Prolyl-leucyl-glycinamide reduces aggression and blocks defeat-induced opioid analgesia in mice

The effects of Prolyl-leucyl-glycinamide (PLG, MIF-1) and the exogenous opiate antagonist naloxone, on aggressive interactions and defeat-induced analgesia were examined in male mice. Both substances reduced the number of bites required to obtain defeat in subordinate mice during aggressive encounters as well as blocking the subsequent defeat-induced analgesia. These results suggest that MIF-1 may function as an endogenous opioid antagonist and have an inhibitory influence on aggression.     

Inhibitory influences of MIF-1 (PLG) and Tyr-MIF-1 (YPLG) on aggression and defeat-induced analgesia in mice

The effects of prolyl-leucyl-glycinamide (MIF-1, PLG), tyrosine-prolyl-leucyl-glycinamide (Tyr-MIF-1, YPLG) and the exogenous opiate antagonist, naloxone, on aggressive interactions and defeat-induced analgesia were examined in male mice. All three substances reduced the number of bites required to obtain defeat in subordinate mice during aggressive encounters, as well as blocking subsequent defeat-induced analgesia. Tyr-MIF-1 had significantly greater inhibitory effects than MIF. These results suggest that both MIF and Tyr-MIF-1 may function as endogenous opioid antagonists and have inhibitory influences on aggression, with the antagonistic effects of Tyr-MIF-1 being more potent than those of MIF-1.     

Study of the antinociceptive effect of tetrahydropapaveroline derivatives. Interaction with opioids

The antinociceptive effect of racemic tetrahydropapaveroline (THP), of its two R(+)- and S(−) enantiomers, of 1-2-dehydro-THP and of 1-carboxy-THP was assessed using different pain tests in mice. None of these drugs possessed a significant activity in the hot-plate and tail-flick tests. However, after i.p. injection, they reduced the number of abdominal writhes induced by phenylbenzoquinone, with ED₅₀ values of 51 ± 7, 73 ± 9 and 79 ± 7 mg/kg for the most potent compounds: 1,2-dehydro-THP, ±THP and -THP, respectively. This activity was not antagonized by naloxone (1 mg/Kg, S.c.). However combination of inactive doses of these three compounds (32 mg/Kg, I.p.) and of morphine (0.5 mg/Kg, S.c.) led to a significant antinociceptive effect (83 to 85 % of reduction of the number of writhes). This synergistic potentiation confirmed with the combination of ±THP (16 mg/Kg, I.p.) and morphine (0.5 mg/Kg, S.c.) was totally inhibited by naloxone (1 mg/Kg, S.c.). These results, although excluding a direct agonistic effect of THP derivatives on opiate receptors, suggest an indirect interaction of these drugs with the endogenous opioid system.     

Effect of naltrindole on the development of physical dependence on morphine in mice: A behavioral and biochemical study

The effect of pretreatment with a δ opioid receptor antagonist, naltrindole (NTI), on the development of physical dependence on morphine was investigated in mice. Several withdrawal signs, an increase in cortical noradrenaline (NA) turnover and a decrease in dopamine (DA) turnover in the limbic forebrain were observed following naloxone challenge in morphine-dependent mice. Pretreatment with NTI (0.3–5 mg/Kg, S.c.) during chronic morphine treatment dose-dependently suppressed the behavioral and biochemical changes after withdrawal. The blocking effects of NTI suggest that δ opioid receptors may play a significant role in modulating the development of physical dependence on morphine.     

