Biomedical subjects

J D Geiger

Publications and source records attributed to J D Geiger.

At least 19 recordsLinked to original sources

PubMed1992

High affinity [3H]ryanodine binding sites in postmortem human brain: regional distribution and effects of calcium, magnesium and caffeine.

The pharmacological properties, regional distribution and autoradiographic localization of [3H]ryanodine binding sites were examined in postmortem human brain. Analyses of binding data from labeled ryanodine titration experiments conducted in frontal cortex revealed a single class of high affinity binding sites with a Kd value of 3.6 nM and a Bmax value of 99 fmol/mg protein. In unlabeled ryanodine titration experiments, Kd and Bmax values were 6.5 nM and 132 fmol/mg protein, respectively. Binding was found to be dependent on free Ca2+ (ED50 value, 89 microM) and was decreased by 35% in the presence of 5 mM Mg2+. This Mg2+ inhibition was abolished by the addition of 10 mM caffeine. Analysis of the regional distribution of [3H]ryanodine binding in membrane preparations revealed high levels of sites in putamen and caudate nucleus, intermediate levels in hippocampus and cortex, and low levels in cerebellum. Autoradiographically, the hippocampus displayed a high density of binding sites in the CA3 region and the dentate gyrus. Ryanodine binding sites in human brain exhibit similar, but not identical binding and pharmacological characteristics to ryanodine receptors previously identified in muscle and more recently in rat and rabbit brain and accordingly may be involved in the regulation of intracellular calcium.

Aged↗

PubMed1992

A novel role for autologous tumour cell vaccination in the immunotherapy of the poorly immunogenic B16-BL6 melanoma.

The growth of immunogenic tumours stimulates the generation of tumour-sensitized, but not functional, pre-effector T cells in the draining lymph nodes. These pre-effector cells can mature into effector cells upon in-vitro stimulation with anti-CD3 and IL-2. In the current study, using a defined, poorly immunogenic tumour, B16-BL6 melanoma, the pre-effector cell response was not evident during progressive tumour growth but was elicited by vaccination with irradiated tumour cells admixed with Corynebacterium parvum. After anti-CD3/IL-2 activation, these cells were capable of mediating the regression of established pulmonary metastases. The efficacy of the vaccine depended on the doses of both tumour cells and the adjuvant. While higher numbers of tumour cells were more effective, an optimal dose (12.5 micrograms) of C. parvum was required. The dose of irradiation was not a critical factor. After vaccination, kinetic studies revealed that the pre-effector cell response was evident 4 days later and declined after 14 days. These observations illustrate the potential role of active immunization in the cellular therapy of cancer.

Animals↗

PubMed1992

Rat brain adenosine deaminase after 2'-deoxycoformycin administration: biochemical properties and evidence for reduced enzyme levels detected by 2'-[3H]deoxycoformycin ligand binding.

Near total inhibition of brain adenosine deaminase (ADA) activity in rats injected with the potent ADA inhibitor 2'-deoxycoformycin (DCF) was previously shown to reduce enzyme activity for up to 50 days during which time the enzyme exhibited reduced sensitivity to in vivo inhibition by DCF. Here, we investigated the biochemical properties of ADA and the basis for its reduced activity after DCF treatment. It was found that much higher doses of DCF were required to inhibit ADA in DCF-treated compared with drug-naive animals. Fourteen days after DCF administration, reduced ADA activity in brain homogenates was due to a decrease in Vmax, rather than to an altered Km of ADA for adenosine. DCF treatment had no effect on Ki values for erythro-9-(2-hydroxy-3-nonyl)adenine inhibition of ADA. The IC50 value for DCF inhibition of ADA in hypothalamus was unchanged. However, the Ki for DCF inhibition of ADA in whole brain increased by fivefold. Sucrose gradient analysis of brain ADA revealed only one corresponding peak of activity and [3H]DCF-labeled ADA in DCF-treated and control rats. A radioligand filtration assay with [3H]DCF was developed to assess the effects of DCF on ADA protein levels. Over a roughly 200-fold range of ADA activities the binding of [3H]DCF was highly correlated with deaminase activity (r = 0.99). In brain tissues taken 8 and 33 days after treatment of rats with DCF, [3H]DCF binding was reduced to 27% and 48% of control levels, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenine↗

PubMed1992

Transport and metabolism of D-[3H]adenosine and L-[3H]adenosine in rat cerebral cortical synaptoneurosomes.

