Electrophysiological properties of neurons contained in the locus coeruleus and mesencephalic nucleus of the trigeminal nerve in vitro

Prenatal exposure to morphine enhances excitability in locus coeruleus neurons

Opioid abuse during pregnancy may have noteworthy effects on the child's behavioral, emotional and cognitive progression. In this study, we assessed the effect of prenatal exposure to morphine on electrophysiological features of locus coeruleus (LC) noradrenergic neurons which is involved in modulating cognitive performance. Pregnant dams were randomly divided into two groups, that is a prenatal saline treated and prenatal morphine-treated group. To this end, on gestational days 11-18, either morphine or saline (twice daily, s.c.) was administered to pregnant dams. Whole-cell patch-clamp recordings were conducted on LC neurons of male offspring. The evoked firing rate, instantaneous frequency and action potentials half-width, and also input resistance of LC neurons significantly increased in the prenatal morphine group compared to the saline group. Moreover, action potentials decay slope, after hyperpolarization amplitude, rheobase current, and first spike latency were diminished in LC neurons following prenatal exposure to morphine. In addition, resting membrane potential, rise slope, and amplitude of action potentials were not changed by prenatal morphine exposure. Together, the current findings show a significant enhancement in excitability of the LC neurons following prenatal morphine exposure, which may affect the release of norepinephrine to other brain regions and/or cognitive performances of the offspring.

Circuit-Specific Early Impairment of Proprioceptive Sensory Neurons in the SOD1G93A Mouse Model for ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease in which motor neurons degenerate, resulting in muscle atrophy, paralysis, and fatality. Studies using mouse models of ALS indicate a protracted period of disease development with progressive motor neuron pathology, evident as early as embryonic and postnatal stages. Key missing information includes concomitant alterations in the sensorimotor circuit essential for normal development and function of the neuromuscular system. Leveraging unique brainstem circuitry, we show in vitro evidence for reflex circuit-specific postnatal abnormalities in the jaw proprioceptive sensory neurons in the well-studied SOD1^G93A mouse. These include impaired and arrhythmic action potential burst discharge associated with a deficit in Nav1.6 Na⁺ channels. However, the mechanoreceptive and nociceptive trigeminal ganglion neurons and the visual sensory retinal ganglion neurons were resistant to excitability changes in age-matched SOD1^G93A mice. Computational modeling of the observed disruption in sensory patterns predicted asynchronous self-sustained motor neuron discharge suggestive of imminent reflexive defects, such as muscle fasciculations in ALS. These results demonstrate a novel reflex circuit-specific proprioceptive sensory abnormality in ALS.SIGNIFICANCE STATEMENT Neurodegenerative diseases have prolonged periods of disease development and progression. Identifying early markers of vulnerability can therefore help devise better diagnostic and treatment strategies. In this study, we examined postnatal abnormalities in the electrical excitability of muscle spindle afferent proprioceptive neurons in the well-studied SOD1^G93A mouse model for neurodegenerative motor neuron disease, amyotrophic lateral sclerosis. Our findings suggest that these proprioceptive sensory neurons are exclusively afflicted early in the disease process relative to sensory neurons of other modalities. Moreover, they presented Nav1.6 Na⁺ channel deficiency, which contributed to arrhythmic burst discharge. Such sensory arrhythmia could initiate reflexive defects, such as muscle fasciculations in amyotrophic lateral sclerosis, as suggested by our computational model.

Brainstem neurons survive the identical ischemic stress that kills higher neurons: insight to the persistent vegetative state

Global ischemia caused by heart attack, pulmonary failure, near-drowning or traumatic brain injury often damages the higher brain but not the brainstem, leading to a ‘persistent vegetative state’ where the patient is awake but not aware. Approximately 30,000 U.S. patients are held captive in this condition but not a single research study has addressed how the lower brain is preferentially protected in these people. In the higher brain, ischemia elicits a profound anoxic depolarization (AD) causing neuronal dysfunction and vasoconstriction within minutes. Might brainstem nuclei generate less damaging AD and so be more resilient? Here we compared resistance to acute injury induced from simulated ischemia by ‘higher’ hippocampal and striatal neurons versus brainstem neurons in live slices from rat and mouse. Light transmittance (LT) imaging in response to 10 minutes of oxygen/glucose deprivation (OGD) revealed immediate and acutely damaging AD propagating through gray matter of neocortex, hippocampus, striatum, thalamus and cerebellar cortex. In adjacent brainstem nuclei, OGD-evoked AD caused little tissue injury. Whole-cell patch recordings from hippocampal and striatal neurons under OGD revealed sudden membrane potential loss that did not recover. In contrast brainstem neurons from locus ceruleus and mesencephalic nucleus as well as from sensory and motor nuclei only slowly depolarized and then repolarized post-OGD. Two-photon microscopy confirmed non-recoverable swelling and dendritic beading of hippocampal neurons during OGD, while mesencephalic neurons in midbrain appeared uninjured. All of the above responses were mimicked by bath exposure to 100 µM ouabain which inhibits the Na⁺/K⁺ pump or to 1–10 nM palytoxin which converts the pump into an open cationic channel.

Therefore during ischemia the Na⁺/K⁺ pump of higher neurons fails quickly and extensively compared to naturally resilient hypothalamic and brainstem neurons. The selective survival of lower brain regions that maintain vital functions will support the persistent vegetative state.

Lesioning noradrenergic neurons of the locus coeruleus in C57Bl/6 mice with unilateral 6-hydroxydopamine injection, to assess molecular, electrophysiological and biochemical changes in noradrenergic signaling

The locus coeruleus (LC) is the major loci of noradrenergic innervation to the forebrain. Due to the extensive central nervous system innervation of the LC noradrenergic system, a reduction in the number of LC neurons could result in significant changes in noradrenergic function in many forebrain regions. LC noradrenergic neurons were lesioned in adult male C57Bl/6 mice with the unilateral administration of 6-hydroxydopamine (6OHDA) (vehicle on the alternate side). Noradrenergic markers were measured 3 weeks later to determine the consequence of LC loss in the forebrain. Direct administration of 6OHDA into the LC results in the specific reduction of noradrenergic neurons in the LC (as measured by electrophysiology, immunoreactivity and in situ hybridization), the lateral tegmental neurons and dopaminergic neurons in the substantia nigra (SN) and ventral tegmental region were unaffected. The loss of LC noradrenergic neurons did not result in compensatory changes in the expression of mRNA for norepinephrine (NE)-synthesizing enzymes. The loss of LC noradrenergic neurons is associated with reduced NE tissue concentration and NE transporter (NET) binding sites in the frontal cortex and hippocampus, as well as other forebrain regions such as the amygdala and SN. Adrenoreceptor (AR) binding sites (α(1)- and α(2)-AR) were not significantly affected on the 6OHDA-treated side compared to the vehicle-treated side, although there is a reduction of AR binding sites on both the vehicle- and 6OHDA-treated side in specific forebrain regions. These studies indicate that unilateral stereotaxic injection of 6OHDA into mice reduces noradrenergic LC neurons and reduces noradrenergic innervation to many forebrain regions, including the contralateral side.

Action potential-triggered somatic exocytosis in mesencephalic trigeminal nucleus neurons in rat brain slices

The neurons in the mesencephalic trigeminal nucleus (MeV) play essential roles in proprioceptive sensation of the face and oral cavity. The somata of MeV neurons are generally assumed to carry out neuronal functions but not to play a direct role in synaptic transmission. Using whole-cell recording and membrane capacitance (C(m)) measurements, we found that the somata of MeV neurons underwent robust exocytosis (C(m) jumps) upon depolarization and with the normal firing of action potentials in brain slices. Both removing [Ca(2+)](o) and buffering [Ca(2+)](i) with BAPTA blocked this exocytosis, indicating that it was completely Ca(2+) dependent. In addition, an electron microscopic study showed synaptic-like vesicles approximated to the plasma membrane in somata. There was a single Ca(2+)-dependent releasable vesicle pool with a peak release rate of 1912 fF s(-1). Importantly, following depolarization-induced somatic exocytosis, GABA-mediated postsynaptic currents were transiently reduced by 31%, suggesting that the somatic vesicular release had a retrograde effect on afferent GABAergic transmission. These results provide strong evidence that the somata of MeV neurons undergo robust somatic secretion and may play a crucial role in bidirectional communication between somata and their synaptic inputs in the central nervous system.

