International Journal of Neuropsychopharmacology (2022) 25(1): 54–63 https://doi.org/10.1093/ijnp/pyab063 Advance Access Publication: 19 September 2021 Regular Research Article Regular Research Article The Kappa Opioid Receptor and the Sleep of Reason: Cortico-Subcortical Imbalance Following Salvinorin-A Genís Ona, Frederic Sampedro, Sergio Romero, Marta Valle, Valle Camacho, Carolina Migliorelli, Miguel Ángel Mañanas, Rosa Maria Antonijoan, Montserrat Puntes, Jimena Coimbra, Maria Rosa Ballester, Maite Garrido, Jordi Riba Human Neuropsychopharmacology Group, Sant Pau Institute of Biomedical Research (IIB-Sant Pau), Barcelona, Spain (Mr Ona); Sant Pau Institute of Biomedical Research (IIB-Sant Pau), Barcelona, Spain (Dr Sampedro); Centro de Investigación en Red-Enfermedades Neurodegenerativas (CIBERNED), Barcelona, Spain (Dr Sampedro); Department of Automatic Control (ESAII), Biomedical Engineering Research Center (CREB), Universitat Politècnica de Catalunya (UPC), Barcelona, Spain (Drs Romero, Migliorelli, and Mañanas); Biomedical Research Networking Center in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Barcelona, Spain (Drs Romero, Migliorelli, and Mañanas); Departament de Farmacologia i Terapèutica, Universitat Autònoma de Barcelona, Barcelona, Spain (Drs Valle and Antonijoan); Nuclear Medicine Department, Hospital de la Santa Creu i Sant Pau, Barcelona, Spain (Dr Camacho); Centre d’Investigació de Medicaments, Sant Pau Institute of Biomedical Research (IIB-Sant Pau), Barcelona, Spain (Dr Antonijoan, Dr Puntes, Dr Coimbra, Dr Ballester, and Ms Garrido); Servei de Farmacologia Clínica, Hospital de la Santa Creu i Sant Pau, Barcelona, Spain (Dr Antonijoan); Centro de Investigación Biomédica en Red de Salud Mental, CIBERSAM, Spain (Drs Antonijoan and Ballester); Blanquerna School of Health Science, Universitat Ramon Llull, Barcelona, Spain (Dr Ballester); Department of Neuropsychology and Psychopharmacology, Maastricht University, Maastricht, the Netherlands (Dr Riba). Correspondence: Frederic Sampedro, PhD, Hospital de Sant Pau Research Institute, Sant Quintí Street number 77, 08041 Barcelona, Spain (fsampedros@ santpau.cat). Abstract Background: The mechanisms through which kappa opioid receptor (KOR) agonists induce psychotomimetic effects are largely unknown, although the modulation of this receptor has attracted attention for its clinical use. In this work, we characterize the neuropharmacological effects of salvinorin-A, a highly selective KOR agonist. Methods: Changes in multimodal electroencephalography, single-photon emission computed tomography, and subjective effects following the acute administration of salvinorin-A are reported. The study included 2 sub-studies that employed a double-blind, crossover, randomized, placebo-controlled design. Results: The electroencephalography measures showed a marked increase in delta and gamma waves and a decrease in alpha waves while subjects were under the effect of salvinorin-A. Regarding single-photon emission computed tomography Received: March 27, 2021; Revised: August 25, 2021; Accepted: September 15, 2021 © The Author(s) 2021. Published by Oxford University Press on behalf of CINP. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and 54 reproduction in any medium, provided the original work is properly cited. The Kappa Opioid Receptor and the Sleep of Reason | 55 Significance Statement Kappa opioid receptors (KOR) remain largely unexplored for clinical use. The mechanisms underlying the effects of KOR agonists, including the associated psychotomimetic effects, remain elusive. In this study, we characterize the neuropharmacology of KOR for the first time, to our knowledge, using a combination of techniques, finding that the hallucinatory state induced by KOR full agonism is associated with a dramatic decrease in regional cerebral blood flow in the cortical areas of the brain. measures, significant decreases in regional cerebral blood flow were detected in multiple regions of the frontal, temporal, parietal, and occipital cortices. Significant regional cerebral blood flow increases were observed in some regions of the medial temporal lobe, including the amygdala, the hippocampal gyrus, and the cerebellum. The pattern of subjective effects induced by salvinorin-A was similar to those observed in relation to other psychotomimetic drugs but with an evidently dissociative nature. No dysphoric effects were reported. Conclusion: The salvinorin-A–mediated KOR agonism induced dramatic psychotomimetic effects along with a generalized decrease in cerebral blood