Endogenous methionine enkephalin may play an anticonvulsant role in the seizure-susceptible El mouse

Viktor Mutt lecture: Peptides can cross the blood-brain barrier

The field of peptides exploded in the 1970's and has continued to be a major area of discovery. Among the early discoveries was that peptides administered peripherally could affect brain functions. This led Kastin to propose that peptides could cross the blood-brain barrier (BBB). Although initially very controversial, Kastin, I, and others demonstrated not only that peptides can cross the BBB, but elucidated many fundamental characteristics of that passage. That work was in large part the basis of the 2022 Viktor Mutt Lectureship. Here, we review some of the early work with current updates on topics related to the penetration of peptides across the BBB. We briefly review mechanisms by which peripherally administered peptides can affect brain function without crossing the BBB, and then review the major mechanisms by which peptides and their analogs have been show to cross the BBB: transmembrane diffusion, saturable transport, and adsorptive transcytosis. Saturable transport systems are adaptable to physiologic changes and can be altered by disease states. In particular, the transport across the BBB of insulin and of pituitary adenylate cyclase activating polypeptide (PACAP) illustrate many of the concepts regarding peptide transport across the BBB.

Anticonvulsant Met-enkephalin analogues containing backbone spacers reveal alternative non-opioid signaling in the brain

Prosthesis of non-critical parts of a polypeptide backbone is an attractive strategy to simplify bioactive peptides. This approach was applied to an opioid neuropeptide, Met-enkephalin, in which two adjacent Gly2-Gly3 residues were replaced with a series of non-peptidic backbone spacers varying in length and/or physicochemical properties. The backbone spacers did not affect the overall structural properties of the analogues, but they did dramatically reduce their affinities and agonist activities toward delta- and mu-opioid receptors. Molecular modeling suggested that the decrease of the affinity of Met-enkephalin to delta-opioid receptor could be accounted for by the loss of a single hydrogen bond. Remarkably, the analogues containing the most isostere spacers retained potent antinociceptive and anticonvulsant properties that were comparable to that of the endogenous peptide. This unexpected high in vivo potency could not be accounted for by an increase in metabolic stability. Moreover, the antiepileptic activity could not be reversed by opioid receptor antagonists. In summary, the results obtained with the analogues containing backbone spacers suggest a novel mechanism for seizure control in the brain that involves alternative non-opioid signaling.

The bacterial endotoxin lipopolysaccharide enhances seizure susceptibility in mice: involvement of proinflammatory factors: nitric oxide and prostaglandins

Central nervous system (CNS) inflammation in cases such as head trauma, infection and stroke has been associated with the occurrence of epileptic seizures. Microglia, the principal immune cells in the brain, readily become activated in response to injury, infection or inflammation. The bacterial endotoxin lipopolysaccharide (LPS) induces the activation of microglia and the production of proinflammatory factors including nitric oxide (NO) and prostaglandins (PGs). We examined the effect of LPS on seizure susceptibility of mice, by using the sensitive test, threshold of clonic seizures induced by i.v. infusion of pentylenetetrazole. LPS decreased the seizure threshold in a dose- and time-dependent manner. Pretreatment of mice with the NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester or cyclooxygenase inhibitor, piroxicam or the opioid receptor antagonist, (-)-naloxone completely reversed the proconvulsant effect of LPS. These results indicate that NO, PGs and endogenous opioid peptides seem to be involved in LPS-induced decrease in seizure threshold.

Enkephalin, PPE mRNA, and PTS-1 in alcohol withdrawal seizure-prone and -resistant mice

Inbred animal strains provide an opportunity to study genetic factors in alcoholism in the absence of environmental factors. Although the concentration of methionine enkephalin (Met-enkephalin) in whole brain has been implicated in the consumption of ethanol, it has not been studied in the brains of alcohol withdrawal seizure-prone (WSP) and withdrawal seizure-resistant (WSR) mice. We compared these concentrations with the levels of preproenkephalin (PPE) mRNA and with the activity of peptide transport system-1 (PTS-1), a brain-to-blood transport system for Met-enkephalin that is affected by ethanol. The concentrations of Met-enkephalin were significantly greater in WSP mice than in WSR mice, whereas synthesis of Met-enkephalin, as reflected by PPE mRNA levels, and transport out of the brain by PTS-1 was not different. These results support a direct link between elevated concentrations of Met-enkephalin in whole brain and proneness to withdrawal-induced seizures. We suggest that the inverse relationship between the consumption of ethanol and proneness to seizures in inbred mice can be explained through their opposite relationships to Met-enkephalin.

