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:The µ-opioid receptor agonist morphine also induced inhibitory effects in both assays, in accordance with previous reports describing the opiate-mediated inhibition of the BSL evoked in mice by excitatory amino acids and neurokinins (Aanonsen et al. 1987; Takahasi et al. 1987; Wilcox 1988).
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