Electroconvulsive therapy: a novel hypothesis for the involvement of purinergic signalling

Abstract

It is proposed that ATP is released from both neurons and glia during electroconvulsive therapy (ECT) and that this leads to reduction of depressive behaviour via complex stimulation of neurons and glia directly via P2X and P2Y receptors and also via P1 receptors after extracellular breakdown of ATP to adenosine. In particular, A₁ adenosine receptors inhibit release of excitatory transmitters, and A_2A and P2Y receptors may modulate the release of dopamine. Sequential ECT may lead to changes in purinoceptor expression in mesolimbic and mesocortical regions of the brain implicated in depression and other mood disorders. In particular, increased expression of P2X7 receptors on glial cells would lead to increased release of cytokines, chemokines and neurotrophins. In summary, we suggest that ATP release following ECT involves neurons, glial cells and neuron–glial interactions acting via both P2 and after breakdown to adenosine via P1 receptors. We suggest that ecto-nucleotidase inhibitors (increasing available amounts of ATP) and purinoceptor agonists may enhance the anti-depressive effect of ECT.

Introduction

Electroconvulsive therapy (ECT), the therapeutic application of electricity to the scalp, has been used in the treatment of psychiatric disorders for more than 60 years [1]. It is now well established and considered one of the most effective methods for treating major depression [2] and catatonic schizophrenia [3]. Major depression is a debilitating condition that includes a range of symptoms and results from the contribution of multiple genetic and environmental factors [4]. It has been claimed that a depressive state can be characterized by abnormalities in the functions of monoaminergic neurotransmission, of the hypothalamic-pituitary-adrenocortical system or of neurotrophin and cytokine activities in genetically predisposed individuals that have endured the impact of stress or infection [5]. There have been numerous studies to elucidate the precise mechanism of action of ECT thought to include generalized seizures, normalization of neuroendocrine dysfunction and increased hippocampal neurogenesis and synaptogenesis [6], but a definitive candidate remains to be found. A multitude of neurotrophic factors, hormones, neuropeptides [7–9] and neurotransmitters and their receptors [10] have all been implicated. The use of ECT has been questioned, due to the experience of side effects in some patients (e.g. retrograde and anterograde amnesia), and its efficacy may depend on the aetiology of the depression and also the placement of the electrodes. We hypothesize that purines, and their receptors may be central to the clinical results observed as a result of ECT. Following the characterization of adenosine (ADO) and adenosine 5′-triphosphate (ATP) receptors, the field of purinergic signalling has become well established [11]. Recent findings suggest that ATP, its breakdown products and receptors are key players in pathological states both in the central nervous system (CNS) [10] and elsewhere [11]. There is also emerging evidence that purines are involved in mood and motivation [10].

Supporting evidence

Modulation of synaptic transmission by ATP and its metabolites

The concept of purinergic neurotransmission was introduced over 40 years ago when ATP was shown to be a transmitter in non-adrenergic, non-cholinergic inhibitory nerves in the guinea pig taenia coli [12]. It has since been shown to be a co-transmitter in both the peripheral and central nervous systems [11]. ATP acts as a fast excitatory neurotransmitter [13] and is taken up by and stored possibly in all secretory and synaptic vesicles [14]. It is known to be stored and co-released with γ-aminobutyric acid, glutamate, noradrenaline and dopamine within the CNS [14].

ATP and its metabolites act upon two classes of receptors that are expressed throughout the CNS. ADO, a breakdown product of ATP, acts upon P1 membrane receptors that are further sub-classified into A₁, A_2A, A_2B and A₃ receptors. ATP acts upon P2 receptors, which are further divided into ionotropic P2X and metabotropic P2Y receptors [11]. There are seven P2X ligand-gated ion channel receptor subtypes and eight P2Y G protein-coupled receptor subtypes [11]. ADO activates P1 receptors leading to activation of mitogen-activated protein kinases and can modulate neuronal and glial signalling [15]. P2X subunits are heterogeneously expressed throughout the CNS [14], and synaptic currents, mediated through activation of P2X receptors, are present in CNS regions, such as the cortex and the hippocampus [11, 14].

