Decreased reward-seeking, often called anhedonia, forms a core symptom of depression. Often, decreased reward-seeking appears as impaired reward anticipation. Stressful experiences precipitate depression and disrupt reward-seeking, but it remains unclear how stress causes anhedonia. To determine how stress alters neural communication, we recorded simultaneous neural activity across limbic brain areas as mice underwent stress and discovered a stress-induced 4 Hz oscillation in the nucleus accumbens (NAc) local field potential (LFP) that predicts the degree of subsequent blunted reward-seeking. This 4 Hz oscillation exhibited strong coherence between the ventral tegmental area (VTA) and the NAc. Through pharmacological inhibition of the VTA, we found that VTA neural activity is necessary for the generation of the 4 Hz oscillation, and extracellular recordings of multi-unit activity in the VTA reveal that VTA neural activity leads the phase of the 4 Hz NAc oscillation. We used transgenic mouse lines to selectively express the inhibitory opsin Archaerhodopsin in dopamine (DA), GABA, and glutamate neurons in the VTA. We combined cell type specific optogenetic inhibition with extracellular single-unit recordings in the VTA and LFP recordings in the NAc to identify the phase-locking of specific cell type spiking with the NAc4 Hz oscillation, as well as to identify the extent to which VTA populations contribute to the generation of the 4 Hz NAc oscillation. We found that VTA GABA neuron firing leads the phase of the 4 Hz NAc oscillation, and that VTA GABA activity is necessary for the generation of the 4 Hz NAc oscillation. This result led us to determine whether rhythmic VTA GABA activity contributes to stress-induced anhedonia. Surprisingly, while previous studies on blunted reward-seeking focused on DA transmission from the VTA to the NAc, we found that VTA GABA neurons mediate stress-induced blunted reward-seeking. Inhibiting VTA GABA neurons during stress disrupts stress-induced NAc oscillations and rescues reward-seeking. By contrast, mimicking this signature of stress by stimulating NAc-projecting VTA GABA neurons at 4 Hz in the absence of stress reproduces both oscillations and blunted reward-seeking. Finally, we found that stress disrupts VTA GABA, but not VTA DA, neural encoding of reward anticipation. Thus, stress elicits rhythmic VTA-NAc GABAergic activity that induces VTA GABA mediated blunted reward-seeking.

京都大学 / 0048 / 新制・課程博士 / 博士(生命科学) / 甲第17931号 / 生博第294号 / 新制||生||38(附属図書館) / 30751 / 京都大学大学院生命科学研究科高次生命科学専攻 / (主査)教授 垣塚 彰, 教授 渡邉 大, 教授 松崎 文雄 / 学位規則第4条第1項該当 / Doctor of Philosophy in Life Sciences / Kyoto University / DFAM

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Effective decision making depends on using memories of past experiences to inform choices in the present. This dissertation examines several ways in which memory is used in decision making, and further aims to establish that one way we adapt to the statistics of our environment is by modifying how we use our memories to guide behavior. In chapters one and two, I focus on how incremental trial-and-error learning and episodic memories of individual events may each contribute to choice. In chapter one, I ask how the brain may arbitrate rationally between these two systems to achieve a balance that maximizes reward. By manipulating the volatility of the environment to affect uncertainty, I show that participants rely on each system in the circumstances to which it is best suited. In chapter two, I then ask how decisions based on these memory systems each depend on striatal dopamine. By studying patients with Parkinson’s disease both on and off their medication, I find that a lack of dopamine alters only incremental learning, and that dopamine replacement remediates this deficit with few effects on the use of episodic memory. Finally, in chapter three, I examine a more difficult class of decisions that require individual memories to be used for planning future action. Using neuroimaging to decode memory access, I find that that the statistical structure of relationships between memories determines when they are used to support planning. Combined, these three chapters suggest that we are capable of flexibly employing multiple forms of memory, with distinct neural mechanisms, to guide a variety of choices.

Female sheep exposed to excess testosterone (T) in utero display symptoms similar to those observed in women with polycystic ovarian syndrome (PCOS). Prenatal T-treated ewes display masculinized sexual behavior and increased food-reward seeking behavior. A neural substrate critical for these goal-directed behaviors is the dopaminergic system in the ventral tegmental area. We have recently shown that in adult ewes dopamine expression in the VTA is increased by prenatal T exposure. In this thesis, I tested the hypothesis that alterations of the VTA dopamine system by prenatal-T are caused via activation of androgen (AR) and/or insulin receptors (IR). Analysis of immunohistochemical staining shows an increase of tyrosine hydroxylase (TH) expression, AR, or IR, along with changes in co-expression of AR/TH and IR/TH. These changes were blocked or reversed by prenatal treatments with flutamide or rosiglitazone, or postnatal treatments with rosiglitazone or metformin, implicating AR and IR in alterations of the VTA

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No description available.

