Ascending reticular activating system in the rat: a 2-deoxyglucose study

From emotional arousal to executive action. Role of the prefrontal cortex

Emotional arousal is caused by the activity of two parallel ascending systems targeting mostly the subcortical limbic regions and the prefrontal cortex. The aversive, negative arousal system is initiated by the activity of the mesolimbic cholinergic system and the hedonic, appetitive, arousal is initiated by the activity of the mesolimbic dopaminergic system. Both ascending projections have a diffused nature and arise from the rostral, tegmental part of the brain reticular activating system. The mesolimbic cholinergic system originates in the laterodorsal tegmental nucleus and the mesolimbic dopaminergic system in the ventral tegmental area. Cholinergic and dopaminergic arousal systems have converging input to the medial prefrontal cortex. The arousal system can modulate cortical EEG with alpha rhythms, which enhance synaptic strength as shown by an increase in long-term potentiation (LTP), whereas delta frequencies are associated with decreased arousal and a decrease in synaptic strength as shown by an increase in long-term depotentiation (LTD). It is postulated that the medial prefrontal cortex is an adaptable node with decision making capability and may control the switch between positive and negative affect and is responsible for modifying or changing emotional state and its expression.

Region- and Layer-Specific Activation of the Higher Order Auditory Cortex Te2 after Remote Retrieval of Fear or Appetitive Memories

The auditory cortex is involved in encoding sounds which have acquired an emotional-motivational charge. However, the neural circuitry engaged by emotional memory processes in the auditory cortex is poorly understood. In this study, we investigated the layers and regions that are recruited in the higher order auditory cortex Te2 by a tone previously paired to either fear or appetitive stimuli in rats. By tracking the protein coded by the immediate early gene zif268, we found that fear memory retrieval engages layers II-III in most regions of Te2. These results were neither due to an enhanced fear state nor to fear-evoked motor responses, as they were absent in animals retrieving an olfactory fear memory. These layers were also activated by appetitive auditory memory retrieval. Strikingly, layer IV was recruited by fear, but not appetitive memories, whereas layer V activity was related to the behavioral responses displayed to the CS. In addition to revealing the layers and regions that are recruited in the Te2 by either fear or appetitive remote memories, our study also shows that the neural circuitry within the Te2 that processes and stores emotional memories varies on the basis of the affective motivational charge of tones.

Brain Systems Underlying Susceptibility to Helplessness and Depression

There has been a relative lack of research into the neurobiological predispositions that confer vulnerability to depression. This article reviews functional brain mappings from a genetic animal model, the congenitally helpless rat, which is predisposed to develop learned helplessness. Neurometabolic findings from this model are integrated with the neuroscientific literature from other animal models of depression as well as depressed humans. Changes in four major brain systems are suggested to underlie susceptibility to helplessness and possibly depression: (a) an unbalanced prefrontal-cingulate cortical system, (b) a dissociated hypothalamic-pituitary-adrenal axis, (c) a dissociated septal-hippocampal system, and (d) a hypoactive brain reward system, as exemplified by a hypermetabolic habenula-interpeduncular nucleus pathway and a hypometabolic ventral tegmental area-striatum pathway. Functional interconnections and causal relationships among these systems are considered and further experiments are suggested, with theoretical attention to how an abnormality in any one system could affect the others.

The higher order auditory cortex is involved in the assignment of affective value to sensory stimuli

The sensory cortex participates in emotional memory but its role is poorly understood. Here we show that inactivation of the higher order auditory cortex Te2 in rats during early memory consolidation impairs remote first- and second-order fear memories but not the association between two neutral cues. Furthermore, Te2 inactivation prevents changes in the valence of such information. Following the presentation of two auditory cues previously paired with either pleasant or painful stimuli, a large percentage of cells responds to both experiences but also a small fraction of neurons responds exclusively to one of them. The latter type of neurons signals the valence rather than the salience or the motor responses associated with the stimuli, and reflects selective associative processes. Pharmacogenetic silencing of memory-activated neurons causes amnesia. Thus, Te2 represents a crucial node for the assignment of the affective value to sensory stimuli and for the storage of such information.

The auditory cortex Te2 represents a key node for the assignment of the affective value to sensory stimuli in rodents. Using pharmacogenetic manipulations, this study shows that in Te2 there are neurons which respond to the emotional valence of sounds and their inactivation impairs emotional memories retrieval.

Auditory cortex involvement in emotional learning and memory

Emotional memories represent the core of human and animal life and drive future choices and behaviors. Early research involving brain lesion studies in animals lead to the idea that the auditory cortex participates in emotional learning by processing the sensory features of auditory stimuli paired with emotional consequences and by transmitting this information to the amygdala. Nevertheless, electrophysiological and imaging studies revealed that, following emotional experiences, the auditory cortex undergoes learning-induced changes that are highly specific, associative and long lasting. These studies suggested that the role played by the auditory cortex goes beyond stimulus elaboration and transmission. Here, we discuss three major perspectives created by these data. In particular, we analyze the possible roles of the auditory cortex in emotional learning, we examine the recruitment of the auditory cortex during early and late memory trace encoding, and finally we consider the functional interplay between the auditory cortex and subcortical nuclei, such as the amygdala, that process affective information. We conclude that, starting from the early phase of memory encoding, the auditory cortex has a more prominent role in emotional learning, through its connections with subcortical nuclei, than is typically acknowledged.

