Diss. (sammanfattning) Stockholm : Karol. inst. / Härtill 7 uppsatser.

The basal ganglia are a collection of sub-cortical nuclei involved in the execution of a range of motor and cognitive behaviours. The striatum is the input nucleus of the basal ganglia, receiving major excitatory innervation from the cerebral cortex and intralaminar thalamic nuclei. The main target of these two pathways are the principal striatal neurons, the medium-sized spiny neurons (MSNs), which are subdivided based on their axonal targets and the expression of molecular markers. Direct pathway neurons project to the output nuclei of the basal ganglia and express the D, dopamine receptor subtype, whereas indirect pathway MSNs project to the output nuclei via the globus pallidus, and express the D2 receptor. The striatum also contains interneurons that are essential in processing information within striatum; the cholinergic interneuron is of particular interest due to its role in reward-related behaviour. The aim of this study was to examine the cortical and thalamic innervation of subtypes of striatal neurons. To examine whether the cortical or thalamic afferents selectively innervate direct or indirect pathway neurons, transgenic mice expressing GFP under either the D, or D2 receptor promoter were used. Striatal sections from these mice were immunostained to reveal the GFP and selective markers of the cortical and thalamic afferents, VGluTI and VGluT2, respectively. A quantitative electron microscopic examination ofsynaptic connectivity was carried out. The results indicate that there is no selectivity of either the cortical or thalamic pathway for D, or D2 expressing MSNs. Thus both direct and indirect pathway MSNs are involved in the processing of both cortical and thalamic information The cortical and thalamic innervation to cholinergic interneurons was also examined. Stimulation of cortex and thalamus in vivo in anaesthetised rats resulted in short-latency excitatory responses in identified cholinergic interneurons, indicative of monosynaptic connections. After recording, cholinergic interneurons were filled with neurobiotin. The synaptic innervation from cortex and thalamus was then examined in two individual, electrophysiologically characterised, and neurochemically identified cholinergic interneurons. One neuron received input from both cortex and thalamus, whereas the other neuron received input from the thalamus only. These results provide anatomical and physiological data illustrating how the excitatory inputs to striatum innervate cholinergic interneurons.

The striatum is the largest nucleus of the basal ganglia, and acts as a point of convergence for thalamic, cortical and midbrain inputs. It is involved in both motor and associative forms of learning, and is composed of spiny projection neurons (SPNs) whose output along the so-called "direct pathway" and "indirect pathway" is modified by the activity of diverse sets of interneurons. Four "classical" or major classes of striatal interneuron can be identified according to the selective expression of the molecular markers parvalbumin (PV), calretinin (CR), nitric oxide synthase (NOS) or choline acetyltransferase (ChAT). Although the interneurons within a class are generally considered to be homogeneous in form and function, there is emerging evidence that some classes encompass multiple types of neuron, and that the heterogeneity in striatal interneurons extends beyond these four classes. Defining the extent of interneuron heterogeneity is important for understanding how the striatum processes distinct, topographically-organized inputs from the cortex and thalamus in order to govern a wide range of behaviors. To address these issues, a combination of immunofluorescence microscopy and stereological cell counting approaches was used in striatal tissue from rat, mouse and non-human primate. This was supplemented by in vivo recording and juxtacellular labelling of single neurons in rat. A first set of experiments showed that secretagogin (Scgn), a calcium-binding protein, is expressed by a large number of interneurons in the dorsal striatum of rat and primate, but not in the mouse. In all species tested, secretagogin was expressed by a subset of PV+ interneurons and a subset of CR+ interneurons in the dorsal striatum, but also labelled a group of interneurons that did not express any of the classical markers of striatal interneurons. A second set of experiments in the rat demonstrated that the selective co-expression of Scgn by PV+ interneurons delineates two topographically-, physiologically- and morphologically-distinct cell populations. These topographical differences in distribution were largely conserved in the primate caudate/putamen. In rats, PV+/Scgn+ and PV+/Scgn- interneurons differed significantly in their firing rates, firing patterns and phase-locking to cortical oscillations. The axons of PV+/Scgn+ interneurons were more likely to form appositions with the somata of direct pathway SPNs than indirect pathway SPNs, whereas the opposite was true for the axons of PV+/Scgn- interneurons. These two populations of GABAergic interneurons provide a potential substrate through which either of the striatal output pathways can be rapidly and selectively inhibited, and in turn mediate the expression of behavioral routines. A third set of experiments showed that CR+ interneurons of the dorsal striatum can be separated into three populations based on their molecular, topographical and morphological properties. Small-sized ("Type 3") CR+ interneurons co-expressed Scgn and were restricted in their distribution towards the rostro-medial poles of the striatum in both rats and primates. In rats, these neurons also expressed the transcription factor SP8, suggesting that they may be newly generated throughout adulthood. Large-sized, ("Type 1") CR+ interneurons did not express Scgn, but could be further distinguished by their expression of the transcription factor Lhx7. Medium-sized ("Type 2") CR+ interneurons did not express Scgn or Lhx7, and had heterogeneous electrophysiological properties in vivo. The expression of Scgn, but not other classical interneuron markers, identified a group of interneurons that were restricted in their distribution towards the ventro-medial aspects of the dorsal striatum. A fourth set of experiments showed that these neurons are also present in the core and the shell of the nucleus accumbens. Unlike the case of dorsal striatum, however, PV+ interneurons and CR+ interneurons of the nucleus accumbens did not co-express Scgn. Moreover, many of the interneuron populations studied had greater densities in the ventral striatum compared to the dorsal striatum, and had quantifiably strong biases in their distribution towards a variety of axes within both the core and the shell of the nucleus accumbens. These data thus highlight some major differences in the constituent elements of the microcircuits of dorsal striatum and nucleus accumbens. In conclusion, these studies have revealed a great deal of molecular, topographical, electrophysiological and structural heterogeneity within the interneuron populations of the striatum. As several of these interneuron populations were not evenly distributed throughout the striatum, this ultimately suggests that the microcircuit of the striatum is specialized according to regions that differ in their cortical, thalamic and dopaminergic inputs.

