Cultured rat sensory neurones express functional tachykinin receptor subtypes 1, 2 and 3

The neuropeptide substance P (SP) is known to play a key role in peripheral nociceptive processes. We investigated the in vitro pharmacological characteristics of functional tachykinin receptors expressed in dorsal root ganglia (DRG) sensory neurones by analysing intracellular free calcium concentration changes induced after stimulation by SP or specific tachykinin agonists. We observed that about 37% of the tested neurones were responsive to either SP or an NK1-, NK2- or NK3-specific agonist. Tachykinin-responsive neurones had a small soma diameter (<20 microm) and were sensitive to capsaicin. These results suggest the presence of NK1, NK2 and NK3 receptors in noxious sensory neurones.

Electrophysiological evidence for multiple glycinergic inputs to neonatal rat sympathetic preganglionic neurons in vitro

The time pattern of glycinergic inhibitory postsynaptic currents (IPSCs) in sympathetic preganglionic neurons was studied in thin transverse spinal cord slices of neonatal (1-10 days postnatal) rats by means of the patchclamp technique. Three time patterns could be distinguished: (i) large events [mostly > 400 pA (30-36 degrees C)] occurring at regular intervals, (ii) small events occurring at irregular intervals, and (iii) small events occurring in transient (1.5-10 s), high-frequency (> 15 Hz) bursts of synaptic activity. The large regular events had uniform kinetics which was consistent with the idea of a proximal site of origin for all of these events. They were reversibly inhibited in amplitude and frequency by extracellular application of a high concentration of acetylcholine (200 microM) or the specific nicotinic acetylcholine receptor agonist dimethylphenylpiperazinium iodide (DMPP; 1 mM), but unaffected by glutamate (100 microM). IPSCs occurring in bursts had slower and less uniform kinetics, suggesting a more diverse site of origin. The frequency of events decreased during a burst. Similar bursts could be induced by extracellular application of glutamate receptor agonists. These results indicate that sympathetic pregnanglionic neurons in a thin, transverse spinal cord slice receive at least two different glycinergic inputs.

Tachykinin neurokinin-1 and neurokinin-3 receptor-mediated responses in guinea-pig substantia nigra: an in vitro electrophysiological study

The effects of tachykinin receptor agonists and antagonists were investigated using intra- and extracellular recordings on spontaneously firing nigral neurons in guinea-pig brain slices. In 70 of 76 electrophysiologically identified dopaminergic neurons, a concentration-dependent increase in firing rate was induced by the selective neurokinin-3 tachykinin agonist senktide and by the natural tachykinin agonists neurokinin B and substance P, with EC50 values of 14.7, 31.2 and 12200 nM respectively. These responses were inhibited in a concentration- and time-dependent manner by the selective non-peptide neurokinin-3 receptor antagonist SR 142801 (1-100 nM; n=23), but neither by its S-enantiomer SR 142806 (100 nM; n=4) nor by selective antagonists of neurokinin-1 (SR 140333) or neurokinin-2 (SR 48968) receptors (both at 100 nM; n=3). The selective neurokinin-1 agonist [Sar9,Met(O2)11]substance P (30-100 nM; n=23) and the selective neurokinin-2 agonist [Nle10]neurokinin A(4-10)(30-100 nM; n=13) were without any effect on dopaminergic cells. In 13 of 21 electrophysiologically identified, presumably GABAergic neurons located in the pars compacta of the substantia nigra, excitatory responses were evoked concentration dependently by substance P and [Sar9,Met(O2)11]substance P, with EC50 values of 18.6 and 41.9 nM respectively. These responses were inhibited by SR 140333 (100 nM; n=3), but neither by its R-enantiomer SR 140603 nor by SR 142801 (both at 100 nM; n=3). Senktide and [Nle10]neurokinin A(4-10) (both at 30-100 nM; n=10) were without effect on these presumed GABAergic neurons. A small population (12%) of pars compacta neurons was insensitive to any of the three selective tachykinin agonists. In the nigral pars reticulata, 12 neurons were recorded which had an electrophysiological profile similar to that of presumed GABAergic neurons in the pars compacta. Of these 12 cells, seven did not respond to any of the selective tachykinin agonists tested, while five were excited by senktide in a concentration-dependent manner (EC50=98.5 nM). Although this value was significantly higher than that found for dopaminergic neurons in the pars compacta, senktide-evoked responses were inhibited by SR 142801 (100 nM; n=3). We conclude that, in the guinea-pig substantia nigra, tachykinins evoke excitatory responses in both dopaminergic and non-dopaminergic neurons; however, the sensitivity to tachykinin agonists (neurokinin-1 versus neurokinin-3) depends on both neuronal type and localization.

Modulation of GABAA receptor-mediated IPSCs by neuroactive steroids in a rat hypothalamo-hypophyseal coculture model

1. We have used the whole-cell configuration of the patch-clamp technique to investigate the effects of neuroactive steroids on GABAA receptor-mediated synaptic transmission between rat hypothalamic neurones and pituitary intermediate lobe (IL) cells grown in coculture. In order to discriminate between possible pre- and postsynaptic sites of action, the effects of neurosteroids on GABAA receptor-mediated synaptic currents (IPSCs) were compared with those of GABAA currents (IGABA) triggered by local application of 50 or 500 microM GABA, which yielded approximately half-maximal and maximal responses, respectively. 2. In primary cultures of rat pituitary IL cells, allopregnanolone (5 alpha-pregnan-3 alpha-ol-20-one) reversibly potentiated IGABA in a dose-dependent manner with a threshold between 0.1 and 1 nM. At a concentration of 10 nM, allopregnanolone increased the response evoked by 50 microM GABA by +21.4 +/- 5.1% (n = 8), but had no effect on IGABA induced by 500 microM GABA. The beta-isomer of allopregnanolone, epipregnanolone (5 beta-pregnan-3 beta-ol-20-one, 10 nM), had no effect on IGABA at any concentration of GABA tested. 3. At concentrations lower than 10 microM, pregnenolone sulphate (5-pregnen-3 alpha-ol-20-one sulphate) did not significantly inhibit IGABA. However, at 10 microM, a systematic reduction of IGABA evoked by 50 and 500 microM GABA was observed, with mean values of -80 and -60%, respectively. This blocking effect was reversible and accompanied by a marked acceleration of decay of GABAA currents during the application of GABA. 4. In isolated pairs of synaptically connected hypothalamic neurones and IL cells, allopregnanolone (10 nM) augmented the mean amplitude of spontaneous IPSCs (sIPSCs) and electrically evoked IPSCs (eeIPSCs) by about 40% and increased the mean frequency of sIPSCs. Allopregnanolone (10 nM) also markedly increased the frequency of miniature IPSCs (mIPSCs) recorded in the presence of TTX (0.5 microM), but without modifying their mean amplitude. Epipregnanolone had no effect on the amplitude or frequency of sIPSCs. Neither epipregnanolone nor allopregnanolone modified the time to peak and decay time constants of GABAergic IPSCs. 5. Pentobarbitone (50 microM), a positive allosteric modulator of GABAA receptors, did not affect the amplitude of sIPSCs or eeIPSCs, but significantly increased the decay time constants of both types of IPSCs. Pentobarbitone had no effect on the frequency of sIPSCs. 6. Pregnenolone sulphate (10 microM) completely and reversibly blocked sIPSCs and eeIPSCs. Progressive block of IPSCs was correlated with a gradual decrease of the mean decay time constant. 7. Our results suggest that, under physiological conditions, allopregnanolone might be a potent modulator of GABAergic synaptic transmission, acting at both pre- and postsynaptic sites. The involvement of pregnenolone sulphate as a modulator of GABAergic IPSCs under physiological conditions is, however, more questionable. The mechanisms of action of both types of neurosteroids are discussed.

Mesencephalic trigeminal neuron responses to gamma-aminobutyric acid

Mesencephalic trigeminal neurons are primary sensory neurons which have cell somata located within the brain stem. In spite of the presence of synaptic terminals on and around the cell somata, applications of a variety of neurotransmitter substances in earlier studies have failed to demonstrate responses. Using intracellular recording in a brain slice preparation, we have observed prominent depolarizations and decreases in input resistance in response to applications of gamma-aminobutyric acid (GABA) in most recorded mesencephalic trigeminal neurons. Those cells failing to respond were located deeply within the slice, and the low responsiveness was shown to be related to uptake of GABA in the slice. The responses were direct, since they remained during perfusion with a low calcium, high magnesium solution that blocks synaptic transmission. The responses were mimicked by the GABA(A) receptor agonist isoguvacine, and blocked by GABA(A) receptor antagonists. The GABA(B) receptor agonist baclofen evoked no changes in membrane potential or input resistance in neurons exhibiting depolarizations with GABA application. Tests of neuronal excitability during GABA applications indicated that the excitatory effects of the depolarization prevail over the depressant effects of the increase in membrane conductance. In situ hybridization histochemistry indicated that the GABA(A) receptors in Me5 cells are comprised of alpha2, beta2 and gamma2 subunits.