Evidence for cellular injury in the midbrain of rats following chronic constriction injury of the sciatic nerve

Complex behavioural disabilities, as well as pain, characterise neuropathic pain conditions for which clinical treatment is sought. In rats, chronic constriction injury (CCI) of the sciatic nerve evokes, allodynia and hyperalgesia as well as three distinct patterns of disability, characterised by changes in social and sleep-wake behaviours: (i) Pain & Disability; (ii) Pain & Transient Disability and (iii) Pain alone. Importantly, the degree of allodynia and hyperalgesia is identical for each of these groups. Social-interactions and sleep-wake behaviours are regulated by neural networks, which converge on the periaqueductal grey (PAG). Rats with Pain & Disability show astrocyte activation restricted to the lateral and ventrolateral PAG. Reactive astrocytes are a hallmark of cell death (apoptosis and necrosis). Quantitative real-time RT-PCR for the mRNAs encoding Bax, Bcl-2, heat shock protein 60 (HSP60), mitogen activated kinase kinase (MEK2) and iNOS was performed on the dorsal midbrains of individual, disability characterised rats, extending our earlier Gene-Chip data, showing a select up-regulation of Bax and MEK2 mRNA, and a down-regulation of HSP60 mRNA, in Pain & Disability rats. The anatomical location of TUNEL and cleaved-caspase-3 immunoreactive profiles in the midbrain was also identified. Rats with Pain & Disability showed: (i) pro-apoptotic ratios of Bax:Bcl-2 mRNAs; (ii) decreased HSP60 mRNA; (iii) increased iNOS and MEK2 mRNAs; (iv) TUNEL-positive profiles in the lateral and ventrolateral PAG; and (v) caspase-3 immunoreactive neurons in the mesencephalic nucleus of the trigeminal nerve. Cell death in these specific midbrain regions may underlie the disabilities characterising this subgroup of nerve-injured rats.

Neurobiology of orofacial proprioception

Primary sensory fibers innervating the head region derive from neurons of both the trigeminal ganglion (TG) and mesencephalic trigeminal nucleus (MTN). The trigeminal primary proprioceptors have their cell bodies in the MTN. Unlike the TG cells, MTN neuronal somata are centrally located within the brainstem and receive synaptic inputs that potentially modify their output. They are a crucial component of the neural circuitry responsible for the generation and control of oromotor activities. Gaining an insight into the chemical neuroanatomy of the MTN is, therefore, of fundamental importance for the understanding of neurobiology of the head proprioceptive system. This paper summarizes the recent advances in our knowledge of pre- and postsynaptic mechanisms related to orofacial proprioceptive signaling in mammals. It first briefly describes the neuroanatomy of the MTN, which is involved in the processing of proprioceptive information from the face and oral cavity, and then focuses on its neurochemistry. In order to solve the puzzle of the chemical coding of the mammalian MTN, we review the expression of classical neurotransmitters and their receptors in mesencephalic trigeminal neurons. Furthermore, we discuss the relationship of neuropeptides and their corresponding receptors in relaying of masticatory proprioception and also refer to the interactions with other atypical neuromessengers and neurotrophic factors. In extension of previous inferences, we provide conclusive evidence that the levels of transmitters vary according to the environmental conditions thus implying the neuroplasticity of mesencephalic trigeminal neurons. Finally, we have also tried to give an integrated functional account of the MTN neurochemical profiles.

Transition between two excitabilities in mesencephalic V neurons

Neurons can make different responses to identical inputs. According to the emerging frequency of repetitive firing, neurons are classified into two types: type 1 and type 2 excitability. Though in mathematical simulations, minor modifications of parameters describing ionic currents can result in transitions between these two excitabilities, empirical evidence to support these theoretical possibilities is scarce. Here we report a joint theoretical and experimental study to test the hypothesis that changes in parameters describing ionic currents cause predictable transitions between the two excitabilities in mesencephalic V (Mes V) neurons. We developed a simple mathematical model of Mes V neurons. Using bifurcation analysis and model simulation, we then predicted that changes in conductance of two low-threshold currents would result in transitions between type 1 and type 2. Finally, by applying specific channel blockers, we observed the transition between two excitabilities forecast by the mathematical model.

Sodium currents in mesencephalic trigeminal neurons from Nav1.6 null mice

Previous studies using pharmacological methods suggest that subthreshold sodium currents are critical for rhythmical burst generation in mesencephalic trigeminal neurons (Mes V). In this study, we characterized transient (I(NaT)), persistent (I(N)(aP)), and resurgent (I(res)) sodium currents in Na(v)1.6-null mice (med mouse, Na(v)1.6(-/-)) lacking expression of the sodium channel gene Scn8a. We found that peak transient, persistent, and resurgent sodium currents from med (Na(v)1.6(-/-)) mice were reduced by 18, 39, and 76% relative to their wild-type (Na(v)1.6(+/+)) littermates, respectively. Current clamp recordings indicated that, in response to sinusoidal constant amplitude current (ZAP function), all neurons exhibited membrane resonance. However, Mes V neurons from med mice had reduced peak amplitudes in the impedance-frequency relationship (resonant Q-value) and attenuated subthreshold oscillations despite the similar passive membrane properties compared with wild-type littermates. The spike frequency-current relationship exhibited reduced instantaneous discharge frequencies and spike block at low stimulus currents and seldom showed maintained spike discharge throughout the stimulus in the majority of med neurons compared with wild-type neurons. Importantly, med neurons never exhibited maintained stimulus-induced rhythmical burst discharge unlike those of wild-type littermates. The data showed that subthreshold sodium currents are critical determinants of Mes V electrogenesis and burst generation and suggest a role for resurgent sodium currents in control of spike discharge.

Participation of sodium currents in burst generation and control of membrane excitability in mesencephalic trigeminal neurons

Subthreshold sodium currents are important in sculpting neuronal discharge and have been implicated in production and/or maintenance of subthreshold membrane oscillations and burst generation in mesencephalic trigeminal neurons (Mes V). Moreover, recent data suggest that, in some CNS neurons, resurgent sodium currents contribute to production of high-frequency burst discharge. In the present study, we sought to determine more directly the participation of these currents during Mes V electrogenesis using the action potential-clamp method. In postnatal day 8-14 rats, the whole-cell patch-clamp method was used to record sodium currents by subtraction in response to application of TTX in voltage-clamp mode using the action potential waveform as the command protocol. We found that TTX-sensitive sodium current is the main inward current flowing during the interspike interval, compared with the h-current (Ih) and calcium currents. Furthermore, in addition to the transient sodium current that flows during the upstroke of action potential, we show that resurgent sodium current flows at the peak of afterhyperpolarization and persistent sodium current flows in the middle of the interspike interval to drive high-frequency firing. Additionally, transient, resurgent, and persistent sodium current components showed voltage- and time-dependent slow inactivation, suggesting that slow inactivation of these currents can contribute to burst termination. The data suggest an important role for these components of the sodium current in Mes V neuron electrogenesis.

