Regulation of rat cortical 5-hydroxytryptamine2A receptor-mediated electrophysiological responses by repeated daily treatment with electroconvulsive shock or imipramine

Cortical influences of serotonin and glutamate on layer V pyramidal neurons

Layer V pyramidal neurons constitute principle output neurons of the medial prefrontal cortex (mPFC)/neocortex to subcortical regions including the intralaminar/midline thalamic nuclei, amygdala, basal ganglia, brainstem nuclei and the spinal cord. The effects of 5-hydroxytryptamine (5-HT) on layer V pyramidal cells primarily reflect a range of excitatory influences through 5-HT_2A receptors and inhibitory influences through non-5-HT_2A receptors, including 5-HT_1A receptors. While the 5-HT_2A receptor is primarily a postsynaptic receptor on throughout the apical dendritic field of 5-HT_2A receptors, activation of a minority of 5-HT_2A receptors also appears to increase spontaneous excitatory postsynaptic currents/potentials (EPSCs/EPSPs) via a presynaptic effect on thalamocortical terminals arising from the midline and intralaminar thalamic nuclei. Activation of 5-HT_2A receptors by the phenethylamine hallucinogen also appears to increase asynchronous release of glutamate upon the layer V pyramidal dendritic field, an effect that is suppressed by 5-HT itself through non-5-HT_2A receptors. Serotonergic hallucinogens acting on 5-HT_2A receptors also appears to increase gene expression of immediate early genes (iEG) and other receptors appearing to induce an iEG-like response like BDNF. Psychedelic hallucinogens acting on 5-HT_2A receptors also induce head twitches in rodents that appear related to induction of glutamate release. These electrophysiological, biochemical and behavioral effects of serotonergic hallucinogens appear to be related to modulating glutamatergic thalamocortical neurotransmission and/or shifting the balance toward 5-HT_2A receptor activation and away from non-5-HT_2A receptor activation. These 5-HT_2A receptor induced responses are modulated by feedback homeostatic mechanisms through mGlu₂, mGlu₄, and mGlu₈ presynaptic receptors on thalamocortical terminals. These 5-HT_2A receptor and glutamatergic interactions also appear to play a role on higher cortical functions of the mPFC such as motoric impulsivity and antidepressant-like behavioral responses on the differential-reinforcement-of low rate 72-s (DRL 72-s schedule). These mutually opposing effects between 5-HT_2A receptor and mGlu autoreceptor activation (e.g., blocking 5-HT_2A receptors and enhancing activity at mGlu₂ receptors) may play a clinical role with respect to currently prescribed or novel antidepressant drugs. Thus, there is an important balance between 5-HT_2A receptor activation and activation of mGlu autoreceptors on prefrontal cortical layer V pyramidal cells with respect to the electrophysiological, biochemical and behavioral effects serotonergic hallucinogenic drugs.

Fluoxetine and stress inversely modify lateral septal nucleus-mpfc neuronal responsivity

Several clinically effective antidepressants increase the neuronal firing rate in the lateral septal nucleus (LSN), a forebrain structure that is anatomically related to medial prefrontal cortex (mPFC) regions. mPFC function is related to depression and the regulation of fear. However, unknown is whether antidepressant treatment or chronic stress modifies the responsivity of neuronal LSN-mPFC connections. We performed single-unit extracellular recordings in the anterior cingulate cortex (ACC) and prelimbic (PL) and infralimbic (IL) regions of the mPFC during stimulation of the LSN in anesthetized male Wistar rats that received fluoxetine (1 mg/kg, 21 days) or were subjected to chronic mild stress (5 weeks). The results were compared with a control group (saline treatment, devoid of behavioral manipulations). Stimulation of the LSN produced an initial excitatory paucisynaptic response, followed by an afterdischarge, characterized by an increase in the neuronal firing rate. Opposite changes were induced by fluoxetine treatment and chronic stress exposure. Peristimulus histograms and unit-activity ratio analyses indicated that LSN-mPFC responsivity differed between fluoxetine treatment and chronic stress exposure. Fluoxetine reduced neuronal responsivity in the LSN-PL and LSN-IL, and stress increased neuronal responsivity in the same regions. In both cases, the changes were more pronounced in the IL region. The lower responsivity of LSN-PL and LSN-IL connections that was produced by fluoxetine may reflect a higher threshold for fear, and lower responsivity of this connection may be related to states of fear. The LSN and mPFC comprise a portion of a limbic-cortical circuit where neuronal responses depend on specific conditions.