Antinociceptive effect of topiramate in models of acute pain and diabetic neuropathy in rodents

This study assesses the antinociceptive effect induced by different dosages of topiramate (TP), an anticonvulsant drug that is orally administered in models of neuropathic pain and acute pain in rats and mice, respectively. Orally administered TP (80 mg/Kg) in mice causes antinociception in the first and second phases of a formalin test, while in doses of 20 and 40 mg/Kg it was only effective in the second phase. TP (80 mg/Kg, p.o) also exhibited antinociceptive action in the hot plate test, however, it did not have an effect in the capsaicin test in mice, nor in the model of neuropathic pain in diabetic rats. The antinociceptive effect caused by TP (80 mg/Kg, p.o) in the formalin test was reversed by prior treatment with naloxone (opioid antagonist), but not with glibenclamide (antagonist of the potassium channel), ondansetron (antagonist of the serotonin 5HT3 receptor) or cyproheptadine (antagonist of the serotonin 5HT2A receptor).The data show that TP has an important antinociceptive effect in the models of nociception induced by chemical (formalin) or thermal (hot plate) stimuli, and that the opioid system plays a part in the antinociceptive effect, as shown by formalin.     

Effect of prolyl-leucyl-glycinamide and α-melanocyte-stimulating hormone on levorphanol-induced analgesia,tolerance and dependence

Prolyl-leucyl-glycinamide (PLG) at a low dose (10 ng/mouse) administered by an intracerebroventricular (i.c.v.) injection did not affect levorphanol analgesia, but PLG at higher doses (10 and 100 μg/mouse) and α-melanocyte-stimulating hormone (α-MSH) (10 ng/ mouse) antagonized levorphanol analgesia. Development of levorphanol tolerance was facilitated by 10 ng/mouse of PLG, unaffected by 10 μg/mouse of PLG, but antagonized by 100 μg/mouse of PLG and 10 ng/mouse of α-MSH. The effect of PLG on levorphanol dependence was assessed by changes in body weight and temperature during naloxone-induced withdrawal. PLG (10 ng/mouse) facilitated the development of levorphanol dependence, but 10 μg/mouse of PLG had no effect. PLG (100 μg/mouse) antagonized development of levorphanol dependence. PLG at doses of 10 and 100 μg/mouse precipitated withdrawal in levorphanol-dependent mice. α-MSH (10 ng/mouse) antagonized development of levorphanol dependence as evidenced by an increase in the ED50 of naloxone required to induce withdrawal jumping. These results indicate that PLG and α-MSH affected levorphanol-induced analgesia, tolerance and dependence in a qualitatively similar manner to their effect on morphine-induced analgesia, tolerance and dependence.     

Putrescine has analgesic activity, in rats

Putrescine, intraperitoneally injected into rats at doses of 200, 300 and 400 mg/Kg, had a dose-dependent analgesic effect (hot plate, 55.5 degrees C), which was not antagonized by naloxone. Analgesia was also obtained by intracerebroventricular injection (1,000 nmoles/rat). Chronic administration (300 mg/Kg/day i.p. for 8 consecutive days) resulted in the development of tolerance. In our experimental conditions, no obvious toxicity was observed. These results may suggest for polyamines a role in nociception, and may disclose a new class of analgesic drugs.     

Further evidence for the role of nitric oxide in maternal aggression: effects of L-NAME on maternal aggression towards female intruders in Wistar rats

It has been shown that nitric oxide (NO) increases aggression in male mice, whereas it decreases aggression in lactating female mice and prairie voles. It is also known that aggression can be exhibited at different levels in rodent species, strain or subtypes. The aims of this study were to investigate the proportion of aggressiveness in Wistar rats, the effect of intraperitoneally administered nonspecific nitric oxide synthase (NOS) inhibitor L-NAME (NG-nitro L-arginine methyl ester) on maternal aggression towards female intruders, and whether these effects are due to NO production or not. Rats were given saline intraperitoneally on the postpartum Day 2 and aggression levels were recorded. The same rats were given 60 mg/kg L-NAME or D-NAME (NG-nitro D-arginine methyl ester) on the postpartum Day 3 and their effects on aggression levels were compared to saline. While L-NAME administration did not cause any differences in the total number of aggressive behavior, aggression duration and aggression intensity, it reduced the proportion of animals showing aggressive behavior. In addition, the latency of the first aggression was significantly increased by L-NAME. In the D-NAME group, however, no significant change was found. Our results have shown that L-NAME reduces maternal aggression towards female intruders in Wistar rats through inhibition of NO production. These results suggest that the role of NO in offensive and defensive maternal aggression shares neural mechanisms.     

