Brain barriers as checkpoints in endocrine regulation of body homeostasis

Physiology of TH transport across brain barriers

Figure 2 illustrates that the BBB and BCSFB possess distinct structural and functional properties that fundamentally shape the pathways through which thyroid hormones access the brain. THs depend on specific peptide transporters to cross the BBB. ¹⁴ Key transporters identified to date include monocarboxylate transporter 8 (MCT8), organic anion-transporting polypeptide 1C1 (OATP1C1), and L-type amino acid transporters (LATs). Among these, MCT8 is the primary transporter, with THs serving as exclusive substrates. ¹⁶ At the BBB, MCT8 expressed in endothelial cells predominantly mediates the uptake of thyroxine (T4) from the circulation. Once inside the CNS, T4 is transported into astrocytes via OATP1C1 and converted to triiodothyronine (T3) by iodothyronine deiodinase 2 (DIO2). T3 is then taken up by neurons through MCT8, where it binds to nuclear receptors and regulates diverse metabolic processes. ¹⁵ OATP1C1, selectively expressed in the CNS, is localized mainly to capillary endothelial cells and the choroid plexus (ChP), and preferentially transports T4 and reverse T3. ³⁴ LAT1 and LAT2 also contribute to TH transport across the BBB and BCSFB. ³⁵ LAT1 is expressed in endothelial cells of the BBB and epithelial cells of the ChP, whereas LAT2 is primarily found in neurons and the ChP. ³⁶

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Figure 2.

Thyroid hormone transport and regulation of brain barriers. Thyroid hormones (T3 and T4) enter the brain through specialized brain barrier interfaces. At the BBB, transport is mediated by specific membrane transporters expressed in brain endothelial cells, including MCT8, OATP1C1, and LAT1/2. The BCSFB and CVOs provide additional routes for hormone exchange between the circulation, cerebrospinal fluid, and brain. Arrows indicate the direction of hormone movement. The right panel summarizes representative alterations in TH transport and barrier function under disease conditions.

Structural and functional analyses have shown that MCT8 and OATP1C1 are the principal, high-affinity TH transporters at the neurovascular interface, and mutations in these genes cause neurological disorders such as Allan–Herndon–Dudley syndrome. ³² Recent evidence has provided compelling in vivo confirmation of the essential role of endothelial MCT8 and OATP1C1 in mediating TH entry into the brain. In a 2025 study by Alevyzaki et al., ¹⁹ mice with conditional knockdown of Mct8 and Oatp1c1 specifically in endothelial cells display region-specific impairments in TH signaling. These animals exhibit reduced brain T3 and T4 levels, diminished expression of TH-responsive genes, and hypomyelination of cortical regions, particularly affecting GABAergic interneuron maturation. However, hippocampal T3 content and TH-regulated transcripts remain relatively preserved compared with that in global double-knockout (DKO) mice. This partial preservation suggests that, in addition to the BBB, alternative MCT8/OATP1C1-dependent routes, such as TH transport through the blood–CSF barrier, may provide a compensatory source of T3 to specific brain regions. Collectively, these findings highlight the roles played by endothelial MCT8 and OATP1C1 as primary pathways for TH entry into the CNS, while recognizing the apparent necessity of additional transport across the BCSFB to sustain complete cerebral TH homeostasis. ¹⁹

The BCSFB, located in the ChP, represents a developmentally early, functionally specialized interface that produces cerebral spinal fluid (CSF) and regulates solute entry into the brain. The ChP epithelium forms tight junctions over a fenestrated, highly vascularized stroma, enabling directional transport; during human gestation, features and transporter programs of the ChP emerge before the BBB and coincide with rising T4 exposure (approximately weeks 13–17), consistent with a role in perinatal TH availability.^37,38

Besides these canonical barriers, CVOs act as specialized neuroendocrine gateways, allowing hormones to directly enter or exit the CNS. In these areas, the BBB is absent or highly permeable, permitting controlled molecular exchange between the blood and neural tissue. ⁷ For example, hypothalamic peptide hormones traverse the median eminence (ME), a type of CVO, to enter the hypophyseal portal circulation and act on the anterior pituitary. ¹⁶ Through this mechanism, thyrotropin-releasing hormone (TRH) stimulates the secretion of thyroid-stimulating hormone (TSH), completing the hypothalamic–pituitary–thyroid feedback loop. ³¹

Alterations in TH transport at brain barriers in disease states

Alterations in TH transport across brain barriers under inflammatory conditions and thyroid dysfunction (including hypothyroidism) are summarized in Table 1, together with the relevant experimental findings and references.

