Alpha-1-adrenergic receptors in the nucleus tractus solitarii region of rats with experimental and genetic hypertension

Alpha2-adrenoceptor and adenosine A1 receptor within the nucleus tractus solitarii in hypertension development

Alpha2-adrenoceptor and A1 adenosine receptor systems within the nucleus tractus solitarii (NTS) play an important role in cardiovascular control. Deregulation of these systems may result in an elevated sympathetic tone, one of the root causes of neurogenic hypertension. The dorsomedial/dorsolateral and subpostremal NTS subnuclei of spontaneously hypertensive rats (SHR) show density changes in both receptors, even at 15 days of age, prior to the onset of hypertension. In addition, adenosine A1 receptors have been specifically reported to modulate alpha2-adrenoceptors in several brain regions, including the NTS, via a PLC-dependent pathway involving cross regulation between sympathetic neurons and astrocytes. The physiological cross talk between these receptor systems is also deregulated in SHR suggesting that alpha2-adrenoceptor and A1 adenosine receptor might be germane to the development of hypertension. In this review, we will focus on these systems within the NTS during development, pointing out some interesting modulations in processes, and chemical changes within specific subnuclei of NTS circuitry, that might have implications for neurogenic hypertension.

Viral vectors as tools for studies of central cardiovascular control

During the last few years physiological genomics has been the most rapidly developing area of physiology. Given the current ease of obtaining information about nucleotide sequences found in genomes and the vast amount of readily available clones, one of the most pertinent tasks is to find out about the roles of the individual genes and their families under normal and pathological conditions. Viral gene delivery into the brain is a powerful tool, which can be used to address a wide range of questions posed by physiological genomics including central nervous mechanisms regulating the cardio-vascular system. In this paper, we will give a short overview of current data obtained in this field using viral vectors and then look critically at the technology of viral gene transfer.

Effect of phenylephrine injected into the nucleus tractus solitarius of Sprague-Dawley rats and spontaneously hypertensive rats

In alpha-chloralose-anesthetized Sprague-Dawley (SD) rats with unilateral nucleus tractus solitarius (NTS) lesions, injection of the alpha(1)-adrenergic receptor agonist phenylephrine into the contralateral NTS dose-dependently increased arterial pressure (AP). Bunazosin (0.1 nmol) or prazosin (0.36 nmol), an alpha(1)-adrenergic receptor antagonist, also increased AP. When injected into the NTS, pre-treatment with phenylephrine (10 nmol) or both antagonists abolished the cardiovascular effects of glutamate and acetylcholine. In contrast, pre-treatment with prazosin or methylatropine did not alter the effect of phenylephrine. Phenylephrine (30 nmol) injected into the NTS abolished aortic depressor nerve (ADN) evoked-responses. The pressor effect of phenylephrine in the NTS was exaggerated in spontaneously hypertensive rats (SHR). These results suggest that when injected into the NTS, the effect of phenylephrine may be due to a baroreflex blockade resulting from direct modulatory actions or non-specific neuronal alterations rather than stimulating the alpha(1)-adrenergic receptor. Additionally, this effect is enhanced in SHR.

Aortic barodenervation up-regulates alpha2-adrenoceptors in the nucleus tractus solitarius and rostral ventrolateral medulla: an autoradiographic study

