Dominant mice show much lower concentrations of methionine-enkephalin in brain tissue than subordinates: cause or effect?

Enkephalin release from VIP interneurons in the hippocampal CA2/3a region mediates heterosynaptic plasticity and social memory

The hippocampus contains a diverse array of inhibitory interneurons that gate information flow through local cortico-hippocampal circuits to regulate memory storage. Although most studies of interneurons have focused on their role in fast synaptic inhibition mediated by GABA release, different classes of interneurons express unique sets of neuropeptides, many of which have been shown to exert powerful effects on neuronal function and memory when applied pharmacologically. However, relatively little is known about whether and how release of endogenous neuropeptides from inhibitory cells contributes to their behavioral role in regulating memory formation. Here we report that vasoactive intestinal peptide (VIP)-expressing interneurons participate in social memory storage by enhancing information transfer from hippocampal CA3 pyramidal neurons to CA2 pyramidal neurons. Notably, this action depends on release of the neuropeptide enkephalin from VIP neurons, causing long-term depression of feedforward inhibition onto CA2 pyramidal cells. Moreover, VIP neuron activity in the CA2 region is increased selectively during exploration of a novel conspecific. Our findings, thus, enhance our appreciation of how GABAergic neurons can regulate synaptic plasticity and mnemonic behavior by demonstrating that such actions can be mediated by release of a specific neuropeptide, rather than through classic fast inhibitory transmission.

Hierarchy in the home cage affects behaviour and gene expression in group-housed C57BL/6 male mice

Group-housed male mice exhibit aggressive behaviour towards their cage mates and form a social hierarchy. Here, we describe how social hierarchy in standard group-housed conditions affects behaviour and gene expression in male mice. Four male C57BL/6 mice were kept in each cage used in the study, and the social hierarchy was determined from observation of video recordings of aggressive behaviour. After formation of a social hierarchy, the behaviour and hippocampal gene expression were analysed in the mice. Higher anxiety- and depression-like behaviours and elevated gene expression of hypothalamic corticotropin-releasing hormone and hippocampal serotonin receptor subtypes were observed in subordinate mice compared with those of dominant mice. These differences were alleviated by orally administering fluoxetine, which is an antidepressant of the selective serotonin reuptake inhibitor class. We concluded that hierarchy in the home cage affects behaviour and gene expression in male mice, resulting in anxiety- and depression-like behaviours being regulated differently in dominant and subordinate mice.

Social dominance rank influences wheel running behavior in mice

Dominance hierarchies within social groups determine resource distribution. Resources, such as food and access to mating partners, can act as reinforcers. The present study examined the effect of social rank on access to wheel running-a reinforcing behavior performed by laboratory animals. Mice were identified as dominant or subordinate and given access to a running wheel access under solitary or social conditions. In the solitary condition, subordinate and dominant mice spent equal amounts of time on the running wheel. In the social condition, when one wheel was present, subordinate mice spent less time on the wheel than did dominant mice. Conversely, when two wheels were present, subordinates spent more time on the wheel than did dominant mice. When mice were given 24h access to one running wheel in the social condition, dominant mice ran more than subordinates during the dark cycle. Subordinate mice did not compensate for the lack of running wheel access by schedule shifting. These results suggest that social rank influences access to reinforcers by behavioral interference rather than by social inhibition.

Use of preproenkephalin knockout mice and selective inhibitors of enkephalinases to investigate the role of enkephalins in various behaviours

RATIONALE

The most simple and efficient method to study the physiological role of enkephalins is to increase the lifetime of these endogenous opioid peptides by inhibiting their inactivating enzymes. Enkephalins are degraded by the concomitant action of two metallopeptidases: neutral endopeptidase (NEP, EC3.4.21.11) and aminopeptidase N (APN, EC3.4.11.2), both enzymes releasing inactive metabolites.

OBJECTIVES

Potent dual inhibitors have been developed, such as RB101. However, NEP and APN have a broad specificity and can cleave various peptides in vitro. Therefore, it was essential to investigate the specific involvement of enkephalins in the various pharmacological responses induced by dual inhibitors.

MATERIALS AND METHODS

We compared the pharmacological responses induced by RB101 in wild-type and preproenkephalin-deficient mice (Penk1-/-) using several behavioural assays.

RESULTS

In all the tests used (hot plate test, force swim test, castor-oil-induced diarrhoea), RB101 induced strong effects in wild-type animals, whereas slight effects were observed in Penk1-/- animals. These residual effects are blocked by pre-administration of the opioid antagonist naloxone, supporting the involvement of the opioid receptors in the responses observed.

CONCLUSIONS

The pharmacological effects induced by dual inhibitors acting on both NEP and APN are mainly due to the protection of the endogenous enkephalins at supraspinal and peripheral levels. It could be speculated that the residual effects observed in Penk1-/- mice after RB101 administration could be due to the direct action of other opioid peptides or through an indirect effect involving the protection of other peptide substrates of NEP or APN, as substance P or angiotensin.

