Nonexocytotic serotonin release tonically suppresses serotonergic neuron activity

The firing activity of serotonergic neurons in raphe nuclei is regulated by negative feedback exerted by extracellular serotonin (5-HT)_o acting through somatodendritic 5-HT1A autoreceptors. The steady-state [5-HT]_o, sensed by 5-HT1A autoreceptors, is determined by the balance between the rates of 5-HT release and reuptake. Although it is well established that reuptake of 5-HT_o is mediated by 5-HT transporters (SERT), the release mechanism has remained unclear. It is also unclear how selective 5-HT reuptake inhibitor (SSRI) antidepressants increase the [5-HT]_o in raphe nuclei and suppress serotonergic neuron activity, thereby potentially diminishing their own therapeutic effect. Using an electrophysiological approach in a slice preparation, we show that, in the dorsal raphe nucleus (DRN), continuous nonexocytotic 5-HT release is responsible for suppression of phenylephrine-facilitated serotonergic neuron firing under basal conditions as well as for autoinhibition induced by SSRI application. By using 5-HT1A autoreceptor-activated G protein–gated inwardly rectifying potassium channels of patched serotonergic neurons as 5-HT_o sensors, we show substantial nonexocytotic 5-HT release under conditions of abolished firing activity, Ca²⁺ influx, vesicular monoamine transporter 2–mediated vesicular accumulation of 5-HT, and SERT-mediated 5-HT transport. Our results reveal a cytosolic origin of 5-HT_o in the DRN and suggest that 5-HT_o may be supplied by simple diffusion across the plasma membrane, primarily from the dense network of neurites of serotonergic neurons surrounding the cell bodies. These findings indicate that the serotonergic system does not function as a sum of independently acting neurons but as a highly interdependent neuronal network, characterized by a shared neurotransmitter pool and the regulation of firing activity by an interneuronal, yet activity-independent, nonexocytotic mechanism.     

Reduced-system analysis of the effects of serotonin on a molluscan burster neuron

The mathematical model described in Bertram (1993) is used to carry out a detailed examination of the manner in which the neurotransmitter serotonin modifies the voltage waveform generated endogenously by burster neuron R₁₅ of Aplysia. This analysis makes use of a reduced system of equations, taking advantage of the slow rate of change of a pair of system variables relative to the others. Such analysis also yields information concerning the sensitivity of the neuron to brief synaptic perturbations. Received: 24 March 1993/Accepted in revised form: 9 June 1993     

A computational study of the effects of serotonin on a molluscan burster neuron

A mathematical model of burster neuron R₁₅ from the abdominal ganglion of Aplysia is presented. This is an improvement over earlier models in that the bursting mechanism is more accurately represented. The improved model allows for simulated application of the neurotransmitter serotonin, which has been reported to have profound effects on the voltage waveform produced by R₁₅. Computational analysis indicates that the serotonin-induced modulation of the waveform can be explained in terms of competition between stationary, bursting, and beating attractors. Analysis also indicates that, as a result of this competition, serotonin increases the sensitivity of the neuron to synaptic perturbations. This may have important consequences with regard to water balance in the Aplysia, particularly during egg laying.     

Sensitivity of a peptide-activated neuron in Aplysia to serotonin and cyclic AMP-relevant agents

Serotonin depolarized Aplysia buccal motoneuron B16. The response could be obtained in high-magnesium/low-calcium medium, indicating a direct effect on B16 and supporting a putative monosynaptic input to B16 from the serotonergic metacerebral neurons. Similar depolarizing effects in high-magnesium/low-calcium medium were obtained in response to 8-bromo cyclic AMP, isobutylmethylxanthine, theophylline and forskolin. Tolbutamide, a putative inhibitor of cyclic AMP-dependent protein kinase, blocked or reversed responses of B16 to egg-laying hormone containing extracts and to serotonin. Serotonin and forskolin significantly increased the cyclic AMP content of buccal ganglia, whereas egg-laying hormone-containing extracts did not.     

Involvement of serotonin system in bulimia

Platelet 3H-imipramine binding was investigated in 8 patients affected by bulimia according to DSM III criteria, and in 7 healthy volunteers. The Bmax +/- SD (fmol/mg protein) was 356 +/- 53 in patients, and 1144 +/- 134 in controls. The Kd +/- SD (nM) was 1.35 +/- 0.44 in patients, and 1.90 +/- 0.72 in controls. There was a significant difference (p less than 0.0001) in Bmax values in the two groups, whereas no significant difference was observed in Kd values. This study suggests the possible involvement of the indoleamine system in bulimia.     

