Localization of [3H]cyclohexyladenosine and [3H]nitrobenzylthioinosine binding sites in rat striatum and superior colliculus

Acetylcholine-dopamine interactions in the pathophysiology and treatment of CNS disorders

Dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area of the midbrain form the nigrostriatal and mesocorticolimbic dopaminergic pathways that, respectively, project to dorsal and ventral striatum (including prefrontal cortex). These midbrain dopaminergic nuclei and their respective forebrain and cortical target areas are well established as serving a critical role in mediating voluntary motor control, as evidenced in Parkinson's disease, and incentive-motivated behaviors and cognitive functions, as exhibited in drug addiction and schizophrenia, respectively. Although it cannot be disputed that excitatory and inhibitory amino acid-based neurotransmitters, such as glutamate and GABA, play a vital role in modulating activity of midbrain dopaminergic neurons, recent evidence suggests that acetylcholine may be as important in regulating dopaminergic transmission. Midbrain dopaminergic cell tonic and phasic activity is closely dependent upon projections from hindbrain pedunculopontine and the laterodorsal tegmental nuclei, which comprises the only known cholinergic inputs to these neurons. In close coordination with glutamatergic and GABAergic activity, these excitatory cholinergic projections activate nicotinic and muscarinic acetylcholine receptors within the substantia nigra and ventral tegmental area to modulate dopamine transmission in the dorsal/ventral striatum and prefrontal cortex. Additionally, acetylcholine-containing interneurons in the striatum also constitute an important neural substrate to provide further cholinergic modulation of forebrain striatal dopaminergic transmission. In this review, we examine neurological and psychopathological conditions associated with dysfunctions in the interaction of acetylcholine and dopamine and conventional and new pharmacological approaches to treat these disorders.

Markers of adenosine removal in normotensive and hypertensive rat nervous tissue

Adenosine mechanisms are altered in brain stem nuclei associated with cardiovascular control in spontaneously hypertensive rats (SHR). Therefore, in the present study we used a number of techniques to compare the binding of the adenosine transport inhibitor [3H]nitrobenzylthioinosine ([3H]NBMPR) as well as adenosine deaminase immunoreactivity (ADA-IR) in brain stems and nodose ganglia of SHR and age-matched normotensive Donryu rats (DRY). Saturation binding revealed a single class of [3H]NBMPR binding sites in the dorsal brain stem of both strains, with Kd and Bmax values of 65 +/- 9 pmol/L and 282 +/- 31 fmol/mg protein, respectively, in SHR and 129 +/- 2 pmol/L and 217 +/- 23 fmol/mg protein in DRY. The Kd for [3H]NBMPR was significantly lower in SHR than in DRY. In competition assays, NBMPR, dilazep, dipyridamole, and adenosine displaced [3H]NBMPR binding, with Kd values of 0.21 +/- 0.04, 57.16 +/- 16.20, 1340 +/- 100, and 87000 +/- 12500 nmol/L, respectively, in DRY and 0.17 +/- 0.04, 28.24 +/- 3.60, 621 +/- 100, and 32000 +/- 6820 in SHR. Kd values for all displacers were lower in SHR; however, only values for dipyridamole and adenosine reached statistical significance. Autoradiography of adenosine transport sites with [3H]NBMPR revealed that unilateral nodose ganglionectomy reduced [3H]NBMPR binding on the denervated side of the nucleus tractus solitarius by 20.6 +/- 1.1% in DRY and 18.7 +/- 2.3% in SHR. The density of [3H]NBMPR binding in nodose ganglia was significantly lower in SHR (0.99 +/- 0.06 Bq/mm2) than in DRY (1.25 +/- 0.08). Immunohistochemical studies demonstrated ADA-IR in the dorsal vagal complex, associated with both nerve cells and fibers. Measurement of ADA-IR in the dorsal vagal complex with an 125I-labeled secondary antibody revealed a significantly higher level of ADA-IR in SHR (122%) than in DRY. In the nodose ganglia, ADA-IR was associated with a population of vagal perikarya. The present study helps provide a molecular explanation for the previously reported impaired cardiovascular responses to intra-nucleus tractus solitarius microinjection of adenosine in hypertensive rats.

