Pharmacological antagonism of facilitatory but not inhibitory effects of serotonin and norepinephrine on excitability of spinal motoneurons.

Serotonin (5HT) and norepinephrine (NE) produced long-lasting facilitation of glutamate-evoked activity of spinal motoneurons when applied iontophoretically with small ejection currents into the ventral horn. The facilitation was usually preceded by a brief period of inhibition at the onset of current application to the monoamine-containing barrels. This inhibition did not outlast the current application. Ejection of hydrogen ions produced only inhibition of glutamate-evoked activity with no subsequent facilitation at current offset. The 5HT antagonists, methysergide and metergoline, blocked the facilitation, but not the inhibition of motoneuron excitability caused by 5HT. Similarly, the alpha-adrenergic antagonists, piperoxane and phentolamine, blocked the facilitatory, but not the inhibitory, effects of NE on excitability of motoneurons. Since the inhibitory effects of 5HT and NE could not be blocked with the antagonists used, and since ejection of hydrogen ions also produced inhibition, non-specific causes for the inhibitory effects of 5HT and NE could not be rejected. However, the facilitatory effects of 5HT and NE on excitability of motoneurons were readily blocked by antagonists and were, therefore, attributed to actions on separate 5HT and NE receptors in the ventral horn.

Alteration of neocortical activity in response to noxious stimulation: participation of the dorsal raphe.

Noxious stimulation induces cortical desynchronization which in turn can suppress epileptic seizures in humans and epileptiform activity in laboratory animals. Recent evidence indicates that serotonin mediates both atropine-resistant cortical desynchronization and the suppression of focal epileptiform activity induced by noxious stimulation. As a large forebrain projection of serotonergic fibres originates from the dorsal raphe nucleus, involvement of this nucleus in altering cortical activity was investigated. Agents known to inhibit serotonergic unit activity including serotonin, (+/-)-8-hydroxy-dipropylaminotetralin, fluoxetine and baclofen, when pressure ejected in the vicinity of the dorsal raphe, prevented cortical desynchronization as well as the suppression of focal epileptiform activity in response to noxious stimulation. From these observations it is concluded that serotonergic neurones of the dorsal raphe can profoundly influence neocortical excitability and this action may underlie the clinical effectiveness of strong or noxious stimuli in suppressing epileptic seizures.

Selective accumulation of hydroxytryptamines by frog tectal neurones.

By means of histofluorescence microscopy, 5,7-dihydroxytryptamine was shown to be taken up by selective populations of brain neurones of the frog, Rana pipiens, following both intracranial administration and in vitro incubation with isolated brain preparations. Presumptive non-aminergic cell bodies of the superficial aspect of tectal lamina 6 exhibited more avid uptake than did putative serotonin perikarya of the raphe complex. Within the tectum, 5,7-dihydroxytryptamine uptake appeared to be restricted to large piriform neurons; in the torus semicircularis, it occurred in a morphologically dissimilar group of scattered cells. The same tectal cell system accumulated 5-hydroxytryptamine and 6-hydroxytryptamine, but not N-acetylserotonin, melatonin, or noradrenaline. 5,7-Dihydroxytryptamine uptake was insensitive to cold or imipramine; however, it was blocked by ouabain at high but not low temperature. At concentrations greater than or equal to 100 microM, 5,7-dihydroxytryptamine-induced fluorescence was sufficiently intense to permit tracing of intratectal dendrites and tectofugal axonal processes projecting to a lateral diencephalic neuropil and an ipsilateral isthmic neuropil. While previous monoamine histofluorescence and immunohistologic studies have not revealed serotonin-containing perikarya in the ranid tectum, our findings demonstrate that lamina 6 piriform projection neurones, presumably lacking indolamine-synthesizing enzymes, possess a striking capability for accumulating hydroxylated tryptamines.

Nicotinic cholinoceptor-mediated excitation in ambigual motoneurons of the rat.

