Brainstem Reticular Nuclei that Project to the Thalamus in Rats: A Retrograde Tracer Study; pp. 1–22

Abstract

The precise nuclear origins of projections from the brainstem reticular formation to the thalamus were identified in rats using two retrograde tracing substances: wheat germ agglutinin-horseradish peroxidase conjugate, and Fluoro-Gold. Injections of these tracers were made into a variety of thalamic nuclei, including the intralaminar nuclei (most of these also involved the lateral part of the mediodorsal nucleus), the central part of the mediodorsal nucleus, the ventrolateral/ ventromedial nuclei, and the ventral posterolateral/ventral posteromedial nuclei. Counts of retrogradely labeled cells were done on a large sample of select cases. The data generated by these cell counts indicate that brainstem reticular projections to the intralaminar/lateral mediodorsal complex are fairly strong, as are those to the ventrolateral/ventromedial nuclear complex. Ascending reticular projections to the mediodorsal nucleus per se are somewhat weaker, while those to the ventrobasal complex (or at least the ventral posterolateral nucleus) are weaker still. As a whole, reticular neurons projecting to the thalamus are by far most numerous in the midbrain, and then decline gradually at successively caudal levels through the pons and medulla. Midbrain reticular groups evincing very strong ascending projections include nucleus reticularis (n.r.) pedunculopontinus (particularly its pars compactus), n.r. cuneiformis and n.r. subcuneiformis (together known as the deep mesencephalic nucleus). Strong thalamic projections arise from the medial part of n.r. pontis oralis, the medial (beta) part of n.r. pontis caudalis, and the mid-pontine dorsomedial tegmental area. Within the medullary reticular formation, a ‘trans-nuclear field’ of neurons encompassing n.r. paragigantocellularis dorsalis and dorsal parts of n.r. gigantocellularis and n.r. parvocellularis was consistently labeled contralateral to the injection site. In general, ascending reticulothalamic projections are largely ipsilateral from midbrain reticular groups, bilateral from pontine reticular groups, and contralateral from medullary reticular groups. Within individual reticular nuclei, the morphology of labeled neurons is identical to that reported previously by this laboratory subsequent to spinal, cortical, or cerebellar tracer injections, thus strengthening our hypothesis that the various brainstem reticular nuclei can be distinguished on the basis of neuronal morphology. As a whole, thalamic-projecting reticular neurons are mostly small or medium-sized cells.

Nitric oxide synthase-containing projections to the ventrobasal thalamus in the rat

“…However, double-labeled neurons were very few. Outside the PPN/LDT complex, reticular projections to the ventrobasal complex were modest, as indicated also in previous studies (Vertes et al 1986;Vertes and Martin 1988;Newman and Ginsberg 1994;and their references). A relatively strong, bilateral thalamopetal projection arises in the deep mesencephalic reticular nucleus, but occasional double-labeled neurons were seen only in the largest injections, such as in case R1663.…”

Cerebellum and Cerebellar Connections

“…The linear nucleus (Fig. 26A,B) was included in the lateral reticular nucleus on the basis of cytological, enzyme histochemical, and anterograde and retrograde axonal tracing studies (Newman and Ginsberg, 1994;Ruigrok, 2004;Fu et al, 2009). The main ventrocaudal portion of the lateral reticular nucleus is located within the ascending spinal fibers of the anterolateral funiculus.…”

Vestibular responses in the macaque pedunculopontine nucleus and central mesencephalic reticular formation

“…Direct projections to the PPN from the vestibular nuclei (Woolf and Butcher, 1989, Hazrati and Parent, 1992, Horowitz et al, 2005), deep cerebellar nuclei (Hazrati and Parent, 1992, Teune et al, 2000), premotor and motor cortices (Matsumura et al, 2000, Aravamuthan et al, 2007), and frontal eye fields (Stanton et al, 1988b, Matsumura et al, 2000) have been shown in primates or rodents. The PPN also projects to multiple thalamic nuclei including the ventral posterior thalamus (Smith et al, 1988, Steriade et al, 1988, Lavoie and Parent, 1994b, Newman and Ginsberg, 1994, Erro et al, 1999) and is reciprocally connected with multiple basal ganglia nuclei (Parent and Smith, 1987, Lavoie and Parent, 1994b, a, Charara et al, 1996). Descending projections include primarily non-cholinergic efferents to the cervical and thoracic spinal cord (Goldsmith and van der Kooy, 1988, Rye et al, 1988, Spann and Grofova, 1989, Skinner et al, 1990a, Aravamuthan et al, 2007) and primarily cholinergic efferents to the pontine and medullary reticular formations (Garcia-Rill and Skinner, 1987, Skinner et al, 1990b, Takakusaki et al, 1996, Whelan, 1996, Jordan, 1998, Garcia-Rill et al, 2001) including projections to a large portion of medullary reticulospinal neurons (Garcia-Rill and Skinner, 1987).…”

“…Like the PPN, the central mesencephalic reticular formation (cMRF, variably known as the deep mesencephalic nucleus and the midbrain reticular formation) receives inhibitory input from the substantia nigra pars reticulata and internal segment of the globus pallidus (Veazey and Severin, 1982, Hay-Schmidt and Mikkelsen, 1992, Rodriguez et al, 2001), sends GABAergic projections to the ventral thalamus (Bowsher, 1975, Veazey and Severin, 1980a, Newman and Ginsberg, 1994, Rodriguez et al, 2001), and sends descending projections to the cervical spinal cord (Veazey and Severin, 1980b, Warren et al, 2008). In addition, the cMRF directly projects to the PPN (Rodriguez et al, 2001).…”