Peter Andreas Löschmann

Substantia nigra regulates action of antiepileptic drugs.

The cholinergic agonist pilocarpine triggers sustained limbic seizures in rodents. Pilocarpine seizures were blocked by systemic administration of benzodiazepines, barbiturates, valproate and trimethadione, while diphenylhydantoin did not affect, and ethosuximide increased the susceptibility of rats to such seizures. This pattern of action of antiepileptic drugs is characteristic for pilocarpine seizures and different from other rodent models of epilepsy. Although the anatomical substrates in the forebrain involved in the expression of anticonvulsant activity are unknown, the basal ganglia are believed to be essential for the motor expression of pilocarpine seizures. Bilateral microinjections into the substantia nigra, a major output station of the basal ganglia, of midazolam (ED50 38.5 nmol; range 29-52 nmol), phenobarbital (ED50 16 nmol; range 7-39 nmol) and trimethadione (ED50 30 nmol; range 16-56 nmol) protected rats against pilocarpine seizures (380 mg/kg i.p.) Diphenylhydantoin (up to 100 nmol) remained inactive, while ethosuximide (ED50 38 nmol; range 22-65.5 nmol) reduced the threshold for pilocarpine seizures, converting subconvulsant doses of pilocarpine (200 mg/kg i.p.) into convulsant ones. The profiles of action of antiepileptic drugs on pilocarpine seizures were similar following intranigral and systemic administration. These observations suggest that the substantia nigra may mediate some actions of antiepileptic drugs.

Effects of forskolin and cyclic nucleotides in animal models predictive of antidepressant activity: interactions with rolipram.

Forskolin, a direct activator of the catalytic subunit of adenylate cyclase (AC), and the cyclic nucleotide analogs dibutyryl cAMP (dBcAMP), 8-bromo cAMP (8-BrcAMP) and dibutyryl cGMP (dBcGMP) were tested for their ability to reverse the hypothermia or hypokinesia of mice depleted of presynaptic endogenous monoamines by pretreatment with reserpine, α-methyl-p-tyrosine and p-chlorophenylalanine. Forskolin and the cAMP analogs decreased the rectal temperature and inhibited locomotor activity in normal mice. In mice depleted of brain monoamines forskolin reversed the hypothermia and hypokinesia; dBcAMP and 8-BrcAMP antagonized the hypothermia but were only marginally effective in reversing the hypokinesia. DBcGMP was inactive. The antihypothermic action of forskolin or salbutamol was enhanced by the novel antidepressant and cAMP selective phosphodiesterase inhibitor rolipram (4RS-[3-cyclopentyloxy-4-methoxy-phenyl]-2-pyrrolidone). As an indirect effect via release of endogenous monoamines stimulating postsynaptic receptors was precluded by the monoamine-depleting pretreatment, forskolin and the cAMP analogs are thought to exert their antidepressant action by directly increasing brain cAMP availability. This is achieved by forskolin via activation of the catalytic subunit of AC and by the cAMP analogs via substitution for cAMP. These findings suggest that antidepressant activity is crucially linked to enhanced cAMP availability within brain effector cells. The successful treatment of endogenously depressed patients with rolipram supports this assumption.

Synthesis and biological effects of NO in malignant glioma cells: modulation by cytokines including CD95L and TGF-β dexamethasone, and p53 gene transfer

Nitric oxide (NO) is thought to play an important role in neurotransmission, inflammation, and regulation of cell death in the mammalian brain. Here, we examined the synthesis and biological effects of NO in human malignant glioma cells. Exposure to cytokines such as interferon (IFN)-γ, tumor necrosis factor (TNF)-α or interleukin (IL)-1β and lipopolysaccharide (LPS) induced NO synthesis in rat C6 and A172 human glioma cells, but not in LN-229, T98G or LN-18 human malignant glioma cells. Induced release of NO involved enhanced expression of inducible NO synthase (iNOS). Failure to detect NO release in the latter cell lines was not overcome by neutralization of endogenous TGF-β or by coexposure to cytokines, LPS, and antioxidants. Apoptosis induced by CD95 ligand (CD95L) did not involve NO formation. Neither NOS inhibitors nor NO donators modulated CD95L-induced apoptosis. Dexamethasone (DEX)-mediated protection of glioma cells from CD95L-induced apoptosis was also independent of DEX effects on NO metabolism. DEX inhibited not only cytokine/ LPS-evoked NO release but also attenuated the toxicity of NO in three of five cell lines. Forced expression of temperature-sensitive p53 val135 in C6 cells in either mutant or wild-type conformation inhibited cytokine/LPS-induced NO synthesis. Further, accumulation of p53 in both mutant or wild-type conformation protected glioma cells from the toxicity of exogenous NO, consistent with a gain of p53 function associated with p53 accumulation. We conclude that resistance to NO-dependent immune defense mechanisms may contribute to the malignant progression of human cancers with p53 alterations, notably those associated with the accumulation of mutant p53 protein.

N-Methyl-D-aspartate antagonists stimulate locomotor activity in monoamine depleted rats: Implications for the therapy of Parkinson’s disease

Recent experimental evidence suggests that the subthalamic and pallidal projections in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treated monkeys are abnormally active. In addition, immunocytochemical and electrophysiological studies have shown that the subthalamic nucleus receives a glutamatergic cortical input and subthalamic neurons projecting to the internal pallidal segment (GPi) themselves are glutamatergic. On the basis of these findings we have hypothesized that glutamate antagonists may be beneficial in the therapy of Parkinson’s disease. To test the validity of this hypothesis in a rodent model of Parkinson’s disease we subjected rats to reserpine (5 mg/kg, i.p.; 24 h) and α-methyl-p-tyrosine (250 mg/kg, i.p.; 3.5 h) treatment and tested whether glutamate antagonists, MK-801 (5-methyl-10,11-dihydro-5H-dibenzo(a,d)cycloheptan-5,10-imine maleate) (0.39–6.25 mg/kg, i.p.), a non-competitive antagonist at the N-methyl-D-aspartate (NMDA) receptor subtype and CPP (3-((±)-2-carboxypiperazin-4-yl)-propyl-1-phosphonate) (0.39–6.25 mg/kg, i.p.), a competitive NMDA antagonist, stimulate locomotor activity. The results show that MK-801 dose-dependently stimulates locomotor activity in monoamine depleted rats and markedly potentiates the action of L-DOPA. CPP also showed stimulatory effects, however its effect was less than MK-801. These observations support the notion that antagonism at NMDA receptors restores locomotion in monoamine depleted animals and raises hopes that adjuvant therapy with NMDA antagonists might improve therapy of patients suffering from Parkinson’s disease.