Extent of bilateral collateralization among pontomesencephalic tegmental afferents to dorsal lateral geniculate nuclei of pigmented and albino rats

Cholinergic cells in the tegmentum send branching projections to the inferior colliculus and the medial geniculate body

The pontomesencephalic tegmentum (PMT) provides cholinergic input to the inferior colliculus (IC) and the medial geniculate body (MG). PMT cells are often characterized as projecting to more than one target. The purpose of this study was to determine whether individual PMT cholinergic cells, (1) innervate the auditory pathways bilaterally via collateral projections to left and right auditory thalamus; or, (2) innervate multiple levels of the auditory pathways via collateral projections to the auditory thalamus and inferior colliculus. We used multiple retrograde tracers to identify individual PMT cells that project to more than one target. We combined the retrograde tracer studies with immunohistochemistry for choline acetyltransferase to determine whether the projecting cells were cholinergic. We found that individual PMT cells send branching axonal projections to two or more auditory targets in the midbrain and thalamus. The collateral projection pattern that we observed most frequently was to the ipsilateral IC and ipsilateral MG. Cells projecting to both MGs were somewhat less common, followed by cells projecting to the contralateral IC and ipsilateral MG. Both cholinergic and non-cholinergic cells contribute to each of these projection patterns. Less often, we found cells that project to one IC and both MGs; there was no evidence for non-cholinergic cells in this projection pattern. It is likely that collateral projections from PMT cells could have coordinated effects bilaterally and at multiple levels of the ascending auditory pathways.

Cholinergic cells of the pontomesencephalic tegmentum: connections with auditory structures from cochlear nucleus to cortex

Acetylcholine (ACh) is a neuromodulator that is likely to play a role in plasticity as well as other phenomena at many sites in the auditory system. The auditory cortex receives cholinergic innervation from the basal forebrain, whereas the cochlea receives cholinergic innervation from the superior olivary complex. Much of the remainder of the auditory pathways receives innervation from the pedunculopontine and laterodorsal tegmental nuclei, two nuclei referred to collectively as the pontomesencephalic tegmentum (PMT). The PMT provides the major source of ACh to the auditory thalamus and the midbrain, and is a substantial source (in addition to the superior olivary complex) of ACh in the cochlear nucleus. Individual cholinergic cells in the PMT often have axon branches that innervate multiple auditory nuclei, including nuclei on both sides of the brain as well as nuclei at multiple levels of the auditory system. The auditory cortex has direct axonal projections to the PMT cells, including cholinergic cells that project to the inferior colliculus or cochlear nucleus. The divergent projections of PMT cholinergic cells suggest widespread effects on the auditory pathways. These effects are likely to include plasticity as well as novelty detection, sensory gating, reward behavior, arousal and attention. Descending projections from the forebrain, including the auditory cortex, are likely to provide a high level of cognitive input to these cholinergic effects. Dysfunction associated with the cholinergic system may play a role in disorders such as tinnitus and schizophrenia.

Pedunculopontine tegmental nucleus lesions impair stimulus--reward learning in autoshaping and conditioned reinforcement paradigms

The role of the pedunculopontine tegmental nucleus (PPTg) in stimulus-reward learning was assessed by testing the effects of PPTg lesions on performance in visual autoshaping and conditioned reinforcement (CRf) paradigms. Rats with PPTg lesions were unable to learn an association between a conditioned stimulus (CS) and a primary reward in either paradigm. In the autoshaping experiment, PPTg-lesioned rats approached the CS+ and CS- with equal frequency, and the latencies to respond to the two stimuli did not differ. PPTg lesions also disrupted discriminated approaches to an appetitive CS in the CRf paradigm and completely abolished the acquisition of responding with CRf. These data are discussed in the context of a possible cognitive function of the PPTg, particularly in terms of lesion-induced disruptions of attentional processes that are mediated by the thalamus.

Characterization of the extent of pontomesencephalic cholinergic neurons' projections to the thalamus: comparison with projections to midbrain dopaminergic groups

We sought to determine whether pontomesencephalic cholinergic neurons which we have been shown previously to project to the substantia nigra and ventral tegmental area also contribute to the thalamic activation projection from the pedunculopontine and laterodorsal tegmental nuclei. Retrograde tracing, immunohistochemical localization of choline acetyltransferase and statistical methods were used to determine the full extent of the cholinergic projection from the pedunculopontine and laterodorsal tegmental nuclei to the thalamus. Progressively larger Fluoro-Gold injections in to the thalamus proportionally labeled increasing numbers of pontomesencephalic cholinergic cells both ipsi- and contralaterally in the pedunculopontine and laterodorsal tegmental nuclei. Multiple large thalamic injections left only a small fraction of the ipsilateral pontomesencephalic cholinergic group unlabeled. This small remainder did not correspond to the populations which project to the substantia nigra and ventral tegmental area, thereby indicating that substantia nigra- and ventral tegmental area-projecting cholinergic neurons must also project to the thalamus. We examined whether there existed any set of cholinergic neurons in the pedunculopontine and laterodorsal tegmental nuclei which did not innervate a thalamic target. The distribution of descending projections of the pedunculopontine and laterodorsal tegmental nuclei demonstrated that the unlabeled remainder cannot correspond to a purely descending group. We also show that substance P-positive cholinergic cells in the laterodorsal tegmental nucleus project to the thalamus. Further studies demonstrated that the small population of cholinergic cells left unlabeled from the thalamus were the smallest sized cholinergic cells, and included two groups of small, light-staining cholinergic cells located in the parabrachial area and central gray, adjacent to the main pedunculopontine and laterodorsal tegmental nuclei cholinergic groups. These small cells, in contrast to thalamic-projecting cholinergic cells, did not stain positively for reduced nicotinamide adenine dinucleotide phosphate-diaphorase. Taken together, these results indicated that all of the reduced nicotinamide adenine dinucleotide phosphate diaphorase-positive/choline acetyltransferase-positive neurons of the pedunculopontine/laterodorsal tegmental nuclei ascend to innervate some portion of the thalamus, in addition to the other targets they innervate. These findings indicate that the diverse physiological and behavioral effects attributed to the activity of pontomesencephalic cholinergic neurons should not be dissociated from their activating effects in the thalamus.

