Actions of serotonin recorded intracellularly in rat dorsal lateral septal neurons

Impact of Vortioxetine on Synaptic Integration in Prefrontal-Subcortical Circuits: Comparisons with Escitalopram

Prefrontal-subcortical circuits support executive functions which often become dysfunctional in psychiatric disorders. Vortioxetine is a multimodal antidepressant that is currently used in the clinic to treat major depressive disorder. Mechanisms of action of vortioxetine include serotonin (5-HT) transporter blockade, 5-HT_1A receptor agonism, 5-HT_1B receptor partial agonism, and 5-HT_1D, 5-HT₃, and 5-HT₇ receptor antagonism. Vortioxetine facilitates 5-HT transmission in the medial prefrontal cortex (mPFC), however, the impact of this compound on related prefrontal-subcortical circuits is less clear. Thus, the current study examined the impact of systemic vortioxetine administration (0.8 mg/kg, i.v.) on spontaneous spiking and spikes evoked by electrical stimulation of the mPFC in the anterior cingulate cortex (ACC), medial shell of the nucleus accumbens (msNAc), and lateral septal nucleus (LSN) in urethane-anesthetized rats. We also examined whether vortioxetine modulated afferent drive in the msNAc from hippocampal fimbria (HF) inputs. Similar studies were performed using the selective 5-HT reuptake inhibitor [selective serotonin reuptake inhibitors (SSRI)] escitalopram (1.6 mg/kg, i.v.) to enable comparisons between the multimodal actions of vortioxetine and SSRI-mediated effects. No significant differences in spontaneous activity were observed in the ACC, msNAc, and LSN across treatment groups. No significant impact of treatment on mPFC-evoked responses was observed in the ACC. In contrast, vortioxetine decreased mPFC-evoked activity recorded in the msNAc as compared to parallel studies in control and escitalopram treated groups. Thus, vortioxetine may reduce mPFC-msNAc afferent drive via a mechanism that, in addition to an SSRI-like effect, requires 5-HT receptor modulation. Recordings in the LSN revealed a significant increase in mPFC-evoked activity following escitalopram administration as compared to control and vortioxetine treated groups, indicating that complex modulation of 5-HT receptors by vortioxetine may offset SSRI-like effects in this region. Lastly, neurons in the msNAc were more responsive to stimulation of the HF following both vortioxetine and escitalopram administration, indicating that elevation of 5-HT tone and 5-HT receptor modulation may facilitate excitatory hippocampal synaptic drive in this region. The above findings point to complex 5-HT receptor-dependent effects of vortioxetine which may contribute to its unique impact on the function of prefrontal-subcortical circuits and the development of novel strategies for treating mood disorders.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

5-Hydroxytryptamine 1B receptors mediate presynaptic inhibition of monosynaptic IPSC in the rat dorsolateral septal nucleus

Effects of 5-hydroxytryptamine (5-HT) on inhibitory synaptic transmission in the rat dorsolateral septal nucleus (DLSN) were examined by conventional intracellular and voltage-clamp recording methods. 5-HT (1-30 microM) depressed the monosynaptic fast IPSC evoked by local stimulation of the DLSN in the presence of DNQX, AP5 and CGP 55845A. CP 93129, a selective 5-HT1B receptor agonist, depressed the fast IPSC. The 5-HT-induced depression of the fast IPSC was attenuated by SB 216641, a selective antagonist for 5-HT1B receptors. 5-HT did not change the inward currents mediated by GABAA receptors, suggesting that 5-HT presynaptically inhibited the fast IPSC. 5-HT and CP 93129 depressed the frequency of miniature fast IPSPs (mIPSPs) without changing their amplitudes. Neither a selective protein kinase A inhibitor, H-89, nor a selective protein kinase C inhibitor, calphostin C, blocked the 5-HT-induced depression of the fast IPSC. N-Ethylmaleimide (NEM) blocked the 5-HT-induced depression of the evoked IPSC. These results suggest that activation of presynaptic 5-HT1B receptors depresses the release of GABA via a pertussis toxin (PTX)-sensitive G-protein in the rat DLSN.

