Brain content of leu5- and met5-enkephalin changes independently during the development of kindling in the rat

Epilepsy: A bibliometric analysis (1968-2020) of the Instituto Nacional de Neurología y Neurocirugía "Manuel Velasco Suarez" in Mexico

The Instituto Nacional de Neurología y Neurocirugía "Manuel Velasco Suárez" (INNN) is one of the main institutions in Latin America treating epilepsy; and bibliometric analysis has an increasing role in analyzing the literature, acting as a Google Maps of medical research. We tracked the scientific output in Scopus and the impact of the institution from its foundation to July 2020 in the field of epilepsy. We roughly separated this group by clinical and experimental approach, identifying core journals, type of article, increase with time, and number of citations. A total of 228 papers, from a total of 3,034 produced by the INNN in that period, were found. Additionally, we identified that neurocysticercosis, pharmacology, genetics, and proteins involved in epilepsy were the most investigated topics. Also, there is a sustained growth in the number of papers per year since 1985. The number of authors per paper ranges from one to 15, and neuroscience journals are the preferred target of researchers, with a predilection for "Epilepsy and Behavior".

Orphanin-FQ/nociceptin inhibits kindling epileptogenesis and enhances hippocampal feed-forward inhibition

The role of Orphanin-FQ/nociceptin in synaptic plasticity was assessed by its potency in modulating kindling epileptogenesis in vivo, and feed-forward inhibition in hippocampal recordings in vitro. In addition, a specific rabbit antiserum against this peptide was obtained and the immunohistochemical distribution of nociceptin was determined in rat brain slices. After the establishment of kindling epilepsy, by daily electrical stimulation of the piriform cortex, the i.c.v. injection of nociceptin, 20 min before the kindling stimulation, was not able to block the generation of the generalized seizures, nor to alter their duration. However, the i.c.v. injection of nociceptin, 20 min before each stimulation along the kindling process, depressed its development in a dose-dependent manner. This effect was specific since the nociceptin antagonist [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2, but not the broad-spectrum opiate antagonist, naloxone, was able to completely block nociceptin actions. The inhibitory role of nociceptin was assessed by in vitro recordings from entorhinal cortex-hippocampal slices. By single pulses applied over the Schaffer collaterals, we found that synaptic transmission was facilitated onto CA1, but using a paired-pulse protocol, we found that nociceptin potentiated feed-forward inhibition. The immunohistochemical data show that nociceptin is expressed in limbic cortical regions, including the piriform cortex and the hippocampus. Our results demonstrate that nociceptin exerts a modulatory role in limbic excitability and suggest that it provides an inhibitory control in the development of epilepsy by possibly inhibiting the spread of excitation through the system, by favoring feed-forward inhibition.

Regional brain IR-Met-, IR-Leu-enkephalin concentrations during progress and full electrical amygdaloid kindling

Using amygdaloid kindling in chronic rats, we were able to observe behavioral, electrographic and IR-Met- and IR-Leu-enkephalin changes throughout the progress of different stages of convulsive activity. Rats presenting the initial stages of kindling, rats presenting the first generalized motor seizure, and rats with at least 10 generalized seizures were sacrificed 24 h after the last stimulus; also rats with at least 10 generalized seizures but sacrificed 21 days after the last seizure were compared with control and sham-operated groups of rats. The IR-Met and IR-Leu enkephalin concentrations in each group were measured in the striatum, amygdala, hypothalamus, medulla oblongata (including pons), hippocampus, mid-brain, spinal cord and cerebral cortex. A progressive increase in IR-Leu-enkephalin in amygdala and hippocampus was observed over the course of kindling. These increases remained until 21 days after rats were fully kindled (at least 10 generalized seizures). We observed increased and decreased concentration of each peptide in different regions. We discussed the regional and the differential effects of each peptide. The increased concentrations in limbic structures were associated with the amygdaloid increased excitability through the kindling process. We suggest that the decreases in concentrations are related with structures involved in the output behavior manifestations produced by kindling stimulation.

Alteration of opioid receptors in seizure-susceptible El mouse brain

The distribution density of opioid receptors in the brain of El mice (seizure-susceptible strain) was examined to determine the relation between seizures and the opioid system. Saturation curves and Scatchard plots of [3H]2-D-alanine-5-D-leucine enkephalin binding revealed that the opioid delta receptor density in adult El mice during interictal periods was significantly increased in the cerebral cortex, hippocampus, and septal area. It was further shown that the concentration of such receptors in 25-day-old El mice that had no seizures was also significantly increased in the hippocampus and septal area, with no changes in apparent affinities, as compared with in the corresponding regions in ddY mice (seizure-nonsusceptible strain; the mother strain of El). Such up-regulation of opioid receptors in the El mouse brain could result from deficits in endogenous opioid peptides, which could be associated with the pathogenesis of seizure diathesis in the El mouse.

