In the hippocampus, two distinct forms of GABAergic inhibition have been identified, phasic inhibitory postsynaptic currents that are the consequence of the vesicular release of GABA and a tonic conductance that is activated by low ambient concentrations of extracellular GABA. It is not known what accounts for the distinct properties of receptors that mediate the phasic and tonic inhibitory conductances. Moreover, the physiological role of the tonic inhibitory conductance remains uncertain because pharmacological tools that clearly distinguish tonic and phasic receptors are lacking. Here, we demonstrate that GABA A receptors that generate a tonic conductance in cultured hippocampal neurons from embryonic mice have different pharmacological properties than those in cerebellar granule neurons or pyramidal neurons in the dentate gyrus. The tonic conductance in cultured hippocampal neurons is enhanced by the benzodiazepine, midazolam, and is insensitive to the inhibitory effects of the competitive antagonist, gabazine (Յ10 M). We also identify penicillin as an uncompetitive antagonist that selectively inhibits the synaptic but not tonic conductance. GABA was applied to hippocampal neurons to investigate the properties of synaptic and extrasynaptic receptors. GABA-evoked current was composed of two components: a rapidly desensitizing current that was blocked by penicillin and a nondesensitizing current that was insensitive to penicillin blockade. The potency of GABA was greater for the penicillin-insensitive nondesensitizing current. Single-channel studies show that the gabazine-insensitive GABA A receptors have a lower unitary conductance (12 pS) than that estimated for synaptic receptors. Thus, specialized GABA A receptors with an apparent higher affinity for GABA that do not readily desensitize mediate the persistent tonic conductance in hippocampal neurons. The receptors underlying tonic and phasic inhibitory conductances in hippocampal neurons are pharmacologically and biophysically distinct, suggesting that they serve different physiological roles.

1. Laminar profiles of the average evoked potentials and current-source-density analysis were used to study the input provided by the medial perforant path (PP) to the hippocampus in the urethan-anesthetized rat. 2. Stimulation of the PP activated an early latency sink in the middle molecular layer of the dentate gyrus (DG) and in the stratum lacunosum-moleculare in CA1. The DG current sink was generated by excitatory synaptic currents activated by the PP on dentate granule cells. In the normal rat, the peak current sink in the DG was typically five times greater than that of CA1. However, the CA1 sink could be distinguished from the DG sink in several ways: 1) it peaked when the DG sink was subsiding; 2) it showed paired-pulse facilitation, whereas the DG sink did not; and 3) in rats in which the DG was lesioned by local colchicine injection, the DG sink was reduced much more than the CA1 sink. 3. The PP afferents to CA1 required a slightly higher stimulus threshold (> 100 microA) for activation than those projecting to the DG granule cells (< 30 microA). The onset latency of the early CA1 sink (2.5 +/- 0.2 ms, mean +/- SE) was also slightly longer than that of the DG sink (1.7 +/- 0.1 ms), suggesting that the axons of entorhinal layer III cells that project to CA1 have a slightly lower conduction velocity than the axons of the layer II cells that project to the DG. 4. The short-latency current sink activated by the PP in the distal dendritic layers of CA1 was likely provided by excitatory currents at the distal apical dendrites of CA1 pyramidal cells. The accompanying current source was mainly confined to stratum radiatum and appeared not to involve the cell body layer. Thus the electrotonic current spread may not be effective enough to depolarize the cell body or axon hillock. Contribution of interneurons to the above source-sink profile is possible, with the provision that these interneurons must have dendritic processes that span strata radiatum and lacunosum moleculare. 5. Extracellular field recordings provided no evidence that PP evoked a short-latency (< 9 ms) CA1-generated population spike, even with the use of micropipettes filled with mM bicuculline. Similarly, unit recordings in CA1 revealed only long-latency (9-17 ms) unit firing after PP stimulation, corresponding to a late, di/trisynaptic excitation of CA1 via the Schaffer collaterals.(ABSTRACT TRUNCATED AT 400 WORDS)

