SUMMARY1. Intracellular recordings from CAI pyramidal cells in the hippocampal slice preparation were used to compare the action of baclofen, a y-aminobutyric acid (GABA) analogue, with GABA.2. Ionophoretic application of GABA or baclofen into stratum (s.) pyramidale evoked hyperpolarizations associated with reductions in the input resistance of the cell. Baclofen responses were easier to elicit in the dendrites than in the cell body layer. 5. The reversal potentials for the somatic GABA and baclofen responses were -70 mV and -85 mV respectively. When the membrane was depolarized, the baclofen response was reduced. This apparent voltage sensitivity was not seen with somatic GABA responses.6. Altering the chloride gradient across the cell membrane altered the reversal potential of the somatic GABA response but not that of the baclofen response. It was extrapolated that a tenfold shift in the extracellular potassium concentration would cause a 48 mV shift in the reversal potential of the baclofen response. Barium ions reduced the baclofen response, but not the GABA response.7. Orthodromic stimulation produced a fast inhibitory post-synaptic potential (i.p.s.p.) and a slow i.p.s.p. The properties of the fast and slow i.p.s.p.s were remarkably similar to those of the somatic GABA and baclofen responses, respectively.8. Application of GABA to the pyramidal cell dendrites evoked, in addition to a depolarization, two types of hyperpolarization. One type of hyperpolarization was * Present address: Merck Sharp and Dohme

Baclofen has been used as an antispastic agent for over a decade, yet its mechanism of action is still not fully understood. While early iontophoretic studies revealed a depression of neuronal activity, more recent studies have emphasized a presynaptic depression of transmitter release, both in the peripheral and central nervous system, possibly resulting from a blockade of calcium channels. Although baclofen is structurally similar to the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), none of its actions seem to be antagonized by the GABA antagonist, bicuculline. However, recent experiments have indicated that baclofen binds to a class of bicuculline-resistant GABA receptors, termed GABAB receptors. Here, we have analysed the action of baclofen on the membrane potential of CA1 hippocampal pyramidal cells in vitro and report that it directly hyperpolarizes these cells in a potent, stereoselective manner which is resistant to bicuculline methiodide. This response is associated with a decrease in neuronal input resistance and may involve an increase in potassium conductance.

SUMMARY1. Experiments were performed on rat hippocampal CAI pyramidal cells in vitro in order to elucidate the origin of the late hyperpolarizing potential, which follows the y-aminobutyric acid (GABA)-mediated inhibitory post-synaptic potential (GABA-i.p.s.p.).2. The late hyperpolarizing potential could be evoked by orthodromic stimulation via stratum radiatum or stratum oriens but not by selective antidromic stimulation.3. The membrane soluble analogue of cyclic AMP, 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br cyclic AMP), which blocks calcium-activated potassium hyperpolarizations (GK(Ca)), did not reduce the late hyperpolarizing potential.4. The enkephalin analogue, (D-ala2-met5)-enkephalinamide (DALA) reversibly reduced both the GABA-i.p.s.p. and the late hyperpolarizing potential.5. The late hyperpolarizing potential and GABA-i.p.s.p. were more sensitive to low doses of the calcium antagonist, cadmium, than the excitatory post-synaptic potential (e.p.s.p.).6. The local application of cadmium to the pyramidal cell layer blocked the antidromic i.p.s.p. but the orthodromically evoked late hyperpolarizing potential was less affected.7. In contrast to the GABA-i.p.s.p., the late hyperpolarizing potential was not reversed by chloride injection and was enhanced, rather than depressed, by bicuculline.8. We conclude that the late hyperpolarizing potential is a bicuculline-resistant i.p.s.p. The unidentified transmitter for this i.p.s.p. is released from feed-forward interneurones primarily onto the dendrites of the pyramidal cell and may act by increasing the potassium permeability of the membrane.9. The epileptiform burst after-hyperpolarization evoked in the presence of GABA antagonists is composed ofat least two components, a long-duration hyperpolarization mediated by GK(Ca) and an earlier and shorter late hyperpolarizing potential.10. Blockade of the GK(ca) by 8-Br cyclic AMP did not alter the duration of epileptiform bursts but did markedly increase the frequency of their occurrence. This suggests that GK(ca) is involved in controlling the interval between bursts.

1 Using grease-gap recordings from the isolated superior cervical ganglion of mouse, rat and guinea-pig, we have compared the depolarization evoked by 5-hydroxytryptamine (5-HT) with that evoked by the selective 5-HT3 receptor agonist 2-methyl-5-HT (2-Me-5-HT). 2 The maximum depolarization induced by 2-Me-5-HT was smaller than that induced by 5-HT in all three species, and particularly in the guinea-pig. 3 The 5-HT2 receptor antagonist ketanserin (1 UM) caused a clear rightward shift of the dose-response curve to 5-HT on the guinea-pig ganglion, but not on the mouse or rat ganglion. Spiperone (0.03pM) had a quantitatively similar action to ketanserin (O.1pM) on the 5-HT dose-response curve of the guinea-pig ganglion. Ketanserin had no significant effect on the dose-response curve to 2-Me-5-HT on any of these ganglia. 4 Using 2-Me-5-HT as the agonist, we determined the pA2 values for two 5-HT3 receptor antagonists. The potency of ICS 205-930 varied by approximately 100 fold between the species and that of (+)-tubocurarine varied by over 1000 fold. The differences in the pA2 values of these compounds varied independently among the species. 5 We conclude that 5-HT3 receptors are present on the superior cervical ganglion from the rat, mouse and guinea-pig, but these receptors may be pharmacologically distinct from each other. In addition, the depolarization of the guinea-pig superior cervical ganglion by low concentrations of 5-HT is largely mediated by ketanserin-sensitive receptors.