The role of serotonin in the control of cerebral activity: studies with intracerebral 5,7-dihydroxytryptamine

Optogenetic activation of DRN 5-HT neurons induced active wakefulness, not quiet wakefulness

There has been a long-standing controversy regarding the physiological role of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) in sleep/wake architecture. Some studies have reported that 5-HT acts as a sleep-promoting agent, but several studies have suggested that DRN 5-HT neurons function predominantly to promote wakefulness and inhibit rapid eye movement (REM) sleep. Furthermore, recent studies have reported that there is a clear neurobiological difference between a waking state that includes alertness and active exploration (i.e., active wakefulness) and a waking state that is devoid of locomotion (i.e., quiet wakefulness). These states have also been shown to differ clinically in terms of memory consolidation. However, the effects of 5-HT neurons on the regulation of these two different waking states have not been fully elucidated. In the present study, we attempted to examine the physiological role of DRN 5-HT neurons in various sleep/wake states using optogenetic methods that allowed manipulation of cell-type specific neuronal activation with high temporal and anatomical precision. We crossed TPH2-tTA and TetO-ChR2(C128S) mice to obtain mice with channelrhodopsin-2 (ChR2) [C128S]-expressing central 5-HT neurons, and we activated DRN-5HT neurons or medullary 5-HT neurons. Optogenetic activation of DRN 5-HT neurons caused rapid transition from non-REM sleep to active wakefulness, not quiet wakefulness, whereas activation of medullary 5-HT neurons did not appear to affect sleep/wake states or locomotor activity. Our results may shed light on the physiological role of DRN 5-HT neurons in sleep/wake architecture and encourage further investigations of the cortical functional connectivity involved in sleep/wake state regulation.

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.

The synchronous activity of lateral habenular neurons is essential for regulating hippocampal theta oscillation

Lateral habenula (LHb) has attracted growing interest as a regulator of serotonergic and dopaminergic neurons in the CNS. However, it remains unclear how the LHb modulates brain states in animals. To identify the neural substrates that are under the influence of LHb regulation, we examined the effects of rat LHb lesions on the hippocampal oscillatory activity associated with the transition of brain states. Our results showed that the LHb lesion shortened the theta activity duration both in anesthetized and sleeping rats. Furthermore, this inhibitory effect of LHb lesion on theta maintenance depended upon an intact serotonergic median raphe, suggesting that LHb activity plays an essential role in maintaining hippocampal theta oscillation via the serotonergic raphe. Multiunit recording of sleeping rats further revealed that firing of LHb neurons showed significant phase-locking activity at each theta oscillation cycle in the hippocampus. LHb neurons showing activity that was coordinated with that of the hippocampal theta were localized in the medial LHb division, which receives afferents from the diagonal band of Broca (DBB), a pacemaker region for the hippocampal theta oscillation. Thus, our findings indicate that the DBB may pace not only the hippocampus, but also the LHb, during rapid eye movement sleep. Since serotonin is known to negatively regulate theta oscillation in the hippocampus, phase-locking activity of the LHb neurons may act, under the influence of the DBB, to maintain the hippocampal theta oscillation by modulating the activity of serotonergic neurons.

Serotonin depletion of supramammillary/posterior hypothalamus nuclei produces place learning deficiencies and alters the concomitant hippocampal theta activity in rats

Hippocampal theta activity is important for the acquisition of spatial information and is strongly influenced and regulated by extra-hippocampal inputs from the synchronising ascending system (SAS), which includes the supramammillary nucleus (SUMn) and the posterior hypothalamic nucleus (PHn). Together these nuclei play an important role in controlling the frequency encoding of theta activity and are innervated by serotonin synapses, which also regulate theta activity and learning abilities. The participation of the SUMn in place learning and modulation of hippocampal theta activity were recently shown; thus, we questioned whether serotonin acting on SUMn/PHn could modulate place learning ability and concurrent hippocampal theta activity. The serotonergic terminals of the SUMn/PHn in rats were lesioned through 5,7-dihydroxytryptamine (5,7-DHT) infusion, and hippocampal theta activity during the Morris water maze test was recorded. Rats in the vehicle group learned the task efficiently and showed learning-related theta changes in the CA1 and dentate gyrus regions throughout the training. The 5-HT-depleted rats were deficient in the Morris water maze task and showed theta activity in the CA1 and dentate gyrus that were unrelated to the processing of learning. We conclude that serotonin can regulate the hippocampal theta activity acting on the SUMn/PHn relay of the SAS and that the influence of 5-HT in these nuclei is required for the learning-related changes in hippocampal theta activity that underlie the successful resolution of the Morris water maze task.

Cortical state and attention

The brain continuously adapts its processing machinery to behavioural demands. To achieve this, it rapidly modulates the operating mode of cortical circuits, controlling the way that information is transformed and routed. This article will focus on two experimental approaches by which the control of cortical information processing has been investigated: the study of state-dependent cortical processing in rodents and attention in the primate visual system. Both processes involve a modulation of low-frequency activity fluctuations and spiking correlation, and are mediated by common receptor systems. We suggest that selective attention involves processes that are similar to state change, and that operate at a local columnar level to enhance the representation of otherwise non-salient features while suppressing internally generated activity patterns.

Muscarinic blockade weakens interaction of gamma with theta rhythms in mouse hippocampus

theta (4-12 Hz) and gamma (40-90) oscillations are prominent rhythms in the mammalian brain. A striking feature of these rhythms, possibly vital to memory encoding, is their specific coordination in a manner that has been termed 'nesting', i.e. the preferred occurrence of bouts of gamma activity during specific phases of theta. Both rhythms are shaped by the neuromodulator acetylcholine, but it is unknown to what degree their coordination is influenced by cholinergic neuromodulation. Here, we investigated the effects of a blockade of muscarinic acetylcholine receptors by atropine on theta and gamma oscillations, and their interaction, in mouse hippocampus in vivo. Multi-site recordings from area CA1 of freely moving mice showed that under control conditions gamma activity was amplitude-modulated at theta frequencies. This coordination of theta and gamma oscillations, as assessed by cross-correlation of theta with the gamma envelope, was prominent in basal and apical dendritic laminae but not in intermediate laminae. It was stronger during active exploration than during awake immobility. Atropine (50 mg/kg intraperitoneal) altered several aspects of the individual and nested rhythms. It rendered theta activity irregular, decreased theta oscillation frequency and reduced gamma power. Atropine also reduced the amplitude-modulation of gamma oscillations at theta frequencies, in part by perturbing the coordination of the rhythms on a short time scale. Thus, our findings demonstrate that phase locking of the amplitude of gamma oscillations to theta in hippocampal area CA1 is partially governed by neuronal elements harbouring muscarinic receptors.

