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Inhibitory processes in the CA3 region of the rat hippocampal slice were studied extracellularly using paired stimuli and with intracellular impalements of pyramidal neurones. As with mossy fibre (MF) or commissural (COMM) conditioning stimuli (Kehl and McLennan 1983), activation of the perforant path (PP) input caused a long-lasting inhibition of test orthodromic population spikes (PSs) evoked by shocks delivered to the fimbria. That at least a portion of this orthodromically-evoked inhibition reflected postsynaptic events was shown by the reduction both of the amplitude of antidromic PSs and the firing rate of spontaneously active single units. Experiments in which the extracellular concentration of chloride was reduced indicated that only an early component of the inhibition was due to a conductance for that anion. The existence of two inhibitory mechanisms distinguishable extracellularly by their sensitivity to bicuculline and manipulation of extracellular ion concentrations was correlated intracellularly with two hyperpolarising peaks occurring approximately 20 and 150 ms following MF, COMM or PP stimuli. The later hyperpolarisation had an equilibrium potential 20-25 mV more negative than the early IPSP, was unaffected by manipulations of extra- or intracellular concentrations of chloride and was associated with a decrease of membrane resistance suggesting that a potassium conductance was involved in its generation. The fact that it was recorded in the absence of any preceding depolarisation, was blocked by drugs acting presynaptically to cause disinhibition (Kehl and McLennan 1985) and, like the early inhibition, was reversibly reduced by hypoxia suggested that the late inhibition/hyperpolarisation was a synaptic phenomenon rather than an intrinsic membrane event. Because the late inhibition/IPSP could be shown to have a lower threshold for activation vis-à-vis the chloride- dependent early inhibition, it is possible that two distinct populations of interneurones mediate these two synaptic events.

The ability of systemic infusion of arachidonic acid and various prostaglandin (PG) compounds to induce luteolysis was examined in the Day 9 pseudopregnant rabbit. Administration of PGF-2 alpha (25 microgram/h for 6 h) elicited a decline in plasma progesterone from mean +/- s.e.m. pretreatment levels of 8.83 +/- 0.54 to 0.54 +/- 0.05 ng/ml on the following day (P less than 0.01). A shorter infusion (25 microgram/h for 3 h) or a lower hourly dose rate (12.5 microgram/h for 6 h) of PGF-2 alpha was ineffectual, suggesting that a dose/duration regimen exists for PGF-2 alpha-mediated luteolysis. PGE-2 (25 microgram/h for 6 h) or sodium arachidonate (667 microgram/h for 6 h) did not significantly affect luteal function. Of the PGF-2 alpha metabolites examined, 13,14-dihydro-PGF-2 alpha was the most effective: it was approximately 4-fold more potent as a luteolysin than PGF-2 alpha since functional regression occurred with infusion of 6.25 microgram/h for 6 h. These data illustrate that systemic infusion of PGF-2 alpha is capable of inducing luteolysis in the rabbit. The metabolite 13,14-dihydro-PGF-2 alpha may also be involved luteolytically in this species.

The actions of kainic acid on excitatory synaptic responses in rat hippocampal slices have been investigated and compared with the effects of other excitatory amino acids. Kainate administered iontophoretically or via the superfusate produced a large and long lasting potentiation of the population spike evoked in the CA1 region by Schaffer collateral-commissural stimulation. This potentiation was associated with a reduction in the field EPSP recorded in the dendritic region (stratum radiatum) but with no change in the presynaptic fibre volley or with any long lasting alteration in the antidromic population spike. The results suggest that one effect of kainate may be to produce dendritic depolarisation in CA1 pyramidal neurones. Kainate in equivalent amounts elicited similar potentiations of the population spike recorded in the dentate gyrus in response to either lateral or medial perforant path stimulation. Smaller amounts of kainate than those required to affect either CA1 or dentate pathways were able to potentiate the mossy fibre-evoked population spike in the CA3 region. Folic acid, which shares kainate's ability to produce seizures and distant brain damage when injected into the brain, elicited similar potentiations of synaptic excitation. Higher doses of folate than of kainate were required which is consistent with its weaker epileptogenic actions in vivo. In contrast, N-methyl-aspartate, ibotenate, L-glutamate and L-aspartate were unable to mimic kainate's potentiating action. In higher doses the substances depressed the population spike for long periods. These data suggest that potentiation of synaptic events may underlie the ability of kainate (and folate) to elicit seizures and distant brain damage.

