All neuropsychopharmaceuticals, including the tryptamines, are potentially neurotoxic depending on the dose at which they are ingested.
Neurotoxicity is, arguaby, a broad definition. According to the NIH, neurotoxicity can be caused by processes ranging from radiation to transplants, and from chemical toxins to cosmetics. NIH defines neurotoxicity as:
Neurotoxicity occurs when the exposure to natural or manmade toxic substances (neurotoxicants) alters the normal activity of the nervous system. This can eventually disrupt or even kill neurons, key cells that transmit and process signals in the brain and other parts of the nervous system. Neurotoxicity can result from exposure to substances used in chemotherapy, radiation treatment, drug therapies, and organ transplants, as well as exposure to heavy metals such as lead and mercury, certain foods and food additives, pesticides, industrial and/or cleaning solvents, cosmetics, and some naturally occurring substances.
LSD is a tryptamine (Shulgin, & Shulgin, 1997), see Fig. 1.
Fig. 1. LSD contains the tryptamine structure (in red). source: Shroomery
Secondly, every compound is toxic when taken in large quantities. For instance, water can kill, referred to as water intoxication, for example as seen in people tripping on MDMA and taking in too much water (Ballantyne, 2007). Oxygen, another key to life, is a deadly gas when taken in artificially high concentrations for too long. Likewise, neuropharmacologically active compounds will hence eventually become neurotoxins. This, because the definition of neurotoxic is indeed quite broad (source: NIH):
Neurotoxicity occurs when the exposure to natural or manmade toxic substances (neurotoxicants) alters the normal activity of the nervous system. This can eventually disrupt or even kill neurons, key cells that transmit and process signals in the brain and other parts of the nervous system.
Thus when you ask
are tryptamines, including LSD [sic] generally neurotoxic?
Then the answer is yes, depending on the dose.
Reversely, then, every neurotoxin becomes non-toxic once taken below a certain threshold dose. If this is what you refer to by 'microdosages', then yes, even the most potent toxin becomes harmless.
To specifically answer your question with regard to LSD and psilocin: LSD is suspected to be neurotoxic at clinically applied dosages (Larsen, 2014), and psilocin can result in convulsions and death follow massive overdose (Gold et al., 2003).
- Ballantyne, Sci Am 2007
- Gold et al., Atlas of Clinical Neurology, Springer. pp 503-24
- Larsen, Hist Psychiatry (2016); 27(2):172-89
- Shulgin & Shulgin, Tihkal - The Continuation, pp. 490-99