Cellular contacts established by Hcrt1/OxA-containing axons in the ventral tegmental area. (A) Hcrt1/OxA-immunoreactivity is seen in large dense-cored vesicles (ldcv), dense-cored vesicles (dcv) and in the cytoplasm of a varicosity (Hcrt1/OxA-V) that makes an asymmetric synapse (curved arrow) with a Fluorogold-labeled dendrite (FG-den). The FG-den is identified by its content in DAB-immunoperoxidase reaction product (black arrows). (B) Hcrt1/OxA-V establishes an asymmetric synapse (curved arrow) with a FG-dendrite that receives convergent input from an unlabeled axon terminal (Ter). (C) A VTA dendrite (Den) receives a synaptic contact (curved arrow) from an Hcrt1/OxA-immunoreactive varicosity (Hcrt1/OxA-V) and an unlabeled Ter. (D) Hcrt1/OxA-T makes an asymmetric synapse (curved arrow) on an unlabeled Den. Hcrt1/OxA-T contains translucent small synaptic vesicles (ssv) near the synaptic specialization while dcv are far from the synapse. (E) Bar graph showing the relative percentage of appositional and synaptic contacts (asymmetric) established by Hcrt1/OxA-boutons with unlabeled- (un) or FG-labeled (FG) dendrites according to the type of axonal bouton (varicosity, diameter < 0.7 μm or axon terminal, diameter ≥ 0.7 μm) in the ventral tegmental area. Mean percentages and standard errors were calculated based on the numbers obtained from 102 Hcrt1/OxA-immunoreactive boutons in 14 vibratome sections from six rat brains. (F) Bar graph showing the percentage distribution of unlabeled and Fluorogold-labeled (FG) dendrites of different sizes receiving asymmetric synapses (n = 22) from Hcrt1/OxA-immunolabeled axonal boutons (terminals and varicosities; total sample: 616 boutons) in the ventral tegmental area. Mean percentages and standard errors were calculated based on the numbers obtained from the synapse-recipient 22 dendrites in 18 ultrathin sections from 6 rat brains. ANOVA (animal X dendritic size) was done to determine in those dendrites significant variations in the formation of asymmetric synapses with respect to their small (<0.5 mm), intermediate (0.5-1.0 mm) or large (>1.0 mm) diameters [*p < 0.05; post hoc Fisher test]. Scale bar, 0.2 μm.