Differential alterations in adenosine A1 and kappa 1 opioid receptors in the striatum in Alzheimer's disease

The alterations in Alzheimer's disease (AD) of two binding sites in the striatum suggested to have a presynaptic localisation have been investigated by quantitative ligand binding autoradiography. Adenosine A1 binding sites labelled with [3H]cyclohexyladenosine (CHA) and kappa 1 opioid binding sites labelled with [3H]U-69593 were studied in adjacent sections of the striatum obtained postmortem from 10 patients with AD and 9 matched controls. In AD, there was a significant reduction of [3H]CHA binding sites in the caudate nucleus (control = 88 +/- 4; AD = 56 +/- 6 pmol/g tissue; mean +/- S.E.M.) and putamen (control = 83 +/- 4; AD = 58 +/- 7 pmol/g). In control subjects, highest levels of [3H]U-69593 binding were localised to patches within the caudate nucleus (9.66 +/- 0.58 pmol/g) with lower levels in the matrix (5.54 +/- 0.48 pmol/g). There was no alteration in [3H]U-69593 binding sites in either the caudate nucleus (patches and matrix) or putamen of AD patients. The activity of choline acetyltransferase (ChAT), determined in the same tissue samples used for autoradiographic analysis, was significantly reduced in AD (control = 124 +/- 11; AD = 64 +/- 14 nmol/h/mg protein). There was a positive correlation between ChAT activity and [3H]CHA binding (r = 0.769), but not [3H]U-69593 binding (r = 0.197). The results indicate that a marked loss of adenosine A1 receptors occurs in the striatum of AD with no loss of kappa 1 opioid receptors, and that the loss of A1 receptors parallels the loss of choline acetyltransferase activity.

Adenosine in vertebrate retina: localization, receptor characterization, and function

1. The uptake of [3H] adenosine into specific populations of cells in the inner retina has been demonstrated. In mammalian retina, the exogenous adenosine that is transported into cells is phosphorylated, thereby maintaining a gradient for transport of the purine into the cell. 2. Endogenous stores of adenosine have been demonstrated by localization of cells that are labeled for adenosine-like immunoreactivity. In the rabbit retina, certain of these cells, the displaced cholinergic, GABAergic amacrine cells, are also labeled for adenosine. 3. Purines are tonically released from dark-adapted rabbit retinas and cultured embryonic chick retinal neurons. Release is significantly increased with K+ and neurotransmitters. The evoked release consists of adenosine, ATP, and purine metabolites, and while a portion of this release is Ca2+ dependent, one other component may occur via the bidirectional purine nucleoside transporter. 4. Differential distributions of certain enzymes involved in purine metabolism have also been localized to the inner retina. 5. Heterogeneous distributions of the two subtypes of adenosine receptors, A1 and A2, have been demonstrated in the mammalian retina. Coupling of receptors to adenylate cyclase has also been demonstrated. 6. Adenosine A1 receptor agonists significantly inhibit the K(+)-stimulated release of [3H]-acetylcholine from the rabbit retina, suggesting that endogenous adenosine may modulate the light-evoked or tonic release of ACh.

The effects of caffeine on ischemic neuronal injury as determined by magnetic resonance imaging and histopathology

The effects of caffeine on ischemic neuronal injury were determined in rats subjected to forebrain ischemia induced by bilateral carotid occlusion and controlled hypotension (50 mmHg for 10 min). High resolution (100 microns) multi-slice, multi-echo magnetic resonance images were obtained daily for three consecutive days post-operatively in sham-operated rats and in rats that received either saline vehicle (controls), a single i.v. injection of 10 mg/kg caffeine 30 min prior to an ischemic insult (acute caffeine group), or up to 90 mg/kg per day of caffeine for three consecutive weeks prior to an ischemic insult (chronic caffeine group). Rats in the control group exhibited enhanced magnetic resonance image intensity in the striatum 24 h after ischemia which increased in the striatum and also appeared in the hippocampus after 48 h, and which began to resolve in both regions by 72 h post-ischemia. Histopathological analysis of each rat following the final magnetic resonance examination showed that ischemic neuronal injury was strictly confined to the brain regions showing magnetic resonance image changes. Acute caffeine rats showed accelerated changes in the magnetic resonance images, with increased hippocampal intensity appearing at 24 h post-ischemia. Although there was magnetic resonance evidence of accelerated injury, quantitative analysis of the histopathological data at 72 h showed no significant difference in the extent of neuronal injury in any brain region between control-ischemic and acute caffeine rats. Nine out of 11 rats in the chronic caffeine group showed no magnetic resonance image changes over the three study days. Chronic caffeine rats had significantly less neuronal damage in all vulnerable brain regions than either of the other groups of ischemic rats. The accelerated ischemic injury in rats treated with an acute dose of caffeine may occur secondary to antagonism of adenosine receptors, whereas protection from ischemic injury following chronic administration of caffeine may be mediated by up-regulation of adenosine receptors.

