Action of caffeine on pyloric motorneurons in the crustacean stomatogastric ganglion

Complex intrinsic membrane properties and dopamine shape spiking activity in a motor axon

We studied the peripheral motor axons of the two pyloric dilator (PD) neurons of the stomatogastric ganglion in the lobster, Homarus americanus. Intracellular recordings from the motor nerve showed both fast and slow voltage- and activity-dependent dynamics. During rhythmic bursts, the PD axons displayed changes in spike amplitude and duration. Pharmacological experiments and the voltage dependence of these phenomena suggest that inactivation of sodium and A-type potassium channels are responsible. In addition, the "resting" membrane potential was dependent on ongoing spike or burst activity, with more hyperpolarized values when activity was strong. Nerve stimulations, pharmacological block and current clamp experiments suggest that this is due to a functional antagonism between a slow after-hyperpolarization (sAHP) and inward rectification through hyperpolarization-activated current (IH). Dopamine application resulted in modest depolarization and "ectopic" peripheral spike initiation in the absence of centrally generated activity. This effect was blocked by CsCl and ZD7288, consistent with a role of IH. High frequency nerve stimulation inhibited peripheral spike initiation for several seconds, presumably due to the sAHP. Both during normal bursting activity and antidromic nerve stimulation, the conduction delay over the length of the peripheral nerve changed in a complex manner. This suggests that axonal membrane dynamics can have a substantial effect on the temporal fidelity of spike patterns propagated from a spike initiation site to a synaptic target, and that neuromodulators can influence the extent to which spike patterns are modified.

Group I, II, and III mGluR compounds affect rhythm generation in the gastric circuit of the crustacean stomatogastric ganglion

We have studied the effects of group I, II, and III metabotropic glutamate receptor (mGluR) agonists on rhythm generation by the gastric circuit of the stomatogastric ganglion (STG) of the Caribbean spiny lobster Panulirus argus. All mGluR agonists and some antagonists we tested in this study had clear and distinct effects on gastric rhythm generation when superfused over combined oscillating or blocked silent STG preparations. A consistent difference between group I agonists and group II and III agonists was that group I agonists acted excitatory. The group I-specific agonists L-quisqualic acid and (S)-3,5-dihydroxyphenylglycine, as well as the nonspecific agonist (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid accelerated ongoing rhythms and could induce gastric rhythms in silent preparations. The group II agonist (2S,1'S, 2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) and the group III agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) slowed down or completely blocked ongoing gastric rhythms and were without detectable effect on silent preparations. The action of L-CCG-I was blocked partially by the group-II-specific antagonist, (RS)-1-amino-5-phosphonoindan-1-carboxylic acid [(RS)APICA], and the group-III-specific antagonist (RS)-alpha-methyl-4-phosphonophenylglycine completely blocked the action of L-AP4. Besides its antagonistic action, the group-II-specific antagonist (RS)APICA had a remarkably strong apparent inverse agonist action when applied alone on oscillating preparations. The action of all drugs was dose dependent and reversible, although recovery was not always complete. In our experiments, the effects of none of the mGluR-specific agonists were antagonized or amplified by the N-methyl-D-aspartate (NMDA)-receptor-specific antagonist D(-)-2-amino-5-phosphonopentanoic acid, excluding the contamination of responses to mGluR agonists by nonspecific cross-reactivity with NMDA receptors. Picrotoxin did not prevent the inhibitory action of L-CCG-I and L-AP4. We conclude that mGluRs, probably similar to those belonging to groups I, II, and III described in mammals, may play a role as modulators of gastric circuit rhythm generation in vivo.

Effects of caffeine on hippocampal pyramidal cells in vitro

The effects of caffeine on the electrophysiological properties of CA1 pyramidal neurones were investigated in the rat hippocampal slice preparation in vitro. A concentration-dependent increase in both the extracellularly recorded excitatory postsynaptic potential (e.p.s.p.) and the population spike resulting from stimulation of the stratum radiatum could be evoked by caffeine with a threshold concentration of 10 microM. Intracellular recordings demonstrate a caffeine-evoked decrease in resting membrane potential, an increase in input resistance, a reduction of the long afterhyperpolarization (a.h.p.) and a decrease in accommodation. The interaction between caffeine and adenosine was investigated on the extracellularly recorded e.p.s.p. The maximal response evoked by caffeine was increased in the presence of adenosine and the adenosine concentration-response curve was shifted to the right in a parallel fashion in the presence of caffeine. It is suggested that the effects of caffeine on hippocampal neurones may be mediated by a decrease of one or more potassium conductance(s), and that adenosine and caffeine may compete for the same electrophysiologically active receptor site on these cells.