Relationships between the superior colliculus and hippocampus: Neural and behavioral considerations

Theories of superior collicular and hippocampal function have remarkable similarities. Both structures have been repeatedly implicated in spatial and attentional behaviour and in inhibitory control of locomotion. Moreover, they share certain electrophysiological properties in their single unit responses and in the synchronous appearance and disappearance of slow wave activity. Both are phylogenetically old and the colliculus projects strongly to brainstem nuclei instrumental in the generation of theta rhythm in the hippocampal EEC

On the other hand, close inspection of behavioural and electrophysiological data reveals disparities. In particular, hippocampal processing mainly concerns stimulus ambiguity, contextual significance, and spatial relations or other subtle, higher order characteristics. This requires the use of largely preprocessed sensory information and mediation of poststimulus investigation. Although collicular activity must also be integrated with that of “higher” centres (probably to a varying degree, depending on the nature of stimuli being processed and the task requirements), its primary role in attention is more “peripheral” and specific in controlling orienting/localisation via eye and body movements toward egocentrically labelled spatial positions. In addition, the colliculus may exert a nonspecific influence in alerting higher centres to the imminence of information potentially worthy of focal attention. Nevertheless, it is noteworthy that collicular and hippocampal lesions produce deficits on similar tasks, although the type of deficit is usually different (often opposite) in each case. Functional overlap between hippocampus and colliculus (i.e., strategically synchronised or mutually interdependent activity) is virtually certain vis-à-vis stimulus sampling, for example in the acquisition of information via vibrissal movements and visual scanning. In addition, insofar as stimulus significance is a factor in collicular orienting mechanisms, the hippocampus — cingulate – cortex — colliculus pathway may play a significant role, modulating collicular responsiveness and thus ensuring an attentional strategy appropriate to current requirements (stimulus familiarity, stage of learning). A tentative “reciprocal loop” model is proposed which bridges physiological and behavioural levels of analysis and which would account for the observed degree and nature of functional overlap between the superior colliculus and hippocampus.

Ultrasonic calls of rats as indicator variables of negative or positive states: acetylcholine-dopamine interaction and acoustic coding

Adult rats produce two distinct types of ultrasonic vocalizations referred to as 22- and 50-kHz calls, respectively. Emission of the respective calls represents signaling a negative or positive state of the rat organism. The signaling has an adaptive value for survival and/or well-being of rats and their social groups. Literature is reviewed from studies on cats and rats, which indicates that the positive or negative states constitute a complex and integrated set of somatic, autonomic, endocrine, affective, and cognitive correlates. The basic states and their correlates are initiated, integrated, and maintained by activity of the subsets of the ascending cholinergic and dopaminergic systems originating from the reticular brainstem core. The cholinergic and dopaminergic systems interact mutually to form a dynamic balance, which is involved in a decision-making process of initiating and maintaining one or the other of these states. Activation of the relevant portion of the ascending cholinergic system invariably induces the negative state and releases 22-kHz calls while activation of the ascending dopaminergic system induces the positive state with 50-kHz calls. The 22- and 50-kHz calls have distinct and mostly non-overlapping acoustic parameters, which ensure unambiguous recognition of the calls and thus, the state of the emitter. The animal may only signal one of the states at any given time and emission of 22- or 50-kHz calls is mutually exclusive. It is postulated, therefore, that these two main types of ultrasonic calls are reliable indicator variables of two opposing states of the adult rat organism: negative or positive.

Opposite metabolic changes in the habenula and ventral tegmental area of a genetic model of helpless behavior

Congenitally helpless rats have been selectively bred to display an immediate helpless response to stress in order to model hereditary brain differences that contribute to depression vulnerability. Differences in regional brain metabolism between congenitally helpless and non-helpless rats were investigated using quantitative cytochrome oxidase histochemistry. The results indicated that congenitally helpless rats had 64-71% elevated metabolism in the habenula and a 25% elevation in the related interpeduncular nucleus. In contrast, helpless rats had 28% reduced metabolism in the ventral tegmental area (VTA) and 14-16% reductions in the basal ganglia and basolateral and central amygdala. The opposite metabolic changes in the habenula and ventral tegmental area may be especially important for determining the congenitally helpless rat's global pattern of brain activity, which resembles the metabolic activity pattern produced by dopamine antagonism.