The relationship between transport and metabolism in synaptoneurosomes was examined to determine the metabolic stability of rapidly accumulated D-[3H]adenosine and L-[3H]adenosine and the degree to which metabolism of the accumulated purines affected measurements of apparent KT and Vmax values for adenosine transport. For D-[3H]adenosine, high- and low-affinity accumulation processes were present. For the high-affinity system an inverse relationship was found between transport reaction times and KT and Vmax values. For incubations of 5, 15, and 600 s, which corresponded to 24, 32, and 76% phosphorylation of accumulated D-[3H]adenosine to nucleotides, apparent KT values were 9.4, 8.4, and 4.5 microM, respectively, and Vmax values were 850, 70, and 12 pmol/min/mg of protein, respectively. Pretreatment with 10 microM erythro-9-(2-hydroxy-3-nonyl)adenine, an adenosine deaminase inhibitor, and 5'-iodotubercidin, an adenosine kinase inhibitor, decreased the phosphorylation of accumulated D-[3H]adenosine to 6% with 5-s and 9% with 15-s incubations. This resulted in significantly higher KT values: 36 microM at 5 s and 44 microM at 15 s. At 10-min incubations in the presence of these inhibitors, metabolism of accumulated D-[3H]adenosine was 32%, and apparent KT and Vmax values at this time were not significantly different from those obtained without inhibitors. For L-[3H]adenosine, apparent KT and Vmax values for 20-s incubations were 38.7 microM and 330 pmol/min/mg of protein, respectively. Metabolism (mainly phosphorylation) of accumulated L-[3H]adenosine was observed only at incubations of greater than 30 s.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenine↗

PubMed1991

[3H]ryanodine binding sites in rat brain demonstrated by membrane binding and autoradiography.

High affinity [3H]ryanodine binding sites were characterized in P1 (crude nuclear), P2 (mitochondrial/synaptosomal) and P3 (microsomal) subcellular fractions of rat brain. Binding in each of the fractions was highest at 37 degrees C and pH 8-9, optimal in the presence of 100 microM Ca2+, 550 microM ATP and 1.0 M KCl, and increased linearly as a function of protein. Saturation analyses revealed a single class of binding sites with mean KD values (nM) of 8.9, 1.6 and 5.7 and Bmax values (fmol/mg protein) of 122, 69 and 106 for the P1, P2 and P3 fractions, respectively. The levels of [3H]ryanodine binding in P1 and P2 fractions of 4 brain regions were fairly uniform while those in P3 fractions were 5-fold greater in cerebral cortex than in the other areas examined. By autoradiography, a high concentration of [3H]ryanodine binding sites was seen in the dentate gyrus and CA3 subregions of the hippocampus. The results suggest that [3H]ryanodine binding sites, perhaps similar to [3H]ryanodine receptors in muscle, are associated with various subcellular structures and are heterogeneously distributed in the CNS.

Animals↗

PubMed1991

Dilazep-induced vasodilation is mediated through adenosine receptors.

Administration of dilazep, an inhibitor of adenosine uptake, significantly reduced systemic arterial blood pressure and increased superior mesenteric arterial conductance without affecting the plasma adenosine levels of femoral arterial or portal venous blood. Administration of a bolus dose of 8-phenyltheophylline (8-PT), an antagonist of adenosine receptors, blocked adenosine-mediated autoregulation of the superior mesenteric artery. After the blockade of adenosine receptors by 8-PT, dilazep did not produce vasodilation. These data suggest that dilazep has a vasodilating effect in vivo that is mediated by adenosine.

Adenosine↗

PubMed1991

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.

Adenosine↗

PubMed1991

L-[3H]adenosine, a new metabolically stable enantiomeric probe for adenosine transport systems in rat brain synaptoneurosomes.

The stereoenantimers D-[3H]adenosine and L-[3H]adenosine were used to study adenosine accumulation in rat cerebral cortical synaptoneurosomes. L-Adenosine very weakly inhibited rat brain adenosine deaminase (ADA) activity with a Ki value of 385 microM. It did not inhibit rat brain adenosine kinase (AK) activity, nor was it utilized as a substrate for either ADA or AK. The rate constants (fmol/mg of protein/s) for L-[3H]adenosine accumulation measured in assays where transport was stopped either with inhibitor-stop centrifugation or with rapid filtration methods were 82 +/- 14 and 75 +/- 10, respectively. Using the filtration method, the rates of L-[3H]adenosine accumulation were not significantly different from the value of 105 +/- 15 fmol/mg of protein/s measured for D-[3H]adenosine transport. Unlabeled D-adenosine and nitrobenzylthiolnosine, both at a concentration of 100 microM, reduced the levels and rates of L-[3H]adenosine accumulation by greater than 44%. These findings suggest that L-adenosine, a metabolically stable enantiomeric analog, and the naturally occurring D-adenosine are both taken up by rat brain synaptoneurosomes by similar processes, and as such L-adenosine may represent an important new probe with which adenosine uptake may be studied.

