Ian M. Devonshire

Neurovascular coupling is brain region-dependent

Despite recent advances in alternative brain imaging technologies, functional magnetic resonance imaging (fMRI) remains the workhorse for both medical diagnosis and primary research. Indeed, the number of research articles that utilise fMRI have continued to rise unabated since its conception in 1991, despite the limitation that recorded signals originate from the cerebral vasculature rather than neural tissue. Consequently, understanding the relationship between brain activity and the resultant changes in metabolism and blood flow (neurovascular coupling) remains a vital area of research. In the past, technical constraints have restricted investigations of neurovascular coupling to cortical sites and have led to the assumption that coupling in non-cortical structures is the same as in the cortex, despite the lack of any evidence. The current study investigated neurovascular coupling in the rat using whole-brain blood oxygenation level-dependent (BOLD) fMRI and multi-channel electrophysiological recordings and measured the response to a sensory stimulus as it proceeded through brainstem, thalamic and cortical processing sites - the so-called whisker-to-barrel pathway. We found marked regional differences in the amplitude of BOLD activation in the pathway and non-linear neurovascular coupling relationships in non-cortical sites. The findings have important implications for studies that use functional brain imaging to investigate sub-cortical function and caution against the use of simple, linear mapping of imaging signals onto neural activity.

Effects of urethane anaesthesia on sensory processing in the rat barrel cortex revealed by combined optical imaging and electrophysiology

The spatiotemporal dynamics of neuronal assemblies evoked by sensory stimuli have not yet been fully characterised, especially the extent to which they are modulated by prevailing brain states. In order to examine this issue, we induced different levels of anaesthesia, distinguished by specific electroencephalographic indices, and compared somatosensory‐evoked potentials (SEPs) with voltage‐sensitive dye imaging (VSDI) responses in the rat barrel cortex evoked by whisker deflection. At deeper levels of anaesthesia, all responses were reduced in amplitude but, surprisingly, only VSDI responses exhibited prolonged activation resulting in a delayed return to baseline. Further analysis of the optical signal demonstrated that the reduction in response amplitude was constant across the area of activation, resulting in a global down‐scaling of the population response. The manner in which the optical signal relates to the various neuronal generators that produce the SEP signal is also discussed. These data provide information regarding the impact of anaesthetic agents on the brain, and show the value of combining spatial analyses from neuroimaging approaches with more traditional electrophysiological techniques.

Neuroscience: Viable Applications in Education?

As a relatively young science, neuroscience is still finding its feet in potential collaborations with other disciplines. One such discipline is education, with the field of neuroeducation being on the horizon since the 1960s. However, although its achievements are now growing, the partnership has not been as successful as first hopes suggested it should be. Here the authors discuss the theoretical barriers and potential solutions to this, which have been suggested previously, with particular focus on levels of research in neuroscience and their applicability to education. Moreover, they propose that these theoretical barriers are driven and maintained by practical barriers surrounding common language and research literacy. They propose that by overcoming these practical barriers through appropriate training and shared experience, neuroeducation can reach its full potential.

Postnatal maturation of endogenous opioid systems within the periaqueductal grey and spinal dorsal horn of the rat

Summary The increase in expression of endogenous opioidergic targets coincides with functional maturation of pain signalling in the third postnatal week of the rat. ABSTRACT Significant opioid‐dependent changes occur during the fourth postnatal week in supraspinal sites (rostroventral medulla [RVM], periaqueductal grey [PAG]) that are involved in the descending control of spinal excitability via the dorsal horn (DH). Here we report developmentally regulated changes in the opioidergic signalling within the PAG and DH, which further increase our understanding of pain processing during early life. Microinjection of the &mgr;‐opioid receptor (MOR) agonist DAMGO (30 ng) into the PAG of Sprague‐Dawley rats increased spinal excitability and lowered mechanical threshold to noxious stimuli in postnatal day (P)21 rats, but had inhibitory effects in adults and lacked efficacy in P10 pups. A tonic opioidergic tone within the PAG was revealed in adult rats by intra‐PAG microinjection of CTOP (120 ng, MOR antagonist), which lowered mechanical thresholds and increased spinal reflex excitability. Spinal adminstration of DAMGO inhibited spinal excitability in all ages, yet the magnitude of this was greater in younger animals than in adults. The expression of MOR and related peptides were also investigated using TaqMan real‐time polymerase chain reaction and immunohistochemistry. We found that pro‐opiomelanocortin peaked at P21 in the ventral PAG, and MOR increased significantly in the DH as the animals aged. Enkephalin mRNA transcripts preceded the increase in enkephalin immunoreactive fibres in the superficial dorsal horn from P21 onwards. These results illustrate that profound differences in the endogenous opioidergic signalling system occur throughout postnatal development.

