D-ala2-methionine-enkephalinamide Research Articles

Effects of opioid receptor activation on cardiovascular responses and extracellular monoamines within the rostral ventrolateral medulla during static contraction of skeletal muscle.

During static muscle contraction, activation of opioid receptors alters the extracellular glutamate concentrations within the rostral ventrolateral medulla (RVLM). In addition, microdialysis of glutamate in the ventrolateral medulla (VLM) increases the release of norepinephrine (NE), dopamine (DA), and serotonin (5-HT). Therefore, we hypothesized that extracellular concentrations of these monoamines as well as cardiovascular responses during static skeletal muscle contraction would be modulated following administration of [D-Ala(2)]methionine enkephalinamide (DAME), an opioid receptor agonist, into the RVLM. Microdialysis of 100 microM DAME into the RVLM of 10 rats significantly (P<0.01) decreased extracellular levels (in pg/10 microl) of NE (from 3.3+/-0.3 to 1.9+/-0.3), DA (from 5.5+/-0.2 to 3.7+/-0.3), and 5-HT (from 6.1+/-0.8 to 3.6+/-0.2) during static exercise. After microdialysis of DAME, the exercise pressor reflex also significantly (P<0.01) decreased mean arterial pressure (MAP) by 13+/-3 mmHg and heart rate (HR) by 16+/-6 bpm, compared with control (MAP=22+/-4 mmHg and HR=31+/-7 bpm). Subsequently, after 30 min microdialysis of naloxone, an opioid receptor antagonist, muscle contraction increased the extracellular monoamine levels (in pg/10 microl, 3.8+/-0.3 NE; 5.2+/-0.3 DA; and 5.5+/-0.4 5-HT) similar to the control groups and evoked a reversal of cardiovascular responses. Similarly, 30 min of microdialyzing naloxone, added to the perfusing medium containing DAME, reversed the attenuating effects of DAME on monoamines, MAP, and HR during a muscle contraction. Furthermore, microdialysis of 100 microM naloxone alone for 30 min potentiated cardiovascular responses and monoamine levels during a muscle contraction. In summary, the present data demonstrates that microdialysis of DAME into RVLM attenuates the exercise pressor reflex mediated increases in MAP, HR and extracellular levels of biogenic monoamines. A subsequent microdialysis of naloxone reversed the effects suggesting that an opioidergic mechanism within RVLM modulates the exercise pressor reflex. Overall, the present study provides further insights into the opioidergic modulation of the exercise pressor reflex.

Rostral ventrolateral medulla opioid receptor activation modulates glutamate release and attenuates the exercise pressor reflex

We previously reported that the administration of [ d-Ala 2]methionine enkephalinamide (DAME), an opioid receptor agonist, into the rostral (RVLM) but not into the caudal ventrolateral medulla (CVLM), attenuated increases in mean arterial pressure (MAP) and heart rate (HR) during static muscle contraction that had been blocked by prior microdialysis of the opioid receptor antagonist, naloxone [Am. J. Physiol. 274 (1998) H139–H146]. In this study, we determine whether this RVLM-mediated opioidergic-modulation of cardiovascular responses is associated with localized changes in extracellular concentrations of glutamate, an excitatory amino acid, using microdialysis techniques in anesthetized rats. Muscle contraction increased MAP and HR by 37±5 mmHg and 23±3 bpm, respectively. Extracellular glutamate concentrations, determined using HPLC–ECD, increased from 0.8±0.2 to 6.6±1.2 ng/5 μl in the bilateral RVLM areas. Microdialysis of DAME (100 μM) for 30 min attenuated the contraction-evoked increases in MAP, HR, and glutamate levels (20±4 mmHg, 10±2 bpm, and 1.8±0.2 ng/5 μl, respectively). After microdialysis of naloxone (100 μM) for 30 min into the RVLM, muscle contraction blocked the attenuations (35±5 mmHg, 26±4 bpm, and 5.8±1.0 ng/5 μl, respectively). Developed muscle tensions were similar throughout the protocol (676±38, 678±37 and 687±37 g, respectively). These results suggest that an opioidergic receptor-mediated mechanism within the RVLM attenuates cardiovascular responses during static exercise via modulating extracellular concentrations of glutamate in the RVLM.