Adrenergic responses in silent and putative inhibitory pacemaker-like neurons of the rat rostral ventrolateral medulla in vitro

Noradrenaline and adrenergic agonists were tested on pacemaker-like and silent neurons of the rat rostral ventrolateral medulla using intracellular recording in coronal brainstem slices as well as in punches containing only the rostral ventrolateral medullary region. Noradrenaline (1-100 microM) depolarized or increased the frequency of discharge of all cells tested in a dose-dependent manner. The noradrenaline-induced depolarization was associated with an apparent increase in cell input resistance at low concentrations and a decrease or no significant change at higher concentrations. Moreover, it was voltage dependent and its amplitude decreased with membrane potential hyperpolarization. Noradrenaline caused a dose-related increase in the frequency and amplitude of spontaneous inhibitory postsynaptic potentials. The alpha 1-adrenoceptor antagonist prazosin (0.5 microM) abolished the noradrenaline depolarizing response as well as-the noradrenaline-evoked increase in synaptic activity and unmasked an underlying noradrenaline dose-dependent hyperpolarizing response associated with a decrease in cell input resistance and sensitive to the alpha 2-adrenoceptor/antagonist yohimbine (0.5 microM). The alpha 1-adrenoceptor agonist phenylephrine (10 microM) mimicked the noradrenaline depolarizing response associated with an increase in membrane resistance as well as the noradrenaline-induced increase in synaptic activity. The alpha 2-adrenoceptor agonists UK-14,304 (1-3 microM) and clonidine (10-30 microM) produced only a small hyperpolarizing response, whereas the beta-adrenoceptor agonist isoproterenol (10-30 microM) had no effect. Baseline spontaneous postsynaptic potentials were abolished by strychnine (1 microM), bicuculline (30 microM) or both. However, only the strychnine-sensitive postsynaptic potentials had their frequency increased by noradrenaline or phenylephrine and they usually occurred with a regular pattern. Tetrodotoxin (1 microM) eliminated 80-95% of baseline spontaneous postsynaptic potentials and prevented the increase in synaptic activity evoked by noradrenaline and phenylephrine. Similar results were obtained in rostral ventrolateral medulla neurons impaled in both coronal slices and punches of the rostral ventrolateral medulla. It is concluded that noradrenaline could play an important inhibitory role in the rostral ventrolateral medulla via at least two mechanisms: an alpha 2-adrenoceptor-mediated hyperpolarization and an enhancement of inhibitory synaptic transmission through activation of alpha 1-adrenoceptors located on the somatic membrane of glycinergic interneurons. Some of these interneurons exhibit a regular discharge similar to the pacemaker-like neurons and might, at least in part, constitute a central inhibitory link in the baroreceptor-vasomotor reflex pathway.

Evidence that GABAA receptor subunit mRNA expression during development is regulated by GABAA receptor stimulation

The expression of six mRNA species (alpha 2, alpha 3, alpha 5, beta 2, beta 3, and gamma 2) encoding for GABAA receptor subunits was followed in cultured early postnatal cortical neurons by in situ hybridization histochemistry. In untreated control cultures it was found that these subunit mRNA expression profiles closely follow those seen during development in vivo. alpha 3, alpha 5, and beta 3 subunit expression declined, alpha 2 expression increased, whereas beta 2 and gamma 2 subunit mRNA expression remained relatively constant. To test the hypothesis that GABAA receptor stimulation regulates these expression profiles, we tested the effect of a GABAA receptor positive modulator, allopregnanolone, and a GABAA receptor noncompetitive antagonist, tert-butylbicyclophosphorothionate (TBPS). It was found that allopregnanolone augmented the rate at which the alpha 3, alpha 5, or beta 3 subunit mRNA expression declined and prevented the increase in alpha 2 subunit mRNA expression. As well, allopregnanolone down-regulated beta 2 subunit mRNA expression. TBPS, on the other hand, up-regulated alpha 3, alpha 5, beta 2, and beta 3 subunit mRNA expression. It also down-regulated the expression of alpha 2 subunit mRNA. Both allopregnanolone and TBPS had no effect on gamma 2 subunit mRNA expression. These results imply that the developmental switchover of GABA receptor subunit mRNA expression is regulated by GABAA receptor activity.

Nicotinic actions on neurones of the central autonomic area in rat spinal cord slices

1. Nicotinic responses and actions on excitatory synaptic activity were studied in eighty-four neurones in the region dorsal to the central canal (lamina X) in transverse thoracolumbar spinal cord slices of neonate (P2-P10) rats by using the whole-cell patch-clamp technique. 2. Neurones (n = 15) labelled with Lucifer Yellow, showed the typical morphology of sympathetic preganglionic neurones (SPNs) in the central autonomic area (CA). Unlabelled neurones of comparable morphology were visually identified and recorded. 3. All neurones recorded responded to the nicotinic acetylcholine receptor (nAChR) agonist, DMPP. Under current-clamp conditions, pressure ejections of DMPP depolarized cells and induced the discharge of action potentials. Tetrodotoxin suppressed action potentials but not DMPP-induced depolarization. 4. Under voltage-clamp conditions at a holding potential (Vh) of -50 mV, DMPP induced a transient inward current (which reversed around 0 mV) and an increase in membrane current noise in 50% of the recorded neurones. In the others, DMPP increased membrane current noise without measurable inward current. The current-voltage relationship showed strong inward rectification at holding potentials more positive than 0 mV. 5. In neurones displaying a detectable current response to DMPP, the following agonist rank order potency could be established: DMPP = nicotine > cytisine > ACh. The DMPP response could be blocked by mecamylamine but was insensitive to methyllycaconite. 6. Pressure application of glutamate induced inward currents in all cells tested at a Vh of -50 mV. This response reversed at 10 mV, displayed a region of negative slope conductance at Vh more negative than -30 mV and was partially blocked by CNQX. Pressure application of DMPP transiently increased the amplitude of the glutamate-induced current in six out of nine cells tested. This potentiation persisted in the presence of tetrodotoxin. 7. Forty per cent of the recorded neurones displayed spontaneous excitatory postsynaptic currents (sEPSCs). At a Vh of -50 mV the sEPSCs had a mean amplitude of -19.3 pA and occurred at a frequency below 0.5 Hz. sEPSCs were blocked by CNQX and inverted around 0 mV. Brief application of DMPP increased the discharge frequency of sEPSCs without affecting their kinetics. Additionally, in some cells DMPP increased mean sEPSC amplitude. 8. Focal electrically evoked EPSCs reversed close to 10 mV and were sensitive to CNQX. They occurred with a constant latency, rise time and a mono-exponential decay time. Application of DMPP decreased the percentage of stimulation failures and increased the amplitude of evoked EPSCs, in all cells tested. 9. It is concluded that neurones in the CA, presumed to be SPNs, have functional nAChRs with activation having two distinct effects: firstly, a direct depolarization of the postsynaptic membrane; and secondly, a facilitation of the excitatory transmission onto these cells. This second effect is achieved by an increase of the size of the glutamate-induced current at the postsynaptic level as well as by an enhancement of the presynaptic release of glutamate.

Recombinant GABAA receptor desensitization: the role of the gamma 2 subunit and its physiological significance

1. The purpose of these investigations was to examine the role that the gamma 2 subunit plays in human GABAA receptor desensitization. Two different recombinant GABAA receptors (alpha 1 beta 3 and alpha 1 beta 3 gamma 2) were compared by measuring the relaxation of whole-cell currents during the application of GABA, isoguvacine or taurine. 2. At concentrations which trigger a maximum response (100-500 microM GABA) the current relaxation usually fitted the sum of two exponentials. For alpha 1 beta 3 subunit receptors these values were tau 1 = 145 +/- 12 ms and tau 2 = 6.3 +/- 2.1 s (means +/- S.E.M.). Receptors consisting of alpha 1 beta 3 gamma 2 subunits desensitized faster: tau 1 = 41.6 +/- 8.3 ms and tau 2 = 2.4 +/- 0.6 s. 3. The Hill slope, determined for each receptor subunit combination, was the same and greater than 1.0, implying two binding steps in the activation of both receptor subunit combinations. 4. For alpha 1 beta 3 subunit receptors the fast desensitization rates were unaltered by reducing the GABA concentration from the EC100 (100 microM) to the approximate EC50 values (10-20 microM), whereas for alpha 1 beta 3 gamma 2 subunit receptors a significant slowing was observed. The fast desensitization disappeared at agonist concentrations below the EC50 for both subunit combinations. In contrast, the slow desensitization appeared at agonist concentrations near the EC20. This rate was dependent on agonist concentration reaching a maximum near the EC60 value of GABA. 5. The fast desensitization rates were unaltered by changing the holding potential of the cell during agonist application. However, for alpha 1 beta 3 gamma 2 subunit receptors the slow desensitization rate increased by approximately 15- to 20-fold over the range of voltages of -60 to +40 mV. This indicates that the gamma 2 subunit makes GABAA receptor desensitization voltage dependent. 6. Recovery from desensitization was also biphasic. The first recovery phase was faster for alpha 1 beta 3 gamma 2 than for alpha 1 beta 3 subunit receptors (0.13 vs. 0.03 s-1, respectively). The second phase of recovery for the two receptors were the same (approximately 0.003 s-1). 7. There was only a poor correlation between agonist potency and the degree or time course of desensitization. Isoguvacine (EC50 approximately to 10 microM) induced biphasic relaxation for both alpha 1 beta 3 and alpha 1 beta 3 gamma 2 subunit receptors (tau 1 = 288.6 +/- 43.3 and 167 +/- 15 ms, and tau 2 = 8.0 +/- 1.9 and 4.4 +/- 0.4 S, respectively, for each subunit combination). Taurine (EC50 approximately 7 mM) usually induced monophasic relaxation for both subunit combinations (tau 2 = 7.1 +/- 1.6 and 23.0 +/- 6.6 s, respectively). 8. A computer model was developed to examine the effect of the gamma 2 subunit on the time course of a synaptic potential. It was found that the gamma 2 subunit theoretically prolongs the time course of a synaptic potential by inducing desensitization more rapidly. The subsequent relaxation of the desensitized receptors through the open state increases Popen (the probability that the GABAA receptor is in an open conducting state) altering the time course of the modelled potential. alpha 1 beta 3 subunit receptors do not desensitize sufficiently rapidly to induce this desensitized state and, therefore, are shorter in time course. These data imply that the physiological role of the gamma 2 subunit is to increase synaptic efficacy by prolonging Popen.

