Pacemaker potentials of serotonergic dorsal raphe neurons: contribution of a low-threshold Ca2+ conductance

Neurotensin excitation of serotonergic neurons in the dorsal raphe nucleus of the rat in vitro

Neurotensin-containing terminals and radioligand binding sites are present in the dorsal raphe nucleus. The purpose of this study was to test, in brain slices containing this nucleus, the effect of neurotensin on the electrical activity of serotonergic neurons. In extracellular recordings, the cells were identified by the ability of the alpha 1-adrenoceptor agonist phenylephrine to induce firing, and serotonin to reduce this effect. After washout of phenylephrine, neurotensin (10 nM to 10 microM) induced a concentration-dependent increase in the firing rate of serotonergic neurons (EC50 = 142 nM; maximum effect approximately 1 microM). The neurotensin excitation, which was mimicked by neurotensin fragments 8-13 but not neurotensin peptide fragment 1-8 and selectively blocked by SR 48692 (100 nM), was observed mainly in the ventral part of the nucleus. Most serotonergic neurons showed marked desensitization to neurotensin, even at low concentrations. The neurotensin response was occluded by supramaximal concentrations of phenylephrine. In intracellular recordings using KCl-containing electrodes, neurotensin induced an inward current associated in some cases with a decrease in apparent input conductance. In conclusion, neurotensin was found to have an excitatory action on serotonergic neurons in the ventral part of the dorsal raphe nucleus, an effect which could be subject to desensitization and was occluded by phenylephrine. This occlusion phenomenon may be important for the physiological role of neurotensin in the dorsal raphe nucleus.

Electrophysiological properties and their modulation by norepinephrine in the ambiguus neurons of the guinea pig

Electrophysiological properties of guinea pig ambiguus (AMB) neurons were studied in a brainstem slice preparation. During subthreshold depolarization AMB neurons displayed an early slow depolarization and a late outward rectification both of which were blocked by replacing Ca2+ with Co2+ in the extracellular solution. AMB neurons showed hyperpolarizing inward rectification which was blocked by extracellular Cs+ and is likely caused by the activation of Ih: In 58% (n = 49) of AMB neurons spike firing was restricted to the early phase of a long-lasting depolarizing current injection (phasic firing). The remaining AMB neurons showed repetitive firing throughout the depolarization (tonic firing). A Ca(2+)-mediated K+ current (IK(Ca)) caused an afterhyperpolarization that followed both single and repetitive spike firing. IK(Ca) also controlled the firing pattern in both types of firing, especially in the phasic firing. Norepinephrine (NE) blocked both the hyperpolarizing inward rectification and the Ca(2+)-dependent AHP. These effects of NE were antagonized by propranolol. It is proposed that the blockade of IK(Ca) and Ih contribute to the improvement of the 'signal-to-noise ratio' by NE in AMB neurons.

Use of the whole-cell patch-clamp method in studies on the role of cAMP in regulating the spontaneous firing of locus coeruleus neurons

The whole-cell patch-clamp technique represents a major advance over conventional intracellular recordings in the study of the modulation of ion channels by intracellular messengers. This report illustrates how application of the whole-cell technique to noradrenergic neurons of the rat locus coeruleus in brain slices has led to the finding that cAMP via its phosphorylation pathway modulates tonic pacemaking in these neurons. In the studies to be described, the particular advantage of the whole-cell technique was that it allowed introduction of macromolecules related to the cAMP pathway (e.g., protein kinase inhibitor and protein kinase A) directly into cells. Furthermore, these studies were carried out in situ, in thick brain slices allowing a direct comparison with a large body of existing extracellular and intracellular data obtained under similar conditions.

Electrophysiology of the neurons in the area of the enkephalinergic magnocellular dorsal nucleus of the guinea-pig hypothalamus, studied by intracellular and whole-cell recordings

