Adenosine and dopamine function in the CNS

TE!i - September 1987 CVol. 83

330 tration and job satisfaction. Here, the captopril group showed a significant improvement compared to a significant deterioration with methyldopa and no change with propranoloi~ On the scale, adjustment well-being which measures areas such as general depression, anxiety, health, positive well-being, self control and vitality, only the captopril group showed an improvement. It is difficult to discard those findings on the basis that the apparent improvement on the quality of life simply reflected a general dissatisfaction with previous treatments. Indeed, such a contrast would be expected to be readily apparent at the end of the one month placebo period which preceded the six month drug treatment period. Furthermore, the patients were not chosen for this study on the basis of their dissatisfaction with the previous tmatment or because of sideefIects associated with previous treatment. It is also difficult to discard those findings as chance events because of their multiplicity and concordance. Finally, this study, designed as it was to capture as many aspects of the ‘quality of life’ as possible, identified a subtle but positive psychopharmacological effect of propranolol, one of the two comparison drugs. The social participation index showed a significant improvement only with proprano1oI. This is concordant with the expected properties of this compound in various social manifestations of anxiety and therefore lends credence to the finding of a mood elevating effect of captopril. On the other hand, most clinicians would agree that patients often do not realize the CNS side-effects of their antihypertensive medication until quite a while after they have been withdrawn from them. Thus the study of Croog ef al.3 is consistent with the possibility that angiotensin II converting enzyme (ACE) inhibitors might elevate mood. Several mechanisms have been proposed to account for the CNS effects of captoprll: a direct i.-siiLuitor~_ effect on the metabolism of enkephalins, endorphins or other peptides’@; an indirect, angiotensin II effect on noradrenergic regulation74; an indirect, angiotensin II effect on the

h~othalamic pituitary adrenal axis by stimulation of adrenocorticotrophic hormone (ACTH) and vasopressin releaselo*l’; and finally an indirect angiotensin II effect on the median eminence and paraventricular nuclei with a secondary influence on the release of other neuropeptides like corticotropin-releasingfactor,witb behavioral effects resulting from glucocorticoid action’. To this list of hypothetical mechanisms we would like to add one more. It is known that plasma angiotensin II exerts central effects on water regulation and blood pressure by acting on circumventricular organs like the neuroh~ophysis and median eminence of the hypothalamus, which are located outside the blood-brain barrier (BBB). Peripherally administered captopril and associated changes in plasma angiotensin II could modulate behavior and affect mood through an action on these circumventricular organs as well. The role of the circumventricular organs and other ACE-rich brain regions might be more important than generally recognized; the distribution of peptidases in brain is such that the regions which are not protected by the BBB endothelium appear to be dramatically enriched in ACE1*. This, in turn, suggests that there are two types of BBB: the specialized endothelium of the BBB and peptidase-rich regions not covered by such an endothelium. The peptidase-rich regions might be subject to biochemical

tuning by peptidase ~hibitors such as captopril and ‘gate’ the passage of peripheral neuropeptides into those regions of the brain from where they would diffuse or be transported to other brain structures. PIERRE E. ETIENNE AND

GEORGE S. ZUBENSKO*

Research Depurtmenf, Ph~uceufica~ Division, Ci&u-Geigy, Summit, h!j 07901, USA, and Western Psychiatric Institute and Clinic, Department of Psychiatry, University of Piftsbu~k, Pittsburgh, PA 15213, USA,

References 1 Zubenko, G. S. and Nixon, R. A. (1984)

