Posted comment on ´Adenosine receptors: expression, function and regulation` by S. Sheth, R. Brito, D. Mukherjea, L.P: Rybak and V. Ramkumar and published in International Journal of Molecular Science 2014 15(2) 2024-2052 doi.org/10.3390/ijms15022024 pmcid:pmc3958836
Adenosine receptors (ARs) are G-protein coupled receptors (GPCR) that mediate the actions of the natural cellular modulator, adenosine. Although Sheth and colleagues outline the properties of the receptors occurring in the peripheral system as well, this summary will only concentrate on the information about those receptors occurring in the brain or in brain cell cultures or slices. In their article, Sheth and colleagues describe the 4 subtypes of adenosine receptors found in the brain and detail their different localizations. The subtype A1R, which exhibits a high affinity for the natural agonist adenosine, is widely distributed on neurons in the cortical, hippocampal and cerebellar areas and can also be found on the glial cell populations such as astrocytes, oligodendrocytes and microglia. In neurons, subtype A1R is localized to the synaptic regions where it modulates the release of neurotransmitters such as glutamate, acetylcholine, serotonin and GABA. The other subtype also showing a high affinity for adenosine is the A2AR. Sheth and colleagues describe this subtype as having a lower distribution than the A1R and as being only localized in the striatal and olfactory bulb areas (on neurons and on glial cells such as microglia and oligodendrocytes and possibly astrocytes) and the hippocampus (at presynaptic areas). The adenosine receptor subtype here modulates the release of the neurotransmitters glutamate, acetylcholine, GABA and noradrenaline. The two other subtypes, A2BR and A3R have a lower affinity for adenosine and the expression of the A2BR is shown to be at low levels on neuronal and glial cells in a wider selection of areas such as the cortex, hippocampus, cerebellum and striatum.
Sheth and colleagues describe the traditional classification of the subtypes of these G-protein coupled receptors by their differing coupling to the adenylyl cyclase (AC) enzyme at the membrane surface. The A1R and A3R are coupled to inhibitory G-proteins (Gi) hence agonist activation leads to a decrease in cyclic adenosine monophosphate (cAMP) levels in the cells. However, A2AR and A2BR are coupled to stimulatory G-proteins (Gs) and hence, agonist activation of these subtypes leads to an increase in cAMP resulting in protein kinase A (PKA) activation and the series of secondary effects attributed to a raised cAMP level. The differing levels of receptors in the brain areas mean that the effect of adenosine can be either stimulatory or inhibitory dependent on the type of receptor present.
Another property of the receptor leading to distinguishing the different subtype populations is, according to Sheth and colleagues, how the receptor population desensitizes on prolonged agonist exposure. In their article, the authors describe a general desensitization mechanism typical for G-protein coupled receptors. Desensitisation in this case involves the phosphorylation of the receptors by G-protein coupled receptor kinases (GPK) which leads to preferential binding of arrestin molecules. This leads to uncoupling of the receptor from the G-protein and an internalization of the arrestin-receptor complex by a clathrin-coated pit dependent endocytosis process. Within the vesicle the receptor undergoes a dephosphorylation process and is re-inserted into the cell membrane to restore agonist sensitivity. In the case of prolonged agonist activation, the internalized receptors are transferred to lysosomes and are degraded thus resulting in a down-regulation of the receptor at the cell surface and a general decreased agonist sensitivity. According to Sheth and colleagues, the adenosine receptor subtypes demonstrate different desensitization process characteristics. In the case of the A1R, this receptor subtype is phosphorylated and internalized slowly (has a half-life of several hours), whereas the A2AR and A2BR undergo the same mechanism, but more rapidly (about an hour) and A3R within minutes. Prolonged agonist at the A1R leads to increased AC activity and a reported desensitization of the insulin dependent glucose transport system which may explain some neuropathological effects seen under these conditions. A high increase in mRNA for the A1R was also observed indicating to the authors that the arrestin binding as a result of the prolonged exposure primes the cell for recovery once the exposure is stopped.