The electrophysiological characteristics of 103 hypothalamic neurons in the area of the guinea-pig enkephalinergic magnocellular dorsal nucleus were studied in a thick slice preparation with sharp microelectrodes (63 neurons) and patch pipettes for whole-cell recordings (40 neurons). Of the sampled cells, 79.6% displayed tetrodotoxin-resistant, calcium-dependent slow-depolarizing potentials when the membrane potential was hyperpolarized to approximately -70 mV (type I neurons). Half of them showed robust slow depolarizing potentials, generating bursts of fast action potentials. In the remaining neurons, the slow-depolarizing potentials did not cause burst-firing action potentials but triggered single action potentials. The other class of neurons (20.4% of the sample: type II neurons) did not exhibit calcium-dependent slow-depolarizing potentials. Resting potential, input resistance and the membrane time constant did not distinguish among the two classes of neurons. Current-voltage relationships were heterogeneous. A transient outward rectification was observed in the two classes. This was not totally blocked by 2 mM 4-aminopyridine but was abolished when using perfusion with cobalt instead of calcium. Input resistance and the time constant were higher when measured in the whole-cell mode but the other electrical parameters and the sampling of the recorded neurons were strikingly similar between the two methods of recording. Intracellular staining of 22 neurons retrogradely labelled from the lateral septum allowed confirmation of their location within the magnocellular dorsal nucleus. The study indicates that the electrical properties of these neurons did not differ from those of neurons found throughout the area explored. It also indicates the presence of distinct electrophysiological types of cells in the magnocellular dorsal nucleus, although the nucleus is composed of a single type of enkephalinergic neuron. It provides a basis for the study of the regulation of activity of the neurons at the origin of an enkephalinergic tractus which is involved in neuroendocrine, psychoneuroendocrine and immune processes.

Whole-cell recordings from visualized C1 adrenergic bulbospinal neurons: ionic mechanisms underlying vasomotor tone

The membrane properties of visually identified, DiI retrogradely labeled bulbospinal neurons of the C1 adrenergic cell group were studied by whole-cell recordings in brainstem slices from 7- to 10-day-old rats. A post-hoc histochemical analysis allowed us to evaluate the electrophysiological properties of the C1 adrenergic neurons, a group of cells known to project to the sympathetic preganglionic neurons. Two types of cells were labeled: pacemaker and non-pacemaker neurons. In voltage-clamp mode, C1 pacemaker neurons exhibited a TTX-sensitive, persistent inward current that was activated between -55 and -50 mV and reached a peak between -40 and -30 mV. This current was significantly larger in the pacemaker neurons as compared to the non-pacemaker neurons and appeared to be a principal conductance driving the C1 pacemaker activity. Two other conductances modulated the frequency of pacemaker discharge: (1) an anomalous rectifier accelerated pacemaker frequency by three synergistic actions: (a) depolarizing it at rest, (b) increasing the slope of the pacemaker potentials, and (c) limiting hyperpolarizing membrane excursions; and (2) an A-type current which had two opposing actions: (a) slowing it by decreasing the slope of the pacemaker potential, and (b) accelerating it by repolarizing the fast action potential. Persistent sodium current functions as the driver potential responsible for the tonic firing pattern of the C1 bulbospinal neurons providing a cellular mechanism responsible for the descending excitatory drive imposed onto sympathetic preganglionic neurons. Thus, it may explain how C1 neurons may function to maintain vasomotor tone or modulate other autonomic functions. This study is the first attempt to analyze voltage-activated membrane conductances of RVLM neurons of known phenotype and axonal connections.

Habenula lesions decrease the responsiveness of dorsal raphe serotonin neurons to cocaine

The median and dorsal (MR and DR) raphe nuclei are the origin of serotonin (5-HT)-containing neurons that innervate the forebrain. Neurons originating in the medial and lateral habenula provide an extensive afferent input to the midbrain that could serve as a negative feedback circuit. The present study was undertaken to establish whether intact habenula nuclei are required to observe the depressant effects of cocaine on the neural activity of 5-HT somata in the DR. To this end, the spontaneous activity of DR 5-HT neurons was assessed in male rats that had previously received bilateral radiofrequency lesions of the habenula complex either 1-4 h (short term) or 7 days (long term) prior to extracellular recordings of single 5-HT neurons of the DR. In rats with short-term lesions, the inhibitory response to cocaine was significantly attenuated. The mean dose to inhibit activity by 50% (ID50) was increased from 0.68 mg/kg in controls to 2.5 mg/kg in lesioned rats. Short-term habenula lesions also significantly decreased the numbers (but not the firing rates) of 5-HT neurons encountered in the DR. In contrast, the dose-response to cocaine as well as the numbers and firing rates of 5-HT neurons found in rats with long-term habenula lesions did not differ from controls. These results suggest that the inhibitory effects of cocaine on DR 5-HT neuronal activity depend in part on the ability of cocaine to affect habenula control of raphe 5-HT function.

