Nervous System Site Of Action Research Articles

Patterns of Hysteresis Between Induction and Emergence of Neuroanesthesia Are Present in Spinal and Intracranial Surgeries.

Recovery of consciousness is usually seen as a passive process, with emergence from anesthesia depicted as the inverse process of induction resulting from the elimination of anesthetic drugs from their central nervous system sites of action. However, that need not be the case. Recently it has been argued that we might encounter hysteresis to changes in the state of consciousness, known as neural inertia. This phenomenon has been debated in neuroanesthesia, as manipulation of the brain might further influence recovery of consciousness. The present study is aimed at assessing hysteresis between induction and emergence under propofol-opioid neuroanesthesia in humans using estimated propofol concentrations in both spinal and intracranial surgeries. We identified the moments of loss (LOR) and recovery of responsiveness (ROR) in 21 craniotomies and 25 spinal surgeries. Propofol was given slowly until loss of responsiveness and stopped at the end of surgery. An opioid was present at induction and recovery. Propofol infused was recorded and plasma and effect-site concentrations were estimated using 2 pharmacokinetic models. Dose-response curves were generated. Estimated propofol plasma and effect-site concentrations were compared to assess hysteresis. Estimated propofol concentrations at LOR and ROR showed hysteresis. Whether for spinal or intracranial surgeries, the EC50 of propofol at which half of the patients entered and exited the state of responsiveness was significantly different. Hysteresis was observed between propofol concentrations at LOR and ROR, in both patients presenting for spinal and intracranial surgeries. Manipulation of the brain does not appear to change patterns of hysteresis, suggesting that neural inertia may occur in humans, in a way similar to that found in animal species. These findings justify performing a clinical study in patients using measured propofol concentrations to assess neural inertia.

Induction of Glucose Intolerance by Acute Administration of Rimonabant

Background/Aims: Rimonabant is a cannabinoid CB1 receptor antagonist. Other CB1 antagonists have biphasic effects on blood glucose levels following acute administration. We therefore tested the effects of rimonabant on glucose tolerance following acute administration. Methods: We tested the effects of oral and intracerebroventricular administration of rimonabant on blood glucose and gastrointestinal transit in mice following oral and intravenous glucose challenge. Results: We found a dose-dependent increase in blood glucose from oral doses of rimonabant of 3 mg/kg and above. WIN55,212-2 (3 mg/kg), a cannabinoid receptor agonist, did not influence blood glucose in the presence or absence of rimonabant. Rimonabant did not induce release of glucose from isolated rat hepatocytes or modify serum insulin concentration in mice. Intracerebroventricular administration of rimonabant caused increases in blood glucose and gastrointestinal transit, suggesting a central nervous system site of action. Increases in blood glucose by rimonabant were partially blocked by the dopamine receptor antagonist haloperidol and significantly blocked by the 5-hydroxytryptophan (5-HT) depleting agent p-CPA and the 5-HT₃ receptor antagonist ondansetron. Conclusions: Rimonabant causes a dose-dependent increase in glucose profile upon glucose challenge partially mediated by the central nervous system control of gastrointestinal carbohydrate absorption through pathways that are modulated by both 5-HT and dopamine.

Buprenorphine added to levobupivacaine enhances postoperative analgesia of middle interscalene brachial plexus block

The aim of this study was to assess whether addition of epineural buprenorphine prolonged postoperative analgesia of middle interscalene brachial plexus block (MIB) with levobupivacaine. One hundred and fifty consenting adult patients, scheduled for shoulder arthroscopic surgery for a rotator cuff tear under MIB with 29.5 ml of 0.75 % levobupivacaine, were randomized to receive additionally either saline or intramuscular buprenorphine 0.15 mg or epineural buprenorphine 0.15 mg. Onset of sensory and motor blocks, duration of postoperative analgesia, and consumption of postoperative analgesics were compared among the groups. There were significant (P < 0.05) differences in the onset and the duration of the sensory block and in the duration of postoperative analgesia. Duration of both sensory block and postoperative analgesia was longer (P < 0.05) in patients who had received epineural buprenorphine (856.1 ± 215.2 and 1,049.7 ± 242.2 min) than in patients who had received intramuscular buprenorphine (693.6 ± 143.4 and 820.3 ± 335.3 min) or saline (488.3 ± 137.6 and 637.5 ± 72.1 min). Requirement of postoperative rescue analgesics was lower in the epineural buprenorphine group than in the other two groups. Few complications occurred from MIB (<1 %) and none from buprenorphine. Epineural buprenorphine prolonged postoperative analgesia of MIB more effectively than intramuscular buprenorphine, which suggests that buprenorphine acts at a peripheral nervous system site of action.

