Effects on sympathetic activity of 8-OHDPAT and clonidine in cat medullary lateral tegmental field

Sex- and age-based differences in the effect of central serotonin on arterial blood pressure regulation

Medullary serotonin (5-hydroxytryptamine; 5-HT) neurons project to multiple autonomic nuclei in the central nervous system (CNS). Infant rats lacking 5-HT have low arterial blood pressure (ABP) in quiet sleep, but the role of 5-HT in ABP regulation across vigilance states in adults has not been studied. We hypothesized that in adults, CNS 5-HT deficiency leads to hypotension mainly in quiet wakefulness (QW) and non-rapid eye movement (NREM) sleep, when 5-HT neurons are active. We tested male and female tryptophan hydroxylase 2 knockout rats (TPH2-/-), specifically deficient in CNS 5-HT, and wild-type (TPH2+/+) controls at 2-3, 5-8, and 12-13 mo of age. Compared with TPH2+/+, mean arterial pressure of 5-8- and 12-13-mo-old (middle-aged) male TPH2-/- rats was significantly elevated (∼10 mmHg) in QW and rapid eye movement (REM) sleep. Middle-aged male TPH2-/- rats also had more frequent extreme hypertensive events during prolonged episodes of REM sleep. Female TPH2-/- had normal ABP. The low- and very-low-frequency components of systolic ABP variability were significantly higher in middle-aged male, but not female, TPH2-/- rats compared with in TPH2+/+ rats, suggesting elevated sympathetic vascular tone in male TPH2-/- rats. However, the hypertension of male TPH2-/- rats was not ameliorated by ganglionic blockade. Hearts and lungs of middle-aged male TPH2-/- rats were significantly heavier than those of TPH2+/+ rats. We show that a loss of CNS 5-HT leads to high ABP only in middle-aged males during wakefulness and REM sleep, possibly due to increased vascular tone. It should be investigated whether elevated ventricular afterload associated with CNS 5-HT deficiency initiates cardiac remodeling or alters pulmonary hemodynamics.NEW & NOTEWORTHY The role of serotonin in arterial blood pressure (ABP) regulation across states of vigilance is unknown. We hypothesized that adult rats devoid of CNS serotonin (TPH2-/-) have low ABP in wakefulness and NREM sleep, when serotonin neurons are active. However, TPH2-/- rats experience higher ABP than TPH2+/+ rats in wakefulness and REM only, a phenotype present only in older males and not females. CNS serotonin may be critical for preventing high ABP in males with aging.

2019 Ludwig Lecture: Rhythms in sympathetic nerve activity are a key to understanding neural control of the cardiovascular system

This review is based on the Carl Ludwig Distinguished Lecture, presented at the 2019 Experimental Biology Meeting in Orlando, FL, and provides a snapshot of >40 years of work done in collaboration with the late Gerard L. Gebber and colleagues to highlight the importance of considering the rhythmic properties of sympathetic nerve activity (SNA) and brain stem neurons when studying the neural control of autonomic regulation. After first providing some basic information about rhythms, I describe the patterns and potential functions of rhythmic activity recorded from sympathetic nerves under various physiological conditions. I review the evidence that these rhythms reflect the properties of central sympathetic neural networks that include neurons in the caudal medullary raphe, caudal ventrolateral medulla, caudal ventrolateral pons, medullary lateral tegmental field, rostral dorsolateral pons, and rostral ventrolateral medulla. The role of these brain stem areas in mediating steady-state and reflex-induced changes in SNA and blood pressure is discussed. Despite the common appearance of rhythms in SNA, these oscillatory characteristics are often ignored; instead, it is common to simply quantify changes in the amount of SNA to make conclusions about the function of the sympathetic nervous system in mediating responses to a variety of stimuli. This review summarizes work that highlights the need to include an assessment of the changes in the frequency components of SNA in evaluating the cardiovascular responses to various manipulations as well as in determining the role of different brain regions in the neural control of the cardiovascular system.

Role of the arterial baroreflex in 5-HT1A receptor agonist-mediated sympathoexcitation following hypotensive hemorrhage

5-HT1A-receptor agonists rapidly restore blood pressure and sympathetic activity in conscious rats subjected to hypotensive hemorrhage. 5-HT1A-receptor activation has also been shown to produce a robust increase in baroreceptor-dependent, pulse-synchronous firing of cardiac sympathetic nerves in anesthetized cats. To determine whether 5-HT1A-receptor agonists reverse hemorrhage-induced suppression of sympathetic activity through facilitation of the arterial baroreflex, the effects of the 5-HT1A-receptor agonist, 8-OH-DPAT, were assessed in male Sprague-Dawley rats subjected to sinoaortic baroreceptor denervation and subsequent hypotensive hemorrhage. 8-OH-DPAT produced rapid pressor and sympathoexcitatory responses in hemorrhaged animals that were attenuated, but not blocked, by sinoaortic denervation (SAD) (+49 +/- 4 vs. +37 +/- 4 mmHg; +165 +/- 30 vs. +92 +/- 24% baseline, P < 0.01). Spectral analysis of sympathetic activity showed that SAD abolished the 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT)-mediated increases in pulse-synchronous (13 +/- 1 vs. 5 +/- 1% total power for intact vs. SAD rats, P < 0.01) and Mayer wave-related bursting (18 +/- 3 vs. 8 +/- 1% total power, P < 0.05). However, 8-OH-DPAT continued to increase total power (+72 +/- 22 vs. -63 +/- 7% prehemorrhage total power, P < 0.05) and power at the respiratory frequency (35 +/- 2 vs. 25 +/- 4% total power) in SAD animals. These data indicate that full expression of the sympathoexcitatory effect of 8-OH-DPAT requires a functional arterial baroreflex. However, a portion of the effect is due to activation of arterial baroreflex-independent sympathetic pathways.

Differential effects of an NMDA and a non-NMDA receptor antagonist on medullary lateral tegmental field neurons

We microiontophoresed an N-methyl-D-aspartate (NMDA) and a non-NMDA receptor antagonist onto medullary lateral tegmental field (LTF) neurons, the naturally occurring discharges of which were correlated to the cardiac-related rhythm in sympathetic nerve discharge (SND) of dialurethane-anesthetized cats. Some of these neurons were classified as sympathoexcitatory, because their firing rate decreased during baroreceptor reflex activation. Microiontophoresis of 1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzo-[f]quinoxaline-7-sulfonamide (NBQX), a non-NMDA receptor antagonist, reduced the mean firing rates of these neurons (51 +/- 8% of control, P < 0.001, n = 20) without affecting their relationship to cardiac-related SND, as indicated by the lack of significant changes in the ratio of peak to background counts in arterial pulse (AP)-triggered histograms of LTF neuronal activity and the AP-LTF coherence value at the frequency of the heartbeat. In contrast, microiontophoresis of D(-)-2-amino-5-phosphonopentanoic acid, an NMDA receptor antagonist, onto LTF neurons reduced the ratio of peak to background counts in AP-triggered histograms to 57 +/- 9% of control (P = 0.002, n = 16) and the AP-LTF coherence value to 25 +/- 10% of control (P = 0.001, n = 10). These data support the view that non-NMDA and NMDA receptors are involved in setting the basal level of activity of LTF sympathoexcitatory neurons and in synchronizing their discharges to the AP, respectively.