In the case of the A2AR, the authors describe the desensitization process as dependent on the length of time the cell is exposed to agonists. In short-term exposure, there is rapid desensitization of the A2AR-stimulated AC activity associated with decreased receptor-Gs coupling and agonist stimulated phosphorylation of the receptor itself. However, longer exposure to the agonist causes a down-regulation of the total receptor number and an up-regulation of alpha subunits of the Gi protein. The authors describe an effective G-protein coupled receptor kinase subtype (GPK2) and found that the mechanism is inhibited by Tumour Necrosis Factor type alpha (TNF-alpha), a pro-inflammatory cytokine. They explained this observation by saying that there is ´novel cross-talk` between the TNK-alpha receptor and the A2AR. It was found that treatment with TNF-alpha led to reduced translocation of GPK2 to the plasma membrane and reduced GPK2 association with the plasma membrane, thus preventing A2AR activity. The A2BR was found to demonstrate the same desensitization and internalization mechanisms as for A2AR (ie. GPK2 and arrestin dependent). Here, the TNF-alpha reduced the agonist-dependent receptor phosphorylation and attenuated the agonist-mediated A2BR desensitization. The authors explained that this action may contribute to the excessive astrocytic activation that occurs in neurodegenerative diseases. The subtype A3R was found by the authors to undergo the same processes as the other subtypes as a result of long-term agonist exposure.
Another property of ARs described by the authors in their article is their ability to form homodimers (ie. with each other) and heterodimers with different adenosine subtypes or receptors of other neurotransmitters. Sheth and colleagues describe the existence of A1R homodimers in the cortex, hippocampal pyramidal cells and cerebellar Purkinje cells. They also said that homodimers present in the cortex could explain the diphasic nature of the effect of small and large doses of caffeine on motor activity. The authors also describe the situations where ARs can form heterodimers with other subtypes. In the case of the A1R/A2AR heterodimer, Sheth and colleagues found that activation of the A2AR by its specific agonist reduced the affinity of the participating partner in the heterodimer ie A1AR for its specific agonist. This was observed by looking at the intracellular calcium ion levels (reduced on pretreatment with A2AR agonist), abolishment of K+ evoked glutamate release and increased GABA uptake. However, this effect was found not to be reciprocal. A cross antagonism and/or physical interaction between the two adenosine receptor subtypes was demonstrated by the A1AR effect being blocked not only by the selective antagonist DPCPX for the A1AR, but also by the SCH 58261 (selective A2AR antagonist) and the effect of A2AR on GABA uptake was blocked by both SCH 58261 and DPCPX. The receptors were shown to be internalized together when exposed to both sets of agonists. The differential responses of the A1R and A2AR agonists on GABA uptake involved the activation of the Gi and Gs proteins respectively and the authors concluded that the presence of these A1R-A2AR heterodimers could increase the complexity by which the two receptors regulate neuronal action at the cell surface. The heterodimer complex of A2AR-A2BR was also found and appears to exist for receptor trafficking and hence, have an effect on regulation of cellular function.
The authors also described in their article the cases of adenosine receptor heterodimers formed with other neurotransmitter receptors. An interaction of adenosine receptors with ATP receptors (classed P2X, P2Y, P2U and P2Z) was demonstrated with the A1AR subtype. P type receptors are also G-protein coupled and P2Y1 can form a heterodimer with A1AR. It was found co-localised in the cell body and dendritic regions in rat cortical neurons, but in the soma and dendrites of slices of cortical, hippocampal and cerebellar neurons. The heterodimer was found to be less effective in inhibiting cAMP formation than with the native A1AR alone.
Other heterodimers formed between adenosine receptors and receptors of other neurotransmitters were found in the striatum. In this area there is also an interaction between A2AR and CB1 receptors (cannabinoid and endocannabinoid as agonists, coupled to Gi proteins). This type of heterodimer when activated by a CB1 agonist leads to reduced agonist induced cAMP accumulation. The effective coupling of the CB1 receptor to the Gi protein requires prior or simultaneous activation of the A2AR. Interaction between ARs and dopamine receptors was also found in this brain area, eg a A2AR-D2R in striatal cultures and further dopamine receptor heterodimers were found in fibroblast cultures and cortical neurons such as in the nucleus accumbens (both A1R-D1R). In the case of the former, it was found that the receptors of this particular type of heterodimer internalized together on activation – an action dependent on the presence of beta-arrestin2 and the gene Akt. Pre-administration with cocaine (an activator of D1R) led to the dissolution of the heterodimeric complex. The heterodimer could also associate with CB1 R to form a trimeric complex.