Eribis Peptide 94 Reduces Infarct Size in Rat Hearts Via Activation of Centrally Located μ Opioid Receptors

Eribis peptide 94 (EP 94) is a novel enkephalin derivative that binds with high potency to μ and δ opioid receptors with less affinity for the κ opioid receptor. This compound has recently been shown to produce an acute reduction in myocardial infarct size in the anesthetized pig and rat partially via an endothelial nitric oxide synthase and KATP channel-dependent mechanism. EP 94 also was found to produce a chronic reduction in infarct size 24 hours postdrug administration via the upregulation of inducible nitric oxide synthase in rats. Despite these findings, no data have emerged in which the opioid receptor subtype responsible for cardioprotection has been identified and the site of action, heart, other peripheral organs, or the central nervous system, has not been addressed. In the current study, EP 94, was administered in 2 divided doses (0.5 μg/kg, intravenously) at 5 and 10 minutes into the ischemic period, and the opioid antagonists were administered 10 minutes before the onset of the 30-minute ischemic period. The selective antagonists used were the μ receptor antagonist CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2), the δ receptor antagonists naltrindole and BNTX (7-benzylidenenaltrexone), and the κ receptor antagonist nor-BNI (norbinaltorphimine). Surprisingly, only CTOP completely blocked the cardioprotective effect of EP 94, whereas naltrindole, BNTX, and nor-BNI had modest but nonsignificant effects. Because there is controversial evidence suggesting that μ receptors may be absent in the adult rat myocardium, it was hypothesized that the protective effect of EP 94 may be mediated by an action outside the heart, perhaps in the central nervous system. To test this hypothesis, rats were pretreated with the nonselective opioid antagonist, naloxone hydrochloride, which penetrates the blood-brain barrier or naloxone methiodide, the quaternary salt of naloxone hydrochloride, which does not penetrate the blood-brain barrier before EP 94 administration. In support of a central nervous system site of action for EP 94, naloxone hydrochloride completely blocked its cardioprotective effect, whereas naloxone methiodide had no effect. These results suggest that EP 94 reduces infarct size (expressed as a percent of the area at risk) in the rat primarily via activation of central μ opioid receptors.

A novel δ opioid receptor‐mediated enhancement of GABAA receptor function induced by stress in ventral tegmental area neurons

Opioid receptors are G-protein-coupled receptors (GPCRs) that modulate synaptic function. Depending upon their nervous system site of action, opioid receptor agonists alter food consumption, pain perception, responses to stress, and drug reward. Opioid receptors signal primarily via G(i/o)-proteins that modulate ion channels to directly inhibit neurons or decrease neurotransmitter release from nerve terminals. Here we report that following stress, activating δ opioid receptors (DORs) on midbrain ventral tegmental area (VTA) neurons causes a novel synaptic effect: the augmentation of GABA(A) receptor (GABA(A)R)-mediated inhibitory postsynaptic currents. Most neurons showing this augmentation were identified as dopaminergic. In addition, in both stressed and unstressed animals, DOR activation decreases GABA(A)R currents in some VTA neurons. Surprisingly, both augmentation and inhibition were also observed when we bypassed the presynaptic terminal by iontophoretically applying GABA, indicating that postsynaptic mechanisms are responsible for both effects. Using a variety of blockers we determined that the augmentation is probably due to insertion of GABA(A)Rs into the synapse by a mechanism that is G-protein independent and mediated by activation of Akt via PI3K. GABA(A)Rs are inserted into the extra-synaptic plasma membrane before trafficking to the synapse, a mechanism consistent with our observation that the DOR-mediated increase in GABA(A)R signalling occurs significantly earlier in iontophoretically applied than in electrically evoked synaptic GABA. This G-protein-independent signalling pathway is not only a novel mechanism of opioid receptor-mediated inhibition, but it also represents the first reported link between activation of a GPCR and insertion of GABA(A)Rs into the plasma membrane.