Generalized cortex activation by the auditory midbrain: Mediation by acetylcholine and subcortical relays

The inferior colliculus (IC) is a critical component of the ascending projection system carrying auditory information from the brainstem to the forebrain. Recent evidence indicates that, in addition to its role in auditory processing, the IC can exert a generalized, modulatory effect on the forebrain by activating the neocortical electrocorticogram (ECoG). Given the sparse direct projections from the IC to the cortex, it appears that the effect of the IC to produce ECoG activation is indirect, mediated by one or several neuromodulatory systems that have diffuse access to the entire cortical mantle. However, the anatomical relays that permit the IC to influence cortical activity have not been elucidated. In the present experiments, electrical stimulation of the IC suppressed slow, large amplitude oscillations in the ECoG of urethane anesthetized rats, replacing them with higher-frequency cortical activation. This effect was blocked by the muscarinic receptor antagonist scopolamine (0.5-1.0 mg/kg, i.p.), suggestive of a critical role of acetylcholine (ACh) release. Consistent with this hypothesis, localized lidocaine infusions (2%, 1 microl) into the cholinergic basal forebrain complex strongly reduced ECoG activation elicited by IC stimulation. To identify additional relays between the IC and basal forebrain, the effects of lidocaine infusions into the superior colliculus, medial prefrontal cortex, midline thalamus, and dorsal raphe were also studied. Inactivation of the superior colliculus and dorsal raphe reduced IC-induced activation, while prefrontal cortex and thalamic infusions were ineffective. Concurrent basal forebrain and raphe inactivation produced effects similar to that of inactivation of the basal forebrain alone, suggesting that these two areas are arranged in series, rather than acting as independent, parallel pathways. These results suggest that the ability of the IC to induce ECoG activation is mediated, in large parts, by the basal forebrain cholinergic system. Consistent with anatomical evidence, the superior colliculus and dorsal raphe appear to provide important links to functionally connect the IC to the basal forebrain, allowing the IC to indirectly access the entire cortical mantle and enhance processing in neocortical networks.

Amplitude, frequency, and phase analysis of hippocampal theta during sensorimotor processing in a jump avoidance task

In the current study, rats implanted with hippocampal recording electrodes were trained to avoid shock in a jump avoidance task, using three separate heights, 28, 33, and 38 cm. The objectives were to measure the progression of Type 2 (immobility-related) hippocampal theta amplitude and frequency recorded during jump avoidances at each of the levels, and determine if there was a consistent relationship between the phase of the Type 1 (movement related) theta jump wave and the moment of movement initiation at each of the jump heights. The results demonstrated that the immobility period prior to the execution of the jump could be divided into two components: a sensory processing period and a movement preparation period. Comparing these two periods, average amplitudes were higher while frequency remained relatively constant during the sensory processing period. During the movement preparation period there was a negative correlation between amplitude and frequency: amplitude declined rapidly and frequency increased rapidly. During the execution of the jump, theta amplitude (Type 1) and frequency were positively correlated, both reaching peak values. The separate analyses of the individual jump heights provided further support for the precise relationships between theta parameters and the magnitude of the required movements. A significant phase preference was demonstrated for the highest jump height, with movement initiation occurring around the trough of theta recorded from the stratum moleculare of the dentate region. The findings supported the conclusion that the generation of hippocampal theta band oscillation and synchrony was related to sensorimotor processing and integration.

Neocortical activation by electrical and chemical stimulation of the rat inferior colliculus: intra-collicular mapping and neuropharmacological characterization

Classic experiments suggested that the midbrain reticular formation plays an important role in the induction and maintenance of high-frequency, low-amplitude activation of the electrocorticogram (ECoG). However, recent studies have shown that generalized activating systems are not restricted to the reticular formation in that non-reticular brain systems (e.g., basal forebrain, amygdala, superior colliculus) can effectively produce ECoG activation. Here, we investigated the role of the inferior colliculus (IC) in regulating ECoG activation in rats. Urethane-anesthetized rats displayed continuous large amplitude ECoG activity with peak power in the delta frequency range (0.5-3.9 Hz). Electrical 100-Hz stimulation (0.1-0.5 mA) of 40/88 (46%) stimulation sites in the IC suppressed low frequency oscillations and induced ECoG activation (>/=50% suppression of peak delta power). Systematic mapping of different IC territories (central nucleus, external and dorsal cortex) revealed that stimulation of all IC parts was equally effective in producing activation. Chemical stimulation of the IC with intra-collicular glutamate infusions (50 mM, 0.5 micro l) produces similar, but more consistent effects, with ECoG activation elicited in eight of nine rats. Pharmacological experiments were carried out in order to identify transmitters that mediate cortical activation in response to IC stimulation. The muscarinic receptor antagonist scopolamine (1 mg/kg, i.p.) reduced, but did not abolish, activation, as did the serotonergic receptor antagonist methiothepin (1 mg/kg, i.p.). A combination of the two drugs produced a complete block of IC-induced ECoG activation. These experiments demonstrate that the IC contains a distributed network, spanning all IC territories, which can participate in regulating the generalized activation state of the rat neocortex. Rather than by some direct cortical projections, IC neurons appear to induce ECoG activation by acting through both cholinergic and serotonergic systems, thought to provide the final effector mechanisms for cortical activation.

Spatial Learning in the 5-HT1B Receptor Knockout Mouse: Selective Facilitation/Impairment Depending on the Cognitive Demand

Age-related memory decline is associated with a combined dysfunction of the cholinergic and serotonergic systems in the hippocampus and frontal cortex, in particular. The 5-HT1B receptor occupies strategic cellular and subcellular locations in these structures, where it plays a role in the modulation of ACh release. In an attempt to characterize the contribution of this receptor to memory functions, 5-HT1B receptor knockout (KO) mice were submitted to various behavioral paradigms carried out in the same experimental context (water maze), which were aimed at exposing mice to various levels of memory demand. 5-HT1BKO mice exhibited a facilitation in the acquisition of a hippocampal-dependent spatial reference memory task in the Morris water maze. This facilitation was selective of task difficulty, showing thus that the genetic inactivation of the 5-HT1B receptor is associated with facilitation when the complexity of the task is increased, and reveals a protective effect on age-related hippocampal-dependent memory decline. Young-adult and aged KO and wild-type (WT) mice were equally able to learn a delayed spatial matching-to-sample working memory task in a radial-arm water maze with short (0 or 5 min) delays. However, 5-HT1BKO mice, only, exhibited a selective memory impairment at intermediate and long (15, 30, and 60 min) delays. Treatment by scopolamine induced the same pattern of performance in wild type as did the mutation for short (5 min, no impairment) and long (60 min, impairment) delays. Taken together, these studies revealed a beneficial effect of the mutation on the acquisition of a spatial reference memory task, but a deleterious effect on a working memory task for long delays. This 5-HT1BKO mouse story highlights the problem of the potential existence of "global memory enhancers."

Superior colliculus stimulation enhances neocortical serotonin release and electrocorticographic activation in the urethane-anesthetized rat

Recent evidence indicates that the superior colliculus (SC), in addition to its functions in sensory detection, also participates in controlling the generalized activation state of the forebrain, as measured by the electroencephalogram (EEG) or electrocorticogram (ECoG). The mechanisms by which the SC modulates forebrain activation are not well understood. By using in vivo microdialysis, we examined the role of serotonin release as a mechanism by which the SC can control neocortical activity in the urethane-anesthetized rat. Electrical 100 Hz stimulation of the SC increased frontal cortex serotonin output to 116, 118, and 140% of baseline levels for stimulation intensities of 0.5, 0.75, and 1.0 mA, respectively. Further, 75% of extracellularly recorded single (putative serotonergic) dorsal raphe neurons increased their discharge rate in response to 100 Hz stimulation of the SC. Stimulation of the SC also suppressed frontal cortex low frequency (1-6 Hz) synchronized ECoG activity, replacing it with high-frequency desynchronization. This activation response was resistant to cholinergic-muscarinic receptor antagonists (atropine, 50 mg/kg; scopolamine, 2 mg/kg), but was reduced or abolished by systemic treatment with the serotonergic receptor antagonists ketanserin (10 mg/kg) or methiothepin (5 mg/kg). These data suggest that efferents from the SC, possibly by an excitatory action on serotonergic dorsal raphe cells, produce an enhanced release of serotonin and ECoG activation in the neocortex. The stimulation of cortical serotonin output may constitute a mechanism by which the SC acts on the forebrain to increase cortical excitability in response to sensory stimuli processed by SC neurons.

GABAergic Interneurons are the Major Postsynaptic Targets of Median Raphe Afferents in the Rat Dentate Gyrus

The termination pattern of median raphe axons was studied in the rat dentate gyrus using Phaseolus vulgaris leucoagglutinin as an anterograde tracer, in combination with postembedding immunostaining for gamma-amino-butyric acid (GABA), and pre-embedding immunostaining for calbindin D28k, parvalbumin and GABA. Postembedding immunogold staining for GABA revealed that the majority (73.7%) of anterogradely labelled median raphe boutons make synaptic contacts with GABA-immunoreactive postsynaptic targets, mainly with dendritic shafts and perikarya. Pre-embedding immunocytochemical double staining for the anterograde tracer and GABA confirmed the electron microscopic results and showed that varicose median raphe axons establish multiple contacts with fusiform interneurons in the hilus and different types of basket cells in the granule cell layer. Some of the innervated cells were shown to contain calbindin D28k, whereas GABAergic interneurons containing another calcium-binding protein, parvalbumin, were never seen to receive multiple contacts from axons of raphe origin. Our results suggest that serotonergic median raphe fibres influence the firing of dentate granule cells via local inhibitory interneurons. The mechanism of using these interneurons with extensive local connections as monosynaptic targets may explain the great efficacy of this pathway in the control of hippocampal electrical activity.

Serotonergic depletion increases conditioned suppression to background stimuli in the rat

Dark Agouti rats were lesioned by intra-ventricular injection of 5,7-dihydroxytryptamine (DHT) and, 2 weeks later, learning was tested in a conditioned suppression of drinking procedure. Lesioned and vehicle-injected control rats were conditioned with a discrete stimulus (tone or light conditioned stimulus, CS) twice paired with footshock (unconditioned stimulus), with or without a 30-s trace interval between these events to produce strong and weak learning conditions (a trace conditioning effect). During this conditioning session, the alternate stimulus (light or tone) was presented continuously in the background. Since the 5,7-DHT lesion also reduced the baseline licking response in the experimental chambers, we used drinking during the first minute, when this non-specific effect was minimal, as the dependent variable. We tested conditioning to target CS and to the alternative experimental background stimulus in exactly the same way in the same rats. We found that a level of serotonergic depletion without any intrinsic action on the trace conditioning effect nevertheless increased conditioning to the alternative background stimulus, irrespective of trace interval or stimulus modality. Thus, for both light and tone stimuli, the effect of serotonergic depletion depended only on the discrete target versus diffuse background role of the stimulus in use. These findings have implications for the modification of human cognition by serotonergic drugs.

Prefrontocortical serotonin depletion results in plastic changes of prefrontocortical pyramidal neurons, underlying a greater efficiency of short-term memory

The prefrontal cortex activity is involved in organizing the short-term memory. Although the involvement of serotonin for an appropriate performance in learning and memory tests is well known, its role is still unclear; as is the cellular basis of short-term memory behavioral performance. Sprague-Dawley rats were stereotactically injected with 1 microg/microl of 5, 7-dihydroxitryptamine to cause a lesion to the dorsal raphe nucleus. Sham-operated or intact rats were also studied as control groups. Before surgery and 20 days post-operatively, each animal was placed in the Biel maze for five consecutive trials. In the pre-treatment test, all three groups decreased significantly the number of errors beginning with the fourth trial. The same occurred in the post-treatment test, except for the experimental group, whose animals committed less errors beginning with the second trial. After behavioral testing, the dorsomedial prefrontal cerebral cortex was dissected out, and the Golgi study of the third-layer pyramidal neurons revealed that the length of both the apical and the basilar dendrites was smaller than that of controls, and that the apical and oblique dendrites had a greater spine density. A major proportion of thin spines was also seen on the basilar and oblique dendrites, and more stubby spines were seen on the apical dendrite. Serotonin depletion in the prefrontal cerebral cortex resulted in cytoarchitectural alterations of the prefrontocortical pyramidal neurons, which may be underlying partially the greater efficiency observed in the short-term memory behavioral performance.

Behavioral effects of anti-muscarinic, anti-serotonergic, and anti-NMDA treatments: hippocampal and neocortical slow wave electrophysiology predict the effects on grooming in the rat

Previous research has shown that hippocampal and neocortical activation accompanies the postural changes occurring during self-grooming in rats but is absent or reduced during the stereotyped components of grooming, including head-washing and licking or biting. Since electrocortical activation is dependent on ascending cholinergic and serotonergic projections, we hypothesized that central muscarinic and serotonergic blockade would disrupt grooming by degrading cerebral control of changes in posture. Consistent with this, we find that systemic injections of scopolamine: (a) markedly reduce the occurrence of adaptive changes in posture during grooming; (b) reduce the probability of transitions from head-washing to body grooming; (c) reduce both the probability and duration of sequences of body grooming; and (d) do not affect the duration of head-washing or the probability of transitions from washing the snout to washing over the top of the head. Destruction of central serotonergic neurons with intracerebral injections of 5,7-dihydroxytryptamine increases the tendency of scopolamine to shorten the duration and increase the number of separate sequences of grooming. Systemic injections of a NMDA antagonist (CGS 19755) also impair grooming behavior. The data show that blockade of muscarinic and glutamatergic transmission impairs instinctive behavior as well as learned behavior and that the behavioral effects of muscarinic and serotonergic blockade are consistent with data obtained from the study of cortical slow wave electrophysiology.

Effects of cholinergic drugs on neocortical EEG and flash-visual evoked potentials in the mouse

The effects of single intraperitoneal injection of two cholinesterase inhibitors, physostigmine (PHY; 0.01, 0.025, 0.05, 0. 1, 0.2 mg/kg) and heptylphysostigmine (HEP; 0.5, 2, 6 mg/kg) on electroencephalographic (EEG) activity and flash visual evoked potentials (f-VEP) in the occipital cortex were compared in DBA/2 mice. EEG spectral analysis of awake periods showed that PHY at all doses and HEP at 2 mg/kg induced an increase of power in the 4.25- to 7-Hz frequency band. Furthermore, PHY at the higher doses and HEP at all doses induced a decrease of power in the 7.25- to 12-Hz frequency band, while the lower doses of PHY (0.01, 0.025 mg/kg) produced an increase of this band. EEG effects elicited by the two drugs were similar, when doses displaying analogous biochemical effects (acetylcholinesterase inhibition) were used (i.e. 0.01 and 0. 025 mg/kg of PHY versus 0.5 and 2 mg/kg of HEP). PHY and HEP induced similar changes in f-VEPs. Amplitudes of early and late components (P1N1, N1P2, P4N4 and particularly N1P3) were enhanced, while amplitudes of middle components were depressed after all doses. The peak latency measures were generally delayed, even though, after the lower doses, a trend to a latency reduction was evident in late components. This finding might indicate a possible effect on stimulus speed diffusion by 'low therapeutic' doses, analogous to the ones used in men. Our data show that both drugs are effective in modifying EEG and f-VEP parameters connected with brain cholinergic function, although in a very narrow dose range.