Serotonin produces an enhanced outward current recorded at rat dorsal lateral septal neurons from the Flinders Sensitive Line of rats, a genetically-selected animal model of depression

Abnormalities in serotonin (5-HT), serotonin receptors, and serotonergic neurons have been reported in studies of brains from patients diagnosed clinically with depression. In this study, we examined a known cellular function of 5-HT(1A) receptor activation in dorsolateral septal nucleus (DLSN) neurons, namely, a concentration dependent 5-HT-induced outward current, and compared basic neuronal membrane properties and activities of DLSN neurons from two known genetic lines of rats. As compared to "control" rats (Flinders Resistant Line, FRL), DLSN neurons from Flinders Sensitive Line of rats (FSL) did not exhibit significant differences in resting membrane potential, membrane input resistance, or changes in typical spontaneous inhibitory or excitatory post-synaptic currents. FSL-rats exhibit a depressive phenotype and have been suggested to be rats with a genetic susceptibility to exhibit depressive behaviors. Exogenous application of 5-HT resulted in expected concentration-dependent outward currents; however, the amplitudes of these currents were enhanced significantly in 50% of DLSN neurons recorded from FSL rats compared to similar results recorded from FRL rats. Our results suggest that within a particular population of DLSN neurons from rats exhibiting a known phenotype of depression a post-synaptic 5-HT(1A) receptor is functionally hyper-responsive compared to controls.

Activation of presynaptic 5-hydroxytryptamine 2A receptors facilitates excitatory synaptic transmission via protein kinase C in the dorsolateral septal nucleus

Effects of 5-hydroxytryptamine (5-HT) on EPSPs and EPSCs in the rat dorsolateral septal nucleus (DLSN) were examined in the presence of GABA(A) and GABA(B) receptor antagonists. Bath application of 5-HT (10 microm) for 5-10 min increased the amplitude of the EPSP and EPSC. (+/-)-8-hydroxy-2-(di-N-propylamino)tetralin hydrobromide (10 microm), an agonist for 5-HT1A and 5-HT7 receptors, did not facilitate the EPSP. alpha-Methyl-5-HT (10 microm), a 5-HT2 receptor agonist, increased the amplitude of the EPSC. Alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine (10 microm) and 6-chloro-2-(1-piperazinyl)pyrazine (10 microm), selective 5-HT2B and 5-HT2C receptor agonists, respectively, had no effect on the EPSP. The 5-HT-induced facilitation of the EPSP was blocked by ketanserin (10 microm), a 5-HT2A/2C receptor antagonist. However, N-desmethylclozapine (10 microm), a selective 5-HT2C receptor antagonist, did not block the facilitation of the EPSP induced by alpha-methyl-5-HT. The inward current evoked by exogenous glutamate was unaffected by 5-HT. 5-HT (10 microm) and alpha-methyl-5-HT (10 microm) increased the frequency of miniature EPSPs (mEPSPs) without changing the mEPSP amplitude. The ratio of the paired pulse facilitation was significantly decreased by 5-HT and alpha-methyl-5-HT. The 5-HT-induced facilitation of the EPSP was blocked by calphostin C (100 nm), a specific protein kinase C (PKC) inhibitor, but not by N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (10 microm), a protein kinase A inhibitor. Phorbol 12,13-dibutyrate (3 microm) mimicked the facilitatory effects of 5-HT. These results suggest that 5-HT enhances the EPSP by increasing the release of glutamate via presynaptic 5-HT2A receptors that link with PKC in rat DLSN neurons.

