Calcium regulates the activity of serotonin-containing dorsal raphe neurons in vitro

Small elevations of calcium ions (15%) significantly depressed the activity of serotonin-containing dorsal raphe neurons by 35% in mouse brain slices in vitro, while large increases in calcium ion concentration (300%) dramatically decreased the incidence of spontaneously active raphe neurons. Neurochemical studies indicated that these effects were not attributable to increased release and metabolism of serotonin. These findings may have implications for the treatment of mood disorders, for which disturbances in both calcium and serotonin metabolism have been demonstrated.     

Effects of baclofen and gamma-aminobutyric acid on different types of medullary respiratory neurons

Effects of baclofen and gamma-aminobutyric acid on medullary respiratory neurons were investigated. Medullary inspiratory neurons of the dorsal and ventral respiratory groups were stimulated by baclofen, 0.5-2 mg/kg, and depressed by doses greater than 4-6 mg/kg. Expiratory neurons were depressed by doses of baclofen which increased phrenic nerve activity. Microelectrophoresis of baclofen (5 mM, pH 3) depressed medullary inspiratory neurons. It is suggested that low i.v. doses of baclofen increase inspiratory activity by disinhibition of medullary neurons whereas higher doses directly depress medullary inspiratory neurons.     

Ionic mechanisms of the modulating effect of serotonin on acetylcholine responses of neurons in the edible snail

Ionic mechanisms of modulating action of serotonin on acetylcholine responses in identified and nonidentified neurons of Helix pomatia were studied using the voltage-clamp method. A 1-3 min exposure to serotonin (10(-5)-10(-4) mol/l) depressed DNa-type, DCl-type and HCl-type of responses to acetylcholine (10(-6)-10(-4) mol/l) without modification of the reversal potential of acetylcholine-induced current. Theophylline produced effects analogous to those of serotonin which evidences for the participation of intracellular processes connected with the cyclase system of cell in modulating the serotonin action on acetylcholine responses. Serotonin produced a shift of the reversal potential and an increase of acetylcholine-induced current in neurons whose acetylcholine responses were associated with changes in the membrane permeability for several kinds of ions. Serotonin inhibited the inward current phase when two-phase acetylcholine responses of DNa-HK-type were observed. Ionic mechanisms of the modulating action of serotonin on acetylcholine responses in tested neurons are discussed on the basis of the data obtained.     

Comparison of Effects of Various Types of NA and 5-HT Agonists on Transmission from Group II Muscle Afferents in the Cat

A number of noradrenaline and serotonin agonists were tested to investigate which of them replicate the depressive actions of monoamines on transmission from group II muscle afferents in the cat spinal cord. The agonists were applied ionophoretically at the two sites at which maximal monosynaptic focal field potentials are evoked from group II afferents-in the intermediate zone and the dorsal horn of the 4th and 5th lumbar segments. Their effects were estimated from changes in the amplitude of the field potentials. The compounds tested fell into three categories according to the site at which they depressed transmission from group II afferents: one category with highly selective actions in the intermediate zone, a second category with similarly selective actions in the dorsal horn, and a third category with non-selective actions. Drugs in the first category included three noradrenaline agonists (tizanidine, B-HT 933 and clonidine), included in the second were five serotonin agonists (8-OH-DPAT, 5-methoxytryptamine, alpha-methyl serotonin, DOI and 2-methyl-serotonin), and in the third two noradrenaline agonists (phenylephrine and isoproterenol) and two serotonin agonists (RU 24969 and 5-carboxamidotryptamine). Field potentials evoked by group I afferents remained unaffected by all but one compound (8-OH-DPAT). Effects of one noradrenaline agonist and one serotonin agonist (tizanidine and 5-methoxytryptamine) were also tested on responses of single extracellularly recorded neurons. Tizanidine depressed responses induced by stimulation of group II afferents in intermediate zone interneurons, but not in dorsal horn neurons, while 5-methoxytryptamine depressed activation of the latter. Tizanidine had no effect on responses evoked by group I afferents, either in intermediate zone interneurons or in the dorsal spino-cerebellar tract neurons of Clarke's column. It is hypothesized that noradrenaline and serotonin released by descending monoaminergic neurons differ in the potency with which they depress transmission from group II afferents to different functional types of neuron. The results suggest that this depression may involve different membrane receptors at different locations, primarily alpha2 adrenoceptors in the intermediate zone/ventral horn and 5-HT1A serotonin receptors in the dorsal horn.     

