5-Hydroxytryptamine produces presynaptic facilitation of cholinergic transmission in rabbit parasympathetic ganglia

Social interactions determine postural network sensitivity to 5-HT

The excitability of the leg postural circuit and its response to serotonin (5-HT) were studied in vitro in thoracic nervous system preparations of dominant and subordinate male crayfishes. We demonstrate that the level of spontaneous tonic activity of depressor and levator motoneurons (MNs) (which control downward and upward movements of the leg, respectively) and the amplitude of their resistance reflex are larger in dominants than in subordinates. Moreover, we show that serotonergic neuromodulation of the postural circuit also depends on social status. Depressor and levator MN tonic firing rates and resistance reflex amplitudes were significantly modified in the presence of 10 mum 5-HT in dominants but not in subordinates. Using intracellular recording from depressor MNs, we show that their input resistance was not significantly different in dominants and subordinates in control conditions. However, 5-HT produced a marked depolarization in dominants and a significantly weaker depolarization in subordinates. Moreover, in the presence of 5-HT, the amplitude of the resistance reflex and the input resistance of MNs increased in dominants and decreased in subordinates. The peak amplitude and the decay phase of unitary EPSPs triggered by sensory spikes were significantly increased by 5-HT in dominants but not in subordinates. These observations suggest that neural networks are more reactive in dominants than in subordinates, and this divergence is even reinforced by 5-HT modulation.

Reciprocal stimulation of decay between serotonergic facilitation and depression of synaptic transmission

Serotonin can produce multiple, contradictory modulatory effects on strength of synaptic transmission in both vertebrate and invertebrate nerve circuits. In crayfish, serotonin (5-HT) can both facilitate and depress transmission to lateral giant escape command neurons; however, which effect is manifest during application, as well as the sign and duration of effects that may continue long after 5-HT washout, may depend on history of application as well as on concentration. We report that protein kinase A (PKA) signaling is essential to the production of facilitation but depression is mediated by non-cAMP/PKA signaling pathways. However, we unexpectedly found that PKA activity is essential for the decay of depression when serotonin is washed out. This, and evidence from the effects of a variety of serotonin application regimens, suggest that facilitatory and depressive states coexist and compete and that the decay of each is dependent on stimulation by the other. A computational model that incorporates these assumptions can account for and rationalize the varied effects of a wide range of serotonin application regimens.

Pharmacology of serotonin as related to anesthesia

Serotonin (5-hydroxytryptamine) is an important biogenic amine that fulfills the role of neurotransmitter and neuromodulator. It has been a focus of interest during the last decade. Its diversity of pharmacologic actions is related to a wide variety of receptors and effector mechanisms. Seven serotonin receptor families have been identified thus far. They are genetically different transmembrane proteins composed of several hundred amino acids. The majority of these are G-protein-coupled, except the 5-HT3 receptors, which are directly ligand gated to fast ion channels. Serotonin is widely distributed in the body within the central and peripheral nervous systems, smooth muscles, and platelets, in particular. Consequently, its effects manifest mainly in these organs and influence a wide variety of neural, vascular, smooth muscle, and platelet functions. (Melatonin, a physiologically active metabolite of serotonin, is also instrumental in affecting many neural and hormonal functions.) Several selective agonists and particularly many selective antagonists have been developed for serotonin, which helped the serotonin receptor subtype classification. Some of these drugs are also used therapeutically in the treatment of migraine (eg, sumatriptan, which is a 5-HT1 receptor agonist), vascular disorders (5-HT2 antagonists), and nausea and vomiting (5-HT3 antagonists, eg, dolasetron, granisetron, ondansetron, and tropisetron), and have been investigated in gastrointestinal motility disorders (5-HT4 antagonists) and behavioral psychopathologies (5-HT1 agonists and 5-HT2-4 antagonists). Serotonin reuptake inhibitors are of particular clinical importance in the treatment of psychological illness. Future use of these drugs is also envisioned in the treatment of certain types of pain syndromes. Awareness of the serotonergic drugs and the recognition of possible drug interactions among drugs that influence serotonergic mechanisms in humans are becoming increasingly important in the practice of anesthesiology.

5-HT2 receptors augment cholinergic nerve-mediated contraction of rat bronchi

5-Hydroxytryptamine (5-HT) potentiated contractions of isolated rat bronchi evoked by electrical field stimulation (EFS). The degree of potentiation caused by 5-HT was dependent upon concentration of the amine present in the tissue bath. The effects of antagonists selective for different subtypes of the 5-HT receptor on potentiation of EFS-induced contractions by 5-HT were examined. Propranolol, a nonselective beta-adrenoceptor antagonist which can act as a 5-HT1 receptor antagonist, did not inhibit the effect of 5-HT on EFS-induced contractile responses. Similarly, 5-HT3 receptor antagonism with MDL 72222 or ICS 205-930, did not inhibit the facilitatory effects of 5-HT. However, ketanserin, mianserin and spiperone, 5-HT2 receptor antagonists, abolished the effects of 5-HT on EFS-induced responses. These latter results suggested that the potentiation was dependent upon activation of 5-HT2 receptors thus additional experiments were conducted using the 5-HT2 receptor agonist, alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT). alpha-Me-5-HT caused a concentration-dependent potentiation of EFS-induced contractile responses comparable to that observed with 5-HT. Concentrations of alpha-Me-5-HT that significantly potentiated EFS-induced contraction were essentially without effect on airway smooth muscle contraction elicited by exogenous acetylcholine. These results are consistent with a role for 5-HT2 receptor activation in mediating the facilitatory effects of 5-HT on cholinergic nerve-mediated responses in airways.