The lowest effective dose of fluoxetine in the forced swim test significantly affects the firing rate of lateral septal nucleus neurones in the rat

The administration of a relatively high dose of antidepressant drugs produces an increased neuronal firing rate of the lateral septal nucleus (LSN) in the rat and a decreased immobility in rats forced to swim. However, it is unknown whether a minimally effective low-dose 21-day treatment with the selective serotonin reuptake inhibitor, fluoxetine, while reducing immobility in the forced swim test, also increases the neuronal firing rate of the LSN in Wistar rats. The total time of immobility decreased with a daily injection of 0.5, 1.0 or 2.0 mg/kg of fluoxetine (p < 0.001), and the lowest dose increasing the latency to the first immobility period (p < 0.0001) was 1.0 mg/kg. Therefore, the action of the 21-day fluoxetine treatment (1.0 mg/kg) on the firing rate of LSN neurones was tested in another group of rats. A total amount of 78 single-unit extracellular recordings was taken from the LSN of eight control rats (n = 40) and eight fluoxetine treated rats (n = 38). The LSN firing rate in the fluoxetine group was double (18.3 +/- 2.5 spikes per 10 s, p < 0.05) that in the control group (7.0 +/- 0.9 spikes per 10 s), and the first order interval of firing proved to be significantly lower in the fluoxetine group compared to the control group (384.3 +/- 22.3 and 639.7 +/- 27.5 ms, respectively; p < 0.05). In conclusion, the increased neuronal tiring rate of the LSN in the animals treated with a low dose of fluoxetine may be associated with an increased motivation to escape from the stressful situation that the forced swim represents.

5-Hydroxytryptamine facilitates spatiotemporal propagation of optical signals in the hippocampal-septal pathway

The role of 5-hydroxytryptamine (5-HT) on the propagation of neuronal excitation in the hippocampal-septal pathway was examined in a brain slice by optical and electrophysiological recording techniques. After electrical stimulation of the fimbrial pathway, optical signals first occurred at the caudal region of lateral septal nucleus (LSN), then propagated toward the rostral region of LSN. All of the evoked optical signals were blocked by tetrodotoxin (TTX). The optical signal that propagated to the LSN was blocked by either the removal of external Ca(2+) or bath-application of 6-cyano-7-nitroquinoxaline-2,3-(1H,4H)-dione (CNQX). Bath-application of 5-HT (1-50 microM) to the LSN for 10 min produced an increase in the propagation area of the optical signal and prolonged the falling phase of the optical signal. Bicuculline blocked the 5-HT-induced facilitation of the optical signal. 8-Hydroxy-di-n-propylamino tetralin (8-OH-DPAT), a selective 5-HT(1A) agonist, mimicked the facilitation of 5-HT. 1-(2-Methoxyphenyl)-4-(4-phthalimidobutyl)piperazine (NAN-190), a 5-HT(1A) antagonist, blocked the facilitation induced by 5-HT. 5-HT enhanced the amplitude of the field potential in septal slices, where the optical signals had been enhanced. These results indicate that 5-HT increases the efficacy of excitatory synaptic transmission in the hippocampal-septal circuit via 5-HT(1A) receptors of LSN neurons.

Characterization of outward currents induced by 5-HT in neurons of rat dorsolateral septal nucleus

