Physical (in)activity-dependent structural plasticity in bulbospinal catecholaminergic neurons of rat rostral ventrolateral medulla

Abstract

Increased activity of the sympathetic nervous system is thought to play a role in the development and progression of cardiovascular disease. Recent work has shown that physical inactivity versus activity alters neuronal structure in brain regions associated with cardiovascular regulation. Our physiological studies suggest that neurons in the rostral ventrolateral medulla (RVLM) are more responsive to excitation in sedentary versus physically active animals. We hypothesized that enhanced functional responses in the RVLM may be due, in part, to changes in the structure of RVLM neurons that control sympathetic activity. We used retrograde tracing and immunohistochemistry for tyrosine hydroxylase (TH) to identify bulbospinal catecholaminergic (C1) neurons in sedentary and active rats after chronic voluntary wheel-running exercise. We then digitally reconstructed their cell bodies and dendrites at different rostrocaudal levels. The dendritic arbors of spinally projecting TH neurons from sedentary rats were more branched than those of physically active rats (P < 0.05). In sedentary rats, dendritic branching was greater in more rostral versus more caudal bulbospinal C1 neurons, whereas, in physically active rats, dendritic branching was consistent throughout the RVLM. In contrast, cell body size and the number of primary dendrites did not differ between active and inactive animals. We suggest that these structural changes provide an anatomical underpinning for the functional differences observed in our in vivo studies. These inactivity-related structural and functional changes may enhance the overall sensitivity of RVLM neurons to excitatory stimuli and contribute to an increased risk of cardiovascular disease in sedentary individuals.

Keywords: neuroplasticity; sedentary; sympathetic nervous system.

Figure 1. Average weekly running data for physically active rats

Daily running by the six active rats used for morphological analysis. Activity of each rat on its in-cage running wheel was recorded with a bicycle computer. Average daily running distance (A) and duration (B) gradually increased with a peak at weeks 6-7 and then decreased from week 7 to 11. Average daily running speed did not vary during the course of the study (not shown). At 12 weeks, the rats received injections of CTB into T10 and were perfused seven days later.

Figure 2. Distribution of bulbospinal catecholaminergic RVLM neurons close to the caudal pole of the facial nucleus

A, Montage of 17 low magnification photomicrographs showing the RVLM very close to the caudal pole of the facial nucleus (FN) from a rat with CTB injections into spinal segment T10 whose activity level was not monitored. Brown TH-immunoreactivity uniformly fills the somata and proximal and distal dendrites of catecholaminergic RVLM neurons. Blue-grey staining due to CTB-immunoreactivity retrogradely transported from the spinal cord is restricted to cell bodies and the portions of proximal dendrites closest to cell bodies. Many of the neurons with immunoreactivity for both TH and CTB occur in a tight cluster (red dashed oval l) that lies laterally within the RVLM. There are occasional CTB-containing neurons in the group of TH-positive neurons that lies medial to the tight cluster (red dashed oval m). Scattered neurons double-stained for TH and CTB (arrows) occur dorsal to the two ventral groups of TH neurons. Two of these double-labeled dorsal neurons (arrows B and C) are shown at higher magnification in panels B and C. Arrowheads, TH-immunoreactive neurons that do not contain CTB-immunoreactivity; py, pyramidal tract. Scale bar, 250 μm. B & C, Higher magnification photomicrographs of the dorsal neurons indicated by arrows B and C in A. In B, blue-grey CTB-immunoreactivity (arrowhead) occurs only around the nucleus of the retrogradely-labeled neuron. In C, labelling for CTB (arrowheads) extends into the proximal dendrites of the bulbospinal neuron. Scale bars, 25 μm.

