Mescaline increases startle responding equally in normal and raphe-lesioned rats

Multiple receptors contribute to the behavioral effects of indoleamine hallucinogens

Serotonergic hallucinogens produce profound changes in perception, mood, and cognition. These drugs include phenylalkylamines such as mescaline and 2,5-dimethoxy-4-methylamphetamine (DOM), and indoleamines such as (+)-lysergic acid diethylamide (LSD) and psilocybin. Despite their differences in chemical structure, the two classes of hallucinogens produce remarkably similar subjective effects in humans, and induce cross-tolerance. The phenylalkylamine hallucinogens are selective 5-HT(2) receptor agonists, whereas the indoleamines are relatively non-selective for serotonin (5-HT) receptors. There is extensive evidence, from both animal and human studies, that the characteristic effects of hallucinogens are mediated by interactions with the 5-HT(2A) receptor. Nevertheless, there is also evidence that interactions with other receptor sites contribute to the psychopharmacological and behavioral effects of the indoleamine hallucinogens. This article reviews the evidence demonstrating that the effects of indoleamine hallucinogens in a variety of animal behavioral paradigms are mediated by both 5-HT(2) and non-5-HT(2) receptors.

Mescaline: excitatory effects on acoustic startle are blocked by serotonin2 antagonists

The ability of serotonin2 (5-HT2) antagonists to block the excitatory effects of mescaline on the acoustic startle reflex were analyzed. Mescaline (20 mg/kg) caused a consistent increase in the amplitude of the acoustic startle reflex. This effect was blocked in a dose-related fashion by the 5-HT2 antagonist ritanserin (ED50 dose = 0.25 mg/kg IP). In contrast, even a high dose of ritanserin (2.0 mg/kg) did not block the excitatory effects of amphetamine on startle. Other 5-HT2 antagonists (ketanserin, cinanserin, LY 53857) also blocked mescaline's effect, whereas the 5-HT1 antagonist pindolol (5 mg/kg) did not. These results support the hypothesis that the behavioral effects of hallucinogens are mediated by agonist actions at 5-HT2 receptors.

Effects of levonantradol on habituation of startle in rats

Levonantradol, a nonopioid analgetic related to cannabinoids, produced a dose-dependent impairment of the habituation of startle response in rats. Startle was elicited by air-puff stimuli and monitored in stabilimeters by a computer. Habituation was assessed by presenting the same tactile stimulus repeatedly at 15-second intervals. While a dose of 0.1 mg/kg reduced startle on all trials, doses ranging from 0.01 to 0.03 mg/kg had no effect on the initial level of reactivity to the first several stimuli but did impair the habituation of startle. This effect was not attributable to the time course of the drug's action and was also found with acoustic stimuli. The relatively inactive enantiomer of levonantradol was ineffective at 0.1 mg/kg. Thus, startle appears to be a sensitive measure of the effects of levonantradol in animals. Since comparable phenomena can be demonstrated in humans by measuring the blink reflex component of startle, direct comparisons can potentially be made across species.

Neurochemical modulation of sensory-motor reactivity: acoustic and tactile startle reflexes

The present review argues that the startle reflex is particularly well suited as a model system to analyze how drugs alter stimulus reactivity and reflex excitability. It then reviews all the literature to date on how drugs or lesions that are thought to alter neurochemical transmitter systems affect acoustic and/or tactile startle. Hypotheses are presented to account for how serotonin, dopamine, norepinephrine, acetylcholine, and opiates modulate startle. Effects on startle plasticity such as habituation, sensitization, and potentiation resulting from prior associative learning are also included.