Penicillin-G induced interictal activity increases both opioid peptide tissue content and in vitro release in the rat brain

Evidence for changes in brain enkephalin contents associated to male rat sexual activity

Indirect evidence suggests that ejaculation might activate endogenous opioid systems, which exert an inhibitory influence on male rat sexual behaviour. The objective of the present study was to search for putative long-term changes in the contents of immunoreactive (IR) Met-enkephalin (IR-Met), Leu-enkephalin (IR-Leu) and opioid octapeptide Met--Arg(6)--Gly(7)--Leu(8) (IR-Oct) in specific brain areas, after the execution of different amounts of sexual activity. Additionally, basal contents of these enkephalins were compared between sexually active (SA) and persistent sexually inactive (SI) rats. Immunoreactivity to enkephalins was determined by radioimmunoanalysis, in the frontal cortex, the hypothalamus and midbrain of SA and SI rats, as well as 24 or 48 h after males had one ejaculation or copulated to exhaustion. Twenty-four hours after sexual activity, there was a generalised increase in enkephalin contents that returned to control values at the 48 h measurement in all brain areas, but the hypothalamus, where IR-Met and IR-Oct remained elevated. No differences in the magnitude of the changes were found between rats that ejaculated once and sexually satiated males. IR-Oct concentration in the hypothalamus of SI rats appeared significantly higher than in SA animals, with no differences in IR-Met and IR-Leu. Results give direct evidence of the activation of endogenous opioid systems by male rat sexual activity. The occurrence of long lasting increases in the contents of IR-Met and IR-Oct in the hypothalamus of rats that copulated was detected. Finally, an intrinsically elevated octapeptide concentration in the hypothalamus of SI rats was found.

Withdrawal from alcohol in withdrawal seizure-prone and -resistant mice: evidence for enkephalin resistance

Methionine enkephalin (Met-enkephalin) functions as an endogenous anticonvulsant. Peptide transport system-1 (PTS-1) is an important regulator of Met-enkephalin levels in brain and transports the peptide from brain to blood. In outbred mice, alcohol dependence is associated with decreased PTS-1 activity and increased levels of Met-enkephalin. In contrast, alcohol withdrawal is associated with recovery of PTS-1 activity, decreased levels of Met-enkephalin, and seizures. In this study, we examined the PTS-1/Met-enkephalin system in two replicates of withdrawal seizure-resistant (WSR) and withdrawal seizure-prone (WSP) mouse lines. We measured levels of preproenkephalin (PPE) mRNA and Met-enkephalin peptide in brain and the activity of PTS-1 during alcohol-naive, -dependent, and -withdrawal states. In alcohol-naive animals, Met-enkephalin levels were higher in WSP than in WSR mice. In alcohol-withdrawal animals, Met-enkephalin levels remained elevated in WSP mice, whereas they increased in WSR mice. Peptide levels were unrelated to levels of PPE mRNA or activity of PTS-1. Factorial analysis showed that proneness to seizures was genetically linked to Met-enkephalin levels in alcohol-naive, -dependent, and -withdrawing mice but not to mRNA levels or PTS-1 activity. Overall, these results may be explained by resistance to enkephalin in WSP mice and suggest that the dysregulation of the PTS-1/Met-enkephalin system contributes to susceptibility to seizures in WSP mice.

Long-term pharmacological kindling increases in vitro release of IR-Met and IR-Leu-enkephalin from amygdala

Met-enkephalin release is increased from amygdala and striatum 1 and 15 days after pharmacological kindling with pentylenetetrazol, following potassium-induced depolarization in vitro via a Ca2+ dependent mechanism. Leu-enkephalin release was only enhanced in amygdala and striatum 1 day after the last seizure. IR-Met-enkephalin amygdala tissue content enhanced 1 and 15 days after seizure. In striatum, we found an IR-Met-enkephalin decrease 35 days after the last stimulus. IR-Leu-enkephalin amygdala tissue content enhanced 1 day after the last seizure, and no significant increases were found in striatum 1, 15 and 35 days after the last seizure. In this paper, we show that opioid peptides release is differentially enhanced in rat brain for several days after the last seizure, thus suggesting that opioid peptides may have a protective action against seizure activity.

Diurnal variations of opioid peptides and synenkephalin in vitro release in the amygdala of kindled rats

Pentylenetetrazol (PTZ) kindling was induced in male Wistar rats (250-300 g) by daily intraperitoneal injections of 35 mg/kg of the convulsant agent. Immunoreactive (IR)-Met-enkephalin (IR-ME), IR-Leu-enkephalin (IR-LE), IR-heptapeptide (IR-HE), IR-octapeptide (IR-OC) and IR-synenkephalin (IR-Syn) in vitro release was measured from amygdala slices 24 h after the last stimulus, in groups of eight rats, every 4 h beginning at 08:00 h. Opioid peptides in vitro release displayed diurnal variations. IR-ME and IR-Syn showed maximal levels before the onset of darkness (16:00 h). IR-LE and IR-OC release was enhanced 4 h later (20:00 h), no changes were detected for IR-HE. These results show that endogenous opioid system (EOS) release displays diurnal variations. The peak for the analysed peptides was reached before and during the dark phase. It is suggested that EOS release enhancement in PTZ-kindled rats, seems to be due to a compensatory mechanism against the excitation induced by the blockade of the GABAergic transmission.

Endogenous opiates: 1995

This article is the eighteenth installment of our annual review of research concerning the opiate system. It includes articles published during 1995 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects. The specific topics covered this year include stress: tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.