Haemodynamic responses to sensory stimulation are enhanced following acute cocaine administration

Cocaine enhances neural activity in response to sensory stimulation, an effect that may play a role in the development of drug craving. However, cocaine-induced sensory enhancement may be difficult to study in humans using neuroimaging if the global increases in baseline haemodynamic parameters, which cocaine produces, interfere with the ability of enhanced sensory-related neural activity to lead to enhanced haemodynamic responses. To investigate the effect of cocaine-induced baseline haemodynamic changes on sensory-related haemodynamic (and electrophysiological) responses, field potential (FP) and haemodynamic responses (obtained using optical imaging spectroscopy and laser-Doppler flowmetry) in the barrel cortex of the anaesthetised rat were measured during mechanical whisker stimulation following cocaine (0.5 mg/kg) or saline administration. During cocaine infusion, the relationship between blood flow and volume transiently decoupled. Following this, cocaine caused large baseline increases in blood flow (133%) and volume (33%), which peaked after approximately 6 min and approached normal levels again after 25 min. During the peak baseline increases, FP responses to whisker stimulation were similar to saline whereas several haemodynamic response parameters were slightly reduced. After the peak, significant increases in FP responses were observed, accompanied by significantly enhanced haemodynamic responses, even though the haemodynamic baselines remained elevated. Hence, the haemodynamic response to sensory stimulation is transiently reduced in the presence of large increases in baseline but, after the baseline peak, enhanced neural responses are faithfully accompanied by enhanced haemodynamic responses. The findings suggest that any cocaine-induced enhancement of sensory-related neural activity in humans is likely to be detectable by neuroimaging.

A dynamic model of neurovascular coupling: Implications for blood vessel dilation and constriction

Neurovascular coupling in response to stimulation of the rat barrel cortex was investigated using concurrent multichannel electrophysiology and laser Doppler flowmetry. The data were used to build a linear dynamic model relating neural activity to blood flow. Local field potential time series were subject to current source density analysis, and the time series of a layer IV sink of the barrel cortex was used as the input to the model. The model output was the time series of the changes in regional cerebral blood flow (CBF). We show that this model can provide excellent fit of the CBF responses for stimulus durations of up to 16 s. The structure of the model consisted of two coupled components representing vascular dilation and constriction. The complex temporal characteristics of the CBF time series were reproduced by the relatively simple balance of these two components. We show that the impulse response obtained under the 16-s duration stimulation condition generalised to provide a good prediction to the data from the shorter duration stimulation conditions. Furthermore, by optimising three out of the total of nine model parameters, the variability in the data can be well accounted for over a wide range of stimulus conditions. By establishing linearity, classic system analysis methods can be used to generate and explore a range of equivalent model structures (e.g., feed-forward or feedback) to guide the experimental investigation of the control of vascular dilation and constriction following stimulation.

Altered neurovascular coupling during information-processing states

Brain imaging techniques rely on changes in blood flow, volume and oxygenation to infer the loci and magnitude of changes in activity. Although progress has been made in understanding the link between stimulus‐evoked neural activity and haemodynamics, the extent to which neurovascular‐coupling relationships remain constant during different states of baseline cortical activity is poorly understood. Optical imaging spectroscopy, laser Doppler flowmetry and electrophysiology were used to measure haemodynamics and neural activity in the barrel cortex of anaesthetized rats. The responses to stimulation of the whisker pad were recorded during quiescence and cortical desynchronization produced by stimulation of the brainstem. Cortical desynchronization was accompanied by increases in baseline blood flow, volume and oxygenation. Haemodynamic responses to low‐frequency whisker stimuli (1 Hz) were attenuated during arousal compared with that observed during quiescence. During arousal it was possible to increase stimulus‐evoked haemodynamics by increasing the frequency of the stimulus. Neural responses to low‐frequency stimuli were also attenuated but to a far lesser extent than the reduction in the accompanying haemodynamics. In contrast, neuronal activity evoked by high‐frequency stimuli (40 Hz) was enhanced during arousal, but induced haemodynamic responses of a similar magnitude compared with that observed for the same high‐frequency stimulus presented during quiescence. These data suggest that there may be differences in stimulus‐evoked neural activity and accompanying haemodynamics during different information‐processing states.