Protective effect of Crocus sativus stigma extract and crocin (trans-crocin 4) on methyl methanesulfonate-induced DNA damage in mice organs

This study was designed to examine the effect of aqueous extract of Crocus sativus stigmas (CSE) and crocin (trans-crocin 4) on methyl methanesulfonate (MMS)-induced DNA damage in multiple mice organs using the comet assay. Adult male NMRI mice in different groups were treated with either physiological saline (10 mL/Kg, intraperitoneal [ip]), CSE (80 mg/Kg, ip), crocin (400 mg/Kg, ip), MMS (120 mg/Kg, ip), and CSE (5, 20, and 80 mg/Kg, ip) 45 min prior to MMS administration or crocin (50, 200, and 400 mg/Kg, ip) 45 min prior to MMS administration. Mice were sacrificed about 3 h after each different treatment, and the alkaline comet assay was used to evaluate the effect of these compounds on DNA damage in different mice organs. The percent of DNA in the comet tail (% tail DNA) was measured. A significant increase in the % tail DNA was seen in nuclei of different organs of MMS-treated mice. In control groups, no significant difference was found in the % tail DNA between CSE- or crocin-pretreated and saline-pretreated mice. The MMS-induced DNA damage in CSE-pretreated mice (80 mg/Kg) was decreased between 2.67-fold (kidney) and 4.48-fold (lung) compared to those of MMS-treated animals alone (p < 0.001). This suppression of DNA damage by CSE was found to be depended on the dose, which pretreatment with CSE (5 mg/Kg) only reduced DNA damage by 6.97%, 6.57%, 7.27%, and 9.90% in liver, lung, kidney, and spleen, respectively (p > 0.05 as compared with MMS-treated group). Crocin also significantly decreased DNA damage by MMS (between 4.69-fold for liver and 6.55-fold for spleen, 400 mg/Kg), in a dose-dependent manner. These data indicate that there is a genoprotective property in CSE and crocin, as revealed by the comet assay, in vivo.     

Behavioral evidence for dopaminergic supersensitivity after chronic haloperidol

Chronic administration for 16 days of haloperidol (in increasing doses up to 20 mg/kg/day) results in a supersensitivity of dopamine receptors. This supersensitivity is manifested by an enhanced stereotypy and aggression in response to small, otherwise ineffective, doses of apomorphine. Maximum aggression is observed 7 days after the last dose of haloperidol when 2.5 mg/Kg of apomorphine is administered. In addition, “wet shakes”, reminiscent of withdrawal from morphine, are observed in these animals after the cessation of the haloperidol administration. These shakes are blocked by morphine. These results may be interpreted to mean that “wet shakes” and drug induced aggression are the results of hyperactivity of the dopaminergic system.     

Effects of nicotine on target biting and resident-intruder attack

The effects of acute administration of nicotine on target biting (defensive) and resident-intruder (offensive) attack of male mice were assessed. In the target biting procedure confined mice received tail shock on a fixed time, 2-min schedule. Under baseline conditions, biting attack directed toward an inanimate target occurred at three distinct rates. A high target biting rate (13.5 +/- 3.8 bites/15 sec) followed shock delivery, an intermediate biting rate (9.6 +/- 4.1 bites/15 sec) occurred during the inter-shock interval, and a low biting rate (1.0 +/- 0.5 bites/15 sec) occurred during a tone stimulus which signalled the impending shock. Nicotine (administered IP, 15 min presession) reduced post-shock and inter-shock interval target biting in a dose-dependent manner (ED50 values estimated at 0.13 and 0.14 mg/kg, respectively) but exerted more variable effects on target biting during the tone. In the resident-intruder paradigm the same mice were exposed to an intruder introduced into its home cage for a 10-min test session. Under baseline conditions, residents directed 20 +/- 3.2 biting attacks toward the intruder during the session with an average latency of 89 +/- 40 sec to the first attack. Nicotine caused a dose-dependent decrease in this attack behavior (ED50 values estimated to be 0.48 and 0.49 mg/kg, respectively). These observations are interpreted to indicate that nicotine has an increased potency at reducing "defensive" aggression.     