No abstract

Summary:Purpose: To determine whether hippocampal afterdischarges (ADs) and excitability changes were induced by γ -aminobutyric acid (GABA) B -receptor blockade in adult, freely moving rats.Methods: A specific GABA B -receptor antagonist CGP35348, CGP55845A, or CGP55699A was injected intracerebroventricularly (i.c.v.), and EEGs and behaviors of rats were analyzed.Results: CGP35348 (56-110 µg, i.c.v.) induced afterdischarges (ADs) ∼60% of the time, starting at the hippocampus or neocortex. Neocortical-onset ADs began with sporadic discharges and were <3 mV. Hippocampal-onset ADs were bilateral, >5 mV, and spread to the entorhinal cortex and amygdala, often ending in a rebound AD and accompanied with stereotypic jumping, forelimb clonus, and wet-dog shakes. The CGP35348-induced hippocampal AD had an onset frequency (5-9 Hz) that was higher than the electrically evoked AD (2-4 Hz). CGP35348 i.c.v. also increased the mean starting frequency of an electrically evoked hippocampal AD from 3.6 Hz to 5.3 Hz. Hippocampal gamma activity (25-80 Hz) increased up to twofold for 30 min after a hippocampal but not a neocortical AD. A single dose of CGP35348 induced repeated ADs of increasing duration. Pairedpulse inhibition of the evoked potentials in CA1, at interpulse interval of <100 ms, was decreased after but not before a hippocampal AD. CGP56999A (i.c.v.) gave results similar to those with CGP35348, whereas CGP55845A (i.c.v.) rarely induced ADs.Conclusions: GABA B -receptor blockade increases seizure susceptibility by reducing AD threshold and increasing the frequency and spread of a hippocampal AD. Hippocampal excitability (based on paired-pulse test) and gamma activity increased after but not before a hippocampal AD. Key Words: CGP35348-Gamma waves-Theta rhythmKindling-Paired-pulse depression.γ -Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the cerebral cortex, and it mediates inhibition through GABA A and GABA B receptors. GABA Areceptor functions are relatively well understood. Suppression of GABA A -receptor function induces seizures (1,2), whereas GABA A -receptor agonists are effective anticonvulsants. In contrast, the participation of GABA B receptors in partial (focal) seizure generation is not well established.Despite their effectiveness against generalized absence seizures (3,4), GABA B -receptor antagonists were found to induce convulsive, presumably partial seizures in rats (5-7). GABA B -receptor antagonists also induced epileptiform activity in vitro (8,9), and one type of GABA Breceptor polymorphism has been associated with tempo- ral lobe epilepsy in humans (10). The GABA B -receptor R1 knockout mice was reported to manifest spontaneous generalized seizures (11,12), but the origin of the seizures was not clear. In studies that induced seizures by GABA Breceptor antagonists, EEG was not used in seizure detection except in one study (5).We are interested in the conditions in which GABA Breceptor blockade may lead to seizures. Although the functions of several types of GABA B receptors...

Paired-pulse depression (PPD), a short-term neural plasticity, was studied in hippocampal CA1 of urethane-anesthetized rats in vivo, using field potential recordings and current source density analysis. PPD was robust when an ipsilateral CA3 (iCA3) conditioning pulse of moderate stimulus intensity was followed 30-200 ms later by a contralateral CA3 (cCA3) test pulse; the ratio of the conditioned (C) to the nonconditioned (NC) response, as measured by the peak excitatory sink at the apical dendrites, ranged from 0.6 to 0.8. An alveus conditioning pulse evoked a large antidromic population spike in CA1 and a modest depression of the CA3-evoked excitatory sink (C/NC ratio of approximately 0.85). High-intensity paired pulses, both delivered to iCA3, also showed PPD of the proximal excitatory sinks; however, paired-pulse facilitation of the dendritic sinks was found at the mid-apical dendrites, >250 microm from the soma. Local injection of GABA(A) antagonist picrotoxin or bicuculline increased the C/NC ratio at IPIs of <150 ms, as well as the ratio of the amplitude of the population spikes (P2/P1; where P2 and P1 are the population spikes evoked by the second and first pulse, respectively). GABA(B) receptor antagonists, CGP35348 given intracerebroventricularly or CGP56999A administered locally, increased C/NC and P2/P1 at IPIs of 150-400 ms. It is concluded that conditioned depression of the excitatory sinks was caused by mainly feedforward and some feedback inhibition at the apical dendrites. GABA(A)-mediated postsynaptic inhibition dominated at early latencies, while GABA(B)-mediated inhibition prevailed at long latencies, probably at both presynaptic and postsynaptic sites. PPD of the excitatory sinks provides a measure of population dendritic inhibition in vivo.