The ability of prostaglandin (PG) E2 treatment to stimulate luteinizing hormone (LH) release was compared in cyclic (4-day) and ovariectomized rats. PGE2 (500 micrograms) was injected sc and plasma LH concentration was determined in serial jugular blood samples. Administration of PGE2 at 13.30 h on each day of the oestrous cycle resulted in a significantly greater increase in plasma LH levels at pro-oestrus and oestrus than at dioestrus 1 and 2. This differential response by the hypothalamo-hypophyseal axis during the oestrous cycle implicates regulatory activity by ovarian steroid feedback. In oestrogen-primed ovariectomized rats, PGE2 administration stimulated a significant increase in plasma LH, but similar treatment in non-primed castrates decreased plasma LH levels. This finding suggests a major role for oestrogen in regulating this response.

1. The macroscopic currents induced in cultured rat melanotrophs by exogenous gamma-aminobutyric acid (GABA) were analysed using the patch clamp recording technique. 2. Using various concentrations of intra- and extracellular chloride it was demonstrated that the conductance activated by GABA was chloride selective. Since these currents were blocked with bicuculline and enhanced with chlordiazepoxide the involvement of GABAA receptors similar to those in the CNS is indicated. 3. When chloride was symmetrically distributed across the membrane the voltage/current relationship was linear; pronounced rectification of GABA mediated currents was evident when there was an asymmetrical distribution of chloride. 4. With concentrations of GABA greater than 10 microM a fading of the current was seen during prolonged (5-10 s) applications. This effect appeared to be due to a decline of conductance rather than a shift of the chloride equilibrium potential. 5. Values for the Hill coefficient derived from dose-response curves suggested that the binding of 2 molecules of GABA to the receptor is required for the activation of the chloride channel. 6. There was no indication of a direct, GABAB receptor-mediated change of conductance.

Long-lasting inhibition (up to 2 s) of the commissurally-evoked response in the CA3 region of hippocampal slices was observed following a mossy fibre or commissural conditioning stimulus. Bicuculline applied iontophoretically or by superfusion (1-5 X 10(-6) M) blocked the early phase (20-40 ms) of the post-stimulus inhibition but either had no effect or potentiated the later inhibition.

Cell-attached and whole-cell recordings were made from adult rat melanotrophs maintained in vitro by standard cell culture techniques. In cell-attached recordings the cells showed small biphasic currents which reflected spontaneous cell firing. Single channel currents often had distinct relaxations and depolarizing currents through single channels could trigger the discharge of an action potential in the cell; both observations are consistent with the high input resistance (1-10 G omega) measured in the whole-cell configuration. The discharge of action potentials occurring either spontaneously or by current injection was eliminated by tetrodotoxin or by removing Na from the external medium. A Na-dependent plateau depolarization which activated near the spike threshold was also seen. In cells exposed to tetrodotoxin and K-channel blocking agents it was possible to evoke a long-lasting (up to 20 s) action potential which was enhanced and reduced, respectively, by Ba and Cd and thus appeared to reflect currents through voltage-activated Ca channels. Small amplitude Ca-dependent depolarizations could also be evoked at membrane potentials as low as -40 mV. In cell-attached and whole-cell recordings 10 mM Ba caused the discharge of tetrodotoxin-insensitive action potentials prior to a maintained depolarization of the membrane. The low threshold for Ca-dependent depolarizations suggest that Ca influx might occur in these cells even at the resting potential. Additionally, both a Ca current and the current underlying the Na-dependent plateau depolarization may influence the rate of cell firing and in doing so further increase Ca influx through voltage-activated channels.(ABSTRACT TRUNCATED AT 250 WORDS)

