Dopaminergic regulation of vestibulo-cerebellar circuits through unipolar brush cells

While multiple monoamines modulate cerebellar output, the mechanistic details of dopaminergic signaling in the cerebellum remain poorly understood. Here we show that Drd1 dopamine receptors are expressed in unipolar brush cells (UBCs) of the cerebellar vermis. Drd1 activation increases UBC firing rate and postsynaptic NMDA receptor-mediated currents. Purkinje neurons directly inhibit Drd1-positive UBCs, forming a recurrent vestibulo-cerebellar circuit. Using anatomical tracing and in situ hybridization, we tested three hypotheses about the source of cerebellar dopamine. We exclude the midbrain dopaminergic nuclei and tyrosine hydroxylase-positive Purkinje cells as potential sources, supporting the possibility of dopaminergic co-release from locus coeruleus (LC) axons. Using an optical dopamine sensor GRABDA, electrical stimulation, and optogenetic activation of LC fibers in the acute slice, we find evidence for monoamine release onto Drd1-expressing UBCs. Altogether, we propose that the LC regulates cerebellar cortex activity by co-releasing dopamine onto UBCs to modulate their response to cerebellar inputs.

Frontal Cortical Monoamine Release, Attention, and Working Memory in a Perinatal Nicotine Exposure Mouse Model Following Kappa Opioid Receptor Antagonism

Perinatal nicotine exposure (PNE) produces frontal cortical hypo-dopaminergic state and attention and working memory deficits consistent with neurodevelopmental disorders such as attention deficit hyperactivity disorder (ADHD). Methylphenidate alleviates ADHD symptoms by increasing extracellular dopamine and noradrenaline. Kappa opioid receptor (KOR) antagonism may be another mechanism to achieve the same results because KOR activation inhibits frontal cortical dopamine release. We administered the selective KOR antagonist norbinaltorphimine (norBNI) (20 mg/kg; intraperitoneal) or methylphenidate (0.75 mg/kg; intraperitoneal) to PNE mouse model and examined frontal cortical monoamine release, attention, and working memory. Both compounds increased dopamine and noradrenaline release but neither influenced serotonin release. Both compounds improved object-based attention and working memory in the PNE group, with norBNI’s effects evident at 2.5 h and 5.5 h but absent at 24 h. Methylphenidate’s effects were evident at 0.5 h but not at 2.5 h. norBNI’s effects temporally coincided with frontal cortical c-Jun N-terminal kinase phosphorylation. norBNI did not alter tissue dopamine content in the nucleus accumbens, offering preliminary support for lack of reinforcement.

Pharmacological Discrimination of Effects of MK801 on Thalamocortical, Mesothalamic, and Mesocortical Transmissions

N-methyl-d-aspartate/glutamate receptor (NMDAR) is one of the major voltage-sensitive ligand-gated cation channel. Several noncompetitive NMDAR antagonists contribute to pathophysiology of schizophrenia and mood disorders; however, the effects of inhibition of NMDAR on several transmitter system have not been well clarified. Thus, this study determined the selective NMDAR antagonist, MK801 (dizocilpine), on thalamocortical, mesothalamic, and mesocortical transmissions associated with l-glutamate, GABA, serotonin, norepinephrine, and dopamine using multiprobe microdialysis. Perfusion with MK801 into the medial prefrontal cortex (mPFC) increased and decreased respective regional releases of monoamine and GABA without affecting l-glutamate. The mPFC MK801-induced monoamine release is generated by the regional GABAergic disinhibition. Perfusion with MK801 into the reticular thalamic nucleus (RTN) decreased GABA release in the mediodorsal thalamic nucleus (MDTN) but increased releases of l-glutamate and catecholamine without affecting serotonin in the mPFC. The RTN MK801-induced l-glutamate release in the mPFC was generated by GABAergic disinhibition in the MDTN, but RTN MK801-induced catecholamine release in the mPFC was generated by activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/glutamate receptor (AMPAR) which received l-glutamate release from thalamocortical glutamatergic terminals in the mPFC. Perfusion with MK801 into the dorsal raphe nucleus (DRN) decreased GABA release in the DRN but selectively increased serotonin release in the MDTN and mPFC. These DRN MK801-induced serotonin releases in the both mPFC and MDTN were also generated by GABAergic disinhibition in the DRN. These results indicate that the GABAergic disinhibition induced by NMDAR inhibition plays important roles in the MK801-induced releases of l-glutamate and monoamine in thalamic nuclei and cortex.

A kinetic account for amphetamine-induced monoamine release

The plasmalemmal monoamine transporters for dopamine, norepinephrine, and serotonin (SERT) are targets for amphetamines. In vivo, amphetamines elicit most, if not all, of their actions by triggering monoamine efflux. This is thought to be accomplished by an amphetamine-induced switch from the forward-transport to the substrate-exchange mode. The mechanism underlying this switch has remained elusive; available kinetic models posit that substrates and cosubstrate Na+ ions bind either in a random or in a sequential order. Neither can account for all reported experimental observations. We used electrophysiological recordings to interrogate crucial conformational transitions associated with the binding of five different substrates (serotonin, para-chloroamphetamine, and the high-affinity naphthyl-propan-amines PAL-287, PAL-1045, and PAL-1046) to human SERT expressed in HEK293 cells; specifically, we determined the relaxation kinetics of SERT from a substrate-loaded to a substrate-free state at various intracellular and extracellular Na+ concentrations. These rates and their dependence on intracellular and extracellular Na+ concentrations differed considerably between substrates. We also examined the effect of K+ on substrate affinity and found that K+ enhanced substrate dissociation. A kinetic model was developed, which allowed for random, but cooperative, binding of substrate and Na+ (or K+). The synthetic data generated by this model recapitulated the experimental observations. More importantly, the cooperative binding model accounted for the releasing action of amphetamines without any digression from alternating access. To the best of our knowledge, this model is the first to provide a mechanistic framework for amphetamine-induced monoamine release and to account for the findings that some substrates are less efficacious than others in promoting the substrate-exchange mode.