Differential Resting-State Functional Connectivity of Striatal Subregions in Bipolar Depression and Hypomania

Bipolar disorder (BP) is characterized by periods of depression (BPD) and (hypo)mania (BPM), but the underlying state-related brain circuit abnormalities are not fully understood. Striatal functional activation and connectivity abnormalities have been noted in BP, but consistent findings have not been reported. To further elucidate striatal abnormalities in different BP states, this study investigated differences in resting-state functional connectivity of six striatal subregions in BPD, BPM, and healthy control (HC) subjects. Ninety medication-free subjects (30 BPD, 30 BPM, and 30 HC), closely matched for age and gender, were scanned using 3T functional magnetic resonance imaging (fMRI) acquired at resting state. Correlations of low-frequency blood oxygen level dependent signal fluctuations for six previously described striatal subregions were used to obtain connectivity maps of each subregion. Using a factorial design, main effects for differences between groups were obtained and post hoc pairwise group comparisons performed. BPD showed increased connectivity of the dorsal caudal putamen with somatosensory areas such as the insula and temporal gyrus. BPM group showed unique increased connectivity between left dorsal caudate and midbrain regions, as well as increased connectivity between ventral striatum inferior and thalamus. In addition, both BPD and BPM exhibited widespread functional connectivity abnormalities between striatal subregions and frontal cortices, limbic regions, and midbrain structures. In summary, BPD exhibited connectivity abnormalities of associative and somatosensory subregions of the putamen, while BPM exhibited connectivity abnormalities of associative and limbic caudate. Most other striatal subregion connectivity abnormalities were common to both groups and may be trait related.

Mean-field modeling of the basal ganglia-thalamocortical system. I Firing rates in healthy and parkinsonian states

Parkinsonism leads to various electrophysiological changes in the basal ganglia-thalamocortical system (BGTCS), often including elevated discharge rates of the subthalamic nucleus (STN) and the output nuclei, and reduced activity of the globus pallidus external (GPe) segment. These rate changes have been explained qualitatively in terms of the direct/indirect pathway model, involving projections of distinct striatal populations to the output nuclei and GPe. Although these populations partly overlap, evidence suggests dopamine depletion differentially affects cortico-striato-pallidal connection strengths to the two pallidal segments. Dopamine loss may also decrease the striatal signal-to-noise ratio, reducing both corticostriatal coupling and striatal firing thresholds. Additionally, nigrostriatal degeneration may cause secondary changes including weakened lateral inhibition in the GPe, and mesocortical dopamine loss may decrease intracortical excitation and especially inhibition. Here a mean-field model of the BGTCS is presented with structure and parameter estimates closely based on physiology and anatomy. Changes in model rates due to the possible effects of dopamine loss listed above are compared with experiment. Our results suggest that a stronger indirect pathway, possibly combined with a weakened direct pathway, is compatible with empirical evidence. However, altered corticostriatal connection strengths are probably not solely responsible for substantially increased STN activity often found. A lower STN firing threshold, weaker intracortical inhibition, and stronger striato-GPe inhibition help explain the relatively large increase in STN rate. Reduced GPe-GPe inhibition and a lower GPe firing threshold can account for the comparatively small decrease in GPe rate frequently observed. Changes in cortex, GPe, and STN help normalize the cortical rate, also in accord with experiments. The model integrates the basal ganglia into a unified framework along with an existing thalamocortical model that already accounts for a wide range of electrophysiological phenomena. A companion paper discusses the dynamics and oscillations of this combined system.

Reduction in the ratio of beta-preprotachykinin to preproenkephalin messenger RNA expression in postmortem human putamen during aging and in patients with status lacunaris. Implications for the susceptibility to parkinsonism

Gamma-aminobutyric acid (GABA)/substance P (SP) neurons and GABA/enkephalin (Enk) neurons in the striatum exert opposing influence on the regulation of movement. The loss of GABA/SP neurons results in hypokinetic disorders (parkinsonism), whereas the loss of GABA/Enk neurons results in hyperkinetic disorders (e.g. chorea). The present study determined age-related changes in the beta-preprotachykinin (the precursor of SP) and preproenkaphalin (the precursor of Enk) messenger RNA (mRNA) ratio in the postmortem human putamen using the reverse transcription-polymerase chain reaction (RT-PCR). The ratio of beta-preprotachykinin to preproenkephalin mRNA expression decreased with age. The reduction in the beta-preprotachykinin/preproenkephalin mRNA ratio was more marked in cases with multiple small infarcts (status lacunaris) in the putamen. These findings may in part explain the susceptibility of the elderly, particularly of those with ischemic changes in the striatum to hypokinetic disorders.

Basal ganglia: functional anatomy and physiology. Part 1

Advances in knowledge about basal ganglia structure and connectivity from 1925 to date are reviewed. Current concepts about neuronal populations, transmitters, and input and output of each of the basal ganglia nuclei are presented. The portrayal by Wilson, in 1925, of the striatum as a simple homogeneous structure has been replaced by the recognition, based on staining characteristics, connectivity, and function, that the neostriatum is compartmentalized into striosomes, matrisomes, and matrix compartments. Electrophysiologic studies have further shown the existence, in the neostriatum, of neuronal clusters that represent basic functional units much like the functional columns described much earlier for the cerebral cortex. Whereas the neostriatum is considered the major receiving area of the basal ganglia, the globus pallidus and substantia nigra pars reticulata constitute the major output nuclei. Combined neuroanatomic and neurophysiologic studies have revealed precise somatotopic organization throughout the basal ganglia system such that the leg, arm, and face areas of the cerebral cortex related to respective topographic areas within the striatum, pallidum, substantia nigra, and subthalamus. The previous concept of an inhibitory role for dopamine on striatal neurons has been modified. It is now acknowledged that dopamine exerts an inhibitory effect on striatal neurons that project to the external pallidum and a facilitatory effect on striatal neurons that project to the internal pallidum and substantia nigra pars reticulata. The previous concept of serial connectivity of the neostriatum (funnel concept) has been replaced by the concept of parallel connectivity. Within the internal connectivity of the basal ganglia, there is a fast system in which the neurotransmitter is gamma-aminobutyric acid (GABA) and a slow system modulated by neuropeptides. The slow system is believed to give identity to an otherwise homogenous GABAergic system.

A possible pathophysiologic substrate of attention deficit hyperactivity disorder

The attention deficit hyperactivity disorder (ADHD) is associated with defective attention and response inhibition and motor restlessness. Inattention, defective response inhibition, and impersistence are more commonly seen in adults with right than with left hemisphere dysfunction. In light of this fact and because children with ADHD not only appear to demonstrate these symptoms but also neglect the left side and have decreased activation of their right neostriatum, we propose that these children have a right hemisphere dysfunction. In addition, because both inattention and defective response inhibition can be seen in children with ADHD and in patients and animals who have frontal lobe and striatal dysfunction, we propose that children with ADHD have dysfunction in a right-sided frontal-striatal system. Motor restlessness may reflect frontal lobe dysfunction due to impairment of the mesocortical dopamine system.