The NR2B-Selective N-Methyl-D-aspartate Receptor Antagonist Ro 25-6981 [(±)-(R*,S*)-α-(4-Hydroxyphenyl)-β-methyl-4-(phenylmethyl)-1-piperidine Propanol] Potentiates the Effect of Nicotine on Locomotor Activity and Dopamine Release in the Nucleus Accumbens