Characterization of functional GABAergic synapses formed between rat hypothalamic neurons and pituitary intermediate lobe cells in coculture: Ca2+ dependence of spontaneous IPSCs

Rat hypothalamic neurons and endocrine cells from the intermediate lobe of the pituitary were grown in dissociated coculture. Neurons positively stained with an antibody against glutamate decarboxylase established apparent contacts with the alpha-melanocyte-stimulating hormone-positive endocrine cells. These sites of contact were intensely labeled with an antibody against the synaptic protein synapsin I and displayed ultrastructural features characteristic of synapses. Using patch-clamp recordings, we have demonstrated that these contacts correspond to functional GABAergic synapses. The synaptic currents were blocked reversibly by bicuculline (5 microM) and SR95531 (5 microM), two competitive antagonists of the GABAA receptor. At a holding potential of -60 mV, spontaneously occurring IPSCs (s-IPSCs) had small amplitudes (10-100 pA), whereas electrically evoked IPSCs (ee-IPSCs) had amplitudes up to 1 nA. The rise times of both types of IPSCs were fast ( < or = 1 msec), and their decaying phases were fitted in most cases with a single exponential function (time constant 50 msec). The amplitude distribution of s-IPSCs did not reveal clear, equally spaced peaks and was little affected by tetrodotoxin, suggesting that most s-IPSCs were miniature IPSCs. Reduction of extracellular calcium concentration to 0.3 mM induced a marked decrease in s-IPSC frequency and revealed a single amplitude peak at 10 pA, suggesting that a single quantum of GABA activates 8-10 GABAA channels. Thus, our preparation might be an interesting model to study different aspects of synapse formation between a central neuron and its target as well as the fundamental mechanisms of synaptic transmission at central synapses.

Sumatriptan inhibits the release of CGRP and substance P from the rat spinal cord

The possible presynaptic action of the anti-migraine drug sumatriptan on primary afferent fibres containing substance P and/or calcitonin gene-related peptide was investigated on superfused rat horizontal spinal cord slices with attached dorsal roots. Electrical stimulation of dorsal roots triggered a significant overflow of both peptides; this could be reduced by sumatriptan in a concentration-dependent manner. As expected from the involvement of 5-HT1B/1.D beta receptors, methiothepin, (-)tertatolol and GR 127,935, but not WAY 100,635, prevented the inhibitors effect of sumatriptan. These data support the idea that the anti-migraine action of sumatriptan may involve, at least in part, a presynaptic inhibitory control of nociceptive (trigeminovascular) substance P- and/or calcitonin gene-related peptide-containing sensory fibres.

Electrophysiological characterization of non-NMDA glutamate receptors on cultured intermediate lobe cells of the rat pituitary

Glutamate is the major excitatory neurotransmitter in the central nervous system, yet little is known about its actions on endocrine cells. We have investigated the membrane effects of glutamate in cultured neonatal rat pituitary intermediate lobe (IL) cells using the whole-cell configuration of the patch-clamp technique. In a standard Na(+)-based extracellular solution, glutamate failed to induce a detectable membrane current at a holding potential (HP) of -60 mV (n = 40). However, when cyclothiazide (50 microM), a benzothiazide that blocks desensitization of alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazole-propanoic acid (AMPA)-type receptors, was added to the extracellular solution, glutamate (0.5-1 mM) induced an inward current at a HP of -60 mV in 65% of the cells tested (n = 72). This response was usually small in amplitude (mean amplitude: 28.6 +/- 37.5 pA, n = 47). The glutamate-induced current reversed polarity close to 0 mV and was reversibly blocked when extracellular Na+ was replaced by the impermeant cation N-methyl-D-glucamine, suggesting that this current was a nonselective cation current. The response to glutamate (1 mM) was reproduced by AMPA (50 microM), kainate (200 microM), and quisqualate (200 microM). N-Methyl-D-aspartate (NMDA, 100 microM) in the presence of 10 microM glycine did not induce any membrane current in cells responding to glutamate (n = 8). The non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM) reversibly inhibited the response to glutamate (0.5 mM) by 85 +/- 14% (n = 7), whereas D(-)-2-amino-5-phosphonopentanoic acid (20 microM), an antagonist of NMDA receptors, had no effect on the glutamate-induced current (n = 3). Moreover, we show that although the amplitude of the glutamate currents was small, the latter induced large (30-mV) membrane depolarizations and triggered the firing of action potentials. Taken together, our results indicate that neonatal rat IL cells possess AMPA-type glutamate receptors that could possibly underlie a fast excitatory glutamatergic synaptic input to these cells.

GABA-induced responses in electrophysiologically characterized neurons within the rat rostro-ventrolateral medulla in vitro

Rostro-ventrolateral medulla (RVL) neurons were recorded using conventional intracellular recording techniques in brain slices maintained in vitro at 32 degrees C and classified into 3 major groups. The first group included neurons having endogenous pacemaker-like (PL) activity with regular firing frequency (mean 8 Hz) and a linear current-voltage relationship (I-V). The second group of neurons were slowly and irregularly firing (IF) or quiescent, presenting membrane potential oscillations and their I-V usually displayed an inward rectification. These neurons had a relatively longer action potential duration. The third group included silent neurons (S) with no apparent membrane oscillations and they differed from the first two groups by having relatively shorter action potential duration and amplitude and lower cell input resistance. When recorded with KCl-filled electrodes, the majority of silent neurons displayed a time-dependent inward rectification. With KAc-filled electrodes, irregular slow hyperpolarizing and depolarizing spontaneous potentials could be recorded primarily on PL and IF neurons, respectively. Moreover, fast spontaneous inhibitory postsynaptic potentials (PSPs) were detected in about 15% of PL and S neurons. They generally exhibited a regular pattern and were depolarizing when KCl-filled electrodes were used for recording. The amplitude of these inhibitory PSPS was reversibly reduced by the GABA A antagonists bicuculline, SR 95531 and picrotoxin. With KAc-filled electrodes, pressure-applied GABA (20 mM) evoked complex responses. In PL neurons, it consisted of a fast hyperpolarization followed by a slower depolarization that were both sensitive to SR 95531 and picrotoxin. The response was terminated by a long-lasting hyperpolarization that was reduced, but not abolished, by the GABA B antagonist CGP 35348. In IF and S neurons, GABA application usually produced a fast followed by a slow monophasic hyperpolarization and depolarization, respectively. The fast component of these responses was sensitive to the GABA A antagonists. Pressure application of isoguvacine (10 mM) always induced monophasic responses in all types of neurons recorded. Baclofen (1-30 mu M) reduced the firing frequency and hyperpolarized PL and IF neurons, an effect that was antagonized by CGP 35348 (50-100 mu M); however, it had little effect on silent neurons. It is concluded that RVL neurons have heterogeneous electrophysiological characteristics. Their predominant synaptic input and GABA responsiveness might be additional criteria to identify the excitatory and inhibitory elements in the RVL circuitry. All neuronal types seem to have functional GABA A and GABA B receptors; however, only a subpopulation is under tonic inhibitory control in vitro, probably from local GABAergic pacemaker interneurons. Our results further emphasize the role of GABA as an important neurotransmitter in the RVL network.

Selective inhibition of high voltage-activated L-type and Q-type Ca2+ currents by serotonin in rat melanotrophs

1. Whole-cell Ca2+ currents (ICa) from cultured rat melanotrophs were identified by their sensitivity to Ca2+ channel blockers, and their modulation by serotonin (5-HT) was studied. All cells displayed high voltage-activated (HVA; > -30 mV) Ca2+ currents. A low voltage-activated (LVA; > -60 mV) Ca2+ current was detected in 92% of the cells. 2. The whole-cell ICa was insensitive to omega-conotoxin GVIA (0.5-1 microM) indicating the absence of N-type Ca2+ channels. 3. At a holding potential (Vh) of -70 mV, the L-type channel blocker nifedipine reduced ICa in a dose-dependent manner with a half-maximal effective concentration (IC50) of 28 nM. The L-type current represented 39% of the total ICa. 4. omega-Agatoxin IVA (omega-Aga IVA) produced a biphasic dose-dependent inhibition of ICa, with IC50 values of 0.4 and 91 nM, indicating the presence of P-type and Q-type Ca2+ channels, which accounted respectively for 16 and 45% of the total ICa. The P-type current was also blocked by synthetic funnel-web spider toxin (sFTX 3.3; 1-10 microM) and was present only in a subpopulation (60-70%) of cells. 5. All cells possessed a Ca2+ current which was resistant to nifedipine (10 microM) and omega-Aga IVA (50 nM). This current was not affected by Ni2+ (40 microM) but was abolished by a low concentration of Cd2+ (10 microM) and by omega-conotoxin MVIIC (1 microM) indicating that it was a Q-type Ca2+ current. 6. 5-HT (10 microM) inhibited the whole-cell ICa in 70% of the cells tested (n = 120) by activating 5-HT1A and 5-HT2C receptors. 5-HT produced either a kinetic slowing of the activation phase (37% of the cells) or a scaling down (14% of the cells) of ICa. In the majority of cells (49%) both types of inhibition were found to coexist. 7. The effects of 5-HT were voltage dependent, rendered irreversible when GTP-gamma-S (30 microM) was present in the pipette solution and abolished by pretreatment of the cells with pertussis toxin (PTX; 150 ng ml-1, 18 h). 8. Low concentrations of omega-Aga IVA (20 nM), which blocked mainly P-type channels, did not reduce the effect of 5-HT on ICa. The scaling down effect of 5-HT on ICa was eliminated in the presence of nifedipine (10 microM) and the kinetic slowing effect of 5-HT persisted after blockade of L- and P-type channels but was abolished by omega-conotoxin MVIIC (1 microM). 9. We conclude that rat melanotrophs possess functional L-, P- and Q-type Ca2+ channels and that 5-HT inhibits selectively L-type and Q-type Ca2+ currents with different modalities. These effects are voltage dependent and mediated by a PTX-sensitive G-protein.

