In vivo microdialysis assessment of extracellular serotonin and dopamine levels in awake monkeys during sustained fluoxetine administration.

Fluoxetine (FLU) rapidly enhances extracellular (EC) serotonin (5-HT) in rodent brain, whereas the antidepressant effects of this drug in humans are typically not observed for 2-3 weeks. Thus, the effects of chronic oral FLU administration on neocortical and hippocampal EC 5-HT, and on caudate EC 5-HT and dopamine (DA), were examined in awake monkeys (Macaca fascicularis) using in vivo microdialysis (10.0 mg/kg; 3, 7, 14, and 21 days). On day 3, 5-HT was significantly increased above baseline levels in hippocampus (HC) and caudate. There was a trend for an increase in neocortex EC 5-HT levels. However, by day 7 5-HT remained significantly elevated only in HC, although 5-HT levels elsewhere had not completely returned to baseline. In contrast, levels of the 5-HT metabolite, 5-HIAA, were significantly reduced in all brain regions at all time points. Caudate DA levels tended to be decreased throughout FLU treatment. Local FLU and K(+) infusion were also used at various times during chronic systemic FLU administration to evaluate changes in functional synaptic regulation. In general, these results, along with the significant decrease in 5-HIAA levels and the tendency for basal EC 5-HT levels to remain modestly elevated only in HC during sustained FLU administration, suggest a reduction in releasable pools of 5-HT. Taken together with the trend for a decrease in caudate EC DA levels, these results do not appear to support the current hypothesis regarding the mechanism of action of SSRI antidepressants-that monoaminergic neurotransmission is progressively augmented during chronic treatment.

The posterior approach to pyloric sonography.

Overlying bowel gas or gastric distension may occasionally hinder the sonographic diagnosis of hypertrophic pyloric stenosis. To address this problem, a novel approach for obtaining posterior views of the pylorus is reported. Utilizing this approach may decrease the incidence of nondiagnostic pyloric ultrasonography.

Distribution of dopamine beta-hydroxylase-like immunoreactive fibers within the shell subregion of the nucleus accumbens.

The nucleus accumbens (Acb) can be divided into distinct subfields, delineated on the basis of histochemical markers as well as by afferent and efferent projection patterns. The shell subregion has reciprocal relationships with a variety of limbic areas and brainstem autonomic structures, and has been suggested to participate in motivation-related processes, including reward, stress, and arousal. The locus coeruleus (LC)-noradrenergic system has similarly been implicated in the modulation of behavioral state and stress-related processes, and previous studies have demonstrated reciprocal projections between the locus coeruleus and Acb shell. To better understand the anatomical substrate through which LC could influence activity within Acb shell, immunohistochemical methods were used to visualize the extent and the distribution of noradrenergic axons within this structure. Coronal sections of rat brain were processed to visualize immunoreactivity for the norepinephrine synthetic enzyme dopamine beta-hydroxylase (DBH), a specific marker for noradrenergic processes. In some cases, alternate sections were processed for immunohistochemical localization of substance P, in order to delineate core, shell, and pallidal compartments. Moderate-to-dense DBH-like immunoreactivity (DBHir) was found in approximately the caudal half of the shell subregion, particularly in caudalmost (septal pole) and ventral zones. The innervation of the septal pole was contiguous with a dense innervation of the bed nucleus of the stria terminalis. Few immunoreactive fibers were observed in the caudate-putamen, Acb core, or rostral Acb shell. Many DBHir fibers within the shell region were highly arborized with numerous varicosities, features indicative of terminal fields. These observations suggest noradrenergic systems might modulate certain processes associated with stress, behavioral state, or reinforcement via actions within the Acb shell.

Locus coeruleus electrophysiological activity and responsivity to corticotropin-releasing factor in inbred hypertensive and normotensive rats.

