Noradrenergic modulation of dendrodendritic inhibition in the olfactory bulb

Ketamine and MK801 attenuate paired pulse inhibition in the olfactory bulb of the rat

We have investigated the effects of the phencyclidine like-compounds ketamine and MK801 on the evoked field potentials of rat olfactory bulb. Low doses of ketamine (3-6 mg/kg) blocked the inhibition of mitral cells by granule cells evoked by stimulation of lateral olfactory tract fibres or by stimulation of olfactory nerve. This blockade was not accompanied by a decrease in granule cell excitation as revealed by field potential recording. MK801 had a similar effect on the inhibition of mitral cells evoked by stimulation of the lateral olfactory tract. As ketamine does not influence the inhibitory action of GABA (Anis et al. 1983) these results suggest that both ketamine and MK801 block inhibition by an action on intrinsic excitatory feed-back circuits in the olfactory bulb.

Norepinephrine and learning-induced plasticity in infant rat olfactory system

Postnatal olfactory learning produces both a conditioned behavioral response and a modified olfactory bulb neural response to the learned odor. The present report describes the role of norepinephrine (NE) on both of these learned responses in neonatal rat pups. Pups received olfactory classical conditioning training from postnatal days (PN) 1-18. Training consisted of 18 trials with an intertrial interval of 24 hr. For the experimental group, a trial consisted of a pairing of unconditioned stimulus (UCS, stroking/tactile stimulation) and the conditioned stimulus (CS, odor). Control groups received either only the CS (Odor only) or only the UCS (Stroke only). Within each training condition, pups were injected with either the NE beta-receptor agonist isoproterenol (1, 20, or 4 mg/kg), the NE beta-receptor antagonist propranolol (10, 20, 40 mg/kg), or saline 30 min prior to training. On day 20, pups received one of the following tests: (1) behavioral conditioned responding, (2) injection with 14C-2-deoxyglucose (2-DG) and exposed to the CS odor, or (3) tested for olfactory bulb mitral/tufted cell single-unit responses to the CS odor. The results indicated that training with either: (1) Odor-Stroke-Saline, (2) Odor-Stroke-Isoproterenol-Propranolol, or (3) Odor only-Isoproterenol (2 mg/kg) was sufficient to produce a learned behavioral odor preference, enhanced uptake of 14C-2-DG in the odor-specific foci within the bulb, and a modified output signal from the bulb as measured by single-cell recordings of mitral/tufted cells. Moreover, propranolol injected prior to Odor-Stroke training blocked the acquisition of both the learned behavior and olfactory bulb responses. Thus, NE is sufficient and may be necessary for the acquisition of both learned olfactory behavior and olfactory bulb responses.

Antidepressant drugs potentiate the alpha 1-adrenoceptor effect in hippocampal slices

The effect of prolonged treatment with antidepressant drugs on the phenylephrine- and norepinephrine (NE)-evoked reaction in hippocampal slices was examined by extracellular recording of the spontaneous activity of CA1 layer neurons. The alpha 1-adrenoceptor agonists, phenylephrine and methoxamine, depressed the neuronal discharges of most of the units tested, while NE evoked both excitatory and inhibitory effects which were blocked by propranolol and phentolamine or prazosin, respectively. Imipramine, mianserin, (+)- and (-)-oxaprotiline administered subchronically (10 mg/kg p.o., twice daily for 14 days, withdrawal 48 h), potentiated the inhibitory reaction to phenylephrine. Mianserin was the only drug tested in the acute dose to effectively augment the reaction to alpha 1-adrenoceptor stimulation. Prolonged administration of mianserin and imipramine attenuated the excitatory effect to NE, which probably reflects beta-receptor down-regulation; however, only mianserin, but not imipramine, enhanced the NE-induced inhibition. The observed potentiation of the alpha 1-adrenoceptor-related inhibitory reaction to phenylephrine produced by antidepressant drugs may reflect the development of the alpha 1-adrenergic system supersensitivity in the hippocampus.