Contrasting phenotypes of putative proprioceptive and nociceptive trigeminal neurons innervating jaw muscle in rat

BACKGROUND

Despite the clinical significance of muscle pain, and the extensive investigation of the properties of muscle afferent fibers, there has been little study of the ion channels on sensory neurons that innervate muscle. In this study, we have fluorescently tagged sensory neurons that innervate the masseter muscle, which is unique because cell bodies for its muscle spindles are in a brainstem nucleus (mesencephalic nucleus of the 5th cranial nerve, MeV) while all its other sensory afferents are in the trigeminal ganglion (TG). We examine the hypothesis that certain molecules proposed to be used selectively by nociceptors fail to express on muscle spindles afferents but appear on other afferents from the same muscle.

RESULTS

MeV muscle afferents perfectly fit expectations of cells with a non-nociceptive sensory modality: Opiates failed to inhibit calcium channel currents (I(Ca)) in 90% of MeV neurons, although ICa were inhibited by GABA(B) receptor activation. All MeV afferents had brief (1 msec) action potentials driven solely by tetrodotoxin (TTX)-sensitive Na channels and no MeV afferent expressed either of three ion channels (TRPV1, P2X3, and ASIC3) thought to be transducers for nociceptive stimuli, although they did express other ATP and acid-sensing channels. Trigeminal masseter afferents were much more diverse. Virtually all of them expressed at least one, and often several, of the three putative nociceptive transducer channels, but the mix varied from cell to cell. Calcium currents in 80% of the neurons were measurably inhibited by mu-opioids, but the extent of inhibition varied greatly. Almost all TG masseter afferents expressed some TTX-insensitive sodium currents, but the amount compared to TTX sensitive sodium current varied, as did the duration of action potentials.

CONCLUSION

Most masseter muscle afferents that are not muscle spindle afferents express molecules that are considered characteristic of nociceptors, but these putative muscle nociceptors are molecularly diverse. This heterogeneity may reflect the mixture of metabosensitive afferents which can also signal noxious stimuli and purely nociceptive afferents characteristic of muscle.

Persistent Sodium Currents in Mesencephalic V Neurons Participate in Burst Generation and Control of Membrane Excitability

The functional and biophysical properties of a persistent sodium current ( INaP) previously proposed to participate in the generation of subthreshold oscillations and burst discharge in mesencephalic trigeminal sensory neurons (Mes V) were investigated in brain stem slices (rats, p7–p12) using whole cell patch-clamp methods. INaPactivated around −76 mV and peaked at −48 mV, with V1/2of −58.7 mV. Ramp voltage-clamp protocols showed that INaPundergoes time- as well as voltage-dependent inactivation and recovery from inactivation in the range of several seconds (τonset= 2.04 s, τrecov= 2.21 s). Riluzole (≤5 μM) substantially reduced INaP, membrane resonance, postinhibitory rebound (PIR), and subthreshold oscillations, and completely blocked bursting, but produced modest effects on the fast transient Na+current ( INaT). Before complete cessation, burst cycle duration was increased substantially, while modest and inconsistent changes in burst duration were observed. The properties of the INaTwere obtained and revealed that the amplitude and voltage dependence of the resulting “window current” were not consistent with those of the observed INaPrecorded in the same neurons. This suggests an additional mechanism for the origin of INaP. A neuronal model was constructed using Hodgkin-Huxley parameters obtained experimentally for Na+and K+currents that simulated the experimentally observed membrane resonance, subthreshold oscillations, bursting, and PIR. Alterations in the model gNaPparameters indicate that INaPis critical for control of subthreshold and suprathreshold Mes V neuron membrane excitability and burst generation.

Excitatory GABAergic synaptic potentials in the mesencephalic trigeminal nucleus of adult rat in vitro

The mesencephalic trigeminal nucleus (MesV) contains the somata of primary afferent neurons innervating masticatory muscle spindles and the periodontal membrane. MesV afferent somata are unique in receiving synaptic inputs. Intracellular recordings in coronal pontine slices from adult rats were made from MesV neurons identified by having Cs-sensitive inward rectification and pseudounipolar morphology. Stimuli near the MesV evoked either a cluster of action potentials superimposed on a postsynaptic potential (PSP) or an antidromic spike at resting membrane potential (RMP). Membrane hyperpolarization revealed that each cluster of action potentials consisted of an antidromic spike and a subsequent PSP. Evoked PSPs in slices and miniature postsynaptic currents (mPSCs) recorded using whole-cell patch in dissociated MesV neurons were resistant to glutamate antagonists and strychnine but were reversibly abolished by 40 microM bicuculline. Superfusion of 1-10 mM GABA decreased input resistance and depolarized the membrane. Reversal potentials for evoked PSPs and GABA-induced depolarizations were similar and close to that for mPSCs which matched the Cl- equilibrium potential. Thus activation of synapses on MesV somata evokes GABAergic PSPs that generate action potentials at RMP in the adult. These data also indicate that primary afferent MesV neurons can act as interneurons in the central control of mastication.

Orexins cause depolarization via nonselective cationic and K+ channels in isolated locus coeruleus neurons

The locus coeruleus (LC) contains noradrenergic neurons that are innervated by orexin (ORX)-like immunoreactive axons and express both orexin receptor-1 and -2. We studied effects of ORX-A and -B (ORX-A/B) on dissociated LC neurons by using whole-cell patch clamp techniques. In current-clamp mode, LC neurons were depolarized by application of ORX-A (10(-7) M) [53% of neurons tested; 9.0+/-0.2 mV (n=5)], or ORX-B (10(-7) M) [38% of neurons tested; 4.0+/-0.1 mV (n=5)]. Firing frequencies of action potentials increased during application [1.1+/-0.2 Hz (n=5) in ORX-A; 0.8+/-0.2 Hz (n=5) in ORX-B] and returned to the control level [0.2+/-0.1 Hz (n=5)] after removal. The ORX-A/B-induced depolarization was well maintained in the presence of TTX (3x10(-7) M), CNQX (10(-6) M) and AP5 (10(-5) M). In voltage-clamp mode, removal of external Na+ suppressed both ORX-A/B-induced currents and shifted their reversal potentials from approximately -45 mV to -60 mV. In addition, ORX-A/B inhibited sustained K+ currents. These results suggest that ORX-A/B increase the firing frequency of LC neurons through the depolarization probably produced by both augmentation of the nonselective cationic conductance and inhibition of the sustained K+ conductance.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Membrane resonance and subthreshold membrane oscillations in mesencephalic V neurons: participants in burst generation

Trigeminal mesencephalic (Mes V) neurons are critical components of the circuits controlling oral-motor activity. The possibility that they can function as interneurons necessitates a detailed understanding of the factors controlling their soma excitability. Using whole-cell patch-clamp recording, in vitro, we investigated the development of the ionic mechanisms responsible for the previously described subthreshold membrane oscillations and rhythmical burst discharge in Mes V neurons from rats ages postnatal day (P) 2-12. We found that the oscillation amplitude and frequency increased during development, whereas bursting emerged after P6. Furthermore, when bursting was initiated, the spike frequency was largely determined by the oscillation frequency. Frequency domain analysis indicated that these oscillations emerged from the voltage-dependent resonant properties of Mes V neurons. Low doses of 4-aminopyridine (<100 microm) reduced the oscillations and abolished resonance in most neurons, suggesting that the resonant current is a steady-state K(+) current (I(4-AP)). Sodium ion replacement or TTX reduced substantially the oscillations and peak amplitude of the resonance, suggesting the presence of a persistent Na(+) current (I(NaP)) that functions to amplify the resonance and facilitate the emergence of subthreshold oscillations and bursting.

