The idea that serotonergic synaptic transmission plays an essential role in the control of mood and the pharmacotherapy of anxiety and depression is one of the cornerstones of modern biological psychiatry. As a result, there is intense interest in understanding the mechanisms controlling the activity of serotonin-synthesizing (serotonergic) neurons. One of the oldest and most durable ideas emerging from this work is that serotonergic neurons are capable of autonomously regulating their own basal firing rate. Serotonergic neurons express on their surface 5-HT1A receptors (autoreceptors) that, when activated, induce the opening of potassium channels that hyperpolarize and thereby inhibit cell firing. Activity-dependent release of serotonin within serotonergic nuclei is thought to activate these autoreceptors, thus completing an autoinhibitory feedback loop. This concept, which was originally proposed in the 1970s, has proven to be enormously fruitful and has guided the interpretation of a broad range of clinical and preclinical work. Yet, remarkably, electrophysiological studies seeking to directly demonstrate this phenomenon, especially in in vitro brain slices, have produced mixed results. Here, we critically review this work with a focus on electrophysiological studies, which directly assess neuronal activity. We also highlight recent work suggesting that 5-HT1A receptor-mediated autoinhibition may play other roles in the control of firing besides acting as a feedback regulator for the pacemaker-like firing rate of serotonergic neurons.

Spinally projecting neurons in the rostral ventrolateral medulla (RVLM) are believed to contribute to pathophysiological alterations in sympathetic nerve activity and the development of cardiovascular disease. The ability to identify changes in the activity of RVLM neurons in conscious animals and humans, especially longitudinally, would represent a clinically important advancement in our understanding of the contribution of the RVLM to cardiovascular disease. To this end, we describe the initial development of manganese-enhanced magnetic resonance imaging (MEMRI) for the rat RVLM. Manganese (Mn ) has been used to estimate in vivo neuronal activity in other brain regions because of both its paramagnetic properties and its entry into and accumulation in active neurons. In this initial study, our three goals were as follows: (1) to validate that Mn enhancement occurs in functionally and anatomically localized images of the rat RVLM; (2) to quantify the dose and time course dependence of Mn enhancement in the RVLM after one systemic injection in conscious rats (66 or 33 mg/kg, intraperitoneally); and (3) to compare Mn enhancement in the RVLM with other regions to determine an appropriate method of normalization of T -weighted images. In our proof-of-concept and proof-of-principle studies, Mn was identified by MRI in the rat RVLM after direct microinjection or via retrograde transport following spinal cord injections, respectively. Systemic injections in conscious rats produced significant Mn enhancement at 24 h (p < 0.05). Injections of 66 mg/kg produced greater enhancement than 33 mg/kg in the RVLM and paraventricular nucleus of the hypothalamus (p < 0.05 for both), but only when normalized to baseline scans without Mn injection. Consistent with findings from our previous functional and anatomical studies demonstrating subregional neuroplasticity, Mn enhancement was higher in the rostral regions of the RVLM (p < 0.05). Together with important technical considerations, our studies support the development of MEMRI as a potential method to examine RVLM activity over time in conscious animal subjects.

Regulation of blood pressure occurs primarily via the activity of bulbospinal rostral ventrolateral medulla (RVLM) neurons that drive sympathetic nerve activity (SNA). Central and peripheral stimuli influence the activity of RVLM neurons and are reflected in their cardiac and respiratory modulation. Dysregulation of RVLM neurons can lead to increased SNA and the development and maintenance of cardiovascular disease. In this study we used spectral (frequency‐domain) analysis to identify cardiac‐ and/or respiratory‐related (CR, RR) activity in barosensitive RVLM neurons (n=22) and splanchnic SNA of 16 Inactin‐anesthetized rats in which we also recorded arterial pressure (AP) and an index of respiration. SNA had RR activity (0.40±0.05; mean ± SE) and/or CR activity (0.63±0.05) as evidenced by a coherence value > 0.1 at the frequency of respiration and the heart rate, respectively, in the corresponding coherence functions. RVLM unit activity was also shown to be RR (0.40±0.14; n=4) or CR (0.41±0.07; n=4). Moreover, RVLM unit activity cohered to SNA at the frequency of the heart rate (0.39±0.08; n=4) or respiration (0.21; n=1). AP‐triggered analysis (time‐domain analysis) revealed CR activity in 4 other RVLM neurons. These data demonstrate our ability to study the relationships among rat RVLM unit activity, SNA, AP, and respiration, which will allow us to explore whether differences in the strengths of these relationships contribute to changes in SNA and AP under different physiological conditions.HL096787