Enzymatic and permeation barrier of [ d-Ala 2]-Met-enkephalinamide in the anterior membranes of the albino rabbit eye

Enzymatic and physical barrier properties of anterior ocular membranes were characterized. The permeation and metabolic degradation of [ d-Ala 2]-methionine enkephalinamide (DAMEA) in the albino rabbit cornea, conjunctiva and sclera were studied in vitro. DAMEA was administered with and without peptidase inhibitors bestatin (aminopeptidase inhibitor) and SCH 39370 (enkephalinase inhibitor). The modified Ussing chambers were used to study the peptide permeation and the samples were analyzed with a novel HPLC method using UV and EC detectors. Sclera was the most permeable membrane to DAMEA, while cornea was almost impermeable to DAMEA. Without inhibitors, the permeability coefficients of DAMEA were 2.7×10 −8 cm/s, 3.1×10 −6 cm/s and 12.5×10 −6 cm/s in the cornea, conjunctiva and sclera, respectively. DAMEA was partly metabolized to tyrosine (Tyr) and tyrosine- d-alanine-glycine (Tyr- d-Ala-Gly). When inhibitors were co-administered with DAMEA, the corneal permeability of intact DAMEA increased 15 times, while conjunctival permeability increased 5.5 times and scleral permeability remained practically unaltered. The formation of metabolites decreased markedly, when the inhibitors were used. Interestingly, when the permeability of DAMEA was compared to permeabilities of polyethylene glycols in different membranes, the permeation was in the same range suggesting that DAMEA permeates through cornea via a paracellular pathway. Both enzymatic and physical barriers were more prominent in the cornea than in the conjunctiva and sclera. Non-corneal pathway of absorption and combined with inhibition of peptidases may be the most viable pathway for ocular peptide administration.

Rostral ventrolateral medullary opioid receptor activation modulates pressor response to muscle contraction

The effects of an opioid agonist, [D-Ala2]methionine enkephalinamide (DAME), administered into the rostral ventrolateral medulla (rVLM) or caudal ventrolateral medulla (cVLM) on cardiovascular responses to isometric muscle contraction were determined in anesthetized rats. A 30-s contraction evoked by tibial nerve stimulation increased mean arterial pressure (MAP) and heart rate (HR) by 34 +/- 6 mmHg and 40 +/- 7 beats/min, respectively, with a developed tension of 322 +/- 30 g, after bilateral insertion of microdialysis probes into the rVLM. Thirty-minute dialysis of DAME (10 and 100 microM) attenuated the contraction-evoked cardiovascular changes dose dependently (10 microM: MAP = 25 +/- 4 mmHg, HR = 27 +/- 3 beats/min, tension = 333 +/- 25 g; 100 microM: MAP = 14 +/- 4 mmHg, HR = 16 +/- 5 beats/min, tension = 330 +/- 34 g). Preadministration of an opioid antagonist, naloxone (100 microM), augmented contraction-evoked MAP and HR responses and blocked effects of 100 microM DAME. Microdialysis of DAME into the cVLM produced no changes in the pressor response to contraction. At end of each experiment, tibial nerve stimulation after neuromuscular blockade evoked no MAP or HR change. Results demonstrate that opioid receptor activation within the rVLM modulates cardiovascular responses to isometric muscle contraction.

Effects of (d-Ala2) methionine enkephalinamide in PMA-induced mouse ear edema

The synthetic opioid peptide (D-Ala2) methionine enkephalinamide (DAMEA) was reported to be an extremely potent inhibitor of the "respiratory burst" of polymorphonuclear neutrophils (PMNs). In the present paper the activity of DAMEA was investigated in the PMA-induced mouse ear edema since there is strong evidence that reactive oxygen species (ROS) are involved in the pathogenesis of this inflammatory reaction. DAMEA inhibited PMA-induced mouse ear edema in a dose-dependent manner between 10(-6) and 10(-9) mol/ear. The well-known anti-inflammatory activity of narcotic analgesics could be explained partly by an inhibition of ROS production.

Proenkephalin a derivatives in lacrimal gland: Occurrence and regulation of lacrimal function

The derivatives of proenkephalin A were measured in acid extracts of rat lacrimal glands by specific radioimmunoassay. Glands from adult male rats contained all four derivatives of the opiate precursor. The content in the gland of proenkephalin A-derived peptides Met 5-enkephalin, Leu 5-enkephalin, Met 5-enkephalin Arg 6-Phe 7 and Met 5-enkephalin Arg 6-Gly 7-Leu 8 indicates tissue specific processing with an enhancement of the heptapeptide. The effect of enkephalins on the activity of adenylate cyclase in lacrimal membranes was measured and compared with the effect of the synthetic enkephalin analogue d-ala 2-methionine enkephalinamide (DALA) that inhibits both lacrimal protein secretion and lacrimal adenylate cyclase. In the presence of the peptidase inhibitors thiorphan and bestatin, the inhibition of forskolin-stimulated adenylate cyclase activity by Met 5-enk, Leu 5-enk, Met 5-enk Arg 6-Phe 7 and DALA were identical. Maximum inhibition was approximately 35% at a dose of 50μ m enkephalin. Addition of the octapeptide, Met 5-enk Arg 6-Gly 7-Leu 8 resulted in decreased adenylate cyclase activity; however, the effect was not statistically significant. Activation of δ opioid receptors by the endogenous enkephalins is indicated by the reversal of adenylate cyclase inhibition in the presence of the δ-receptor antagonist ICI 174864. The data support the physiological significance of in vitro inhibition of lacrimal secretion by DALA and indicate a possible role for endogenous enkephalins in lacrimal function.

Respiratory burst inhibition in human neutrophils by ultra-low doses of [D-Ala 2]methionine enkephalinamide

An ultra-low dose (10 −14 M) of opioid peptide [D-Ala 2]methionine enkephalinamide (DAMEA) is found to exert an inhibitory effect on the production of reactive oxygen species (respiratory burst) in human neutrophils. The validity of this phenomenon has been verified in a series of studies that comprised 30 experiments. The inhibition has proved to be statistically significant (P<0.001). The dose-response dependence of the effect (10 −15−10 −9 M) followed a characteristic biphasic pattern (with the maximum effect at ultra-low doses). An opioid antagonist, naloxone partially blocks the inhibitory effect, which indicates that the DAMEA action is at least partially mediated by opioid receptors.

Effects of [D-Ala 2] Met-enkephalinamide, a Met-enkephalin analog, on delayed response by squirrel monkeys

Squirrel monkeys were tested on an indirect spatial delayed response task after subcutaneous injections of either physiological saline, 100, or 500 microgram/kg [D-Ala 2] methionine enkephalinamide (DAME). During Experiment 1 monkeys completed two-choice delayed response problems with 0-, 6-, 12- and 18- sec delays; DAME significantly enhanced performance at 18-sec delays but not for the shorter delay. During Experiment 2, monkeys performed a nine-choice spatial delayed response task with 0-, 4- and 8-sec delays. In Experiment 2, low but not high doses of DAME impaired performance. Memory functions represented by a bias-free performance measure paralleled percentage correct in both experiments. Therefore, DAME effects, in enhancing and in impairing performance, were not attributable to changes in the relative number of systematic spatial errors (positional win-shift; lose-stay, position perseveration, and position preference).

Opioid modulation of feeding and drinking in fowls.