Properties of native P2X receptors in rat trigeminal mesencephalic nucleus neurones: lack of correlation with known, heterologously expressed P2X receptors

Trigeminal mesencephalic nucleus (MNV) neurones express functional P2X receptors. In order to determine the molecular identity of the P2X receptors in this nucleus we have used whole cell patch clamp recording of P2X receptor-mediated currents to determine the pharmacological properties of the receptors, and have compared them with those of cloned P2X receptor subunits. The purine nucleotides ATP (300 microM), ATP-gamma-S (30 microM) and alphabetameATP (300 microM) evoked inward currents in all MNV neurones whereas alphabetameADP (300 microM) did not. betagammame-L-ATP (300 microM) evoked only a small ( approximately 20 pA) current in 3 out of 6 MNV neurones. The P2X receptor antagonist TNP-ATP (10 nM-10 microM) and raised extracellular Ca(2+) (8 and 30 mM) reduced, but did not abolish, the current evoked by ATP-gamma-S. The current remaining in TNP-ATP was insensitive to blockade by raised Ca(2+). These properties suggest that MNV neurones do not express homomeric P2X(3), P2X(4) or P2X(6) receptors. Whilst the TNP-ATP-insensitive ATP-gamma-S-evoked current has many characteristics similar to both homomeric P2X(2) and P2X(5) receptors, its insensitivity to blockade by raised Ca(2+) is difficult to reconcile with the receptor being a P2X(2) or P2X(5) homomeric channel. More likely, the receptor is a heteromer that comprises either or both of these subunits. The TNP-ATP-sensitive component of the ATP-gamma-S-evoked current is dissimilar to known cloned homomeric or heteromeric P2X receptors.

Hyperpolarization-activated cationic currents (Ih) in neurones of the trigeminal mesencephalic nucleus of the rat

1. We studied the voltage-dependent current activated by membrane hyperpolarization in sensory proprioceptive trigeminal mesencephalic nucleus (MNV) neurones. 2. Membrane hyperpolarization (from -62 to -132 mV in 10 mV steps) activated slowly activating and non-inactivating inward currents. The hyperpolarization-activated currents could be described by activation curves with a half-maximal activation potential (V ) of -93 mV, slope (k) of 8.4 mV, and maximally activated currents (Imax) of around 1 nA. The reversal potential of the hyperpolarization-activated currents was -57 mV. 3. Extracellular Cs+ blocked hyperpolarization-activated currents rapidly and reversibly in a concentration-dependent manner with an IC50 of 100 microM and Hill slope of 0.8. ZD7288 (1 microM; 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride), the compound developed as an inhibitor of the cardiac hyperpolarization-activated current (If), also blocked the hyperpolarization-activated currents in MNV neurones. Extracellular Ba2+ (1 mM) did not affect hyperpolarization-activated currents. We tested whether the hyperpolarization-activated currents contribute to the somatic membrane properties of MNV neurones by performing some experiments using current-clamp recording. In such experiments application of Cs+ (1 mM) produced no effect on neuronal resting membrane potentials. 4. During the course of our experiments we noticed that activating ATP-gated non-selective cation channels (P2X receptors) caused an inhibition of Ih associated with a V shift of 10 mV in the hyperpolarizing direction. This P2X receptor-mediated inhibition of Ih was blocked in recordings made with the rapid calcium chelator BAPTA (11 mM) in the pipette solution. 5. We conclude that the current activated by membrane hyperpolarization in MNV neurones is Ih on the basis of its similarity to Ih observed in other neuronal preparations. Activation of Ih can account for the anomalous time-dependent inward rectification that has previously been described in MNV neurones.

Membrane properties of dissociated trigeminal mesencephalic neurons of the adult rat

Electrophysiological properties of pseudounipolar trigeminal mesencephalic (Me5) neurons, dissociated from the rat brain, were studied under current-clamp conditions using the whole-cell configuration. Almost all Me5 neurons (37/38, 97%) exhibited a rapid adaptation in response to long depolarizing current pulses. Another firing type, slowly-adapting, was observed in only 3% of neurons (1/38). Most Me5 neurons (42/43) generated an overshooting action potential without a hump on the falling phase, and the remaining neuron (1/43) showed an action potential with a small hump. The action potential of Me5 neurons was reversibly blocked by 1 microM tetrodotoxin (TTX) or by removing Na+ from the bathing medium. When the outward K+ current was suppressed, two types of Ca2+ spikes were revealed. According to characteristic thresholds and sensitivity to inorganic (Ni2+, Cd2+) and organic (nifedipine, omega-conotoxin GVIA) Ca2+ channel blockers, these Ca2+ spikes were identified as T-type LTS (low-threshold spike) and L-type HTS (high-threshold spike). Also, a time-dependent inward rectification was observed in all Me5 neurons. It is concluded that the majority of Me5 neurons are of the rapidly-adapting type and generate a TTX-sensitive Na+ spike with negligible contribution of Ca2+, showing that the electrophysiological properties of Me5 neurons are more similar to those of CNS neurons than to those of PNS ganglion cells which have similar morphological features to Me5 neurons.

Dopamine-immunoreactivity in the rat mesencephalic trigeminal nucleus: an ultrastructural analysis

The ultrastructure and distribution of dopaminergic boutons within the rat mesencephalic trigeminal (Me5) nucleus was examined with the use of electronmicroscopic immunocytochemistry. A total of 5102 boutons, comprising axosomatic and axodendritic synaptic terminals as well as non-synaptic boutons (or varicosities), located in the ventrocaudal portion of Me5 was analysed. Approximately 20% of these boutons were dopamine-immunoreactive. Morphological analysis showed that the dopaminergic synaptic terminals, axodendritic as well as axosomatic, were exclusively of the S- and G-bouton type; they contained, respectively, small spherical vesicles or small pleomorphic vesicles in combination with large granular dense-cored vesicles. All dopaminergic varicosities in the Me5 were of the G-bouton type. Quantitative analysis revealed that most of the dopaminergic synaptic terminals in the Me5 nucleus contacted dendrites, while only a minority (12%) contacted Me5 somata. This dopaminergic somatic input comprised about half (52%) of the total axosomatic input on Me5 neurons. The present results and previous findings with respect to the prominent serotonergic component of the axosomatic input to Me5 neurons indicate that dopamine and serotonin account for most of the axosomatic input in the ventrocaudal part of the Me5 nucleus. In fact, the present results seem to support previous observations regarding the existence of a population of afferent neurons in which dopamine and serotonin are colocalized.

ATP-gated cation channels (P2X purinoceptors) in trigeminal mesencephalic nucleus neurons of the rat

1. We have investigated whether receptors for ATP exist on neurones of the trigeminal mesencephalic nucleus (MNV) of the rat using whole-cell and outside-out patch-clamp recording in coronal brainstem slices. 2. With whole-cell recording, the batch application of ATP, adenosine 5'-O-[3-thiotriphosphate] (ATP gamma S) and alpha,beta methylene adenosine triphosphate (alpha beta meATP) caused concentration-dependent inward currents in all cells tested (holding potential, -62 mV), with EC50 values of 437, 15 and 254 microM, respectively. All three agonist-evoked currents developed rapidly (rise time, approximately 10-25 s), desensitized slowly (over approximately 20-50 s), cross-desensitized with each other, were associated with an increase in membrane conductance and were attenuated by the application of suramin (30 microM). 3. The inward current evoked by ATP decreased as the membrane potential was made less negative and had a zero current potential of +1.0 +/- 3.7 mV. The current-voltage relationship showed marked inward rectification. 4. Brief flickery single-channel openings could be resolved in response to ATP (3 microM) in outside-out membrane patches. Unitary current at -82 mV was -1.81 +/- 0.2 pA, which corresponds to a unitary conductance of 22 pS. 5. We conclude that proprioceptive MNV neurones contain ATP-gated cation channels. Such P2X purinoceptors may be involved in the processing of proprioceptive information, thus suggesting a potentially important physiological role of ATP.

