration. The IOP response to 50 and 165 micrograms of R-PIA demonstrated that the ocular hypotensive response to R-PIA was dose-related; however, no initial hypertension was observed at these lower doses. The ocular response to R-PIA was primarily unilateral with only a small reduction in contralateral IOP at 1 hour observed in animals treated with 500 micrograms. No significant change in pupil diameter was observed with any dose of R-PIA. Pretreatment with the adenosine antagonist CPT (10 mg/kg; i.p.) significantly inhibited the ocular hypotensive response to R-PIA. However, pretreatment with the cyclooxygenase inhibitor indomethacin (50 mg/kg; i.p.) did not alter the change in IOP induced by R-PIA. The administration of R-PIA once a day for five days demonstrated that tolerance does not develop in rabbits with repeated administration. These data demonstrate that the adenosine A1 agonist R-PIA can lower IOP. The unilateral nature and the inhibition by CPT supports the idea that this response is mediated by adenosine receptors located in the eye.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intrathecal injection of the adenosine receptor agonists R-phenylisopropyl-adenosine and N-ethylcarboxamide-adenosine on nociception and motor function in the rat

R-phenylisopropyl-adenosine, which has an affinity for the adenosine A1 receptor higher than that for the A2 receptor, and N-ethylcarboxamide-adenosine, which has near equal affinity for the A1 and A2 receptors, were injected intrathecally into rats to evaluate differences in antinociceptive effect and motor impairment. Using the tail-immersion test, both compounds had antinociceptive effects. Motor function was evaluated during spontaneous movement in a free space. N-ethylcarboxamide-adenosine rapidly impaired motor function even after low intrathecal doses. R-phenylisopropyl-adenosine also induced motor impairment, but only after high intrathecal doses, and onset was much slower. These results suggest that the receptor selectivity of R-phenylisopropyl-adenosine is diminished at higher doses and that the motor impairment is an A2-receptor-mediated effect. A selective A1 receptor agonist, e.g., R-phenylisopropyl-adenosine, which produces a good antinociceptive effect without motor impairment, is more promising as a drug of possible use for the future treatment of clinical pain.

The role of spinal and brainstem adenosine receptors in the modulation of the volume-evoked micturition reflex in the unanesthetized rat

The pharmacology of the spinal, supraspinal and peripheral adenosine receptor subtypes (A1, A2) and their influence on the volume-evoked micturition reflex (VEMR) was studied in a chronic unanesthetised rat model by cystometrography after intrathecal (i.t.), intracerebroventricular (i.c.v.) and intravenous (i.v.) injection. Intrathecally administered A1 adenosine agonist: N6-(L-2-phenylisopropyl)adenosine (R-PIA) and A2 adenosine agonist: 5'-(N-ethylcarboxamido)adenosine (NECA) were equally active with 1.0 nmol reliably producing an increase in the volume necessary to induce the VEMR. At a higher dose (3 nmol), a long-lasting blockade of the VEMR was produced by both agonists. These effects were reversed following intraperitoneal injection of caffeine, an adenosine antagonist. This inhibition of the VEMR outlasted the spinal antinociceptive action which we have previously reported for these two agonists. Contrary to the spinal effect of these agonists, i.c.v. (0.3-3 nmol) and i.v. (100-1000 nmol) injections of R-PIA and NECA resulted in a significant decrease in the volume required to evoke the VEMR. We conclude that at the spinal level a xanthine-sensitive adenosine receptor(s) inhibits the VEMR. Based on several indirect lines of evidence, we speculate that these effects are not mediated by an action on primary afferent input or directly on preganglionic neurons, but on an excitatory interneuronal link.

Modification of the rat adipocyte A1 adenosine receptor-adenylate cyclase system during chronic exposure to an A1 adenosine receptor agonist: alterations in the quantity of GS alpha and Gi alpha are not associated with changes in their mRNAs