The spontaneously hypertensive rat (SHR) and its normotensive progenitor, the Wistar-Kyoto rat (WKY), have been shown to be differentially responsive to the behavioral and endocrine effects of both stress and corticotropin-releasing factor (CRF), both of which increase locus coeruleus (LC) electrophysiological activity. However, the effect of central administration of CRF in these rat strains has yet to be examined. In the present studies, LC electrophysiological responsivity to intracerebroventricular infusions of CRF was assessed in SHR, an inbred strain of WKY rats (the WKY[LJ] rat), and an outbred normotensive rat strain, Sprague-Dawley (SD) rats. Spontaneous LC discharge rate, mean arterial blood pressure and heart rate were also examined. LC activity was increased to the same extent in the three rat strains in response to a 3 microg dose of CRF. However, WKY(LJ) rats showed an exaggerated LC in response to a 1 microg dose of CRF in comparison to the other rat strains tested at this dose. Spontaneous discharge rates of individual LC neurons were lower in both SHR and WKY[LJ] rats than in SD rats. Further, the variability of the discharge rates of LC neurons was greater in WKY[LJ] rats than in the other two strains. These results indicate that the WKY[LJ] rat may provide a useful model for assessing the role of sensitivity to CRF in stress responsiveness.

Enhancement of behavioral and electroencephalographic indices of waking following stimulation of noradrenergic beta-receptors within the medial septal region of the basal forebrain.

Previous studies in halothane-anesthetized rat documented potent electroencephalographic (EEG) modulatory actions of the locus coeruleus (LC) noradrenergic system, with LC neuronal activity causally related to the maintenance of EEG activity patterns associated with enhanced arousal/alertness. Recent studies, also in halothane-anesthetized rat, demonstrated that the region of the basal forebrain encompassing the medial septum/vertical limb of the diagonal band of Broca (MS) is a site at which noradrenergic efferents act to influence EEG state via actions at beta-receptors. These and other observations are consistent with the hypothesis that the LC noradrenergic system participates in the modulation of behavioral state. However, the degree to which this system modulates EEG state in the absence of anesthesia and to what extent such actions are accompanied by behavioral modulatory actions remain to be determined. The current studies examined whether small infusions of isoproterenol (ISO), a beta-adrenergic agonist, into MS alter behavioral, EEG, and electromyographic (EMG) measures of sleep and waking in the resting, undisturbed rat. These infusions resulted in a significant increase in time spent awake, defined by both behavioral and EEG/EMG measures, and in the nearly complete suppression of REM sleep. EEG/EMG responses either coincided with or preceded behavioral responses by 10-320 sec. The pattern of behavioral responses observed following MS-ISO infusions was qualitatively similar to that associated with normal waking. Infusions of vehicle into MS or ISO into sites adjacent to MS did not elicit consistent alterations in behavioral state. These results suggest that the LC noradrenergic system exerts potent behavioral and EEG-activating effects via actions of norepinephrine at beta-receptors located within MS.

Modulation of forebrain electroencephalographic activity in halothane-anesthetized rat via actions of noradrenergic beta-receptors within the medial septal region.

The locus coeruleus (LC)-noradrenergic system modulates forebrain electroencephalographic (EEG) activity in halothane-anesthetized rat. For example, unilateral enhancement of LC neuronal activity increases cortical EEG (ECoG) and hippocampal EEG (HEEG) indices of arousal bilaterally (Berridge and Foote, 1991). Conversely, bilateral suppression of LC discharge activity increases EEG measures of sedation (Berridge, et al., 1993b). The EEG-activating effects of LC stimulation appear to involve noradrenergic beta-receptors (Berridge and Foote, 1991). Two candidate sites at which LC efferents could influence ECoG and HEEG are the medial septum/vertical limb of the diagonal band of Broca (MS) and the substantia innominata/nucleus basalis of Meynert (SI). To determine whether norepinephrine mediates such actions within either of these regions, the EEG effects of small infusions of a beta-agonist or antagonist into MS or SI were examined in halothane-anesthetized rat. Unilateral infusions (150 nl) of the beta-agonist isoproterenol (ISO) (3.75 microg, 17 nmol) into MS, but not SI (150-450 nl), elicited robust bilateral activation of ECoG and HEEG. Infusions of glutamate (0.5 microg, 3.0 nmol) into either MS or SI elicited bilateral ECoG and HEEG activation. Neither vehicle infusions into MS nor infusions of ISO into regions adjacent to MS altered forebrain EEG activity. Bilateral, but not unilateral, MS infusions of the beta-antagonist timolol (3.75 microg, 8.7 nmol) decreased EEG indices of arousal in the lightly anesthetized preparation. Power spectral analyses provided quantitative confirmation of these qualitative observations. These results indicate that under these experimental conditions, noradrenergic efferents, presumably arising from LC, modulate forebrain EEG state via actions at beta-receptors located within MS. The results presented in the accompanying report extend these observations to the unanesthetized preparation and incorporate additional measures of behavioral state.

