Caffeine and the Regulation of Locomotor Activity / Summary

In addition to modulating dopaminergic processes, caffeine shows neuroprotective efficacy in animal models of PD. In mice with neurotoxin-induced damage, caffeine acts in a dose-dependent manner to reduce the loss of endogenous dopamine and the dopamine transporter in mouse striatum. Striatal A_2AAR has been proposed to promote glutamate release and glia-mediated inflammation, which contribute to the pathogenesis of PD. Studies showing a significant reduction in risk of PD in populations with regular caffeine intake support a neuroprotective effect of caffeine against the development or exacerbation of PD. In an animal model, the neuroprotective effects of caffeine were mimicked by A_2AAR but not A₁AR antagonists, suggesting that A_2AAR blockade predominates in the antiPD effect of caffeine.

Caffeine and the Regulation of Locomotor Activity / Caffeine and PD

The influence of caffeine on locomotion is dose-dependent. High doses of caffeine (100 mg/kg) are either ineffective or reduce locomotor activity in both rats and mice; this effect has been attributed to rapid development of tolerance. A₁AR blockade promotes D₁R-mediated enhancement of locomotion in rats, and this effect is attenuated by prior chronic treatment with caffeine but not with a selective A_2AAR antagonist. Furthermore, long-term administration of a specific A_2AAR antagonist did not induce tolerance to caffeine. These findings led to the proposal that tolerance to caffeine is associated with chronic occupancy of A₁AR and that A₁AR activity is reduced and A_2AAR function is retained during the generation of caffeine tolerance.

Caffeine and the Regulation of Locomotor Activity / part 2

Other mechanisms also contribute to the general behavioral stimulation induced by caffeine. Studies with adenosine receptor KO mice indicate that A_2AAR, but not A₁AR, are involved in the sleep-promoting effect of adenosine. Caffeine administration reduces the hypnotic effects of alcohol in mice by means of A_2AAR blockade,and caffeine-induced psychomotor stimulation is deficient in A_2AAR KO mice, supporting the contention that caffeine-induced psychomotor stimulation occurs through A_2AAR antagonism. Finally, by blocking A₁AR, caffeine attenuates adenosine-mediated inhibition of mesopontine cholinergic neurons in the brainstem, thereby increasing neuronal firing rate, stimulating prefrontal cortex, and promoting arousal.

Caffeine and the Regulation of Locomotor Activity / part 1

The methylxanthine drug caffeine is a well-known psychostimulant chemical that promotes behaviors such as vigilance, attention, arousal, and locomotor activity. Caffeine is a competitive inhibitor of cyclic nucleotide phosphodiesterase (an enzyme that catalyzes cAMP degradation). However, caffeine's major mechanism of action in brain is nonselective blockade of adenosine re-ceptors.³⁰ Caffeine blocks postsynaptic A₁AR and A_2AAR, causing attenuation of adenosinergic neurotransmission and relative amplification of dopaminergic neurotransmission. These effects enhance both D₁R-mediated inhibition (via the direct pathway) and D₂R-mediated attenuation in stimulation (via the indirect pathway) on GPi-SNr-mediated inhibition of thalamus, thereby increasing thalamic stimulation of cortex and promoting locomotion.

Adenosine-dopamine Receptor Interactions in the Regulation of Locomotor Activity

Psychomotor activation is regulated in striatum via both the direct and indirect pathways. A₁AR-D₁R and A_2AAR-D₂R colo-calize as heterodimers on the MSN of the direct and indirect pathways, respectively. Molecular interactions within the complex allow one receptor of the pair to readily affect its counterpart. DR-mediated neurotransmission ultimately promotes behavioral activation, as reflected by increased voluntary locomotor activity. Stimulation of A₁AR and A_2AAR respectively counteracts D₁R- and D₂R-mediated neurotransmission, tending to reduce locomotor activity, and antagonism of either of these 2 AR promotes locomotor activity.

Strategies for Evaluating Adenosine and Dopamine Interactions in Brain

Pharmacologic manipulation and genetic deletion of adenosine and dopamine receptors frequently are used in rodents to investigate the specific physiologic functions and interactions of adenosine and dopamine receptor subtypes. Agents that bind to but do not stimulate receptors (that is, antagonists) prevent or reduce the binding of endogenous stimulatory ligands, thereby allowing behavioral studies under the condition of temporary deficiency of receptor-mediated physiologic activity. The duration of antagonist occupancy of the receptor depends on the combined effects of the drug concentration and its receptor binding affinity as compared with those of endogenous ligands. Receptor function resumes after the antagonist dissociates, thereby allowing binding of the endogenous ligand. Based on their binding affinities for specific receptors, antagonists can be categorized as selective or nonselective. For example, caffeine is a nonselective adenosine receptor antagonist, with similar binding affinities for both AjAR and A_2AAR. In contrast, as a result of their higher affinities, the chemicals 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and SCH58261 are selective antagonists for A₁AR and A_2AAR, respectively.

Adenosine and Dopamine: Dopamine / part 2

Therefore, somatodendritic and nerve terminal autoreceptors act synergistically to reduce dopaminergic transmission, and both are more sensitive to agonists than are postsynaptic dopamine receptors. Stimulation of D₂ autoreceptors reduces AC activity, calcium influx, and neuronal excitability and promotes membrane hyperpolarization and potassium efflux.Functions of autoreceptors include inhibition of dopaminergic neuronal firing, inhibition of dopamine synthesis, promotion of dopamine uptake by DAT located on neuronal membranes and cytosolic vesicles, and enhanced DAT expression.

