Inhibitory action of adenosine and adenosine analogs on neurotransmission in the olfactory cortex slice of guinea pig - structure-activity relationships

The postsynaptic potential (PSP) was recorded from thin slices of the olfactory cortex of the guinea pig. Application of adenosine and adenine nucleotides such as 5'-ATP, 5'-ADP and 5'-AMP in the incubation medium, depressed the amplitude of the PSP without altering the presynaptic fiber potential. The other purine and pyrimidine derivatives had no inhibitory effect. The inhibitory action of adenosine and adenine nucleotides on the PSP were manifest at concentrations of 5 microM-1 mM. Adenosine, 5'-ATP, 5'-ADP and 5'-AMP were equipotent in evoking depression of PSPs. Inhibition occurred within 10-20 sec after administration of the agents and the depressant effect disappeared rapidly after the removal of the compounds from the medium. Theophylline reversed and prevented the inhibition produced by adenosine and adenine nucleotides. To test the structure-activity relationships of these compounds, adenosine analogs and adenine nucleotide derivatives were applied to the medium. The 6-aminopurine riboside (adenosine radical) was found to be essential for inhibitory action on the PSP. Among adenosine analogs, the presence of at least one hydrogen atom in the amino group at the 6-position of the purine, and the OH group at the 2'-position of the ribose was essential for inhibitory activity.     

Effects of emodin on synaptic transmission in rat hippocampal CA1 pyramidal neurons in vitro

Rhubarb extracts provide neuroprotection after brain injury, but the mechanism of this protective effect is not known. The present study tests the hypothesis that rhubarb extracts interfere with the release of glutamate by brain neurons and, therefore, reduce glutamate excitotoxicity. To this end, the effects of emodin, an anthraquinone derivative extracted from Rheum tanguticum Maxim. Ex. Balf, on the synaptic transmission of CA1 pyramidal neurons in rat hippocampus were studied in vitro. The excitatory postsynaptic potential (EPSP) was depressed by bath-application of emodin (0.3-30 microM). Paired-pulse facilitation (PPF) of the EPSP was significantly increased by emodin. The monosynaptic inhibitory postsynaptic potential (IPSP) recorded in the presence of glutamate receptor antagonists (DNQX and AP5) was not altered by emodin. Emodin decreased the frequency, but not the amplitude, of the miniature EPSP (mEPSP). The inhibition of the EPSP induced by emodin was blocked by either 8-CPT, an adenosine A1 receptor antagonist, or by adenosine deaminase. These results suggest that emodin inhibits the EPSP by decreasing the release of glutamate from Schaffer collateral/commissural terminals via the activation of adenosine A1 receptors in rat hippocampal CA1 area and that the neuroprotective effects of rhubarb extracts may result from decreased glutamate excitotoxicity.     

C-Jun N-terminal kinase regulates adenosine A1 receptor-mediated synaptic depression in the rat hippocampus

Adenosine A1 receptors are ubiquitous mediators of presynaptic inhibition of neurotransmission in the central nervous system, yet the signalling pathway linking A1 receptor activation and decreased neurotransmitter release remains poorly resolved. We tested the contribution of c-Jun N-terminal kinase (JNK) to adenosine A1 receptor-mediated depression of field excitatory postsynaptic potentials (fEPSPs) in area CA1 of the rat hippocampus. We found that inhibition of JNK with SP600125 or JNK inhibitor V, but not an inactive analogue, attenuated the depression of fEPSPs induced by adenosine, hypoxia, and the A1 receptor agonist N⁶-cyclopentyladenosine (CPA). In contrast, the JNK inhibitor SP600125 did not inhibit GABA_B-mediated synaptic depression. In support of our electrophysiological findings, Western blot analysis showed that A1 receptor stimulation resulted in a transient increase in JNK phosphorylation in the membrane fraction of hippocampal lysates. The total amount of JNK in the membrane fraction was unchanged by CPA treatment. The increase in phosphorylated JNK induced by A1 receptor stimulation was blocked by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), indicating that A1 receptors specifically activate JNK in the hippocampus. Together with functional data indicating that JNK inhibition decreased CPA-induced paired pulse facilitation, these results suggest that JNK activation is necessary for adenosine A1 receptor-mediated synaptic depression occurring at a presynaptic locus The adenosine A1 receptor-JNK signalling pathway may represent a novel mechanism underlying inhibition of neurotransmitter release in the CNS.     