Observations of serotonin and FMRFamide-like immunoreactivity in palp sensory structures and the anterior nervous system of spionid polychaetes

Evidence suggests that ciliated sensory structures on the feeding palps of spionid polychaetes may function as chemoreceptors to modulate deposit-feeding activity. To investigate the probable sensory nature of these ciliated cells, we used immunohistochemistry, epi-fluorescence, and confocal laser scanning microscopy to label and image sensory cells, nerves, and their organization relative to the anterior central nervous system in several spionid polychaete species. Antibodies directed against acetylated alphatubulin were used to label the nervous system and detail the innervation of palp sensory cells in all species. In addition, the distribution of serotonin (5-HT) and FMRFamide-like immunoreactivity was compared in the spionid polychaetes Dipolydora quadrilobata and Pygospio elegans. The distribution of serotonin immunoreactivity was also examined in the palps of Polydora cornuta and Streblospio benedicti. Serotonin immunoreactivity was concentrated in cells underlying the food groove of the palps, in the palp nerves, and in the cerebral ganglion. FMRFamide-like immunoreactivity was associated with the cerebral ganglia, nuchal organs and palp nerves, and also with the perikarya of ciliated sensory cells on the palps.     

Domains of calcium channels as dissipative structures in a simulated neuron

The lateral distribution of open calcium channels of the fluid-mosaic membrane were investigated in a phenomenological model of a cylinder-shaped nerve cell. The local density of the channels changed due to their lateral diffusion and voltage- and calcium-dependent conformation transitions between open and closed states. Domains with an increased steady density of the open calcium channels were created as a result of action of intracellular calcium on its own channels, increasing the probability of the open state of the latter. These spatially nonuniform distributions of the channels are considered dissipative structures emerged in the active nonlinear medium at the expense of energy of active transport.Neirofiziologiya/Neurophysiology, Vol. 26, No. 2, pp. 99–107, March–April, 1994.     

Axonal transport of [3H]serotonin in an identified neuron of Aplysia californica

The distribution of (Na+ + K+) ATPase over the plasma membranes of the proximal convoluted tubule from canine renal cortex has been determined. Ultrathin frozen sections of this tissue were stained with rabbit antibodies to this enzyme and ferritin-conjugated goat antirabbit gamma-globulin. It is demonstrated that high concentrations of this enzyme uniformly line the intercellular spaces of this epithelium. The consequences of this observation are discussed in terms of the low resistant tight junctions of these tubules and the isotonic fluid transport which they support. Furthermore, antibodies to (Na+ + K+) ATPase recognize an antigen on the luminal surfaces of the tubules within the brush border. It is proposed that the enzyme is present in this region of the plasma membrane as well, although at much lower concentration. To further substantiate this conclusion, a brush border fraction has been purified from rabbit kidney and been shown to contain significant (Na+ + K+) ATPase. These results contradict earlier conclusions about the location of (Na+ + K+) ATPase in this tissue.     

Some system considerations of neuron pools with feedback

This paper describes an approach to the analysis of the inputoutput relationships present in a neuron pool that receives a number of inputs. These inputs consist of primary inputs to the neuron pool and inputs resulting from feedback of information from the neuron pool as well. Multiple input-output relationships are obtained in terms of the synaptic weightings of the inputs, the membrane response characteristics of the neurons and the conduction delays on the feedback pathways.y=(I+MHD) ^-1 ·MFx is the explicit representation of the cell pool behavior assuming quasi-linear conditions, wherey is the output vector of cell responses,I is the identity matrix,M is the response matrix of the cells,H is the feedback synaptic weighting matrix,D is the delay matrix,F is the input weighting matrix, andx is the input vector. It is shown that a solution to this formulation exists, is unique, is stable, and can be computed by specified algorithms. An insight gained from this formulation suggests that the output of each cell in the pool is related to virtually all of the inputs to the pool and the outputs of all cells in the pool.Supported by NSF Grant GK 38301 and US-PHS Grant NS 08470.Program in Bioengineering, Electrical and Computer Engineering Department, The University of Michigan.Computer Information and Control Program, and Department of Electrical and Computer Engineering, The University of Michigan.     

