Cholinergic axons in the rat ventral tegmental area synapse preferentially onto mesoaccumbens dopamine neurons - PubMed (original) (raw)

Cholinergic axons in the rat ventral tegmental area synapse preferentially onto mesoaccumbens dopamine neurons

Natalia Omelchenko et al. J Comp Neurol. 2006.

Abstract

Cholinergic afferents to the ventral tegmental area (VTA) contribute substantially to the regulation of motivated behaviors and the rewarding properties of nicotine. These actions are believed to involve connections with dopamine (DA) neurons projecting to the nucleus accumbens (NAc). However, this direct synaptic link has never been investigated, nor is it known whether cholinergic inputs innervate other populations of DA and gamma-aminobutyric acid (GABA) neurons, including those projecting to the prefrontal cortex (PFC). We addressed these questions by using electron microscopic analysis of retrograde tract-tracing and immunocytochemistry for the vesicular acetylcholine transporter (VAChT) and for tyrosine hydroxylase (TH) and GABA. In tissue labeled for TH, VAChT(+) terminals frequently synapsed onto DA mesoaccumbens neurons but only seldom contacted DA mesoprefrontal cells. In tissue labeled for GABA, one-third of VAChT(+) terminals innervated GABA-labeled dendrites, including both mesoaccumbens and mesoprefrontal populations. VAChT(+) synapses onto DA and mesoaccumbens neurons were more commonly of the asymmetric (presumed excitatory) morphological type, whereas VAChT(+) synapses onto GABA cells were more frequently symmetric (presumed inhibitory or modulatory). These findings suggest that cholinergic inputs to the VTA mediate complex synaptic actions, with a major portion of this effect likely to involve an excitatory influence on DA mesoaccumbens neurons. As such, the results suggest that natural and drug rewards operating through cholinergic afferents to the VTA have a direct synaptic link to the mesoaccumbens DA neurons that modulate approach behaviors.

J. Comp. Neurol. 494:863-875, 2006. (c) 2005 Wiley-Liss, Inc.

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Figures

Figure 1

Figure 1

Light microscopic images of rat coronal brain sections. Panels A and B show injections of FG into the NAc or PFC. The corresponding retrograde transport of FG to the VTA is shown in panels C and D. Inserts illustrate the transport of FG into soma and dendrites (arrows indicate the same cells). Panels E-G illustrate peroxidase labeling for VAChT in the VTA and adjacent interpeduncular nucleus (IP). Although VAChT+ fibers are notably denser in the IP than in the VTA, VAChT axons in the latter structure viewed at higher magnification are markedly beaded and appear to form chains of varicosities (small arrows in G). Abbreviations: ac, anterior commissure; cc, corpus callosum; fm, forceps minor; fr, fasciculus retroflexus; LV, lateral ventricle; ml, medial lemniscus; mp, mammillary peduncle; rs, rhinal sulcus. Scale bar represents 1 mm in A-B, 500 μm in C-D, 62.5 in inserts, 125 μm in E-F, and 31.25 μm in G.

Figure 2

Figure 2

Electron micrographs of the rat VTA demonstrating immunoperoxidase labeled VAChT-positive axon terminals (VT) forming synapses with asymmetric (white arrow) or symmetric (black arrows) morphology onto unlabeled dendrites (ud). One VT terminal also shows immunogold-silver labeling for GABA (GABA/VT in C), and a GABA-labeled dendrite appears in the adjacent neuropil (GABA-d in B). Scale bar, 0.33 μm.

Figure 3

Figure 3

Electron micrographs showing asymmetric (white arrows) or symmetric (black arrows) synapses formed by VAChT-positive terminals (VT) onto dendrites singly labeled by immunogold-silver for TH (TH-d) or GABA (GABA-d). The GABA-d in B receives additional synaptic input from a GABA-labeled terminal (GABA-t). The TH-d in C is shown in a serial section in D to verify the TH labeling. Scale bar, 0.33 μm.

Figure 4

Figure 4

Electron micrographs of the VTA from rats receiving FG injections into the NAc. VAChT-immunoreactive terminals (VT) form asymmetric (white arrows) or symmetric (black arrows) synapses onto soma (s) or dendrites (d) dually labeled by immunoperoxidase for FG and immunogold-silver for TH (FG+TH) or GABA (FG+GABA). In C, the VT synapses onto a FG+TH soma, as evidenced by the presence of a nucleus (n). The FG+GABA-d in F contains a low level of GABA labeling and FG concentrated in a lysosome (arrowhead). The serial section in G shows the synapse formed by the VT. Scale bar, 0.33 μm for A-C and E-G; 0.5 μm for D.

Figure 5

Figure 5

Electron micrographs of the VTA from rats receiving FG injections into the PFC. VAChT-labeled axon terminals (VT) form asymmetric (white arrows) or symmetric synapses (black arrows) onto dendrites dually-labeled for FG and TH (FG+TH-d) or FG and GABA (FG+GABA-d) or singly-labeled for FG (FG-d). In B, sparse immunogold-silver labeling for TH is indicated by small arrows. Arrowheads indicate diffuse FG in the cytoplasm (B) or concentrated in a lysosome (D). Scale bar, 0.33 μm for A, B, D; 0.40 μm for C.

Figure 6

Figure 6

Schematic drawing comparing the asymmetric (presumed excitatory) synapses in white and symmetric (presumed modulatory/inhibitory) synapses in black formed by cholinergic axons presumably coming from the mesopontine tegmentum onto identified cell populations of DA (D) and GABA (G) VTA neurons. The thickness of cholinergic axons is weighted to reflect the approximate frequency of the connections observed in the present study.

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