Rat Brain Steady State Levels of Cyclic Nucleotides as an Endpoint of Lsd-Like Hallucinogen Effects.

Abstract

LSD-like hallucinogens have long been considered to form a distinct pharmacological class mainly because of similar subjective effects and the occurrence of a degree of cross tolerance. The class is represented by different chemical structural categories such as derivatives of lysergic acid, tryptamine and amphetamine. The present studies compared the effects of hallucinogens representing different chemical categories on a mammalian CNS in vivo endpoint. 3,4-methylene-dioxyamphetamine (MDA), N,N-dimethyltryptamine (DMT) and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) were compared for their effects on the concentration of cyclic nucleotides in rat brain. This research strategy as a means of studying mechanisms of neuronal response to hallucinogens was considered especially appropriate in view of the importance of examining the effects of these drugs in brain as opposed to peripheral systems. In measuring brain steady state levels of 3',5'-cyclic adenosine and guanosine monophosphate (cyclic AMP and cyclic GMP, respectively) it is necessary to rapidly inactivate critical brain enzymes. The technique of rapid heating of brain by microwave irradiation was used here as a means of sacrifice and its validity was tested in the early studies. Preliminary studies in which cyclic AMP was measured in regions of adult rat brain at intervals following dosages of MDA and DMT showed that no changes occurred in cyclic AMP levels when rats were killed by microwave irradiation. This is in contrast to an earlier literature report of increased brain cyclic AMP following various hallucinogens in animals sacrificed by decapitation. A serotonin (5-HT) sensitive adenylate cyclase is reportedly present in neonatal rat colliculi. A large body of data indicates that brain 5-HT is important in the mediation of hallucinogen effects. Accordingly, representative tryptamine and amphetamine hallucinogens, 5-MeODMT and MDA, respectively were compared for their effects on steady state levels of cyclic nucleotides in neonatal rat colliculi. Neither MDA or 5-MeODT, which is also a central 5-HT receptor agonist, nor chemically induced increases in brain 5-HT concentrations had any effect on cyclic AMP concentrations in neonatal colliculi or remaining rat brain. In newborn rat colliculi, a dosage of 0.05 mmol/kg 5-MeODMT, ip, increased the concentration of cyclic GMP by nearly 2 fold at 10 minutes after injection. Concentrations of cyclic GMP in brain minus colliculi were unchanged by 5-MeODMT and cyclic GMP levels were not elevated in colliculi of rats at 7 days of age or older. MDA (0.10 mmol/kg) which induced a behavioral state in the neonatal rat indistinguishable from that caused by 5-MeODMT, failed to affect concentrations of cyclic GMP in newborn rat colliculi or brain minus colliculi. Pretreatment of neonates with methysergide (0.05 mmol/kg, ip), and 5-HT receptor antagonist, 20 minutes before 5-MeODMT effectively blocked the increase in collicular cyclic GMP concentration. The effect of methysergide suggests a serotonergic mediation of the cyclic GMP increase in response to 5-MeODMT. The absence of an effect of MDA may indicate that the cyclic GMP increase is not an essential correlate of LSD-like hallucinogen activity. Further study of the neonatal rat collicular cyclic GMP response may establish a new in vivo endpoint of CNS 5-HT receptor stimulation.