Enkephalin may mediate euphoria and drive-reduction reward

Opiate modification of intracranial self-stimulation in the rat

Studies were conducted to confirm the involvement of central opiate receptors in the expression of opiate modulation of intracranial self-stimulation (ICSS). Biphasic, dose-related changes in ICSS responding are described following IP administration of morphine sulfate (1-25 mg/kg) and levorphanol tartrate (LEV, 0.5-5 mg/kg). Similar patterns of response modification are reported following intraventricular (IVt) administration of LEV (0.01-0.2 muMoles) LEV's enantiomorph, dextrorphan, was not found to elicit comparable effects after either IP or IVt administration. Both the facilitatory and the depressant phases of LEV's action were antagonized by naltrexone (10 microgram, IVt), which had no apparent effect on ICSS by itself. Complete tolerance developed to the suppression of responding by 2.5 mg/kg LEV (IP) but not to the facilitatory effect of 0.5 mg/kg (IP), during a 5-day course of administration. The implications of these results for opiate reinforcement theory are discussed and possible mechanisms are advanced.

The enkephalins. Peptides with morphine-like activity

Two penta-peptides with morphine-like properties have recently been found in the nervous system and gastrointestinal tract. These peptides known as enkephalins are unevenly distributed in the brain, the highest concentration being in the globus pallidus. The enkephalins are found to have analgesic properties and are also anti-tussive, anti-diarrheal and able to stimulate release of prolactin and growth hormone. Synthetic analogues of the enkephalins containing specific D-amino acid substitutions have been developed and show a wide spectrum of activities.

Effects of morphine and naloxone on separation distress and approach attachment: evidence for opiate mediation of social affect

In order to determine the relationship between endorphins and social attachment, the effects of morphine (an opiate agonist) and naloxone (an opiate antagonist) on various indices of attachment in guinea pigs were studied. In infants, crying or separation-induced distress vocalizations were significantly decreased by single injections of low morphine doses (0.25, .050 and 0.75 mg/kg) in a dose-dependent manner. Naloxone (1mg/kg1 reliably increased separation distress vocalizations in both juvenile and adult guinea pigs. Therefore, similar to opiate withdrawal symptoms, separation distress appeared to be alleviated by morphine and potentiated by naloxone. As for approach attachment, offspring/maternal proximity-maintenance time was significantly decreased by morphine (1.0, 2.5 and 5.0 mg/kg), suggesting that opiates may be capable of replacing a function normally subserved by endorphins in reinforcing attachments. These data support the hypothesis that an endorphin-based addiction-like process may underlie the maintenance of social attachments, and that separation distress may reflect a state of endogenous "endorphin withdrawal".

Epileptic properties of leucine- and methionine-enkephalin: comparison with morphine and reversibility by naloxone

Morphologically similar epileptic seizures were recorded from the cortex of rats after injections into the lateral ventricle of 100 microgram of leucine-enkephalin, methionine-enkephalin, and morphine. Seizures were either greatly attenuated or blocked completely by prior systemic administration of naloxone (10 mg/kg). These findings suggest that such seizures result from an interaction of these compounds with opiate receptors in the brain. The epileptogenic potency of the enkephalins was illustrated by the observation that seizures and other pathological manifestations could still be elicited by doses as low as 10 microgram. Leucine-enkephalin was seen to have greater epiliptic potency than methionine-enkephalin. At doses of 1 microgram both enkephalins typically evoked cortical spindles resembling those seen in drowsy animals. Enkephalin-induced analgesia was seen in only one animal at the 100 microgram dose. Results obtained with repeated injections of morphine suggest that the epileptogenic effect of opiates may be subject to either tolerance or potentiation, depending on the prior occurrence of seizures. A synthesis of the present findings with several other lines of evidence suggests both that endogenous enkephalins play some role in normal mechanisms of reward, and that, when regulatory processes are disturbed, they may contribute as well to the elaboration of certain epileptic phenomena.

Different brain areas mediate the analgesic and epileptic properties of enkephalin

Single injections of 120 micrograms of methionine-enkephalin were made into various midbrain and forebrain structures in the rat. Analgesia was observed after injections into or near the ventral, caudal midbrain periaqueductal gray matter. Seizures and other pathological electroencephalogram (EEG) changes were seen with injections into or near the forebrain dorsomedial nucleus of the thalamus. No animals with midbrain injection sites showed EEG changes, and none with forebrain injection sites were analgesic. These data, taken together with other lines of evidence, suggest that enkephalin-induced analgesia and enkephalin-induced seizures are mediated by opiate receptors that are located in different brain areas and that are pharmacologically different.

Effect of substance P on medial forebrain bundle self-stimulation in rats following intracerebral administration

The effect of Substance P infused intracerebrally via chronically implanted electrode-cannulae on self-stimulation induced from the same site was studied in rats. Substance P caused a significant depression of self-stimulation at 60 and 120 microgram/rat. Morphine infused into this site also caused significant depression of self-stimulation, but the doses were considerably lower than those of Substance P (5 and 10 microgram/rat). Pretreatment with naloxone, a narcotic antagonist, significantly antagonized the effects of Substance P on self-stimulation. It is proposed that Substance P modulates self-stimulation by the release of an endogenous morphine-like substance, but the possibility of a direct effect of Substance P was not ruled out.