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Table 1.

Alterations in hormone transport across brain barriers in disease states.

Hormone	Transport mechanism	Alteration in disease states	References
TH	MCT8 in ECs of BBB OATP1C1 in ECs of BBB and choroid plexus LATs (LAT1/LAT2) in BBB and BCSFB CVOs	Inflammation: Acute—↓ MCT8, OATP1C1, LAT1 Recovery phase—↑ MCT8, OATP1C1, LAT1 Hypothyroidism: ↑ BBB transport of neutral amino acids, glucose, and amino acids without leakage; increased transport capacity and endothelial membrane fluidity ↑ OATP1C1 Hyperthyroidism: ↓ OATP1C1	39,40 34,41
Insulin	Insulin receptor-mediated transcytosis (IR-A, IR-B) Clathrin-mediated endocytosis	Diabetes: ↑ Insulin uptake and binding in BECs CSF glucose reduction delayed Obesity/high-fat diet: ↑ Insulin receptor ↓ IR-mediated transcytosis ↓ IR internalization and signaling, altered IR-A/total IR ratio Alzheimer’s disease: ↓ IR mRNA and protein in brain microvessels ↓ Receptor phosphorylation and Akt signaling Aging: ↓ Insulin transport Inflammation: ↓ Insulin transport ↓ IR-mediated transcytosis	86,87 88–90 91–96 97–99
GCs	OATP influx in ECs of BBB P-gp efflux in ECs of BBB OATP2 influx in BCSFB CVOs	Oxidative stress: Abl/Src kinase activation → P-gp internalization (via caveolin-1 phosphorylation) ↓ P-gp efflux → ↑ GC accumulation in brain Pregnancy/fetal BBB: Antenatal GC → ↑ P-gp in fetal brain endothelium (timing dependent) → long-term BBB transport alterations	115–117 21
Ghrelin	GHSR GHSR-independent CVOs	Obesity: ↓ Transport Fasting: ↑ Transport ↑ Fenestrated capillaries and tanycytes ↑ Median eminence permeability Chronic social stress: ↑ Ghrelin entry	26,139,144,145 26,144,145 146,147

GCs: glucocorticoids; TH: thyroid hormones.

Tight junction integrity and permeability

A TH imbalance can affect the BBB through two primary mechanisms: first, by compromising the integrity of tight junctions, and second, by modulating the expression and distribution of specific nutrient transporters within brain barriers. ⁴³

Key BBB structural components, including collagen IV, integrin αVβ3, F-actin, claudin-5, and PECAM-1, were reported to be downregulated and disorganized in a rat model of propylthiouracil (PTU)-induced hypothyroidism, together with NHS-biotin leakage into brain tissue, indicating impaired barrier function and compromised BBB integrity.^44,45 In a canine model of hypothyroidism, levels of vascular endothelial growth factor (VEGF), a well-known promoter of vascular permeability, and S-100β, a biomarker of astrocytic brain injury, were markedly elevated at 12 and 18 months following iodine-131 treatment, reflecting increased BBB permeability in the hypothyroid state. ²³ Furthermore, serum levels of S-100β also showed a delayed increase at 18 months, occurring later than in the CSF. This delay indicates that elevated serum S-100β levels result from changes in BBB permeability, which permit the leakage of S-100β from the brain into the bloodstream. ⁴⁶

In patients with overt hypothyroidism, CSF levels of albumin and immunoglobulin G (IgG) are elevated, suggesting that reduced TH levels may compromise BBB integrity and alter protein transport. ⁴⁷ Preclinical studies have also suggested a relationship between TH deficiency and BBB permeability. Studies in Mct8/Dio2-knockout mice, which exhibit TH transporter dysfunction, show significant impairments in neurovascular unit integrity and increased transcytotic flux. ⁴⁸ Collectively, human and animal studies suggest a potential association between TH disorders and BBB disruption.