Properties of the 5-hydroxytryptamine (5-HT)-induced current (I(5-HT)) were examined in neurons of rat dorsolateral septal nucleus (DLSN) by using whole cell patch-clamp techniques. I(5-HT) was associated with an increase in the membrane conductance of DLSN neurons. The reversal potential of I(5-HT) was -93 +/- 6 (SE) mV (n = 7) in the artificial cerebrospinal fluid (ACSF) and was changed by 54 mV per decade change in the external K(+) concentration, indicating that I(5-HT) is carried exclusively by K(+). Voltage dependency of the K(+) conductance underlying I(5-HT) was investigated by using current-voltage relationship. I(5-HT) showed a linear I-V relation in 63%, inward rectification in 21%, and outward rectification in 16% of DLSN neurons. (+/-)-8-Hydroxy-dipropylaminotetralin hydrobromide (30 microM), a selective 5-HT(1A) receptor agonist, also produced outward currents with three types of voltage dependency. Ba(2+) (100 microM) blocked the inward rectifier I(5-HT) but not the outward rectifier I(5-HT). In I(5-HT) with linear I-V relation, blockade of the inward rectifier K(+) current by Ba(2+) (100 microM) unmasked the outward rectifier current in DLSN neurons. These results suggest that I(5-HT) with linear I-V relation is the sum of inward rectifier and outward rectifier K(+) currents in DLSN neurons. Intracellular application of guanosine-5'-O-(3-thiotriphosphate) (300 microM) and guanosine-5'-O-(2-thiodiphosphate) (5 mM), blockers of G protein, irreversibly depressed I(5-HT). Protein kinase C (PKC) 19-36 (20 microM), a specific PKC inhibitor, depressed the outward rectifier I(5-HT) but not the inward rectifier I(5-HT). I(5-HT) was depressed by N-ethylmaleimide, which uncouples the G-protein-coupled receptor from pertussis-toxin-sensitive G proteins. H-89 (10 microM) and adenosine 3',5'-cyclic monophosphothioate Rp-isomer (300 microM), protein kinase A inhibitors, did not depress I(5-HT). Phorbol 12-myristate 13-acetate (10 microM), an activator of PKC, produced an outward rectifying K(+) current. These results suggest that both 5-HT-induced inward and outward rectifying currents are mediated by a G protein and that PKC is probably involved in the transduction pathway of the outward rectifying I(5-HT) in DLSN neurons.

5-Hydroxytryptamine-induced outward currents mediated via 5-HT(1A) receptors in neurons of the rat dorsolateral septal nucleus

Effects of 5-hydroxytryptamine (5-HT) on neurons of the rat dorsolateral septal nucleus (DLSN) were examined by intracellular and whole-cell patch-clamp recording techniques. An outward current was induced by 5-HT (1-100 microM) in a concentration-dependent manner. The EC(50) for 5-HT was 4.8 microM. Also, 8-OH-DPAT (10-100 microM) produced the outward current an EC(50) of 17 microM. Amplitudes of the outward currents produced by 5-HT (100 microM) and 8-OH-DPAT (100 microM) were 117+/-4 (n=6) and 58+/-8 pA (n=6), respectively. Fluvoxamine (200 nM), a specific serotonin re-uptake inhibitor, enhanced the 5-HT (1 microM)-induced outward current: the EC(50) for 5-HT was 0.5 microM in the presence of fluvoxamine (200 nM). L-694247 (100 microM) and CP 93129 (100 microM) also produced outward currents with amplitudes of 33+/-3 (n=4) and 18+/-5 pA (n=4), respectively in DLSN neurons. DOI (100 microM) and RS 67333 (100 microM) did not produce outward currents. NAN-190 shifted, in a parallel manner, the concentration-response relationship of 5-HT to the right. The Lineweaver-Burk plot of the concentration-response curve showed that NAN-190 depressed the 5-HT-induced current in a competitive manner. The current-voltage relationship indicates that the 5-HT-induced current reversed polarity at a potential close to the equilibrium potential of K(+). Ba(2+) (100 microM-1 mM) partially depressed the outward current produced by 5-HT. These results suggest that 5-HT induces multiple K(+) currents via 5-HT(1A) receptors in DLSN neurons.

Septo-hippocampal drug interactions in post-trial memory processing

To determine if serotonin and GABA regulate post-trial memory processing of the cholinergic projection from the septum to the hippocampus, mice were trained on footshock avoidance in a T-maze. Immediately after training, drugs were injected into the septum, hippocampus or both. Retention was tested 1 week after training and drug administration. Ketanserin, a serotonin type 2 receptor antagonist at a dose of 0.5 ng, had no measurable effect on retention, but it reduced the dose of bicuculline, in the septum, or arecoline in the hippocampus that was needed to improve retention. DOI, a serotonin type 2 receptor agonist at a dose of 2.5 ng, had the opposite effect of increasing the doses of bicuculline and arecoline needed to improve retention. Bicuculline, a GABA(A) receptor antagonist at a dose of 0.1 pg, did not affect retention when injected alone into the septum, but it reduced the dose of arecoline needed to improve retention in the hippocampus. Muscimol, a GABA(A) receptor agonist at a dose of 5 ng, injected into the septum, increased the dose of arecoline needed to improve retention. The results of this study are compatible with models that propose that serotonin innervation from the median raphe drives GABA interneurons in the medial septum that synapse on cholinergic neurons projecting to the hippocampus.

