Involvement of serotonin in mediation of inhibition of substantia nigra neurons by noxious stimuli

Dorsal Raphe Dual Serotonin-Glutamate Neurons Drive Reward by Establishing Excitatory Synapses on VTA Mesoaccumbens Dopamine Neurons

Dorsal raphe (DR) serotonin neurons provide a major input to the ventral tegmental area (VTA). Here, we show that DR serotonin transporter (SERT) neurons establish both asymmetric and symmetric synapses on VTA dopamine neurons, but most of these synapses are asymmetric. Moreover, the DR-SERT terminals making asymmetric synapses on VTA dopamine neurons coexpress vesicular glutamate transporter 3 (VGluT3; transporter for accumulation of glutamate for its synaptic release), suggesting the excitatory nature of these synapses. VTA photoactivation of DR-SERT fibers promotes conditioned place preference, elicits excitatory currents on mesoaccumbens dopamine neurons, increases their firing, and evokes dopamine release in nucleus accumbens. These effects are blocked by VTA inactivation of glutamate and serotonin receptors, supporting the idea of glutamate release in VTA from dual DR SERT-VGluT3 inputs. Our findings suggest a path-specific input from DR serotonergic neurons to VTA that promotes reward by the release of glutamate and activation of mesoaccumbens dopamine neurons.

Prenatal maternal depression and child serotonin transporter linked polymorphic region (5-HTTLPR) and dopamine receptor D4 (DRD4) genotype predict negative emotionality from 3 to 36 months

Prenatal maternal depression and a multilocus genetic profile of two susceptibility genes implicated in the stress response were examined in an interaction model predicting negative emotionality in the first 3 years. In 179 mother-infant dyads from the Maternal Adversity, Vulnerability, and Neurodevelopment cohort, prenatal depression (Center for Epidemiologic Studies Depressions Scale) was assessed at 24 to 36 weeks. The multilocus genetic profile score consisted of the number of susceptibility alleles from the serotonin transporter linked polymorphic region gene (5-HTTLPR): no long-rs25531(A) (LA: short/short, short/long-rs25531(G) [LG], or LG/LG] vs. any LA) and the dopamine receptor D4 gene (six to eight repeats vs. two to five repeats). Negative emotionality was extracted from the Infant Behaviour Questionnaire-Revised at 3 and 6 months and the Early Child Behavior Questionnaire at 18 and 36 months. Mixed and confirmatory regression analyses indicated that prenatal depression and the multilocus genetic profile interacted to predict negative emotionality from 3 to 36 months. The results were characterized by a differential susceptibility model at 3 and 6 months and by a diathesis-stress model at 36 months.

Neural mechanism underlying hyperalgesic response to orofacial pain in Parkinson's disease model rats

To investigate the neural mechanism of pain originating from the orofacial region in PD patients, we used PD model rats produced by unilateral injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. We investigated effects of nigrostriatal lesions on the behavioral response (face rubbing) to formalin injection into the upper lip. We also examined expression of c-Fos and phosphorylated extracellular signal-regulated kinase (pERK) in the trigeminal spinal subnucleus caudalis (Vc) and expression of c-Fos in the periaqueductal gray matter (PAG). Face rubbings following formalin injection showed a biphasic profile, with the first phase for the first 5 min and the second phase from 10 to 90 min. Rats with 6-OHDA lesions showed increased face rubbings in the second phase when formalin was injected ipsilaterally to the lesion, and c-Fos expression in the Vc increased. When formalin was injected contralaterally, face rubbings were reduced in the first phase, however, expression levels of c-Fos and pERK in the Vc were unchanged. No significant difference was found in c-Fos expression in the PAG between 6-OHDA- and saline-injected rats. These results suggest that unilateral dopamine depletion in the nigrostriatal pathway may be involved in hypersensitivity to noxious stimulation delivered to the orofacial region.