Am. I. Psyckiatr. 141,110-111 2 Deicken, R. F. (1986) Biol. Psyckiatr. 21, 1425-1428 3 Crook, S. H., Levine, S., Byron, B., BulFitt, C. J., Jenkins, C. D., Klerman, G. L., Wiliiamson,G. H. andTesta,M. A. (1986) N. Et@. j. Med. 314,1657-X64 4 Stine, S. M., Yang, H-Y. T. and Costa, E. (1980) Bruin Res. 188,295--299 5 Schwartz, J. C., de la Baume, S., Yi, C. C., Chaillet, I?., Marcais-Collazo, H. and Costentin, J. (1982) Prog. Neuro-Psyckopka~acal. &al. Psyckiatr. 6,66%67l 6 Gillrnan, M. A. and Sandyk, R. (1985) Am. 1. Psyckiatr. 142,270 7 Roth, R. H. (1972) Proc. Am. SW. Exp. Biol. 31,1358-1364 8 Mendelsohn, F. A., Q&ion, R. and Saavedra, J. M. (1984) Proc. Nat1 Acad. Sci. USA 81,1575-1579 9 Strittmatter, S. M., Lo, M. M., Javitch, J. A. and Snyder, S. H. (1984) Proc. Natl Acad. Sci. USA 81, X599-1603 10 Maran, J. W. and Yates, E. F. (1977) Am. I. Pkysiol. 233, E273-E285 11 Ramsay, D. J., Keif, K. C., Sharp, M. C. and Shinsako, J. (1978) Am. 1. Physiol. ^. 1,* __* .Y&,IWO-K/l 12 Snyder. S. H. (1986) in Second Co~o9u~um in Eiologicai Sciences (Vol. 463) (BurrelI, C. D. and Strand, F. L., eds), pp. 21-30, New York Academy of Sciences

A~e~~$ine and dqpamine function in the CNS A large body of evidence now exists to support a role for the purine riboside, adenosine, in mammalian CNS function**2. However, unlike the more classical neurotransmitters, it appears that receptor density, rather than adenosine availability per se, reflects the actions of this neuromodulator since relatively high concentrations of the purine occur in most mammalian tissues. This fact has tended to contribute to a skepticism as to the physiologIca relevance of adenosinel. However, the purine has potent effects {10m9lo4 M) on various biochemical

and elect~physiological processes in brain tissue acting via two types of receptor, A1 and Az, either indirectly via inhibition of the release of transmitters such as acetylcholjne, GABA, glutamate, doparnine, noradrenaline and 5HT or directly via actions at both pre- and postsynaptic loci2. Consistent with these actions, the xanthine adenosine antagonists, caffeine and theophylline, have been shown to increase cell firing and transmitter release*. Despite the apparent plethora of effects on transmitter release, there is considerable evidence

TIPS -- September 1987 [Vol. 81 available to support a -unique relationship between high affinity A2 receptors and dopamine systems in rat striatum. Early studies showing that methylxanthines could induce and/or potentiate rotation in rats with unilateral striatal lesitins3-5 have been augmented with more recent finding&’ that adenosine agonists are efficacious in animal models predictive of antipsychotic activity. Purine nucleosides can also inhibit dopamine synthesis and release both in vim and in ~ifr&~~ and can decrease locomotor activity6J*, actions opposite to those of methylxanthine adenosine antagonists’I*12. Following unilateral lesioning of the substantia nigra, xanthines can induce contralateral tuming13, an effect that may be related to alterations in dopamine releaser4 and can induce self-mutilatory behaviour in food-deprived rats15. The effects of caffeine are dependent on the presence of dopamine, since 6-hydroxydopamine (6 OHDA) lesions can block the effects of xanthines on locomotor activityll. Conversely, intrastriatal administration of the non-selective adenosine agonist, NECA (5’Nethylcarboxamido adenosine), can produce similar effects in rats treated with dopamine agonists such as apomorphine16, while B-OHDA-treatment in neonates can increase susceptibili~ to selfmutilation behaviors”. Given the known deficiency in purine metabolism in Lesch-Nyhan syndrome, a self-mutilation syndrome involving an X-linked disorder, a logical extrapolation might be to assume that the effects of adenosine agonists and antagonists in rat models reflect the etiology underlying the human condition. However, while Lesch-Nyhan is associated with a decrease in the enzyme, HRGPT (hypoxanthineguanine phosphoribosyl transferase) and an associated decrease in striatal dopamine levels”, the selfmutilation syndrome seen following caffeine administration, involves an increase in this enzyme”. To further confuse the issue, the dopamhe agonists apomorphine, pemoline and methampetamine can also induce self-mutilation in animal models’“. In fact, the dopaminergic damage seen in LeschNyhan syndrome is similar to that s2en in methamphetamine-

