Nigral innervation of cholinergic and glutamatergic cells in the rat mesopontine tegmentum: Light and electron microscopic anterograde tracing and immunohistochemical studies

Abstract

The substantia nigra (SN) has long been known as an important source of afferents to the pedunculopontine tegmental nucleus (PPN). However, it has not been established which of the chemospecific cell populations receive this synaptic input. We sought to address this issue by a correlative light and electron microscopic approach that combines anterograde tracing of nigral efferents with pre-embedding choline acetyltransferase (ChAT) and/or glutamate (Glu) immunohistochemistry. Following large bilateral injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) in the SN, the labeled nigrotegmental fibers were concentrated in a small area of the mesopontine tegmentum which contained very few ChAT-immunoreactive (ChAT-ir) cell bodies. However, strands of fine varicose fibers penetrated to adjacent regions of the PPN which harbored numerous cholinergic perikarya. The anterogradely labeled boutons were often seen in the proximity of ChAT-ir perikarya and dendrites, but the majority (82-93%) established symmetric synaptic junctions with noncholinergic profiles. In the pars dissipata of the PPN (PPNd), one-third of the labeled terminals synapsed onto noncholinergic perikarya and primary dendrites, while in the pars compacta of the PPN (PPNc) axosomatic synapses were rare. The possibility that the perikarya receiving a rich synaptic input from the SN are glutamatergic was tested in experiments combining anterograde transport of biotinylated tracers biocytin and dextran-amine (BDA) with glutamate immunohistochemistry. In double-labeled sections, Glu-ir perikarya within the terminal plexus of nigrotegmental fibers were surrounded by synaptic terminals. The PPNd also contained retrogradely BDA-labeled neurons which were contacted by anterogradely labeled terminals. These results indicate that although a small subpopulation of cholinergic neurons in the mesopontine tegmentum receive direct synaptic input from the SN, the primary target of nigrotegmental fibers are glutamatergic cells in the PPNd. Our results also provide ultrastructural evidence that some nigrotegmental fibers innervate pedunculonigral neurons.

Morphological study of the tegmental pedunculopontine nucleus, substantia nigra and subthalamic nucleus, and their interconnections in rat organotypic culture

“…Previous in vivo projection studies in the rat, utilizing a combined anterograde tracing technique with ChAT immunohistochemistry, have shown that fibers originating from the SNr do not terminate in the areas with a cluster of cholinergic neurons, (e.g., PPN) but project heavily to the adjacent MEA, which was mostly void of cholinergic neurons (Rye et al 1987). Grofova and Zhou (1998) have also reported a similar finding, which indicated that nigrotegmental fibers terminate in a small area of the MPT that is almost void of ChAT-ir neurons (i.e., MEA). Their ultrastructural study has demonstrated that a majority of boutons from the nigrotegmental fibers established symmetrical synaptic contacts with non-cholinergic profiles.…”

“…DA cells in the SNc project mainly to the striatum (Faull and Mehler 1978;Vandermaelen et al 1978;Beckstead et al 1979;Bentivoglio et al 1979; Van der Kooy 1979) and with fewer to the STN (Canteras et al 1990;Hassani et al 1997) and the PPN (Grofova and Zhou 1998). The SNc receives cholinergic and glutamatergic inputs from the PPN/MEA, STN and the cerebral cortex (Clarke et al 1987;Beninato and Spencer 1988;Gould et al 1989;Naito and Kita 1994;Futami et al 1995;Takakusaki et al 1996) and GABAergic inputs from the striatum and globus pallidus .…”

Pedunculopontine tegmental nucleus neurons provide reward, sensorimotor, and alerting signals to midbrain dopamine neurons

“…The PPTg is known to receive inputs from many brain areas (Matsumura, 2000; Mena-Segovia et al, 2004), but dominant inputs originate from basal ganglia nuclei, particularly the substantia nigra pars reticulata (SNr) (Beckstead and Frankfurter, 1982) (Noda and Oka, 1986; Takakusaki et al, 2003) and the globus pallidus internal segment (GPi) (Shink et al, 1997; Rolland et al, 2011), where neurons encode sensorimotor signals strongly modulated by expected rewards (Hikosaka et al, 2006; Joshua et al, 2009). SNr and GPi neurons have GABAergic inhibitory connections to PPTg neurons (Noda and Oka, 1986; Scarnati et al, 1987; Granata and Kitai, 1991), particularly non-cholinergic (Shink et al, 1997) or glutamatergic (Grofova and Zhou, 1998) neurons, although it is unknown whether the SNr/GPi-recipient PPTg neurons project to SNc-DA neurons. In addition, some DA neurons in the SNc or VTA innervate the PPTg (Rolland et al, 2009).…”

How vital is sleep in Huntington’s disease?

“…This includes the striatum as, although it is not considered to be a key part of the sleep-wake process (see above), studies involving striatal lesions have nevertheless suggested a possible involvement with sleep [23,100,173]. This effect may be mediated through the outflow of the basal ganglia to critical thalamic nuclei and the PPT [56,77]. Significant volume reductions in the brainstem, a key structure involved in the regulation of sleep, have been reported in HD patients [140], with some evidence of such reductions worsening with disease severity and increasing UHDRS motor score [72].…”