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:BDNF has been shown to interact with several neurotrans-
mitter systems, including the DA (Spenger et al., 1995), serotonin (5-HT) (Lyons et al., 1999; Rumajogee et al., 2002), and NPY systems (Barnea and Roberts, 2001; Nawa et al., 1993, 1994).
Category: