Neural correlates of distinct levels of predatory threat in dorsal periaqueductal grey neurons.

The dorsal periaqueductal grey (PAG) is an important site for integrating predatory threats. However, it remains unclear whether predator-related activation in PAG primarily reflects threat itself and thus can distinguish between various degrees of threat, or rather reflects threat-oriented behaviours, with the PAG potentially orchestrating different types of defensive repertoire. To address this issue, we performed extracellular recording of dorsal PAG neurons in freely behaving rats and examined neuronal and behavioural responses to stimulus conditions with distinct levels of predatory threat. Animals were sequentially exposed to a nonthreatening stimulus familiar environment (exposure to habituated environment) and to a novel nonthreatening stimulus (i.e., a toy animal-plush) and to conditions with high (exposure to a live cat), intermediate (exposure to the environment just visited by the cat, with remnant predator scent), and low (exposure on the following day to the predatory context) levels of predatory threat. To test for contributions of both threat stimuli and behaviour to changes in firing rate, we applied a Poisson generalized linear model regression, using the different predator stimulus conditions and defensive repertoires as predictor variables. Analysis revealed that the different predator stimulus conditions were more predictive of changes in firing rate (primarily threat-induced increases) than the different defensive repertoires. Thus, the dorsal PAG may code for different levels of predatory threat, more than it directly orchestrates distinct threat-oriented behaviours. The present results open interesting perspectives to investigate the role of the dorsal PAG in mediating primal emotional and cognitive responses to fear-inducing stimuli.

Frequency matters: how changes in hippocampal theta frequency can influence temporal coding, anxiety-reduction, and memory.

Hippocampal theta frequency is a somewhat neglected topic relative to theta power, phase, coherence, and cross-frequency coupling. Accordingly, here we review and present new data on variation in hippocampal theta frequency, focusing on functional associations (temporal coding, anxiety reduction, learning, and memory). Taking the rodent hippocampal theta frequency to running-speed relationship as a model, we identify two doubly-dissociable frequency components: (a) the slope component of the theta frequency-to-stimulus-rate relationship ("theta slope"); and (b) its y-intercept frequency ("theta intercept"). We identify three tonic determinants of hippocampal theta frequency. (1) Hotter temperatures increase theta frequency, potentially consistent with time intervals being judged as shorter when hot. Initial evidence suggests this occurs via the "theta slope" component. (2) Anxiolytic drugs with widely-different post-synaptic and pre-synaptic primary targets share the effect of reducing the "theta intercept" component, supporting notions of a final common pathway in anxiety reduction involving the hippocampus. (3) Novelty reliably decreases, and familiarity increases, theta frequency, acting upon the "theta slope" component. The reliability of this latter finding, and the special status of novelty for learning, prompts us to propose a Novelty Elicits Slowing of Theta frequency (NEST) hypothesis, involving the following elements: (1) Theta frequency slowing in the hippocampal formation is a generalised response to novelty of different types and modalities; (2) Novelty-elicited theta slowing is a hippocampal-formation-wide adaptive response functioning to accommodate the additional need for learning entailed by novelty; (3) Lengthening the theta cycle enhances associativity; (4) Even part-cycle lengthening may boost associativity; and (5) Artificial theta stimulation aimed at enhancing learning should employ low-end theta frequencies.

The within-subject application of diffusion tensor MRI and CLARITY reveals brain structural changes in Nrxn2 deletion mice.

Background: Of the many genetic mutations known to increase the risk of autism spectrum disorder, a large proportion cluster upon synaptic proteins. One such family of presynaptic proteins are the neurexins (NRXN), and recent genetic and mouse evidence has suggested a causative role for NRXN2 in generating altered social behaviours. Autism has been conceptualised as a disorder of atypical connectivity, yet how single-gene mutations affect such connectivity remains under-explored. To attempt to address this, we have developed a quantitative analysis of microstructure and structural connectivity leveraging diffusion tensor MRI (DTI) with high-resolution 3D imaging in optically cleared (CLARITY) brain tissue in the same mouse, applied here to the Nrxn2α knockout (KO) model. Methods: Fixed brains of Nrxn2α KO mice underwent DTI using 9.4 T MRI, and diffusion properties of socially relevant brain regions were quantified. The same tissue was then subjected to CLARITY to immunolabel axons and cell bodies, which were also quantified. Results: DTI revealed increases in fractional anisotropy in the amygdala (including the basolateral nuclei), the anterior cingulate cortex, the orbitofrontal cortex and the hippocampus. Axial diffusivity of the anterior cingulate cortex and orbitofrontal cortex was significantly increased in Nrxn2α KO mice, as were tracts between the amygdala and the orbitofrontal cortex. Using CLARITY, we find significantly altered axonal orientation in the amygdala, orbitofrontal cortex and the anterior cingulate cortex, which was unrelated to cell density. Conclusions: Our findings demonstrate that deleting a single neurexin gene (Nrxn2α) induces atypical structural connectivity within socially relevant brain regions. More generally, our combined within-subject DTI and CLARITY approach presents a new, more sensitive method of revealing hitherto undetectable differences in the autistic brain.

Down-regulation of hippocampal BDNF and Arc associated with improvement in aversive spatial memory performance in socially isolated rats.

Rats deprived of social contact with other rats at a young age experience a form of prolonged stress that leads to long-lasting changes in behavioral profile. Such isolation is thought to be anxiogenic for these normally gregarious animals, and the abnormal reactivity of isolated rats to environmental stimuli is thought to be a product of prolonged stress. We now show that isolation of rats at weaning reduced immobility time in the forced swim test, decreased sucrose intake and preference, and down-regulated both brain-derived neurotrophic factor (BDNF) and activity-regulated cytoskeletal associated protein (Arc) in the hippocampus. In the Morris water maze, isolated rats showed a reduced latency to reach the hidden platform during training, indicative of an improved learning performance, compared with group-housed rats. The cumulative search error during place training trials indicated a reliable difference between isolated and group-housed rats on days 4 and 5. The probe trial revealed a significant decrease of the average proximity to the target location in the isolated rats suggesting an improvement in spatial memory. Isolated rats also showed an increase in the plasma level of corticosterone on the 5th day of training and increased expression of BDNF and Arc in the hippocampus on both days 1 and 5. These results show that social isolation from weaning in rats results in development of depressive-like behavior but has a positive effect on spatial learning, supporting the existence of a facilitating effect of stress on cognitive function.