Further changes in neurochemistry in pregnant/parous females (e.g., changes in neuropeptides, opioids, neurotransmitters, etc.; Bridges, Felicio, Pellerin, Stuer, & Mann 1993 ; Keverne & Kendrick, 1990 ; Kinsley & Bridges, 1988 ), may render neurons from a pregnant or lactating female substantially different in form and function than those taken from NULL females.
Source: wiktionary