Noradrenaline modifies the spontaneous spiking activity of red nucleus neurons in the rat by activation of alpha 2- and beta-adrenoceptors

We have investigated the effects of noradrenaline (NA) on the spontaneous firing activity of red nucleus (RN) neurons recorded extracellularly in anesthetized rats by using an in vivo electrophysiological technique. Microiontophoretic applications of NA (5-100 nA for 30 s) modified the background firing rate in 99 out of 124 neurons and three different patterns of response were observed in distinct cells. In 61% of the responding neurons NA decreased the mean firing rate, whereas 22% of the neurons responded to NA application with an increase of their spiking activity; in a smaller group of cells (17%) NA exerted a biphasic inhibitory/excitatory effect on the spontaneous firing rate. The effects of NA were reversible and dose-dependent. From histological examination, the neurons responding to NA with a purely inhibitory effect were scattered throughout the RN. On the other hand, the neurons responding to NA with an excitation were found to be more numerous in the dorso-medial part of the RN, whereas the neurons in which NA induced biphasic effects appeared to be segregated in the outer lateral portion of the RN. The alpha 2-adrenoceptor antagonist yohimbine completely blocked the inhibitory effect of NA but was unable to antagonize the excitatory response. In addition, the inhibitory effect of NA was mimicked by clonidine, a selective agonist of alpha 2-adrenoceptors; clonidine had no effect on those cells which responded to NA with an increase of the mean firing rate. The excitatory effect of NA was mimicked by the beta-receptor agonist isoprenaline and was antagonized by timolol, a selective antagonist of beta-adrenoceptors. Isoprenaline was ineffective in those cells in which NA exerted inhibitory responses. Taken together, our results indicate that the inhibitory effect of NA on the firing activity of rat RN neurons were mediated by alpha 2-adrenoceptors, whereas beta-adrenoceptors were responsible for the excitatory effects.     

Systemically administered cocaine alters stimulus-evoked responses of thalamic somatosensory neurons to perithreshold vibrissae stimulation

A modular organization of bands enriched in high concentrations of D2 receptors are observed throughout the rostral to caudal aspects of the temporal cortex of the normal human at postmortem, but are most frequently observed in the inferior and superior temporal cortices [S. Goldsmith, J.N. Joyce, Dopamine D2 receptors are organized in bands in normal human temporal cortex, Neuroscience 74 (1996) 435-451]. In the tissue derived at postmortem from Alzheimer's disease cases (AD), these D2 receptor-enriched modules were found to be largely absent at rostral and mid-levels of the temporal cortex. Regions exhibiting this loss of receptor binding also showed a marked reduction in the number of pyramidal neurons stained for D2 mRNA. In addition, the AD material exhibited numerous thioflavin-positive plaques and tangle-filled extraneuronal (ghost) pyramidal neurons that were D2 mRNA-negative. Regions that are the earliest affected and most susceptible to classical AD pathology are also most sensitive to the loss of D2 receptors. These results, along with our previous data [J.N. Joyce, C. Kaeger, H. Ryoo, S. Goldsmith, Dopamine D2 receptors in the hippocampus and amygdala in Alzheimer's disease, Neurosci. Lett. 154 (1993) 171-174; H. Ryoo, J. N. Joyce, The loss of dopamine D2 receptors varies along the rostrocaudal axis of the hippocampal complex in Alzheimer's disease, J. Comp. Neurol. 348 (1994) 94-110], indicate that specific pathways enriched with D2 receptors, including that within modules of higher order association cortices of the temporal lobe and continued through segregated pathways within the parahippocampus and hippocampus, are particularly susceptible to the loss in AD. These dopamine D2 receptor-enriched modules may play an important role in the reciprocal activity of large groups of neurons in these high-order association cortical regions. Hence, the loss of the D2 receptor-enriched modules in Alzheimer's disease contributes to disturbances in information processing in these high-order association cortices, and may promote the cognitive and non-cognitive impairments observed in Alzheimer's disease.     