Brain structures involved in the behavioral stimulant effect of central serotonin release

Drugs such as p-chloroamphetamine or a combination of tranylcypromine and tryptophan release serotonin in the central nervous system and produce a behavioral serotonin syndrome. However, in the presence of methysergide or following destruction of descending spinal serotonergic projections by 5,7-dihydroxytryptamine, central serotonin release produces hyperlocomotion. This supports the hypothesis that release of serotonin in the brain promotes locomotion but that the expression of this effect can be blocked by concomitant intraspinal effects of serotonin release. Hyperlocomotion induced by serotonin release is attenuated or blocked by: (a) pretreatment with p-chlorophenylalanine; (b) acute surgical lesions of the basal diencephalon; (c) chronic lesions of the ventromedial midbrain tegmentum by local injection of 5,7-dihydroxytryptamine; and (d) acute surgical decortication. Medial decortication tends to be more effective then lateral decortication. Hyperlocomotion produced by methamphetamine is also attenuated or blocked by acute basal diencephalic lesions or decortication. It is suggested that ascending serotonergic and dopaminergic projections collaborate in the generation of spontaneous voluntary motor activity.

The effects of cholinergic drugs on rat neocortical high-voltage spindles in ketanserin-treated rats

To investigate the roles of the cholinergic system and 5-HT2 receptors in the modulation of thalamocortical oscillations, we studied the effects of systemic (s.c.) administration of anticholinesterases (physostigmine, tetrahydroaminoacridine) and muscarinic acetylcholine receptor agonists (pilocarpine, oxotremorine) on spontaneous thalamically generated rhythmic neocortical high-voltage spindles in adult rats pretreated with either saline or ketanserin, a 5-HT2 receptor antagonist. Ketanserin at 20.0 mg/kg increased the number of high-voltage spindles. In saline-treated rats, tetrahydroaminoacridine 3.0 and 9.0 mg/kg was able to decrease high-voltage spindles, whereas in ketanserin 20.0 mg/kg-treated rats only the highest dose of tetrahydroaminoacridine (9.0 mg/kg) decreased high-voltage spindles. Both doses of physostigmine, 0.12 and 0.36 mg/kg, decreased high-voltage spindles in both saline and ketanserin 20.0 mg/kg-treated rats. Lower doses of tetrahydroaminoacridine (1.0 mg/kg) and physostigmine (0.06 mg/kg) were ineffective in both saline- and ketanserin 20.0 mg/kg-treated rats. Pilocarpine 3.0 mg/kg and oxotremorine 0.1 and 0.9 mg/kg decreased high-voltage spindles in saline-treated rats. However, in rats treated with ketanserin 20.0 mg/kg, only the lower doses of pilocarpine (0.3 and 1.0 mg/kg) and oxotremorine (0.03 mg/kg) were able to decrease the high-voltage spindles. The results suggest that activation of the cholinergic system and activation of 5-HT2 receptors have additive effects in the suppression of thalamocortical oscillations and related neocortical high-voltage spindles in rats, thus maintaining effective information processing in thalamocortical networks.

The role of brain noradrenaline in cortical activation and behavior: a study of lesions of the locus coeruleus, medial thalamus and hippocampus-neocortex and of muscarinic blockade in the rat

Local injection of 6-hydroxydopamine in the locus coeruleus resulted in a 90% depletion of noradrenaline (NA) in the cerebral cortex as assessed by high-pressure liquid chromatography. This NA depletion had no effect on scopolamine-resistant hippocampal rhythmical slow activity and only an occasional effect on scopolamine-resistant neocortical low voltage fast activity. However, NE depletion resulted in a slight deficit in a behavioral swim-to-platform test and increased the deficit produced on the test by systemic treatment with scopolamine. Large surgical lesions of the medial thalamus or hippocampal formation plus posterior neocortex greatly increased the behavioral deficit produced by scopolamine. It is concluded that ascending noradrenergic projections play only a modest and possibly indirect role in the control of electrocortical activation and that a number of different brain lesions increase the behavioral impairment produced by central muscarinic blockade.

5-Hydroxytryptamine (5-HT) agonists: effects on neocortical slow wave activity after combined muscarinic and serotonergic blockade

In freely-moving rats treated with a combination of reserpine (10 mg/kg, i.p.) and scopolamine (5 mg/kg, i.p.), neocortical low voltage fast activity (LVFA) associated with continuous multiunit activity (MUA) was abolished and replaced by 2-6 Hz large irregular slow activity (LISA) above 1.5 mV associated with a burst-suppression pattern of MUA. Administration of the monoamine oxidase inhibitor pargyline (50-100 mg/kg, i.p.) completely suppressed 2-6 Hz LISA and restored normal-appearing LVFA and continuous MUA. The 5-hydroxytryptamine (5-HT) receptor agonists quipazine (0.5-20 mg/kg, i.p.), (+/-)-DOI (0.1-5 mg/kg, s.c.), and buspirone (0.1-10 mg/kg, i.p.), but not 8-hydroxy-2-(di-n-propylamine) tetraline (8-OH-DPAT, 0.05-0.8 mg/kg, s.c.) and RU 24969 (1-30 mg/kg, i.p.), produced a partial suppression of 2-6 Hz LISA and restored some lower voltage activity (< 1 mV) above 6 Hz associated with continuous MUA. However, as opposed to pargyline, no receptor agonist tested restored continuous, normal-appearing LVFA. Even though agonists at 5-HT receptors can produce some activation of neocortical slow wave activity after combined cholinergic and serotonergic blockade, this effect is not equivalent to that observed after restoration of endogenous 5-HT transmission.

Modulation of rat neocortical high-voltage spindle activity by 5-HT1/5-HT2 receptor subtype specific drugs

To investigate the role of serotonin (5-hydroxytryptamine; 5-HT) receptors in the modulation of rat thalamocortical oscillations, we studied the effects of 5-HT1/5-HT2 receptor subtype specific drugs on neocortical high-voltage spindle activity in adult male rats. A 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.03, 0.1, 0.3 and 1.0 mg/kg s.c.), had no effect on neocortical high-voltage spindle activity. Furthermore, a mixed 5-HT1/5-HT2 receptor antagonist, methysergide (1.0, 5.0 and 15.0 mg/kg i.p.), had no effect, whereas a non-specific mixed 5-HT1/5-HT2 receptor antagonist, methiothepin (0.2, 1.0 and 5.0 mg/kg i.p.), significantly increased neocortical high-voltage spindles. Of the 5-HT2 receptor antagonists, ritanserin (0.1, 1.0 and 5.0 mg/kg s.c.) had no effect, whereas ketanserin (1.0, 5.0 and 20.0 mg/kg s.c.) increased neocortical high-voltage spindles, but only at the highest dose used. A 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.5, 1.0 and 2.0 mg/kg s.c.), at the two highest doses significantly decreased neocortical high-voltage spindle activity, and this effect was blocked by the 5-HT2 receptor antagonists, ketanserin (1.0, 5.0 and 20.0 mg/kg s.c.) and ritanserin (1.0 and 5.0 mg/kg s.c.), as well as by methiothepin (0.2, 1.0 and 5.0 mg/kg i.p.) and methysergide (1.0, 5.0 and 15.0 mg/kg i.p.). Furthermore, unilateral intrathalamic infusions, but not intrahippocampal control infusions, of DOI (10 and 50 micrograms/1.0 microliters/rat) decreased neocortical high-voltage spindle activity and systemic administration of ketanserin (20.0 mg/kg s.c.) completely blocked this effect. The present results suggest that (1) the serotonergic system modulates rat thalamocortical oscillations as measured by neocortical high-voltage spindle activity, (2) activation of 5-HT2 receptors, possibly located in the thalamus, with a specific 5-HT2 receptor agonist, DOI, causes a reduction in rat neocortical high-voltage spindle activity.