Passage of peptides across the blood-brain barrier: pathophysiological perspectives

Blood-borne peptides are capable of affecting the central nervous system (CNS) despite being separated from the CNS by the blood-brain barrier (BBB), a monolayer comprised of brain endothelial and ependymal cells. Blood-borne peptides can directly affect the CNS after they cross the BBB by nonsaturable and saturable transport mechanisms. The ability of peptides to cross the BBB to a meaningful degree suggests that the BBB may act as a modulatory pathway in the exchange of informational molecules between the brain and the peripheral circulation. The permeability of the BBB to peptides is a regulatory process affected by developmental, physiological, and pathological events. This regulation sets the stage for the relation between peptides and the BBB to be involved in pathophysiological events. For example, some of the classic actions of melanocortins on the CNS are explained by their abilities to cross the BBB, whereas aspects of feeding and alcohol-related behaviors are associated with the passage of other specific peptides across the BBB. The BBB should no longer be considered a static barrier but should be recognized as a regulatory interface controlling the exchange of informational molecules, such as peptides, between the blood and CNS.

HPLC/RIA analysis of bioactive methionine enkephalin content in the seizure-susceptible El mouse brain

We previously reported a deficit of methionine enkephalin-like immunoreactivity (ME-LI), in the cerebral cortex, septal area, hippocampus, and striatum and the abnormal metabolism of opioid peptides in the hippocampus and striatum of seizure-susceptible El mice, which are involved in the pathogenesis of seizures. However, these findings suggest that the ME-LI does not necessarily reflect the bioactive methionine enkephalin (ME). Herein, we measured the biologically active peptide, ME excluding cross-reactive substances by using HPLC coupled with radioimmunoassay to clarify the abnormal function of enkephalinergic neurons in the El mouse brain. The ME content in 25-day-old El mice that had no seizures was significantly decreased in the hippocampus and septal area, as compared with corresponding regions in ddY mice (seizure-nonsusceptible; the mother strain of El). At the age of 50 days when El mice displayed abortive seizures, this content in both stimulated El[s] and nonstimulated El[ns] was significantly reduced in the septal area and cerebral cortex. At the age of 150 days when El mice exhibit tonic-clonic seizures, this content in both El[s] and El[ns] was significantly reduced in the septal area, cerebral cortex and striatum. These findings were generally compatible with our previous findings. This study further supports our hypothesis that a deficit of anticonvulsant endogenous ME, in the cerebral cortex, septal area, and hippocampus of seizure-susceptible El mice play an important role in the pathogenesis of seizures.

Metabolic profile of opioid peptides differs in the hippocampus and striatum of seizure-susceptible E1 mice

We previously suggested that a deficit of anticonvulsant endogenous methionine enkephalin, in the cerebral cortex, septal area, hippocampus, and striatum of seizure-susceptible El mice plays a role in the pathogenesis of seizures. To determine whether a hypofunction of enkephalinergic neuron may be due to metabolic abnormalities of opioid peptides in the El mouse brain, we measured methionine enkephalin-like immunoreactivity (ME-LI) of 50 fractions eluted by high performance liquid chromatography obtained from those four regions of the brain of El and seizure-nonsusceptible ddY mice (the mother strain of El mice). We observed the same ME-LI patterns of 50 fractions in the cerebral cortex and septal area in El and ddY mice, whereas exhibited differing ME-LI patterns in the hippocampus and striatum in the two stains. Different ME-LI patterns may imply the difference in the metabolic profile of opioid peptides. Thus, an abnormal metabolism of opioid peptides in the hippocampus and striatum of the El mouse may be involved in the pathogenesis of seizures.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.