In addition to direct and paired release from neurons, there is increasing evidence that purines play a significant role in glial neurotransmission [gliotransmission] [16]. Immunocytochemical studies have demonstrated that vesicular ATP and glutamate are contained within astrocytes [16]. Release of ATP from astrocytes, for example from in vitro culture preparations [17] and acute slices [18], results in neuronal excitation through activation of P2X receptors (for review, see [19]). Activation of P2X7 receptors on these neurons can also cause an enhancement of AMPA receptor surface expression, and a resulting increase in miniature excitatory post-synaptic currents [16]. ADO, produced as a result of hydrolysis from ATP, can give rise to an inhibitory response through activation of A₁ receptors [17] (see Fig. 1). Interestingly, the synthesis and release of neurotrophins, cytokines and chemokines in glial cells is also controlled by purine receptors. All four ADO (P1) receptor subtypes and P2X7 and P2Y receptors have been shown to be expressed on glial cells [11, 19]. These receptors have varying affinities to their ligands, and it is possible that different receptor subtypes are selectively activated in a concentration-dependent manner. Varying concentrations of extracellular purines can therefore precisely regulate the release of different neurotrophins or chemokines from glial cells.

Fig. 1.

Schematic hypothesis of purinergic signalling in electroconvulsive therapy. During electroconvulsive therapy, ATP is released from astrocytes, neurons and vascular endothelial cells. The ATP acts upon P2X7, P2Y₁ receptors and also P1 receptors after the extracellular breakdown of ATP to ADO. Activation of P2X7 and ADO receptors on glial cells results in increased release of cytokines, chemokines and neurotrophins, which act upon neurons in regions involved in the control of mood and motivation. Stimulation of P2Y receptors may modulate the release of dopamine (DA), glutamate (Glut) and nitric oxide (NO) all implicated in control of mood

Purinergic signalling in mood and motivation

ATP

The stimulation of P2X and P2Y receptors has been implicated in sensitization, reward and motivation [20]. Stimulation of P2 receptors in rats has demonstrated extended periods of novelty-induced locomotion indicative of an anxiolytic effect [21]. Intracerebroventricular injections of a P2Y₁ receptor agonist into rats has demonstrated an anxiolytic effect, which is in close relationship with the ensuing formation and release of dopamine [21] and nitric oxide [22], known to modulate noradrenaline, serotonin, dopamine and glutamate, the major neurotransmitters involved in the neurobiology of major depression [23]. It has been shown that P2Y₁ receptor agonists induce an antidepressant effect in a rat model of depression, which was reversed by P2Y₁ receptor antagonists [24]. The authors showed further that P2Y₁ receptor knockout mice displayed decreased depression-like behaviour. P2 receptors of the mesolimbic-mesocortical system, probably of the P2Y₁ subtype, are involved in the release of transmitters such as dopamine and glutamate, which are responsible for the generation and pattern behaviour following motivation-related stimuli [25]. P2X7 receptor knockout mice exhibited an antidepressant-like profile [26]. Additionally, gene studies in patients with recurrent major depressive illness [27] and bipolar-effective disorder [28] have demonstrated a specific single nucleotide polymorphism involving the P2X7 receptor. Changes in the expression of the P2X7 receptor are not unique to disorders of mood; there is a large body of evidence to suggest increased expression in microglia and other cell types in multiple CNS disorders [29–31]. Depression is considered to be associated with central inflammation [32] and P2X7 receptor occupation leads to release of inflammatory cytokines [33]. The role of the P2X7 receptor in depression and its involvement with central inflammation requires further study.

Adenosine

Adenosingeric activity has been implicated in mania, aggression and panic disorder [34, 35]. ADO is also thought to interact with other potent mood regulators: the psychotomimetic phencyclidine and alcohol [10]. ADO and dopamine receptors share an extensive co-localisation within the forebrain regions implicated in mood and motivational processes, and there is considerable evidence that A_2A receptor activation is able to influence dopaminergic function [36] and hence mediate goal-directed behaviour [37]. There is attenuation of psychostimulant-induced behavioural responses in mice lacking A_2A receptors [38]. A_2A and dopamine receptor knockout mice exhibit decreased preference and consumption of ethanol and saccharin [37], more so than those of D1 knockout mice. In A₁ receptor knockout mice, there is an increased state of anxiety [39]. Indeed, selective stimulation of the A₁ receptor in rats impairs the acquisition of fear conditioning [40]. ADO has been demonstrated to act as an antidepressant when either given by intraperitoneal or intracerebroventricular routes in mouse models of depression [41], acting via A₁ and A_2A receptors, and high doses of caffeine, a non-selective ADO antagonist, can produce anxiety, irritability and agitation [42]. There is conflicting evidence to suggest that both A_2A receptor agonists [41] and antagonists [43] have an antidepressant effect in mouse models of depression; this conflict may reflect that where A_2A receptors are claimed to have an antidepressant effect, this is believed to occur only following an interaction between A₁ and A_2A receptors [41]. Indirect evidence for the role of ADO in major depression is also suggested by the finding that serum activity of adenosine deaminase (a T-cell-associated enzyme) was decreased in patients with major depression, with an inverse relationship between the enzyme activity and the severity of the depression [44]. This supports a role for ADO having a depressant effect, since less enzyme results in greater ADO concentrations. These results also suggest that decreased enzyme activities might reflect that depressed patients may have a greater tendency to immune dysfunction.