Pharmacological glycerol-3-phosphate acyltransferase inhibition decreases food intake and adiposity and increases insulin sensitivity in diet-induced obesity

Storage of excess calories as triglycerides is central to obesity and its associated disorders. Glycerol-3-phosphate acyltransferases (GPATs) catalyze the initial step in acylglyceride syntheses, including triglyceride synthesis. We utilized a novel small-molecule GPAT inhibitor, FSG67, to investigate metabolic consequences of systemic pharmacological GPAT inhibition in lean and diet-induced obese (DIO) mice. FSG67 administered intraperitoneally decreased body weight and energy intake, without producing conditioned taste aversion. Daily FSG67 (5 mg/kg, 15.3 μmol/kg) produced gradual 12% weight loss in DIO mice beyond that due to transient 9- to 10-day hypophagia (6% weight loss in pair-fed controls). Continued FSG67 maintained the weight loss despite return to baseline energy intake. Weight was lost specifically from fat mass. Indirect calorimetry showed partial protection by FSG67 against decreased rates of oxygen consumption seen with hypophagia. Despite low respiratory exchange ratio due to a high-fat diet, FSG67-treated mice showed further decreased respiratory exchange ratio, beyond pair-fed controls, indicating enhanced fat oxidation. Chronic FSG67 increased glucose tolerance and insulin sensitivity in DIO mice. Chronic FSG67 decreased gene expression for lipogenic enzymes in white adipose tissue and liver and decreased lipid accumulation in white adipose, brown adipose, and liver tissues without signs of damage. RT-PCR showed decreased gene expression for orexigenic hypothalamic neuropeptides AgRP or NPY after acute and chronic systemic FSG67. FSG67 given intracerebroventricularly (100 and 320 nmol icv) produced 24-h weight loss and feeding suppression, indicating contributions from direct central nervous system sites of action. Together, these data point to GPAT as a new potential therapeutic target for the management of obesity and its comorbidities.

CHO cell expression, long‐term stability, and primate pharmacokinetics and brain uptake of an IgG–paroxonase‐1 fusion protein

Paraoxonase (PON)-1 is the most potent human organophosphatase known, but recombinant forms of human PON1 have been difficult to produce owing to poor secretion by host cells. In the present investigation, human PON1 is re-engineered as an IgG-PON1 fusion protein. The 355 amino acid human PON1 is fused to the carboxyl terminus of the heavy chain of a chimeric monoclonal antibody (MAb) against the human insulin receptor (HIR), and this fusion protein is designated HIRMAb-PON1. The HIRMAb part of the fusion protein enables brain penetration of the PON1, which was considered important, because organophosphate toxicity causes death via a central nervous system site of action. A high producing line of stably transfected Chinese hamster ovary (CHO) cells secreting the HIRMAb-PON1 fusion protein in the absence of serum or lipid acceptors was cloned. The bioreactor generated fusion protein was purified to homogeneity with low impurities by protein A affinity chromatography and anion exchange chromatography. The HIRMAb-PON1 fusion protein was stable as a sterile liquid formulation stored at 4°C for at least 1 year. The plasma pharmacokinetics (PK) of the HIRMAb-PON1 fusion protein was evaluated in Rhesus monkeys, which is the first PK evaluation of a recombinant PON1 protein. The fusion protein was rapidly removed from blood, primarily by the liver. The blood-brain barrier permeation of the HIRMAb-PON1 fusion protein was high and comparable to other HIRMAb fusion proteins. Re-engineering human PON1 as the HIRMAb fusion protein allows for production of a stable, field-deployable formulation of the enzyme that is brain-penetrating.