Effects of enucleations and visual deprivation on adenosine A1 and A2 receptor binding in superior colliculus of rat

Adenosine receptors of A1 and A2 subtypes on retino-collicular projections in rat were studied using [3H]cyclohexyladenosine ([3H]CHA), [3H]N-ethylcarboxamidoadenosine ([3H]NECA), and [3H]CGS 21680. Unilateral enucleation significantly reduced by 51% [3H]CHA binding to A1 sites, and by 42% the A1 component of [3H]NECA binding in contralateral SC. The binding of [3H]CGS 21680 and [3H]NECA to A2 sites was not significantly affected by enucleation procedures. No significant reductions in adenosine receptor binding were observed in SC from rats exposed to a dark environment for three weeks. The reductions in A1 but not A2 adenosine receptor binding strongly suggests that A1 sites are presynaptically located on retinal projections and are vulnerable to deafferentiation-, but not visual deprivation-induced decreases.

Activation of adenosine A1 receptor by N6-(R)-phenylisopropyladenosine (R-PIA) inhibits forskolin-stimulated tyrosine hydroxylase activity in rat striatal synaptosomes

We investigated the effect of the relatively selective A1 adenosine receptor agonist N6-(R)-phenylisopropyladenosine (R-PIA) on tyrosine hydroxylase activity (TH) of synaptosomes obtained from rat striatum. TH activity was assayed in supernatant obtained following sonication and centrifugation of the tissue preincubated with the test compounds. R-PIA produced a modest decrease of basal enzyme activity, but significantly reduced the activation of the enzyme by submaximal (0.1-0.5 microM) concentrations of forskolin (FSK) a stimulator of adenylate cyclase. The IC 50 value of R-PIA was 17 nM and the maximal inhibition corresponded to 30-40% decrease of the enzyme activity stimulated by FSK. The S-isomer of PIA failed to affect TH activity under control and stimulated conditions. Moreover, the inhibitory effect of R-PIA was completely antagonized by 8-cyclopentyl- 1,3 -dimethylxanthine, an adenosine receptor blocker. R-PIA inhibited both basal and FSK-stimulated adenylate cyclase activity. These results indicate that in striatal dopaminergic terminals TH activity can be modulated in an inhibitory manner by activation of presynaptic A1 adenosine receptors.

The cellular localization of adenosine receptors in rat neostriatum

Using quantitative autoradiography of ligand binding sites combined with lesions of specific neuronal pathways, the cellular locations of A1 and A2 adenosine receptors, as well as a third binding site for the adenosine receptor ligand, [3H]N-ethylcarboxamidoadenosine, and a nucleoside transporter were investigated in rat neostriatum. Intrastriatal kainic acid administration resulted in the loss of 50% of A1 adenosine receptors and virtually abolished ligand binding to A2 receptors. A small reduction in [3H]cyclohexyladenosine binding to striatal A1 receptors was found after lesioning the corticostriatal input. A2 receptor sites were unaffected by this treatment. Destruction of dopaminergic neurons using 6-hydroxydopamine or the raphestriatal serotoninergic input using 5,7-dihydroxytryptamine affected neither A1 nor A2 binding sites. These results indicate the localization of both A1 and A2 adenosine receptors on neurons intrinsic to the neostriatum and probably postsynaptic to the dopaminergic input. In addition, a binding site for [3H]N-ethylcarboxamidoadenosine which is not affected by the adenosine receptor agonist, R-phenylisopropyladenosine, was also partly abolished after kainic acid injection. In contrast, no significant change in the binding of the nucleoside transporter ligand, [3H]nitrobenzylthioinosine, was observed after any lesions, indicating the widespread association of this site with various cell types.