Innervation of VIP-immunoreactive neurons by the ventroposteromedial thalamic nucleus in the barrel cortex of the rat

We investigated the synaptic terminals of fibers originating in the ventroposteromedial thalamic nucleus (VPM) and projecting to the main input layers (IV/III) of the rat posteromedial barrel subfield. It was our aim to determine whether or not the subpopulation of vasoactive intestinal polypeptide (VIP)-immunoreactive neurons in these layers are directly innervated by the sensory thalamus. Anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and immunohistochemistry for VIP were combined for correlated light and electron microscopic examination. Columns of cortical tissue were well defined by barrel-like patches of PHA-L-labeled fibers and boutons in layers IV and III. Within these columns VIP-immunoreactive perikarya were located mainly in supragranular layers. Marked perikarya were also seen in infragranular layers, but their immunoreactivity was often weaker. Granular layer IV, which is the main terminal field for thalamic fibers, contained fewer VIP neurons than supragranular layers. In the light microscope, however, PHA-L-labeled fibers appeared to contact the somata or proximal dendrites of 60-86% of the layer IV VIP neurons . By contrast, only 18-35% of the VIP neurons in the supragranular layers, which receive a moderately dense projection from the VPM, appeared to be contacted. PHA-L-labeled boutons were seen close to 13-25% of infragranular VIP-positive cells. Electron microscopy showed that thalamic fibers formed at most four asymmetric synapses on a single layer IV, VIP-positive neuron. Although the proportion of VIP-positive neurons with labeled synapses was lower in supragranular layers, most of them shared multiple asymmetric synapses with labeled thalamic fibers. Up to six labeled synapses were seen on individual VIP neurons in layer III. We conclude that subpopulations of VIP-immunoreactive neurons, located in layers IV, III, and II are directly innervated by the VPM. These neurons may be involved in the initial stages of cortical processing of sensory information from the large, mystacial vibrissae. Since VIP is known to be colocalized with the inhibitory transmitter GABA, it is likely that VIP neurons participate in the shaping of the receptive fields in the barrel cortex.     

Facilitation of miniature GABAergic currents by ruthenium red in neonatal rat hippocampal neurons

The whole cell configuration of the patch-clamp technique was used to study the modulation gamma-aminobutyric acid (GABA)-mediated postsynaptic currents by ruthenium red in CA3 hippocampal neurons in slices obtained from postnatal (P) days P6-P10 old rats. In the presence of kynurenic acid (1 mM), ruthenium red (100 microM) completely blocked stimulus-elicited GABA-mediated postsynaptic currents and reduced by 50% the amplitude of the spontaneous ones. Ruthenium red (100 microM) increased the frequency but not the amplitude of miniature GABAergic currents recorded in the presence of tetrodotoxin (1 microM) and kynurenic acid (1 mM), an effect that was prevented by heparin (100 microM). Ruthenium red did not modify the kinetics of miniature postsynaptic currents and the currents induced by exogenous application of GABA (10 microM) in the presence of tetrodotoxin, suggesting that its action was presynaptic in origin. The effects of ruthenium red on quantal GABA release was independent of external calcium. In a nominally Ca2+-free solution the potentiating effect induced by this polyvalent cation on miniature postsynaptic currents was still present. Intracellular calcium stores were not involved in ruthenium red action, because this polyvalent cation was able to facilitate miniature currents also in the presence of thapsigargin (10-20 microM). These results indicate that ruthenium red has a dual action on GABA release from GABAergic interneurons: it reduces the amplitude of spontaneous events and increases the frequency of miniature currents. The former effect is calcium-dependent, whereas the latter is calcium independent.     

Antinociceptive effect of intrathecally administered pituitary adenylate cyclase activating polypeptide (PACAP) on the rat formalin test

Diclofenac (0.5-2 mM) dose- and time-dependently reduces the viability of isolated hepatocytes. This effect cannot be counteracted by the calcium channel blockers diltiazem (0.05-0.1 mM) and verapamil (0.05-0.5 mM), the calmodulin antagonist calmidazolium (0.01 mM) or Quin 2-AM (0.1 mM), an intracellular calcium chelating agent. On the contrary, verapamil even accentuates the toxic effects of diclofenac. It is concluded from these results, that diclofenac causes cell damage by other mechanisms than calcium overload.     

