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

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.

Responses to 5-HT in morphologically identified neurons in the rat substantia gelatinosa in vitro

Bath application of 5-HT (1-1000 muM) induced a tetrodotoxin (TTX)-resistant outward current at the holding membrane potential (V(H)) of -50 mV in 104/162 (64.2%) of substantia gelatinosa (SG) neurons from the rat spinal cord in vitro. The 5-HT-induced outward current was suppressed by an external solution containing Ba(2+), or a pipette solution containing Cs(2)SO(4) and tetraethylammonium. It was reversed near the equilibrium potential of the K(+) channel. The response to 5-HT was abolished 30 min after patch formation with a pipette solution containing guanosine-5-O-(2-thiodiphosphate)-S. The 5-HT-induced outward current was mimicked by a 5-HT(1A) agonist, (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide, and suppressed by a 5-HT(1A) antagonist, WAY100635, suggesting the 5HT(1A) receptor-mediated activation of K(+) channels in the outward current. In 11/162 (6.8%) SG neurons, 5-HT produced an inward current, which was mimicked by a 5-HT(3) agonist, 1-(m-chlorophenyl)-biguanide (mCPBG). The 5-HT-induced outward currents were observed in vertical cells (21/34) and small islet cells (11/34), while inward currents were induced in islet cells (1/5) and small islet (4/5) cells, but not in vertical cells. It is known that most vertical cells and islet cells in the SG are excitatory (glutamatergic) and inhibitory interneurons, respectively, while small islet cells consist of both excitatory and inhibitory neurons. Bath application of 5-HT or mCPBG increased the amplitude and the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs), but no neurons showed a decrease in sIPSC. Furthermore, frequency, but not amplitude, of miniature IPSCs increased with perfusion with 5-HT in the presence of TTX. These findings, taken together, suggest that 5-HT induces outward currents through 5-HT(1A) receptors in excitatory SG neurons. These findings also suggest that the inward currents are post- and presynaptically evoked through 5-HT(3) receptors, probably in inhibitory neurons.

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.