1. D-ala2-methionine enkephalinamide (DME), the stable analogue of met-enkephalin (an opioid agonist), stimulated food intake of immature hens in the first 30 min after intracerebroventricular injection (2 and 8 micrograms/kg), but had no effect on either food or water intake when injected intravenously (15 and 60 micrograms/kg). 2. Naloxone (an opioid antagonist) had no effect on food intake after either intracerebroventricular (50 and 200 micrograms/kg) or intravenous (1 and 4 mg/kg) injection, but inhibited water intake in the second 30 min after intravenous injection. 3. Water intake was not measured after the intracerebroventricular injections of DME and naloxone. 4. Both feeding and drinking were inhibited in a dose-related way in the 7 h after intramuscular injection of nalmefene (0.2, 0.4, 0.8 and 1.6 mg/kg), a more potent and longer-lasting antagonist than naloxone. 5. These data are compared with published results from similar work with birds and mammals. It is concluded that central release of endogenous opioids may reinforce both feeding and drinking in fowls, but whereas opioid blockage affects feeding more than drinking in pigeons and quail, the opposite appears to be the case in fowls.

Effects of intrathecal antagonists on the antinociception, hypotension, and bradycardia produced by intravenous administration of [D-Ala2]-methionine enkephalinamide (DALA) in the rat.

DALA is a synthetic pentapeptide that produces inhibition of the tail-flick reflex evoked by radiant heat, as well as hypotension and bradycardia. Two experiments examined the effects of administration of various receptor antagonists into the subarachnoid space of the lumbar spinal cord on the antinociception produced by IV administration of DALA. Experiment 1 showed that intrathecal administration of 30 micrograms of phentolamine produced a significant reduction in the antinociceptive effect of DALA, while naloxone (30 micrograms), methysergide (30 micrograms), or vehicle control had no effect. Experiment 2 showed that intrathecal administration of combinations of either phentolamine and methysergide, or phentolamine and naloxone, were no more effective in reducing the antinociceptive effect of DALA than administration of phentolamine alone. These data demonstrate the involvement of descending noradrenergic systems in the production of antinociception by IV DALA. Further, they indicate that the antinociception produced by DALA is independent of a direct spinal action of the drug.

The effect of intracerebroventricular D-ALA 2 methionine enkephalinamide and naloxone on cardiovascular parameters in the cat

Lathers and Schraeder (1) have shown that autonomic dysfunction is associated with epileptogenic activity induced by pentylenetetrazol while Vindrola et al (2) found increased D-ALA 2 methionine-enkephalinamide (DAME) levels in the rat brain after pentylenetetrazol-induced epileptogenic activity. Thus, this study was designed to determine whether DAME could have contributed, at least in part, to the autonomic dysfunction associated with pentylenetrazol-induced epileptogenic activity in the study of Lathers and Schraeder (1). DAME (500 ug/ kg, icv) was given to 9 cats anesthetized with alpha chloralose. Epileptogenic activity and hypotension occurred in all cats (maximum fall ranging from 6 to 46 mmHg; duration of 6 to 35 min). In 6 cats the heart rate decreased, in 2 it increased, and in 1 it showed little or no change. The duration of heart rate changes varied from 18 to 76 min. Naloxone (100 ug/kg, iv) was given to 6 cats after DAME. Naloxone suppressed or abolished the epileptogenic activity in all 6 cats, reversed the DAME-induced hypotension and increased the heart rate in 3 cats, decreased it in 2, and produced no change in 1. These results indicate that DAME may produce epileptogenic activity and cardiovascular changes through an action on central opiate receptors. It is hypothesized that: 1) increased levels of DAME inhibit the release of gamma aminobutyric acid; 2) this then increases vagal bradycardia and hypotension; and 3) step 2 causes an imbalance in peripheral autonomic cardiac neural discharge which may cause arrhythmias and/or sudden unexplained death in the epileptic person.