flow and electric activity within the cerebral cortex. Keywords: EEG, hallucinogens, Kappa opioid receptor, opioids, salvinorin-A, SPECT Introduction Opioid receptors belong to the large family of G protein-coupled 2019), although strategies have been developed in an effort to receptors, and they are expressed by central and peripheral avoid these undesired effects (e.g., the use of peripherally re- neurons (Stein, 2016). They were named for their interactions stricted compounds). The mechanism through which selective with morphine, codeine, and other active principles found in KOR agonists induce dysphoria may involve the recruitment of opium, a medicinal product made with the latex of the plant β-arrestin and the activation of p38 mitogen-activated protein Papaver somniferum. Despite opium having been used to treat kinase. However, the mechanism through which psychotomim- pain and other symptoms for thousands of years, it was not etic effects are induced remains unknown. until the 1970s that a group of membrane proteins with an af- In human studies involving KOR agonists, hallucinations of finity to morphine was discovered. Three subtypes were iden- monsters, disturbances of space and time, racing thoughts, and tified through binding studies and bioassays: µ (MOR), δ (DOR), feelings of body distortion, among other effects, have been re- and k (KOR) receptors (Brownstein, 1993). ported (Kumor et al., 1986; Pfeiffer et al., 1986). Recent clinical Except in the case of endomorphins, most endogenous opi- trials conducted by our group (Maqueda et al., 2015, 2016), in oids are not selective for one of the opioid receptors. This is be- which salvinorin-A, a potent hallucinogenic non-nitrogenous di- cause opioid receptors share structural similarities, and they terpene that shows high selectivity for KOR (Roth et al., 2002) was tend to form homomeric and heteromeric complexes with other used, have shown similar results. In these more recent studies, opioid and non-opioid receptors. In this way, the effect of an potent hallucinogenic effects were reported, consisting of opioid ligand is commonly the result of an interaction between changes in the perception of time, depersonalization, derealiza- different receptor complexes. In contrast, synthetic opioid pep- tion, incapacity to interact with surroundings, and out-of-body tides and alkaloids show selectivity for specific opioid receptors, experiences, among other effects. Although such experiences and they have been used to characterize the pharmacological are all commonly classified as “hallucinogenic,” some study par- properties of opioid receptors (Feng et al., 2012; Valentino and ticipants specified their uniqueness compared with effects in- Volkow, 2018). Although all three opioid receptors modulate pain duced by other hallucinogenic drugs (Maqueda et al., 2016). This and analgesia, MOR has been studied most extensively. MOR may suggest a unique pattern of hallucinogenic effects elicited agonists not only have potent analgesic effects, but they also by the KOR agonism (Maqueda et al., 2015) in contrast with the induce euphoria, respiratory depression, and physical and psy- effects of classical serotonergic psychedelics, which are medi- chological dependence (Al-Hasani and Bruchas, 2011). To have ated by 5-HT2A receptors (Vollenweider et al., 1998; Kyzar et al., potent analgesics without these side effects, several ligands 2017; Holze et al., 2020). The study of salvinorin-A effects using with varying potencies, efficacies, and selectivity profiles for neuroimaging techniques has been highly challenging given the other opioid receptors have been designed. DOR agonists have fast onset and short duration of effects. An electroencephalog- anxiolytic, neuroprotective, and analgesic effects, but their ac- raphy (EEG) study involving salvinorin-A reported a reduction in tivation may lead to convulsions, whereas KOR agonists have beta waves (Ranganathan et al., 2012). However, the participants analgesic, antidepressant, and neuroprotective effects, but dys- in that study, who had previously used salvinorin-A, reported phoric and psychotomimetic effects are also common (Stein, that the effect was equivalent to 20%–30% of the effect obtained 2016). in recreational contexts, suggesting the possibility that the re- In recent years, interest in KOR agonists for the treatment searchers used a less efficient vaporization method, leading of pain has increased considerably (Snyder et al., 2018) given to lower absorption. Another salvinorin-A neuroimaging study the side effects associated with MOR agonists, such as eu- used functional magnetic resonance imaging (fMRI) to ana- phoria and addiction. KOR ligands have also shown promising lyze functional connectivity (FC) changes (Doss et al., 2020). It results for the treatment of depression and drug dependence was found that the peak effects were associated with within- (Butelman et al., 2012; Taylor and Manzella, 2016). However, as network decreases and between-network increases in static FC. noted, the selective activation of KOR is associated with dys- The attenuation of the default mode network (DMN) FC was one phoria and psychotomimetic effects (Clark and Abi-Dargham, of the most notable effects. 