Whole-cell patch-clamp recordings from visualized bulbospinal neurons in the brainstem slices

The purpose of this study was to develop a method for electrophysiological characterization of retrogradely labeled bulbospinal neurons in the specific cytoarchitectonic regions in the brainstem slices. Several days after the spinal cord was injected with the carbocyanine dye, DiI, retrogradely labeled bulbospinal neurons were visualized by epifluorescence optics in the brainstem slices with the aid of a silicon intensifier tube (SIT) camera. Labeled somata were routinely seen in the caudal raphe nuclei, rostroventral medial and lateral portions of the medulla, the A5 group and in other medullary sites known to project to the spinal cord. Electrophysiological properties of the DiI-labeled neurons were assessed by whole-cell recordings using micropipettes filled with biocytin. The slices were subsequently processed for dual visualization of biocytin and serotonin or a marker for noradrenergic neurons, tyrosine hydroxylase (TH). The electrophysiological properties of bulbospinal neurons were correlated with their morphology and neurochemical content. This technique may be useful in other areas of CNS for studying morphology, neurochemical content and physiology of retrogradely labeled neurons.

Is the nucleus raphe dorsalis a target for the peptides possessing hypnogenic properties?

Several peptides exhibiting hypnogenic properties when administered i.p., i.v. or i.c.v. are now known. No data, however, are available concerning their targets in the brain. In the present work we hypothesize that the nucleus raphe dorsalis (nRD) may be one such target since it contains 2 sleep permissive components that must be influenced for sleep to occur. One of these components is serotoninergic in nature and gates the occurrence of ponto-geniculo-occipital (PGO) waves. The other, of unknown nature, influences tonic sleep phenomena. For hypnogenic peptides, a putative mechanism permitting the triggering and maintenance of sleep might consist of influencing both the above components. In the present work, 3 hypnogenic substances, CLIP (corticotropin-Like intermediate lobe peptide), VIP (vasoactive intestinal polypeptide) and DSIP (delta sleep inducing peptide), were injected into the nRD in order to determine whether these compounds still induce sleep by local administration. To verify that such local injections do not spread outside the nRD, radiolabelled CLIP and VIP were also injected. Autoradiograms obtained with either labeled CLIP or VIP indicate that these compounds, injected in a 0.2 microliter volume, do not spread outside the nRD. The sleep data obtained confirm that CLIP, at a dose of 10 ng, induces an increase in duration of paradoxical sleep (PS); this effect is observed only for injection sites located in the dorsolateral part of the nRD, an area where CLIP immunoreactive (IR) fibers are present. VIP, at a dose of 100 ng, also increases PS duration, whereas at 10 ng, only slow wave sleep duration is increased. In this case, the positive injection sites are scattered throughout the entire nRD as are the VIP-IR fibers. With DSIP, no sleep effect was found whatever the dose used or the site injected; in the same manner, no DSIP-IR fibers have been located in this structure. These data suggest that the nRD is a target for the expression of the hypnogenic properties of CLIP and VIP, but not for DSIP. The nature of the possible mechanisms permitting such expression are discussed.

Medial vestibular neurons are endogenous pacemakers whose discharge is modulated by neurotransmitters

1. Neurons in the medial vestibular nucleus (MVN), recorded in a rat brain slice preparation, exhibit a highly regular, high-frequency (5- to 35-Hz) spontaneous discharge. The rhythmic firing rate was constant (< 5% variation) and sustained for a long time (maximum observation, 4 hr). 2. The rhythmic firing was evident even in neurons (n = 15) completely isolated from exogenous input fibers, suggesting that it is due to an endogenous pacemaker property. When recorded intracellularly, the discharge was found to be associated with a smooth, concave pacemaker prepotential, and the rate of firing was reduced in proportion to applied hyperpolarizing current, indicating that these are pacemaker discharges. 3. This conclusion is supported by the observation that perfusion with a low-calcium/high-magnesium Krebs-Ringer solution, which completely and reversibly blocks all synaptic transmission, did not abolish the spontaneous discharge. The low-calcium/high-magnesium solution increased spontaneous firing in some neurons and decreased in others, suggesting that the firing is synaptically modulated and the synaptic influence is tonically active. 4. Application of kynurenate (10 mM), an antagonist of the excitatory amino acid receptors, gradually reduced neuronal discharges in most neurons (22 of 25), while the addition of 10 mM sucrose as an osmotic control had no effect. Depression of neuronal discharges reached its minimum (an average of 60% of the control level) and was maintained at that level until gradually washed out. The rhythmic firing pattern persisted in all neurons even after the excitatory receptors were blocked. 5. When the GABAA receptor antagonist, bicuculline (20 microM), was applied, elevation of neuronal discharges was evident in most neurons (30 of 32) tested. The firing increased gradually, with a final control level of 130% (121-160%). In contrast, the GABAB receptor antagonist, phaclofen (20 microM and 100 microM), had no effect in most neurons (19 of 23) tested. Further, the excitatory and inhibitory action could be detected on the same neuron when bicuculline and kynurenate were both evaluated (n = 10). 6. These results indicate that the spontaneous discharge of MVN neurons is due to an endogenous pacemaker under the tonic influence of both inhibitory and excitatory transmitter actions. The bicuculline-sensitive GABAA receptors and the kynurenate-sensitive glutamate receptors both mediate the tonic modulation.