Earlier findings have shown that alpha2-adrenoceptors in the nucleus tractus solitarius and rostral ventrolateral medulla modulate baroreflexes. The present study investigated whether attenuation of baroreflexes induced by surgical interruption of aortic baroafferents is related to an alteration of alpha2-adrenoceptor binding in these regions of the brainstem. In vitro autoradiography was utilized to assess the density and binding dissociation constant (affinity) of alpha2-adrenoceptors in the rostral ventrolateral medulla and in the middle and rostral portions of the nucleus tractus solitarius of aortic-barodenervated and sham-operated rats. Compared to sham operation, aortic barodenervation caused an acute rise in mean arterial pressure and heart rate and a significant reduction in baroreflex sensitivity. Two days later, mean arterial pressure and heart rate of conscious aortic-barodenervated rats subsided to sham-operated levels, whereas the baroreflex sensitivity remained significantly (P<0.05) reduced when measured by phenylephrine (0.55+/-0.08 vs 1.26+/-0.07 ms/mmHg) or nitroprusside (0.43+/-0.06 vs 1.01+/-0.09ms/mmHg). Examination of brainstem coronal sections obtained from separate groups of rats 48 h after surgery and preincubated with [3H]rauwolscine (0.5-16 nM) revealed that labeling of alpha2 binding sites was saturable and of high affinity. Scatchard analysis of the saturation isotherms obtained from the three brain areas of sham-operated rats showed an uneven distribution of alpha2 binding sites; the rostral nucleus tractus solitarius exhibited the highest density and lowest affinity. Aortic barodenervation caused region-dependent changes in the binding activity of alpha2-adrenoceptors. These changes comprised significant (P<0.05) increases in the density of alpha2-adrenoceptors in the middle nucleus tractus solitarius (436+/-60 vs 240+/-50 fmol/mg protein) and rostral ventrolateral medulla (350+/-67 vs 194+/-35 fmol/mg protein) compared with sham-operated rats; no significant changes occurred in the rostral nucleus tractus solitarius. The affinity of alpha2 binding sites was not changed by aortic barodenervation in any of the three brain regions. These findings suggest that attenuation of baroreflexes produced by aortic barodenervation coincides with up-regulation of alpha2-adrenoceptors in brainstem areas that play critical roles in the control of cardiovascular functions.

Short-term aortic barodenervation diminishes alpha 1-adrenoceptor reactivity in rat aortic smooth muscle

Our previous studies have shown that aortic baroreceptor denervation elicits acute increases in blood pressure and significant elevations of sympathetic activity and peripheral vascular resistance. This study investigated the short-term (3 and 48 h) effect of aortic barodenervation and associated sympathetic hyperactivity on the functional activity of alpha 1-adrenoceptors in rat aortic smooth muscle. Compared with sham operation, aortic barodenervation caused acute rises in blood pressure and heart rate and reductions in baroreflex sensitivity. Blood pressure and heart rate remained elevated when measured in conscious aortic barodenervated rats 3 h after surgery but subsided to sham-operated levels at 48 h; the baroreflex sensitivity, however, remained attenuated. Hexamethonium (0.5-4 mg/kg, i.v.) elicited significantly (P < 0.05) greater depressor responses in conscious aortic barodenervated rats than in sham-operated rats at both 3 and 48 h, suggesting a higher sympathetic activity in denervated rats. Exposure of aortic rings from aortic barodenervated and sham-operated rats to cumulative addition of phenylephrine (alpha 1-adrenoceptor agonist, 3 x 10(-8)-1 x 10(-4) M) resulted in concentration related contractile responses that were similar in the two groups of rats at 3 h in contrast to significantly (P < 0.05) smaller contractions in rings from denervated rats at 48 h. The maximum contraction developed (Emax) at 48 h showed approximately 50% reduction in rings from aortic barodenervated compared with sham-operated rats (239 +/- 16 vs. 558 +/- 15 mg tension/mg tissue). The pA2 value for prazosin (alpha 1-adrenoceptor antagonist) was not altered by aortic barodenervation at 3 h but showed significant (P < 0.05) increases, compared with sham-operated values, at 48 h. It is concluded that short-term aortic barodenervation results in an elevation of sympathetic activity that coincides with reduced responsiveness of aortic smooth muscle to alpha 1-adrenoceptor activation. The aortic barodenervation-induced alpha 1-adrenoceptor desensitization is not a result of decreased receptor affinity but may involve an alteration of receptor density or in the post-receptor activation events.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.