5-HT2A receptors (the primary target of LSD and most classic psychedelics) are expressed in the lower urinary tract, as are other serotonin receptor subtypes. Lots of literature on this. Thus, the urge to void is likely, at least in part, due to a direct interaction between LSD and your urinary tract.
For example:
Adv Ther. 2015 Oct;32 Suppl 1:3-15.
Regulatory Effects of 5-Hydroxytryptamine Receptors on Voiding Function.
A growing body of evidence suggests that 5-hydroxytryptamine (5-HT; serotonin)
has both physiological and pathological functions in the lower urinary tract. A
wide variety of 5-HT receptor subtypes are variably expressed in different
organs, both peripheral and central. On urinary bladder smooth muscle, 5-HT1A,
5-HT2, 5-HT3, and 5-HT7 subtypes could function as postjunctional receptors.
Postjunctional 5-HT2 receptors induce detrusor contraction of the bladder body.
5-HT1A is suggested to have a similar effect to 5-HT2, while 5-HT3 might suppress
detrusor contraction evoked by direct muscle stimulation. Postjunctional 5-HT7 is
reported to induce relaxation of the bladder neck, which might be required for
efficient voiding. 5-HT1A, 5-HT2A, 5-HT2C, 5-HT3, 5-HT4, and 5-HT7 subtypes also
could act as prejunctional receptors in autonomic excitatory nerve terminals.
5-HT2A, 5-HT2C, 5-HT3, 5-HT4, and 5-HT7 subtypes facilitate the neurogenic
contraction of the detrusor by enhancing cholinergic or purinergic transmission,
whereas 5-HT1A receptors might inhibit the release of acetylcholine in the
detrusor. Furthermore, 5-HT1D could be involved in the suppression of ATP release
from the urothelium, aiding visceral sensation of the urinary bladder. In the
central pathways controlling the micturition reflex, 5-HT1A, 5-HT2A, and 5-HT7
are involved in regulation of bladder and urethral sphincter activities. Their
functions, especially that of 5-HT1A, vary in a species- and site (spinal or
supraspinal)- dependent manner. In addition to urinary bladder, 5-HT could be
involved in prostate contraction and cell proliferation. Evidence indicates that
5-HT receptor subtypes may be novel therapeutic targets for lower urinary tract
symptoms.
DOI: 10.1007/s12325-015-0240-2
PMID: 26391372 [Indexed for MEDLINE]