Electrophysiological, behavioural and biochemical evidence for activation of brain noradrenergic systems following neurokinin NK3 receptor stimulation

The objective of the present in vitro and in vivo experiments was to examine the involvement of neurokinin NK3 receptors in the regulation of the noradrenergic function in gerbils and guinea-pigs. Application of senktide, a peptide NK3 receptor agonist, on guinea-pig locus coeruleus slices increased the firing rate of presumed noradrenergic neurons (EC50 = 26 nM) in a concentration-dependent manner. Given i.c.v., senktide (0.5-2 micrograms) and (MePhe7)neurokinin B (1-10 micrograms), another NK3 receptor agonist, reduced exploratory behaviour in gerbils in a dose-dependent manner (2 micrograms of senktide producing a 50% reduction of locomotor activity and rearing). In vivo microdialysis experiments in urethane-anaesthetized guinea-pigs showed that senktide (2-8 micrograms i.c.v.) induced a dose-dependent increase in norepinephrine release in the medial prefrontal cortex. The electrophysiological, behavioural and biochemical changes elicited by senktide were concentration- or dose-dependently reduce by SR 142801, the selective non-peptide NK3 receptor antagonist. In the locus coeruleus slice preparation, complete antagonism of senktide (30 nM) was observed with 50 nM of SR 142801, while injected i.p. (0.1-1 mg/kg) it abolished the senktide-induced norepinephrine release in guinea-pigs. In gerbils, SR 142801 (1-10 mg/kg i.p.) reversed the reduction of exploratory behaviour induced by senktide (1 microgram). By contrast, the 100-fold less active enantiomer, SR 142806, did not exert any antagonism in these models. Finally, the reduction of exploratory behaviour in gerbils was found to be reversed by prazosin (0.25-2.56 micrograms/kg i.p.) and to some extent by clonidine, drugs known to depress noradrenergic function. All these experiments strongly support the hypothesis that brain noradrenergic neurons can be activated by stimulation of neurokinin NK3 receptors.

Differences in the effects of tachykinin NK1 receptor antagonists: neuronal versus smooth muscle tissues

The effects of three tachykinin NK1 receptor antagonists (L-668,169, (+/-)-RP 67580, and (+/-)-CP 96.345) were examined for their ability to antagonise responses evoked by substance P O-methyl ester (a selective NK1 receptor agonist) in isolated neuronal tissue (rat superior cervical ganglia and guinea-pig locus coeruleus) and smooth muscle tissues (rat urinary bladder and guinea-pig ileum longitudinal muscle/myenteric plexus). (+/-)-RP 67580 was similarly effective in antagonising responses in both rat superior cervical ganglia and urinary bladder (estimated pKa value = 7.4 for both tissues); however, (+/-)-CP 96,345 was 50-fold less effective in antagonising responses in guinea-pig locus coeruleus than in ileum longitudinal muscle/myenteric plexus (estimated pKa values = 7.6 and 9.3 respectively). It is suggested that the differential effects of (+/-)-CP 96,345 may reflect the existence of a population of NK1 receptors within guinea-pig locus coeruleus that are less sensitive to the effects of this NK1 receptor antagonist.

Effects of the central analgesic tramadol and its main metabolite, O-desmethyltramadol, on rat locus coeruleus neurones

1. Tramadol is a centrally acting analgesic with low opioid receptor affinity and, therefore, presumably additional mechanisms of analgesic action. Tramadol and its main metabolite O-desmethyltramadol were tested on rat central noradrenergic neurones of the nucleus locus coeruleus (LC), which are involved in the modulation of nociceptive afferent stimuli. 2. In pontine slices of the rat brain the spontaneous discharge of action potentials of LC cells was recorded extracellularly. (-)-Tramadol (0.1-100 microM), (+)-tramadol (0.1-100 microM), (-)-O-desmethyl-tramadol (0.1-100 microM) and (+)-O-desmethyltramadol (0.01-1 microM) inhibited the firing rate in a concentration-dependent manner. (+)-O-desmethyltramadol had the highest potency, while all other agonists were active at a similar range of concentrations. 3. (-)-Tramadol (10, 100 microM) was less inhibitory in brain slices of rats pretreated with reserpine (5 mg kg-1, 5 h before decapitation) than in controls. 4. The effect of (-)-tramadol (10 microM) was abolished in the presence of the alpha 2-adrenoceptor antagonist, rauwolscine (1 microM), whilst that of (+)-O-desmethyltramadol (0.3 microM) virtually disappeared in the presence of the opioid antagonist, naloxone (0.1 microM). (+)-Tramadol (30 microM) and (-)-O-desmethyl-tramadol (10 microM) became inactive only in the combined presence of naloxone (0.1 microM) and rauwolscine (1 microM). 5. In another series of experiments, the membrane potential of LC neurones was determined with intracellular microelectrodes. (-)-Tramadol (100 microM) inhibited the spontaneous firing and hyper-polarized the cells; this effect was abolished by rauwolscine (1 microM). (+)-O-desmethyltramadol (10 microM)had a similar but somewhat larger effect on the membrane potential than (-)-tramadol. The (+)-O-desmethyltramadol-(10 microM) induced hyperpolarization was abolished by naloxone (0.1 microM).6. The hyperpolarizing effect of noradrenaline (30 microM) was potentiated in the presence of (-)-tramadol(100 microM), but not in the presence of (+)-O-desmethyltramadol (10 microM). There was no potentiation of the noradrenaline (30 microM) effect, when the cells were hyperpolarized by current injection to an extent similar to that produced by (-)-tramadol (100 microM).7. Both noradrenaline (100 microM) and (- )-tramadol (100 microM) decreased the input resistance.8. The results confirm that the analgesic action of tramadol involves both opioid and non-opioid components. It appears that (-)-tramadol inhibits the uptake of noradrenaline and via a subsequent increase in the concentration of endogenous noradrenaline indirectly stimulates alpha2-adrenoceptors. (+)-0-desmethyltramadol seems to stimulate directly opioid micro-receptors. The effects of (+)-tramadol and(-)-O-desmethyltramadol consist of combined micro-opioid and alpha2-adrenergic components.

Effects of potassium channel openers and their antagonists on rat locus coeruleus neurones

1. Intracellular recordings were obtained from a pontine slice preparation of the rat brain containing the locus coeruleus (LC). Two openers of ATP-sensitive potassium (K(ATP)) channels, RO 31-6930 (10 microM) and cromakalim (100 microM) decreased the spontaneous discharge of action potentials without altering their amplitude or duration. Neither compound changed the resting membrane potential. 2. Of two K(ATP) channel blockers, tolbutamide (300 microM) increased the firing rate, while glibenclamide (3 microM) only tended to do so. In addition, both compounds antagonized the effect of RO 31-6930 (10 microM). Neither glibenclamide (3 microM) nor tolbutamide (300 microM) altered the resting membrane potential. 3. Tetrodotoxin (0.5 microM) depressed the firing, but did not influence the inhibitory action of RO 31-6930 (10 microM). The excitatory amino acid antagonist, kynurenic acid (500 microM), did not change the spontaneous discharge of action potentials. 4. Small shifts (2-4 mV) of the membrane potential by hyper- or depolarizing current injections markedly decreased and increased the firing rate, respectively. 5. Noradrenaline (100 microM) hyperpolarized the cells and decreased their input resistance. This effect was not antagonized by glibenclamide (3 microM) or tolbutamide (300 microM). Ba2+ (2 mM), a blocker of both ATP-sensitive and inwardly rectifying potassium channels, abolished the effects of RO 31-6930 (10 microM) and noradrenaline (100 microM). 6. These data suggest that K(ATP) channels are present on the noradrenergic LC neurones, but are not coupled to alpha 2-adrenoceptors.

