Kevin S. Lee

The anticonvulsive action of adenosine: a postsynaptic, dendritic action by a possible endogenous anticonvulsant.

Neural afterdischarges generated in the presence of penicillin or low extracellular calcium concentrations were found to be inhibited by adenosine in the rat hippocampus in vitro. This anticonvulsant effect of adenosine is observed in the absence, as well as in the presence, of chemical synaptic transmission and apparently occurs at a postsynaptic site which is most sensitive in the apical dendritic region of the CA1 pyramidal cells. The methylxanthine theophylline antagonizes the effect of adenosine; and, the anticonvulsant action of the L-isomer of the adenosine analogue phenylisopropyladenosine (PIA) is substantially more potent than the D-isomer, findings which are characteristic of an A1 type adenosine receptor. The endogenous release of adenosine may therefore serve to tonically reduce the tendency for repetitive discharge in CA1 pyramidal cells via an interaction with a high affinity A1 receptor which appears to be preferentially localized in the apical dendrites.

An A1-adenosine receptor, characterized by [3H]cyclohexyladenosine binding, mediates the depression of evoked potentials in a rat hippocampal slice preparation

In slices of rat hippocampus, adenosine and several adenosine derivatives depressed evoked neuronal responses to afferent stimulation. The nanomolar potency of adenosine derivatives and their relative effectiveness indicate that the depression of evoked potentials is mediated via an A1-adenosine receptor. A remarkable similarity was found between the relative potencies of nucleoside derivatives with respect to their electrophysiological effects and to their inhibition of high affinity [3H] cyclohexyladenosine ([3H]CHA) binding to rat brain membranes. We conclude that the [3H] CHA binding site in rat brain membranes represents a physiological receptor of the A1-type.

Rapid down regulation of hippocampal adenosine receptors following brief anoxia

Adenosine A1 receptors, as demonstrated by [3H]cyclohexyladenosine (CHA) binding to cryostat sections of the brain, were studied utilizing quantitative autoradiographic techniques. A brief period of global CNS anoxia resulted in the rapid and persistent down regulation of [3H]CHA binding sites in the hippocampus but not in the neocortex or striatum. The density of adenosine A1 receptors in a given brain region has previously been shown to be a critical factor in determining the strength of the inhibitory action of adenosine. Since the down regulation of these sites is correlated temporally with the onset of hyperactivity following transient anoxia, it is suggested that a reduction in the strength of the neuromodulatory action of adenosine contributes to the postanoxic hyperactivity of CA1 pyramidal cells and perhaps to their selective vulnerability.

Regulation of the strength of adenosine modulation in the hippocampus by a differential distribution of the density of A1 receptors

Correlative studies examining: (1) the binding of [3H]cyclohexyladenosine to A1 adenosine receptor sites, and (2) the depressive action of adenosine on evoked activity, were performed in the hippocampal formation. A greater number of A1 sites was observed in the dorsal versus ventral aspect of the hippocampus, and a larger depression of evoked potentials by adenosine was obtained in the dorsal aspect. These observations suggest that a differential density of A1 receptors might control the magnitude of modulatory action by adenosine on hippocampal circuitry.

Sustained enhancement of evoked potentials following brief, high-frequency stimulation of the cerebral cortex in vitro

Abstract An in vitro slice preparation of the cerebral cortex and hippocampus was utilized to examine the effect of brief, tetanic stimulation on cortical evoked potentials. Following repetitive stimulation, a sustained enhancement of the amplitude of the calcium-sensitive portion of the evoked waveform was observed while the calcium-resistant portion was unchanged.

1,3-Dipropyl-8-cyclopentylxanthine (DPCPX) inhibition of [3H]N-ethylcar☐amidoadenosine (NECA) binding allows the visualization of putative non-A1 adenosine receptors

The binding of the adenosine receptor agonists, [3H]N-ethylcarboxamidoadenosine (NECA) and [3H]cyclohexyladenosine (CHA) to membrane preparations and to cryostat sections of the rat brain was examined. The xanthine derivative, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) was ca. 500-fold more effective at A1 than at A2 sites. [3H]CHA binding to A1 adenosine receptors was virtually eliminated by the inclusion of DPCPX (50 nM), while [3H]NECA binding was only partially inhibited. The pattern of DPCPX-insensitive [3H]NECA binding sites was strikingly different from that of A1 receptors and is believed to represent an association with A2 type adenosine receptors and perhaps another or several, previously undescribed non-A1 sites.

