Evidence for a projection from the perireticular thalamic nucleus to the dorsal thalamus in the adult rat and ferret

Abstract

During early development, the perireticular thalamic nucleus is very large (i.e. has many cells) and has a strong projection to the dorsal thalamus and to the cerebral neocortex. By adulthood, the nucleus has much reduced in size and only a few cells remain. It is not clear whether these perireticular cells that remain into adulthood maintain their connections with the dorsal thalamus and with the neocortex. This study examines this issue by injecting neuronal tracers into various nuclei of the dorsal thalamus (dorsal lateral geniculate nucleus, medial geniculate complex, ventroposteromedial nucleus, lateral posterior nucleus, posterior thalamic nucleus) and into different areas of the neocortex (somatosensory, visual, auditory). After injections of tracer into the individual nuclei of the rat and ferret dorsal thalamus, retrogradely-labelled perireticular cells are seen. In general, after each injection, the retrogradely-labelled perireticular cells lie immediately adjacent to a group of retrogradely-labelled reticular cells. For instance, after injections into the medial geniculate complex, perireticular cells adjacent to the auditory reticular sector are retrogradely-labelled, whilst after an injection into the dorsal lateral geniculate nucleus, retrogradely-labelled perireticular cells adjacent to the visual reticular sector are seen. By contrast, injections of tracer into various areas of the rat and ferret neocortex result in no retrogradely-labelled cells in the perireticular nucleus. Thus, unlike during perinatal development when perireticular cells project to both neocortex and dorsal thalamus, perireticular cells in the adult seem to project to the dorsal thalamus only: the perireticular projection to the neocortex appears to be entirely transient.

Organization in the auditory sector of the cat's thalamic reticular nucleus

“…An auditory sector in the cat's PRN is defined here by the location of its cells that project to the MGv, MGdr, MGd, or MGm. Like in the rat (Mitrofanis et al, 1995), this sector is confined to a small caudoventral region of the PRN that lies opposite the auditory sector of the TRN. In the cat, the efferent connections of the PRN with the MGv and the MGdr are sparse, whereas those with the MGd and the MGm are robust.…”

“…In the cat, the efferent connections of the PRN with the MGv and the MGdr are sparse, whereas those with the MGd and the MGm are robust. The PRN also contains sectors that are related to visual and somatosensory nuclei of the dorsal thalamus (Mitrofanis et al, 1995;Crabtree, 1996).…”

Immunocytochemical and ultrastructural study of the rat perireticular thalamic nucleus during postnatal development

“…Both PV and GABA were demonstrated in the PRT of adult cats and ferrets (Clemence and Mitrofanis, 1992), whereas we here provide evidence that in rats, PV-ir neurons do not contain GABA. As in rats, the neurons of the RT contain both PV and GABA throughout the postnatal development (De Biasi et al, 1986Celio, 1990;Frassoni et al, 1991), our results indicate that the PV-ir neurons of the PRT represent a distinct population from those of the RT, as suggested also from their different projections (Mitrofanis and Baker, 1993;Adams and Baker, 1995;Mitrofanis et al, 1995) and from our ultrastructural investigation (see below).…”

“…In particular, PRT neurons projecting to the cortex were localized mainly in the dorsal part of the IC (Adams and Baker, 1995), where we never found PV-ir neurons, and those projecting to the adult rat thalamus were detected only in the caudalmost (auditory) region of the IC (Mitrofanis et al, 1995), where we found only rare PV-ir neurons. Finally, Earle and Mitrofanis (1996) described, by means of quantitative methods on Nissl-stained sections, an almost complete loss of PRT cells by P15.…”

“…As for the functional significance of this group of neurons, neuroanatomical studies have reported the existence of connections of the rat PRT with the dorsal thalamus and the cerebral cortex in early postnatal ages (Mitrofanis and Baker, 1993;Adams and Baker, 1995;Mitrofanis et al, 1995;Earle and Mitrofanis, 1996). The existence of these connections, together with the strategic position of the PRT in the IC and its large size in early developmental stages, have led to the suggestion that the PRT might play a role in the Grant sponsor: Ministero dell'Università e della Ricerca Scientifica e Tecnologica (MURST 40%).…”