Frank C. Tortella

Repeated electroconvulsive shock or chronic morphine treatment increases the number of 3HDALA2, DLEU5enkephalin binding sites in rat brain membranes

Experiments were performed in rats to evaluate the possible mechanisms responsible for the pharmacological cross-sensitization observed between repeated electroconvulsive shock (ECS) and chronic morphine administration. Repeated daily ECS for 9 days as well as chronic morphine pellet implantation resulted in a significant increase in the number of 3H-DADLE binding sites (Bmax values of 231 and 196 fmoles/mg protein, respectively). By contrast, single ECS, repeated sham ECS, and placebo pellet-treated rats all had significantly lower Bmax values (approx. 170 fmoles/mg protein). Affinities were not significantly altered by these treatments (Kd values between 3.1 and 4.0 nM). These data may link pharmacological cross-sensitization (repeated ECS and chronic morphine treatment) with a functional increase in the number of available opioid receptors.

Comparison of the anticonvulsant effects of opioid peptides and etorphine in rats after icv administration.

The effect of acute icv administration of β-endorphin (5–160 μg), D-ala2-D-leu5-enkephalin (DADL; 5–160 μg), D-ala2-met-enkephalinamide (DAME; 10–160 μg), and etorphine (0.05–1.6 μg) on brain excitability was studied by measuring flurothyl seizure thresholds in rats. Each test compound produced a behavioral stupor characterized by muscle rigidity, exophthalmos, and the absence of spontaneous movement. Wet-dog shakes occured only after injection of the opioid peptides. All four compounds produced a dose-related increase in seizure threshold. Naloxone antagonized the behavioral and anticonvulsant effects; the increase in seizure threshold induced by β-endorphin was the most resistant to naloxone. These results indicate that the opioid peptides, in addition to their known EEG epileptogenic potential, are also anticonvulsant in the rat, thus raising the possibility of a dual action for the opioid peptides on central nervous system excitability.

Studies on opioid peptides as endogenous anticonvulsants

In both normal and hypophysectomized rats, electroconvulsive shock (ECS) produced a significant postictal rise in seizure threshold (S.T.) to flurothyl, a volatile convulsant. This ECS-induced increase in S.T. was markedly attenuated by naloxone (10 mg/kg s.c.), which itself did not alter basal S.T. A dose of bicuculline (0.1 mg/kg i.p.) which was slightly proconvulsant in the flurothyl test did not significantly alter the postictal rise in S.T. produced by ECS. With hypophysectomized rats, there was a 24% increase in basal S.T. to flurothyl challenge. Hypophysectomy had no influence on the postictal rise in S.T. produced by ECS, nor on the attenuation that occurs with naloxone. We propose that the postictal rise in S.T. reflects a change occurring centrally which prevents a static convulsive state, possibly through brain opioid peptides acting as endogenous anticonvulsants.

Studies on the excitatory and inhibitory influence of intracerebroventricularly injected opioids on seizure thresholds in rats

The influence of centrally administered meperidine, normeperidine and pentazocine on the excitability of brain was studied by measuring the threshold for flurothyl-induced convulsions in rats. All three opioids are reported to lower seizure thresholds when given subcutaneously to rats in this test. Dose-and time-dependent changes in the seizure threshold occurred after intracerebroventricular injection of pentazocine (10-160 micrograms), meperidine (25-150 micrograms) and normeperidine (50-150 micrograms). Rapid increases in the seizure threshold were associated with pentazocine and meperidine, whereas a slowly developing decrease in the threshold was caused by normeperidine. Naloxone (10 mg/kg, s.c.) antagonized the anticonvulsant effect of meperidine (but not that of pentazocine) and enhanced the proconvulsant effect of normeperidine. Thebaine (25-150 micrograms), which had no marked influence on the seizure threshold when given intracerebroventricularly, lowered the threshold after subcutaneous injection of 12.5 and 25 mg/kg. This effect was not altered by injection of naloxone. These results show that centrally administered opioids can act on excitatory or inhibitory systems that regulate seizure mechanisms in the rat brain. Furthermore both naloxone-sensitive and naloxone-insensitive components are involved. Meperidine, pentazocine and thebaine have different actions on the seizure threshold after intracerebroventricular, as opposed to subcutaneous, administration. This work has, therefore, identified the route of administration as a critical variable in the effect of opioids on the seizure threshold in rats.

A comparison of the anticonvulsant effects of two systemically active enkephalin analogues in rats

FK-33,824 (Tyr-D-Ala-Gly-MePhe-Met(O)-ol) and metkephamid (Tyr-D-Ala-Gly-Phe-N(Me)Met-CONH2; LY 127623) are two parenterally active synthetic analogues of the endogenous morphinomimetic pentapeptide, [Met5]-enkephalin. Acute s.c. administration of each analogue raised the seizure threshold in a dose-related manner in rats challenged with flurothyl, a volatile convulsant. The anticonvulsant action was antagonized by a low dose of naloxone (0.10 mg/kg s.c.). FK-33,824 and metkephamid can therefore be classified with typical mu-receptor agonists such as morphine and etorphine in this procedure.

Eeg and behavioral effects of ethylketocyclazocine, morphine and cyclazocine in rats: Differential sensitivities towards naloxone ☆

Abstract The effects of ethylketocyclazocine were compared with those of morphine and cyclazocine on the cortical EEG and spontaneous behavior of unrestrained, male Sprague-Dawley rats. Intraperitoneal administration of ethylketocyclazocine (2.5 mg/kg), morphine (10 mg/kg), and cyclazocine (2.5 mg/kg) each induced alterations in the EEG and behavior of rats characterized by high-voltage synchrony associated with behavioral stupor during wakefulness. Both ethylketocyclazocine and morphine produced biphasic EEG and behavioral profiles consisting of an initial phase of EEG synchrony and stupor followed by a period of EEG activation and behavioral arousal. In contrast, the profile of cyclazocine was not biphasic but was distinguished by intermittent periods of behavioral excitation and EEG desynchronization. When the rats were pretreated with naloxone (0.01–10 mg/kg, s.c. at −10 min), there was a dose-related antagonism of the duration of EEG synchrony and stupor produced by the three analgesics. Ethylketocyclazocine was the most sensitive to naloxone (AD 50 dose of naloxone = 0.035 mg/kg) and cyclazocine was the least sensitive (AD 50 = 2.1 mg/kg). Antagonism of the drug-induced increase in latency to slow-wave sleep was dose-related for ethylketocyclazocine and morphine, but not for cyclazocine. The ability to identify differences between these three analgesics, based on their respective EEG and behavioral profiles and their sensitivities to naloxone, is consistent with current theories of multiple opiate receptors mediating certain pharmacological effects of opioids.

Pituitary opioid involvement in ECS-postictal electrogenesis and behavioral depression in rats

The role of pituitary opioids in electroconvulsive shock (ECS)-induced postictal electrogenesis and behavioral depression was investigated in sham-hypophysectomized and hypophysectomized rats. These animals were divided into two subgroups and injected SC with either saline or naloxone (3 mg/kg) 10 min prior to transauricular ECS. Sham-hypophysectomized rats given saline responded to a single ECS with a 65 +/- 18% (s.e.) increase in postictal electrogenesis and a behavioral depression lasting 3840 +/- 530 sec. Naloxone significantly antagonized both the postictal increase in EEG voltage output and behavioral depression. Hypophysectomy by itself was without effect on EEG patterns and only partially attenuated the ECS-induced electrogenesis and postictal depression (31.9 +/- 9% and 2360 +/- 511 sec, respectively). However, in hypophysectomized rats, naloxone did not further antagonize these effects of ECS. Thus, it appears that pituitary opioids may, at least in part, mediate postictal electrogenesis and behavioral depression. Alternatively, since hypophysectomy only partially attenuates these phenomena, central or nonpituitary opioid peptide systems may be involved. In view of the observed decrease in responsiveness to naloxone in hypophysectomized rats, nonopioid systems cannot be ruled out as contributors to the opioid-like effects of ECS in these animals.

