Serotonergic Feedback Control

Serotonin regulation of behavior via large-scale neuromodulation of serotonin receptor networks

Although we understand how serotonin receptors function at the single-cell level, what role different serotonin receptors play in regulating brain-wide activity and, in turn, human behavior, remains unknown. Here, we developed transcriptomic-neuroimaging mapping to characterize brain-wide functional signatures associated with specific serotonin receptors: serotonin receptor networks (SRNs). Probing SRNs with optogenetics-functional magnetic resonance imaging (MRI) and pharmacology in mice, we show that activation of dorsal raphe serotonin neurons differentially modulates the amplitude and functional connectivity of different SRNs, showing that receptors' spatial distributions can confer specificity not only at the local, but also at the brain-wide, network level. In humans, using resting-state functional MRI, SRNs replicate established divisions of serotonin effects on impulsivity and negative biases. These results provide compelling evidence that heterogeneous brain-wide distributions of different serotonin receptor types may underpin behaviorally distinct modes of serotonin regulation. This suggests that serotonin neurons may regulate multiple aspects of human behavior via modulation of large-scale receptor networks.

Cognition and serotonin in Parkinson's disease

Cognitive impairment affects up to 80% of patients with Parkinson's disease (PD) and is associated with poor quality of life. PD cognitive dysfunction includes poor working memory, impairments in executive function and difficulty in set-shifting. The pathophysiology underlying cognitive impairment in PD is still poorly understood, but there is evidence to support involvements of the cholinergic, dopaminergic, and noradrenergic systems. Only rivastigmine, an acetyl- and butyrylcholinesterase inhibitor, is efficacious for the treatment of PD dementia, which limits management of cognitive impairment in PD. Whereas the role of the serotonergic system in PD cognition is less understood, through its interactions with other neurotransmitters systems, namely, the cholinergic system, it may be implicated in cognitive processes. In this chapter, we provide an overview of the pharmacological, clinical and pathological evidence that implicates the serotonergic system in mediating cognition in PD.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Serotonin actions within the prelimbic cortex induce anxiolysis mediated by serotonin 1a receptors

BACKGROUND:

Serotonin plays an important role in the regulation of anxiety, acting through complex modulatory mechanisms within distinct brain structures. Serotonin can act through complex negative feedback mechanisms controlling the neuronal activity of serotonergic circuits and downstream physiologic and behavioral responses. Administration of serotonin or the serotonin 1A receptor agonist, (±)-8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT), into the prefrontal cortex, inhibits anxiety-like responses. The prelimbic area of the prefrontal cortex regulates serotonergic neurons within the dorsal raphe nucleus and is involved in modulating anxiety-like behavioral responses.

AIMS:

This study aimed to investigate the serotonergic role within the prelimbic area on anxiety- and panic-related defensive behavioral responses.

METHODS:

We investigated the effects of serotonin within the prelimbic area on inhibitory avoidance and escape behaviors in the elevated T-maze. We also extended the investigation to serotonin 1A, 2A, and 2C receptors.

RESULTS:

Intra-prelimbic area injection of serotonin or 8-OH-DPAT induced anxiolytic effects without affecting escape behaviors. Previous administration of the serotonin 1A receptor antagonist, WAY-100635, into the prelimbic area counteracted the anxiolytic effects of serotonin. Neither the serotonin 2A nor the serotonin 2C receptor preferential agonists, (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) and 6-chloro-2-(1-piperazinyl) pyrazine (MK-212), respectively, affected behavioral responses in the elevated T-maze.

CONCLUSION:

Facilitation of serotonergic signaling within the prelimbic area of rats induced an anxiolytic effect in the elevated T-maze test, which was mediated by local serotonin 1A receptors. This inhibition of anxiety-like defensive behavioral responses may be mediated by prelimbic area projections to neural systems controlling anxiety, such as the dorsal raphe nucleus or basolateral amygdala.

Increased response to a 5-HT challenge after discontinuation of chronic serotonin uptake inhibition in the adult and adolescent rat brain

Little is known about the effects of chronic fluoxetine on 5-HT transmission in the adolescent brain, even though it is acknowledged that the neuroplasticity of the brain during childhood and adolescence might influence the neurobiological mechanisms underlying treatment response. Also, possible ongoing effects on monoamine function following drug discontinuation are unidentified. We therefore examined the chronic effects of fluoxetine on extracellular 5-HT and dopamine concentrations in the medial prefrontal cortex and studied their responsiveness to an acute 5-HT challenge after a one-week washout period, both in adolescent and adult rats. Noradrenaline was measured in adult animals only. Fluoxetine increased 5-HT to 200-300% of control and DA and NA to 150% of control. Although there were no lasting effects of chronic fluoxetine on basal monoamine levels, we observed a clear potentiating effect of previous treatment on the fluoxetine-induced increase in extracellular 5-HT and, to a lesser extent, extracellular DA. No differential effect was found for noradrenaline. Age-at-treatment did not influence these results. So, after cessation of chronic fluoxetine treatment 5-HT responsiveness remains heightened. This may be indicative of the continuing presence of 5-HT receptor desensitization, at least until one week after drug discontinuation in rats. No apparent age-at-treatment effects on extracellular monoamine concentrations in the medial prefrontal cortex were detected, but age-related differences in 5-HT transmission further down-stream or in the recovery processes cannot be ruled out.