Reciprocal cross-desensitization of locus coeruleus electrophysiological responsivity to corticotropin-releasing factor and stress.

While acutely administered corticotropin-releasing factor (CRF) and acute stress each activate neurons of the locus coeruleus (LC), desensitization to both develops with repeated treatment. The present experiments were designed to investigate whether cross-desensitization develops between CRF and stress. Because acute hemodynamic stress caused by intravenous infusion of sodium nitroprusside increases LC electrophysiological discharge rate via a CRF-dependent mechanism, it was hypothesized that repeated CRF administration would cause desensitization to the effect of this stressor on LC. For a complementary experiment, it was hypothesized that repeated stress, which presumably results in the repeated release of endogenous CRF, would result in desensitization to subsequent exogenous CRF. The results of the first experiment showed that repeated intracerebroventricular (i.c.v.) administration of CRF caused a significant attenuation of the sodium nitroprusside-induced increase in LC discharge rate seen in naive rats, although this pretreatment actually potentiated the decrease in blood pressure produced by sodium nitroprusside. In the second experiment, either one or eight sessions of white-noise stress attenuated the effect of CRF on LC activity 24 h after the last stress exposure, and this attenuation was more pronounced following eight sessions of stress than following one session. In a test of the specificity of this effect, stress-induced desensitization did not generalize to the LC electrophysiological response to clonidine (i.c.v.). One week following the last of eight sessions of stress, LC responsivity to CRF had recovered to control levels. These experiments demonstrate reciprocal cross-desensitization between CRF and stress using LC electrophysiological responsivity as an assay. This modifiability of the interaction between CRF and the LC may represent the operation of mechanisms mediating adaptive responding to stress.

Effects of pretreatment with corticotropin-releasing factor on the electrophysiological responsivity of the locus coeruleus to subsequent corticotropin-releasing factor challenge.

Both acute central administration of exogenous, and stress-induced release of endogenous corticotropin-releasing factor result in electrophysiological activation of the noradrenergic neurons constituting the locus coeruleus. The present experiments were designed to examine whether single (1) or repeated (8) intracerebroventricular pretreatment with exogenous corticotropin-releasing factor would alter locus coeruleus electrophysiological responsivity to subsequent corticotropin-releasing factor challenge in rats. A single corticotropin-releasing factor (3 microg) pretreatment significantly attenuated challenge-induced locus coeruleus activation 24 and 72, but no 96 h later, while a single vehicle pretreatment had no significant effect on the response to subsequent challenge at any pretreatment-to-test interval. Repeated pretreatment with either corticotropin-releasing factor or vehicle completely attenuated locus coeruleus response to challenge 24 h after the final pretreatment. Seventy-two hours after the last vehicle pretreatment, challenge resulted in a significant increase in locus coeruleus activity, though the response was less than in naive controls. Challenge continued to produce no effect on locus coeruleus activity in repeated corticotropin-releasing factor-pretreated rats at this (72 h) time point. One week (168 h) after the cessation of repeated pretreatment, challenge resulted in a significant increase in locus coeruleus activity which was equal to that of naive controls in vehicle-pretreated rats, but reduced by comparison to controls in corticotropin-releasing factor-pretreated rats. Basal discharge rates of locus coeruleus neurons 24 h after the last repeated corticotropin-releasing factor pretreatment were significantly less than in naive controls. Thus, the failure of challenge to increase neuronal activity in these rats was not due to a "ceiling" effect caused by elevated tonic discharge rate. Repeated vehicle pretreatment produced a functional change similar to that produced by exogenous corticotropin-releasing factor administration. One hypothesis is that repeated vehicle pretreatment was stressful and caused the repeated release of endogenous corticotropin-releasing factor. This hypothesis was tested by determining whether locus coeruleus neurons remained responsive to challenge following repeated administration of a corticotropin-releasing factor antagonist. Thus, the effect if repeated pretreatment with the antagonist, [D-Phe, Nle, Calpha MeLeu]CRF was also examined. Challenge resulted ina significant increase in discharge rate 24 h after the final antagonist pretreatment, providing support for the hypothesis. Additionally, in rats repeatedly pretreated with vehicle, carbachol challenge induced an increase in locus coeruleus activity equal to that induced in naive controls. These results indicate that prior exposure to corticotropin-releasing factor, or the repeated mild stress of vehicle infusions, reduces locus coeruleus responsiveness to corticotropin-releasing factor, and reveal that the relationship between these two neurotransmitter systems is modifiable. This altered relationship may contribute to stress-related affective disorders in which both systems have been implicated.