Membrane properties of identified guinea-pig paraventricular neurons and their response to an opioid mu-receptor agonist: evidence for an increase in K+ conductance

Intracellular recordings were obtained from neurons in the paraventricular nucleus (PVN) of guinea-pig hypothalamic slices. Passive and active properties of the neurons were determined, and when possible, recorded neurons were injected with biocytin. The effects of a mu-receptor opioid agonist [D-Ala2, Nme-Phe4, Gly5-ol]enkephalin (DAGO) were studied in order to determine which types of cells have mu receptors and to test the hypothesis that an increase in K+ conductance causes mu-receptor-mediated inhibition in the PVN. The recorded cells inside the PVN were divided into two groups, primarily on the basis of the presence or absence of a low threshold Ca2+ spike (LTS). In one group of neurons, LTS potentials could not be evoked (non-LTS cells, n = 42). In another group of neurons (n = 35), LTS potentials with one or two Na+ spikes could be initiated with depolarizing pulses superimposed on steady hyperpolarizing currents; however, these neurons did not fire robust bursts (i.e. non-bursting LTS cells). The mean time constant of non-LTS cells (19.9 +/- 1.6 ms; mean +/- SEM) was significantly shorter than that of non-bursting LTS cells (26.7 +/- 2.1 ms). There were no differences in the mean resting membrane potential, spike amplitude, spike duration, input resistance, spike threshold and pattern of synaptic inputs between the two groups. Intracellular labeling with biocytin combined with cresyl violet counter-staining demonstrated that the two types of cells were located in the PVN.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and functional expression of a member of the low voltage-activated calcium channel family

Oscillatory firing patterns are an intrinsic property of some neurons and have an important function in information processing. In some cells, low voltage-activated calcium channels have been proposed to underlie a depolarizing potential that regulates bursting. The sequence of a rat brain calcium channel alpha 1 subunit (rbE-II) was deduced. Although it is structurally related to high voltage-activated calcium channels, the rbE-II channel transiently activated at negative membrane potentials, required a strong hyperpolarization to deinactivate, and was highly sensitive to block by nickel. In situ hybridization showed that rbE-II messenger RNA is expressed in regions throughout the central nervous system. The electrophysiological properties of the rbE-II current are consistent with a type of low voltage-activated calcium channel that requires membrane hyperpolarization for maximal activity, which suggests that rbE-II may be involved in the modulation of firing patterns.

Ionic dependence of a slow inward tail current in rat dorsal raphe neurones

1. The mechanism underlying a large slow inward tail current was studied in serotonergic dorsal raphe (DR) neurones. The tail current is most easily observed under conditions of suppressed K+ channel outward currents and follows the activation of a calcium current. This current may underlie a slow after-depolarizing potential (ADP) which follows action potentials observed in acutely isolated DR neurones. 2. The after-hyperpolarizing potential (AHP) following action potentials which should reverse at EK (the reversal potential for potassium) becomes an ADP at less negative potentials than expected due to contamination by the slow inward tail current. 3. DR neurones were acutely isolated enzymatically; the ADP in current clamp and the tail current underlying it in voltage clamp were studied using the patch clamp method. When the external Na+ was replaced with TEA or choline the slow inward tail current was completely abolished. Blocking K+ channels from the inside of the cell membrane with 40 mM TEACl or large concentrations of internal Cs+ also blocked the slow inward tail current. 4. The tail current proved to be independent of calcium influx or intracellular calcium release as it was not affected by inorganic calcium channel blockers or caffeine. 5. The tail grew exponentially upon lengthening the depolarizing test pulse and appeared to reverse close to 0 mV indicating that the current was carried by a nonselective cation conductance. Removal of external Na+ and replacement with Li+ ions reversibly blocked the tail current by 77%. 6. The data rule out several mechanisms for the generation of the current, namely: a calcium-activated chloride conductance, a calcium-activated non-selective cation conductance, a Na(+)-Ca2+ exchange pump current or a sodium-activated K+ conductance. 7. The slow tail current may be explained by postulating an inward movement of Na+ through a channel which is blocked by high concentrations of external TEA and Li+ or internal Cs+ or 40 mM TEA.