Patch-clamp characterization of nicotinic receptors in a subpopulation of lamina X neurones in rat spinal cord slices

1. Nicotinic acetylcholine receptors (nAChRs) on lamina X neurones in neonate (P1-P12) rat transverse thoracolumbar spinal cord slices were studied using the whole-cell patch-clamp technique. These visually selected neurones are located dorsal to the central canal, mainly in the ventral half of the dorsal commissure. 2. Pressure application of the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP) (1 mM) induced a rapid depolarization on which action potentials are superimposed. 3. At -50 mV, DMPP (1 mM), pressure ejected for 100 ms, induced a fast inward current with a mean amplitude of -280 pA (n = 28) in 90% of the neurones recorded. Superfusion of tetrodotoxin (TTX), a solution containing 0 Ca(2+)-high Mg2+, CdCl2 or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) did not abolish the DMPP-induced current, which confirmed a direct postsynaptic effect of DMPP on recorded neurones. 3. The current-voltage (I-V) relationship for DMPP-induced current exhibited a reversal potential of 0 mV (NaCl outside, potassium gluconate inside) and a strong inward rectification. 4. The DMPP-induced responses were blocked by mecamylamine, hexamethonium and d-tubocurarine (dTC) but were insensitive to alpha-bungarotoxin and methyllycaconitine (MLA). 5. We conclude that lamina X neurones located dorsally to the central canal possess nicotinic acetylcholine receptors. Activation of these nicotinic receptors results in depolarization and generation of action potentials. These receptors may be involved in the modulation of the somato- and viscerosensory transmission.

GABAB receptors negatively regulate transcription in cerebellar granular neurons through cyclic AMP responsive element binding protein-dependent mechanisms

GABAB receptors affect short-term signalling in various cell types. However, nothing is known about possible long-term effects on transcription. To analyse such effects in the CNS, we studied GABAB receptor-mediated gene regulation in primary cultures of cerebellar granule neurons. Transcription was followed using a chloramphenicol acetyl transferase reporter gene driven by the minimal cyclic AMP-responsive element (TGACGTCA). Transcription was stimulated by activation of both the cyclic AMP (forskolin: 5 x 10(-6) M) and the Ca2+ dependent (KCl: 30 mM) pathways (-)-Baclofen (10(-6) M to 10(-4) M), a specific GABAB receptor agonist, reduced by 50-70% the transcriptional stimulation evoked by both forskolin and KCl, whereas isoguvacine, a GABAA receptor agonist, was without effect. Moreover, the GABAB antagonist CGP 35348 abrogated the inhibitory effects of both GABA and baclofen, indicating that GABAB receptors were specifically implicated in this response. Measurements of cyclic AMP levels suggested that (-) baclofen inhibits forskolin-initiated transcription by reducing cyclic AMP production. Direct transcriptional activation, via the cyclic AMP pathway, by overexpression of the catalytic subunit of the cyclic AMP-dependent protein kinase, was not significantly altered by (-) baclofen. This indicates again that (-) baclofen-dependent inhibitory mechanisms operate upstream of cyclic AMP-dependent protein kinase at the level of second messenger formation. Further, we used a yeast transcriptional activator GAL4-cyclic AMP-responsive element binding protein to analyse whether GABAB receptor-mediated inhibition of cyclic AMP-responsive element transcription implicated the transacting factor cyclic AMP-responsive element binding protein. We show that the negative effects of (-) baclofen implicate this transcription factor and this holds good for both the forskolin and KCl-stimulated pathways. The results indicate that GABAB receptors negatively regulate cyclic AMP-responsive element binding protein-mediated transcription in the CNS.

Kinetics of A-currents in sympathetic preganglionic neurones and glial cells

Transient outward currents (A-currents; IA) were recorded in sympathetic preganglionic neurones (SPNs) and glial cells of the intermediolateral cell column (IML) by whole-cell recordings in rat spinal cord slices. In both cell types IA activated at around -45 mV and the time-course of decay was monoexponential, but faster in glial cells than in neurones. In both cases decay time constants displayed the same increase with depolarization above -30 mV. In neurones, the activation curve was shifted to more negative value along the voltage axis and was steeper than the activation curve for glial cells whereas inactivation curves were similar. Recovery from inactivation followed a double and monoexponential decay in neurones and glial cells, respectively.

Electrophysiological evidence for oxytocin receptors on sympathetic preganglionic neurones--an in vitro study on the neonatal rat

The action of oxytocin (0.01-1 microM) on sympathetic preganglionic neurones was studied by intracellular recording in slices of neonatal rat thoracic spinal cord. In 85% of the cells superfusion induced a slow tetrodotoxin-insensitive depolarization accompanied by the appearance or increase in frequency of repetitive discharges. Oxytocin also caused some cells to switch from silent neurones to spontaneously active ones. These effects were reversibly blocked by a specific oxytocin antagonist.

Excitatory postsynaptic currents and glutamate receptors in neonatal rat sympathetic preganglionic neurons in vitro

1. We obtained whole cell patch-clamp recordings from visually identified sympathetic preganglionic neurons (SPNs) in thin (200-300 microns) transverse spinal cord slices of neonatal rats (1-14 days postnatal). Exogenous application of glutamate (100 microM), N-methyl-D-aspartate (NMDA; 100 microM), kainate (100 microM), quisqualate (1 microM), and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 50 microM) induced inward currents at a holding potential of -30 mV. 2. Excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation either in the dorsal horn or the lateral funiculus. They reversed at 1.2 +/- 4.6 (SD) mV and could in most cases (49 of 51) be separated into two components. 3. In the presence of DL-2-amino-5-phosphonovalerate (10-40 microM) the current-voltage (I-V) relationship of the remaining EPSC was linear. When stimulated in the lateral funiculus, its rise time (10-90%) and the time constant of the monoexponential decay were 1.6 +/- 1.0 and 5.5 +/- 2.7 ms, respectively. By contrast, when stimulated in the dorsal horn, this component had a rise time (10-90%) of 3.0 +/- 0.8 ms and a decay time constant of 13.7 +/- 7.6 ms. 4. We studied the NMDA receptor-mediated component of the EPSCs after superfusion of 6-cyano-7-nitroquinoxaline-2,3-dione (5 microM). The I-V relationship of this component had a region of negative slope conductance between -30 and -80 mV, which was abolished in Mg(2+)-free saline. The rise time (10-90%) ranged from 3.3 to 9.5 ms and the decay was biexponential. Both decay time constants increased with depolarization. Mg(2+)-free saline reduced this voltage sensitivity. 5. At a membrane potential of -80 mV and in 1 mM extracellular Mg2+, the NMDA receptor-mediated component represented 74.8 +/- 11.2% of the total charge carried by the EPSCs evoked by stimulation in the dorsal horn. In contrast, when stimulated from the lateral funiculus, 28.9 +/- 18.9% of the total charge carried during the EPSC was mediated by the NMDA receptor-mediated component. The contribution of the NMDA receptor-mediated component increased in both cases with depolarization. In addition, in 2 of 18 SPNs the EPSC evoked in the dorsal horn was exclusively carried by NMDA receptors. 6. We conclude that L-glutamate or a related substance mediates the fast excitatory input onto SPNs. Viscerosomatic and supraspinal inputs form synapses with different topographical locations on the SPN.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal change of glycinergic IPSC decay in sympathetic preganglionic neurons

The characteristics of glycinergic inhibitory postsynaptic currents (IPSCs) in sympathetic preganglionic neurons (SPNs) of neonatal rats were studied by whole-cell recordings in transverse spinal cord slices. In relation to postnatal age, the decay time constants of these currents decreased without a comparable effect on their rise time. This may result from alpha-subunit switching of the glycine receptor and/or increased glycine uptake during this period of postnatal life. The kinetics of glycinergic IPSCs were also temperature- and voltage-dependent. Whereas, compared with room temperature, rise and decay of the events were faster at more physiological temperature, only the decay increased upon depolarization. Visual identification of SPNs was confirmed by intracellular staining and comparison with retrogradely labeled SPNs.

