Cortico-cortical connectivity is influenced by levodopa in tremor-dominant Parkinson's disease.

Resting tremor is the most common presenting motor symptom in Parkinson's disease (PD). The supplementary motor area (SMA) is a main target of the basal-ganglia-thalamo-cortical circuit and has direct, facilitatory connections with the primary motor cortex (M1), which is important for the execution of voluntary movement. Dopamine potentially modulates SMA and M1 activity, and both regions have been implicated in resting tremor. This study investigated SMA-M1 connectivity in individuals with PD ON and OFF dopamine medication, and whether SMA-M1 connectivity is implicated in resting tremor. Dual-site transcranial magnetic stimulation was used to measure SMA-M1 connectivity in PD participants ON and OFF levodopa. Resting tremor was measured using electromyography and accelerometry. Stimulating SMA inhibited M1 excitability OFF levodopa, and facilitated M1 excitability ON levodopa. ON medication, SMA-M1 facilitation was significantly associated with smaller tremor than SMA-M1 inhibition. The current findings contribute to our understanding of the neural networks involved in PD which are altered by levodopa medication and provide a neurophysiological basis for the development of interventions to treat resting tremor.

Reduced Cerebellar Brain Inhibition Measured Using Dual-Site TMS in Older Than in Younger Adults.

Dual-site transcranial magnetic stimulation (TMS) can be used to measure the cerebellar inhibitory influence on the primary motor cortex, known as cerebellar brain inhibition (CBI), which is thought to be important for motor control. The aim of this study was to determine whether age-related differences in CBI (measured at rest) were associated with an age-related decline in bilateral motor control measured using the Purdue Pegboard task, the Four Square Step Test, and a 10-m walk. In addition, we examined test re-test reliability of CBI measured using dual-site TMS with a figure-of-eight coil in two sessions. There were three novel findings. First, CBI was less in older than in younger adults, which is likely underpinned by an age-related loss of Purkinje cells. Second, greater CBI was associated with faster 10-m walking performance in older adults, but slower 10-m walking performance in younger adults. Third, moderate intraclass correlation coefficients (ICCs: 0.53) were found for CBI in younger adults; poor ICCs were found for CBI (ICC: 0.40) in older adults. Together, these results have important implications for the use of dual-site TMS to increase our understanding of age- and disease-related changes in cortical motor networks, and the role of functional connectivity in motor control.

Test Re-test Reliability of Dual-site TMS Measures of SMA-M1 Connectivity Differs Across Inter-stimulus Intervals in Younger and Older Adults.

Dual-site transcranial magnetic stimulation (TMS) is a promising tool to measure supplementary motor area and primary motor cortex (SMA-M1) connectivity in younger and older adults, and could be used to understand the pathophysiology of movement disorders. However, test re-test reliability of dual-site TMS measures of SMA-M1 connectivity has not been established. We examined the reliability of SMA-M1 connectivity using dual-site TMS in two sessions in 30 younger and 30 older adults. For dual-site TMS, a conditioning pulse delivered to SMA (140% of active motor threshold) preceded a test pulse delivered to M1 (intensity that elicited MEPs of ~1 mV) by inter-stimulus intervals (ISI) of 6 ms, 7 ms, and 8 ms. Moderate intraclass correlation coefficients (ICC) were found for SMA-M1 connectivity at an ISI of 7 ms in younger (ICC: 0.69) and older adults (ICC: 0.68). Poor ICCs were found for SMA-M1 connectivity at ISIs of 6 ms and 8 ms in both age groups (ICC range: 0.01-0.40). We report evidence for stable measures of SMA-M1 connectivity at an ISI of 7 ms in both age groups. These findings are foundational for future research developing evidence-based interventions to strengthen SMA-M1 connectivity to improve bilateral motor control in older adults and populations with movement disorders.

Coming of Age in Ireland: the Twilight Zone!

Aim To describe the healthcare needs of adolescent patients inhabiting the 'seventh age of childhood' in our region with a view towards future workforce and infrastructure planning. Methods This is a retrospective descriptive study of patients aged between 14 and 16 years presenting to each of the six hospitals in our hospital group over a 10 year period (01.07.2006-1.07.2016) using electronic databases. Results There were 10,992 hospital admissions, 41,456 outpatient appointments and an average of 1,847 attendances per year at our Emergency Department in this age group. Seventeen percent (n=1,873) of patients were admitted to age appropriate wards. Only 11.3% (n=1,242) of our cohort were admitted under the care of a Paediatrician. Conclusion The Irish healthcare agenda needs to be advanced to ensure the optimal health for this valuable, yet vulnerable generation. Further investment will help shape the fledgling discipline of 'adolescent health' in Ireland.

Zona incerta: Substrate for contralateral interconnectivity in the thalamus of rats.

