Peptide drug delivery into the central nervous system

Multifunctional Polymeric Nanoplatforms for Brain Diseases Diagnosis, Therapy and Theranostics

The blood-brain barrier (BBB) acts as a barrier to prevent the central nervous system (CNS) from damage by substances that originate from the blood circulation. The BBB limits drug penetration into the brain and is one of the major clinical obstacles to the treatment of CNS diseases. Nanotechnology-based delivery systems have been tested for overcoming this barrier and releasing related drugs into the brain matrix. In this review, nanoparticles (NPs) from simple to developed delivery systems are discussed for the delivery of a drug to the brain. This review particularly focuses on polymeric nanomaterials that have been used for CNS treatment. Polymeric NPs such as polylactide (PLA), poly (D, L-lactide-co-glycolide) (PLGA), poly (ε-caprolactone) (PCL), poly (alkyl cyanoacrylate) (PACA), human serum albumin (HSA), gelatin, and chitosan are discussed in detail.

Analgesic efficacy and safety of DALDA peptide analog delivery to the brain using oil-in-water nanoemulsion formulation

PURPOSE

The main objective of this study was to develop and evaluate therapeutic efficacy and safety following systemic delivery of a peptide analgesic into the CNS using an oil-in-water nanoemulsion system.

METHODS

We have formulated a safe and effective, omega-3 rich polyunsaturated fatty acid containing oil-in-water nanoemulsion formulation, for encapsulating and delivering chemically-modified DALDA, a potent mu-opioid peptide analogue, to the CNS. One of the challenges with CNS delivery is the lack of a non-invasive bioanalytical technique to confirm CNS uptake and therapeutic efficacy. Using blood oxygen-level dependent (BOLD) functional magenetic resonance imaging (fMRI), we provide quantitative evidence of nanoemulsion-based delivery and analgesic activity of DALDA analogue in capsaicin-induced awake rat model of pain.

RESULTS

Nanoemulsion formulation effectively encapsulated the modified analgesic peptide and demonstrated efficacy in the capsaicin- pain induced functional magnetic resonance imaging model in rodents. Preliminary safety evaluations show that the nanoemulsion system was well tolerated and did not cause any acute negative effects.

CONCLUSIONS

Overall, these results show tremendous opportunity for the development of modified peptide analgesic-encapsulated nanoemulsion formulations for CNS delivery and therapeutic efficacy.

CNS delivery and pharmacokinetic evaluations of DALDA analgesic peptide analog administered in Nano-sized oil-in-water emulsion formulation

PURPOSE

Although neuro-active peptides are highly potent as central nervous system (CNS) therapeutics, their systemic delivery across the blood-brain barrier (BBB) is limited due to lack of permeability in the brain and rapid systemic metabolism. In this study, we aimed at enhancing the brain delivery and stability of chemically modified [D-Arg(2), Lys(4)]-dermorphin-(1-4)-amide)] (DALDA) peptide to achieve prolonged analgesic effects.

METHODS

The C8-DALDA peptide analog was encapsulated in an oil-in-water nanoemulsion formulation made specifically with oils rich in omega-3 rich polyunsaturated fatty acid (PUFA) to enhance CNS availability. The nanoemulsion formulation was administered systemically in CD-1 mice and qualitative and quantitative biodistribution was evaluated. We have also examined the effect of curcumin, which is known to down-regulate efflux transporters and inhibit systemic metabolism, on the pharmacokinetic properties of the peptide.

RESULTS

Qualitative and quantitative biodistribution and pharmacokinetic studies in mice clearly demonstrated improved plasma and brain exposure of modified DALDA when administered in nanoemulsion, thereby providing an exciting opportunity towards improved efficacy and/or lowered dose of the peptide. The various dosing regimens tested for modified DALDA solution and curcumin nanoemulsion directed towards a novel combination strategy for improved systemic delivery of peptides across the BBB.

CONCLUSIONS

Encapsulation of the drug in PUFA nanoemulsion is an effective strategy for delivery of peptides. This work provides a novel combination strategy for improved delivery of peptides to the brain.