Calcium influx through neuronal-type nicotinic acetylcholine receptors present on the neuroendocrine cells of the porcine pars intermedia

The properties of neuronal-type nicotinic acetylcholine receptors (nAChRs) present on the neuroendocrine cells of the porcine pars intermedia of the pituitary were studied in intact single cell using measurements of the free intracellular Ca2+ concentration ([Ca]i) with the calcium-sensitive dye fura 2. Local application of an extracellular solution containing 50 mM K+ or of the selective nAChR agonist, 1,1-dimethyl-4-phenylpiperazinium (DMPP) depolarised the cells and induced an elevation in [Ca]i. The effect of DMPP on [Ca]i was dose dependent (EC50 = 6 microM), reversibly blocked by d-tubocurarine and strictly dependent on the concentration of extracellular Ca2+. The calcium channel blocker Cd2+ (100 microM) reversibly blocked 80% of the response induced by 50 mM K+, whereas it reduced the DMPP response by only 50%. In the absence of extracellular Na+, DMPP no longer depolarised the cells but still increased [Ca]i. The rise in [Ca]i under these conditions represented 41% of the control response, i.e. in the presence of external Na+. Thus activation of nAChRs induced an elevation in [Ca]i which was in part independent of cell depolarisation. This was confirmed by recording simultaneously, under whole-cell voltage-clamp, a rise in [Ca]i associated with the inward nicotinic current. During prolonged application of the agonist (50 s), the amplitude of the nicotinic current decayed rapidly to a very low plateau level reflecting nAChR desensitisation. However, photometric experiments performed on intact non-dialysed cells revealed the presence of a slowly decaying phase in [Ca]i throughout the application of DMPP. This suggests the persistence of a substantial Ca2+ influx during prolonged exposure to the agonist. Taken together, our results show that stimulation of nAChRs induces an influx of Ca2+ which elevates [Ca]i. This phenomenon is due to activation of voltage-dependent Ca2+ channels and to Ca2+ entry through the nAChR.

Insulin-like growth factor-I stimulates c-fos and c-jun transcription in PC12 cells

We analyzed the effects of insulin-like growth factor-I (IGF-I), a polypeptide growth factor which exerts mitogenic effects via specific membrane receptors. The control of IGF-I on c-fos and c-jun transcription was studied in PC12 cells. Gel mobility shift assays with a labeled AP1 consensus binding sequence (TRE: TGACTCA) showed an increase in specific binding upon trIGF-treatment. Gene transfer studies revealed that the increase in AP1 binding is functional since IGF-I stimulates transcription from a reporter gene containing the minimal TRE linked to the chloramphenicol acetyl transferase (CAT) reporter gene. To further characterize the molecular mechanism by which IGF-I increases AP1 activity, we analysed the transcription regulation of c-fos and c-jun using reporter genes containing the respective promoters or specific regulatory elements. Deletion studies with the c-jun promoter, showed that IGF-I stimulates c-jun transcription via a cis acting element(s) localized within the 132 base pairs prior to the transcription start site; possibly the AP1 like element TGACATCA. Similar studies revealed that c-fos stimulation by IGF-I requires the presence of a regulatory sequence spanning the dyad symmetry element (DSE) and the fos AP1-like sequence (FAP). Further experiments using specific elements linked to the minimal unresponsive c-fos promoter, showed that the DSE is the main target for c-fos induction by IGF-I.

Mobilization of intracellular calcium by substance P in a human astrocytoma cell line (U-373 MG)

Variations in intracellular free calcium concentration (delta[Ca2+]i) were measured in intact and isolated human astrocytoma cells (U373 MG) loaded with fura-2 acetoxymethylester. Microperfusion of 50 nM substance P (SP), applied for 1 s, increased [Ca2+]i by 351 nM from a stable basal level of [Ca2+]i of 26 nM. The peak delta[Ca2+]i induced by SP was dose dependent with a threshold of 10(-3) nM, an ED50 of 1.3 nM and a maximal effect for concentrations of SP greater than 100 nM. The NK1 receptor agonist, [Sar9Met(O2)11]SP, mimicked the effect of SP, while the NK2 and NK3 selective receptor agonists, [N1(10)]NKA(4-10) and senktide, respectively, had no effect. The delta[Ca2+]i induced by SP was unaffected by 100 microM cadmium or by removal of extracellular calcium ions. Caffeine up to 30 mM had no effect on [Ca2+]i. In contrast, thapsigargin increased resting [Ca2+]i by 92 nM and reduced the delta[Ca2+]i induced by SP. A pertussis treatment (500 ng/ml-24 h) did not modify the delta[Ca2+]i induced by SP. We conclude that SP, acting on a NK1 receptor, mobilizes cytosolic calcium from an intracellular calcium pool which can be partially depleted by thapsigargin.

SR 140333, a novel, selective, and potent nonpeptide antagonist of the NK1 tachykinin receptor: characterization on the U373MG cell line

The effects of a novel nonpeptide NK1 tachykinin receptor antagonist, SR 140333, on the functional consequences of NK1 receptor activation in a human astrocytoma cell line, U373MG, were investigated. Radioligand binding conducted with 125I-Bolton-Hunter substance P revealed a competitive inhibition by SR 140333 and its R enantiomer SR 140603 with Ki values of 0.74 and 7.40 nM, respectively. The NK1-selective agonist, [Sar9,Met(O2)11]-substance P, stimulated the formation of inositol phosphates with an EC50 of 3.8 x 10(-9) M. SR 140333 blocked the stimulatory effect of this agonist (10(-7) M) with an IC50 of 1.6 x 10(-9) M, whereas the effect of another NK1 agonist, septide (EC50 = 1.5 x 10(-8) M) was antagonized with an IC50 of 2.1 x 10(-10) M. Enhancement of [3H]taurine release by [Sar9,Met(O2)11]-substance P (EC50 = 7.4 x 10(-9) M) was also inhibited by SR 140333 with an IC50 of 1.8 x 10(-9) M. SR 140603 was 10-fold less potent than SR 140333 in inhibiting inositol monophosphate formation and [3H]taurine release. The calcium mobilization induced by [Sar9,Met(O2)11]-substance P (10(-8) M) was totally prevented by 10(-8) M SR 140333. Patch-clamp experiments showed that SR 140333 depressed the outward current evoked by 5 x 10(-8) M [Sar9, Met(O2)11]-substance P with an IC50 of 1.3 x 10(-9) M. The expression of c-fos was stimulated by [Sar9,Met(O2)11]substance P with an EC50 of 2.5 x 10(-10) M, an effect that was also inhibited by SR 140333 with an IC50 of 1.1 x 10(-9) M.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of oxytocin binding sites in the thoracic and upper lumbar spinal cord of the adult and postnatal rat: a histoautoradiographic study

Oxytocin binding sites were detected by autoradiography on films and emulsion-coated sections in the spinal cord of adult and postnatal rats from C8 to L2, using a highly selective 125I-labelled oxytocin antagonist. Oxytocin binding sites were detected on all transverse sections in the dorsal horn, where labelling was scattered over laminae I and II. The autonomic areas, i.e. the intermediolateral cell column, the central grey (lamina X) and the nucleus intercalatus were labelled. Binding in the intermediolateral cell column was most frequently observed on sections from T9 to T11 in adult and T7 to T8 in postnatal rats. In this location, oxytocin binding sites were highly concentrated on cell bodies of putative sympathetic preganglionic neurons; however, not all of these cells were labelled. Diffuse labelling occurred on the dorsal part of the central grey, mainly between T8 and L2. Isolated labelled cells belonging to the nucleus intercalatus were scattered between the central canal and the intermediolateral cell column. In addition, oxytocin binding sites were found on some motoneurons of the lateral group of T12-T13, but only in postnatal rats. The distribution of oxytocin binding sites in the rat spinal cord coincides with that of the oxytocin innervation and strongly suggests a modulatory role of this peptide in sensory and autonomic functions.

Subpopulations of neonatal rat sensory neurons express functional neurotransmitter receptors which elevate intracellular calcium

We have attempted to identify which subpopulations of rat sensory neurons possess functional neurotransmitter receptors which elevate the free concentration of intracellular calcium. Subpopulations of sensory neurons were identified using three accepted criteria: (i) the distribution and proportion of neurons with differing somatic diameters; (ii) the expression of substance P-like immunoreactivity; and (iii) the responsiveness of each neuron to capsaicin. The total neuronal population was primarily grouped into three classes according to somatic diameter and defined as small- (< 17 microns), intermediate- (17-25 microns) and large- (> 25 microns) sized neurons. It was not possible to distinguish between small and intermediate-sized neurons since a similar percentage of each class expressed substance P-like immunoreactivity or sensitivity to capsaicin. Large-sized neurons did not possess these characteristics and, therefore, represented a distinct neuronal population. In single, intact neurons of differing diameter, the ability of a variety of receptor agonists to elevate the free concentration of intracellular calcium was determined using the calcium-sensitive indicator, Fura-2. Local application of capsaicin, adenosine, bradykinin, ATP and substance P elevated the resting level of the free concentration of intracellular calcium in small and intermediate-sized neurons. The large-sized neurons were unresponsive to these receptor agonists with the exception of ATP. The response to ATP was relatively transient in nature and did not differ between neurons of differing somatic diameter.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptic- and agonist-induced chloride currents in neonatal rat sympathetic preganglionic neurones in vitro

