Cholesterol as a risk factor for dementia and cognitive decline: a systematic review of prospective studies with meta-analysis

The relationships between total serum cholesterol (TC) and dementia and between TC and cognitive decline were investigated in a systematic review of 18 prospective studies. Follow-ups ranged from 3 to 29 years, and included a total of 14,331 participants evaluated for Alzheimer disease (AD), 9,458 participants evaluated for Vascular dementia (VaD), 1,893 participants evaluated for cognitive decline, and 4,793 participants evaluated for cognitive impairment. Compatible results were pooled using meta-analysis. Consistent associations between high midlife TC and increased risk of AD, and high midlife TC and increased risk of any dementia were found. There was no evidence supporting an association between late-life TC and AD, or between late-life TC and any dementia. No study reported a significant association between TC (measured in midlife or late-life) and VaD. An association between high midlife TC and cognitive impairment was found but there was only weak evidence for an association between TC and cognitive decline. Two of seven studies reporting data on the interaction between TC and apolipoprotein e4-allele had significant effects. Results suggest the effect of TC on dementia risk occurs in midlife but not late-life, and that there may be different cardiovascular risk factor profiles for AD and VaD. Results from additional studies involving long-term follow-up of midlife samples will allow for clarification of the association between age, TC and risk of specific types of dementia. These data are required to inform recommendations of modulation of cholesterol to reduce or delay dementia risk.

Thinking on your back: Solving anagrams faster when supine than when standing

There is potentially less locus coeruleus-noradrenergic system activity when lying down than when standing, an effect expected to develop via a difference in baroreceptor load. Furthermore, there is evidence implying that locus coeruleus-noradrenergic system activity impairs attempts to solve anagrams. Consistent with these ideas, we found that subjects solved anagrams significantly faster when supine than when standing. With anagrams characterized as insight problems, our finding suggests that insight may be influenced by body posture.

Convergence of parvocellular and magnocellular information channels in the primary visual cortex of the macaque

We investigated whether responses of single cells in the striate cortex of anaesthetized macaque monkeys exhibit signatures of both parvocellular (P) and magnocellular (M) inputs from the dorsal lateral geniculate nucleus (dLGN). We used a palette of 128 isoluminant hues at four different saturation levels to test responses to chromatic stimuli against a white background. Spectral selectivity with these isoluminant stimuli was taken as an indication of P inputs. The presence of magnocellular inputs to a given cortical cell was deduced from its responses to a battery of tests, including assessment of achromatic contrast sensitivity, relative strengths of chromatic and luminance borders in driving the cell at different velocities and conduction velocity of their retino-geniculo-cortical afferents. At least a quarter of the cells in our cortical sample appear to receive convergent P and M inputs. We cannot however, exclude the possibility that some of these cells could be receiving a convergent input from the third parallel channel from the dLGN, namely the koniocellular (K) rather than the P channel. The neurons with convergent P and M inputs were recorded not only from supragranular and infragranular layers but also from the principal geniculate input recipient layer 4. Thus, our results challenge classical ideas of strict parallelism between different information streams at the level of the primate striate cortex.

Repeated local administration of noradrenaline or saline inhibits thermal hyperalgesia in pain-sensitized human skin

AIMS

Noradrenaline increases thermal hyperalgesia in skin sensitized to heat by the topical application of capsaicin. The aim of this study was to determine whether desensitization to the hyperalgesic effects of noradrenaline would develop after repeated local administrations of noradrenaline in the skin of the forearm.

METHODS

Noradrenaline and saline were administered to the forearm by iontophoresis (200 microA, 2 min, over a surface area of 3.1 cm(2)) two times per day for 4-10 days in 19 healthy subjects. The adequacy of the desensitization procedure was evaluated by measuring noradrenaline-induced vasoconstriction with laser Doppler fluxmetry. Thresholds and pain ratings to heat were then investigated at treated and control sites before and after the topical application of capsaicin, and after the iontophoresis of noradrenaline.

RESULTS

At previously untreated sites blood flow was 49 +/- 14% (+/- 95% confidence intervals) lower than flow at reference sites after the iontophoresis of noradrenaline. Vascular signs of adrenergic desensitization developed after 4-5 days of repeated local administration of noradrenaline in the majority of subjects. In those whose vessels constricted after the acute administration of noradrenaline, the adrenergic response averaged 23 +/- 15% at the desensitized site compared with 61 +/- 9% at previously untreated sites (P < 0.001). However, similar signs developed after repeated iontophoreses of saline (adrenergic response 7 +/- 16% compared with 58 +/- 15% at previously untreated sites, P < 0.001). Both the noradrenaline and saline treatments inhibited thermal hyperalgesia after the topical application of capsaicin. Heat-pain thresholds averaged 43.2 +/- 2.5 degrees C and 43.0 +/- 2.3 degrees C at the noradrenaline and saline pretreated sites compared with 41.4 +/- 2.7 degrees C at the control site (P < 0.05 and P < 0.06, respectively). On a 0-10 scale, heat-pain ratings to a 7 s, 45 degrees C stimulus averaged 3.8 +/- 1.6 and 3.5 +/- 1.7 at the noradrenaline and saline pretreated sites compared with 5.3 +/- 1.6 at the control site (P < 0.05). After the iontophoresis of noradrenaline heat-pain ratings increased 1.6 +/- 1.4 at the site pretreated with saline (P < 0.05) compared with only 0.4 +/- 1.0 at the site pretreated with noradrenaline (not significant), consistent with local adrenergic desensitization.