Locus coeruleus-induced modulation of forebrain electroencephalographic (EEG) state in halothane-anesthetized rat.

The effects of reversible enhancement or suppression of locus coeruleus (LC) neuronal discharge activity on forebrain electroencephalographic (EEG) activity have been previously examined in two series of experiments in halothane-anesthetized rats. Unilateral enhancement of LC activity increased EEG measures of arousal in frontal cortex and hippocampus. The EEG effects of LC activation were blocked by intracerebroventricular pretreatment with the noradrenergic beta-antagonist, propranolol. Bilateral, but not unilateral, suppression of LC activity substantially increased EEG measures of sedation/anesthesia in cortex and hippocampus. In all experiments: a) EEG responses were only observed following changes in LC activity levels; b) onset of EEG responses closely followed changes in LC neuronal activity; c) recovery of EEG responses closely followed the recovery of LC neuronal activity. The present report integrates these previous results and considers their implications for the hypothesis that the LC may be an important modulator of behavioral state and/or state-dependent processes. Together, the two series of experiments yield complementary observations that have implications for LC function that are not apparent when considering each series in isolation.

Effects of systemic clonidine on auditory event-related potentials in squirrel monkeys.

Event-related potential (ERP), electroencephalographic (EEG), and behavioral data were collected from squirrel monkeys (Saimiri sciureus) in a 90-10 auditory oddball paradigm. Background or target tones were presented once every 2 s, and responses to the targets were rewarded. ERPs were recorded from epidural electrodes following systemic administration of clonidine (0.1 mg/kg) or a saline placebo. EEG power spectral and behavioral performance were assessed simultaneously as indices of behavioral state. Clonidine significantly decreased the area and increased the latency of a P300-like potential. The amplitude and areas of the earlier P1, N1, and P2 components and a later slow wave-like potential were not reduced, nor were ther latencies altered. Clonidine produced increased EEG power in the alpha range (7.5-12 Hz) and decreased power in the upper beta range (20-40 Hz) but did not affect performance in the oddball task. Because two major effects of clonidine are to substantially reduce activity in the noradrenergic nucleus locus coeruleus (LC) and to reduce norepinephrine (NE) release from axons, the present results support the hypothesis that the LC and its efferent projection system are important in modulating the activity of P300-like potentials.

Locus coeruleus neuronal activity in awake monkeys: relationship to auditory P300-like potentials and spontaneous EEG.

These experiments were designed to test the hypothesis that novel auditory stimuli lead to phasic and/or tonic increases in locus coeruleus (LC) cell firing, which may be a necessary condition for the occurrence of P300 potentials. Event-related potentials (ERPs) and LC unit activity were simultaneously recorded from three awake macaque monkeys exposed to an auditory "oddball" paradigm. Oddball stimuli resulted in probability-sensitive potentials resembling the human P3a component. Twenty-five percent (3/12) of LC units showed small phasic enhancements of LC firing after infrequent but not frequent tones. A comparison between histograms elicited by the two types of stimuli revealed significant effects of stimulus sequence. This pattern suggested a slight activation by rare tones, followed by a brief inhibition of firing in the subsequent trial. These data suggest that changes in LC activity during oddball paradigms are subtle, heterogeneous, and influenced by the subject's level of arousal and vigilance.