1. By using the whole-cell recording configuration of the patch-clamp technique in a spinal cord slice preparation, we have made recordings from visually identified neurones in the lateral horn of the thoracic and lumbar spinal cord of neonatal rats (newborn to 14 days postnatal). 2. Some of the recorded neurones were labelled with the fluorescent dye Lucifer Yellow (n = 27). Their morphology was typical for sympathetic preganglionic neurones (SPNs). Based on the size of the cell soma and the electrophysiological properties, unlabelled neurones were also regarded as SPNs. 3. Spontaneous synaptic activity of different patterns could be observed in 73% of the recorded neurones (n = 106). It reversed at the chloride equilibrium potential (ECl) and could be reversibly blocked by strychnine (1-10 microM), but not by bicuculline (10 microM) or SR95531 (5-10 microM). 4. Synaptic activity could be elicited by focal electrical stimulation in the vicinity of the recorded neurone. These evoked synaptic events exhibited features similar to the spontaneous synaptic activity. 5. Application of glycine (100 microM-1 mM) by a fast microperfusion system induced a chloride current in twenty-seven out of thirty cells tested. The currents were reversibly blocked by strychnine (1-10 microM), but were only weakly sensitive to bicuculline (10 microM). Stability of current responses to glycine was increased by inclusion of ATP (4 mM) in the intracellular medium. 6. Application of gamma-aminobutyric acid (GABA; 100 microM-1 mM) by the fast microperfusion system induced a chloride current in all twenty neurones tested. These currents were reversibly blocked by bicuculline (10 microM). Strychnine (1-10 microM) blocked this current only weakly. Run-down of GABA-induced currents was prevented to a great extent by inclusion of ATP (4 mM) in the pipette. 7. These results suggest that the inhibitory synaptic activity recorded from SPNs in thin, transverse slices of neonatal rat spinal cord is mediated by glycine receptor-gated Cl- channels. GABAA receptor-gated Cl- channels might be activated by inputs from other spinal segments and/or descending pathways from higher brain regions.

Serotonin inhibits Ca2+ currents in porcine melanotrophs by activating 5-HT1C and 5-HT1A receptors

1. We have investigated the effect of serotonin (5-HT) on Ca2+ currents in cultured porcine pituitary intermediate lobe (IL) cells. Electrophysiological recordings were performed in the whole-cell configuration of the patch-clamp technique. All membrane currents other than Ca2+ currents were blocked pharmacologically and by ionic substitution. 2. Two types of Ca2+ currents were recorded in IL cells, differing by their activation and inactivation properties. The first type of Ca2+ current was activated at membrane potentials more positive than -60 mV and had a transient time course during the 100 ms depolarizing voltage steps. The properties of this current correspond to those of the T-type or low-voltage-activated Ca2+ current. The second type of Ca2+ current had a threshold for activation between -30 and -20 mV and showed no sign of inactivation with time during the voltage steps. The properties of this current are similar to those of the L-type or high-voltage-activated Ca2+ current. 3. Current to voltage (I-V) relationships obtained either by conventional 100 ms voltage steps from a holding potential (VH) of -100 mV to various test potentials or by 800 ms voltage ramps from -100 to +50mV matched one another closely and showed two inward current humps corresponding to the activation of the T-type and L-type Ca2+ currents respectively. The ramp protocol was used to characterize the effect of 5-HT on the Ca2+ current I-V relationship. 4. 5-HT (100nM to 50 microM) reversibly inhibited the amplitude of the Ca2+ current triggered by 100 ms voltage jumps from a Vh of -100 mV to a test potential of 0 mV. 5. The effect of 5-HT was dose dependent with a threshold between 10 and 100 nM and a maximal effect at 10 microM. At a concentration of 10 microM, the average inhibition of Ca2+ current by 5-HT was 18.3 +/- 6.5% (n = 27). 5-HT inhibited Ba2+ current in a similar fashion. 6. When examining the effect of 5-HT on Ca2+ current I-V relationships, we observed a reversible inhibition of the high-threshold component corresponding to the L-type Ca2+ current. We never observed any effect of 5-HT on the T-type current. 7. The effect of 5-HT (10 microM) was antagonized to various extents by mianserin (1 microM) but not by ketanserin (0.1 microM), suggesting the involvement of 5-HT1C receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Caffeine-induced Ca2+ release inhibits GABAA responsiveness in rat identified native primary afferents

It has been shown previously that an increase in cytoplasmic Ca2+ concentration depresses the GABA-A response. However, little attention has been paid to the Ca2+ source involved. In the present study, we show that the Ca2+ increase triggered by caffeine-induced Ca2+ release from the intracellular pool inhibits the GABA-A response, whereas Ca2+ influx through voltage-activated Ca2+ channels has no effect on this response.

Acetylcholine synthesis in a primary culture of porcine intermediate lobe cells

Abstract Previous pharmacological studies with the pituitary gland have suggested that acetylcholine (ACh) might be involved in the regulation of intermediate lobe (IL) function. Whether ACh is endogenous to the IL cells or provided from an extrinsic source is unclear. The present experiments tested the possibility that the endocrine cells of the IL may be a source of ACh by measuring certain cholinergic markers in a primary culture of dissociated porcine cells. The endogenous ACh content was readily measurable in both the freshly dissociated IL cells and in 2- or 4-day primary cultures. Choline acetyltransferase activity was also measurable in the freshly dissociated and cultured IL cells and was reduced by 53% in the presence of a specific inhibitor, napthylvinylpyridine (50 muM). IL cells incubated in the presence of [(14)C]choline (1 muM) were able to synthesize [(14)C]ACh and the accumulation of the new ACh was inhibited by hemicholinium-3 (30 muM), a competitive inhibitor of high affinity choline uptake at cholinergic nerve terminals. In conclusion, these results demonstrate that the endocrine cells of the IL are capable of synthesizing and storing ACh.

Modulation of GABA-gated chloride currents by intracellular Ca2+ in cultured porcine melanotrophs

1. The modulatory role of intracellular Ca2+ concentration ([Ca2+]i) on gamma-aminobutyric acid type A (GABAA) receptor-gated Cl- currents was investigated in dialysed and intact cells of cultured porcine pituitary intermediate lobe (IL) cells using the patch-clamp technique. In order to isolate Ca2+ and Cl- currents all other membrane currents were blocked pharmacologically. Isoguvacine, a specific GABAA receptor agonist, was used to activate selectively GABAA receptor-mediated whole-cell and single-channel Cl- currents. 2. In the whole-cell recording (WCR) configuration inward Ca2+ currents triggered before and/or during the application of isoguvacine (100 microM), did not inhibit the GABAA receptor-mediated response. This lack of effect of calcium currents was obtained in all situations tested, i.e. when the intracellular Ca2+ concentration was only weakly buffered (0.5 mM-EGTA in the pipette solution), not buffered at all (no EGTA added to the pipette solution) or when the resting [Ca2+]i was buffered at 10(-7) M (pCa 7) with internal EGTA. 3. At pCa 7, simultaneous application of isoguvacine (100 microM) and caffeine (10 mM) resulted in a 47 +/- 15% reduction of the whole-cell GABAA response. In the same conditions, a ten times lower concentration of caffeine (1 mM), induced a transient increase of the GABAA response which turned into a steady-state inhibition during the subsequent applications. 4. At pCa 7, when isoguvacine (100 microM) was applied together with 3Me-His2-TRH (50 nM), a potent analogue of the calcium-recruiting thyrotrophin-releasing hormone, the GABAA receptor-gated Cl- current was increased by 40 +/- 8%. In the absence of the Ca2+ chelator EGTA in the pipette solution, either potentiating or inhibitory effects of 3Me-His2-TRH on the GABAA response were observed. 5. If a high concentration (18 mM) of the calcium chelator EGTA was included in the pipette solution, caffeine and 3Me-His2-TRH had markedly lower effects on the GABAA response than those observed at pCa 7, suggesting that the effect of both substances was mediated by an increase in [Ca2+]i. 6. In the absence of extracellular Ca2+, the effects of caffeine and 3Me-His2-TRH were not significantly different from those obtained in the presence of Ca2+ (5 mM), suggesting that Ca2+ influx was not the major route for increasing [Ca2+]i. 7. In the cell-attached (CA) configuration, the presence of isoguvacine (3-5 microM) in the pipette solution triggered the opening of channels displaying multiple current levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Bicuculline blocks nicotinic acetylcholine response in isolated intermediate lobe cells of the pig

1. The effect of bicuculline on nicotinic acetylcholine (ACh) responses in isolated intermediate lobe (IL) cells of the pig was investigated by use of patch-clamp techniques. Bicuculline was found to reduce ACh-evoked whole-cell currents (IACh) in all cells tested (n = 40). 2. The blocking effect of bicuculline on IACh was dose-dependent, the concentration producing half-maximal blockade being 43.8 microM. 3. The blockade of IACh by bicuculline was not voltage-dependent at membrane potentials above -60 mV, but a slight voltage-dependence was observed at holding potentials (HP) of -80 and -100 mV. 4. The inhibitory effect of bicuculline on IACh was partially competitive at a HP of -60 mV. 5. Neither SR 95531, a pyridazinyl gamma-aminobutyric acid derivative, nor t-butylbicyclophosphorothionate (TBPS) blocked IACh in IL cells. 6. It is concluded that bicuculline interacts directly with the ACh receptor-ionophore complex on porcine IL cells.