Effect of intrathecally administered local anesthetics on protein phosphorylation in the spinal cord

To elucidate the biochemical mechanisms of spinal anesthesia, we studied the effects of procaine and tetracaine on protein phosphorylation in the mouse spinal cord. Mice were injected intrathecally with either procaine, tetracaine (67 mM/approximately 2%, 10 microL, N = 5/drug), or saline (N = 4/group). Five minutes after injection, animals were killed with a guillotine, and the spinal cord was removed. The caudal 3-cm cord segment was homogenized and centrifuged, and an aliquot of the supernatant was used for phosphorylation assays. Calcium-dependent phosphorylation was initiated by incubating the samples in buffer containing [gamma-32P]ATP at 37 degrees for 30 min. The proteins were electrophoresed using slab gel and two-dimensional electrophoresis, and phosphorylated proteins were visualized by autoradiography. The data demonstrated that spinal anesthesia changes the phosphorylation state of five endogenous substrate proteins with apparent molecular masses of 130 (protein-a), 105 (protein-b), 55 (protein-c), 47 (protein-d), and 33 (protein-e) kDa. In two-dimensional electrophoresis, protein-a resolved into two proteins (a1 and a2). Analysis of variance of the densitometric data suggested a significant effect for the treatment (F(2,16) 735, P < 0.00005). Post hoc comparisons with the saline-treated controls, using the Newman-Keuls test, indicated that local anesthetics significantly affected phosphoproteins (P < 0.05) except for protein-al in the tetracaine-treated group. Further characterization of these phosphoproteins should aid in determining their role in the signal transduction cascade affected by spinal anesthesia.     

Induction of reactive oxygen species in neurons by haloperidol

Haloperidol (HP) is widely prescribed for schizophrenia and other affective disorders but has severe side effects such as tardive dyskinesia. Because oxidative stress has been implicated in the clinical side effects of HP, rat primary cortical neurons and the mouse hippocampal cell line HT-22 were used to characterize the generation of reactive oxygen species (ROS) and other cellular alterations caused by HP. Primary neurons and HT-22 cells are equally sensitive to HP with an IC50 of 35 microM in the primary neurons and 45 microM in HT-22. HP induces a sixfold increase in levels of ROS, which are generated from mitochondria but not from the metabolism of catecholamines by monoamine oxidases. Glutathione (GSH) is an important antioxidant for the protection of cells against HP toxicity because (1) the intracellular GSH decreases as the ROS production increases, (2) the exogenous addition of antioxidants, such as beta-estradiol and vitamin E, lowers the level of ROS and protects HT-22 cells from HP, and (3) treatments that result in the reduction of the intracellular GSH potentiate HP toxicity. The GSH decrease is followed by the increase in the intracellular level of Ca2+, which immediately precedes cell death. Therefore, HP causes a sequence of cellular alterations that lead to cell death and the production of ROS is the integral part of this cascade.     

Facilitation of L-type calcium current in thalamic neurons

We have studied facilitation of the L-type calcium current in neurons acutely isolated from the ventrobasal nucleus of the rat thalamus. Currents were recorded after pretreatment with 1 microM omega-conotoxin GVIA and 5 microM omega-conotoxin MVIIC, to better isolate L-current. Long, strong depolarizations induced slow tail currents at negative voltages, but did not affect currents at voltages where channels were strongly activated. The initial peak tail current was not measurably increased. The time course of recovery from facilitation paralleled the time course of the tail current, indicating that facilitation does not outlast channel closing. The kinase inhibitors staurosporine and H-7 and the phosphatase inhibitor okadaic acid had no significant effect on L-current facilitation compared with control, but facilitation was greater with H-7 than with okadaic acid. The guanosine 5'-triphosphate (GTP) analogs GTP-gamma-S and GDP-beta-S did not affect facilitation. We conclude that L-current facilitation in thalamic neurons does not result from Ser/Thr phosphorylation, although phosphorylation may modulate facilitation. This form of facilitation differs kinetically and pharmacologically from facilitation induced by activation of G protein-coupled receptors.     

Changes in the impulse activity of neurons of the human thalamic ventrolateral nucleus during voluntary movement

Functional differences were revealed in evoked activity of two types (A and B) of units of the human thalamic ventro-lateral nucleus (VL). Collective activities of these polyfunctional neurons were selectively related to triggering and execution phases of movement. Common character of dynamics of the responses seems to be due to similar polyfunctional nature as well as to the functional role of these two complementary elements in the motor signal transmission. The collective activities reflect in the VL the integrative processes related to processing and programming of generalised parameters of motor signals, but unrelated to performance of a concrete motor act.     

