OXYTOCIN-INDUCED EXCITATION OF NEURONES IN THE RAT CENTRAL AND MEDIAL AMYGDALOID NUCLEI M. G. TERENZI a AND C. D. INGRAM b * a Department of Physiological Sciences, Federal University of Santa Catarina, Florianopolis SC 88040-900, Brazil b School of Neurology, Neurobiology and Psychiatry, University of Newcastle, Medical School, Newcastle NE2 4HH, UK Abstract—Central oxytocin plays an important role in regu- lating emotionality. The amygdala expresses gonadal ste- roid-sensitive oxytocin binding sites in both the central and medial sub-nuclei, although the densities markedly differ be- tween these nuclei. These studies examined the in vitro elec- trophysiological effects of oxytocin in the two amygdaloid nuclei and compared responses in female rats in different reproductive states (virgin, pregnant and lactating). Oxytocin (10 9 –10 6 M) caused a concentration-dependent increase in the firing rate of 20 –36% of the neurones in both nuclei. Although autoradiographic studies using the oxytocin recep- tor antagonist [ 125 I]d(CH 2 ) 5 [Tyr(Me) 2 ,Thr 4 ,Orn 8 ,Tyr-NH 2 9 ]-va- sotocin showed a higher density of binding in the central nucleus of the amygdala than medial nucleus of the amyg- dala, neurones in the central nucleus of the amygdala had a much lower sensitivity to oxytocin: equivalent responses obtained with 10 6 M in the central nucleus of the amygdala and 10 8 M in the medial nucleus of the amygdala, and neu- rones in the central nucleus of the amygdala were insensitive to concentrations below 10 6 M. Furthermore, repeated appli- cations of oxytocin induced homologous desensitization in the central nucleus of the amygdala, but not medial nucleus of the amygdala—a single application of oxytocin producing long duration suppression of responses. This indicates that oxytocin has contrasting modes of action in the amygdala. Studies made across the reproductive cycle showed that lactating animals exhibited a larger proportion of oxytocin- responsive neurones in the medial nucleus of the amygdala and a smaller proportion in the central nucleus of the amyg- dala, compared with virgin or pregnant animals, indicating a peripartum shift in relative activation within the amygdala. However, changes in responses were not accompanied by changes in the density of oxytocin binding sites. These data show that oxytocin has a markedly different efficacy on neu- ronal activation in the central and medial sub-nuclei of the amygdala. The relative shift in excitatory responses between these two nuclei may underlie some of the neuroendocrine, behavioral and anxiolytic effects which have been ascribed to oxytocin in the periparturient rat. © 2005 Published by Elsevier Ltd on behalf of IBRO. Key words: limbic system, emotionality, peptide neurotrans- mission, lactation, pregnancy, electrophysiology. Oxytocin has widespread neurotransmitter functions in the CNS, acting through selective receptors (Barberis and Tri- bollet, 1996). These receptors are particularly abundant in regions of the limbic system (Insel, 1985, 1990; de Kloet et al., 1986; Freund-Mercier et al., 1987; Tribollet et al., 1990; Krémarik et al., 1991, 1993, 1995; Insel et al., 1992; Patchev et al., 1993; Veinante and Freund-Mercier, 1997) where oxytocin has been suggested to participate in the modulation of neuroendocrine reflexes (Richard et al., 1991; Ingram et al., 1995), sexual receptivity (Caldwell et al., 1986; Bale et al., 2001), and social and maternal behaviors (Pedersen and Prange, 1979; Insel, 1990; Ferris et al., 1992; Carter et al., 1992; Champagne et al., 2001; Ferguson et al., 2001; Lubin et al., 2003; Bosch et al., 2004). More recently the central actions of oxytocin have been strongly implicated in the control of anxiety and re- sponses to stress (Uvnäs-Moberg et al., 1994; McCarthy et al., 1996; Windle et al., 1997a, 2004; Neumann et al., 2000a,b; Bale et al., 2001; Neumann, 2002; Mantella et al., 2003; Amico et al., 2004). Some of these effects have been considered to be mediated through the amygdala complex, where oxytocin binding sites have been de- scribed in three sub-nuclei: the central (CeA) and medial (MeA) nuclei and, to a lesser extent, in the basomedial nucleus (Freund-Mercier et al., 1987; Krémarik et al., 1993; Condés-Lara et al., 1994; Veinante and Freund- Mercier, 1997). Although the presence of these binding sites suggests that oxytocin may have important modula- tory effects on the activity of amygdala neurones, to date only a single study has determined the electrophysiological effects of oxytocin in the amygdala (Condés-Lara et al., 1994). In that study recordings made from male rats in vivo showed that iontophoretic application of oxytocin could excite approximately 50% of neurones throughout the amygdala complex, although no clear distinction was drawn between responses in the different sub-nuclei. One characteristic of the oxytocin binding sites in the amygdala is that, in female rats, their density of expression varies both with the levels of gonadal steroids, particularly estrogens (de Kloet et al., 1986; Tribollet et al., 1990; Patchev et al., 1993; Krémarik et al., 1995), and with the reproductive status of the animal (Young et al., 1997). Furthermore, the CeA and MeA are extensions of the bed nuclei of the stria terminalis (BST), with which they have extensive reciprocal connections and which together have been considered as the ‘extended amygdala’ (Alheid et al., 1995). The density of oxytocin binding in the BST also *Corresponding author. Tel: +44-191-222-8210; fax: +44-191-222-5227. E-mail address: c.d.ingram@ncl.ac.uk (C. D. Ingram). Abbreviations: aCSF, artificial cerebrospinal fluid; ANOVA, analysis of variance; BST, bed nuclei of the stria terminalis; BSTm, medial nu- cleus of the bed nuclei of the stria terminalis; BSTo, oval nucleus of the bed nuclei of the stria terminalis; BSTp, principal nucleus of the bed nuclei of the stria terminalis; CeA, central nucleus of the amygdala; MeA, medial nucleus of the amygdala; NMDA, N-methyl-D-aspartate; OTA, d(CH 2 ) 5 [Tyr(Me) 2 ,Thr 4 ,Orn 8 ,Tyr-NH 2 9 ]-vasotocin; sp/s, spikes/s; VLS, ventrolateral septum; VMH, ventromedial hypothalamus. Neuroscience 134 (2005) 345–354 0306-4522/05$30.00+0.00 © 2005 Published by Elsevier Ltd on behalf of IBRO. doi:10.1016/j.neuroscience.2005.04.004 345