The authors suggested that the role of adenosine receptors in neuropathological diseases such as Parkinson`s disease and Huntingdon`s Chorea could be attributed to the presence of the homodimeric and heterodimeric forms of the receptor. Another factor contributing to their neuropathological influence is their association with transcription factors such as nuclear factors which regulate the expression of proteins in the neurons. The authors proposed that certain forms of the adenosine receptor complex could act as sensors of cellular oxidative stress which is known to be indicated by the activity certain transcription factors such as NF-kB. This particular transcription factor regulates the expression of the ARs particularly A1R and A2AR, ie. those that have a high affinity for the natural agonist. A1 R was found to be positively regulated by oxidative stress brought about by the excessive level of reactive oxygen species (ROS) within the cell. Treatment of the cell with the nerve growth factor (and others) led to a three-fold decrease in A2AR expression within 3 days and this observation was explained by the location of the NF-kB consensus sites on the A2AR gene promotor. In the case of the A1R then NF-kB not only regulated the expression of the receptor, but also caused a deficit in A1R/Gi protein – an observation associated with increased neuronal apoptosis. The authors also described the cellular situation in hypoxia regarding adenosine activity. In this case, hypoxia was found to be associated with increased adenosine levels and an up-regulation of the A2AR, but also a desensitisation of A1R which was linked to a decreased density of the A1R (an observation under dispute).
Sheth and colleagues also discussed the involvement of adenosine receptors and some normal and abnormal physiological processes such as sleep, the development of certain cancers and in the protection against hearing loss. Adenosine has been shown to be involved in the sleep-wake cycle in differing ways. Adenosine is known to promote sleep, but an increase in the forebrain level is linked to prolonged wakefulness. Increasing adenosine via decreasing adenosine deaminase function leads to a deeper sleep and higher slow-wave activity within sleep. Inhibition of the adenosine intracellular uptake by inhibiting the transporter protein also leads to symptoms similar to sleep deprivation. The actions of adenosine in sleep were attributed to the A1R where A1R agonist administration leads to increased sleep and antagonists to increased wakefulness. Sleep deprivation was shown to be linked to an increase in A1R density in the basal forebrain which the authors suggested could be responsible for the subsequent sleep re-bound. The increased levels of adenosine observed in wakefulness however were attributed to the astrocytes being the source of the agonist. Adenosine in this case was released by a SNARE-dependent exocytosis. An interesting observation according to the authors was that experiments using a dominant negative SNARE protein led to a lower level of memory deficit induced by sleep deprivation compared to controls. This suggested to them that adenosine plays a role in memory deficits observed with sleep deprivation. The conflicting roles of adenosine in sleep is further shown by the studies using A1R and A2AR knock-out mice who show that both types of receptor are involved in both mediating the sleep suppressing role and arousal action of caffeine. Further studies indicate that A1R expression and normal sleep patterns should be regarded as dissociated and that they mediate a physiological drive following sleep deprivation.
The role of adenosine receptors and hearing loss prove more conclusive of an adenosine role. The authors explained that the cochlea expresses 3 subtypes of ARs in different cells and that A1R confers protection against hearing loss whereas A2AR activation exacerbates cisplatin induced ototoxicity. Both agonists of the A1R and antagonists of the A2AR are used to treat ototoxicity. The mechanism by which adenosine acts involves a reduction in adenosine levels and adenosine uptake inhibitors, while theophylline and adenosine deaminase is increased. Application of adenosine to inner hair cells causes an increase in intracellular calcium ion levels demonstrating an A1R link. Oxidative stress by activation of NF-kB was shown to lead to enhanced transcriptional activity of the A1R gene. The authors suggested that feedback regulation could increase a cyto-protective activity of the A1R in response to oxidative stress caused by noise exposure or therapeutic agents. ROS was also found to increase inflammatory processes in the cochlea by activating NF-kB. The oxidative stress in the cochlea was said to contribute to the inflammatory process by activating signal transducers and the activator of transcription 1 (STAT1) transcription factor which can couple the activation of transient receptor potential vanilloid receptor (TRPV)-1 to the induction of inflammation. Down-regulation of STAT1 ameliorates cisplatin-induced ototoxicity in rats and therefore, it was suggested that the otoprotective actions of the A1R against cisplatin ototoxicity possibly involves inhibition of both NF-kB and STAT1 transcription factors.
