Elsevier

Neuropharmacology

Volume 39, Issue 7, June 2000, Pages 1156-1167
Neuropharmacology

Purinergic modulation of [3H]GABA release from rat hippocampal nerve terminals

https://doi.org/10.1016/S0028-3908(99)00237-3Get rights and content

Abstract

The hippocampal GABAergic system is assumed not to be a target for purine modulation. We have now confirmed that neither adenosine A1 and A3 receptor nor nucleotide P2 or P4 receptor activation modified the K+-evoked [3H]GABA release from hippocampal synaptosomes. However, activation of adenosine A2A receptors with CGS 21680 (10 nM) or HENECA (30 nM) facilitated GABA release by 32% and 21%, respectively. These effects were prevented by the A2A antagonist, ZM 241385 (20 nM). A2A receptors may activate adenylate cyclase and protein kinase A since CGS 21680 (10 nM) facilitation was partially prevented by 8-bromo-cAMP (1 mM), forskolin (10 μM) and HA-1004 (10 μM). Protein kinase C may also be recruited, since chelerythrine (6 μM) and phorbol-12,13-didecanoate (250 nM) attenuated CGS 21680 (10 nM) facilitation of [3H]GABA release. ω-Agatoxin-IVA (200 nM) occluded CGS 21680 facilitation suggesting the involvement of P-type calcium channels. Thus, the adenosine A2A receptor system appears to be one of the first presynaptic neuromodulatory systems able to enhance the evoked release of GABA from hippocampal nerve terminals.

Introduction

Purines are important neuromodulatory substances in the hippocampus. Stimulation of neuronal preparations causes the release of purines, such as ATP, adenosine and diadenosine polyphosphates (Pintor and Miras-Portugal, 1995, Cunha et al., 1996a) in parallel with the release of neurotransmitters (e.g. Jo and Schlichter, 1999). Adenosine tonically inhibits hippocampal neuronal excitability by acting on inhibitory A1 receptors and can also activate facilitatory A2A receptors, enhancing hippocampal neurotransmitter release (Cunha et al., 1994a) and synaptic transmission (Cunha et al., 1994b). A1 Receptor mRNA is mostly expressed in granular and pyramidal cell bodies, with a pattern similar to that of A2A receptor mRNA although the latter is more diffuse (Cunha et al., 1994a). A1 Receptors are mostly located presynaptically and in axons (Swanson et al., 1995, Cunha et al., 1996b) and their density is higher than that of A2A receptors in the hippocampus (Cunha et al., 1996b). In spite of the different densities of A1 and A2A receptors, modulation of synaptic transmission and neurotransmitter release by adenosine in the hippocampus depends on a balance between A1 and A2A receptor activation (Cunha et al., 1994a, Cunha et al., 1994b). Adenosine can also activate two other receptors (A2B and A3) which have a diffuse and low density distribution in the hippocampus and that have not been demonstrated so far to modulate neurotransmitter release. ATP can also modulate neuronal excitability in the hippocampus, either directly through activation of P2 receptors (Pankratov et al., 1998) or indirectly upon its extracellular catabolism into adenosine through an ecto-nucleotidase cascade (Cunha et al., 1996a). Diadenosine polyphosphates also modulate synaptic transmission in the hippocampus (Klishin et al., 1994) either through P2 or P4 receptors (Pintor and Miras-Portugal, 1995) or through P1 receptors upon extracellular catabolism by ecto-nucleotidases (Pintor and Miras-Portugal, 1995).

Synaptic transmission in the hippocampus is processed by excitatory pathways tightly regulated by a network of inhibitory interneurons (Buckmaster and Soltesz, 1996). Purines can modulate neuronal excitability in the hippocampus either by directly modulating excitatory circuits, as seems to be the case in adenosine A1 inhibition (Lambert and Tyler, 1991, Yoon and Rothman, 1991), or by modification of GABAergic function. Although the modulation of inhibitory transmission is of fundamental importance for information processing in the hippocampus, it is generally assumed that the GABAergic system is not a target for purinergic modulation, since adenosine A1 agonists failed to affect inhibitory transmission in hippocampal slices (Lambert and Tyler, 1991, Yoon and Rothman, 1991). In the present work, we directly investigated the involvement of different purinergic receptors in the modulation of GABA release from hippocampal nerve terminals. We found that activation of adenosine A2A receptors enhances the evoked release of GABA via P-type calcium channel activation in a cAMP- and PKC-dependent manner.