Lipopolyamine-mediated transfection allows gene expression studies in primary neuronal cells

A simple and efficient transfection technique based on lipopolyamine-coated DNA that can be used for gene transfer in cerebellar granular neurons is described. Gene transfer is achieved by exposure of cells to a DNA/lipid complex obtained by simple mixing of lipopolyamine and plasmid DNA. This procedure may represent a general tool of physiological investigations in primary cells. We show that the promoters of the introduced chimera genes are regulated by their respective trans-acting factors and may be modulated via membrane receptors and second messengers. This procedure has no noticeable toxic effects, nor does it seem to interfere with complex physiological behavior like neuronal differentiation.

Electrophysiological study of tert-butylbicyclophosphorothionate-induced block of spontaneous chloride channels

The action of TBPS (tert-butylbicyclophosphorothionate) on spontaneous chloride channels recorded from porcine pars intermediate lobe cells in primary culture has been studied. This compound, which binds specifically to the gamma-aminobutyric acidA (GABAA) receptor complex, is known as a channel-gating (non-competitive) GABA antagonist. The present results show that TBPS reduces spontaneous chloride channel activity in a dose-dependent manner, with an IC50 equal to 55 nM, which is a value comparable to its affinity for the GABAA binding sites. Single-channel analysis revealed that TBPS affects neither the amplitude nor the open time of these spontaneous channels but prolongs the longer closed times, resulting in a dramatic decrease in opening probability.

Nicotinic acetylcholine receptors in porcine hypophyseal intermediate lobe cells

1. Acetylcholine (ACh) was found to depolarize isolated porcine intermediate lobe cells maintained in primary cells culture. We investigated the ACh-induced responses in both whole-cell and cell-attached configurations of the patch-clamp technique. 2. From noise analysis of ACh-evoked whole-cell currents, we estimated an elementary conductance of 20 pS and a channel open duration of about 1.7 ms at -60 mV. From single-channel recordings, we obtained a slope conductance of 26 pS and a mean open time of 1.8 ms at membrane potentials between -60 and -80 mV. 3. ACh-evoked responses were blocked by d-tubocurarine (d-TC), hexamethonium and mecamylamine, but were insensitive to alpha-bungarotoxin. These characteristics define a neuronal type of nicotinic receptors. 4. The whole-cell current induced by ACh showed a strong inward rectification with no outward current being obtained. This phenomenon was observed when the intracellular ion is either sodium or caesium, and even when Ca2+ and Mg2+ were totally removed from the intracellular medium. 5. ACh-gated channels in intermediate lobe cells were cation selective and were permeable to Na+ and Cs+. In Ca2(+)-free extracellular solution, single-channel conductances were much larger (46 pS) than in the presence of 2 mM-Ca2+ (26 pS). 6. The possibility of an excitatory cholinergic control of intermediate lobe cells is discussed.

Pharmacological characterization of the aminopyridazine SR 95639A, a selective M1 muscarinic agonist

In order to design a selective M1 muscarinic agonist, we synthesized SR 95639A (morpholinoethylamino-3-benzocyclohepta-(5,6-c)-pyridazine, dihydrochloride), a semi-rigid analogue of the aminopyridazine antidepressant drug minaprine. SR 95639A displaced [3H]pirenzepine from its binding sites in rat hippocampal membranes with an IC50 value of 0.27 microM. It only weakly displaced [3H]N-methylscopolamine from cerebellar, cardiac and ileal membranes (10-48 microM), and, up to 100 microM, did not interact with the main other receptors of the rat brain. In rat isolated sympathetic ganglia, SR 95639A induced dose-dependent depolarizations which were antagonized by pirenzepine, and dose dependently suppressed the M current. These latter effects were also pirenzepine-sensitive. After i.p. or oral treatment in mice, SR 95639A never induced the classical cholinergic syndrome, up to lethal doses. Finally, SR 95639A (i.p. and p.o.) antagonized contralateral rotations induced by intrastriatal injection of pirenzepine, in mice. These results suggest that SR 95639A is a selective agonist at central muscarinic M1 receptors and may represent a useful tool for further characterization of the nature and function of muscarinic receptor subtypes.

Spontaneous spiking activities of porcine pars intermedia cells: effects of thyrotropin-releasing hormone

The electrophysiological properties of porcine melanotrophs in primary culture were studied with patch clamp techniques. In the cell-attached (c/a) configuration, extracellular spikes were recorded but in only 47 of the 259 cells examined; active cells were more frequently found in short-term cultures (48 h, 22 out of 71). In the whole-cell (WCR) configuration, the mean resting potential was -46.6 mV and the mean input resistance 2.41 G omega. In the current clamp mode, of the 99 cells recorded, 39.4% displayed spontaneous spiking activities, 14.1% spiked when a depolarizing current was applied, and 46.5% were silent. Two patterns of spontaneous activity were recorded. The first consisted of rapid membrane depolarizations (mean duration: 16.0 ms) which reached a mean value of +8.9 mV. These action potentials occurred either at random intervals or at a mean frequency of 1.19 Hz. Tetrodotoxin (TTX), a sodium channel blocker, completely abolished these action potentials. The second pattern consisted of regular bursts (mean duration: 1.69 s and mean frequency: 0.15 Hz) of spikes with smaller amplitudes (culminating at -9.5 mV) and greater durations (79.7 ms). This pattern could be recorded in the presence of TTX in the external medium. In the c/a configuration, thyrotropin-releasing hormone (TRH) and (3Me-His2)-TRH enhanced spike frequency, whereas histidyl-proline-diketopiperazine, a metabolite of TRH, had no effect. In WCR, out of the 27 cells tested, TRH (5.10(-9)-5.10(-8) M) induced firing in 4 quiescent cells and increased the frequency of action potentials in 4 spontaneously active cells. This was usually, but not necessarily, preceded by a hyperpolarization (n = 5). TRH (5.10(-8) M) enhanced the secretion of melanocyte-stimulating hormone (alpha MSH) from perfused isolated melanotrophs by 62.8 +/- 9.8% (n = 9) over basal levels. (3Me-His2)-TRH and (Pro3)-TRH mimicked the TRH response, whereas histidyl-proline-diketopiperazine was without effect.

Thyrotropin-releasing hormone stimulates porcine melanotrope cells in primary culture

The action of thyrotropin-releasing hormone (TRH) on melanotrope cells maintained in primary culture was studied with biochemical and electrophysiological techniques. TRH effects on polyphosphoinositide (PPI) breakdown was measured in [3H]myoinositol labelled cells maintained in suspension for 24 hours or in primary culture. TRH (50 nM) or its potent analogue (3Me-His2)-TRH increased total PPI levels by 50-125% in separate experiments after 30 min of treatment whereas corticotropin-releasing hormone (CRF) was without effect. The effect of TRH was dose-dependent (ED50 = 5 nM), the maximal effect being reached with 50 nM TRH. Using the patch-clamp technique in the cell-attached configuration spikes were recorded extracellularly. In 6 of the 13 cells tested, (3Me-His2)-TRH (10 nM) elicited an increase in the spontaneous spiking rate. Furthermore, TRH (50 nM) increased melanocyte-stimulating hormone (alpha-MSH) secretion 2-fold after 8 h of treatment. These results suggested that TRH activated phospholipase C and electrical activity in melanotrope cells; the resulting phosphoinositide breakdown and increase in intracellular free Ca2+ ultimately led to a stimulation of hormone release.

Small-conductance chloride channels activated by calcium on cultured endocrine cells from mammalian pars intermedia

Porcine intermediate lobe (IL) endocrine cells maintained in primary culture have been studied using patch-clamp derived configurations to record unitary activity on outside-out vesicles. Solutions were devised so as to record Cl current in isolation and to fix cytoplasmic Ca concentration [Ca]i between 0.1 microM and 3 microM. Between [Ca]i 0.5 and 1 microM, the chloride permeability was restricted to single events with a small amplitude, that varied linearly with the membrane potential. Mean slope conductance of this chloride channel was 2.5 pS. Single channel analysis yielded two mean open time values of 10 and 55 ms at -80 mV. Relaxations of chloride currents on outside-out patches was examined at different [Ca]i. Relaxation was negligible at 0.15 microM [Ca]i, whereas at higher [Ca]i, the current exhibited relaxation in response to voltage jumps the kinetic of which could be fitted by two exponentials. At 0.5 microM [Ca]i, the fast relaxation time constant was shown to be voltage insensitive with a value of about 10 ms. The slow relaxation time constant had a mean value of 75 ms at -60 mV and increased with membrane depolarization with a twofold change over 120 mV. Another voltage effect was to favour the slow opening mode at the more depolarized potentials: the ratio of fast to slow relaxations being 5:1 at -60 mV as compared to 1:1 at +80 mV). Finally the estimated probability of opening (po) linearly increased with voltage. po displayed a bell-shaped dependence on [Ca]i, so that full activation of the channels was not achieved.