Modulation of nutrient transport systems

Altered peripheral TH levels can also modulate BBB transporter systems. For example, in methimazole-induced hypothyroid rats, brain uptake of D-β-hydroxybutyrate (BHB) is significantly reduced compared to controls, whereas the uptake of other nutrients, such as hexoses and basic amino acids, remains unchanged. ⁴⁹ Because BHB crosses the BBB via the monocarboxylate transporter (MCT) system, which includes the key thyroxine transporter, MCT8, these findings indicate that hypothyroidism may impair MCT function. Importantly, TH replacement was shown to restore BHB uptake, highlighting the reversible impact of a TH imbalance on BBB transport dysfunction.

In severe neonatal hypothyroidism, induced by maternal PTU treatment, BBB transport of glucose and amino acids was shown to markedly increase in the absence of evident barrier leakage, indicating transporter upregulation. ⁵⁰ Furthermore, T3 treatment alleviated these changes in nutrient transport across the BBB. In a subsequent study using the same rat model, transport capacity was enhanced and endothelial membrane fluidity was increased, suggesting that hypothyroidism modifies BBB kinetics and exposes latent transport pathways. ⁵¹ The alterations observed in disease states are also illustrated in Figure 2 in comparison with normal conditions.

Effects beyond the BBB

Beyond the BBB, other brain barrier structures, including the BCSFB and the BLB, may also be involved in TH disorders. For example, an analysis of CSF in patients with Hashimoto’s encephalopathy revealed albuminocytological dissociation (ACD) in 75%–80% of cases, indicating breakdown of the BCSFB. ⁵² In addition to TH imbalance, Hashimoto’s encephalopathy is thought to have an autoimmune basis, with antithyroid antibodies and other autoantibodies contributing to vascular and inflammatory injury, and the high frequency of ACD supports the notion that BCSFB dysfunction is a component of its pathophysiology. ⁵³ Although direct evidence for TH effects on the BLB is lacking, THs are critical for auditory system development and function. TH imbalances have been linked to sensorineural hearing loss, likely through disruption of auditory pathway maturation and ion channel expression in the inner ear. ⁵⁴ Experimentally induced hypothyroidism impairs cochlear development, alters endocochlear potential, and leads to degeneration of hair cells and spiral ganglion neurons, suggesting that TH‑dependent signaling in the cochlea and its microvasculature is essential for maintaining BLB integrity and normal hearing. These findings suggest that, beyond their known effects, TH imbalances may also contribute to auditory dysfunction by compromising the integrity of the BLB.^55,56 These findings suggest that, beyond their known effects, TH imbalances may also contribute to auditory dysfunction by compromising the integrity of the BLB. Further investigation is warranted to elucidate the potential link between TH imbalance and BLB integrity, with the aim of preventing auditory complications in patients with TH-related disorders.

Despite these insights, most studies on TH transporter regulation and brain barrier function are outdated, with limited recent mechanistic data. Considering the significant impact of BBB dysfunction on neurodegenerative diseases and mood disorders, future research should focus on elucidating how THs regulate barrier transport and integrity at the molecular level.

Physiology of insulin transport across brain barriers

The IR is a transmembrane glycoprotein that functions as a pivotal receptor tyrosine kinase, mediating intracellular signaling cascades in response to insulin binding. Within the CNS, IRs are expressed on multiple cell types, including neurons, pericytes, astrocytes, and endothelial cells.^61,62 Among these, transcriptomic analyses indicate that endothelial cells exhibit the highest IR expression levels in the human brain. ⁶³ Structurally, the IR is a heterotetramer composed of two extracellular α-subunits and two transmembrane β-subunits, the latter possessing intrinsic tyrosine kinase activity essential for downstream signaling. ⁶⁴ The α-subunit exists in two isoforms, IR-A and IR-B, distinguished by the presence of an additional exon in the IR-B variant. ⁶⁵ While both isoforms are expressed in astrocytes and endothelial cells, neurons predominantly express the IR-A isoform.^66,67 Insulin transport at the BBB is saturable and specific, indicating the involvement of receptor-mediated mechanisms.^20,68,69 It has long been assumed that insulin crosses the BBB primarily through receptor-mediated transcytosis involving the IR. Transcytosis typically involves three major steps: (1) receptor-mediated endocytosis at the luminal surface of brain BECs, (2) vesicle transfer across the cytoplasm, and (3) exocytosis at the abluminal surface. ⁷⁰

However, the specific nature of the endocytic pathway remains controversial. Endocytosis in BECs can occur through clathrin-coated pits, caveolae, or macropinocytotic vesicles. ⁷¹ Studies suggest that the mechanism of insulin endocytosis may vary depending on vascular bed and anatomical region. For example, clathrin-mediated endocytosis predominates in the microvasculature, whereas caveolin-1-dependent pathways are more prominent in macrovascular regions, such as the aorta.^72,73 This regional heterogeneity suggests that insulin trafficking mechanisms vary across brain regions and physiological conditions. A study using isolated mouse brain microvessels reported that clathrin-mediated mechanisms are primarily responsible for insulin binding at the luminal surface, whereas caveolin-mediated endocytosis may play a role in insulin transport in specific brain regions, such as the hypothalamus. ¹⁸ Furthermore, genetic and pharmacologic inhibition of caveolin-1 impairs insulin signaling in BECs,^74,75 whereas clathrin-mediated uptake is disrupted by hyperinsulinemia, which reduces clathrin-coated pit formation and insulin receptor internalization. ⁷⁶

Despite extensive evidence for endocytosis, it remains unclear whether the internalized vesicles actually complete transcytosis and deliver insulin into the brain parenchyma. Recent in vivo studies using IR-deficient endothelial cells (EndoIR-KO) and pharmacological IR antagonists (S961) have shown that insulin can still cross the BBB in the absence of IR signaling, challenging the concept that IR-mediated transcytosis is the exclusive delivery pathway.^77,78 These observations have prompted the hypothesis that insulin also utilizes non-IR-mediated transport pathways across the BBB, though the identity and functional relevance of such mechanisms remain poorly defined. Consistent with this possibility, a surface biotinylation-based proteomic study in a human cerebral microvascular endothelial cell line (hCMEC/D3) that readily detected transferrin receptor internalization (positive control) failed to detect internalization of the IR. While this does not exclude a role for IR-mediated transport in vivo, it underscores potential limitations of IR involvement in BBB insulin uptake and highlights the need to further explore alternative transport mechanisms. ⁷⁹

Transport through the BCSFB is considerably slower and less efficient than that across the BBB, underscoring the dominant route of the latter in insulin entry into the CNS. Insulin transport into the brain occurs primarily across the BBB, with only a limited contribution by the BCSFB.^80,81 Blood-borne radiolabeled insulin has also been shown to bind to CVOs, such as the ME and area postrema,^82–84 supporting limited signaling at these specialized sites, but providing little evidence that this represents a major route for insulin entry into the brain parenchyma. Interestingly, a recent study has identified IRs in tanycytes of the hypothalamic arcuate nucleus—a CVO—where they appear to gate feeding-state-dependent regulation of Agouti-related protein (AgRP)-expressing neurons. ⁸⁵ Importantly, in vivo studies using radiolabeled insulin coupled with electrophoresis-based detection techniques have confirmed that intact insulin crosses the BBB without undergoing degradation, providing direct evidence for functional trans-barrier transport. ¹⁸

Taken together, the current evidence indicates that multiple mechanisms contribute to insulin entry into the brain. Although evidence supports a contribution of IR-mediated transcytosis, insulin has also been detected independently of IR signaling, pointing to alternative, yet unidentified, pathways. ¹⁸ Identifying these alternative transporters remains critical for elucidating the regulation of brain insulin entry in health and disease. These physiological mechanisms are summarized in Figure 3.

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Figure 3.

Insulin transport and regulation of brain barriers. Insulin produced by the pancreas reaches the central nervous system through interactions with brain barrier interfaces. At the BBB, insulin binds to IRs expressed on brain endothelial cells and undergoes receptor-associated uptake and intracellular trafficking. Arrows in the BBB panel illustrate proposed transport routes and highlight the uncertainty regarding insulin delivery into the brain parenchyma. In addition to the BBB, insulin can interact with other barrier interfaces, including the BCSFB and CVOs. The right panel summarizes representative alterations in insulin transport and brain barrier function in disease states such as diabetes, obesity, aging, and Alzheimer’s disease.

Alterations in insulin transport at the brain barriers in disease states

Pathological conditions differentially regulate insulin transport across the BBB, either enhancing or impairing its delivery to the brain parenchyma. Disease-associated alterations in insulin transport across brain barriers are summarized in Table 1. These alterations likely reflect disease-specific changes in transporter expression, receptor activity, and/or barrier integrity. The following examples illustrate the diverse regulatory patterns observed in metabolic, neurodegenerative, aging-related, and inflammation-related disorders.

Regulation of tight junction integrity

Insulin exerts multifaceted effects on brain barrier function, extending beyond its classical metabolic role in modulating BBB structure, endothelial signaling, and nutrient transport. IR signaling is a key determinant of BBB integrity: upon insulin binding to the IR extracellular α-subunit, the β-subunit undergoes autophosphorylation, activating downstream pathways such as Akt and the mitogen-activated protein kinase (MAPK) cascade. ¹⁰⁰ Knockdown of IRs in hCMEC/D3 cells results in reduced expression of the tight junction proteins, claudin-5, occludin, and zonula occludens-1 (ZO-1), and glucose transporter 1 (GLUT1); increased expression of inflammatory adhesion molecules, including intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1); and impaired barrier integrity. ¹⁰¹ Conversely, insulin stimulation of BECs protects BBB function by activating the IR/AKT2/forkhead box protein O1 (FOXO1) signaling pathway, which promotes claudin-5 expression and enhances barrier integrity. ¹⁰² However, pathological conditions can reverse these protective effects. Chronic hyperinsulinemia disrupts tight junctions, increases permeability, and promotes neurovascular complications^102,103; an insulin deficiency, as observed in streptozotocin-induced diabetes, also similarly elevates BBB permeability and reduces or mislocalizes tight junction proteins. ¹⁰⁴ This barrier disruption is linked to elevated matrix metalloproteinase-2 (MMP-2) and MMP-9 activity, as supported by partial restoration of BBB integrity following pharmacologic inhibition of these enzymes. ¹⁰⁵ In addition, prolonged hyperinsulinemia promotes oxidative stress within the cerebrovascular endothelium, further compromising IR signaling and barrier function. This combination of receptor desensitization and reactive oxygen species (ROS) accumulation may exacerbate BBB dysfunction in metabolic disease states. ⁹⁰ Furthermore, chronic hyperinsulinemia impairs endothelial nitric oxide synthase (eNOS) signaling by reducing insulin-stimulated phosphorylation at Ser1177, a key activation site, thereby diminishing eNOS activity and contributing to cerebrovascular endothelial dysfunction. ⁷⁴

Modulation of nutrient transport systems

In addition to structural and metabolic regulation, insulin regulates nutrient transport at the BBB. ¹⁸ Early studies demonstrated that insulin enhances the transport of neutral amino acids, such as tyrosine and tryptophan, by modulating competitive substrate availability,^106,107 and increases the blood-to-brain delivery of molecules including leptin and azidothymidine via facilitative transporters.^86,108 IR knockdown selectively reduces GLUT1 expression, whereas LAT1 and transferrin receptor levels remain unchanged, indicating that IR-mediated insulin transport modulates specific transporter networks rather than broadly altering nutrient uptake. ¹⁰¹ Interestingly, despite these regulatory effects on transporter expression, glucose uptake into the brain parenchyma remains largely insulin-independent and is primarily mediated by GLUT1. ¹⁰⁹ EndoIR-KO studies confirm that, although insulin signaling is delayed in brain regions such as the hypothalamus and hippocampus, overall brain glucose uptake is unaffected. ¹¹⁰ This suggests that insulin dynamically regulates BBB metabolic capacity and transporter expression without being essential for basal glucose delivery.

