Otto A. Orsingher

Effect of cannabidiol and of other Cannabis sativa compounds on hippocampal seizure discharges

The natural Cannabis sativa compounds, cannabidiol, cannabinol, δ9- and δ8-tetrahydrocannabinol, in that order of potency, decreased the susceptibility of rat dorsal hippocampus to seizure discharges caused by afferent stimulation. The drugs were effective following both intraperitoneal injection and topical application. They were more active, on a dose basis, than the well-known antiepileptic agents mysoline and diphenylhidantoin. Within the dose range effective in depressing hippocampal seizures, they had no effect on hippocampal evoked responses. This suggested that they might act by interfering with K+ release from hippocampal cells, which is known to be the causative factor in hippocampal seizures. This point was investigated using cannabidiol, which was found to effectively block the release of K+ from the hippocampus caused by afferent stimulation.

Effects of Alpha-Methyl Tyrosine and Adrenergic Blocking Agents on the Facilitating Action of Amphetamine and Nicotine on Learning in Rats*

Abstractdl-amphetamine sulphate (2 mg/kg) and nicotine (0.2 mg/kg) showed a facilitatory action on the acquisition of a conditioned response in a shuttle-box by rats and this was reversed by pretreatment with α-MT (30 mg/kg).Pretreatment with dibenamine (10 mg/kg) impaired the action either of amphetamine or nicotine. Nethalide (5–10 mg/kg) exerted a partial protection on the depressant effect produced by the interaction between dibenamine and nicotine.Animals treated with α-MT (30 mg/kg) and kept in the cold (4–6° C for 3 h) also showed a depressed learning capacity. dl-Dopa (200 mg/kg) provided a partial protection on the depressive effects caused by the interaction of α-MT with amphetamine, nicotine or cold. It is suggested that the facilitatory learning action of amphetamine and nicotine involves a common adrenergic mechanism. The depressant effects of amphetamine, nicotine or cold after α-MT treatment are attributed to depletion of “functional pools” of catecholamines.

Brain dopamine and noradrenaline levels in rats submitted to four different aversive behavioral tests

Rats were trained to perform shuttle responses to a buzzer in four different situations: Pseudoconditioning (buzzers and footshocks presented at random), classical conditioning (buzzers and footulus paired on every trial), avoidance without stimulus pairing (buzzer-shock intervals varied at random, shocks contingent upon the nonemission of a shuttle response to the preceding buzzer), and standard two-way avoidance (buzzers paired to shocks, but the latter omitted every time there was a shuttle to the buzzer). Animals were killed immediately after the last trials and the noradrenaline and dopamine content of their hypothalamus, amygdala, caudate nucleus, and nucleus accumbens was determined. There were falls of dopamine content in the caudate and accumbens and falls of noradrenaline levels in all structures except the caudate after the pseudoconditioning test. Noradrenaline levels were normal, and dopamine levels were partially recovered, in the animals submitted to the other training situations. Thus learning factors (stimulus pairing and/or the avoidance contingency) offset the depleting influence of footshocks per se on both catecholamines in at least the structures studied.

Reactivity of amphetamine in perinatally undernourished rats: Behavioral and neurochemical correlates

Adult rats deprived at perinatal age and then rehabilitated on balanced chow were treated with a multiple amphetamine (AMPH) schedule (2 mg/kg/48 hr) and submitted, on days of injections, to an open-field test. Throughout 11 sessions, deprived rats showed a progressive increase of locomotor activity as compared with controls. Stereotyped activity evaluated during the AMPH treatment did not differ between control and deprived animals. No differences were detected in basal values of the dopaminergic function measured in naive control and deprived animals. By the end of the multiple AMPH treatment, a reduction of striatal DA and DOPAC levels together with a lower apparent DA turnover rate was detected in deprived animals. Besides, DA receptor binding was significantly increased in striatum from deprived rats as compared with controls. These results demonstrate that a repeated AMPH treatment, that was unable to alter the normal behavior of control rats, produced in early undernourished animals a progressive sensitization to AMPH effects, in addition to significant changes in the striatal dopaminergic function.

Decreased reactivity to the anticonflict effect of diazepam in perinatally undernourished rats

Offspring of rats were submitted to a protein deprivation dietary treatment from the third week of gestation until 50 days of age, and later nutritionally rehabilitated for at least 90 days. In the punished lever-pressing conflict test, undernourished animals exhibited a decreased reactivity to the anticonflict effect of a 3 mg/kg dose of diazepam as compared to control animals. This decreased reactivity to diazepam in a shock-induced conflict test indicates that functional alterations in the GABAergic transmission might contribute to the state of hypersensitivity to stressful or aversive situations present in undernourished animals.