The A1-adenosine receptor (A1AR) adenylate cyclase system in rat adipocytes undergoes heterologous desensitization following chronic in vivo exposure to an A1AR agonist (+)-N6-(R-phenylisopropyl)adenosine [J. Biol. Chem. 262:841-847 (1987)]. This desensitization involves an absolute increase in adenylate cyclase activity and a refractoriness to receptor ligands that are inhibitory to adenylate cyclase. In this study, receptor changes were characterized using an A1AR antagonist radioligand, [3H]8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl] oxy]phenyl]-1,3-dipropyl xanthine. Saturation binding studies demonstrated a 47% decrease in total A1AR density without a change in KD. Agonist competition studies revealed a decreased percentage of receptors, from 55% to 35%, in the high affinity state following desensitization. An increase in GS alpha of 49% was found by Western blotting using specific GS alpha antibodies. Further, an antibody that recognizes Gi alpha 1 adn Gi alpha 2 was used to quantitate these subtypes of Gi alpha and both were decreased by 59% following desensitization. However, when an antibody that recognizes Gi alpha 3 was used, no change in Gi alpha 3 was found, demonstrating, in this case, differential regulation of Gi alpha subtypes. The mechanisms responsible for changes in GS alpha and Gi alpha were studied by measuring the levels of their mRNAs from normal and desensitized adipocytes. Using either labeled cDNAs (Gi alpha 2, Gi alpha 3) or oligonucleotides (GS alpha, Gi alpha 1), Northern analysis demonstrated that mRNAs for GS alpha and all three isoforms of Gi alpha are present in adipocytes but that there are no changes in the levels of any of these transcripts following desensitization. These data suggest that desensitization of the A1AR-adenylate cyclase system involves a down-regulation of A1ARs and an additional loss of A1AR agonist high affinity sites. Further, an increase in GS alpha, a decrease in Gi alpha 1 and Gi alpha 2, and no change in Gi alpha 3 were found. The regulation of GS alpha and the subtypes of Gi alpha in this system does not occur by altering the levels of their respective transcripts.

Rat adipose tissue adenosine sensitivity and adenosine content after subcutaneous administration of N6-(phenylisopropyl)adenosine

Injections of N6-(phenylisopropyl)adenosine, a nonmetabolizable adenosine A1 receptor agonist, desensitized rat adipocytes to the drug in 20 h. Lipolysis stimulated by 2 mumol/l DL-isoproterenol was inhibited markedly less by N6-(phenylisopropyl)adenosine in adipocytes isolated from treated than control animals (P less than 0.01). Glucose uptake was more responsive to N6-(phenylisopropyl)adenosine in adipocytes from control than treated animals (P less than 0.02). Adenosine content was the same in adipose tissue of control and treated animals. The number of adenosine binding sites was not significantly lower in treated compared with control animals (1580 +/- 279 and 1988 +/- 575 fmol/mg protein; mean +/- SEM). There was no change in receptor affinity (Kd = 10 nmol/l in both groups). There was no decrease in the amounts of the inhibitory guanine nucleotide binding protein (Gi) alpha subunits as studied by pertussis toxin catalyzed ADP-ribosylation. It is concluded that desensitization to N6-(phenylisopropyl)adenosine can be observed without changes in the adenosine receptor status or decrease in the amount of inhibitory guanine nucleotide binding protein and that adipose tissue adenosine content is not changed by the agonist treatment.

Differing effects of apical and basolateral adenosine on colonic epithelial cell line T84

Adenosine and its agonists, 5'-(N-ethylcarboxamido)adenosine and N6-(D-2-phenylisopropyl)-adenosine, induced a sustained increase in chloride secretion when added to either the apical or basolateral aspect of monolayers of the human colonic epithelial cell line T84. Secretion was induced with identical kinetics by addition to both sides, but apical addition was less potent. The rank order of potency of the agonists on either side was consistent with the presence of an adenosine A2-receptor, but the apical and basolateral receptors differed in both their ability to stimulate increases in adenosine 3',5'-cyclic monophosphate, and their susceptibility to down-modulation by chronic exposure to 5'-(N-ethylcarboxamido) adenosine in culture.

Effects of adenosine analogs alone and in combination with caffeine in the squirrel monkey

Effects of the adenosine analogs R-N6-phenylisopropyl-adenosine (R-PIA) and 5'-N-ethylcarboxamidoadenosine (NECA), alone and in combination with caffeine, were studied in squirrel monkeys trained to respond under multiple fixed-interval fixed-ratio schedules of reinforcement. Both drugs produced dose-related decreases in rates of responding, with little difference between effects in the two components. NECA was about ten times more potent than R-PIA in producing these effects, the order of potency suggests that these effects may be mediated by actions at A2-adenosine receptors. Effects of either drug were antagonized by caffeine. Caffeine when administered alone increased rates of responding. The increases in response rates produced by caffeine were altered by R-PIA only at doses of R-PIA that alone decreased response rates. Effects of caffeine were either enhanced or attenuated by doses of NECA that were inactive when administered alone. These results do not support the notion that increases in rates of behavior, e.g., psychomotor-stimulant effects, produced by caffeine are due to its antagonist actions at adenosine receptors.

