“…For example, subthreshold doses of enkephalin and NE have synergistic effects that, when combined, produce analgesia (Meert & Dekock, 1994; Yaksh & Reddy, 1981). Numerous studies with rodent models of acute and chronic pain have suggested that adrenal chromaffin cells implanted in the intrathecal space and in the periaqueductal gray reliably produce very large, long-lasting analgesic effects (Czech & Sagen, 1995; Hama & Sagen, 1993; Hama & Sagen, 1994; Hama, Unnerstall, Siegan, & Sagen, 1995; Ortegaalvaro et al, 1994; Pappas & Sagen, 1988b; Sagen, 1992; Sagen, 1997; Sagen, Kemmler, & Wang, 1991; Sagen, Ortega, & Pappas, 1994; Sagen & Pappas, 1987, 1988a ; Sagen, Pappas, & Perlow, 1986; Sagen, Pappas, & Perlow, 1987a; Sagen, Pappas, & Pollard, 1986; Sagen & Wang, 1990; Sagen, Wang, & Pappas, 1990; Sagen, Wang, Tresco, & Aebischer, 1993; Siegan & Sagen, 1995, 1997; Wang & Sagen, 1994, 1995). Xenogeneic adrenal chromaffin cells encapsulated in immunoisolatory polymer membranes have also been reported to produce substantial analgesic effects (Decosterd et al, 1998; Sagen et al, 1993).…”