Chemoanatomical organization of the noradrenergic input from locus coeruleus to the olfactory bulb of the adult rat

The locus coeruleus contains noradrenergic neurons which project widely throughout the CNS. A major target of locus coeruleus projections in the rat is the olfactory bulb (Shipley et al.: Brain Res. 329:294-299, '85) but the organization of the projections within the bulb has not been systematically examined. In this study, the laminar distribution and densities of locus coeruleus-noradrenergic fibers in the main and accessory olfactory bulbs were determined with anterograde tracing and immunocytochemical techniques. Following iontophoretic injections of 1% wheat germ agglutinin-horseradish peroxidase into the locus coeruleus, the densest anterograde label in the accessory olfactory bulb was observed in the external plexiform layer, granule cell layer, and especially in the internal part of the mitral cell layer. Virtually no label was observed in the glomerular layer. In the main olfactory bulb, labelled axons were observed in the granule cell layer, in the internal and external plexiform layers, occasionally in the mitral cell layer, and least often in the glomerular layer. Noradrenergic fibers in the olfactory bulb were identified by using immunocytochemistry with an antibody to dopamine-beta-hydroxylase. Laminar patterns and densities of noradrenergic innervation were determined with quantitative image analysis. In the accessory olfactory bulb, the densest innervation was in the innermost portion of the mitral cell layer followed by the granule cell layer, the superficial part of the mitral cell layer, and the external plexiform layer. The density of fibers in the glomerular layer was least. The laminar pattern of noradrenergic fiber distribution in the main olfactory bulb was similar to that in accessory olfactory bulb. The present studies demonstrate that locus coeruleus-noradrenergic fibers terminate preferentially in the internal plexiform, granule cell, and external plexiform layers. This suggests that the major influence of the locus coeruleus input to both the main and accessory the olfactory bulbs is on the predominant neuronal element in those layers, the granule cells. Additional studies are needed to resolve how this input influences specific olfactory bulb circuits.

Neural basis of olfactory memory in the context of pregnancy block

In mice, only strange male pheromones block pregnancy; pheromones of the familiar male with which the female has mated have the capacity to block pregnancy but are ineffective with the consort female. Hence, some form of recognition/memory to the stud male is formed at mating. By infusing lignocaine locally into the accessory olfactory bulb and second order olfactory synapses in the medial nucleus of the amygdala, this study localizes changes that occur in the accessory olfactory bulb at mating to be subsequently important in preventing the stud male's pheromones from blocking pregnancy. Further attention is focused on the dendrodendritic synapses between mitral and granule cells in the accessory olfactory bulb. Blockade of the GABA receptors (granule to mitral cell synapse) in the accessory bulb without mating, but in the presence of male pheromones, prevents any male from blocking pregnancy. Conversely inhibition of protein kinase C, a second messenger system activated by excitatory amino acids (mitral to granule cell synapse), in the accessory bulb during a 4-h period after mating permits all male pheromones including the stud's to activate pregnancy block. While blockade of protein kinase C activity during the critical exposure time for memory formation prevents memory formation, infusions of a protein synthesis inhibitor (anisomycin) are without effect. However, protein synthesis inhibition in the accessory olfactory bulb in the late phase of the critical exposure time (3-6 h after mating) does prevent memory formation. These studies show that changes in synaptic plasticity in the accessory olfactory bulb following mating are critical to recognition of the stud male's pheromones, hence preventing these from subsequently blocking pregnancy.

Noradrenergic modulation of olfactory bulb excitability in the postnatal rat

Noradrenergic centrifugal inputs to the rat olfactory bulb mainly terminate on granule cells, which are inhibitory interneurons. In the mature bulb, norepinephrine suppresses granule cell activity, thus increasing the excitability of the primary output neurons of the bulb. However, since the majority of granule cells develop postnatally, the effectiveness of noradrenergic inputs to the bulb during development is unclear. The present report describes the postnatal development of noradrenergic modulation of olfactory bulb function by examining the effects of noradrenergic beta-receptor agonists and antagonists on paired-pulse inhibition at the granule cell/mitral cell reciprocal synapse. The results demonstrate that noradrenergic modulation of olfactory bulb excitability emerges during the first postnatal week in the rat. These results suggest that noradrenergic centrifugal control of olfactory bulb activity appears early during postnatal development, and thus is capable of playing an important role in pup responses to olfactory cues early in life.

