The Mesoscopic Connectome of the Cholinergic Pontomesencephalic Tegmentum

Abstract

The pontomesencephalic tegmentum, comprising the pedunculopontine nucleus and laterodorsal tegmental nucleus, is involved in various functions via complex connections; however, the organizational structure of these circuits in the whole brain is not entirely clear. Here, combining viral tracing with fluorescent micro-optical sectional tomography, we comprehensively investigated the input and output circuits of two cholinergic subregions in a continuous whole-brain dataset. We found that these nuclei receive abundant input with similar spatial distributions but with different quantitative measures and acquire similar neuromodulatory afferents from the ascending reticular activation system. Meanwhile, these cholinergic nuclei project to similar targeting areas throughout multiple brain regions and have different spatial preferences in 3D. Moreover, some cholinergic connections are unidirectional, including projections from the pedunculopontine nucleus and laterodorsal tegmental nucleus to the ventral posterior complex of the thalamus, and have different impacts on locomotion and anxiety. These results reveal the integrated cholinergic connectome of the midbrain, thus improving the present understanding of the organizational structure of the pontine-tegmental cholinergic system from its anatomical structure to its functional modulation.

Simultaneously cholinergic projection in Ascending and Descending Circuits from Midbrain

“…The fibers of reconstructed neurons widely distributed in the whole-brain and some neurons even sent divergent fibers to cortical areas, cerebellum and medulla simultaneously ( Figure 1F ). These fibers covered the main projection areas of PTCNs (Peilin Zhao et al, 2022), which indicated these reconstructed neurons present the morphological characterization of individual cholinergic neurons in the midbrain.…”

“…DA neurons in the ventral tegmental area (VTA) (Beier et al, 2015) and different types of neurons in the dorsal raphe (DR) (Xu et al, 2021) mainly send their fibers to ascending areas, while the mesencephalic locomotor region modulates the ascending and descending circuits to participate in different functions via two separate group glutamatergic neurons (Ferreira-Pinto et al, 2021). However, cholinergic neurons in the midbrain send abundant axonal projections both in ascending and descending circuits (Henrich Martin et al, 2020; Peilin Zhao et al, 2022), and their projection patterns have not been systematically characterized until now.…”

The regulation of the pedunculopontine tegmental nucleus in sleep–wake states

“…It receives inputs from the hypocretin/orexin (ORX) neurons of the posterior LH and histaminergic (HA) neurons of the tuberomammillary nucleus (TMN), serotonergic inputs from the dorsal raphe nucleus (DRN), noradrenergic input from the locus coeruleus (LC), cholinergic input from the contralateral PPTg [ 8 , 19 – 26 ]. Furthermore, the PPTg receives projections from the motor cortex, the basal ganglia, substantia nigra pars reticulate (SNr) and globus pallidus internus (GPi), the subthalamic nucleus (STN), and the deep cerebellar nuclei [ 27 , 28 ]. The PPTg also activates and innervates thalamocortical neuron activity through the dorsal pathway directly in thalamus by acting on nicotinic and muscarinic receptors, and indirectly in the thalamocortical neurons by acting on M2 receptors to promote the release of excitatory neurotransmitters, thus activating the cortex and maintaining cortical excitability [ 14 , 29 , 30 ].…”

“…On average, there are five collaterals and most of them have ascending trajectories along dorsal and ventral streams [ 7 , 45 ], and ascending ventral innervation is located in the basal ganglia and extends to the hypothalamic region of the BF. There are shreds of evidence showing that the synapses formed by cholinergic axons are located in the cerebral cortex, the cerebral nuclei, the thalamus, the hypothalamus, the midbrain, the pontine reticular formation (PnO), the medulla and the cerebellum [ 28 ]. Therefore, the level of collateralization in cholinergic neurons is high, which innervate the structures of the midbrain, forebrain, and lower brainstem.…”

“…Mena–Segovia et al confirmed that the PPTg plays a role in the cortical transition from a slow-wave, low arousal state to a high arousal state [ 7 ]. The acetylcholine-releasing nucleus of the PPTg is mainly aimed at the BF, but also to the relay nucleus and reticular cells of the thalamus [ 28 ]. This pathway is essential to stimulate the transmission of thalamic signals to the cerebral cortex.…”