Effects of baclofen and γ-aminobutyric acid on different types of medullary respiratory neurons

Effects of baclofen and γ-aminobutyric acid on medullary respiratory neurons were investigated. Medullary inspiratory neurons of the dorsal and ventral respiratory groups were stimulated by baclofen, 0.5–2 mg/kg, and depressed by doses greater than 4–6 mg/kg. Expiratory neurons were depressed by doses of baclofen which increased phrenic nerve activity. Microelectrophoresis of baclofen (5 mM, pH 3) depressed medullary inspiratory neurons. It is suggested that low i.v. doses of baclofen increase inspiratory activity by disinhibition of medullary neurons whereas higher doses directly depress medullary inspiratory neurons.     

Cytochemical and pharmacological studies on polysensory neurons in the primate frontal cortex

Neurons of the squirrel monkey postarcuate cortex respond to a number of different sensory modalities. The pharmacological responses of these neurons to putative neurotransmitters, administered by microiontophoresis, was studied. Norepinephrine and serotonin had a powerful inhibitory action on the vast majority of cells at relatively low iontophoretic currents. The inhibitory action of norepinephrine was potentiated by low doses of desmethylimipramine; higher doses produced a direct slowing of discharge.Acetylcholine had mixed effects, exciting and inhibiting approximately equal numbers of neurons. The threshold current for acetylcholine responses was considerably higher than that for norepinephrine or serotonin.Treatment of freeze-dried slices of postarcuate cortex with paraformaldehyde vapor, after uptake of alpha-methyl norepinephrine or 6-hydroxytryptamine, showed an extensive plexus of fine fluorescent axons with characteristic varicosities, throughout the cortex. Many of the varicosities surrounded the soma of neurons.Light microscopic autoradiography of polysensory cortex, after subdural injection or incubation with tritiated norepinephrine or serotonin, revealed many grains along neuronal soma and major dendrites. With electron microscopy after this same treatment, autoradiographic grains were localized largely to fine unmyelinated axons and to small terminals making synaptic contact with the soma and large dendrites.After chronic treatment with 6-hydroxydopamine, fibers within the polysensory cortex which take up alpha-methyl norepinephrine or tritiated norepinephrine are greatly reduced in number, whereas fibers which take up 6-hydroxytryptamine or tritiated serotonin are still plentiful. These electrophysiological and cytochemical studies support the existence of norepinephrine and serotonin-containing inhibitory pathways to neurons of the postarcuate cortex of squirrel monkey.     

α-Methyltyrosine blocks methylamphetamine-induced degeneration in the rat somatosensory cortex

Neurochemical and histological studies suggest that methylamphetamine (MA) administered continously or in high doses is toxic to dopaminergic and serotonergic nerve terminals. Degeneration of the dopaminergic or serotonergic cell bodies themselves has not been reported, however. In the present study, administration of a single 100 mg/kg dose of MA was toxic to a subpopulation of neurons in the somatosensory cortex, an area of the brain which does not contain catecholaminergic or serotonergic cell bodies. This dose of MA also produced a long-lasting depletion of serotonin (5-HT) but not norepinephine in the somatosensory cortex. Dompamine levels in the somatosensory cortices of control animals were virtually undetectable and therefore were not studied further. Administation of α-methyltyrosine (α-MT), a catecholamine synthesis inhibitor, prior to the injection of MA blocked both the depletion of 5-HT and the degeneration of cortical perikarya produced by MA alone. Since the MA-induced depletion of 5-HT and the MA-induced degeneration of cortical perikarya are correlated, we suggest that the serotonergic system may be involved in the toxic effects of MA on the cortical neurons.     

