The localization of adrenoceptors and opiate receptors in regions of the cat central nervous system involved in cardiovascular control

Descending noradrenergic pathways involved in the A5 depressor response

The objective of the present study was to analyze the anatomical basis of the A5 depressor response and to test if the putative neurotransmitter noradrenaline is involved in the response. Two approaches were used; one was neuroanatomical and the other was pharmacological. First, the retrograde transport method in which two fluorescent markers (Fast blue and rhodamine microspheres) was used in combination with the indirect immunofluorescence technique to establish that A5 catecholamine neurons project to both the spinal cord and the region of the nucleus tractus solitarii (NTS). Second, we analyzed the effects of 6-hydroxydopamine (6-OHDA) lesions of the spinal cord and/or NTS area on the A5 depressor response. This response was elicited by a 80-nl microinjection of L-glutamate (500 mM) into the A5 region in pentobarbital anesthetized rats; it was characterized by a decrease in blood pressure and heart rate. After destruction of various noradrenergic terminal fields we have found that intraspinal injections of 6-OHDA caused a 30% reduction in the blood pressure component of the A5 depressor response and a transient depression of the bradycardic response. This result suggests that only a small portion of the A5 depressor response depends on the descending A5 spinal pathway. Injections of 6-OHDA into the NTS region caused a transient depression of the A5 depressor response, and by 7-14 days postinjection, the response returned to normal. After combined 6-OHDA injections into the spinal cord and NTS area, the blood pressure and heart rate components of the A5 depressor response were reduced to 80% of the control level at 3 days postinjection. By 14 days, even with severe depletion of noradrenaline in the spinal cord (96%) and a moderate depletion of noradrenaline in the NTS (50%), the A5 response was restored to about 80% of its original magnitude, suggesting some type of functional recovery occurs in this system. Third, the blood pressure decrease elicited by L-glutamate stimulation of the A5 cell group was unaffected by pharmacological blockade of the heart. In addition, this response appeared to be normal in rats that had both their autonomic supply to the heart blocked pharmacologically and their spinal cord noradrenaline levels depleted (14 days after intraspinal 6-OHDA injections). These data suggest that the major A5 depressor response operates mainly by inhibition of the sympathetic outflow involved in control of total peripheral resistance and that this system is controlled by a descending spinal pathway which probably does not use noradrenaline as a neurotransmitter.

A comparison of the effects of doxazosin and alfuzosin with those of urapidil on preganglionic sympathetic nerve activity in anaesthetised cats

Preganglionic sympathetic nerve activity, blood pressure, heart rate and femoral arterial conductance were recorded in anaesthetised, paralysed cats. Urapidil, doxazosin and alfuzosin were infused i.v. for 1 h into different animals at the rate of 2 mg kg-1 h-1. All three drugs caused a fall in thoracic preganglionic sympathetic nerve activity along with blood pressure. Although urapidil had a greater hypotensive action than doxazosin and alfuzosin its sympathoinhibitory action was delayed and weaker. Since all three drugs are alpha 1-adrenoceptor antagonists it appears that antagonism at this receptor type may cause central sympathoinhibition as well as a decrease in total peripheral resistance. However, the different effects of urapidil suggest that its action on central sympathetic tone and therefore its hypotensive action cannot be due entirely to its ability to block alpha 1-adrenoceptors.

Autoradiographic localization of alpha-adrenoceptors, muscarinic acetylcholine receptors and opiate receptors in the heart

Using in vitro autoradiography the distribution of [3H]rauwolscine, [3H]prazosin and [3H]quinuclidinyl benzilate binding sites has been demonstrated in cardiac tissue taken from the cat and rat. A similar distribution of both alpha 1- and alpha 2-adrenoceptor sites was seen but the distribution of muscarinic acetylcholine sites was markedly different. alpha-Adrenoceptors were present predominantly in ventricular muscle whereas muscarinic acetylcholine receptors exhibited a greater density in atrial tissue compared to ventricular muscle. Opiate receptors were absent from cardiac tissue.