Astrocytic adenosine kinase regulates basal synaptic adenosine levels and seizure activity but not activity-dependent adenosine release in the hippocampus

Adenosine is an endogenous inhibitor of excitatory synaptic transmission with potent anticonvulsant properties in the mammalian brain. Given adenosine's important role in modulating synaptic transmission, several mechanisms exist to regulate its extracellular availability. One of these is the intracellular enzyme adenosine kinase (ADK), which phosphorylates adenosine to AMP. We have investigated the role that ADK plays in regulating the presence and effects of extracellular adenosine in area CA1 of rat hippocampal slices. Inhibition of ADK activity with 5′-iodotubercidin (IODO; 5 μM) raised extracellular adenosine, as measured with adenosine biosensors, and potently inhibited field excitatory post-synaptic potentials (fEPSPs) in an adenosine A₁R-dependent manner. In nominally Mg²⁺-free aCSF, which facilitated the induction of electrically-evoked epileptiform activity, adenosine biosensor recordings revealed that seizures were accompanied by the transient release of adenosine. Under these conditions, IODO also inhibited the fEPSP and greatly suppressed epileptiform activity evoked by brief, high-frequency stimulation. During spontaneous seizures evoked by the A₁R antagonist CPT, adenosine release was unaffected by IODO. This suggests that ADK activity does not limit activity-dependent adenosine release. On the basis of strong ADK immunoreactivity in GFAP-positive cells, astrocytes are likely to play a key role in regulating basal adenosine levels. It is this action of ADK on the basal adenosine tone that is permissive to seizure activity, and, by extension, other forms of activity-dependent neuronal activity such as synaptic plasticity.     

Inhibitory effect of adenosine on transmission in sympathetic ganglia

Summary The effect of bath-applied adenosine on transmission in the isolated superior cervical ganglion of the rat was investigated. The compound post ganglionic action potential was recorded as an index of ganglionic transmission. Adenosine and 2-chloroadenosine were equipotent in producing a dose-dependent inhibition of the amplitude of the compound action potential. At the highest concentration tested (1 mM) adenosine and 2-chloroadenosine produced about 30% decrease in the amplitude of the compound action potential. This inhibitory effect was antagonized by theophylline (1 and 100 M) which by itself had no significant effect on ganglionic transmission. The adenosine uptake blocker dipyridamole (1 and 100 M) failed to potentiate the inhibitory action of adenosine. Both 4-aminopyridine (20 M) and high frequencies of stimulation (3, 10 and 20 Hz) were effective in nearly completely abolishing the inhibitory effect of adenosine on ganglionic transmission.The results suggest that the inhibitory effect of adenosine on ganglionic transmission may be the result of activation of presynaptic adenosine receptors in the ganglion.     

The low KM-phosphodiesterase inhibitor denbufylline enhances neuronal excitability in guinea pig hippocampus in vitro