Designing prodrugs for the treatment of Parkinson's disease

INTRODUCTION

Current Parkinson's disease (PD) therapy is essentially symptomatic, and l-Dopa (LD), is the treatment of choice in more advanced stages of the disease. However, motor complications often develop after long-term treatment, and at this point physicians usually prescribe adjuvant therapy with other classes of antiparkinsonian drugs, including dopamine (DA) agonists, catechol-O-methyl transferase (COMT) or monoamine oxidase (MAO)-B inhibitors. In order to improve bioavailability, the prodrug approach appeared to be the most promising, and some antiparkinsonian prodrugs have been prepared in an effort to solve these problems.

AREAS COVERED

This review discusses the evidence of progress in PD therapy, mainly focused on prodrug approach for treatment of this neurological disorder. Several derivatives were studied with the aim of enhancing its chemical stability, water or lipid solubility, as well as diminishing the susceptibility to enzymatic degradation. Chemical structures mainly related to LD, DA and dopaminergic agonists are also reviewed in this paper.

EXPERT OPINION

In order to strengthen the pharmacological activity of antiparkinsonian drugs, enhancing their penetration of the blood-brain barrier (BBB), different approaches are possible. Among these, the prodrug approach appeared to be the most promising, and many prodrugs have been prepared in an effort to optimize physicochemical characteristics. In addition, novel therapeutic strategies based on formulations linking dopaminergic drugs with neuroprotective agents, increasing LD striatal levels and offering sustained release of the drug without any fluctuation of brain concentration, offer promising avenues for development of other effective new treatments for PD.

Anticonvulsant neuropeptides as drug leads for neurological diseases

Anticonvulsant neuropeptides are best known for their ability to suppress seizures and modulate pain pathways. Galanin, neuropeptide Y, somatostatin, neurotensin, dynorphin, among others, have been validated as potential first-in-class anti-epileptic or/and analgesic compounds in animal models of epilepsy and pain, but their therapeutic potential extends to other neurological indications, including neurodegenerative and psychatric disorders. Disease-modifying properties of neuropeptides make them even more attractive templates for developing new-generation neurotherapeutics. Arguably, efforts to transform this class of neuropeptides into drugs have been limited compared to those for other bioactive peptides. Key challenges in developing neuropeptide-based anticonvulsants are: to engineer optimal receptor-subtype selectivity, to improve metabolic stability and to enhance their bioavailability, including penetration across the blood–brain barrier (BBB). Here, we summarize advances toward developing systemically active and CNS-penetrant neuropeptide analogs. Two main objectives of this review are: (1) to provide an overview of structural and pharmacological properties for selected anticonvulsant neuropeptides and their analogs and (2) to encourage broader efforts to convert these endogenous natural products into drug leads for pain, epilepsy and other neurological diseases.

Progress in drug delivery to the central nervous system by the prodrug approach

This review describes specific strategies for targeting to the central nervous system (CNS). Systemically administered drugs can reach the brain by crossing one of two physiological barriers resistant to free diffusion of most molecules from blood to CNS: the endothelial blood-brain barrier or the epithelial blood-cerebrospinal fluid barrier. These tissues constitute both transport and enzymatic barriers. The most common strategy for designing effective prodrugs relies on the increase of parent drug lipophilicity. However, increasing lipophilicity without a concomitant increase in rate and selectivity of prodrug bioconversion in the brain will result in failure. In these regards, consideration of the enzymes present in brain tissue and in the barriers is essential for a successful approach. Nasal administration of lipophilic prodrugs can be a promising alternative non-invasive route to improve brain targeting of the parent drugs due to fast absorption and rapid onset of drug action. The carrier-mediated absorption of drugs and prodrugs across epithelial and endothelial barriers is emerging as another novel trend in biotherapeutics. Several specific transporters have been identified in boundary tissues between blood and CNS compartments. Some of them are involved in the active supply of nutrients and have been used to explore prodrug approaches with improved brain delivery. The feasibility of CNS uptake of appropriately designed prodrugs via these transporters is described in detail.