Medullary substrates of descending spinal inhibition activated by intravenous administration of [ d-Ala 2]methionine enkephalinamide in the rat

Two experiments were performed in pentobarbital-anesthetized rats to provide information about the locus of the descending inhibitory system(s) in the brainstem engaged by [ d-Ala 2]methionine enkephalinamide (DALA) activation of vagal afferents. In Expt. 1, local anesthetic block of either the medial medullary nucleus raphe magnus (NRM) or the combined NRM-lateral medullary reticular formation (MRF) region failed to affect DALA-induced inhibition of the tail-flick reflex or reflex bradycardia. The hypotensive action of DALA was marginally enhanced in rats with a combined NRM-MRF local anesthetic block. However, inhibition of the tail-flick reflex produced by electrical stimulation in the NRM-MRF was eliminated by the local anesthetic. In Expt. 2, local anesthetic blocks of regions at the level of and caudal to obex were effective in blocking DALA-induced inhibition of the tail-flick reflex. These regions included lateral sites approximately 0.4 mm rostral to obex, to −2.4mm caudal to obex, and midline sites approximately −2.4 to −3.2mm caudal to obex. These findings suggest that brainstem areas mediating DALA-induced antinociception are lateral to the midline at the level of the obex. The spinopetal efferents mediating antinociception then course medially and ventrally, and finally course bilaterally in the dorsolateral funiculi before terminating in the spinal dorsal horn.

D-Ala2]-methionine enkephalinamide (DALA): characterization of antinociceptive, cardiovascular, and autonomic nervous system actions in conscious and pentobarbital-anesthetized rats.

The antinociceptive, cardiovascular, and autonomic nervous system actions of [D-Ala2]-methionine enkephalinamide (DALA) were assessed in conscious and pentobarbital-anesthetized rats. Intravenous administration of DALA inhibited the tail-flick reflex evoked by noxious radiant heat in both conscious and pentobarbital-anesthetized rats. In general, the magnitude of the inhibition was not significantly affected by the presence of pentobarbital-anesthesia. DALA also induced hypotension and bradycardia in these rats, but these responses were significantly attenuated by pentobarbital-anesthesia. Arterial blood flows were initially decreased and vascular resistances increased in mesenteric, renal, and hindquarter beds following DALA administration, but these parameters rapidly returned to baseline levels except in the hindquarter bed where flows increased significantly above baseline levels. All of the changes in blood flows were significantly greater in conscious compared to pentobarbital-anesthetized rats. In intact pentobarbital-anesthetized rats, DALA induced inhibition of renal sympathetic nerve activity. However, DALA induced a pressor response with a brief increase in renal sympathetic nerve activity in rats with bilateral vagotomy. Similarly, in the conscious rat with ganglionic blockade, DALA induces a brief pressor response. These outcomes indicate that the brief hypotension observed in the intact rat following administration of DALA is probably the net outcome of a large vagally-induced decrease in sympathetic tone and a small increase in sympathetic tone of either neurogenic or peripheral origin. These outcomes are discussed in terms of cardiovascular-somatosensory interactions.

Blood pressure responses of conscious rats to intravenous administration of enkephalin derivatives (D-ala 2 methionine and leucine enkephalinamide, and methionine and leucine enkephalinamide)

The present study was designed to determine the blood pressure (BP) responses of conscious rats given intravenous (IV) injections of enkephalin derivatives (D-ala 2-methionine enkephalinamide, DAMEA; D-ala 2-leucine enkephalinamide, DALEA; methionine enkephalinamide, MEA; leucine enkephalinamide, LEA) and the receptor mechanisms mediating the resultant change in BP. IV injection of 1.6–16.0 nmoles of DAMEA or DALEA caused a transient but potent decrease in mean arterial pressure (MAP) and mean heart rate (MHR). LEA and MEA (16.0 nmoles) given IV produced slight pressor responses, which were not associated with concomitant tachycardia whereas 48 nmoles of MEA elicited a hypotensive effect accompanied by a fall in MHR. Pretreatment studies whereby various receptor antagonists (naloxone, diprenorphine, phentolamine, D-L-propranolol or atropine) were given IV 5 min before subsequent IV administration of DAMEA, DALEA, MEA or LEA (16 nmoles) showed that naloxone, diprenorphine and atropine blocked the depressor and bradycardic effects of DALEA and DAMEA. Naloxone and phentolamine suppressed the pressor reponse of both MEA and LEA (16.0 nmoles) while diprenorphine blocked the rise in MAP to only MEA. The results show that DAMEA and DALEA mediate their depressor actions in conscious rats via a negative chronotropic effect through an interaction of muscarinic cholinergic receptors on the myocardium. It is suggested that the pressor response of MEA and LEA may be produced via an α-receptor mediated effect on the peripheral vasculature to cause vasoconstriction.