Summary The actions of the phosphodiesterase inhibitor denbufylline on the excitability of hippocampal neurons were investigated by means of extracellular and intracellular recordings. Denbufylline, which has been shown to selectively inhibit a low K_M, Ca²⁺/calmodulin-independent phosphodiesterase isozyme, concentration-dependently increased the amplitude of the extracellularly recorded CAI population spike evoked by electrical stimulation of the Schaffer collateral/commissural pathway. Concentration-response-curves yielded an EC₅₀ for denbufylline of 0.76 M. In comparison, the nonselective phosphodiesterase inhibitor 3-isobutyl-lmethylxanthine (IBMX) also produced an increase in the amplitude of the population spike. From the concentration-response-curve, which was steeper than that of denbufylline, an EC₅₀ for IBMX of 1.04 M was obtained. However, despite their similar EC₅₀ values, denbufylline was found to be significantly more potent at lower concentrations (<- 300 nM) than IBMX.Intracellular recordings from CAI pyramidal cells revealed postsynaptic actions of denbufylline (300 nM) as indicated by a small drug-induced depolarization (2 – 5 mV) associated with an increase in membrane input resistance by 10–20%. In addition, denbufylline blocked the accommodation of trains of action potentials evoked by the injection of depolarizing current pulses.The results suggest i) that accumulation of adenosine-3,5-monophosphate (CAMP) in the postsynaptic cell and/or in the presynaptic terminal produced by blockade of phosphodiesterases leads to enhanced synaptic transmission in the CAI area of the hippocampus and ii) that a low K_M, Ca ²⁺/calmodulin-independent cAMP-phosphodiesterase is an important component involved in the regulation of the intracellular cAMP level at synapses of central nervous system neurons. Send offprint requests to B. Sutor at the above address     

Effect of adenosine on sinoatrial and ventricular automaticity of the guinea pig

Summary In isolated spontaneously beating right ventricular strips and right atrial preparations of guinea pigs adenosine was found to exert a concentration-dependent suppressing effect on the pacemaker activity. Responsiveness to adenosine was approximately 30-fold higher in ventricular than in atrial preparations. A decrease in the rate of slow diastolic (phase 4) depolarization of Purkinje and sinoatrial nodal fibers proved to be a major determinant of the adenosine-induced alteration in pacemaker activity. It is suggested that adenosine might exert its depressant effect on ventricular automaticity via direct excitation of purine receptors located in the specialized pacemaker fibres of the ventricular tissue.This work was supported by the Scientific Research Council, Ministry of Public Health, Hungary, Grant No. 2-06-0101-02-2/SzPreliminary report of this work was presented at the 7th International Congress of Pharmacology, Paris, 1978     

Substance P stimulates inhibitory synaptic transmission in the guinea pig basolateral amygdala in vitro

To determine the physiological role of tachykinin NK1 receptors in the basolateral nucleus of the amygdala (BLN) we have studied the electrophysiological effects of substance P (SP) in the absence and presence of selective tachykinin receptor antagonists in guinea pig brain slices. Recordings were made from two populations of neurones; spiny pyramidal and stellate neurones, both thought to be projection neurones. Activation of NK1 receptors with SP increased the frequency of spontaneous inhibitory postsynaptic potentials in the majority of cells. This effect was blocked by bicuculline or tetrodotoxin, but not ionotropic glutamate receptor antagonists. The enhanced synaptic activity induced by SP was antagonised by the NK1 receptor antagonist L-760,735 but not by the less active enantiomer L-781,773 or the NK3 receptor antagonist L-769,927. Thus in the basolateral nucleus of the guinea pig amygdala, NK1 receptor activation preferentially stimulates inhibitory synaptic activity. Consistent with this observation, immunohistochemistry revealed NK1 receptor immunoreactivity to be largely restricted to a subset of GABA interneurones. These studies support a physiological role for SP in the regulation of pathways involved in the control of emotional behaviour.     

Excitatory and inhibitory action of GABA on synaptic transmission in slices of guinea pig superior colliculus

The laminar distribution of gamma-aminobutyric acid (GABA) in the superior colliculus of the guinea pig was analyzed. The superficial gray layer, especially the upper half, was found to contain the highest amount, 37.4 +/- 1.1 mmol/kg dry. To investigate the role of GABA in the superficial gray layer, the effect of GABA on neurotransmission in the superficial gray layer was studied in superior colliculus slices in a perfusion system. Bath-applied GABA, 100 microM-1 mM, enhanced the amplitude of the postsynaptic field potential (PSP) in the superficial gray layer dose dependently and at concentrations above 1 mM it depressed the PSP dose dependently. A similar response pattern was obtained with muscimol (0.1-10 microM, enhancement; greater than 10 microM, depression). However, (-)-baclofen only inhibited the PSP dose dependently (0.1-1 microM). The excitatory effect of GABA was enhanced at concentrations of nipecotic acid less than 0.5 mM. Bicuculline 1 microM shifted the dose-response curve of GABA to the right and the excitatory effect was also enhanced. These results indicate that GABA has a dual action on neurotransmission in the superficial gray layer: an excitatory effect, possibly mediated by GABAA receptors and an inhibitory effect mediated by both GABAA and GABAB receptors.     