Effects beyond the BBB

Overall, studies of insulin’s actions at brain barriers have largely focused on the BBB, with comparatively little attention given to the BCSFB, BLB, or CVOs. It is well established that chronic hyperinsulinemia disrupts tight junctions, potentiates inflammatory signaling, promotes MMP-mediated protein degradation, and induces oxidative stress; these processes are likely to similarly compromise the BCSFB, which relies on junctional proteins to maintain barrier integrity.^80,87,104 Moreover, diabetes has been shown to impair the BLB through hyperglycemia and insulin resistance, resulting in microenvironmental perturbations and sensory dysfunction.^111,112 Taken together, these findings suggest that, beyond its classical metabolic roles, insulin not only traverses brain barriers but also exerts significant influence on their maintenance, offering a broader perspective on its role in brain barrier physiology (Figure 3).

Physiology of GC transport across brain barriers

As highly lipophilic steroid hormones, GCs have traditionally been presumed to enter the CNS via passive diffusion. ¹¹⁰ However, accumulating evidence indicates that GCs not only diffuse into the CNS, but also interact with several transport systems at the BBB and ChP—including influx transporters, such as organic anion transporting polypeptide (OATP), and the efflux transporter, P-glycoprotein (P-gp)—in both mice and humans.^{109,111–114} By comparing hormone distribution in wild-type and Abcb1a-deficient mice (which lack the gene encoding the efflux transporter P-glycoprotein at the BBB and are commonly used to investigate transporter-mediated drug and hormone transport across the BBB) after systemically administering cortisol and corticosterone, Mason et al. demonstrated that P-gp impedes the entry of cortisol, but not corticosterone, into the mouse brain. ¹¹⁴ In contrast, Uhr et al. reported that both cortisol and corticosterone penetration are modulated by P-gp, based on studies in mice lacking both Abcb1a and Abcb1b. ¹¹² Additionally, these mice exhibit chronically reduced corticosterone levels and altered stress-related behaviors, indicating the relevance of P-gp in glucocorticoid homeostasis and HPA axis regulation. ¹¹⁵

GC transport also occurs in the pituitary gland, a CVO and a critical component of the HPA axis.^109,113 In situ brain perfusion studies with radiolabeled corticosterone and cortisol in mice have demonstrated the presence of influx mechanisms—including OATP2-mediated transport—that contribute to GC accumulation in the pituitary gland. ¹⁰⁹ Notably, no significant differences in corticosterone or cortisol distribution were observed in the pituitary gland between Abcb1a/b^−/− and wild-type mice, suggesting that the efflux transporter P-gp does not influence GC transport at this site.^109,113 These results highlight that the facilitative influx mechanism has the potential role in modulating GC levels and regulating the HPA axis.

Additionally, GC transport occurs across another CNS interface: the BCSFB. Similar to the case in CVOs, both corticosterone and cortisol are taken up into the ChP via a saturable influx mechanism involving multiple transporters, including OATP2. ¹⁰⁹ In humans, systemically administered cortisol has been shown to cross the blood into the CSF, indicating that cortisol can traverse the BCSFB. ¹¹⁶

Taken together, these observations indicate that, not only are GCs able to enter the CNS via passive diffusion, their access is also actively regulated by transporter systems, including OATPs and P-gp, at the BBB, CVOs, pituitary, and BCSFB. This allows precise control over steroid distribution under physiological conditions and suggests potential relevance in pathological states, with implications for brain barrier function and CNS homeostasis (Figure 4).

