Intrathecal melanin-concentrating hormone reduces sympathetic tone and blocks cardiovascular reflexes

Melanin-concentrating hormone (MCH) is a neuropeptide that acts to increase feeding behavior and decrease energy expenditure. The role of MCH in central cardiorespiratory regulation is still poorly understood. Experiments were conducted on urethane-anesthetized, vagotomized, and artificially ventilated male Sprague-Dawley rats ( n = 22) to ascertain whether MCH modulates sympathetic vasomotor tone, as well as barosympathetic, chemosympathetic, and somatosympathetic reflexes at the level of the spinal cord. Intrathecal injection of 10 μl of MCH produced a dose-dependent hypotension, bradycardia, and sympathoinhibition. Peak response was observed following administration of 1 mM MCH, causing a decrease in mean arterial pressure of 39 ± 2 mmHg ( P < 0.001), splanchnic sympathetic nerve activity of 78 ± 11% ( P < 0.001), and heart rate of 87 ± 11 beats per minute (bpm) ( P < 0.01). The two peaks of the somatosympathetic reflex were decreased by intrathecal MCH, 7 ± 3% ( P < 0.01) and 31 ± 6% ( P < 0.01), respectively, and the spinal component of the reflex was accentuated 96 ± 23% ( P < 0.05), with respect to the baseline for MCH, compared with the two peaks and spinal component of the somatosympathetic reflex elicited following saline injection with respect to the baseline for saline. MCH decreased the sympathetic gain to 120 s of hyperoxic hypercapnea (10% CO2 in 90% O2) and to 10–12 s poikilocapneic anoxia (100% N2) from 0.74 ± 0.14%/s to 0.23 ± 0.04%/s ( P < 0.05) and 16.47 ± 3.2% to 4.35 ± 1.56% ( P < 0.05), respectively. There was a 34% decrease in gain and a 62% decrease in range of the sympathetic baroreflex with intrathecal MCH. These data demonstrate that spinal MCH blunts the central regulation of sympathetic tone and adaptive sympathetic reflexes.

Patterning of somatosympathetic reflexes reveals nonuniform organization of presympathetic drive from C1 and non-C1 RVLM neurons

To determine the organization of presympathetic vasomotor drive by phenotypic populations of rostral ventrolateral medulla (RVLM) neurons, we examined the somatosympathetic reflex (SSR) evoked in four sympathetic nerves together with selective lesions of RVLM presympathetic neurons. Urethane-anesthetized (1.3 g/kg ip), paralyzed, vagotomized and artificially ventilated Sprague-Dawley rats ( n = 41) were used. First, we determined the afferent inputs activated by sciatic nerve (SN) stimulation at graded stimulus intensities (50 sweeps at 0.5–1 Hz, 1–80 V). Second, we recorded sympathetic nerve responses (cervical, renal, splanchnic, and lumbar) to intensities of SN stimulation that activated A-fiber afferents (low) or both A- and C-fiber afferents (high). Third, with low-intensity SN stimulation, we examined the cervical SSR following RVLM microinjection of somatostatin, and we determined the splanchnic SSR in rats in which presympathetic C1 neurons were lesioned following intraspinal injections of anti-dopamine-β-hydroxylase-saporin (anti-DβH-SAP). Low-intensity SN stimulation activated A-fiber afferents and evoked biphasic responses in the renal, splanchnic, and lumbar nerves and a single peak in the cervical nerve. Depletion of presympathetic C1 neurons (59 ± 4% tyrosine hydroxylase immunoreactivity profiles lesioned) eliminated peak 2 of the splanchnic SSR and attenuated peak 1, suggesting that only RVLM neurons with fast axonal conduction were spared. RVLM injections of somatostatin abolished the single early peak of cervical SSR confirming that RVLM neurons with fast axonal conduction were inhibited by somatostatin. It is concluded that unmyelinated RVLM presympathetic neurons, presumed to be all C1, innervate splanchnic, renal, and lumbar but not cervical sympathetic outflows, whereas myelinated C1 and non-C1 RVLM neurons innervate all sympathetic outflows examined. These findings suggest that multiple levels of neural control of vasomotor tone exist; myelinated populations may set baseline tone, while unmyelinated neurons may be recruited to provide actions at specific vascular beds in response to distinct stressors.