alpha-Methyltyrosine blocks methylamphetamine-induced degeneration in the rat somatosensory cortex

Neurochemical and histological studies suggest that methylamphetamine (MA) administered continuously or in high doses is toxic to dopaminergic and serotonergic nerve terminals. Degeneration of the dopaminergic or serotonergic cell bodies themselves has not been reported, however. In the present study, administration of a single 100 mg/kg dose of MA was toxic to a subpopulation of neurons in the somatosensory cortex, an area of the brain which does not contain catecholaminergic or serotonergic cell bodies. This dose of MA also produced a long-lasting depletion of serotonin (5-HT) but not norepinephrine in the somatosensory cortex. Dopamine levels in the somatosensory cortices of control animals were virtually undetectable and therefore were not studied further. Administration of alpha-methyltyrosine (alpha-MT), a catecholamine synthesis inhibitor, prior to the injection of MA blocked both the depletion of 5-HT and the degeneration of cortical perikarya produced by MA alone. Since the MA-induced depletion of 5-HT and the MA-induced degeneration of cortical perikarya are correlated, we suggest that the serotonergic system may be involved in the toxic effects of MA on the cortical neurons.     

A fixed-dose clinical trial of fluoxetine in outpatients with major depression

Fixed daily doses of 20 mg, 40 mg, or 60 mg of fluoxetine, a highly specific serotonin reuptake inhibitor, were given to 84 depressed outpatients in a double-blind, placebo-controlled, randomized 6-week trial. The 20-mg dose produced improvement of depression in the moderate-severe depression group as expressed in significant reductions of scores on the Hamilton Rating Scale for Depression (p greater than or equal to .007) and the Patient Global Impressions scale (p greater than or equal to .011), and the 20-mg dose caused fewer side effects than did the higher doses. A mildly depressed group of patients showed no improvement at any dose level of fluoxetine.     

Toxic effects of MDMA on central serotonergic neurons in the primate: importance of route and frequency of drug administration

This study compared the toxic effects of oral versus subcutaneous and single versus multiple doses of 3,4-methylenedioxymethamphetamine (MDMA) on central serotonergic neurons in non-human primates. Orally administered MDMA was approximately one-half as effective as subcutaneously administered drug. Multiple doses were more effective than single doses, but a single 5 mg/kg dose of MDMA given orally still produced a long-lasting depletion of serotonin in the monkey brain. These results indicate that when MDMA is given to monkeys in a manner similar to that employed by humans, it exerts toxic effects on central serotonergic neurons. This suggests that humans using MDMA may be at risk for incurring central serotonergic neuronal damage.     

Interplexiform, horizontal and bipolar-like cells of the rabbit retina take up exogenous serotonin during early developmental stages

A subpopulation of amacrine cells which have the ability to take up exogenous serotonin occur in the rabbit retina at all stages during postnatal development. However, during the initial stages of the retina's development some serotonin-accumulating neurons are also associated with the outer plexiform layer in the form of interplexiform and/or bipolar and/or horizontal cells. These neurons could have an additional function of their own or simply be misplaced serotonin-accumulating neurons destined to degenerate in the process of synaptogenesis.     