Section snippets

Nerve terminal preparation

A synaptosomal fraction from the rat hippocampus was prepared as previously described (Cunha et al., 1997a). Briefly, male Wistar rats (130–140 g) were decapitated after anesthesia under halothane atmosphere, following the Portuguese guidelines for handling experimental animals. The brains were rapidly removed into ice-cold 0.32 M sucrose solution containing 1 mM EDTA, 1 mg/ml bovine serum albumin, and 5 mM HEPES buffered to pH 7.4 with 1 M NaOH. The brain was cut longitudinally and the two

Results

The stimulation of hippocampal synaptosomes with K+ (20 mM) for 2 min caused a nearly 3-fold increase in [3H]GABA release with an S2/S1 ratio of 0.74±0.02 (Fig. 1). The average basal release of 3H]GABA was 0.11±0.01% (n=25) and the peak of [3H]GABA release induced by K+ (20 mM) during S1 was 0.31±0.03% (n=25) of total tritium retained by the synaptosomes (Fig. 1). The K+-induced [3H]GABA release was mostly calcium-dependent, since omission of calcium from the extracellular medium and addition

Discussion

The present study shows that adenosine A2A receptors facilitate GABA release from hippocampal nerve terminals. The association of adenosine A2A receptors with GABAergic neurons was hypothesized based on in situ hybridization and on autoradiography studies with the prototypical A2A receptor agonist, [3H]CGS 21680, in the hippocampus (Cunha et al., 1994b) and in the medial septum (Cunha et al., 1995) which provides a major cholinergic and GABAergic projection into the hippocampus (Wainer et al.,

Acknowledgments

The technical assistance of M.D.Constantino is greatly acknowledged. RAC thanks A. Alves-Rodrigues for critically reviewing the manuscript. Supported by Fundação para a Ciência e Tecnologia (SAU/14012/1998).

References (54)

  • G. Nemoz et al.

    Selective inhibition of one of the cyclic AMP phosphodiesterases from rat brain by the neurotropic compound rolipram

    Biochem. Pharmacol.

    (1985)
  • J.W. Phillis

    Inhibitory action of CGS 21680 on cerebral cortical neurons is antagonized by bicuculline and picrotoxin-is GABA involved?

    Brain Res.

    (1998)
  • J. Pintor et al.

    P2 purinergic receptors for diadenosine polyphosphates in the nervous system

    Gen. Pharmacol.

    (1995)
  • J.C. Poncer et al.

    Either N- or P-type calcium channels mediate GABA release at distinct hippocampal inhibitory synapses

    Neuron

    (1997)
  • S.M. Thompson et al.

    Presynaptic inhibition in the hippocampus

    Trends Neurosci.

    (1993)
  • D. Ulrich et al.

    Purinergic inhibition of GABA and glutamate release in the thalamus: implications for thalamic network activity

    Neuron

    (1995)
  • B.H. Wainer et al.

    Cholinergic and non-cholinergic septohippocampal pathways

    Neurosci. Lett.

    (1985)
  • H.L. Wang et al.

    Vasoactive intestinal polypeptide enhances the GABAergic synaptic transmission in cultured hippocampal neurons

    Brain Res.

    (1997)
  • Agostinho, P., Caseiro, P., Rego, A.C., Duarte, E.P., Cunha, R.A., Oliveira, C.R., 1999. Adenosine modulation of...
  • R.A. Barraco et al.

    Evidence for presynaptic adenosine A2a receptors associated with norepinephrine release and their desensitization in the rat nucleus tractus solitarius

    J. Neurochem.

    (1995)
  • R.M. Broad et al.

    A1, but not A2A, adenosine receptors modulate electrically stimulated [14C]acetylcholine release from rat cortex

    J. Pharmacol. Exp. Ther.

    (1996)
  • P.S. Buckmaster et al.

    Neurobiology of hippocampal inter-neurons: a workshop review

    Hippocampus

    (1996)
  • S.P. Burke et al.

    Regulation of glutamate and aspartate release from slices of the hippocampal CA1 area: effects of adenosine and baclofen

    J. Neurochem.

    (1988)
  • M. Capogna et al.

    Presynaptic enhancement of inhibitory synaptic transmission by protein kinases A and C in the rat hippocampus

    J. Neurosci.

    (1995)
  • G. Chen et al.

    Adenosine modulation of calcium currents and presynaptic inhibition of GABA release in suprachiasmatic and arcuate nucleus neurons

    J. Neurophysiol.

    (1997)
  • R.A. Cunha et al.

    Excitatory and inhibitory effects of A1 and A2A adenosine receptor activation on the electrically-evoked [3H]acetylcholine release from different areas of the rat hippocampus

    J. Neurochem.

    (1994)
  • R.A. Cunha et al.

    Purinergic modulation of the evoked release of [3H]acetylcholine from the hippocampus and cerebral cortex of the rat: role of the ecto-nucleotidases

    Eur. J. Neurosci.

    (1994)
  • Cited by (0)

    View full text