Inhibition of adenosine accumulation into guinea pig ventricle by benzodiazepines

Several benzodiazepines were compared with dipyridamole for their ability to inhibit the nucleoside transporter specific accumulation of [³H]adenosine into guinea pig ventricle. Diazepam inhibited adenosine accumulation but was considerably less potent than dipyridamole. Lorazepam and flurazepam also inhibited accumulation, but were less potent than diazepam. Coronary vasodilation by diazepam or by dipyridamole probably is mediated via potentiation of adenosine following inhibition of the nucleoside transport system.     

Evidence for an A2 adenosine receptor in guinea pig lung

Summary Adenosine receptors in guinea pig lung were characterized by measurement of cyclic AMP formation and radioligand binding. 5-N-Ethylcarboxamidoadenosine (NECA) increased cyclic AMP levels in lung slices about 4-fold over basal values with an EC₅₀ of 0.32 mol/l. N⁶-R-(–)-Phenylisopropyladenosine (R-PIA) was 5-fold less potent than NECA. 5-N-Methylcarboxamidoadenosine (MECA) and 2-chloroadenosine had EC₅₀-values of 0.29 and 2.6 mol/l, whereas adenosine and inosine had no effect. The adenosine receptors in guinea pig lung can therefore be classified as A₂ receptors. Several xanthine derivatives antagonized the NECA-induced increase in cyclic AMP levels. 1,3-Diethyl-8-phenylxanthine (DPX; K _i 0.14 mol/l) was the most potent analogue, followed by 8-phenyltheophylline (K _i 0.55 mol/l), 3-isobutyl-1-methylxanthine (IBMX; K _i 2.9 mol/l) and theophylline (K _i 8.1 mol/l). In contrast, enprofylline (1 mmol/l) enhanced basal and NECA-stimulated cyclic AMP formation. In addition, we attempted to characterize these receptors in binding studies with [³H]NECA. The K _D for [³H]NECA was 0.25 mol/l and the maximal number of binding sites was 12 pmol/mg protein. In competition experiments MECA (K _i 0.14 mol/l) was the most potent inhibitor of [³H]NECA binding, followed by NECA (K _i 0.19 mol/l) and 2-chloroadenosine (K _i 1.4 mol/l). These results correlate well with the EC₅₀-values for cyclic AMP formation in lung slices. However, the K _i-values of R-PIA and theophylline were 240 and 270 mol/l, and DPX and 8-phenyltheophylline did not compete for [³H]NECA binding sites. Therefore, a complete characterization of A₂ adenosine receptors by [³H]NECA binding was not achieved. In conclusion, our results show the presence of adenylate cyclase-coupled A₂ adenosine receptors in lung tissue which are antagonized by several xanthines.     

Age-dependence in the homeostatic upregulation of hippocampal dendritic spine number during blocked synaptic transmission