Serotonin and dopamine: unifying affective, activational, and decision functions

Serotonin, like dopamine (DA), has long been implicated in adaptive behavior, including decision making and reinforcement learning. However, although the two neuromodulators are tightly related and have a similar degree of functional importance, compared with DA, we have a much less specific understanding about the mechanisms by which serotonin affects behavior. Here, we draw on recent work on computational models of dopaminergic function to suggest a framework by which many of the seemingly diverse functions associated with both DA and serotonin-comprising both affective and activational ones, as well as a number of other functions not overtly related to either-can be seen as consequences of a single root mechanism.

Patterns of FOS expression in the spinal cord and periaqueductal grey matter of 6OHDA-lesioned rats

A less well-known feature of Parkinson disease is that up to 40% of patients experience distinct sensory disturbances, including hyperalgesia and chronic pain. There is a limited understanding of the neural mechanisms that generate these symptoms, however. This study explores the patterns of Fos expression (a well-known marker for changes in cell activity) in the spinal cord and periaqueductal grey matter (PaG), two major sensory (nociceptive) centers, of hemiParkinsonian rats. The medial forebrain bundle (mfb; major tract carrying dopaminergic nigrostriatal axons) was injected with either 6OHDA or saline (controls). A week later, some rats were subjected to mechanical stimulation (pinching) of the hindpaw for 2 h, whereas others received no stimulation. Thereafter, brains were processed using routine tyrosine hydroxylase (marker for dopaminergic cells) or Fos immunocytochemistry. In the PaG, there were many more Fos(+) cells in the 6OHDA-lesioned than in the Control group, in both the stimulation and, in particular, the non-stimulation cases. In the spinal cord, there were also more Fos(+) cells in the 6OHDA-lesioned than in the Control group, but in the stimulation cases only. Overall, the results show distinct changes in Fos expression in the spinal cord and PaG of 6OHDA-lesioned rats, suggesting a substrate for some of the abnormal sensory (nociceptive) circuits that may be evident in parkinsonian cases.

Reward-dependent modulation of neuronal activity in the primate dorsal raphe nucleus

The dopamine system has been thought to play a central role in guiding behavior based on rewards. Recent pharmacological studies suggest that another monoamine neurotransmitter, serotonin, is also involved in reward processing. To elucidate the functional relationship between serotonin neurons and dopamine neurons, we performed single-unit recording in the dorsal raphe nucleus (DRN), a major source of serotonin, and the substantia nigra pars compacta, a major source of dopamine, while monkeys performed saccade tasks in which the position of the target indicated the size of an upcoming reward. After target onset, but before reward delivery, the activity of many DRN neurons was modulated tonically by the expected reward size with either large- or small-reward preference, whereas putative dopamine neurons had phasic responses and only preferred large rewards. After reward delivery, the activity of DRN neurons was modulated tonically by the received reward size with either large- or small-reward preference, whereas the activity of dopamine neurons was not modulated except after the unexpected reversal of the position-reward contingency. Thus, DRN neurons encode the expected and received rewards, whereas dopamine neurons encode the difference between the expected and received rewards. These results suggest that the DRN, probably including serotonin neurons, signals the reward value associated with the current behavior.

Behavioral responses and Fos activation following painful stimuli in a rodent model of Parkinson's disease