Pupillary effects of leucine and methionine enkephalin in rats after intraperitoneal administration

Changes in pupil size after peripheral administration of met-enkephalin, leu-enkephalin, or morphine were studied in the rat. With a simple pupillographic technique, the pupil diameter of male, S.D. rats (250--300 g) was measured by a series of photographs taken every 60 sec for at least 45 min after the last drug injection. Morphine (8 mg/kg, SC) caused mydriasis characterized by rapid and marked fluctuations of pupil size. Mydriasis also occurred after leu-enkephalin (5 and 10 mg/kg, IP) and met-enkephalin (20 mg/kg, IP) Both peptides induced morphine-like fluctuations. When given 15 min after morphine, leu-enkephalin (5 and 10 mg/kg) increased the mydriatic effect of morphine from 172 percent of control to 224 and 272 percent, respectively. Met-enkephalin (20 mg/kg, but not 10 mg/kg) also enhanced the mydriatic response of morphine, to 244 percent of control. These interactions appear to represent simple addition rather than potentiation. The effects of both peptides were reversed by naloxone (1 mg/kg, SC), suggesting an opiate receptor interaction for the pupillary effect of the enkephalins. The rat pupil thus provides one of the few in vivo models permitting quantification of enkephalin action after parenteral administration.

Opioids and Stress: Effects Upon Nociception, Behavior, and Autonomic Function Following Electroconvulsive Shock, Circulatory Shock, or Central Nervous System Injury

Since the pioneering work of Selye defining the global response to stress, many investigators throughout the world have confirmed that endocrine, autonomic, psychologic, metabolic, immune and other biologic systems are altered by behavioral or physiologic stressors. The enhanced release or inhibition of many biologic mediators by stressful situations has well-defined functional consequences, affecting behavioral and physiologic endpoints. However, many of these mediators do not play an important role in normal biologic functions, and their actions can only be discerned following their release in dyshomeostatic situations, such as those typified by the stress of “fight or flight” reactions. The endogenous opioid systems belong to this category of stress mediators.

EEG ALTERATIONS INDUCED IN RATS BY ETHYLKETOCYCLAZOCINE, CYCLAZOCINE, AND MORPHINE: THE DIFFERENTIAL EFFECTS OF NALOXONE

Intraperitoneal administration of ethylketocyclazocine, cyclazocine, and morphine each produced alterations in the spontaneous EEG and behavior of the rat characterized by high-voltage, slow-frequency waves associated with behavioral stupor during wakefulness. The initial effect of ethylketocyclazocine was an immediate high-voltage, slow-frequency EEG associated with an intense behavioral stupor. In contrast, the earliest EEG response to cyclazocine and morphine administration was the appearance of intermittent slow-wave bursts associated with a light behavioral stupor. The effects of the three agonists were antagonized in a dose-related manner by pretreatment with naloxone. Ethylketocyclazocine was the most sensitive to naloxone challenge (AD50 = 0.035 mg/kg of naloxone, s.c.) and cyclazocine, the least sensitive (AD50 = 2.1 mg/kg). Antagonism of the drug-induced, increased latency to slow-wave sleep was dose-related for ethylketocyclazocine and morphine. There was a weak, but not dose-related, antagonism of cyclazocine-induced suppression of slow-wave sleep in the rat. Although ethylketocyclazocine, cyclazocine, and morphine produced similar EEG changes, the compounds could be clearly distinguished on the basis of their differential sensitivities towards naloxone. This latter finding is, therefore, consistent with current theories of multiple receptors mediating certain effects of opioids.