331 induced neurotoxici~= i.e. a degeneration of nerve terminals with cell bodies in the substantia nigra remaining morphologically intact*sz21.While somewhat contradictory, these data do support an intimal relationship between dopamine and adenosine systems in the CNS. The effects of caffeine on dopamine-mediated behaviours further support this relationship. The xanthine can potentiate the effects of apomo~hine, amphetamine, methylphenidate and cocaine on locomotor activity22, while the effects on locomotor activity when given alone, are accompanied by increases in the release of both dopamine and norepinephrineI*. Caffeine can also potentiate the effects of dopamine agonists in reversing reserpine-induced suppression of locomotor activity but by itself, is ineffective in aversing the behavioural effects of ~~~d~~~~~~~~~~~

sine agonists and antagonists on dopamine function. Alternatively, newer evidence2 showing adenosine-mediated alterations in Ca2+ flux may be of considerable importance in this context. This role is consistent with observed effects ‘of methybumthines on dopamine release. For instance, 6-OHDA, ar-methyltyrosine and rese@ne effectively attenuate the behavioral effects of caffein21**17.ln contrast, amphetamine-induced hyperactivity is mediated via a Ca2+independent release of newly synthesized or recently taken up dopamine, that is attenuated by 6OHDA and ar-methyltyrosine but not by reserpine treatment28. The fact that several adenosine receptor agonists can affect iocomotor activity and apomorphineinduced climbing behaviour in a manner similar to classical dopamine antipsychotics?’ provides a rationale for further study of these interesting, albeit confusing, findings. The data derived from these inhibitor, ~-methyl~sine23, can behavioural studies however, am indicative that the adenosine agonblock the locomotor stimulatory ists presently available are more effects of caffeine; interestingly, like haloperidol than atypicalneumthe latter compound can facilitate leptics such as clozapine. the levodopa-induced reversal of Given the paucity of dissimilar the behavioural effects of 01chemical structur2s and the conmethyltransferase23. Caffeine also servative efforts in medicinal causes transient increases in substantia nigra dopamine 12~21s~~ chemistry that have typified the study of adenosine neuroeffector and in addition to potentiating the systems to date’, it still remains to effects of direct dopamine agonists be determined whether agonists and releasing agents, methylxantruly selective for the Aa receptor thines have direct dopamine-like (the most selective known comactions, inducing rotation in rats pound, Z-phenylamino adenosine unilaterally lesioned in the sub[CV 18081 has five-fold selectivity stantia nigra*‘. for the A2 recep&or and an ICm Adenosine analogs, including value of 120nM ) devoid of the NECA, increase serum prolactin hypotensive actions of ~IIOWII levelsID, again consistent with an adenosine agonists* may indeed inhibition of dopamine release, an have potential as novel antieffect antagonized by the methylpsychotic agents. Furthermore, xanthines and by dopamine agonthe evidence linking adenosine to ists and potentiated by dopamine depression, analgesia, anxiety, antagonists. Since neither purines sedation and epilepsy’,* and the nor xanthines interact dire&l importance of dopamine in CNS with striatal dopamine receptors 2Y, function, further suggests that the actions of endogenous adenocompounds rno~~ing the eff2cts sine on dopamine function appear of adenosine may be useful in the to be modulatory in nature. treatment of other CNS disorders. Lesioning studies with the The availability of novel, potent excitoxins, kainate and quinolinand selective chemical entities ate, and with 6-OHDA have will allow evaluation of these passhown that adenosine receptors sibilities. are probab!y present on intemeurons or glia rather than dopMICHAEL F. JARvIS amine terminals26,27. Thus it is &ND MICHAEL WILI&i.Ms iikeiy that a trans-synaptic negative feedback loop within the extraDrug DiscoveryDivision, Research Depaement, ~~~~~ce~ticut Division, CUBA-GEIGY pyramidal system might explain Summit, New ~erse~~~$~~,UsA. the observed effects of adeno-

TX% - September 1987 /Vol. SJ

332 References 1

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Micha&,E. K.

and Myers, S. L. (1979) Life Sci. 24, X8320Q2 9 Myers, S. and Pugsley, T. A. (1%) Brain