Altered receptive fields and sensory modalities of rat VPL thalamic neurons during spinal strychnine-induced allodynia

Altered receptive fields and sensory modalities of rat VPL thalamic neurons during spinal strychnine-induced allodynia. J. Neurophysiol. 78: 2296-2308, 1997. Allodynia is an unpleasant sequela of neural injury or neuropathy that is characterized by the inappropriate perception of light tactile stimuli as pain. This condition may be modeled experimentally in animals by the intrathecal (i.t.) administration of strychnine, a glycine receptor antagonist. Thus after i.t. strychnine, otherwise innocuous tactile stimuli evoke behavioral and autonomic responses that normally are elicited only by noxious stimuli. The current study was undertaken to determine how i.t. strychnine alters the spinal processing of somatosensory input by examining the responses of neurons in the ventroposterolateral thalamic nucleus. Extracellular, single-unit recordings were conducted in the lateral thalamus of 19 urethan-anaesthetized, male, Wistar rats (342 +/- 44 g; mean +/- SD). Receptive fields and responses to noxious and innocuous cutaneous stimuli were determined for 19 units (1 per animal) before and immediately after i.t. strychnine (40 microgram). Eighteen of the animals developed allodynia as evidenced by the ability of otherwise innocuous brush or air jet stimuli to evoke cardiovascular and/or motor reflexes. All (3) of the nociceptive-specific units became responsive to brush stimulation after i.t. strychnine, and one became sensitive to brushing over an expanded receptive field. Expansion of the receptive field, as determined by brush stimulation, also was exhibited by all of the low-threshold mechanoreceptive units (14) and wide dynamic range units (2) after i.t. strychnine. The use of air jet stimuli at fixed cutaneous sites also provided evidence of receptive field expansion, because significant unit responses to air jet developed at 13 cutaneous sites (on 7 animals) where an identical stimulus was ineffective in evoking a unit response before i.t. strychnine. However, the magnitude of the unit response to cutaneous air jet stimulation was not changed at sites that already had been sensitive to this stimulus before i.t. strychnine. The onset of allodynia corresponded with the onset of the altered unit responses (i.e., lowered threshold/receptive field expansion) for the majority of animals (9), but the altered unit response either terminated concurrently with symptoms of allodynia (6) or, more frequently, outlasted the symptoms of allodynia (10) as the effects of strychnine declined. The present results demonstrate that the direct, receptor-mediated actions of strychnine on the spinal processing of sensory information are reflected by changes in the receptive fields and response properties of nociceptive and nonnociceptive thalamic neurons. These changes are consistent with the involvement of thalamocortical mechanisms in the expression of strychnine-induced allodynia and, moreover, suggest that i.t. strychnine also produces changes in innocuous tactile sensation.     

Relationships between striatin-containing neurons and cortical or thalamic afferent fibres in the rat striatum. An ultrastructural study by dual labelling

The processing and localization of Plasmodium falciparum rhoptry-associated protein 1 (RAP-1) products were examined using polyclonal and monoclonal antibodies raised to a recombinant protein containing residues 1-294 of RAP-1. Immunoblot and epitope mapping results with antibodies that selectively bound epitopes in the RAP-1 products Pr86, p82, and p67 showed that p82 and p67 are formed from Pr86 by progressive removal of epitopes from the amino-terminus of the RAP-1 coding sequence. The capacity of Pr86 to form complexes was revealed after size fractionation of parasite proteins radiolabeled in the presence of brefeldin A to prevent processing of Pr86. Fractions containing complexed Pr86 also contained the RAP-2 product p39 and the RAP-3 product p37, suggesting that Pr86, p39 and p37 may form complexes similar to complexes previously reported for p82 and p67 with p39 or p37. Immunofluorescence localization and immunoblot studies revealed that Pr86 is present in the rhoptries, but only transiently, and that it is not detected in segmenting schizonts or extracellular merozoites. p67 and p82, on the other hand, were shown to be major RAP-1 components in purified merozoites. Neither p67 nor p82 were relocalized from the intracellular rhoptries to the merozoite surface under conditions that promoted relocalization of the rhoptry protein PF83/apical membrane antigen 1. These results suggest that processing of Pr86 begins after Pr86 complexes are transported to the forming rhoptries and that two site-selective processing reactions occur in the rhoptries, a rapid cleavage of Pr86 to p82 and a delayed cleavage of p82 to p67. Since p67 is missing from ring-stage parasites (Howard et al., Am J Trop Med Hyg, 1984;33:1055 59), the present results indicate there is a narrow time during which p67 may play a role in merozoite invasion of erythrocytes.     

