The mechanism of hypertension and bradycardia following lesions of the caudal ventrolateral medulla in the rabbit: the role of sympathetic nerves, circulating adrenaline, vasopressin and renin

The central autonomic nervous system: conscious visceral perception and autonomic pattern generation

The overall organization of the peripheral autonomic nervous system has been known for many decades, but the mechanisms by which it is controlled by the central nervous system are just now coming to light. In particular, two major issues have seen considerable progress in the past decade. First, the pathways that provide visceral sensation to conscious perception at a cortical level have been elucidated in both animals and humans. The nociceptive system runs in parallel to the pathways carrying visceral sensation from the cranial nerves and may be considered in itself a component of visceral sensation. Second, structures in the central nervous system that generate patterns of autonomic response have been identified. These pattern generators are located at multiple levels of the central nervous system, and they can be combined in temporal and spatial patterns to subserve a wide range of behavioral needs.

Angiotensin converting enzyme inhibition improves baroreflex-induced noradrenaline spillover responses in rabbits with heart failure

Impaired baroreflex function is a characteristic feature of congestive heart failure (CHF), although the mechanism is obscure. This study examined the hypothesis that activation of the renin-angiotensin system contributes to baroreflex dysfunction in CHF. The acute effects of an angiotensin converting enzyme inhibitor, enalaprilat, on baroreflex-mediated changes in heart rate (HR), total and renal noradrenaline (NA) spillover rates were examined in conscious rabbits with doxorubicin-induced cardiomyopathic CHF. Studies were performed under resting conditions and in response to changes in mean arterial pressure (MAP) induced by sodium nitroprusside and phenylephrine infusions. Seven saline-treated (normal group) and 11 doxorubicin-treated rabbits (1 mg/kg administered intravenously twice weekly) were studied after 4 and 6 weeks' treatment. Five CHF rabbits received saline (C group) and 6 enalaprilat infusion (ACEI group) during each study period. After 4 weeks of doxorubicin, baroreflex-HR responses were normal, whereas baroreflex-NA spillover responses were enhanced. Enalaprilat infusion shifted the HR-MAP curve downwards to the left but had no effect on the NA spillover-MAP curves. After 6 weeks of doxorubicin, when CHF was established, baroreflex-HR and NA spillover curves were depressed. At this stage, enalaprilat had little effect on the HR-MAP curve but restored towards normal the NA spillover-MAP curves. The results suggest that the endogenous renin-angiotensin system contributes to attenuated baroreflex responses when CHF is established.

Cardiovascular effects of microinjections of quipazine into nuclei of the medulla oblongata in anaesthetized cats: comparison with L-glutamate

Unilateral microinjections of quipazine (0.9 micrograms in 50 nl) into the subretrofacial nucleus produced hypertension and a slight tachycardia associated with an increase in renal sympathetic nerve activity. Microinjections of quipazine lateral, caudal or rostral to this nucleus failed to alter blood pressure and heart rate. Similarly, microinjections of l-glutamate (3 nmol in 15 nl) into the subretrofacial nucleus elicited hypertension, tachycardia and renal sympatho-excitation. The magnitude of the pressor response to quipazine was smaller than the response elicited by l-glutamate but its duration was longer. Microinjections of quipazine into the lateral tegmental field at l-glutamate hypertensive sites failed to alter arterial blood pressure and heart rate. In contrast, microinjections of quipazine into the caudal ventrolateral medulla or into the nucleus tractus solitarii produced hypotension and sympatho-inhibition. These effects were prevented by microinjections of the 5-HT2 receptor antagonists, LY 53857 or BW 501C. The present results indicate that stimulation of 5-HT2 receptors of the subretrofacial nucleus produces hypertension and sympatho-excitation whereas stimulation of 5-HT2 receptors in the caudal ventrolateral medulla and in the nucleus tractus solitarii produces hypotension and sympatho-inhibition.

Localization of the main noradrenergic neuron groups in the pons and medulla of the rabbit and the importance of cathodal lesions for prolonged survival

Methods for stereotaxically localizing the major noradrenergic (NA) cell groups (i.e. A1, A2, A5 and A6 + A7) in the rabbit are described. Using a modified Kopf head holder we used surface landmarks including the obex for making lesions of the A1 and A2 cells in the medulla. Localization of the pontine cell groups was done by mapping intracerebral structures including the facial nerve for A5 and the motor nucleus of the trigeminal nerve for A6 + A7. In the initial experiments we made A1 lesions by passing anodal currents through stainless steel electrodes, which was associated with pulmonary oedema, neurological complications and a high mortality. This syndrome was probably related to toxic effects of ferric ion deposition, and disappeared when cathodal currents were employed. We have now made 106 bilateral cathodal lesions in the different groups, with a 20% intraoperative mortality. But virtually all survivors remained indefinitely in clinically good condition for the 2-4 weeks duration of our experiments. In 65 of these rabbits we achieved greater than 75% of NA cell destruction (average 84%). From the cardiovascular viewpoint 'non-specific' damage by the lesions was relatively small, except after A2 lesions where there was some impairment in the baroreceptor-heart rate reflex, though a considerable amount of residual function remained.

Role of renal nerve activity, plasma catecholamines and plasma vasopressin in cardiovascular responses to intracisternal neurotoxins in the rabbit

We have examined the acute (0-3 h) effect of intracisternally administered 5,7-dihydroxytryptamine (DHT) and 6-hydroxydopamine (6-OHDA) on blood pressure, heart rate, renal nerve activity, plasma adrenaline, plasma noradrenaline and plasma vasopressin in conscious rabbits. The increase in blood pressure seen following 5,7-DHT treatment was associated with increases in adrenaline and vasopressin levels and renal nerve activity throughout the response. The increase in blood pressure which followed 6-OHDA administration was associated with an increase in renal nerve activity alone. These findings indicate that the rise in blood pressure elicited by these drugs involves an increase in sympathetic nerve activity. The absence of a rise in vasopressin levels during the response to 6-OHDA suggests that the rise in blood pressure seen in these animals is due entirely to a bulbospinal sympathoexcitatory pathway.