A difference in glutamate-decarboxylase activity between isolated and grouped mice

Olfaction, GABAergic neurotransmission in the olfactory bulb, and intermale aggression in mice: modulation by steroids

A model to explain individual differences in mice for the propensity to attack male conspecifics is proposed. In the first part of the paper, the relation between olfaction and intermale aggression is discussed emphasizing the importance of olfactory cues provided by the opponent and their subsequent processing by the attacking male. The physiological role of GABA in the olfactory pathway is presented in the second part of the paper. The third part investigates the possible modulating action of steroids on the GABA-A receptor complex, intermale aggression, and olfaction. We hypothesize that at least part of the individual differences in the propensity to attack may be explained by a differential olfactory recognition and discrimination of the opponent as a stranger through a differential processing threshold of the olfactory cues provided by the urine of the opponent. A possible modulation of this threshold by steroids, especially testosterone, is also discussed.

Inhibitory influence of dihydroergosine on the aggressiveness of rats and mice

Fifty mg/kg of ergot alkaloid dihydroergosine (DHESN) inhibited the mouse-killing behavior of isolated male rats, while 10 mg/kg did not. This effect of DHESN (50 mg/kg) lasted for 24 hr. When an additional injection of DHESN (50 mg/kg) was given to mouse-killer rats 6 days following the first, the mouse-killing behavior was again inhibited. The effect of the second drug injection also persisted for 24 hr and was accompanied by an increased concentration of 5-HT in the raphe nuclei and hypothalamus and by a decreased concentration of GABA in the olfactory bulbs. DHESN also inhibited aggressiveness in isolated mice. Two hr following the administration of 10 mg/kg DHESN the fighting was inhibited in 46% of pairs tested, while 50 mg/kg abolished it completely. The effect of 50 mg/kg lasted 24 hr. These results, showing the antiaggressive effects of DHESN, support our previous suggestion that DHESN might presumably be a new antidepressant, and suggest that besides the serotoninergic, the GABA-ergic system might also be involved in the modification of behavior induced by this drug.

Changes in neurotransmitter amino acids content in several CNS areas from aggressive and non-aggressive bull strains

Neurotransmitter amino acid levels (glutamate, aspartate, GABA, and glycine) of crude synaptosomal fractions from several non-limbic CNS areas (pons, ventral tegmentum, midbrain reticular formation, fastigial nucleus of cerebellum, posterior colliculus and anterior colliculus) showed significant differences between aggressive Spanish fighting-bull and non-aggressive Friesian strains. The most unequal distribution was observed in neurotransmitter amino acids, while the non-transmitter amino acids (serine, isoleucine, leucine, threonine and alanine) showed minor and uneven changes. The results suggest a possible relationship between changes in amino acid neurotransmitter levels and the aggressive behavior observed in the aggressive breed.

Neuronal transmitter sensitivity after social isolation in rats

After 3 and 12 months of isolation rearing of rats, sensitivity of single neurons to microiontophoretically applied transmitters as well as neuronal spontaneous activity are analysed in striatum, cortex, hippocampus, locus coeruleus and nucleus raphis medialis. It is demonstrated that regulation of transmitter sensitivity, as well as spontaneous activity, depend on isolation duration, therefore showing time dynamics. Sensitivity changes of single neurons to distinct transmitters are not uniform and simultaneous in all areas but are structure specific. After 3 months of isolation an enhanced dopamine sensitivity is observed only in the striatum, whereas in the cortex the dopamine sensitivity is increased after 12 months isolation. A diminished response of single neuron activity to serotonin is demonstrable after 3 months of isolation in striatum and nucleus raphis. The observed changes are discussed in connection with biochemically and pharmacologically demonstrated changes in isolation. The complex patterns of neurobiological changes characterizing the isolation syndrome are emphasized.

Differential distribution of neurotransmitter amino acids from the limbic system of aggressive and non-aggressive bull strains

The amino acid content of crude synaptosomal fractions from the limbic system and related CNS regions showed significant differences between the aggressive Spanish fighting-bull and the non-aggressive Friesan bull breeds. Neurotransmitter amino acids (glutamate, aspartate, GABA and glycine) were the most unequally distributed. A higher ratio of excitatory to inhibitory neurotransmitter amino acids was always found in all the CNS regions studied in the aggressive breed. The concentrations of five non-transmitter amino acids (threonine, alanine, serine, leucine and isoleucine) showed minor variations between both studied bull strains and cannot be ascribed to differences in central energy metabolism. The results are explained in terms of a possible relationship between the amino acid neurotransmitter levels and the innate aggressiveness of the Spanish fighting-bull.

Influence of environment on GABA receptors in muricidal rats

The influence of environment (isolation) on GABA receptor numbers ( [3H]-muscimol binding sites) and affinities was investigated in specific limbic areas known to be involved with the development of muricide. Olfactory bulbs of isolated rats were found to have identical numbers of [3H]-muscimol binding sites whether or not they were muricidal. However, in the olfactory bulbs of the aggregated animals a greater than two-fold increase was found in numbers of [3H]-muscimol binding sites in those rats that were muricidal. In the amygdala muricidal rats had a 32-34% increase in [3H]-muscimol binding sites over non-muricidal rats regardless of environment. In the septum non-muricidal rats had fewer [3H]-muscimol binding sites than muricidal rats and although the trend was evident, statistical vigor was seen only in the aggregated animals. Neither muricide nor isolation significantly influenced [3H]-muscimol binding numbers in the hypothalamus. GABA Ki values were examined in all brain regions and found to be the same in the isolated and aggregated animals whether or not they were muricidal. We concluded that environment appears to influence apparent GABA receptor numbers in the olfactory bulbs and septum whereas muricidal behavior correlates well with an increase in apparent number of GABA receptors in the amygdala. GABA receptors in the hypothalamus were not influenced by either environment or aggression.

Sex differences in the response of rats to drugs affecting GABAergic transmission

The administration of diazepam 1.0 mg/kg decreased the level of plasma corticosterone in female but not in male Wistar rats. Picrotoxin, another drug affecting GABAergic transmission, also brought about an increase of plasma corticosterone in both sexes. However, in order to achieve a plasma corticosterone increase of similar magnitude (more than 500%) a threefold higher dose of picrotoxin had to be given to males. When the convulsive properties of picrotoxin were tested, it became evident that the dose of picrotoxin (2.5 mg/kg) which was subconvulsive in male was almost 100% convulsive in female rats. The existing sex differences in the response of rats to drugs affecting GABAergic transmission might have possible implications in the treatment of GABA system dysfunction.

Changes in neurotransmitter amino acids and protein in CNS areas of mice subjected to differential housing conditions

The amino acid and protein content of mice exposed to enriched, restricted and impoverished environments have been studied in six discrete CNS areas. Differences between enriched and either restricted or impoverished groups were found whereas no difference was observed between restricted and impoverished ones. In the first case, a significant increase for aspartate was found in spinal cord, whereas glutamate significantly decreased in colliculi and cerebral cortex. Similarly, glycine increased in cerebral cortex and decreased in colliculi and pons-medulla, and gamma-aminobutyrate (GABA) increased in spinal cord, pons-medulla and cerebellum and decreased in thalamus-hypothalamus. No changes in concentrations of five non-transmitter amino acids (serine, threonine, alanine, isoleucine, leucine) were observed. Significant increases of the protein concentration in cerebellum and spinal cord were found. The changes were due to enrichment, not to aggregation conditions. The results corroborate the proposed plasticity of the aminoacidergic system.

Behavioral responses to apomorphine and amphetamine in differentially housed mice

The climbing response to apomorphine (AP, 0.075-3.0 mg/kg) and the motor activity response to amphetamine (AMP, 0,3-5.0 mg/kg) were determined in grouped or socially isolated mice. After 4 weeks of differential housing commencing at 5 weeks of age, the individually housed mice showed an increased response only to low doses of these drugs. The responses of the group-housed mice at 5 or 9 weeks of age were identical, ruling out an aging component to the differential responsiveness. Also, the response of separate groups of naive mice to acute treatments of saline, AP (0.15 and 3.0 mg/kg), and AMP (0.3 and 5.0 mg/kg) was examined at 1, 2, 3, and 4 weeks of differential housing. With respect to age-matched group-housed mice, a significantly greater climbing response to AP (0.15 mg/kg) and a significantly greater locomotor activity response to AMP (0.3 mg/kg) were seen after 1 and 3 weeks of individual housing. There were no significant differences in the behavioral responses to the higher doses of AP and AMP.

Effects of n-di-propylacetate on aggressive behavior and brain GABA level in isolated mice

n-di-Propylacetate (nDPA, valproate) a GABA-T inhibitor, injected IP at the dose of 300 mg/kg antagonized agonistic behavior of isolated DBA/2 mice in a time-dependent fashion in parallel to an increase of GABA levels in olfactory bulb, striatum, posterior colliculus and septum. After 75 min, aggressive responses were higher than those after 15 to 45 min and significantly lower in comparison with those of saline injected mice. After 120 min aggressive behavior was not different from that of control mice. The concentration of GABA in the striatum and olfactory bulb returned to control value 75 and 120 min after drug administration, respectively. After 120 min GABA levels in posterior colliculus and septum were lower than those after 15 to 75 min, although significantly higher in comparison with those of saline injected mice. The results are discussed in terms of the possible involvement of olfactory bulb and striatum in GABA-mediated control of isolation-induced aggressive behavior in mice.

Isolation reduces midbrain tryptophan hydroxylase activity in mice

Male C57 mice were divided into two experimental groups: "Isolated" and "Grouped". Isolated mice were housed individually for 30 or 45 days; Grouped mice were housed five per cage. Midbrain tryptophan hydroxylase activity was determined at the end of the isolation period. Isolated mice showed 35% less tryptophan hydroxylase activity than grouped mice (P less than 0.001). Prolongation of the isolation (45 vs 30 days) did not further reduce the enzyme activity. The changes in tryptophan hydroxylase activity may be related to behavioral changes induced by isolation.

