It is a well established fact that caffeine is dopaminergic in effect even though it does not bind directly to any dopamine "receptors". It looks like aspirin is also dopaminergic. As you can see from the study, using aspirin alone required doses equivalent do 5g - 6g in a human. However, in a combination with caffeine in a human dose of about 50mg - 75mg much lower doses of aspirin were effective and synergized with caffeine. An aspirin dose of as low as a human equivalent of 500mg combined with the low dose caffeine displayed additive dopaminergic effects for as long as 6 hours.
http://www.ncbi.nlm.nih.gov/pubmed/41063
"...Both aspirin at a high dose (400 mg kg-1) and caffeine (5 mg kg-1) induced hyperactivity in the DA rat, but lower doses of aspirin were without effect. Caffeine-induced hyperactivity was brief (2 h) but that due to aspirin was evident from 1--6 h after dosing. Co-administration of the two drugs caused long-lasting hyperactivity, even with doses of aspirin which had no stimulant effects themselves. Absorptive and metabolic effects did not appear to play a major role in the interaction. The most likely effect is that of salicylate on catecholamine utilization in the central nervous system, which is compounded in the presence of a phosphodiesterase inhibitor."
Here is some info on the dopaminergic effects of caffeine by itself:
http://www.ncbi.nlm.nih.gov/pubmed/9364486
http://www.ncbi.nlm.nih.gov/pubmed/25871974
http://www.ncbi.nlm.nih.gov/pubmed/41063
"...Both aspirin at a high dose (400 mg kg-1) and caffeine (5 mg kg-1) induced hyperactivity in the DA rat, but lower doses of aspirin were without effect. Caffeine-induced hyperactivity was brief (2 h) but that due to aspirin was evident from 1--6 h after dosing. Co-administration of the two drugs caused long-lasting hyperactivity, even with doses of aspirin which had no stimulant effects themselves. Absorptive and metabolic effects did not appear to play a major role in the interaction. The most likely effect is that of salicylate on catecholamine utilization in the central nervous system, which is compounded in the presence of a phosphodiesterase inhibitor."
Here is some info on the dopaminergic effects of caffeine by itself:
http://www.ncbi.nlm.nih.gov/pubmed/9364486
http://www.ncbi.nlm.nih.gov/pubmed/25871974