Adenosine and Dopamine: Dopamine

Dopamine. Dopamine is both a neurotransmitter and a precursor of the neurotransmitters epinephrine and norepinephrine. Dopamine is synthesized from the amino acid tyrosine, which is transported across the blood-brain barrier and taken up by dopaminergic neurons. In the first and rate-limiting step in dopamine synthesis, the enzyme tyrosine hydroxylase (TH) converts tyrosine to 3,4-dihydroxy-L-phenylalanine (l-DOPA). l-DOPA, in turn, is converted into dopamine by L-aromatic amino acid decarboxylase. Because the levels of tyrosine in the brain normally exceed the processing capacity of TH, increasing the availability of tyrosine in the brain does not significantly increase dopamine synthesis. Instead, the amount and activity of TH determine the rate of dopamine biosynthesis.

Adenosine and Dopamine / part 2

Adenosine receptor subtypes present in the brain include A₁, A_2A, A_2B, and A₃. The binding affinities of A_2BAR and A₃AR are in the micromolar range, making it unlikely that they play a major role in neurotransmission under physiologic conditions. In contrast, the ligand-binding affinities of A₁AR and A_2AAR are both in the physiologically relevant nanomolar range. As a result, A₁AR and A_2AAR are viewed as the predominant targets of caffeine-induced AR antagonism in brain. A₁AR and A_2AAR are distributed differently throughout the brain. A₁AR are highly expressed in hippocampus, cortex, and cerebellum, with relatively low expression in the basal ganglia, whereas A_2AAR are expressed primarily in striatum, nucleus accumbens, and olfactory tubercle.

Adenosine and Dopamine: Synthesis, Regulation, and Receptors

Adenosine. Adenosine is an endogenous purine nucleoside that is generated extracellularly from adenine nucleotides like ATP and intracellularly from 5'-AMP or S-adenosyl-homocysteine. Adenosine can be converted into inosine by adenosine deaminase or back into 5'-AMP by adenosine kinase (Figure 3). Adenosine kinase is important in regulation of the basal intracellular levels of adenosine, whereas adenosine deaminase lowers the increased adenosine that is generated during neuronal excitation. The normal concentration of adenosine in brain varies widely based on metabolic state, ranging between 25 and 250 nM in rat. Physiologic extracellular concentrations of adenosine in the brain are sufficient to generate sustained tonic activation of A₁AR and A_2AAR.

Locomotor-regulatory Circuits in Brain / part 2

About 95% of striatal neurons are GABAergic medium spiny neurons (MSN). The striatum receives glutamatergic neuronal projections from cortex and dopaminergic projections from SNc. Dopamine receptors can be either excitatory (D₁R) or inhibitory (D₂R) and thus differentially modulate neurotransmission in response to endogenous dopamine. Presynaptic glutamate and dopamine receptors modulate neurotransmitter release, whereas postsynaptic receptors on soma and dendrites regulate neuronal excitability.

Locomotor-regulatory Circuits in Brain / part 1

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.

Abbreviations / part 2

Figure 1. Locomotor-regulatory circuits in rodent brain.

Adenosine is produced both intra- and extracellularly from many types of cells and neurons. Four adenosine receptor (AR) subtypes are expressed in brain (A₁, A_2A, A_2B, and A₃). However, only A₁AR and A_2AAR are physiologically important in striatal regulation of behavior, largely because of their relative abundance and affinity for binding agonists. Presynaptically, A₁AR and A_2AAR colocalize on glutamatergic nerve terminals, where they jointly modulate glutamate release. Postsynaptically, dimers of A₁AR with D₁R are present on neurons of direct pathways, whereas A_2AAR with D₂R are on neurons of indirect pathways.

Adenosine and Dopamine Receptor Interactions in Striatum and Caffeine-induced Behavioral Activation

Abbreviations: 6-HDA, 6-hydroxydopamine; AjAR, type 1 adenosinergic receptor; A_2AAR, type 2A adenosinergic receptor; AC, adenylyl cyclase; AMPA, a-amino-3-hydroxy-5-methylisoxazole-4-propionic acid; aMPT, a methylparatyrosine; AR, adenosine receptor; CNS, central nervous system; CREB, cAMP response element binding protein; D₁R, type 1 dopaminergic receptor; D₂R, type 2 dopaminergic receptor; DAT, dopamine transporter; DPCPX, 8-cyclopentyl-1,3-dipropylxanthine; ERK, extracellular signal-related kinase; GABA, y-aminobutryic acid; GPe, globus pallidus externa; G protein, guanine nucleotide binding protein; GPi, globus pallidus interna; IEG, immediate early gene; KO, knock-out; l-DOPA, 3,4-dihydroxy-L-phenylalanine; mGluR, metabotropic glutamate receptor; MPTP, 1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine; MSN, medium spiny neurons; NMDA, N-methyl-D-aspartic acid; PD, Parkinson disease; PKA, protein kinase A; SNc, substantia nigra pars compacta; SNr, substantia nigra pars reticulata; TH, tyrosine hydroxylase; VTA, ventral tegmental area