Antiparkinson prodrugs

Parkinson’s disease (PD) is a progressive, neurodegenerative disorder which involves the loss of dopaminergic neurons of the substantia nigra pars compacta. Current therapy is essentially symptomatic, and L-Dopa (LD), the direct precursor of dopamine (DA), is the treatment of choice in more advanced stages of the disease. Substitution therapy with LD is, however, associated with a number of acute problems. The peripheral conversion of LD by amino acid decarboxylase (AADC) to DA is responsible for the typical gastrointestinal (nausea, emesis) and cardiovascular (arrhythmia, hypotension) side effects. To minimize the conversion to DA outside the central nervous system (CNS) LD is usually given in combination with peripheral inhibitors of AADC (carbidopa and benserazide). In spite of that, other central nervous side effects such as dyskinesia, on-off phenomenon and end-of-dose deterioration still remain. The main factors responsible for the poor bioavailability and the wide range of inter- and intra-patient variations of plasma levels are the drug’s physical-chemical properties: low water and lipid solubility, resulting in unfavourable partition, and the high susceptibility to chemical and enzymatic degradation. In order to improve the bioavailability, the prodrug approach appeared to be the most promising and some LD prodrugs have been prepared in an effort to solve these problems. We report here a review of progress in antiparkinson prodrugs, focusing on chemical structures mainly related to LD, DA and dopaminergic agonists.

Neuropeptides as possible targets in sleep disorders

Insomnia and hypersomnia are frequent sleep disorders, and they are most often treated pharmacologically with hypnotics and wake-promoting compounds. These compounds act on classical neurotransmitter systems, such as benzodiazepines on GABA-A receptors, and amfetamine-like stimulants on monoaminergic terminals to modulate neurotransmission. In addition, acetylcholine, amino acids, lipids and proteins (cytokines) and peptides, are known to significantly modulate sleep and are, therefore, possibly involved in the pathophysiology of some sleep disorders. Due to the recent developments of molecular biological techniques, many neuropeptides have been newly identified, and some are found to significantly modulate sleep. It was also discovered that the impairment of the hypocretin/orexin neurotransmission (a recently isolated hypothalamic neuropeptide system) is the major pathophysiology of narcolepsy, and hypocretin replacement therapy is anticipated to treat the disease in humans. In this article, the authors briefly review the history of neuropeptide research, followed by the sleep modulatory effects of various neuropeptides. Finally, general strategies for the pharmacological therapeutics targeting the peptidergic systems for sleep disorders are discussed.

Rapid antidepressive-like activity of specific glycogen synthase kinase-3 inhibitor and its effect on beta-catenin in mouse hippocampus

BACKGROUND

Inhibition of glycogen synthase kinase-3 (GSK-3) is thought to be a key feature in the therapeutic mechanism of several mood stabilizers; however, the role of GSK-3 in depressive behavior has not been determined. In these studies, we evaluated the antidepressive effect of L803-mts, a novel GSK-3 peptide inhibitor, in an animal model of depression, the mouse forced swimming test (FST).

METHODS

Animals were intracerebroventricularly injected with L803-mts or with respective control peptide (cp) 1 hour, 3 hours, or 12 hours before their subjection to FST.

RESULTS

Animals administered L803-mts showed reduced duration of immobility at all three time points tested, as compared with cp-treated animals. Expression levels of beta-catenin, the endogenous substrate of GSK-3, increased in the hippocampus of L803-mts-treated animals by 20%-50%, as compared with cp-treated animals.

CONCLUSIONS

Our studies show, for the first time, that in-vivo inhibition of GSK-3 produces antidepressive-like behavior and suggest the potential of GSK-3 inhibitors as antidepressants.

Central nervous system effects of thyrotropin-releasing hormone and its analogues: opportunities and perspectives for drug discovery and development

Besides its well-known endocrine role in the thyroid system, thyrotropin-releasing hormone (L-pyroglutamyl-L-histidyl-L-prolinamide) has been long recognized as a modulatory neuropeptide. After a brief overview of the extrahypothalamic and receptor distribution, and of the neurophysiological, neuropharmacological and neurochemical effects of this tripeptide, this review discusses efforts devoted to enhance therapeutically beneficial central nervous system effects via structural modifications of the endogenous peptide. An enormous array of maladies affecting the brain and the spinal cord has been a potential target for therapeutic interventions involving agents derived from thyrotropin-releasing hormone as a molecular lead. Successful development of several centrally active analogues and recent accounts of efforts aimed at improving metabolic stability, selectivity and bioavailability are highlighted.

