Effects of caffeine on hippocampal pyramidal cells in vitro

Caffeine blocks absence seizures in the tottering mutant mouse

The neurological tottering mutant mouse is characterized by frequent "absence" seizures accompanied by bilateral synchronous spike and wave EEG bursts. Under anesthesia, adult homozygous tottering mice were implanted with permanent epidural electrodes, and at least 7 days elapsed before electrocorticograms in unrestrained mice were scored for seizure incidence and duration. Caffeine (5, 10, 15 mg/kg, n = 8) injected intraperitoneally (i.p.) at the fourth hour of 8-h recording sessions significantly (p less than 0.001 for 10 and 15 mg) decreased seizure incidence as compared with control saline injections. Spike and wave bursts were eliminated during the 30 min after injection and reached 50% preinjection levels between the first and the second hour after injection. Another central nervous system (CNS) stimulating drug, amphetamine (1 mg/kg; n = 5), under identical conditions failed to decrease seizure incidence in this mutant.

Adenosine antagonists increase spontaneous and evoked transmitter release from neuronal cells in culture

To examine the role played by endogenous adenosine in the modulation of transmitter release in the CNS, the effect of adenosine antagonists has been studied. Two systems have been used: EPSPs recorded from pyramidal cells in organotypic hippocampal cultures; and release of newly synthesized [3H]glutamate from cerebellar granule cells in dissociated culture. Bath application of 0.1-1 microM 8-phenyltheophylline (8-PT) reversibly increased both the number and size of spontaneous EPSPs and caused bursting activity in some cells. This effect was blocked by the glutamate antagonist gamma-D-glutamylglycine (DGG) (1 mM) but not by atropine (10 microM) or bicuculline (100 microM). Another adenosine antagonist isobutylmethylxanthine (IBMX, 10 microM) had a similar effect to 8-PT. Spontaneous activity in pyramidal cells and that induced by adenosine antagonists was blocked by the adenosine agonist 2-chloroadenosine (2-CA) (0.2-20 microM). 8-PT (10 microM) markedly potentiated K+-stimulated release of newly synthesized glutamate, and also enhanced basal glutamate release. The agonist (-)-phenylisopropyladenosine ((-)-PIA, 2 microM) which is relatively selective for A1 receptors, reduced by 19 +/- 5% the 8-PT-induced enhancement, and reduced K+-stimulated glutamate release in the absence of 8-PT to a similar extent. In contrast 5'-N-ethylcarboxamido adenosine (NECA, 2 microM), which is a relatively selective A2 agonist, slightly enhanced glutamate release. From these results it is likely that 8-PT potentiates glutamate release in both systems by blocking the effect of endogenous adenosine on presynaptic A1 receptors.

Effects of 2-chloroadenosine on hippocampal GABA content and turnover

The effects of 2-Chloroadenosine (CADO), a stable analog of adenosine, on GABA turnover rate and GABA content in the rat hippocampus in vivo have been studied. The intracerebroventricular injection of CADO reduced the GABA turnover rate in the hippocampus, as estimated from the rate of GABA accumulation after inhibition of GABA transaminase (GABA-T) by aminooxyacetic acid (AOAA). The effect of CADO on AOAA-induced accumulation of GABA in the hippocampus was blocked by the intraperitoneal injection of the adenosine receptor antagonist caffeine. Furthermore, CADO at the dose of 5 micrograms per ventricle produced a significant decrease in GABA content in the hippocampus. Our results support the hypothesis that adenosine exerts inhibitory effects on GABAergic circuits in the hippocampus.

Adenosine inhibits epileptiform activity arising in hippocampal area CA3

The ability of adenosine and structurally-related compounds to inhibit epileptiform activity induced by bicuculline in the CA3 region of the hippocampal slice of the rat was examined. Bath application of all purinoceptor agonists tested reduced the frequency of generation of burst potentials. Analysis of dose-response curves yielded the following IC50 values: adenosine, 1.5 microM; 2-chloroadenosine, 0.144 microM; 5'-(N-ethyl)carboxamidoadenosine, 30.2 nM; L-phenylisopropyladenosine, 12.1 nM; cyclohexyladenosine, 7.9 nM. Theophylline (30 microM) increased the rate of bursting and antagonized the effect of exogenous adenosine. Dipyridamole (0.03-1 microM) reduced the occurrence of burst firing. In slices untreated with bicuculline, theophylline (30 microM) and adenosine deaminase (10 micrograms ml-1) induced bursting activity. These results demonstrate that purinoceptor agonists can suppress epileptiform activity in the hippocampus and suggest that adenosine may act as an endogenous anticonvulsant.

Effects of histamine on hippocampal pyramidal cells of the rat in vitro

The actions of bath applied histamine on CA1 pyramidal cells were investigated in hippocampal slices of the rat. Histamine caused a) a slight depolarization but no significant change in resting membrane conductance; b) an abbreviation of long afterhyperpolarizations after single action potentials, bursts of action potentials or TTX resistant spikes; c) a loss of accommodation of firing. In the presence of TEA or barium, histamine prolonged and increased the size and number of the slow TTX resistant spikes. A depolarizing plateau which follows such spikes was also increased by histamine, but the population spike was increased. The frequency of spontaneous chloride dependent potentials, which reflect interneurone firing, was also increased. These effects considerably outlasted histamine application and were mimicked by the H2-agonist impromidine but not the H1-agonist thiazolethylamine, and blocked by the H2-antagonists cimetidine and metiamide but not the H1-antagonists mepyramine or the beta-antagonist propranolol. It is concluded that histamine, by activating H2-receptors, antagonizes a calcium mediated potassium conductance in hippocampal pyramidal cells without affecting calcium current. By this mechanism histaminergic afferent fibres could effectively regulate cortical responsiveness by selectively potentiating large excitatory inputs of target neurones.

Does corticotropin releasing factor act via a calcium-dependent mechanism?

The widespread distribution of CRF in the brain suggests a possible role of this peptide as a neuromodulator. An interaction with calcium-dependent conductances, as observed in hippocampal slices, could explain the action of CRF in different brain structures. In the present study, the calcium antagonist, verapamil, was found to block the excitatory action of CRF on neuronal discharge activity. This supports the hypothesis that CRF blocks a calcium-dependent potassium conductance, which represents a resting component of neuronal activity, thereby enhancing ion fluxes into the cell.