Differences in ME-LI and VIP-LI in discrete brain regions of seizure-naive and seizure-experienced El mice

In an attempt to elucidate the relationship between endogenous methionine-enkephalin (ME) and vasoactive intestinal polypeptide (VIP) with generalized seizures, we determined regional brain levels of ME-like and VIP-like immunoreactivity (ME-LI and VIP-LI) in El mice during and after seizures induced by repeated tossing stimulation. The levels of ME-LI in the striatum and hippocampus of seizure-naive El mice (El-) were lower than those of the control ddY mice, the mother strain of El mice. Conversely, the level of VIP-LI in the medulla oblongata and pons of El- was higher than that of ddY mice. The level of ME-LI in the striatum of seizure-experienced El mice (El+) killed 96 hours after three consecutive seizures was high, while levels of VIP-LI in the striatum and hypothalamus were low, in comparison to those of El- mice. A detailed time-course study revealed that seizures in El mice caused (1) significant decreases in levels of ME-LI in the striatum and hippocampus during seizures, (2) a significant decrease of VIP-LI content in the striatum 3 hours after seizures, and (3) a significant increase in hypothalamic VIP-LI 9 hours after seizures. These observations suggest that ME and VIP may play some role in El mouse seizures.

Epileptogenesis and muscular hypertonic postictal phenomena induced by naloxone in intact cats

Epileptogenesis produced by repeated i.p. administration of naloxone chloride and sensory stimulation (photoacoustic stimulation at 1, 3, 10, and 15 Hz) every 15 min was studied in freely moving cats. The repeated administration of naloxone provoked some behavioral manifestations that resemble those produced by electrical amygdaloid kindling. Photoacoustic stimulation accentuated the manifestations. All the animals presented generalized behavioral seizures when total naloxone administration reached 80 mg/kg. None of the animals demonstrated postictal depression. These results suggest an inhibitory role of endogenous opioids and/or GABA in epileptogenesis.

An in vitro autoradiographic analysis of mu and delta opioid binding in the hippocampal formation of kindled rats

Recent studies have shown that opioid peptide levels are altered in hippocampal formation of kindled animals. We therefore studied the distributions of mu and delta opioid binding sites in hippocampal formation of kindled and control rats using quantitative in vitro autoradiography. Animals received daily stimulations of the amygdala until they experienced 3 class 5 seizures. Paired control animals underwent implantation of electrodes but were not stimulated. Mu binding sites were labeled with 125I-FK-33824. Twenty-four hours after the last kindled seizure, mu binding was decreased by 32% in stratum pyramidale of CA1 and stratum radiatum of CA2 and by 17-27% throughout most of the rest of CA1, CA2, and CA3. Few, if any, differences were seen between kindled and control animals at 7 or 28 days after the last kindled seizure. Delta binding sites were labeled with 125I-[D-Ala2,D-Leu5]enkephalin in the presence of the morphiceptin analog PL-032. Twenty-four hours after the last kindled seizure, delta binding was decreased only in stratum moleculare of the dentate gyrus. Seven days after the last kindled seizure, delta binding was decreased by 11-17% throughout CA1, CA3, and the dentate gyrus. At 28 days after the last seizure, however, no differences were found between kindled and control animals. Since the decreases in mu and delta opioid binding are transient, they are unlikely to be the molecular basis of the permanent kindling phenomenon. Rather, these changes in opioid binding may represent responses to repeated seizures.

Effect of opiates on the parameters of seizures in rats with full amygdaloid-kindled convulsions

The effect on the parameters of seizures of opiates, administered in doses used clinically for analgesic effects, was studied in rats with full amygdaloid-kindled seizures. The largest dose of fentanyl studied (100 micrograms/kg) had a pronounced inhibitory effect on kindled seizures: severity of seizures, duration of seizures and duration of afterdischarge were significantly reduced to 36, 40 and 37% of controls, respectively, and the latency of seizures was significantly increased to 168% of untreated animals. The largest dose of pentazocine (16 mg/kg) also significantly inhibited the duration of seizures and duration of afterdischarge. Morphine (1-4 mg/kg) and meperidine (4-16 mg/kg) had a tendency to inhibit the duration of seizure and afterdischarges but did not significantly affect any of the measured parameters of seizures. Fentanyl, meperidine and pentazocine resulted in a lowering, whereas morphine caused a slight elevation, of the threshold for initiation of kindled seizures. The data suggest that fentanyl, in relatively small doses, may cause an inhibition of the intensity of behavioural and electrographic seizures but, paradoxically, an increased sensitivity to induction of seizures in rats with full amygdaloid-kindled seizures.

Convulsions may alter the specificity of kappa-opiate receptors

Morphine, a mu-opiate agonist, and ethylketazocine, a kappa-opiate agonist, produce distinct behavioral, pharmacologic, and biochemical effects. In the mouse, large doses of morphine produce convulsions that are usually lethal and that cannot be blocked by naltrexone, whereas ethylketazocine produces nonlethal clonic convulsions that can be blocked by naltrexone. Moreover, mice made tolerant to morphine failed to show cross-tolerance to ethylketazocine, suggesting that the convulsions induced by these drugs are not mediated via a common opioid mechanism. Following a series of electroconvulsive shocks, both morphine and ethylketazocine produced clonic convulsions that were not lethal and that could be blocked by naltrexone. Furthermore, electroconvulsive shock-treated animals made tolerant to morphine-induced convulsions showed cross-tolerance to ethylketazocine. These data suggest that electroconvulsive shock may alter kappa-opioid systems in such a way as to allow mu-agonists to be functional at these sites.

