Research ReportThe supragenual nucleus: A putative relay station for ascending vestibular signs to head direction cells
Introduction
Head direction (HD) cells discharge as a function of the animal's head direction in the horizontal plane and are independent of the rat's location in the environment as well as ongoing behavior. Evidence suggests that these cells participate in navigation and spatial cognition (Muller et al., 1996). The circuitry responsible for the generation and maintenance of the HD signal has been extensively studied, and HD cells have been identified in the postsubiculum (Taube et al., 1990), retrosplenial cortex (Chen et al., 1994, Cho and Sharp, 2001), lateral dorsal thalamic nucleus (Mizumori and Williams, 1993), anterior dorsal thalamic nucleus (Taube, 1995), and lateral mammillary nucleus (Blair et al., 1998, Stackman and Taube, 1998).
Vestibular information plays an essential role in the generation of HD signal. Recent lesion studies (Stackman and Taube, 1997, Stackman et al., 2002) indicated that inactivation of labyrinthine inputs abolished the direction-related activity of HD cells. Unfortunately, knowledge of the putative paths conveying vestibular information to the circuitry responsible for the generation and maintenance of the HD signal is still very fragmentary.
In this circuitry, the lateral mammillary nucleus plays a critical role to generate HD cell activity in upstream thalamic targets, such as the anterior dorsal thalamic nucleus (Blair et al., 1998). Vestibular information to the lateral mammillary nucleus is thought to be conveyed by the dorsal tegmental nucleus (Bassett and Taube, 2001, Sharp et al., 2001), which is classically known to provide a dense projection to this mammillary region (Guillery, 1957, Groenewegen and Van Dijk, 1984, Liu et al., 1984, Hayakawa and Zyo, 1990). Importantly, the dorsal tegmental nucleus contains head angular velocity cells likely to be involved in the generation of the HD signal (Bassett and Taube, 2001, Sharp et al., 2001).
In a recent study, by using viral transneuronal tracing method, the supragenual nucleus (SG) emerged as a putative relay of vestibular inputs to the lateral mammillary nucleus (Brown et al., 2005). The SG forms part of the periventricular gray surrounding the floor of the fourth ventricle and lies directly dorsal to the genu of the facial nerve. It is composed of round or ovoid medium-sized neurons, intensely stained with Nissl staining methods. In the present study, using anterograde and retrograde tract-tracing methods, we investigated the contribution of the SG in conveying vestibular inputs to the HD cell-related circuitry. Of particular relevance, the results from our study indicate the SG as an important relay station for vestibular information to the circuitry responsible for the generation and maintenance of the HD signal.
Section snippets
Fluorogold injections into the SG
In the present study, we first examined the pattern of retrograde labeling in the vestibular nuclei after Fluorogold injection in the SG. As illustrated in Fig. 1, in three experiments, the tracer deposit was centered in the SG with a faint tracer spreading over the adjacent caudal end of the nucleus incertus (indicated in Fig. 1 as a light gray area). In one case, a light tracer spreading also included the rostral tip of the nucleus prepositus (Fig. 1A). All these experiments yielded a similar
Discussion
The present findings support the idea that the SG works as an important relay station for vestibular inputs to the circuitry responsible for the generation and maintenance of the HD signal. First, using anterograde and retrograde tract-tracing methods, we were able to show that the medial vestibular nucleus provides a direct projection to the SG.
The results of the present retrograde axonal tract-tracing experiments indicate that vestibular inputs to the SG arise from the medial vestibular
Animals and housing
Adult male Wistar rats (n = 15), weighing about 250 g and obtained from the local breeding facilities, were used in the present study. The animals were kept under controlled temperature (23°C) and illumination (12-h cycle) in the animal quarters, and had free access to water and standard laboratory diet (Nutrilab CR1, Nuvital Nutrientes, Ribeirão Preto, SP, Brazil). Experiments were carried out in accordance with the National Institute of Health Guide for the Care and Use of Laboratory Animals
Acknowledgments
This research was supported by grants from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, no. 01/14039-9) and Conselho Nacional de Desenvolvimento Científico e Tecnologico (CNPq, no. 300360/2003-7) awarded to N.S.C.
References (28)
- et al.
Role of the lateral mammillary nucleus in the rat head direction circuit: a combined single unit recording and lesion study
Neuron
(1998) - et al.
Does the vestibular system contribute to head direction cell activity in the rat?
Physiol. Behav.
(2002) - et al.
Afferent and efferent connections of the medial, inferior and lateral vestibular nuclei in the cat and monkey
Brain Res.
(1983) - et al.
Efferent connections of the dorsal tegmental region in the rat, studied by means of anterograde transport of the lectin Phaseolus vulgaris-leucoagglutinin (PHA-L)
Brain Res.
(1984) - et al.
Neural connections of the pontine reticular formation, which connects reciprocally with the nucleus prepositus hypoglossi in the rat
Neuroscience
(1999) - et al.
The dorsal tegmental nucleus: an axoplasmic transport study
Brain Res.
(1984) - et al.
Efferent connections of the vestibular nuclei in the rat: a neuromorphological study using PHA-L
Brain Res. Bull.
(2002) - et al.
Head direction cells: properties and functional significance
Curr. Opin. Neurobiol.
(1996) - et al.
Neural correlates for angular head velocity in the rat dorsal tegmental nucleus
J. Neurosci.
(2001) - et al.
Polysynaptic pathways from vestibular nuclei to the lateral mammillary nucleus of the rat: substrates for vestibular input to head direction cells
Exp. Brain Res.
(2005)