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Intracellular recording in the in vitro hippocampal slice was utilized to examine the effects of nimodipine and nifedipine on CA1 pyramidal cell excitability. The excitatory postsynaptic potential (EPSP) elicited by a single stimulus in stratum radiatum was enhanced by nifedipine as evidenced by increases in EPSP amplitude, area and slope. Threshold for synaptically-evoked somatic action potentials was decreased following either nifedipine or nimodipine application, often resulting in spontaneous action potential activity. A secondary, late EPSP-like event appeared in the intracellular recordings during and following bath application of nimodipine, and was associated with burst-like activity in field potential recordings. In accordance with the hydrophobic nature of these compounds, extensive washout in normal Krebs' solution failed to reverse their effects, but nifedipine's actions were photolabile. These results indicate that dihydropyridines can enhance synaptic efficacy in the CA1 region of the hippocampus.

Intra-axonal recordings were obtained in vitro from frog sciatic nerve axons. Adjusting whole nerve stimulation current to just subthreshold for the impaled axon elicited triphasic threshold changes in that axon. Threshold changes were determined by direct intra-axonal current application. Graded subthreshold depolarizations were present in many axons during the passage of action potentials in surrounding axons. When nerve branches were stimulated and axon recordings were obtained from the main nerve trunk, both branches, the one that activated the impaled axon and the one that did not, elicited threshold changes in the impaled axon. These data indicate on a cellular level that impulse activity in an intact nerve bundle can modulate the excitability of adjacent nonactivated fibers.

Inputs from the substantia nigra (SN) to the caudate nucleus (Cd) and the effects of electrophoretic dopamine on Cd neurones were studied in cats anaesthetized with pentobarbitone with intracellular techniques. Single shock electrical stimulation of the SN or the medial forebrain bundle (MFB) induced monosynaptic EPSPs in Cd neurones. Dopamine depolarized Cd neurones and chlorpromazine suppressed the SN or MFB induced EPSPs. Appropriate controls indicate that the drug effects were not artifactual. Some recorded neurones were identified by procion yellow dye injection as the medium-size intrinsic Cd neurones.

A sucrose gap chamber was used to study the effect of gamma-aminobutyric acid (GABA) on normal and regenerating rat peripheral nerve fibers. Sciatic nerves and dorsal roots were depolarized by GABA and the GABAA receptor subtype agonist muscimol, but not by the GABAB receptor subtype agonist baclofen. Ventral root fibers were not affected by these agents, suggesting a selective presence of axonal GABA receptors on sensory fibers of mammalian peripheral nerves. Regenerating dorsal and ventral root fibers were studied 13 to 20 days following nerve crush. The regenerated dorsal root fibers were depolarized by GABA or muscimol, but the regenerated ventral root fibers were not. These results indicate that GABA receptors are selectively present on normal mammalian sensory axons, and are reestablished on regenerated sensory axons.

Antidromically and synaptically activated spike and synaptic potentials in the nucleus reticularis tegmenti pontis (NRTP) of the cat were recorded intracellularly. The antidromic firing of the NRTP neurone is composed of IS-SD spikes with short duration and short spike-after-hyperpolarization. Membrane resistance and firing patterns were studied by applying depolarizing and hyperpolarizing current through the recording electrode. Findings indicate that the NRTP neurone has a relatively high membrane resistance and is capable of firing at a high frequency. Single shock stimulation of the interpositus (IP) and lateral nucleus (LN) of the cerebellum, brachium conjunctivum (BC), red nucleus (RN) and cerebral peduncle (CP) induced monosynaptic EPSPs in the NRTP neurones. Superior vestibular nucleus (SVN) stimulation induced monosynaptic IPSPs. Collision tests showed that (1) NRTP neurones are activated by the axons of IP and LN neurones which travel through BC, (2) these axons also send collaterals to RN, and (3) there is convergence of cerebellar, cerebral and brain stem inputs to a single NRTP neurone.

Sections of dorsal columns of the spinal cord were removed from rats of various ages and studied in a sucrose gap chamber. The potassium channel blocking agent, 4-aminopyridine (4-AP), led to a pronounced increase in the area of the compound action potential of immature dorsal column axons. During the course of maturation this effect was attenuated but not lost. Occlusion experiments indicate that the 4-AP-elicited increase in area of the response was likely the result of multiple action potential discharge, an effect also present in dorsal root but not ventral root fibers (Bowe et al. 1985). These results indicate that the 4-AP-elicited changes in action potential characteristics previously described for sensory fibers in the peripheral nervous system are also present in the central nervous system extensions of these axons.

Intracellular recordings were obtained from caudate (Cd) nucleus efferent neurons in the cat. A supernormal period, as characterized by increased excitability and conduction velocity, was found for the Cd efferent fibers following either a conditioning antidromic action potential, or a single orthodromic action potential induced by direct current application through the recording microelectrode. These data indicate that the impulse activity of a single fiber can modulate excitability and conduction velocity independent of surround impulse activity.