Cardiovascular effects of microinjections of adenosine analogs into the fourth ventricle of rats

Rats were implanted with chronic indwelling cannulae into the posterior region of the fourth ventricle. After recovery from surgery, acute experiments on blood pressure were conducted under urethane anesthesia. The blood pressure and heart rate responses following administration of two adenosine analogs, NECA and L-PIA were examined. Microinjections of both analogs produced dose-dependent reductions in blood pressure and heart rate. NECA was approximately 20-fold more potent than L-PIA in reducing blood pressure and depressing heart rate. The cardiovascular effects of both analogs were antagonized by parenteral injections of caffeine. These findings show that microinjections of analogs of adenosine into the fourth ventricle can influence areas of the central nervous system involved in cardiovascular control.

Central effects of adenosine analogs on blood pressure and heart rate in the mouse

Mice implanted with chronic indwelling cannulae were injected into the lateral cerebral ventricle with two adenosine analogs, NECA and L-PIA, and the effects on blood pressure and heart rate recorded. Both analogs produced dose-related reductions in blood pressure and heart rate. NECA exhibited approximately 10 fold more potency than L-PIA on mean arterial blood pressure. The effects of both drugs on blood pressure and heart rate were antagonized by parenteral injections of caffeine. These results show that injections of adenosine analogs into the lateral ventricle of mice can influence the areas of the central nervous system involved in the control of cardiovascular function.

Antagonism by theophylline of respiratory inhibition induced by adenosine

The effects on respiration of an analogue of adenosine, L-2-N6-(phenylisopropyl)adenosine (PIA), and of the methylxanthine, theophylline, were determined in 19 vagotomized glomectomized cats whose end-tidal PCO2 was kept constant by means of a servo-controlled ventilator. Integrated phrenic nerve activity was used to represent respiratory output. Our results show that PIA, whether given systemically or into the third cerebral ventricle, depressed respiration. Systemically administered theophylline stimulated respiration. Theophylline given intravenously, or into the third ventricle not only reversed the depressive effects of previously administered PIA but caused further increases of respiration above the control level. Prior systemic administration of theophylline blocked both respiratory and hypotensive effects of subsequently administered PIA. Effects of either agent on medullary extracellular fluid pH did not explain the results. We conclude that the adenosine analogue PIA, acts to inhibit neurons in the brain that are involved in the control of respiration and that its effects are blocked by theophylline. We suggest that adenosine acts as a tonic modulator of respiration and that theophylline stimulates breathing by competitive antagonism of adenosine at neuronal receptor sites.

Behavioral effects of caffeine, N6-(L-phenylisopropyl) adenosine and their combination in the squirrel monkey

The behavioral effects of caffeine, N6-(L-phenylisopropyl)adenosine (PIA) and the two drugs in combination were studied in squirrel monkeys. Monkeys responded by pressing a lever under different schedules of reinforcement involving either food or electric shock. The effects of caffeine depended primarily on the type of schedule that controlled responding and were largely independent of the type of consequent event. When responding was maintained at high rates under fixed-ratio schedules of either food presentation or stimulus-shock termination, caffeine (1-56 mg/kg) typically decreased responding in a dose-dependent manner. When responding was maintained at moderate rates under fixed-interval schedules involving either event, intermediate doses of caffeine (3-10 mg/kg) slightly increased responding, whereas higher doses decreased responding. When responding was maintained under a fixed-interval schedule of stimulus-shock termination and concurrently suppressed by response-produced shock, intermediate doses of caffeine (3-30 mg/kg) markedly increased responding. PIA (0.03-1 mg/kg) only decreased responding under all schedules. Caffeine (1-30 mg/kg) antagonized the rate-decreasing effects of PIA under each schedule. PIA (0.03-1 mg/kg) reduced or eliminated the rate-increasing effects of intermediate doses of caffeine under the fixed-interval schedules but did not attenuate the rate-decreasing of high doses of caffeine under any schedule. Thus, the rate-increasing and rate-decreasing effects of caffeine differed in their susceptibility to modification by PIA.