Pharmacological characterization of rapidly accumulated adenosine by dissociated brain cells from adult rat

Mechanically dissociated brain cells from adult rats were used to study biochemically and pharmacologically their capacity to accumulate rapidly [3H]adenosine. The assay, which used an inhibitor-stop method to prevent further uptake into cells, was characterized with respect to protein and optimal substrate concentrations, and incubation times that ranged from 5 to 180 s. The accumulation of [3H]adenosine using 15-s incubation periods, conditions under which less than 10% of accumulated [3H]adenosine was metabolized, was best described kinetically by a two-component system with Km and Vmax values for the high-affinity component of 0.8 microM and 6.2 pmol/mg protein/15 s and for the low-affinity component 259 microM and 2,217 pmol/mg protein/15 s, respectively. The potencies with which nucleosides, adenosine deaminase resistant adenosine receptor agonists, and nucleoside uptake inhibitors competed for these uptake components were determined. Of the nucleosides examined, adenosine was the "preferred" substrate for the uptake site. The Ki value of adenosine for the high-affinity component was 10.7 microM. Inosine and uridine competed for a single lower affinity uptake system: Ki values were 142 and 696 microM, respectively. Nucleoside uptake inhibitors--nitrobenzylthioinosine, dipyridamole, and dilazep--were the most potent inhibitors of [3H]adenosine accumulation tested: the Ki values for the high-affinity system were 0.11, 1.3, and 570 nM, respectively. The adenosine analogs S-phenylisopropyladenosine, R-phenylisopropyladenosine, and cyclohexyladenosine inhibited the high-affinity component with Ki values of 2.3, 9.3, and 14.5 microM, respectively. N-Ethylcarboxamidoadenosine competed for a single lower affinity uptake system: Ki, 292 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine uptake site heterogeneity in the mammalian CNS? Uptake inhibitors as probes and potential neuropharmaceuticals

Inhibitors of adenosine uptake or transport have been used clinically for some time in certain cardiovascular diseases. More recently, some of them have also been investigated for possible clinical use in combination with antimetabolites based on the observed heterogeneity of nucleoside transport in mammalian tumor cells. Such a heterogeneity of adenosine uptake and uptake sites has now also been suggested in the mammalian CNS. The aim of this article is, therefore, to review the present status of our knowledge of adenosine uptake in the mammalian CNS, compare it with our far more advanced knowledge of nucleoside transport in other mammalian cells and suggest direction of future research. The possible implications for the development of adenosine uptake inhibitors as adenosinergic neuropharmaceuticals will be discussed based on our knowledge of the physiological function of adenosine in the CNS.

Adenosine uptake and [3H]nitrobenzylthioinosine binding in developing rat brain

The ontogenesis of adenosine transport sites as labelled with [3H]nitrobenzylthioinosine ([3H]NBI) was examined using radioligand binding and membrane preparations from whole brain and 4 brain regions of rats between the postnatal ages of one day through to adulthood. In whole brain, cerebral cortex and cerebellum, [3H]NBI binding was two-fold higher in 6-day-old than in 50-day-old rats. In contrast, [3H]NBI binding was higher in adults than in one-day-old rats by 4-fold in hypothalamus and 8-fold in superior colliculus. In cortex and hypothalamus, the levels of [3H]NBI binding in newborn and adult rats were reflected by changes in Bmax and not Kd values. As a measure of the utility of [3H]NBI as a probe for identifying functional adenosine transport sites, we examined [3H]NBI binding to and [3H]adenosine accumulation by intact brain cells prepared from adult and newborn rats. For [3H]NBI binding to brain cells from adult rats, the values of Kd were 0.092 nM and of Bmax were 274 fmol/mg protein. For newborns, slightly higher Kd and Bmax values were observed; 0.2 nM and 395 fmol/mg protein, respectively. [3H]Adenosine accumulation was higher in brain cells from one-day-old than from adult rat brains. Kinetically this uptake was best described by a two-component model: the Vmax values for the high- and low-affinity uptake, and the Km value for the high-affinity component in one-day-old rats were greater than in adults.(ABSTRACT TRUNCATED AT 250 WORDS)

Subcellular, regional and immunohistochemical localization of adenosine deaminase in various species

