The placebo effect represents a fascinating example of how cognition can influence the physiology of the brain and body. The expectation of therapeutic benefit elicited by a placebo given in the guise of active medication has been proposed to be a form of reward expectation, and is associated with activation of brain reward circuitry. Prominent placebo effects occur in Parkinson’s disease (PD), where the expectation of symptom improvement stimulates dopamine release in the striatum. In the work described in this dissertation, positron emission tomography with [¹¹C] raclopride was used to investigate the relationship between the strength of expectation of benefit and the degree of dopamine release in PD, and how this relationship corresponds to current models of dopamine function in reward. Chapter 3 describes a pilot study conducted in patients who had undergone subthalamic nucleus deep-brain stimulation (STN-DBS) in which we examined how awareness of stimulator status (ON or OFF) affected synaptic dopamine levels compared to when subjects were blind. No difference was detected between conditions; however, it proved to be difficult to maintain blinding due to the profound effects of STN-DBS. Chapter 4 describes the development of the methodology for the analysis of high-resolution PET data, in which we utilized the combined efforts of neuroscience and imaging physics to optimize the analysis of [¹¹C] raclopride PET data. In Chapter 5, I describe the use of verbal instructions to manipulate patients’ expectations in order to investigate how the likelihood of receiving levodopa influenced dopamine release when the patients were in fact given placebo. Placebo-induced dopamine release was differentially modulated by expectation in the dorsal and ventral striatum: dopamine release in the putamen was related monotonically to expected reward value, whereas dopamine released in the ventral striatum reflected the uncertainty of benefit or the salience of the expectation. The placebo effect in PD therefore involves at least two related but separate mechanisms: the expectation of benefit itself, which is scaled to reflect the value of the drug to the patient and is mediated by nigrostriatal dopamine, and the uncertainty or salience of benefit that is mediated by mesolimbic dopamine.

The activity of neurons in the prefrontal cortex (PFC) may underlie working memory processes in the brain. Both the performance of working memory tasks and the activity of PFC neurons are modulated by dopamine. The goal of the present thesis was to gain insight into the neural basis of working memory by studying the PFC, and the DA system in the PFC, from both a behavioral and cellular perspective. The functional contribution of the PFC to working memory processes in the rat was assessed in Chapter 2 of the present thesis using memory-based foraging tasks on an 8-arm radial maze. The results of these studies indicated that lidocaine-induced inactivations of the PFC selectively disrupted the ability to use mnemonic information to guide foraging, but not the ability to acquire or retain such information. The ability to use mnemonic information to guide foraging was also disrupted by microinjection of a D1 but not D2 receptor antagonist into the PFC. Chapters 3-5 investigated how PFC neurons process synaptic inputs to their dendrites to produce spike output. The intrinsic membrane properties and synaptic responses at the soma and dendrites of deep layer PFC pyramidal neurons were recorded using sharp intracellular or whole-cell patch-clamp techniques in a brain-slice preparation. Different passive and active membrane properties of the soma and dendrites of PFC neurons were observed. The distal dendrites of PFC neurons responded most effectively to strong, highly coincident synaptic inputs. Ca²⁺currents near the soma both amplified the effects of these inputs and modulated the spike output pattern. Spike output at the soma was also controlled by the interplay of slowly-inactivating Na⁺ and K⁺ currents. Chapter 6 investigated the modulation of PFC neurons by DA. DA or a D1 but not D2 receptor agonist increased the evoked firing of PFC neurons via a D1- mediated modulation of slowly-inactivating Na⁺ and K⁺ currents. Concurrently, D1 receptor activation reduced burst firing in PFC neurons, due to a attenuation of Ca²⁺ currents. D1 receptor activation also increased both GABA[sub A] IPSPs and NMDA EPSPs. The final chapter of this thesis integrated these data into a cellular model of PFC function and its modulation by DA. It is proposed that DA may tune PFC neurons such that they respond selectively to strong synchronized inputs from other cortical areas. In the presence of DA, working memory processes mediated by the PFC may be influenced selectively by stimuli of behavioral significance.

No description available.

