Effects of tryptophan on serotonin in nerve endings.

—Preparations of crude synaptosome fractions (P₂) from the telencephalon and from the diencephalon plus optic lobes of the pigeon and from the telencephalon of the rat were used to study the effects of l -tryptophan on (a) the levels of serotonin (5-HT), norepinephrinc (NE) and dopamine in nerve endings and (b) the release of radioactive 5-HT, NE and dopamine from nerve endings. The level of 5-HT was significantly higher (P < 0–05) in the P₂ fraction isolated from the telencephalon of pigeons given intramuscular injections of 300mg/kg of l -tryptophan in comparison to control values (1.11 ± 0.09 vs 0.74 ± 0.13 nmol/g original tissue wt). A smaller but not statistically significant increase in 5-HT was noted in the P₂ fractions isolated from the diencephalon plus optic lobes of pigeons given injections of l -tryptophan. In vitro studies using preparations of synaptosomes (from both pigeon and rat) labelled with [³H]5–HT demonstrated that 1.0 mm -l -tryptophan caused a 30% increase (P < 0.05) in the release of [³H]5-HT over control values. This effect by l -tryptophan was blocked when a decarboxylase inhibitor was added to the medium. Tryptophan had no effect on the levels of NE or dopamine in these nerve endings nor did it have any effect on the release of these two amines from these preparations of synaptosomes. The results are discussed in terms of the role of serotonin in producing depression in pigeons working on a certain learned behavioural task.     

Effects of 5-hydroxytryptophan on serotonin in nerve endings

—Preparations of synaptosomes (P₂) from the telencephalon and from the diencephalon plus optic lobes of the pigeon and from the telencephalon of the rat were used to study the effects of 5-hydroxytryptophan (5-HTP) on (a) the levels of serotonin (5-HT) in nerve endings and (b) the release of 5-HT from nerve endings. The levels of 5-HT were significantly higher (3.21 × 0.35 nmol/g original tissue weight) in the P₂ fraction isolated from the telencephalon of pigeons given intramuscular injections of 50mg/kg of d ,l -5-HTP in comparison to control values (1.42 ± 0.07). A similar twofold increase was observed with the P₂ fraction isolated from the diencephalon plus optic lobes. In addition, the levels of 5-HTP and 5-hydroxyindoleacetic acid also increased significantly in these P₂ fractions isolated from pigeons given d ,l -5-HTP injections in comparison to values obtained for pigeons given saline injections. In vitro studies using preparations of synaptosomes (from both pigeon and rat) labelled with [³H]5-HT indicated that 0.10 mil l -5-HTP increased the release of [³H]5-HT twofold over control values. A concentration as low as 0.001 mm l -5-HTP was tested on the P₂ fraction from the telencephalon of the pigeon and was found to significantly increase the release of [³H]5-HT over control values. This effect by l -5-HTP was blocked if a decarboxylase inhibitor was added to the medium. l -5-HTP at a concentration of 1.5 mm had no apparent effect on the release of [³H]norepinephrine or [³H]dopamine from synaptosomes prepared from the telencephalon of the rat or pigeon. The results are discussed in terms of the role of serotonin in producing certain types of behavioral depressions exhibited by pigeons and rats given injections of 5-HTP.     

The ultrastructure of the individual nerve endings of the tonic fibers in the frog

A regular diminution of specific areas of synaptic contacts and active zones (AZs) along the terminals towards the distal parts has been first shown. The discontinuity was found in the terminal nerve branch apposition to the muscle fiber as a result of periodic intercalation of the Schwann cell and its processes into the synaptic gap. A significant variability in shapes of active zones lengthways the terminal is revealed. It is suggested that all these structural features may reflect the functional properties of synapses on the tonic muscle fibres.     

