Role of dorsal medullary reticular formation in maintenance of vasomotor tone in rats

Chronic Pain-Associated Cardiovascular Disease: The Role of Sympathetic Nerve Activity

Chronic pain affects many people world-wide, and this number is continuously increasing. There is a clear link between chronic pain and the development of cardiovascular disease through activation of the sympathetic nervous system. The purpose of this review is to provide evidence from the literature that highlights the direct relationship between sympathetic nervous system dysfunction and chronic pain. We hypothesize that maladaptive changes within a common neural network regulating the sympathetic nervous system and pain perception contribute to sympathetic overactivation and cardiovascular disease in the setting of chronic pain. We review clinical evidence and highlight the basic neurocircuitry linking the sympathetic and nociceptive networks and the overlap between the neural networks controlling the two.

Cardiovascular responses to chemical stimulation of the lateral tegmental field and adjacent medullary reticular formation in the rat

Relatively few studies have been done to characterize cardiovascular responses to the chemical stimulation of sites located in the medullary lateral tegmental field (LTF) and most of them have been carried out in anesthetized animals. Our experiments were carried out in decerebrated, artificially ventilated, adult male Wistar rats. In the LTF, two types of cardiovascular responses were elicited. One type consisted of pressor responses accompanied by bradycardia. Such responses were elicited from a region 0.4 mm caudal to 0.8 mm rostral to the calamus scriptorius (CS); maximum responses were elicited from a site 0.6 mm rostral to the CS, 1.2 mm lateral to the midline and 1.2 mm deep from the dorsal medullary surface. Another type consisted of pressor responses without any change in heart rate; such responses were elicited from a region 1-1.6 mm rostral to the CS. Nucleus ambiguus (nAmb) and dorsal motor nucleus of the vagus (nDMX) and the reticular formation surrounding these areas were the main sites from which bradycardia (accompanied by either no or small changes in BP) was elicited. In the nAmb, maximum bradycardia was elicited from a site 0.6 mm rostral to the CS, 1.8 mm lateral to the midline and 2.4 mm deep from the dorsal medullary surface. In the nDMX, most prominent bradycardic responses were elicited at 0-0.6 mm rostral to the CS, and 0.6 mm lateral to the midline and 1 mm deep from the dorsal medullary surface. Cardiovascular effects elicited from sites in other well-known areas, such as the rostral ventrolateral medullary pressor area (RVLM) and caudal ventrolateral medullary depressor area (CVLM), and the nucleus tractus solitarius (nTS) were also included for comparison of different responses. These results are expected to prove useful in studies in which the microinjection technique is used to characterize cardiovascular responses.

Medullary lateral tegmental field: an important source of basal sympathetic nerve discharge in the cat

We used blockade of excitatory amino acid (EAA) neurotransmission in the medullary lateral tegmental field (LTF) and rostral ventrolateral medulla (RVLM) to assess the roles of these regions in the control of inferior cardiac sympathetic nerve discharge (SND) and mean arterial pressure (MAP) in urethan-anesthetized, baroreceptor-denervated cats. Bilateral microinjection of a non-N-methyl-D-aspartate (NMDA)-receptor antagonist [1,2,3, 4-tetrahydro-6-nitro-2,3-dioxobenzo-[f]quinoxaline-7-sulfonamide (NBQX)] into the LTF significantly decreased SND to 46 +/- 4% of control (as demonstrated with power-density spectral analysis) and MAP by 16 +/- 6 mmHg. In contrast, bilateral microinjection of an NMDA-receptor antagonist [D(-)-2-amino-5-phosphonopentanoic acid (D-AP5)] into the LTF did not decrease SND or MAP. These results demonstrate that the LTF is an important synaptic relay in the pathway responsible for basal SND in the cat. Bilateral microinjection of NBQX or D-AP5 into the RVLM significantly decreased power in SND to 48 +/- 5 or 61 +/- 5% of control, respectively, and reduced MAP by 15 +/- 2 or 8 +/- 4 mmHg, respectively. These data indicate that EAA-mediated synaptic drive to RVLM-spinal sympathoexcitatory neurons accounts for a significant component of their basal activity.

Rhythmic sympathetic nerve discharges in an in vitro neonatal rat brain stem-spinal cord preparation

To understand the origination of sympathetic nerve discharge (SND), I developed an in vitro brain stem-spinal cord preparation from neonatal rats. Ascorbic acid (3 mM) was added into the bath solution to increase the viability of preparations. At 24 degrees C, rhythmic SND (recorded from the splanchnic nerve) was consistently observed, but it became quiescent at <16 degrees C. Respiratory-related SND (rSND) was discernible and was well correlated with C(4) root activity. Power spectral analysis of SND revealed a dominant 2-Hz oscillation. In most preparations (86%), such oscillation was persistent, whereas it only slightly reduced its magnitude after isolation from the brain stem. The removal of neural structures rostral to the superior cerebellar artery (equivalent to the level of facial nuclei) reduced rSND, increased tonic SND, but did not affect the temporal coupling between SND and C(4) root activity. Our data suggest a prominent contribution of SND from the neural mechanisms confined within the neonatal rat spinal cord. This ascorbic acid-enhanced in vitro preparation is a very useful model to study neural mechanisms underlying sympathorespiratory integration.