Organisation of the reticular thalamic projection to the intralaminar and midline nuclei in rats

This study examines the projection of the reticular thalamic nucleus to the classic "nonspecific" dorsal thalamic nuclei of rats. Individual nuclei of the intralaminar (central-lateral, paracentral, central-medial, parafascicular) and the midline (reuniens/rhomboid, parataenial) nuclear groups, together with the reticular nucleus itself, were injected with the neuronal tracers biotinylated dextran or fluorescent latex microspheres (red or green). Reticular cells projecting to the intralaminar and midline nuclei are limited largely to the rostral pole of the nucleus. Within the rostral pole, most reticular cells projecting to the intralaminar and midline nuclear groups are found in largely distinct sectors; cells that project to the intralaminar nuclei tend to lie more laterally, whereas those projecting to the midline nuclei lie more medially within the pole. Among the individual nuclei of both the intralaminar and midline nuclear groups, however, the segregation is far less distinct. For instance, the reticular cells that project to the intralaminar central-lateral, central-medial, paracentral, and parafascicular nuclei are intermixed completely on the lateral edge of the rostral pole. After separate injections of different colored latex microspheres into individual intralaminar nuclei, the incidence of double-labelled reticular cells is about 37%, a percentage much higher than among the "specific" dorsal thalamic nuclei (< 1%). All the above-mentioned results refer to the reticular labelling seen on the side ipsilateral to the injection. After separate injections into the intralaminar central-medial nucleus, the midline nuclei, and the reticular nucleus itself, we also see a very small group of reticular cells labelled on the contralateral side. In general, our results indicate that the reticular projection to the intralaminar and midline nuclei is far more diffuse than the reticular projection to the specific dorsal thalamic nuclei.

The pedunculopontine nucleus--auditory input, arousal and pathophysiology

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

Two types of interneuron in the dorsal lateral geniculate nucleus of the rat: a combined NADPH diaphorase histochemical and GABA immunocytochemical study

The rationale for this study was to provide a comprehensive light microscopical description of the morphology of diaphorase-reactive neurons and neuropil elements in the dorsal lateral geniculate nucleus (dLGN) of the rat. An additional objective was to quantitatively assess whether a subpopulation of the diaphorase-reactive neurons, previously shown to be GABA-immunoreactive, constitute a distinct type of local-circuit neuron in the rat dLGN. Diaphorase activity was localised in a population of predominantly bipolar fusiform neurons. These cells were weak to moderately stained and possessed the morphological features of intrinsic inhibitory neurons, previously called class B neurons in the rat dLGN. Quantitative estimates indicated that the diaphorase-reactive neurons constituted approximately 10% of the total neuron composition of the dLGN. The majority (about 83%) of the diaphorase-reactive cells were located in the lateral half of the nucleus. In addition, a dense plexus of diaphorase-reactive varicose fibres was found throughout the dLGN lying between the oriented fibre bundles coursing dorsoventrally through the LGN. Diaphorase-reactive punctae were found to be closely associated with the somata and proximal dendritic segments of nonreactive neurons and also with the stained proximal dendritic segments of diaphorase-reactive dLGN neurons. The source of the diaphorase-reactive fibres in the dLGN was unknown. Evidence suggests, however, that they are of extrinsic origin. The GABA-immunoreactive nature of the diaphorase neurons in the dLGN was demonstrated by colocalising GABA immunoreactivity within the somata of diaphorase-reactive cells. The majority (> 90%) of diaphorase-reactive dLGN neurons were GABA-immunopositive. Also present was a distinct population of GABA-immunopositive neurons that were not diaphorase-reactive. In this study, cells that were solely GABA-immunopositive have been called class B1 neurons, while cells that were both diaphorase-reactive and GABA-immunoreactive have been called class B2 neurons. Size-frequency distributions of somatic profile areas established that the two populations of GABA-immunoreactive neuron were significantly different. Class B1 neurons constituted 57%, with class B2 cells representing 43% of all GABA-immunostained neurons in the rat dLGN. The characteristic morphological features, neurochemical identity and frequency of the diaphorase-reactive neurons in the rat dLGN indicate that they represent a subpopulation of inhibitory interneurons with the ability to affect intrinsic dLGN operations and thalamocortical interactions using the neuromodulator nitric oxide.