Behavioral and electroencephalographic effects of the adenosine1 agonist, L-PIA

Manipulation of adenosine kinase affects sleep regulation in mice

Sleep and sleep intensity are enhanced by adenosine and its receptor agonists, whereas adenosine receptor antagonists induce wakefulness. Adenosine kinase (ADK) is the primary enzyme metabolizing adenosine in adult brain. To investigate whether adenosine metabolism or clearance affects sleep, we recorded sleep in mice with engineered mutations in Adk. Adk-tg mice overexpress a transgene encoding the cytoplasmic isoform of ADK in the brain but lack the nuclear isoform of the enzyme. Wild-type mice and Adk(+/-) mice that have a 50% reduction of the cytoplasmic and the nuclear isoforms of ADK served as controls. Adk-tg mice showed a remarkable reduction of EEG power in low frequencies in all vigilance states and in theta activity (6.25-11 Hz) in rapid eye movement (REM) sleep and waking. Adk-tg mice were awake 58 min more per day than wild-type mice and spent significantly less time in REM sleep (102 ± 3 vs 128 ± 3 min in wild type). After sleep deprivation, slow-wave activity (0.75-4 Hz), the intensity component of non-rapid eye movement sleep, increased significantly less in Adk-tg mice and their slow-wave energy was reduced. In contrast, the vigilance states and EEG spectra of Adk(+/-) and wild-type mice did not differ. Our data suggest that overexpression of the cytoplasmic isoform of ADK is sufficient to alter sleep physiology. ADK might orchestrate neurotransmitter pathways involved in the generation of EEG oscillations and regulation of sleep.

Increase of waking and reduction of NREM and REM sleep after nitric oxide synthase inhibition: prevention with GABAA or adenosine A1 receptor agonists

The effect of N(G)-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of enzyme nitric oxide synthase (NOS), on spontaneous sleep during the light period, was studied in adult rats implanted for chronic sleep recordings. L-NAME was injected by subcutaneous (s.c.) route or was infused directly into the dorsal raphe nucleus (DRN). Subcutaneous (46.0--185.0 micromol/kg) administration of L-NAME increased waking (W), slow wave sleep (SWS) and rapid-eye-movement sleep (REMS) latency, whereas SWS, REMS and the number of REM periods were reduced. Direct application of L-NAME into the DRN (0.37--1.1 micromol) induced an increment of W and a reduction of SWS and REMS. Values corresponding to SWS and REMS latency, and the number of REM periods remained within control levels. Subcutaneous administration of the GABA(A) receptor agonist muscimol (1.7--3.5 micromol/kg) or the adenosine A(1) receptor agonist L-PIA [L(-)N(6)-(2-phenylisopropyl)adenosine] (0.1--0.3 micromol/kg) induced slight but inconsistent changes of W, light sleep (LS), SWS and REMS that did not attain significance. Pretreatment with muscimol (1.7--3.5 micromol/kg, s.c.) or L-PIA (0.1--0.3 micromol/kg, s.c.) antagonized the increase of W and reduction of SWS and REMS induced by s.c. (92.0 micromol/kg) or intra-DRN (0.74 micromol) administration of L-NAME. However, neither muscimol nor L-PIA prevented the increase of REMS latency induced by L-NAME 92.0 micromol/kg, s.c. Our findings tend to indicate that the change of behavioral state observed after systemic or intra-DRN administration of L-NAME is partly related to the reduction of GABA and adenosine at critical sites in the CNS.

Extracellular adenosine levels in neostriatum and hippocampus during rest and activity periods of rats

Adenosine is an inhibitory modulator in the mammalian brain with a possible role in sleep regulation, which is mainly indicated by pharmacological studies showing that adenosine or its analogs can induce sedation and sleep, whereas adenosine antagonists, like caffeine and theophylline, are potent behavioral and neuronal stimulants. In contrast to these pharmacological findings, data on endogenous adenosine in relation to sleep and waking are sparse. Therefore, we have now used in vivo microdialysis to investigate the extracellular levels of adenosine in the neostriatum and hippocampus of freely moving rats. Adenosine was monitored over a time course of 24 h, during which the animals were exposed to a 12 h day/night rhythm with lights-off from 19.00 to 07.00. In this lights-off period, i.e. the rats' active period, the maximal levels of neostriatal and hippocampal extracellular adenosine were higher than during the lights-on period. In contrast to the neostriatum, extracellular levels of hippocampal adenosine tended to increase towards the end of the lights-off period, reaching its maximal level at 07.00, and decreasing again within the following hour. The changes of hippocampal adenosine levels were related to behavior, since significant increases in "sleep-like" behavior, as well as decreases in overall movements and consummatory behavior, were observed when adenosine levels had reached their maxima in the hippocampus; no such relationship was found with respect to the neostriatum. These results are in keeping with a role of endogenous adenosine in the regulation of sleep and wakefulness, and point to a specific role of adenosine in the hippocampus. They also raise the possibility that adenosine may be involved in different behavioral processes dependent on the area of the brain, as well as the type of adenosine receptor involved. Finally, given the known evidence for neuroprotective actions of adenosine, its accumulation in the hippocampus as a function of behavioral activity may serve to prevent or repair the neural degenerative consequences of such activity. It is proposed that adenosine's sleep-promoting effects result from its signalling to cease behavioral activity in order to prevent excessive activity-related changes, and thus allow other restorative sleep-related processes to take over.