The purpose of this study was to determine if ambigual oesophageal motoneurons of the rat possess functional nicotinic cholinoceptors. In urethane anaesthetized rats, acetylcholine (20-50 pmol) delivered micropneumophoretically from multibarrelled pipettes to the compact formation of the nucleus ambiguus produced either synchronous or propulsive oesophageal contractions which were fully and reversibly blocked by dihydro-beta-erythroidine (8-10 pmol) but were resistant to D-tubocurarine and hexamethonium (10-20 pmol). 1,1-Dimethyl-4-phenyl-piperazinium but not muscarine (8 pmol) exerted an analogous agonist action. Ejection of glutamate at the same sites produced similar oesophageal responses which were, however, resistant to dihydro-beta-erythroidine. Acetylcholine applied 5-15 s prior to glutamate transiently facilitated the glutamate-evoked response. The facilitatory effect of acetylcholine was replicated by 1,1-dimethyl-4-phenyl-piperazinium but not muscarine and inhibited by dihydro-beta-erythroidine. Physostigmine, applied either intra-ambigually (10-20 pmol) or by intravenous injection (0.15-0.3 mumol/kg), enhanced both acetylcholine and glutamate-evoked responses. In brainstem transverse slices, application of acetylcholine and glutamate to quiescent ambigual neurons of the compact formation resulted in a rapid membrane depolarization associated with an increased membrane conductance and spiking. Under voltage clamp, both acetylcholine and glutamate elicited a net inward current. The depolarizing response of these neurons to acetylcholine was blocked by dihydro-beta-erythroidine (0.5-2 pmol), hexamethonium (0.2 mM) and D-tubocurarine (10 microM) and persisted in the presence of tetrodotoxin (10(-6) M) or Mn2+ (5 mM) in the bathing medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous and network bursts induced by N-methyl-D-aspartate and magnesium free medium in the CA3 region of the hippocampal slice.

The epileptogenic properties of N-methyl-D-aspartate and magnesium-free medium were investigated in the CA3 region of the hippocampal slice preparation in the rat. Bath application of N-methyl-D-aspartate (5-10 microM) or magnesium-free medium induced both spontaneous and stimulus-evoked bursts. Both endogenous and network bursts were generated, the former always preceding the latter. The paroxysmal depolarizing shift underlying the network bursts generated by N-methyl-D-aspartate and magnesium-free medium resembled a giant excitatory postsynaptic potential with a reversal potential near 0 mV and a synaptic input in the apical dendrites above the mossy fibre zone. In the presence of N-methyl-D-aspartate or magnesium-free medium, population bursts were synchronized by activating single CA3 neurons. N-methyl-D-aspartate receptor antagonists prevented the development of N-methyl-D-aspartate-induced spontaneous and stimulus-evoked bursts. However, the only N-methyl-D-aspartate receptor antagonist effective in preventing such bursts in magnesium-free medium was DL-3-[(+/-)-2-carboxypiperazin-4-yl-]-propyl-1-phosphonic acid. Endogenous bursting in the CA3 region has not been observed with other convulsants and thus may reflect the novel voltage dependence of the N-methyl-D-aspartate receptor gated ionic channel. N-methyl-D-aspartate receptors may also partially contribute to the excitatory interaction between CA3 neurons and thereby account for the synchronization of the population observed when activating single CA3 neurons.

Somatostatin regulates excitatory amino acid receptor-mediated fast excitatory postsynaptic potential components in vagal motoneurons.