Differential threshold for long-term potentiation in the hippocampus of rats with inborn high or low learning capacity

Threshold of stimulation frequency in the perforant path to induce long-term potentiation (LTP) in dentate gyrus was determined in hippocampal slices obtained from two different lines of rats inbred for 30 generations according to their performance in an avoidance escape test in a shuttle box. High-performance (HP) rats were defined as those giving at least 70% conditioned responses (CRs) and low-performance (LP) rats as those giving less than 15% CRs. LTP was defined as a 30% or more increase in the amplitude of the evoked population spike (PS), lasting at least 20 min. Stimulation frequency threshold was determined by stimulating with a train of pulses of 0.5 ms duration during 1 s. The same slice was stimulated with trains of increasing frequency from 5 to 400 Hz, each train separated by an interval of at least 20 min. HP rats showed a lower threshold (13 +/- 4 Hz) than LP rats (92 +/- 42 Hz) for the induction of LTP; there were no differences in the magnitude of LTP. The greater learning ability of HP rats may be related to the plasticity of hippocampal synaptic transmission.

Lack of tolerance to the anxiolytic effect of diazepam and pentobarbital following chronic administration in perinatally undernourished rats

Adult female rats, undernourished at perinatal age, were evaluated for anxiolytic action in the plus-maze test after acute and chronic administration of diazepam (DZP) and pentobarbital (PTB). Deprived (D) rats chronically treated with vehicle showed an increased anxiety as compared with control (C) animals. A single intraperitoneal (i.p.) administration of DZP (1 mg/kg) or PTB (7.5 mg/kg) produced similar anticonflict effect in both C and D rats. Tolerance to the anxiolytic effect of DZP and PBT developed in C rats after a 15-day administration schedule, whereas no tolerance was observed in D animals. Drug disposition was not altered after chronic treatment either in C or in D rats. Gamma-aminobutyric acid (GABA)-mediated chloride uptake in microsacs of cerebral cortex of naive D rats was decreased as compared with naive C rats. After chronic DZP administration (1 mg/kg/day i.p. for 15 days), GABA-mediated 36Cl- influx in brain cortex microsacs of C rats did not change; however, GABA efficacy was increased in microsacs of D animals. In addition, chronic DZP treatment induced GABA-benzodiazepine uncoupling in brain cortex of C rats, but not in D animals, as assessed by chloride uptake in microsacs. Chronic PTB treatment (7.5 or 30 mg/kg/day i.p. for 15 days) did not modify GABA stimulation or GABA-PTB interaction in cortical microsacs of C or D rats.

Perinatally protein-deprived rats and reactivity to anxiolytic drugs in the plus-maze test: An animal model for screening antipanic agents?

Adult rats submitted to a protein deprivation schedule at perinatal age (from 14th day of fetal life until 50 days of age) and then recovered on balanced chow (D rats) were assayed in the elevated plus-maze test for anticonflict effects of diazepam and drugs with therapeutic efficacy in panic disorders as compared with controls (C rats). Diazepam and alprazolam showed a similar anticonflict effect in D rats than in C rats. In contrast, buspirone, which was ineffective in C rats at a wide dosage range, showed a significant anticonflict effect on D rats at 0.3 mg/kg. Neither propranolol, desipramine, nor phenelzine treatment (10 mg/kg/day during 3-7 days) induced anticonflict effect in C rats. Conversely, these treatments fostered a significant and selective anxiolytic effect on D rats. Such results underscore long-lasting alterations caused by early undernutrition, namely, changes in reactivity to the drugs assayed. In addition, perinatally deprived rats may represent a useful animal model for studying potential antipanic agents.

Inhibition of catecholamine biosynthesis and memory processes.

Rats injected with 60 mg/kg of α-methyl tyrosine (α-MT) immediately after a training in a shuttle box impaired retention of conditioned avoidance response (CAR). Dl-Dopa (200 mg/kg) administered 5, 30, 60, and 120 min after α-MT treatment counteracted the depressive effect of this drug during the retention test; at longer times it was ineffective. Post-trial injections of 600 mg/kg of diethyldithiocarbamate (DDC) impaired retention of CAR; at a lower dose (300 mg/kg) it had no effect. Administration of α-MT (60 mg/kg) immediately after passive avoidance training lowered memory in females but not in males.These results suggest that noradrenaline (NA) is required for memory consolidation processes for about 2 h after training.

A physiclogical difference in the hippocampus of rats with a low inborn learning ability

Rats with a low ability to learn conditioned avoidance responses in a shuttle-box were in-bred. After five or six generations, a fairly homogeneous population was obtained consisting of bad learners both in the shuttle-box and in a Lashley III maze. When the hippocampus of these bad-learner rats was perfused with high potassium solutions, it was observed that more stimuli were needed, under each potassium concentration, in order to elicit a seizure, than in normal animals. This was apparently the result of a lower release of potassium per stimulus into the extracellular space, in as much as other possibilities (low sensitivity to the epileptogenic effect of potassium, large extracellular space, high Na-K-Mg-ATPase activity) were excluded. This reduced potassium release by stimulation in the hippocampus of bad-learner rats was not due to a lower potassium gradient across cell membranes, and therefore, could in principle be attributed to a defect in the property of local neural membranes (pre- and/or postsynaptic) to increase potassium conductance when stimulated.The present data fit with the hypothesis advanced previously, that the hippocampus plays a role in learning through heterosynaptic interactions mediated by the release, and subsequent accumulation of potassium.