Role of thalamic and cortical neurons in augmenting responses and self-sustained activity: dual intracellular recordings in vivo

Progressively increasing (augmenting) responses are elicited in thalamocortical systems by repetitive stimuli at approximately 10 Hz. Repeated pulse trains at this frequency lead to a form of short-term plasticity consisting of a persistent increase in depolarizing synaptic responses as well as a prolonged decrease in inhibitory responses. In this study, we have investigated the role of thalamocortical (TC) and neocortical neurons in the initiation of thalamically and cortically evoked augmenting responses. Dual intracellular recordings in anesthetized cats show that thalamically evoked augmenting responses of neocortical neurons stem from a secondary depolarization (mean onset latency of 11 msec) that develops in association with a diminution of the early EPSP. Two nonexclusive mechanisms may underlie the increased secondary depolarization during augmentation: the rebound spike bursts initiated in simultaneously recorded TC cells, which precede by approximately 3 msec the onset of augmenting responses in cortical neurons; and low-threshold responses, uncovered by hyperpolarization in cortical neurons, which may follow EPSPs triggered by TC volleys. Thalamic stimulation proved to be more efficient than cortical stimulation at producing augmenting responses. Stronger augmenting responses in neocortical neurons were found in deeply located (<0.8 mm, layers V-VI) regular-spiking and fast rhythmic-bursting neurons than in superficial neurons. Although cortical augmenting responses are preceded by rebound spike bursts in TC cells, the duration of the self-sustained postaugmenting oscillatory activity in cortical neurons exceeds that observed in TC neurons. These results emphasize the role of interconnected TC and cortical neurons in the production of augmenting responses leading to short-term plasticity processes.     

Single midline thalamic neurons projecting to both the ventral striatum and the prefrontal cortex in the rat

The midline thalamic nuclei have been known to send projection fibres to the ventral striatum and the autonomic/limbic-associated areas of the prefrontal cortex. In the present study, we sought to determine whether or not single midline thalamic neurons project both to the ventral striatum and to the cerebral cortical areas. Experiments were performed on chloral hydrate-anaesthetized male Sprague Dawley rats; two fluorescent retrograde tracers were centred on the medial or lateral part of the nucleus accumbens--the major part of the ventral striatum--and the medial or lateral prefrontal viscerolimbic cortex. Our retrograde double-labelling study revealed that a subset of midline thalamic neurons send projection fibres to both the nucleus accumbens and the cerebral cortex. Such neurons projecting to both targets were principally identified in the paraventricular thalamic nucleus. The majority of the dually-labelled neurons in the paraventricular thalamic nucleus projected to the lateral part of the nucleus accumbens and the medial wall of the prefrontal cortex. Dually-labelled neurons were additionally found in other midline nuclei, including the paratenial, intermediodorsal, rhomboid, and reuniens nuclei, as well as in the medial part of the parafascicular thalamic nucleus. Dually-projecting neurons identified in the present study may represent a potential link between the limbic striatum and the viscerolimbic-associated cortex, thus suggesting that non-discriminative information relayed to the prefrontal cortex might exert an influence through the same neurons on the nucleus accumbens implicated in affective behaviour.     

