Activation of the noradrenergic projection from locus coeruleus reduces the excitatory responses of anterior cingulate cortical neurones to substance P

Intrinsic organization of the corpus callosum

The corpus callosum-the largest commissural fiber system connecting the two cerebral hemispheres-is considered essential for bilateral sensory integration and higher cognitive functions. Most studies exploring the corpus callosum have examined either the anatomical, physiological, and neurochemical organization of callosal projections or the functional and/or behavioral aspects of the callosal connections after complete/partial callosotomy or callosal lesion. There are no works that address the intrinsic organization of the corpus callosum. We review the existing information on the activities that take place in the commissure in three sections: I) the topographical and neurochemical organization of the intracallosal fibers, II) the role of glia in the corpus callosum, and III) the role of the intracallosal neurons.

Context-dependent relationships between locus coeruleus firing patterns and coordinated neural activity in the anterior cingulate cortex

Ascending neuromodulatory projections from the locus coeruleus (LC) affect cortical neural networks via the release of norepinephrine (NE). However, the exact nature of these neuromodulatory effects on neural activity patterns in vivo is not well understood. Here, we show that in awake monkeys, LC activation is associated with changes in coordinated activity patterns in the anterior cingulate cortex (ACC). These relationships, which are largely independent of changes in firing rates of individual ACC neurons, depend on the type of LC activation: ACC pairwise correlations tend to be reduced when ongoing (baseline) LC activity increases but enhanced when external events evoke transient LC responses. Both relationships covary with pupil changes that reflect LC activation and arousal. These results suggest that modulations of information processing that reflect changes in coordinated activity patterns in cortical networks can result partly from ongoing, context-dependent, arousal-related changes in activation of the LC-NE system.

Intracallosal neuronal nitric oxide synthase neurons colocalize with neurokinin 1 substance P receptor in the rat

The corpus callosum (cc) contains nitric oxide (NO)-producing neurons. Because NO is a potent vasodilator, these neurons could translate neuronal signals into vascular responses that can be detected by functional brain imaging. Substance P (SP), one of the most widely expressed peptides in the CNS, also produces vasomotor responses by inducing calcium release from intracellular stores through its preferred neurokinin 1 (NK1) receptor, thus inducing NO production via activation of neuronal NO synthase (nNOS). Single- and double-labeling experiments were performed to establish whether NK1-immunopositive neurons (NK1IP -n) are found in the rat cc and the extent of NK1 colocalization with nNOS. NK1IP -n were seen to constitute a large neuronal population in the cc and had a distribution similar to that of nNOSIP neurons (nNOSIP -n). NK1IP -n were numerous in the lateral cc and gradually decreased in the more medial portions, where they were few or absent. Intracallosal NK1IP -n and their dendritic trees were intensely labeled, allowing classification into four morphological types: bipolar, round, polygonal, and pyramidal. Confocal microscopic examination demonstrated that nearly all NK1IP -n contained nNOS (96.43%) and that 84.59% of nNOSIP -n co-expressed NK1. These data suggest that the majority of intracallosal neurons can release NO as a result of the action of SP. A small proportion of nNOSIP -n does not contain NK1 and is not activated by SP; these neurons may release NO via alternative mechanisms. The possible mechanisms by which intracallosal neurons release NO are also reviewed.

Antisocial personality and bipolar disorder: interactions in impulsivity and course of illness

Antisocial personality disorder (ASPD) and bipolar disorder are both characterized by impulsive behavior, increased incarceration or arrest, addictive disorders and suicidal behavior. These characteristics appear more severe in the combined disorders. Individuals with ASPD who also have bipolar disorder have higher rates of addictive disorders and suicidal behavior and are more impulsive, as measured by questionnaires or behavioral laboratory tests. Those with bipolar disorder who have ASPD have higher rates of addictive, criminal and suicidal behavior, earlier onset of bipolar disorder with a more recurrent and predominately manic course and increased laboratory-measured, but not questionnaire-rated, impulsivity. These characteristics may result in part from differential impulsivity mechanisms in the two disorders, with bipolar disorder driven more by excessive catecholamine sensitivity and ASPD by deficient serotonergic function.

Locus coeruleus activation shortens synaptic drive while decreasing spike latency and jitter in sensorimotor cortex. Implications for neuronal integration

Chronic recording of locus coeruleus (LC) neurons in rat and monkey have pointed out that brief, phasic LC discharges, but not sustained activity, are specifically related to salient stimuli and attention. However, the sensory consequences of phasic activation of the noradrenergic system by a brief conditioning stimulation of the LC have not been fully investigated. This study examined the effect of LC activation on synaptic and neuronal responses to a tactile stimulus in the sensorimotor cortex of the anaesthetized rat, by analysing the fine temporal structure of sensory discharges and current source-density profiles recorded from the same electrodes. LC stimulation, with minimal EEG effects, consistently reduced the synaptic input in layers IV and V-VI, by decreasing the amplitude and duration of short-latency current sinks, but not the slope of their early rising phase. Simultaneously, most multiple and single unit excitatory responses were shortened by the suppression of their late component after 25-30 ms, whereas robust temporal facilitation of the early discharge was found for spike latency mean and variance, spike timing and synchronization to the stimulus, but leaving the number of spikes unaffected. These two apparently opposite effects on the synaptic drive and neuronal response are reminiscent of the noradrenergic depression of afferent synaptic potentials observed with an increased neuronal excitability in vitro. They are interpreted as a noradrenergic sharpening of thalamocortical processing consistent with a presumed role of synchronous discharges in perception that would depend on activated states, particularly when LC activity is correlated with vigilance or attention.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.

Noradrenergic neurons expressing substance P receptor (NK1) in the locus coeruleus complex: a double immunofluorescence study in the rat

By using a double immunofluorescence method we examined the distribution of noradrenergic neurons expressing substance P receptor (NK1) or neuromedin K receptor (NK3) in the rat brainstem. The distribution of SPR-like immunoreactive (-LI) neurons completely overlapped that of tyrosine hydroxylase (TH)-LI neurons in the locus coeruleus (A6), ventrolateral and lateral reticular formation of pons (A5 and A7). Partially overlapping distribution of SPR- and TH-LI neurons were found in certain regions of the medulla oblongata (A1-A4). Neurons showing both SPR- and TH-like immunoreactivities, however, were only found in the locus coeruleus complex (A5-A7): 100% of these TH-LI neurons displayed SPR-like immunoreactivity. Neurons showing both NKR- and TH-like immunoreactivities were not detected in the aforementioned areas of brainstem. The present study has provided morphological evidence for direct physiological modulation of noradrenergic neurons by tachykinins through SPR in locus coeruleus complex (A5-A7).

Ultrastructural evidence that substance P neurons form synapses with noradrenergic neurons in the A7 catecholamine cell group that modulate nociception

Potent antinociception can be produced by electrical stimulation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group and this effect is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. Microinjection of substance P near A7 neurons also produces antinociception that is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. These observations suggest that substance P produces antinociception by activating noradrenergic A7 neurons. However, it is not known whether this effect of substance P is produced by a direct or an indirect action on A7 neurons. Although light microscopic studies have demonstrated the existence of both substance P-containing axon terminals and neurokinin-1 receptors in the region of the A7 cell group, it is not known whether substance P terminals form synapses with noradrenergic A7 neurons. These experiments used double-labeling immunocytochemical methods and electron microscopic analysis to determine whether substance P-containing axons form synapses with noradrenergic neurons in the A7 cell group. Pre-embedding immunocytochemistry, combined with light and electron microscopic analysis, was used to provide ultrastructural evidence for synaptic connections between substance P-immunoreactive terminals labeled with immunoperoxidase and tyrosine hydroxylase-immunoreactive A7 neurons labeled with silver-enhanced immunogold. Tyrosine hydroxylase labeling was found in perikarya and dendrites in the A7 region, and substance P labeling was found in axons and synaptic terminals. Substance P-labeled terminals formed asymmetric synapses with tyrosine hydroxylase-labeled dendrites, but only a few of these were present on tyrosine hydroxylase-labeled somata. Substance P-labeled terminals also formed asymmetric synapses with unlabeled dendrites, and many unlabeled terminals formed both symmetric and asymmetric synapses with tyrosine hydroxylase-labeled dendrites. These results demonstrate that substance P neurons form a significant number of synapses with the dendrites of noradrenergic A7 neurons and support the conclusion that microinjection of substance P in the A7 cell group produces antinociception by direct activation of spinally projecting noradrenergic neurons.

Phasic activation of the locus coeruleus enhances responses of primary sensory cortical neurons to peripheral receptive field stimulation

In the present study we examined the effects of phasic activation of the nucleus locus coeruleus (LC) on transmission of somatosensory information to the rat cerebral cortex. The rationale for this investigation was based on earlier findings that local microiontophoretic application of the putative LC transmitter, norepinephrine (NE), had facilitating actions on cortical neuronal responses to excitatory and inhibitory synaptic stimuli and more recent microdialysis experiments that have demonstrated increases in cortical levels of NE following phasic or tonic activation of LC. Glass micropipets were used to record the extracellular activity of single neurons in the somatosensory cortex of halothane-anesthetized rats. Somatosensory afferent pathways were activated by threshold level mechanical stimulation of the glabrous skin on the contralateral forepaw. Poststimulus time histograms were used to quantitate cortical neuronal responses before and at various time intervals after preconditioning burst activation of the ipsilateral LC. Excitatory and postexcitatory inhibitory responses to forepaw stimulation were enhanced when preceded by phasic activation of LC at conditioning intervals of 200-500 ms. These effects were anatomically specific in that they were only observed upon stimulation of brainstem sites close to (>150 micron) or within LC and were pharmacologically specific in that they were not consistently observed in animals where the LC-NE system had been disrupted by 6-OHDA pretreatment. Overall, these data suggest that following phasic activation of the LC efferent system, the efficacy of signal transmission through sensory networks in mammalian brain is enhanced.

A study of tachykinin-immunoreactivity in the cat visual cortex

The localization of tachykinin-immunoreactivity in the cat visual cortex (area 17) was investigated using immunohistochemical methods. Strong laminar specificity was observed, with immunoreactivity highest in layer V, followed by layers I, VI, II and III, and the lowest density in layer IV. Most of the immunoreactive product was localized in neuronal processes. A few immunopositive cell bodies were also present. The immunopositive neurons were non-pyramidal, multipolar, or bipolar in shape, and mostly found in layer V. There were particularly dense immunopositive fibers and varicosities around somata in layer V. These may represent tachykinin-containing presynaptic terminals (boutons). The results provide anatomical evidence that tachykinins may primarily affect layer V neurons in the cat visual cortex.

In vivo noradrenaline release evoked in the anteroventral thalamic nucleus by locus coeruleus activation: an electrochemical study

The anteroventral thalamic nucleus is innervated by noradrenergic terminals exclusively originating in the locus coeruleus, a densely packed cell group located in the dorsotegmental part of the pons. In urethane-anaesthetized rats, electrical stimulations of locus coeruleus axons (dorsal noradrenergic bundle; 14 Hz, 20 s) evoked a rapid increase in the signal (catechol oxidation current) measured within the anteroventral thalamic nucleus by the use of carbon fibre electrodes combined with electrochemistry. This effect was reproducible and immediately reversible. Evoked changes in this current were found to be due to oxidation of noradrenaline released from terminals. The amplitude of the evoked noradrenaline release varied non-linearly with the frequency of stimulation. We investigated the influence of locus coeruleus activation on noradrenaline release measured in the anteroventral thalamic nucleus every second by means of differential pulse amperometry: (i) chemical activation of locus coeruleus by local injection of glutamate (0.2-0.8 nmol) immediately and consistently evoked noradrenaline release in a dose-dependent manner; and (ii) peripheral stimulation of the sciatic nerve (20 s)--known to enhance the firing rate of locus coeruleus neurons-evoked a noradrenaline release similar to that produced by a stimulation of the dorsal noradrenergic bundle at 8-10 Hz. Pharmacological and kinetic characteristics of the noradrenaline release were the same for central or peripheral stimulation of locus coeruleus neurons. Our results indicate that in vivo electrochemistry, because of its sensitivity and its high space and time resolution, is well suited for studies of evoked noradrenaline release from locus coeruleus terminals. This approach allowed us to describe the characteristics of central noradrenaline release evoked by central and peripheral stimulations of short duration. In particular, we observed a very close relationship between impulse flow and evoked noradrenaline release.

A comparison of the development of neuropeptide and MAP2 immunocytochemical labeling in the macaque visual cortex during pre- and postnatal development

The appearance of Substance P (SP) and Neuropeptide Y (NPY) has been studied using light microscopic immunocytochemical labeling throughout the complete developmental span of Macaca nemestrina monkey striate cortex. In the adult, 80% of the NPY+ neurons occur in the white matter (WM) and most of the remainder are medium to large multipolar neurons in layer 2. Fibers occur in all layers except 4C and are very numerous, given the relatively small number of NPY+ cell bodies. NPY+ neurons first were seen at embryonic day (E) 75. Most neurons were in the intermediate zone (IZ), but a few were in the immature cortical plate (CP). An adult-like distribution was present by E125 for neurons and by birth for fibers, but fiber staining intensity and number increased to postnatal year 1 (P1yr). In adult cortex, numerous SP+ nonpyramidal neurons were present in layers 2-6 and WM, but SP+ fibers were surprisingly infrequent. During development, significant numbers of SP+ neurons were not seen in the CP until E113-125. Later prenatal ages had a prominent plexus of SP+ cell bodies and fibers at the layer 5/6 border. This plexus disappeared by P12wk due to either down-regulation of SP or cell death. SP+ neurons in IZ/WM were very sparse until birth after which they increased in number and staining intensity up to P1yr, suggesting a postnatal up-regulation of SP in a preexisting WM subpopulation. Cell densities were determined for SP, NPY, and the neuron-specific marker microtubule-associated protein 2 (MAP2) to clarify the developmental dynamics of IZ/WM neurons. MAP2+ cell densities in WM peaked around birth and then declined 20% in the outer half and 77% in the inner half of WM. SP+ cell density rose 57% from birth to P20wk and then declined 20% into adulthood. NPY+ cell density was fairly constant prenatally and then rose 300% by adulthood. Neuropeptide cell density changes took place predominantly in the outer WM. These data indicate that cell death does occur in the general population of monkey striate cortical WM neurons. In contrast, both SP+ and NPY+ cells are characterized by minimal cell death and a late expression of neuropeptides which causes an increase in neuropeptide+ cell density in postnatal WM.

Antinociception induced by microinjection of substance P into the A7 catecholamine cell group in the rat

Stimulation of neurons in the ventromedial medulla produces antinociception that is mediated in part by indirect activation of pontospinal noradrenergic neurons. Substance P-containing neurons located in the ventromedial medulla project to the A7 catecholamine cell group and may serve as an excitatory link between these two cell groups. Thus, the antinociception induced by stimulation of the neurons in ventromedial medulla may be mediated by substance P released from these projections which activates spinally projecting noradrenergic neurons in the A7 cell group. This hypothesis was tested by determining whether microinjection of various doses of substance P into the A7 cell group of the rat could induce antinociception. The results indicated that substance P induced dose-dependent antinociception that was more pronounced in the hindlimb ipsilateral to the microinjections. This observation is consistent with anatomical observations that noradrenergic A7 neurons project predominantly to the ipsilateral spinal cord dorsal horn. Moreover, the antinociceptive effects of substance P microinjection appear to be mediated at least in part by activation of spinally projecting noradrenergic neurons in the A7 cell group, because intrathecal injections of the alpha-2 noradrenergic antagonists yohimbine and idazoxan blocked these antinociceptive effects. The results of these experiments support the hypothesis that the antinociception induced by stimulation of neurons in the ventromedial medulla is mediated in part by activation of substance P-containing neurons that project to, and activate, spinally projecting noradrenergic neurons located in the A7 catecholamine cell group.

A role for norepinephrine in arousal, emotion and learning?: limbic modulation by norepinephrine and the Kety hypothesis