Somatostatin is considered to be a brain neurotransmitter/neuromodulator; however, there is little concrete information on how this peptide contributes to generation of synaptic potentials in the mammalian central nervous tissue. Recently, a well-defined somatostatin-containing pathway has been traced from the subnucleus centralis of the solitarial complex to the compact formation of the nucleus ambiguus. Moreover, we have demonstrated both in vivo and in vitro that somatostatin enhances glutamate but inhibits acetylcholine excitation of ambigual motoneurons, suggesting involvement of this peptide in central oesophagomotor transmission. The availability of a brainstem slice containing this pathway has allowed us to characterize an excitatory amino acid receptor-mediated excitatory postsynaptic potential in compact formation neurons. This excitatory postsynaptic potential is unusual because its rising phase involves activation of N-methyl-D-aspartate receptors. Here we report that somatostatin participates in ambigual excitatory postsynaptic potential generation by permitting expression of the N-methyl-D-aspartate receptor-mediated component, thereby regulating fast information transfer in this pathway.

Loss of cortical serotonin2A signal transduction in senescent rats: reversal following inhibition of protein kinase C.

Using grease gap recordings, age-related changes in serotonin2A receptors were assessed in sensorimotor regions of the cortex by examining serotonin-induced facilitation of the N-methyl-D-aspartate depolarization in cortical wedges prepared from young adult (3-6 months) and senescent (22-34 months) Fisher 344 rats. Serotonin (10-100 microM) facilitated the N-methyl-D-aspartate depolarization in wedges from young adult rats in a concentration-dependent manner, whereas no facilitation was observed in wedges from senescent rats. Similar results were obtained when +/- 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, a mixed serotonin2A and serotonin2C receptor agonist, was substituted for serotonin. In contrast, agonists at alpha 1A-adrenoceptors, metabotropic glutamate receptors and muscarinic cholinoceptors facilitated the N-methyl-D-aspartate depolarization in wedges from both young adult and senescent rats. Chelerythrine and staurosporine, inhibitors of protein kinase C, but not concanavalin A, myo-inositol or calmodulin antagonists, restored the serotonin facilitation in wedges from senescent animals. In situ hybridization histochemistry revealed that serotonin2A receptor messenger RNA was present in layers II-VI of the cortex, with the highest density of silver grains located in layers III and V of both young adult and senescent rats. Detailed examination of layer V showed that silver grains were significantly higher than background only over pyramidal cells. We conclude that serotonin2A receptors are expressed by pyramidal cells in both young adult and senescent rats and that serotonin acts directly on these receptors to facilitate the N-methyl-D-aspartate depolarization. Moreover, in senescent rats, signal transduction at cortical serotonin2A receptors involved with facilitation of the N-methyl-D-aspartate response is compromised as a result of protein kinase C activation.

Effects of hypomagnesia on histamine H1 receptor-mediated facilitation of NMDA responses.

1. The ability of histamine to facilitate the N-methyl-D-aspartate (NMDA) induced depolarization of cortical projection neurones was examined by use of grease-gap recording. 2. Histamine (1 to 15 microM) reversibly facilitated the NMDA-induced depolarization yielding a bellshaped concentration-response relationship. The peak enhancement was 167% above the control at 10 microM histamine. Desensitization was present in 4 out of 5 slices on second exposure 40 min following the first exposure. 3. Histamine did not alter the depolarization induced by 10 microM kainate. 4. The histamine-induced facilitation persisted in the presence of tetrodotoxin, but was reduced in a concentration-dependent manner by diphenhydramine (IC50 = 7.6 nM). Cyproheptadine (10 nM) also reduced the facilitation, whereas ranitidine (200 nM) and thioperamide (10 nM) were ineffective in this regard. 5. Histamine (10 microM) facilitated the NMDA (25 microM)-induced depolarization in nominally Mg(2+)-free medium. The magnitude of the facilitation was smaller than that observed in Mg(2+)-containing medium (17% above the control) and desensitization was not observed. This facilitation was not reduced by cyproheptadine (10 nM) or diphenhydramine (1 microM). 6. We conclude that histamine facilitates the NMDA depolarization at cortical neurones via two distinct mechanisms. One mechanism involves activation of the histamine H1 receptor and is sensitive to Mg2+. The second mechanism is independent of histamine cell surface receptor activation and may reflect a direct action of histamine at the NMDA receptor.