An immunocytochemical and autoradiographic investigation of the serotoninergic innervation of trigeminal mesencephalic and motor nuclei in the rabbit

The results of a previous experiment suggest that the cell bodies of many jaw closing muscle spindle afferents in the trigeminal mesencephalic nucleus of the rabbit are phasically inhibited during fictive mastication. The aim of this study was to investigate one possible neurotransmitter system that could be involved in this modulation, serotonin, by use of receptor autoradiography techniques and immunofluorescence combined with retrograde labelling of masseteric spindle afferents and motoneurons. A second objective was to compare the serotonin innervation of neurons in the trigeminal mesencephalic nucleus with that of masseteric motoneurons. Serotoninergic fibres were seen surrounding labelled masseteric spindle afferents, as well as unlabelled neurons, in the trigeminal mesencephalic nucleus. These fibres were close to the cell bodies and sometimes to the axon hillocks of the neurons. Although it has been reported that many neurons of the trigeminal nucleus are multipolar in some species, none of the labelled spindle afferent in this study had more than one process. Throughout the motor trigeminal nucleus, serotonin fibres were found in close proximity with cell bodies and with the proximal portions of axons and dendrites of labelled and unlabelled motoneurons. Serotonin fibres were also seen adjacent to cell bodies and processes of efferent neurons in cell group k. Autoradiography with several tritiated ligands was used to reveal the presence of receptors for serotonin as well as its uptake sites. Only serotonin2 receptors were found to be abundant in the trigeminal mesencephalic nucleus. The motor nucleus and cell group k contained serotonin2 and serotonin3 receptors, as well as serotonin uptake sites. Serotonin1A receptors appear to be absent from both nuclei. The findings suggest that release of serotonin from fibres in close proximity to trigeminal primary afferent somata could modify the transmission of action potentials from muscle spindle receptors during mastication through an action on serotonin2 receptors. In the motor nucleus and cell group k, serotonin may alter neuronal properties through actions on at least two receptor subtypes (serotonin2 and serotonin3).

Central connections of trigeminal primary afferent neurons: topographical and functional considerations

This article reviews literature relating to the central projection of primary afferent neurons of the trigeminal nerve. After a brief description of the major nuclei associated with the trigeminal nerve, the presentation reviews several early issues related to theories of trigeminal organization including modality and somatotopic representation. Recent studies directed toward further definition of central projection patterns of single nerve branches or nerves supplying specific oral and facial tissues are considered together with data from intraaxonal and intracellular studies that define the projection patterns of single fibers. A presentation of recent immunocytochemical data related to primary afferent fibers is described. Finally, several insights that recent studies shed on early theories of trigeminal input are assessed.

Depolarization of rat locus coeruleus neurons by adenosine 5'-triphosphate

Intracellular recordings were performed in a pontine slice preparation of the rat brain containing the locus coeruleus. The enzymatically stable P2-purinoceptor agonist alpha,beta-methylene ATP increased the firing rate without altering the amplitude or shape of action potentials; the afterhyperpolarization following a spike was not changed either. When locus coeruleus neurons were hyperpolarized by current injection in order to prevent spontaneous firing, alpha,beta-methylene ATP produced depolarization and a slight increase in the apparent input resistance. A combined application of kynurenic acid and bicuculline methiodide failed to alter the alpha,beta-methylene ATP-induced depolarization, and tetrodotoxin only slightly depressed it. A gradual shift of the membrane potential by hyperpolarizing current injection led to a corresponding decrease, but no abolition or reversal of the alpha,beta-methylene ATP effect. In the hyperpolarized region, the current-voltage curve of alpha,beta-methylene ATP came into close approximation with, but did not cross, the control curve. Elevation of the external K+ concentration, or the intracellular application of Cs+ by diffusion from the microelectrode, depressed the response to alpha,beta-methylene ATP; external tetraethylammonium was also inhibitory. External Ba2+ and Cs+ had no effect or only slightly decreased the alpha,beta-methylene ATP-induced depolarization. A low Na+, or a low Ca2+ high Mg2+ medium, as well as the presence of Co2+ in the medium, markedly reduced or even abolished the depolarization by alpha,beta-methylene ATP. ATP itself did not produce consistent changes in the membrane potential or input resistance. However, in the presence of the P1-purinoceptor antagonist 8-cyclopentyl-1,3-dipropylxanthine, ATP consistently increased the firing rate and evoked an inward current. In conclusion, P2-purinoceptor activation appears to depolarize locus coeruleus neurons by inhibiting a persistent potassium current, and at the same time opening calcium-sensitive sodium channels or calcium-sensitive non-selective cationic channels.

Blockade of alpha 2-adrenoceptors increases opioid mu-receptor-mediated inhibition of the firing rate of rat locus coeruleus neurones

In pontine slices of the rat brain, the frequency of spontaneous action potentials of locus coeruleus (LC) neurones was recorded extracellularly. Noradrenaline 0.1-100 mumol/l, UK 14,304 0.01-100 nmol/l, [Met5]-enkephalin 1-10,000 nmol/l and [D-Ala2,D-Leu5]enkephalin 0.1-1,000 nmol/l, all depressed the firing rate. Rauwolscine 1 mumol/l antagonized the effects of both noradrenaline and UK 14,304, but potentiated the effects of [Met5]enkephalin and [D-Ala2, D-Leu5]enkephalin. Idazoxan 1 mumol/l acted in a similar manner. Prazosin 1 mumol/l did not change the effects of either noradrenaline or [Met5]enkephalin. Naloxone 0.1 mumol/l antagonized both [Met5]enkephalin and [D-Ala2, D-Leu5]enkephalin, but failed to alter the effects of either noradrenaline or UK 14,304. Rauwolscine, idazoxan and prazosin, all 1 mumol/l, as well as naloxone 0.1 mumol/l, did not influence the firing rate when given alone. Desipramine 1 mumol/l inhibited the discharge of action potentials in a rauwolscine-antagonizable manner. Noradrenaline 10 mumol/l produced the same depression of firing, both in the presence of noradrenaline 1 mumol/l and [Met5]enkephalin 0.03 mumol/l. Likewise, the effect of [Met5]enkephalin 0.3 mumol/l was the same, irrespective of whether it was added to a medium containing [Met5]enkephalin 0.03 mumol/l or noradrenaline 1 mumol/l. The spontaneous activity of LC neurones is inhibited by somatic alpha 2-adrenoceptors and opioid mu-receptors. We suggest that the two receptors interact with each other at a site located between themselves and not in the subsequent common signal transduction system.

Inhibitory adenosine A1-receptors on rat locus coeruleus neurones. An intracellular electrophysiological study

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

Neurotransmitters and neuropeptides within the mesencephalic trigeminal nucleus of the rat: an immunohistochemical analysis

In order to determine which neurotransmitters and neuropeptides are utilized by the neurons of the mesencephalic trigeminal nucleus and by the fibres making synaptic contact with these primary sensory cells, we have set up an immunohistochemical study using antibodies against 17 major neurotransmitters and neuropeptides in the rat. Apart from some intracellular immunostaining for glutamate, no immunoreactivity to any of the tested neurotransmitters and neuropeptides could be detected inside mesencephalic nucleus of the trigeminal nerve neurons. Our immunohistochemical observations indicate that mesencephalic nucleus of the trigeminal nerve neurons receive input from various nerve fibres that appear to utilize serotonin, GABA, dopamine, noradrenaline (and likely glutamate) as transmitters. The innervation appeared randomly distributed over all mesencephalic nucleus of the trigeminal nerve neurons. The presence of substance P, cholecystokinin, vasoactive intestinal polypeptide, bombesin/gastrin releasing peptide, [Leu]enkephalin and neuropeptide Y observed in some fibres that contact with mesencephalic nucleus of the trigeminal nerve neurons, presumably reflect the co-existence of these peptides with one of the neurotransmitters.