Quantitative evaluation of the properties of a pyridazinyl GABA derivative (SR 95531) as a GABAA competitive antagonist. An electrophysiological approach

We have investigated the effects of an aryl-aminopyridazine derivative of GABA (SR 95531) on dose-response curves of GABA-induced depolarizations from dorsal root ganglion neurones recorded intracellularly. The reversible shift to the right of the dose-response curves in a parallel fashion and the dissociation constant (KB) value of 0.13 +/- 0.02 microM (n = 15) indicate that this compound is a potent competitive GABAA antagonist. The competitive nature of SR 95531-induced antagonism was confirmed by single channel analysis. In excised membrane patches from bovine chromaffin cells (outside out configuration), 0.2-0.5 microM SR 95531 did not alter the mean open time of GABA-activated channels and did not introduce further short closing gaps within bursts. Whole cell recordings from cultured nodose ganglion neurones indicated that SR 95531 (10 microM) did not modify significantly any of the 3 types of calcium currents already reported in sensory neurones. This result might be of importance for further studies of presynaptic GABA actions on transmitter release.

Spontaneous and GABA-evoked chloride channels on pituitary intermediate lobe cells and their internal Ca requirements

On porcine intermediate lobe (IL) endocrine cells, spontaneously opening chloride channels have been studied and compared to GABA-A activated chloride channels. Elementary currents were recorded mainly from outside-out patches excised from IL cells maintained in culture for 1-4 weeks. Spontaneous inward currents were observed in Cs-loaded cells after replacing Na in the extracellular medium by the impermeant ion choline. This activity, at an internal calcium concentration of 10(-8) M corresponded to a channel for chloride ions with a main conductance level of 26 pS, and substates around 11 pS. The sequence of permeabilities to halides was I greater than Br greater than Cl. These conductance characteristics were common to the GABA-operated channels which also showed a main conductance substate of 23-31 pS. The open time of the 26 pS level mostly encountered in spontaneous activity, was distributed along two modes: one, the most frequent, around 1 ms, and the other around 4 ms. This latter mode was the predominant one observed during GABA and isoguvacine applications but in addition a bursting activity of 19 ms duration was also seen. Specific GABA-A receptor antagonists (bicuculline and SR42641, 1 microM) blocked activity evoked by GABA (1-10 microM), but did not affect spontaneous events. These spontaneous Cl events were only observed in a restricted range of internal Ca concentrations, i.e. between 1 nM and 0.1 microM, and were practically abolished at Cai 1 microM. The GABA-induced activity of Cl channels was also Ca-sensitive, being reduced when Cai reached 1 microM.

Intracellular effectors and modulators of GABA-A and GABA-B receptors: a commentary

The inhibitory neurotransmitter GABA activates two receptor subtypes that can be distinguished by their pharmacology. The GABA-A site is competitively antagonized by bicuculline and exclusively coupled to a chloride channel. The GABA-B receptor, for which baclofen is the only specific agonist, is resistant to bicuculline inhibition and, depending upon its localization, will activate K currents and/or inhibit Ca currents. Both electrophysiological and biochemical approaches have been applied to the study of each receptor. The membrane and intracellular components that to date have been implicated in GABA-B activation are discussed: G proteins, adenylate cyclase and intracellular calcium levels. This latter factor is also discussed with respect to GABA-A receptor action.

Electrophysiological study of SR 42641, a novel aminopyridazine derivative of GABA: antagonist properties and receptor selectivity of GABAA versus GABAB responses

A new arylamino-pyridazine gamma-aminobutyric acid (GABA) derivative, SR 42641, has been tested for its ability to antagonize the actions of GABA on mammalian sensory neurones. SR 42641 and bicuculline reversibly decreased GABAA-induced depolarizations and currents recorded intracellularly from dorsal root ganglion neurons (DRG). Dose-response curves were shifted to the right in a parallel fashion. KB values (determined under voltage clamp conditions) were respectively 0.12 +/- 0.05 and 0.38 +/- 0.08 microM. Similar values were obtained with current clamp recording conditions. The study of the GABA-induced Cl- current under voltage-clamp conditions did not show any voltage-dependency of the antagonist effect of SR 42641. In nodose ganglion neurones, SR 42641 (0.4-4.5 microM) did not alter the (-)-baclofen-induced shortening of the calcium component of action potentials. At concentrations higher than 10 microM, SR 42641 itself prolonged calcium-dependent action potentials. Patch-clamp recordings from DRG cultured neurones indicated that SR 42641 did not affect the calcium current responsible for sustained calcium entry into cells. We conclude that SR 42641 is a potent competitive GABA antagonist, specific for the GABAA receptor. It does not act at the level of the chloride ionophore.

Low concentrations of GABA reduce accommodation in primary afferent neurons by an action at GABAB receptors

The pattern of accommodation of spike activity during sustained membrane depolarization was investigated in primary afferent neurons recorded intracellularly in vitro in the rat. We show that gamma-aminobutyric acid (GABA) and baclofen reduce accommodation in some fast conducting dorsal root ganglion neurons. This effect was restricted to those A delta cells with axons displaying a rather fast conduction velocity (15-25 m/s). GABA-induced blockade of accommodation was not observed in large A beta neurons. Pharmacological studies with baclofen, as opposed to isoguvacine, indicate that this effect is due to GABAB receptors activation. The effect is also shown to be resistant to bicuculline antagonism. In slow conducting afferents, GABAB receptor activation is known to shorten the CA2+ component of action potentials. By contrast, no such component was observed in the A delta cells studied. Furthermore, Ca2+-activated K+ conductances are not implicated in the reduction of accommodation caused by GABAB receptor activation. In conjunction with the actual knowledge about the distribution of GABA receptors on primary afferents, our result indicates that GABAA and GABAB receptors coexist on all categories of A delta and C primary afferents in the rat.

Ionic conductances related to GABA action on secretory and biosynthetic activity of pars intermedia cells

We have examined the action of GABA on the electrical, secretory and synthetic activities of rat and porcine intermediate lobe (IL) cells in primary culture. Chloride and calcium currents were investigated using patch-clamp techniques. A chloride current activated by 1-100 microM isoguvacine, a specific GABA-A agonist, and antagonised by bicuculline and SR 95103 was recorded at the whole cell and single channel level current. Whole cell calcium currents were investigated and shown to be reduced by 40 microM cadmium, zero external calcium and 10 microM baclofen, a specific GABA-B receptor agonist. Both GABA-B receptor activation and use of calcium deficient medium inhibited peptide release from IL cells. Finally, pro-opiomelanocortin (POMC) mRNA levels were measured using a hybridization technique. Removal of calcium from the culture medium or long-term (48 hr) incubation with 10 microM GABA or muscimol (a mixed GABA-A and GABA-B agonist) significantly reduced POMC mRNA levels.

GABAA and GABAB receptors on porcine pars intermedia cells in primary culture: functional role in modulating peptide release

A primary culture of porcine pars intermedia cells with particularly high yields has been developed. The cells, grown in monolayers, secrete the pro-opiomelanocortin-derived peptide alpha-melanocyte-stimulating hormone over several weeks. The patch-clamp technique has been used to demonstrate the presence of gamma-aminobutyrateA (GABAA) receptors on the cells. GABA or the selective GABAA receptor agonist isoguvacine produced a depolarizing increase in chloride conductance that desensitized rapidly. The response was antagonized by bicuculline and by the aminopyridazine derivative of GABA (SR 95103), a novel GABAA receptor antagonist. The effects of specific agonists for each receptor were tested on peptide release from cells maintained in a perfusion system. Isoguvacine (10 microM) potentiated Ba2+-evoked release of alpha-melanocyte-stimulating hormone, whereas (-)-baclofen (50 microM) decreased both basal and stimulated hormone release. This negative effect on peptide secretion was reproduced when GABA (50 microM) was perfused in the presence of bicuculline (10 microM) to block GABAA receptor activation. The possible mechanisms underlying these GABAA and GABAB effects on stimulus-secretion coupling in this neuroendocrine model are discussed.

Synthesis and activity of 5-(aminomethylene)-1,3-cyclohexanediones: enolic analogues of gamma-aminobutyric acid

Eight 1,3-cyclohexanediones with an aminoalkyl side chain in the 5-position were synthesized as rigid enolic analogues of GABA (gamma-aminobutyric acid). Biochemical investigations about their abilities to displace [3H]GABA and [3H]baclofen [beta-(p-chlorophenyl)-gamma-aminobutyric acid] in binding studies or to inhibit the high-affinity sodium-dependent GABA uptake showed that these compounds were generally devoid of affinity for the two GABA receptors and for the GABA carrier. Only compound 1 exhibited a weak affinity in the GABA-A binding experiments (IC50 = 6.5 X 10(-5) M). Graphic computer modeling was applied in an attempt to explain this activity in comparison to some reference GABA agonists. Electrophysiological studies on dorsal root ganglia (DRG) also excluded agonistic or antagonistic properties on GABA-A or GABA-B receptor models but pointed out an atypical prolongation of Ca2+-dependent action potential for compound 1.

Properties of the GABA receptors located on spinal primary afferent neurones and hypophyseal neuroendocrine cells of the rat

Electrophysiological techniques have been used to study the pharmacological characteristics of GABA receptors in two in vitro preparations likely to provide the ionic basis for GABAergic inhibition of excitation-secretion coupling. The shortening of Ca2+ spikes duration by GABAB receptors was shown to occur in slow conducting dorsal root ganglion cells, independently of marked depression of inward calcium currents. Ion-selective electrodes (K+ or Ca2+) were used to show the presence of both GABAA and GABAB receptors on the neurosecretory terminals and gland cells from hypophyseal neuro-intermediate lobe (NIL). In this latter preparation, potentiation of hormone release was observed under GABAA receptor activation, whilst inhibition was seen with GABAB agonists.