In rats that had been treated neonatally with 6-hydroxydopamine (6OHDA) to deplete striatal dopamine more than 95%, a single injection of p-chloroamphetamine (pCA) (5 or 10 mg/kg, i.p.) resulted in depletion of striatal and hippocampal serotonin at 1 week to a similar extent as in control rats. These findings suggest that striatal dopamine is not essential to the long-lasting depletion of brain serotonin by pCA in rats.

Presynaptic and postsynaptic actions of dextroamphetamine (DEX) were studied on dopamine (DA) sensitive neurons of the subesophageal ganglion of the garden snail Helix aspersa utilizing standard microelectrode techniques. Dextroamphetamine (5.5 × 10-7-10-4 M) produced effects on DA-sensitive neurons similar to that caused by DA (5.5 × 10-7-10-4 M). On cells excited by DA, surfused DEX (5.5 × 10-7 M) caused an excitation that could be blocked by chlorpromazine (0.5-1 × 10-6 M) or haloperidol (0.5-1 × 10-6 M). Elevating the extracellular Mg2+ from 4 to 20 mM reduced the depolarization caused by DEX from 11 to 2.5 mV without affecting the response to DA. The response remaining is attributed to a direct response to DEX on DA receptors. Surfused DEX caused an inhibition of cells inhibited by DA. Both DA and DEX effects were selectively blocked by dihydroergotamine (0.5-1 × 10-6 M). Elevating the [Mg2+] decreased the hyperpolarization caused by DEX from 11 to 3 mV without affecting the DA response. The effect of elevated magnesium in decreasing responses to surfused DEX suggests that the primary action of DEX is at the nerve terminal to cause DA release. Iontophoretic application of DEX caused minimal excitation or inhibition of DA neurons. This is attributed to the fact that DA receptors at the site of drug application are not associated with synaptic innervation. The response obtained with iontophoretically applied DEX suggest a weak direct action on DA receptors.

In rat, the neonatal destruction of nigrostriatal dopamine (DA) neurons by intracerebral administration of 6-hydroxydopamine entails dramatic changes in serotonin (5-hydroxytryptamine, 5-HT) as well as DA function. Most striking is the 5-HT hyperinnervation of the adult neostriatum, associated with increases in density of various 5-HT receptor subtypes and enhanced neuronal responsiveness to the iontophoretic application of 5-HT and its 5- HT(1B/2C) and 5-HT(2A/2C) receptor agonists, m-chlorophenylpiperazine and iododimethoxyphenylaminopropane. The topographical distribution of these changes is consistent with up-regulation and/or increased production and transport of 5-HT(1B) and 5-HT(2A) receptors by the neostriatal projection neurons, as confirmed for the 5-HT(2A) receptor in a recent in situ hybridization study. It is interesting that this study has also shown that increases in both 5-HT(2A) binding and mRNA level were abolished by chronic pretreatment with the DA agonists, apomorphine and SKF 38393, suggesting a regulatory influence of DA in the expression of this 5-HT receptor. D1 receptor binding is known to be slightly reduced in the rostral neostriatum of these rats, a down-regulation apparently imputable to a reduced rate of synthesis of the receptor. In contrast, D2 receptor binding is increased throughout the DA-denervated and 5-HT-hyperinnervated neostriatum, perhaps due to some post-transcriptional modifications. Stereotyped and motor behaviors induced by systemic treatment with D1 and D2 agonists are markedly enhanced in these rats (behavioral supersensitivity), although priming is commonly required to unmask a latent D1 supersensitivity. In the case of oral activity, however, overt behavioral supersensitivity is induced by D1 as well as D2 agonists. Moreover, there is overt supersensitivity of oral activity in response to the 5-HT receptor agonist m- chlorophenylpiperazine, which is presumably imputable to 5-HT(2C) receptors and may be demonstrated even in the absence of supersensitivity to D1 receptor agonist. 5-HT adaptations, therefore, seem to play a role not only in the abnormal spontaneous behavior, but also in the behavioral supersensitivity to 5-HT as well as DA receptor agonists in these rats.