Prodrugs to enhance central nervous system effects of the TRH-like peptide pGlu-Glu-Pro-NH2

Potential prodrugs for the TRH-like tripeptide pGlu-Glu-Pro-NH(2) were synthesized either by esterifying the Glu side-chain of the parent peptide in solution with alcohols in the presence of resin-bound dicyclohexylcarbodiimide or by solid-phase peptide chemistry. Affinities of these ester prodrugs to lipid membranes as predictors of the transport across the blood-brain barrier were compared by immobilized artificial membrane chromatography, and prodrug activation was tested in the brain tissue of experimental animals. Esters of pGlu-Glu-Pro-NH(2) with long-chain primary alcohols emerged as potentially useful prodrugs to improve the central nervous system activity of pGlu-Glu-Pro-NH(2) upon systemic administration, as revealed by the enhancement of analeptic activity in mice.

The diverse roles of specific GLP-1 receptors in the control of food intake and the response to visceral illness

Intracerebroventricular administration of glucagon-like peptide-1 (7-36) amide (GLP-1) reduces food intake and produces symptoms of visceral illness, such as a conditioned taste aversion (CTA). The central hypothesis of the present work is that separate populations of GLP-1 receptors mediate the anorexia and taste aversion associated with GLP-1 administration. To test this hypothesis, we first compared the ability of various doses of GLP-1 to induce anorexia or CTA when administered into either the lateral or fourth ventricle. Lateral and fourth ventricular GLP-1 resulted in reduction of food intake at similar doses, whereas only lateral ventricular GLP-1 resulted in a CTA. Such data indicate that both hypothalamic and caudal brainstem GLP-1 receptors are likely to participate in the ability of GLP-1 to reduce food intake. We also hypothesized that the site that must mediate the ability of GLP-1 to induce visceral illness is in the central nucleus of the amygdala (CeA). Administration of 0.2 or 1.0 microg of GLP-1 (7-36) but not the inactive GLP-1 (9-36) resulted in a strong CTA with no accompanying anorexia. In addition, bilateral CeA administration of 2.5 microg of a GLP-1 receptor antagonist before intraperitoneal administration of the toxin lithium chloride resulted in a diminished CTA. Together, these data indicate that separate GLP-1 receptor populations mediate the multiple responses to GLP-1. These results indicate that GLP-1 is a flexible system that can be activated under various circumstances to alter the ingestion of nutrients and/or produce other visceral illness responses, depending on the ascending pathways of the GLP-1 system that are recruited.

In vitro stability and in vivo pharmacokinetic studies of a model opioid peptide, H-Tyr-D-Ala-Gly-Phe-D-Leu-OH (DADLE), and its cyclic prodrugs

In vitro stability and in vivo pharmacokinetic studies of a model opioid peptide, H-Tyr-D-Ala-Gly-Phe-D-Leu-OH (DADLE), and its cyclic prodrugs (acyloxyalkoxy-based cyclic prodrug of DADLE, coumarinic acid-based cyclic prodrug of DADLE, and oxymethyl-modified coumarinic acid-based cyclic prodrug of DADLE) were conducted. The enzymatic stability of DADLE and its prodrugs in various biological media was determined at 37 degrees C in the presence and absence of paraoxon, a known esterase inhibitor. The prodrugs exhibited metabolic stability to exo- and endopeptidases, and esterase-catalyzed bioconversion of the prodrugs to DADLE was observed. For pharmacokinetic studies in rats, various biological samples (blood, bile, urine, and brain) were collected after i.v. administration of DADLE and its prodrugs. The samples were analyzed by high-performance liquid chromatography with tandem mass spectrometric detection, and the conversion from the prodrugs to intermediates to DADLE was monitored. The prodrugs exhibited similar pharmacokinetic properties and showed improved stability compared with DADLE in rat blood. This increased stability led to higher plasma concentrations of DADLE after i.v. administration of the prodrugs compared with i.v. administration of DADLE alone. In terms of elimination pathways, metabolism by endopeptidases was the major route for DADLE elimination, whereas rapid biliary excretion was the major route of elimination for the prodrugs. The rapid elimination of the prodrugs by the liver and the formation of stable intermediates after esterase hydrolysis limited the bioconversion efficiencies of the prodrugs to DADLE after i.v. administration. The substrate activity of the prodrugs for efflux transporters (e.g., P-glycoprotein) in the blood-brain barrier significantly restricted their access to the brain.

Characterization of the efflux transporter(s) responsible for restricting intestinal mucosa permeation of an acyloxyalkoxy-based cyclic prodrug of the opioid peptide DADLE

PURPOSE

To elucidate the efflux transporter(s) responsible for restricting the permeation of an acyloxyalkoxy-based cyclic prodrug of the opioid peptide DADLE (AD) through Caco-2 cell monolayers.