56 | International Journal of Neuropsychopharmacology, 2022 The aim of this study is to reveal functional changes in the The HRS has been widely used to assess the psychological ef- brain associated with the hallucinogenic effects of the KOR fects of hallucinogenic drugs, including in previous studies con- agonism. Two independent, complementary studies were con- ducted by our group (Riba et al., 2003; Maqueda et al., 2015, 2016). ducted with healthy volunteers. Sub-study 1 assessed spontan- It includes 6 sub-scales: somesthesia, reflecting somatic effects; eous brain electrical oscillations using EEG while individuals affect, showing sensitivity to emotional and affective responses; were under the effect of salvinorin-A, whereas subjective meas- cognition, describing modifications in thought processes or con- ures and single-photon emission computed tomography (SPECT) tent; perception, measuring visual, auditory, gustatory, and ol- were used in sub-study 2 to characterize both clinical effects factory experiences; volition, indicating the volunteer’s capacity and changes in regional cerebral blood flow (rCBF) following the to willfully interact with his/her “self” and/or the environment; acute administration of salvinorin-A. and intensity, which reflects the strength of the overall experi- ence. A validated Spanish version of the questionnaire was used (Riba et al., 2001). MATERIALS AND METHODS Self-administered VAS were used to retrospectively rate peak effects during the session. Volunteers retrospectively indicated Ethics the intensity of the drug effects using a chart with 100 horizontal This study was conducted in accordance with the Declarations lines placed 1 mm apart (from 0, no effects, to 100, extremely in- of Helsinki and its updates concerning experimentation on hu- tense effects). There were 10 labeled lines for the VAS measures. mans, and it was approved by the Santa Creu i Sant Pau hospital’s “Global intensity” indicated the overall intensity of the experi- ethics committee and the Spanish Ministry of Health. All partici- ence. “Good effects” indicated any effect the volunteer perceived pants gave their written informed consent prior to participation. as positive. “I liked the experience” indicated the degree of en- joyment of the experience. “I would like to take the substance again” indicated willingness to take the substance again. “Bad Study Design, Sample, and Assessments effects” indicated any effect the volunteer perceived as negative. Volunteers with previous experience with hallucinogens (at “Loss of contact with the body” indicated dissociation between least 10 previous experiences and no history of adverse effects) mind and body. “Extracorporeal experiences” indicated the in- were recruited for both sub-studies. Participants underwent a tensity of the sensation of being out-of-body. “Modification in clinical interview, physical examination, and laboratory testing, time perception” indicated modifications of the perception of including hematology and urinalysis. Subjects with a history of time. “Changes in dimensionality” indicated alterations in the psychiatric disorders, pregnancy, illness, or any substance de- perception of the dimensionality of the body. “Loss of contact pendence were excluded. Twenty-four people participated in with external reality” indicated separation from surroundings. sub-study 1 (EEG), while 20 people participated in sub-study 2 “Visual phenomena” indicated visual modifications with eyes (SPECT). While in sub-study 1 the experimental feasibility was open or closed. “Auditory phenomena” indicated the intensity of tested and only EEG measures were collected, the SPECT meas- possible sounds/noises/voices/music that the volunteer attrib- ures for sub-study 2 were complemented by the collection of uted to drug effects. subjective measures. One subject was removed from sub-study 1 as a consequence