Indiana University-Purdue University Indianapolis (IUPUI) / The sympathetic nervous system strongly modulates the contractile and electrical function of the heart. The anatomical underpinnings that enable a spatially and temporally coordinated dissemination of sympathetic signals within the cardiac tissue are only incompletely characterized. In this work we took the first step of unraveling the in situ 3D microarchitecture of the cardiac sympathetic nervous system. Using a combination of two-photon excitation fluorescence microscopy and computer-assisted image analyses, we reconstructed the sympathetic network in a portion of the left ventricular epicardium from adult transgenic mice expressing a fluorescent reporter protein in all peripheral sympathetic neurons. The reconstruction revealed several organizational principles of the local sympathetic tree that synergize to enable a coordinated and efficient signal transfer to the target tissue. First, synaptic boutons are aligned with high density along much of axon-cell contacts. Second, axon segments are oriented parallel to the main, i.e., longitudinal, axes of their apposed cardiomyocytes, optimizing the frequency of transmitter release sites per axon/per cardiomyocyte. Third, the local network was partitioned into branched and/or looped sub-trees which extended both radially and tangentially through the image volume. Fourth, sub-trees arrange to not much overlap, giving rise to multiple annexed innervation domains of variable complexity and configuration. The sympathetic network in the epicardial border zone of a chronic myocardial infarction was observed to undergo substantive remodeling, which included almost complete loss of fibers at depths >10 µm from the surface, spatially heterogeneous gain of axons, irregularly shaped synaptic boutons, and formation of axonal plexuses composed of nested loops of variable length. In conclusion, we provide, to the best of our knowledge, the first in situ 3D reconstruction of the local cardiac sympathetic network in normal and injured mammalian myocardium. Mapping the sympathetic network connectivity will aid in elucidating its role in sympathetic signal transmisson and processing.

Indiana University-Purdue University Indianapolis (IUPUI) / Gender differences in the basal function of autonomic cardiovascular control are well documented. Consistent baroreflex (BRx) studies suggest that women have higher tonic parasympathetic cardiac activation compared to men. Later in life and concomitant with menopause, a significant reduction in the capacity of the BRx in females increases their risk to develop hypertension, even exceeding that of age-matched males. Loss of sex hormones is but one factor. In female rats, we previously identified a distinct myelinated baroreceptor (BR) neuronal phenotype termed Ah-type, which exhibits functional dynamics and ionic currents that are a mix of those observed in barosensory afferents functionally identified as myelinated A-type or unmyelinated C-type. Interestingly, Ah-type afferents constitute nearly 50% of the total population of myelinated aortic BR in female but less than 2% in male rat. We hypothesized that an afferent basis for sexual dimorphism in BRx function exists. Specifically, we investigated the potential functional impact Ah-type afferents have upon the aortic BRx and what changes, if any, loss of sex hormones through ovariectomy brings upon such functions. We assessed electrophysiological and reflexogenic differences associated with the left aortic depressor nerve (ADN) from adult male, female, and ovariectomized female (OVX) Sprague-Dawley rats. Our results revealed sexually dimorphic conduction velocity (CV) profiles. A distinct, slower myelinated fiber volley was apparent in compound action potential (CAP) recordings from female aortic BR fibers, with an amplitude and CV not observed in males. Subsequent BRx studies demonstrated that females exhibited significantly greater BRx responses compared to males at myelinated-specific intensities. Ovariectomy induced an increased overall temporal dispersion in the CAP of OVX females that may have contributed to their attenuated BRx responses. Interestingly, the most significant changes in depressor dynamics occurred at electrical thresholds and frequencies most closely aligned with Ah-type BR fibers. Collectively, we provide evidence that, in females, two anatomically distinct myelinated afferent pathways contribute to the integrated BRx function, whereas in males only one exists. These functional differences may partly account for the enhanced control of blood pressure in females. Furthermore, Ah-type afferents may provide a neuromodulatory pathway uniquely associated with the hormonal regulation of BRx function.