Homeostatic regulation of spine number in mature hippocampal neurons results in more dendritic spines when synaptic transmission is blocked, providing a mechanism to compensate for diminished synaptic input. It is unsettled whether blockade of synaptic transmission also elevates spine number during development. To address this question, synaptic transmission was blocked in rat hippocampal slices during critical developmental stages of spine formation at postnatal days (P) 6-P22 and compared to adults. CA1 pyramidal cells were labeled with DiI and maintained for 5 h in one of three conditions, control artificial cerebrospinal fluid (ACSF), block media containing synaptic transmission antagonists in ACSF, or block media containing synaptic transmission antagonists in a nominally calcium-free ACSF with high magnesium. Slices were fixed in mixed aldehydes, sectioned, and the lateral dendrites were imaged in stratum radiatum with confocal microscopy. Dendritic spine density was quantified per unit length of dendrite. At P6-7 there were only a few protrusions emerging from the dendrites, which were predominantly filopodia-like in appearance. At both P11-12 and P15-16 there was a mixture of dendritic spines and filopodia-like structures. By P20-22 dendritic spines predominated and spine density was about 82% of the adult level. Dendritic spine density increased during blocked synaptic transmission at P20-22 as in adults, but was unchanged during blockade at younger ages. When extracellular calcium was nominally zero, dendritic spine density further increased on P20-22 dendrites as in adults. In contrast, spine density decreased along P11-12 dendrites under the nominally zero calcium condition. Under control conditions, dendritic protrusions were longer at P6-7 than at all other ages, which did not differ from one another. When synaptic transmission was blocked, dendritic protrusions further elongated at P6-7 only. Under the nominally zero calcium condition with blocked synaptic transmission, dendritic protrusions shortened at P11-12 only. These findings reveal age-dependent changes in the manifestation of homeostatic control of dendritic spines that could be mediated by maturational changes in mechanisms regulating postsynaptic calcium.     

Inhibitory adenosine A1-receptors on rat locus coeruleus neurones

Summary Intracellular recordings were performed in ₁-pontine slice preparation of the rat brain containing the locus coeruleus (LC). Adenosine (100, 300 mol/l) and its structural analogues, namely (–)-N⁶-(R-phenyliso-propyl)-adenosine (R-PIA; 3 – 30 mol/l) and S-PIA (10, 30 mol/l), as well as 5-N-ethylcarboxamido-adenosine (NECA; 3–30 mol/l) inhibited the firing rate of spontaneous action potentials and produced hyperpolarization; their rank order of potency was RPIA - NECA > S-PIA > adenosine. When applied by superfusion, all agonists strongly desensitized the LC cells; the hyperpolarization never surmounted 6 mV. Upon pressure ejection of adenosine 10 mmol/l from ₁- micropipette positioned close to an LC neurone, the membrane potential was raised by 14 mV and the apparent input resistance decreased by 20%. When the membrane potential was hyperpolarized by current injection to ₁- similar extent as adenosine did, the fall in input resistance was only 7%. The adenosine uptake inhibitor S-(p-nitrobenzyl)-6-thioguanosine (NBTG) 30 mol/l decreased the frequency of action potentials alone; on simultaneous bath-application with adenosine 300 mol/l it potentiated the hyperpolarization caused by the purine derivative. 8-Cyclopentyl-1,3-dipropylxanthine (CPDPX) 0.1 mol/l had no effect on its own, but it antagonized both R-PIA 30 mol/l and NBTG 30 mol/l. A higher concentration of CPDPX (1 mol/l) facilitated the spontaneous firing. In conclusion, both exogenous and endogenous adenosine activates somatic and/or dendritic A₁-receptors of LC neurones leading to an enhancement of potassium conductance and thereby to ₁- decreased firing rate and ₁- hyperpolarization. Send offprint requests to P. Illes at the above address     

Modification of adenosine modulation of synaptic transmission in the hippocampus of aged rats

We compared the modulation of synaptic transmission by adenosine A(1) receptors in the hippocampus of aged (24 months) and young adult rats (6 weeks). The adenosine A(1) receptor agonist, N(6)-cyclopentyladenosine, was less potent (P:<0.05) to inhibit synaptic transmission in aged (EC(50)=53 nM) than young adult (EC(50)=14 nM) hippocampal slices, these effects being prevented by the A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). In contrast with the lower effect of the A(1) receptor agonist, it was observed that blockade of A(1) receptors with DPCPX (50 nM), or removal of endogenous extracellular adenosine with adenosine deaminase (2 u ml(-1)), caused a more pronounced disinhibition of synaptic transmission in aged rats. Also consistent with a more intense A(1) receptor-mediated inhibitory tonus by endogenous adenosine in aged rats was the finding that to fully prevent the depression of synaptic transmission induced by 3 min hypoxia, a higher concentration of DPCPX was required in slices from aged (100 nM) than from young (50 nM) rats. It is concluded that in hippocampal slices of aged rats the efficiency of A(1) receptors to modulate synaptic transmission is reduced, but this may be compensated by an enhanced inhibitory tonus by endogenous adenosine.     

