Dopamine autoreceptor roles in striatal function and behavior Dopaminergic transmission in striatum is involved in action selection, movement initiation, reward, reinforcement and responses to drugs of abuse. Dopamine (DA) produces its physiological actions through the activation of 5 receptor subtypes. Within the striatum, two subtypes, D1 and D2, predominate. The D2 receptor has a variety of physiological roles within the striatum. This receptor is also the major target for antipsychotic drugs, and levels of D2 expression have been related to liability for substance abuse disorders. However, D2 receptors are present on a variety of neuronal subtypes within the striatum. Receptors are present postsynaptically on striatal medium spiny neurons and interneurons. Presynaptic D2 receptors are present on glutamatergic afferents coming from the neocortex and dopaminergic afferents from the Substantia Nigra pars compacta. These latter receptors, known as autoreceptors, are intriguing in that they provide feedback inhibitory control of DA release in striatum, often opposing the effects of the postsynaptic D2 receptors. Sorting out the actions of the D2 receptors on different striatal cellular elements has been difficult in past studies due to the lack of suitably specific pharmacological reagents that can separately target the different receptors. We have worked with Dr. Marcelo Rubinstein, who developed mice in which the D2 receptor was selectively eliminated from midbrain dopaminergic neurons using a Cre recombinase action on a loxP-flanked D2 receptor construct (termed autoDrd2KO mice). We performed voltammetric studies to examine DA release in the dorsal striatum of these mice. Increased DA release was observed in striatum in the autoDrd2KO mice, that appeared to be linked to increased activity of the enzyme tyrosine hydroxylase (the rate-limiting enzyme for DA synthesis). This finding supports the idea that D2 autoreceptors inhibit DA synthesis through actions on the enzyme. The normal inhibitory effect of D2 activation on DA release was also lost in the autoDrd2KO mice, providing additional support for the loss of autoreceptor feedback inhibition. When short trains of stimulation were given to activate DA release over a period of several seconds, there is normally an initial surge of DA release followed by a rapid decrease in neurotransmitter levels, reaching a sustained steady state level. Application of a D2 receptor antagonist allows for sustained high levels of DA release, and this effect was mimicked in the autoDrd2KO mouse striatum. Furthermore, D2 antagonist treatment had no effect on the sustained high DA levels produced by repeated afferent stimulation in the autoDrd2KO mouse striatum. These findings indicate that an important autoreceptor role is to curtail DA release during short phasic bursts of afferent activity, preventing excessive DA release. It appears that the D2 receptor is the predominant, if not exclusive, DA autoreceptor, at least in mouse striatum. The autoDrd2KO mice exhibited hyper-locomotion in a novel environment, with sustained higher levels of activity even when mice were familiarized with the environment. The mice were hyper-sensitive to treatment with D2 agonist, most likely due to enhanced effect of postsynaptic receptor actions. The autoDrd2KO mice were also more sensitive to movement inhibition produced by the D2 antagonist haloperidol, suggesting a role for autoreceptors in limiting effects of antipsychotic D2-targeted drugs. The autoDrd2KO mice were hypersensitive to locomotor activation by cocaine, and showed increased place preference for cocaine. In addition, these mice showed greater persistence in operant tasks motivated by food reward. These findings indicate that D2 autoreceptors play key roles in limiting hyperactivity and response to antipsychotic drugs. In addition, the evidence from this study indicates that these autoreceptors regulate the effects of natural rewards and drugs of abuse, perhaps limiting abuse liability. In ongoing studies we are examining the effects of drugs used to treat attention deficit hyperactivity disorder (ADHD) in the hyperdopaminergic/hyperactive autoDrd2KO mice. These mice may serve as a useful model for aspects of ADHD, allowing us to determine how drugs such as methylphenidate (Ritalin) alter striatal function to alleviate ADHD symptoms. In future studies we can use the loxP-flanked D2 mice developed by Dr. Rubinstein to examine physiological and behavioral roles of D2 receptors on other cellular elements within the striatum. Dissecting the roles in instrumental learning of CB1 receptors on different striatal afferents Instrumental learning allows animals to acquire new behaviors to adapt to environmental demands. Research in behavioral psychology has identified two separable processes that take place during instrumental learning. One process, termed action-outcome (A-O, also known as goal-directed) learning, involves behavior that is sensitive to the proximal outcome of the action. The learning process known as stimulus-response (S-R, or habitual) learning is expressed as behavior that is driven by the context, independent of the proximal outcome. Previous studies from our laboratory group indicated that CB1 endocannabinoid receptors play a crucial role in S-R learning. We have now examined roles of CB1 receptors present on different cellular elements within striatum in A-O and S-R instrumental learning. To do this we have employed mice expressing loxP-flanked CB1 receptor genes, and interbred them with mice expressing the cre recombinase in different cortical and striatal neuronal subtypes. Mice are then trained on an instrumental lever-pressing task in two separate environments, using reward schedules that foster A-O learning (random ratio training) or S-R learning (random interval training). Using a tamoxifen-inducible Cre mouse, we have been able to knock out CB1Rs in the neocortex. Knockout animals learned the lever pressing task with performance equivalent to wildtype mice. However, regardless of training schedule, the cortical CB1R knockout mice showed evidence of A-O and not S-R learning. This finding suggests that receptors expressed on cortical projection neurons play a crucial role in S-R habitual learning. The orbitofrontal cortex is one subcortical region that likely plays a role in arbitrating between A-O and S-R control of behavior. Using local injections of tamoxifen, we can explore the role of OFC CB1 receptors in these forms of learning. Current experiments are aimed at determining the participation of CB1 receptors at synapses in striatum, including corticostriatal glutamatergic and intrastriatal GABAergic synapses, in instrumental learning. We are also recording the activity of single neurons in the OFC as well as dorsomedial and dorsolateral striatum in mice as they learn and perform the instrumental bar-pressing task under the random ratio and random interval schedules. The use of chronically implanted multielectrode arrays allows us to examine multiple neurons in the same brain region simultaneously. The unique experimental design in which animals perform in separate contexts with separate schedules on the same day, allows us to examine the same neuron in the two different training situations. Findings in wildtype mice indicate that neurons in these brain regions show different patterns of lever-press related firing in the different training contexts, as well as differential responses when the outcome retains its value or is devalued by pre-feeding. Experiments are ongoing to determine how loss of CB1 receptors alters neuronal activity in relation to task performance.
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