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375,393-197

10 Stewart. S. F. and Pugsley, T. A. (1985) Naunyn-Sckmied. Arch. Phsnnacol. 331,

X40-14s II Erinoff, L. and Snodgrass, S. rrQ86) Pkarmocof. Biockem. Beknu. 24, 103% 1345 12 Ftedhoim. 8. %. and HedotiSt. P. 11980) Bi&h~m. $hamwcol. 29, l&S&~ 13 Fredhobn, 8. B., Herrera-Mars&wit& M., Jonzan, B., Lindstrom, K. and U. (1983) Pkannacof. Ungerstedt, Biochrm. Brhnv, 19,535-541 14 Govoni, S., Petkov, V. V., Montefusco, O., M&safe, C.. Battaini, F., Spd~. I?. and Trabucchi, M. (1984) J. Pkamz. Phormacof. 36.458-460 15 Ferrer, I., Costed. M. and Crisolia, S. (1982) FEBS Left. X41,275-278 16 Green, R. D., Roudfit, H. and Yeung, S.-M. H. (lQ82) ScfPtrce 2X8,59-61 17 Breese. G. R.. Baumeister. A. A.. Emerick, S. &., F@er G. D. and Mue&, R. A, (1984) Pharmacof. B&kern. Behav. 21, 4591461 18 tlavd. K. G.. Homvkiewicz. 0.. DavidSh&nak,i., Frale$ i., Goldstein, M., Shinuya, M., Kelly, W. and FOX, i. (1981) N. Eng. 1. Med. 305, 1X&

son: i.,

Rapid sampling ofb~ulu~~ signals using a simple assemblylanguage luup Many computer programs that control the collection of data in the pharmacology laboratory utilize timing devices to trigger data colIection. The use of a timing device (such as a clock card) to generate a signal f’inferrupt’) provides a convenient and ftexibge means of precise@ controlling data co&&on, The extra time that is required by the computer to process an interrupt is small, but can Emit the data collectian rate in certain applications. The enhanced Apple He, for

example, is limited to about 3000 samples per second when responding to clock-generated interrupt signals. Same pharmacological applications require fast data sampling to permit analysis of rapidly changing signais. Our interest in analysing the cardiac action potential required approximately lf000 samples per second to obtain adequate sampling during the Fast (phase 0) rise of: the action potential. Such fast sampling of spontaneous activity in excitable tissue may also be required in applications such as the andXysis of spontaneous neuronal activity or of ion channel activity in patch clamp experiments. Short programs written in assembly language are abte to control data coEection at rates that are limited by the speed of the digitizing process, and can approach 50000 samples per second. Assembly languageprogramming Pz%?@amsrepresent directions for the computer to follow in solving a problem. Languages such as BASICand FORTRAN areconvenient for the programmer but

19 Minana, M. U., Portoh, M., Jorda, A. and Grisolia, S. f19843 f. ~~~ch~rn. 43, 15%-1560 20 Mulier, K. and Nyhan, W. (1982) pkurmacuf. Biochem. Brhau. 16,957~3’63 21 Recaurte, G.. Guillety, R. and Se&den, L. (lQS2) Brain Res. 235,93-103 2.2 Wald&h, B. (1975) Acfn Phatm. Toxicol /SuppI+ IV). l-23 23 White, B. C., Simpson, C. C.. Adams, J. E. and Harkins, D. (1978) Nrumpknrmacofogy 17, Sll-513 24 Zielke, H. R. and Zielke, C. L. (1986) Life Sci. 39, 565-572 25 Watanabe. H. and Uramoto. H. llQ86)

27 J&s, &Z. F. a&d ~B~ams, M (1Q86) Sot. Neu~osci. At&r. 12,800 28 Moore, K. 11978) in bamboos of Psychopka~macoiogy fvot. 2) (Iversen, L. L., Iversen, S. D. and Snyder, S. H., eds), pp. 48-98, plenum Press 29 &runs, R. F., Lu, G. H. and Pugs&y, T. A, (1986) Mol. Pkonnucof. 29,331-346

are less efficient in terms of computer memory and speed. Assembiy language programs are more difficu& to write, but permit fast direct control of each of the microprocessor elements. Assembly language programs have the format shown in the proparn print-out. ThreeletterinstrueCons (such as LDA for load aecumulator) are used to control the microprocessor. The program is assembled into machine code, which is displayed following the memory locations in the left-mast cofumn.

In order to collect digitized data rapidly from the cardiac action potential, an assembiy language loop was used (Fig. 1). An Interactive Microware Model AI13 fast A/D converter was chosen because of its fast response (19 c&s)and its selectable voltage ranges, which permit direct use on signals from laboratory equipment. Although the AI13 is capable of collecting 12bit A/D vahzes, this program convets digitized vahz~s to b-bits to conserve memory. The program co&&s vafues at 88 ps intervals; this permits analysis of the rate af rise of the action potential in right atrial cells. Storing a code value in a tnem~ ory location of the A/D card begins the digitizing process (lines 1213). The four most significant bits are collected, shifted to the leftand stored (lines N-25). The eight least