Dual projections of single cholinergic and aminergic brainstem neurons to the thalamus and basal forebrain in the rat

Compelling evidence indicates that cholinergic basal forebrain neurons are strongly activated during waking, and concurrently thalamic spindle activity is suppressed and thalamocortical sensory transmission is facilitated. Both thalamus and basal forebrain are known to receive projections from brainstem cholinergic and aminergic neuronal pools that are involved in wake/sleep regulation. The present study addressed the question of whether single cholinergic and aminergic neurons contributed to both of these ascending projections, by using two fluorescent retrograde tracers combined with immunofluorescence. Cholinergic neurons projecting to both the basal forebrain and thalamus were found in the pedunculopontine and laterodorsal tegmental nuclei, representing an average of 8.0% of the total cholinergic cell population in these nuclei. Serotonergic neurons with dual projections were observed in the dorsal, median and caudal linear raphe nuclei, accounting for a mean of 4.7% of total serotonergic neurons in these nuclei. Relatively few noradrenergic neurons (2.0%) in the locus ceruleus projected to both target structures, and a very small subpopulation of histaminergic neurons (1.5%) in the tuberomammillary hypothalamic nucleus had dual projections. Of all brainstem neurons with dual projections, cholinergic and serotonergic neurons accounted for an overwhelming majority, with noradrenergic followed by histaminergic neurons representing the remaining minority. These data suggest that through dual projections, cholinergic and aminergic brainstem neurons can concurrently modulate the activity of neurons in the thalamus and basal forebrain during cortical arousal.

Modulation of high-threshold Ca current and spontaneous postsynaptic transient currents by phorbol 12,13-diacetate, 1-(5-isoquinolinesulfonyl)-2-methyl piperazine (H-7), and monosialoganglioside (GM1) in CA1 pyramidal neurons of rat hippocampus in vitro

Phorbol esters, which activate protein kinase C (PKC), enhance synaptic transmission in the CA1 subfield of hippocampus, both in situ and in vitro. The increase in synaptic transmission could be the consequence of enhanced Ca influx into nerve terminals, and perhaps a more general increase in voltage-dependent Ca currents. The effects of phorbol 12,13-diacetate (PDAc) on the high-voltage activated (HVA) Ca currents, as well as spontaneous transient currents were therefore investigated by intracellular recording in hippocampal slices. PDAc selectively augmented, by 45% +/- 10%, the early peak of the HVA Ca current (but not its sustained component), and also spontaneous inhibitory postsynaptic currents. The inactive phorbol ester, 4 alpha-PDAc, had no comparable effects. The actions of PDAc were reversible on prolonged washing, and they were antagonized by the PKC inhibitors (1-(5-isoquinolinesulfonyl)-2-methyl piperazine (H-7) and monosialoganglioside (GM1). In addition, GM1, which also activates the Ca/calmodulin-dependent kinase, enhanced spontaneous excitatory postsynaptic currents, while inhibiting the IPSCs. It is concluded that activation of PKC increases HVA (probably N-type) Ca current and facilitates ongoing GABAergic IPSCs.

Trans-ACPD-induced phosphoinositide hydrolysis and modulation of hippocampal pyramidal cell excitability do not undergo parallel developmental regulation

The selective metabotropic glutamate receptor agonist, trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD), stimulates phosphoinositide hydrolysis and elicits a number of electrophysiological responses in the hippocampus. If these effects are mediated by the same receptor subtype, they should undergo parallel developmental regulation. Therefore, we compared the phosphoinositide hydrolysis response and the electrophysiological responses to trans-ACPD at two different developmental stages. Trans-ACPD-stimulated phosphoinositide hydrolysis was significantly greater in hippocampal slices from immature (6-11-day-old) rats than from adults. In contrast, trans-ACPD elicited decreases in spike frequency adaptation and in the amplitude of the slow afterhyperpolarization in roughly equal percentages of immature and adult CA1 pyramidal cells. Similar results were obtained using the putative endogenous agonist, glutamate. These data support the hypothesis that certain electrophysiological effects of trans-ACPD are mediated by a metabotropic glutamate receptor that is distinct from the phosphoinositide hydrolysis-linked glutamate receptor.