We have shown previously that the zona incerta (ZI), a small nucleus deriving from the ventral thalamus, has extensive ipsilateral connections with the higher order and intralaminar nuclei of the dorsal thalamus and that there are many ipsilateral interconnections between the different cytoarchitectonic sectors of the ZI. In this study, we explore the contralateral connections that the ZI has with its opposing nucleus as well as with the other nuclei of the thalamus. Injections of biotinylated dextran or cholera toxin subunit B were made into each of the different ZI sectors (rostral, dorsal, ventral, and caudal) and into intralaminar and higher order dorsal thalamic nuclei of Sprague-Dawley rats by using stereotaxic coordinates. Brains were fixed in aldehyde and processed using standard methods. Our results show that, after injections limited to a given ZI sector, labelled terminal-like elements and cells were seen across the other sectors of the ZI of the contralateral side. Furthermore, after each of these ZI injections, labelling was seen in the intralaminar (e.g., parafascicular, central lateral, and central medial) and higher order (e.g., posterior thalamic, lateral posterior, and lateral dorsal) nuclei of the contralateral side. These patterns of labelling were confirmed after tracer injections into intralaminar and higher order nuclei; after such injections, labelling was seen in the contralateral ZI. In all cases, there was labelling on the ipsilateral side as well, and this was generally heavier than on the contralateral side. Overall, our results indicate that there is a network of interconnections between the ZI of both sides of the thalamus and that the ZI has contralateral connections with the intralaminar and higher order nuclei. Hence, the ZI furnishes a substrate that spreads activity to both sides of the brain.

Evidence for a visual subsector within the zona incerta.

Here we examine the patterns of connections between the zona incerta (ZI) of the thalamus and the major visual centers of the rat brain, namely the retina, dorsal lateral geniculate nucleus (LGd), superficial layers of the superior colliculus (SCs), and occipital cortex (Ocl). Injections of the tracers biotinylated dextran or cholera toxin subunit b were made into each of these centers, as well as ZI itself, by using stereotaxic coordinates. Rat brains were then aldehyde-fixed and processed using standard methods. We show that the retina, LGd, SCs, and Ocl all have connections with ZI; moreover, that each of these connections make a very distinct territory or subsector within the most lateral ZI regions. This subsector of connectivity with the visual centers does not respect the well-defined cytoarchitectonic sectors of ZI, being made up of small zones in the dorsal, ventral, and caudal sectors. Often, a distinctive "horse-shoe" pattern is evident, particularly after retinal and Ocl injections. Tracer injections into topographically distinct regions of the LGd. SCs, or Ocl results in no shift in the spatial location of labelling within ZI; after each injection, labelling is always seen within the lateral edge of the nucleus. Labelled terminals and cells are seen after LGd and SCs injections, while only labelled terminals are seen after retinal and Ocl injections. Although the precise function of this novel visual subsector is not known, these early findings suggest that ZI may be in a position to integrate visual information together with the other somatosensory, motor, and visceral information that it receives.

Dorsal thalamic connections of the ventral lateral geniculate nucleus of rats.

In order to understand better the organisation of the ventral lateral geniculate nucleus of the ventral thalamus, this paper has examined the patterns of connections that this nucleus has with various nuclei of the dorsal thalamus in rats. Injections of biotinylated dextran or cholera toxin subunit B were made into the parafascicular, central lateral, posterior thalamic, medial dorsal, lateral dorsal, lateral posterior, dorsal lateral geniculate, anterior, ventral lateral, ventrobasal and medial geniculate nuclei of Sprague-Dawley rats and their brains were processed using standard tracer detection methods. Three general patterns of ventral lateral geniculate connectivity were seen. First, the parafascicular, central lateral, medial dorsal, posterior thalamic and lateral dorsal nuclei had heavy connections with the parvocellular (internal) lamina of the ventral lateral geniculate nucleus. This geniculate lamina has been shown previously to receive heavy inputs from many functionally diverse brainstem nuclei. Second, the visually related dorsal lateral geniculate and lateral posterior nuclei had heavy connections with the magnocellular (external) lamina of the ventral lateral geniculate nucleus. This geniculate lamina has been shown by previous studies to receive heavy inputs from the visual cortex and the retina. Finally, the anterior, ventral lateral, ventrobasal and medial geniculate nuclei had very sparse, if any, connections with the ventral lateral geniculate nucleus. Overall, our results strengthen the notion that one can package the ventral lateral geniculate nucleus into distinct visual (magnocellular) and non-visual (parvocellular) components.

Evidence for extensive inter-connections within the zona incerta in rats.

We have examined whether there is an extensive system of inter-connections between the functionally distinct sectors of the zona incerta (ZI) of the thalamus. Unilateral injections of biotinylated dextran were made into each of the different incertal sectors (rostral, dorsal, ventral and caudal) of Sprague-Dawley rats by using stereotaxic coordinates. Our results show that after separate injections limited to each of the incertal sectors, many labelled terminals and cells were seen across the different sectors of the ipsilateral, as well as the contralateral side, with the heaviest labelling being on the side ipsilateral to the injection. Thus, these results suggest that the ZI is in a position to integrate an extensive array of afferents from many functionally diverse neural centres of the brain.

