Effects of psilocybin on hippocampal neurogenesis and extinction of trace fear conditionning
CATLOW B.J., SONG S., PAREDES D.A., KIRSTEIN C.L., SANCHEZ-RAMOS J.
Experimental Brain Research, 2013, 228, 481-491.
Doi : 10.1007/s00221-013-3579-0
Drugs that modulate serotonin (5-HT) synaptic concentrations impact neurogenesis and hippocampal
(HPC)-dependent learning. The primary objective is to determine the extent to which psilocybin (PSOP) modulates neurogenesis and thereby affects acquisition and extinction of HPC-dependent trace fear conditioning. PSOP, the 5-HT2A agonist 25I-NBMeO and the 5-HT2A/C antagonist ketanserin were administered via an acute intraperitoneal injection to mice. Trace fear conditioning was measured as the amount of time spent immobile in the presence of the conditioned stimulus (CS, auditory tone), trace (silent interval) and post-trace interval over 10 trials. Extinction was determined by the number of trials required to resume mobility during CS, trace and post-trace when the shock was not delivered. Neurogenesis was determined by unbiased counts of cells in the dentate gyrus of the HPC birthdated with BrdU co-expressing a neuronal marker. Mice treated with a range of doses of PSOP acquired a robust conditioned fear response. Mice injected with low doses of PSOP extinguished cued fear conditioning significantly more rapidly than high-dose PSOP or saline-treated mice. Injection of PSOP, 25I-NBMeO or ketanserin resulted insignificant dose-dependent decreases in number of newborn neurons in hippocampus. At the low doses of PSOP that enhanced extinction, neurogenesis was not decreased, but rather tended toward an increase. Extinction of “fear conditioning” may be mediated by actions of the drugs at sites other than hippocampus such as the amygdala, which is known to mediate the perception of fear. Another caveat is that PSOP is not purely selective for 5-HT2A receptors. PSOP facilitates extinction of the classically conditioned fear response, and this, and similar agents, should be explored as potential treatments for post-traumatic stress disorder and related conditions.
Keywords : Neurogenesis · Psilocybin · Serotonin · Hippocampus · Learning · Memory · Trace conditioning
Psilocybin (4-phosphoryloxy-N,N-dimethyltryptamine, PSOP) was first isolated from Psilocybe mexicana, a mushroom from Central America by Albert Hofmann in 1957 and soon after produced synthetically in 1958 (Hofmann et al. 1958a, b). PSOP is an indole hallucinogen with potential clinical applications in the treatment for anxiety disorders, obsessive–compulsive disorder, major depression and cluster headaches (Moreno et al. 2006; Grob et al. 2011; Young 2013). PSOP is dephosphorylated in the body and converted into the active metabolite psilocin (4-hydroxy- N,N-dimethyltryptamine) which exerts psychoactive effects by altering neurotransmission through serotonin (5-HT) receptors 5-HT1A, 5-HT1D, 5-HT2A and 5-HT2C, but binds to 5-HT2A receptors with high affinity (Ki = 6 nM) (McKenna et al. 1990; Passie et al. 2002). 5-HT2A receptors are highly expressed throughout the hippocampus (HPC) in the dentate gyrus (DG), hilus, cornu ammonis (CA) 1 and CA3 and are colocalized on GABAergic neurons, pyramidal and granular cells (Luttgen et al. 2004; Morilak et al. 1993, 1994; Pompeiano et al. 1994; Shen and Andrade 1998; Cornea-Hebert et al. 1999). 5-HT2A receptor downregulation is thought to be an adaptive process triggered by chronic administration of selective 5-HT uptake inhibitors (SSRIs) (Eison and Mullins 1996).
Antidepressant medications which elevate 5-HT over prolonged periods such as SSRIs have been shown to enhance hippocampal neurogenesis (Malberg et al. 2000). Neurogenesis occurs throughout the life span in the adult brain, the subventricular zone (SVZ) and the subgranular zone (SGZ) in the DG (Altman 1962, 1969; Altman and Das 1965). The proliferation and survival of neural progenitors in the adult HPC can be influenced by a variety of stimuli including stress, age, drugs of abuse, physical activity and depression (Malberg et al. 2000; Van et al. 1999; Kempermann et al. 1998; Gould et al. 1992). The involvement of 5-HT in the regulation of neurogenesis may be mediated through different 5-HT receptor subtypes expressed on cells in the neurogenic microniche (Barnes and Sharp 1999).
