A possible role for the striatum in the pathogenesis of the cognitive symptoms of schizophrenia

Abstract

The cognitive symptoms of schizophrenia are largely resistant to current treatment and are thus a life-long burden of the illness. Studies of cognitive symptoms have commonly focused on prefrontal cortex because of its demonstrated importance for executive function and working memory--key components of the deficit. The role of striatal-cortical circuitry and therefore the striatum itself has received much less attention. Here we review longstanding evidence that the striatum and its cortical connections are critical for complex cognition and discuss emerging evidence of the striatum's potential involvement in cognitive symptoms. Finally, we suggest how mouse models might test ideas about the contribution of early striatal dysfunction to the cognitive symptoms of schizophrenia.

Figure 1. Tasks Used to Measure Prefrontal-Cortex-Dependent Cognitive Executive Function in Humans

Figure 2. The Dopamine Hypothesis of Schizophrenia

(A) Organization of the nigrostriatal and mesocortical midbrain dopaminergic projections. The dopaminergic midbrain neurons topographically project to the striatum but with an inverse dorsal-to-ventral organization. The mesocortical projections arise from the dorsal and medial dopamine cells. (B) The original dopamine hypothesis of schizophrenia. The original dopamine hypothesis proposed that a global hyperactivity of the dopaminergic projections in the brain may lead to the symptoms of schizophrenia. (C) The revised dopamine hypothesis of schizophrenia. The revised dopamine hypothesis proposed that a hyperactive nigrostriatal dopaminergic projection leads to positive symptoms but a hypoactive mesocortical projection is responsible for cognitive and negative symptoms. (D) Results of brain imaging studies in schizophrenia. Brain imaging studies show that an increase in amphetamine-induced dopamine release in the striatum is highest in the associative striatum. Moreover, the density and occupancy of striatal D2 receptors is increased in drug-free or drug-naive patients. PFC, prefrontal cortex; dSTR, dorsal striatum; vSTR, ventral striatum; VTA, ventral tegmental area; SNc, substantia nigra pars compacta; SNr, substantia nigra pars reticulata; D2R, dopamine D2 receptor.

Figure 3. Three Potential Cellular Mechanisms by which Increased Striatal Dopamine Transmission via D2 Receptors Could Affect Striatal Function in Patients with Schizophrenia

(A) Heterosynaptic inhibition of corticostriatal terminals. It has been shown that activation of D2 receptors at the presynaptic terminal of the incoming cortical projections inhibits corticostriatal glutamate release. Heterosynaptic inhibition is selective for the least active terminals, thereby filtering out less active inputs (Bamford et al., 2004). Because PET imaging does not have the resolution to determine which exact cells contribute to the elevated binding potential of D2 receptors in the striatum, D2 receptors could be upregulated in the corticostriatal terminals. This in combination with increased dopamine release may lead to a hypereffective heterosynaptic inhibition that may prevent the transmission of important cognitive information from the cortex to the striatum. (B) Inhibition of medium spiny neurons of the direct pathway. Both D1 and D2 receptors are present on medium spiny neurons, which represent 95% of striatal neurons and are the main output neurons of the striatum. D1 receptors are preferentially present on medium spiny neurons that project directly to the substantia nigra pars reticularis and the internal segment of the globus pallidus. D1 receptor activation enhances excitability of these cells. In contrast, D2 receptors are preferentially expressed on medium spiny neurons that project indirectly to the substantia nigra pars reticularis and the internal segment of the globus pallidus. D2 receptor activation reduces excitability of these cells (Surmeier et al., 2007). Upregulation of D2 receptors may therefore affect the balance between direct and indirect pathways and alter the striatal output of the incoming cortical information. (C) Global inhibition of medium spiny neurons. Activation of D2 receptors on the cholinergic interneurons diminishes acetylcholine release from these cells. The released acetylcholine affects glutamatergic and dopaminergic terminals. Acetylcholine generally enhances the glutamatergic excitation of medium spiny neurons (Surmeier et al., 2007). Increased activation of D2 receptors on cholinergic interneurons may therefore globally reduce the excitability of the striatum. Although the mechanism for the reduced metabolic activity observed in patients with schizophrenia is unknown, reduced cholinergic activity could be a contributing factor. MSN, medium spiny neuron; ChAT, choline-acetyl-transferase as a marker for cholinergic interneurons; GLU, glutamate; DA, dopamine.

Figure 4. Mechanisms on the Circuit Level by which Increased Striatal Dopamine Transmission via D2 Receptors Could Lead to Cognitive Deficits

(A) Striatal disruption. The striatum and cortex are anatomically and functionally linked via several parallel frontostriatal loops. Neuronal information encoding working memory and executive function may be altered by increased D2 receptor signaling at the level of the medium spiny neuron in the associative striatum, which is one synapse down-stream of the dorsolateral prefrontal cortex. (B) Striatal modulation of prefrontal function via the dopaminergic system. The striatum harbors the main input to the dopaminergic neurons of the ventral tegmental area (VTA) and substantia nigra (SN). By modulating the activity of neurons in the VTA, it may modulate the dopaminergic input of the prefrontal cortex. In mice with upregulated D2 receptors in the striatum, prefrontal dopamine turnover is decreased and D1 receptor activation increased. Since optimal D1 receptor activation in the prefrontal cortex is crucial for working memory, this could be a mechanism by which striatal D2 receptor overactivation may affect prefrontal function. (C) Striatal modulation of cortical function via the thalamus. The striatum projects back to the prefrontal cortex via the thalamus and may thereby modulate activity in the prefrontal cortex (for simplicity only the direct pathway via the substantia nigra reticularis is drawn). These mechanisms on the circuit level could already arise during development. In mice, selective upregulation of D2 receptors in the striatum during development leads to prefrontal-dependent cognitive deficits that cannot be reversed by switching off the additional receptors in the adult animal. This suggests that early upregulation of D2 receptors during development leads to compensatory changes potentially in the discussed circuits that cannot be easily reversed in the adult animal.