Serotonin (5-HT) and noradrenalin (NA) increase persistent inward currents mediated by sodium and calcium channels on the dendrites of motoneurons. The ability of 5-HT and NA to modulate these channels depends on the distributions of 5-HT and NA synapses. Recent studies of the distribution of 5-HT and NA synapses on motoneurons innervating the neck muscle splenius reported that these synapses are rare on the somata and have a strong bias to dendrites with small diameters. It is unknown whether this distribution pattern represents a general principle of organization (1) for all motoneuron groups or (2) for all types of modulators. To address the first question, we have examined the distribution of 5-HT and NA synapses on flexor motoneurons, which unlike extensor motoneurons, are not able to generate self-sustained discharges known to involve the activation of persistent inward currents. To answer the second question, we have mapped the distribution dopamine (DA) synapses. The dendrites of motoneurons that innervate the neck flexor rectus capitis anterior (RCA) were stained. Synapses containing 5-HT, NA and DA were identified using immunohistochemical techniques. Observations based on five RCA motoneurons indicate that the average densities of 5-HT and NA contacts are 2.3 and 1.4 times less dense than the average densities of 5-HT and NA contacts on splenius motoneurons, respectively. Moreover, pairs of 5-HT contacts and pairs of NA contacts were found to be 3.0 and 1.8 times closer together on splenius compared to RCA motoneurons, respectively. These observations may reflect the inability of flexor motoneurons to generate self sustained discharges. Similar to splenius motoneurons, 5-HT and NA synapses were found to preferentially innervate dendrites with diameters less than 2 µm. Thus, 5-HT and NA synapses facilitate channels in regions where excitatory or inhibitory signals undergo the largest attenuations. DA synapses on the dendritic tree were sparse (0.2 and 0.1 contacts per 1000 µm2), suggesting that the actions of DA synapses are confined to local regions on the dendritic tree. These results highlight that motoneurons do not all share the same intrinsic properties, and the distribution of modulatory synapses have a crucial role in determining these properties. / Thesis (Master, Neuroscience Studies) -- Queen's University, 2011-09-28 15:17:54.335

Psychological stressors such as social stress and bullying are prevalent in today’s society. Disorders such as PTSD, depression and social anxiety disorder can be either caused or exacerbated by social stress and treatment options are not always effective in providing relief for these disorders. Our laboratory studies a form of social stress termed conditioned defeat, whereby a defeated Syrian hamster no longer displays species-typical territorial aggression but instead is submissive and defensive toward an intruder in its own cage. We hypothesized that the nucleus accumbens is a necessary component of the circuit mediating the acquisition and expression of conditioned defeat and that dopamine is necessary within the nucleus accumbens for inducing memory processes as well as expression of behavioral responses to stressful situations. We also hypothesized that defeat activates dopaminergic and/or nondopaminergic neurons in the ventral tegmental area (VTA) and that dopamine released by neurons projecting from the VTA to the nucleus accumbens and basolateral amygdala (BLA) increases neuronal activation of these structures during defeat. We found that dopamine, but not GABA, modulates memory of social defeat within the nucleus accumbens. However, GABA does affect the expression of behavioral responses to social defeat. Defeat also increased Fos activation of non-dopaminergic neurons, but it did not increase activation of dopaminergic neurons. Baclofen infusion into the VTA prior to defeat, which was hypothesized to specifically inhibit dopaminergic neurons, did not affect Fos activation within the nucleus accumbens and the basolateral amygdala. These experiments determined that dopamine does modulate memory of social defeat within the nucleus accumbens, but it is currently unclear what the source of this dopamine is. Future experiments are planned to determine this source of dopamine that could be a target of treatment for disorders that are caused or exacerbated by social stress.

While selective serotonin (5-HT) reuptake inhibitors (SSRIs) are utilized as the first-line strategy in treating depression, new approaches are still desired. Using in vivo electrophysiological techniques, the effects of co-administration of bupropion with the SSRI escitalopram on the firing rate of dorsal raphe 5-HT and locus coeruleus norepinephrine (NE) neurons were investigated. Escitalopram significantly decreased the firing of 5-HT and NE neurons at day 2. The 5-HT firing rate, unlike that of NE, recovered after the 14-day escitalopram regimen. Bupropion did not increase 5-HT firing but decreased that of NE after 2 days. Following 14-day bupropion, 5-HT firing was markedly enhanced, and NE firing was back to baseline. Co-administration of escitalopram and bupropion doubled 5-HT firing after 2 and 14 days, whereas NE neurons were inhibited after 2, but partially recovered after 14 days. Although sustained bupropion administration did not alter the sensitivity of 5-HT1A receptors in hippocampus, the tonic activation of postsynaptic 5-HT1A receptors was enhanced in 14-day bupropion-treated rats to a greater extent than in the 2-day and control rats. The function of terminal 5-HT1B autoreceptors was not changed. The inhibitory action of α2-adrenergic receptors on 5-HT terminals was, however, diminished. The function of terminal α2-adrenergic autoreceptors was also attenuated in rats given bupropion for 14 days. Administration of the antidepressant trazodone suppressed the 5-HT firing at day 2, which recovered to baseline following 14 days. Prolonged trazodone-administration enhanced the tonic activation of postsynaptic 5-HT1A receptors in hippocampus, and decreased the function of terminal 5-HT1B autoreceptors. Finally, a novel psychotropic agent asenapine showed potent antagonistic activity at 5-HT2A, D2, and α2-adrenoceptors. Asenapine, however, acted as a partial agonist at 5-HT1A receptors in dorsal raphe and hippocampus. Overall, the therapeutic effects of various antidepressants may be, at least in part, due to the enhancement of 5-HT and/or NE neurotransmission.