In Parkinson's disease (PD), the motor dysfunction caused by degeneration of the nigrostriatal pathway is often associated with alterations of pain perception. This is likely related to the role that the nigrostriatal system may play in the processing of noxious, somatosensory stimuli. To further address this issue, we used a rodent model of PD, based on the unilateral, intrastriatal injection of neurotoxin 6-hydroxydopamine (6-OHDA). We investigated the effects of the nigrostriatal lesion on behavioral responses to pain tests designed to explore different aspects of nociception, such as the formalin test and the tail flick test; we also explored modifications in the expression of Fos protein, a marker of neuronal activation, in supraspinal nuclei involved in the integration of pain perception and stress-related behavior. Rats bearing the nigrostriatal lesion showed complex alterations in pain perception, including hyperalgesic responses to the tonic, inflammatory pain elicited by formalin injection, but only when the stimulus was delivered ipsilaterally to the lesion. This phenomenon was associated with delayed responses to the phasic, thermal stimulus induced by the tail flick test. The hyperalgesic response to the formalin test was accompanied by reduced Fos expression in the paraventricular nucleus of the hypothalamus, which is part of a network (the medial pain system) that mediates motivational-affective aspects of pain. Our results confirm that a unilateral alteration of central dopaminergic transmission disrupts the neural mechanisms underlying proper integration of painful stimuli, particularly in the hemibody ipsilateral to the dopaminergic denervation.

GABAergic projection from the ventral tegmental area and substantia nigra to the periaqueductal gray region and the dorsal raphe nucleus

Previous studies have shown that neurons in the ventral tegmental area (VTA) and substantia nigra (SN) project to the ventrolateral periaqueductal gray (PAGvl) and dorsal raphe nucleus (DR). Research has also shown that stimulation of neurons in the VTA/SN elicits cardiovascular depressor responses that are mediated by a projection to the PAGvl/DR. Anatomic and physiological experiments were done in the present study to determine the neurochemical identity of the VTA/SN projection to the PAGvl/DR. Experiments were done to characterize the origin and chemical nature of this projection by combining cholera toxin B tracing with immunofluorescence for the 67K isoform of glutamic acid decarboxylase (GAD) and tyrosine hydroxylase. The PAGvl/DR region was found to receive a substantial input from neurons in the VTA, SN, and deep mesencephalic nucleus. The DR was preferentially innervated by neurons in the VTA, whereas the PAGvl was preferentially innervated by neurons in the SN. A proportion of neurons in the VTA and the reticular portion of the SN found to project to the PAGvl/DR were GAD positive. In addition, experiments were done in urethane-anesthetized rats to determine whether injections of a gamma-aminobutyric acid (GABA) antagonist in the region of the PAGvl/DR attenuated the cardiovascular depressor responses produced by glutamate stimulation of the VTA/SN. Injections of the GABA-blocking agent picrotoxin (2.5 nmol, 500 nl) into the PAGvl/DR eliminated the cardiovascular responses from stimulation of the VTA/SN (0.01 M, 50 nl). The results of the present investigation provide evidence for a GABAergic projection from the VTA/SN to the PAGvl/DR. This projection may be an important regulator of the PAGvl/DR, an area of the midbrain involved in the production of behavioral and physiological responses to pain and stress.

5-HT2 receptors exert a state-dependent regulation of dopaminergic function: studies with MDL 100,907 and the amphetamine analogue, 3,4-methylenedioxymethamphetamine

The highly selective 5-HT2 receptor antagonist, MDL 100,907, was used to explore the role of serotonin in the stimulation of dopaminergic function produced by the amphetamine analogue 3,4-methylenedioxymethamphetamine (MDMA). MDL 100,907 blocked MDMA-stimulated dopamine synthesis in vivo without affecting basal synthesis. The long-term deficits in 5-HT concentrations believed to be a consequence of MDMA-induced dopamine release were also blocked by MDL 100,907 over the same dose range. In vivo microdialysis confirmed that 5-HT2 receptor blockade with MDL 100,907 attenuated MDMA-induced increases in extracellular concentrations of striatal dopamine. In contrast to its effect on MDMA-induced synthesis, MDL 100,907 did not alter dopamine synthesis stimulated by haloperidol or reserpine. In vivo dopamine release produced by haloperidol was also unaffected by MDL 100,907. The results suggest a permissive role for 5-HT2 receptors in the activation of the dopamine system which occurs during states of high serotonergic activity or during conditions of elevated dopamine efflux with high D2 receptor occupancy.