Ghrelin: Central Nervous System Sites of Action in Regulation of Energy Balance

Ghrelin, a peptide hormone secreted by the stomach, has been shown to regulate energy homeostasis by modulating electrical activity of neurons in the central nervous system (CNS). Like many circulating satiety signals, ghrelin is a peptide hormone and is unable to cross the blood-brain barrier without a transport mechanism. In this review, we address the notion that the arcuate nucleus of the hypothalamus is the only site in the CNS that detects circulating ghrelin to trigger orexigenic responses. We consider the roles of a specialized group of CNS structures called the sensory circumventricular organs (CVOs), which are not protected by the blood-brain barrier. These areas include the subfornical organ and the area postrema and are already well known to be key areas for detection of other circulating hormones such as angiotensin II, cholecystokinin, and amylin. A growing body of evidence indicates a key role for the sensory CVOs in the regulation of energy homeostasis.

Cannabinoid-1 receptor inverse agonists: current understanding of mechanism of action and unanswered questions

Rimonabant and taranabant are two extensively studied cannabinoid-1 receptor (CB1R) inverse agonists. Their effects on in vivo peripheral tissue metabolism are generally well replicated. The central nervous system site of action of taranabant or rimonabant is firmly established based on brain receptor occupancy studies. At the whole-body level, the mechanism of action of CB1R inverse agonists includes a reduction in food intake and an increase in energy expenditure. At the tissue level, fat mass reduction, liver lipid reduction and improved insulin sensitivity have been shown. These effects on tissue metabolism are readily explained by CB1R inverse agonist acting on brain CB1R and indirectly influencing the tissue metabolism through the autonomic nervous system. It has also been hypothesized that rimonabant acts directly on adipocytes, hepatocytes, pancreatic islets or skeletal muscle in addition to acting on brain CB1R, although strong support for the contribution of peripherally located CB1R to in vivo efficacy is still lacking. This review will carefully examine the published literature and provide a perspective on what new tools and studies are required to address the peripheral site of action hypothesis.

Centrally acting drugs for erectile dysfunction: Do they have a future?

Initial placebo-controlled trials with sublingual apomorphine showed promising results for the drug as an option in the pharmacologic management of erectile dysfunction (ED). More recent studies propose poor erectile effects by apomorphine sublingual in patients with diabetes and fewer benefits than sildenafil in patients with ED. In June this year, the European Medicines Agency declared that due to commercial reasons, the marketing authorization for Uprima (Abbott Laboratories, Abbott Park, IL) was not renewed, and the drug is no longer available in the European Union. This does not mean that receptor functions in the central nervous system are uninteresting pharmacologic targets for ED. Experiences with apomorphine sublingual in humans should be acknowledged for a more careful preclinical and clinical characterization of agents with a central nervous system site of action. This article focuses on information obtained from human trials of central acting drugs for ED.

Differential Anaesthetic Effects following Microinjection of Thiopentone and Propofol into the Pons of Adult Rats: A Pilot Study

Identifying the central nervous system sites of action of anaesthetics is important for understanding the link between their molecular actions and clinical effects. The aim of the present pilot study was to compare the anaesthetic effect of bilateral microinjections of propofol and thiopentone (both 200 microg/microl, in Intralipid and 0.9% saline respectively) into a recently discovered anaesthetic-sensitive region in the rat brainstem, the "mesopontine tegmental anaesthetic area" (MPTA). Microinjections (1 microl per side) were made into the MPTA of fifteen male Sprague-Dawley rats. The effect of each agent on spontaneous behaviour, postural control and nociceptive responsiveness was subjectively assessed according to established criteria. The main finding was that thiopentone induced an "anaesthesia-like" state, including complete atonia and loss of righting ability, in 20% of the subjects. Overall, thiopentone significantly reduced postural control and had a moderate antinociceptive effect compared to saline microinjections (P < 0.01 and 0.05, respectively, Wilcoxon test). In contrast, propofol did not induce "anaesthesia" in any animal tested, although a similar antinociceptive effect to that of thiopentone was observed (P < 0.05, Wilcoxon test). In summary, propofol and thiopentone have different effects when microinjected into the MPTA. While both agents reduced reflex withdrawal to a nociceptive stimulus, only thiopentone induced an "anaesthesia-like" state.

