A plethora of neurological and psychiatric diseases are associated with dysregulation or insufficient maintenance of monoamine neurotransmitter (dopamine [DA], serotonin [5-HT], norepinephrine [NE]) homeostasis1,2,3. These conditions include, but are not limited to, depression1,2, schizophrenia2, anxiety2, addiction4, menopause5,6,7, pain8, and Parkinson's disease3. For instance, several rat models of menopause have shown that the dysregulation or reduction of monoamines within the hippocampus, prefrontal cortex, and striatum may be associated with both depression and cognitive decline, which is seen in women experiencing menopause. The dysregulation of monoamines in these models have been extensively examined using HPLC-ECD, although the studies did not discriminate between measured neurotransmitter content versus neurotransmitter release5,6,7. Monoamines are classically released into the extracellular space through Ca2+-dependent vesicular release9, and are recycled back through their respective plasma membrane re-uptake system (dopamine transporter, DAT; serotonin transporter, SERT; norepinephrine transporter, NET)10,11. Conversely, data suggests that these transporters are able to release or efflux monoamines, since drugs of abuse such as amphetamine (AMPH) and 3,4-Methylenedioxymethamphetamine (MDMA) are known to release DA and 5-HT, respectively through their transporter systems12,13,14,15,16,17. Thus, a proper mechanistic understanding of monoamine release dynamics is crucial for developing specific and targeted pharmacotherapies.
A wide range of techniques have been employed to study monoamine release such as Fast Scan Cyclic Voltammetry (FSCV)18, in vivo microdialysis13, imaging19, preincubation with radiolabeled monoamines20, optogenetics, and more recently, genetically encoded fluorescent sensors and photometry21,22. FSCV and in vivo microdialysis are the primary techniques used for studying monoamine release. FSCV is used to study the stimulated exocytotic release of, primarily, DA in acute brain slices and in vivo23. Because FSCV uses electrodes to stimulate or evoke release, the primary source of neurotransmitter release is Ca2+-dependent vesicular release18,24,25,26,27,28,29,30,31. In vivo microdialysis coupled with HPLC measures changes in extracellular neurotransmitter levels using a probe placed in a brain area of interest13,32. Similar to FSCV, a major limitation to in vivo microdialysis is the difficulty in determining the source of neurotransmitter release: Ca2+ dependent vesicular release or transporter dependent. Noteworthy, both methods allow for the direct measurement of monoamine release. Through the recent advancement of optogenetics, research demonstrates detection of 5-HT and DA release in a short timespan with exquisite cell-type specificity21,22. However, these strategies require complex and costly techniques and equipment, and indirectly measure monoamine release, specifically through monoamine binding to receptors. Further, radiolabeled monoamines are also used for studying monoamine dynamics. Radiolabeled monoamines may be preloaded into various model systems such as heterologous cells overexpressing each monoamine transporter20,33,34,35,36,37,38,39,40, primary neurons20, synaptosomes33,39,41,42, and acute brain slices43,44. However, radioactivity poses potential harm to the experimenter, and the tritium-labeled analytes may not faithfully recapitulate endogenous monoamine dynamics45,46. Superfusion systems combined with off-line detection methods such as HPLC-ECD have allowed for the detection of monoamines from multiple tissue sources. Here, this protocol provides as an optimized and low-cost, simple, and precise method using acute brain slices to directly measure endogenous basal and stimulated monoamine release.
Acute brain slices allow for testing mechanistic hypotheses, primarily as they preserve the in vivo anatomical microenvironment, and maintain intact synapses47,48,49,50,51,52. In a few studies, acute brain slices or chopped brain tissue have been used in conjunction with a superfusion technique using KCl to stimulate Ca2+ mediated release53,54,55,56. Superfusion systems have been critical to advance the field's understanding of neurotransmitter release mechanisms, including monoamines. However, these systems are relatively expensive, and the number of chambers available for tissue analysis ranges from 4-12. In comparison, the method presented here is inexpensive, allows the measurement of 48 tissue samples, and may be refined to use up to 96 tissue samples. Each well within the 48-well plate contains tissue holders that use filters to separate the released product from the tissue, and released monoamines are then collected and analyzed by HPLC-ECD. Importantly, this method allows for the simultaneous measurement of 5-HT, DA, and NE release from different brain areas such as the prefrontal cortex, the hippocampus, and the dorsal striatum after treatment with pharmacological agents that modulate monoamine release. Thus, the experimenter can answer multiple questions using an inexpensive multi-well system that increases the number of samples tested and thereby reducing the number of animals used.