Central neurons which utilize acetylcholine as a neurotransmitter affect a number of important brain functions such as processing of sensory information, arousal, and learning and memory. Loss of cortical cholinergic innervation can result in severe behavioral deficits. A cholinergic deficit is also partly responsible for the dementia of Alzheimer's Disease. The studies proposed here will extend previous work of this laboratory on the synaptic organization of central cholinergic pathways that affect transmission in the cerebral cortex, either directly or via subcortical circuitries. These studies will utilize somatosensory pathways in the rat brain as a model system to identify anatomically the synaptic sites at which cholinergic transmission might modulate the processing of sensory information. The specific pathway to be studied is the mystacial vibrissal system which conveys tactile information that is essential for exploratory behavior in this species. Preliminary evidence from this laboratory suggests that both forebrain and brainstem cholinergic cell groups influence sensory pathways at several levels of the neuraxis including the midbrain, thalamus, and cortex. The proposed studies will utilize correlated light and electron microscopic analyses to identify such connections. First, putative sensory afferents to cholinergic neurons of the brainstem pedunculopontine tegmental nucleus (PPTn) will be studied by injecting the retrograde tracer wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) into the PPTn. Identified retrogradely- labeled sensory relay nuclei will then be injected in a second set of experiments with anterograde tracers including WGA-HRP and phaseolus vulgarus leukoagglutinin. The resulting anterogradely- labeled sensory axons and terminals will be co-visualized with choline acetyltransferase immunoreactive neurons of the PPTn. Correlated light and electron microscopic analyses will verify synaptic contacts between anterogradely-labeled sensory axons and PPTn neurons. Second, cholinergic PPTn projections to the ventrobasal thalamus will be studied by placing simultaneous injections of anterograde tracers in the PPTn and retrograde tracers in the barrel field of somatosensory cortex. Light and ultrastructural visualization of anterogradely-labeled fibers and retrogradely-labeled cells will permit identification of cholinergic-PPTn inputs onto thalamic relay cells responsible for transmission of vibrissal tactile information. Third, tissue from the same animals, which contains retrogradely-labeled cholinergic and noncholinergic basal forebrain neurons will be processed in a similar manner to identify putative PPTn-basal forebrain connections. Finally, cortical targets of basal forebrain projections to the barrel field of somatosensory cortex will be identified by anterograde tracing of these projections combined with either Golgi impregnation of cortical neurons, GABA immunocytochemistry, or retrograde tracing from known cortical- cortical and cortico-fugal pathways. The results of these studies will provide a detailed understanding of a specific information- processing system in which the modulatory role of cholinergic transmission can be studied in detail at both an anatomical and physiological level.
