A major subpopulation of midbrain 5-hydroxytryptamine (5-HT) neurons expresses the vesicular glutamate transporter 3 (VGLUT3) and co-releases 5-HT and glutamate, but the function of this co-release ...is unclear. Given the strong links between 5-HT and uncontrollable stress, we used a combination of c-Fos immunohistochemistry and conditional gene knockout mice to test the hypothesis that glutamate co-releasing 5-HT neurons are activated by stress and involved in stress coping. Acute, uncontrollable swim stress increased c-Fos immunoreactivity in neurons co-expressing VGLUT3 and the 5-HT marker tryptophan hydroxylase 2 (TPH2) in the dorsal raphe nucleus (DRN). This effect was localized in the ventral DRN subregion and prevented by the antidepressant fluoxetine. In contrast, a more controllable stressor, acute social defeat, had no effect on c-Fos immunoreactivity in VGLUT3-TPH2 co-expressing neurons in the DRN. To test whether activation of glutamate co-releasing 5-HT neurons was causally linked to stress coping, mice with a specific deletion of VGLUT3 in 5-HT neurons were exposed to acute swim stress. Compared to wildtype controls, the mutant mice showed increased climbing behavior, a measure of active coping. Wildtype mice also showed increased climbing when administered fluoxetine, revealing an interesting parallel between the behavioral effects of genetic loss of VGLUT3 in 5-HT neurons and 5-HT reuptake inhibition. We conclude that 5-HT-glutamate co-releasing neurons are recruited by exposure to uncontrollable stress. Furthermore, natural variation in the balance of 5-HT and glutamate co-released at the 5-HT synapse may impact stress susceptibility.
The recent development of genetically encoded fluorescent neurotransmitter biosensors has opened the door to recording serotonin (5-hydroxytryptamine, 5-HT) signaling dynamics with high temporal and ...spatial resolution
. While this represents a significant step forward for serotonin research, the utility of available 5-HT biosensors remains to be fully established under diverse
conditions. Here, we used two-photon microscopy in awake mice to examine the effectiveness of specific 5-HT biosensors for monitoring 5-HT dynamics in somatosensory cortex. Initial experiments found that whisker stimulation evoked a striking change in 5-HT biosensor signal. However, similar changes were observed in controls expressing green fluorescent protein, suggesting a potential hemodynamic artifact. Subsequent use of a second control fluorophore with emission peaks separated from the 5-HT biosensor revealed a reproducible, stimulus-locked increase in 5-HT signal. Our data highlight the promise of 5-HT biosensors for
application, provided measurements are carried out with appropriate optical controls.
Depression is the most common neuropsychiatric co-morbidity in Parkinson’s disease (PD). The underlying mechanism of depression in PD is complex and likely involves biological, psychosocial and ...therapeutic factors. The biological mechanism may involve changes in monoamine systems, in particular the serotonergic (5-hydroxytryptamine, 5-HT) system. It is well established that the 5-HT system is markedly affected in the Parkinsonian brain, with evidence including pathological loss of markers of 5-HT axons as well as cell bodies in the dorsal and median raphe nuclei of the midbrain. However, it remains unresolved whether alterations to the 5-HT system alone are sufficient to confer vulnerability to depression. Here we propose low 5-HT combined with altered network activity within the basal ganglia as critically involved in depression in PD. The latter hypothesis is derived from a number of recent findings that highlight the close interaction between the basal ganglia and the 5-HT system, not only in motor but also limbic functions. These findings include evidence that clinical depression is a side effect of deep brain stimulation (DBS) of the subthalamic nucleus (STN), a treatment option in advanced PD. Further, it has recently been demonstrated that STN DBS in animal models inhibits 5-HT neurotransmission, and that this change may underpin depressive-like side effects. This review provides an overview of 5-HT alterations in PD and a discussion of how these changes might combine with altered basal ganglia network activity to increase depression vulnerability.
► Depression is the most common neuropsychiatric co-morbidity in Parkinson’s disease. ► Low 5-HT and altered basal ganglia activity may be critical to develop depression. ► STN DBS has shown to inhibit 5-HT neurotransmission in animal models. ► Inhibited 5-HT transmission may underlie STN DBS depressive-like side effects.
