1. Introduction: Peptide Neuromodulators in Cognitive Research
Short regulatory peptides have attracted sustained interest in neuroscience because, unlike many small-molecule agents, several appear to act as neuromodulators that influence endogenous signalling cascades rather than acting as blunt receptor agonists or antagonists. Three peptides recur frequently in the experimental literature on cognition and neuroprotection: Semax, an analogue of a fragment of adrenocorticotropic hormone (ACTH); Selank, an analogue of the endogenous immunopeptide tuftsin; and Dihexa, a peptidomimetic derived from the brain peptide angiotensin IV. This review summarises the published preclinical and, where available, clinical findings for each compound, with an emphasis on the molecular mechanisms reported in peer-reviewed studies. It is presented strictly as a summary of existing research for laboratory use; no claims of clinical benefit are made or implied.
A common thread across these three peptides is their reported engagement with neurotrophic and growth-factor systems. Brain-derived neurotrophic factor (BDNF) and hepatocyte growth factor (HGF) are recurrent themes, as are downstream processes such as dendritic spine formation and synaptic remodelling. Studying how short peptides interact with these pathways is of considerable interest to researchers investigating synaptic plasticity, neuronal survival and the molecular basis of learning and memory in model systems.
2. Semax: An ACTH(4-10) Heptapeptide Analogue
Semax is a synthetic heptapeptide (Met-Glu-His-Phe-Pro-Gly-Pro) constructed around the 4-10 fragment of adrenocorticotropic hormone, to which a C-terminal Pro-Gly-Pro tripeptide has been appended to improve metabolic stability. The native ACTH(4-10) sequence had long been associated with behavioural effects in animal models that were independent of the hormone’s adrenal actions, which motivated the development of a more stable analogue for research into learning and memory paradigms.
One of the most cited mechanistic observations concerns the relationship between Semax and BDNF. In a study of rat basal forebrain, Semax was reported to bind specifically within forebrain tissue and to increase levels of BDNF protein, positioning the peptide as a candidate modulator of neurotrophin signalling rather than a direct receptor ligand (Dolotov et al., 2006). BDNF is a central regulator of synaptic plasticity, and any agent reported to influence its expression is naturally of interest to researchers studying the molecular substrates of memory.
Beyond neurotrophins, the experimental literature has examined Semax at the level of the transcriptome. Genome-wide expression analyses in a rat model of focal cerebral ischaemia reported that the peptide modulated the expression of genes associated with immune-system and vascular-system processes, suggesting a broad, multi-pathway profile rather than a single discrete target (Medvedeva et al., 2014). More recent RNA-sequencing work in a cerebral ischaemia-reperfusion model reported that Semax was associated with suppression of inflammation-related gene expression alongside changes in genes linked to neurotransmission, reinforcing the view that its reported neuroprotective profile in these models is distributed across multiple gene networks (Filippenkov et al., 2020). These studies are conducted in animal and cell-based systems and characterise gene-expression responses; they do not establish clinical outcomes.
3. Selank: A Tuftsin Analogue with Anxiolytic and Immune Research Profiles
Selank is a synthetic heptapeptide based on the endogenous tetrapeptide tuftsin, an immunomodulatory fragment derived from the heavy chain of immunoglobulin G. As with Semax, a stabilising sequence was added to the parent peptide to extend its half-life. Selank has been studied predominantly in the context of anxiety-related behaviour and immune signalling in animal models, and a smaller body of clinical observation exists in the Russian literature.
A notable feature of the Selank research is that its reported anxiolytic-type effects in animal models do not appear to depend on direct binding to benzodiazepine sites. Instead, gene-expression studies have reported that Selank administration alters the expression of genes involved in GABAergic neurotransmission, offering a candidate molecular route by which the peptide could influence inhibitory signalling without acting as a classical GABA-A receptor ligand (Volkova et al., 2016). This indirect, expression-level mechanism is frequently contrasted with conventional anxiolytic pharmacology in the research literature.
On the clinical-observation side, a controlled study in patients with generalised anxiety disorder and neurasthenia compared Selank with a benzodiazepine comparator and reported broadly comparable anxiolytic outcomes alongside changes in markers such as serum enkephalin-degrading activity (Zozulia et al., 2008). This work is reported here purely as a summary of a published clinical study; it is not a recommendation, and the present document makes no therapeutic claim. As a tuftsin-derived peptide, Selank also retains research interest at the interface of the nervous and immune systems, an area where peptide signalling between the two systems remains an active question.
