Age-at-Injury Determines the Extent of Long-Term Neuropathology and Microgliosis After a Diffuse Brain Injury in Male Rats

Traumatic brain injury (TBI) can occur at any age, from youth to the elderly, and its contribution to age-related neuropathology remains unknown. Few studies have investigated the relationship between age-at-injury and pathophysiology at a discrete biological age. In this study, we report the immunohistochemical analysis of naïve rat brains compared to those subjected to diffuse TBI by midline fluid percussion injury (mFPI) at post-natal day (PND) 17, PND35, 2-, 4-, or 6-months of age. All brains were collected when rats were 10-months of age (n = 6–7/group). Generalized linear mixed models were fitted to analyze binomial proportion and count data with R Studio. Amyloid precursor protein (APP) and neurofilament (SMI34, SMI32) neuronal pathology were counted in the corpus callosum (CC) and primary sensory barrel field (S1BF). Phosphorylated TAR DNA-binding protein 43 (pTDP-43) neuropathology was counted in the S1BF and hippocampus. There was a significantly greater extent of APP and SMI34 axonal pathology and pTDP-43 neuropathology following a TBI compared with naïves regardless of brain region or age-at-injury. However, age-at-injury did determine the extent of dendritic neurofilament (SMI32) pathology in the CC and S1BF where all brain-injured rats exhibited a greater extent of pathology compared with naïve. No significant differences were detected in the extent of astrocyte activation between brain-injured and naïve rats. Microglia counts were conducted in the S1BF, hippocampus, ventral posteromedial (VPM) nucleus, zona incerta, and posterior hypothalamic nucleus. There was a significantly greater proportion of deramified microglia, regardless of whether the TBI was recent or remote, but this only occurred in the S1BF and hippocampus. The proportion of microglia with colocalized CD68 and TREM2 in the S1BF was greater in all brain-injured rats compared with naïve, regardless of whether the TBI was recent or remote. Only rats with recent TBI exhibited a greater proportion of CD68-positive microglia compared with naive in the hippocampus and posterior hypothalamic nucleus. Whilst, only rats with a remote brain-injury displayed a greater proportion of microglia colocalized with TREM2 in the hippocampus. Thus, chronic alterations in neuronal and microglial characteristics are evident in the injured brain despite the recency of a diffuse brain injury.

Basal Ganglia Responses to Electrical Stimulation of the Posterior Hypothalamic Nucleus

Electrical or chemical stimulation of the posterior hypothalamic nucleus (PH) elicits highly adaptive locomotion, demonstrating both evidence of flexibility and variety in exhibited motor behaviours. However, the neural substrates of PH stimulation elicited behavioural changes are poorly understood. The basal ganglia are postulated to be critically involved in the process of action selection in conjunction with thalamo-cortical systems. The present study examines changes in basal ganglia activities in response to the high-frequency stimulation of the PH. Under urethane anaesthesia, ensemble and single-unit recordings were obtained from the striatum (STR), globus pallidus externa (GPe), entopeduncular nucleus (EP), subthalamic nucleus (STN) and the substantia nigra pars reticulata (SNr). Upon PH stimulation, increases in firing rates were observed in the STR, GPe, and STN, a decrease was observed in the SNr and no changes were seen in the EP. The increase in spike rate in the STR and GPe was dependent on the stimulation intensity but not duration. Despite the differences in the direction of firing changes during PH stimulation, all examined areas including those not part of the basal ganglia demonstrated an elevated spiking rate upon stimulus train termination. Taking into account the known anatomical connections between the PH and the basal ganglia, it is hypothesized responses seen during PH stimulus trains are mediated through thalamic and cortical relays whereas the overall post-stimulus excitatory response is related to the impact of the PH on brainstem arousal systems.

