Anecdotal evidence suggests that alkaloids contained within ayahuasca brew, a traditional South American plant-based tea made from the Banisteriopsis caapi vine and the Psychotria viridis shrub, when used in combination with reconsolidation therapy (RT), may be a novel treatment for psychiatric disorders such as post-traumatic stress disorder (PTSD). Although the mechanism of action underlying this effect remains unclear, we hypothesised that it may be due to effects on neuronal calcium homeostasis. We therefore sought to determine if the ayahuasca-derived harmala alkaloids, harmine and harmaline, modulated depolarisation-induced calcium entry in differentiated neuroblastoma-derived SH-SY5Y cells.

SH-SY5Y cells, initially plated at a density of 1×10⁶ cells on glass coverslips for 24 hours, were treated with retinoic acid (10μM) to induce differentiation into a neuronal phenotype. Cells were used for experimentation after the cells had been incubated for further 5-10 days at 37°C, in the presence or absence of either harmine (1μM) or harmaline (1μM). Prior to experimentation, cells were loaded with the non-ratiometric calcium-sensitive dye, Calbryte 520 AM (4μM), for one hour. Coverslips containing the cells were then placed under a microscope and superfused (2ml/min) with normal HEPES-buffered saline solution (HBSS) for 10 minutes before depolarisation was induced by superfusing on a 50mM K⁺-containing HBSS solution for 5 mins, followed by a 15 minute washout period. Calcium responses (F/F₀) in individual cells were recorded and measured using the calcium imaging software, Winfluor (John Dempster, Strathclyde University). Both the percentage peak depolarisation-induced calcium response (relative to baseline), and area under the curve (AUC; calculated using Graphpad Prism) for 300 seconds after calcium influx was initiated in untreated (control) neurons, were compared to calcium responses generated in the presence of the harmala alkaloids. Statistical analyses were conducted using one way ANOVA with Dunnett’s multiple comparisons test with values of P < 0.05 considered statistically significant. All data are expressed as mean ± S.D.

Our results demonstrated that 50mM K⁺-HBSS increased intracellular calcium above basal levels by 26.6 ± 15.8%, with an average AUC of 6303.8 ± 4230.6 (arbitrary units (a.u.); n=36). Harmine (1mM) significantly reduced both the peak depolarisation-induced calcium response by 31.4% (18.3 ± 8.0% above basal calcium; n = 30; P < 0.05) and the corresponding AUC by 39.0% (3846.6 ± 1567.0 a.u.; n = 30; P < 0.01) relative to responses in untreated cells. Similarly, harmaline (1mM) also significantly reduced both the peak depolarisation-induced calcium response and AUC to an even greater extent than harmine, by 54.4% (12.1 ± 10.3% above basal calcium; n = 29; P < 0.0001) and 63.7% (2289.3 ± 2218.9; n = 29; P < 0.0001), respectively, relative to responses in untreated cells.

Our hypothesis that one possible means by which ayahuasca-derived compounds may alter neuronal function by modulating neuronal calcium homeostasis is supported by our finding that chronic treatment with the ayahuasca-derived harmala alkaloids, harmine and harmaline, significantly reduces depolarisation-induced calcium entry into neuron-like SH-SY5Y cells.