A gradient of synaptic efficacy and its presynaptic basis in the cercal system of the cockroach.

1. The cerci of the cockroach Periplaneta americana bear longitudinal columns of wind-sensitive receptors which provide excitatory inputs to the giant interneurons (GIs) of the abdominal nerve cord. By using sound stimuli, we showed that spikes were more easily induced in the GIs from the most proximal than from the most distal receptors of the same column. 2. This was not due to a greater responsiveness of proximal sensilla to tones but to stronger synaptic connections; for the 3 largest GIs, the amplitude of the monosynaptic unitary EPSP tended to be all the higher as the stimulated sensillum was more proximal in each column. 3. The differences in EPSP size were due, at least partly, to presynaptic factors: a statistical analysis of the amplitude fluctuations of single-fibre EPSPs, showed that the amount of transmitter released per presynaptic impulse was larger for proximal than for distal sensory neurons in each column. 4. These differences in synaptic strength were correlated with differences in the structure of the afferent terminals. The location, the size and the shape of the axonal arbors are nearly the same for all sensory neurons of the same column, but proximal neurons arborize more profusely, and the terminal arbor of distal neurons is generally characterized by dorsal clusters of varicosities. 5. During postembryonic development, a decrease in the connection strength of 2 identified cercal neurons was accompanied by a retraction of ramifications on the medial side of their axonal arbor. 6. Possible mechanisms involved in the genesis and the remodelling of the gradient of synaptic strength are discussed in the light of available data and hypotheses relative to the development of ordered afferent connections.

[Effect of difluorodichloromethane (FC 12) on the cardiac cellular electric activity of mammals]. / Action du difluorodichloraméthane (FC 12) sur l'activité électrique cardiaque de mammifère au niveau cellulaire.

The cardiotoxicity of high concentrations of inhaled difluorodichloromethane (FC 12) has now been acknowledged. In the present study, the effects of FC 12 on the electrical activity of cells in the atrial and ventricular myocardium of anesthetized rats were recorded with "flexibly mounted" intracellular microelectrodes. The major phenomena observed in both types of cells were: a distinct decrease in the diastolic potential, a decrease inthe amplitude of the action potential, modifications in the shape of the action potential. Analysis of the simultaneously obtained electrocardiogram specifies the rhythm abnormalities which consist of an important decrease in the atrio-ventricular conduction and changes in the myocardial excitability. The cardiotoxicity of FC 12 is assumed to affect passive or active transmembrane ionic movements. Possible mechanisms are suggested here.

Postsynaptic effects of 4-aminopyridine in the sixth abdominal ganglion of the cockroach.

The synaptic action of 4-aminopyridine (4-AP) was studied in the sixth abdominal ganglion of the cockroach Periplaneta americana L. The drug 4-AP did not modify the affinity of cholinergic receptors. At concentrations above 10(-4)M, 4-AP depolarized the postsynaptic membranes even after blockage of muscarinic and nicotinic receptors by antagonistic substances.

Synaptic transmission in the sixth ganglion of the cockroach: action of 4-aminopyridine.

1. Study was made of the action of 4-aminopyridine (5 X 10(-5) M) on synaptic transmission in the last abdominal ganglion of Periplaneta americana. The 'oil-gap' technique was used to record postsynaptic events in a single giant axon. 2. 4-AP quickly increased the 'background' of postsynaptic activity, which consisted of 'spontaneous' unitary EPSPs and IPSPs. Postsynaptic spikes were also propagated. 3. Both evoked EPSPs (stimulation of cercal nerve XI) and evoked IPSPs (stimulation of cercal nerve X) were greatly increased in amplitude although their duration (half-time) was unaltered. 4. 4-AP triggered presynaptic action potentials in the cercal nerves (recorded with external electrodes). These 'antidromic' potentials appeared singly or sometimes repetitively, especially after electrical stimulation of the cercal nerves. They were often in monosynaptic correlation with unitary EPSPs. 5. Neither the resting potential nor the postsynaptic membrane resistance was modified. 6. There were no changes in the equilibrium potentials of the ions involved in postsynaptic events. 7. The results may be essentially explained by an increase in transmitter release after 4-AP treatment, which may be partly the result of a rise in presynaptic terminal excitability, and partly the result of a lengthening of the presynaptic action potentials.

The pharmacology of an insect ganglion: actions of carbamylcholine and acetylcholine.

1. Methods for presenting dose-response data for the ganglionic actions of cholinergic agonists (e.g. carbamylcholine) are compared, using the mannitol-gap technique for electrophysiological recording of synaptic events at the cercal nerve, giant fibre synapse of the sixth abdominal ganglion of the cockroach Periplaneta americana. 2. At concentrations around 10(-5)M, carbamylcholine has no effect on ganglionic polarization but potentiates the monosynaptic EPSP. At 10(-4)M and higher concentrations, ganglionic depolarization is accompanied by a reduction of EPSP. 3. Pretreatment with eserine (10(-6) M) considerably shifts the dose-response curve for acetylcholine so that synaptic transmission is consistently sensitive to 10(-6) M acetylcholine.

[The effect of 4-aminopyridine of the synaptic activity in the last abdominal ganglion of the cockroach (Periplaneta americana)]. / Modifications par la 4-aminopyridine de l'activité synaptique au niveau du dernier ganglion abdominal de la Blatte (Periplaneta americana)

Synaptic action of 4-aminopyridine (4-AP) has been studied in the sixth abdominal ganglion of the cockroach. Our results show that 4-AP has a strong excitatory effect on the ganglion and its seems that it depolarizes as well the pre and postsynaptic membranes. The threshold concentration is about 10(-5) M.