Here we show that cocaine interrupted CBF in some arteriolar branches for over 45min and this effect was exacerbated with repeated cocaine administration. In addition we show that cocaine produced marked decreases in CBF (e.g., ~70%) shortly after acute cocaine administration (2–3min) and that the magnitude and recovery differed between vessels, showing faster recovery in arterioles (~5min) than in venules (~12min) and revealing marked variability and pulsatility in capillaries (recovery varied from 4–20min). These findings provide evidence that acute cocaine elicits cerebral microischemic dysfunction that seems to get exacerbated with repeated cocaine administration. It also uncovers significant heterogeneity in the cerebrovascular responses to cocaine, highlighting the importance of separately assessing vessels of different calibers. Our findings were possible due to the enhanced capabilities of μOCA/μODT, which demonstrates its value as a novel and more sensitive tool for investigating neurovascular toxicity by drugs or other insults.
Our cocaine findings are relevant since stroke is one of the most serious clinical complications associated with cocaine abuse. Indeed cocaine is a main risk factor for stroke among young abusers.19
Though it was hypothesized that cocaine-induced cerebral microischemia was involved in some of the clinical complications seen in cocaine abusers, there was no data to support this. Here we show evidence of long-lasting CBF interruptions in cerebral microvessels (>45min) that are exacerbated with repeated cocaine administrations. Specifically, 3 sequential cocaine doses induced greater changes than those induced after a single dose, which is clinically relevant since cocaine when abused is repeatedly administered in binges and rarely used as a single administration.20
Thus a sensitized response of cerebral microvessels to repeated cocaine administration could contribute to cocaine’s neurotoxic effects. More specifically, the long-lasting interruption in flow observed in some of the vessels, if it is exacerbated with repeated cocaine use could result in microischemic dysfunction and if prolonged could lead to neuronal death and loss of function. We had previously used Doppler OCT to show decreases in CBF after acute cocaine,18, 21, 22
but the limited resolution and sensitivity did not allow us to measure the effects of cocaine on capillary beds. In the current study the enhanced capabilities of μOCA/μODT allowed us to document cocaine-induced microischemic events in capillaries and to show marked differences in the responses to cocaine between arterioles, venules and capillaries in the cerebrovascular networks (–). Of these the capillaries showed the greatest variability and pulsatility upon intravenous cocaine administration, and the terminal arterioles (~77%) seemed more vulnerable to cocaine-elicited microischemia than terminal venules (~23%).
The mechanisms underlying cocaine vasoactive effects are likely to reflect in part its dopaminergic effects. Indeed there is evidence of dopamine terminals in close contact with arterioles and capillaries in cortical tissue that when stimulated results in vasoconstriction.23
Studies on isolated cerebral arterioles have shown that application of cocaine or its metabolites induced vasoconstriction corroborating a direct effect of cocaine on blood vessels as opposed to indirect effects secondary to neuronal actions24
. Moreover, vasoconstriction from cocaine was prevented by haloperidol, which suggests the involvement of dopamine (D2) receptors in cocaine induced vasoconstriction24
. There is also evidence of dopamine transporter expression (target of cocaine’s effects) in cerebral blood vessels in the brain25
. However, it is also possible that the local anesthetic effects of cocaine may contribute to its vasoactive actions26
Our findings also demonstrate the enhanced capabilities of our μOCA/μODT tool for simultaneously rendering angiographic (μOCA) and quantitative CBF (μODT) images of 3D cerebrovascular networks with capillary details comparable to those by MPM. Specifically, we incorporate ultrahigh-resolution OCT for improving spatial resolution (~3μm) and PIM (based on FFT analysis in lateral direction14, 15, 18
) for optimizing phase detection sensitivity (≤10μm/s, Supplemental Tab. S1
), and show that the new μOCA/μODT platform offers several unique capabilities that are highly relevant to brain functional studies, yet lacking in current imaging modalities (e.g., MPM, OCA). This technique, based on intrinsic Doppler effect (i.e., tracker free), enables time-lapse imaging of the dynamic responses to brain functional activation () and disease progression12
. It extends the image depth of MPM (~300μm) to 700μm~1mm and the vastly increased FOV (e.g., 2×2×1mm3
) is crucial for mapping cerebral microvascular network effects. Noteworthily, μODT is uniquely capable of CBF quantization in both capillaries and branch vessels (–), which provides more sensitive physiological changes (e.g., microischemia) in the local microvascular networks than μOCA (–). Additionally, it allowed us to separate venous and arterial vasculatures and thus to study their respective physiological responses to various functional and pharmacological interventions (–).
A limitation in our study was that the mice had to be anesthetized (as is the case for most rodent imaging studies), which raises concerns of potential interactions between cocaine and the anesthetic agent. However, we specifically chose isoflurane since in a prior study addressing the influence of anesthetic drugs on cocaine’s effects we showed that the findings from the isoflurane-anesthetized rodents agreed with those reported in human subjects27
and more recently with those reported in awake macaques.28
Moreover, isoflurane did not uncouple cocaine’s effects on CBF from those in oxygen metabolism, which suggests that at the doses used to anesthetize the mice, isoflurane did not disrupt the autoregulation of CBF. Also to control for potential confounds secondary to cocaine-induced peripheral vascular effects28
we monitored the mean arterial blood pressure (MABP) throughout the experiments. Although MABP decreased in response to cocaine (Supplementary Fig. S10
), this effect was modest (MABP>70mmHg) and short lasting (<5min), suggesting that neither the immediate () nor the long-lasting () decreases in CBF following cocaine administration were due to cocaine’s peripheral effects. In addition, the measured apparent CBF comprises artifacts (e.g., underestimation) due to Doppler angle effect, especially when the angle θ→90°. The error can be accurately corrected; however, angle correction of the entire CBF network is challenging because of high correction errors for flows with θ→90° and limited sensitivity for capillary beds (see Supplementary Fig. S8
and Tab. S2
for details). It should also be noted that due to limited temporal resolution of 3D μODT (e.g., 1min), the imaged CBF change in response to cocaine (e.g., ) was confounded with the inherent CBF fluctuation over time (e.g., basal ΔCBF(t: t<0)
variations in ). Although this change in larger vessel (e.g., >ϕ50μm) was negligible (~8%) compared to the changes induced by cocaine (e.g., 50%), the influence was more obvious in smaller vessels and capillaries (see Supplementary Fig. S7
In summary, we provide evidence that cocaine induced cerebral microischemic changes that in some vessels were long lasting (>45min) and were exacerbated with repeated administration. This could underlie some of the neurologic deficits reported in cocaine abusers ranging from mild and transient facial paralysis, to severe and irreversible tetraplegia.29
We also show evidence of the enhanced capabilities of μOCA/μODT for studying the dynamic responses of cerebral microvessels to drugs and other insults.