Electrophysiological properties of neurones in the region of the paraventricular nucleus in slices of rat hypothalamus

1. Neurones in the region of the hypothalamic paraventricular nucleus (PVN) of the rat were studied with intracellular recording in the coronal slice preparation. Three types of hypothalamic neurones were distinguished according to their membrane properties and anatomical positions. Lucifer Yellow or ethidium bromide was injected intracellularly to determine the morphology of some recorded cells. 2. The most distinctive electrophysiological characteristic was the low-threshold depolarizing potentials which were totally absent in type I neurones, present but variable in type II neurones and very conspicuous in type III neurones. Type II neurones generally showed relatively small low-threshold depolarizations (26.5 +/- 2.2 mV) which generated at most one to two action potentials. Type III neurones, on the other hand, generated large low-threshold potentials (40.3 +/- 2.8 mV) which gave rise to bursts of three to six fast action potentials. Deinactivation of the low-threshold conductance in both type II and type III neurones was voltage- and time-dependent. Low-threshold potentials persisted in TTX (1-3 microM) but were blocked by solutions containing low Ca2+ (0.2 mM) and Cd2+ (0.5 mM), suggesting they were Ca(2+)-dependent. 3. Type I neurones had a significantly shorter membrane time constant (14.5 +/- 1.7 ms) than those of type II (21.6 +/- 1.7 ms) and type III neurones (33.8 +/- 4.4 ms). Input resistance and resting membrane potential did not differ significantly among the cell groups. 4. Current-voltage (I-V) relations were significantly different among the three cell types. Type I neurones had linear I-V relations to -120 mV, while type III neurones all showed marked anomalous rectification. I-V relations among type II neurones were more heterogeneous, although most (75%) had linear I-V curves to about -90 to -100 mV, inward rectification appearing at more negative potentials. 5. Type I neurones generated fast action potentials of relatively large amplitude (64.2 +/- 1.1 mV, threshold to peak) and long duration (1.1 +/- 0.1 ms, measured at half-amplitude); the longer duration was due to a shoulder on the falling phase of the spike. Type II neurones had large spikes (66.5 +/- 1.6 mV) of shorter duration (0.9 +/- 0.1 ms) with no shoulder. Type III neurones had relatively small spikes (56.1 +/- 2.2 mV) of short duration (0.8 +/- 0.1 ms) with no shoulder. 6. The three cell populations showed different patterns of repetitive firing in response to depolarizing current pulses. Type I neurones often generated spike trains with a delayed onset and little spike-frequency adaptation.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiologic characteristics of basal forebrain neurons in vitro

Our data show that different cell types recorded in vitro can be identified by their intrinsic membrane properties. One type of neuron, namely S-AHP cells, have the ability to fire single action potentials in a rhythmic fashion following sufficient membrane depolarization. The rate is apparently controlled by several voltage-dependent conductances. S-AHP cells are normally quiescent at their resting potentials but will discharge once threshold is reached (-55 to -60 mV). Importantly, S-AHP (or F-AHP) cells will not convert into burst-firing neurons merely with changes in membrane potential. On the other hand, burst-firing cells have the ability to switch to a repetitive-firing pattern following membrane depolarization. All of these data provide a first step in an understanding of the firing rates of basal forebrain neurons, however, our results must be consolidated with existing in vivo studies for a more general understanding of basal forebrain function. Comparing our data to an in vivo preparation of the MS/nDB with synaptic afferents surgically removed may be one approach to correlating in vitro and in vivo studies. Vinogradova et al. (1980) used single unit recording techniques in unanesthetized chronic rabbits and compared the firing rates of cells before and after deafferentation. These authors reported a preservation of burst-firing neurons (25% of the cells) after deafferentation but with a significant reduction in the mean frequency of bursts. In addition a higher percentage of regularly firing cells also occurred following deafferentation (Vinogradova et al., 1980). It is interesting to speculate that these regularly firing cells may correspond to S-AHP cells in our in vitro studies, and some of the burst-firing units may correspond to the burst-firing cells we record in slices. Nevertheless, the in vivo data strongly suggests that endogenous regular spiking as well as rhythmic burst capabilities are present in some MS/nDB cells, however, the firing rates of most MS/nDB neurons are strongly influenced by synaptic afferents (see also Vinogradova et al., 1980; 1987). The endogenous activity in vivo can be explained, in part, by the intrinsic properties elucidated in our in vitro studies. How the synaptic afferents control MS/nDB circuitry and integrative output is premature to speculate without a more thorough understanding of the synaptic mechanisms involved. It is possible that future in vitro studies will help define these mechanisms and again contribute to an understanding of basal forebrain function.