Morphological evidence for selective modulation by serotonin of a subpopulation of dorsal horn cells which possess the neurokinin-1 receptor

Serotonin selectively depresses transmission of nociceptive information through the spinal dorsal horn but the mechanisms of this depression are poorly understood. In this study we report that serotonin-containing axons form basket-like clusters which are intimately woven around cell bodies and proximal dendrites of a subpopulation ( approximately 50%) of laminae III/IV neurons which possess the neurokinin-1 receptor. Statistical analysis confirms that cells belonging to this subpopulation have significantly higher numbers of serotoninergic contacts on proximal dendrites when compared with the population of neurokinin-1 cells that are not associated with clusters (mean +/- SD = 13+/-5.8 and 5+/-2.9, respectively). Neurokinin-1 cells in laminae III/IV project to regions of the brain which are involved in nociceptive processing and are likely to be activated predominantly by nociceptive input. The concentration of serotoninergic axons around proximal regions of some of these cells indicates that serotonin may have a powerful influence on transmission through this pathway. This type of arrangement could be a morphological correlate for at least part of the selective antinociceptive actions of serotonin.     

Modulation of Responses of Four Types of Feline Ascending Tract Neurons by Serotonin and Noradrenaline

Modulation of responses of four types of ascending tract cells by noradrenaline and serotonin was compared in order to investigate how information forwarded by these cells may be gated by monoaminergic tract neurons. Spinocervical tract, postsynaptic dorsal column and dorsal spinocerebellar tract neurons located in Clarke's column and in the dorsal horn were identified by their axonal projections. Noradrenaline and serotonin were applied ionophoretically close to a selected neuron, and their effects were tested on extracellularly recorded responses of this neuron to electrical stimulation of low-threshold skin afferents and group II muscle spindle afferents. The modulatory actions of noradrenaline and serotonin were estimated from changes in the number of responses evoked by 30 successive stimuli, the minimal latencies of these responses, and their firing frequency. All four populations of ascending tract neurons investigated were modulated by serotonin and noradrenaline, but not in the same way. The responses were most often depressed by noradrenaline and facilitated by serotonin, but in some types of neuron they were affected in the same direction. Transmission from low-threshold skin and group II muscle afferents changed in the same direction in some types of neuron but in the opposite direction in other types. The results indicate that transfer of information from skin and group II muscle afferents to supraspinal centres may be gated by descending monoaminergic pathways in a highly differentiated manner, and is adjusted to the requirements of various behavioural situations.     

Effects of high-dose methamphetamine administration on serotonin uptake sites in rat brain measured using [3H]cyanoimipramine autoradiography

Repeated administration of high doses of methamphetamine (15 mg/kg given for 5 doses over 24 h) resulted in long-term decreases in the binding of [3H]cyanoimipramine ([3H]CN-IMI) to serotonin uptake sites measured using quantitative autoradiography. Seven days after termination of drug administration decreases were seen in 23 of 28 regions examined. This is consistent with previous studies indicating that methamphetamine and related amphetamines are neurotoxic to serotonin neurons. Significant decreases were still present in many brain areas on the same dosage schedule 30 days after drug administration. However, the number of areas affected was considerably less, consistent with some regrowth of serotonin neurons. At a lower dosage (7.5 mg/kg on the same schedule) no effects on [3H]CN-IMI binding were seen. The results of this study provide support for the serotonergic neurotoxicity of repeated methamphetamine administration in rats. They also show that the neurotoxicity is highly regional and dose dependent.     

Effects of monoamines on interneurons in four spinal reflex pathways from group I and/or group II muscle afferents

Effects of locally applied serotonin (5-HT) and noradrenaline (NA) were tested on extracellularly recorded responses of single spinal interneurons in deeply anaesthetized cats. These effects were tested on: (i) interneurons mediating reciprocal inhibition from group Ia afferents; (ii) interneurons mediating non-reciprocal inhibition from group Ia and Ib afferents; (iii) intermediate zone interneurons co-excited by group I and II afferents; and (iv) dorsal horn interneurons excited by group II afferents. Effects of monoamines were tested on responses evoked at latencies compatible with monosynaptic coupling. Responses evoked by group Ia and/or Ib muscle afferents were facilitated in all of the tested interneurons both by NA and 5-HT. Responses evoked by group II muscle afferents were depressed in the majority of the interneurons but were facilitated in some of them. 5-HT depressed these responses in all dorsal horn interneurons and in one subpopulation of intermediate zone interneurons, while it facilitated them in another subpopulation of intermediate zone interneurons. NA depressed them in all intermediate zone interneurons and in one subpopulation of dorsal horn interneurons, while it facilitated them in another subpopulation of dorsal horn interneurons. The results of this study lead to the conclusions that: (i) modulation of synaptic actions of muscle spindle and tendon organ afferents on spinal interneurons by 5-HT and NA is related to both the type of the afferent and the functional type of the interneuron; and that (ii) 5-HT and NA counteract each others' actions on some interneuronal types but mutually enhance them on the others.     