Enhancement of adenosine A1 receptor functions by benzoylthiophenes in guinea pig tissues in vitro

Previous reports on a series of benzoylthiophenes, including PD 81,723 {2-amino-4,5-dimethyl-3-(3-trifluoromethyl-benzoyl)thiophene}, have shown specific enhancement of agonist binding at the adenosine A₁ receptor. We have studied the effects of two substituted benzoylthiophenes, PD 78,416 {thieno[2,3-c]pyridine-6(5H)-carboxylic acid, 2-amino-3-benzoyl-4,7-dihydro-ethyl ester} and RS-74513-000 {2-amino-4-ethyl-5-methyl-3-(3-trifluoro-methyl-benzoyl) thiophene} on response elicited by adenosine A₁ receptors in isolated guinea pig left atrium and ileum.In the electrically paced left atrium, PD 78,416 antagonized negative inotropic effect elicited by the agonist CPA {N⁶-cyclopentyladenosine} with a pK_B value of 6.2 ± 0.2 (n = 4) . At a low concentration which had no antagonistic effect (0.1 M), PD 78,416 enhanced the effect of CPA. The concentration-response curve to CPA was shifted leftward by 5.1 fold (95% confidence limits 2.4–11.2). In field stimulated isolated ileum, PD 78,416 (0.1, 0.3, 1 M) did not enhance or antagonize effects of CPA. At concentrations above 1 M, PD 78,416 decreased electrically induced contraction. This effect was not sensitive to adenosine deaminase and was not antagonized by the A₁ antagonist CPX {8-cyclopentyl-1,3-dipropyl-xanthine} (1 M).Unlike PD 78,416, RS-74513-000 (0.01, 0.1, 1, 3, 10 M) did not antagonize or enhance effects of CPA in the left atrium. However, effects of CPA in ileum were enhanced by RS-74513-000 (1 and 3 M). Maximum enhancement was observed at 3 M; the concentration-response curve to CPA was shifted leftward by 3.2 fold (95% confidence limits 2.4–4.2). Higher concentrations of RS-74513-000 (10 and 30 M) decreased electrically induced contraction, this effect was not reversed by CPX. These findings confirmed that functional effects of A₁ adenosine receptor may be enhanced by substituted benzoylthiophenes in vitro. The differential effect of PD 78,416 and RS-74513-000 on cardiac and ileal A₁ receptors suggests that it may be possible to design selective enhancers for cardiac and neural functions.     

Adenine nucleotides and synaptic transmission in the in vitro rat hippocampus

1 The effects of adenosine and various derivatives were examined in the in vitro hippocampal slice preparation from rat.

2 The amplitudes of extracellularly recorded field potentials from the CA1 region were depressed by adenosine, and this effect could be antagonized by methylxanthines. Because presynaptic field potentials were unaffected by adenosine, while the field e.p.s.p. was depressed, adenosine would appear to act at a synaptic site to depress transmission.

3 Adenosine deaminase, which breaks down adenosine to inosine, increased the amplitude of synaptic responses, while hexobendine, which blocks reuptake of adenosine, had a depressant effect. This strongly suggests that the endogenous release of adenosine from the hippocampal slice preparation is sufficient to exert a tonic inhibitory influence on the amplitude of synaptic responses.

4 Cyclic adenosine 3′,5′-monophosphate (cyclic AMP) and its dibutyryl derivative had depressant effects on the amplitude of field responses which were blocked by theophylline, suggesting that they are able to act at the extracellular adenosine receptor. (-)-Isoprenaline (which raises tissue cyclic AMP levels), and the 8-p-chlorophenylthio derivative of cyclic AMP both increased the amplitude of population spike responses, and these effects were not blocked by theophylline, suggesting that the physiological effects of adenosine are not mediated via a cyclic AMP-dependent mechanism.