of the artifact rejection procedure, and 1 EEG Data Collection and Processing subject was removed from sub-study 2 due to technical prob- lems that occurred while administering the substance, which Vigilance-controlled EEG readings were recorded at baseline resulted in the subject experiencing no psychoactive effects. (for 3 minutes with eyes closed, 3 minutes before salvinorin-A Additional details on the study participants are provided in the vaporization), 0 minutes, +3 minutes, and +6 minutes after drug supplementary Information file. administration. EEG recordings were acquired via a BrainAmp Both sub-studies were conducted following a double-blind, amplifier (Brain Products GmbH, Gilching, Germany) using 19 randomized, placebo-controlled design involving two experi- electrodes according to the international 10/20 system (Fp1, mental sessions that were one week apart. Volunteers were Fp2, F7, F3, Fz, F4, T3, C3, Cz, C4, T4, T5, P3, Pz, P4, T6, O1, and not allowed to take any medications or illicit drugs in the two O2) referenced to averaged mastoids. Vertical and horizontal weeks prior to beginning or during the sub-studies. The placebo electrooculograms were also recorded for ocular artifact mini- or a fully psychoactive dose of 1 mg of vaporized pure (>99%) mization. Signals were recorded with a sample frequency of 100 salvinorin-A, as based on a previous trial (Maqueda et al., 2015), Hz and analogically band-pass filtered between 0.1 and 45 Hz. was administered by inhalation. Based on previous studies EEG data was re-referenced to the common average. In previous (Maqueda et al., 2015, 2016), we expected the experience elicited studies with 10/20 systems, standardized LORETA (sLORETA) by this dose to be remarkably intense. The psychoactive effects provided a solution with a localization error distance from the of salvinorin-A have a fast onset (approximately 1 minute) and source of 1.90 ± 4.36 mm and a spread of 7.07 ± 0.37 mm (Song short duration (10–15 minutes) and are characterized by vivid vi- et al., 2015). sions with eyes closed, changes in dimensionality and time per- A 2-step procedure was applied to filter artifacts from EEG ception, and a complete blockage of external stimuli (Maqueda data. First, we applied an ocular contamination reduction pro- et al., 2015, 2016). cess based on blind source separation (Romero et al., 2008). EEG signals were segmented into 5-second epochs. In the second stage, we applied an automatic rejection procedure fo- Subjective Measures cused on saturation, muscle, and movement artifacts (Anderer Psychological effects elicited by the administration of et al., 1992). A minimum of 6 artifact-free epochs was required salvinorin-A were measured in sub-study 2 using the hallu- to include a time-point EEG recording in the analysis. If the cinogen rating scale (HRS) questionnaire and various visual ana- high-artifact time-point recording was the baseline, all of the logue scales (VAS). volunteers would have been excluded from the analysis. The Kappa Opioid Receptor and the Sleep of Reason | 57 The sLORETA technique was applied to estimate 3D source Montreal Neurological Institute standard anatomical space, and distribution of the intracerebral current density function Gaussian smoothing using a kernel of 12 × 12 × 12 mm full-width from the voltage values recorded by the scalp EEG electrodes. at half maximum. sLORETA estimates a particular solution of the non-unique EEG inverse solution restricted to 6239 cortical gray matter voxels Statistical Analyses with a spatial resolution of 0.125 cm3 according to a digitized head model from Montreal Neurological Institute (Pascual- Post-salvinorin-A changes in subjective measures were as- Marqui, 2002). In the first step, the current density values were sessed using paired t test analyses, for which a P < .001 after estimated based on the EEG cross-spectral matrix and then Bonferroni correction (.05/6 = .008 for the HRS questionnaire squared for each voxel in the classical frequency bands: delta and .05/12 = .004 for VAS measures) was considered significant. (1.3–3.5 Hz), theta (3.5–7.5 Hz), alpha (7.5–13 Hz), beta (13–35 Hz), Results are reported after Bonferroni correction. and gamma (35–40 Hz). Statistical EEG differences between drug and control groups MATLAB and LORETA software were used to analyze the EEG were evaluated by paired-sample t tests computed for the signals. The MATLAB statistical toolbox was used for statistical baseline-corrected and log-transformed LORETA power values analyses. in each voxel and for each frequency