Serotonergic input to cholinergic neurons in the substantia innominata and nucleus basalis magnocellularis in the rat

Sargassum swartzii extracts ameliorate memory functions by neurochemical modulation in a rat model

Recently, considerable attention has been paid to drug exploration from natural sources for treating memory loss, a major manifestation of various neurodegenerative diseases. Increasing evidences implicate brain serotonin metabolism in learning and memory, supporting the notion that targeting 5-HT (5-hydroxytryptamine) and its receptors would be beneficial in the treatment of cognitive disorders. In the present study, behavioral and neurochemical effects were examined following administration of Sargassum swartzii extracts in albino Wistar rats. Increase in spatial working memory and recognition memory was exhibited by the seaweed-treated rats as compared to controls. Plasma tryptophan, brain 5-HT, and 5-hydroxyindoleacetic acid levels were measured using HPLC-ECD, and a significant increase in brain 5-HT metabolism was observed in the seaweed-treated rats. The increase in memory functions following repeated administration of S. swartzii extracts is suggested to be due to the increased serotonergic neurotransmission in the brain of seaweed-treated rats.

Attention-deficit/hyperactivity disorder: neurophysiology, information processing, arousal and drug development

In this review, we draw on literature from both animal and human neurophysiological studies to consider the neurochemical mechanisms underlying attention-deficit/ hyperactivity disorder (ADHD). Psychophysiological and neuropsychological research is used to propose possible etiological endophenotypes of ADHD. These are conceptualized as patients with distinct cortical-arousal, information-processing or maturational abnormalities, or a combination thereof, and how the endophenotypes can be used to help drug development and optimize treatment and management. To illustrate, the paper focuses on neuro- and psychophysiological evidence that suggests cholinergic mechanisms may underlie specific information-processing abnormalities that occur in ADHD. The clinical implications for a cholinergic hypothesis of ADHD are considered, along with its possible implications for treatment and pharmacological development.

Anticataleptic effects of 5-HT(1B) receptors in the globus pallidus

The globus pallidus occupies an important position in the indirect pathway of the basal ganglia. Being a monoamine neurotransmitter, 5-HT is involved in mediating many physiological functions and pathophysiological processes in several movement disorders. Morphological studies have revealed that the globus pallidus receives serotonergic innervation arising from the raphe nuclei, mainly the dorsal raphe nucleus. A high level of 5-HT and 5-HT(1B) receptors were detected in the globus pallidus. In the present study, bilateral microinjection of 5-HT or 5-HT(1B) receptor agonist, CP-93129, into the globus pallidus significantly alleviated the symptoms of rigidity caused by haloperidol. To further elucidate 5-HT(1B) receptor-induced anticatalepsy, in vivo extracellular recordings were performed to examine the effects of 5-HT(1B) receptor activation on the firing activity of the globus pallidus neurons under the presence of haloperidol. Micro-pressure ejection of 5-HT or CP-93129 increased the spontaneous firing rate of the pallidal neurons. Furthermore, by using immunohistochemistry, positive staining of 5-HT(1B) receptor was observed in the globus pallidus neurons. Taken together, the present findings provide evidence that activation of 5-HT(1B) receptor may exert anticataleptic effects by increasing the activity of pallidal neurons.

Neuropathological changes in the nucleus basalis in schizophrenia

The nucleus basalis has not been examined in detail in severe mental illness. Several studies have demonstrated decreases in glia and glial markers in the cerebral cortex in schizophrenia, familial bipolar disorder and recurrent depression. Changes in neocortical neuron size and shape have also been reported. The nucleus basalis is a collection of large cholinergic neurons in the basal forebrain receiving information from the midbrain and limbic system, projecting to the cortex and involved with attention, learning and memory, and receives regulation from serotonergic inputs. Forty-one cases aged 41-60 years with schizophrenia or major depressive disorder with age-matched controls were collected. Formalin-fixed paraffin-embedded coronal nucleus basalis sections were histologically stained for oligodendrocyte identification with cresyl-haematoxylin counterstain, for neuroarchitecture with differentiated cresyl violet stain and astrocytes were detected by glial fibrillary acid protein immunohistochemistry. Cell density and neuroarchitecture were measured using Image Pro Plus. There were larger NB oval neuron soma in the combined schizophrenia and major depression disorder groups (p = 0.038), with no significant change between controls and schizophrenia and major depression disorder separately. There is a significant reduction in oligodendrocyte density (p = 0.038) in the nucleus basalis in schizophrenia. The ratio of gemistocytic to fibrillary astrocytes showed a greater proportion of the former in schizophrenia (18.1 %) and major depressive disorder (39.9 %) than in controls (7.9 %). These results suggest glial cell abnormalities in the nucleus basalis in schizophrenia possibly leading to cortical-limbic disturbance and subcortical dysfunction.

Selective anterograde tracing of nonserotonergic projections from dorsal raphe nucleus to the basal forebrain and extended amygdala

The dorsal raphe nucleus (DRN) contains both serotonergic and nonserotonergic projection neurons. Retrograde tracing studies have demonstrated that components of the basal forebrain and extended amygdala are targeted heavily by input from nonserotonergic DRN neurons. The object of this investigation was to examine the terminal distribution of nonserotonergic DRN projections in the basal forebrain and extended amygdala, using a technique that allows selective anterograde tracing of nonserotonergic DRN projections. To trace nonserotonergic DRN projections, animals were pretreated with nomifensine, desipramine and the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), 7 days prior to placing an iontophoretic injection of biotinylated dextran amine (BDA) into the DRN. In animals treated with 5,7-DHT, numerous nonserotonergic BDA-labeled fibers ascended to the basal forebrain in the medial forebrain bundle system. Some of these labeled fibers crossed through the lateral hypothalamus, bed nucleus of the stria terminalis, and substantial innominata. These fibers entered the amygdala through the ansa peduncularis and ramified within the central and basolateral amygdaloid nuclei. Other fibers entered the diagonal band of Broca and formed a dense plexus of labeled fibers in the dorsal half of the intermediate portion of the lateral septal nucleus and the septohippocampal nucleus. These findings demonstrate that the basal forebrain and extended amygdala receive a dense projection from nonserotonergic DRN neurons. Given that the basal forebrain plays a critical role in processes such as motivation, affect, and behavioral control, these findings support the hypothesis that nonserotonergic DRN projections may exert substantial modulatory control over emotional and motivational functions.

Neurokinin-1 receptors in cholinergic neurons of the rat ventral pallidum have a predominantly dendritic distribution that is affected by apomorphine when combined with startle-evoking auditory stimulation

Cholinergic neurons of the basal forebrain are implicated in startle reflex inhibition by a prior weak stimulus often referred to as prepulse inhibition (PPI) and used as an index of sensorimotor gating deficits in schizophrenia. Gating deficits can be produced in rodent models by acute systemic administration of apomorphine, a non-selective dopamine D1 and D2 receptor agonist that also affects trafficking of neurokinin-1 (NK(1)) receptors induced by startle evoking auditory stimulation (AS) in midbrain neurons. We used electron microscopic immunolabeling of NK(1) receptors and the vesicular acetylcholine transporter (VAchT) to test the hypothesis that the subcellular distributions of these receptors in cholinergic neurons of the rat ventral pallidum are subject to a similar regulation. In vehicle controls, NK(1) immunogold was often seen near cytoplasmic endomembranes in somata and large dendrites, but was more equally distributed in cytoplasmic and plasmalemmal compartments of medium dendrites, and principally located on the plasma membrane of small dendrites. These labeling patterns appeared to be largely independent of whether the NK(1) receptor was co-expressed with VAchT, however only the medium and small VAchT-labeled dendrites showed significant treatment-specific differences in NK(1) immunogold distributions. The NK(1) receptor immunogold particle density on the plasma membrane of medium cholinergic dendrites was significantly enhanced by combined apomorphine and AS, while neither alone affected either the plasmalemmal density or the equality of the plasmalemmal and cytoplasmic distributions of NK(1) receptors in these dendrites. Small cholinergic dendrites showed a significant AS-induced increase in both the plasmalemmal and cytoplasmic density of NK(1) gold particles, and an apomorphine-induced disruption of the preferential plasmalemmal targeting of the NK(1) receptors. These results provide ultrastructural evidence that NK(1) receptors in cholinergic neurons of the ventral pallidum have subcellular locations and plasticity conducive to active involvement in dopamine-dependent sensorimotor processing.

