Evoked surface-positive potentials in isolated mammalian olfactory cortex

Neurons, Receptors, and Channels

Here, I recount some adventures that I and my colleagues have had over some 60 years since 1957 studying the effects of drugs and neurotransmitters on neuronal excitability and ion channel function, largely, but not exclusively, using sympathetic neurons as test objects. Studies include effects of centrally active drugs on sympathetic transmission; neuronal action and neuroglial uptake of GABA in the ganglia and brain; the action of muscarinic agonists on sympathetic neurons; the action of bradykinin on neuroblastoma-derived cells; and the identification of M-current as a target for muscarinic action, including experiments to determine its distribution, molecular composition, neurotransmitter sensitivity, and intracellular regulation by phospholipids and their hydrolysis products. Techniques used include electrophysiological recording (extracellular, intracellular microelectrode, whole-cell, and single-channel patch-clamp), autoradiography, messenger RNA and complementary DNA expression, antibody injection, antisense knockdown, and membrane-targeted lipidated peptides. I finish with some recollections about my scientific career, funding, and changes in laboratory life and pharmacology research over the past 60 years.

Topiramate hyperpolarizes and modulates the slow poststimulus AHP of rat olfactory cortical neurones in vitro

1. The effects of the novel antiepileptic drug topiramate (TPM) were investigated in rat olfactory cortex neurones in vitro using a current/voltage clamp technique. 2. In 80% of recorded cells, bath application of TPM (20 microm) reversibly hyperpolarized and inhibited neuronal repetitive firing by inducing a slow outward membrane current, accompanied by a conductance increase. The response was reproducible after washout, and was most likely carried largely by K(+) ions, although other ionic conductances may also have contributed. 3. In 90% of cells, TPM (20 microm) also enhanced and prolonged the slow (Ca(2+)-dependent) poststimulus afterhyperpolarization (sAHP) and underlying slow outward tail current (sI(AHP)). This effect was due to a selective enhancement/prolongation of an underlying L-type Ca(2+) current that was blocked by nifedipine (20 microm); the TPM response was unlikely to involve an interaction at PKA-dependent phosphorylation sites. 4. The carbonic anhydrase (CA) inhibitor acetazolamide (ACTZ, 20 microm) and the poorly membrane permeant inhibitor benzolamide (BZ, 50 microm) both mimicked the membrane effects of TPM, in generating a slow hyperpolarization (slow outward current under voltage clamp) and sAHP enhancement. ACTZ and BZ occluded the effects of TPM in generating the outward current response, but were additive in producing the sAHP modulatory effect, suggesting different underlying response mechanisms. 5. In bicarbonate/CO(2)-free, HEPES-buffered medium, all the membrane effects of TPM and ACTZ were reproducible, therefore not dependent on CA inhibition. 6. We propose that both novel effects of TPM and ACTZ exerted on cortical neurones may contribute towards their clinical effectiveness as anticonvulsants.

Olfactory bulb repetitive stimulations reveal non-homogeneous distribution of the inhibitory processes in the rat piriform cortex

Optical signals were recorded in the in vivo rat piriform cortex in response to a burst of seven electrical stimulations (100 ms interval) delivered in the olfactory bulb. Based on the recorded responses, three types of signal could be identified according to the relative amplitude of their monosynaptic and disynaptic components. The dysynaptic component had a larger (type 1) or an equal amplitude (type 2) compared with the monosynaptic one. Type 3 exhibited only the monosynaptic component. Type 1 represented 96% of the first response. The second response was characterized by an increase in type 3 signals (39%). The remaining type 1 signals were lower in amplitude when compared with the first response. The responses to the last five stimulations did not differ from one another but were different from the first two (type 1, 74%; type 2, 7.8%; type 3, 18.2% on average). The spatial distribution of these three types of signal was analysed by dividing the piriform cortex into several areas. These areas were not homogeneous in the percentage of each signal type: the percentage of type 3 signals was highest (approximately 30%) in the area near the lateral olfactory tract and < 10% in the most posterodorsal area. Thus the level of inhibition remained high in some piriform areas whereas it decreased rapidly in others, suggesting that the inhibitory processes were not homogeneously distributed in the whole piriform cortex. Functional implications are discussed.

Persistent muscarinic excitation in guinea-pig olfactory cortex neurons: involvement of a slow post-stimulus afterdepolarizing current

The persistent excitatory effects of the muscarinic agonist oxotremorine-M were investigated in guinea-pig olfactory cortex neurons in vitro (28-30 degrees C) using a single-microelectrode current-clamp/voltage-clamp technique. In 40% of recorded cells (type 1), bath-application of oxotremorine-M (2-10 microM; 1-2 min) induced a strong membrane depolarization, an increase in input resistance and a sustained neuronal discharge lasting over 30 min following agonist washout. A large depolarizing stimulus applied during the action of oxotremorine-M, evoked a slow post-stimulus afterdepolarization (approximately 10-15 mV) lasting approximately 30 s. Injection of steady negative current at the peak of this response produced a slow repolarization of the membrane potential (half-time approximately 0.6 min) towards a plateau level ("hyperpolarization recovery"); these effects of oxotremorine-M were slowly reversed on washout or by application of atropine (1 microM). In a second population of neurons (type 2; 39% of total), oxotremorine-M produced a large depolarization, a resistance increase and repetitive firing that did not persist after agonist washout; these neurons failed to generate a prominent slow afterdepolarization on stimulation, and showed no hyperpolarization recovery effect. Their resting membrane properties were not significantly different from those of type 1 cells. The remaining proportion of cells (type 3) elicited little or no muscarinic response to oxotremorine-M and no slow afterdepolarization; these cells showed characteristics spike fractionation (pre-potentials) during an evoked train of action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

The ontogeny of repetitive firing and its modulation by norepinephrine in rat neocortical neurons

The postnatal ontogeny of electrical properties was studied in rat sensorimotor cortical neurons (P6 to adult) using intracellular recording in an in vitro slice preparation. Many action potential properties and input resistance changed during the first 4 postnatal weeks. Repetitive firing behavior also changed during the first postnatal month. Spike-frequency adaptation was much stronger in immature neurons. At 1 week postnatal, the majority of cortical neurons would only fire for less than 200 ms regardless of the intensity of long depolarizing current injections. These cells were normal in other parameters and could fire throughout a depolarizing current injection in the presence of inorganic calcium channel blockers or norepinephrine (NE), suggesting that the inability to fire was not due to injury. The frequency with which we encountered cells with this extreme adaptation decreased with age. Spike-frequency adaptation in immature neurons appears to be primarily controlled by Ca-dependent K+ conductances as in mature neurons. In mature and immature neurons, three afterhyperpolarizations (AHPs) could be distinguished by their rate of decline. The fast AHP followed repolarization of a single spike and was only partially Ca- and K-dependent. The medium duration AHP was Ca-dependent and apamin-sensitive and the slow AHP was partially Ca-dependent and not blocked by apamin. NE decreased the slow Ca-dependent AHP via beta-adrenergic receptors. This effect of NE on AHPs appeared qualitatively similar throughout postnatal development. NE had a proportionately greater effect in younger neurons, however, due to their relatively larger slow AHP. The quantitative differences of NE's action on the slow AHP (sAHP) led to a qualitative difference in NE's effect on firing behavior. The effects of NE on firing behavior may therefore be greater during times critical for cortical maturation.