Excitatory actions of serotonin on GABAergic neurons of the medial septum and diagonal band of Broca

The physiological and pharmacological actions of serotonin (5-HT) on neurons in the medial septum and diagonal band of Broca (MSDB) were examined using extracellular and intracellular recording techniques in an in vitro rat brain-slice preparation. In addition to previously described inhibitory effects, novel excitatory actions of 5-HT on GABA-type cells were observed. In intracellular recordings with KCl-containing electrodes, bath-applied 5-HT induced a bicuculline and tetrodotoxin-sensitive increase in the number of reverse IPSPs in both cholinergic- and noncholinergic-type neurons (presumably GABAergic). In brain slices where all structures neighboring the MSDB, including the lateral septum, had been excised, a similar increase in 5-HT-induced IPSPs occurred, indicating that 5-HT-induced IPSPs in both cholinergic- and noncholinergic-type neurons originate from GABAergic neurons within the MSDB itself. Accordingly, GABA-type neurons in the MSDB were found to be directly excited by 5-HT. MDL 100,907, a selective 5-HT2A antagonist, blocked 5-HT-induced excitations in a majority of neurons (58%). ICS 205-930, a 5-HT3/5-HT4 antagonist, or mianserin, a nonselective 5-HT antagonist, blocked most MDL-resistant responses, indicating a role for multiple 5-HT receptor subtypes. This study also provides the first electrophysiological evidence for synaptic interactions between 5-HT-activated GABAergic neurons and cholinergic neurons and amongst GABAergic neurons in the MSDB. The implications of the findings vis-à-vis intraseptal circuitry and septohippocampal circuitry are discussed.

Synaptic inputs to GABAA and GABAB receptors originate from discrete afferent neurons

gamma-Aminobutyric acid (GABA) inhibits neurons by acting at GABAA and GABAB receptors but it is not known whether the two receptors are associated with discretely separate afferent inputs or whether GABA released from a single presynaptic neuron activates both receptors. Intracellular recordings were used to show that, in the lateral amygdala and ventral tegmental area of the rat, distinct sets of GABA-containing neurons provide the synaptic input to GABAA and GABAB receptors. Synaptic potentials resulting from GABAA receptor activation (blocked by bicuculline) and from GABAB receptor activation (blocked by 2-hydroxysaclofen) occurred spontaneously but as unrelated events. Furthermore, the two components of evoked synaptic potentials were differentially inhibited by agonists acting presynaptically (muscarine and 5-hydroxytryptamine). The finding that GABA acting at GABAA and GABAB receptors originates from distinct sets of presynaptic fibers suggests that two groups of GABA-containing neurons might be generally distinguishable in the mammalian nervous system.

Facilitation of dopamine release in vivo by serotonin agonists: studies with microdialysis

Using microdialysis, changes in extraneuronal levels of dopamine (DA), and the metabolites of DA and serotonin (5-HT), were monitored concurrent with perfusion of 5-HT1 agonists into the anterior striata of anesthetized rats. Perfusion of 5-HT facilitated DA release in a dose dependent manner, and to a greater extent than any other agonist tested. Extraneuronal DA levels increased 34% with perfusion of 0.04 nmol 5-HT and 18-fold with perfusion of 4.0 nmol 5-HT. Perfusion with multiple doses of either 1-(m-chlorophenyl)piperazine (m-CPP) or trifluoromethylphenylpiperazine (TFMPP) also resulted in a dose-dependent facilitation of DA release with a 40% increase in extracellular DA produced by either 0.4 nmol m-CPP or 10.0 nmol TFMPP. A 50-fold increase in DA followed 40.0 nmol m-CPP, while 160 nmol TFMPP enhanced DA 11-fold. Local application of either 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H indole succinate (RU24969) or 8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) (2.0 nmol perfused over 20 min) increased extracellular DA by 300 and 40%, respectively. RU24969 (2.0 nmol) also facilitated DA release following systemic pretreatment with 8-OH-DPAT (100 micrograms/kg). Perfusion with fenfluramine to release endogenous 5-HT also increased extraneuronal DA in a dose-dependent manner, and this facilitation was prevented by pretreatment with the 5-HT reuptake inhibitor fluoxetine. The facilitation of DA release by 0.4 nmol 5-HT was reduced by pretreatment with the 5-HT1 antagonist pindolol (4.0 nmol). These results suggest that serotonergic innervation of the anterior striatum may exert a facilitatory influence on DA release.