band at different time points. To correct for multiple comparisons, a nonparametric permutation test based on the theory of randomization was ap- Neuroimaging Acquisition and Processing plied (Nichols and Holmes, 2002). Voxel intensity nonparametric Brain 99mTc-D,L-hexamethylene-propyleneamine oxime SPECT permutation test calculates a critical t-value by means of a scans were conducted prior to and following the administration random sample of all possible permutations in order to estimate of salvinorin-A using a 2-headed General Electric HELIX gamma the distribution of a maximum t-statistic. camera equipped with fan-beam collimators. This technique is Finally, to examine post-salvinorin-A rCBF changes, a particularly appropriate given the extremely short nature of the SPECT voxel-wise paired t test was performed, using propor- psychotropic effects elicited by this drug, whereby the peak lasts tional scaling for global cerebral tracer uptake (Riba et al., 2006). for only 1 to 5 minutes following inhalation. Contrasts were selected to identify the set of voxels in which Subjects were trained to perform an uninterrupted inhalation significant increases or decreases in tracer uptake were found of 30 seconds to absorb all of the vapor. Immediately following following the administration of the drug compared with pla- salvinorin-A inhalation (+25 seconds after the end of inhalation, cebo. Only brain regions showing voxel-wise P < .001 and family- i.e., at +55 seconds after the beginning of inhalation), the tracer wise error correction for multiple comparisons (cluster-level was injected into each volunteer to precisely capture cerebral P < .05 using random field theory as implemented in SPM) were blood flow during the drug’s peak effect. Acquisition duration considered significant. was roughly 50 minutes to achieve an appropriate signal-to- noise ratio. The initial tracer uptake and distribution, which re- flects the rCBF at the time of injection, remain unchanged for at RESULTS least 2 hours, independent of rCBF variations occurring after the Sub-study 1 fixation time. This property of quick uptake and prolonged sta- bility is particularly indicated in this case, where the powerful A total of 24 participants (11 male, 13 female) were recruited, effects of salvinorin-A are too short-lived to induce them and aged between 21 and 42 years (M = 33 years), with previous ex- acquire brain scans using other techniques. Volunteers’ scans perience in the use of hallucinogenic drugs. were acquired using a headrest and a 128 × 128 × 128 image matrix in 3-degree steps. Acquisition duration was roughly 50 EEG Data Acquisition and Analysis minutes, and images were subsequently reconstructed using fil- tered back projection with the application of Metz filtering and This study assessed the effect of a single dose of salvinorin-A attenuation-correction according to the Chang method, with a (1 mg) using EEG measures. After the artifact rejection procedure, factor of 0.075. we established a minimum of 6 artifact-free epochs (30 seconds Neuroimaging analyses were performed using the SPM8 soft- each) for each recording to obtain sufficient signal information ware package (https://www.fil.ion.ucl.ac.uk/spm/). Briefly, the to represent each time point. For this reason, 1 volunteer was original SPECT images underwent the following preprocessing excluded from the analysis. Figure 1 shows the time course for steps: DICOM to NIFTI conversion, spatial normalization to the main salvinorin-A effects measured by EEG. Alterations of Figure 1. Averaged time courses (19 electrodes and 24 volunteers) of the absolute power (Y axis) in the delta, alpha, and gamma bands after placebo (blue) and salvinorin-A (SA; red) administrations. Shaded areas depict the SD of the mean in each time point. Significant changes are indicated by **P < .01 and *P < .05. 58 | International Journal of Neuropsychopharmacology, 2022 absolute powers were highest in the first 3 minutes following dramatic and generalized decrease in rCBF in cortical areas, the start of inhalation. including mainly the frontal, temporal, and parietal lobes. Small Neural sources associated with EEG frequency bands were decreases were also observed in regions of the occipital lobe, such estimated after placebo and salvinorin-A administrations. as the calcarine sulcus. Increases in rCBF were located at the Figure 2 shows statistical LORETA images corresponding to medial temporal lobe (especially in the amygdala, hippocampus, suprathreshold regions found for delta, alpha, and gamma and parahippocampal