4. Dihexa: An Angiotensin IV-Derived Peptidomimetic
Dihexa is a small peptidomimetic developed from angiotensin IV (Ang IV), a metabolite of the brain renin-angiotensin system that had been linked to learning and memory through a distinct receptor subtype. Earlier work established that the brain Ang IV-AT4 receptor system contributes to spatial learning in rodent models, with AT4 ligands modulating acquisition of spatial tasks (Wright et al., 1999). This finding provided the rationale for engineering metabolically stable Ang IV analogues that could be studied as research tools for cognition.
Dihexa emerged from that medicinal-chemistry effort. In a characterisation study of metabolically stabilised Ang IV analogues, Dihexa was reported to be serum-stable and to penetrate biological barriers, and was evaluated in rodent cognitive paradigms as a research compound (McCoy et al., 2013). Readers should note that this primary report carries an editorial expression of concern relating to figure integrity and should be read with that context in mind; it has not been formally retracted at the time of writing.
The mechanistic hypothesis most associated with Dihexa concerns the hepatocyte growth factor (HGF) and its receptor c-Met. Reviews of the brain renin-angiotensin system have proposed that Ang IV-derived peptides intersect with the HGF/c-Met system, a growth-factor pathway implicated in dendritic arborisation and synaptogenesis, providing a candidate framework for the synaptogenic activity reported for these molecules in cell-culture studies (Wright et al., 2013). This positions Dihexa, in the research literature, as a putative facilitator of growth-factor signalling rather than a conventional receptor agonist.
5. Comparative Mechanisms
Although the three peptides arise from very different parent molecules — a pituitary hormone fragment, an immunoglobulin-derived immunopeptide and a renin-angiotensin metabolite — the research literature describes a degree of mechanistic convergence on neurotrophic and growth-factor signalling. Semax is most strongly associated with BDNF and neurotrophin-related gene expression (Dolotov et al., 2006), while Dihexa is associated with the HGF/c-Met axis and downstream synaptic remodelling (Wright et al., 2013). Selank occupies a somewhat different niche, with its reported effects centring on GABAergic gene expression and immune-nervous-system signalling (Volkova et al., 2016).
A second shared characteristic is the apparent reliance on indirect, expression-level or facilitatory mechanisms. Rather than acting as direct, high-affinity agonists at a single classical receptor, each peptide is reported in the literature to modulate endogenous signalling systems — neurotrophin synthesis, growth-factor pathway facilitation or transcriptional regulation of neurotransmission genes. The transcriptome-wide studies of Semax illustrate how broad such effects can be, spanning immune, vascular and neurotransmission gene networks (Medvedeva et al., 2014) (Filippenkov et al., 2020). This multi-pathway character complicates direct mechanistic comparison but is itself a consistent theme across the peptide-neuromodulator literature.
6. Research Applications
Within laboratory settings, these peptides are studied chiefly as tools for probing the molecular biology of cognition and neuronal resilience. Semax features prominently in transcriptomic and proteomic studies of cerebral ischaemia models, where it is used to interrogate gene-expression responses to neuroprotective intervention (Medvedeva et al., 2014) (Filippenkov et al., 2020). Selank is employed in behavioural-pharmacology paradigms examining anxiety-related behaviour and in studies of GABAergic gene regulation (Volkova et al., 2016). Dihexa, and the broader class of Ang IV analogues, are used as research probes of the AT4 and HGF/c-Met systems and their relationship to synaptogenesis and spatial learning in animal and cell-culture models (McCoy et al., 2013) (Wright et al., 1999).
For researchers, the value of these compounds lies in their relatively well-characterised hypothesised mechanisms, which allow targeted experimental questions about neurotrophin and growth-factor signalling. Detailed compound-specific profiles — including sequence, analytical data and the underlying literature — are summarised on the individual Semax, Selank and Dihexa research pages.
7. Limitations and Research-Use-Only Note
The evidence base for all three peptides is dominated by preclinical work in animal and in-vitro systems, with clinical data limited in scope and, in several cases, originating from a small number of research groups. Some primary reports — notably within the Ang IV/Dihexa literature — carry editorial expressions of concern, and findings across the field have not been uniformly replicated in large, independent studies. Mechanistic models such as BDNF facilitation, HGF/c-Met activation and GABAergic gene modulation are best regarded as well-supported hypotheses derived from specific experimental contexts rather than settled fact, and effects observed in rodent or cell-culture systems do not necessarily translate to other species or to human physiology.
The materials and information described here are summarised for scientific and educational purposes only and relate to in-vitro and laboratory research. The compounds discussed are research chemicals intended exclusively for laboratory research use. They are not medicines, are not approved for human or veterinary use, and nothing in this review constitutes medical advice, a therapeutic claim, or any guidance on administration or dosing. Researchers are responsible for ensuring that all work is conducted in accordance with applicable United Kingdom legislation, institutional approvals and good laboratory practice.