Action of 5-hydroxytryptamine in facilitating N-methyl-D-aspartate depolarization of cortical neurones mimicked by calcimycin, cyclopiazonic acid and thapsigargin.

1. The ability of calcimycin, cyclopiazonic acid and thapsigargin to facilitate the N-methyl-D-aspartate (NMDA)-mediated depolarization of cortical projection neurones was investigated by use of grease-gap recording and the results compared with the facilitation that results from activation of 5-hydroxytryptamine2A receptors. 2. Calcimycin (0.25 to 3 microM), cyclopiazonic acid (5 to 30 microM), and thapsigargin (10 to 300 nM) reversibly facilitated the NMDA (50 microM)-induced depolarization in the presence of tetrodotoxin. The concentration-response relationships were bell-shaped with a mean enhancement of 550% for calcimycin (1 microM) and approximately 400% for cyclopiazonic acid (20 microM) and thapsigargin (100 nM). At the highest concentration of each agent tested, no facilitation was observed. 3. Chlorpromazine (1 microM) partially restored a facilitation at 3 microM calcimycin and 300 nM thapsigargin. Myo-inositol (10 mM) and 100 nM staurosporine were both ineffective in this regard. 4. The depolarization elicited by 10 microM quisqualate or 5 microM kainate was not facilitated by 10 microM cyclopiazonic acid. 5. Calcimycin (0.5 microM), cyclopiazonic acid (20 microM), and thapsigargin (100 nM) elicited a significant facilitation in the presence of an antagonist cocktail consisting of D,L-2-amino-3-phosphonopropionic acid, prazosin, ritanserin, and scopolamine, although the magnitude of the facilitation was reduced. 6. Facilitation of the NMDA depolarization elicited by both 30 microM 5-hydroxytryptamine and 10 microM phenylephrine was eliminated in nominally Mg(2+)-free medium. In contrast, the facilitation induced by 0.5 microM calcimycin remained intact. 7. Bis-(o-aminophenoxy)-ethane-N,N,N,N, tetraacetic acid aminoethoxy (50 microM) or perfusion with nominally Ca(2+)-free medium eliminated facilitation of the NMDA depolarization induced by 30 microM 5-hydroxytryptamine and 100 nM thapsigargin. 8. The facilitation induced by both 30 microM 5-hydroxytryptamine and 1 microM calcimycin was reduced in a concentration-dependent manner by nifedipine (1 to 10 microM). 9. Calcimycin, cyclopiazonic acid and thapsigargin facilitate the NMDA depolarization in a manner which closely mimics the facilitation induced by 5-hydroxytryptamine. It is concluded that enhancement of the NMDA depolarization at cortical projection neurones results from an elevation of Ca2+ in the cytosol and that several sources of Ca2+ contribute to the facilitation.

Characterization of metabotropic glutamate receptor-mediated facilitation of N-methyl-D-aspartate depolarization of neocortical neurones.