Morphine responsiveness and seizure proneness

Previous research demonstrated that following amygdala kindling, animals showed a heightened sensitivity to morphine's convulsive effects and an exaggerated Straub tail response. These effects were evident to 3 months after their last convulsion and could be blocked by naloxone pretreatment. The present paper extends these findings by demonstrating that animals given metrazol or electroshock (ECS) convulsions also showed an enhanced morphine response that was blocked by naltrexone. Both metrazol- and ECS-treated animals convulsed in response to doses of morphine that produced little or no effect in control animals. In addition, it was shown that brain damage induced by electrode implantation or neocortex penetration by skull screws also increased an animal's sensitivity to morphine even in the absence of prior convulsions. This effect, however, could not be blocked by naltrexone. Finally, as opiate receptors vary with the diurnal rhythm, we determined that following amygdala kindling, animals are more sensitive to morphine's convulsive action during their dark phase when receptor number and sensitivity are highest. The results indicated that seizure proneness, whether induced by a history of prior convulsions or brain damage, increased sensitivity to morphine. This effect may be due to a change in opiate receptors only when prior convulsions have occurred.

Activating endogenous opioid systems by electroconvulsive shock or footshock stress inhibits recurrent kindled seizures in rats

Electroconvulsive shock (ECS) significantly decreased the behavioral manifestations of seizures elicited by amygdaloid stimulation in kindled rats. This anticonvulsant effect was significantly reduced by the opiate antagonist, naloxone, and by the development of morphine tolerance. A form of footshock stress known to cause opioid-mediated analgesia had a similar anticonvulsant effect, whereas another form causing non-opioid analgesia did not. These results suggest that the anticonvulsant effects of ECS and stress are mediated by the release of endogenous opioids.

Cataleptic effects of opiates following intrathecal administration

Intrathecal (i.t.) administration of D-ala2-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 micrograms. 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.

Pentylenetetrazol kindling produces a long-lasting elevation of IR-Met-enkephalin but not IR-Leu-enkephalin in rat brain

Pentylenetetrazol (PTZ) kindling was induced in male Wistar rats by daily i.p. injections of 40 mg/kg of the convulsant agent. IR-Met-enkephalin and IR-Leu-enkephalin were quantified in several brain regions 16 days after the last stimulus. In addition other rats received another PTZ dose on the sixteenth day and the samples were analyzed 1 and 24 h later. The results showed a long-lasting elevation in amygdala, septum, hypothalamus and hippocampus of IR-Met-enkephalin only. These brain structures also showed a decrease of IR-Met-enkephalin 1 h after the PTZ-induced seizure, but reached newly elevated levels 24 h later. IR-Leu-enkephalin levels only showed a decrease in striatum 1 h after PTZ-induced convulsions. The data suggest that Met-enkephalin is related to permanent changes in brain function induced by PTZ kindling, while Leu-enkephalin may depend on the occurrence of epileptic seizures.

Pro- and anticonvulsant actions of morphine and the endogenous opioids: involvement and interactions of multiple opiate and non-opiate systems

The proconvulsant actions of high doses of systemic morphine are probably mediated by 3 different systems. One of them produces non-convulsant electrographic seizures and can be activated separately from the others both by intracerebroventricular injections as well as microinjections into discrete subcortical areas. The enkephalins and beta-endorphin, when administered to the same loci, produce similar effects. Pharmacological evidence suggests that specific opiate receptors of the delta-subtype mediate the epileptiform effects produced by this system. The second system mediating proconvulsant effects of systemic morphine is not mediated by stereo-specific opiate receptors. It produces behavioral convulsions, and the GABA-ergic system has been implicated in its action. A third proconvulsant action of systemic morphine can be activated separately from the other two systems by administering this compound with other convulsive agents or manipulations. Specific mu-type opiate receptors are implicated in this effect. In addition to potent proconvulsant effects, systemic morphine also has anticonvulsant properties which are mediated by specific opiate mu-receptors. The conditions under which morphine acts as a proconvulsant rather than an anticonvulsant agent are, as yet, not understood.

Brain content of immunoreactive [Leu5]enkephalin and [Met5]enkephalin after pentylenetetrazol-induced convulsions

Wistar rats were injected daily with convulsant doses of PTZ (40 mg/kg). The brain content of immunoreactive (IR) enkephalins was measured after repetitive clonic convulsions and after the 'kindling' state was reached. In both instances we found a significant increase of IR [Leu5]enkephalin and IR [Met5]enkephalin in septum, striatum and amygdala 24 h after PTZ administration. These results support the suggestion that the peptide increase occurs with repetitive convulsions and not only after the 'kindling' state is reached.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.