GABA and glutamate-like immunoreactivity in processes presynaptic to afferents from hair plates on the proximal joints of the locust leg

Hair plate afferents from coxal group 1 on the meso- and prothoracic legs of the locust were backfilled with cobalt salts or HRP for light and electron microscopy. The distribution of the terminal branches of the afferents is described from wholemount preparations and from 150 microns thick slices through the ganglion. Identified branches from the slices were sectioned for electron microscopy and examined for the presence of input and output synapses. Both were found in close proximity on small-diameter varicose branches in all parts of the arborization. Immunocytochemistry using antibodies against GABA and glutamate was used to try to identify putative transmitters in processes presynaptic to the afferents. Ninety-three percent of processes presynaptic to the hair plate afferents were clearly immunoreactive for GABA and only 7% appeared unlabelled. Most neuronal processes in the vicinity of afferent terminals were also immunoreactive for GABA, but a small number of glutamate-immunoreactive processes were found in intimate contact with afferents and one of these was demonstrated to be presynaptic. Processes postsynaptic to the afferents were of small diameter (mean = 0.28 micron) and were not found to be immunoreactive for GABA.

Effect of nifedipine on neurons of guinea pig celiac ganglion

The effect of nifedipine on electrophysiological membrane properties and nicotinic neurotransmission of guinea pig celiac ganglion neurons was studied using intracellular recordings in vitro. Nifedipine in concentrations of 0.1-10 microM did not affect membrane potential, membrane input resistance or the amplitude and duration of action potentials induced by intracellular current injection. Higher doses of nifedipine (0.1-1 mM) significantly reduced the amplitude and extended the duration of action potentials induced by intracellular current injection. Superfusion of the ganglia with nifedipine in concentrations of 0.1-10 microM significantly inhibited nicotinic fast excitatory postsynaptic potentials (f-EPSPs) and orthodromic action potentials evoked by nerve stimulation. This depressant effect of nifedipine on synaptic transmission was eliminated with high Ca2+ (12.5 mM). Nifedipine (10 microM) did not affect the postsynaptic effect of exogenous acetylcholine (ACh), but significantly reduced the quantal content but not the quantal size of evoked f-EPSPs in a low Ca2+ (0.5 mM), high Mg2+ (5.5 mM) Krebs solution. Nifedipine in concentration of 10 microM did not affect afterspike hyperpolarization (AH) and post-tetanic hyperpolarization (PTH), which have been recognized to be generated mainly by an increase of calcium-dependent potassium conductance. Higher doses of nifedipine (0.1-1 mM) significantly depressed AH and PTH. These experimental results suggest that nifedipine in concentrations of 0.1-10 microM exerts an inhibitory effect on nicotinic neurotransmission without affecting the membrane properties of the guinea pig celiac ganglion neurons. This inhibitory effect of nifedipine on synaptic transmission may result from blocking L-type calcium channels and reducing the quantal release of ACh from the presynaptic nerve terminals.

5-Hydroxytryptamine depresses reticulospinal excitatory postsynaptic potentials in motoneurons of the lamprey

Application of 5-hydroxytryptamine (5-HT) to the lamprey spinal cord in vitro reversibly depressed the chemical component of excitatory post-synaptic potentials recorded intracellularly in motoneurons and evoked by stimulation of single reticulospinal Müller cells. The depression could be produced either by local application of small volumes of 10 mM 5-HT to the surface of the spinal cord or by bath-application of 1 or 10 microM 5-HT. No effect on the input resistance of the postsynaptic cells or their sensitivity to glutamate, the suspected transmitter at this synapse, could be detected, suggesting the possibility of a presynaptic action of 5-HT at this synapse in the lamprey.

Alpha 2-adrenoceptors mediate the inhibition of cholinergic transmission in parasympathetic ganglia of the rabbit urinary bladder

Intracellular recordings were made from neurons of vesical parasympathetic ganglia (VPG) isolated from the rabbit urinary bladder and maintained, in vitro. Bath-application of norepinephrine (NE, 500 nM-5 microM) caused a hyperpolarizing response at the postsynaptic membrane of VPG neurons in a concentration-dependent manner. NE blocked the action potential elicited by an orthodromic stimulation of preganglionic (pelvic) nerve fibers. At a relatively low concentration (5-100 nM), NE depressed the fast excitatory postsynaptic potential (EPSP), without producing the hyperpolarization. NE (100 nM) produced 49 +/- 17% (N = 5) decrease in the amplitude of the fast EPSP. NE did not depress the acetylcholine (ACh) potential produced by iontophoretic application of ACh to the ganglion cells. NE did not affect the amplitude of the miniature EPSP, while it reduced the frequency of miniature EPSPs. These results suggest that NE inhibits the nicotinic transmission in the rabbit VPG, probably reducing the ACh release from presynaptic nerve terminals. Epinephrine (1 microM) was more potent than NE (1 microM) in producing the hyperpolarization as well as the blockade of the fast EPSP amplitude. Isoproterenol was ineffective as an agonist for these inhibitory adrenoceptors. Clonidine mimicked the effect of NE on the fast EPSP. Yohimbine and idazoxan antagonized both the inhibition of the fast EPSP and the hyperpolarization produced by NE. These results suggest that alpha 2-adrenoceptors are responsible for the inhibition of the neuronal activity in parasympathetic ganglia of the rabbit urinary bladder. Immunohistochemical study demonstrated the presence of tyrosine hydroxylase (TH)-labelled neuronal elements in the VPG. They were a small proportion of principal neurons, their dendrites, and many varicose fibers.(ABSTRACT TRUNCATED AT 250 WORDS)