In 1791 Luigi Galvani revealed the electrical nature of nerve impulses in frogs’ neuromuscular joints, when he published his masterwork De viribus electricitatis in motu musculari commentarius [1]. Strongly influenced by Galvani’s work, the scientific community defended the concept of bio-electricity up to the beginning of the 20th century, when Ramón y Cajal’s illustrations questioned the possibility of electrical transmission between communicating neurons through perceptible intervals. Afterwards, Otto Loewi’s experiments in vagus nerve preparations led to the discovery of the first neurotransmitter, acetylcholine [2]. This was the beginning of the neurotransmitter concept, an endogenous chemical substance capable of ensuring the correct transmission of information between one neuron and another cell in the body. Recognition of glutamate as the main excitatory neurotransmitter only occurred in the 1950s, when Jeffrey Watkins contributed to this field by identifying distinct types of ionotropic glutamate receptors. [3], of which AMPA type receptors are the main mediators of rapid excitatory transmission in the central nervous system.

By allowing variation of the response capacity of the postsynaptic neuron to the glutamate released by the afferent neuron, regulation of the AMPA function ensures significant changes in the efficiency of synaptic transmission that are inherent to the maturation of neural networks with development, as well as to synaptic plasticity phenomena. In addition, these receptors are key molecules in excitotoxicity situations, which hypoxic or ischemic insults are examples of. In those situations, the increase in extracellular concentration of glutamate and the superactivation of its receptors encourages the unregulated flow of calcium and activates cell death cascades. Unfortunately, the neuroprotective potential of AMPA antagonists in animal models of brain ischemia has not found parallel in clinical trials carried out so far. [4]. As an alternative, strategies for the pharmacological modulation of the AMPA function may be of interest from a therapeutic stance. Accordingly, the present work aimed to investigate the possibility of adenosine-mediated modulation of the postsynaptic AMPA component, because it is an ubiquitous neuromodulator whose multiple actions ensure the balance between incoming and used energy substrates in excitable tissues [5]. To this effect, patch-clamp recordings of currents mediated by AMPA receptors were made in pyramidal cells of the CA1 area of the hippocampus.

Particularly prone to reflect changes in synaptic efficiency according to levels of activity, this neural population is also particularly susceptible to death by apoptosis following ischemia. We have noted that using an adenosine A2A receptor agonist (CGS21680) lead to a significant increase in the amplitude of AMPA currents, which was lost in the presence of a protein kinase A inhibitor (PKA). It is known that the PKA phosphorylation of AMPA receptors enables regulating neuronal membrane traffic. Resorting to biotinylation trials to selectively evaluate the membrane receptor levels, we found that the latter increased following administering CGS21680. Afterwards, and in order to ascertain the functional impact of this modulation, we used a high frequency afferent stimulation protocol capable of evoking long term potentiation (LTP) of the synaptic transmission, considered to be the cellular substrate in the formation of new memories. We noted that this form of plasticity was facilitated after a brief course of treatment with CGS21680. In addition, blockade of the A2A receptors reduced, by itself, the magnitude of the LTP, which suggests that it is an endogenous regulatory mechanism. Accordingly, it is possible to infer that in situations of sudden increase of extracellular adenosine caused by exacerbated neuronal activity, the activation of A2A receptors facilitates the PKA phosphorylation of AMPA receptors, which increases their availability in membrane reserves and corresponding recruitment to synapse, thus strengthening synaptic transmission.

Curiously, these conditions of exacerbated neuronal activity and the extracellular accumulation of adenosine are also present in the initial stages of ischemic insults. We also know that exposure to transitory hypoxic/ischemic insults brings about a persistent increase in synaptic efficiency that shares many of the mechanisms inherent to LTP expression, including a functional gain of the postsynaptic AMPA component. With the aim of investigating the involvement of A2A receptors in this process, we compared the effects of brief ischemic insults [6] in the presence and absence of a receptor selective antagonist (SCH58261). As noted by other authors, the inhibition of synaptic transmission caused by the insult was followed by a gradual recovery of the postsynaptic response, up to values significantly higher than the initial ones. This type of ischemia-induced plasticity was totally abolished both by the previous blockade of A2A receptors and by the antagonism of a subclass of AMPA receptors permeable to calcium. Thus, these results reveal a new mechanism of adenosine neuromodulation, and also demonstrate the presence of new common features between physiological and “pathological” forms of plasticity. In this context, it will be of utmost interest to clarify the role played by ischemia-induced plasticity processes, given that, whether they contribute to aggravating damages caused by excitotoxicity or are involved in the formation of new synaptic contacts able to recover some of the lost circuits in regions most affected by ischemic accidents, is still subject to controversy. From the viewpoint of the latter hypothesis, it may also be important to invest in neurogenerative strategies that use drugs that modulate (adaptive) plasticity triggered by ischemia.

The results described above are part of the research carried out as part of my PhD thesis, which was recently submitted to FCT for appreciation. The work was supervised by Professors Joaquim Alexandre Ribeiro and Ana Maria Sebastião, to whom I express my deep gratefulness. For a detailed viewing of the results, please see Dias et al., 2010
(here)

 

Raquel Baptista Dias
Institute of Pharmacology and Neurosciences, Faculty of Medicine
Neuroscience Unit, Institute of Molecular Medicine
rdias@fm.ul.pt
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Bibliografia

[1] Galvani L (1791) De viribus electricitatis in motu musculari commentarius. Bon Sci Art Inst Acad Comm 7:363–418
[2] Loewi O (1921) Über humorale Übertragbarkeit der Herznerven-wirkung. Pflügers Archiv189: 239-242
[3] Curtis DR, Watkins JC (1961) Analogues of glutamic and gamma-amino-n-butyric acids having potent actions on mammalian neurones. Nature 191:1010-1011
[4] Mattes H, Carcache D, Kalkman HO, Koller M (2010) alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) antagonists: from bench to bedside. J Med Chem 53: 5367-5382
[5] Sebastião AM, Ribeiro JA (2000) Fine-tuning neuromodulation by adenosine. Trends Pharmacol Sci 21: 341-634
[6] Rossi DJ, Oshima T, Attwell D (2000) Glutamate release in severe brain ischaemia is mainly by reversed uptake. Nature 403: 316-321