Mapping Pavlovian conditioning effects on the brain: blocking, contiguity, and excitatory effects

Pavlovian conditioning effects on the brain were investigated by mapping rat brain activity with fluorodeoxyglucose (FDG) autoradiography. The goal was to map the effects of the same tone after blocking or eliciting a conditioned emotional response (CER). In the tone-blocked group, previous learning about a light blocked a CER to the tone. In the tone-excitor group, the same pairings of tone with shock US resulted in a CER to the tone in the absence of previous learning about the light. A third group showed no CER after pseudorandom presentations of these stimuli. Brain systems involved in the various associative effects of Pavlovian conditioning were identified, and their functional significance was interpreted in light of previous FDG studies. Three conditioning effects were mapped: 1) blocking effects: FDG uptake was lower in medial prefrontal cortex and higher in spinal trigeminal and cuneate nuclei in the tone-blocked group relative to the tone-excitor group. 2) Contiguity effects: relative to pseudorandom controls, similar FDG uptake increases in the tone-blocked and -excitor groups were found in auditory regions (inferior colliculus and cortex), hippocampus (CA1), cerebellum, caudate putamen, and solitary nucleus. Contiguity effects may be due to tone-shock pairings common to the tone-blocked and -excitor groups rather than their different CER. And 3) excitatory effects: FDG uptake increases limited to the tone-excitor group occurred in a circuit linked to the CER, including insular and anterior cingulate cortex, vertical diagonal band nucleus, anterior hypothalamus, and caudoventral caudate putamen. This study provided the first large-scale map of brain regions underlying the Kamin blocking effect on conditioning. In particular, the results suggest that suppression of prefrontal activity and activation of unconditioned stimulus pathways are important neural substrates of the Kamin blocking effect.

c-Fos expression in the auditory pathways related to the significance of acoustic signals in rats performing a sensory-motor task

Neuronal activity was established in the auditory pathways in relation to behavioural response and cognitive information processing during a sensory-motor acoustic learning. Rats were trained in three consecutive phases. The first phase was an association between an auditory stimulus and a food reward; the second phase a simple discrimination between two sounds of different frequency components, and the third phase a more complex discrimination involving both spectral and spatial sound dimensions. Auditory stimuli were bursts of complex sounds lasting 500 ms. Neuronal activity related to the behaviourally relevant stimuli was established in 20 "learning" rats undergoing this protocol, which were progressively sacrificed at the beginning, middle and end of each phase. For comparison, activity was also established in four "control" rats exposed to the same stimuli delivered pseudo-randomly, thus carrying no behavioural meaning. Neuronal activity was assessed immunocytochemically using the functional marker Fos. To establish a baseline, two rats were unexposed to controlled acoustic stimulation ("unstimulated" rats). In the superior olivary complex (SOC), inferior colliculus (IC) and medial geniculate body (MGB), the number of Fos-like immunopositive cells was comparable in "learning" and "control" animals, but higher than in the "unstimulated" rats. In the auditory cortex (AC), most prominently in the secondary area Te2, the number of Fos-like positive cells differed between "learning" and "control" rats, suggesting that the auditory cortical areas may be involved in the encoding of the behavioural significance of the acoustic stimuli.

Disorders of the reticular activating system

The organization of components of the reticular activating system and their role in sleep-wake mechanisms and arousal are described. A functional model is proposed based on known neuroanatomical and neurophysiological findings. The involvement of these elements of the reticular activating system in various neurological and psychiatric disorders is discussed. A series of hypotheses are advanced to account for the role of these nuclei in such diverse disorders as schizophrenia, post-traumatic stress disorder, REM behavior disorder, Parkinson's disease and narcolepsy. This line of reasoning suggests that, when neurological or psychiatric disorders manifest symptoms related to arousal and sleep-wake control, disturbances of elements of the reticular activating system must be considered responsible.

Decreased habituation of midlatency auditory evoked responses in Parkinson's disease

The P1 midlatency auditory evoked potential was studied in patients with Parkinson's disease and compared to that in age-matched controls. Habituation of the potential was determined by using a two-click stimulus paradigm in which the stimuli were presented at 250-, 500-, and 1,000-ms interstimulus intervals. Results showed that habituation of the P1 potential had a statistically significant decrease at the 250-ms and 500-ms interstimulus intervals in patients with Parkinson's disease compared to normal controls. The degree of decreased habituation was found to increase with severity of the disease such that stage 5 patients showed greater decreases in habituation compared to stage 4, as did stage 4 compared to stage 3. These findings may be explained by the presence of a dysregulation of sensory processing, possibly by elements of the reticular activating system, including the pedunculopontine nucleus, in Parkinson's disease.

Slow wave sleep is accompanied by release of certain amino acids in the thalamus of cats

To determine whether EEG synchronization in sleep has a metabolic equivalent, we investigated state-dependent changes in extracellular concentrations of amino acids. In vivo microdialysis studies were performed in the ventroposterolateral (VPL) nuclei of the thalamus of cats during natural slow wave sleep (SWS), waking (W) and carbachol-induced paradoxical sleep (PS) like episodes. About two-fold increases in aspartate, glutamate, asparagine, glycine, alanine and gamma-aminobutyric acid (GABA) were observed in SWS compared with control samples collected in W, but serine increased to 487 +/- 211%. In the PS-like state, glutamine increased and GABA decreased. These results suggest changes in intracellular processes reflected by amino acid release in the thalamus, specific to slow wave generation in EEG during natural sleep.

