Hypotensive spinal serotonergic effect. Are S1 or S2 receptors involved?

Excitatory and indirect inhibitory actions of 5-hydroxytryptamine on sympathetic preganglionic neurones in the neonate rat spinal cord in vitro

The action of 5-hydroxytryptamine (5-HT) on sympathetic preganglionic neurones (SPN) was studied by intracellular recordings in thin slices of neonatal rat spinal cord in vitro. Superfusion of 5-HT (1-270 microM) to SPN caused a concentration dependent slow depolarization or inward current and an increase in synaptic activity consisting of both EPSPs and IPSPs. The slow depolarization was still present after superfusion with TTX. Similar effects were seen during superfusion with 5-carboxamidotryptamine (5-CT) or alpha-methyl-5-hydroxytryptamine (alpha-me-5-HT). A comparison with the potency of 5-HT was made for 5-CT or alpha-me-5-HT on the same neurone by determining the magnitude of the slow depolarization to different concentrations of agonist. This showed that the apparent potency of the agonists was 5-CT > 5-HT > alpha-me-5-HT even in the presence of fluoxetine, a 5-HT uptake inhibitor. The 5-HT-induced slow depolarization was partially blocked by ketanserin but full recovery was not observed. The results suggest that the excitatory action of 5-HT on SPN is mediated via an atypical 5-HT2 receptor or a 5-HT1C-like receptor. The 5-HT-induced IPSPs were reversibly blocked by superfusion with strychnine, suggesting they were mediated by glycine.

Activation of 5-HT2 receptors inhibits the evoked release of [3H]noradrenaline in the rat spinal cord

1. The participation of 5-HT2 receptors in the modulation of the evoked release of [3H]noradrenaline from rat spinal cord slices has been examined. 2. In rat spinal cord slices preincubated with [3H]noradrenaline, the alpha 2-receptor agonist clonidine (10(-6) mol/l) decreased the release of tritium evoked by field stimulation (600 pulses at 5 Hz, 20 mA, 2 msec), while the alpha 2-antagonist yohimbine (10(-6) mol/l) increased it. 3. The 5-HT2/5-HT1c agonist DOI (3 x 10(-7) mol/l) decreased the evoked release of tritium, an effect which was prevented by ketanserin (10(-7) mol/l). 4. It is suggested that in addition to presynaptic alpha 2-adrenoceptors, there are 5-HT2 receptors which modulate the release of noradrenaline in the rat spinal cord.

5-Hydroxytryptamine, substance P and thyrotropin-releasing hormone synapses in the intermediolateral cell column of the rat thoracic spinal cord

Serotonin-, substance P-, and thyrotropin-releasing hormone-immunoreactive profiles were studied in the intermediolateral cell column at the thoracic level of the rat spinal cord with light- and electron-microscopic immunocytochemistry. For each transmitter, a dense immunoreactive deposit was observed with the light microscope. At ultrastructural level, morphologically identified synapses amounted to 47% of all serotonergic varicosities, to 49% for substance P and 50% for thyrotropin-releasing hormone. Synapses appeared both symmetrical and asymmetrical. In each case, these synapses were mainly axodendritic (98%). These synaptic connections could mediate the physiological influence of these 3 substances in the spinal cord on the cardiovascular system.

Autoradiographic mapping of 5-HT1, 5-HT1A, 5-HT1B and 5-HT2 receptors in the rat spinal cord

The distribution of 5-HT1, 5-HT1A, 5-HT1B and 5-HT2 receptors in the rat spinal cord was investigated with quantitative autoradiography. Receptors were labeled respectively with [3H]serotonin (5-[3H]HT),8- hydroxy-2-[N-dipropylamino-3H]tetralin (8-OH-[3H]DPAT), [125I]iodocyanopindolol and [3H]ketanserin. It is shown that 5-HT1, 5-HT1A and 5-HT1B receptors are distributed within the spinal cord according to a rostro-caudal gradient. Both 5-HT1 and 5-HT1A receptors are mainly present in the dorsal horn and 5-HT1B is present throughout the spinal cord, exhibiting high densities in the caudal-most part of the dorsal horn in lamina X and in the sacral parasympathetic area. On the other hand, 5-HT2 receptors are shown mostly in the thoracic sympathetic area and in the thoracic ventral horn; the dorsal horn exhibits few 5-HT2 receptors. The differential involvement of 5-HT through different receptors in nociception, autonomous nervous system control and motility are discussed.

Examination of the effects of some 5-HT2 receptor antagonists on central sympathetic outflow and blood pressure in anaesthetised cats

Thoracic preganglionic sympathetic nerve activity, blood pressure, heart rate and femoral arterial conductance were recorded in anaesthetised, paralysed cats. Cumulative dose response curves were constructed for the 5-HT2 antagonists cinanserin, ritanserin, cyproheptadine, methiothepin, metergoline and ICI 169,369. These antagonists showed differing effects on the above parameters. Methiothepin and cyproheptadine caused hypotension and sympathoinhibition at low and high doses, whilst metergoline and ritanserin caused these effects at high doses, above 1 mg kg-1. Cinanserin and ICI 169,369 did cause large transient changes in heart rate, blood pressure and sympathetic outflow. However, all the antagonists except ICI 169,369 and methiothepin caused an increase in femoral arterial conductance which was not associated with hypotension. It was therefore concluded that using the above antagonists it was difficult to attribute changes in blood pressure and central sympathetic outflow to blockade of 5-HT2 receptors. However, it is suggested that 5-HT2 receptors may be involved in the control of skeletal muscle and/or skin vascular beds.