The effect of microinfusions of drugs into the accessory olfactory bulb on the olfactory block to pregnancy

Female mice which have mated and are subsequently exposed to the odour (pheromones) of a strange male undergo hormonal changes resulting in a block to their pregnancy. The fact that the stud male's odours can also block pregnancies, that is other than his own, implies the formation of a memory or some form of recognition process by the female for this male's pheromones at the time of mating. The purpose of this study was to evaluate the effect of microinfusions of drugs which interfere with neural transmission, into the accessory olfactory bulbs. This was carried out immediately after mating over a 4-h period during which the "memory" to the stud male's pheromones is formed. Infusions of the alpha-blocker, phentolamine, blocked the formation of the olfactory memory, while the GABA receptor blocker, bicuculline, itself blocked pregnancy, but was without effect on memory formation. Protein synthesis inhibition or calpain inactivation in the accessory bulb was without effect on memory formation at any of the doses used. These studies demonstrate that GABAergic transmitter blockade in the accessory olfactory bulb at the time of mating can prevent subsequent blastocyst implantation some 3 days later, while alpha-noradrenergic blockade can prevent the formation of an olfactory memory to the stud male.

Effects of intrabulbar injections of 6-hydroxydopamine on ethyl acetate odor detection in castrate and non-castrate male rats

The function of norepinephrine-containing neurons which project to the olfactory bulb is poorly understood. Although there has been suggestion that norepinephrine (NE) may modulate general olfactory sensitivity by attenuating the inhibitory feedback of granule cells upon mitral and tufted cells, behavioral indices of olfactory sensitivity have not been measured in animals with depletions of bulbar NE. The present experiment used computerized olfactometry and signal detection methodology to assess the odor detection performance of castrate and non-castrate male rats to a range of perithreshold concentrations of ethyl acetate following 6-hydroxydopamine (6-OHDA) depletion of bulbar NE. Such depletion had no significant influence on odor detection performance at any of the odorant concentrations examined in either castrate or non-castrate animals, as indexed by the non-parametric sensitivity measure SI. This observation implies that general olfactory sensitivity is unaltered by major depletion of intrabulbar NE, but does not preclude the possibility that NE modulates sensitivity to select odorants or odorant mixtures, or alters detection ability under atypical states of arousal.

Norepinephrine decreases synaptic inhibition in the rat hippocampus

The effects of norepinephrine (NE) on inhibitory synaptic potentials were studied on CA1 pyramidal neurons in the hippocampal slice in vitro. Norepinephrine caused the appearance of multiple population spikes in the CA1 region of the hippocampal slice, reminiscent of the actions of gamma-aminobutyric acid (GABA) antagonists. Intracellular recording revealed that NE causes a marked and reversible reduction in inhibitory postsynaptic potentials (IPSPs) recorded in CA1 pyramidal cells. This reduced IPSP results in a larger intracellular excitatory postsynaptic potential (EPSP), which can cause the cell to fire more than one action potential. This disinhibitory effect of NE appears to be mediated by an alpha-receptor, and occurs at a site presynaptic to the pyramidal cell, since NE does not change the reversal potential of the IPSP nor does it affect the amplitude or the reversal potential of iontophoretic GABA responses. In addition to reducing evoked IPSPs, NE causes an increase in the frequency of spontaneous IPSPs, suggesting that inhibition of interneuronal firing may not account for this disinhibitory action of NE.

Microdialysis measurement of oxytocin, aspartate, gamma-aminobutyric acid and glutamate release from the olfactory bulb of the sheep during vaginocervical stimulation

Concentrations of oxytocin and amino acids were measured in microdialysis samples taken from the olfactory bulbs of 5 conscious, oestrogen-treated ewes before, during and after a 10-min period of vaginocervical stimulation. Results showed that vaginocervical stimulation induced significant increases in concentrations of oxytocin, aspartate, gamma-aminobutyric acid (GABA) and glutamate in dialysis samples. Concentrations of other amino acids measured were not affected. These findings show that vaginocervical stimulation produces significant changes in neurochemical release in the olfactory bulbs of sheep. Such changes may be involved in the induction of maternal behaviour of the olfactory recognition of offspring.

Ultrastructural localization of beta-adrenergic receptor-like immunoreactivity in the cortex and neostriatum of rat brain