Temperature-dependent blockade of nucleocytoplasmic transport of newly synthesized RNA in neurons

This study evaluates the temperature sensitivity of transport of recently synthesized RNA from the nucleus to the cytoplasm (nucleocytoplasmic transport) in CNS neurons. Rat hippocampal slices were incubated with [3H]uridine for 1 h to label recently synthesized RNA. Slices were then fixed immediately or maintained at 27 degrees C or 37 degrees C for chase intervals of 3, 4.5, and 6 h to allow for nucleocytoplasmic transport of recently synthesized RNA. The time-dependent translocation of recently synthesized RNA was evaluated autoradiographically. At the end of the 1 h pulse at either 27 degrees C or 37 degrees C, the label was localized exclusively over nuclei. In slices maintained at 37 degrees C, labeling expanded to cover the cell body and proximal dendrites. However, in slices that were labeled and maintained at room temperature, labeling remained confined to the nucleus. In slices that were pulse-labeled at room temperature, and then transferred to 37 degrees C medium, cytoplasmic labeling increased as a function of time. Nucleocytoplasmic transport of RNA in cultured rat hippocampal neurons showed a comparable temperature sensitivity. The inhibition of nucleocytoplasmic transport of RNA at room temperature provides an opportunity to evaluate neuronal function when no new RNA molecules can reach the cytoplasm.

O2 tension in adult and neonatal brain slices under several experimental conditions

Brain tissue O2 tension (pO2) was measured in brainstem slices of adult and neonatal rats using carbon fiber polarographic microelectrodes. These studies were performed in order to examine the relation between pO2 and a variety of experimental conditions including temperature, distance from slice surface, brain region, animal age, tissue thickness and ambient O2 levels. Baseline brain tissue pO2 was inversely proportional to temperature and depth from slice surface. White matter had a much higher pO2 than gray matter. Tissue thickness and animal age had major effects on tissue pO2. In slices of 800 microns thick at 37 degrees C, for example, brain tissue pO2 in the adult dropped to 0 mm Hg at a depth of 200-300 microns, but remained above 45 mm Hg throughout neonatal (3-10 days) slices, when O2 tension in the perfusate was about 600 mm Hg. In thicker neonatal slices (1500 microns), pO2 decreased also to 0 mm Hg in deep areas. An N2 environment produced a rapid reduction in pO2 to 0 mm Hg within 15 s, and O2 levels of 21, 10 and 5% induced graded pO2 minima and graded latencies to reach each pO2 nadir. We conclude that: (1) tissue thickness has a major effect on tissue pO2 level: pO2 can reach zero if the slice is thicker than 600 microns in the adult and 1500 microns thick in the neonate; (2) pO2 level is higher in neonatal brain tissue at all ambient O2 concentrations than in the adult; and (3) graded hypoxia produces patterned and graded reductions in tissue pO2.

Profiles of percent reduction of cytochromes in guinea pig hippocampal brain slices in vitro

Percent reduction profiles of cytochromes (cyt.) aa3, b and c were investigated in bloodless guinea pig hippocampal brain slices of 400, 600 and 800 microns in thickness ranging in temperature from 22 to 37 degrees C. The extent of the percent reduction of cytochromes was compared with the generation of orthodromic potentials elicited by the stimulation of the stratum radiatum, and the cessation of the potentials was found to be correlated with the extent of the percent reduction of the cytochromes. In the case of 400 microns slices, they were found to be in normoxia both from the extent of the percent reduction levels of cytochromes and from the generation of orthodromic responses over a range in temperature. In the case of 600 microns slices, those incubated under temperatures of 22 to 32 degrees C were not in hypoxia from the levels of cytochrome reduction and the production of a field potential. However, slices at 37 degrees C were in hypoxia because of cyt. c levels approached those of cyt. b and the orthodromic response was suppressed. In 800 microns slices, those at 22-27 degrees C were in normoxia; however, slices maintained at 32-37 degrees C were in hypoxia because the levels of cyt. c reduction closely approximated those of cyt. b at 32 degrees C whereas those of cyt. aa3, b and c were almost the same as at 37 degrees C. Moreover, the orthodromic field potential was not evoked.

Muscarinic suppression of the evoked N-wave by oxotremorine-M recorded in the guinea-pig olfactory cortex slice

The effect of the muscarinic agonist oxotremorine-M has been studied on the surface-negative field potential (N-wave) evoked by orthodromic stimulation of the lateral olfactory tract in slices of guinea-pig olfactory cortex. Bath-application of oxotremorine-M (5-80 microM) or carbachol (10-300 microM) produced a reversible depression of the N-wave amplitude without affecting the lateral olfactory tract compound action potential. Oxotremorine-M was approximately 5 times more potent than carbachol in this respect, and the effects of both agonists were competitively blocked by telenzepine (5-100 nM), a selective M1-receptor antagonist. In contrast, methoctramine or AF-DX 116, two 'cardioselective' M2-receptor antagonists, had little or no blocking effect on the agonist responses. It is suggested that oxotremorine-M (like carbachol) inhibits the evoked field potential by activating presynaptic M1-type muscarinic receptors in the olfactory cortex slice.

Direct inhibition by opioid peptides of neurones located in the ventromedial nucleus of the guinea pig hypothalamus

In slices of guinea pig brain, intracellular recordings were obtained from neurones of the ventromedial nucleus of the hypothalamus (VMH). [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO), an agonist selective for mu-opioid receptors, caused an inhibition of spontaneous firing activity and a membrane hyperpolarization. This effect was reversible, concentration-dependent and could be blocked by naloxone. DAGO directly inhibited VMH neurones since its effect persisted when the slice was perifused with a solution that blocks synaptic transmission. The hyperpolarization induced by DAGO was associated with a marked decrease in membrane input resistance and it was reversed in polarity at membrane potentials 30-40 mV more negative than the resting potential. A chloride current did not contribute to the hyperpolarization brought about by DAGO. We conclude that DAGO inhibits VMH neurones, probably by opening membrane potassium channels.