Carrier-mediated and carrier-independent release of serotonin from isolated central nerve endings

The release of serotonin elicited by Ca²⁺-dependent stimuli (depolarization, ionophore A23187) from rat brain synaptosomes previously labelled with the radioactive indoleamine was not affected by the presence of the serotonin carrier blocker chlorimipramine. In contrast, other releasing stimuli, such as superfusion with a Na⁺-free medium or exposure to various releasing drugs (fenfluramine, p-chloroamphetamine, tryptamine and mianserin, both in normal Krebs-Ringer medium and in low-Na⁺ medium), evoked efflux of serotonin from nerve endings which was prevented by chlorimipramine. The results indicate that serotonin can be released from central nerve endings by two mechanisms, differentially affected by the blockade of the membrane carrier system: the characteristics of the Ca²⁺-dependent release are compatible with an exocytotic mechanism, whereas the release induced by lack of Na⁺ or by phenylethylamines and tryptamine appears to occur by outward transport mediated by the membrane carrier.     

Calciumm-dependent release of endogenous serotonin,dopamine and norepinephrine from nerve endings

The release of endogenous serotonin, dopamine, norepinephrine and 5-hydroxyindoleacetic acid was studied in static incubations of synaptosome (P₂) preparations from the telencephalon of the rat. Elevated potassium medium specifically stimulated the release of the biogenic monoamines while the deaminated metabolite of serotonin was not effected. The release of the monoamines was also sensitive in part to the presence of calcium in the incubation medium.     

Changes in the ultrastructure of Merkel's nerve endings of the sinus hair of rats after denervation

Merkel nerve endings of sinus hairs in the upper lip of the rat were observed after having cut the nervus infraorbitalis. The nerve terminal of the Merkel nerve ending has already been degenerated 24 hours after denervation and was phagocytised by neighbouring keratinocytes. The Merkel cells did not change in structure after having lost their nerve terminal even within 169 days after nerve crush. Their position in the stratum basale of the sinus hairs remained constant; the number, size and position of the osmiophilic granules in the cytoplasm of the Merkel cells did not change. These results may show, that Merkel cells are not modified keratinocytes. The possibilities of their origin are discussed.     

Characteristics of electrical activity in different sections of nerve endings in the frog

In experiments on frog sartorius neuromuscular preparations, the evoked electrical responses of nerve endings were recorded by extracellular microelectrodes. It was shown that in proximal parts of the nerve ending, the three-phase response (+ - +) with a high amplitude negative phase occurred due to motor nerve stimulation. With movement of the extracellular electrode in distal direction a certain increase of the initial positive phase and a significant decrease of the negative one were observed. At the end of the terminal that response transformed to the monophasic one (+). On local iontophoretic application of tetrodotoxin (TTX) to the recording site two components of the nerve ending response were revealed: TTX-insensitive and TTX-sensitive. A significant decrease of the TTX-sensitive component occurred along the course of the nerve ending. That component was absent from the distal synaptic areas. It is concluded that in frog nerve ending, the action potential propogates with decrement while depolarization of the end parts of the terminal is passive in nature.     

Primary afferent responses of a crustacean mechanoreceptor are modulated by proctolin, octopamine, and serotonin

Modulation of sensory responses recorded intracellularly in primary sensory afferents of a crustacean proprioceptor is described. The neuropeptide proctolin enhances the sensory response, whereas the bioamines octopamine and serotonin depress it. The lobster oval organ of the second maxilla, a simple stretch receptor lacking centrifugal control, provides a useful model for studies on nonsynaptic modulation at peripheral sensory loci. Its three large afferents, X, Y, and Z, were prepared for intracellular recording and tested under five experimental conditions: (1) when fully rested, (2) when adapted to maintained stretch and firing tonically, (3) when showing reduced responses after habituation to repetitive stimulation, (4) not stretched but depolarized with current injections, (5) after TTX blockade. The results, taken together, indicate that conductances contributing to the overall amplitude of the receptor potential are major targets for modulators. Thus proctolin increased receptor potential amplitudes with consequent augmentation of spiking, whereas serotonin and octopamine depressed the receptor potentials, often to subthreshold levels with loss of spiking. Octopamine was a less potent agent than serotonin and failed to act upon fibers under TTX blockade. Fibers Y and Z consistently showed sensitivity to the modulators tested. The largest fiber, X, typically was resistant to proctolin, octopamine, and serotonin. Threshold concentrations of 10(-10)-10(-11) M determined in vitro are well below the circulating levels for serotonin and octopamine found in vivo. Proctolin, however, is usually not detectable in the hemolymph, and it is suggested that a significant site of proctolin release may be the oval organ itself.     