Pharmacology of nicotinic receptor-mediated inhibition in rat dorsolateral septal neurones

1. Intracellular electrophysiological techniques were employed to investigate the effects of nicotinic receptor stimulation on rat dorsolateral septal nucleus (DLSN) neurones in a submerged rat brain slice preparation. 2. Acetylcholine (in the presence of the muscarinic antagonist, atropine), nicotine or dimethylphenylpiperazinium (DMPP), applied either by pressure ejection or superfusion, produced predominantly a membrane potential hyperpolarization. 3. Following concentration-response comparisons, DMPP appeared to exhibit fewer desensitizing properties and greater efficacy than nicotine with half-maximal hyperpolarizing responses attainable at 3 and 10 microM, respectively. 4. Pharmacological analyses revealed that the agonist-induced membrane hyperpolarization was sensitive to antagonism by mecamylamine (50-100 microM) and neuronal bungarotoxin (0.2-0.3 microM), but not alpha-bungarotoxin (0.5-1.0 microM), curare (10-50 microM) or dihydro-beta-erythroidine (50-100 microM). 5. Hyperpolarizing responses to DMPP were found to reverse near the equilibrium potential for potassium and were sensitive to changes in extracellular potassium concentration as predicted by the Nernst equation. Under single-electrode voltage clamp, application of DMPP produced an outward current (75-100 pA) which approached reversal at around -88 mV. These findings indicated that the hyperpolarizing response to nicotinic receptor stimulation was mediated by changes in membrane permeability to potassium. 6. DMPP-induced membrane hyperpolarization resulted from a direct action on postsynaptic DLSN neurones since the response persisted under conditions of superfusion with calcium-free/high-magnesium media or tetrodotoxin; both conditions blocked orthodromically induced neurotransmission. The hyperpolarizing response remained unaltered in TTX but was diminished in calcium-free/high-magnesium media. Further studies revealed blockade of the DMPP response following intracellular injection of EGTA. This response was also sensitive to antagonism by various calcium-dependent potassium channel blockers including apamin, barium and tetraethylammonium. 7. Our studies reveal a novel class of CNS nicotinic receptor whose action upon stimulation by an agonist results in a membrane hyperpolarization via a calcium-dependent increase in potassium ion conductance.

Serotonin receptor heterogeneity and the role of potassium channels in neuronal excitability

Intracellular recordings in vitro from a variety of central neuronal types have shown both inhibition and excitation to be modulatory consequences of serotonin (5-HT) receptor activation. These responses can be seen in isolation or in some cases (e.g. hippocampal pyramidal cells) as a complex biphasic combination of hyperpolarisation followed by depolarisation, suggesting overall control of neuronal excitability may be dependent on the interaction between activation of more than one post-synaptic receptor and/or mechanism. Our studies have confirmed the 5-HT evoked depolarisation of rat facial motorneurones (FM's) and the hyperpolarisation seen in presumed serotonergic neurones of the dorsal raphe nucleus (DRN) to be the result of opposite effects on K+ ion permeability. Suppression of a resting K+ conductance leads to depolarisation while activation leads to hyperpolarisation. The same mechanisms appear to be responsible for the 5-HT evoked responses in hippocampal pyramidal cells but in addition there is also a suppression of a Ca++ dependent K+ conductance responsible for the long spike after hyperpolarisation (AHP). Data from the hippocampus and DRN indicate the 5-HT induced hyperpolarisation to be sensitive to Pertussis Toxin (PTX) and irreversibly mimicked by GTP gamma S, a non-hydrolysable analogue of GTP, suggesting the involvement of a G protein in K+ channel activation. The mechanism of K+ channel closure is less clear as it is unaffected by PTX or activation of adenylate cyclase, however there is indirect evidence that the phosphoinositide pathway may be involved from the cloned 5-HT1C receptor which also closes a K+ channel in cell lines. The results show that hyperpolarisation evoked by 5-HT in the hippocampus and DRN to be mimicked and blocked by 5-HT1A agonists and antagonists. However, the depolarisations in the hippocampus and FM's are mediated by site-dependent receptors with profiles which do not fit into the current 5-HT receptor subtype classification.