The basal ganglia are a set of subcortical nuclei in the forebrain of vertebrates that are highly conserved among mammals. Classically, dysfunction in the basal ganglia has been linked to motor abnormalities. However, it is now widely recognized that in addition to their role in motor behavior, these set of nuclei play a role in reinforcement learning and motivated behavior as well as in many diseases that present with abnormal motivation. In this dissertation, I first provide a review of the literature that describes the current state of research on the basal ganglia and the background for the original studies I later present. I describe the anatomy and physiology of the basal ganglia, including how structures are interconnected to form two parallel pathways, the direct and the indirect pathways. I further review published studies that have investigated how the basal ganglia regulate motor behavior and motivation. And finally, I also summarize findings on how disruption in basal ganglia circuitry function has been linked to a number of neuropsychiatric diseases, with special focus on the symptoms of schizophrenia. I then present original data and discuss the results of three studies investigating basal ganglia function and behavior. In the first study, I investigated the bridging collaterals, axon collaterals of direct-pathway medium spiny neurons (dMSNs) in the striatum that target the external segment of the globus (GPe), the canonical target of indirect-pathway medium spiny neurons (iMSNs). Previous work in the Kellendonk laboratory has linked these collaterals to increased dopamine D2 receptor (D2R) function and increased striatal excitability, as well as to abnormal locomotor response to stimulation of the direct pathway. I expanded on these findings by first demonstrating that bridging collaterals form synaptic contacts with GPe cells. I was also able to generate a viral vector to selectively increase excitability in specific populations of MSNs. I used this virus to show that chronically increasing excitability of the indirect pathway, but not the direct pathway, leads to a circuit-level change in connectivity by inducing the growth of bridging collaterals from dMSNs in the GPe. I also confirmed that increased density of bridging collaterals are associated with an abnormal locomotor response to stimulation of striatal dMSNs and further demonstrated that chronic pharmacologic blockade of D2Rs can rescue this abnormal locomotor phenotype. Furthermore, I found that motor training reverses the enhanced density of bridging collaterals and partially rescue the abnormal locomotor phenotype associated with increased collaterals, thereby establishing a new link between connectivity in the basal ganglia and motor learning. In the second study, I conducted a series of experiments in which I selectively increased excitability of the direct or indirect pathway in specific striatal sub-regions that have been implicated in goal-directed behavior, namely the DMS and NA core. I found that this manipulation was not sufficient to induce significant effects in different behavioral assays of locomotion and motivation, including the progressive ratio and concurrent choice tasks. These findings also suggest that increased bridging collateral density does not have a one-to-one relationship with the motivational deficit of D2R-OEdev mice, as previously hypothesized. In the third and final study, my original aim was to determine whether the motivational deficit of D2R-OEdev mice, induced by upregulation of D2Rs in the striatum, could be reversed by acutely activating Gαi-coupled signaling in the indirect pathway in these animals. I found that this manipulation increased motivation in D2R-OEdev mice but also in control littermates. This effect was due to energized behavioral performance, which, however, came at the cost of goal-directed efficiency. Moreover, selective manipulation of MSNs in either the DMS or NA core showed that both striatal regions contribute to this effect on motivation. Further investigation aimed at understanding how Gαi-coupled signaling affects striatal circuit function revealed that activating a Gαi-coupled receptor did not lead to a significant change in somatic MSN activity in vivo or to a change in neuronal excitability in vitro. In contrast, the GPe, which receives monosynaptic inhibition from the indirect pathway, showed disinhibited activity when Gαi signaling was activated in striatal iMSNs. In addition, as drug therapies for psychiatric diseases are not usually given acutely but involve long-term, continuous administrations, I also studied how chronically decreasing function of iMSNs would affect behavior. I showed that chronically activating a Gαi-coupled receptor in iMSNs does not lead to a measurable effect on locomotion or motivation, a behavioral desensitization response that can be recovered within 48 h and may be due to receptor desensitization to the drug or circuit-level compensation to a chronic decrease in iMSN function. Finally, I conclude this dissertation with a general discussion addressing how the findings from each study can be related to each other to provide a more complete understanding of how basal ganglia function regulate behavior, how disruption in the basal ganglia can underlie neuropsychiatric disease, and how strategies to target basal ganglia function should be employed to treat disorders of motivation. I conclude this dissertation by proposing new avenues of research for further exploring my findings.