Adenosine↗

PubMed1991

Improved high-pressure liquid chromatographic-fluorometric assay for measurement of adenosine in plasma.

A high-pressure liquid chromatographic system using fluorescence detection was characterized for the determination of adenosine levels in plasma from anesthetized cat, rat, dog, mouse, rabbit, and guinea pig. The optimal concentration of chloroacetaldehyde necessary to convert physiological levels of adenosine to its fluorescent derivative 1,N6-ethenoadenosine (ethenoadenosine) was in excess of 220 mM. As little as 0.2 pmol of ethenoadenosine could be measured, and detection was linear up to 200 pmol. Derivatization of adenosine into ethenoadenosine was highly dependent on temperature and time. However, ethenoadenosine showed thermal instability in that levels dropped sharply after 30 min at 100 degrees C, 2 h at 80 degrees C, and 24 h at 55 degrees C. Adenosine nucleotides were extracted from plasma samples with an efficiency of greater than 91% to prevent adenosine formation from ATP and AMP that would have otherwise occurred during the derivatization procedure. Plasma levels (microM) of adenosine in venous blood were 0.31 in dog, 0.54 in cat, 0.71 in guinea pig, 1.03 in mouse, 1.04 in rat, and 1.68 in rabbit. Plasma levels of adenosine in arterial blood were not significantly different from levels in venous blood. This method can be used to measure even very low levels of adenosine without interference from nucleotide breakdown.

Adenosine↗

PubMed1991

Dilazep potentiation of adenosine-mediated superior mesenteric arterial vasodilation.

The present study investigated the effects of dilazep, an inhibitor of adenosine uptake, on adenosine-mediated vasodilation in vivo. Intravenous and intraportal venous infusions of exogenous adenosine (0.04-1.0 mg/kg/min) did not recirculate to cause increases in superior mesenteric arterial conductance (SMAC) or arterial plasma adenosine levels except at the higher doses tested (0.4-1.0 mg/kg/min). After administration of dilazep, however, even low doses (0.04-0.1 mg/kg/min) of exogenous adenosine significantly increased SMAC and elevated arterial plasma adenosine concentration. The increased adenosine levels were highly correlated with the increased percentage of change of SMAC and values for Rmax and EC50 were 193.4 +/- 27.3% change of SMAC and 2.8 +/- 1.3 microM, respectively. Administration of bolus doses of 8-phenyltheophylline abolished the ability of dilazep to potentiate vasodilation, but did not affect isoproterenol-induced relaxation. Together, these results suggest that potentiation of the vasodilating effect of exogenous adenosine by dilazep is mediated through inhibition of adenosine uptake in vivo which increases the availability of plasma adenosine to act on adenosine receptors.

Adenosine↗

PubMed1990

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.

Adenosine↗

PubMed1990

Adenosine transport systems on dissociated brain cells from mouse, guinea-pig, and rat.

The kinetics and sodium dependence of adenosine transport were determined using an inhibitor-stop method on dissociated cell body preparations obtained from mouse, guinea-pig and rat brain. Transport affinity (KT) values for the high affinity adenosine transport systems (KT(H] were significantly different between these three species; mean +/- SEM values were 0.34 +/- 0.1 in mouse, 0.9 +/- 0.2 in rat, and 1.5 +/- 0.5 microM in guinea-pig. The KT values for the low affinity transport system (KT(L) were not different between the three species. Brain cells from rat displayed a significantly greater maximal capacity to accumulate [3H]adenosine (Vmax) than did mouse or guinea-pig for the high affinity system, or than did mouse for the low affinity system. When sodium chloride was replaced in the transport medium with choline chloride, the KT(H) values for guinea-pig and rat were both increased by approximately 100%; only in rat did the change reach statistical significance. The sodium-dependence of adenosine transport in mouse brain was clearly absent. The differences between KT(H) values in mouse and those in guinea-pig or rat were accentuated in the absence of sodium. The differences in kinetic values, ionic requirements, and pharmacological characteristics between adenosine transporters in CNS tissues of mouse, guinea-pig and rat may help account for some of the variability noted among species in terms of their physiological responses to adenosine.

Adenosine↗

PubMed1990

2'-Deoxycoformycin inhibition of adenosine deaminase in rat brain: in vivo and in vitro analysis of specificity, potency, and enzyme recovery.

