Cyclic adenosine monophosphate and norepinephrine: effects on transmembrane properties of cerebellar Purkinje cells

Neck muscle fatigue impacts plasticity and sensorimotor integration in cerebellum and motor cortex in response to novel motor skill acquisition

The cerebellum undergoes neuroplastic changes in response to motor learning. Healthy human individuals demonstrate reduced cerebellar inhibition (CBI) following motor learning. Alterations in neck sensory input due to muscular fatigue are known to impact upper limb sensorimotor processing, suggesting that neck fatigue may also impact cerebellum to motor cortex (M1) pathways in response to motor learning. Therefore, this study aimed to determine whether cervical extensor muscle (CEM) fatigue alters CBI in response to motor learning. We examined 16 participants (8 CEM fatigue and 8 CEM control). A double cone transcranial magnetic stimulation (TMS) coil stimulated the ipsilateral cerebellar cortex 5 ms before application of contralateral test stimuli of the M1 to the right first dorsal interosseous muscle. Cerebellar-M1 activity curves were established pre- and post-motor skill acquisition (consisting of tracing sinusoidal-pattern waves with the index finger) and following either the CEM fatigue or control intervention. The control group showed greater cerebellar disinhibition than the fatigue group following motor skill acquisition (P < 0.006), while the fatigue group showed similar levels of CBI pre- and post-motor skill acquisition. Both groups improved in accuracy following acquisition (P = 0.012) and retention (P = 0.007), but the control group improved significantly more (17% at acquisition and 22% at retention) versus lower (6% and 9%) improvements for the fatigue group. Lessened cerebellar disinhibition in the CEM fatigue versus control group, coupled with diminished motor learning, suggests that CEM fatigue affects the cerebellar-M1 interaction, influencing the cerebellum's ability to adjust motor output to acquire and learn a novel motor task.NEW & NOTEWORTHY Normally motor learning decreases cerebellar inhibition (CBI) to facilitate learning of a novel skill. In this study, neck fatigue before motor skill acquisition led to less of a decrease in CBI and significantly less improvement in performance accuracy relative to a control group. This study demonstrated that neck fatigue impacts the cerebellar-motor cortex interaction to distal hand muscles, a highly relevant finding due to the altered neck postures and fatigue accompanying increased technology use.

Locus coeruleus: From global projection system to adaptive regulation of behavior

The brainstem nucleus locus coeruleus (LC) is a major source of norepinephrine (NE) projections throughout the CNS. This important property was masked in very early studies by the inability to visualize endogenous monoamines. The development of monoamine histofluorescence methods by Swedish scientists led to a plethora of studies, including a paper published in Brain Research by Loizou in 1969. That paper was highly cited (making it a focal point for the 50th anniversary issue of this journal), and helped to spark a large and continuing set of investigations to further refine our understating of the LC-NE system and its contribution to brain function and behavior. This paper very briefly reviews the ensuing advances in anatomical, physiological and behavioral aspects of the LC-NE system. Although its projections are ubiquitously present throughout the CNS, recent studies find surprising specificity within the organizational and operational domains of LC neurons. These and other findings lead us to expect that future work will unmask additional features of the LC-NE system and its roles in normative and pathological brain and behavioral processes. This article is part of a Special Issue entitled SI:50th Anniversary Issue.

Movement-related discharge in the cerebellar nuclei persists after local injections of GABA(A) antagonists

Limb movement-related neurons in the cerebellar nuclei (CN) typically produce bursts of discharge in association with movement. Consequently, given the inhibitory nature of the Purkinje cell (PC) projection to CN, it is puzzling that only a minority of movement-related PCs pause; the majority burst. Some of the movement-related CN activity may be the result of excitation from collaterals of mossy and climbing fiber projections to the cerebellar cortex. The only other input to CN is diffuse and neuromodulatory, from locus ceruleus and raphe nuclei. To investigate the role of the excitatory mossy fiber input, single units in CN were recorded in macaque monkeys during the performance of reaching and manipulation tasks, before and after blocking the PC input with local microinjections of GABA(A) antagonists (bicuculline or SR95531). After these injections, the movement-related modulation of CN discharge was greater and began earlier, compared with the modulation in the preinjection group of neurons. These observations indicate that an important excitatory drive is provided by extracerebellar inputs to CN, most likely from collaterals of mossy fibers. PCs may serve primarily to regulate this activity, by either pausing or bursting as necessary.

beta-adrenergic receptor-mediated presynaptic facilitation of inhibitory GABAergic transmission at cerebellar interneuron-Purkinje cell synapses

Norepinephrine (NE) has been shown to elicit long-term facilitation of GABAergic transmission to rat cerebellar Purkinje cells (PCs) through beta-adrenergic receptor activation. To further examine the locus and adrenoceptor subtypes involved in the NE-induced facilitation of GABAergic transmission, we recorded inhibitory postsynaptic currents (IPSCs) evoked by focal stimulation with paired-pulse (PP) stimuli from PCs in rat cerebellar slices by whole cell recordings and analyzed the PP ratio of the IPSC amplitude. NE increased the IPSC amplitude with a decease in the variance of the PP ratio, which was mimicked by presynaptic manipulation of the transmission caused by increasing the extracellular Ca(2+) concentration, confirming that the presynaptic adrenergic receptors are responsible for the facilitation. Pharmacological tests showed that the beta(2)-adrenoceptor antagonist, ICI118,551, but not the beta(1)-adrenoceptor antagonist, CGP20712A, blocked the NE-induced IPSC facilitation, suggesting that the beta(2)-adrenoceptors on cerebellar interneurons, basket cells (BCs), mediate the noradrenergic facilitation of GABAergic transmission. Double recordings were performed from BCs and PCs to further characterize the regulation of the GABAergic synapses. First, on-cell recordings from BCs showed that the beta-agonist isoproterenol (ISP) increased the frequencies of the spontaneous spikes in BCs and the spike-triggered IPSCs in PCs recorded with the whole cell mode. The amplitude of the spike-triggered IPSCs decreased or increased depending on the individual GABAergic synapses examined. Forskolin invariably increased both the amplitude and the frequency of the spike-triggered IPSCs. Double whole cell recordings from BC-PC pairs showed that ISP mainly caused an increase in the amplitude of the IPSCs evoked in the PCs by an action current in the BCs produced in response to voltage steps from -60 to -10 mV. Our data suggest that the noradrenergic facilitation of GABAergic transmission in the rat cerebellar cortex is mediated, at least in part, by depolarization and action potential discharges in the BCs through activation of the beta(2)-adrenoceptors in BCs coupled to intracellular cyclic AMP formation.