A microiontophoretic study of acetylcholine effects in the inferior colliculus of horseshoe bats: implications for a modulatory role

The effects of acetylcholine (ACh) in processing acoustical information in the inferior colliculus (IC) of awake horseshoe bats (Rhinolophus rouxi) were examined with single cell recordings and microiontophoresis. Cholinergic agonists, acetylcholine and carbachol raised the stimulus evoked discharge in 37% and suppressed responses in 16% of the sample. They did not alter the shapes of tuning curves and rate-intensity functions but the latter showed parallel shifting. The nicotinic antagonist, hexamethonium raised neuronal activity in 52% of neurons without affecting discharge patterns. The nonspecific muscarinic antagonist atropine was mostly inhibitory (62% of units) and caused changes in temporal discharge patterns by affecting the tonic response component. The selective muscarinic ml antagonist pirenzepine, also had an inhibitory effect (37% of units) and predominantly influenced the tonic response component. The selective m2 antagonist, gallamine however produced mainly excitatory effects (64% of units) and changed temporal discharge patterns by adding tonic response components. These findings may indicate a differential pre- and postsynaptic synaptic distribution of m1/m2 receptors in the inferior colliculus as reported for other brain structures. The results indicate that ACh plays a neuromodulatory transmitter role in the auditory midbrain by setting the level of neuronal activity. Its exact function in particular behavioral contexts remains to be determined, since the origin of cholinergic innervation of the mammalian IC is still unclear.

Regional brain effects of sodium azide treatment on cytochrome oxidase activity: a quantitative histochemical study

The objective of the present study was to determine if regional variation in brain cytochrome oxidase activity was observed following systemic administration of sodium azide. An image analysis system calibrated with internal standards of known cytochrome oxidase activity was used to quantify cytochrome oxidase in histochemically stained brain sections. Rats receiving chronic infusion of sodium azide (400 micrograms/hr), which were sacrificed after two weeks, showed a substantial decrease in brain cytochrome oxidase activity over those infused with saline. All of the 22 regions sampled from telencephalic, diencephalic, and mesencephalic levels, showed a significant activity reduction which ranged between 26% and 37%. The regions that appeared significantly more vulnerable to the sodium azide effects were the mesencephalic reticular formation and the central amygdala, which displayed the largest decrease in activity. In addition, interregional correlations of activity showed a deeply modified pattern of correlative metabolic activity between hippocampal, amygdaloid and cortical areas after azide treatment. The regional effects found were consistent with azide-induced learning and memory dysfunctions.

The pedunculopontine nucleus--auditory input, arousal and pathophysiology

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

Functional anatomy of the thalamic centrolateral nucleus as revealed with the [14C]deoxyglucose method following electrical stimulation and electrolytic lesion

The effects of electrical stimulation and electrolytic lesion of the thalamic intralaminar centrolateral nucleus were studied in the rat brain by means of the quantitative autoradiographic [14C]deoxyglucose method. Unilateral electrical stimulation of the centrolateral nucleus induced: (i) local increase in metabolic activity within the stimulated centrolateral nucleus and the ipsilateral thalamic mediodorsal nucleus, (ii) metabolic depression in all layers of the ipsilateral frontal cortex, (iii) bilateral increase in glucose consumption within the periaqueductal gray, pedunculopontine nucleus, and pontine reticular formation, and (iv) contralateral metabolic activation in the deep cerebellar nuclei. The unilateral electrolytic lesion of the thalamic centrolateral nucleus elicited metabolic depressions in several distal brain areas. The metabolic depression elicited in the mediodorsal, ventrolateral, and lateral thalamic nuclei, as well as in the caudate nucleus, the cingulate, and the superficial layers of forelimb cortex were ipsilateral to the lesioned side. The metabolic depression measured in the medulla and pons (medullary and pontine reticular formation, periaqueductal gray, locus coeruleus, dorsal tegmental, cuneiformis, raphe and pedunculopontine tegmental nuclei), the cerebellum (molecular and granular layers of the cerebellar cortex, interpositus and dentate nuclei), the mesencephalon (substantia nigra reticulata, ventral tegmental area and deep layers of the superior colliculus), the diencephalon (medial habenula, parafascicular, ventrobasal complex, centromedial and reticular thalamic nuclei), the rhinencephalon (dentate gyrus and septum), the basal ganglia (ventral pallidum, globus pallidus, entopeduncular and accumbens nuclei) and the cerebral cortex (superficial and deep layers of the frontal and parietal cortex, deep layers of the forelimb cortex) were bilateral. These functional effects are discussed in relation to known anatomical pathways. The bilateral effects induced by the centrolateral nucleus lesion reflect an important role of the centrolateral nucleus in the processing of reticular activating input and in the interhemispheric transfer of information. The cortical metabolic depression induced by centrolateral nucleus stimulation indicates the participation of this nucleus in attentional functions.