Relationship between injection duration, transporter occupancy and reinforcing strength of cocaine

Among drugs that can function as positive reinforcers, slower occupancy of central nervous system sites of action has been associated with diminished reinforcing strength. The present study examined the relative reinforcing strength of cocaine, and the rate of in vivo dopamine transporter binding, as a function of injection duration. Rhesus monkeys ( N=5) were allowed to self-administer cocaine under a progressive-ratio schedule with doses injected over different times (10–600 s). An ex vivo dopamine transporter binding assay was used to examine kinetics of in vivo transporter occupancy by cocaine injected over the same times in rats. Cocaine was a weaker reinforcer, and dopamine transporter binding rate decreased, with slower injections. Maximum transporter binding was the same across injection durations. These results support the hypothesis that slower onset of action is associated with a slower transporter occupancy and diminished reinforcing strength. Relative strength as a reinforcer may not be determined by maximum occupancy, at least not exclusively.

Role of the hypothalamic-pituitary-adrenal axis in the suppression of luteinizing hormone release by delta-9-tetrahydrocannabinol.

The ability of cannabinoids to affect anterior pituitary luteinizing hormone (LH) secretion has been largely attributed to a central nervous system site of action, however, the mechanism(s) by which cannabinoids alter LH release remains unclear. In the present study, the acute administration of delta-9-tetrahydrocannabinol (THC) produced a dose-related suppression of plasma LH and stimulation of adrenocorticotropin (ACTH) levels in ovariectomized female rats. To determine if activation of the hypothalamic-pituitary-adrenal axis was involved in the ability of THC to inhibit LH release, female rats were either pretreated with the corticotropin-releasing hormone (CRH) receptor antagonist, alpha-helical CRH, or were adrenalectomized prior to acute THC administration, in order to assess the roles of CRH and corticosterone in the ability of THC to suppress LH secretion. A low dose of THC (0.5 mg/kg b.w., iv) produced a decrease in plasma LH levels at 20 and 40 min posttreatment in ovariectomized, sham adrenalectomized rats. However, in adrenalectomized animals, plasma LH levels were suppressed at 40 min and remained decreased at 80 min following THC administration. Thus, the duration of LH suppression following THC treatment was significantly increased in adrenalectomized versus sham adrenalectomized rats (p < 0.05). Furthermore, pretreatment with the CRH receptor antagonist, alpha-helical CRH (100 micrograms/5 microliters, icv), 30 min before THC administration, attenuated the ability of a high THC dose (1.0 mg/kg) to inhibit LH release in ovariectomized rats. Together, these results demonstrate that THC has significant effects on LH and ACTH secretion in ovariectomized rats and suggest that THC-induced CRH activation, but not corticosterone release, plays a role in the suppression of LH release by cannabinoids.

Acute Sympathoinhibitory Actions of Metformin in Spontaneously Hypertensive Rats

Chronic treatment with the antihyperglycemic agent metformin prevents hypertension in spontaneously hypertensive rats. This effect has been ascribed to normalization of plasma insulin levels. However, whether metformin affects arterial pressure via changes in sympathetic nerve activity is unknown. Therefore, the objective of this study was to examine whether acute administration of metformin produces changes in mean arterial pressure, heart rate, or efferent renal sympathetic nerve activity in spontaneously hypertensive rats. Rats were anesthetized with alphaxalone-alphadolone (Saffan), paralyzed with pancuronium, and artificially ventilated. Intravenous administration of metformin (0, 1, 10, 100 mg/kg) produced dose-dependent reversible decreases in mean arterial pressure, heart rate, and efferent renal sympathetic nerve activity that were not affected by arterial or cardiopulmonary baroreceptor denervation, nitric oxide synthase inhibition by N(omega)-nitro-L-arginine methyl ester, or cyclooxygenase inhibition by indomethacin. Metformin given into the lateral cerebral ventricle (250, 500, 1000 microg) produced dose-dependent decreases in mean arterial pressure, heart rate, and efferent renal sympathetic nerve activity in doses that caused no changes when given intravenously. The sympathoinhibitory response to intracerebroventricular administration of metformin was not affected by alpha2-adrenoceptor blockade by intracerebroventricular yohimbine. We conclude that metformin has acute sympathoinhibitory effects (decreased arterial pressure, heart rate, and efferent renal sympathetic nerve activity) that are produced by a direct central nervous system site of action.

Effect of acute NaCl depletion on NaCl-sensitive hypertension in borderline hypertensive rats.