Electrophysiological properties of hypoglossal motoneurons of guinea-pigs studied in vitro

Intracellular recordings were made from the hypoglossal nuclear complex in brain slices from guinea-pigs. Retrograde transport of horseradish peroxidase from the tongue confirmed the identity of the visually identified hypoglossal nucleus. Eighteen neurons were stained by intracellular electrophoresis of Lucifer Yellow through the recording pipette. Two types of neurons were encountered, motoneurons with maximal discharge rates of 90 Hz and another type with maximal discharge rates of 250 Hz. Motoneurons were prevalent in the hypoglossal nucleus and the other type prevailed in the adjoining nucleus prepositus hypoglossi. In both nuclei the two types were mixed. Antidromic spikes elicited from hypoglossal root fibres had initial segment and somatodendritic components. Electrical stimulation of the reticular matter dorsolateral to the hypoglossal nucleus elicited excitatory postsynaptic potentials and strychnine sensitive inhibitory postsynaptic potentials. Motoneurons responded to depolarizing current pulses with a train of spikes. The initial spike interval was much shorter than the rest and fast adaptation occurred over three to four intervals. Slow adaptation was most prominent when the neuron was depolarized and discharged at a high rate. High threshold calcium spikes were evoked by depolarizing pulses when sodium spikes were blocked by tetrodotoxin and the potassium conductance reduced by tetraethylammonium bromide. Motoneurons discharged in a single range, inflections on the frequency-current plot being absent. Spikes and spike trains evoked by depolarizing pulses were followed by afterhyperpolarizations with fast and slow parts. The fast phase was eliminated by tetraethylammonium bromide, possibly because the delayed rectifier was involved. A calcium dependent potassium conductance was probably involved in the slow phase, because it was sensitive to inorganic calcium blockers. The amplitude of the afterhyperpolarization following trains of spikes depended on the frequency of the preceding spikes. At constant frequency, the amplitude depended, in addition, on the strength of stimuli arising from different hyperpolarized potentials. Afterdepolarizing potentials were absent. Lissajous plots of double ramp current stimulation showed anomalous rectification between resting potential and spike threshold. The rectification was sensitive to inorganic calcium blockers. Subthreshold responses showed initial sags and rebound responses in all healthy cells and these were eliminated by caesium. Barium, substituted for calcium, unleashed a depolarizing plateau potential sensitive to tetrodotoxin, indicating the presence of a persistent sodium conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of purinoceptor agonists on electrophysiological properties of rat mesencephalic trigeminal neurones in vitro

Intracellular recordings were performed in a midpontine slice preparation of the rat brain containing the mesencephalic trigeminal nucleus (MTN). In spite of the previous demonstration of an adenosine deaminase-containing plexus terminating on this nucleus, adenosine, adenosine 5'-triphosphate (ATP), alpha,beta-methylene ATP (alpha,beta-meATP) and 2-methylthio ATP all failed to influence the membrane potential or input resistance of the MTN cells. Moreover, there was no apparent change in the shape of action potentials in the presence of these drugs, and the accomodation of the firing rate to depolarizing pulses was not affected either.

Electrophysiological responses of trigeminal root ganglion neurons in vitro

The membrane electrical properties of neurons and their responses to endogenous compounds or other neuroactive substances were investigated in vitro with intracellular recording techniques in slices of trigeminal root ganglia of guinea-pigs. The mean resting membrane potential of these neurons was -60 mV. Intracellular injections of hyperpolarizing current pulses evoked time-dependent rectification with varying degrees of dependence on membrane voltage in 107 of 110 neurons. Membrane potential oscillations were observed following the termination of the hyperpolarizing pulses and after similar injections of depolarizing current. This phenomenon appeared to be voltage-dependent at levels that were subthreshold for spike genesis; the more pronounced oscillations were evident at the more depolarized levels and were insensitive to tetrodotoxin applications. Two groups of neurons could be distinguished on the basis of certain characteristics in their action potentials. The majority exhibited short duration (0.6 ms) spikes with mean amplitude of 72 mV in response to intracellular depolarizing current. The brief (3 ms) afterhyperpolarizations that followed such spikes were blocked by intracellular injections of Cs+ or by bath applications of tetraethylammonium. Action potentials in the minority group exhibited a hump in their repolarization phase. The humped spikes had a mean peak amplitude of 78 mV and a longer duration (2 ms). Both the duration (6 ms) and the amplitude (16 mV) of the afterhyperpolarization were significantly greater in this latter group of neurons. Some fast spikes were easily blocked whereas others, including humped spikes, were resistant to tetrodotoxin (10(-6) M). Spikes which were resistant, were also not affected by perfusion with Co2+ (10(-3) M) and were reduced in amplitude during perfusion with Na+-deficient solution. Bath applications of S-glutamate (10(-4)-10(-2) M) depolarized only two of ten neurons by less than 3 mV. Similarly, 5-hydroxytryptamine produced a small depolarization in only two of thirteen neurons. Perfusion of gamma-aminobutyrate (10(-5)-10(-2) M) resulted in an increase in input conductance that waned despite continued application and was associated with a depolarization (2-14 mV) in 44/50 neurons. In some neurons, gamma-aminobutyrate application enhanced their repetitive firing ability, possibly as a result of the increased oscillatory behavior of the membrane at certain depolarized potentials. The effects of gamma-aminobutyrate were blocked by the GABAA-receptor antagonist, bicuculline (10(-4) M) but were unaffected by the GABAB-receptor agonist, baclofen (10(-4) M).(ABSTRACT TRUNCATED AT 400 WORDS)

Kappa opioid receptor activation depresses excitatory synaptic input to rat locus coeruleus neurons in vitro

Intracellular recordings were made from neurons of the rat locus coeruleus contained within a brain slice maintained in vitro. When applied to the slice in known concentrations the selective kappa opioid receptor agonist trans-(+)-3,4-dichloro-N-methyl-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide methane sulphonate (U50488) (0.01-1 microM) produced a concentration-dependent depression of the excitatory post-synaptic potential evoked by electrical stimulation of afferent inputs to the locus coeruleus. This effect was antagonized by naloxone with an apparent dissociation equilibrium constant (Kd) of 28 nM. U50488 did not completely abolish the EPSP. Over the same concentration range U50488 had no effect on the resting membrane potential, input resistance or action potential waveform of locus coeruleus neurons, nor did U50488 depress the depolarization produced by local application of L-glutamic acid. The mu opioid receptor agonists [D-Ala2, NMe Phe4, Gly-ol5] enkephalin (0.003-1 microM) and [D-Ala2, NMe Phe4, Met(O)5] enkephalinol (0.003-1 microM) caused a membrane hyperpolarization concomitant with a fall in neuronal input resistance. These effects were concentration-dependent and antagonized by naloxone with an apparent Kd of 1.5 nM. Mu agonists also caused a depression of the tetrodotoxin resistant action potential. An in vitro autoradiographic study of [3H]bremazocine binding within the locus coeruleus revealed that, although the majority of binding appears to be to mu sites, a significant proportion was displaceable by unlabelled U50488 and thus represented kappa binding sites. The possibility that kappa opioid receptors may be located pre-synaptically within the locus coeruleus, and that activation of these receptors depresses excitatory synaptic input, is discussed.

