Quantitative Study of the Antagonistic Effect of (-)-Cis-1-Methyl-7-[[4-(2, 6-Dichlorophenyl)Piperidin-1-Yl]Methyl]-6,7,8,9-Tetrahydro-5H-Benzocyclohepten- 5-Ol (Sb-612111)on Nociceptin/Orphanin Fq-Mediated Potassium Channel Activation in Rat Periaqueduct