Preferential subcortical collateral projections of pedunculopontine nucleus-targeting cortical pyramidal neurons revealed by brain-wide single fiber tracing.

The pedunculopontine nucleus (PPN) is a heterogeneous midbrain structure involved in various brain functions, such as motor control, learning, reward, and sleep. Previous studies using conventional tracers have shown that the PPN receives extensive afferent inputs from various cortical areas. To examine how these cortical axons make collateral projections to other subcortical areas, we used a dual-viral injection strategy to sparsely label PPN-targeting cortical pyramidal neurons in CaMKIIα-Cre transgenic mice. Using a high-speed volumetric imaging with on-the-fly-scan and Readout (VISoR) technique, we visualized brain-wide axonal projections of individual PPN-targeting neurons from several cortical areas, including the prelimbic region (PL), anterior cingulate area (ACA) and secondary motor cortex (MOs). We found that each PPN-projecting neuron had a unique profile of collateralization, with some subcortical areas being preferential targets. In particular, PPN-projecting neurons from all three traced cortical areas exhibited common preferential collateralization to several nuclei, with most neurons targeting the striatum (STR), lateral hypothalamic area (LHA) and periaqueductal gray (PAG), and a substantial portion of neurons also targeting the zona incerta (ZI), median raphe nucleus (MRN) and substantia nigra pars reticulata (SNr). Meanwhile, very specific collateralization patterns were found for other nuclei, including the intermediate reticular nucleus (IRN), parvicellular reticular nucleus (PARN) and gigantocellular reticular nucleus (GRN), which receive collateral inputs almost exclusively from the MOs. These observations provide potential anatomical mechanisms for cortical neurons to coordinate the PPN with other subcortical areas in performing different physiological functions.

Reciprocal interactions between the basolateral amygdala and infralimbic and prelimbic regions of the mPFC: Actions of diazepam.

The basolateral amygdala (BLA) and prelimbic (PL) and infralimbic (IL) cortices are interconnected structures that participate in the regulation of fear. Unknown are the reciprocal functional influence of these regions on neuronal responsivity and the action of an anxiolytic drug. We performed multiunit activity recordings from one neuronal pool while applying electrical stimulation to another neuronal pool. In the same experimental session, PL-BLA and IL-BLA sequences of stimulation-recording were applied, followed by the inverse sequence (BLA-PL and BLA-IL). Using this procedure, we obtained information from the same neuronal pool that was stimulated and recorded. Using peristimulus histograms, we analyzed neuronal responsivity. In the saline-treated group, the PL-BLA stimulation-recording sequence produced an inhibitory response. The inverse sequence, BLA-PL, produced an excitatory response. For BLA-IL and IL-BLA stimulation-recording sequences, a minimal response was observed in the saline control group. Diazepam minimally affected responsivity of the PL-BLA and BLA-IL connections. Diazepam blocked the initial excitatory response of the IL-BLA connection. Under control conditions, the PL and BLA appeared to regulate each other. Under the action of diazepam, the IL exerted an inhibitory influence on the BLA. Because of the well-known actions of this anxiolytic drug on the BLA, this combined action may result in the synergistic control of fear.