Metabolic activation of the brain of young rats after exposure to environmental complexity

Autoradiography with 14C-2-deoxyglucose was used to determine brain metabolic activity during the quiet period that follows after daily exploratory experiences in new complex environments. Eight 1-month-old, male Tryon rats were selected from two litters. Pairs of littermates matched by body weight were assigned to one of two conditions: rats housed individually in small home cages as the "impoverished condition," or rats exposed twice daily to changing and complex environments of two large cages with inanimate objects and conspecifics as the "enriched condition." After 4 days, rats were injected with 2-deoxyglucose, placed individually in a home cage and left undisturbed for 90 min until sacrificed. The brains of "enriched" rats were heavier than their "impoverished" littermates, and showed a global trend for metabolic enhancement. They also showed significantly greater amounts of 2-deoxyglucose in occipital cortex (27%), hippocampal subiculum (36%), and nucleus accumbens (40%).

Functional mapping of the rat brain during drinking behavior: a fluorodeoxyglucose study

Autoradiographic techniques using the radiolabeled glucose analog [14C]2-fluoro-2-deoxy-D-glucose (FDG) were used to map the functional activity in the CNS during drinking behavior. Rats were trained to drink water during a 1-h session each day. Half of the rats were injected with FDG and allowed to drink, while the other half were satiated prior to FDG injection. Uptake of FDG for drinking and control groups of rats was quantified in 60 brain structures from frontal cortex to cervical spinal cord. The largest percent increase in activity (96%) during drinking was in the lateral hypothalamus. Limbic structures with significant metabolic increases included the lateral septum (48%), lateral habenula (44%), and nucleus accumbens (32%). Thalamic nuclei activated included intralaminar (60%), zona incerta (51%), ventroposteromedial (50%), anterior ventral (47%), and dorsal medial (40%). Other structures with increases were the caudal caudate nucleus (53%) and the spinal trigeminal nucleus (45%). The findings were interpreted in light of related metabolic mapping studies of the effects of orofacial stimulation, dehydration, ingestion, arousal, and reward. It was concluded that this FDG study revealed primarily the involvement of structures linked to rewarding and arousal components of motivated drinking behavior, as well as sensorimotor correlates of the orofacial stimulation. The findings provide the first comprehensive functional map of brain systems related to drinking behavior in adult animals.

Effect of low amphetamine doses on cardiac responses to emotional stress in aged rats

In young Wistar rats conditioned emotional stress can be characterized by a learned bradycardiac response to an inescapable footshock. In aged rats this bradycardiac response is attenuated and accompanied by suppressed behavioral arousal in response to novelty. In the present study, cardiac responses to emotional stress and behavioral reactivity to a novel experience in an open field were tested in aged and young rats under the influence of a low dose of d-amphetamine (AMPH, 0.5 mg/kg IP). AMPH administration in 27-month-old rats reinstated the bradycardiac response to emotional stress, while it failed to influence the resting heart rate in the home cage. Age-associated differences in open-field ambulation, present in drug-free conditions, were antagonized by low doses of AMPH (0.25-1.0 mg/kg). It is concluded that enhanced arousal by aminergic stimulation with AMPH in the aged rat invoked cardiac and behavioral response patterns resembling those at younger ages.

Metabolic activation of the rat visual system by patterned light and footshock

Autoradiography with [14C]2-deoxy-D-glucose was used to examine metabolic changes in the visual system of hooded rats exposed to patterned light or to darkness following footshock. Primary retinorecipient structures (superficial layer of the superior colliculus and the dorsal lateral geniculate nucleus) showed a response to light but not to shock. Higher visual sites showed two different shock effects. First, in darkness the intermediate grey layer of the superior colliculus was suppressed by the shock. Second, in the lateral posterior nucleus and primary visual cortex, the footshock led to significant enhancement of the metabolic responses to the patterned light. The findings suggest that footshock-induced arousal has significant modulatory effects on the operations of higher visual centers of behaving rats.

Functional mapping of the rat brain during vocalizations: a 2-deoxyglucose study

Autoradiographic [14C]2-deoxyglucose (2-DG) procedures were used to map the functional activity in the CNS during vocalizations elicited by electrical stimulation of the midbrain reticular formation (MRF) in behaving rats. Following injection of 2-DG, rats received MRF stimulation through stainless steel electrodes over 90 min. Yoked controls received 2-DG injection followed by playback of the recorded vocalizations. Relative differences in peak isotope uptake (gray/white matter ratios) in 22 structures related to vocalization were compared between the two groups. The major findings were localized to hypothalamus, midbrain and brainstem structures. Significant increases in 2-DG uptake were noted in the following structures in MRF stimulated rats: dorsolateral central gray (PAG), MRF, lateral hypothalamus (LH), ventromedial hypothalamus (VmH), paraventricular nucleus (PVN) and nucleus ambiguus (NA). Cortical structures were not activated during MRF stimulation. The PAG and NA are known to be important relays in the production of vocalizations. MRF stimulation, therefore, activates the motor output pathways for vocalization, but does not appear to activate cortical and limbic motivational centers.

