Involvement of different mesotocin (oxytocin homologue) populations in sexual and aggressive behaviours of the brown anole

Abstract

The oxytocin (OT) family of neuropeptides are known to modulate social behaviours and anxiety in mammals and birds. We investigated cell numbers and neural activity, assessed as Fos induction, within magnocellular and parvocellular populations of neurons producing the OT homologue mesotocin (MT, Ile(8)-oxytocin). This was conducted within the male brown anole lizard, Anolis sagrei, following agonistic or courtship encounters with a conspecific. Both neurons colocalizing and not colocalizing corticotropin-releasing factor (CRF) were examined. Parvocellular neurons of the paraventricular nucleus exhibited a positive correlation between courtship frequency and Fos colocalization, regardless of whether they produce just MT or MT + CRF. Magnocellular populations showed only trends towards positive relationships with courtship and no cell populations showed aggression-related Fos induction. These findings are novel because they demonstrate the involvement of MT neurons in male social behaviour, especially in reptiles for whom the involvement of MT in social behaviour was previously unknown.

Keywords: aggression; anole lizard; corticotropin-releasing factor; courtship; mesotocin; oxytocin.

Figure 1.

Photomicrographs of immunofluorescent colocalization of MT (green), CRF (purple), Fos (red) and DAPI (a general nuclear stain, blue) in 50 μm coronal brain sections of the male brown anole brain. Present are whole brain sections (a1–a3), with insets of the hypothalamus (b1–b3). Further insets (c1–c3) demonstrate (at top) neurons with all four colours displayed, and then separate displays of MT, CRF and Fos, respectively. Three rostro-caudal points are represented: (a1–c1) SON, (a2–c2) rostral PVN and (a3–c3) caudal PVN. Yellow diagonal arrows indicate example magnocellular MT + CRF neurons. White horizontal arrows indicate magnocellular MT-only neurons. Red diagonal arrows indicate the smaller and less immunoreactive parvocellular MT + CRF neurons. Blue diagonal arrows indicate parvocellular MT-only neurons.

Figure 2.

Correlations between social behaviours and neural measures. The frequency of sexual behaviours during a courtship trial correlates positively with Fos colocalization within both (a) MT-CRF neurons and (b) MT-only neurons in the PVN. See text for statistical results.