We have shown that blockade of central glial glutamate uptake selectively activates a forebrain circuit involved in the regulation of mood and induces a phenotype with some features of depression. By implication these findings suggest a relationship between a lack of glial cells observed in humans with depression and clinical dysregulation of mood. Here, a lack of astrocytic glial glutamate uptake results in diminished reward value, impaired spatial memory, and activation of brain regions that are associated with motivation. Disruption of glial glutamate uptake is one component of a complex circuit that may mediate anhedonia and cognitive impairment. This finding may provide insight not only into the etiology of depression, but also into the mechanisms underlying the symptoms of other neuropsychiatric illnesses that share these symptoms.
Intracranial DHK increased the ICSS thresholds (T0), the minimum frequency that would sustain responding, suggesting that blocking astrocytic glutamate uptake attenuates brain stimulation reward. This effect is consistent with the induction of an anhedonic-like or dysphoric-like state; two constructs from animal models that represent prominent symptoms of depression (ie, diminished pleasure and depressed or negative mood, respectively). These findings are in agreement with recent work suggesting that glial cells may be involved in the induction or mediation of depressive-like states. For instance, prefrontal cortical glial ablation reportedly decreases sucrose intake, increases response to novelty, and increases immobility in the forced swimming test, effects consistent with a depressant-like effect (Banasr and Duman, 2008
). Further, blockade of amygdalar astrocytic glutamate uptake has been reported to decrease social exploration and disrupt circadian rhythms, effects that are also consistent with depression-like symptoms (Lee et al, 2007
DHK had an effect on the maximal response rate although this was not statistically significant. This effect could be indicative of a performance impairment (Carlezon and Chartoff, 2007
), as it has been previously noted that manipulations that interfere with lever pressing, such as muscle relaxation and increasing lever weight, results in lower asymptotic response rates (Miliaressis et al, 1986
). It has also been shown that reductions in stimulation intensity or increases in response requirements (eg, changing from an FR1 to an FR10 schedule of reinforcement) can decrease the maximum response rates (Do Carmo et al, 2009
), suggesting a relationship between motivation and performance. Examination of frequency–rate curves (eg, ) shows that both rightward and downward shifts were characteristic of DHK's effect. Together these data indicated that there was a decrease in reward efficacy that could have been accompanied by motor sedation. Consequently, we examined the effects of DHK on locomotor activity, where no signs of sedation were observed. Collectively these data suggest that the threshold-increasing and maximal rate-reducing effects of DHK were not an artifact of performance impairment, but rather the result of reductions in the reward efficacy of the brain stimulation.
Because ICSS provides a relative index of reward (eg, changes in response to rewarding brain stimulation) it is difficult to distinguish between manipulations that induce anhedonia and those that induce dysphoria, as both produce decreases in reward efficacy. In humans, too, although anhedonia and dysphoria may appear together, they represent distinct symptoms that can be observed to varying degrees in different episodes of clinical depression or related illnesses. Place conditioning can be helpful in clarifying the presence of each state because dysphoric effects can be detected as avoidance, without the need for a relative comparison to a reward state. In the present experiments, DHK treatment yielded no significant change in the aversion scores, suggesting that DHK does not provoke discomfort and avoidance of the DHK-paired chamber (ie, no conditioned place aversion). Interpreting these data in combination with the ICSS results make it likely that the effects observed in ICSS were the result of the induction of anhedonia without dysphoria. It should be noted that a lack of an observable effect does not prove that no effect exists, so it is possible that under different conditions DHK would produce dysphoria.
DHK treatment yielded a unique pattern of activation in discrete brain areas that are associated with depression and reward (Hercher et al, 2009
; Nestler and Carlezon, 2006
; Phillips et al, 2003
), without inducing activation in areas that have not been explicitly associated with depression and/or reward. More specifically, c-Fos expression was increased in the AcbC, AcbSh, BL, Ce, IL, DRV, dDG, and LC, but not in the dSTr and VP. In addition to reductions in the volume of some of these mood/reward-related regions (for review see Phillips et al, 2003
), specific cellular changes have also been reported in depressed patients (for review see Hercher et al, 2009
). A reduction in glial cells or associated markers has been noted in the prefrontal cortex (Cotter et al, 2001
; Öngür et al, 1998
; Rajkowska et al, 1999
), hippocampus (Müller et al, 2001
), and amygdala (Bowley et al, 2002
; Hamidi et al, 2004
). By contrast, reductions in neurons or their markers have been noted in the the prefrontal cortex (Rajkowska et al, 1999
), dorsal raphe nucleus (Baumann et al, 2002
), and locus coeruleus (Arango et al, 1996
). To be clear, the raphe nucleus and the locus coeruleus have not yet been examined for glial abnormalities. These regions are part of a large interconnected circuit with projections from the prefrontal cortex to the ventromedial striatum (Öngür and Price, 2000
) and raphe nucleus, and projections from the locus coeruleus to the raphe nucleus (Marcinkiewicz et al, 1989
), as well as reciprocal connections between the medial prefrontal cortex, amygdala, and the enorhinal cortex (Öngür and Price, 2000
). Because these regions are so highly connected, it is not possible at this time to distinguish between c-Fos expression resulting from the direct effects of blocking glial glutamate uptake in a given locus and expression that is an indirect consequence of activation through projections. Nonetheless, these data begin to characterize how glial deficits can produce depressive-like effects and serve to highlight important candidate regions for further study.