The borderline hypertensive rat (BHR) is the first generation offspring of a mating between a female spontaneously hypertensive rat and a male normotensive Wistar-Kyoto rat. With increased dietary NaCl intake, the BHR develops hypertension and augmented cardiovascular and renal responses to acute environmental stress. This investigation sought to examine the role of extracellular fluid volume (ECFV) in these changes. Three groups of 16-week-old BHR were studied: (1) rats on a 1% NaCl diet for 12 weeks; (2) rats on an 8% NaCl diet for 12 weeks; and (3) rats on an 8% NaCl diet for 12 weeks plus furosemide (50 mg/kg, i.p., twice daily) for the preceding 2 days. Rats were chronically instrumented for the measurement of mean arterial pressure (MAP), heart rate and renal hemodynamic, excretory and sympathetic nerve activity responses to acute environmental stress (acute air jet stress). ECFV was measured as inulin space. BHR fed an 8% NaCl diet had increased MAP, urinary sodium excretion and ECFV compared with those fed a 1% NaCl diet; they also exhibited augmented pressor, tachycardic, renal sympathetic nerve excitatory and antinatriuretic responses to acute environmental stress. When 8% NaCl-diet BHR were treated with furosemide for 2 days, arterial pressure, urinary sodium excretion, ECFV and cardiovascular and renal responses to acute environmental stress returned to values seen in 1% NaCl-diet BHR. The hypertension and increased cardiovascular and renal responses to acute environmental stress produced by increased dietary NaCl intake in BHR derive from a central nervous system site of action via a mechanism(s) related to ECFV and/or sodium.

Effect of vasotocin on locomotor activity in bullfrogs varies with developmental stage and sex

Although neurohypophysial peptides are present in many regions of the developing and adult bullfrog ( Rana catesbeiana) brain, the function of these peptides remains unclear. To investigate possible behavioral actions, we examined locomotor activity following peptide injection in bullfrogs at various developmental stages. An intraperitoneal (ip) injection of arginine vasotocin (AVT) in tadpoles (stages V, X, or XVII) produced an immediate and dose-dependent inhibition of locomotor activity. On the other hand, AVT stimulated activity when administered ip to juvenile or adult female bullfrogs, but did not influence activity in juvenile or adult males. The minimum effective dose of AVT, when injected directly into the brain of tadpoles, was 100-fold less than that observed when injected ip, suggesting a central nervous system site of action for this peptide. A vasopressin receptor antagonist (d(CH 2) 5[Tyr(Me) 2]AVP administered ip or icv) significantly increased locomotor activity in tadpoles, compared to controls. Oxytocin, vasopressin, and AVP 4–9 inhibited activity in tadpoles while mesotocin, des Gly(NH 2)AVP, and pressinoic acid had no significant effect. Injection of PGF 2α also significantly decreased activity levels in tadpoles. However, pretreatment of tadpoles with indomethacin, a prostaglandin synthesis inhibitor, did not prevent the behavioral effects of AVT, suggesting that prostaglandin synthesis is not required for this response. In summary, AVT influenced locomotor activity in bullfrog tadpoles and female frogs. This effect shifted during development from an inhibitory action in tadpoles to a stimulatory effect in metamorphosed female frogs. The effect of AVT on juvenile and adult frog locomotion was sexually dimorphic, as this peptide altered female behavior but not male behavior.

Central nervous system site of action of bombesin to elevate plasma concentrations of catecholamines

To identify the central nervous system site of action of bombesin to elevate plasma concentrations of catecholamines, this peptide has been injected into numerous brain ventricular and parenchymal sites. Low doses of bombesin (1–10 ng) injected into the region of the rostral nucleus tractus solitarius (NTS) elicited an elevation of plasma catecholamines greater than those observed following an injection of bombesin into other brain regions. Bombesin-induced (10 ng) elevation of plasma epinephrine but not norepinephrine was prevented by co-administration of somatostatin-28 (100 ng). Mean arterial pressure (MAP) and heart rate (HR) were measured following injection of bombesin into the NTS. Bombesin injected into the NTS resulted in prolonged decreases in HR without significantly altering MAP. These studies demonstrate that bombesin injected into the dorsal medulla resulted in significant changes of plasma catecholamine levels and HR. Based on these actions of bombesin and the neuroanatomic distribution of bombesin-like peptide, it is suggested that this peptide may play an important role in regulation of sympatho-adrenal and cardiac functions.