On the innervation of trigeminal mesencephalic primary afferent neurons by adenosine deaminase-containing projections from the hypothalamus in the rat

The localization and sources of adenosine deaminase-containing structures in the mesencephalic nucleus of the trigeminal nerve of the rat was studied using indirect immunofluorescence or immunoperoxidase immunohistochemical staining techniques for adenosine deaminase in combination with retrograde fluorescent tracing or lesion methods. The majority of large mesencephalic neurons were engulfed by a dense adenosine deaminase-immunoreactive plexus. Immunostaining was often punctate surrounding neuronal profiles or sometimes had the appearance of varicose fibers coursing over the neuronal surface. Occasionally, immunostained axons were found travelling towards and contacting mesencephalic neurons. Mesencephalic neuronal somas surrounded by immunofluorescence staining for adenosine deaminase were simultaneously labelled with fast blue after injections of this dye into the temporalis or masseter muscles of mastication. Injections of fast blue into the mesencephalic nucleus resulted in fast blue labelling of adenosine deaminase-immunoreactive neurons in the tuberal, caudal and postmammillary caudal magnocellular nuclei of the hypothalamus. Ablation of these hypothalamic nuclei caused a near total depletion of adenosine deaminase-immunostained fibers in the mesencephalic nucleus including those associated with mesencephalic neurons. It is concluded that adenosine deaminase-containing neurons in the posterior hypothalamus innervate mesencephalic primary sensory neurons, which are known to convey proprioceptive input to trigeminal motor nuclei controlling jaw muscles. The possibility is considered that the hypothalamus, via a direct action on these sensory neurons, may exert automatic control over jaw movements related to aggressive attack, defensive or feeding behavior. In addition, it appears that mesencephalic neurons may provide an ideal model system for electrophysiological investigations of the neurotransmitter(s) utilized by adenosine deaminase-containing hypothalamic projections.

Acetylcholine acts on m2-muscarinic receptors to excite rat locus coeruleus neurones

Intracellular recordings were made from neurones of the rat locus coeruleus in vitro. Acetylcholine increased the firing rate by depolarizing the membrane; both muscarinic and nicotinic antagonists partially reduced this effect. In hexamethonium, pirenzepine shifted the acetylcholine dose-response curve to the right with an equilibrium dissociation constant of 200 nM, indicating an interaction with a m2-type of muscarinic receptor.

Basal firing rate of rat locus coeruleus neurons affects sensitivity to ethanol

Intracellular recordings were made from spontaneously active rat locus coeruleus (LC) neurons in a totally submerged brain slice preparation. Bath application of ethanol (ETOH) (1-60 mM) inhibited the spontaneous firing of LC neurons. These ETOH concentrations are equal to or below ETOH concentrations found in the brain during mild to moderate intoxication. The basal frequency of spontaneous firing of LC neurons ranged from 0.4-7 Hz. For 9 LC neurons which showed complete block of firing by ETOH, the latency to block was found to be directly related to the logarithm of the firing rate (correlation coefficient 0.94). This relationship was not secondary to a relationship between membrane potential and latency to block since for the same 9 neurons, membrane potential and latency to block were not significantly correlated. We conclude that the basal firing rate of a neuron can affect its sensitivity to the inhibitory effects of ETOH.

Membrane properties of rat locus coeruleus neurones

Intracellular recordings were made from neurones in the locus coeruleus contained within a slice cut from rat pons and maintained in vitro. Most neurones fired action potentials spontaneously at frequencies of between 1 and 5 Hz; this did not arise from spontaneous synaptic input but appeared to result from endogenous properties of the membrane conductances. Under voltage clamp at potentials near threshold for action potential generation (-55 mV) there was a persistent inward calcium current. This current became less with membrane hyperpolarization and was abolished at about -70 mV. Two potassium currents were observed. The first had properties similar to that generally described as the "fast" potassium current (IK,A); it flowed transiently (for about 200 ms) when the membrane potential passed from about -65 to -45 mV, and was blocked by 4-aminopyridine. The second was a calcium-activated potassium current (IK,Ca); it flowed for several seconds following a burst of calcium action potentials. Spontaneous and evoked action potentials had both tetrodotoxin-sensitive and tetrodotoxin-resistant components. The latter was apparently due to calcium entry. The potential changes occurring during the spontaneous firing of locus coeruleus neurones could be reconstructed qualitatively from the ionic conductances observed. The membrane properties of the locus coeruleus neurones were remarkably uniform; however, about 5% of cells impaled within the region of the locus coeruleus were electrophysiologically distinct. These atypical cells had short duration action potentials, did not fire spontaneously and had large spontaneous depolarizing synaptic potentials.

Hyperpolarizing and age-dependent depolarizing responses of cultured locus coeruleus neurons to noradrenaline

The electrical activity and responses to noradrenaline (NA) of locus coeruleus (LC) neurons have been studied in organotypic cultures using intracellular recording. Most LC neurons were predominantly quiescent, though occasional bursts of activity were observed; a few cells were tonically active at rates of 0.5-5/s. In most cells tested, iontophoretic application of NA evoked responses which were initially hyperpolarizing, sometimes followed by a depolarizing phase and frequently followed by a period of increased excitatory synaptic activity. The enhanced synaptic activity appeared to be an indirect effect since it was blocked by bath application of tetrodotoxin (TTX). In the presence of TTX, responses to NA of all but one cell were simple hyperpolarizations or biphasic (hyperpolarization/depolarization) responses. The presence of the depolarizing component appeared to be age-dependent, since it was frequently observed in cultures grown in vitro for less than 26 days, while neurons in older cultures exhibited only hyperpolarizing responses. If such age-dependent depolarizing responses are present in vivo, they would represent a unique example of a transmitter response which is present only during a transient developmental phase.

Noradrenaline-mediated synaptic inhibition in rat locus coeruleus neurones

Intracellular recordings were made from neurones in the nucleus locus coeruleus (l.c.) in slices of rat pons maintained in vitro. Focal electrical stimulation to the slice surface within the region of the l.c. evoked a synaptic depolarization followed by a hyperpolarization. These potentials were graded with stimulus intensity and were abolished in calcium-free and/or high-magnesium solutions. The nature of the hyperpolarizing synaptic potential (i.p.s.p.) was investigated. The i.p.s.p. amplitude decreased as the membrane was artificially made more negative and reversed at -114 mV. This reversal potential shifted to less negative potentials in solutions of elevated potassium ion content as predicted by the Nernst equation. The i.p.s.p. was potentiated in amplitude and its time course was prolonged by desmethylimipramine (DMI). Yohimbine (100 nM) and phentolamine (100 nM) reversibly abolished the i.p.s.p. and did not change the synaptic depolarization. Noradrenaline hyperpolarized all l.c. neurones tested, whether applied by perfusion (1-30 microM) or by pressure ejection from a micropipette placed in the solution near the recording site. The noradrenaline-induced hyperpolarization was accompanied by an increase in conductance and it reversed in polarity at -104 mV. The reversal potential of the noradrenaline hyperpolarization became less negative when the potassium ion content was increased. The noradrenaline-induced hyperpolarization was potentiated by DMI and was antagonized by yohimbine and phentolamine in the same concentrations which blocked the i.p.s.p. The results support the notion that l.c. neurones can release noradrenaline onto the somadendritic membrane of other l.c. neurones and thereby provide local feed-back inhibition.

Substance P excitation of rat locus coeruleus neurones

We have made extracellular recordings from locus coeruleus neurones contained in a pontine slice preparation. The in vitro technique permits application of known concentrations of drugs in the perfusion medium. Substance P excites locus coeruleus neurones in a concentration-dependent manner. The activity of substance P was found to reside in the C terminal region of the molecule. The relative potency of substance P and related peptides suggests that the substance P receptors present within the locus coeruleus are predominately of the substance P-P type. The synthetic analogue [D-Pro2,D-Trp7.9]substance P was not found to antagonise the action of substance P on locus coeruleus neurones.

Opiate activation of potassium conductance inhibits calcium action potentials in rat locus coeruleus neurones

Opiates act on mu-receptors to increase the potassium conductance of rat locus coeruleus neurones. Opiates also depress the rate of rise and peak amplitude of calcium action potentials in these cells. The action of opiates on calcium action potentials was prevented by two procedures which blocked the opiate-induced potassium current, intracellular caesium and extracellular barium. This indicates that the opiate reduction in calcium entry is secondary to an increased potassium current.