Corticotropin-releasing factor in the locus coeruleus mediates EEG activation associated with hypotensive stress.

Although corticotropin-releasing factor (CRF) is thought to act as a neurotransmitter to activate the locus coeruleus (LC) during hypotensive stress, the consequences of LC activation by CRF are unknown. In the present study a hypotensive challenge that activated rat LC neurons also produced cortical electroencephalographic (EEG) correlates of arousal. Selective, bilateral LC inactivation by local clonidine infusion prevented EEG activation associated with hypotension. Additionally, bilateral LC infusion of CRF antagonists prevented both LC and EEG activation by this challenge. These results indicate that CRF, acting as a neurotransmitter to activate LC during stress, has a powerful of modulatory influence over global forebrain electrophysiological activity.

The locus coeruleus as a site for integrating corticotropin-releasing factor and noradrenergic mediation of stress responses.

Anatomic and electrophysiologic studies have provided evidence that CRF meets some of the criteria as a neurotransmitter in the noradrenergic nucleus, the locus coeruleus (LC), although some of the criteria have yet to be satisfied. Thus, immunohistochemical findings suggest that CRF innervates the LC, but this must be confirmed at the ultrastructural level. CRF alters discharge activity of LC neurons and these effects are mimicked by some stressors. Moreover, the effects of hemodynamic stress on LC activity are prevented by a CRF antagonist. However, it has not been demonstrated that stimulation of CRF neurons that project to the LC activates the LC or that the effects of such stimulation are prevented by a CRF antagonist. The role of CRF in LC activation by stressors other than hemodynamic stress needs to be determined. It could be predicted that the effects of CRF neurotransmission in the LC during stress would enhance information processing concerning the stressor or stimuli related to the stressor by LC target neurons. One consequence of this appears to be increased arousal. Although this may be adaptive in the response to an acute challenge, it could be predicted that chronic CRF release in the LC would result in persistently elevated LC discharge and norepinephrine release in targets. This could be associated with hyperarousal and loss of selective attention as occurs in certain psychiatric diseases. Manipulation of endogenous CRF systems may be a novel way in which to treat psychiatric diseases characterized by these maladaptive effects.

Effects of social deprivation in prepubescent rhesus monkeys: immunohistochemical analysis of the neurofilament protein triplet in the hippocampal formation.

Social deprivation during early postnatal life has profound and long-lasting effects on the behavior of primates, including prolonged and exaggerated responses to stress as well as impaired performance on a variety of learning tasks. Although the cellular changes that underlie such alterations in behavior are unknown, environmentally induced psychopathology may involve morphologic or biochemical changes in select neuronal populations. The hippocampal formation of both socially deprived and socially reared prepubescent rhesus monkeys was selected for immunocytochemical investigation because of its association with the behavioral stress response and learning. Immunocytochemical analysis using antibodies specific for the neurofilament protein triplet was performed since these proteins are modified within degenerating neurons in a variety of neurodegenerative disorders. Results from optical density measurements indicate an increase in the intensity of non-phosphorylated neurofilament protein immunoreactivity in the dentate gyrus granule cell layer of socially deprived monkeys in comparison with that of socially reared animals, suggesting that early social deprivation may result in an increase in the amount of non-phosphorylated neurofilament protein in these cells. This phenotypic difference in dentate granule cells between differentially reared monkeys supports the notion that specific subpopulations of neurons in brain regions that subserve complex behaviors may undergo long-term modifications induced by environmental conditions. Furthermore, the data suggest that constitutive chemical components related to structural integrity may be as susceptible to early environmental manipulations as the more traditionally viewed measures of cellular perturbations, such as neurotransmitter dynamics, cell density and the establishment of connectivity. The observed modifications may serve as an anatomical substrate for behavioral abnormalities that persist in later life.