1. Facilitation of the N-methyl-D-aspartate (NMDA) receptor-mediated depolarization of cortical neurones induced by metabotropic glutamate receptor (mGluR) agonists in the presence of tetrodotoxin has been examined by use of grease-gap recording. 2. Quisqualate (1-2 microM) and 10 to 100 microM 1S,3R-I-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) facilitated the NMDA-, but not the kainate-induced depolarization with an EC50 of 16 microM for 1S,3R-ACPD. The facilitation induced by quisqualate was reduced, but not blocked, by 4 microM 6-cyano-7-nitroquinoxaline-2,3-dione. 3. D,L-2-Amino-3-phosphonopropionic acid and D,L-2-amino-4-phosphonobutyric acid antagonized the 1S,3R-ACPD facilitation in a non-competitive manner with IC50 values of 0.24 microM and 4.4 microM respectively. 4. Homologous desensitization of the 1S,3R-ACPD induced facilitation was not observed. The facilitation was not altered by 10 nM staurosporine or 3 microM phorbol diacetate. 5. Substitution of 20 microM 8-bromo-cyclic adenosine monophosphate, 20 microM 8-bromo-cyclic guanosine monophosphate, or 10 microM arachidonic acid for 1S,3R-ACPD did not induce facilitation of the NMDA response. However, the 1S,3R-ACPD facilitation was potentiated by 10 mM myo-inositol and exhibited heterologous desensitization following exposure to 100 microM 5-hydroxytryptamine. 6. The 1S,3R-ACPD-induced facilitation persisted in both 10 microM nifedipine and nominally Ca(2+)-free medium and was only gradually eliminated following addition of 100 microM bis-(-o-aminophenoxy)-ethane-N,N,N,N-tetraacetic acid in Ca(2+)-free medium. Facilitation of the NMDA response induced by carbachol, but not phenylephrine, was also observed in nominally Ca(2+)-free medium. Perfusing 50 microM bis-(-aminophenoxy)-ethane-N,N,N,N-tetraacetic acid aminoethoxy eliminated the 1S,3R-ACPD facilitation. 7. These experiments have shown that mGluR agonists selectively facilitate the NMDA depolarization of cortical wedges, most likely by activating one or more mGluR subtypes that couple to phospholipase C. We conclude the facilitation results from a Ca(2+)-sensitive mechanism dependent on activation of phospholipase C and release of internal Ca2+. The facilitation is not contingent on activation of protein kinase C or entry of Ca2+ through nifedipine-sensitive Ca2+ channels.

Ca2+ mobilizing agents mimic serotonin 5-HT2A facilitation of N-methyl-D-aspartate depolarization.

Serotonin activation of 5-HT2A receptors facilitates depolarization of neocortical neurons by N-methyl-D-aspartate (NMDA). Using grease-gap recordings from an in vitro cortical wedge preparation we have examined whether agents which raise the concentration of intracellular Ca2+ mimic the facilitation. Perfusing A23187, cyclopiazonic acid or thapsigargin selectively facilitate the NMDA depolarization of cortical neurons in a concentration-dependent manner. Buffering intracellular Ca2+ by perfusing BAPTA-AM eliminates the serotonin, cyclopiazonic acid and thapsigargin induced facilitation. We conclude that a rise in intracellular Ca2+ is both necessary and sufficient to account for facilitating the NMDA depolarization following activation of 5-HT2A receptors.

Fictive oesophageal peristalsis evoked by activation of muscarinic acetylcholine receptors in rat nucleus tractus solitarii.

The aim of the present study was to determine if muscarinic acetylcholine receptor-mediated peristaltic rhythmogenesis in the rat oesophagus is a central motor program that can be generated without peripheral sensory support. In anaesthetized male Sprague-Dawley rats, pressure-ejection of glutamate (10-20 pmol) and muscarine (5-10 pmol) in the sub-nucleus centralis of the nucleus tractus solitarii (NTSC) evoked monophasic pressure waves and rhythmic oesophageal peristalsis, respectively, but did not change mean arterial blood pressure or respiration. Application of muscarine (50-100 pmol) to the NTS extraventricular surface evoked rhythmic multi-unit burst discharges in the compact formation of the nucleus ambiguus (AMBC) that led to oesophageal peristalsis in a phase-locked manner. Evoked rhythmic AMBC activity persisted during neuromuscular blockade with curare, although the peak frequency of individual bursts was decreased. In a brainstem slice preparation, intracellular and whole cell patch recordings from AMBC neurones during focal stimulation of the NTSC region with muscarine revealed rhythmic depolarizing waves that showed a pattern similar to that of rhythmic oesophageal peristalsis. The present findings support the concept that medullary circuits comprising premotor neurones of the NTSC are intrinsically capable of generating rhythmic oesophagomotor output, but are subject to a powerful modulation by peripheral sensory feedback.

Facilitation of spinal motoneurone excitability by 5-hydroxytryptamine and noradrenaline.