Modulation of responses of feline ventral spinocerebellar tract neurons by monoamines.

Ventral spinocerebellar tract neurons located in laminae V-VII of cat lumbar spinal cord were tested for the effects of ionophoretically applied monoamines and receptor selective agonists. Extracellularly recorded responses, monosynaptically evoked by group I afferents in a muscle nerve, were compared before, during, and after ionophoresis. They were analyzed with respect to changes in the number of evoked spikes and in the latency. Both serotonin (5-HT) and noradrenaline (NA) were found to facilitate responses of all neurons tested. Ionophoresis of three serotonin subtype receptor agonists (5-carboxamidotryptamine maleate, 5 methoxytryptamine HCl, and alpha-methyl 5-hydroxytryptamine) and of two NA receptor agonists (phenylephrine and isoproterenol) likewise had a facilitatory effect. However, three other 5-HT receptor agonists (8-hydroxy-dipropylaminotetraline hydrobromide), 2-methyl 5-hydroxytryptamine, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl and two NA receptor agonists (tizanidine and clonidine) had the opposite effect because they depressed responses of the tested neurons. These results show that information forwarded by means of the ventral spinocerebellar tract may be modulated by monoamines and that several receptor subtypes, located pre- or postsynaptically, may be involved. The results also demonstrate that transmission by means of group I muscle afferents may not only be facilitated by monoamines but also depressed by selective receptor subtype activation.     

The role of GABA in the regulation of the dopamine/tyrosine hydroxylase-containing neurons of the rat retina

Dopamine (DA) and gamma-aminobutyric acid (GABA) are putative neurotransmitters in two separate populations of amacrine neurons in the mammalian retina. Pharmacological studies have been conducted to determine if GABA neurons regulate the neuronal activity of the neurons that secrete DA. Tyrosine hydroxylase (TH) activity, a biochemical indicator of changes in activity of DA/TH-containing neurons, was low in dark-adapted retinas and high in light-exposed retinas. Muscimol (a GABA receptor agonist) produced a dose-related, biphasic effect on the light-evoked activation of TH, when the drug was injected into the vitreous (intravitreal injection) of dark-adapted rats. At low doses, (35 and 60 pmol) muscimol enhanced the light-evoked activation of TH, but at higher doses (greater than or equal to 120 pmol) it inhibited the light-evoked increase in enzyme activity. Muscimol had no significant effect on the TH activity of dark-adapted retinas. GABA antagonists, bicuculline and picrotoxin, produced effects on TH activity that were dependent on both dose and light-exposure. At low doses (0.4-0.5 nmol), bicuculline and picrotoxin both inhibited the light-evoked activation of TH, but had no effect on TH activity of the dark-adapted retinas. At a higher dose (2.0 nmol), both antagonists increased TH activity in the dark-adapted retina and attenuated the further activation of the enzyme by light. Rat retinas were dissociated into suspensions of viable cells in order to investigate the direct effects of muscimol and picrotoxin on the DA/TH-containing cells. The process of dissociating dark-adapted retinas resulted in an apparent activation of TH. Incubation of the cells with muscimol resulted in a decrease of TH activity in a concentration-dependent manner. Picrotoxin antagonized the inhibitory effect of muscimol, but had no effect when incubated alone. The biphasic effects of GABA agonists and antagonists in vivo suggest that a certain subpopulation of GABA neurons are involved in the activation of the DA/TH-containing neurons by photic stimulation, while another subpopulation of GABA neurons produce a tonic inhibition of the DA/TH-containing neurons in darkness. The experiments with retinal cell suspensions indicate that the tonic inhibition is probably mediated by synapses of GABA neurons directly onto the DA/TH-containing cells.     