METHODS

The cellular permeation characteristics of AD were investigated using Caco-2 cells, Madin-Darby canine kidney wild-type II cells (MDCK-WT), MDCK cells transfected with the human MDR1 gene (MDCK-MDR1), and MDCK cells transfected with the human MRP2 gene (MDCK-MRP2). These cells were grown as monolayers onto microporous membranes. The disappearance of AD from the donor side and its appearance on the receiver side were monitored by high-performance liquid chromatography. The substrate activity of AD for P-glycoprotein (P-gp) was determined using GF120918, a known P-gp specific inhibitor. The substrate activity of AD for MRP2 was determined by using cyclosporin A, a known MRP2 and P-gp inhibitor.

RESULTS

In Caco-2 cells, the ratio of the apparent permeability coefficients (Papp) of AD flux measured in the basolateral (BL) to apical (AP) direction vs. the flux in the AP-to-BL direction (Papp BL-to-AP/ Papp AP-to-BL) was 99. In the presence of 2 microM GF120918 or 25 microM cyclosporin A. the Papp BL-to-AP/Papp AP-to-BL ratio was decreased to 11. In MDCK-WT, MDCK-MDR1, and MDCK-MRP2 cells, the Papp BL-to-AP/Papp AP-to-BL ratios of AD were 4.7, 10, and 5.8, respectively. A mixture of GF120918 (2 microM) and cyclosporin A (25 microM) decreased the Papp BL-to-AP/Papp AP-to-BL ratios of AD in MDCK-WT, MDCK-MDR1, and MDCK-MRP2 cells to 1.2,1.8, and 2.3, respectively.

CONCLUSIONS

These data suggest that AD is a much better substrate for P-gp than MRP2 and that the restricted permeation of this cyclic prodrug in Caco-2 cells and in the intestinal mucosa probably is due primarily to its substrate activity for P-gp.

Integration of mass spectrometry into early-phase discovery and development of central nervous system agents

The early-phase discovery and development of useful central nervous system (CNS) agents present ample opportunities to exploit mass spectrometry and provide detailed compound/mixture characterization, or to make the process faster and/or more economic. Neuropeptide FF antagonists and centrally active thyrotropin-releasing hormone analogues were used as specific examples in this work. We evaluated the characterization of focused libraries of peptide derivatives by electrospray ionization, tandem mass spectrometry and liquid chromatography/tandem mass spectrometry on a quadrupole ion trap and nanoelectrospray on a Fourier transform ion cyclotron resonance mass spectrometer. Immobilized artificial-membrane chromatography was employed as a model to predict/rank new agents against lead compounds for their potential to reach the central nervous system in pharmacologically significant amounts. Measuring brain concentrations in rodents after the intravenous administration of test compounds was used as an in vivo approach, and we took advantage of microdialysis sampling that furnished samples without interfering tissue matrix and afforded the estimation of extracellular concentrations in a localized part of the brain. Overall, making atmospheric-pressure ionization mass spectrometry an integral part of the process has played a major role in increasing throughput, selectivity, specificity and detection sensitivity and thereby providing useful information about the extent or mechanism of transport and metabolic activation/inactivation in early-phase discovery and development of CNS agents.

Uptake and efflux of the peptidic delta-opioid receptor agonist

P-glycoprotein (P-gp) and organic anion transporting polypeptides (Oatp) are expressed at the blood-brain barrier (BBB). There is little functional evidence for Oatp-mediated transport at the BBB. The peptidic delta opioid-receptor agonist [D-penicillamine(2,5)]-enkephalin (DPDPE) is a substrate of mdr1a P-gp and Oatp2. The present study evaluated the influence of these transporters on brain uptake of DPDPE by in situ perfusion in mice. Brain uptake was increased approximately 12-fold in mice lacking P-gp in the BBB, but the P-gp inhibitor dexverapamil did not increase uptake in P-gp-competent mice. In P-gp-deficient mice, DPDPE uptake was saturable (K(m) approximately 24 mM), and was inhibited by dexverapamil and the Oatp2 substrates digoxin, estradiol-17beta-glucuronide and fexofenadine. These results confirm P-gp-mediated efflux of DPDPE, and suggest functional uptake transport of DPDPE by Oatp, at the murine BBB.