Parallel processing and selection of the responses to serotonin during reinnervation of an identified leech neuron

In an attempt to define the mechanism of synaptic specificity, we have been studying pairs of identified leech neurons isolated in tissue culture. The cultured neurons reform specific synapses when paired with appropriate partners in the absence of other cell types. In recent studies, we have examined in detail the reformation of a serotoninergic synapse between the Retzius cell and one of its targets, the pressure sensitive (P) cell. The P cell in vivo and its soma in vitro have two types of responses to serotonin (5-HT). From voltage clamp analysis of cultured P cells, we demonstrated the parallel activation of chloride (gCls) and monovalent cation (gCations) channels coupled to distinct receptor subtypes and gated by separate second messengers. Only gCls was activated by 5-HT released from the presynaptic Retzius cell both in vivo and in vitro. This demonstrates the remarkable specificity of the reformation of this synapse in culture since only the correct 5-HT receptor subtype is activated. An 80% reduction of gCations was observed in P cells that had failed to be innervated by Retzius cells in culture, suggesting that gCations may be lost prior to synapse formation. Retzius cells depleted of 5-HT also reduced gCations in the paired P cells and incubating single P cells in 5-HT did not reduce gCations. In addition, aldehyde-fixed Retzius cells were able to selectively reduce gCations when paired with P cells. We conclude that the loss of gCations was due to contact between the neurons. The early clearing of counter-effective receptor subtypes may be a prelude to synapse formation.     

Protective action of serotonin on changes in the ultrastructure of the Retzius neuron induced by acetylcholine

A combined action of acetylcholine and serotonin is demonstrated to produce, in ultrastructure of the Retzius neuron of the leech, changes similar to those resulted from synaptic activation. Nevertheless, acetylcholine alone produces much deeper morphological shifts. A conclusion is made that serotonin not only retards impulse activity of the neuron, but it "slows down" development of rather great changes in its ultrastructure.     

Alteration of serotonin system by polychlorinated biphenyls exposure

Although commercial production of polychlorinated biphenyls (PCBs) was banned in 1979, PCBs continue to be an environmental and health concern due to their high bioaccumulation and slow degradation rates. In fact, PCBs are still present in our food supply (fish, meat, and dairy products). In laboratory animals, exposure to single PCB congener or to mixtures of different congeners induces a variety of physiological alterations. PCBs cross the placenta and even exposure at low level is harmful for the foetus by leading to neurodevelopment alterations. Serotonin system which regulates many physiological functions from platelet activation to high cerebral processes and neurodevelopment is one of the targets of PCBs toxicity. The effects of PCBs exposure on serotonin system have been investigated although to a lesser extent compared to its effect in other neurotransmitter systems. This review provides a summary of the results concerning the impact of PCBs exposure (in vitro and in vivo) on serotonin system. Further research is needed to correlate specific deficits with PCB-induced changes in the serotonin system.     

The relations between neurite development and the subcellular structures of hippocampal neuron somata

The relations between neurite development and the subcellular structures of the hippocampal neuron somata have been studied with atomic force microscopy (AFM). The conformation of the neuron was achieved by the synapse-like structures found by AFM scanning along a neurite of the cell. Hippocampal neuron somata were divided into two or three subcellular parts by one or two horizontal grooves. The upper parts increased while the middle and the lower parts decreased with the number and the length of the neurites and the formation of the neurosynapse-like structures. When neurites sufficiently developed, the middle parts were lost and the lower parts became very small. Mitosis inhibitors could prevent the formation of such subcellular structures of hippocampal neuron somata, which was accompanied by the loss of ability to form synapse-like structures. These results suggest that the upper parts are responsible for neuritogenesis while the middle and the lower parts only have indirect effect on it.     

Extending the mirror neuron system model,I

The paper introduces mirror neuron system II (MNS2), a new version of the MNS model (Oztop and Arbib in Biol Cybern 87 (2):116–140, 2002) of action recognition learning by mirror neurons of the macaque brain. The new model uses a recurrent architecture that is biologically more plausible than that of the original model. Moreover, MNS2 extends the capacity of the model to address data on audio-visual mirror neurons and on the response of mirror neurons when the target object was recently visible but is currently hidden.