We sought to quantitatively examine the processes containing beta-adrenergic receptor-like immunoreactivity (beta-AR-LI) in the cerebral cortex and neostriatum using a previously characterized rabbit antiserum to frog erythrocyte beta-ARs under optimized immunolabeling conditions. Quantitative assessments of the laminar distribution of beta-AR-LI in the cortex was achieved by computer-assisted image analysis of immunoautoradiographs and by quantitative electron microscopic analysis of peroxidase-antiperoxidase (PAP) labeling in aldehyde-fixed sections and unfixed synaptosomes. In the somatosensory and anterior cingulate cortical areas, light microscopy of aldehyde-fixed sections immunolabeled by the PAP method revealed small (0.5-1.0 micron) punctate processes in all layers. In the deeper layers, rims of immunoreactivity around the plasmalemma of a population of neuronal perikarya and processes were also observed. By immunoautoradiography, labeling was seen in distinct, laminar distributions resembling the reported autoradiographic patterns using radioligands. By electron microscopy, the immunoreactive profiles in all cortical layers were primarily thick and thin postsynaptic densities (PSDs), comprising 4% of all identifiable PSDs in fixed sections and 12% in unfixed synaptosomal preparations. Also labeled were saccules of smooth endoplasmic reticulum and pinocytotic vesicles in dendrites, glial processes and lightly myelinated axons. In the neostriatum, the density of autoradiographic immunoreactivity was equivalent to the heavily labeled laminae of the cerebral cortex. Immunoreactivity detectable by light microscopy included punctate processes and rims of perikarya, as was seen in the cerebral cortex. The PAP reaction was shown by electron microscopy to be localized to the cytoplasmic surface of plasmalemma of a few proximal dendrites, but was most prominently associated with PSDs of dendritic spines. Preadsorption of the antiserum with a partially purified beta-AR preparation abolished all detectable immunoreactivity. These results provide further support for the specificity of the antiserum for beta-ARs, and are the first quantitative ultrastructural evidence for association of beta-AR-LI with PSDs in the cerebral cortex. The neostriatum, whose major catecholaminergic innervation is dopaminergic, and not noradrenergic, is also confirmed to exhibit high levels of beta-AR-LI within subcellular structures analogous to those seen in the cerebral cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Excitation of rat prefrontal cortical neurons by dopamine: an in vitro electrophysiological study

The effects of dopamine (DA) on prefrontal pyramidal neurons were studied in vitro on rat cerebral cortex slices using intracellular recordings. Pyramidal neurons were first identified by Lucifer yellow and some of their basic bioelectrical properties were analysed. At resting potential, white matter stimulation mainly evoked depolarizing inhibitory postsynaptic potentials (IPSPs) which reversed between -60 and -50 mV and were almost totally abolished by bicuculline. Furthermore, pyramidal cells often exhibited spontaneous depolarizing IPSPs abolished by bicuculline. Under tetrodotoxin (TTX) this synaptic noise was partly blocked suggesting that it was due both to the spontaneous firing of presynaptic gamma-aminobutyric acid (GABA)ergic neurons and to a spontaneous quantal release from these afferent fibers. In pyramidal cells, DA enhanced the number of spikes evoked by depolarizing current pulses, and the input resistance was increased by 10-20%. DA also clearly increased the inhibitory synaptic noise. This effect was blocked by fluphenazine. In contrast, evoked IPSPs were not consistently affected by DA. Taken altogether, these results suggest, that in the prefrontal cortex, dopamine has a mild excitatory effect on both pyramidal cells and GABAergic interneurons impinging on them.

Locus coeruleus stimulation modulates olfactory bulb evoked potentials

Field-evoked potentials from the main olfactory bulb in response to stimulation of the olfactory nerve and lateral olfactory tract were measured without and with conditioning stimulation of the locus coeruleus noradrenergic system. The locus coeruleus conditioning stimulus suppressed or inhibited the late components of the olfactory bulb potential evoked by orthodromic olfactory nerve stimulation; this inhibitory effect was suppressed by the microinjection of the alpha-adrenergic blocker prazosin into the olfactory bulb. Results indicate that noradrenergic fibers projecting from the locus coeruleus exert modulatory influences on neuronal networks underlying orthodromic evoked responses in the main olfactory bulb.

Abrupt decrease in synaptic inhibition in the postnatal rat olfactory bulb

Olfactory bulb responses to paired-pulse stimulation of the lateral olfactory tract were examined in urethane-anesthetized rats, aged 5 days to adult. Brief inter-pulse intervals resulted in a depression of test responses at all ages. The magnitude of this depression decreased dramatically between postnatal days 19 and 20 to approach adult levels. Longer inter-pulse intervals resulted in a facilitation of test response amplitude in adult animals. This facilitation was evident at adult levels by postnatal day 10. These results suggest that both inhibitory and facilitatory synaptic mechanisms appear early in the course of rat olfactory bulb development. Furthermore, presumed granule cell-mediated inhibition is present at unusually high levels in the developing bulb, decreasing sharply between days 19 and 20.