Identification of C1 presympathetic neurons in rat rostral ventrolateral medulla by juxtacellular labeling in vivo

The rostral ventrolateral medulla (RVLM) contains barosensitive, bulbospinal neurons that provide the main supraspinal excitatory input to sympathetic vasomotor preganglionic neurons. However, the phenotype of the critical RVLM cells has not been conclusively determined. The goal of the current study was to identify the proportion of electrophysiologically defined, putative, presympathetic RVLM neurons that are C1 cells. We used a juxtacellular labeling technique to individually fill spontaneously active, barosensitive, bulbospinal RVLM neurons with biotinamide following electrophysiological characterization in chloralose-anesthetized rats. To determine whether these neurons could be classified as C1 cells, the biotinamide-labeled cells were processed for detection of tyrosine hydroxylase. The majority of barosensitive bulbospinal RVLM neurons were tyrosine hydroxylase immunoreactive (TH-ir; 28 of 39). All of the barosensitive bulbospinal RVLM neurons with axonal conduction velocities in the C fiber range (<1 m/second) were TH-ir (n = 16), whereas faster conducting cells (1 to 7 m/second) were either lightly TH-ir (n = 12) or not detectably TH-ir (n = 11). Adjacent respiratory-related RVLM units labeled with biotinamide were not detectably TH-ir (n = 10). To verify that TH-ir cells were indeed adrenergic, a subset of barosensitive bulbospinal cells labeled with biotinamide were examined for phenylethanolamine N-methyltransferase immunoreactivity (PNMT-ir). Three slowly conducting cells had detectable PNMT-ir, and two fast-conducting cells had no detectable PNMT-ir. These results indicate that the majority of bulbospinal RVLM neurons with putative sympathoexcitatory function are C1 cells.     

Depletion of NOS activity in the rat dentate gyrus neurons by DSP-4 and protection by deprenyl

Insulin resistance and hyperinsulinemia have been linked with essential hypertension. Age-associated increases in glucose intolerance and hypertension are also well established. To clarify the influence of aging on the insulin sensitivity, euglycemic hyperinsulinemic glucose clamp technique was carried out in 41 normotensive subjects and 42 patients with essential hypertension. The subjects of these groups were divided into two subgroups: young (< 40 years old) and middle-elderly (> or = 40 years old). Insulin sensitivity was assessed as M-value, the rate at which glucose must be infused to maintain a basal blood glucose level. In normotensive subjects, the young subgroup had a significantly higher M-value than did the middle-elderly subgroup. There was a significant negative correlation between age and M-value in normotensive subjects. On the other hand, there was no significant difference in M-value between the young and middle-elderly subgroups in the patients with essential hypertension. The age did not correlate with M-value in the hypertensive group. The normotensive subjects showed a significantly lower M-value than the hypertensive patients in the young group, but not in the middle-elderly group. These results indicate that 1) insulin sensitivity declines with age in normotensive subjects and that 2) insulin sensitivity is already diminished in the early stage of hypertension, and no further decrease in insulin sensitivity occurs with aging in essential hypertensive patients.     

Speed of Ca2+ channel modulation by neurotransmitters in rat sympathetic neurons

We have measured the onset and recovery speed of inhibition of N-type Ca2+ channels in adult rat superior cervical ganglion neurons by somatostatin (SS), norepinephrine (NE), and oxotremorine-M (oxo-M, a muscarinic agonist), using the whole cell configuration of the patch-clamp method with 5 mM external Ca2+. With a local perfusion pipette system that changed the solution surrounding the cell within 50 ms, we applied agonists at various times before a brief depolarization from -80 mV that elicited I(Ca). At concentrations that produced maximal inhibition, the onset time constants for membrane-delimited inhibition by SS (0.5 microM), NE (10 microM), and oxo-M (20 microM) were 2.1, 0.7, and 1.0 s, respectively. The time constants for NE inhibition depended only weakly on the concentration, ranging from 1.2 to 0.4 s in the concentration range from 0.5 to 100 microM. Inhibition by oxo-M (20 microM) through a different G-protein pathway that uses a diffusible cytoplasmic messenger had a time constant near 9 s. The recovery rate constant from membrane-delimited inhibition was between 0.09 and 0.18 s(-1), significantly higher than the intrinsic GTPase rate of purified G protein Go, suggesting that Ca2+ channels or other proteins in the plasma membrane act as GTPase activating proteins. We also measured the rate of channel reinhibition after relief by strong depolarizing prepulses, which should reflect the kinetics of final steps in the inhibition process. In the presence of different concentrations of NE, reinhibition was four to seven times faster than the onset of inhibition, indicating that the slowest step of inhibition must precede the binding of G protein to the channel. We propose a kinetic model for the membrane-delimited NE inhibition of Ca2+ channels. It postulates two populations of receptors with different affinities for NE, a single population of G proteins, and a single population of Ca2+ channels. This model closely simulated the time courses of onset and recovery of inhibition and reinhibition, as well as the dose-response curve for inhibition of Ca2+ channels by NE.     