Sheth and colleagues also described the role of adenosine in some cancers. Although these are not related to brain function, they will form part of this summary because of their importance and link to general adenosine functioning in the brain. Studies on the link between adenosine and cancer show differing results, eg. debatable differences are observed in the expression and function of A1R in breast cancer, with high A1R expression being seen in human colorectal adenocarcinoma and human leukemia. The source of the increased expression was said to be activated astrocytes and microglia. However, other studies found that A1R demonstrates anti-tumour effects, eg. A1R activation increases apoptosis by activating caspases in human colon cancer cells. Over-expression of the A2AR was observed in several cancer cell lines. It was found to stimulate cell proliferation, migration and tube formation, but again was also found to inhibit tumour growth and angiogenesis in other studies by activating caspases to induce apoptosis. Reports of the action of A2BR in cancer appear more consistent with it being pro-angiogenic with receptor activation leading to neovascularization through the production of vascular endothelial growth factor (VEGF) and the release of the pro-angiogenic growth factor, interleukin-8. The receptor A3AR like the A1R also demonstrates differing observations with over-expression being shown in different types of cancer cells, eg. prostate and breast carcinoma, but again demonstrating anti-tumour actions in others, eg. growth of melanomas and prostate cancer cells inhibited by A3AR agonist administration. This activity is explained by an increase in the natural killer (NK) cell activity which promotes killing of the tumour cells. In prostate cancer cells, activation of A3AR leads to a suppression of high levels of ROS generated by these cells – an action involving the inhibition of NADPH oxidases.
Sheth and colleagues concluded their article by saying that because of its demonstrated modulatory cellular roles a better understanding of the physiology and functioning of adenosine and adenosine receptors is necessary. Such knowledge could aid the development of new therapies for treatment of some neuropathological diseases.
This article is interesting because it again describes how a little known molecule can have a significant effect on neuronal functioning and synaptic performance. Neuroscience has spent many years concentrating on the popular neurotransmitters such as acetylcholine and glutamate, but as more and more is known then it is becoming clearer that the absolute workings observed at the neuronal and synaptic level are extremely complex and consist of thousands of different elements all of which have their own structure, mechanisms and influencing factors. This article attempts to describe one such small element and here we discuss how this element can influence the neuronal and synaptic workings as a whole. Although the authors in this article give evidence of adenosine`s roles in the human body we will discuss in this blog comment only those observations relating to the brain and here, the complexity of the effect of adenosine is dependent on different mechanisms.
One of the fundamental differences in its activity is adenosine`s link to the level of cAMP within the cell. Such a link is important because of the well-known prolific effects of cAMP as a secondary messenger. A rise in cellular cAMP can lead to protein kinase activation resulting in the general effects of phosphorylation of serine/threonine residues and effects on for example, glycogen metabolism, stimulation of the expression of specific genes (relating to phosphorylation of the transcriptional activitator cAMP-response element binding protein, CREB) and the closing of potassium channels.. In the case of adenosine, this neuromodulation takes the form of a rise in cAMP which is linked to neurotransmitter release. Therefore, the cAMP response is dependent on the structure of the adenosine receptor itself. Adenosine receptors are known to be G-protein receptors and therefore, the observation of adenosine effects on cAMP means that these G-proteins are linked to adenylyl cyclase function. In the case of adenosine, certain subtypes of receptor (eg. A2AR and A2BR) are linked to stimulatory G-proteins (Gs) and result in excitatory responses and increased cellular cAMP. However, other subtypes (A1R and A3R) are linked to inhibitory G-proteins (Gi) and agonist activation here leads to synaptic inhibition and a decrease in cAMP. This decrease in cAMP means that neurotransmitter release is inhibited or lower. Therefore, the effect of the agonist adenosine in each brain area eg. on neurotransmitter release can be dependent on the type and number of adenosine receptor subtypes present on the neurons or glial cells within that area. And this is observed since certain areas such as the cortex, hippocampus and cerebellum have high distribution of the inhibitory A1R and other areas eg. the striatum and olfactory bulb a high distribution of the excitatory A2AR.
Since G-proteins can also be linked to ion gated channels, the excitatory or inhibitory modulating effect of adenosine can depend on the link between receptor subtype present and associated ion channel. Adenosine receptors have been reported to be linked to potassium ion and calcium ion channels and hence, stimulatory action (ie. through the A2AR and A2BR) is likely to be linked to potassium ion channel shut-down or non-functioning and the inhibitory action (ie. through the A1R and A3R) with potassium ion channel opening. Again the overall effect of adenosine at the neuron or glial cell will depend on the subtype of receptor present and its attached G-protein.