No inotropic action of enkephalins or enkephalin derivatives on electrically-stimulated atria isolated from lean and obese rats.

Inotropic actions of the endogenous enkephalins, leucine enkephalin [( Leu] enkephalin) and methionine enkephalin [( Met] enkephalin), and derivatives, [D-Ala2-methionine] enkephalinamide (DAMEA) and [D Ala2-leucine]enkephalinamide (DALEA) were tested, alone or in combination with noradrenaline (NA), (+/-)-isoprenaline or carbachol, on electrically-stimulated atria excised from Sprague-Dawley, fatty, Zucker (fa/fa) and lean, hooded heterozygous (Fa/fa) rats. [Met] enkephalin, [Leu] enkephalin, DAMEA and DALEA (4 X 10(-7)M to 4 X 10(-4)M) caused no significant changes in atrial tension in any group compared to pre-injection control values or those following the infusion of Krebs-Henseleit control solution. NA and isoprenaline (10(-7) to 10(-6)M) caused significant, dose-related increases in atrial tension in each of the three strains of rats tested with the Fa/fa group showing the greatest change and fastest rate of tension development. [Met] enkephalin, [Leu] enkephalin, DAMEA or DALEA (4 X 10(-6)M) infused concurrently with NA or isoprenaline (10(-6)M) evoked atrial tension changes within each group that were not different from those observed when NA or isoprenaline was administered alone. Carbachol (10(-9) and 10(-8)M) caused a dose-related decrease (10% and 30-40%, respectively, from pre-injection control values) in atrial tension in auricles excised from all three groups. Again, infusion of [Met] enkephalin, [Leu] enkephalin, DAMEA or DALEA (4 X 10(-6)M) together with carbachol (10(-8)M) did not affect atrial tension changes of auricles isolated from any group compared to when carbachol was given alone. The results indicate that the endogenous pentapeptides, [( Met] or [Leu] enkephalin), or derivatives (DAMEA and DALEA) do not affect atrial tension of electrically-stimulated auricles isolated from Sprague-Dawley, fa/fa or Fa/fa rats. In addition, these pentapeptides do not modify the positive inotropic actions of NA or isoprenaline or the negative inotropic effects of carbachol. It is suggested that in vivo, the enkephalins or enkephalin derivatives do not have a direct action on the heart to alter myocardial contractility.

Behavioural and spectrum power effects of opioid peptides in chicks

The effects of intracerebroventricular administration of β-endorphin, dermorphin, D-ala2-methionine-enkephalinamide (DALA) and pentazocine, on behaviour, electrocortical activity and ECoG spectrum power in young chicks were studied. β-endorphin, dermorphin and DALA produced dose-dependent behavioural sedation and sleep accompanied by electrocortical high voltage slow wave activity and increase in total voltage power with a predominant increase in the lower bands of ECoG spectrum. Dermorphin was found significantly more potent than β-endorphin and DALA since it was active at much lower doses. Behavioural and ECoG sleep evoked by opioid peptides were rapidly reversed by naloxone showing that these effects are mediated through an activation of specific opioid receptors. Doses of pentazocine equimolar with doses of dermorphin and β-endorphin producing sleep did not affect behaviour and spectrum power whereas higher doses produced opposite effects, i.e., behavioural stimulation, pecking and increase in locomotor activity.

Analgesic cross-tolerance between morphine and opioid peptides.

