Neurochemical effects of LSD in the MPFC of conscious rats
Lee, David Jin
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Hallucinogenic compounds have been used widely throughout human history and across cultures in philosophical and religious contexts, and are also subject to continued abuse in our society. These compounds, naturally occurring in plants and mushrooms (e.g. mescaline and psilocybin) or synthesized (e.g., lysergic acid diethylamide [LSD] and phencyclidine [PCP]), exert a wide range of psychoactive effects, and can also mimic the symptoms of psychotic disorders in humans. Understanding the neurochemical effects of LSD may provide insight into the mechanism that underlies both the drug-induced and clinical hallucinogenic states. Currently, 14 different serotonin (5-HT) receptor subtypes have been cloned, identified, and grouped into 7 different families (5-HT 1-7 ). The development of drug discrimination, a method that uses operant conditioning procedures to study drug-induced interoceptive cues, in animals and the discovery of specific 5-HT receptor ligands have helped identify the central role of 5-HT 2A receptors in mediating the interoceptive cue of LSD and hallucinogens belonging to the indoleamine (psilocybin) and phenethylamine (e.g., mescaline, DOM, DOI) structural classes. The drug-induced head-twitch response (HTR) in rodents – described as resembling a "wet dog shake" behavior – also has been used in addition to the drug discrimination method to characterize the mechanism of action of these drugs. As in drug discrimination, the 5-HT 2A receptor has been shown to mediate the HTR induced by LSD and the indoleamine and phenethylamine hallucinogens. The prefrontal cortex, an area with a high density of 5-HT 2A receptors, has been suggested to be involved in the mechanism of action of LSD and other serotonergic hallucinogens and the pathogenesis of schizophrenia. The present investigation sought to determine, using a combination of behavioral (drug discrimination, HTR) and biochemical (microdialysis) methods, the neurochemical processes occurring in the mPFC that could be mediating the actions of LSD in the conscious rat. Microdialysis experiments were conducted to determine the effect of LSD on levels of dopamine (DA), 5-HT, and glutamate in the mPFC of conscious rats. Head-twitch experiments were conducted to determine the role of glutamate release in the mPFC on the HTR in rats, extending work previously conducted in our laboratory using drug discrimination. LSD decreases mPFC 5-HT levels in drug-naive conscious rats. The present investigation evaluated the effect of systemic and local administrations of LSD on 5-HT levels in the mPFC of conscious rats. Systemic administration of LSD decreased dialysate levels of 5-HT, consistent with the findings in the literature for the phenethylamine hallucinogen, DOI. However, in contrast to the local effects of DOI (which has been shown to increase cortical 5-HT levels), retrodialysis of LSD had no effect on 5-HT levels. These results suggest that systemic administration of LSD, like DOI, mainly activates 5HT 2A receptors outside the mPFC to indirectly decrease 5-HT release in the mPFC. This would mean that, if LSD exerts any part of its interoceptive effects via activation of 5HT 2A receptors in the mPFC, the activation of those receptors must occur by direct binding by LSD given the net decrease in mPFC 5-HT levels induced by systemic LSD. Furthermore, the local effects of LSD may not be identical to that of DOI. A possible explanation for this difference is that LSD has a more complex receptor activation profile than DOI. LSD, but not DOI, is an agonist at 5-HT 1A receptors in addition to 5-HT 2A receptors. There is evidence that the 5HT 1A receptor oppose 5HT 2A receptor-mediated effects on 5-HT release. For example, co-application of the 5HT 1A receptor agonist BAY X 3702 has been reported to block the local DOI-induced increase in 5-HT release. In contrast to drug-naive rats, rats used in drug discrimination studies are subject to chronic treatment with the training drug. A chronic drug administration experiment was conducted to determine whether a drug history causes desensitization to the effects of LSD. A group of rats were administered a 3-week history of repeated LSD injections designed to mimic the schedule of drug exposure in a drug discrimination study. A group of rats receiving chronic saline injections served as a control to the chronic LSD injections. No desensitization was detected, indicating that the effects of LSD and other 5-HT 2A receptor agonists observed in drug discrimination studies may generalize to the drug naive state. LSD does not affect levels of DA in the mPFC of conscious rats. In drug discrimination, a late dopaminergic phase of LSD stimulus control has been reported. The ability of systemically administered LSD to induce DA release in the mPFC was studied using microdialysis. If DA release was mediating the late dopaminergic component of LSD, it was expected that a behaviorally active dose of LSD would induce dopamine release around 90 minutes after administration. No such DA release was detected in response to LSD, in contrast to a robust increase in DA levels after PCP and methamphetamine administrations. This result does not support the hypothesis that DA release in the mPFC mediates the dopaminergic component of the interoceptive cue of LSD. LSD does not affect levels of glutamate in the mPFC of conscious rats. Based on electrophysiological and behavioral evidence, glutamate release in the mPFC has been proposed to mediate the effects of LSD. Systemic and local administration of LSD (via retrodialysis into the mPFC) had no impact on glutamate levels in the mPFC of conscious rats; in contrast, systemic administration of dmethamphetamine and local administration of the glutamate transport blocker TBOA reliably increased glutamate levels (positive controls). Endogenous glutamate clearance was ruled out as a factor in masking LSD-induced increases in glutamate levels. These results do not support the hypothesis that LSD induces the release of glutamate in the mPFC of conscious rats. Glutamate release in the mPFC does not mediate LSD-induced HTR. The head-twitch response in rodents, induced by LSD and other hallucinogens via the 5-HT 2A receptor, has been shown to be modulated by glutamatergic drugs, including group II/III metabotropic glutamate (mGlu 2/3 ) receptor ligands. As in drug discrimination, glutamate release has been proposed to mediate the HTR induced by LSD and other hallucinogens. A series of experiments was conducted to test the hypothesis that LSD-induced HTR is mediated by glutamate release in the mPFC. Systemic co-administration the mGlu 2/3 agonist LY379268 resulted in suppression of LSD-induced HTR, confirming the serotonergic-glutamatergic interaction in the mechanism of action of LSD. Microinjection of LSD into the mPFC also elicited the HTR. However, co-microinjection of LSD and LY379268 into the mPFC did not result in inhibition of LSD-induced HTR, suggesting that glutamate release in the mPFC is not necessary for LSD-induced HTR. The results from the glutamate microdialysis and HTR experiments do not support the hypothesis that glutamate release in the mPFC plays a central role in the mechanism of action of LSD. Other brain regions, such as the somatosensory cortex or the anterior cingulate cortex, that have been reported to be sites for the glutamatergic actions of LSD or similar hallucinogens (e.g. DOI) may be candidate locations for future studies into the mechanism by which LSD and glutamatergic neurotransmission interact.