Metabotropic glutamate receptor agonists alter neuronal excitability and Ca2+ levels via the phospholipase C transduction pathway in cultured Purkinje neurons

Selective agonists for metabotropic glutamate receptor (mGluR) subtypes were tested on mature, cultured rat cerebellar Purkinje neurons (> or = 21 days in vitro) to identify functionally relevant mGluRs expressed by these neurons and to investigate the transduction pathways associated with mGluR-mediated changes in membrane excitability. Current-clamp recordings (nystatin/perforated-patch method) were used to measure the membrane response of Purkinje neurons to brief microperfusion pulses (1.5 s) of the group I (mGluR1/mGluR5) agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (300 microM), quisqualate (5 microM), and (R,S)-3,5-dihydroxyphenylglycine (50-500 microM). All group I mGluR agonists elicited biphasic membrane responses and burst activity in the Purkinje neurons. In addition, the group I mGluR agonists produced alterations in the active membrane properties of the Purkinje neurons and depressed the OFF response after hyperpolarizing current injection. In parallel microscopic Ca2+ imaging experiments, application of the group I mGluR agonists to fura-2-loaded cells elicited increases in intracellular Ca2+ in both the somatic and dendritic regions. The group II (mGluR2/mGluR3) agonist (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (10 microM) and the group III (mGluR4/mGluR6/mGluR7/mGluR8) agonists L(+)-2-amino-4-phosphonobutyric acid (1 mM) and O-phospho-L-serine (200 microM) had no effect on the membrane potential or intracellular Ca2+ levels of the Purkinje neurons. The cultured Purkinje neurons, but not granule neurons or interneurons, showed immunostaining for mGluR1alpha in both the somatic and dendritic regions. All effects of the group I mGluR agonists were blocked by (+)-alpha-methyl-4-carboxyphenylglycine (1 mM), an mGluR antagonist. Furthermore, the phospholipase C inhibitor 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H -pyrrole-2,5-dione (2 microM) blocked the group I mGluR agonist-mediated electrophysiological response and greatly attenuated the Ca2+ signal elicited by group I mGluR agonists, particularly in the dendrites. The inactive analogue 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-2, 5-pyrrolidine-dione (2 microM) was relatively ineffective against the electrophysiological response and Ca2+ signal. These results indicate that functional group I mGluRs (but not group II or III mGluRs) can be activated on mature Purkinje neurons in culture and result in changes in neuronal excitability and intracellular Ca2+ mediated through phospholipase C. These data obtained from a defined neuronal type, the Purkinje neuron, confirm biochemical and molecular studies on the transduction mechanisms of group I mGluRs and show that this transduction pathway is linked to neuronal excitability and intracellular Ca2+ release in the Purkinje neurons.

Interaction of glutamatergic and adrenergic inputs of cortical neurons during conditioning

Background and evoked activities of sensorimotor cortex neurons have been examined on learning cats with conditioned placing reaction before, during and after iontophoretic application of synaptically active drugs. It was shown that glutamate exerted not only a direct excitatory effect on the cortical neurons during its application, but also developed modulatory influences on background and evoked impulse activity after cessation of application in the subsequent 10-20 min. Adrenergic influences on the activity of neocortical neurons evoked by application of adrenomimetic drugs were complex and consisted of at least two different types. Noradrenaline depressed background and particularly evoked activity of many neurons through beta1-adrenoreceptors. At the same time, activation of beta2-adrenoreceptors was accompanied by facilitation of background and evoked activity during application and 10-20 min after its cessation, as was shown in experiments with alupent. Co-application of glutamate and alupent improved facilitation of impulse response evoked by conditioned stimuli. It was concluded that beta1- and beta2-adrenergic inputs to neocortical neurons are involved in plasticity changes of glutamate inputs of some cortical neurons.

Activation of metabotropic glutamate receptors induces an outward current which is potentiated by methylxanthines in rat cerebellar Purkinje cells

The responses of slice-cultured Purkinje cells to trans-DL-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD) were examined by intracellular recording techniques and fura-2 microfluorometry. Bath-application of t-ACPD (100 microM, 30 s), a selective agonist of metabotropic glutamate receptors (mGluRs), to Purkinje cells voltage-clamped near their resting potential -65 to -60 mV) consistently induced a transient inward current, followed by a slower outward current (Iout). This outward current was characterized by a linear current-voltage relationship in the range from -130 to -60 mV and accompanied by a significant decrease in membrane conductance. The extrapolated reversal potential of Iout was positive to 0 mV. When t-ACPD was applied for 60 s or more it became apparent that Iout emerged in parallel to the wash-out of t-ACPD. Microfluorometric fura-2 measurements in combination with electrophysiological recordings were used to assess the relation between Iout and intracellular free calcium concentration ([Ca2+]i). In contrast to the inward current that was associated with a transient elevation in [Ca2+]i. Iout was not correlated with an elevated [Ca2+]i. When t-ACPD was applied in the presence of caffeine (5 mM), Iout was reversibly enhanced in amplitude. Caffeine affected neither the t-ACPD-induced calcium signal nor the resting [Ca2+]i. While longer applications of caffeine alone induced outward currents with a current-voltage relationship similar to that of Iout, short applications (30 s) of caffeine had no detectable effect per se but still were effective in enhancing Iout when applied in conjunction with t-ACPD. 3-Isobutyl-1-methylxanthine (IBMX, 0.5 mM), a more selective and potent phosphodiesterase inhibitor than caffeine, exhibited caffeine-like effects at a 10-fold lower concentration. We propose that Iout is generated by a transient inhibition of an inward current that is tonically active at rest and largely voltage-independent in the range tested. Our observations provide evidence for an involvement of cyclic nucleotide second messenger systems in the regulation of this current.

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.

Noradrenergic and cholinergic modulations of corticocerebellar activity modify the gain of vestibulospinal reflexes

In addition to mossy fibers and climbing fibers, the cerebellar cortex receives noradrenergic and cholinergic afferents. Since the Purkinje (P) cells of the cerebellar vermis (culmen) respond to roll tilt of the animal with a discharge pattern that is out of phase with respect to that of the related lateral vestibular neurons, thus exerting a facilitatory influence on the gain of the vestibulospinal (VS) reflex, we tested the effects of local microinjection into the anterior vermis of noradrenergic and cholinergic agents on these reflexes. In decerebrate cats, unilateral microinjection in the paramedial zone B of the culmen of 0.25 microliters of small doses of alpha 1-, alpha 2-, and beta-noradrenergic agonists (i.e., metoxamine, clonidine, and isoproterenol, respectively) increased the response gain (in impulses/second per deg) of the EMG response of the ipsilateral and to some extent also of the contralateral triceps brachii to animal tilt (at 0.15 Hz, +/- 10 degrees). On the other hand local injection of the corresponding antagonists (i.e., prazosin, yohimbine, and propranolol) either decreased the gain of the ipsilateral triceps brachii to labyrinth stimulation or else prevented the occurrence of the effects induced by the corresponding agonists. An increase in gain of the VS reflexes was also elicited in other experiments by unilateral microinjection either of the nonselective cholinergic agonist carbachol or of the anticholinesterase eserine sulfate. Thus, the effects could be produced by increasing the naturally present amount of acetylcholine. Further experiments indicated that a bilateral increase in the response gain of the triceps brachii to labyrinth stimulation occurred after microinjection of a selective muscarinic (bethanechol) or nicotinic agonist (nicotine), while just the opposite result was obtained after microinjection of the corresponding muscarinic (scopolamine) and nicotinic (hexamethonium, D-tubocurarine) blockers. The effects of the noradrenergic and cholinergic agonists, which persisted for about two hours after the injection, were site specific and dose dependent. It appears, therefore, that the noradrenergic and cholinergic afferents to the cerebellar vermis intervene in the gain regulation of the VS reflexes, possibly by increasing the amplitude of modulation of the P cells to labyrinth stimulation.

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.

Noradrenergic agents into the cerebellar anterior vermis modify the gain of vestibulospinal reflexes in the cat

The noradrenergic (NA) afferent projection to the cerebellar cortex, which originates mainly from the locus coeruleus (LC), may act on the target neurons by utilizing both alpha- and beta-adrenoceptors. Experiments performed in decerebrate cats have shown that unilateral injection into the vermal cortex of the cerebellar anterior lobe of 0.25 microliter of the alpha 1-adrenergic agonist metoxamine or the alpha 2-agonist clonidine (at 2-8 micrograms/microliters of saline) as well as of the non-selective beta-agonist isoproterenol (at 8-16 micrograms/microliters) decreased the postural activity in the ipsilateral forelimb, while the extensor tonus either remained unmodified or slightly increased on the contralateral side. The same agents also increased the gain of the vestibulospinal (VS) reflexes elicited by recording the multiunit EMG responses of the ipsilateral and the contralateral triceps brachii to roll tilt of the animal (at 0.15 Hz, +/- 10 degrees), leading to sinusoidal stimulation of labyrinth receptors. The crossed effects were more prominent for the alpha 2- than for the alpha 1- and beta-agonists. Only slight changes in the phase angle of the responses were observed. The effects described above appeared 5-10 min after the injection, reached the peak values after 15-30 min and disappeared within 2 h. The effective area was located within the third and/or the fourth folium of the culmen rostral to the fissura prima, 1.4-1.8 mm lateral to the midline. This area corresponded to zone B of the cerebellar cortex, which projects to the ipsilateral lateral vestibular nucleus (LVN), on which it exerts a prominent inhibitory influence. In fact, monopolar stimulation of this area with three negative pulses (at 300/sec) performed prior to the local injection inhibited the spontaneous EMG activity of the ipsilateral triceps brachii. The effects described above were dose-dependent; injection of an equal volume of saline was ineffective. All changes in posture and reflexes elicited by metoxamine or clonidine were impaired by previous injection into the same corticocerebellar area of the corresponding alpha 1- or alpha 2-adrenergic antagonist prazosin or yohimbine, respectively (0.25 microliters at 8-16 micrograms/microliters). However, cross-interactions between alpha 1- and alpha 2-adrenergic agonists and antagonists were also observed. In fact, injection of the alpha 2-adrenergic antagonist yohimbine prevented the occurrence of all the metoxamine effects, while administration of the alpha 1-adrenergic antagonist prazosin prevented the occurrence of the ipsilateral, but not of the contralateral effects induced by clonidine injection.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Increased activity of tyrosine hydroxylase in the cerebellum of the X-irradiated dystonic rat

The exposure of the cephalic end of rats to repeated doses of X-irradiation (150 rad) immediately after birth induces a long-term increase in the noradrenaline (NA) content of cerebellum (CE) (+ 37.8%), and a decrease in cerebellar weight (65.2% of controls), which results in an increased NA concentration (+ 109%). This increase in the neurotransmitter level is accompanied by a dystonic syndrome and histological abnormalities: Purkinje cells (the target cells for NA afferents to CE) fail to arrange in a characteristic monolayer, and their primary dendritic tree appears randomly oriented. The injection of reserpine 0.9 and 1.2 mg/kg ip to adult rats for 18 h depletes cerebellar NA content in both controls (15.7 +/- 4 ng/CE and 2.8 +/- 1.5 ng/CE, respectively) and X-irradiated rats (17.1 +/- 1 ng/CE and 8.3 +/- 2 ng/CE, respectively). The activity of tyrosine hydroxylase (TH) in CE of adult rats, measured by an in vitro assay, is significantly increased in neonatally X-irradiated animals when compared to age-matched controls (16.4 +/- 1.4 vs 6.32 +/- 0.6 nmol CO2/h/mg prot., p less than 0.01). As observed for NA levels, a net increase in TH activity induced by the ionizing radiation is also measured: 308.9 +/- 23.8 vs 408.2 +/- 21.5 nmol CO2/h/CE, p less than 0.01 (controls and X-treated, respectively). These results suggest that X-irradiation at birth may induce an abnormal sprouting of noradrenergic afferents to CE. The possibility that these changes represent a response of the NA system to the dystonic syndrome is discussed.

Messenger molecules in the cerebellum

As data accumulate, the mammalian brain reveals its complex and subtle synaptic mechanisms. In the simplest system, a neurotransmitter binds to the receptor portion of a molecular complex incorporating an ion channel and thus alters the membrane potential, leading to excitatory or inhibitory effects. In more complex systems, receptors are coupled to second messenger systems to generate signals of longer duration and to modulate more diverse molecular mechanisms. The cerebellar cortex has a relatively simple wiring diagram with the primary neurotransmitter of most inhibitory and excitatory synapses well established. The second messenger signalling systems are more complex and those of the cerebellar output, the Purkinje cells, are the best characterized. More recently, molecules that might act as neuromodulators, carrying messages between neurons and between neurons and glial cells, have been identified, such as endothelin and nitric oxide. The classic neurotransmitters and novel neuromodulators, together with second messenger-activated trophic factors, can interact in complex ways; in this review Christopher Ross, David Bredt and Solomon Snyder discuss how studies of cerebellar circuitry and biochemistry are revealing such interrelations.

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.

Participation of cyclic adenosine monophosphate and beta-adrenergic receptors in the facilitatory effect of noradrenaline on the response of cultured cerebellar neurons to glutamate

For the purpose of examining possible involvement of the cyclic adenosine monophosphate (cAMP) system and adrenergic receptors in the modulatory effect of noradrenaline (NA) on the glutamate-induced depolarizing response, the effects of dibutyryl cAMP (DBcAMP), forskolin, theophylline, clonidine, isoproterenol and propranolol were intracellularly investigated in the cerebellar neurons cultured from chick embryos. Not only NA-induced hyperpolarization and increase in input resistance but also the facilitatory effect of NA on the glutamate response were mimicked by DBcAMP and isoproterenol. This facilitatory effect of DBcAMP was enhanced by theophylline or forskolin, while that of isoproterenol was antagonized by propranolol. Clonidine suppressed glutamate-induced depolarization. These results that the enhancing action of NA on the responsiveness of cultured cerebellar neurons to excitatory amino acids is mediated by beta-adrenergic receptors and the intracellular cAMP system.

New evidence for a gating action of norepinephrine in central neuronal circuits of mammalian brain

Many previous studies have examined the effects of norepinephrine (NE) on neuronal responsiveness to synaptic inputs and putative transmitter substances and have described differential depressant actions of NE on stimulus evoked versus spontaneous discharge such that the "signal to noise" ratio of threshold responses was increased. In the present studies, similar experimental strategies employing a combination of microiontophoresis, single unit recording and afferent pathway stimulation in intact anesthetized and brain tissue slice preparations have revealed noradrenergic "gating" actions whereby weak or subthreshold synaptic stimuli can evoke threshold neuronal responses in the presence of iontophoretically applied NE or following electrical stimulation of the locus coeruleus. Overall, these results suggest that potentially threshold excitatory and inhibitory synaptic inputs may normally arrive at central neurons but appear weak or absent except during behavioral conditions favoring the synaptic release of NE. As such, these findings provide evidence that signal to noise ratio may not be the only potential modulatory action expressed by NE in noradrenergic target circuits of the mammalian brain.

Noradrenaline excites neurons in the guinea pig cerebellar vermis in vitro

Noradrenaline (NA) was applied to the solution bathing the cavy cerebellar vermis in vitro and the responses of 98 neurons were recorded extracellularly. Two thirds (23/35) of the responses were excitations and the remaining third were inhibitions. The lowest concentration of NA with which responses could be obtained was 10(-11) M NA. Responses were generally transient and occurred with a mean latency of 61 +/- 8 sec. The excitation was generally direct as most responses (9/11) survived synaptic blockade. The excitations were thought to be mediated by alpha 1 receptors because they could be mimicked by phenylephrine and antagonised by prazozin.

Electroencephalographic manifestations of grand mal epilepsy in Africans: observation of relative rarity of interictal abnormalities

Interictal EEG records of Nigerian epileptic patients living in Lagos and clearly diagnosed as having grand mal epilepsy were compared with those of British epileptic patients with a similar diagnosis. Interictal EEG abnormalities (spike and wave discharges, (SW), photoparoxysmal discharges (PPD), and nonspecific changes) were significantly less common in Nigerian patients (incidence 15.7%) compared with British patients (incidence 52.9%) irrespective of age or sex of the patients. These findings may be related to ethnic or geographical factors. Rarity of PPD in Africans has previously been attributed to ethnic factors. Other studies have demonstrated an influence of geographical factors, specifically sunshine. In papio papio, such a geographical factor was shown to influence occurrence of PPD. In human epileptic subjects, environmental sunshine has been found to inversely influence the occurrence of PPD as well as SW. Besides possible ethnic factors, the relative rarity of interictal EEG abnormalities in Africans with grand mal epilepsy may be related to the larger amount of sunshine in the tropics.

Effects of noradrenaline on the responsiveness of cultured cerebellar neurons to excitatory amino acids

The effects of noradrenaline (NA) on the responsiveness of cultured cerebellar neurons to excitatory amino acids were intracellularly investigated. NA applied to external medium to a final concentration of 10 microM or lower slightly decreased the firing frequency of spontaneous spikes, induced a small hyperpolarization or slightly increased the input resistance of Purkinje cells. In addition, bath-applied NA was found to enhance the depolarizations induced by iontophoretically applied glutamate and aspartate but to a smaller extent for the latter. These direct and modulating effects of NA were also observed when NA was applied by iontophoresis. The sites sensitive to iontophoresed NA were found to be not uniformly distributed but localized in restricted regions on individual Purkinje cells. The enhancement by NA of the glutamate or aspartate response was blocked by beta-adrenergic antagonists, propranolol or pindolol, and extracellularly applied cAMP mimicked the NA action. These results suggest the possibility that NA physiologically modulates excitatory amino acid-mediating synaptic transmission in the cerebellum probably by acting on beta-rather than alpha-adrenergic receptors.

Production of an antiserum against cyclic nucleotide phosphodiesterase and its use for the immunocytochemical demonstration of this enzyme in rat cerebellum

A procedure for the separation of cyclic AMP phosphodiesterase from a commercially available preparation and for raising antibodies against this enzyme in rabbits is described. An antiserum thus obtained was used for the immunocytochemical detection of cyclic nucleotide phosphodiesterase in rat cerebellum. The molecular layer, the granular layer and the cerebellar white matter exhibited different degrees of immunoreactivity. Only a few cell bodies (possibly glial cells) were stained. Most of the antigenic sites were present in the neuropil of the molecular layer and around Purkinje cells. Cerebellar glomeruli, sites of synaptic interactions between mossy fibres, Golgi cells and granule cells, were also stained by this antiserum. Control reactions using preimmune serum were consistently negative.

Electrophysiological actions of norepinephrine in rat lateral hypothalamus. I. Norepinephrine-induced modulation of LH neuronal responsiveness to afferent synaptic inputs and putative neurotransmitters

The present studies were conducted as part of an ongoing investigation of the effects of norepinephrine (NE) in neuronal circuits of the mammalian brain. In this report, we describe noradrenergic actions in the lateral hypothalamus (LH), an area which has been implicated in the central integration of cardiovascular regulatory mechanisms, fluid balance and ingestive behaviors. Microiontophoretically applied NE was interacted with extracellularly recorded responses of LH neurons to iontophoretically applied putative neurotransmitters gamma-aminobutyric acid (GABA), acetylcholine (ACh) and glutamate (Glu); and activation of known input pathways from the reticular thalamus (RT) and the lateral preoptic area (LPO). Peri-event histograms of cell responses were computed before, during and after NE microiontophoresis (5-50 nA) and used to quantitatively evaluate monoamine-induced effects on spontaneous and stimulus evoked activity of LH neurons. In 16 of 23 LH neurons, RT-stimulus-induced inhibition was markedly prolonged from a mean of 28.3 +/- 4.8 ms to 44.7 +/- 5.2 ms, during iontophoretic application of NE. In 22 of 38 LH cells, LPO-stimulus-induced excitatory responses were enhanced above control levels during NE administration. In further tests, inhibitory responses of LH cells to iontophoretic pulses of GABA were potentiated during NE administration in 69% (24 of 35) of the cases tested. ACh-induced excitation was potentiated in 9 of 21 cells. In 4 of these cases, otherwise subthreshold doses of ACh caused marked increases in cell firing during the period of NE administration. By contrast, Glu-evoked excitation was antagonized by NE iontophoresis in 65.5% (17 of 26) of LH cells tested. These findings indicate that, as in other noradrenergic target regions of the CNS, NE can facilitate synaptically mediated responses of LH neurons. Taken together these observations suggest that NE may play an important regulatory role in the synaptic transfer of information within LH circuits, and consequently exert considerable influence over the homeostatic functions mediated by this structure.

Noradrenaline- and vasoactive intestinal peptide-containing neuronal systems in neocortex: functional convergence with contrasting morphology

Neurotransmitter-specific anatomical techniques have provided a tool to define the morphological constraints within which a given neurotransmitter will exert its cellular actions. Biochemical and electrophysiological approaches have revealed the nature of these cellular actions for several neurotransmitters. Furthermore, by using purified preparations and tissue cultures a certain degree of resolution has been achieved by which the cell type, where a neurotransmitter's effect takes place, can be determined. In this article we review these aspects for noradrenaline and vasoactive intestinal peptide, two neurotransmitters of the cerebral cortex contained within neuronal systems that present strikingly different morphologies. Nevertheless, noradrenaline and vasoactive intestinal peptide share certain cellular actions and can interact synergistically. The experimental evidence accumulated to date indicates that noradrenaline- and vasoactive intestinal peptide-containing neurons can influence three general cell types of the cerebral cortex, i.e. (i) other neurons, (ii) astrocytes and (iii) cells of the vasculature. This diversity in cellular partners supports the notion that noradrenaline and vasoactive intestinal peptide can be released from neurons at conventional synapses as well as at extrasynaptic sites, thus suggesting the co-existence of two modes of release within the same neuron.

Electrophysiological correlates of presynaptic opiate receptor activation: reduction in norepinephrine-mediated inhibition from the locus coeruleus

Inhibitory responses of rat cerebellar Purkinje cells to locus coeruleus (LC) stimulation and iontophoresis of norepinephrine (NE) were examined before and after administration of morphine to determine whether the inhibitory modulation of NE release by opiates results in a functional impairment in noradrenergic synaptic action. Administration of morphine systemically (0.2-1.2 mg/kg, i.v.) or by iontophoresis reduced inhibitions in Purkinje firing elicited by LC stimulation without affecting depressions in activity induced by postsynaptic applications of NE. This antagonistic effect of morphine on LC-induced inhibition was reversed or prevented by naloxone and mimicked by administration of levorphanol but not dextrorphan. Morphine increased the excitatory response of Purkinje cells to monosynaptic input from the parallel fibers, whereas it blocked gamma-aminobutyric acid-induced inhibitions in firing via a non-opiate receptor-mediated mechanism. These results demonstrate that morphine interferes with synaptic inhibition derived from the LC and suggest that this may occur via activation of presynaptic opiate receptors residing on noradrenergic nerve terminals.

Noradrenergic innervation of the cerebellar cortex in normal and in Purkinje cell degeneration mutant mice: evidence for long term survival following loss of the two major cerebellar cortical neuronal populations

Purkinje cell degeneration mutant mice were examined during the course of Purkinje cell death (26 and 35 days old) and at 3, 5, 9 and 12 months of age. Glyoxylic acid fluorescence histochemistry for catecholamines was used to investigate possible alterations or reorganization of the noradrenergic fibers from the coeruleo-cerebellar system in response to the degeneration of two major cell types in the cerebellar cortex, of which one, the Purkinje cell, is reported to be the major target neuron. In control mice, noradrenergic fibers traveled in linear and tortuous profiles through the granule cell layer, formed pericellular arrays alongside Purkinje cell somata, and branched profusely into both radially oriented and longitudinally oriented chains. The density of noradrenergic varicosities diminished in the molecular layer, there was with age. In the mutants, concomitant with the progressive shrinkage of the molecular layer, there was a progressive increase in the density of noradrenergic varicosities. This was most conspicuous at 9 and 12 months of age, at which time the molecular layer has been depleted not only of Purkinje cell dendrites, but also of parallel fibers. Noradrenergic fibers in these zones formed dense parallel bundles of varicose profiles whose density reached 621.3 +/- 122.8% (mean +/- SD, n = 4) at 9-12 months of age, compared with age-matched controls. Neurochemical measurement of norepinephrine content in whole cerebellum of the Purkinje cell degeneration mutants revealed no change compared with age-matched controls. We conclude that noradrenergic innervation persists in the cerebellar cortex despite the death of Purkinje cells and most of the granule cells. Although we found an increased density of varicosities in the molecular layer of mutant mice, progressing with age, we believe that this can be explained on the basis of the resultant geometry of the altered cerebellar cortex. It appears that the health of the environment surrounding the noradrenergic fibers in cerebellar cortex has little influence on their anatomical integrity.

Immunocytochemically defined astroglia from fetal, newborn and young adult rats express beta-adrenergic receptors in vitro

Autoradiography of radioligand binding was used to assess the expression of beta-adrenergic receptors (beta-AR) by immunocytochemically identified astroglia cultured from the cerebral cortices of rats 16 days in gestation through 28 days postnatal (DPN). Polygonal astroglia isolated from animals at each age examined were found to exhibit large numbers of beta-AR. In contrast, only low levels of beta-AR could be detected on process-bearing astroglia and fibroblasts. Quantitative analysis showed that there was an increase in the density of beta-AR on polygonal astroglia between 16 days in gestation and 1 DPN. This increase in beta-AR receptor density was present whether the cells were grown for long periods of time in culture (8-22 days) or for short periods of time in culture (1-5 days). The results also suggest that differences in the level of receptor expression between cells grown in short-term and long-term culture may be due in part to culture methodology.

Forskolin effects on longitudinal myometrial strips from the pregnant rat: relationship with membrane potential and cyclic AMP

The effects of forskolin on tension, membrane potential and cyclic AMP accumulation were studied in longitudinal myometrial strips from pregnant rats. 0.1 microM forskolin reduced the amplitude of spontaneous contractions by decreasing the frequency of action potential discharge without a change in resting potential or cyclic AMP accumulation. Forskolin, 1.0 microM, abolished contractions and action potentials, hyperpolarized the membrane and increased cyclic AMP accumulation. Ouabain, 1 mM, depolarized the muscle and increased resting tension. Ouabain reduced potential change produced by forskolin but did not prevent the relaxation or cAMP accumulation. Therefore changes in membrane potential are not prerequisite for the inhibitory actions of forskolin. The cyclic AMP-related relaxation may result primarily from intracellular events that remove calcium from the contractile elements.

Calcitonin-induced anorexia in rats: evidence for its inhibitory action on lateral hypothalamic chemosensitive neurons

Effects of a synthetic derivative of eel calcitonin, [Asu1,7]ECT on feeding behavior, and its direct action on the neuronal activity of lateral hypothalamic area (LHA) were studied in rats. Food intake was significantly reduced in a dose-dependent manner after intra-third ventricular injection of [Asu1,7]ECT (0.2-1.0 U/rat). The neuronal activity of LHA neurons, especially the glucose-sensitive neurons, was inhibited by electrophoretic application of [Asu1,7]ECT. The inhibition was accompanied by a hyperpolarization of the membrane by about 5-7 mV with an increase in the membrane resistance (5.0-8.7%). This effect was also shown to be independent of noradrenergic or serotonergic mechanisms. Phosphodiesterase inhibitors, such as 3-isobutyl-1-methylxanthine and papaverine augmented the inhibitory response, whereas nicotinic acid blocked it. These results suggest that the anorexia caused by [Asu1,7]ECT is mediated through a direct inhibition of chemosensitive neuronal activity in the LHA, caused by an increase in the intracellular level of cyclic 3',5'-adenosine monophosphate.

Responses of morphologically identified cortical neurons to intracellularly injected cyclic AMP

Intracellular injections of cyclic adenosine monophosphate (cAMP) and horseradish peroxidase (HRP) were made in neurons of the motor cortex of awake cats. Eighty-six percent of injected cells responded to cAMP and HRP with a rapid decrease in input resistance. The decreases in input resistance occurred immediately after injection and began to return toward baseline 2 to 3 min later. The decreases were significantly greater than the small decreases in input resistance normally seen in uninjected cells held for 2 min or more after penetration and exceeded comparably small decreases in input resistance seen after control injections of 5'-AMP plus HRP. Pyramidal cells of layer V were identified as responding to cAMP with a decreased input resistance. A spiny stellate cell of layer III and a pyramidal cell of layer VI were also identified that showed similar responses. Increased rates of discharge were also observed after penetration with electrodes containing cAMP, but significant changes in input resistance were not found in association with the increased rates of discharge. After pressure injection of cAMP, the rates of discharge decreased toward more normative values. Our findings indicate that cAMP has an effect on cortical neurons similar to that found in some types of invertebrate (molluscan) neurons and dissimilar to the effect of cyclic guanosine monophosphate.

Actions of noradrenaline recorded intracellularly in rat hippocampal CA1 pyramidal neurones, in vitro

CA1 pyramidal neurones were studied in rat in vitro hippocampal slices using standard intracellular and single-electrode voltage-clamp recording techniques to examine the actions of noradrenaline (NA). NA had two different effects on the resting membrane potential of pyramidal neurones; either a hyperpolarization accompanied by a decrease in membrane input resistance, or less commonly, a depolarization accompanied by an increase in input resistance. In many cells, both effects, a hyperpolarization followed by a depolarization were observed. The depolarization was mediated by a noradrenergic beta-receptor. The hyperpolarization was more difficult to characterize, but may result from alpha-receptor activation. NA reduced the amplitude and duration of the slow calcium-activated potassium after-hyperpolarization (a.h.p.) that follows depolarization-induced action potentials. This action of NA was mediated by beta 1-noradrenergic receptors. NA, in the presence of tetrodotoxin and tetraethylammonium, reduced the a.h.p. without reducing the size of the calcium action potential which preceded it. This was unlike the action of the calcium channel blocker, cadmium, which reduced the calcium action potential and the a.h.p. in parallel. Furthermore, NA did not reduce the amplitude of calcium or barium currents recorded under voltage clamp after blockade of potassium currents. A functional consequence of this blockade of the calcium-activated a.h.p. was a reduction of the accommodation of action potential discharge such that the excitatory responses of the neurone to depolarizing stimuli, such as glutamate application or current passed through the recording electrode, were enhanced. We conclude that the effects of NA on calcium-activated potassium conductance and on resting membrane potential can interact to increase the signal-to-noise ratio of hippocampal pyramidal neurone responsiveness.

Cyclic adenosine 3',5'-monophosphate mediates beta-receptor actions of noradrenaline in rat hippocampal pyramidal cells

Intracellular recordings were made from rat hippocampal CA1 pyramidal neurones in the in vitro slice preparation to study the actions of cyclic adenosine 3',5'-monophosphate (cyclic AMP). Application of the membrane permeant analogue of cyclic AMP, 8-Br cyclic AMP caused a small depolarization of the resting membrane potential accompanied by an increase in membrane input resistance and also reduced the amplitude of depolarization-evoked calcium-activated potassium after-hyperpolarizations (a.h.p.s.). 8-Br cyclic AMP reduced calcium-activated a.h.p.s but did not reduce calcium action potentials in these cells. 8-Br cyclic AMP also reduced action potential frequency accommodation. The effects of 8-Br cyclic AMP were not mimicked by cyclic AMP applied extracellularly but were imitated by intracellular injections of cyclic AMP. Activation of the endogenous adenylate cyclase of pyramidal cells either by intracellular injection of the stable guanosine 5'-triphosphate (GTP) analogue guanylyl-imidodiphosphate, or by extracellular application of forskolin, reduced the a.h.p. and accommodation. Reducing phosphodiesterase activity with application of either 3-isobutyl-1-methylxanthine or Ro20-1724 reduced the amplitude of the a.h.p. and potentiated the a.h.p.-blocking action of noradrenaline. Reducing adenylate cyclase activity by application of SQ22,536 slightly increased the amplitude of the (a.h.p.) and reduced the a.h.p.-blocking action of noradrenaline. We conclude that the beta-receptor actions of NA on hippocampal CA1 pyramidal cells are mediated by intracellularly produced cyclic AMP.

Cerebellar cyclic nucleotides and the development of convulsion, with reference to the anticonvulsant activity of diazepam

Dibutyryl cyclic GMP (DbcGMP) or dibutyryl cyclic AMP (DbcAMP) given to rats intracerebellarly in a dose of 200 nmol/head produced electroencephalographic convulsive changes. Intracerebellar (i.c.) administration of lower doses (100 nmol/head) of DbcGMP and DbcAMP and 200 nmol/head of norepinephrine (NE) and glutamate (Glu) facilitated the pentylenetetrazol (PTZ)-induced convulsions. Diazepam (100 nmol/head, i.c.) suppressed the PTZ-induced convulsions. GABA (400 nmol/head, i.c.) did not affect the PTZ-induced convulsions. These results suggest that DbcGMP, DbcAMP and Glu inhibit seizure control mechanisms in the cerebellum, and that one of the sites of the anticonvulsant action of diazepam is located in the cerebellum.

Electrophysiological assay and characterization of central adrenoceptors: techniques and neuropharmacology

Several single unit electrophysiological studies that have investigated central adrenoceptors are reviewed. The techniques and paradigms employed to electrophysiologically assay such adrenoceptors are discussed. Several regions of the brain, e.g., the nucleus locus coeruleus, the dorsal raphe nucleus, the lateral geniculate nucleus, and the cerebellar Purkinje neurons, are examined in detail, with reference to the nature of the adrenoceptor(s) located on these neurons. From the studies reviewed, it can be concluded that single unit electrophysiological recordings provide a valuable and powerful assay for adrenoceptors. Modification of this technique to study adrenoceptors from awake (behaving) or chronically treated animals is likely to result in significant advances in our understanding of mechanisms contributing to neuroreceptor plasticity.

Two different mechanisms control inhibition of spike discharge in neurons of cat motor cortex after stimulation of the pyramidal tract

Intracellular recordings were made from neurons of the motor cortex of awake cats while the pyramidal tract (PT) was stimulated at the level of the facial nucleus. In some neurons IPSPs of 35-120 ms peak latency were recorded that diminished in size or reversed with hyperpolarizing current. During these IPSPs a decrease in input resistance reflective of a conductance increase was measured. More often, however, PT stimulation produced IPSPs with comparable latencies that increased in size with hyperpolarizing current. These IPSPs diminished with depolarizing current, and in some instances they appeared to reverse with strong depolarization. During these IPSPs an increased input resistance reflective of a decreased conductance was measured. The results indicate that two different mechanisms control rapid inhibition of spike discharge in neurons of the motor cortex after PT stimulation.

Regression analysis of catecholamine utilization in discrete hypothalamic and forebrain regions of the male rat: effects of thyroidectomy

The effects of thyroidectomy (4 weeks) on dopamine (DA) and noradrenaline (NA) turnover rates were determined by means of regression analysis. The disappearance of catecholamine (CA) fluorescence (using quantitative histofluorimetry) after tyrosine hydroxylase inhibition (alpha-methyl-DL-p-tyrosine methyl ester) has been investigated in discrete hypothalamic and forebrain DA and NA nerve terminal systems of the male rat. A time-dependent monophasic CA fluorescence disappearance was observed in all CA nerve terminal systems of the sham-operated and thyroidectomized rats. In the thyroidectomized rat, DA turnover in the anterior nucleus accumbens and in the medial and lateral palisade zones of the median eminence (ME) was reduced while DA turnover in the posterior nucleus accumbens was increased as compared to control rats. Furthermore, NA turnover was increased in the paraventricular hypothalamic nucleus (PA) and reduced in the dorsomedial hypothalamic nucleus (DM) and in the 'border zone' (lateral hypothalamus). Radioimmunoassay of hormones in serum demonstrated marked increases in TSH levels and reduced concentrations of GH, prolactin, corticosterone, triiodothyronine and thyroxine. The reduced DA turnover in the external layer of the ME and the increased NA turnover in the PA may indicate an inhibitory dopaminergic mechanism in the ME and a facilitatory noradrenergic mechanism in the PA in the regulation of TSH secretion. These mechanisms seem to interact with thyroid hormones. The reduced NA turnover demonstrated in the DM and in the border zone may be related to the lowering of growth hormone levels and pulsatility caused by thyroidectomy. Finally, the DA nerve terminal systems in the anterior and posterior parts of the nucleus accumbens are differently regulated by changes in the brain-pituitary-thyroid axis.

Noradrenergic hyperinnervation of the trigeminal sensory nuclei

Administration of 6-hydroxydopamine to neonatal rats results in a permanent increase in the norepinephrine content in several brainstem areas. To assess the physiological effects of this hyperinnervation, we studied the noradrenergic inhibition of transmission of sensory information through the principal sensory and rostral spinal trigeminal nuclei. Unit activity produced by tactile stimulation of the face was recorded extracellularly from trigeminal sensory neurons in normal and hyperinnervated rats. The noradrenergic neurons projecting to the trigeminal sensory nuclei (locus coeruleus and the region of the lateral lemniscus) were stimulated 40 ms prior to delivery of a tactile stimulus to the face, producing complete inhibition. The interstimulus interval was then increased in 100 ms increments until the sensory response returned to control values. Compared with controls, the duration of inhibition was 30% longer in hyperinnervated rats and 25% shorter in rats depleted of catecholamines with reserpine and alpha-methyl-p-tyrosine. While the beta-adrenergic blocker, propranolol, had no effect on the duration of inhibition in normal animals, the mean latency of response to tactile stimulation was decreased from 15.3 to 10.4 ms. Propranolol given to hyperinnervated rats decreased the latency of the response to tactile stimulation from 15.1 to 9.1 ms and decreased the duration of inhibition by 40% compared with untreated hyperinnervated rats, suggesting an alteration in numbers or sensitivity of beta-receptors. Since the drug treatment never eliminated the inhibition due to locus coeruleus stimulation, there is also a non-noradrenergic component. We conclude from these observations that noradrenergic hyperinnervation is not completely counteracted by receptor down regulation.

Cultured cerebellar neurons: endogenous and exogenous components of Purkinje cell activity and membrane response to putative transmitters

Modified explant cultures of fetal rat cerebellum were developed for electrophysiological and pharmacological studies, at the membrane level, of Purkinje neurons. The goals of the present series of experiments were to identify possible endogenous and exogenous components to the electrical activity of Purkinje neurons, to assess the sensitivity of these neurons to putative excitatory and inhibitory neurotransmitters, and to characterize the membrane response to the transmitters. Intracellular recordings were made from Purkinje neurons, identified on a morphological basis, using conventional electrophysiological techniques. Virtually all Purkinje neurons displayed spontaneous activity. A contribution of both endogenous and exogenous components to the spontaneous activity was indicated by alterations in the pattern and amount of activity when the membrane potential was varied and by the characteristics of the individual potentials themselves. Several types of activity were considered to be endogenous: the most common type consisted of pacemaker-like potentials which generated a pattern of firing similar to that characterized as simple spike activity in previous in vivo studies; another type of endogenous activity consisted of large membrane depolarizations that evoked one or two spikes. These depolarizing responses were similar to the membrane response generated by climbing fiber input to Purkinje cells in vivo. The exogenous components to the spontaneous activity consisted of synaptic potentials including excitatory (EPSPs) and inhibitory (IPSPs) synaptic potentials and biphasic EPSP/IPSPs. Several putative transmitters thought to mediate these synaptic potentials were tested by focal micropressure application to determine if they could mimic the action of the endogenous transmitters. The putative transmitter glutamate depolarized the cultured Purkinje neurons and evoked action potentials, characteristics which were displayed by the excitatory synaptic potentials. The putative inhibitory transmitter GABA hyperpolarized the cultured Purkinje neurons and depressed activity, characteristics which were displayed by the inhibitory synaptic potentials. The putative inhibitory transmitters glycine and taurine were ineffective. Norepinephrine, the transmitter mediating the inhibitory input from the locus coeruleus to Purkinje neurons, was also tested. When applied in the microM range, NE effects were variable. When applied in the mM range, NE depressed the spontaneous activity in a manner suggestive of a presynaptic action.