Recent electrophysiological studies, measurements of 5-HT synthesis and in vivo voltammetry recordings of 5-HT metabolism have suggested that serotoninergic neurones in the median raphe (MR) are less ...sensitive to 5-HT1A autoreceptor stimulation relative to those in the dorsal raphe (DR). To further study the putative differences in regulation between ascending 5-HT projections from the raphe nuclei we have used microdialysis to measure the release of 5-HT in ventral hippocampus, globus pallidus, dorsal hippocampus, frontal cortex, nucleus accumbens and medial septum, following systemic administration of the specific 5-HT1A agonist 8-OH-DPAT. The results show that the baseline output of 5-HT was similar in each of the areas studied. While 8-OH-DPAT decreased dialysate levels of 5-HT in all areas, the inhibition of 5-HT release seen in globus pallidus was significantly less marked compared to that observed in the other five regions. The results indicate that 5-HT1A autoreceptor-mediated control of 5-HT release is functional in all of the brain areas studied, including those receiving a preferential 5-HT innervation from the DR and MR. We find little evidence in support of the idea that brain 5-HT neuronal projections are heterogenous with respect to 5-HT1A autoreceptor regulation of 5-HT release; the globus pallidus, however representing a possible exception to this.
The development of pharmacological magnetic resonance imaging (phMRI) has presented the opportunity for investigation of the neurophysiological effects of drugs in vivo. Psilocin, a hallucinogen ...metabolised from psilocybin, was recently reported to evoke brain region-specific, phMRI signal changes in humans. The present study investigated the effects of psilocin in a rat model using phMRI and then probed the relationship between neuronal and haemodynamic responses using a multimodal measurement preparation. Psilocin (2 mg/kg or 0.03 mg/kg i.v.) or vehicle was administered to rats (N = 6/group) during either phMRI scanning or concurrent imaging of cortical blood flow and recording of local field potentials. Compared to vehicle controls psilocin (2 mg/kg) evoked phMRI signal increases in a number of regions including olfactory and limbic areas and elements of the visual system. PhMRI signal decreases were seen in other regions including somatosensory and motor cortices. Investigation of neurovascular coupling revealed that whilst neuronal responses (local field potentials) to sensory stimuli were decreased in amplitude by psilocin administration, concurrently measured haemodynamic responses (cerebral blood flow) were enhanced. The present findings show that psilocin evoked region-specific changes in phMRI signals in the rat, confirming recent human data. However, the results also suggest that the haemodynamic signal changes underlying phMRI responses reflect changes in both neuronal activity and neurovascular coupling. This highlights the importance of understanding the neurovascular effects of pharmacological manipulations for interpreting haemodynamic neuroimaging data.
•The pharmacological MRI (phMRI) response to psilocin is characterised in rats.•Region specific increases and decreases in phMRI signals are observed.•Underlying neuronal and haemodynamic changes are also measured in vivo.•Evidence for altered neuronal-haemodynamic (neurovascular) coupling by psilocin.•phMRI interpretation should consider neurovascular as well as neuronal drug effects.
Deep brain stimulation (DBS) of the fornix has gained interest as a potential therapy for advanced treatment-resistant dementia, yet the mechanism of action remains widely unknown. Previously, we ...have reported beneficial memory effects of fornix DBS in a scopolamine-induced rat model of dementia, which is dependent on various brain structures including hippocampus. To elucidate mechanisms of action of fornix DBS with regard to memory restoration, we performed c-Fos immunohistochemistry in the hippocampus. We found that fornix DBS induced a selective activation of cells in the CA1 and CA3 subfields of the dorsal hippocampus. In addition, hippocampal neurotransmitter levels were measured using microdialysis before, during and after 60 min of fornix DBS in a next experiment. We observed a substantial increase in the levels of extracellular hippocampal acetylcholine, which peaked 20 min after stimulus onset. Interestingly, hippocampal glutamate levels did not change compared to baseline. Therefore, our findings provide first experimental evidence that fornix DBS activates the hippocampus and induces the release of acetylcholine in this region.
Adding reward-concurrent cues to a rat gambling task (rGT) increases risky choice. This cued version of the task may reflect an “addiction-like” cognitive process, more similar to human gambling than ...the uncued task. Serotonergic drugs that target 5-HT2 receptors alter mechanisms linked to impulse control. However, relatively little is known regarding the impact of such agents on either risky decision making, or the ability of conditioned stimuli to bias the choice process, despite potential relevance to addiction development and treatment. The aim of this study was to determine the effects of SB 242,084 and M100907, selective antagonists at the 5-HT2C and 5-HT2A receptors respectively, as well as the selective 5-HT2C receptor agonist Ro-60-0175, on performance of both cued and uncued versions of the rGT. SB 242,084 significantly and dose-dependently increased choice of the most optimal option in the cued rGT only, despite concurrently increasing impulsive responses made prematurely on both the cued and uncued rGT. M100907 and Ro-60-0175 did not alter risky decision making, but nevertheless produced the expected decrease in premature responses on both task variants. These findings demonstrate that the 5-HT2 receptor-mediated regulation of risky decision making and motor impulsivity can be pharmacologically dissociated and further show that the presence of highly salient reward-paired cues critically alters the neurochemical regulation of the choice process. Importantly, these results suggest that 5-HT2C receptor antagonists may be of use in disrupting maladaptive patterns of decision making.