The level of naturally occurring adenosine can also play a part in the effect of adenosine on cellular function and this can be seen by looking at the affinity of adenosine for each receptor subtype. Both A1R and A2AR exhibit high affinity for the agonist which means that lower levels of agonist are necessary for action to occur than the other two subtypes (A2BR and A3R). Therefore, this means that one subtype needs a low level of agonist for its inhibitory effect (A1R) and the other a low level of agonist for its excitatory effect (A2AR). The subtypes A2BR and A3R both demonstrate low affinity for the agonist and therefore, need to be in higher cellular levels for an effect on the cAMP action to occur. Therefore, not only does location and distribution of the receptor subtypes matter, but also the level of naturally occurring agonist.
Another factor that can affect the action of the adenosine receptor subtypes is their ability to interact with other receptors (either adenosine receptors itself or receptors of other neurotransmitters) to form monodimers, homodimers or heterodimers. Since each form has different characteristics, the level of each within a brain area or even more specifically the neuronal synapse can lead to a different action and sensitivity to adenosine. For example, the heterodimeric A1R-A2AR responds to A2AR activation by decreasing the affinity of adenosine for the A1R, although the effect is not reciprocal. This means that the overall effect of a high level of adenosine is excitatory – the presence of the A1R has been essentially ´neutered`. The heterodimer forms with A2AR and CB1R (Gs and Gi proteins) leads to decreased cAMP production (ie. the excitatory effect of the A2AR is essentially ´neutered`) and A2AR with DA receptors with an effect linked to Parkinson`s disease. The effect of these homodimeric and heterodimeric forms can be explained by, for example, the changes to the phospholipid membrane fluidity affecting the binding and action of its components and the influences of the interaction of the two receptors on the quartenary protein structures of the ´complex` that could favour one binding over another.
Therefore, bearing in mind the structure of the adenosine receptor and its affinity for agonist we can see that adenosine can have an excitatory or inhibitory effect at the cell level. In the case of the excitatory effect, adenosine is likely to exert its action through the high affinity receptor, A2AR. One possible version of the mechanism employed here is that both the neuronal synaptic receptors and the astrocytes are involved since both are present and contain functional AR. The neuronal action potential leads to increased calcium ion concentration as normal and a release of synthesised adenosine (or ATP) either via adenosine transporters (passive or Na+ dependent – a mechanism that fits in with sodium ion changes occurring with depolarisation) or from the attachment of vesicles containing ATP and other neurotransmitters in the normal exocytotic mechanisms associated with the action potential and neurotransmitter release. Since extracellular adenosine is related to intracellular adenosine and ATP concentration, therefore its release reflects the metabolic demand of cell.
In this version of neuronal excitation, the released adenosine (or ATP) then binds to either presynaptic neuronal A2ARs or astroglial A2ARs, but in both cases the binding leads to the activation of G-protein related adenylyl cyclase (Gs protein) and cAMP production and increased neurotransmitter release. In the case of presynaptic receptors, the raised cAMP produces the same effects as other neurotransmitters (eg. increased neurotransmitter release) so that the released neurotransmitter can then bind to the post-synaptic receptors along with the neurotransmitter released directly from the action of depolarisation. Neurotransmitters can also bind to neurotransmitter receptors on astroglial cells leading to gliotransmission and neurotransmitter release that can subsequently bind post-synaptically. The effects can be antagonised by the action of caffeine and theophylline which are both A2AR dependent. Hence, the excitatory response of such a set of neurons is enhanced by the presence of adenosine since the effect does not directly cause nerve transmission. Therefore, the mechanism is ideal for areas where frequency of firing is not enough to cause transmission of the signal directly.