Quantitative effects of some muscarinic agonists on evoked surface-negative field potentials recorded from the guinea-pig olfactory cortex slice

1. The effects of muscarinic receptor agonists on the electrically-evoked surface-negative field potential (N-wave) were measured in the guinea-pig olfactory cortex slice maintained in vitro. 2. Bath-superfusion of (+/-)-muscarine, acetylcholine (ACh), carbachol (CCh), or methacholine (MCh) (10-200 microM) produced reversible, dose-dependent depressions of the N-wave (ACh and MCh effects were observed in the presence of 10 microM neostigmine). The order of potencies (based on agonist dose causing 50% field depression: IC50) was: ACh greater than or equal to muscarine greater than CCh greater than MCh. All four agonists depressed the field potential by 100% at doses greater than 500 microM. 3. Pilocarpine and bethanechol were weak agonists and only produced measurable effects at high doses (1-2 mM). Neither agonist evoked a maximum response at doses up to 10 mM. 4. The muscarinic ganglion stimulant, McN-A-343 yielded inconsistent results, depressing the field potential in some slices, but having no effect in others. Pre-application of a conditioning dose (100 microM) of McN-A-343 reduced subsequent responses to CCh, suggesting possible partial agonist properties. 5. Oxotremorine (up to 100 microM) did not depress the field potential, but it reversibly antagonized the effects of CCh. 6. It is concluded that reproducible, quantifiable responses to muscarinic agonists can be evoked in the olfactory cortex slice. We suggest this preparation may be useful for conducting pharmacological studies of 'intact' central muscarinic receptors.

Pitfalls in the use of brain slices

In vitro brain slices are the preparation of choice for the detailed examination of local circuit properties in mammalian brain. However it is the investigator's responsibility to verify that the circuits under investigation are indeed confined within the boundaries of the functional region of the slice used. The medium in which the slice is maintained is under the full control of the investigator. This places the burden on the investigator to ensure that: (1) the properties of the medium are fully under control; (2) the effects of the medium on the slice are known; (3) the conditions under which the slice is being maintained bear some reasonable relation to those it enjoys (or endures) in vivo. Generalizations to in vivo conditions must be made with caution. If at all possible, similar studies (perhaps less extensive, due to the greater technical difficulties) should be done in vivo to provide a basis for comparison. Investigators using drugs should be aware of, and respect, the basic pharmacological principles cited in the text. In particular, the substantial freedom the investigator has in defining the extracellular medium should not be abused.

Calcium-dependent potassium conductance in guinea-pig olfactory cortex neurones in vitro

1. Guinea-pig olfactory cortex neurones in vitro (23-25 degrees C) were voltage clamped by means of a single-micro-electrode sample-and-hold technique. 2. Under current clamp at the resting potential (approximately -80 mV), brief depolarizing stimuli evoked trains of action potentials with little visible after-potential. However, in 90% of recorded cells held at membrane potentials between -70 and -45 mV, depolarizing current pulses evoked a slow after-hyperpolarization (a.h.p.) (approximately 8 mV) lasting several seconds and accompanied by an increase in input conductance. 3. The outward membrane current underlying the a.h.p. was revealed either by switching rapidly to voltage clamp at the end of a spike train ('hybrid' clamp) or by applying brief depolarizing commands from potentials between -60 to -45 mV. The tail current showed a distinct rising phase (time to peak approximately 1 s) and exponential decay (tau approximately 3 s) and was suppressed by removal of external Ca2+, or adding Co2+ (1-2 mM), Cd2+ (200 microM) or Mg2+ (6 mM). The a.h.p. current reversal potential was -96 mV in 3 mM-K+ medium. 4. Low concentrations (1-2 microM) of muscarine, carbachol, oxotremorine or the muscarinic ganglion stimulant, McN-A-343 (1-10 microM) reduced the a.h.p. current and leak conductance and induced a steady inward current, without affecting M-current (IM) relaxations. IM inhibition generally required higher (greater than 10 microM) agonist concentrations, although oxotremorine remained ineffective at up to 50 microM. 5. The a.h.p. current was reduced by noradrenaline and tetraethylammonium (TEA), but not by apamin or tubocurarine. Apart from TEA, these agents had no effect on IM. 6. Addition of tetrodotoxin (TTX, 1 microM) or removing external Na+ depressed the a.h.p. current amplitude recorded under voltage clamp. The residual tail current could be further reduced by adding Cd2+ or muscarinic agonists. 7. Repolarizing tail currents induced following positive voltage commands consisted mainly of IM and slow a.h.p. current with little evidence of a 'fast' Ca2+-activated K+ current (IC). 8. It is concluded that the slow a.h.p. current that underlies the post-burst after-hyperpolarization of olfactory neurones, is a Ca2+-dependent K+ current distinct from IM. It is suggested that the cholinergic modulation of this current (rather than IM) may provide a more subtle control of cell excitability in cortical neurones.

Ca-channel blockers and the electrophysiology of synaptic transmission of the guinea-pig olfactory cortex

Slices of guinea-pig olfactory cortex have been used to compare the potency of various Ca-blockers on the electrophysiology of synaptic transmission. Listed in the order of potency, the divalent cations Cd2+, Ni2+, Mn2+, Co2+, La3+ and Mg2+ depressed synaptic transmission. The organic Ca-blockers, nifedipine or nimodipine or verapamil and diltiazem were ineffective up to 0.01 mmol/l. Verapamil, D600 or diltiazem (0.1-0.3 mmol/l) depressed both synaptic transmission and the sodium-mediated presynaptic action potential. These results reaffirm the idea that 'organic Ca-antagonist' do not block all Ca-channels in brain and the high Cd2+ sensitivity suggests the Ca-channels in post- and presynaptic membranes have dissimilar pharmacological profiles.

pO2-dependent distribution of potassium in hippocampal slices of the guinea pig

The distribution of extracellular K+-concentration (cK+s) in 200-1000-micron thick hippocampal slices was studied with ion-selective microelectrodes. In ca. 500-micron thick slices cK+s increased from the surface to the innermost layers by ca. 2 mmol/liter if the pO2 of the bath (pBO2) ranged from 300-600 mm Hg and if the temperature was 28 degrees C. In thicker slices and lowered pO2-values further elevations of cK+s were observed. In vital slices thinner than 500 micron cK+s-values exceeded the potassium-concentration of the bath (cK+B) only when pBO2 was markedly lowered. When pBO2 was reincreased in such thin slices, cK+s rapidly declined and often decreased transiently below ck+B. Similar undershoots of cK+s were observed when cK+B was lowered from high to normal levels. The rapid decline was blocked by hypoxia, ouabain, antimycine and a temperature of 18 degrees C. A stepwise rise of cK+B also caused rapid changes of cK+s in vital thin slices. The rates of changes, however, were hardly affected e.g. by a transient hypoxia. Diffusion did not contribute significantly to these steep changes of cK+s. These rapid distribution modes were widely missing in slices thicker than 500 micron. Therefore in such preparations, the extracellular microenvironment of neurons may markedly differ from the ionic concentrations in the bath.