II. N-acetyl human β-endorphin-(1–31) alleviates the morphine withdrawal syndrome in rodents: A comparative study with clonidine

The potential effect of intracerebroventricular (icv) N-acetyl human β-endorphin-(1–31) on morphine dependence was examined in mice and rats. Animals were rendered tolerant-dependent by subcutaneous (sc) implantation of an oily suspension (10 ml/Kg mouse and 3 ml/Kg rat) containing 0.1 g/ml of morphine. After 72 h of chronic morphine, 1 mg/Kg sc naloxone precipitated in both species a withdrawal syndrome that was moderate in animals pretreated with the acetylated derivative of β-endorphin. Doses of 28 fmols/rat or 80 fmols/mouse N-acetyl human β-endorphin-(1–31) reduced the number of animals presenting the jumping behaviour, as well as the number of jumps recorded. Moreover, less than half of the rats presented the other withdrawal signs evaluated: squeak on touch, diarrhoea, chattering, chewing, ptosis and body shakes. This activity could be observed when N-acetyl human β-endorphin was injected 1 h to 24 h before naloxone; longer intervals resulted in a significant loss of this activity. The ₂ agonist clonidine given icv at pmol-nmol doses decreased the incidence of morphine withdrawal syndrome. Combinations of these two substances generally did not produce any further enhancement of the effects of clonidine and N-acetyl β-endorphin when used alone. Icv injections of the antagonist of ₂-adrenoceptors yohimbine prevented both clonidine and N-acetyl β-endorphin-(1–31) from reducing the jumping behaviour displayed by morphine-abstinent mice. It is suggested that N-acetyl β-endorphin produces this alleviation of the morphine withdrawal syndrome by improving the efficiency of ₂-mediated agonist effects after acting on a neural substrate that is distinct from the μ opioid receptor binding site.     

Participation of 5-HT1A-receptors in regulating various types of aggressive behavior

We studied the effects of selective agonists of 5-HT1A receptors 8-OH-DPAT and flesinoxan on aggressive behavior of C57BL/6 male mice in the "resident-intruder" test and on defensive aggression of Norway rats toward man. 8-OH-DPAT (0.4 mg/kg, i.p.) significantly reduced the intermale aggression in mice and defensive aggression in rats (0.1-0.5 mg/kg, i.p.). In the dose of 0.5 mg/kg, flesinoxan inhibited the aggressive behavior in mice. These results suggest that activation of 5-HT1A receptors reduces different kinds of affective aggression. The results are discussed in terms of interaction between the well-known anxiolytic effects of 5-HT1A agonists and their antiaggressive properties.     

What can animal aggression research tell us about human aggression?

Research on endocrinological correlates of aggression in laboratory animals is implicitly motivated by an expectation that the results of such studies may be applicable to human aggression as well. Research with a focus on the stimulus antecedents of aggression, its response characteristics, and its outcomes suggests a number of detailed correspondences between offensive aggression in laboratory rodents and human angry aggression. These include resource (including status and territory) competition as motives that are particularly elicited by conspecific challenge situations and, when the aggression is successful, outcomes of reduction of challenge and enhancement of resource control and status. Although the response characteristics of human aggression have been dramatically altered by human verbal, technological, and social advancements, there is some evidence for targeting of blows, similar to a well-established pattern for offensive aggression in many nonhuman mammals. Finally, for people as well as for nonhuman mammals, fear of defeat or punishment is a major factor inhibiting the expression of offensive aggression. While defensive aggression has been very little researched in people, it may represent a different phenomenon than angry aggression, again providing a parallel to the offense-defense distinction of laboratory rodent studies.     