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.

Local GABAergic modulation of noradrenaline release in the rat olfactory bulb measured on superfused slices

Ascending noradrenergic projections from locus coeruleus reaching the rat olfactory bulb (OB) synapse onto gamma-aminobutyric acid (GABA)-ergic interneurons. We tested the hypothesis that these interneurons could locally modulate noradrenaline (NA) release. On superfused OB slices, GABA (10, 100 and 1000 microM) enhanced potassium-evoked release of newly accumulated [3H]NA. This raises the possibility that GABAergic interneurons could establish a functional link between olfactory deutoneuron activity and NA release in the rat OB.

Odor detection performance of rats following d-amphetamine treatment: a signal detection analysis

The effects of d-amphetamine sulfate (0.2, 0.4, 0.8, and 1.6 mg/kg SC) on the odor detection performance of 16 adult male Long Evans rats was assessed using high precision olfactometry and a go/no-go operant signal detection task. The drug or saline was administered every 3rd day in a counterbalanced order, with the injections occurring 5 min before each 260-trial test session. Relative to saline, enhanced detection performance to the target stimulus (ethyl acetate), as measured by a non-parametric signal detection index (SI), was observed following administration of 0.2 mg/kg of the drug, whereas decreased detection performance was observed following administration of 1.6 mg/kg of the drug. Significant increases in the responsivity index (RI) occurred at the higher drug dosages for the lower odorant concentrations. In addition, small but statistically significant increases in the latency to respond in the presence of the odor (i.e., S+ response latency) were present at the higher drug dosages. Overall, these data suggest that (a) odor detection performance is enhanced by low doses of amphetamine, (b) odor detection performance is depressed by moderate doses of amphetamine, and (c) drug-related alterations in response criteria occur following the administration of moderate doses of amphetamine.

An analysis of the effects of excitatory amino acid receptor antagonists on evoked field potentials in the olfactory bulb

The effects of excitatory amino acid antagonists on extracellular field potentials in the olfactory bulb produced by lateral olfactory tract stimulation were analysed in vivo. The compounds tested D-2-amino-5-phosphonovalerate, L-(+)2-amino-4-phosphonobutyrate, gamma-D-glutamylglycine, L-glutamic acid diethylester and cis-2,3-piperidine dicarboxylic acid, were administered by brain dialysis. Of the compounds tested, only cis-2,3 piperidine-dicarboxylic acid and gamma-D-glutamylglycine were able to suppress the synaptic excitation of granule cells. This pharmacological profile suggests the involvement of non-N-methyl-D-aspartate receptors. However, the suppression was accompanied by a reduction in the amplitude of the presynaptic volley. A second finding was that D-2-amino-5-phosphono-valerate and gamma-D-glutamyl glycine attenuated granule cell mediated inhibition of mitral cells, suggesting the involvement of N-methyl-D-aspartate-sensitive receptors. The possibility that mitral cells and that either centrifugal fibres, or an intrinsic olfactory bulb feedback loop might use an excitatory amino acid as its neurotransmitter is therefore discussed.

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.

Multimodal sensory discrimination deficits in Korsakoff's psychosis

A consistent impairment in odor identification was observed among a group of 21 amnesic patients, diagnosed as having Korsakoff's psychosis. In a subsequent study of eight Korsakoff and matched alcoholic control subjects, a comparable olfactory deficit was again demonstrated, as well as impairment in color discrimination and auditory perception. No such deficit was observed for a picture identification task designed to control for the non-sensory demands of the olfactory test. Stepwise multiple regression analysis showed a significant correlation between odor identification scores and the concentration of the primary metabolite of norepinephrine in lumbar cerebrospinal fluid. The data demonstrate a consistent coincidence between memory impairment and deficient sensory perception among patients with Korsakoff's psychosis.

Neurochemical aspects of depression: the past and the future?

The role of aberrant neurochemical substrates in the etiology of depression and the neurochemical mechanisms of antidepressant therapies have been the subjects of many hypotheses in the last 30 years. Pharmacological studies of early antidepressant drugs indicated that brain monoamines were significantly affected by these drugs and these led to the formulation of the biogenic amine hypothesis of depression. Although this hypothesis has been of heuristic value in the study of drug mechanisms and has provided a basis for screening drugs for antidepressant potential, deficiencies in it have become apparent. Neuroanatomical and neurochemical considerations favour the view that brain noradrenaline and serotonin systems may serve as bias adjusting systems for each other and numerous other neural systems. As a consequence of such a relationship, a primary defect in some other neural system would appear amplified in measurements of serotonin or noradrenaline. A possible site for this primary defect may be in membrane composition and function. Recent studies have found that typical and other antidepressant therapies have a pronounced effect on membrane lipids. Thus, in view of the important functions of membrane lipids and the fact that they have been linked to the initiation and development of a number of other disease processes, it is now suggested that consideration be given to them as playing primary causal roles in the etiology of depression and as a site of action for antidepressant drugs.

Surprisingly rich projection from locus coeruleus to the olfactory bulb in the rat

The brainstem nucleus, locus coeruleus (LC), is the major, if not the sole, source of noradrenergic (NE) innervation of the telencephalon. It is generally held that LC neurons project diffusely to the entire neuroaxis and this had been the basis for theories that postulate 'general' functions (sleep, attention, learning, etc.) for LC. We report that at least 40% of all LC neurons project to the olfactory bulb; the projection is almost 10 times greater than to any other part of the cerebral cortex. This unsuspectedly rich LC-olfactory bulb connection is consistent with current theories which implicate LC neurons in heightened sensory vigilance and trophic regulation of connectional development and plasticity.

Long-term potentiation in the dentate gyrus: effects of noradrenaline depletion in the awake rat

The chronic rat preparation was utilized to study the effects of noradrenaline (NA) depletion on field potentials recorded from the hilus of the fascia dentata. Both single pulses and high-frequency trains were applied to the perforant path (PP). The effects of NA depletion on baseline responses as well as on long-term potentiation (LTP) were examined. Reduced NA levels resulted in an increase in the population spike amplitude and a depression of the population excitatory postsynaptic potential (EPSP). Depleted animals showed significantly higher levels of LTP of the population EPSP, but reduced levels of population spike LTP (measured 13-15 min after tetanization). There were, however, no differences in LTP levels 1 week after the potentiation tests. These results demonstrate that NA levels do not affect that component of LTP which can persist for several weeks.

The role of noradrenaline in tuning and dopamine in switching between signals in the CNS

Neuronal catecholaminergic activity modulates central nervous function. Specifically noradrenaline can exert a tuning or biassing function whereby the signal to noise ratio is altered. Dopamine activity may promote switching between inputs and outputs of information to specific brain regions. It has been ten years since evidence for a tuning function was advanced for noradrenaline and in the last 5 years the switching hypothesis for dopamine has been tentatively put forward. Recent studies are reviewed to show that while catecholamine activity contributes to neural interactions in separate brain regions that give rise to the organization of different functions, their working principles may be common between species and independent of the nucleus of origin. Behavioral examples are discussed and an attempt is made to integrate this with evidence from intracellular recording studies. It is suggested that the tuning principle in noradrenergic systems is particularly important for the formation of associations and neural plasticity (interference control) and that the switching principle of dopaminergic systems modulates the timing, time-sharing and initiation of responses (program-control).

Olfactory bulb excitability selectively modified in behaving rats after local 6-hydroxydopamine treatment

Previous studies have shown that the olfactory bulb (OB) responsiveness was selectively enhanced towards food odor in free-moving hungry rats. this modulation of the olfactory input depended on the action of centrifugal fibers. The present study investigated the possible involvement of the noradrenergic (NA) projections from the locus coeruleus to the OB in this adaptative control of the OB excitability. After plugging of the olfactory lumen, 17 rats received discrete injections of 6-hydroxydopamine solution (10 micrograms of the salt; 40 nmol free base) into one OB (6-OHDA bulb) and the vehicle into the contralateral side (control bulb); 10 sham operated rats received intrabulbar isotonic saline solution injected bilaterally (sham bulbs). In the 12 6-OHDA bulbs with the noradrenaline content significantly reduced, the mean dopamine and noradrenaline levels were, respectively, 90% and 34% of the control values. One week after treatment, the average irritability score was significantly higher in the 6-OHDA treated animals than in the sham-operated ones. The main electrophysiological effect of the treatment was to amplify the selective enhancement of OB responsiveness to food odor normally occurring in hungry animals; this effect was restricted to the 6-OHDA bulbs. The results discussed considering the possible involvement of NA fibers in selective attention to odors at the OB level.