Behavioral effects of GABA in the hippocampal formation: functional interaction with histamine

Some behavioral effects of GABA in the hippocampus and its probable interaction with histamine in adult male rats were studied. Four experiments were performed. In Expt. 1, rats were implanted unilaterally into the ventral hippocampus and they were microinjected with increasing doses of GABA. Five minutes later the following behavioral scores were measured in a holeboard: (1) locomotion, (2) head-dipping and (3) rearing. Results showed that GABA induced an increase in locomotion and a decrease in the frequency of long-lasting rears. In Expt. 2, the implanted rats were microinjected into the ventral hippocampus with Gamma-vinyl-GABA (GVG), an inhibitor of the metabolizing enzyme of GABA and picrotoxin and bicuculline, both antagonists of GABA. The following behaviors were measured later in the holeboard: (1) locomotion, (2) head-dipping frequency, (3) rearing activity and (4) grooming frequency. Results showed that GVG also increased the locomotor activity and this effect was antagonized by picrotoxin and bicuculline. In Expt. 3 the brain endogenous levels of GABA were measured in rats microinjected with GVG. Results showed that the GVG injection into the hippocampus augmented the endogenous levels of GABA. In Expt. 4 the implanted rats were microinjected into the hippocampus with GVG and histamine. Behavioral scores were measured later in the holeboard. Results showed that the increase in locomotion induced by GVG was blocked by the administration of histamine. Present results show that GABA may be involved in some hippocampal-mediated behaviors and suggest a histamine-GABA link in the final expression of these behaviors.

Learning-related activation in the auditory system of the rat produced by long-term habituation: a 2-deoxyglucose study

Autoradiography with [14C]2-deoxyglucose (2-DG) was used to examine the functional activity of the rat auditory system during long- and short-term habituation of the acoustic startle reflex. The data showed that presentation of the acoustic stimulus to long-term habituated rats resulted in a learning-related metabolic enhancement that was significantly greater than the response evoked by the same acoustic stimulus in the inexperienced rats. This enhancement was localized to brainstem and midbrain auditory nuclei and no significant changes occurred at thalamocortical levels of the auditory pathway. The largest difference in 2-DG uptake between long- and short-term habituated rats was in the lateral superior olivary nucleus (LSO). The LSO activation suggests that olivocochlear efferents may operate in a central feedback control of peripheral auditory input during long-term habituation. Findings of enhanced metabolism from the cochlear nuclei to the central nucleus of the inferior colliculus indicated that active processes of neuronal plasticity take place in the lower auditory system during long-term habituation. The results provide the first demonstration of how a nonassociative learning experience such as long-term habituation modifies the metabolic activity of the auditory system. The findings support the conclusion that auditory responses of behaving animals to acoustic stimuli are dependent not only on the physical parameters of a stimulus, but also on its learned behavioral significance.

Neural substrates for long-term habituation of the acoustic startle reflex in rats: a 2-deoxyglucose study

Autoradiography with [14C]2-deoxyglucose was used to examine the functional activity of the rat brain during long- and short-term habituation of the acoustic startle reflex. Long-term habituated rats, when compared to short-term rats, showed an enhanced metabolic activation of the auditory system, with the exclusion of thalamocortical levels. Regional metabolic increases were also located within the cerebellum and its major input-output structures. In contrast, the midbrain reticular formation and its ascending thalamocortical activating system showed a widespread metabolic suppression. The findings provide the first map of structures in a mammalian brain with learning-related metabolic alterations dependent on long-term habituation.

Effect of heroin-conditioned auditory stimuli on cerebral functional activity in rats

Cerebral functional activity was measured as changes in distribution of the free fatty acid [1-14C]octanoate in autoradiograms obtained from rats during brief presentation of a tone previously paired to infusions of heroin or saline. Rats were trained in groups of three consisting of one heroin self-administering animal and two animals receiving yoked infusions of heroin or saline. Behavioral experiments in separate groups of rats demonstrated that these training parameters imparts secondary reinforcing properties to the tone for animals self-administering heroin while the tone remains behaviorally neutral in yoked-infusion animals. The optical densities of thirty-seven brain regions were normalized to a relative index for comparisons between groups. Previous pairing of the tone to heroin infusions irrespective of behavior (yoked-heroin vs. yoked-saline groups) produced functional activity changes in fifteen brain areas. In addition, nineteen regional differences in octanoate labeling density were evident when comparison was made between animals previously trained to self-administer heroin to those receiving yoked-heroin infusions, while twelve differences were noted when comparisons were made between the yoked vehicle and self administration group. These functional activity changes are presumed related to the secondary reinforcing capacity of the tone acquired by association with heroin, and may identify neural substrates involved in auditory signalled conditioning of positive reinforcement to opiates.