Hypotensive effect of naloxone on high blood pressure induced by stress in the rat

A naloxone-reversible enhancement of systolic blood pressure (BP) was induced in rats by application of three different types of stressor, i.e. intense light and sound, cold and foot-shock. In the case of labile high BP provoked by short-term isolation, the opiate antagonist naloxone (1 mg/Kg, i.p.) was also found to reverse hypertension. Naltrexone (2.5 mg/Kg, i.p.) also diminished high BP readings of briefly isolated rats. Conversely, blockade of the opiate receptor with naloxone did not alter elevated BP in cases of established hypertension (spontaneously hypertensive rats, deoxycorticosterone (DOCA)-salt rats and long-term isolated rats). These data can be taken as an evidence of opioid involvement at the onset of high BP readings induced by stress. However, once hypertension becomes established, the opioid system appears to recover its silent features.     

Pharmacological activities of the MIF-1 analogues Pro-Leu-Gly, Tyr-Pro-Leu-Gly and pareptide

The pharmacological activities of the related free acid analogues of MIF-1, Pro-Leu-Gly (PLG) and Tyr-Pro-Leu-Gly (YPLG), were investigated because of the possibility that they may be formed during the digestion of milk and wheat proteins in vivo. The amino acid sequences-Tyr-Pro-Leu-Gly- and -Pro-Leu-Gly- are present in proteins from these foods. Chronic administration of either PLG (0.25 mg/kg, SC, BID) or the control substance, pareptide (0.25 mg/kg, SC, BID), antagonized the development of tolerance to the cataleptic effects of haloperidol in mice. The effect of YPLG (0.25 mg/kg, SC, BID) on the development of this tolerance was borderline and not statistically significant. Nanomolar concentrations of PLG, YPLG, and pareptide each increased the in vitro binding of ³H-apomorphine to rat striatal receptors. In this in vitro system, bell-shaped dose response curves were observed for each peptide. The effects of these peptides on tolerance development and apomorphine binding are similar to those previously reported for MIF-1 and demonstrate that amidation at the carboxyl terminus is not required for biological activity.     

Vasopressin differentially modulates aggression and anxiety in adolescent hamsters administered anabolic steroids

Adolescent Syrian hamsters (Mesocricetus auratus) treated with anabolic/androgenic steroids display increased offensive aggression and decreased anxiety correlated with an increase in vasopressin afferent development, synthesis, and neural signaling within the anterior hypothalamus. Upon withdrawal from anabolic/androgenic steroids, this neurobehavioral relationship shifts as hamsters display decreased offensive aggression and increased anxiety correlated with a decrease in anterior hypothalamic vasopressin. This study investigated the hypothesis that alterations in anterior hypothalamic vasopressin neural signaling modulate behavioral shifting between adolescent anabolic/androgenic steroid-induced offensive aggression and anxiety. To test this, adolescent male hamsters were administered anabolic/androgenic steroids and tested for offensive aggression or anxiety following direct pharmacological manipulation of vasopressin V1A receptor signaling within the anterior hypothalamus. Blockade of anterior hypothalamic vasopressin V1A receptor signaling suppressed offensive aggression and enhanced general and social anxiety in hamsters administered anabolic/androgenic steroids during adolescence, effectively reversing the pattern of behavioral response pattern normally observed during the adolescent exposure period. Conversely, activation of anterior hypothalamic vasopressin V1A receptor signaling enhanced offensive aggression in hamsters exposed to anabolic/androgenic steroids during adolescence. Together, these findings suggest that the state of vasopressin neural development and signaling in the anterior hypothalamus plays an important role in behavioral shifting between aggression and anxiety following adolescent exposure to anabolic/androgenic steroids.     

