BDNF/TrkB Signaling in Intracardiac Ganglia Modulates Cardiac Parasympathetic Tone

Figure 1. Neural crest-specific BDNF deletion increases resting heart rate without affecting baseline cardiac function. (A) Representative M-mode echocardiographic recordings from control and neural crest-specific BDNF knockout (ncBDNF KO) mice. (B) Quantification of the resting heart rate. Data are shown as box-and-whisker plots with individual values, n = 6. Statistical comparisons were performed using unpaired two-tailed Student’s t-test (Welch’s correction where appropriate). p < 0.05 was considered statistically significant.

Figure 2. VAChT immunostaining identifies cholinergic intracardiac ganglia in adult mouse heart. Representative low- and high-magnification images of cardiac sections immunolabeled for vesicular acetylcholine transporter (VAChT), a marker of cholinergic neurons. Low-magnification imaging (left) shows the anatomical localization of an intracardiac ganglion (arrow) within the epicardial fat at the atrial base. Higher magnification (right) reveals clustered VAChT-positive neuronal somata within the ganglion, confirming the cholinergic identity of intrinsic cardiac neurons. Scale bars: 1 mm (left), 250 μm (right).

Figure 3. TrkB is predominantly associated with the satellite glial compartment of murine intracardiac ganglia. Representative immunofluorescence images of an intracardiac ganglion labeled for the satellite glial marker S100 (A), TrkB (B), and the neuronal marker MAP2 (C), with merged channels shown in (D). TrkB immunoreactivity displays a prominent perineuronal distribution and is predominantly associated with the S100-positive glial network surrounding MAP2-positive neuronal somata.

Figure 4. BDNF-positive neurons reside within a synaptically organized intracardiac ganglionic microenvironment. Representative immunofluorescence images of murine intracardiac ganglia labeled for TrkB (A), S100 (B), BDNF (D,G), and the presynaptic vesicle-associated protein synaptophysin (Syn; (E,H)). TrkB and S100 display similar perineuronal distribution patterns within ganglionic structures (C), whereas BDNF immunoreactivity is predominantly localized within neuronal somata (D,G). Double immunolabeling for BDNF and the synaptic vesicle-associated protein synaptophysin revealed distinct localization patterns within intracardiac ganglia (F,I). Synaptophysin immunoreactivity was observed in presumed presynaptic processes surrounding BDNF-positive neuronal cell bodies (F,I). Synaptophysin-positive nerve fibers were also detected in the adjacent atrial tissue (panel (I), white arrows), where BDNF immunoreactivity was absent. Similarly, TrkB immunoreactivity was not detected in atrial Schwann cells (panel (C), white arrows) or nerve fibers.

Figure 5. Acute BDNF infusion modulates cardiac function in isolated perfused hearts. (A) Isolated-perfused heart scheme, BDNF or vehicle administration. (B) Heart rate (HR); (C) left ventricular developed pressure (LVDevP); (D) rate-pressure product (RPP), expressed as percentage change from baseline, was not significantly altered by BDNF; (E) maximal dP/dt, expressed; and (F) minimal dP/dt. All parameters expressed as percentage change from baseline after 10 min from infusion of BDNF or Vehicle. Data are shown as box-and-whisker plots with individual values, n = 4–6. Comparisons between groups were performed using the non-parametric Mann–Whitney U test. Statistical significance is indicated in the individual panels.

Figure 6. Muscarinic blockade abolishes the chronotropic effect of BDNF while not affecting its positive inotropic actions in isolated perfused hearts. (A) Experimental setting: Langendorff-perfused mouse hearts (constant flow) and protocols (baseline, atropine, and BDNF administration). (B) Heart rate (HR); (C) left ventricular developed pressure (LVDevP); (D) rate-pressure product (RPP); (E) maximal dP/dt and (F) minimal dP/dt, at baseline, after atropine, and after atropine + BDNF. Data are shown as paired individual values (gray lines) with mean ± SEM, n = 8. Comparisons across experimental conditions were performed using a non-parametric repeated-measures ANOVA (Friedman test). Statistical significance is indicated in the individual panels.

Figure 7. Cholinesterase inhibition potentiates the bradycardic effect of BDNF in isolated perfused hearts. (A) Isolated-perfused heart scheme, neostigmine and BDNF administration. (B) Heart rate (HR); (C) left ventricular developed pressure (LVDevP); (D) rate-pressure product (RPP); (E) maximal dP/dt; (F) minimal dP/dt. Data are shown as paired individual values (gray lines) with mean ± SEM, n = 5. Comparisons across experimental conditions were performed using a non-parametric repeated-measures ANOVA (Friedman test). Statistical significance is indicated in the individual panels.

Figure 8. Conceptual model for BDNF-dependent neuro-glial modulation of parasympathetic signaling within intracardiac ganglia. Within intracardiac ganglia (ICG), BDNF released by cholinergic neurons is proposed to act locally on TrkB-expressing satellite glial cells, establishing a tripartite neuro-glial signaling unit. Through TrkB-dependent mechanisms, glial cells may modulate the ganglionic microenvironment by regulating ionic homeostasis, gliotransmitter release, metabolic support, or secondary trophic signaling, ultimately facilitating acetylcholine (ACh) release from postganglionic parasympathetic neurons. Increased cholinergic transmission at the neuroeffector junction enhances activation of muscarinic receptors in sinoatrial node pacemaker cells, resulting in reduced heart rate and increased heart rate variability.