1. Kety hypothesized that forebrain norepinephrine acted to selectively enhance cell firing in neurons receiving environmental inputs during affectively important events. He further suggested that norepinephrine could act to induce a 'persistent facilitation' of the inputs which accompany affectively important events and would thus promote a memory for these events. 2. The electrophysiological actions of norepinephrine at the time Kety proposed his hypothesis were thought to be inhibitory. More recent evidence has demonstrated that norepinephrine in neocortex and cerebellum enhances both excitatory and inhibitory evoked activity much as Kety proposed. This has been termed norepinephrine neuromodulation. 3. The locus coeruleus-norepinephrine system which gives rise to neocortical and cerebellar norepinephrine innervation also innervates, even more densely, areas of the limbic system. A review of norepinephrine actions, particularly in limbic cortex, indicates that locus coeruleus-norepinephrine also enhances transmission of evoked inputs in these structures. 4. A long-lasting enhancement of evoked inputs by locus coeruleus-norepinephrine has been demonstrated in the hippocampus. This long-lasting enhancement of inputs is reviewed in detail since it appears to directly support Kety's hypothesized 'persistent facilitation' effect of norepinephrine. It is suggested that norepinephrine-induced long-lasting enhancement may underlie numerous demonstrations of norepinephrine-dependent memory and neural plasticity in the forebrain. 5. The relationship of norepinephrine neuromodulation to possible candidate mechanisms and to activation of specific norepinephrine receptors is briefly discussed.

Glutamate-induced activation of rat locus coeruleus increases CA1 pyramidal cell excitability

In anesthetized rats, injections of a 0.5 mM glutamate solution into the locus coeruleus (LC) reversibly increased the amplitude of the population spike evoked in CA1 by stimulation of the Schaffer-commissural fiber tract. This effect was absent in N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4)-treated, noradrenaline (NA)-depleted animals. The excitatory postsynaptic potential recorded in the stratum radiatum was unaffected following the glutamate injections. Systemic administration of the NA-uptake inhibitor desipramine also produced an increase in population spike amplitude. The findings demonstrate that activation of LC neurons increases pyramidal cell excitability in vivo.

Pharmacological characterization of the receptor mediating the adrenergic inhibition of responses to substance P in the cingulate cortex

The excitatory responses of neurones in the anterior cingulate cortex of the rat to iontophoretically applied substance P (SP) are reduced by noradrenaline (NA) applied iontophoretically or released from noradrenergic pathways. In order to determine the receptor involved in this inhibitory effect we have studied the effects of a number of receptor-specific adrenergic agonists and antagonists on responses of cingulate neurones to SP in rats anaesthetized with chloral hydrate. Low iontophoretic currents (0-15 nA) of NA, adrenaline and the beta-agonist, clenbuterol, all strongly reduced responses to SP. Isoprenaline was also effective but less consistently so, although problems were experienced with its iontophoretic release from micropipettes. The alpha 1-agonists, phenylephrine and methoxamine were also able to reduce responses to SP. However, this reduction required higher iontophoretic currents (15-60 nA) and was associated with depressant effects on baseline firing rate. The alpha 2-agonist clonidine was only weakly active at high currents and this too was associated with depression of baseline firing. Similar weak effects were noted with dopamine. The inhibitory effects of NA on SP responses were convincingly blocked or reversed by the beta-antagonist, practolol, but not by the alpha 1-antagonist, prazosin. The reduction of SP responses by phenylephrine was also blocked by practolol but unaffected by prazosin. Finally, reduction of SP excitations by activation of the coeruleocortical pathway was also blocked by practolol applied iontophoretically to the cortical cells. These results are consistent with the hypothesis that the effect of NA on SP responsiveness in the cingulate cortex is mediated by beta-adrenoreceptors.

Neuronal sensitivity to substance P is increased after repeated treatment with tranylcypromine, carbamazepine or oxaprotaline, but decreased after repeated electroconvulsive shock

The responsiveness of neurones in the cingulate cortex of the rat to the excitatory effects of iontophoretically applied substance P (SP) was determined after chronic administration of several drugs known to possess antidepressant actions. The agents tested were tranylcypromine, carbamazepine and oxaprotiline [(+) and (-)isomers]. Twenty-four to 36 hr after the last of 14 consecutive daily treatments there was an increase in neuronal sensitivity to substance P with all three drugs. The two isomers of oxaprotaline were equally active in inducing this change in sensitivity. None of the agents significantly altered responses to substance P after a single acute treatment. Responsiveness to substance P was also tested one day after the last of 14 daily treatments with electroconvulsive shock. In this case there was a marked reduction of the sensitivity of cingulate neurones to substance P with no apparent change in responsiveness to acetylcholine. A single shock treatment did not detectably alter responses to substance P.