Modulation of iNOS activity in age-related cardiac dysfunction

Based on the role of inducible nitric oxide synthase (iNOS) in heart failure, we hypothesized that the elevated expression of iNOS compared to young mice in the myocardium contributes to the age-related decline of left ventricular (LV) function. Cardiac iNOS mRNA and protein expression was singularly identified in old, wild type (WT) male mice (I6-month) and not in young WT male mice (6-month). Characterized with in vivo pressure-volume loops analysis, an age-related LV dysfunction was found in the old WT mice. The LV dysfunction of the aged mice was modified to that of the younger mice by the specific iNOS inhibitors, aminoguanidine (AMG, 10 mg/Kg, i.v. or infusion, n = 15) and S-methyl-isothiourea (MITU, 3 mg/Kg, i.v. n = 7), and declined with L-arginine (10 mg/Kg, i.v. n = 7). All three drugs had no effects on the LV function of young WT mice or old iNOS knockout (KO) mice. The NOx and cGMP levels were significantly higher only in the old WT mice (n = 6) and cGMP levels decreased to normal with AMG administration. In conclusion, these results suggested that the iNOS/NO/cGMP pathway may contribute to ventricular dysfunction during the aging process and that inhibition of iNOS activity significantly improved heart function in aged mice.     

Effects of mammalian FMRF-NH2-related peptides and IgG from antiserum against them on aggression and defeat-induced analgesia in mice

The effects of two endogenous mammalian FMRFamide (Phe-Met-Arg-Phe-NH2)-related peptides, an octapeptide F8Fa (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2) and an octadecapeptide A18Fa (Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Pro-Gln-Arg-Phe-NH2 ), and IgG from serum against them on the responses to aggression and defeat-induced analgesia were examined in subordinate mice in "resident-intruder" pairings. Intracerebroventricular (ICV) administrations of F8Fa and A18Fa (0.10-10 micrograms) reduced, in a dose-dependent manner, the number of bites to obtain defeat in the subordinate mice during the agonistic encounters, as well as attenuating defeat-induced analgesia, with F8Fa having a greater inhibitory effect than A18Fa. Peripheral administration of naloxone (1.0 mg/kg) had a similar inhibitory effect on the number of bites to defeat and the level of defeat-induced analgesia. In contrast, ICV administrations of F8Fa-IgG and A18Fa-IgG antisera increased the number of bites to defeat and augmented the levels of defeat-induced analgesia, with F8Fa-IgG having a greater effect than A18Fa-IgG. These results provide further evidence that the peptides, F8Fa and A18Fa, are involved in the modulation of opioid-mediated analgesia accompanying biological stressors and suggest that these endogenous FMRF-NH2-related peptides may also be associated with the expression of opioid-sensitive components of aggressive behavior.     

Brain 2-phenylethylamine as a major mediator for the central actions of amphetamine and methylphenidate.

2-Phenylethylamine (PEA) was measured in rabbit brain by gas-liquid chromatography. D-Amphetamine sulfate (0.65 mg/Kg) initially reduced brain PEA levels to one-third of its usual content (30 min) and subsequently doubled brain PEA (4 hr). Brain PEA levels were reduced (30 min) and subsequently increased (ten-fold at 4 hr) by D-amphetamine sulfate (13 mg/Kg); tolerance to these two effects was observed in rabbits treated for three days with D-amphetamine. Methylphenidate HCl (30 mg/Kg) but not L-amphetamine sulfate (0.65 mg/Kg and 13 mg/Kg) induced a small, non-significant lowering of brain PEA (30 min) followed by a marked augmentation (4 hr) of brain PEA content. D-Amphetamine (30 min or 4 hr prior) increased the recovery of labeled PEA from the brain of rabbits injected intraventricularly with labeled phenylalanine, and reduced the recovery of labeled PEA after its intraventricular injection, suggesting that D-amphetamine accelerates both the synthesis and the disposition of brain PEA. Pretreatment with α-methyldopa (which depletes PEA and other brain amines) or with α-methyldopa hydrazine (which selectively reduces brain PEA content by inhibiting decarboxylase in peripheral tissues only) markedly reduced the CNS effects of D-amphetamine (behavioral stimulation in mice and rabbits, anti-convulsant effect in mice); these decarboxylase inhibitors enhanced the amphetamine-like effects induced by PEA in mice pretreated with a monoamine oxidase inhibitor. The ability of PEA depleters to selectively block the stimulant effects of D-amphetamine, together with the close structural and pharmacological similarities between amphetamine and PEA, and marked influence of amphetamine administration upon PEA brain levels, synthesis and metabolism, suggest to us that many of the central actions of amphetamine may be mediated by endogenous PEA.