Figure 3. Bulbospinal catecholaminergic neuron from the RVLM of a sedentary rat

A, Montage of 8 photomicrographs showing a section through the RVLM located 150 μm caudal to the caudal pole of the facial nucleus (FN-1) from the medulla of a sedentary rat (Sed). The section contains many neurons that are immunoreactive for both CTB (blue-grey) and TH (brown), one of which is indicated by arrow B+C. Scale bar, 250 μm. Inset, Drawing of FN-1 showing architectonic landmarks. B, Montage of 26 photomicrographs showing the double-labelled neuron indicated by arrow B+C in A. Scale bar, 50 μm. The cell body of the neuron contains a network of blue-grey staining (arrow), indicating that the neuron has retrogradely transported CTB from the spinal cord. Ventral to the reconstructed neuron, there is another retrogradely labelled neuron showing faint TH-immunoreactivity (arrowhead). Scale bar, 50 μm. C, Digital reconstruction of the neuron from A and B. Scale bar, 100 μm.

Figure 4. Bulbospinal catecholaminergic neuron from the RVLM of a physically active rat

A, Montage of 5 photomicrographs showing a section through the RVLM located 150 μm rostral to the caudal pole of the facial nucleus (FN+1) from the medulla of a rat that ran voluntarily on an in-cage running wheel for 11 weeks. The section contains many neurons that are immunoreactive for both CTB (blue-grey) and TH (brown), one of which is indicated by arrow B+C. Scale bar, 250 μm. Inset, Drawing of FN+1 showing architectonic landmarks. B, Montage of 11 photomicrographs showing the double-labelled neuron indicated by arrow B+C in A. The neuron contains a network of blue-grey staining (arrow) in its cell body and in a thick proximal dendrite, indicating that the neuron has retrogradely transported CTB from the spinal cord. Two other retrogradely labelled neuron (stars) that lacks TH-immunoreactivity lie near to the bulbospinal TH neuron. Scale bar, 50 μm. C, Digital reconstruction of the neuron in B. Scale bar, 50 μm.

Figure 5. (In)activity-related differences in morphology of dendrites of bulbospinal catecholaminergic RVLM neurons based on Sholl analyses

Morphological data on dendrites obtained via Sholl analyses demonstrated that the length and complexity of the dendrites of TH-positive, CTB-positive RVLM neurons is greater in sedentary rats than in active rats. Data were obtained at 10 μm intervals beginning 20 μm away from the center of each cell body. Differences in the number of dendritic intersections (A) and dendritic length (B) between sedentary and active rats occurred from 50 to 370 micrometers away from the center of the cell body. *, p<0.05.

Figure 6. (In)activity-related differences in morphology of dendrites of bulbospinal catecholaminergic RVLM neurons based on branch order

Morphological differences between the dendrites of TH-positive, CTB-positive RVLM neurons from sedentary and active rats were not consistent for all types of dendrite. A, The number of primary dendrites was similar between sedentary and active rats, but sedentary rats had more higher-order dendrites relative to active rats. B, Sedentary rats had shorter primary dendrites than active rats, but higher-order dendrites were longer. C, In sedentary rats, 1^st to 4^th order dendrites had more branches than in active rats. *, p<0.05, individual differences between sedentary and active animals.

Figure 7. (In)activity-related differences in morphology of dendrites and cell bodies of bulbospinal catecholaminergic RVLM neurons based on rostrocaudal position

Morphological differences between TH-positive, CTB-positive RVLM neurons from sedentary and active rats depended on the position of the neurons relative to the facial nucleus (FN). Each section analyzed was 150 micrometers thick and section FN0 contained the caudal pole of FN. Equal numbers of CTB/TH neurons were reconstructed in eight consecutive sections of the ventrolateral medulla from both groups (i.e. 9.6 ± 0.2 vs. 9.2 ± 0.4 neurons/section in sedentary versus physically rats, respectively). A, In sedentary rats, TH-positive, CTB-positive RVLM neurons had increased branching in sections from 300 micrometers caudal to 450 micrometers rostral of FN0. B, In both groups, cell body size of TH-positive, CTB-positive RVLM neurons was slightly larger in the most rostral section compared to other sections. C, There was no difference in the number of primary dendrites at any level. *, p<0.05, individual differences between sedentary and active animals. ^#, p<0.05, main effect of rostrocaudal level on soma area.