Muscarinic depression of evoked surface-negative field potentials recorded from guinea-pig olfactory cortex in vitro

The action of some cholinergic drugs has been studied on the field potentials evoked by orthodromic stimulation of the lateral olfactory tract (LOT) in guinea-pig olfactory cortex slices maintained in vitro. A reversible depression of the electrically evoked surface-negative field potential (N-wave) was seen following superfusion of muscarine (10-200 microM) or mixed-agonist choline esters but not nicotinic agonists. This depression was blocked by atropine and pirenzepine, but not d-tubocurarine or by antagonists active at gamma-aminobutyric acid or adenosine receptors. Little effect of muscarinic agonists was observed on the compound action potential recorded from the LOT, or on pial surface DC potential. A possible presynaptic site of action of muscarinic agonists in the olfactory cortex is discussed.

Membrane resistivity estimated for the Purkinje neuron by means of a passive computer model

A multicompartment passive electrotonic computer model is constructed for the cerebellar Purkinje cell of the guinea-pig. The model has 1089 coupled compartments to accurately represent the morphology of the Purkinje cell. In order that the calculated behavior of the model fit the published electrophysiological observations of somatic and dendritic input conductance, the neural membrane resistivity must be spatially non-uniform. The passive electrical parameter values for which the model best fits the observations of input conductances, pulse attenuation and current-clamp voltage transients are rm,dend = 45,740 omega cm2, rm,soma = 760 omega cm2, ri = 225 omega cm and cm = 1.16 microF/cm2 (the membrane and cytoplasm specific resistivities and membrane specific capacitance, respectively). The model with these parameter values is electrically compact, with electrotonic length X = 0.33 and dendritic dominance ratio p = 0.44. Analysis of the calculated voltage transient of the multicompartment model by the methods of equivalent-cylinder cable theory is shown to result in very different and unreliable conclusions. The significance for neuronal function of the estimated electrical parameter values is discussed. The possible effect of active conductances on these conclusions is assessed.

The action of cholinomimetic substances on impulse conduction in the habenulointerpeduncular pathway of the rat in vitro

The effects of some cholinomimetic substances and their antagonists on the peak height of compound action potentials recorded from the terminal region of the habenulointerpeduncular pathway have been studied using a rat brain slice preparation. Carbachol and acetylcholine (ACh) depressed the peak height of the compound action potential and increased the latency to peak. The nicotinic agonists nicotine and dimethylphenylpiperazinium depressed the peak height of the compound action potential while muscarine and glutamate had no effect. The depressant effect of carbachol was blocked by the nicotinic antagonists hexamethonium, mecamylamine and d-tubocurarine but not by atropine. Physostigmine enhanced the effects of ACh and, to a lesser extent, carbachol. In the presence of physostigmine, carbachol or ACh initiated a spontaneous oscillation of the amplitude of the compound action potential which was Ca2+ dependent and was blocked by mecamylamine. It is concluded that depression of the amplitude of the compound action potential is due to activation of presynaptic nicotinic receptors. The results are discussed with reference to possible cholinergic mechanisms in the habenulointerpeduncular pathway.

Chemical transmission in the rat interpeduncular nucleus in vitro

We have used a rat brain-slice preparation to study the effects of some cholinomimetic and amino acid agonists and antagonists on the discharge frequency of neurones in the interpeduncular nucleus (i.p.n.), and on the response of these neurones to electrical stimulation of their main excitatory input, the fasciculus retroflexus of Meynert (f.r.m.). A high proportion of i.p.n. neurones were excited by carbachol, acetylcholine (ACh) and muscarine, but methacholine was less effective. The amino acids L-glutamate and L-aspartate were highly effective stimulants of i.p.n. neurones. The responses to ACh or carbachol were greatly reduced by the nicotinic blocking agents hexamethonium, d-tubocurarine and mecamylamine but only slightly reduced by atropine. The response to muscarine was abolished by low doses of atropine. Alpha-Bungarotoxin did not block the response of i.p.n. neurones to f.r.m. stimulation or to cholinomimetic agonists. The response of i.p.n. neurones to f.r.m. stimulation was not appreciably affected by high doses of nicotinic antagonists or atropine nor was there any enhancement of the response by physostigmine. The amino acid antagonists gamma-D-glutamylglycine (gamma DGG) and 2-amino phosphonovalerate (2-APV) were effective blockers of the response to f.r.m. stimulation and preferentially reduced responses to aspartate while having little effect on responses to glutamate or cholinomimetic agonists. It is concluded that ACh is an unlikely candidate for transmitter in this pathway despite abundant neurochemical evidence in its favour. It is more likely that the transmitter is an excitatory amino acid, probably an aspartate-like substance.

Sympathetic preganglionic neurons in the isolated spinal cord of the neonatal rat

A preparation of the isolated spinal cord of the neonatal rat was developed for the study of sympathetic preganglionic neurons (PGNs). PGNs were identified for extracellular single unit recording by their location and by antidromic activation by ventral root stimulation. PGNs could be synaptically activated by stimulation of the dorsal root and spinal pathways. Spontaneous firing was observed in 18% of the PGNs. The average firing rate was 1 Hz with a range of 0.3 to 2 H z. PGNs (and motoneurons) were visualized by incubating ventral roots in horseradish peroxidase (HRP) solutions. The location and morphology of PGNs were similar to those reported in studies using adult animals. Primary afferent fibers were visualized by incubating dorsal roots in HRP solutions. Dorsal root projections appeared mature in the neonatal rat. Primary afferents did not appear to project directly to PGNs. It is concluded that PGNs are viable in this preparation and that spinal sympathetic systems are relatively mature in the neonatal rat.

Calcium-activated outward current in voltage-clamped hippocampal neurones of the guinea-pig

Slow clamp currents were recorded from CA1 and CA3 pyramidal neurones in slices of guinea-pig hippocampus maintained in vitro, using a single micro-electrode sample-and-hold technique. Depolarizing voltage commands evoked a time- and voltage-dependent outward current which was suppressed by removing external Ca or by adding Cd (0.5 mM) or Mn (5 mM). This Ca-dependent current (Ic) was not reduced by muscarinic agonists (unlike IM) but was greatly reduced by 5-20 mM-tetraethylammonium (TEA). Repolarizing IC tail currents reversed at -73 +/- 5 mV in 3 mM-K solution. The reversal potential became about 30 mV more positive on raising [K]o to 15 mM. No clear change in current amplitude or tail-current reversal potential occurred on adding Cs (2 mM), reducing [Cl]o from 128 to 10 mM, or replacing external Na with Tris. The underlying conductance GC was activated at membrane potentials positive to -45 mV. At -32 mV GC showed an approximately exponential increase with time, with a time constant of approximately 0.6 sec at 26 degrees C. Repolarizing tail currents declined exponentially with time, the time constant becoming shorter with increasing negative post-pulse potentials. When the clamp was switched off at the end of a depolarizing command of sufficient amplitude and duration to activate IC, a membrane hyperpolarization to -73 mV ensued, of similar amplitude and decay time to that following spontaneous action potentials. It is concluded that the clamp current observed in these experiments is probably the Ca-activated K current thought to contribute to the post-activation after-hyperpolarization in hippocampal neurones.