2'-Deoxycoformycin (DCF), a potent inhibitor of adenosine deaminase (ADA), is increasingly used as a tool to investigate adenosine metabolism and neuromodulation. To advance further the usefulness of DCF for studies of purines in the CNS, we determined the inhibitory potency of this compound against ADA and adenylate deaminase (AMPDA) in brain, the rate of ADA recovery in various brain regions after single or repeated intraperitoneal DCF administrations, and the effect of DCF on several neurotransmitter synthetic enzymes. In vitro, the Ki values for inhibition of ADA and AMPDA were found to be 23 pM and 233 microM, respectively. In vivo, DCF inhibited ADA with ED50 values ranging from 155 to 280 micrograms/kg at 2 h posttreatment, and 98% inhibition was achieved with 1 mg/kg. AMPDA activity was not affected by doses up to 5.0 mg/kg. In contrast to the greater than 95% inhibition of ADA seen 1 day after DCF at 5 mg/kg, the effectiveness of a second similar DCF treatment on the activity that had recovered by 14 days was dramatically reduced. Eight days after DCF treatment with doses of 5-50 mg/kg, the degree of ADA activity recovery in 10 brain regions examined was similar; it averaged 35% of control values at the low dose but showed some heterogeneity, ranging from 15 to 54% of control values, at the higher doses. Forty days after treatment with a single dose of 5 mg/kg, ADA activity recovered by 68-78% of control values in brain regions with normally high levels of activity and by 44-59% of control values in other regions. The activities of choline acetyltransferase, glutamic acid decarboxylase, and histidine decarboxylase (an enzyme colocalized with ADA in hypothalamic neurons) were unaffected by DCF treatment, a result suggesting the lack of a generalized neurotoxic effect. The very low doses of DCF required for ADA inhibition in vivo are consistent with the high potency of this drug against ADA in vitro, and any physiological effects observed at low doses might therefore be ascribed to inhibition of ADA.

AMP Deaminase↗

PubMed1990

Binding of the adenosine A2 receptor ligand [3H]CGS 21680 to human and rat brain: evidence for multiple affinity sites.

A new radiolabeled adenosine receptor agonist, 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadeno sin e (CGS 21680), apparently specific for high-affinity binding sites of the A2 subtype in rat brain, was used to identify and pharmacologically characterize adenosine receptors in human brain. The binding of [3H]CGS 21680, as determined by standard radioligand binding technique in the presence of exogenously added adenosine deaminase, reached equilibrium after 40 min at 25 degrees C. In saturation studies, a single class of high-affinity binding sites with values for KD of 22 +/- 0.5 nM and Bmax of 444 +/- 63 fmol/mg of protein were observed. Similar binding characteristics were observed regardless of whether rapid filtration or centrifugation was used to separate bound versus free ligand. Of the 14 brain regions examined, [3H]CGS 21680 binding was highest in putamen, followed by globus pallidus and caudate nucleus. The level of [3H]CGS 21680 binding in these areas of basal ganglia was identical to 5'-N-[3H]ethylcarboxamidoadenosine ([3H]NECA) binding in the presence of 50 nM N6-cyclopentyladenosine (CPA). The rank order of agonist potencies as determined by a series of competition experiments was NECA greater than or equal to CGS 21680 greater than 2-chloroadenosine greater than N6-(R)-phenylisopropyladenosine greater than N6-cyclohexyladenosine greater than N6-(S)-phenylisopropyladenosine. This potency order was the same for the binding of [3H]CGS 21680 to rat, and of [3H]NECA in the presence of 50 nM CPA to rat and human, brain membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine↗

PubMed1989

Adenosine analogs inhibit gastric acid secretion.

The potencies with which four adenosine deaminase-resistant analogs of adenosine affected the volume, pH and acid output of basal gastric acid secretions were examined in unanesthetized rats with chronic indwelling gastric cannulas. All four adenosine receptor agonists, R-phenylisopropyladenosine (R-PIA), S-phenylisopropyladenosine (S-PIA), N-ethylcarboxamide adenosine (NECA), and 2-chloroadenosine (CADO) significantly decreased gastric acid output in a dose-dependent manner. The rank order of potency was NECA, R-PIA greater than CADO greater than S-PIA. NECA and R-PIA were approximately equipotent in reducing gastric acid output. The levels of gastric acid output tended to increase at the lowest doses of the agonists. NECA decreased the volume of gastric secretion, whereas R-PIA had no effect on volume, but significantly increased the pH of the secretions. Valid measurements of pH in NECA-treated rats were not always obtainable because of near total inhibition of gastric secretions. S-PIA did not significantly affect volume, but increased pH at the higher doses tested. CADO decreased volume, but did not affect pH. These results indicate that adenosine analogs regulate not only the hydrogen ion concentration, but also the volume of gastric secretions.