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Figure 4.

GC transport and regulation of brain barriers. GCs, produced by the adrenal glands, access the central nervous system through multiple brain barrier interfaces. At the BBB, GC entry occurs primarily through passive diffusion but can also be modulated by transporter systems expressed in brain endothelial cells, including OATPs and the efflux transporter P-gp. Arrows in the BBB panel indicate the proposed transport route between blood and brain parenchyma. In addition to the BBB, GCs can interact with other barrier interfaces, including the BCSFB and CVOs, contributing to hormone exchange between the circulation, CSF, and brain. The right panel summarizes representative alterations in GC transport and brain barrier function under pathological or developmental conditions.

Alterations in GC transport at the brain barriers in disease

GC transport into the CNS is altered under pathological conditions. Disease- and development-associated alterations in GC transport across brain barriers are summarized in Table 1. Under pathological conditions associated with acute oxidative stress, such as ischemia–reperfusion injury and inflammatory insults, exposure to reactive oxygen species (e.g. H₂O₂) activates Abl and Src kinases at the BBB. These kinases phosphorylate caveolin‑1 on tyrosine‑14, driving internalization of P‑gp together with caveolin‑1 and rapidly decreasing P‑gp‑mediated cortisol efflux, thereby increasing GC accumulation in the brain. ¹¹⁵ Given the widespread clinical use of GCs, these finding emphasize the importance of understanding how pathological conditions modify GC transport across the BBB.

Developmental regulation of GC transport at brain barriers

During physiological development, glucocorticoid exposure can modulate BBB properties, particularly in late gestation. Antenatal GC administration in pregnancies at risk of preterm delivery promotes lung maturation but also affects fetal brain microvessels, where cortisol and synthetic GCs such as dexamethasone increase P‑gp expression and activity in a gestational age‑dependent manner.^116,117 In guinea pig BECs, GC treatment in late gestation and early postnatal life upregulates Abcb1 mRNA and P‑gp protein, likely through changes in vesicular trafficking (e.g. enhanced Rab‑dependent recycling), leading to a persistent increase in efflux capacity at the developing BBB. ¹¹⁶ This sustained elevation of P‑gp may reduce fetal and neonatal brain exposure to both endogenous GCs and xenobiotics, with potential long‑term consequences for neurodevelopment and drug response. ²¹ These observations indicate that GC transport capacity is dynamically regulated during both disease and development, with implications for CNS exposure and barrier function.

Local metabolism of GC at brain barriers

Beyond transporter-mediated mechanisms, glucocorticoid exposure at brain barriers is also regulated by local steroid metabolism. The enzymes 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) and 11β-HSD2 interconvert active cortisol and inactive cortisone, thereby modulating the availability of biologically active glucocorticoids at the neurovascular interface. ¹¹⁸ 11β-HSD1 primarily functions as an 11β-reductase that regenerates cortisol from cortisone and amplifies intracellular glucocorticoid signaling. ¹¹⁹ This enzyme is widely expressed in forebrain regions such as the cortex and hippocampus and is also present in glial cells including microglia, where inflammatory activation can further upregulate 11β-HSD1 expression and enhance local cortisol regeneration. ¹²⁰

In contrast, 11β-HSD2 catalyzes the oxidative inactivation of cortisol to cortisone and is highly expressed during brain development, where it functions as a protective metabolic barrier limiting excessive glucocorticoid exposure to developing neural cells. ¹²¹ At brain barrier interfaces such as the BBB and BCSFB, differential expression of 11β-HSD isoforms is thought to regulate local cortisol–cortisone conversion and thereby influence the fraction of active glucocorticoid that reaches the CNS parenchyma. ¹²²

Through this enzymatic interconversion, brain barriers may not only influence the physical passage of glucocorticoids but also modulate the proportion of hormone that remains biologically active after entering the CNS. This metabolic layer of regulation therefore likely complements passive diffusion and transporter-mediated pathways in shaping glucocorticoid exposure within the brain, potentially influencing brain barrier physiology and function.