gyrus) and the cerebellum. bands at the first 3 minutes after salvinorin-A administration as compared with the controls. Significant increases in delta and Discussion gamma bands were located at the temporal and occipital lobes, respectively. Decreases in the alpha band were restricted to the The aim of this study was to characterize the neuropharmaco- cingulate gyrus, precuneus, and superior parietal lobe. logical effects of salvinorin-A, a non-nitrogenous diterpene with high selectivity to KOR. As a consequence of the acute adminis- tration of salvinorin-A, drastic changes in subjective effects, as Sub-study 2 well as EEG and SPECT measures, were registered. A sample of 20 participants (16 male, 4 female) was recruited, Regarding the subjective measures used in sub-study 2, the aged between 23 and 46 years (M = 35.1 years), with previous ex- highest scores in HRS were obtained for the scales of intensity perience in the use of hallucinogenic drugs. and volition. While the former concerns a very intense experi- ence, the latter concerns a drastic interruption in contact with both the self (body sensations, awareness) and one’s surround- Subjective Measures ings. Among the scales with lower scores, we find affect, which HRS—Mean scores obtained in the subscales of the HRS suggests the low emotional content of the experience. While this questionnaire for salvinorin-A and control conditions are shown pattern is different from that found in the case of psilocybin or in Figure 3. The scores for all subscales differed significantly ketamine (Gouzoulis-Mayfrank et al., 2005; Griffiths et al., 2011), between conditions, showing clear psychoactive effects induced by some similarities exist with the pattern of effects obtained from salvinorin-A. The subscales that showed higher post-salvinorin-A using N,N-dimethyltryptamine (DMT) or ayahuasca (Riba et al., increases were intensity, volition, cognition, and perception. 2003). The battery of VAS showed the highest scores for “I liked the experience” and “I would like to take the substance again,” VAS—Mean scores for all VAS are shown in Figure 4. Similar to suggesting the mostly positive effects of salvinorin-A. This is the HRS, all items showed significantly higher scores following in contrast with most KOR agonists, for which a pattern of dys- salvinorin-A administration than in the control condition. The phoric effects is generally reported (Kumor et al., 1986). However, highest scores were obtained for “I liked the experience,” “I would it is possible that more dysphoric effects might be found if like to take the substance again,” “good effects,” and “changes in salvinorin-A were administered to naïve subjects (Butelman and dimensionality.” Kreek, 2015). Other scales for which high scores were obtained include “visual effects,” “modifications in dimensionality,” and “loss of contact with external reality.” While alterations in visual SPECT stimuli are common with hallucinogenic drugs, modifications Changes in rCBF produced by the acute administration of in dimensionality and the loss of contact with external reality salvinorin-A are reported in Figure 5 (voxel-wise P < .001 and are less common, except in the case of high doses of ketamine, cluster-level family-wise error corrected P < .05). There was a which also induce out-of-body experiences (Morgan et al., 2009). Figure 2. Three-dimensional LORETA statistical maps from the first 3 minutes after the acute inhalation of salvinorin-A (SA) compared with placebo for the delta (1.3–3.5 Hz), alpha (7.5–13 Hz), and gamma (35–40 Hz) frequency activities. Cold and hot colors indicate significant decreases and increases after voxel intensity nonparametric correction for multiple comparisons. The Kappa Opioid Receptor and the Sleep of Reason | 59 Figure 3. Comparison of scores in subscales of hallucinogen rating scale (HRS) questionnaire after the administration of salvinorin-A (SA) and placebo. ** P < .001. Figure 4. Comparison of scores in visual analogue scales (VAS) after the administration of salvinorin-A (SA) or placebo. *P = .001; **P < .001. The EEG measures showed an increase in delta waves (de Araujo et al., 2012), so the enhanced activity of gamma (mostly associated with human NREM sleep) along the tem- waves could be attributed to being immersed in the intense poral and parietal cortices and in limbic regions, such as the visual imagery induced by salvinorin-A. Other EEG studies parahippocampal gyrus. This is in accordance with the sub- using serotonergic hallucinogens reported contradictory re- jective effects of