Central Nervous System Sites of Action of an Enkephalin Analogue, (D‐Met2, Pro5)‐Enkephalinamide, and Naloxone on the Secretion of Five Anterior Pituitary Hormones of Male Rats

Abstract Central nervous system sites of action of opioid peptides on pituitary hormone secretion were investigated. One nmol of an enkephalin analogue, (D-Met(2), Pro(5))-enkephalinamide, and 10 nmol of the opiate antagonist naloxone were injected into ten different regions of the brain of conscious male rats and their effect on the release of five anterior pituitary hormones tested. The injections were made through a special injection cannula which was inserted into the brain through a guide cannula fixed on the skull and implanted into the brain 5 to 7 days earlier. Both compounds injected into the medial septum, medial preoptic area and hypothalamic paraventricular nucleus affected prolactin, growth hormone and luteinizing hormone (LH) secretion. The enkephalin analogue stimulated prolactin and growth hormone and inhibited LH release. Naloxone induced the opposite effect. Drugs given into the hypothalamic ventromedial nucleus caused changes in plasma prolactin and growth hormone levels. Enkephalinamide increased and naloxone decreased plasma concentrations of both hormones. Administration of the compounds into the dorsal raphe area resulted in alterations of prolactin and LH release, the analogue caused elevation of prolactin and inhibition of LH release, whereas the opiate antagonist resulted in opposite changes. Only an LH response was obtained from the hypothalamic dorsomedial nucleus and a growth hormone response from the central amygdala. Also in these cases the enkephalin analogue decreased LH and elevated growth hormone plasma levels, and naloxone brought about a rise in LH and a diminution of growth hormone concentration. None of the regions were effective in inducing a clear-cut adrenocorticotrophin or follicle-stimulating hormone response. The parietal cortex, medial amygdala and the dentate gyrus were entirely ineffective sites. The findings suggest that in the brain there are multiple sites of action of opioids on pituitary trophic hormone secretion and the effective sites are not identical in terms of pituitary hormone response.

Evidence for GABA involvement in stress-induced inhibition of male amphibian sexual behavior

The behavioral effects of GABA analogs were investigated to determine whether GABAergic neurotransmission is involved in the stress-induced inhibition of masculine sexual behaviors in rough-skinned newts ( Taricha granulosa). Injections of bicuculline, a GABA antagonist, stimulated male sexual behaviors in a dose-dependent fashion, and the minimum effective dose was 40-fold less when administered centrally rather than systemically, suggesting a central nervous system site of action. Injections of muscimol, a GABA agonist, suppressed reproductive behaviors in male newts, and this inhibition lasted at least 5 hr and was proportional to the dose of muscimol administered. The inhibitory effects of muscimol on newt sexual behaviors could be reversed by a single 100-μg ip injection of arginine vasotocin. The inhibitory effects of confinement stress or corticosterone (CS) injections on newt sexual behaviors were blocked by pre-treatment of newts with mercaptopropionic acid, a blocker of GABA synthesis. As well, bicuculline prevented the inhibition of sexual behavior induced by CS injection. These results support the conclusion that, in a male amphibian, the GABAergic system is involved in the inhibitory mechanisms regulating sexual behaviors and that CS mediates the stress-induced inhibition of sexual behaviors through the GABAergic system.

Blood-brain barrier: Mechanisms of peptide regulation and transport

Delivery of peptide and protein drugs to the central nervous system from the blood is subject to the permeability limitations imposed by the blood-brain barrier. Recent development of an in vitro model permits the opportunity to explore fundamental blood-brain barrier characteristics at the cellular and biochemical level. In part, information generated from such studies will be necessary to develop strategies for delivering blood-borne therapeutic peptide and proteins across the blood-brain barrier to central nervous system sites of action. Reviewed here are applications of the in vitro model in studies of the biochemistry of both peptide regulation of transcellular vesicular pathways and transmembrane transport of peptides.