Pedunculopontine tegmental nucleus of the rat: Cytoarchitecture, cytochemistry, and some extrapyramidal connections of the mesopontine tegmentum

Abstract

The pedunculopontine tegmental nucleus (PPTn) was originally defined on cytoarchitectonic grounds in humans. We have employed cytoarchitectonic, cytochemical, and connectional criteria to define a homologous cell group in the rat. A detailed cytoarchitectonic delineation of the mesopontine tegmentum, including the PPTn, was performed employing tissue stained for Nissl substance. Choline acetyltransferase (ChAT) immunostained tissue was then analyzed in order to investigate the relationship of cholinergic perikarya, dendritic arborizations, and axonal trajectories within this cytoarchitectonic scheme. To confirm some of our cytoarchitectonic delineations, the relationships between neuronal elements staining for ChAT and tyrosine hydroxylase were investigated on tissue stained immunohistochemically for the simultaneous demonstration of these two enzymes. The PPTn consists of large, multipolar neurons, all of which stain immunohistochemically for ChAT. It is present within cross-sections that also include the A-6 through A-9 catecholamine cell groups and is traversed by catecholaminergic axons within the dorsal tegmental bundle and central tegmental tract. The dendrites of PPTn neurons respect several nuclear boundaries and are oriented perpendicularly to several well-defined fiber tracts. Cholinergic axons ascend from the mesopontine tegmentum through the dorsal tegmental bundle and a more lateral dorsal ascending pathway. A portion of the latter terminates within the lateral geniculate nucleus. It has been widely believed that the PPTn is reciprocally connected with several extrapyramidal structures, including the globus pallidus and substantia nigra pars reticulata. Therefore, the relationships of pallidotegmental and nigrotegmental pathways to the PPTn were investigated employing the anterograde autoradiographic methodology. The reciprocity of tegmental connections with the substantia nigra and entopeduncular nucleus was investigated employing combined WGA-HRP injections and ChAT immunohistochemistry. The pallido- and nigrotegmental terminal fields did not coincide with the PPTn, but, rather, were located just medial and dorsomedial to it (the midbrain extrapyramidal area). The midbrain extrapyramidal area, but not the PPTn, was reciprocally connected with the substantia nigra and entopeduncular nucleus. We discuss these results in light of other cytoarchitectonic, cytochemical, connectional, and physiologic studies of the functional anatomy of the mesopontine tegmentum.

Regional neural activity within the substantia nigra during peri-ictal flurothyl generalized seizure stages

“…Postictal states can be compared to movement arrest, using similar suppressive mechanisms during motor seizure termination. Decreased uptake in the PPTg is consistent with lack of inhibitory GABAergic regulation, e.g., from the SNR, which represents one of most important afferent projections of the PPTg (Rye et al, 1987;Spann and Grofova, 1991). This in turn results in an increase of cholinergic or glutamatergic activity within the PPTg, which is reflected by increased activity in the STN, one of the main ascending projections of PPTg (Nakano et al, 2000).…”

Deep Brain Stimulation for Movement Disorders of Basal Ganglia Origin: Restoring Function or Functionality?

“…The PPN is a very heterogeneous structure, consisting of a caudolateral pars compacta (PPNc) and an anteromedial pars dissipata. Cholinergic cells predominate in PPNc, but PPNc and anteromedial pars dissipata also contain large populations of GABAergic or glutamatergic neurons [77][78][79]. The input and output relationships of the various neuron groups in the PPN have not been precisely determined, but it is known that the nucleus gives rise to projections to the basal ganglia, thalamus, basal forebrain, reticular formation, and spinal cord [69,74,[80][81][82][83][84][85][86][87][88][89][90][91][92][93], thus being, at the same time, part of the extended basal ganglia family of nuclei [74], and a conduit of descending basal ganglia outputs.…”

Relationships between thalamostriatal neurons and pedunculopontine projections to the thalamus: a neuroanatomical tract-tracing study in the rat

“…The reciprocal connections existing between the PPTg and the basal ganglia have been extensively studied (Saper and Loewy 1982;Jackson and Crossman 1983;Rye et al 1987;Lee et al 1988), with emphasis on their relationship with the SN (Beninato and Spencer 1987;Clarke et al 1987;Scarnati et al 1987b;Beninato and Spencer 1988;Gould et al 1989;Spann and Grofova 1991;Lavoie and Parent 1994). The pedunculonigral projection may be an indirect way of modulating striatal activity, as PPTg nigral afferents synapse onto dopaminergic nigrostriatal neurons (Tokuno et al 1988;Bolam et al 1991;Hernández-López et al 1992).…”

“…The most caudal brain nucleus receiving projections from the basal ganglia is the pedunculopontine tegmental nucleus (PPTg). Anterograde and retrograde tract-tracing procedures and neurophysiological studies have reported the existence of a close and reciprocal relationship between the PPTg and several basal ganglia structures such as the entopeduncular nucleus (EP), the subthalamic nucleus (STh) and the substantia nigra (SN) (Saper and Loewy 1982;Jackson and Crossman 1983;Scarnati et al 1987a;Rye et al 1987;Spann and Grofova 1989;Lavoie et al 1990;Lavoie and Parent 1994a). Basal ganglia structures lack direct access to pontomedullary or spinal nuclei and the PPTg has therefore been considered a link between the basal ganglia and the brainstem and spinal cord.…”