Adenosine receptor density and the depression of evoked neuronal activity in the rat hippocampus in vitro

The relationship between the heterogeneous distribution of A-1 adenosine receptors and the capacity of adenosine to depress neuronal activity was examined in the rat hippocampus. Utilizing autoradiographic techniques, the distribution of A-1 adenosine receptors was assessed by the binding of [3H]cyclohexyladenosine ([3H]CHA) to cryostat sections of rat brain. The apical dendritic region of CA1 showed a differential distribution of adenosine receptors between the stratum radiatum and stratum lacunosum/moleculare. The physiological relevance of this binding difference was studied in the hippocampal slice by examining the capacity of adenosine to depress evoked potentials in these two strata. It was observed that the receptor differences correlated with differential sensitivities to adenosine modulation of the evoked potentials. These data suggest that receptor density, as shown by binding techniques, may provide not only a qualitative but also a quantitative map of the sites of adenosine action.

Heterogeneity of binding sites for N-ethylcar☐amido[3H]adenosine in rat brain: Effects of N-ethylmaleimide

Binding sites for N-ethylcarboxamido[3H]adenosine (NECA) in rat brain membranes and cryostat sections were examined in relation to their sensitivities to displacement by unlabeled NECA and R(-)-phenylisopropyladenosine (R-PIA). In membrane fractions from cerebral cortex, cerebellum, hippocampus and striatum, nanomolar concentrations of these adenosine receptor agonists displaced [3H]NECA such that R-PIA was more effective than NECA, consistent with the presence of an A1-adenosine receptor. At concentrations of displacing agent higher than 1 microM, R-PIA was unable to displace [3H]NECA further in cerebral cortex, cerebellum and hippocampus. In striatum, a second R-PIA-sensitive component of [3H]NECA binding was evident which was more sensitive to NECA than to R-PIA, i.e. it showed the characteristics of an A2-adenosine receptor. In striatum, however, R-PIA was also unable to displace [3H]NECA binding completely. Similar results were obtained in quantitative autoradiographic studies. Preincubation of cryostat sections with N-ethylmaleimide (NEM) abolished both the A1- and R-PIA-insensitive binding components such that both NECA and R-PIA were able to displace [3H]NECA binding completely. The remaining sites showed IC50 values of 0.13 and 3.68 microM for NECA and R-PIA, respectively. These A2-like [3H]NECA binding sites had a highly specific distribution in the brain, being concentrated in the striatum, nucleus accumbens and olfactory tubercle. The results indicate the presence in brain tissue of at least 3 classes of [3H]NECA binding sites, an A1-site, an A2-site and a third, unidentified R-PIA-insensitive site.

Modulation of an inhibitory circuit by adenosine and AMP in the hippocampus.

Abstract The action of adenosine or AMP on the efficacy of a recurrent inhibitory loop was examined utilizing the hippocampal slice preparation. A reversible attenuation of paired-pulse inhibition was produced by micromolar concentrations of these compounds. Thus, in addition to its well-described capacity for reducing the strength of excitatory circuitry, adenosine appears capable of attenuating inhibitory circuitry.

A Combined In Vivo/In Vitro Study of the Presynaptic Release of Adenosine Derivatives in the Hippocampus

Abstract: To investigate the release of adenine compounds from defined neuronal pathways, we employed a hippocampal slice preparation in which a selective‐loading of the releasable pools was achieved in vivo with the aid of axonal transport. By injecting radioactive adenosine stereotaxically into the entorhinal cortex, the major afferent system to the dentate gyrus (the perforant path) was loaded within 20–36 h, at which time the rats were killed and hippocampal slices were prepared. The efflux of radioactive material, as recovered from the perfusate and measured in a scintillation counter, was found to be significantly increased in response to electrophysiologically controlled stimulation of the perforant path but not to stimulation of an alternative fiber tract, the fimbria. These findings provide supportive and more direct evidence for an activation‐coupled release of adenosine derivatives from presynaptic sites in the central nervous system.