Functional mapping of the brainstem during centrally evoked bradycardia: a 2-deoxyglucose study

Autoradiographic 2-deoxy-[14C] glucose (2-DG) procedures were used to map the functional activity of the brainstem during bradycardia elicited in awake rats by stimulation of the deep mesencephalic nucleus of the midbrain reticular formation (MRF). Quantitative determinations of 2-DG uptake in 46 brainstem structures of MRF-stimulated rats were compared to those of control rats without stimulation. This paper is the first 2-DG study to map the brainstem structures involved in a heart rate response evoked by central stimulation. The structures activated in the midbrain, caudal to the stimulation site, are part of the reticular formation and the central gray. The greater focuses of labeling were concentrated on the lateral aspects of the deep mesencephalic nucleus and on the lateral divisions of the midbrain central gray. The remaining structures activated during bradycardia were all located in the caudal medulla. The largest increase was observed in the caudal nucleus ambiguus. Significant increases were also found in the dorsal motor nucleus of vagus and in the nucleus of the solitary tract. The region of the caudal inferior olive showed a small increase in 2-DG uptake, whereas structures like the raphe magnus and parvocellular reticular nucleus showed a tendency to reduce 2-DG uptake levels in the stimulated rat. It was concluded that bradycardia induced centrally by MRF stimulation may be mediated by well-defined brainstem descending pathways, direct and indirect, between the activated regions of the midbrain and the various medullary nuclei known to induce bradycardia upon electrical stimulation. The results suggest that the midbrain central gray and reticular formation may play a role as intermediates in an indirect hypothalamus-medullary circuitry for bradycardia. In addition, descending MRF information and afferent baroreceptor inputs appear to exert their inhibitory influences on heart rate via a common set of neuroanatomical substrates in the medulla.

Stimulus-specific long-term habituation of visually guided orienting behavior toward prey in toads: a 14C-2DG study

The regional distribution of cerebral glucose utilization, revealed by the 14C-2DG technique, was compared between (i) toads after stimulus-specific long-term habituation of the orienting response toward a repeatedly presented prey dummy ('habituation group') and (ii) non-habituated toads, readily orienting toward the repetitively presented prey stimulus ('naive group'). In the 'habituation group', the ventral medial pallium (vMP), a certain portion of the preoptic area (PO), and the dorsal hypothalamus (dHYP) showed a statistically significant increase in 14C-2DG-uptake; decrease was observed in the ventral layers of the optic tectum (vOT), a portion of the tegmental reticular formation (RET), the ventral cerebellum (vCB), and the striatum (STR). The results suggest that stimulus-specific long-term habituation of prey-catching affects both, components of the stimulus-response mediating circuit (e.g., involving OT), and structures extrinsic to it, (e.g., vMP, PO, dHYP), which may belong to a modulatory circuitry. Bilateral lesions to vMP strongly delay habituation. Our results are suggesting that damping of the adequate behavioral motor response during habituation involves active inhibitory processes of a modulatory system that develops in strength during stimulus repetition so as to suppress response output, which basically supports Sokolov's hypothesis (1975).

Projections of brainstem core cholinergic and non-cholinergic neurons of cat to intralaminar and reticular thalamic nuclei

We combined the retrograde transport of wheat germ agglutinin conjugated with horseradish peroxidase with choline acetyltransferase immunohistochemistry to study the projections of cholinergic and non-cholinergic neurons of the upper brainstem core to rostral and caudal intralaminar thalamic nuclei, reticular thalamic complex and zona incerta in the cat. After wheat germ agglutinin-horseradish peroxidase injections in the rostral pole of the reticular thalamic nucleus, the distribution and amount of retrogradely labeled brainstem neurons were similar to those found after tracer injection in thalamic relay nuclei (see preceding paper). After wheat germ agglutinin-horseradish peroxidase injections in the caudal intralaminar centrum medianum-parafascicular complex, rostral intralaminar central lateral-paracentral wing, and zona incerta, the numbers of retrogradely labeled brainstem neurons were more than three times higher than those found after injections in thalamic relay nuclei. The larger numbers of horseradish peroxidase-positive brainstem reticular neurons after tracer injections in intralaminar or zona incerta injections results from a more substantial proportion of labeled neurons in the central tegmental field at rostral midbrain (perirubral) levels and in the ventromedial part of the pontine reticular formation, ipsi- and contralaterally to the injection site. Of all retrogradely labeled neurons in the caudal midbrain core at the level of the cholinergic peribrachial area and laterodorsal tegmental nucleus, 45-50% were also choline acetyltransferase-positive after the injections into central lateral-paracentral and reticular nuclei, while only 25% were also choline acetyltransferase-positive after the injection into the centrum medianum-parafascicular complex. These findings are discussed in the light of physiological evidence of brainstem cholinergic mechanisms involved in the blockade of synchronized oscillations and in activation processes of thalamocortical systems.

Cardiac, ventilatory and behavioural arousal responses evoked by electrical brain stimulation in the goldfish (Carassius auratus)

Goldfish (Carassius auratus) were fitted with intracranial stainless steel microelectrodes for electrical evocation of behavioural arousal and its cardiac and ventilatory correlates. Behaviour was monitored on a videosystem and ECG electrodes and a buccal catheter were implanted to monitor physiological responses. Thresholds for responses were described in relation to the current spread likely to excite CNS tissue. Two types of responses were obtained. These were (A) cardiac and ventilatory responses alone, apparently due to stimulation of primary sensory pathways and (B) these responses and behavioural arousal responses which were elicited at higher thresholds. These latter, more complete expressions of arousal resulted from stimulation of the Dm/Dc region of the telencephalon, the dorsal diencephalon and the midbrain tegmentum. Response thresholds were higher and physiological response magnitudes lower in the midbrain tegmentum compared to the forebrain regions.

Cardiovascular and respiratory changes elicited by stimulation of rat superior colliculus

Stimulation of the rat superior colliculus can produce either orienting or defensive movements, which if elicited by natural stimuli would be accompanied by cardiovascular changes. To assess whether cardiovascular changes might also be mediated by the superior colliculus, blood pressure and heart rate were measured in Saffan-anaesthetised rats while the dorsal midbrain was systematically explored with electrical and chemical stimulation. Electrical stimulation (10 sec trains of 0.3 msec 100 Hz cathodal pulses, 50 microA) within the superficial and intermediate layers of the rostral superior colliculus transiently lowered blood pressure without affecting heart rate. In contrast sites within the deep layers, and in adjacent periaqueductal grey and midbrain tegmentum, gave pressor responses accompanied by a variety of heart-rate changes, that usually included a period of bradycardia. A roughly similar distribution was obtained with the cell-stimulant bicuculline (200 or 500 nl, 490 microM), though sodium L-glutamate (200 nl, 0.05 or 1.0 M) was ineffective. These results suggest that (a) cardiovascular responses can be produced by stimulation of the rat superior colliculus; (b) their nature depends on the location of the stimulation; and (c) they may be mediated in part by cells differentially sensitive to glutamate and to bicuculline. In addition, in some animals respiratory responses were measured stethographically. Short-latency increases in thoracic girth, often accompanied by increases in respiratory rate and depth, were elicited by electrical stimulation from 61% of the collicular sites examined, and by microinjection of glutamate from 56% of collicular sites. These data suggest that (a) cells within the superior colliculus are capable of influencing respiration; (b) given the widespread distribution of responsive sites within the superior colliculus, the respiratory changes may be preparatory for both approach and defensive movements; (c) the collicular cells that affect respiration may be different from those that influence blood pressure, because the latter are relatively insensitive to microinjection of glutamate.

Catecholamines and attention. I: Animal and clinical studies

One important function of the catecholamine innervation of the cerebral cortex may be the control of attention. Of particular interest are the catecholamine projections to the cerebral cortex from the reticular formation, namely the dopamine neurons of the ventral tegmentum of the midbrain and the noradrenergic neurons of the locus coeruleus in the upper pons. Animal studies implicate noradrenaline and dopamine in a wide range of attention-related behaviours involving search and exploratory activity, distractibility, response rate, discriminability and the switching of attention. Most human studies come from the clinical literature relating to schizophrenia, Parkinson's disease and attention deficit disorder. An association has been claimed in each of these conditions between abnormal catecholamine activity (in particular dopamine) and attentional dysfunction. In particular, difficulty with the attachment of appropriate responses to environmental stimuli, akin to those observed in animals with lesions to central dopamine pathways, indicates a role for dopamine in response selection processes. Overall, the animal and human studies reviewed indicate a role for central noradrenaline and dopamine in the early and late processing of information, respectively.

Midbrain reticular stimulation produces patterns of metabolic activation and suppression in the cerebellum and vestibular nuclei: a 2-deoxyglucose study

Autoradiographic 2-deoxyglucose (2-DG) procedures were used to map the activity in the cerebellum and vestibular nuclei during electrical stimulation of the midbrain reticular formation (MRF) in unrestrained rats. The major finding was a large increase in 2-DG uptake observed in the flocculus of MRF-stimulated rats. The peak of labeling in the flocculus was greater than any other peak of labeling measured in the cerebellum of MRF-stimulated or control rats. Structures showing significant decreases in 2-DG uptake included the 3 deep cerebellar nuclei and the 3 vestibular nuclei. The most pronounced suppressive effects of MRF stimulation were on the medial and lateral vestibular nuclei. The changes in metabolic activity revealed by 2-DG provide a first anatomical demonstration of: the activating effects of MRF stimulation on the flocculus; and the suppressive effects of MRF stimulation on deep cerebellar and vestibular nuclei. The observed patterns of metabolic activation and suppression were correlated with the known electrophysiological properties of the structures affected by MRF stimulation. The findings are consistent with specific effects of MRF stimulation on floccular-vestibular-visual interactions that may be disruptive to learning functions such as adaptability of the vestibulo-ocular reflex. The effects of MRF stimulation on the deep cerebellar nuclei are also consistent with a potential disruption of somatomotor learning-related activities in these nuclei. The results support the existence of MRF mechanisms for the modulation of integrative sensory-motor functions in the cerebellum.