A sedentary lifestyle, a key risk factor in cardiovascular disease (CVD), is linked to increased sympathetic nerve activity (SNA). Control of SNA occurs via bulbospinal catecholaminergic (C1) and non‐C1 neurons primarily in the RVLM. We recently reported that sedentary, but not physically active rats, exhibit increased dendritic branching in bulbospinal C1 RVLM neurons in a specific caudal to rostral pattern. However, regionally specific increases in RVLM neuronal activation have not yet been associated with increased dendritic branching. In many brain regions, manganese (Mn2+), a paramagnetic ion, has been used as a contrast agent to detect neuronal activity via calcium channel uptake. The purpose of this study was to develop MeMRI for in vivo studies of the RVLM, with the hypothesis that sedentary rats would have increased RVLM neuronal activity and greater Mn2+ uptake. One sedentary and one active rat (12 wks wheel running) were administered Mn2+ chloride (66mg/kg, IP), and a control rat was given saline. After 24 hours, rats were imaged (7T, Bruker Avance) and saline values were subtracted from Mn2+ injected animals. Uptake of Mn2+ in the RVLM was greater in the sedentary versus active rat (e.g. 808 vs. 327 avg voxel intensity, respectively). Notably, only the sedentary rat had a pattern of increased uptake in the caudal to rostral RVLM, very similar to our structural studies. These preliminary studies suggest that increased dendritic branching in more rostral RVLM neurons may account for enhanced RVLM neuronal activity and the growing prevalence of CVD in sedentary individuals. Grant Funding Source: Supported by HL096787‐PJM; VA 1l01RX001095‐01‐AGH

A sedentary lifestyle has been linked to the development of cardiovascular disease (CVD). The RVLM is believed to contribute to CVD through direct activation of sympathetic preganglionic neurons. Our previous work showed that sedentary rats exhibit enhanced sympathoexcitation after glutamate activation of the RVLM compared to active counterparts. We have also recently shown increased dendritic arborization in spinally‐projecting C1 RVLM neurons from sedentary rats. We hypothesized that the RVLM exhibits increased excitability due to structural and functional differences in sedentary versus active rats (14 weeks wheel running). To our knowledge, we are the first group to have performed in vivo single unit recordings from putative presympathetic RVLM neurons in sedentary and active rats to examine functional differences in presympathetic RVLM neurons. Preliminary results from Inactin‐anesthetized rats indicate that individual presympathetic RVLM neurons from sedentary rats appeared to have higher resting firing rates (14.0±5.1 Hz, n=4) compared to active rats (8.7±3.0 Hz, n=4). RVLM neurons also appeared to have greater responses to acute increases in blood pressure (∆‐11.4±5.3 Hz vs. ∆‐5.6±1.4 Hz, respectively), but decreases in blood pressure did not appear to elicit different responses (∆+3.1±1.4 Hz vs. ∆+2.9±0.3 Hz). Though our current sample size does not yet permit us to draw definitive conclusions, these preliminary data support our hypothesis and suggest that a sedentary lifestyle may increase sympathetic nerve activity by causing an increase in the activity of individual presympathetic RVLM neurons. Grant Funding Source: HL096787, WSU Rumble Fellowship