Evidence for a large projection from the zona incerta to the dorsal thalamus.

In an effort to understand better how the zona incerta may influence neocortical activity, this study has examined the patterns of projection that this nucleus has to the dorsal thalamus, the "gateway" to the neocortex. To this end, Sprague-Dawley rats were anaesthetised with Ketamil (100 mg/kg) and Rompun (10 mg/kg), and injections of biotinylated dextran or cholera toxin subunit B (CTB) were made into various dorsal thalamic nuclei, including the primary relay (dorsal lateral geniculate, medial geniculate, ventral posterior), association (lateral dorsal, lateral posterior, posterior thalamic), and intralaminar (central lateral, parafascicular) nuclear groups, by using stereotaxic coordinates. Brains were aldehyde fixed and processed with standard methods. Our results show that there is a large projection from the zona incerta to the dorsal thalamus. This projection does not blanket all nuclei of the dorsal thalamus but, rather, shows a clear preference for some nuclei over others. After CTB or dextran injections into the primary relay nuclei, very few cells are labelled in the zona incerta. After similar injections are made into the association or intralaminar nuclei, however, many more labelled incertal cells are seen. There are some differences in the distribution of labelled cells within the zona incerta after injections into the association nuclei compared with injections into the intralaminar nuclei. The association nuclei relate strongly to the ventral sector, whereas the intralaminar nuclei relate strongly to the dorsal sector of the zona incerta. After each of these injections into the dorsal thalamus, labelled terminals are seen in the zona incerta also, and their distribution mirrors the distribution of the labelled incertal cells described above. Thus, in summary, our results indicate that the zona incerta has a large and preferential projection to the dorsal thalamus, in particular from the association and intralaminar nuclei. Through this dorsal thalamic projection, the zona incerta is in a position to influence large areas of the neocortex.

Specificity of projection among cells of the zona incerta.

We have examined whether individual cells of the zona incerta of the thalamus have widespread projections across the brain. Double injections of different coloured fluorescent latex beads (red or green) were made, in various combinations, into regions of neocortex, dorsal thalamus or brainstem of Sprague-Dawley rats. These regions were chosen since they have been shown previously to receive projections from the zona incerta. We also made injections of different coloured beads into different regions of these same brain centres (ie, distinct cortical areas or individual dorsal thalamic and brainstem nuclei). In general, our results show that cells of the zona incerta have projections limited to one of these brain centres only. We saw very few double-labelled incertal cells after double injections of different coloured latex beads into either the neocortex/dorsal thalamus, neocortex/brainstem or dorsal thalamus/brainstem. Further, we show that within each of these brain centres, the projection patterns of individual incertal cells is rather restricted, since double injections of different coloured beads into separate regions of the same centre resulted in few double-labelled incertal cells. Taken together, these results suggest a very clear specificity of projection among cells of the zona incerta. Thus, although the cells of the zona incerta receive a plethora of inputs from many sources, it appears that its cells have a very clear and focussed output to distinct regions of the brain.

Patterns of connections between zona incerta and brainstem in rats.

To understand better the organisation of zona incerta of the thalamus, this study has examined the patterns of connections that this nucleus has with various nuclei of the brainstem. Injections of biotinylated dextran or cholera toxin subunit B were made into the dorsal raphe, midbrain reticular nucleus, pedunculopontine tegmental nucleus, periaqueductal grey matter, pontine reticular nucleus, substantia nigra, superior colliculus, and ventral tegmental area of Sprague-Dawley rats, and their brains were processed by using standard tracer-detection methods. In general, our results show that zona incerta forms the major zone in the thalamus where these ascending brainstem axons terminate and from which descending axons that travel back to these same brainstem centres originate. These incertal inputs and outputs are limited largely to a distinct sector of zona incerta, the dorsal sector. An exception to this pattern is evident in the incertal projection to the deep layers of the superior colliculus; this projection, unlike all of the others, arises from cells in the ventral sector of zona incerta. Our results also show little evidence for a well-defined topography of projection between the brainstem and the zona incerta. For instance, small injections into each brainstem nucleus result in labelled terminals and in cells spread throughout much of the dorsal sector of zona incerta, with no local zone of concentration within the sector. Again, an exception to this pattern is seen in the incertal projection to the superior colliculus. This projection, unlike the others, shows a clear topographical organisation: A medial-lateral shift in the injection site in the colliculus results in a lateral-medial shift in the position of labelled cells in zona incerta. Curiously, even though the incertal projection to the colliculus appears to be mapped, the collicular projection back to zona incerta is not mapped. In conclusion, then, a number of brainstem nuclei (except for the deep collicular layers) have strong and overlapping connections within the same sector of zona incerta. This convergence of many functionally diverse brainstem afferents within zona incerta places this nucleus in a strategic position to sample the general activity of the brainstem and, perhaps, acts as a relay of this information to higher centres, such as the dorsal thalamic relay nuclei and the cerebral hemispheres.