One of the goals in studying the electrical properties of neurosecretory cells is to relate their electrical activity to the process of secretion. A central question in these studies concerns the role of transmembrane calcium ion flux in the initiation of the secretory event. With regard to the secretory process in pituitary cells, several research groups have addressed this question in vitro using mixed primary anterior pituitary cell cultures or clonal cell lines derived from pituitary tumours. Other workers, including ourselves, have used homogeneous cell cultures derived from the pituitary intermediate lobes of rats to examine the characteristics of voltage-dependent conductances, the contribution of these conductances to action potentials and their role in stimulus-secretion coupling. Pars intermedia (PI) cells often fire spontaneous action potentials whose frequency can be modified by the injection of sustained currents through the recording electrode. In quiescent cells action potentials can also be evoked by the injection of depolarizing current stimuli. At around 20 degrees C these action potentials have a duration of about 5 ms. Although most of the inward current during action potentials is carried by sodium ions, a calcium ion component can be demonstrated under abnormal conditions. Voltage-clamp experiments have revealed that the membrane of these cells contains high-threshold, L-type, Ca2+ channels and low-threshold Ca2+ channels. Since hormone release from PI cells appears not to be dependent on action potential activity but does depend on external calcium ions, it is not clear what role these Ca2+ channels play in stimulus-secretion coupling in cells of the pituitary pars intermedia. One possibility is that the low-threshold Ca2+ channels are more important to the secretory process than the high-threshold channels.

Population spikes evoked in CA3 pyramidal cells of rat hippocampal slices by stimulation of the fimbria are subject to an early and a late inhibition following activation of the perforant path or the mossy fibres. The early inhibition is known to be GABA-mediated, and is blocked by addition of bicuculline to the superfusing medium; however the late inhibition is bicuculline-insensitive. Both inhibitions are reduced by the addition of (+/-)-baclofen or noradrenaline to the medium; the early inhibition only is blocked by D-Ala-D-Leu-enkephalinamide while the late inhibition is preferentially reduced by kainate. These data together with the results in the preceding paper suggest that both inhibitions are synaptically mediated, possibly by two distinct types of interneurone, one GABAergic and a second which may release an unidentified transmitter.

The actions of the neurotoxic amino acids folate and kainate have been compared on ortho-and antidromic responses evoked in CA1, CA3 and the dentate gyrus of slices of rat hippocampus maintained in vitro. Both in CA1 and the dentate gyrus superfusion of these acids caused an increase in amplitude of the population spike discharging from an excitatory postsynaptic potential which either remained unaffected or was reduced. In the CA3 region kainate and folate had broadly similar actions to enhance the probability of cell firing to synaptic excitation, and also caused epileptiform discharges to occur spontaneously or in response to electrical stimulation. Spontaneous and evoked population bursts in CA3 did not persist in low calcium/high magnesium medium indicating their dependence on intact synaptic transmission; spontaneously occurring bursts in CA1 were eliminated with the latter treatment or when the axonal connections between it and CA3 were cut. Following folate superfusion the commissural-evoked response in CA3 showed large and variable shifts of the latency which were dependent on the stimulus intensity and its timing after a spontaneous population discharge. Although all of the effects of folate were reproduced by bicuculline, no evidence for a decreased recurrent inhibition in CA1 was obtained although this was observed with kainate. The finding that folate and kainate produced their effects in the absence of a detectable effect on the antidromic population spike suggests a mechanism of action other than neuronal depolarization. The implications of these data for the neurotoxic mechanism(s) and the receptor homologies of folate and kainate are discussed.