Chronic postnatal monoamine oxidase inhibition affects affiliative behavior in rat pupso Sofia Blazevic, Mirna Merkler, Dora Persic, Dubravka Hranilovic ⁎ University of Zagreb, Faculty of Science, Department of Biology, Rooseveltov trg 6, HR-10 000 Zagreb, Croatia abstract article info Article history: Received 26 September 2016 Received in revised form 12 December 2016 Accepted 14 December 2016 Available online 15 December 2016 Monoamine neurotransmitters serotonin (5-HT), dopamine (DA), and noradrenaline (NA) act as important mod- ulators of mammalian brain development and represent neurobiological substrates of affiliative behavior reflected in rat pups as a tendency to huddle or produce ultrasonic vocalizations (USV) when separated from the nest. Monoamines are metabolized through oxidative deamination catalyzed by the mitochondrial enzyme monoamine oxidase (MAO). In this study, we examined the consequences of postnatal MAO inhibition on affiliative behavior in rat pups. Pups received daily injections of either an irreversible non-selective MAO inhibitor tranylcypromine (TCP) or saline, from post-natal day (PND) 1 to PND 22. Quantitative and qualitative compo- nents of USV were analyzed on PNDs 10, 13 and 16 in order to determine the level of separation-induced anxiety and the modality of vocal communication. In comparison to control pups, TCP-treated pups displayed higher cor- tical 5-HT, DA and NA levels, higher peripheral 5-HT concentration, lower body mass throughout the pre- weaning period, higher isolation-induced drop in body temperature, and reduced total number of calls. Further- more, they produced lower pitched calls of longer average duration without a preferable waveform. Our results demonstrate that chronic MAO inhibition by TCP primarily affects 5-HT concentrations, but also raises central catecholamine levels. They further indicate that disturbed monoaminergic homeostasis during early postnatal development leads to decreased weight-gain, compromised thermoregulation, and altered affiliative behavior in pre-weaning pups as reflected in reduced separation anxiety and inadequate vocal communication. Finally, they suggest a need for thorough examination of the potential effects of TCP and other monoamine inhibitors on the developing human brain. © 2016 Elsevier Inc. All rights reserved. Keywords: Tranylcypromine MAO Serotonin Catecholamines Ultrasonic vocalization Separation anxiety 1. Introduction Monoamine neurotransmitters act as important neuromodulators of mammalian brain development, and alterations in serotonin (5-HT), dopamine (DA), and noradrenaline (NA) neurotransmission during pre- natal and early postnatal periods are considered to affect the develop- ment of essential cortical circuits required for functional neuronal signaling and the resulting behavioral outcomes (Benes et al., 2000; Thompson and Stanwood, 2009). Monoamines, together with neuropeptides and endogenous opioids, represent neurobiological substrates of affiliative behavior. NA is in- volved in early postnatal offspring and maternal social learning (Nelson and Panksepp, 1998), the DA reward system promotes maternal behavior (Stoesz et al., 2013), while 5-HT regulates neural net- works necessary for motivation, memory, sensory processing, and other integral processes of social interaction (Insel and Winslow, 1998). In mammalian infants, affiliation is reflected in tendency to grasp, huddle, or produce a high-pitched, species-specific alarm cry when separated from their family group. Affiliation of a pre-weaning rat with its dam and littermates is dom- inantly mediated by thermal and tactile sensory inputs, and removal of a pup from its social group elicits behavioral activation, including ultra- sonic vocalization (USV), which induces maternal search and retrieval behavior (Insel and Winslow, 1998; Nelson and Panksepp, 1998). Growing evidence has implicated the role of monoamines in the modu- lation of USV. Dopamine receptor agonists (Dastur et al., 1999) and re- uptake inhibitors (Kehoe and Boylan, 1992) are shown to significantly reduce USV. Noradrenaline has been reported to have the opposite ef- fect as both, the α 2 receptor agonists (Hård et al., 1988) and NA reup- take inhibitors (Winslow and Insel, 1990a) increase USV. 5-HT influence on USV seems to be more complex: 5-HT enhancers acting on the 5-HT transporter (Hodgson et al., 2008; Olivier et al., 1998; Winslow and Insel, 1990b) as well as 5-HT 1A and 5-HT 2 receptor Pharmacology, Biochemistry and Behavior 153 (2017) 60–68 Abbreviations: 5-HT, 5-hydroxytryptamine (serotonin); DA, dopamine; NA, noradrenaline; USV, ultrasonic vocalization; MAO, monoamine oxidase; TCP, tranylcypromine; PND, post-natal day; ADR, adrenaline; LFC, latency to first call; TNC, total number of calls; ACD, average call duration; FME, frequency of maximal energy. ⁎ Corresponding author at: Division of Animal Physiology, Department of Biology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, HR-10 000 Zagreb, Croatia. E-mail address: dubravka@biol.pmf.hr (D. Hranilovic). http://dx.doi.org/10.1016/j.pbb.2016.12.008 0091-3057/© 2016 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Pharmacology, Biochemistry and Behavior journal homepage: www.elsevier.com/locate/pharmbiochembeh