Physiology of ghrelin transport across brain barriers

Ghrelin can cross the BBB of the mouse brain in both periphery-to-brain and brain-to-periphery directions through saturable transport mechanisms, ¹³⁴ which have been studied in both directions using radio-labeled human ghrelin. ¹³⁵ Ghrelin exerts its effects by binding to growth hormone secretagogue receptor-1a (GHSR-1a), which is abundantly expressed in the CNS. ¹³⁶ However, studies using radiolabeled ghrelin have shown that BBB transport rates do not differ between wild-type and GHSR-null mice, suggesting that ghrelin crosses the BBB through a GHSR-independent mechanism. ¹³⁴

Ghrelin can also enter the brain via CVOs, which lack a conventional BBB and possess fenestrated capillaries. ¹⁷ Using real-time in vivo imaging, Schaeffer et al. demonstrated that intravenously (i.v.)-administered fluorescently labeled ghrelin (F-ghrelin) rapidly and passively extravasates through the fenestrated vasculature of the ME, a representative CVO. ²⁶ These fenestrated capillaries project toward the arcuate nucleus (ARC) of the ventromedial hypothalamus, facilitating rapid diffusion of ghrelin and mediating orexigenic effects.^26,137–140 After i.v. injection, F-ghrelin was observed in ME tanycytes, specialized ependymal cells lining the floor of the third ventricle that regulate molecular exchange between the blood, CSF, and hypothalamus, indicating the involvement of these cells in transporting ghrelin into the brain.^137,139 In vitro studies of tanycytes obtained from neonatal rats demonstrated that these cells internalize fluorescently labeled ghrelin via clathrin-mediated endocytosis. ¹⁴¹ Ghrelin transport has also been observed in the area postrema, another CVO with fenestrated capillaries, although higher peripheral concentrations of F-ghrelin were required for detectable uptake in this region.^137,138

In addition, ghrelin can be transported through the BCSFB, composed of the ChP and hypothalamic tanycytes, to reach the brain via the CSF. ¹⁴⁰ Peripherally administered F-ghrelin is detected in epithelial cells of the ChP and is selectively internalized into the CSF, predominantly in a GHSR-dependent manner, although GHSR-independent uptake has also been observed. ¹⁴² Once in the CSF, ghrelin diffuses rapidly and passively into periventricular brain regions to exert its central effects.^140,142 Within the CSF, F-ghrelin is subsequently internalized by hypothalamic tanycytes via clathrin-mediated endocytosis, contributing to a reduction in CSF ghrelin levels and shortening the duration of its central effects. ¹⁴¹

Collectively, these findings indicate that ghrelin transport into the brain involves multiple brain barrier systems: across the BBB via GHSR-independent mechanisms, through CVOs via tanycytes and clathrin-mediated endocytosis, and at the BCSFB via epithelial cell uptake. This suggests an integral role of brain barriers in regulating ghrelin access to the CNS. Building on this understanding, further investigations into how pathological conditions affect brain barrier function may provide new insights into the mechanisms linking ghrelin signaling with barrier integrity and CNS homeostasis (Figure 5).

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Figure 5.

Ghrelin transport and regulation of brain barriers. Ghrelin, a peptide hormone primarily produced in the stomach, can access the central nervous system through multiple brain barrier interfaces. At the BBB, ghrelin crosses the endothelium through a saturable transport mechanism, although the molecular identity of the transport system remains unclear. Arrows in the BBB panel indicate the proposed transport route into the brain parenchyma. In addition to the BBB, ghrelin can enter the brain through CVOs, which contain fenestrated capillaries, and through the BCSFB at the choroid plexus. These routes allow circulating ghrelin to reach the CSF and hypothalamic regions. The right panel summarizes representative alterations in ghrelin transport and brain barrier function under physiological or pathological conditions, including aging, obesity, fasting, and brain injury.

Alterations in ghrelin transport at brain barriers in disease states

Metabolic and stress-related conditions can alter ghrelin transport across brain barriers. Disease- and condition-associated alterations in ghrelin transport across brain barriers are summarized in Table 1.