Electrophysiological properties of frog spinal dorsal horn neurons and their responses to serotonin: an intracellular study in the isolated hemisected spinal cord

Frog dorsal horn neurons and their responses to serotonin (5-HT) were studied in intracellular recordings from isolated hemisected spinal cords. Electrophysiological properties were comparable to those of mammals. Bath application of 5-HT (10-50 microM) increased the excitability and caused membrane depolarizations in 7/14 cells, reduced or abolished activity and caused hyperpolarizations in 4 neurons, exerted a biphasic effect in two cells, and produced no detectable change in one neuron. The multiple effects of 5-HT may be mediated through different 5-HT receptor subtypes.

Serotonin attenuates a slow inhibitory postsynaptic potential in rat hippocampal neurons

Activity of hippocampal neurons was recorded in an in vitro slice preparation. Topical application of serotonin produced hyperpolarization, blockade of a slow afterhyperpolarization which follows a burst discharge and blockade of a slow inhibitory postsynaptic potential. The slow inhibitory postsynaptic potential evoked by stimulation of the apical dendritic region of the hippocampus is more sensitive to serotonin than the membrane potential or conductance. The effects of serotonin on the inhibitory postsynaptic potentials are blocked by the 5-HT1a antagonist spiperone, and not by mianserin, a 5-HT2 antagonist. The attenuation of the inhibitory postsynaptic potentials is not accompanied by a change in postsynaptic reactivity to GABA or baclofen. Serotonin blocks repetitive large inhibitory postsynaptic potentials evoked in hippocampal neurons by topical application of 4-aminopyridine. Putative interneurons are more sensitive to topical application of serotonin than pyramidal neurons. Fenfluramine, a serotonin releaser mimics the effects of topical application of serotonin indicating that synaptically released serotonin can produce the changes in membrane potential and reactivity to afferent stimulation. It is suggested that serotonin attenuates slow inhibitory postsynaptic potentials by inhibiting feed forward inhibitory interneurons which impinge upon the recorded pyramidal neurons.

Serotonin and local circuits in rat hippocampus

Serotonin produces several actions in rat hippocampal neurons recorded in an in vitro slice preparation. These include activation of a voltage-independent potassium current which results in the hyperpolarization of the recorded cells, the blockage of a calcium-dependent slow potassium current underlying the slow after-hyperpolarization (AHP) which follows a burst firing and generation of a slow late depolarization associated with the blockage of a persistent potassium current. An additional effect that we detected recently involves a blockage of inhibitory postsynaptic potentials (IPSP), primarily the slow IPSP, likely to be mediated by activation of potassium currents through a gamma-aminobutyric acid (GABA)-B receptor. The reduction in the IPSPs is produced by a lower concentration of serotonin than that required to produce the hyperpolarization. The lack of reduced sensitivity to GABA indicates that the effect of serotonin on the IPSPs might be presynaptic. Indeed, serotonin has a much larger effect on putative interneurons than on pyramidal neurons. It is suggested that serotonin can enhance reactivity of the hippocampus to afferent stimulation by reducing slow inhibitory synaptic potentials while at the same time reducing the spontaneous activity of the recorded neurons. These effects of serotonin develop relatively late (2-3 weeks postnatally), along with the late appearance of the slow inhibitory potentials.