Effects of chronic administration of adenosine A1 receptor agonist and antagonist on spatial learning and memory

Spatial memory acquisition in Morris water maze was tested in C57BL/6 mice. Animals were injected once daily with different doses of either N6-cyclopentyladenosine (CPA) or 8-cyclopentyl-1,3-dipropylxanthine (CPX). Drugs were administered for 9 days either concurrently with water maze testing (drugs injected 1 h after each trial), or prior to the entire block of trials. In the latter case, 1 day without injections preceded water maze experiments. Chronic administration of CPA resulted in a significant, dose-dependent reduction of target latencies, rapid development of spatial preference, and the absence of animals unable to perform the task. CPX treated animals did not show significant performance changes, and failed to develop spatial preference. Locomotor disturbances were not the cause of the observed effects. Our results indicate that chronic treatment with agents acting at adenosine A1 receptors results in behavioral effects that are significantly different from those observed following their acute administration. Therefore, particular caution is required in development of adenosine-based strategies targeted at neurodegenerative or cognitive disorders in which chronic treatment is advocated.

Circadian rhythm of cortical and striatal adenosine receptors

In order to investigate diurnal variations in binding parameters of A1 and A2 adenosine receptors, Kd and Bmax were calculated in mice that had been housed under controlled light-dark cycles for 4 weeks (light on from 7.00 to 19.00 h). A1 cortical receptors were labelled by N6-cyclohexyl-[3H]adenosine, and A1 and A2 striatal receptors by 5'-N-ethylcarboxamido[8-3H]adenosine. Significant differences were found for Bmax values measured at 3-h intervals across a 24-h period. Cyclic variations of the number of binding sites were shown, with a minimum number of A1 and A2 receptors during the light period and a maximum during the dark period. The amplitude for cortical A1 receptors, between 03.00 and 18.00 h, was 39%, while for striatal A1 receptors, between 03.00 and 15.00 h, was 92%. The amplitude for the A2 striatal receptors, between 03.00 and 15.00 h, was 147%. No substantial rhythm was found in the Kd values. These differences in the amplitude could suggest a different physiological modulation of the two adenosine receptor subtype moiety which could reflect a physiologically-relevant mechanism by which adenosine exerts its modulatory role in the central nervous system.

Differential effects of various xanthines on pentylenetetrazole-induced seizures in rats: an EEG and behavioural study

The present study deals with the EEG (electroencephalogram) and behavioural effects of a subconvulsant dose (30 mg/kg i.p.) of pentylenetetrazole in freely moving rats pretreated (100 mg/kg p.o., 1 h before pentylenetetrazole) with two classic (theophylline and caffeine) and two new (enprofylline and isbufylline) xanthines. In rats treated with vehicle, pentylenetetrazole caused a slight desynchronization of the EEG, characterized by periods of 'wave discharges', and 'spike-and-wave discharge complexes'. In rats pretreated with xanthines (theophylline or caffeine) pentylenetetrazole produced a dramatic increase in ictal seizures with the appearance of continuous spikes; concomitantly animals experienced myoclonic jerks (100%) and in some cases (ca. 20%) the animals died. In contrast, in enprofylline-pretreated rats, pentylenetetrazole induced only brief periods of wave discharges and spike-and-wave discharge complexes whose duration was significantly reduced compared to that of controls, although these discharges were associated with mild epileptic behaviour. When isbufylline-pretreated rats were challenged with pentylenetetrazole, the EEG was characterized by a short run of wave discharges (whose duration was shorter than that of other groups). No enprofylline- or isbufylline-treated rats developed seizures or died. In conclusion, only xanthines with strong adenosine A1 receptor antagonism (theophylline and caffeine) markedly enhance the EEG and behavioural effects of a subconvulsive dose of pentylenetetrazole. The present experimental approach could be used to evaluate the pro-convulsive potential of new xanthine derivatives.

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)