Lateral tegmental field involvement in the central sympathoinhibitory action of 8-OH-DPAT

Effects on sympathetic activity of 8-OHDPAT and clonidine in cat medullary lateral tegmental field

This study was designed to test the hypothesis that 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) and clonidine reduce sympathetic nerve discharge (SND) and mean arterial pressure (MAP), in part by actions in the medullary lateral tegmental field (LTF). We microinjected these drugs bilaterally into the LTF of baroreceptor-innervated and -denervated cats anesthetized with Dial-urethane. Neither drug altered SND (as quantified by using power spectral analysis) or MAP when injected into the LTF of baroreceptor-denervated cats. However, cardiac-related power in SND was significantly increased to 148 +/- 12 (mean +/- SE) and 149 +/- 5% of control by microinjections of 8-OHDPAT (n = 5) and clonidine (n = 5), respectively, in baroreceptor-innervated cats whose MAP was kept constant; there was no change in 0- to 6-Hz power or total power. SND was significantly reduced by microinjection of these drugs into the rostral ventrolateral medulla of baroreceptor-innervated and -denervated cats. In conclusion, although 8-OHDPAT and clonidine did not reduce SND when injected into the LTF, they acted in this region to facilitate baroreceptor reflex control of SND, as evidenced by a selective increase in cardiac-related power.

Rapid and long duration tolerance to the vagal bradycardic effects of 5-HT1A receptor agonists

Administration of a single dose of the 5-HT1A receptor high intrinsic agonist U-93385E (either 0.3, 1.0 or 3.0 mg/kg, i.v.) results in a 20-30% decrease in heart rate. In contrast, cumulative dosing of U-93385E (0.01-3.0 mg/kg, i.v.) failed to lower heart rate in the spinal cat. Similarly, infusion of 1 mg/kg of U-93385E over a 2 h period failed to lower heart rate and prevented a bradycardic effect of a single bolus dose of U-93385E or flesinoxan. In contrast, the alpha 2-receptor agonist clonidine decreased heart rate in animals receiving the U-93385E infusion. Single bolus doses of flesinoxan or U-93385E failed to decrease heart rate in cats treated for 7 days with U-93385E (3 mg/kg, b.i.d.) and then saline for 3 days. Similarly, U-93385E failed to lower heart rate 12 days following a 14 day infusion of U-93385E (1 mg/kg per day). These data indicate that a rapid and long duration tolerance develops to the vagal bradycardia produced by 5-HT1A receptor agonists.

Evidence against the involvement of the nucleus tractus solitarii in the sympatholytic effect of 8-hydroxy-2-(di-n-propylamino)tetralin in the cat

In different animal species, microinjections of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) into the nucleus tractus solitarii failed to alter arterial blood pressure and sympathetic nerve activity; however, the cardiovascular effects (hypotension, bradycardia, reduction in sympathetic nerve activity) of intravenous administration of 8-OH-DPAT were significantly reduced after blockade of the nucleus tractus solitarii by kainic acid as well as after blockade of the lateral tegmental field by kainic acid. The aim of the present study was to clarify these conflicting results. In the anesthetized cat, inhibition of neurotransmission in the nucleus tractus solitarii by bilateral microinjections of either muscimol (1 nmol in 50 nl) or kynurenic acid (2.5 nmol in 50 nl) suppressed the baroreceptor reflex and abolished the synchronism between the renal sympathetic bursts; however, these procedures did not alter the dose-related hypotension, bradycardia and sympatho-inhibition elicited by cumulative doses of 8-OH-DPAT (1-30 micrograms/kg i.v.). Moreover complete electrolytic destruction of the nucleus tractus solitarii, assessed by a complete loss of the baroreceptor reflex and the cardiac-related bursts of the sympathetic nerves, failed to alter the inhibitory effects of i.v. 8-OH-DPAT. Bilateral microinjections of muscimol into the lateral tegmental field induced a decrease of mean arterial blood pressure, heart rate and renal nerve activity (by respectively -35 +/- 13 mm Hg, -30 +/- 16 beats/min and -53 +/- 14%) and greatly reduced the effects of subsequent i.v. administration of 8-OH-DPAT. The present data indicate that the nucleus tractus solitarii does not play a dominant role in the central action of 8-OH-DPAT whereas they confirm our previous results showing that the lateral tegmental field is involved in this action and in the mecanisms regulating sympathetic tone. The results also suggest that kainic acid lesions are not restricted to the region in which the neurotoxic agent is injected.

Serotoninergic and nonserotoninergic neurons in the medullary raphe system have axon collateral projections to autonomic and somatic cell groups in the medulla and spinal cord

Fluorescent double retrograde-tracing studies combined with fluorescent immunostaining for serotonin were carried out to determine the potential patterns of divergence in axonal projections to autonomic and somatic motor sites from medullary raphe and parapyramidal neurons. Injections (20-60 nl) of combinations of fluorescent retrograde tracers (Fast Blue, fluoro-gold, green latex microspheres, Diamidino Yellow) were made into the intermediolateral cell column (IML) of the spinal cord and the brainstem lateral tegmental field or ventral horn of the lumbar spinal cord of male Wistar rats. The animals were perfused after a 7-10-day survival period, and the brains were removed, sectioned (50 microns), and immunostained for serotonin. Following injections of different retrograde-tracer substances into the IML of the thoracic cord and the ventral horn of the lumbar cord, 36% of the neurons with axon collateral projections to the IML and the lumbar ventral horn were serotoninergic. Following injections of different retrograde-tracer substances into the IML and the lateral tegmental field, 26% of the neurons with axon collateral projections to the IML and the lateral tegmental field were serotoninergic. Many of the medullary neurons with projections to the lateral tegmental field and the lumbar cord were located dorsal and lateral to those neurons with projections to the IML. The results indicate that serotoninergic and nonserotoninergic neurons of the midline raphe system and parapyramidal region have axon collateral branches to the IML and the lateral tegmental field or the IML and the lumbar ventral horn. These projection neurons may form the anatomical substrate for the integration of autonomic and somatic motor activity.