Putative role of ECT on purinergic signalling in mood

We hypothesize that high levels of ATP are released following ECT, from microglia [19], neurons [45] and vascular endothelial cells [11], resulting in improvement in depressive symptoms through direct stimulation of pre- and post-synaptic purinoceptors that modulate neuronal and glial signalling (see Fig. 1). Pronounced increases in brain ADO levels in mice following electroconvulsive shock in rats has been shown [46], which would occur following breakdown of released ATP. In cases where ECT does not alleviate depression, this may reflect different aetiologies of the depression and also the placement of the electrodes and the amount of ATP released following the seizure. Sequential ECT may herald changes in pre- and post-synaptic purinergic receptor profiles, and it is these changes that are fundamental to the long-term benefits of ECT in major depressive illness. For instance, P2Y₁ receptors inhibit long-term depression of rat prefrontal cortex neurons [47], and synaptic plasticity may underlie the long-term benefits of ECT in depression. An important aspect of the physiological effect of ECT is the induction of a seizure-like state, and it is recognized that seizures give rise to increased levels of extracellular ATP [2, 11, 48, 49]. We suggest that this released pool of ATP following ECT is derived, in part, from astrocytes through connexin hemichannels [50] and directly from neurons throughout the brain. ATP is also released from astrocytes via connexin or pannexin hemichannels, ABC transporters, maxi ion channels or by vesicular release [19, 51, 52]. There is also evidence that ATP may be released from vascular endothelial cells in addition to neurons and glial cells [53]. Once released into the extracellular space, ATP and its breakdown product ADO have a multitude of targets (see Fig. 1), which can improve mood and motivation. Extracellular ATP is co-released in conjunction with dopamine [54, 55] within the mesolimbic-mesocortical system, and ATP may reduce anxiety and depression through stimulation of P2X7 receptors, the gene for which has been associated with major depressive disorder [27]. ECT is associated with an increase of ADO (after breakdown of ATP) and up-regulation of ADO A₁ receptors in the brain [56]. Activation of pre- and post-synaptic A₁ receptors by ADO can cause inhibition of NMDA receptor activation [57], which in turn has been shown to exert an antidepressant action in both pre-clinical and clinical studies[58]. At the simpler level, increased concentrations of ADO as a result of ATP hydrolysis may relieve depression [44]. Released ATP may also stimulate glial cells to release neurotransmitters, neurotrophins, cytokines and chemokines [59–61] (see Fig. 1), which can give rise to long term changes in neuronal circuitry, such as synaptic plasticity (long-term potentiation/long-term depression) and up-regulation/down-regulation of receptors [62, 63] (i.e. more long-term effects of ECT). The use of ecto-nucleotidase inhibitors (e.g. ARL 67156 (Sigma)) to increase extracellular ATP may be a potential novel therapeutic strategy to supplement ECTs long-term effects [64, 65]. P2X7 receptor agonists, such as the potent agonist 2′,3′-O-(benzoyl-4-benzoyl)-ATP, may also offer therapeutic potential.

Future developments

The effect of ATP release following ECT is multifaceted; neuronal, glial and neuronal-glial signalling is implicated. The proposed hypothesis for this mechanism has the advantage that the tools are available so that every step can be tested both in vivo and in vitro. Very sensitive ATP assay techniques are now available, as well as some selective purinoceptor subtype agonists and antagonists. Increase of extracellular ATP during ECT by the use of ecto-nucleotidase inhibitors as well as purinoceptor agonists may be worth consideration to enhance the anti-depressive effects of ECT.