5 Since adenosine is not the transmitter at this CA1 pyramidal cell synapse, but is apparently present in the extracellular compartment in sufficient concentrations to affect the synaptic physiology of this region, this provides strong evidence in favour of the concept of a neuromodulatory role for adenosine in the central nervous system.

     

Triethyltin exposure suppresses synaptic transmission in area CA1 of the rat hippocampal slice

To examine the effects of TET on the electrophysiology of area CA1 of hippocampus, hippocampal slices were obtained from adult hooded rats and were maintained in vitro using standard techniques. Stimulating and recording electrodes were placed in the Schaffer collaterals and CA1 pyramidal cell body layer, respectively. Following baseline measurements, slices were exposed to either 0, 1, 3, 6, or 10 μM TET in the incubating medium. Both pyramidal cell excitability and recurrent/feedforward inhibition were suppressed in a dose-dependent manner within 3 hr postexposure. The evoked population spike and population excitatory postsynaptic potential (EPSP) were suppressed significantly by 2 hr postexposure for 1 and 3 μM TET exposures, and by 45 min postexposure for 6 and 10 μM exposures. A similar dose-dependency was observed for the suppression of recurrent/feedforward inhibition in hippocampal CA1. A second procedure tested the specificity of TET effects to axonal conduction of Schaffer collaterals. Both the stimulating and recording electrode were placed in the Schaffer collaterals so that both the Schaffer collateral population fiber volley and the CA1 pyramidal cell population EPSP could be recorded. TET exposure suppressed pyramidal cell EPSPs without significantly affecting the amplitude of Schaffer collateral fiber volleys. The results support the view that acute TET exposure suppresses synaptic transmission in area CA1 of hippocampus.     

Adenosine analogs mediating depressant effects on synaptic transmission in rat hippocampus: Structure-activity relationships for the N6 subregion

Summary Previous studies have shown that analogs of adenosine with substituents upon the N⁶-nitrogen (e.g., N⁶-[1-phenyl-2(R)-propyl]adenosine; R-PIA) are often much more potent than the parent compound in activating adenosine receptors, particulary those of the A₁ subtype. The present investigation characterized the potencies of a number of N⁶-substituted adenosine analogs in depressing excitatory synaptic transmission in slices of rat hippocampus, an electrophysiological response mediated by receptors of the A₁ subtype. These potencies correlated well with previously reported affinities of these analogs for A₁ receptor sites in brain, but not with coronary vasodilation in the dog heart, an A₂ receptor mediated response. Analogs with alkyl or aryl substituents at the N⁶ position were generally more potent than adenosine, although analogs with a tertiary carbon attached directly to the N⁶-nitrogen were usually only weakly active. Although it has been suggested that there may be a subregion of the A₁ receptor with some specificity for aryl groups, these experiments did not suggest that this was the case. Analogs with chiral centers attached to the N⁶-nitrogen usually displayed stereoselectivity, with R-isomers more potent than the S-isomers. The mechanism underlying this selectivity appeared to be both a facilitating effect of alkyl substituents in the propyl C1 position of R-PIA, and a hindering effect of substituents in the position normally occupied by the hydrogen attached to propyl C2 of R-PIA. These results indicate that although there are some similarities in terms of requirements for activity at A₁ and A₂ receptors, differences between the N⁶ sub-regions of these receptors are sufficient to permit the development of selective analogs for these two receptor sites.     

Interaction of diazepam with synaptic transmission in the in vitro rat hippocampus

Summary Diazepam (5×10^–8–10^–6 M) was found to augment recurrent inhibition of pyramidal neuron firing in a dose-dependent manner in rat hippocampal slices. To determine possible loci of this effect, diazepam was locally administered by pressure ejection from a micropipette, while recording action potentials from single inhibitory (basket) interneurons. Diazepam induced reversible and reliable increases in interneuron firing in response to stimulation of Schaffer collateral and commissural afferents. Taken together with previous electrophysiological reports, these data suggest that benzodiazepines may augment central inhibition by increasing either the excitability of inhibitory interneurons, or by increasing the strength of excitatory afferents to these cells.