Serotonergic modulation of plasticity of the auditory cortex elicited by fear conditioning

In the awake big brown bat, 30 min auditory fear conditioning elicits conditioned heart rate decrease and long-term best frequency (BF) shifts of cortical auditory neurons toward the frequency of the conditioned tone; 15 min conditioning elicits subthreshold cortical BF shifts that can be augmented by acetylcholine. The fear conditioning causes stress and an increase in the cortical serotonin (5-HT) level. Serotonergic neurons in the raphe nuclei associated with stress and fear project to the cerebral cortex and cholinergic basal forebrain. Recently, it has been shown that 5-HT(2A) receptors are mostly expressed on pyramidal neurons and their activation improves learning and memory. We applied 5-HT, an agonist (alpha-methyl-5-HT), or an antagonist (ritanserin) of 5-HT(2A) receptors to the primary auditory cortex and discovered the following drug effects: (1) 5-HT had no effect on the conditioned heart rate change, although it reduced the auditory responses; (2) 4 mm 5-HT augmented the subthreshold BF shifts, whereas 20 mm 5-HT did not; (3) 20 mm 5-HT reduced the long-term BF shifts and changed them into short-term; (4) alpha-methyl-5-HT increased the auditory responses and augmented the subthreshold BF shifts as well as the long-term BF shifts; (5) in contrast, ritanserin reduced the auditory responses and reversed the direction of the BF shifts. Our data indicate that the BF shift can be modulated by serotonergic neurons that augment or reduce the BF shift or even reverse the direction of the BF shift. Therefore, not only the cholinergic system, but also the serotonergic system, plays an important role in cortical plasticity according to behavioral demands.

Acute hypercarbic gas exposure reveals functionally distinct subpopulations of serotonergic neurons in rats

Although increasing evidence suggests that anatomically defined subpopulations of serotonergic neurons have unique stress-related functional properties, the topographical distribution of the serotonergic neurons involved in responses to stress-related stimuli have not been well-defined. Inspiration of air containing elevated concentrations of carbon dioxide (CO(2); hypercarbic gas exposure) at high concentrations activates both hypothalamic-pituitary-adrenal axis and sympathetic responses in rats and humans. In order to determine the effects of acute hypercarbic gas exposure on subpopulations of topographically organized serotonergic neurons, conscious adult male rats were placed in flow cages and exposed to either atmospheric air or increasing environmental CO2 concentrations (from baseline concentrations up to 20% CO2) for 5min. The presence of immunoreactivity for the protein product of the immediate-early gene c-fos was used as a measure, at the single cell level, of functional cellular responses within subpopulations of serotonergic, noradrenergic and adrenergic neurons. Rats exposed to hypercarbic gas had increased numbers of c-Fos/tryptophan hydroxylase immunoreactive (ir) and c-Fos/tyrosine hydroxylase-ir neurons in specific topographically organized subdivisions of brainstem nuclei, compared to control rats. Within serotonergic cell groups (B1-B9), the most striking effects occurred in a subpopulation of large, multipolar serotonergic neurons within the ventrolateral periaqueductal grey and ventrolateral part of the dorsal raphe nucleus, a region implicated in serotonin-dependent suppression of stress-induced sympathetic outflow and serotonin-dependent inhibition of 'fight or flight' behaviour. These findings have important implications for understanding the role of serotonergic systems in the modulation of stress-related physiology and behaviour and stress-related neuropsychiatric disorders.

Opposing Electrophysiological Actions of 5-HT on Noncholinergic and Cholinergic Neurons in the Rat Ventral Pallidum In Vitro

The ventral pallidum in rat is a basal forebrain structure that contains neurons that project in the limbic striatopallidal circuitry and magnocellular cholinergic corticopetal neurons. Because 5-hydroxytryptamine (5-HT) terminals on dorsal raphe projections form close appositions with these neurons, we made patch-clamp recordings in immature rat brain slices to determine whether they are modulated by postsynaptic 5-HT receptors. Inward currents were predominantly induced by 5-HT in noncholinergic neurons, which were distinguished from cholinergic neurons by immunohistochemical and electrophysiological criteria. The inward current induced by 5-HT was mimicked and occluded when adenylyl cyclase was stimulated with forskolin, and was almost abolished when h-currents in noncholinergic neurons were blocked with cesium. Consistent with 5-HT7 receptor activation of h-curents by cAMP in other brain regions, we found inward currents were mimicked by the mixed 5-HT1/5-HT7 agonists 5-methoxytryptamine, and by 5-carboxamidotryptamine (5-CT), which was more potent than 5-HT. In contrast, 5-HT1 preferring 8-OH-DPAT was a weak partial agonist, and the 5-HT1–selective antagonist pindolol had no effect. However, despite this profile, antagonists that bind at the 5-HT7 receptor only partly reduced the agonist inward current (SB-269970 and clozapine), or had no effect (mianserin and pimozide). We found in cholinergic neurons that 5-HT predominantly induced hyperpolarizing currents, which were carried by potassium channels, and were smaller than currents induced by 8-OH-DPAT and 5-CT. We conclude from this study that ascending 5-HT projections from the dorsal raphe could have direct and opposite effects on the activities of neurons within the limbic striatopallidal and cholinergic corticopetal circuitry in the ventral pallidum.