Effect of acute and chronic electroconvulsive shock on 5-hydroxytrypamine 6 receptor immunoreactivity in rat hippocampus

Electroconvulsive shock (ECS) induces not only an antidepressant effect but also adverse effects such as amnesia. One potential mechanism underlying both the antidepressant and amnesia effect of ECS may involve the regulation of serotonin (5-hydroxytryptamine) 6 (5-HT6) receptor, but less is known about the effects of acute ECS on the changes in 5-HT6 receptor expression in the hippocampus. In addition, as regulation of 5-HT receptor expression is influenced by the number of ECS treatment and by interval between ECS treatment and sacrifice, it is probable that magnitude and time-dependent changes in 5-HT6 receptor expression could be influenced by repeated ECS exposure. To explore this possibility, we observed and compared the changes of 5-HT6 receptor immunoreactivity (5-HT6 IR) in rat hippocampus at 1, 8, 24, or 72 h after the treatment with either a single ECS (acute ECS) or daily ECS for 10 days (chronic ECS). We found that acute ECS increased 5-HT6 IR in the CA1, CA3, and granule cell layer of hippocampus, reaching peak levels at 8 h and returning to basal levels 72 h later. The magnitude and time-dependent changes in 5-HT6 IR observed after acute ECS were not affected by chronic ECS. These results demonstrate that both acute and chronic ECS transiently increase the 5-HT6 IR in rat hippocampus, and suggest that the magnitude and time-dependent changes in 5-HT6 IR in the hippocampus appear not to be influenced by repeated ECS treatment.

New approaches to the pharmacological management of major depressive disorder

Despite effective and safe therapies for major depressive disorder (MDD), the current arsenal of antidepressant therapies does not fully satisfy the needs of patients or physicians. Many patients are only partial responders or are treatment resistant and side effects interfere with compliance. The majority of antidepressants directly affect monoamine neurotransmission within the central nervous system. Moving beyond this mechanism has been a challenge because of the lack of knowledge about the underlying etiology and pathophysiology of MDD. Provided in this report is a review of some of the major new advances in MDD research that suggest the possibility of novel and improved future therapeutic options. Emphasis is placed on studies of unipolar, but not bipolar, depression. New therapies include dual and triple monoamine uptake inhibitors, non-conventional antidepressants such as tianeptine, and a number of augmentation strategies. In addition, studies are underway on a number of mechanisms of action that might yield the next therapeutic advance. These include agents that interact with endocannabiniod systems, examination of natural products, and compounds that influence neuropeptide systems such as galanin and melanin-concentrating hormone, and growth and neurotrophic factors. Epigenetic mechanisms involving histone modification are also being explored. An area of intensive investigation is glutamate neurotransmission. Data support the hypothesis that NMDA receptor antagonists are effective in MDD individuals resistant to conventional therapies. The potential of metabotropic glutamate receptors as novel targets is also discussed. Accumulating evidence supports the idea that amplification of AMPA receptor function is a critical link in the transduction processes involved antidepressant effects.

Cortical 5-hydroxytryptamine2A-receptor mediated excitatory synaptic currents in the rat following repeated daily fluoxetine administration

Down-regulation of 5-hydroxytryptamine(2A) (5-HT(2A)) receptors has been a consistent effect induced by most antidepressant drugs. The evidence for down-regulation of 5-HT(2A) receptor binding following subchronic treatment with fluoxetine and other selective serotonin reuptake inhibitors (SSRIs) is mixed. The question of 5-HT(2A) receptor sensitivity during chronic administration of antidepressants is important since activation of 5-HT(2A) receptors is associated with impulsivity. Continued activation of 5-HT(2A) receptors may functionally oppose activation of other non-5-HT(2A) receptors in the prefrontal cortex associated with the clinical efficacy of SSRI treatment. Therefore, the effects of repeated daily administration of fluoxetine (10 mg/kg, i.p. x 3 weeks) on pharmacologically characterized electrophysiological response mediated by 5-HT(2A) receptor activation, 5-HT-induced excitatory postsynaptic currents (EPSCs), in rat prefrontal cortical slices was examined. The concentration-response curve for 5-HT-induced EPSCs was unchanged following subchronic fluoxetine treatment. This subchronic fluoxetine treatment failed to modify electrophysiological responses to AMPA in layer V pyramidal cells as well. These findings would be consistent with the hypothesis that blockade of 5-HT(2A) receptors may enhance the effects of SSRIs or serotonin/norepinephrine reuptake inhibitors (SNRIs).