salvinorin-A, which are commonly described sults, such as decreases in the power density of alpha and as involving a dream-like state, usually with a loss of phys- delta frequency bands (dos Santos et al., 2016). This suggests ical awareness (Addy, 2012; Maqueda et al., 2015). A marked that different mechanisms of action underlie the hallucino- decrease in alpha waves in the parietal and temporal lobes gens’ psychotomimetic effects. Indeed, it is well known that was also reported, possibly associated with the potent psy- salvinorin-A does not induce psychoactive effects through the choactive effects of salvinorin-A and the subsequent loss of 5-HT2A receptors like classic psychedelic drugs do. Notably, a inhibitory control. Gamma waves showed enhanced activity recent EEG study (Timmermann et al., 2019) is the only one to mainly in the visual areas of the brain. These waves are asso- report similar findings. In this study, the authors reported re- ciated with selective attention to sensory stimuli (Fries et al., ductions in alpha and beta waves and increases in delta and 2001; Bichot et al., 2005). With eyes closed following the admin- theta waves following the acute administration of DMT. This istration of hallucinogenic drugs, the activation of visual brain is consistent with the abovementioned similarities found be- regions is similar to that observed when seeing a real image tween subjective measures for both drugs. 60 | International Journal of Neuropsychopharmacology, 2022 Figure 5. Decreases (above) and increases (below) in regional cerebral blood flow (rCBF) after the acute administration of salvinorin-A (SA). Only regions surviving voxel-wise P < .001 and cluster-level family-wise error correction for multiple comparisons (P < .05) are displayed. The administration of salvinorin-A was associated with a but further research should clarify this finding. Regarding the widespread cortical decrease in rCBF in the frontal, parietal, tem- parietal lobe, the precuneus and the supramarginal gyrus were poral, and occipital lobes. Post-salvinorin-A increases in rCBF the main regions to show decreases in CBF. It has been pro- were found in the cerebellum and in the medial regions of the posed that the precuneus might be involved in autobiographical temporal lobes of both hemispheres, especially in the amygdala, memory and consciousness (Knauff et al., 2003; Cavanna, 2007; the hippocampus, and the parahippocampal gyrus, where there Freton et al., 2014). Furthermore, other neuroimaging studies is a high density of KORs. This pattern differs again from that have shown that the precuneus is among the first regions to be observed in other neuroimaging studies in which serotonergic deactivated in other states of consciousness. Decreased activity hallucinogens were administered. For instance, a SPECT study in this region, during both slow-wave and REM sleep phases, has involving mescaline reported a “hyperfrontal pattern,” the in- been reported in PET studies (Braun et al., 1997; Andersson et al., verse situation (Hermle et al., 1992). Similarly, another SPECT 1998), which may be related to the increased activity of delta study involving ayahuasca observed increases in CBF in the waves. Another PET study found that the precuneus is among anterior insula, the inferior frontal gyrus, and the anterior cin- the first regions to show activity when individuals recover con- gulate, among other regions, with no region showing decreases sciousness (Laureys et al., 1999). Additionally, it should be noted in CBF (Riba et al., 2006). A positron emission tomography (PET) that the precuneus is a key node of the DMN. According to pre- study offered similar results, showing increases in the cerebral vious research (Carhart-Harris et al., 2012; Palhano-Fontes et al., metabolic rate of glucose in the frontomedial and frontolateral 2015; Müller et al., 2018), decreased functional connectivity of cortices, among other cortical regions, following the acute ad- the DMN is a common consequence of the acute administration ministration of psilocybin (Vollenweider et al., 1997). Conversely, of various hallucinogens, although there are some contradictory Carhart-Harris et al. (2012) reported only decreases in CBF fol- findings in this regard (Carhart-Harris et al., 2017; Müller et al., lowing acute administration of psilocybin. Those decreases were 2020). registered mainly in the thalamus and in the anterior cingu- Regarding the occipital lobe, the calcarine sulcus was the late and posterior cingulate cortices. However, it is challenging main region to