Effects of locus coeruleus inactivation on electroencephalographic activity in neocortex and hippocampus.

The effects of inhibition of locus coeruleus neuronal discharge activity on cortical and hippocampal electroencephalographic activity were examined in halothane-anesthetized rats. A combined recording/infusion probe was used to place 35-150-nl infusions of the alpha 2-noradrenergic agonist, clonidine (1 ng/nl) which inhibits locus coeruleus neuronal discharge activity, immediately adjacent to the locus coeruleus. The recording electrode allowed verification and quantification of the electrophysiological effects of these infusions. Simultaneously, electroencephalographic activity was recorded from sites in frontal neocortex and dorsal hippocampus and subjected to power spectrum analyses. Neither cortical nor hippocampal electroencephalographic activity was substantially affected following unilateral locus coeruleus inactivation. In contrast, bilateral clonidine infusions that completely suppressed locus coeruleus neuronal discharge activity in both hemispheres altered cortical and hippocampal electroencephalographic status. The cortical response to bilateral LC inhibition was characterized by a shift from low-amplitude, high-frequency to large-amplitude, slow-wave activity. Additionally, theta-dominated activity in the hippocampus was replaced with mixed frequency activity. The onset of these changes in forebrain electroencephalographic activity was coincident with the complete bilateral inhibition of locus coeruleus neuronal discharge activity. The resumption of pre-infusion electroencephalographic patterns closely followed recovery of locus coeruleus neuronal activity or could be induced with systemic administration of the alpha 2-noradrenergic antagonist, idazoxan. Clonidine infusions placed 800-1200 microns from the locus coeruleus were less effective at inducing a complete suppression of locus coeruleus activity. These infusions either did not completely inhibit locus coeruleus discharge (35 nl infusions), or did so with a longer latency to complete locus coeruleus inhibition and a shorter duration of inhibition (150 nl infusions). Changes in forebrain electroencephalographic activity occurred only following the complete bilateral suppression of locus coeruleus neuronal discharge activity. These electroencephalographic responses closely followed or coincided with the onset of complete bilateral locus coeruleus inhibition and persisted throughout the period during which bilateral LC neuronal discharge activity was completely absent (60-240 min). Recovery of electroencephalographic patterns was coincident with the reappearance of locus coeruleus discharge activity. These results suggest that the clonidine-induced changes in forebrain electroencephalographic activity were dependent on the complete bilateral suppression of locus coeruleus discharge activity, and that under the present experimental conditions the locus coeruleus/noradrenergic system exerts a potent and tonic activating influence on forebrain electroencephalographic state. These results support the hypothesis that this system may be an important modulator of behavioral state and/or state-dependent processes.

Distribution of corticotropin-releasing factor-like immunoreactivity in squirrel monkey (Saimiri sciureus) amygdala.

Previous anatomical studies of corticotropin-releasing factor (CRF)-like immunoreactivity in rat brain have reported prominent clustering of neuronal elements containing this peptide within the amygdala. The highest concentrations of both CRF-positive cells and fibers were evident in the central nucleus, an observation consistent with the putative role of this peptide in autonomic and endocrine regulation. In addition, lower densities of CRF-positive somata and processes have been noted in other amygdaloid nuclei. However, the distribution of CRF-like immunoreactivity in the amygdala has not been described for any primate species. Such a description would be of interest since substantial differences in the distribution of CRF in rodent and primate have been reported for other brain regions. The present study uses immunohistochemical methods, with a polyclonal antiserum directed against the human form of CRF, to determine the distribution of this peptide in non-colchicine-treated monkeys (Saimiri sciureus). Within the amygdaloid complex, the most numerous and concentrated collections of CRF-positive neurons were seen in the basal and lateral nuclei. The highest densities of CRF-positive fibers and terminals were seen in the lateral and central amygdaloid nuclei. Moderately dense plexuses of CRF-positive fibers also were seen in layer Ia of the periamygdaloid cortex, nucleus of the lateral olfactory tract, anterior and posterior cortical nuclei, and the medial nucleus. Thus, the distribution of CRF-like immunoreactivity differs substantially in monkey and rat amygdala. Since CRF-positive perikarya in monkey are most prominent in nuclei with pronounced interconnections with neocortex, these differences may be an integral component of the increased cortical development that characterizes the primate brain.

