Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Secondly, this effect was shown to be selectively due to CCK2 activation (Ladurelle et al. 1997) through mechanisms involving δ-opioid (Daugé et al. 1999) and D1-dopamine receptors (Ladurelle et al. 1993) that could be blocked by the selective antagonists naltrindole (a δ antagonist), and SCH23,390.
Category: