Noradrenergic potentiation of cerebellar Purkinje cell responses to GABA: evidence for mediation through the beta-adrenoceptor-coupled cyclic AMP system

Opposite regulation by the beta-adrenoceptor-cyclic AMP system of synaptic plasticity in the medial and lateral amygdala in vitro

The effects of beta-adrenoceptor activation on short-term potentiation in the medial and lateral amygdala were investigated using rat brain slice preparations in vitro. Application of tetanic stimulation (100 pulses at 100 Hz) induced only short-term potentiation under normal recording conditions. In the medial amygdala, when the same tetanic stimulation was applied in the presence of a beta-adrenoceptor agonist, isoproterenol, short-term potentiation was significantly enhanced and long-term potentiation was induced. Phenylephrine, an alpha-adrenoceptor agonist, did not affect short-term potentiation. The short-term potentiation-enhancing effect of isoproterenol was mimicked by forskolin, an adenylate cyclase activator, and was blocked by Rp-adenosine-3',5'-cyclic-monophosphothioate, an inhibitor of cyclic AMP-dependent protein kinase. On the other hand, in the lateral amygdala, isoproterenol suppressed short-term potentiation. The short-term potentiation-suppressing effect of isoproterenol was mimicked by forskolin, and was blocked by Rp-adenosine-3',5'-cyclic-monophosphothioate. These results suggest that the beta-adrenoceptor-cyclic AMP system plays a role in facilitating the induction of long-term potentiation in the medial amygdala, but suppresses synaptic plasticity in the lateral amygdala.

8-Bromo-cAMP mimics beta-adrenergic sensitization of GABA responses to ethanol in cerebellar Purkinje neurons in vivo

Previous studies in our laboratory indicated that electrophysiological responses of cerebellar Purkinje neurons to GABA were not routinely potentiated by ethanol (EtOH), and the potentiation was not large when it occurred. In the presence of beta-adrenergic agonists, such as isoproterenol, however, GABA inhibitions became sensitive to potentiation by EtOH in nearly every Purkinje neuron tested. beta-adrenergic receptor activation alone also modulates (potentiates) GABA responses on Purkinje neurons, and this has been reported to be mediated by a cAMP second messenger system. Herein, we report that the membrane-permeable cAMP analog, 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP), but not the membrane-impermeable cAMP, can also modulate GABA responses and that EtOH potentiates this facilitatory action of 8-Br-cAMP. These effects are not likely caused by adenosine receptor mechanisms, because this 8-bromoadenosine mediated modulation and sensitization was observed in the presence of systemic theophylline. These data suggest that the beta-adrenergic modulation and sensitization to EtOH of cerebellar Purkinje neuron GABA responses occur via a cAMP second messenger mechanism.

Modulation of GABAA receptor function by G protein-coupled 5-HT2C receptors

Two classical neurotransmitters, 5-hydroxytryptamine (5-HT) and GABA, coexist in neurons of the medulla oblongata, and activation of 5-HT receptors modulates GABAA receptor function in neurons of the ventral tegmental area, substantia nigra and cerebellum. We now report that activation of 5-HT2C receptors produces a long-lasting (20-90 min) inhibition of GABAA receptors in Xenopus oocytes coexpressing both types of receptors 5-HT2C receptors caused a approximately 60% decrease in the GABAA receptor Emax without affecting the EC50 or Hill coefficient. Intracellular microinjection of 500 microM BAPTA blocked, whereas microinjection of inositol 1,4,5-triphosphate mimicked the inhibitory action of 5-HT2C receptors. The inhibition was independent of the GABAA receptors subunit composition; receptors containing alpha 2 beta 1, alpha 1 beta 1 gamma 2L, and alpha 2 beta 1 gamma 2S were inhibited to the same extent by 5-HT2C receptor activation. Moreover, GABAA receptors composed of wild-type alpha 2 plus mutant beta 1(S409A) subunits were inhibited to the same extent as wild-type receptors. The nonspecific protein kinase inhibitor, staurosporine, and the inhibitor of serine/threonine protein phosphatases, calyculin A, did not block the inhibitory effects of 5-HT2C receptors. The results with these inhibitors, taken together with those obtained with GABAA receptors with different subunit compositions, suggest that protein kinases or serine/threonine phosphatases are not involved in this GABAA receptor modulatory process. Thus, we propose that 5-HT2C receptors inhibit GABAA receptors by a Ca(2+)-dependent, but phosphorylation independent, mechanism and that 5-HT and GABA may act as cotransmitters to regulate neuronal activity. Furthermore, disruption of the cross-talk between these receptors may play a role in the anti-anxiety actions of 5-HT2 receptor antagonists.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Methamphetamine facilitates ethanol-induced depressions in cerebellar Purkinje neurons of prazocin- or DSP4-treated rats

Methamphetamine (MA) and ethanol (EtOH) are two commonly abused drugs. Previous behavioral studies indicated that MA may synergistically alter EtOH responses. In the present study, we found that local application of MA did not potentiate ethanol-induced depressions of the spontaneous activity of Purkinje neurons in urethane-anesthetized rats. We and others previously found that, in cerebellar Purkinje neurons, EtOH and gamma-amino-butyric acid (GABA)-mediated depressions can be enhanced by norepinephrine (NE) acting via beta-adrenergic receptors while these responses are decreased by activation of alpha-adrenergic receptors. In the present experiment, after blocking alpha-adrenergic receptors with prazocin, MA significantly enhanced EtOH responses in most of neurons studied. It has been reported that MA may directly and indirectly enhance alpha-adrenergic and beta-adrenergic receptor-mediated responses. The present study may suggest that MA can negatively modulate (antagonize) the depressant effects of ethanol via the alpha-adrenergic receptor, which oppose the positive modulatory mechanism (potentiation of EtOH depression) via actions of the beta-adrenergic receptors. We found that lesioning NE neurons with N-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4), a selective noradrenergic neurotoxin, enhance the MA-facilitated ethanol responses, suggesting that this action of MA may not require NE. Since it has been reported that MA increases serotonin (5-HT) and catecholamine release from their nerve terminals, MA may potentiate EtOH depressions by facilitating the release of NE and 5-HT. Taken together, our data suggested that MA may modulate EtOH responses via catecholaminergic and serotonergic mechanisms in cerebellar Purkinje neurons.

Erosion of inhibition contributes to the progression of low magnesium bursts in rat hippocampal slices

1. Bathing slices of rat hippocampus in media containing no magnesium ions results in epileptic discharges that originate in hippocampal area CA3. These discharges increase in severity gradually over periods of hours. 2. The progression of epileptic activity was much slower than the equilibration of extracellular magnesium activity and the resulting increase in strength of monosynaptic NMDA receptor-mediated excitation. Its time course matched that of a progressive decrease in pharmacologically isolated, evoked GABAA receptor-mediated inhibitory postsynaptic current (IPSC) in the CA3 pyramidal cells. Conductance decreased to 37 +/- 6% of control values after 4 h. Responses to exogenous GABA application decreased to 52 +/- 12%. 3. Maximal IPSC conductance in 0 mM extracellular Mg2+ ([Mg2+]o) also decreased gradually when epileptic activity was abolished by bath application of 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 50 microM D-2-amino-5-phosphonovaleric acid (D-APV) throughout the 4 h incubation period. It reached 61 +/- 8% of control values, a significantly smaller decline than that seen after 4 h of epileptic activity. 4. The decrease in mean IPSC conductance only partially reversed when the recording electrode contained 100 mM Mg2+. Complete recovery of IPSC strength occurred when electrodes also contained either 50 mM MgATP or 20 mM BAPTA. Reintroduction of 1 mM [Mg2+]o rapidly abolished epileptic activity and caused a slow, partial increase in IPSC conductance. 5. In the presence of 1 mM [Mg2+]o, GABAA receptor-mediated inhibition had to decrease to 17 +/- 11% of control values, in the presence of 4-7 microM bicuculline, to reach threshold for epileptic activity. 6. These data demonstrate a postsynaptic decrease in GABAA receptor-mediated inhibition in the in vitro low magnesium model of epilepsy. We propose that the apparent leaching of intracellular Mg2+ ([Mg2+]i) from cells leads to loss of ATP and consequent partial dephosphorylation of the GABAA receptor and that this is exacerbated by epileptic activity.

Effects of systemic and local ethanol on responses of rat cerebellar Purkinje neurons to iontophoretically applied norepinephrine and gamma-aminobutyric acid

The goal of the present study was to determine the effect of acute ethanol (ETOH), administered intraperitoneally or electro-osmotically, on norepinephrine (NE) induced increases in gamma-aminobutyric acid (GABA) mediated inhibition of single cerebellar Purkinje neurons (P-cells). Male Sprague-Dawley rats (230-370g) were anesthetized with halothane and implanted with an intraperitoneal catheter for systemic administration of ETOH (1.0-1.5 g/kg) prior to the recording session. Extracellular activity of single P-cells was recorded before and after iontophoresis of GABA and NE using five-barrel glass micropipettes. GABA was administered at the recording site by microiontophoretic pulses before, during and after continuous iontophoretic application of NE. Spontaneous discharge, GABA responses and NE-GABA interactions in P-cells were monitored for each experiment before and 1-1.5 h following systemic administration of ETOH. As in our previous reports administration of NE, at low ejection currents (10-60 nA), augmented GABA mediated suppression of P-cell spontaneous discharge. Between 10 and 60 min after injection of ETOH, this NE induced augmentation of GABA inhibition was further potentiated. This potentiation involved increases in both the magnitude and the duration of the GABA inhibition observed after NE alone. NE-induced augmentation of GABA inhibition persisted for 2-13 min longer after ETOH administration than in the pre-ETOH control period. Local electro-osmotic application of ETOH, which resulted in strong depression of spontaneous activity and caused small increases in GABA-mediated inhibition, did not directly potentiate NE-induced augmentation of GABA action. These results indicate that NE-mediated augmentation of GABA inhibition of P-cell activity is potentiated following systemic, but not local, ETOH administration.

Noradrenergic enhancement of GABA-induced input resistance changes in layer V regular spiking pyramidal neurons of rat somatosensory cortex

Previous in vivo studies have shown that microiontophoretic application of norepinephrine (NE) and isoproterenol (ISO) can enhance gamma-aminobutyric acid (GABA)-induced depressant responses of rat somatosensory cortical neurons. In the present investigation we have examined the transmembrane electrophysiological events which are associated with interactions between NE and GABA in layer V pyramidal neurons of rat barrel field cortex. Intracellular recordings were made from electrophysiologically identified cells in a superfused cortical tissue slice preparation before, during and after bath or microdrop application of GABA, NE and ISO, alone or in combination. GABA application produced a small depolarization from resting membrane potential associated with a reduction (22%) in input resistance. NE and ISO (10-100 microM) also produced in some cases small membrane depolarizations (1-4 mV) but little concomitant changes in input resistance. Simultaneous application of NE with GABA potentiated amino acid-induced changes in input resistance in 4 cases and antagonized (n = 4) or had no effect (n = 4) on GABA-associated membrane events in 8 other cases. When the alpha-blocker, phentolamine (20 microM), was added to the medium, NE-induced enhancement of the GABA response was observed in 3 of 5 cases (60%), suggesting both, a beta-adrenergic mediation and a possible alpha-receptor masking of this noradrenergic-potentiating action. Consistent with this interpretation was the finding that the beta-agonist, ISO (10-100 microM), produced net increases in GABA-induced input resistance changes in 64% of cases tested (9 of 14). The potentiating effect of NE and ISO was mimicked by the adenyl cyclase activator, forskolin (n = 2), and a membrane permeant analog of cyclic-AMP, 8-bromo-cyclic AMP (n = 3); and could also be demonstrated when the GABAA agonist muscimol (0.5-1 microM) was substituted for GABA. The reversal potential for GABA and GABA + NE remained the same. These findings suggest that previous demonstrations of NE-potentiating effects on GABA inhibition may be mediated by beta-receptor/cyclic-AMP-linked actions on mechanisms which regulate GABAA receptor-induced membrane conductance changes.

Localization of glutamatergic neurons in the dorsolateral pontine tegmentum projecting to the spinal cord of the cat with a proposed role of glutamate on lumbar motoneuron activity

Glutamate is considered to be a major excitatory neurotransmitter in the central nervous system. The presence of glutamate-like immunoreactive neurons in the rodent locus coeruleus has been reported previously. In this study we used both immunohistochemical and electrophysiological techniques to answer two major questions: (1) Is there any glutamate-like immunoreactivity in the catecholaminergic coeruleospinal system of the cat? (2) What is the physiological role, if any, of glutamate in descending locus coeruleus control of spinal motoneurons? Following injections of rhodamine-labeled latex microspheres or Fast Blue into the seventh lumbar segment of the spinal cord of the cat, retrogradely labeled cells were found throughout the rostrocaudal extent of the dorsolateral pontine tegmentum. They were primarily observed in the nucleus locus coeruleus and the Kolliker-Fuse nucleus. Some labeled cells were also present in the nucleus subcoeruleus and, to a lesser extent, in the parabrachial nuclei. Data from immunohistochemical studies indicate that 86% of all dorsolateral pontine tegmentum neurons that project to the spinal cord contain glutamate-like immunoreactivity, and 77% co-contain both glutamate- and tyrosine hydroxylase-like immunoreactivity. Electrical stimulation (four pulses of 500 microseconds duration at 500 Hz; intensity = 50-200 microA) of the locus coeruleus, in decerebrate cats, consistently induced lumbar motoneuron discharges recordable ipsilaterally as ventral root responses. These motoneuronal responses were reversibly antagonized following chemical inactivation of noradrenergic locus coeruleus neurons by local infusion of the alpha 2-adrenergic agonist clonidine, suggesting the locus coeruleus neurons to be the main source of evoked ventral root responses. Additionally, the evoked ventral root responses were reversibly reduced by 34.20 +/- 4.45% (mean +/- S.E.M.) upon intraspinal injections of the non-N-methyl-D-aspartate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, into the ventral horn of seventh lumbar spinal cord segment (three to four injections, 20 nmol in 0.2 microliter of 0.1 M Tris-buffered saline for each injection). Similar volumes of vehicle injections had no significant effect on the locus coeruleus-evoked ventral root responses. These ventral root responses were also partially blocked (62.30 +/- 11.76%) by intravenous administration of the alpha 1-adrenergic receptor antagonist prazosin (20 micrograms/kg). In the light of several anatomical reports of noradrenergic and glutamatergic terminals in close contact with spinal motoneurons, our present findings suggest that the locus coeruleus-evoked ventral root response probably involves the synaptic release of both norepinephrine and glutamate onto lumbar motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

gamma-Aminobutyric acidA receptor function is modulated by cyclic GMP

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the vertebrate nervous system. Modulatory effects of intracellular ATP on the GABA response in isolated bullfrog dorsal root ganglion neurons were examined using whole-cell voltage clamp. Investigation of the plausible mechanisms ATP might utilize to regulate the GABA response led to the discovery that intracellular cyclic GMP may play an important role in modulating inhibitory neurotransmission. This modulatory effect of cyclic GMP is likely to be mediated via a cyclic GMP-dependent protein kinase.

Ethanol inhibits the uptake of exogenous norepinephrine from the extracellular space of the rat cerebellum

Rapid chronoamperometric recordings using nafion-coated carbon fiber electrodes coupled with pressure-ejection of drugs were used to investigate the effects of ethanol on norepinephrine (NE)-containing nerve terminals in the urethane-anesthetized Fischer 344 rat. Local application of ethanol from a double-barrel micropipette did not produce detectable changes in extracellular levels of NE in the rat cerebellar cortex. However, when ethanol was applied prior to local application of NE, it was seen to inhibit the uptake of NE from the extracellular space. These results were compared to the effects seen from the local application of a known high-affinity uptake inhibitor, nomifensine. Nomifensine was found to inhibit the extracellular uptake of NE in rat cerebeller cortex similar to ethanol. Our results support the hypothesis that one effect of ethanol on the noradrenergic system of the rat cerebellum is an alteration in the uptake of NE into NE-containing nerve endings. In addition, the present data concerning ethanol-induced inhibition of NE clearance or uptake support our previous electrophysiological studies in which we found that ethanol can potentiate the modulatory effects of beta-agonists on GABA responses of cerebellar Purkinje neurons.

Immunochemical characterization of the beta 2 subunit of the GABAA receptor

To date three beta subunits of the GABAA receptor have been identified in rat brain as a result of cDNA library screening. The beta 2 subunit has been reported to have a wide distribution in rat brain based on in situ hybridization studies quantifying beta 2 mRNA. To study the beta 2 subunit more directly, we have raised a polyclonal antibody to a synthetic peptide representing residues 315-334 of the intracellular loop of the beta 2 subunit. The antibody, which had been affinity-purified, recognized the beta 2 peptide but did not immunolabel homologous beta 1 and beta 3 subunit peptides, indicating that this antibody is specific for the beta 2 subunit of the receptor. In western blots of the purified receptor, the antibody recognized a major diffuse band of 54-58 kDa and exhibited minor labeling of lower-molecular-mass polypeptides. In western blots of cortex homogenate, the antibody exhibited nervous system-specific labeling of a 55-kDa band that comigrated with the 55-kDa band of the purified receptor. Quantitative immunolabeling of this 55-kDa polypeptide permitted direct determination of the relative amounts of the beta 2 subunit in different brain regions. The brainstem contained the highest relative specific activity of the beta 2 subunit, followed by the inferior colliculus, olfactory lobe, and cerebellum. Lower levels of immunolabeling were seen in hypothalamus, hippocampus, thalamus, and cortex.

Age-dependent expression, phosphorylation and function of neurotransmitter receptors: pharmacological implications

In recent years, a number of experimental controversies have arisen in the pharmacological sciences literature. Two possibly related examples concern the role of phosphorylation in receptor regulation and the occurrence of paradoxical effects of neurally active drugs in patients. In this article, Ruth Lanius and colleagues review these two issues and suggest that some of the recently reported age-dependent aspects of neurotransmitter receptor regulation and function may explain drug action. An appreciation of the dynamic interactions involved in receptor regulation, especially during development, may offer novel perspectives on neural function.

Protein kinase A-mediated phosphorylation reduces only the fast desensitizing glycine current in acutely dissociated ventromedial hypothalamic neurons

Modulation of glycine receptor-ionophore complex by internally perfused cyclic AMP was investigated and compared to that of GABA in the acutely dissociated ventromedial hypothalamic neurons using whole-cell and outside-out patch-clamp techniques. Cyclic AMP significantly reduced both GABA- and glycine-gated chloride currents. The reduction in glycine-induced chloride current was specific in that only the fast-desensitizing one gated by high concentrations of glycine (30-100 microM) was affected. Cyclic AMP did not modulate the non-desensitizing current induced by lower concentrations (6-10 microM). Addition of N-[-2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride, a protein kinase A inhibitor, did not have a significant effect on its own but prevented the attenuation of fast desensitizing glycine current induced by cyclic AMP. Both the reversal potential and inactivation kinetics of glycine current were not affected by the activation of protein kinase A, suggesting that cyclic AMP-mediated attenuation is not due to an enhancement of desensitization. In outside-out patch studies intracellular perfusion of cyclic AMP reduced the open probability of the 100 microM glycine-activated channels without affecting that of the 6 microM glycine-activated channels. In conclusion, cyclic AMP selectively modulates the channel open frequency of the glycine receptor when activated at higher concentrations through a protein kinase A-mediated phosphorylation.

Beta-adrenergic enhancement of inhibitory synaptic activity in rat cerebellar stellate and Purkinje cells

1. Using the tight-seal whole-cell recording technique, we studied the effects of noradrenaline (NA) on the spontaneous inhibitory synaptic currents (IPSCs) of stellate and Purkinje cells in rat cerebellar slices. 2. In both types of cells, NA (10 microM) induced a marked increase in the frequency of the IPSCs. This effect was observed both in the absence and in the presence of TTX in the saline bathing the cerebellar slices. 3. The NA-induced increase in frequency of IPSCs and miniature IPSCs (mIPSCs) in the two cell types was mimicked by bath applications of isoprenaline (10 microM) and of the adenylyl cyclase activator forskolin (20 microM). Neither phenylephrine nor clonidine changed the frequency of IPSCs in stellate or Purkinje cells. 4. In stellate cells, the beta-agonists and forskolin had variable effects on the amplitudes of both IPSCs and mIPSCs. None of these compounds altered the amplitude of mIPSCs in Purkinje cells. 5. The responses to local applications of GABA to Purkinje cells were unchanged by bath applications of beta-adrenergic agonists or forskolin. A decrease in the response to GABA after treatment with these agents was observed in half the stellate cells examined. 6. We conclude that the major effect of NA on stellate and Purkinje cells is an increase in the frequency of occurrence of spontaneous inhibitory synaptic currents. This action is exerted through the activation of beta-adrenergic receptors and is probably mediated by an intracellular mechanism involving cAMP. The beta-adrenergic modulation of IPSC frequency takes place at the presynaptic level and may involve a change in the process of transmitter release.

Regulation of growth-associated protein 43 (GAP-43) messenger RNA associated with plastic change in the adult rat barrel receptor complex

Plastic change occurs in the adult rat barrel receptor complex following peripheral deafferentation by removal of facial vibrissae (vibrissectomy) and can be prevented by prior depletion of brain norepinephrine. Growth-associated protein (GAP-43, B50, F1, pp46), a marker for synaptic reorganization, increases in the barrel cortex of adult rats following both peripheral and central deafferentation. Here we followed changes in GAP-43 mRNA expression in the barrel receptor system following vibrissectomy. Adult rats had unilateral total vibrissectomy with sparing of the central (C3) vibrissa. By in situ hybridization, GAP-43 mRNA first increased at 24h (9%, P < 0.05) in the ipsilateral trigeminal complex. Levels remained elevated (up to 25% of the unlesioned side) over the next 6 days, decreased to 88% at 7 days and returned to control levels at 14 days. Contralateral barrel cortex levels of GAP-43 mRNA increased by 14% at 4-5 days remained elevated through 7 days and returned to control levels by 14 days. Increased GAP-43 mRNA levels 6 days after vibrissectomy were reproduced by complete transection of the infraorbital nerve and were blocked by depletion of brain norepinephrine. No change occurred in ventrobasal thalamus GAP-43 mRNA at any time. Dot blot and Northern blot hybridizations of GAP-43 mRNA after vibrissectomy showed a 43% increase in the ipsilateral trigeminal complex and a 16% increase in the contralateral barrel cortex at 3 days and an 84% increase in ipsilateral trigeminal and 50% increase in contralateral barrel cortex GAP-43 mRNA at 6 days, respectively. Thus, deafferentation-induced plasticity in the barrel pathway depends upon norepinephrine and is associated with increase in both GAP-43 mRNA and protein suggesting that this may involve a structural change.

Phosphorylation of amino acid neurotransmitter receptors in synaptic plasticity

The precise regulation of synaptic efficacy in the mammalian central nervous system is fundamental for learning, memory, motor control and sensory processing, as well as synaptogenesis. Currently, the molecular mechanisms underlying synaptic plasticity involved in these crucial processes are topics of intense investigation. The modulation of neurotransmitter receptors has received considerable attention, since these receptors mediate signal transduction at the postsynaptic membranes of chemical synapses. Over the past several years, evidence has suggested that protein phosphorylation of neurotransmitter receptors is a common mechanism for the regulation of receptor function. In this reaction, protein kinases catalyse the transfer of a highly charged phosphate moiety from ATP to serine, threonine or tyrosine residues of a neurotransmitter receptor, thereby altering the charge and/or conformation of the receptor and regulating its function. Phosphorylation of neurotransmitter receptors is reversible, can occur rapidly, and might result in prolonged changes in receptor function. Thus, this modification might play an important role in both short- and long-term changes in synaptic transmission.

Effects of noradrenaline on the firing rate of vestibular neurons

The effects of microiontophoretic noradrenaline on the firing rate of neurons located in the vestibular complex have been studied in anaesthetized rats. Eighty-five per cent of the neurons tested in all the vestibular nuclei modified their background firing rate upon noradrenaline application, generally by reducing it (86% of them). In few cases inhibitions were followed by a rebound. Responses were dose-dependent. No significant difference was found between vestibular neurons projecting to the spinal cord and those delivering their fibres to the oculomotor complex. Phentolamine, an alpha-adrenergic antagonist, blocked the noradrenaline-evoked inhibitions, whereas beta-adrenergic antagonist timolol was ineffective or enhanced them. Furthermore, responses were blocked by yohimbine, an alpha 2-adrenergic antagonist, and mimicked by clonidine, an alpha 2-adrenergic agonist, in the majority of neurons. In few cases prazosin, an alpha 1-adrenergic antagonist, was able to antagonize weak inhibitions and phenylephrine, an alpha 1-adrenergic agonist, to evoke an inhibitory effect blocked by prazosin. Isoproterenol, a beta-adrenergic agonist was totally ineffective on the neuronal firing rate. It is concluded that noradrenaline can modify the level of neuronal activity in the vestibular complex by acting mostly, but not exclusively, through alpha 2-adrenergic receptors. An influence of noradrenergic systems on the vestibular function by a direct action of noradrenaline inside the vestibular nuclei is proposed.

Modulation of GABAA receptor-activated current by norepinephrine in cerebellar Purkinje cells

Previous studies employing extracellular single-unit recording in the intact cerebellum have demonstrated that norepinephrine can potentiate GABA-induced suppression of Purkinje cell spike activity. However, many issues related to the nature of this modulatory phenomenon remain to be resolved. Using whole-cell patch clamp recording, the present study investigated the effect of norepinephrine on GABA-activated membrane currents (IGABA) in solitary Purkinje cells isolated from neonatal rat cerebella following acute dissociation. Exposure of Purkinje cells to norepinephrine at a concentration which, by itself, had no obvious effect on Purkinje cell membrane conductance, consistently augmented IGABA. The catecholamine also potentiated GABA-gated chloride currents as well as muscimol-induced currents in Purkinje cells. Thus, the facilitating effect of norepinephrine on IGABA was attributed to an interaction between norepinephrine and the GABAA receptor-mediated chloride conductance. The effect of norepinephrine could be mimicked by isoproterenol as well as by 8-bromo cAMP, suggesting that a beta-receptor-mediated, cAMP-dependent cascade may underlie the observed heteroreceptor interaction. Our results establish the existence of a postsynaptic mechanism by which norepinephrine, through activation of the beta-adrenoceptor, may modulate GABAA receptor function in cerebellar Purkinje cells. This study provides the groundwork for a detailed investigation into the cascade of membrane and intracellular events underlying such a synergistic modulatory interaction at the cellular and subcellular levels.

Functional modulation of GABAA receptors by cAMP-dependent protein phosphorylation

gamma-Aminobutyric acidA (GABAA) receptors are ligand-gated ion channels that mediate inhibitory synaptic transmission in the central nervous system. The role of protein phosphorylation in the modulation of GABAA receptor function was examined with cells transiently transfected with GABAA receptor subunits. GABAA receptors consisting of the alpha 1 and beta 1 or the alpha 1, beta 1, and gamma 2 subunits were directly phosphorylated on the beta 1 subunit by adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase (PKA). The phosphorylation decreased the amplitude of the GABA response of both receptor types and the extent of rapid desensitization of the GABAA receptor that consisted of the alpha 1 and beta 1 subunits. Site-specific mutagenesis of the serine residue phosphorylated by PKA completely eliminated the PKA phosphorylation and modulation of the GABAA receptor. In primary embryonic rat neuronal cell cultures, a similar regulation of GABAA receptors by PKA was observed. These results demonstrate that the GABAA receptor is directly modulated by protein phosphorylation and suggest that neurotransmitters or neuropeptides that regulate intracellular cAMP levels may modulate the responses of neurons to GABA and consequently have profound effects on synaptic excitability.

Electrophysiological actions of VIP in rat somatosensory cortex

Electrophysiological and biochemical studies suggest that VIP may exert a facilitating action in the neocortical local circuitry. In the present study, we examined the actions of VIP and VIP + norepinephrine (NE) on somatosensory cortical neuron responses to direct application of the putative transmitters acetylcholine (ACh) and gamma-aminobutyric acid (GABA). Spontaneous and transmitter-induced discharges of cortical neurons from halothane-anesthetized rats were monitored before, during and after VIP, NE and VIP + NE iontophoresis. In 57 VIP-sensitive cells tested, VIP application (5-70 nA) increased (n = 18), decreased (n = 36) or had biphasic actions (n = 3) on background firing rate. In a group of 20 neurons tested for NE + VIP, the combined effect of both peptide and bioamine was predominantly (70%) inhibitory. On the other hand, inhibitory and excitatory responses of cortical neurons to GABA (11 of 15 cases) and ACh (10 of 18 cases), respectively, were enhanced during VIP iontophoresis. Concomitant application of VIP and NE produced additive (n = 2) or more than additive (n = 3) enhancing effects on GABA inhibition. NE administration reversed or enhanced further VIP modulatory actions on ACh-induced excitation. These findings provide electrophysiological evidence that NE and VIP afferents may exert convergent influences on cortical neuronal responses to afferent synaptic inputs such that modulatory actions are anatomically focused within the cortex.

Noradrenergic potentiation of excitatory transmitter action in cerebrocortical slices: evidence for mediation by an alpha 1 receptor-linked second messenger pathway

Considerable evidence from intact, anesthetized preparations suggests that norepinephrine (NE) can modulate the efficacy of synaptic transmission within local circuits of the mammalian neocortex; i.e. both iontophoretic application of NE and activation of the coeruleocortical pathway are capable of facilitating cortical neuronal responses to non-noradrenergic synaptic inputs and putative transmitter agents. In the present study, the effects of NE on somatosensory cortical neuronal responses to putative excitatory transmitters were characterized using in vitro tissue slice preparations. Somatosensory unit responses to iontophoretic pulses of acetylcholine (ACh) or glutamate (Glu) (10-60 nA; 5-25 s duration) were examined before, during and after a period of continuous NE (1-35 nA; 4-25 min duration) microiontophoresis. Quantitative analysis of per-event histograms indicated that both Glu- and ACh-evoked excitatory discharges were routinely (Glu 94%, n = 54; ACh 67%, n = 9) potentiated above control levels during NE administration. In 8 cells, NE revealed robust excitatory discharges to otherwise subthreshold iontophoretic doses of Glu. The alpha-specific agonist, phenylephrine, mimicked (n = 3), NE-induced potentiation of Glu-evoked discharges whereas the alpha antagonist phentolamine blocked (n = 5) enhancement of these responses. Moreover, activation of protein kinase C by iontophoretic application of phorbol 12,13-diacetate (5-15 nA, n = 4) mimicked the potentiating actions of NE on Glu-evoked excitatory responses. Results from other experiments further indicated that these facilitating actions of NE on Glu-evoked responses do not involve beta receptor activation or intracellular increases in cyclic AMP. In summary, these results demonstrate that NE can facilitate cortical neuronal responses to threshold and subthreshold level applications of putative excitatory transmitter agents. Moreover, it appears that, unlike noradrenergic facilitating influences on GABA-induced inhibition, these actions are mediated by an alpha adrenoceptor mechanism which may be linked to intracellular activation of protein kinase C. Overall, these findings reinforce the idea that noradrenergic modulatory actions on excitatory and inhibitory neuronal responses may involve the activation of separate receptor-linked second messenger systems.

Ethanol potentiation of GABA-induced electrophysiological responses in cerebellum: requirement for catecholamine modulation

In this study, we confirmed that microiontophoretically applied norepinephrine (NE) and isoproterenol potentiate the depressant effects of locally-applied gamma-aminobutyric acid (GABA) on cerebellar Purkinje neurons of anesthetized rats. Although ethanol (EtOH) does not reliably or efficaciously potentiate GABA-induced depressions of neuronal activity, we found that systemic or locally-applied EtOH does markedly potentiate GABA-induced inhibitions of Purkinje neuron firing rate if that response is concomitantly modulated by NE or isoproterenol. This study suggests that the EtOH sensitivity of the GABA mechanism of electrophysiological responses in the cerebellar cortex is regulated by the neuromodulatory effect of beta-adrenergic receptor activation.

Subtypes of adrenergic receptors and intracellular mechanisms involved in modulatory effects of noradrenaline on glutamate

We have previously reported that the response of cultured chick cerebellar neurons to glutamate is enhanced by noradrenaline (NA) or isoproterenol and suppressed by clonidine. The present study was carried out to further specify the adrenergic receptor subtypes involved in the facilitatory effect of NA or isoproterenol and the suppressive effect of clonidine, and to examine the intracellular mechanisms underlying these modulatory effects of NA. The clonidine effect, which was mimicked by NA iontophoresed with large ejecting currents, was blocked by yohimbine and tolazoline (alpha 2 antagonists) and also by dibutyryl cyclic AMP or forskolin which augmented the glutamate response by itself. Prazosin, an alpha 1 receptor antagonist did not block the clonidine effect. NA- or isoproterenol-induced facilitation, which was mimicked by denopamine (beta 1 agonist), was antagonized by acebutolol (beta 1 antagonist) and not by ICI 118,551 (beta 2 antagonist). Pretreatment of neurons with pertussis toxin for more than 24 h blocked the suppressive action of clonidine without affecting the facilitatory action of isoproterenol. Furthermore, intracellular injection of GDP beta S inhibited the modulatory effects of either clonidine or isoproterenol. These results indicate that the facilitatory and inhibitory modulatory effects of NA may be mediated by beta 1 and alpha 2 receptors linked to cAMP systems, respectively, and the former is coupled with the stimulatory G protein (Gs) and the latter is with the inhibitory G protein (Gi).

Changes in VOR adaptation after local injection of beta-noradrenergic agents in the flocculus of rabbits

Noradrenaline (NA) has been implicated as a neuromodulator in plasticity, presumably facilitating adaptive processes. Since the flocculus receives noradrenergic afferents, and ablation of the flocculus interferes with the normal adaptive changes in the VOR gain, experiments were performed to find out whether bilateral injection of monoaminergic substances into the flocculus of rabbits could modify the adaptive changes of the VOR. The visual world surrounding the rabbit was oscillated in opposite direction to the platform on which the rabbit was mounted, which resulted in an adaptive increase in the VOR gain; this adaptation was measured either in light or in darkness. Floccular injection of the beta-agonist isoproterenol did not greatly affect the adaptation of the VOR measured in light. In darkness, however, the increase in gain after injection of isoproterenol was larger than during normal adaptation. The beta-antagonist sotalol reduced the adaptation of the VOR gain significantly in light as well as in darkness. In a control condition without pressure for adaptation (only intermittent testing of the VOR gain over a period of 2.5 h), the gain of the VOR was not significantly affected by similar injections of beta-adrenergic agents. We conclude that the noradrenergic system facilitates the adaptation of the VOR gain to retinal slip in rabbits without affecting the VOR gain directly. At least part of this influence is exerted through beta-receptors located in the cerebellar flocculus.

Second messenger-mediated actions of norepinephrine on target neurons in central circuits: a new perspective on intracellular mechanisms and functional consequences

Ever since the initial demonstration of a widespread distribution of noradrenergic fibers to functionally diverse regions of the mammalian forebrain, there has been considerable interest in determining the electrophysiological effects of norepinephrine (NE) on individual neurons within these target areas. While early studies showed that NE could directly inhibit cell firing via increased intracellular levels of cyclic AMP, more recent work has revealed a spectrum of noradrenergic actions, which are more accurately characterized as neuromodulatory. More specifically, numerous experimental conditions have been described where NE at levels subthreshold for producing direct depressant effects on spontaneous firing can facilitate neuronal responses to both excitatory and inhibitory synaptic stimuli. The goal of this report is to review recent evidence which suggests that the various modulatory actions of NE on central neurons result from the activation of different adrenoceptor-linked second messenger systems. In particular, we have focused on the candidate signal transduction mechanisms that may underlie NE's ability to augment cerebellar and cortical neuronal responsiveness to GABAergic synaptic inputs. The consequences of such NE-induced changes in synaptic efficacy are considered not only with respect to their influences on feature extraction properties of individual sensory cortical neurons but also with regard to the potential impact such actions would have on the signal processing capabilities of a network of noradrenergically innervated cortical cells.

Modulatory actions of norepinephrine on neural circuits

A spectrum of studies has been conducted on a single aspect of NE function in which, through a beta-one receptor activation, NE appears to mediate a degree of physiological control over the gain of GABA mediated inhibition. It is significant that this single effect has been observed in numerous interrelated preparations ranging from single isolated Purkinje cells from young rats to adult Purkinje cells in awake locomoting rats. With respect to the functional conse-quences of these effects, our best current speculation as to "what NE does" is that NE acts to regulate the strength of these tuned gating mechanisms in both cerebral and cerebellar cortices. There are numerous unanswered questions raised by the past work. One pressing issue is - when and for what reason in normal function does the modulation take place? When does NE release normally occur (is it phasic or tonic), and which of the demonstrated actions appears and for how long in relation to period of receptor activation? Does NE release cause the circuit to "react" to conditions which need "improved neurocomputation" or does NE stabilize the circuit to react predictably in the face of stress? Finally, what is the molecular sequence of events between receptor activation and an alteration of GABA receptor channel opening? What additional molecular control mechanisms exist and how can the diverse inhibitory and modulatory phenomena be reconciled, both short and long term? Issues are defined which need to be clarified at all levels of the current skeleton of basic understanding. Our prediction is that pursuit of these issues will benefit from an exchange of insight gained from investigations at all levels.

