DSIP: study models, terminology and open questions

This study summary walks through the key papers: the original characterisation, the reviews that followed, a small human sleep laboratory study and the open questions that remain. It belongs to the study summaries in the article library, which report what each paper measured and the limits the authors stated.

It covers published research only and does not describe any use of DSIP in people or animals.

Where DSIP came from

The peptide was described by Schoenenberger, Maier, Tobler, Wilson and Monnier (1978, Pflügers Archiv). Their group isolated it from dialysate of cerebral venous blood taken from rabbits during electrical stimulation of the intralaminar thalamic area, a procedure intended to induce sleep. Amino acid analysis and sequencing gave the nine-residue chain, which they named delta sleep-inducing peptide.

The original rabbit study

The 1978 paper did more than sequence the peptide. The team synthesised DSIP along with five possible breakdown fragments, two nonapeptide analogues with one or two amino acids exchanged, and a related tripeptide. All nine synthetic peptides were infused into the brain ventricles of rabbits and tested under double-blind conditions, with 61 rabbits used in total including controls.

• The measurement. EEG signals from the frontal neocortex and limbic cortex were analysed by fast Fourier transform.
• The reported result. Synthetic DSIP increased mean EEG delta activity by 35% in both regions compared with animals given control solution or any of the other eight peptides.
• The isomer finding. Only the pure alpha-aspartyl form of DSIP was highly active. Its beta-aspartyl isomer, which has the same mass, was not.

That last point is easy to overlook and matters for anyone working with the material. Two forms of the same sequence, indistinguishable by a simple mass check, behaved differently. Why that calls for more than one analytical method is explained in orthogonal identification.

What the first reviews reported

A review by Graf and Kastin (1984, Neuroscience and Biobehavioral Reviews) gathered the early work. It reported that DSIP was said to induce mainly delta sleep in rabbits, rats, mice and humans, while in cats the effect on REM sleep was more pronounced.

• A U-shaped response. Activity followed a U-shaped curve for both dose and timing of infusion, so more was not reliably more.
• Detection by antibody. DSIP-like material was found by radioimmunoassay and immunohistochemistry in rat brain, by radioimmunoassay in peripheral rat organs and in the plasma of several mammals.
• A long list of other effects. The review also listed reported effects on electrophysiology, brain neurotransmitter levels, circadian and locomotor patterns and hormone levels.

A small human sleep laboratory study

A double-blind study by Bes, Hofman, Schuur and Van Boxtel (1992, Neuropsychobiology) enrolled 16 adults with chronic insomnia in a matched-pairs, parallel-group design. Participants spent five consecutive nights in the laboratory: one for adaptation, one for baseline, then three nights during which half received DSIP and half a glucose placebo.

• Measures. Sleep structure was recorded by polysomnography, alongside subjective sleep quality and tiredness.
• Results. Objective sleep efficiency was higher and sleep latency shorter with DSIP than with placebo. One measure of subjective tiredness fell in the DSIP group.
• The authors’ reading. They judged the significant effects weak and partly explained by a chance change in the placebo group. No other measure, including subjective sleep quality, changed.

The authors concluded that short-term DSIP was not likely to be of major benefit in chronic insomnia. With 16 participants and three treatment nights, it is a small study, and its result should be read at that scale.

What DSIP research does not establish

A mini-review by Kovalzon and Strekalova (2006, Journal of Neurochemistry) calls DSIP “a still unresolved riddle”. The authors write that the link between DSIP and sleep was never further characterised, partly because no DSIP gene, protein or receptor was isolated, and that the sleep factor hypothesis is extremely poorly documented.

• No isolated gene or receptor. Without a precursor protein or a receptor, it is unclear how or whether the body makes and responds to DSIP.
• Structurally unusual. The review notes that DSIP’s structure differs from any other known peptide family.
• The signal may be a relative. The authors propose that a DSIP-like peptide, not DSIP itself, may be responsible for at least part of the immunoreactivity and activity reported.
• Analogues behaved differently. In their own earlier rabbit and rat work, certain artificial DSIP analogues promoted slow-wave sleep while DSIP itself did not.
• Results are hard to combine. Different species, assays, analogues and infusion routes make cross-study comparison unreliable.

How we handle evidence this mixed is set out in how we write.

Checking a DSIP certificate of analysis

Given the isomer finding in the original paper and the reliance on antibody assays in later work, identity data matters more for DSIP than the purity figure alone. These are the points to look for.

• Sequence stated as Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu (WAGGDASGE)
• Molecular formula and weight consistent with the free peptide, about 849 g/mol
• Identity confirmed by mass spectrometry, not by HPLC retention time alone
• Chromatogram showing individual impurity peaks, including any isomer-related peaks
• Counter-ion or salt form named, since it changes the weight
• Peptide content or water content stated alongside purity
• Lot number on the certificate identical to the lot on the vial label

Isomers of the aspartate residue share the same mass, so a matching mass alone does not rule them out. The methods that can are covered in orthogonal identification.

Frequently asked questions