An evaluation of in vivo voltage-sensitive dyes: pharmacological side effects and signal-to-noise ratios after effective removal of brain-pulsation artifacts

In the current study, we investigated pharmacological side effects and signal-to-noise ratios (SNRs) of two commonly used voltage-sensitive dyes (VSDs): the blue dye RH-1691 (1 mg/ml) and the red dye di-4-ANEPPS (0.1 mg/ml), applied in vivo to the rat barrel cortex. Blue dyes are often favored over red dyes in in vivo studies due to their apparent superior SNR, partly because their fluorescence spectrum is farther away from the hemoglobin absorption spectrum, making them less prone to heartbeat-associated brain-pulsation artifacts (BPA). We implemented a previously reported template-based BPA removal algorithm and evaluated its applicability to di-4-ANEPPS before comparing characteristics of the two dyes. Somatosensory-evoked potentials (SEPs) were also recorded. Whereas SEPs recorded before and after application of di-4-ANEPPS failed to exhibit demonstrable differences, RH-1691 caused a significant and prolonged increase in SEP amplitude for several hours. In contrast, neither dye influenced the spontaneous cortical activity as assessed by the spectral content of the EEG. Both dyes turned out to be strikingly similar with respect to changes in fractional fluorescence as a function of SEP response amplitude, as well as regarding shot noise characteristics after removal of the BPA. Thus there is strong evidence that the increased SNR for RH-1691 is a consequence of an artificially increased signal. When applying an appropriate BPA removal algorithm, di-4-ANEPPS has proven to be suitable for single-trial in vivo VSD imaging (VSDI) and produces no detectable neurophysiological changes in the system under investigation. Taken together, our data argue for a careful re-evaluation of pharmacological side effects of RH-1691 and support the applicability of di-4-ANEPPS for stable single-trial in vivo VSDI recordings.

Cocaine preferentially enhances sensory processing in the upper layers of the primary sensory cortex.

Sensory systems are believed to play an important role in drug addiction, particularly in triggering craving and relapse, and it has been shown in previous studies that administration of cocaine can enhance evoked responses in the primary sensory cortex of experimental animals. Primary sensory cortex comprises a multi-layered structure to which a variety of roles have been assigned; an understanding of how cocaine affects evoked activity in these different layers may shed light on how drug-associated sensory cues gain control over behavior. The aim of the present study was to examine how cocaine affects whisker sensory responses in different layers of the primary sensory (barrel) cortex. Field potential and multi-unit activity were recorded from the cortex of anesthetized rats using 16 channel linear probes during repetitive (air puff) stimulation of the whiskers. In control conditions (under saline, i.v.), responses strongly adapted to the repeated sensory stimulation. Following an i.v. injection of cocaine (0.5 mg/kg, i.v.), this adaptation was strongly attenuated, giving each stimulus a more equal representation and weight. Attenuation of adaptation was more marked in the upper cortical layers in both field potential and multi-unit data. Indeed, in these layers, not only was adaptation attenuated but multi-unit response amplitudes under cocaine exceeded those under saline for stimuli occurring early in the train. The results extend our previous findings concerning the enhancement by cocaine of primary sensory responses. Insofar as enhanced neural responses equate to enhanced stimulus salience, the results indicate that cocaine may play a previously under-appreciated role in the formation of associations between drug and drug-related environmental cues by enhancing stimulus salience. The associative process itself may be assisted by a preferential action in the upper cortical layers, thought to be involved in learning and plasticity.

Environmental enrichment differentially modifies specific components of sensory-evoked activity in rat barrel cortex as revealed by simultaneous electrophysiological recordings and optical imaging in vivo.

Environmental enrichment of laboratory animals leads to multi-faceted changes to physiology, health and disease prognosis. An important and under-appreciated factor in enhancing cognition through environmental manipulation may be improved basic sensory function. Previous studies have highlighted changes in cortical sensory map plasticity but have used techniques such as electrophysiology, which suffer from poor spatial resolution, or optical imaging of intrinsic signals, which suffers from low temporal resolution. The current study attempts to overcome these limitations by combining voltage-sensitive dye imaging with somatosensory-evoked potential (SEP) recordings: the specific aim was to investigate sensory function in barrel cortex using multi-frequency whisker stimulation under urethane anaesthesia. Three groups of rats were used that each experienced a different level of behavioural or environmental enrichment. We found that enrichment increased all SEP response components subsequent to the initial thalamocortical input, but only when evoked by single stimuli; the thalamocortical component remained unchanged across all animal groups. The optical signal exhibited no changes in amplitude or latency between groups, resembling the thalamocortical component of the SEP response. Permanent and extensive changes to housing conditions conferred no further enhancement to sensory function above that produced by the milder enrichment of regular handling and behavioural testing, a finding with implications for improvements in animal welfare through practical changes to animal husbandry.