The cerebellar norepinephrine system: inhibition, modulation, and gating

A series of studies has been conducted to determine the mode of action on the cerebellar cortical circuitry of the norepinephrine (NE)-containing afferents from the locus coeruleus. NE has been known to exert an "inhibitory" action on the background firing observed in Purkinje cells, due presumably to a shift in conductances favoring hyperpolarization. An additional independent action at low threshold appears to be an enhancement of GABA, the inhibitory transmitter of cerebellar interneurons. Recent whole-cell patch-clamp studies on isolated Purkinje cells indicate that exposure to NE increases the chloride current caused by transient pulses of GABA applied iontophoretically. NE applied to Purkinje cells in the parafloccular lobule during stimulation by moving visual patterns revealed the capacity either to "gate" signals initially not expressed, or to amplify the gain of phasic excitations. The control of emergent circuit functions may be the functional consequence of the multiple modulatory functions of NE.

Modification of noradrenergic innervation in the cerebellum of mutant rats with Purkinje cell degeneration (jaundiced Gunn rats)

In heterozygous (Jj) and homozygous Gunn rats (jj), cerebellar noradrenergic innervation was examined using immunohistochemical, neurochemical and electrophysiological techniques. Immunohistochemical analysis using an antiserum against tyrosine hydroxylase (TH) revealed a marked enhancement in immunoreactivity largely in the granular layer and the whole nuclei in the jj cerebellum, resulting from an increase in TH-immunoreactive varicose fibers forming synapse-like structures on the somata and dendrites of granule cells or nuclear neurons. The concentration of norepinephrine in both the cortical and nuclear regions of the jj cerebellum was significantly higher than that in the control, whereas no significant difference of this total amount was observed between the jj and Jj cerebella. Injection of norepinephrine into the Jj cerebellar nuclei reduced the firing rate of spontaneous unitary discharges of neurons in the interpositus nucleus. These findings suggest that the the jj cerebellum causes an enhancement of the noradrenergic innervation which may possibly be one of its characteristic alterations.

Modulation of rat cortical area 17 neuronal responses to moving visual stimuli during norepinephrine and serotonin microiontophoresis

The present study was conducted to examine the actions of norepinephrine (NE) and serotonin (5-HT) on the multiphasic, visually evoked discharges of cells recorded from the visual cortex (area 17) of anesthetized Long-Evans pigmented rats. Visual responses of 51 cells, evoked by computer controlled presentation of moving visual stimuli, were examined before, during and after low level microiontophoretic application of NE (1-55 nA) or 5-HT (1-50 nA). Drug-induced changes in stimulus-evoked and spontaneous discharges were quantitatively assessed by computer analysis of peri-event histograms. In the majority of cases tested, NE produced a net enhancement of visually evoked responses by facilitating excitatory and inhibitory components of stimulus-bound discharges. By contrast, 5-HT tended to suppress stimulus-evoked excitation and inhibition in many cases to the extent that neurons were no longer responsive to appropriate visual stimuli. In selected cases we were able to demonstrate additional effects of NE and 5-HT on response threshold, direction selectivity and discrimination of receptive field borders. For example, in some cells NE was capable of revealing evoked responses to visual stimuli which were previously ineffective in eliciting stimulus-bound discharges. In other instances, changes in cell activity evoked by stimulus movement across the visual field were accentuated during NE application in such a way that unit discharges at receptive field borders were more sharply defined in comparison to control conditions. 5-HT, on the other hand, was capable of decreasing the contrast between spontaneous and visually evoked discharge at receptive field boundaries. In summary, these results suggest that endogenously released NE and 5-HT may modulate, by complimentary actions, the magnitude of responses of visual cortical neurons to afferent synaptic inputs. Moreover, these monoaminergic projection systems may also have the capacity to modify the threshold of detection of afferent signals within a neuronal network as well as alter feature extraction properties of the circuit.

Modification of the visual response properties of cerebellar neurons by norepinephrine

Extracellular recordings were conducted in the paraflocculus of anesthetized Long-Evans pigmented rats to determine how ionotophoresis of norepinephrine (NE) affects the responsiveness of individual Purkinje cells and interneurons to presentations of visual stimuli within their visual receptive fields. Presentations of moving or stationary visual stimuli during the control (pre-NE) period elicited simple spike excitations or inhibitory responses in slightly more than one-half (55%, n = 32) of the cells tested (20 of 38 Purkinje cells, 12 of 20 interneurons). The predominant effect of NE iontophoresis was to improve visually evoked responses in those neurons which showed modulations in their simple spike discharge to control presentations of visual stimuli. A clear enhancement of visual responses by NE (i.e., absolute increase over control) was observed in 18 of the units, and in 12 of the 14 remaining cells, reductions in stimulus-bound discharge during catecholamine iontophoresis were accompanied by much larger depressions in background activity, resulting in a net enhancement in the ratio of signal-to-noise. NE differentially affected responses to stimulus movement in the preferred and non-preferred direction in one-third of these neurons, such that directional selectivity was increased. However, the orientation bias of individual units was unchanged by NE. Iontophoretic application of the beta-adrenergic antagonist sotalol but not the alpha-adrenergic antagonist phentolamine blocked these facilitating noradrenergic effects. An additional feature of noradrenergic action was revealed in tests conducted in 26 cells which did not respond to control presentations of visual stimuli. Iontophoresis of NE resulted in the elicitation of visual responses in 11 of these units, suggesting the possibility that NE might act in some cases to gate the efficacy of subliminal synaptic input conveyed by classical afferent channels. It is proposed that an important aspect of noradrenergic action within local cerebellar circuits might be to refine the receptive field properties of individual neuronal elements and thereby improve information flow through the cerebellum.