The effects of iontophoretic application of 5-hydroxytryptamine (5-HT) and noradrenaline (NA) on lumbar motoneurone excitability were examined. 5-HT and NA produced long-lasting changes in motoneurone excitability as revealed by decreased threshold for glutamate-evoked motoneurone action potentials, increased rate of motoneurone firing evoked by suprathreshold amounts of glutamate and increased amplitude of ventral root and dorsal root evoked motoneurone field potentials. Neither 5-HT nor NA directly evoked motoneurone action potentials in the absence of other excitatory input. The 5-HT antagonist, methergoline, reduced glutamate-evoked motoneurone activity and prevented 5-HT, but not NA, facilitation of glutamate-evoked activity. These results suggest that 5-HT and NA enhance the effects of excitatory inputs to spinal motoneurones by actions on separate receptors.

Myo-inositol reduces serotonin (5-HT2) receptor induced homologous and heterologous desensitization.

The effect of myo-inositol was examined on 5-HT2 receptor mediated facilitation of NMDA depolarization of rat neocortical neurons in vitro. Myo-inositol (1-10 mM) potentiated the 5-HT facilitation, the potentiation increasing linearly with log 5-HT concentration. Myo-inositol also eliminated 5-HT induced heterologous desensitization of muscarinic and alpha 1-adrenergic receptor mediated facilitation. Our findings suggest that 5-HT induced homologous and heterologous desensitization results in part from depleting phosphoinositide substrate.

Long-lasting potentiation of the dentate gyrus population spike by norepinephrine.

The effects of iontophoretically applied norepinephrine (NE) on the dentate gyrus field potential evoked by perforant path stimulation were examined. NE potentiated the population spike by 20-400%, whereas the population EPSP was rarely increased. At 39% of the potentiated sites NE application resulted in long-lasting potentiation (LLP) suggesting a role for NE in long-term hippocampal plasticity.

Antagonism of spontaneous and evoked bursts by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in the CA3 region of the in vitro hippocampus.

Superfusion of hippocampal slices with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) antagonized kainate-induced bursts and bursts of unknown origin in the CA3 region. CNQX also increased the latency of and eventually blocked evoked bursts which persist following kainate washout. In contrast, D-(-)-2-amino-7-phosphonoheptanoic acid did not alter burst latency or block bursts unless applied subsequent to CNQX. We conclude that the quisqualate type receptor has a prominent role in burst generation with a smaller contribution from N-methyl-D-aspartate receptors.

Activation of NMDA receptors is necessary for fast information transfer at brainstem vagal motoneurons.

The involvement of N-methyl-D-aspartate (NMDA) excitatory amino acid subtype receptors in synaptically driven excitatory responses of ambigual motoneurons was investigated in vivo and in vitro. In urethane-anaesthetized rats, fictive oesophageal peristalsis evoked by topical application of muscarine (0.05-0.5 nmol) to the dorsal surface of the solitarial complex (NTS) was reversibly blocked by ipsilateral intraambigual injection of DL-2-amino-7-phosphonoheptanoic acid (AP-7, 0.5-1.5 nM) and (+-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 0.5-1.5 nM). In brainstem sagittal slices, post-synaptic potentials were recorded from neurons of the compact formation of the nucleus ambiguus (AMBc). Stimulation of presumptive NTS afferents elicited a complex excitatory postsynaptic potential (EPSP) which usually consisted of both a high-threshold fast (HTF) and a low-threshold slow (LTS) component. Bath perfusion with AP-7 (30-50 microM) and CPP (50 microM) selectively blocked the HTF without affecting the LTS component, while kynurenate (1 mM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5-10 microM) nonselectively suppressed both components. With sufficient stimulus strength, the EPSP generated a single spike arising from the HTF component. AP-7 (50 microM) either blocked the spike or increased the firing threshold. Furthermore, at the resting membrane potential, bath-applied NMDA induced a net inward current (269 +/- 189 pA) which had a negative slope in the range of -95 to -35 mV. In conclusion, NMDA receptors participate in solitario-ambigual synaptic transmission under physiological conditions and activation of these receptors is necessary for functional information transfer in this pathway.