Ro 15-1788 is a potent antagonist of benzodiazepines in the olfactory cortex slice

The olfactory cortex slice from the guinea pig has been used to test the benzodiazepine antagonist, Ro 15-1788. Bath application of muscimol has a GABA-mimetic effect on the resting input conductance of these neurones. Benzodiazepines increase the potency of muscimol and increase the duration of postsynaptic inhibitory conductance. To measure the effect of muscimol, the input conductance was measured either directly using intracellular microelectrodes or by measuring its effect on the amplitude of the evoked compound potentials recorded from the slice surface after stimulating the lateral olfactory tract. The potentiation of postsynaptic inhibition produced by benzodiazepines was measured indirectly by their effect on the amplitude of the second of two evoked compound potentials. All of the potentiating effects of diazepam, clonazepam, flurazepam and chlordiazepoxide were blocked by Ro 15-1788 (0.01-10 mumol/l). Ro 15-1788 up to a concentration of 10 mumol/l had no effect on any of the synaptic or electrical responses when applied alone. General anaesthetics which also potentiate inhibition were unaffected by Ro 15-1788. It is concluded that Ro 15-1788 is a highly potent and specific benzodiazepine antagonist in this preparation.

Fast inward-rectifying current accounts for anomalous rectification in olfactory cortex neurones

The somatic membrane of guinea-pig olfactory cortex neurones in vitro (23 degrees C) was voltage clamped by means of a single-micro-electrode sample-and-hold technique. In most cells the current-voltage (I-V) relationship showed inward (anomalous) rectification with increasing hyperpolarization beyond the resting potential (ca. -80 mV). Under current-clamp conditions a time-dependent 'sag' of the hyperpolarizing electrotonic potentials was observed following an initial overshoot. No depolarizing after-potential was seen on return to the resting potential. Inward rectification was activated between -100 and -110 mV (irrespective of pre-set resting potential) and increased the membrane input conductance by up to three-fold. The rectification was unaffected by tetrodotoxin or Cd2+. Under somatic voltage clamp, hyperpolarization beyond -110 mV activated a rapid inward relaxation fitted by a single exponential. The relaxation time constant (tau on) decreased e-fold for a 40 mV hyperpolarization. (Typical values: 28 ms at -110 mV declining to 13 ms at -140 mV; external K+ concentration 3 mM, 23 degrees C). More extreme hyperpolarizations evoked a slower 'inactivation' phase (tau = 40-60 ms). A transient outward-decaying 'tail' current reflecting deactivation of inward rectification was seen on stepping from -140 mV to more positive potentials. tau off became slower with hyperpolarization. The tail current disappeared at a potential close to the expected VK but was rarely inverted to an inward-decaying tail. It is proposed that the fast inward-rectifying current of olfactory neurones (If.i.r.) is a K+ current analogous to the anomalous K+ rectifier of marine egg and frog muscle membranes. The behaviour of the inward rectifier was dependent on external K+ concentration in accordance with the unique 'V--VK' dependence of classical anomalous rectification; however, of several agents tested (external Cs+, Ba2+, Rb+, Tl+ or tetraethylammonium), only Cs+ and Ba2+ blocked If.i.r. in a time- and voltage-dependent manner. The effect of tetraethylammonium resembled that of an increase in external K+. The possible contribution of the inward rectifier to the passive cell membrane properties is discussed.

PO2-profiles in hippocampal slices of the guinea pig

For a detailed analysis of the oxygen supply of hippocampal slices, tissue PO2 (Pt,O2) was recorded polarographically in the neural layers of thick and thin slice preparations from the guinea pig. The experiments showed that the Pt,O2-gradients were extremely steep in the outer zones of vital slices. In an air equilibrated salt solution the surface PO2 was reduced to less than 50% within ca. 25 micron. Minimum values were measured at a depth of ca. 150 micron. A rise of temperature lowered the oxygen supply in the deeper layers of the excised tissue. An elevation of the surface PO2 hardly improved Pt,O2 in the deep structures, because the O2-consumption of the hippocampal slices increased with rising PO2.

Some effects of excitatory amino acid receptor antagonists on synaptic transmission in the rat olfactory cortex slice

A study has been made of the effects of a series of excitatory amino acid receptor antagonists on the field potentials evoked on electrical stimulation of the lateral olfactory tracts of olfactory cortex slices perfused in vitro. The antagonists studied included (+/-)-2-amino-5-phosphonovaleric acid, a potent, specific antagonist of N-methyl-D-aspartate (NMDA) receptors, gamma-D-glutamylglycine, an antagonist of NMDA and kainate receptors and (+/-)-cis-2,3-piperidine dicarboxylic acid and 2-amino-4-phosphonobutyric acid, drugs which in addition to antagonizing NMDA and kainate receptors also block responses to quisqualic acid. From the patterns of effects of the drugs it is proposed that quisqualate and NMDA but not kainate receptors are involved in mediating excitatory transmission in the olfactory cortex; quisqualate receptors are located at the lateral olfactory tract - superficial pyramidal cell synapse whereas NMDA receptors are present at the synapses of the superficial pyramidal cell collaterals with the deep pyramidal cell dendrites and/or at the synapses of the pyramidal cell collaterals and inhibitory interneurones. The results are discussed in terms of possible presynaptic and/or postsynaptic sites of antagonist action.

Baclofen: effects on evoked field potentials and amino acid neurotransmitter release in the rat olfactory cortex slice

A study has been made of the in vitro effects of (+/-)- and (-)-baclofen on the evoked field potentials and release of endogenous amino acid neurotransmitter candidates (aspartate, glutamate, GABA and possibly taurine) which accompany electrical stimulation of the excitatory input to the olfactory cortex slice, the lateral olfactory tract. Baclofen appears to reduce the excitatory input to the GABA-utilizing inhibitory interneurones; this action was manifest as a drug-induced abolition of the field potential known as the P-wave (IC50 for (-)-baclofen, 1.7 +/- 0.4 microM) together with a simultaneous reduction in the synaptically evoked release of aspartase and glutamate from the cut surface of slices. Both these actions of baclofen exhibited concentration dependence and stereospecificity and were not antagonized by picrotoxin (25 microM) thereby suggesting that they are directly related. The consequence of this action of baclofen was the abolition of GABA-mediated presynaptic and postsynaptic inhibition together with their respective field potential correlates, the late N- and I-waves. (+/-)-Baclofen (5 and 25 microM) also inhibited the potassium-evoked release of aspartate and glutamate from small cubes of tissue but, except at a high concentration (1 mM), had no effect on GABA release. Baclofen (up to 1 mM) did not affect transmission either at the lateral olfactory tract-superficial pyramidal cell synapse, a site where aspartate is the likely neurotransmitter, or at the superficial pyramidal cell collateral-deep pyramidal cell excitatory synapse. It is proposed that: (i) the actions of baclofen on the olfactory cortex are the result of inhibition of aspartate and glutamate release, probably from deep pyramidal cell collaterals; and (ii) not all neurones utilizing excitatory amino acids as their neurotransmitters are subject to the inhibitory action of baclofen.