Optimization of whole-brain rabies virus tracing technology for small cell populations

The lateral hypothalamus (LH) is critically involved in the regulation of homeostatic energy balance. Some neurons in the LH express receptors for leptin (LepRb), a hormone known to increase energy expenditure and decrease energy intake. However, the neuroanatomical inputs to LepRb-expressing LH neurons remain unknown. We used rabies virus tracing technology to map these inputs, but encountered non-specific tracing. To optimize this technology for a minor cell population (LepRb is not ubiquitously expressed in LH), we used LepRb-Cre mice and assessed how different titers of the avian tumor virus receptor A (TVA) helper virus affected rabies tracing efficiency and specificity. We found that rabies expression is dependent on TVA receptor expression, and that leakiness of TVA receptors is dependent on the titer of TVA virus used. We concluded that a titer of 1.0–3.0 × 107 genomic copies per µl of the TVA virus is optimal for rabies tracing. Next, we successfully applied modified rabies virus tracing technology to map inputs to LepRb-expressing LH neurons. We discovered that other neurons in the LH itself, the periventricular hypothalamic nucleus (Pe), the posterior hypothalamic nucleus (PH), the bed nucleus of the stria terminalis (BNST), and the paraventricular hypothalamic nucleus (PVN) are the most prominent input areas to LepRb-expressing LH neurons.

Activation of thalamo-cortical circuits with posterior hypothalamic nucleus deep brain stimulation

The posterior hypothalamic nucleus (PH) has extensive anatomical connections to motor, cognitive, visceral, and homeostatic areas of the brain and serves as a crucial subcortical modulator of behaviour. Previous studies have demonstrated that deep brain stimulation (DBS) of this area can lead to powerful activation of motor behaviour, overcoming two rodent models of parkinsonian akinesia by increasing neocortical excitability. However, it is unclear how the PH may mediate this increase in neocortical excitability. In the present study, we examined the role of the thalamus in the PH-DBS mediated increase in neocortical excitability. In urethane anaesthetized animals, we demonstrate that PH-DBS elicits increased spiking activity in the motor thalamus (VL) that receives direct afferents from the PH that precedes the increase in spiking activity in the corresponding motor cortex. In contrast, in the somatosensory thalamus (VPM) where PH afferents are sparse at best, PH-DBS did not elicit an increase in thalamic activity despite of a slight increase in the corresponding somatosensory cortical spiking. Current source density analyses suggest a thalamo-cortical mechanism for motor cortex activation whereas a cortico-cortical activation mechanism is involved in somatosensory cortical activation. Inactivation of the VL resulted in the abolition of motor cortex spiking despite of the persistence of desynchronized field potential activity. Collectively, these data suggest indirect orthodromic activation of PH output fibres to the thalamus mediates increased neocortical excitation, which may spread through cortico-cortical connections and lead to an increase in integrated, non-stereotypical motor behaviour.

Histomorphological Studies on the Different Nuclei of Hypothalamus of Indian Buffalo

Background: Hypothalamus is an integral part of the hypothalamo-hypophyseal ovarian axis. It contains several small nuclei that have been implicated in several specialized functions. It is the master endocrine gland because it regulates the activity of the pituitary. The present investigation was planned to elucidate the histomorphological details of different nuclei in the hypothalamus of Indian buffaloes. Methods: The hypothalami of buffaloes (n =52) were collected from local abattoirs and Teaching Veterinary Clinical Complex, GADVASU, Ludhiana. The tissue samples were collected from three levels i.e. the supraoptic, tuberal and the mamillary region of the hypothalamus and processed for paraffin sectioning and the sections were stained with various stains for histomorphological studies. Result: The well-defined cell groups or nuclei were identified both in the coronal and sagittal sections of the hypothalamus in all the reproductive phases. Eight nuclei were distinctly recognized in buffalo hypothalamus at three levels in the coronal sections of the hypothalamus. At the level of optic chiasma, the nuclei were paraventricular nucleus (PVN), supraoptic nucleus (SON) and preopticnucleus (PON); at the tuberal region, there were arcuate nucleus (AN) and ventromedial nucleus (VMN) and at the mammillary region, the lateral mammillary nucleus (LMN), medial mammillary nucleus (MMN) and posterior hypothalamic nucleus (PHN) were identified. Most of the nuclei consisted of a loose or compact heterogeneous collection of neurons. The neurons of the different nuclei were of variable shape. The cytoplasm contained a varied amount of Nissl material. The neurosecretory substance was peripheral in position. Small to large-sized blood vessels were observed in between the neuronal substance. The size of the neurons varied during different stages of reproduction.