Injections of muscimol into the median raphe nucleus produce hippocampal theta rhythm in the urethane anesthetized rat

It has previously been shown that serotonergic [5-hydroxytryptamine (5-HT)] neurons of the median raphe nucleus (MR) are critically involved in the control of the hippocampal electroencephalogram (EEG). Activation of MR 5-HT neurons desynchronizes the hippocampal EEG, whereas inhibition of MR 5-HT activity produces hippocampal theta rhythm. The MR contains an intrinsic population of gamma-aminobutyric acid (GABA) containing neurons that synapse on 5-HT cells of the MR. The present study examined the effects on the hippocampal EEG of injections of the GABAA agonist muscimol hydrobromide into the MR. Low doses of muscimol (0.5 microgram) produced hippocampal theta rhythm at a mean latency of 6.81 min and for a mean duration of 23.6 min. Higher doses (1.0 microgram and 3.0 micrograms, respectively) produced theta at mean latencies of 2.24 min and 3.2 min and for mean durations of 31.84 min and 24.88 min. Injections of muscimol into regions adjacent to the MR generated theta at significantly longer latencies or were without effect. The present results indicate that MR injections of muscimol produce theta by inhibiting the activity of MR 5-HT neurons. It is concluded that MR GABAergic systems, via their influence on MR 5-HT cells, serve an important role in the control of the hippocampal EEG.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Urethane reduces contraction to 5-hydroxytryptamine (5-HT) and enhances the action of the 5-HT antagonist ketanserin on the rat thoracic aortic ring

The general anesthetic urethane (ethyl carbamate) is widely used in electrophysiological in vivo experiments. However, its pharmacological effects are poorly understood. Here, the effects of urethane on in vitro contractile responses of the rat thoracic aortic ring preparation were investigated. Bath application of 5-HT produced a concentration-dependent contractile response (EC50 = 4.3 x 10(-6) M). Urethane (11.2 mM = 1 mg/ml) shifted the concentration-response curve (CRC) for 5-HT to the right (EC50 = 1.7 x 10(-5) M) and decreased the maximal contraction by 30.8%. The CRC for NA (EC50 = 7.2 X 10(-9)M) was also shifted to the right by urethane (EC50 = 1.4 X 10(-8)M), but the shift of the 5-HT-CRC was twice that of the NA-CRC (3.95 vs. 1.95). The CRC to KCl was shifted rightwards only slightly by urethane (ratio 1.27) and the maximal contraction to KCl was not affected. The CRC to replacement of CaCl2 (0.1-10 mM) to KCl-depolarized vessels in a Ca(2+)-free Krebs solution was unaffected by urethane. Ketanserin (10(-9)M) antagonized the contraction to 5-HT, and a combination of ketanserin and urethane was markedly more effective than either drug alone, decreasing the maximal contraction by 58%. Antagonism of NA contraction by prazosin (5 X 10(-8)M) was not increased by addition of urethane. The urethane dose used here approximates blood and brain concentrations required to produce anesthetic effects in mammals. It is possible that reductions in 5-HT transmission and, to a lesser extent, in NA transmission, but not blockade of Ca2+ or K+ channels, may contribute to the anesthetic effect of urethane. In addition, the action of the selective 5-HT2 antagonist ketanserin is clearly altered by urethane. These findings are important to consider when urethane is used for in vivo neurophysiological investigations, particularly when 5-HT mechanisms are involved.

Effect of injury on the bi-affinity alpha 1-adrenoreceptor binding in rat brain in vivo

Focal freezing lesions in rats cause a widespread decrease of cortical glucose utilization in the lesioned hemisphere, probably as a reflection of depressed cortical activity. The noradrenergic neurotransmitter system was implicated in these alterations when it was demonstrated that prazosin, a specific norepinephrine (NE) antagonist at alpha 1-adrenergic receptors, prevented their development. In normal rat brain, specific binding of [125I]HEAT [(+/-)2-(3-[125I]iodo-4-hydroxyphenyl)-ethyl-aminomethyl-tetralone], another selective alpha 1-adrenoreceptor ligand, was demonstrated in vivo at sites consistent with the alpha 1A- and alpha 1B-adrenoreceptor subtypes. In the present study, the effect of a freezing lesion on specific binding of [125I]HEAT in rat brain in vivo was determined three days after traumatization when cortical glucose use suggested the greatest degree of functional depression. The steady-state volumes of distribution of [125I]HEAT three days after injury were significantly increased in all the cortical areas of the lesioned hemisphere, but not in the subcortical structures. Injury did not modify the binding affinities for HEAT. However, a statistically significant increase in the number of low-affinity binding sites for this ligand was demonstrated in all cortical areas of the lesioned hemisphere, but not in subcortical structures. The traumatization did not modify Bmax estimates for the high-affinity binding of HEAT. The results support the hypothesis that changes in the noradrenergic system are of functional importance in brain injury and that at least some effects of injury are mediated by alpha 1B-adrenergic receptors.

Some general anesthetics reduce serotonergic neocortical activation and enhance the action of serotonergic antagonists

In urethane-anesthetized rats, neocortical LVFA induced by 100 Hz electrical stimulation of the median raphe area or by tail pinching was completely eliminated by a combination of scopolamine (5 mg/kg, IP) and p-chlorophenylalanine (500 mg/kg/day x 3, IP), providing evidence that LVFA is dependent on cholinergic-muscarinic and serotonergic inputs to the neocortex in urethane-anesthetized as well as in freely moving rats. The serotonergic receptor antagonists ketanserin and methiothepin (1-10 mg/kg, IP) also produced a dose-dependent blockade of LVFA in urethane-anesthetized rats, and eliminated virtually all LVFA when combined with scopolamine. A combination of diethyl ether anesthesia and scopolamine completely eliminated all neocortical LVFA without additional antiserotonergic treatment, and a combination of chloral hydrate anesthesia and scopolamine similarly blocked all LVFA in about 50% of the rats tested. In the remaining chloral hydrate-anesthetized rats, the residual LVFA could be eliminated by the serotonergic antagonist ritanserin (10 mg/kg, IP). As shown previously, in nonanesthetized rats treated with scopolamine, LVFA can be maintained by a serotonergic input to the neocortex. The present data suggest that some general anesthetics reduce or completely abolish this serotonergic LVFA. Further, the serotonergic antagonists used here exert much stronger antiserotonergic effects in rats anesthetized with urethane or chloral hydrate than in freely moving rats. Therefore, studies of serotonergic transmission or antagonist action, especially in the neocortex, in anesthetized rats may not be applicable to the waking state.

Effects of several 5-HT1A agonists on hippocampal rhythmical slow activity in unanesthetized rats

We examined the effect of 5-hydroxytryptamine (5-HT)1A agonists on walking related, atropine-resistant, rhythmical slow activity (wr-RSA) of the hippocampus in rats. Selective 5-HT1A agonists, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), flesinoxan, buspirone and ipsapirone significantly decreased the power value of 7-9 Hz band activity and the median frequency of wr-RSA. The order of potency was 8-OH-DPAT > flesinoxan = buspirone in power reduction. The 5-HT1A antagonists, (-)pindolol, (-)propranolol and spiperone, inhibited the effect of 8-OH-DPAT on wr-RSA. Pretreatment with parachlorophenylalanine did not abolish the effect of 8-OH-DPAT. These results indicate that 5-HT1A agonists reduce both power and median frequency values of wr-RSA through activation of post-synaptic 5-HT1A receptors in the forebrain in unanesthetized rats, in vivo.

Pharmacological suppression of the median raphe nucleus with serotonin1A agonists, 8-OH-DPAT and buspirone, produces hippocampal theta rhythm in the rat

The effects on the hippocampal electroencephalogram of microinjections of procaine hydrochloride and the serotonin1A agonists, 8-OH-DPAT and buspirone, into the median raphe nucleus were examined in the urethane anesthetized rat. Injections of procaine, 8-OH-DPAT or buspirone into the median raphe nucleus produced a change in the hippocampal electroencephalogram from a spontaneous desynchronized pattern to a synchronized pattern (theta rhythm) within short latencies and for long durations post-injection. Procaine was shown to elicit theta at a mean latency of 52 s and for a mean duration of 21.75 min; buspirone at a mean latency of 2 min and for a mean duration of 34.5 min. A dose dependent relationship was observed between 8-OH-DPAT injections and latencies but not durations. Small doses (0.5 micrograms) of 8-OH-DPAT produced theta at a mean latency of 1.33 min and large doses (3.0 micrograms) at a mean latency of 1.17 min. 8-OH-DPAT injections generated theta for a mean duration of 62 min. Injections of each of these substances into structures dorsal, lateral or rostrocaudal to the median raphe (dorsal raphe nucleus, pontine reticular formation, caudal linear nucleus or raphe pontis, respectively) failed to generate theta or in a few cases produced theta at very long latencies (> 24 min). Saline injections in the median raphe nucleus or control structures were without effect. The demonstration that agents injected into the median raphe nucleus that inhibit its activity (procaine and serotonin1A agonists) produce theta indicate that serotonin-containing median raphe neurons normally suppress theta or are involved in the control of hippocampal desynchronization. The present findings are consistent with previous work showing that median raphe nucleus stimulation desynchronizes the hippocampal electroencephalogram and that median raphe nucleus lesions produce constant theta, but are at odds with the proposal that serotonergic mechanisms may play a role in the generation of the theta rhythm.

Hippocampal theta rhythm in behaving rats following ibotenic acid lesion of the septum

The effects of ibotenic acid lesion of the septum were studied in rats implanted with chronically indwelling electrodes and septal cannula. Each rat served as its own control and the properties of the hippocampal theta rhythm were studied before and after ibotenic acid and control saline infusion into the medial septal area. Ibotenic acid preferentially killed neurons in the lateral septum, and significantly attenuated the hippocampal theta rhythm about 50% bilaterally, at both surface and deep electrodes. The coherence and the phase of the theta rhythm at the CA1 apical dendrites, with respect to a superficial electrode, also declined significantly after ibotenic acid lesion. Pilocarpine (25 mg/kg i.p.) induced a theta rhythm of 7-9 Hz during immobility in the lesioned rats that was significantly higher in frequency than that induced in intact rats (4-6 Hz). In lesioned rats, the theta rhythm during tail pinch under urethane anesthesia was largely abolished, and the theta during walking was attenuated by atropine sulfate (50 mg/kg i.p.). Phencyclidine (10 mg/kg i.p.) or parachlorophenylalanine (PCPA) alone, which was inferred to abolish an atropine-resistant theta input, did not affect the power of the walking theta rhythm in either the lesioned or the normal rat. It was concluded that the theta in the behaving rats after ibotenic acid lesion in the septum has a strong atropine-sensitive component, and that it is not predominantly atropine-resistant, as suggested previously. The lack of PCPA effect on the theta phase in intact and lesioned rats also suggested a different view of the atropine-resistant theta in hippocampal region CA1. One possible mechanism of the atropine-resistant theta at the distal dendrites of pyramidal cells may result from rhythmic inhibition by stratum lacunosum-moleculare interneurons which may be activated by either serotonergic or cholinergic inputs.

Transcallosal evoked potentials: behavior-dependent modulation by muscarinic and serotonergic receptors

In chronically prepared rats, unilateral single pulse stimulation (35-80 microA) of the deep neocortex produced an evoked potential in the contralateral hemisphere. The evoked potential consisted of an initial negative component associated with multiunit discharge, and a subsequent positive component associated with multiunit suppression. Duration and amplitude of both evoked potential components were suppressed during movement relative to immobility. Scopolamine enhanced both evoked potential components, but the effect on the late component was apparent only during movement and not immobility. Ketanserin prolonged the duration of the late potential component during immobility but not movement. Methiothepin enhanced the duration and amplitude of both components, but these effects were largely dependent on the concurrent background neocortical slow wave activity. Urethane anesthesia enhanced the early and abolished the late potential components. Tail pinching reversed this effect of urethane. Muscarinic and serotonergic receptors may modulate both the excitatory and inhibitory responses of neocortical neurons to inputs from the contralateral neocortex.

Serotonergic influence on olfactory learning in the neonate rat

The role of the serotonergic innervation of the olfactory bulb was examined in neonate rat pups (Sprague-Dawley) by utilizing an olfactory learning paradigm (Sullivan, McGaugh, & Leon, 1991; Sullivan, Wilson, & Leon, 1989). One hundred fifty nanoliters of the neurotoxin 5,7-dihydroxytryptamine (5,7-dHT, 10 micrograms/microliters) was injected into each anterior olfactory nucleus of Postnatal Day 2 (PND 2) pups in order to selectively deplete the serotonergic (5-HT) innervation of the olfactory bulb. On PND 8, control, sham-operated, or 5-HT-depleted pups were presented with stroke (unconditioned stimulus) paired with peppermint odor (conditioned stimulus) or were presented with peppermint alone. Other pups remained naive, i.e., they were not presented with either stroke or odor. The following day, the pups were placed on a mesh screen above two boxes containing either fresh bedding or bedding with peppermint odor for five 1-min trials. Control and sham-operated pups that were previously trained by the odor/stroke paradigm spent significantly more time over the peppermint odor than similarly trained 5-HT-depleted pups, pups trained using the odor only paradigm, or naive pups. Immunocytochemistry verified that the 5,7-dHT injections specifically depleted the 5-HT innervation of the olfactory bulb and left the 5-HT innervation of more caudal levels of the neuraxis (e.g., piriform cortex) intact. The locomotor behavior of the pups was not affected by the 5-HT depletions. This study is the initial investigation to suggest that 5-HT innervation to the olfactory bulb is important in the acquisition or expression of olfactory-based learned behavior in the neonate rat.

Pharmacological and immunohistochemical evidence for serotonergic modulation of cholinergic nucleus basalis neurons

Identified electrophysiologically by low threshold bursts and transient outward rectification, cholinergic nucleus basalis neurons were recorded and labelled intracellularly in guinea-pig basal forebrain slices. By means of a triple labelling immunofluorescent technique, serotonin-immunoreactive fibres were visualized in close proximity to the soma and dendrites of the biocytin-labelled, choline acetyl transferase (ChAT)-immunoreactive cells. By bath application, 5-hydroxytryptamine (5-HT) produced a direct hyperpolarization of the identified cells which was mimicked by 5-HT1A receptor agonists, suggesting that it may inhibit the tonic firing but also modulate the low threshold bursting of the cholinergic nucleus basalis neurons.