Membrane characteristics of visual cortical neurons in in vitro slices

In in vitro slices prepared from the visual cortex of cats a total of 15 neurons were studied for their membrane properties by passing currents through an intracellular microelectrode and by electrical stimulation of the white matter. Passive membrane properties (mean +/- S.D.): resting membrane potential, 58 +/- 10 mV; input resistance, 42.1 +/- 34.0 M omega; time constant, 9.0 +/- 3.6 ms. Some neurons showed a linear relationship between intensity of injected DC current and frequency of thereby induced discharges.

The effects of chlordiazepoxide on synaptic transmission and amino acid neurotransmitter release in slices of rat olfactory cortex

The rat olfactory cortex slice has been used to investigate the effects of chlordiazepoxide on evoked field potentials and the release of endogenous amino acid neurotransmitters (aspartate, glutamate, GABA and possibly taurine) which accompany electrical stimulation of the lateral olfactory tract. When single, low frequency stimuli were employed, chlordiazepoxide (2 microM-1 mM) depressed the amplitude of the field potential correlate of the depolarizing actions of the lateral olfactory tract excitatory transmitter (aspartate?) although aspartate release was unaffected. The field potential correlate of GABA-mediated presynaptic inhibition (late N-wave) was also depressed in amplitude but low drug concentrations (between approximately 2 and 50 microM) increased its peak duration . Effects of chlordiazepoxide on evoked inhibition were analyzed by giving paired stimuli such that the second stimulus occurred during the field potentials evoked by the first stimulus. Chlordiazepoxide (1-20 microM) increased the depression in amplitudes of the presynaptic massed action potential and late N-wave evoked by the second of a pair of stimuli compared with those evoked by the first stimulus suggesting that presynaptic inhibition was potentiated. These effects of chlordiazepoxide were accompanied by a significant reduction in aspartate release from the lateral olfactory tract terminals. Moreover, the drug effects on presynaptic inhibition and aspartate release were antagonized by picrotoxin (5 microM). On the other hand, chlordiazepoxide (1-50 microM) had no significant effect on postsynaptic inhibition. The results are discussed in terms of both the sites (presynaptic or postsynaptic) and mechanisms of action of chlordiazepoxide.

Patterns of endogenous amino acid release from slices of rat and guinea-pig olfactory cortex

A study has been made of the effects of depolarizing stimuli on the release of endogenous amino acid neurotransmitter candidates (aspartate, glutamate, GABA and taurine) from in vitro preparations of rat and guinea pig olfactory cortex. Exposure of small cubes of olfactory cortex tissue from either species to potassium chloride (50 mM) was accompanied by a calcium-dependent release of aspartate, glutamate and GABA. A similar release pattern was evoked by protoveratrine A (100 muM) although the release was largely calcium-independent. Neither agent led to increased release of taurine. Electrical stimulation of the excitatory input (lateral olfactory tract) of freshly prepared, synaptically intact olfactory cortex slices of both species induced significant release of aspartate and GABA from the uncut pial surface and of aspartate, GABA and glutamate from the cut surface. Evoked taurine release occurred from both surfaces of rat olfactory cortex slices but no release was detected from guinea pig olfactory cortex slices. These patterns of release were unaffected by changes in stimulus frequency and were mimicked by protoveratrine A (100 muM) applied to one or other surface. Preincubation of slices from rats for 2 led to loss of tissue amino acids and to changes in their release patterns; the presence of glutamine (5 mM) during preincubation prevented the loss of amino acids but did not alter their pattern of release. Because of the close similarities between both the electrophysiological properties and the patterns of amino acid release it is concluded that there is probably an identity of amino acid neurotransmitters (aspartate, glutamate and GABA) in rat and guinea pig olfactory cortex. The role of taurine in the rat olfactory cortex is unknown but would seem unlikely to be that of a neurotransmitter. The results are discussed: (i) in terms of the cellular origins of the released amino acids; and (ii) wit respect to apparent experimental discrepancies which have appeared in the literature.

Epileptiform burst afterhyperolarization: calcium-dependent potassium potential in hippocampal CA1 pyramidal cells

Synaptic excitation of hippocampal cells during blockade of synaptic inhibition results in an epileptiform "burst" potential followed by a prolonged afterhyperpolarization. This afterhyperpolarization resembles the one that is seen after the epileptic interictal spike and that is considered of critical importance in preventing seizure development. The afterhyperpolarization produced in the presence of y-aminobutyric acid antagonists is associated with a conductance increase and is inhibitory. It can occur in an all-or-none fashion after a burst, is independent of chloride, and is depressed by barium. The afterhyperpolarization has a reversal potential of (-86) millivolts, and the reversal potential is strongly dependent on the extracellular concentration of potassium. The afterhyperpolarization appears to be an intrinsic, inhibitory potassium potential mediated by calcium. This finding has implications for understanding the cellular mechanisms of epilepsy.

Time-course of declining electrical activity in guinea-pig olfactory cortex after olfactory bulb removal

An electrophysiological study has been made of the degeneration of the afferent axons of the lateral olfactory tract (L.O.T.) which give rise to excitatory synapses throughout the olfactory cortex. These fibres were severed by removing one or both olfactory bulbs. Evoked potentials were recorded from slices of olfactory cortex in vitro prepared at various times after bulbectomy. Up to 1.6 days later, all potentials were similar to those of the unoperated side. Between 1.6 and 1.8 days there was a complete loss of evoked potential from both L.O.T. axons and postsynaptic neurones, which occurred synchronously at all points along a 5-6 mm length of the tract.

Convulsants antagonise inhibition in the olfactory cortex slice

The effects of some gamma-aminobutyric acid (GABA) antagonists and other "convulsants" have been tested on neurones of the isolated olfactory cortex slice preparation of the guinea-pig using single cell intracellular and gross extracellular recording techniques. Bicuculline, picrotoxin, strychnine, leptazol, bemegride, and also theophylline and d-tubocurarine all increased the duration and amplitude of the excitatory postsynaptic potential, thereby producing a seizure-like discharge. These drugs reduced and shortened the peak conductance increase during the inhibitory postsynaptic potential (i.p.s.p.) in normal solution and after the i.p.s.p. had been prolonged by the presence of a barbiturate. The results suggested that these drugs antagonise synaptic inhibition through a common mechanism, perhaps by reducing the effect of neurally released GABA.