Dendritic arbors of neurons from different regions of the rat thalamic reticular nucleus share a similar orientation

Neurons in different regions of the rat thalamic reticular nucleus were labeled with biotin dextran amine and reconstructed. When viewed in coronal section, some neurons had a radial dendritic tree while others had dorso-ventrally elongated arbors. When rotated, all the neurons had a planar, disc-shaped dendritic field with the dendrites orientated parallel to the long axis of the nucleus. We conclude that all thalamic reticular nucleus neurons have a similar dendritic morphology and orientation.     

Enantioselective blockade of T-type Ca2+ current in adult rat sensory neurons by a steroid that lacks gamma-aminobutyric acid-modulatory activity

A number of steroids seem to have anesthetic effects resulting primarily from their ability to potentiate currents gated by gamma-aminobutyric acidA (GABAA) receptor activation. One such compound is (3alpha,5alpha, 17beta)-3-hydroxyandrostane-17-carbonitrile [(+)-ACN]. We were interested in whether carbonitrile substitution at other ring positions might result in other pharmacological consequences. Here we examine effects of (3beta,5alpha, 17beta)-17-hydroxyestrane-3-carbonitrile [(+)-ECN] on GABAA receptors and Ca2+ channels. In contrast to (+)-ACN, (+)-ECN does not potentiate GABAA-receptor activated currents, nor does it directly gate GABAA-receptor mediated currents. However, both steroids produce an enantioselective reduction of T-type current. (+)-ECN blocked T current with an IC50 value of 0.3 microM with a maximal block of 41%. (+)-ACN produced a partial block of T current (44% maximal block) with an IC50 value of 0.4 microM. Block of T current showed mild use- and voltage-dependence. The (-)-ECN enantiomer was about 33 times less potent than (+)-ECN, with an IC50 value of 10 microM and an amount of maximal block comparable to (+)-ECN. (+)-ECN was less effective at blocking high-voltage-activated Ca2+ current in DRG neurons (IC50 value of 9. 3 microM with maximal block of about 27%) and hippocampal neurons. (+)-ECN (10 microM) had minimal effects on voltage-gated sodium and potassium currents in rat chromaffin cells. The results identify a steroid with no effects on GABAA receptors that produces a partial inhibition of T-type Ca2+ current with reasonably high affinity and selectivity. Further study of steroid actions on T currents may lead to even more selective and potent agents.     

Noradrenalin enhances the activity of cochlear nucleus neurons in the rat

The cochlear nucleus of rats is heavily innervated by noradrenergic fibres from the locus coeruleus. The physiological meaning of this innervation is poorly understood. Therefore, iontophoretically applied noradrenalin was tested on single neurons of the cochlear nucleus in urethane-anaesthetized rats. Iontophoresis of noradrenalin had a dual effect. During application noradrenalin led to moderate inhibition of tone-evoked activity in 37% of the tested neurons. In contrast, approximately 20-30 s after the onset of iontophoresis a long-lasting increase in discharge activity was found in most neurons. Data from iontophoresis of the alpha1-receptor agonist phenylephrine and the alpha2-receptor agonist clonidine suggest that the fast moderate inhibition is mediated by alpha2-receptors while the pronounced long-lasting elevated neuronal firing is mediated by alpha1-receptors. However, these data do not exclude the possibility that part of the response to noradrenalin is also mediated by beta-receptors. Electrical stimulation of the locus coeruleus resulted in an increase in discharge activity comparable with iontophoresis of noradrenalin or phenylephrine. Thus, activation of the locus coeruleus predominantly increases spontaneous and tone-evoked neuronal firing in the cochlear nucleus of the rat. This alpha-receptor-mediated enhanced discharge activity may serve to increase the sensitivity of acoustic processing mechanisms or to lower the threshold for short-latency acoustic reflexes.