Rodents show a variety of both genetic and behavioral features that are similar to that of humans, and are used widely to study CNS responses under both physiologic and pathologic conditions and to test responses to drugs. Behavioral studies with rodents, compared with those in people, allow scientists to control of many factors that could affect study results (for example, diet, housing conditions, ambient temperature). Like that in other mammals, the rodent striatum is characterized by a high degree of axonal collateralization that allows striatal neurons to deliver similar efferent outputs to a number of target brain loci. Functionally, the striatum of rodents and other mammals responds similarly to neurotransmitters and exogenous agents, such as neurotropic drugs. These similarities provide the fundamental rationale for using rodents to study the effect of receptor interactions on striatal neurotransmission.
The dorsal striatum, thalamus, and (ultimately) cortex are linked by means of the globus pallidus (GP; Figure 1). In primates, the GP is divided into internal (GPi) and external (GPe) segments, whereas in nonprimates, the GP and entopeduncular nucleus are considered homologous to the primate GPe and GPi, respec-tively. Two major locomotor-regulatory circuits in the brain involve these structures and are known as the direct and indirect pathways (Figure 2). In the direct pathway, striatal projections directly inhibit GPi and substantia nigra pars reticulata (SNr), whereas the indirect pathway projects sequentially through the GPe and subthalamic nucleus to influence SNr. Neurons arising in GPe and GPi are y-aminobutryic acid (GABA)-ergic and inhibit downstream neuronal groups (Figure 1). Eventually the indirect pathway excites, whereas the direct pathway inhibits, the common targets SNr and GPi. GABAergic projections from Gpi and SNr in turn inhibit thalamic neurons that send excitatory glutamatergic projections to the premotor cortex. Therefore, the direct and indirect pathways have opposing effects on cortical stimulation (Figure 2), and the 2 pathways jointly regulate locomotion by modulating the activity of the Gpi and SNr. see more about asthma inhalers
The dorsal striatum, thalamus, and (ultimately) cortex are linked by means of the globus pallidus (GP; Figure 1). In primates, the GP is divided into internal (GPi) and external (GPe) segments, whereas in nonprimates, the GP and entopeduncular nucleus are considered homologous to the primate GPe and GPi, respec-tively. Two major locomotor-regulatory circuits in the brain involve these structures and are known as the direct and indirect pathways (Figure 2). In the direct pathway, striatal projections directly inhibit GPi and substantia nigra pars reticulata (SNr), whereas the indirect pathway projects sequentially through the GPe and subthalamic nucleus to influence SNr. Neurons arising in GPe and GPi are y-aminobutryic acid (GABA)-ergic and inhibit downstream neuronal groups (Figure 1). Eventually the indirect pathway excites, whereas the direct pathway inhibits, the common targets SNr and GPi. GABAergic projections from Gpi and SNr in turn inhibit thalamic neurons that send excitatory glutamatergic projections to the premotor cortex. Therefore, the direct and indirect pathways have opposing effects on cortical stimulation (Figure 2), and the 2 pathways jointly regulate locomotion by modulating the activity of the Gpi and SNr. see more about asthma inhalers
Figure 2. Direct and indirect striatal locomotor-regulatory pathways.
The 2 pathways are themselves regulated predominantly by reciprocal inhibitory interactions between A1AR and D1R on neurons of the direct pathway and between A2AAR and D2R on neurons of the indirect pathway.
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