We observed robust activation of the dentate gyrus of the hippocampus after DHK treatment that was accompanied by deficits in hippocampus-dependent spatial navigation. Although little is known about effects in mammals, astrocyte trafficking of glutamate has been heavily implicated in learning and memory consolidation in chicks (for review see Gibbs et al, 2008
) and the hippocampus has been extensively studied with respect to depression. In patients with unremitted depression, hippocampal volumes are decreased (Sheline et al, 2003
) and numerous cognitive deficits have been documented (Clark et al, 2009
). Impaired spatial navigation, which requires the hippocampus, is among these cognitive deficits (Gould et al, 2007
). Likewise, animal models of depression show major alterations in hippocampal plasticity and structure (Duman, 2004
; Kim et al, 2006
) that can be accompanied by impaired spatial navigation (Song et al, 2006
; Sun and Alkon, 2004
). The present results are in agreement with these many findings and extend them to suggest that similar mechanisms involving astrocyte dysfunction may underlie both cognitive and reward-related symptoms in depression.
One might suspect that DHK-induced cognitive impairment could underlie the decreased responding in the ICSS paradigm and a lack of conditioned place aversion in the place conditioning paradigm. Some evidence suggests otherwise. There is a large published literature showing that many drugs that are well known to impair spatial learning and memory (eg, MK-801; Whishaw and Auer, 1989
) actually facilitate, rather than impair, ICSS responding (Corbett, 1989
). Furthermore, the unique timing of the effects in this paradigm diminishes the likelihood that learning or memory impairment decreased the responding for rewarding brain stimulation. The frequencies were presented in a descending order for 50
s each. Here, rats responded normally for the highest frequencies, which were presented first, and anhedonia was shown by a lack of responding at lower frequencies. As such, the change in responding for a given frequency occurred during the 5-s timeout and the 0.5-s non-contingent stimulus train that came between trials. This normal initial responding coupled with the rapid transition from responding to not responding suggests that the rats were able to recall the task. For place conditioning, there is evidence in the literature that rats seek novelty (eg, reference Hughes, 1968
). If DHK disrupted memory, we would expect the DHK-paired chamber to be novel relative to the vehicle-paired chamber. By extension, we would expect the rats to prefer the DHK-paired environment, for its novelty, relative to the vehicle-paired chamber. The observation that the rats spent equal amounts of time in the DHK- and saline-paired chambers suggests that each was equally familiar, and supports the hypothesis that DHK neither produced dysphoria nor disrupted place conditioning.
Here we suggest an important role for astrocytes in the etiology of anhedonia and cognitive dysfunction in depression. However, these symptoms are not unique to depression as they are shared by a number of neuropsychiatric disorders, including, but not limited to; bipolar disorder, schizophrenia, and Parkinson's disease (Andreasen and Olsen, 1982
; Burdick et al, 2007
; Cassidy et al, 2000
; Gorwood, 2008
; Ibarretxe-Bilbao et al, 2009
; Isella et al, 2003
; Kraus and Keefe, 2007
). Of particular interest is bipolar disorder, for which an unspecified reduction in glial cell density and number has also been reported (Öngür et al, 1998
; Rajkowska, 2002
; Rajkowska et al, 2001
). Abnormalities in specific glial cell subtypes have been strongly implicated in schizophrenia and Parkinson's disease, as well, although lack of astrocytes has not been suggested in these disorders (Bernstein et al, 2009
; Mena and Garcia de Yebenes, 2008
). Nevertheless, there is a suggested relationship between anhedonia, cognitive dysfunction, and glial cells in a number of neuropsychiatric disorders. Thus, our findings may have implications beyond depression. Indeed, a current active discussion in psychiatry is underway as to whether psychiatric disorders are best conceived as separate illnesses with separate pathophysiologies or as illnesses along several dimensions of disorder, such as mood or cognitive disorder, with overlapping underlying pathologies in particular neurotransmitter (eg, glutamate) or cellular (eg, glial) systems shared to varying degrees among the illnesses.
DHK treatment provided a model to mimic some aspects of what might occur clinically as a consequence of a paucity or dysfunction of glial cells. This treatment appeared to induce anhedonia, absent dysphoria, and cognitive impairment suggesting that a lack of astrocytic glutamate uptake could have a causal role in producing specific symptoms of depression. Furthermore, DHK induced the activation of a limited number of brain areas that are thought to be key in the regulation of mood. These findings broaden our understanding of the underlying pathophysiology of depression, highlight possible novel sites of action for the development of new treatments, and may be important for other neuropsychiatric disorders that, like depression, include anhedonia and cognitive impairment as symptoms.