Inherent hyporesponsiveness to methylxanthine-induced behavioral changes associated with supersensitivity to 5'-N-ethylcarboxamidoadenosine (NECA).

Two inbred mouse strains, SWR and CBA, differed significantly in their susceptibility to acute dose dependent theophylline- and caffeine-induced stimulation of locomotor activity. The efficacy of both methylxanthines was reduced in the SWR strain compared to the CBA strain. When brain levels of theophylline were determined at a dose (32 mg/kg IP) which gave maximal behavioral separation of the two strains, no significant differences were found between them (SWR levels 12.5 +/- 1.9, CBA levels 14.3 +/- 1.7 micrograms/g wet weight brain). The dose dependent ability of several adenosine agonists (N6-cyclohexyladenosine, (-)-N6-phenylisopropyladenosine, 5'-N-ethylcarboxamidoadenosine) to depress locomotor activity was investigated. SWR mice were found to be significantly more sensitive to NECA-induced depression of locomotor activity and the NECA-induced hypothermia than were CBA mice (respective ED50 values for inhibition of activity, 11.6 and 30.5 nmoles/kg IP). No differences were found in brain [3H]-NECA levels at doses which produced marked differences in behavioral effects between the two strains. The differences in adenosine agonist sensitivity between the strains were both agonist- and behavior-specific. These data indicate that an inherited alteration in behavioral responsiveness to methylxanthine administration can be inversely associated with inherent alterations in susceptibility to the action of specific adenosine analogs. An adenosine A-2 receptor sub-class may be involved in these changes in in vivo pharmacological susceptibility to the action of both methylxanthines and adenosine agonists on locomotor activity.