2-Chloroadenosine↗

PubMed1989

Sodium-dependent uptake of nucleosides by dissociated brain cells from the rat.

Sodium-dependent 3H-labeled nucleoside transport was studied using a mixed population of dissociated brain cells from adult rats. The accumulation of [3H]adenosine during brief (15-s) incubation periods was significantly greater in the presence of 110 mM Na+ than in its absence. This occurred at substrate concentrations that ranged from 0.25 to 100 microM. Similar findings were observed for the rapid accumulation of [3H]uridine. Kinetically, the rapid accumulation of [3H]adenosine in both the absence and the presence of Na+ was best described by a two-component system. In the presence of Na+, the KT and Vmax values for the high-affinity affinity component were 0.9 microM and 8.9 pmol/mg of protein/15 s, and those for the low-affinity component were 313 microM and 3,428 pmol/mg of protein/15 s, respectively. In the absence of Na+, the KT value for the high-affinity component was significantly higher (1.8 microM). [3H]Uridine accumulation was best described kinetically by a one-component system that in the presence of Na+ had KT and Vmax values of 1.0 mM and 2.6 nmol/mg of protein/15 s, respectively. As was found for [3H]adenosine, in the absence of Na+, the KT value was significantly higher (1.8 mM). The sodium-dependent transport of [3H]adenosine was inhibitable by ouabain and 2,4-dinitrophenol. Of the three nucleoside transport inhibitors tested, only nitrobenzylthioninosine demonstrated high affinity and selectivity in blocking the sodium component. Thus, high-affinity sodium-dependent nucleoside transport systems, in addition to facilitated diffusion systems, exist on brain cells from adult rats.

Adenosine↗

PubMed1988

Distinct adenosine deaminase-containing inputs to the substantia nigra from the striatum and tuberomammillary nucleus.

Immunohistochemical, neuroanatomical and lesion methods were used to investigate the projections of adenosine deaminase immunoreactive (ADA-IR) neurons in the striatum (caudate/putamen) and hypothalamus to the substantia nigra (SN). Striatal ADA-IR neurons were distributed within two zones; anteriorly in the medial and ventromedial extreme of the head and body of the striatum, and posteriorly in the tail of the striatum. The posterior hypothalamus contained ADA-positive neurons which were confined to the tuberomammillary nucleus (TM). The SN was devoid of ADA-positive neurons, but contained two distinct types of ADA-IR fiber terminations. One type was confined to bands located at the ventrolateral and dorsomedial borders of the pars reticulata and consisted of fine puncta. The other type was distributed throughout the SN and consisted of long, beaded fibers. Injections of the retrograde tracer Fluoro-gold (FG) into the SN gave rise to FG-labelling of significant numbers of ADA-IR neurons in both the striatum and TM. Medial SN injections preferentially labelled ADA-IR neurons in the anterior striatum and lateral SN injections labelled posterior ADA-IR striatal neurons. Kainic acid lesions of the anterior medial striatum selectively abolished the punctate ADA-IR band in the medial SN and left the long, ADA-IR nigral fibers in an apparently hypertrophied state. Despite depletion of ADA-IR neurons in the striatum by kainic acid, ADA activity increased significantly at striatal lesion sites. The results suggest that the SN receives two topographically segregated fine terminal fields from striatal ADA-IR neurons, and a substantial innervation from ADA-IR neurons in the TM as well. These findings add to the heterogeneous chemical composition of nigral afferents and are discussed in the context of adenosine neuromodulatory mechanisms in the striatonigral system.

Adenosine Deaminase↗

PubMed1988

Adenosine deaminase-'like' immunoreactivity in cerebellar Purkinje cells of rat.

The localization of adenosine deaminase (ADA) throughout the rat CNS was investigated immunohistochemically with 5 different affinity purified polyclonal antibody preparations against ADA. Except in the cerebellum, identical immunostaining patterns of ADA-immunoreactivity were observed with all 5 antibodies. In the cerebellum, Purkinje cells and presumptive swellings on their axons were immunostained by only one of the 5 antibodies. Preabsorption with purified ADA abolished immunostaining with all 5 antibodies. Biochemical tests showed that the Km for substrate, the specific activity of immunoprecipitable enzyme and the isozymic characteristics of cerebellar ADA was not different from ADA in whole brain or several other brain regions examined. The atypical immunohistochemical behavior of cerebellar ADA, despite its biochemical similarity to ADA elsewhere in the brain, suggests that the enzyme in the cerebellum has some unique features which must be taken into account when considering its possible role in regulating the neuromodulatory actions of adenosine in the cerebellar cortex.

Adenosine Deaminase↗