Serotonin activates fifteen known receptors, many of which are expressed in the DG (Tecott et al. 1993; Vilaro et al. 1996; Djavadian et al. 1999; Clemett et al. 2000; Kinsey et al. 2001). Acute activation of the 5-HT1 receptor has been shown to increase cell proliferation in the DG, whereas repeated inhibition of the receptor results in diminished neurogenesis (Klempin et al. 2010). Interestingly, both acute and repeated stimulation of the 5-HT2 receptor attenuate proliferation and neuronal survival (Klempin et al. 2010). Studies investigating the effects of 5-HT2 receptor subunits on proliferation and neurogenesis in the DG have found that acute activation of the 5-HT2A/C receptor via 2,5-dimethoxy 4-iodo-amphetamine (DOI), the 5-HT2C receptor agonist RO 600175 or the 5-HT2C receptor antagonist SB-206553 had no effect on cell proliferation, whereas the 5-HT2A/C receptor antagonist ketanserin produced a 63 % decrease in BrdU incorporation (Banasr et al. 2004). Similarly, other investigators reported that acute ketanserin decreased proliferation, but chronic ketanserin increased proliferation in the DG (Jha et al. 2008). Additionally, no effect on proliferation was observed after DOI or lysergic acid diethylamide (LSD) was administered either acutely or once daily for seven consecutive days (chronic) (Jha et al. 2008). Administering LSD and PSOP by giving daily doses has been shown to produce
rapid tolerance to the drug and results in a selective downregulation of the 5-HT2A receptor (Buckholtz et al. 1985, 1990).
The serotonergic system has been implicated in hippocampal (HPC)-dependent learning. Administration of SSRIs produces alterations in performance on learning tasks that require the HPC (Flood and Cherkin 1987; Huang et al. 2004). In a knockout (KO) mouse model, central 5-HT–deficient mice developed heightened contextual fear conditioning which was reversed by intracerebroventricular microinjection of 5-HT (Dai et al. 2008). An impairment in learning on the Morris water maze was observed in 5-HT1A
KO mice along with functional abnormalities in the HPC (Sarnyai et al. 2000). Activation of 5-HT1A receptors in the medial septum alters encoding and consolidation in a HPC-dependent memory task (Koenig et al. 2008). In addition, LSD facilitated learning of a brightness discrimination reversal problem (King et al. 1972, 1974).
Evidence suggests that performance on HPC-dependent learning tasks is influenced by neurogenesis in the DG of the HPC (Van et al. 2002; Gould et al. 1999a, b; Nilsson et al. 1999; Shors et al. 2001, 2002). This was elegantly demonstrated by Shors et al. by treating animals with methylazoxymethanol acetate (MAM), an antimitotic agent which eradicates the progenitor cell population in the DG before testing mice on HPC-dependent and HPCindependent learning tasks (Shors et al. 2001, 2002). MAM-treated animals had significantly fewer BrdU+ cells in the SGZ but showed no impairment in the spatial navigation task (HPC-dependent) or delay eyeblink conditioning task (HPC-independent), demonstrating that the hippocampal progenitor cell population is not essential for these particular
tasks (Shors et al. 2001, 2002). In contrast, MAM severely impaired performance on trace fear conditioning and trace eyeblink conditioning, providing evidence for the involvement of progenitor cells in the DG in trace classical conditioning.
The present study investigated the role of PSOP through the 5-HT2A receptor on hippocampal neurogenesis and a HPC-dependent learning task, trace fear conditioning. The effects of single-dose injections of PSOP, the 5-HT2A agonist 25I-NBMeO and the 5-HT2A/C antagonist ketanserin on the survival and phenotypic fate of progenitor cells in the DG were assessed using immunofluorescence techniques. The effects of single doses of PSOP on trace fear conditioning were chosen for this study because it has previously been demonstrated to be a HPC-dependent learning paradigm.