Serotonin-like actions of quipazine and CPP on spinal motoneurones.

The actions of iontophoretically applied quipazine (QPZ) and 6-chloro-2-[1-piperazinyl]-pyrazine (CPP) were compared with those of serotonin (5-HT) on rat spinal motoneurones. QPZ and CPP qualitatively resembled 5-HT in that both facilitated single unit activity evoked by glutamate. Like 5-HT, the facilitation they produced could be antagonized by metergoline or methysergide. These observations are compatible with the suggestion that the actions of QPZ and CPP are mediated by 5-HT receptors. In rats pretreated with the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), QPZ and CPP remained effective in facilitating the glutamate evoked activity, whereas p-chloroamphetamine (PCA), a known releaser of 5-HT, was without effect. In contrast, PCA produced a long lasting facilitation in untreated rats. These data, taken together, suggest that QPZ and CPP are direct agonists at 5-HT receptors, but do not preclude the possibility that they might also act indirectly.

Multiple mechanisms of serotonin 5-HT2 receptor desensitization.

Desensitization of serotonin 5-HT2 receptor-mediated enhancement of the N-methyl-D-aspartate (NMDA) depolarization was studied in rat cortical neurons. Serotonin and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) induced long term desensitization. Staurosporine, a nonspecific protein kinase C inhibitor, potentiated the serotonin and DOI facilitation, suggesting acute desensitization was operative. In the case of DOI, long term desensitization was prevented by staurosporine. Activators of protein kinase C abolished the serotonin facilitation, an action prevented by staurosporine. Concanavalin A potentiated the facilitation at 100 microM, but not 30 microM serotonin, suggesting these receptors undergo dose dependent internalization. Calmodulin antagonists prevent long term desensitization induced by serotonin. The depolarization induced by NMDA alone was not altered by staurosporine, protein kinase C activators, concanavalin A or calmodulin antagonists. Serotonin at 100 microM, but not 30 microM, induced heterologous desensitization of phenylephrine and carbachol induced facilitation of the NMDA depolarization. We conclude that serotonin 5-HT2 receptors both induce and undergo several forms of desensitization.

Activation of 5-HT2 receptors facilitates depolarization of neocortical neurons by N-methyl-D-aspartate.

The interaction between serotonin and excitatory amino acid agonists at rat neocortical neurons was investigated using the grease-gap recording method. Depolarization evoked by 50 microM N-methyl-D-aspartate was dose dependently facilitated by serotonin (5-HT) (1 to 100 microM) giving a bell-shaped dose-response curve with maximum enhancement at 30 microM. In contrast, quisqualate and kainate depolarizations were not enhanced. Subnanomolar concentrations of methysergide, ritanserin and spiperone, but not ICS 205-930, attenuated the 5-HT enhancement, compatible with 5-HT2, but not 5-HT1 or 5-HT3 receptor subtype involvement. Enhancement was observed with 5-HT2 receptor agonists, whereas 5-HT1 receptor subtype agonists had either no effect (1B and 1C) or reduced (1A) the N-methyl-D-aspartate depolarization. Scopolamine and prazosin reduced the N-methyl-D-aspartate depolarization and blocked facilitation induced by carbachol and phenylephrine, but not that due to 5-HT. Tetrodotoxin reduced the N-methyl-D-aspartate depolarization, but the facilitation by 5-HT persisted. Activators of protein kinase C (phorbol diacetate and 1-oleoyl-2-acetyl-sn-glycerol) did not mimic the serotonin facilitation. We conclude that serotonin enhances N-methyl-D-aspartate depolarization of rat cortical neurons through activation of 5-HT2 receptors, however the cellular mechanism underlying the facilitation remains to be established.