Immunohistochemical and subcellular fractionation techniques were employed to compare the cellular and subcellular localization of adenosine deaminase (ADA) in various brain regions of several mammalian species. A relatively restricted distribution of ADA-immunoreactive neurons in rat brain was previously reported. Mouse brain exhibited a pattern similar in many respects to rat and, in addition, contained intensely immunostained neurons in lateral habenula and hippocampus. Glial immunostaining was absent or very light in rat but evident in mouse. Prominent immunoreactive fibers and neurons were observed in hamster spinal cord and anterior hypothalamus, respectively. ADA-immunostaining in guinea-pig was localized to presumptive fibers in the superficial layers of spinal cord dorsal horn and to glial cells throughout the brain. Demonstration of specific immunostaining in rabbit was not possible. ADA activity was far more heterogeneously distributed in rat and most brain areas in guinea-pig and rabbit contained up to 5-fold and 10-fold higher levels of activity, respectively, compared with rat. Crude synaptosomal (P2) fractions of rat cortex contained a greater proportion of ADA activity than those of rabbit cortex. Within rat, relatively high activity was found in P2 fractions of whole hypothalamus, cerebellum, and hippocampus. ADA activity was greater in P2 fractions of rat anterior compared with whole hypothalamus and the greatest proportion of the enzyme in this fraction was localized to purified synaptosomes. The large variations in the activity and cellular location of ADA in the animals examined suggest species differences in mechanisms governing adenosine metabolism in brain and possible differences in the relationships between cellular metabolism, ADA and the neuroregulatory role of adenosine in the CNS.

Development of adenosine-dependent cyclic AMP accumulation in the avian optic tectum

The present work shows the existence of adenosine-dependent cyclic adenosine monophosphate (AMP) accumulation in the chick optic tectum. When tecta from 18-day-old embryos were incubated with the phosphodiesterase inhibitor IBMX and RO 20-1724, the cyclic AMP level increased from 39.2 to 73.3 and 285.5 pmol/mg protein, respectively. The high level obtained with RO 20-1724 could be inhibited by increasing concentrations of IBMX or by adenosine deaminase, but not by dipyridamole. 2-Chloroadenosine promoted a dose-dependent cyclic AMP accumulation in tecta incubated with RO 20-1724 and adenosine deaminase. This effect was blocked by IBMX and varied substantially during the development of the tissue. The degree of stimulation increased after day 11 of incubation, attaining maximal levels on day 14. The effect of 2-chloroadenosine remained constant until day 18, a period when both the protein content and the basal cyclic AMP levels are increasing in the developing tectum. The cyclic AMP increase elicited by 2-chloroadenosine was greatly reduced in tecta from 20-day-old embryos and 2-day-old chicks. The putative transmitters glutamate and glycine and the neurotransmitter analogs isoproterenol and carbachol had no stimulatory effect on the cyclic AMP accumulation of tecta from 10- and 17-day-old embryos.

Endogenous adenosine and adenosine receptors localized to ganglion cells of the retina

Using specific sensitive antisera against adenosine, we have immunocytochemically localized endogenous adenosine to specific layers of rat, guinea pig, monkey, and human retina. Highest adenosine immunoreactivity was observed in ganglion cells and their processes in the optic nerve fiber layer. Substantial staining was also found throughout the inner plexiform layer and in select cells in the inner nuclear layer. Adenosine A1 receptors, labeled with the agonists L-[3H]phenylisopropyladenosine and 125I-labeled hydroxy-phenylisopropyladenosine, were autoradiographically localized. The highest levels of binding sites occurred in the nerve fiber, ganglion cell, and inner plexiform layers of the retina in all the species examined. The distribution of adenosine A1 receptor sites closely parallels that of retinal neurons and fibers containing immunoreactive adenosine. These results suggest a role for endogenous adenosine as a coneurotransmitter in ganglion cells and their fibers in the optic nerve.

Colocalization of (3H)-adenosine accumulation and GABA immunoreactivity in the chicken and rabbit retinas

Using combined autoradiography and immunohistochemistry, we have compared (3H)-adenosine accumulation and GABA immunoreactivity in the chicken and rabbit retinas. Colocalization of the two markers was observed in a subset of amacrine cells and in certain cell bodies in the ganglion cell layer in both species and in a few horizontal cells in the chicken retina. Cells that contained only (3H)-adenosine or GABA were also seen. The degree of colocalization differed greatly between the two species. The results demonstrate a morphological relationship between the adenosine and GABA systems and provides information on the possible anatomical substrates underlying at least some types of functional interactions.