CONCLUSIONS

We conclude that repeated iontophoreses of noradrenaline or saline inhibit vasoconstriction to noradrenaline, and also inhibit increases in thermal hyperalgesia evoked by capsaicin. The release of endogenous stores of noradrenaline by iontophoretic currents might contribute to these effects.

Vascular and nociceptive effects of localized prolonged sympathetic blockade in human skin

Supersensitivity to noradrenaline contributes to certain vascular disorders (e.g., hypertension) and chronic neuropathic pain conditions (e.g., complex regional pain syndrome). We aimed to develop a procedure for inducing adrenergic supersensitivity that could be used to investigate the role of catecholamines in these clinical conditions. In the first study, three doses of guanethidine were administered by iontophoresis to separate small patches of skin in the forearm of healthy human volunteers. Four to five hours later. the vasoconstrictor response to the adrenergic releasing agent tyramine was inhibited in a dose-dependent manner by iontophoretic pretreatment with guanethidine, indicating that guanethidine had depleted endogenous adrenergic stores. In a second study, guanethidine and saline were administered by iontophoresis four times over approximately 2 weeks at separate sites in the forearm. One to two days after the final pretreatment, vasoconstriction to the iontophoresis of a weak dose of noradrenaline was enhanced at sites pretreated with guanethidine. To investigate the effect of guanethidine pretreatment on thermal hyperalgesia. the experimental sites were sensitized to heat by the topical application of 0.6% capsaicin. Both before and after the application of capsaicin, the heat-pain threshold and heat-pain ratings to suprathreshold stimulation were similar at sites pretreated for 2 weeks with guanethidine or saline. However, after the iontophoresis of noradrenaline, thermal hyperalgesia was greater at the guanethidine-pretreated site than the saline pretreated site. These observations indicate that prolonged depletion of adrenergic stores by guanethidine induces adrenergic supersensitivity in cutaneous vessels, and that adrenergic supersensitivity enhances thermal hyperalgesia in the presence of noradrenaline.

Facilitating laser Doppler measurements of cutaneous adrenergic vasoconstriction: a comparison of methods

Detecting vasoconstriction in the skin of the human forearm is often difficult because cutaneous perfusion is usually low, even in the absence of vasoconstrictive agents. However, flow through "vasoconstricted" skin can be detected by increasing the background skin blood flow. The aim of the current study was to evaluate local warming and postocdusive reactive hyperemia as techniques for increasing background flow to facilitate laser Doppler measurements of cutaneous vasoconstriction in the forearm. Noradrenaline introduced by iontophoresis into the forearm of healthy volunteers initially reduced the peak hyperemic response (mean decrease in hyperemia 21 +/- 7%, p <0.01), compared with a saline control. However, vasoconstriction was unrelated to the dose of noradrenaline and was not detectable when the test was repeated, possibly because the peak of the normal hyperemic response decreased with repeated occlusions. When the forearm was warmed to 42 degrees C, noradrenaline introduced by iontophoresis produced dose-dependent vasoconstriction relative to saline control sites and skin not having undergone iontophoresis (greatest mean decrease 64 +/- 7%, p <0.001). Therefore, local warming is recommended over reactive hyperemia for facilitating laser Doppler recordings of cutaneous vasoconstriction in the human forearm.

Noradrenaline provokes axon reflex hyperaemia in the skin of the human forearm

High doses of noradrenaline increase the sensitivity of skin to heat, presumably by sensitizing nociceptive afferent fibres. Since activation of these fibres increases local blood flow by an axon reflex mechanism, noradrenaline might simultaneously evoke adrenoceptor-mediated vasoconstriction and axon reflex vasodilatation. To investigate this possibility, noradrenaline was introduced into the skin of the human forearm by iontophoresis, and changes in blood flow were monitored in nearby skin. Increases in blood flow were greater near the site of noradrenaline iontophoresis than near the site of saline iontophoresis. Since the response was limited in its distribution to a few centimeters from the site of iontophoresis, hyperaemia was most likely due to a local mechanism rather than a spinal or supraspinal reflex. In addition, pretreatment of the skin with a local anaesthetic cream inhibited the increase in flow, indicating that the response was mediated neurally. These findings suggest that activation of nociceptive afferents by noradrenaline provokes axon reflex hyperaemia.