Calretinin is present in non-pyramidal cells of the rat hippocampus--III. Their inputs from the median raphe and medial septal nuclei

The subcortical innervation of a recently described subpopulation of non-pyramidal neurons, containing the calcium binding protein, calretinin, was investigated in the rat hippocampus using the anterograde tracer Phaseolus vulgaris-leucoagglutinin and double immunocytochemistry for calretinin and serotonin at the light and electron microscopic levels. Our results show that the GABAergic component of the septohippocampal pathway and the serotonergic raphe afferents establish multiple synaptic contacts with the calretinin-immunoreactive interneurons. The majority of the targets of both pathways were spine-free calretinin neurons known to innervate the dendritic region of the principal cells, but the GABAergic septal pathway was found to terminate also on the spiny neurons of stratum lucidum of the CA3 region and in the dentate hilus. The present results demonstrate that the serotonergic raphe-hippocampal and the GABAergic septohippocampal pathways are able to modulate dendritic inhibition of principal cells via calretinin-containing GABAergic interneurons.

Alpha 2 modulation of type 1 and type 2 hippocampal theta in the freely moving rat

In order to determine the brain sites involved in the noradrenergic modulation of hippocampal theta, the alpha 2 agonist detomidine was infused into one of three brain sites in the freely moving animal: hippocampus, median raphe nucleus (MRN), or locus coeruleus (LC). Bilateral hippocampal recording electrodes were implanted in 45 animals. Infusions of detomidine into the hippocampus produced an attenuation of type 1 (movement) theta. Detomidine infused into the MRN produced a release of type 2 (immobility) theta, although having no effect on type 1 theta. No significant changes were noted in the hippocampal EEG activity following infusions of detomidine into the LC. Infusions of Xylocaine into each of the above brain sites were ineffective in eliciting changes in hippocampal EEG activity. No behavioral effects were noted following infusions of detomidine or Xylocaine. The present results suggest that the attenuation of type 1 theta occurs at the hippocampus, while the release of type 2 theta occurs at the MRN.

Behavior-related cortical activity and swim-to-platform performance in the aged rat

Aged rats (26 months) usually retained normal acetylcholine-dependent and serotonin-dependent forms of neocortical low-voltage fast activity and serotonin-dependent hippocampal rhythmical slow-wave activity. In a simple swim-to-platform test, aged rats (23 and 26 months) performed normally in acquisition and in retention over a 7-day period. The results are discussed in relation to the common assumption that aged rats provide a valid model of human senile dementia.

Serotonin-dependent cerebral activation: effects of methiothepin and other serotonergic antagonists

In scopolamine-treated (5.0 mg/kg, s.c.) rats hippocampal rhythmical slow activity (RSA) and neocortical low voltage fast activity (LVFA) occur only in close correlation with head movements, spontaneous changes in posture, or locomotion (Type I behavior). Previous work indicates that such scopolamine-resistant RSA and LVFA are dependent on ascending serotonergic projections. A test of 9 serotonergic antagonists (methiothepin; ritanserin; ketanserin; pizotifen; mianserin; pirenperone; ICS-205-930; metoclopramide; methysergide) showed that methiothepin produces a partial reduction in RSA and LVFA in scopolamine-treated rats, while the other antagonists are completely inactive over a wide range of doses. It may be that serotonergic cerebral activation depends on both 5-HT1 and 5-HT2 receptors.

The extrinsic modulation of hippocampal theta depends on the coactivation of cholinergic and GABA-ergic medial septal inputs

The long trains of theta field activity recorded from the hippocampal formation of urethane-anesthetized rats are thought to be primarily dependent on cholinergic afferents originating in the medial septum/vertical limb of the diagonal band of Broca (MS/vDBB). Recent anatomical studies have revealed the existence of a septal GABA-ergic input to the hippocampal formation which synapses mainly on intrinsic GABA-ergic interneurons. The present work investigated the possibility that some form of interaction between cholinergic and GABA-ergic MS/vDBB inputs might be required for the generation of hippocampal theta field and cellular activities in urethane-anesthetized rats. Reversible inactivation of the MS/vDBB completely abolished theta field and theta-on cell activities, but "released" theta-off cells. The theta field and theta-on cell activities induced by direct intrahippocampal microinfusions of carbachol were also abolished by MS/vDBB inactivation. We speculated that septal suppression was producing two effects: 1) removing excitatory, cholinergic input; and 2) removing inhibitory control of hippocampal GABA-ergic interneurons, thereby increasing the overall level of hippocampal inhibition. Sequential administration of both carbachol and the GABA-A antagonist, bicuculline, resulted in theta-like oscillations similar to those seen in hippocampal slices bath perfused with carbachol alone. Thus, following MS/vDBB inactivation hippocampal GABA-ergic systems are overactive; this enhances intrinsic inhibition and blocks carbachol theta. By reducing the overall level of inhibition in the hippocampus with bicuculline, it is possible to reinstate its oscillatory properties. Conversely, increasing the level of inhibition in the hippocampus (with muscimol) results in the abolishment of theta field activity and the discharges of both theta-on and theta-off cells. Based on these findings we are proposing that cholinergic and GABA-ergic systems originating in the MS/vDBB act synergistically to modulate hippocampal theta. Cholinergic projections provide the afferent excitatory drive for hippocampal theta-on cells and septal GABA-ergic projections act to reduce the overall level of inhibition by inhibiting hippocampal GABA-ergic interneurons (hippocampal theta-off cells). Both activities must be present for the generation of hippocampal theta field and cellular activities. The balance between the cholinergic and GABA-ergic systems may determine whether hippocampal synchrony (theta) or asynchrony (LIA) occurs.

DSP-4, a noradrenergic neurotoxin, produces more severe biochemical and functional deficits in aged than young rats

The present study examines the effects of noradrenergic lesions (either DSP-4 i.p. or 6-hydroxydopamine (6-OHDA) into the dorsal noradrenergic bundle on biochemical (noradrenaline (NA), dopamine (DA), serotonin (5-HT) and choline acetyltransferase (ChAT) activity) and cortical EEG (quantitative EEG (qEEG) and high-voltage spindle (HVS)) activity in young and aged rats. Near complete 6-OHDA NA lesions, but not partial DSP-4 NA lesions, increased HVS activity in young rats. DSP-4 and 6-OHDA lesions produced no significant changes in the 5-HT or DA levels or in the ChAT activity in young rats. In some of the aged rats, DSP-4 produced similar biochemical and HVS effects, as it induced in young rats. In the remainder of the aged rats, NA levels were greatly and 5-HT levels slightly decreased. DA levels and ChAT activity were unaltered in either set of aged rats. HVS activity was increased only in that group of aged rats with the greatly lowered NA content. These results suggest that: (1) some of the aged rats are more sensitive to DSP-4 treatment than young adult rats; and (2) NA depletions have to be complete to produce an increase in HVS activity in young and aged rats.

Effects of raphe stimulation on hippocampal and neocortical activity and behaviour

In chronically prepared rats, electrical stimulation (100 Hz, 0.1 ms pulses) of the dorsal raphe nucleus, some sites in the median raphe nucleus, and adjoining regions of the midbrain produced locomotion accompanied by hippocampal rhythmical slow activity (RSA) and neocortical low voltage fast activity (LVFA). Both the behaviour and the cerebral waveforms persisted after injection of scopolamine HBr (5 mg/kg, s.c.). Median raphe stimulation usually produced behavioural freezing or an unnatural forced movement accompanied by RSA and LVFA. The behavioural response and the LVFA were not affected by scopolamine but scopolamine eliminated the RSA, replacing it with a low amplitude irregular (suppressed) pattern. p-Chlorophenylalanine (PCPA, 500 mg/kg/day x 3, i.p.) reduced the RSA and LVFA normally present during walking after scopolamine but did not reduce the hippocampal suppression produced by median raphe stimulation in scopolamine-treated rats. Hippocampal suppression and LVFA in response to median raphe stimulation were also present in urethane (1.0-1.5 g/kg, i.p.) anesthetized rats, whether pretreated with PCPA or not. Stimulation at most other midbrain sites produced RSA and LVFA in the urethane condition. RSA was abolished in the urethane plus scopolamine condition. The data support the view that scopolamine-resistant RSA and LVFA are dependent on serotonergic projections. The hippocampal suppression produced by median raphe stimulation may be dependent on non-serotonergic neurotransmission.