show a relevant decrease in CBF. Interestingly, to compare these findings with ours given the distinct mech- some previous studies involving hallucinogenic drugs found no anism of action and subjective effects elicited by salvinorin-A. effects in this region (Vollenweider et al., 1997; Riba et al., 2006; The main frontal regions showing decreases in CBF were the left Carhart-Harris et al., 2016), whereas others found an increase in prefrontal cortex, the superior frontal gyrus, and the premotor activity in this region (de Araujo et al., 2012; Carhart-Harris et al., cortex, regions rich in KORs. It can be speculated that the de- 2016). This should raise questions about the different mechan- crease in CBF in these regions could explain the marked loss of isms through which hallucinatory experiences can be induced, contact with reality and body awareness reported by volunteers, as it has been proposed that psychedelic-induced hallucinations The Kappa Opioid Receptor and the Sleep of Reason | 61 may be produced by the hyperactivation of visual areas of the associated with alterations in this receptor system (Clark and brain; however, this is not applicable in the case of salvinorin-A. Abi-Dargham, 2019). Beyond gaining an improved understanding Moreover, activity in the calcarine sulcus has been considered of these pathological processes, the study of KOR signaling also essential to producing mental images (Ganis et al., 2004), so it offers an opportunity to elucidate the mechanisms of other su- is challenging to explain how most study participants could perior functions, such as consciousness. In particular, it seems experience intense visual hallucinations while their visual cor- that KOR full agonists act as a kind of switch that completely tices remained hypoactive. Again, further research using other interrupts perception and cognitive functions, inducing unique neuroimaging techniques should shed light on this issue. and bizarre hallucinations. The mechanism through which this The increase in CBF registered in the medial temporal lobe, phenomenon occurs, a drastic decrease in cortical rCBF, is rem- especially in the amygdala, suggests differential mechanisms iniscent of the famous etching made by the Spanish painter with serotonergic hallucinogens. This appears to be the case Francisco Goya: “the sleep of reason produces monsters.” as, following acute administration of both psilocybin and ly- sergic acid diethylamide, reduced activity and connectivity of Supplementary Materials the amygdala were observed (Kraehenmann et al., 2015; Mueller et al., 2017). The experience elicited by a high dose of salvinorin-A Supplementary data are available at International Journal of is much more abrupt and intense than that elicited by regular Neuropsychopharmacology (IJNPPY) online. doses of serotonergic hallucinogens. We speculate that this may explain the increase of CBF in this region, as the amygdala is Acknowledgments implicated in emotional salience. Moreover, dynamics between This paper is dedicated to the memory of our dear colleague and the corticofrontal regions and amygdala could also be involved, the senior author of this paper, Jordi Riba, who recently passed since attenuation of the activity of the amygdala has been asso- away. ciated with enhanced activation in some regions of the frontal This study was partially supported by the Ministry of Economy cortex (Ochsner et al., 2002; Urry et al., 2006). The cerebellum and Competitiveness (MINECO), Spain, under contract DPI2017- also showed an increase in CBF. This could be interpreted in the 83989-R. CIBER-BBN is an initiative of the Instituto de Salud context of well-known connections between the limbic system Carlos III, Spain. The authors thank Dr. M.T. Colomina for her and the cerebellum (Blatt et al., 2013). Additionally, previously supervision and the contributions to the manuscript. mentioned patterns that are similar to those found during sleep could also be related to this finding, as the cerebellum has been found to be increasingly active during sleep (Dang-vu et al., Conflict of Interest 2008). Nevertheless, we cannot be certain about these state- None. ments given the nature of the data obtained. Among the limitations of this study, the small sample sizes used for both sub-studies should be mentioned. Moreover, the fact that References participants had previous experience with hallucinogens could be Addy PH (2012) Acute and post-acute behavioral and psycho- related to the absence of dysphoric effects. 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