The preservation of nerve cells in rat neostriatal slices maintained in vitro: a morphological study

The in vitro preservation of neurons in 300 micron thin neostriatal slices, which are routinely used for electrophysiological studies, was examined by light and electron microscopy and was compared to 700 micron thick neostriatal slices. The thin slices displayed well-preserved cells after up to 5 h of incubation. This finding correlated well with whether electrical activity could be recorded. In cross-section, the thin slices consisted of three layers: the inner layer contained many intact cells (80%) and was sandwiched between the outer layers where deteriorating cells predominated. In contrast to the thin slices, the thick slices (700 micron) displayed no layering of intact cells in cross-section. Instead, the majority of cells throughout these thick slices was swollen (98%), with only small patches of intact cells. Two types of deteriorating cells were apparent: swollen cells and dark (pycnotic) cells. The proportion of swollen cells increased with incubation time. In the thin slices this swelling occurred in the outer layers with the middle layer of intact cells remaining relatively unchanged over long incubation periods, whereas all cells in the thick slices were swollen after 2 h of incubation. Dark cells were localized to the outer portion of both slices and the number of such dark cells did not change with incubation time.

Intracellularly-recorded effects of glutamate and aspartate on neurones in the guinea-pig olfactory cortex slice

The effects of bath-applied glutamate, aspartate (and some related amino acids) on neurones of the guinea pig olfactory cortex slice were recorded intracellularly. Neurones were activated either by intracellularly-applied current or orthodromically by stimulating the lateral olfactory tract. In response to orthodromic stimuli several neurones displayed a late hyperpolarizing potential (LHP) after the usual sequence of EPSP, spike and IPSP. Glutamate and aspartate evoked 3 types of response: (a) a depolarization with apparent increase in input conductance; (b) a depolarization with no detectable conductance change; and (c) a hyperpolarization with conductance increase. Some possible mechanisms by which these 3 response-types could be generated are discussed. Depolarizations evoked by the glutamate analogue, kainate, were usually irreversible. Our results emphasize that glutamate and aspartate can evoke a variety of neuronal responses from olfactory cortex neurones. Several of these responses were previously undetected in experiments based on extracellular recordings.

Release of endogenous amino acid neurotransmitter candidates from rat olfactory cortex slices: possible regulatory mechanisms and the effects of pentobarbitone

A study has been made of the regulation of evoked release of the amino acid neurotransmitter candidates (aspartate, GABA and taurine) from rat olfactory cortex slices. The effects of pentobarbitone (10-1000 microM) on release have also been assessed. Release of aspartate, the presumed excitatory transmitter of some of the lateral olfactory tract fibres, is reduced by muscimol (10 microM) and this effect is antagonized by picrotoxin (15 microM): it is concluded that presynaptic GABA receptors may modulate aspartate release. Low concentrations of pentobarbitone also reduce aspartate release, but this effect is picrotoxin-insensitive. Release of GABA, the presumed transmitter of inhibitory interneurones, is reduced by muscimol (10 microM) and this effect is antagonized by picrotoxin (15 microM): it is suggested that GABA release may be regulated by presynaptic autoreceptors. Pentobarbitone significantly increases release of GABA when slices are synaptically activated although the mechanism of this effect is unclear. Release of taurine, not hitherto considered a neurotransmitter in this brain area, is depressed by muscimol (10 microM) and pentobarbitone and increased by picrotoxin (15 microM). Results are discussed in terms of (i) mechanisms of regulation of amino acid release in the olfactory cortex, (ii) effects of pentobarbitone on release and (iii) the compatibility of the present results with previously published electrophysiological studies.

Influence of cellular transport on the interaction of amino acids with gamma-aminobutyric acid (GABA)-receptors in the isolated olfactory cortex of the guinea-pig

1 Freshly cut guniea-pig olfactory cortex slices contained 2.2 mmol gamma-aminobutyric acid (GABA)/kg tissue weight. This declined during in vitro incubation at 25 degrees C in the absence of exogenous GABA, but increased to 6.95 mmol/kg after 1.5 h incubation in 1 mM GABA. 2 Uptake of [3H]-GABA (1 microM) was inhibited by 1 mM (+/-)-nipecotic acid (-83%), beta-amino-n-butyric acid (BABA) (-59%), L-2,4-diaminobutyric acid (DABA) (-63%), (+/-)cis-3-aminocyclohexane carboxylic acid (ACHC) (-53%), and 3-aminopropanesulphonic acid (3-APS) (-26%), but was increased by beta-alanine (BALA) (+23%). 3 Autoradiographs showed steep concentration gradients of radioactivity across slices incubated for short periods in [3H]-GABA. 4 Efflux of [3H]-GABA from pre-loaded slices was accelerated strongly by nipecotic acid, BABA, DABA and ACHC but weakly or not all by BALA or 3-APS. 5 Nipecotic acid (1 mM) potentiated the surface-depolarization of the slice produced by GABA but not that produced by 3-APS. 6 The depolarizing actions of DABA, BABA, nipecotic acid and ACHC, but not that of 3-APS or BALA, were potentiated when the endogenous GABA content of slices was raised. 7 It is concluded that: (a) the depolarizing action of exogenous GABA is limited by cellular uptake; (b) surface-depolarizations produced by nipecotic acid, DABA, BABA and ACHC may be mediated by the release of GABA; and (c) neuronal, rather than glial, transport systems are responsible for these effects.

Responses of the guinea-pig isolated olfactory cortex slice to gamma-aminobutyric acid recorded with extracellular electrodes

1. Potential changes between the pial and cut surfaces of slices of guinea-pig olfactory cortex in vitro produced by gamma-aminobutyric acid (GABA) were recorded with extracellular electrodes. 2. GABA, superfused over the pial surface (0.1 to 10 mM), produced a pial-negative potential deflection, accompanied by inhibition of the postsynaptic response to lateral olfactory tract (LOT) stimulation. 3. This effect was replicated by the following compounds (potency relative to GABA = 1, in brackets): 3-aminopropanesulphonic acid (5.3), epsilon-aminovaleric acid (0.07), beta-alanine (0.07), beta-amino-nibutyric acid 0.05), epsilon-aminocaproic acid, alpha-amino-isobutyric acid, L-leucine (less than 0.02). 4. L-Glutamate (1 to 10 mM) produced a very large surface negative shift, with relatively less synaptic inhibition. Glycine (1 to 10 mM) produced less surface negatively, accompanied by synaptic inhibition. 5. Responses to GABA were antagonized more effectively than those to glycine by bicuculline (3 to 30 micrometer) and picrotoxin (1 to 30 micrometer). Strychnine (1 to 10 micrometer) incompletely inhibited responses to glycine. 6. It is concluded that, while the locus within the slice of these effects is uncertain, the preparation may be useful for testing the interaction of drugs with cerebral GABA receptors.