Modulation of swimming activity in the medicinal leech by serotonin and octopamine

1. The monoamines serotonin (5-HT) and octopamine (OA) enhance the expression of swimming activity in the medicinal leech (Willard, 1981; Belanger and Orchard, 1988). We explored further the effects of these monoamines and related agents on swimming activity observed in isolated leech nerve cords. 2. We confirmed that swimming activity is induced reversibly following exposure of the nerve cord to 5-HT (50 microM); the half-maximal rate of swimming activity develops in about 15 min. Swimming activity returns to control levels about 30 min after drug washout. 3. Swim-induction by 5-HT is blocked by the presence of 10 microM cyproheptadine (a 5-HT antagonist). 4. Although apparently less effective than 5-HT, OA application to nerve cords also induced swimming activity. 5. Depletion of endogenous amines from nerve cords by acute exposure to reserpine (10-150 microM) blocked stimulus-evoked swimming activity within 4 hr. 6. Subsequent application of 5-HT (50 microM) or OA (100 microM) reinstated stimulus-evoked swimming and induced repeated episodes of non-triggered swimming activity. 7. Application of cAMP and cAMP analogs, as well as phosphodiesterase inhibitors (theophylline and IBMX), mimicked the effects of the monoamines, suggesting that 5-HT and OA may activate swimming activity by increasing neuronal cAMP. 8. We obtained episodes of swim-like activity from individual, isolated ganglia exposed to 5-HT or OA. Such episodes were usually brief, with variable cycle period. 9. We conclude that individual nerve cord ganglia contain the complete neuronal circuitry required to generate the rudiments of swimming activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of serotonin and octopamine on behavioral arousal in the crayfish

1. In the crayfish, behavioral arousal is known to elicit walking and to enhance compensatory eye movements.2. To see if serotonin and octopamine modulate arousal, we measured their effects on walking and eye movements in tethered crayfish, Procambarus clarkii. Serotonin strongly suppresses both walking and eye movements.3. In contrast, octopamine elicits an arousal-like state of continuous jittery leg movements and increased eye movements.4. Serotonin's effect on arousal is uncertain, but octopamine remains a plausible modulator of behavioral arousal.     

Behavioral functions of serotonin and octopamine: some paradoxes of comparative physiology

Evaluation of recent data on monoaminergic control of invertebrate behavior suggests that behavioral functions of serotonin in molluscs and octopamine in insects are remarkably similar. Specifically, in the respective taxa, these monoamines are responsible for activation of food searching and intense locomotion, increase in food consumption and general activity, enhancement of cardial and respiratory rhythms, facilitation of learning, sensitization of sensory circuits. At the same time, in insects, behavioral effects of serotonin are opposite to those of octopamine. It seems thus that two monoamines have exchanged their behavioral roles in the two major invertebrate taxa. Possible reasons of this paradoxical inversion touches inevitably upon basic questions of signal molecular evolution.     