Targeting drugs to the brain by redox chemical delivery systems

Chemical delivery systems (CDSs) based on the redox conversion of a lipophilic dihydropyridine to an ionic, lipid-insoluble pyridinium salt have been developed to improve the access of therapeutic agents to the central nervous system. A dihydropyridinium-type CDS or a redox analog of the drug is sufficiently lipophilic to enter the brain by passive transport, then undergoes an enzymatic oxidation to an ionic pyridinium compound, which promotes retention in the CNS. At the same time, peripheral elimination of the entity is accelerated due to facile conversion of the CDS in the body. This review discusses chemical, physicochemical, biochemical, and biological aspects in relation to the principles and practical implementation of the redox brain-targeting approach to various classes of drugs. Representative examples to the brain-enhanced delivery of neurotransmitters, steroids, anticonvulsants, antibiotics, antiviral, anticancer and antidementia agents, and neuropeptides and their analogs are presented in detail. In vivo and in vitro studies and preliminary clinical data of several novel derivatives have been promising, which could lead to a practical use of the redox CDSs after proper pharmaceutical development. The investigations accentuate the need for considering physicochemical, metabolic, and pharmacokinetic properties in designing of carrier systems that are able to target drugs into the central nervous system.

Synthesis and biological studies of glycosyl dopamine derivatives as potential antiparkinsonian agents

A new approach to deliver dopamine into the central nervous system, based on the use of D-glucose as transportable agent, has been studied. Glycosyl dopamine derivatives bearing the sugar moiety linked to either the amino group or the catechol ring of dopamine through amide, ester or glycosidic bonds were synthesised as potential antiparkinsonian agents. Studies on the binding to dopamine D2 receptor, in vitro stability, and locomotive effect in mice of the synthetic glycoconjugates are reported.

Neuropeptides in drug research

Neuropeptides have been a subject of considerable interest in the pharmaceutical industry over the last 20 years or more. Many drug discovery teams have contributed to our understanding of neuropeptide biology but no significant drugs that act selectively upon neuropeptide receptors have yet emerged from the clinic. There are, however, a plethora of clinically useful drugs that act at other classes of neurotransmitter and neuromodulator receptors, many of them discovered over the last 20 years. Nevertheless, we think that the future for the discovery of novel drugs acting at neuropeptide receptors looks bright for two reasons: (1) there has been a substantial increase in our understanding of the function of neuropeptides; and (2) high-throughput screening (HTS) against neuropeptide receptors has now begun to yield many interesting drug-like molecules, rather than peptides, that have the potential to become clinically useful drugs. The objective of this review is to summarise our current understanding of specific areas of neuropeptide biology and pharmacology in the CNS as well as the PNS. We will also speculate on where we think the new generation of neuropeptide agonists and antagonists could emerge from the clinic.

Do centrally administered neuropeptides access cognate receptors?: an analysis in the central corticotropin-releasing factor system

To determine the extent to which centrally administered corticotropin-releasing factor (CRF) activates neurons that express CRF receptors (CRF-Rs), we followed the kinetics and distribution (relative to those of CRF-Rs) of Fos induction seen in response to intracerebroventricular (icv) injection of the peptide (1-10 microg). CRF provoked widespread Fos expression: its strength was dose-related, it peaked at 2 hr after injection, and it was antagonized in a dose-dependent manner by coinjection of CRF-R antagonists. The activation pattern closely mimicked the distribution of CRF-R1 mRNA, in including widespread Fos induction throughout the cortical mantle, in cell groups involved in sensory information processing, and in the cerebellum and several of its major afferents and targets. Dual labeling revealed extensive correspondence of CRF-stimulated Fos-immunoreactivity (Fos-ir) and CRF-R1 mRNA at these and other loci. Unique sites of CRF-R2 expression were relatively unresponsive to CRF but were more so after icv administration of urocortin (UCN), a new mammalian CRF-related peptide. Both CRF and UCN elicited activational responses in cell groups that are involved in central autonomic control but that express neither CRF-R, including the central amygdaloid and paraventricular hypothalamic nuclei, and brainstem catecholaminergic cell groups. The results support an ability of CRF-related peptides in the ventricular system to access receptor-expressing cells directly but leave open questions as to the basis for the recruitment of central autonomic structures, many of which have been identified as stress-related sites of CRF action.