Neuronal responses of the globus pallidus to systemic administration of d-amphetamine: investigation of the involvement of dopamine, norepinephrine, and serotonin

Systemic administration of d-amphetamine (d-AMP) caused significant increases in the unit activity of spontaneously firing neurons in the rat globus pallidus. Intravenous injection of 0.2 mg/kg of d-AMP produced an average increase of 32% while a cumulative dose of 6.4 mg/kg of d-AMP increased cell firing 81% above base line control. The excitatory effects of d-AMP on pallidal cells were effectively blocked and reversed by haloperidol. Increasing intravenous doses of l-amphetamine (l-AMP), totaling 12.8 mg/kg, caused a slight excitation of pallidal neurons. The average maximum increase was 18.3%. Minor rate-elevating effects were also observed after systemic administration of desmethylimipramine and clonidine. The serotonin uptake inhibitor, fluoxetine, produced varied changes in firing frequencies. Pretreatment with reserpine and alpha-methyl-p-tyrosine significantly attenuated the d-AMP-induced increase in pallidal activity. These results suggest that dopamine plays a prominent role in mediating the stimulatory effects of d-AMP on the firing rates of a population of globus pallidus cells in gallamine-paralyzed rats.     

Atypical indolamine-immunoreactive cell groups in the dorsal myelencephalon of the rat

The presence of atypical indolamine-immunoreactive (IAI) neurones in the dorsal myelencephalon of the rat was demonstrated by means of peroxidase-antiperoxidase (PAP) immunocytochemistry. Besides the area postrema, two other regions, viz. the solitary complex and the superficial rostral cuneate fascicle, were found to contain neuronal perikarya displaying a normally weak staining which was markedly enhanced after monoaminoxidase (MAO) inhibition. In contrast to immunoreactive cells of the periaqueductal central gray, the dorsal myelencephalic IAI neurons were undetectable after serotonin synthesis inhibition with p-chlorophenylalanine (PCPA), as were immunoreactive terminal neuropils in most brainstem areas. However, sequential treatment with the MAO inhibitor, iproniazid, completely reversed the PCPA-induced suppression of perikaryal immunoreactivity and partially restored axonal staining. None of the atypical cell groups displayed a detectable formaldehyde-induced specific histofluorescence. Since brain levels of tryptamine are likely to increase significantly after MAO inhibitor/PCPA treatment, and furthermore tryptamine can be assumed to cross-react with serotonin, it is suggested that the observed atypical IAI neurons may represent either a subpopulation of serotoninergic neurons; previously postulated true tryptamine neurons; or non-indolaminergic neurons endowed with a selective uptake mechanism for serotonin or tryptamine. These results corroborate the view that different types of indolamine neurons exist in the rat brainstem. Moreover, they underscore the need for cautious interpretation of serotonin neuron mapping studies involving the use of MAO inhibitors.     

Prostaglandins and the functional specificity of central neurons in Helix pomatia

Effects of intra- and extracellular injected prostaglandins (PG) E2 and F2 alpha on chemosensitivity to transmitter substances as well as on electrical activity of identified neurons were studied in the snail Helix pomatia. Prostaglandin E2 was found to amplify as a rule the electrical activity of identified neurons and prostaglandin F2 alpha was found to inhibit it. Prostaglandin E2 usually depressed the reactions of neurons to serotonin, and F2 alpha -- to acetylcholine. Effects of prostaglandins depended on the level of functional activity of neurons and the way of their application (intra- or extracellular). Possible molecular mechanisms of the discovered effects, their relation to cyclic purine nucleotides, calcium ions and specific proteins are discussed.