Key Aspects of Neurovascular Control Mediated by Specific Populations of Inhibitory Cortical Interneurons

Inhibitory interneurons can evoke vasodilation and vasoconstriction, making them potential cellular drivers of neurovascular coupling. However, the specific regulatory roles played by particular interneuron subpopulations remain unclear. Our purpose was therefore to adopt a cell-specific optogenetic approach to investigate how somatostatin (SST) and neuronal nitric oxide synthase (nNOS)-expressing interneurons might influence the neurovascular relationship. In mice, specific activation of SST- or nNOS-interneurons was sufficient to evoke hemodynamic changes. In the case of nNOS-interneurons, robust hemodynamic changes occurred with minimal changes in neural activity, suggesting that the ability of blood oxygen level dependent functional magnetic resonance imaging (BOLD fMRI) to reliably reflect changes in neuronal activity may be dependent on type of neuron recruited. Conversely, activation of SST-interneurons produced robust changes in evoked neural activity with shallow cortical excitation and pronounced deep layer cortical inhibition. Prolonged activation of SST-interneurons often resulted in an increase in blood volume in the centrally activated area with an accompanying decrease in blood volume in the surrounding brain regions, analogous to the negative BOLD signal. These results demonstrate the role of specific populations of cortical interneurons in the active control of neurovascular function.

A novel method for classifying cortical state to identify the accompanying changes in cerebral hemodynamics

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We classified brain state using a vector-based categorisation of neural frequencies.

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Changes in cerebral blood volume (CBV) were observed when brain state altered.

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During these state alterations, changes in blood oxygenation were also found.

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State dependent haemodynamic changes could affect blood based brain imaging.

Background

Many brain imaging techniques interpret the haemodynamic response as an indirect indicator of underlying neural activity. However, a challenge when interpreting this blood based signal is how changes in brain state may affect both baseline and stimulus evoked haemodynamics.

New method

We developed an Automatic Brain State Classifier (ABSC), validated on data from anaesthetised rodents. It uses vectorised information obtained from the windowed spectral frequency power of the Local Field Potential. Current state is then classified by comparing this vectorised information against that calculated from state specific training datasets.

Results

The ABSC identified two user defined brain states (synchronised and desynchronised), with high accuracy (∼90%). Baseline haemodynamics were found to be significantly different in the two identified states. During state defined periods of elevated baseline haemodynamics we found significant decreases in evoked haemodynamic responses to somatosensory stimuli.

Comparison to existing methods

State classification – The ABSC (∼90%) demonstrated greater accuracy than clustering (∼66%) or ‘power threshold’ (∼64%) methods of comparison.

Haemodynamic averaging – Our novel approach of selectively averaging stimulus evoked haemodynamic trials by brain state yields higher quality data than creating a single average from all trials.

Conclusions

The ABSC can account for some of the commonly observed trial-to-trial variability in haemodynamic responses which arises from changes in cortical state. This variability might otherwise be incorrectly attributed to alternative interpretations. A greater understanding of the effects of cortical state on haemodynamic changes could be used to inform techniques such as general linear modelling (GLM), commonly used in fMRI.

Administration of a gonadotropin releasing hormone analogue in oligozoospermic infertile males

This study investigated the stimulatory potential of a superactive gonadotropin releasing hormone analogue in idiopathic oligozoospermia. In a double-blind trial, 19 men were randomized to receive buserelin (D-Ser(TBU)6-GnRH ethylamide) in one of two dosages or saline, twice weekly, for twelve weeks. Treatment did not lead to a significant increase in serum concentrations of the pituitary gonadotropins or in sperm concentrations. However, in the dosages and schedules investigated, there was no inhibitory effect of the analogue. Further assessment of this approach is suggested.

Assay of endotoxin by the hypothermic response of mice

Escherichia coli endotoxin injected into CMRL mice in a 23° environment produced a fall in rectal temperature at 3 hr post-injection. Endotoxin given i.v. produced a greater hypothermia than if injected i.p. Pretreatment of the mice with pertussis vaccine, BCG or Corynebacterium parvum substantially increased their responsiveness to endotoxin. SJL mice were more sensitive to endotoxin than CMRL mice and showed hypothermias as great as -4·9°. Endotoxins from Salmonella typhi and Bordetella pertussis had a similar activity to Esch. coli endotoxin, but Shigella flexneri was less active. There was no evidence to support the idea that endotoxin responsiveness depends on the mice having previous environmental exposure to the corresponding bacterium e.g. in the gut flora. The relative endotoxic activities of 1 batch each of pertussis, TAB and cholera vaccines were assayed by the hypothermias produced in SJL mice. Cholera vaccine was twice as potent as TAB which was 4 times more active than pertussis. The index of precision of the mouse hypothermia test (0·32-0·65) is not inferior to that of the standard rabbit pyrogenicity test (about 0·6). The test should be useful for comparing the endotoxic activities of preparations such as bacterial vaccines which may be rich in endotoxin.