GABAergic influences on defensive fighting in rats

The involvement of GABAergic mechanisms in shock-induced defensive fighting in rats was investigated in a series of three experiments. In Experiment 1, sodium n-dipropylacetate (100-200 mg/kg) failed to produce significant behavioural change whilst gamma-vinyl-GABA (100-200 mg/kg) induced a selective and dose-dependent reduction in fighting. In Experiment 2, although inconsistent behavioural effects were obtained with (+)-bicuculline (0.25-4 mg/kg), a biphasic influence on defensive fighting was observed with picrotoxin (0.125-2 mg/kg). The inhibitory effect on fighting, induced by the highest dose of picrotoxin, was related to motor impairment. In Experiment 3, muscimol reduced fighting at doses above 0.25 mg/kg with motor disruption evident only at the highest dose used (1 mg/kg). A dose-dependent inhibition of defensive fighting was observed with 1-baclofen (0.15-1.2 mg/kg) which, at the highest dose tested, also impaired motor coordination. None of the compounds tested significantly altered shock thresholds. Results are discussed in relation to the hypothesized inhibitory role of GABA in the mediation of aggressive behaviours.

Effect of valproic acid on brain serotonin metabolism in isolated and grouped rats

The effect of the GABA transaminase inhibitor valproic acid (DPA) on serotonin (5HT) metabolism of different brain regions were studied in both grouped and isolated rats. One hour after DPA injection 5HT levels in the amygdala were increased in grouped and isolated rats. In the hypothalamus of grouped rats, changes in 5HT metabolism were also found. The alteration in 5HT metabolism in grouped rats was reversed 150 min after injection of DPA. At this same time, a large and significant increase in 5HT turnover was observed in all brain areas examined in isolated rats. It can be concluded that prolonged isolation induces a differential sensitivity to the effects of DPA leading to differences in 5HT metabolism: the drug effect being more intense in isolated rats.

gamma-Aminobutyric acid in brain areas of isolated aggressive or non-aggressive inbred strains of mice

In order to investigate the effects of social isolation on aggressive behavior and GABA levels in different brain areas, inbred mice of the C57 Bl/6 and the DBA/2 strains were housed individually over a period of 8 weeks. Social isolation induced a clear increase of aggressive responses only in the DBA/2 strain and a decrease of GABA levels in septum, striatum, olfactory bulb and posterior colliculus in both the C57Bl/6 and in the DBA/2 strains. An increase of neurotransmitter concentration was observed in amygdala of DBA mice. DBA mice when compared to C57 mice showed significantly lower levels of GABA in olfactory bulb and striatum. These results are discussed in light of several previous studies which have pointed out a correlation between a deficiency of GABA mediated inhibition in some brain areas and different kinds of aggressive behavior as well as the possibility of a blockade of aggressive behavior by potentiation of GABAergic mediated inhibition. A possible suggestion emerging from our results is that the aggressive responses exhibited by isolated DBA mice but not by isolated C57 mice may be related to lower levels of the inhibitory neurotransmitter in the olfactory bulb and striatum.

Sodium valproate: kinetic profile and effects on GABA levels in various brain areas of the rat

1. The kinetic profile of sodium valproate (VPA) and the GABA levels were studied in discrete brain areas of the rat after an i.p. injection of 200 mg/kg. The results were discussed comparatively with GABA-T and GAD activities reported in the literature. 2. VPA was rapidly distributed in brain areas; its concentrations, its kinetic parameters and the GABA levels after the drug administration were not uniform in the different brain areas studied. 3. The results showed a particular relation of the VPA to the olfactory bulbs; in this specific area the VPA effect on GABA level was stronger; the VPA apparent half life of elimination was longest; the VPA apparent disappearance rate constant was smallest; the initial GABA level was higher; the activities of GABA-T and GAD were higher than in other brain areas studied except the hypothalamus. 4. These data were correlated with the role of the olfactory bulbs in the behaviour of the rodents.

Involvement of the GABAergic system on shock-induced aggressive behavior in two strains of mice

The effects of drugs that antagonize or potentiate the action of brain gamma-aminobutyric acid (GABA) on shock-induced aggressive behavior in mice were investigated. In previous studies it has been shown that in C57 BL/6 strain shock-induced aggressive behavior is absent up to the 10th week of age and rises to the highest intensity after the 20th week, while at the same ages aggressive responses are lowest or absent in DBA/2 strain. GABA antagonist, picrotoxin and glutamic acid decarboxylase (GAD) inhibitor, D, L-allylglycine induced aggressive responses in non-aggressive 10 week old C57 BL/6 and 20 week old DBA/2 mice. GABA agonist muscimol hydrobromide, and GABA-T inhibitor sodium n-dipropylacetate inhibited aggressive responses in 20 week old C57 BL/6 mice. These effects were not related to changes in shock sensitivity and motor activity. The results strongly suggest that the GABAergic system is involved in the control of shock-induced aggressive behavior in mice and that this control is related to developmental and genetic factors.