Catestatin in rat RVLM is sympathoexcitatory, increases barosensitivity, and attenuates chemosensitivity and the somatosympathetic reflex

The fundamental role and corollary effects of neuropeptides that govern cardiorespiratory control in the brain stem are poorly understood. One such regulatory peptide, catestatin [Cts, human chromogranin A-(352–372)], noncompetitively inhibits nicotinic-cholinergic-stimulated catecholamine release. Previously, we demonstrated the presence of chromogranin A mRNA in brain stem neurons that are important for the maintenance of arterial pressure. In the present study, using immunofluorescence histochemistry, we show that Cts immunoreactivity is colocalized with tyrosine hydroxylase in C1 neurons of the rostral ventrolateral medulla (RVLM, n = 3). Furthermore, we investigated the effects of Cts on resting blood pressure, splanchnic sympathetic nerve activity, phrenic nerve activity, heart rate, and adaptive reflexes. Cts (1 mM in 50 nl or 100 μM in 50–100 nl) was microinjected into the RVLM in urethane-anesthetized, vagotomized, ventilated Sprague-Dawley rats ( n = 19). Cardiovascular responses to stimulation of carotid baroreceptors, peripheral chemoreceptors, and the sciatic nerve (somatosympathetic reflex) were analyzed. Cts (1 mM in 50 nl) increased resting arterial pressure (28 ± 3 mmHg at 2 min postinjection), sympathetic nerve activity (15 ± 3% at 2 min postinjection), and phrenic discharge amplitude (31 ± 4% at 10 min postinjection). Cts increased sympathetic barosensitivity 40% (slope increased from −0.05 ± 0.01 before Cts to −0.07 ± 0.01 after Cts) and attenuated the somatosympathetic reflex [1st peak: 36% (from 132 ± 32.1 to 84.0 ± 17.0 μV); 2nd peak: 44% (from 65.1 ± 21.4 to 36.6 ± 14.1 μV)] and chemoreflex (blood pressure response to anoxia decreased 55%, sympathetic response decreased 46%). The results suggest that Cts activates sympathoexcitatory bulbospinal neurons in the RVLM and plays an important regulatory role in adaptive reflexes.

Galanin microinjection into rostral ventrolateral medulla of the rat is hypotensive and attenuates sympathetic chemoreflex

Galanin is present in neurons in the brain that are important in the control of arterial pressure, and intracisternal administration of galanin evokes hypotension, but the site of action is unknown. In urethane-anesthetized, vagotomized mechanically ventilated Sprague-Dawley rats ( n = 34), we investigated the effects of microinjecting galanin (1 mM, 50 nl, 50 pmol) into the rostral ventrolateral medulla on resting splanchnic sympathetic nerve activity, arterial pressure, heart rate, and phrenic nerve activity. Second, we determined the effect of microinjecting galanin into the rostral ventrolateral medulla on the cardiovascular response to stimulation of central and peripheral chemoreceptors, arterial baroreceptors, and the somatosympathetic reflex. Galanin caused a prolonged reduction in resting splanchnic sympathetic nerve activity (−37.0 ± 7.2% of baseline), mean arterial pressure (−17.0 ± 3.5 mmHg), and heart rate (−25.0 ± 9.1 beats/min). Galanin increased the sympathoinhibitory response to aortic depressor nerve stimulation by 51.8%, had no effect on the somatosympathetic reflex, and markedly attenuated the effect of hypercapnia and hypoxia on arterial pressure (by 65% and 92.4% of control, respectively). These results suggest a role for galanin neurotransmission in the integration of the cardiovascular responses to hypoxia, hypercapnia, and the sympathetic baroreflex in the rostral ventrolateral medulla. The data suggest that galanin may be an important peptide in the homeostatic regulation of chemosensory reflexes.

The somato-sympathetic reflex in the spontaneous hypertensive rats

In chloralose anesthetized and paralyzed spontaneously hypertensive rats (SHR) a somatosympathetic reflex in the cervical sympathetic trunk elicited by a single electrical shock to forelimb afferent fibres in the median nerve was recorded. It has been shown that the somatosympathetic reflex consists of two responses and following silent period. The А-response evoked by the somatic myelinated afferent fibres stimulation, and C-respon.se elicited by the both stimulation of myelinated and unmyelinated afferent fibres. The silent period occurred with the myelinated fibres stimulation. Its duration was proportional to the electrical shock amplitude. The А-response consisted of four waves, the three of them formed early and late responses.