Physiological control of emotion-related behaviors by endogenous enkephalins involves essentially the delta opioid receptors

The endogenous pentapeptide enkephalins bind to the mu and delta opioid receptors, with a slightly higher affinity for the latter. It remains a controversy regarding the respective physiological role of mu and delta opioid receptors in the control of emotion and motivation. One of the difficulties to investigate this problem is the low tonic extracellular release of enkephalins in various brain structures. To overcome this problem the synaptic levels of these pentapeptides were enhanced by inhibition of enzymes involved in their catabolism with the selective inhibitor H3N-CH(CH2-CH2-S-CH3)-CH2-S-S-CH2-CH(CH2phi)-CONH-CH(CH2phi)-COOCH2phi (RB101). This compound was shown to increase the extracellular levels and lifetime of endogenous enkephalins. Similar responses were obtained in wild-type and mu opioid receptor knockout mice following RB 101 administration in behavioral tests measuring locomotor activity, anxiety (elevated O-maze), and motivation (forced swim test and conditioned suppression of motility). In contrast, RB 101 led to antinociceptive responses only in wild-type animals using hot plate and tail immersion tests. These results clearly demonstrate the critical role of delta opioid receptors activated by the endogenous opioid peptides, in the physiological control of emotion- and motivation-related behaviors. In contrast, antinociceptive modulation, at least with respect to thermal nociceptive stimuli, involves enkephalin-activated mu opioid receptors. These findings could open new perspectives in the treatment of mood disorders using either inhibitors of enkephalin catabolism or delta opioid agonists.

Distribution of methionine and leucine enkephalin neurons within the social behavior circuitry of the male Syrian hamster brain

Enkephalin plays a role in the social behaviors of many species, but no corresponding role for this peptide has been investigated in the male Syrian hamster, a species in which brain nuclei controlling social behaviors have been identified. Previous studies have shown the distribution of dynorphin and beta-endorphin throughout social behavior circuits within the male hamster brain. To date, the only studies of enkephalin in the hamster brain address the distribution of this peptide in the olfactory bulb and hippocampus. The present study provides a complete map of enkephalinergic neurons within the forebrain and midbrain of the male Syrian hamster and addresses the question of whether enkephalin immunoreactive (Enk-ir) cells are found within brain regions relevant to male hamster social behaviors. Following immunocytochemistry for either methionine enkephalin (met-enkephalin) or leucine enkephalin (leu-enkephalin), we observed enkephalin localization consistent with data that have previously been reported in the rat, with notable exceptions including lateral septum, ventromedial nucleus of the hypothalamus and cingulate gyrus. Additionally, met- and leu-enkephalin localization patterns largely overlap. Consistent with the post-translational processing of preproenkephalin, met-enkephalin was more abundant than leu-enkephalin both within individual cells (darker staining), and within given brain nuclei (more met-enkephalin immunoreactive cells). Two exceptions were the posterointermediate bed nucleus of the stria terminalis, containing more neurons heavily labeled for leu-enkephalin, and the main olfactory bulb, where only met-enkephalin was observed. Of most interest for this study was the observation of Enk-ir cells and terminals in areas implicated in both sexual and agonistic behaviors in this species.

Reduced aggression in AMPA-type glutamate receptor GluR-A subunit-deficient mice

The importance of AMPA-type glutamate receptors has been demonstrated in neuronal plasticity and in adaptation to drugs of abuse. We studied the involvement of AMPA receptors in social interaction and anxiety and found that in several paradigms of agonistic behavior naïve male mice deficient for the GluR-A subunit- containing AMPA receptors are less aggressive than wild-type littermates. GluR-A deficient mice and wild-type littermates exhibited similar basic behavior and reflexes as monitored by observational Irwin's test, but they tended to be less anxious in elevated plus-maze and light-dark tests. Maternal aggression or male-female encounters were not affected which suggests that male hormones are involved in the expression of suppressed aggressiveness. However, testosterone levels and brain monoamines can be excluded and found to be similar between GluR-A deficient and wild-type littermates. The reduced AMPA receptor levels caused by the lack of the GluR-A subunit, and measured by a 30% reduction in hippocampal [3H]-S-AMPA binding, seem to be the reason for suppressed male aggressiveness. When we analyzed mice with reduced number of functional AMPA receptors mediated by the genomic introduced GluR-A(Q582R) channel mutation, we observed again male-specific suppressed aggression, providing additional evidence for GluR-A subunit-containing AMPA receptor involvement in aggression.

Pain responses, anxiety and aggression in mice deficient in pre-proenkephalin

Enkephalins are endogenous opioid peptides that are derived from a pre-proenkephalin precursor protein. They are thought to be vital in regulating many physiological functions, including pain perception and analgesia, responses to stress, aggression and dominance. Here we have used a genetic approach to study the role of the mammalian opioid system. We disrupted the pre-proenkephalin gene using homologous recombination in embryonic stem cells to generate enkephalin-deficient mice. Mutant enk-/- animals are healthy, fertile, and care for their offspring, but display significant behavioural abnormalities. Mice with the enk-/- genotype are more anxious and males display increased offensive aggressiveness. Mutant animals show marked differences from controls in supraspinal, but not in spinal, responses to painful stimuli. Unexpectedly, enk-/- mice exhibit normal stress-induced analgesia. Our results show that enkephalins modulate responses to painful stimuli. Thus, genetic factors may contribute significantly to the experience of pain.

Endogenous opiates: 1990

This paper, an examination of works published during 1990, is thirteenth in a series of our annual reviews of the research involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence, eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, pregnancy, development, and aging; immunological responses; and other behavior.