The importance of timing for the action of alpha 1- or beta-agonists: possible relationship with the density of [3H]prazosin and [3H]dihydroalprenolol binding sites.

Exploratory behaviour was measured in the open-field at 8.00 h and 20.00 h with naive rats and rats treated with an alpha 1-agonist (phenylephrine, 25 micrograms i.c.v.) or beta-agonists (salbutamol, 2.5 mg/kg; clenbuterol 0.05 mg/kg i.p.). The spontaneous activity was similar at both times, yet the response to a alpha 1-as well as to beta-stimulation was significantly enhanced at 20.00 h. The density of cortical [3H]prazosin and [3H]dihydroalprenolol ([3H]DHA) binding sites at 20.00 h was enhanced by 19% and 28% respectively, in comparison with the density measured at 8.00 h.