p-Chloroamphetamine blocks physostigmine-induced memory enhancement in rats with unilateral nucleus basalis lesions

The present experiment examined whether p-chloroamphetamine (PCA), a serotonergic releasing/depleting agent, would block the memory-enhancing effect of physostigmine in rats with N-methyl-D-aspartic acid (NMDA)-induced unilateral lesions of the nucleus basalis of Meynert (uni-nbM). Six groups of subjects with uni-nbM lesions in addition to an isolated sham-operated control group were included. Subjects were trained and tested 72 h later on a one-trial passive avoidance task. Thirty minutes before training, rats with uni-nbM lesions were injected with either 1.0 or 5.0 mg/kg PCA or saline. Immediately after training, approximately half the subjects in each group were injected with either saline or 0.06 mg/kg physostigmine. Animals in the sham group received saline injections. Saline-injected animals with uni-nbM lesions performed poorly at test, a deficit that was reversed with physostigmine. Pretraining injections of PCA blocked physostigmine's memory-enhancing effect, although motor impairment during training may have contributed to decrements in test performance in animals injected with 5.0 mg/kg. Subjects were killed about 10 days later and their frontal cortices examined for choline acetyltransferase (ChAT). Results from the neurochemical analysis revealed that the lesion decreased ChAT levels and that the injection of 1.0 mg/kg PCA exaggerated this lesion-induced depletion. Implications for the interaction between acetylcholine and serotonin are discussed.

Acquisition, retention, and recall of memory after injection of RS67333, a 5-HT(4) receptor agonist, into the nucleus basalis magnocellularis of the rat

The serotonin 5-HT4 subtype receptor is predominantly localized into anatomical structures linked to memory and cognition. A few experimental studies report that the acute systemic administration of selective 5-HT4 agonists has ameliorative effects on memory performance, and that these effects are reversed by contemporary administration of 5-HT4 receptor antagonists. To verify whether this procognitive action occurs via the activation of the cholinergic nucleus basalis magnocellularis (NBM)-cortical pathways, we examined the effects of RS67333, a selective partial agonist of the 5-HT4 receptor, on rat performance in a place recognition task upon local administration of the drug into the NBM area. The intra-NBM administration of RS67333 enhances the acquisition (200-500 ng/0.5 microL) and the consolidation (40-200 ng/0.5 microL) of the place recognition memory. These effects are reversed by pretreatment with the selective 5-HT4 receptor antagonist RS39604 (300 ng/0.5 microL). Conversely, the recall of memory is not affected by the 5-HT4 agonist. Our results indicate that 5-HT4 receptors located within the NBM may play a role in spatial memory and that the procognitive effect of RS67333 is due, at least in part, to the potentiation of the activity of cholinergic NBM-cortical pathways.

Peripheral 8-OH-DPAT and scopolamine infused into the frontal cortex produce passive avoidance retention impairments in rats

The present study determined whether peripheral injections of the 5HT(1A) agonist (8-OH-DPAT), scopolamine infusions into the frontal cortex, or a combination of both drug treatments would produce impairments in rats trained on passive avoidance. Using a 2x2 design, rats were infused with either bacteriostatic water or 30 microg/1 microl of scopolamine HCl into the frontal cortex 30 min before being trained on passive avoidance. This was followed by injections (ip) of either 0.1% ascorbic acid/bacteriostatic water or 30 microg/kg of 8-OH-DPAT 15 min later. All subjects were tested for retention 72h later. At test, the initial latency to enter into the black shocked compartment and the total time spent in the white safe compartment (TTW) were recorded. Analysis of the latency data indicated that scopolamine and 8-OH-DPAT, when administered singly or in combination, produced amnesia for the task. Assessment of TTW scores, however, revealed that of the three drug-treated groups, only animals treated with 8-OH-DPAT alone tended to avoid the previously shocked black compartment and spend more time in the white safe compartment. These data indicate that either stimulating 5-HT(1A) or blocking frontal cortical muscarinic receptors at training impairs passive avoidance performance and that the deficit following the latter treatment is somewhat more extensive. Implications for the role frontal cortical muscarinic and 5HT(1A) receptors play in learning and memory are discussed.