Serotonin receptor activation reduces calcium current in an acutely dissociated adult central neuron

The release of serotonin (5-HT) from the terminals of serotonergic (raphe) neurons is under inhibitory feed-back control. 5-HT, acting on raphe cell body autoreceptors, also mediates inhibitory postsynaptic potentials as a result of release from collaterals from neighboring raphe neurons. This may involve a ligand (5-HT)-gated increase in the membrane potassium conductance, leading to a decrease in action potential frequency, which could indirectly reduce calcium influx into nerve terminals. In this report we demonstrate that 5-HT can also directly reduce calcium influx at potentials including and bracketing the peak of calcium current activation. Using acutely isolated, patch-clamped dorsal raphe neurons, we found that low concentrations of 5-HT and the 5-HT1A-selective agonist 8-OH-DPAT reversibly decrease whole-cell calcium current. This effect is antagonized by the putative 5-HT1A-selective antagonist NAN 190. Hence, the inhibition of calcium current may serve a physiological role in these cells and elsewhere in the brain.

Transient low-threshold Ca2+ current triggers burst firing through an afterdepolarizing potential in an adult mammalian neuron

In a variety of mammalian neurons, a brief depolarization generates an afterdepolarizing potential that triggers the firing of a short series or burst of action potentials. Although such burst firing is thought to contribute to the processing of neural information, the ionic currents that underlie this phenomenon have not been established. In whole-cell patch-clamp experiments on dorsal root ganglion neurons, we have found that the current that underlies this type of burst firing is a transient low-threshold (T-type) Ca2+ current. The data suggest that the T-type Ca2+ current may play an important role in the processing of information in the nervous system by virtue of its ability to elicit burst firing in neurons.

Post-natal development of burst firing behavior and the low-threshold transient calcium current examined using freshly isolated neurons from rat dorsal root ganglia

Burst firing was triggered by an afterdepolarizing potential (ADP) in whole-cell recordings from neurons recently isolated from dorsal root ganglia (DRG) of adult rats, less frequently in neurons from 12-day-old rats, and not at all in neurons from 1-day-old rats. Both the ADP and voltage-activated transient (T-type) Ca2+ current which generates the ADP were present in neurons at all postnatal ages; however, the amplitude of the ADP and of the T-type current were greatest in neurons from adults, smaller in neurons from 12-day animals, and smallest in neurons from 1-day animals. These observations suggest that the increase in amplitude of the T-type current with age leads to the generation of an ADP of increased amplitude and those properties of the ADP which develop with age may contribute to or generate burst firing behavior.

Intracellular studies in the facial nucleus illustrating a simple new method for obtaining viable motoneurons in adult rat brain slices

In general, it has been difficult to preserve electrophysiologically viable motoneurons in brain slices from adult mammals. The present study describes a new method for obtaining viable motoneurons in the facial nucleus of adult rat brain slices. The essence of the method was to use a modified artificial cerebrospinal fluid (ACSF) in which NaCl was replaced initially by sucrose; the modified ACSF was used during 1) preparation and 2) a 1 hr recovery period. The rationale for the modification is discussed in terms of the proposed acute neurotoxic effects of passive chloride entry and subsequent cell swelling and lysis. The actual recordings were made only after switching back to normal ACSF. Use of this method yielded large numbers of viable motoneurons that were suitable for intracellular recording; no motoneurons survived when normal ACSF (i.e., with NaCl) was used during slice preparation. A survey of some electrophysiological and pharmacological properties of facial motoneurons in this preparation, by means of current-clamp and voltage-clamp recording, revealed close similarities to the properties of adult motoneurons previously observed in vivo (e.g., time-dependent inward rectification, apamin-sensitive afterhyperpolarization, and serotonin-induced slow depolarization).