The δ-opioid receptor agonist SNC80 [(+)-4-[α(R)-α-[(2S, 5R)4-allyl-2,5-dimethyl-1-piperazinyl]-(3-methoxybenzyl)-N, N-diethylbenzamide] synergistically enhances the locomotor-activating effects of some psychomotor stimulants, but not direct dopamine agonists, in rats