Cyto- and chemoarchitecture of the sensory trigeminal nuclei of the echidna, platypus and rat Ken W.S. Ashwell a, * , Craig D. Hardman a , George Paxinos b a Department of Anatomy, School of Medical Sciences, The University of New South Wales, New South Wales, Sydney 2052, Australia b Prince of Wales Medical Research Institute, The University of New South Wales, New South Wales, Sydney 2052, Australia Received 7 June 2005; received in revised form 9 August 2005; accepted 22 August 2005 Available online 29 September 2005 Abstract We have examined the cyto- and chemoarchitecture of the trigeminal nuclei of two monotremes using Nissl staining, enzyme reactivity for cytochrome oxidase, immunoreactivity for calcium binding proteins and non-phosphorylated neurofilament (SMI-32 antibody) and lectin histochemistry (Griffonia simplicifolia isolectin B4). The principal trigeminal nucleus and the oralis and interpolaris spinal trigeminal nuclei were substantially larger in the platypus than in either the echidna or rat, but the caudalis subnucleus was similar in size in both monotremes and the rat. The numerical density of Nissl stained neurons was higher in the principal, oralis and interpolaris nuclei of the platypus relative to the echidna, but similar to that in the rat. Neuropil immunoreactivity for parvalbumin was particularly intense in the principal trigeminal, oralis and interpolaris subnuclei of the platypus, but the numerical density of parvalbumin immunoreactive neurons was not particularly high in these nuclei of the platypus. Neuropil immunoreactivity for calbindin and calretinin was relatively weak in both monotremes, although calretinin immunoreactive somata made up a large proportion of neurons in the principal, oralis and interpolaris subnuclei of the echidna. Distribution of calretinin immunoreactivity and Griffonia simplicifolia B4 isolectin reactivity suggested that the caudalis subnucleus of the echidna does not have a clearly defined gelatinosus region. Our findings indicate that the trigeminal nuclei of the echidna do not appear to be highly specialized, but that the principal, oralis and interpolaris subnuclei of the platypus trigeminal complex are highly differentiated, presumably for processing of tactile and electrosensory information from the bill. # 2005 Elsevier B.V. All rights reserved. Keywords: Parvalbumin; Calbindin; Calretinin; Neurofilament protein; Echidna; Platypus 1. Introduction The monotremes are a remarkable group of mammals from Australia and New Guinea, which are represented in modern times by the echidnas of Australia and New Guinea (family Tachyglossidae) and the platypus of eastern Australia (family Ornithorhynchidae) (Griffiths, 1978; Musser, 2003). The length of time since the divergence of the prototherian lineage (leading to the modern monotremes) and the therian line (leading to modern marsupial and placental mammals) is not known with certainty, but is at least 125 million years (Griffiths, 1978; Musser, 2003). This lengthy period of separate evolution has provided opportunities for the emergence of unique neural solutions to behavioural problems in the two lineages. Both of the Australian monotremes (the short beaked echidna – Tachyglossus aculeatus and the platypus – Ornithorhynchus anatinus) have been reported to use electroreception for the detection of their arthropod prey. This electroreception is reported to make use of the trigeminal sensory pathways, with specialized receptors in the skin of the beak or bill (Iggo et al., 1985; Scheich et al., 1986; Gregory et al., 1987, 1988, 1989; Andres et al., 1991; Manger and Hughes, 1992; Manger and Pettigrew, 1996; Proske et al., 1998) and transmission of electrosensory information to the brainstem by all three divisions of the trigeminal nerve (Andres et al., 1991; Manger and Pettigrew, 1996). While considerable attention has been devoted to the physiology and fine structure of the peripheral electrosensory apparatus and several studies have examined the cortical organization with either electrophysiological or anatomical approaches (Lende, 1964; Bohringer and Rowe, 1977; Welker and Lende, 1980; Iggo et al., 1992; Krubitzer et al., 1995; Manger et al., 1996; Hassiotis et al., 2004, 2005), no recent studies have been made of the anatomy or physiology of the central trigeminal sensory pathways which presumably serve the analysis of this electrosensory information. The trigeminal www.elsevier.com/locate/jchemneu Journal of Chemical Neuroanatomy 31 (2006) 81–107 * Corresponding author. Tel.: +61 2 9385 2482; fax: +61 2 9385 8016. E-mail address: k.ashwell@unsw.edu.au (Ken W.S. Ashwell). 0891-0618/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jchemneu.2005.08.007