Distribution and ultrastructure of pyramidal tract endings in the cat rhombencephalon

The distribution and ultrastructure of terminals of corticofugal fibers in the cat rhombencephalon were investigated under the optical and electron microscopes at different periods (2–6 days) of experimental degeneration evoked by destruction of the sensomotor cortex. It was shown by the Fink–Heimer method that most degenerating fibers are distributed in the reticular nuclei of the pons and medulla. Massive degeneration of corticofugal fibers also was observed in the nuclei of the dorsal columns (nuclei of Goll and Burdach). Most of the degenerating (the "pale" type of degeneration) axo-dendritic and axo-somatic synapses in the gigantocellular reticular nucleus and the nucleus of Goll retained spherical vesicles. Small endings were found on the branches of the dendrites in which degenerative changes were of the "dark" type. The topography of the degenerating elements and axo-axonal synapses was studied in large areas of sections by the coordinate grid method. The dimensions of most degenerating axons in the gigantocellular reticular nucleus were greater (1.5 µ) than those of the degenerating axons (0.5 µ) in the nucleus of Goll. Most endings of pyramidal fibers and axo-axonal synapses are located in the central part of the nucleus of Goll at a depth of 0.5–1.2 mm from the brain surface. The results are discussed in connection with electrophysiological studies of the mechanisms of cortical control over unit activity of the reticular formation of the brain stem and nuclei of the dorsal columns.     

Effects of combined treatment with mephedrone and methamphetamine or 3,4-methylenedioxymethamphetamine on serotonin nerve endings of the hippocampus

Aims

Mephedrone is a stimulant drug of abuse with close structural and mechanistic similarities to methamphetamine and 3,4-methylenedioxymethamphetamine (MDMA). Although mephedrone does not damage dopamine nerve endings it increases the neurotoxicity of amphetamine, methamphetamine and MDMA. The effects of mephedrone on serotonin (5HT) nerve endings are not fully understood, with some investigators reporting damage while others conclude it does not. Presently, we investigate if mephedrone given alone or with methamphetamine or MDMA damages 5HT nerve endings of the hippocampus.

Main methods

The status of 5HT nerve endings in the hippocampus of female C57BL mice was assessed through measures of 5HT by HPLC and by immunoblot analysis of serotonin transporter (SERT) and tryptophan hydroxylase 2 (TPH2), selective markers of 5HT nerve endings. Astrocytosis was assessed through measures of glial fibrillary acidic protein (GFAP) (immunoblotting) and microglial activation was determined by histochemical staining with Isolectin B4.

Key findings

Mephedrone alone did not cause persistent reductions in the levels of 5HT, SERT or TPH2. Methamphetamine and MDMA alone caused mild reductions in 5HT but did not change SERT and TPH2 levels. Combined treatment with mephedrone and methamphetamine or MDMA did not change the status of 5HT nerve endings to an extent that was different from either drug alone.

Significance

Mephedrone does not cause toxicity to 5HT nerve endings of the hippocampus. When co-administered with methamphetamine or MDMA, drugs that are often co-abused with mephedrone by humans, toxicity is not increased as is the case for dopamine nerve endings when these drugs are taken together.     

MAO activity in serotonergic endings of rat major cerebral arteries

The present work studies the existence of monoamine oxidase (MAO) activity in serotonergic endings present in rat major cerebral arteries. Enzymatic activity was appraised in vivo by serotonin (5-HT) accumulation or 5-hydroxyindole acetic acid (5-HIAA) disappearance with time after systemic administration of MAO inhibitors. Pargyline (75 mg/Kg, ip) brought about significant 5-HT increase and 5-HIAA decrease in major cerebral arteries 30 and 60 min after its administration. Clorgyline (75 mg/Kg, ip) also induced 5-HT enhancement and 5-HIAA decline in these arteries 30 and 60 min after its injection. However, treatment with deprenyl (75 mg/Kg, ip) only evoked a significant 5-HT increase at 60 min. When either clorgyline (5 mg/Kg, ip) or deprenyl (5 mg/Kg, ip) were administered 5-HT and 5-HIAA levels remained unaffected. Two weeks after performing electrolytical lesion of dorsal raphe nucleus and 60 min after clorgyline (75 mg/Kg, ip) injection 5-HT and 5-HIAA levels appeared significantly reduced in cerebral arteries and striatum when compared to sham-lesioned controls. These results suggest that MAO-A isoform acting on endogenous 5-HT is present in rat major cerebral arteries and is located in nerve endings of fibers arising from dorsal raphe nucleus.