Oscillation of interspike interval length in substantia nigra dopamine neurons: effects of nicotine and the dopaminergic D2 agonist LY 163502 on electrophysiological activity.

The rates and patterns of discharge activity exhibited by 16 spontaneously active substantia nigra pars compacta dopamine neurons were studied in halothane-anesthetized rats using three types of quantitative measures: 1) mean discharge rates, 2) population characteristics of interspike interval samples, and 3) interspike interval time-series measures which were used to examine patterns in the ordering of interspike intervals. The mean discharge rate of these 16 cells was 2.9 +/- 0.3 spikes/sec, and each cell was classified as bursting (25% of the cells) or non-bursting (75%). The distribution of interspike intervals of non-bursting neurons were more normally distributed. Time-series analyses (raw time-series plots, return maps, and phase portraits) revealed a substantial oscillatory tendency in the magnitudes of consecutive interspike intervals in these neurons under baseline conditions: Successive interspike intervals tended to alternate between short and long durations, although short bursts often occurred. Under baseline conditions, these cells exhibited both multispike bursts and consecutive long intervals less frequently than would have been predicted by chance ordering of the interspike intervals. These results imply that there are mechanisms acting to reduce the probability of these types of events. Locally infused nicotine enhanced discharge rates in these neurons. Burst firing increased in four neurons, while five neurons did not show any change in burst firing. LY 163502 induced significant decreases in both discharge rate and bursting activity in all cells tested. The variation coefficient, skew, and kurtosis of the interspike interval distributions were not consistently altered by either drug. The local infusion of either nicotine or LY 163502 decreased the oscillatory phenomenon seen in the baseline condition. Neither the nicotine or LY 163502 time-series data exhibited a larger proportion of long-short and short-long pairs (relative to the median interval) than would be expected by chance. It is hypothesized that these neurons have intrinsic mechanisms, made manifest under anesthesia, which induce oscillations in interspike interval length. The oscillatory effect of these mechanisms can be overridden by tonic increases in either excitatory or inhibitory tone.

Localization of corticotropin-releasing factor-like immunoreactivity in monkey olfactory bulb and secondary olfactory areas.

Electrophysiological and anatomical observations suggest that terminals of olfactory bulb mitral cells ending in rat primary olfactory cortex exert certain postsynaptic effects via an excitatory amino acid neurotransmitter. Recent anatomical studies have shown that several peptides, most notably corticotropin-releasing factor (CRF) (Imaki et al., '89) Brain Res., 496: 35-44), are also localized within rat olfactory bulb projection neurons, thus raising the possibility that there is a peptide cotransmitter in this system. In contrast to the availability of data for rodents, very little is known about the distribution of peptides and other putative transmitters in the olfactory systems of primate species. In the present study, sections through the olfactory bulb and its target areas were obtained from two monkey species (Saimiri sciureus and Macaca fascicularis) and processed for immunohistochemistry with a well-characterized polyclonal antiserum directed against the human form of CRF. Virtually identical results were obtained in the two species. Within the olfactory bulb, nearly all mitral and many tufted cells contained CRF-like immunoreactivity. CRF-positive fibers were seen within the olfactory tract and olfactory stria, which contain the axons of mitral and tufted cells. Within the anterior olfactory nucleus and layer Ia of the olfactory tubercle and piriform cortex, immunoreactivity was seen within fine processes, as well as in coarse, varicose fibers and isolated puncta. CRF-positive cells were seen within layer III of the olfactory tubercle and piriform cortex. Immunoreactive fibers and varicosities were also seen within olfactory-recipient regions of the amygdala and entorhinal cortex. These observations suggest that CRF may act as a transmitter and/or neuromodulator in primate olfactory system.