However, adenosine can also have an inhibitory effect. This effect is elicited through the A1R, where the adenylyl cyclase effect is inhibitory due to the presence of the Gi protein. In this case, the mechanism of adenosine action can be hypothesised as beginning just like in the case of its excitatory action with the production of adenosine and its release via transporters or vesicle endocytosis. However, in the case of adenosine having an inhibitory effect, the adenosine released binds to high affinity A1R on presynaptic neurons or astroglial cells. In the case of the presynaptic binding, ion gated channels could be opened and potassium ions enter the cell as in the normal stages of cell transmission dependent on depolarisation. The presence of the Gi protein means that cAMP production is decreased. Therefore, the overall effect of the action of adenosine is that neurotransmitter release is decreased (eg. as observed with glutamate, 5HT, Ach and GABA). This results in decreased neurotransmitter binding to pre-synaptic neuronal cells and decreased neurotransmitter release. Adenosine binding to the A1R on astroglial cells has the same effect.
However, it is interesting to note that in areas where GABA is released that the inhibitory effect of GABA is enhanced by the presence of adenosine receptors. This can be explained by the action of GABA itself which switches on chloride ion channels and leads to the hyperpolarisation of cells, whereas adenosine merely switches the cell off by removing the membrane potential differences caused by the ion concentration differences. This inhibitory process can also be seen in the SC-CA1 hippocampal area, but here a different mechanism is employed. In this case, astrocytic calcium ions trigger the release of ATP and subsequent presynaptic binding. Therefore in general, inhibition occurs through potassium ion gating into the presynaptic neuron leading to the removal of the membrane potential difference caused by ion concentration differences and resulting in the dissolution of firing, or it by adenosine inhibiting adenylyl cyclase action which decreases cAMP concentration and its secondary effects resulting in decreased neurotransmitter release.
These excitatory and inhibitory actions of adenosine and its receptor subtypes can be put to good effect and sleep is a good example of the balancing act of these two effects. Neural activity leads to increased levels of adenosine in the awake state and in the prolonged awake state the increased level comes from astrocytes. It is known that the activity of the A2 antagonists, caffeine and theophylline, lead to maintaining the awake state, but adenosine itself can promote sleep and also its level decreases during sleep. This ´conflicting` action of adenosine can be explained by looking at the location and distribution of adenosine receptor subtypes. In the case of wakefulness, some areas are needed to be excitatory and therefore adenosine A1Rs are blocked and A2ARs are active. Some areas though need to exhibit inhibited adenosine activity. Here, the A1Rs are active with A2ARs blocked or non-existent. In the case of sleep, the reverse occurs ie. adenosine requirements mean that A2AR are inhibited (or blocked – supported by the action of caffeine and theophylline which are both A2AR antagonists) and the inhibitory action of the A1R dominates.
Therefore, the action of the naturally occurring adenosine in brain areas can be explained by the location and distribution of the various subtypes of its own receptor on the neurons and astroglial cells present and the mechanisms involved relating to the G-proteins associated with them. Since adenosine has a modulatory role in the neuronal synapse, then anywhere where the capability to produce and release adenosine exists and its receptors occur then this area can be affected by it. The presence of A1R will enhance inhibition or decrease excitation in an excitatory neuron and the presence of A2AR will enhance excitation or decrease inhibition in an inhibitory neuron and sleep is a natural example of when these types of mechanisms are brought into play. The authors of the article, Sheth and colleagues, also showed how adenosine receptors are involved in other brain functions and neuropathological diseases. However, can we say that adenosine could be used to exert a therapeutic effect through its modulating activity? The answer is it might be possible to aid the action of one brain area or another by enhancing or inhibiting its natural level, but administration of adenosine or another agonist would have to be very location specific and an intensive understanding of the interplay between brain areas would be required. However, with the increasing accuracy of selective drug administration and the improving knowledge and imaging of interconnectivity between brain areas then this might be a possibility for the future and adenosine agonists and antagonists may become important in the treatment of some neuropathological diseases such as Parkinson`s disease.
Since we`re talking about the topic………
…it is believed that there is an over-excitability of certain hippocampal areas in Alzheimer`s disease. Would an exploration of A1R and A2AR populations and an upregulation of the A1R in these areas have a positive effect in combating this disease effect?
….it is known that in Parkinson`s disease the brain area substantia nigra shows decreased activity due to a decreased density of dopamine receptors. It has been shown that dopamine receptors can be linked to adenosine receptors in heterodimeric complexes and therefore, could the activity of the dopamine receptor be enhanced not just be administering dopamine agonist, but also by administering adenosine agonists? Again, is selectivity of administration to this area alone important or could the knowledge of interconnectivity between the substantia nigra and other brain areas be used to administer adenosine agonists to these other areas which may be easier to administer to?