Rationale Adding reward-concurrent cues to a rat gambling task (rGT) increases risky choice. This cued version of the task may reflect an "addiction-like" cognitive process, more similar to human ...gambling than the uncued task. Serotonergic drugs that target 5-HT.sub.2 receptors alter mechanisms linked to impulse control. However, relatively little is known regarding the impact of such agents on either risky decision making, or the ability of conditioned stimuli to bias the choice process, despite potential relevance to addiction development and treatment. Objectives The aim of this study was to determine the effects of SB 242,084 and M100907, selective antagonists at the 5-HT.sub.2C and 5-HT.sub.2A receptors respectively, as well as the selective 5-HT.sub.2C receptor agonist Ro-60-0175, on performance of both cued and uncued versions of the rGT. Results SB 242,084 significantly and dose-dependently increased choice of the most optimal option in the cued rGT only, despite concurrently increasing impulsive responses made prematurely on both the cued and uncued rGT. M100907 and Ro-60-0175 did not alter risky decision making, but nevertheless produced the expected decrease in premature responses on both task variants. Conclusions These findings demonstrate that the 5-HT.sub.2 receptor-mediated regulation of risky decision making and motor impulsivity can be pharmacologically dissociated and further show that the presence of highly salient reward-paired cues critically alters the neurochemical regulation of the choice process. Importantly, these results suggest that 5-HT.sub.2C receptor antagonists may be of use in disrupting maladaptive patterns of decision making.
Lithium, commonly used to treat bipolar disorder, potentiates the ability of the muscarinic agonist pilocarpine to induce seizures in rodents. As this potentiation by lithium is reversed by the ...administration of myo-inositol, the potentiation may be mediated by inhibition of inositol monophosphatase (IMPase), a known target of lithium. Recently, we demonstrated that ebselen is a ‘lithium mimetic’ in regard to behaviours in both mice and man. Ebselen inhibits IMPase in vitro and lowers myo-inositol in vivo in the brains of mice and men, making ebselen the only known inhibitor of IMPase, other than lithium, that penetrates the blood-brain barrier. Our objective was to determine the effects of ebselen on sensitization to pilocarpine-induced seizures and neural activity. We administered ebselen at different doses and time intervals to mice, followed by injection of a sub-seizure dose of pilocarpine. We assessed seizure and neural activity by a subjective seizure rating scale, by monitoring tremors, and by induction of the immediate early gene c-fos. In contrast to lithium, ebselen did not potentiate the ability of pilocarpine to induce seizures. Unexpectedly, ebselen inhibited pilocarpine-induced tremor as well as pilocarpine-induced increases in c-fos mRNA levels. Both lithium and ebselen inhibit a common target, IMPase, but only lithium potentiates pilocarpine-induced seizures, consistent with their polypharmacology at diverse molecular targets. We conclude that ebselen does not potentiate pilocarpine-induced seizures and instead, reduces pilocarpine-mediated neural activation. This lack of potentiation of muscarinic sensitization may be one reason for the lack of side-effects observed with ebselen treatment clinically.
Genetic variation in the human serotonin transporter (5-HTT) has been linked to altered fear learning but the data are inconsistent and the mechanism is unclear. The present study investigated ...conditioned aversive learning in 5-HTT knockout (KO) mice while simultaneously recording neural network activity (theta oscillations) and hemodynamic responses (tissue oxygen delivery) from the amygdala, a brain region necessary for forming fearful memories. Conditioned aversive learning was measured using a discrimination learning task in which one auditory cue was paired with foot-shock, whereas a second auditory cue was not. Compared with wild-type mice, 5-HTTKO mice exhibited faster discrimination learning. This effect was associated with stronger theta frequency oscillations and greater hemodynamic changes in the amygdala in response to both the emotionally relevant cues and the unconditioned foot-shock stimulus. Furthermore, hemodynamic responses to the unconditioned stimulus predicted behavioral discrimination performance the following day. Acute pharmacological 5-HTT blockade in wild-type mice produced a similar effect, to the extent that administration of citalopram during the fear conditioning sessions enhanced fear memory recall. Collectively, our data argue that loss of 5-HTT function enhances amygdala responsivity to aversive events and facilitates learning for emotionally relevant cues.
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