Brain injury: new insights into neurotransmitter and receptor mechanisms

The studies reviewed here represent a continuing search for mechanisms which play a role in neurological disturbances resulting from brain injury. Focal cortical freezing lesions in rats were shown to cause a widespread decrease in local cerebral glucose utilization (LCGU) in cortical areas of the lesioned hemisphere and this was interpreted as reflecting a depression of cortical activity. Such an interpretation was supported by the finding that in lesioned brain reduction of cerebral metabolism by pentobarbital and isoflurane was limited by the metabolic depression that has already occurred as a result of injury and by the demonstration that the energy status and substrate (glucose) supply in the cortical areas in the injured brain have not been compromised at the time when LCGU was decreased. Both the serotonergic and the noradrenergic neurotransmitter systems were implicated in functional alterations associated with injury. Cortical serotonin (5-HT) metabolism was increased throughout the lesioned hemisphere and complete inhibition of 5-HT synthesis with p-chlorophenylalanine ameliorated the decrease in cortical LCGU, interpreted as reflecting cortical functional depression. Cortical norepinephrine metabolism was bilaterally increased in focally injured brain, while prazosin, a selective alpha 1-noradrenergic receptor blocker, normalized cortical LCGU in the lesioned hemisphere. Low-affinity in vivo binding of [125I]HEAT, another selective alpha 1-receptor ligand, was specifically increased in cortical areas of the lesioned hemisphere at the time of the greatest depression in LCGU, suggesting that alpha 1-adrenoreceptors may be of functional importance in injured brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of serotonergic denervation on the density and plasticity of brain muscarinic receptors in the rat

This study investigated whether serotonergic lesion may affect density, sensitivity, and plasticity of muscarinic receptors in hippocampus and cerebral cortex. Intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) in rats produced a 90% reduction in cortical and hippocampal 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents. In these brain areas, the 5,7-DHT lesion did not affect the overall density of muscarinic receptors or those of M1 and non-M1 muscarinic receptor subtypes as assayed using [3H]N-methylscopolamine ([3H]NMS), [3H]pirenzepine, and [3H]NMS in the presence of pirenzepine, respectively. In addition, the binding of the muscarinic agonist [3H]oxotremorine-M (OXO-M), taken as an indirect index of coupling efficiency of non-M1 receptors with G-proteins, did not change significantly in cortex and hippocampus of 5,7-DHT-lesioned rats. Similarly, carbachol-induced accumulation of [3H]inositol phosphates (InPs) in hippocampal miniprisms showed no significant differences between tissues from 5,7-DHT-lesioned and sham-operated rats. In sham-operated rats, an intraperitoneal (i.p.) injection of scopolamine (10 mg/kg once daily) during 21 days caused an increased density of [3H]NMS binding sites in cortex (+20%) and hippocampus (+26%). This up-regulation was restricted to non-M1 receptors subtypes. In 5,7-DHT-lesioned rats, chronic scopolamine failed to modify significantly the density of cortical or hippocampal M1 or non-M1 receptors. These results suggest 1) that 5-HT denervation did not affect the density and sensitivity of muscarinic receptors and 2) that the ability of cortical and hippocampal non-M1 receptors to up-regulate following repeated injection of scopolamine requires the integrity of 5-HT neurons terminating in these brain structures.

Phenoxybenzamine reduces mortality associated with intracerebral injections of excitatory neurotoxins

Intracerebral injections of kainic acid produce gross hematuria, renal cortical necrosis, and an associated high mortality. Hematuria is virtually eliminated and mortality significantly reduced by pretreatment with phenoxybenzamine.

Septal GABAergic neurons innervate inhibitory interneurons in the hippocampus of the macaque monkey

The septohippocampal projection was visualized in three Macaca mulatta monkeys by anterograde transport of Phaseolus vulgaris leucoagglutinin. Following injections of the lectin into the medial septal nucleus, P. vulgaris leucoagglutinin-labelled fibres were found in the hippocampal complex, mainly in stratum oriens of the CA1 subfield, throughout the CA3 subfield, and in the hilus and stratum moleculare of the dentate gyrus. The majority of labelled axons were varicose, and formed multiple contacts with cell bodies and dendrites of calbindin D28k- and parvalbumin-immunoreactive non-pyramidal cells. GABA immunoreactivity of P. vulgaris leucoagglutinin-labelled axons and their postsynaptic targets was investigated by sectioning varicose axon segments for correlated light and electron microscopy, and processing alternate ultrathin sections for postembedding immunogold staining for GABA. All P. vulgaris leucoagglutinin-labelled boutons examined were GABA-immunoreactive and the majority of them formed symmetrical synapses with GABA-immunoreactive cell bodies and dendrites. The results demonstrate that a GABAergic septohippocampal pathway exists in the monkey, and, similar to the rat, terminates on different types of GABAergic neurons, including the parvalbumin- and calbindin D28k-containing non-pyramidal cells.

The effects of alpha 2 agonists and antagonists on hippocampal theta activity in the freely moving rat

The alpha 2 agonists detomidine and xylazine were given to freely moving rats implanted with bilateral hippocampal recording electrodes. These alpha 2 agonists produced virtually continuous type 2 (immobility) theta. Type 1 (movement) theta was attenuated with these drugs. Subsequent administration of atropine sulfate eliminated all theta in the hippocampus. Other animals were given the alpha 2 antagonist tolazoline. This drug had no effect on type 1 theta but animals showed an increase in the production of type 2 theta during sensory stimulation. These type 2 theta responses habituated both within and between trials. Tolazoline was ineffective as a blocking agent for the effects of detomidine. Preadministration of atropine sulfate blocked the effects of detomidine on type 2 theta but not on the elimination of type 1 theta. Results are discussed in terms of alpha 2 modulation of the cholinergic and serotonergic systems.

Animal models of human amnesia and dementia: hippocampal and amygdala ablation compared with serotonergic and cholinergic blockade in the rat

The behavioral effects of combined bilateral hippocampal and amygdala ablation (previously proposed as a model of human global amnesia) were compared to those seen with central blockade of the ascending cholinergic and serotonergic projections (a possible model of human global dementia) in male hooded rats. Rats were prepared with: (a) bilateral surgical lesions of the hippocampus and amygdala; (b) pharmacological blockade of central cholinergic and serotonergic function by systemic injections of scopolamine and p-chlorophenylalanine; and (c) neurotoxic lesions of the rostrally projecting serotonergic nuclei in the brainstem using intracerebral injections of 5,7-dihydroxytryptamine, later combined with scopolamine. The behavioral tests used were: an open field test, a swim-to-platform test, and a Lashley III maze. In all 3 tests, rats with either the neurotoxin lesions plus scopolamine or p-chlorophenylalanine plus scopolamine treatment showed greater impairments in comparison with controls than did the combined lesion group. These results indicate that simultaneous blockade of central serotonergic and cholinergic transmission has a greater effect on some aspects of the organization of behavior than large surgical lesions of the hippocampus and amygdala.