Depolarization of neurones in the isolated olfactory cortex of the guinea-pig by gamma-aminobutyric acid

1 Effects of gamma-aminobutyric acid (GABA) on single neurones in slices of guinea-pig olfactory cortex maintained in vitro were recorded with single intracellular microelectrodes. The average resting potential of 52 cells was -75 mV and apparent input resistance ranged from 20 to 200 MOmega.2 Superfusions of GABA over the slice invariably depolarized the neurones and reduced their input resistance. The minimum effective concentration was 50 to 200 muM.3 The reversal potential for the depolarization produced by 0.1 mM GABA (E(g)) was -66 +/- 2 mV. At concentrations >0.1 mM the reversal potential became progressively more positive (-55 to -50 mV).4 Reduction of external chloride, with isethionate as the substitute anion, increased the amplitude of the depolarization.5 GABA reduced the amplitude of the excitatory postsynaptic potential produced by lateral olfactory tract stimulation, and occluded or reversed the subsequent depolarizing recurrent inhibitory postsynaptic potential.6 Action potentials elicited by injection of depolarizing current or by focal antidromic stimulation were slowed and reduced in amplitude by GABA.7 The effects of GABA on membrane conductance (potency = 1) were duplicated by 3-aminopropanesulphonic acid (potency = 20), beta-alanine (0.5), beta-amino-n-butyric acid (0.5), glycine (0.3) and L-2,4-diaminobutyric acid (0.2). For a given conductance change, 3-aminopropanesulphonic acid, glycine and beta-alanine produced less depolarization than did GABA.8 It is concluded that the action of GABA on the neurones is compatible with a role in mediating recurrent postsynaptic inhibition.

Excitability increase of neurons in olfactory cortex slices of the guinea pig after penicillin administration

The mechanism underlying the effects of penicillin on slices of the olfactory cortex of the guinea pig was examined. In a previous report it was shown that penicillin increases the amplitude of the presynaptic action potential, the population EPSP and, more strongly, the population responses of the postsynaptic cells. Moreover, the postsynaptic population responses increased in number and suggested strong repetitive firing. These results were confirmed in the present study. Analysis of stimulus-response relationships suggested that the enhancement of the postsynaptic response was due to an increase in excitability of the postsynaptic neurons by penicillin. The amplitude changes of the presynaptic action potential and the EPSP were probably largely, if not completely, due to an increase in resistance of the bathing fluid. It was found that the changes in population responses paralleled to a large extent changes in cell discharge. In addition, penicillin was found to induce spontaneous firing of the postsynaptic cells. The changes in cell discharge were consistent with an increase in excitability of the postsynaptic cells.

Depression of evoked potentials in brain slices by adenosine compounds

1 A study has been made of the action of adenosine on surface slices of guinea-pig olfactory cortex in vitro. 2 With extracellular recordings from the pial surface and stimulation of the presynaptic input, the lateral olfactory tract (LOT) generated a monosynaptic negative wave representing dendritic excitatory potentials. This negative wave was depressed by bath application of 1 micron adenosine with increasing effect up to 1 mM. Adenosine 5'--triphosphate (ATP), adenosine 5'-monophosphate (AMP) and cyclic adenosine 3',5'-monophosphate (cyclic AMP) had similar depressant actions. Adenine and guanosine were very weak depressants. 3 Theophylline concentrations in the range 10 micron to 3 mM progressively antagonized the action of adenosine. 4 Dibuyryl cyclic AMP (100 micron) and agents which increase intracellular cyclic AMP were not depressants, suggesting that the action of adenosine was not cyclic AMP-mediated. 5 Intracellular recordings confirmed the depressant effect of adenosine on excitatory potentials generated by LOT stimulation and also showed that postsynatpic action potentials and the membrane of the soma were unaffected by adenosine. 6 Since presynaptic action potentials were also unaffected by adenosine, these experiments suggest that adenosine reduces excitatory transmission at LOT synapses and fortifies the idea that adenosine has a 'neurohumoral' action.

A depolarizing inhibitory potential in neurones of the olfactory cortex in vitro

1. Stable intracellular recordings were obtained from neurones in slices of the guinea-pig olfactory cortex maintained in vitro. 2. Single stimuli applied to the lateral olfactory tract (l.o.t.) produced an excitatory post-synaptic potential (e.p.s.p.) usually generating a single spike. 3. The e.p.s.p. was followed by a long (200-500 msec) after-depolarization (l.a.d.) of peak amplitude 5-16 mV. This was accompanied by a very large conductance increase and was associated with an inhibition of the intracellularly recorded e.p.s.p. and of spike generation. 4. The l.a.d. was more susceptible than the e.p.s.p. to depression by (i) repetitive l.o.t. stimulation and (ii) raising external [Mg2+]. The l.a.d. could be generated without a preceding spike. 5. At an average resting membrane potential of -74 mV the average reversal potential for the l.a.d. (El.a.d.) was -63 mV.El.a.d. became more positive on reducing [Cl-]out or on using KCl-filled electrodes. 6. It is concluded that the l.a.d. represents a Cl- -mediated inhibitory post-synaptic potential, generated through deep-lying recurrent inhibitory loops.

Field potentials, inhibition and the effect of pentobarbitone in the rat olfactory cortex slice

1. Field potentials were evoked in slices of rat olfactory cortex by stimulating the lateral olfactory tract. In addition to previously described components of the wave-form, a further distinct surface-negative potential of low amplitude and long duration (I-wave) has been described. 2. Pentobarbitone, at concentrations of 10(-5) M and above, markedly enhanced enhanced the amplitude and duration of the I-wave with only minimal effect on other components of the field potential. 3. The I-wave was reversibly reduced by the GABA antagonists bicuculline and picrotoxin and was also attenuated at rapid rates of stimulation. Low chloride medium usually caused a transient increase in amplitude of the I-wave followed by a gradual reduction, suggesting that a chloride-mediated depolarization was involved. 4. Evoked inhibition, which was most probably post-synaptic, occurred in parallel with the I-wave. This was monitored as a suppression of, or increase in latency of the population spike evoked by a second stimulus at appropriate intervals after the first. Pentobarbitone substantially increased the duration of the post-synpatic inhibition, without obvious changes in the presynaptic inhibitory phenomenon associated with antidromic firing in the lateral olfactory tract. 5. It is proposed that the I-wave is the field potential representation of a population depolarizing i.p.s.p. and that the main action of pentobarbitone is to enhance this inhibition.