In vertebrate and invertebrate systems, the role of glia-neuron interactions during development and behavior is becoming apparent. Recent studies have been aimed at characterizing glial-expressed proteins that affect the modulation of activities traditionally thought to be regulated by the neuron itself. The soil nematode Caenorhabditis elegans has recently emerged as an important invertebrate model to study glial roles in nervous system function and development. My dissertation work focuses on the characterization of HLH-17, a C. elegans basic helix-loop-helix transcription factor that is strongly and constitutively expressed in the glial cells that associate with four of the cephalic (CEP) neurons in the head of the animal. The CEP neurons are four of eight dopaminergic neurons with well characterized roles in the modulation of a number of behavioral activities in the worm. Although HLH-17 is required for neither the specification nor the development of the CEPsh glia or the CEP neurons, it does have a defined role during dopamine responses. We show that HLH-17 functions upstream of the dopamine receptors DOP-1, DOP-3 and the dopamine transporter DAT-1 to affect DA-dependent behaviors. Also, our microarray analyses provide preliminary evidence that HLH-17 targets factors responsible for receiving and transducing signaling molecules that are involved in the modulation of synaptic events in the worm nervous system. Together these results point to a role for HLH-17 in glia-neuron interactions in C. elegans. My dissertation studies therefore provide further support for the role of glial-expressed proteins in the regulation of activities mediated by the nervous system.

Dopamine (DA) is a key catecholamine neurotransmitter involved in motor control, cognition, and neuroendocrine regulation. Reduced DA transmission is associated with Parkinson’s disease, depression, and anhedonia. An overexpression of the dopamine transporter in mice (DAT-tg) results in a 40% reduction in extracellular DA, and can be classified as a genetic model of hypodopaminergia. Reserpine treatment depletes extracellular DA, and is a pharmacological model of hypodopaminergia. The aim of this study was to determine morphological and proteomic changes to medium spiny neurons (MSNs), which receive dopaminergic input, as a consequence of reduced DA transmission. To achieve this, MSNs were fluorescently labelled using a diolistics method and immunofluorescence. There were no observable changes to morphology or proteomic profile of MSNs in DAT-tg animals. Reserpine treatment resulted in reduced spine density in MSNs. DAT-tg animals may present a level of DA depletion that is below the threshold to induce morphological changes to MSNs.

Dopamine (DA) is a key catecholamine neurotransmitter involved in motor control, cognition, and neuroendocrine regulation. Reduced DA transmission is associated with Parkinson’s disease, depression, and anhedonia. An overexpression of the dopamine transporter in mice (DAT-tg) results in a 40% reduction in extracellular DA, and can be classified as a genetic model of hypodopaminergia. Reserpine treatment depletes extracellular DA, and is a pharmacological model of hypodopaminergia. The aim of this study was to determine morphological and proteomic changes to medium spiny neurons (MSNs), which receive dopaminergic input, as a consequence of reduced DA transmission. To achieve this, MSNs were fluorescently labelled using a diolistics method and immunofluorescence. There were no observable changes to morphology or proteomic profile of MSNs in DAT-tg animals. Reserpine treatment resulted in reduced spine density in MSNs. DAT-tg animals may present a level of DA depletion that is below the threshold to induce morphological changes to MSNs.

Environmental stimuli associated with drug use are related to drug craving and relapse. The mechanism of cue-induced craving is thought to involve the release of dopamine (DA) in brain regions associated with reward and habit formation. The aim of the study was to investigate the role of DA in cue-induced craving in tobacco-dependent smokers using Positron Emission Tomography (PET) and a picture cue paradigm. Tobacco-associated cues were capable of eliciting significantly greater subjective reports of craving relative to neutral cues in tobacco smokers (n=6) in a neuroimaging environment. Using this cue paradigm and [11C]-(+)-PHNO PET (n=6), a non-significant trend towards a greater decrease in binding potential, indicative of dopamine release, was shown in selected brain regions of interest. These findings are similar to findings in cocaine-dependent individuals and suggest the involvement of dopamine in the response to smoking-associated cues in tobacco-dependent individuals.