The analgesic effect of intracerebroventricular administration of morphine, ketocyclazocine, [D-ala2]-methionine enkephalinamide (DAM), [D-ala2-D-leu5]-enkephalin (DADLE), leuenkephalin, metenkephalin, and beta-endorphin on acetic acid-induced abdominal writhing (AAW) was investigated in naive and morphine-tolerant mice. It was found that the relative potencies of a series of opioids are different in naive and morphine-tolerant groups. In naive animals, the order of potency (ED50, nmol) was beta-endorphin greater than morphine = DAM greater than DADLE greater than ketocyclazocine = leuenkephalin = metenkephalin. The morphine-tolerant animals were cross-tolerant to ketocyclazocine and to all the peptides studied; DAM and beta-endorphin exhibited the highest degree of tolerance. In morphine-tolerant animals, the order of potency was morphine = DADLE = beta-endorphin greater than DAM = ketocyclazocine = metenkephalin greater than leuenkephalin. The results indicate that endogenous opioid systems may be affected by tolerance development to morphine.

Differential behavioral effects induced by intrathecal microinjection of opiates: Comparison of convulsive and cataleptic effects produced by morphine, methadone, and d-Ala 2-methionine-enkephalinamide

Intrathecal (i.t.) microinjection of 400 μg of morphine in rats induces convulsive activity restricted to the hindlimbs. This activity is potentiated rather than antagonized by naltrexone, is potentiated in animals pretreated over 6 days with increasingly higher systemic doses of morphine, and is potentiated and prolonged by high thoracic spinalization. Similar to morphine, convulsive spinal activity could also be elicited with i.t. injection of the non-opiate convulsant penthylenetetrazol (PTZ). However, methadone, d-ala 2-methionine-enkephalinamide (DALA), or naltrexone, injected i.t. at equimolar doses with morphine did not produce similar convulsive behavior. DALA and methadone rather produced pronounced spinal catalepsy. It was concluded: (1) that the spinal convulsive action of morphine is not mediated by specific opiate receptors; and (2) that the spinal cord may be one of the sites where high doses of systemically administered morphine may produce convulsions.

Cataleptic effects of opiates following intrathecal administration

Intrathecal (i.t.) administration of d-ala 2-methionine-enkephalinamide (DALA) and methadone induces pronounced hindlimb catalepsy in rats. DALA is more potent than methadone in inducing this behavior and catalepsy induced by both compounds is partially blocked by prior treatment with naltrexone. Intrathecal morphine induces convulsive hindlimb activity at doses of 25, 100 and 400 μg. However, only the highest dose produces indications of cataleptic behavior. It is suggested that whereas the convulsive behavior induced by i.t. morphine is not mediated by specific opiate receptors, catalepsy induced by all 3 compounds is.

Peptide and non-peptide opioid-induced hyperthermia in rabbits

Intracerebroventricular administration of all three prototype non-peptide opioid receptor (μ, χ and σ) agonists, morphine, ketocyclazocine and N-allyl-normetazocine (SKF 10, 047) induced hyperthermia in rabbits. Similar administration of peptide opioids like β-endorphin (BE), methionine-enkephalin (ME) and its synthetic analogue d-ala 2-methionine-enkephalinamide (DAME) also caused hyperthermia. As expected, the synthetic enkephalin DAME was more potent than the parent enkephalin. Of the three anion transport systems (iodide, hippurate and liver-like or L) present in the choroid plexus, it is suggested that only the L transport system seems to be important to ventricular inactivation of BE and DAME since iodipamide (an inhibitor of the L transport system) augmented the hyperthermia produced by BE and DAME. Prostaglandins (PG) and norepinephrine (NE) were not involved in peptide and non-peptide opioid-induced hyperthermia because a PG synthesis inhibitor, indomethacin, and an α-adrenergic receptor blocker, phenoxybenzamine, had no thermolytic effect on them. Likewise cAMP was not required since a phosphodiesterase inhibitor, theophylline, did not accentuate the hyperthermia due to peptide and non-peptide opioids. Naloxone-sensitive receptors were involved in the induction of hyperthermia by morphine. BE, ME and DAME since naloxone attentuated them. In contrast, the hyperthermic response to ketocyclazocine and SKF 10, 047 were not antagonized by naloxone.