The effects of lesions to noradrenergic projections from the locus coeruleus and lateral tegmental cell groups on conditioned taste aversion in the rat

Lesions of the coeruleo-cortical noradrenergic projections caused marked cortical noradrenaline depletions but were not associated with deficits in the acquisition or extinction of a conditioned taste aversion (CTA). Lesions of lateral tegmental noradrenergic projections resulted in marked hypothalamic noradrenaline depletions, enhanced neophobia to the novel taste of saccharin, unimpaired acquisition but prolonged extinction of the CTA. However, when animals with lateral tegmental noradrenergic lesions received extensive preconditioning exposure to saccharin, acquisition of the CTA was attenuated and extinction was more rapid than in controls. Alterations in CTA learning and extinction following lesions of the lateral tegmental noradrenergic system appear to reflect alterations in the way that animals with lesions react toward the hedonic aspects of taste-related stimuli rather than alterations in associational or attentional mechanisms.     

Changes in body weight and food-related behaviour induced by destruction of the ventral or dorsal noradrenergic bundle in the rat

Three experiments contrasted the effects of 6-hydroxydopamine-induced lesions of the ventral noradrenergic and dorsal noradrenergic projections, predominantly to hypothalamus and cortex, respectively, upon body weight changes and food-related behaviour in the rat. In general, ventral noradrenergic bundle lesions enhanced weight gain and these effects were exaggerated by the provision of palatable cheese to the standard chow diet. In contrast, lesions of the dorsal noradrenergic bundle produced minor changes in body weight. Associated with the effects of ventral noradrenergic bundle lesions were hyperphagia, enhanced suppression of intake of food adulterated with quinine, (at high concentration), a small attenuation of food neophobia, and enhanced acquisition, but not performance, of the eating response to tail-pinch stimulation. These ventral noradrenergic bundle lesions failed to alter basal activity levels, amphetamine anorexia or the diurnal pattern of eating or activity. In contrast, lesions of the dorsal noradrenergic bundle did not produce either hyperphagia or enhanced rejection of food adulterated with quinine. However, there was a strong attenuation of food neophobia and a retarded acquisition (but unimpaired performance) of eating in response to tail-pinch stimulation.

The results are discussed in connection with previous studies of ventral and dorsal noradrenergic bundle lesions, with the effects of ventromedial hypothalamic lesions and with the underlying behavioural and physiological processes that mediate these contrasting effects of different neuroanatomical patterns of central noradrenaline depletion.     

Central noradrenaline depletion attenuates amphetamine-induced locomotor behavior

Male rats were given 6-hydroxydopamine-induced lesions of the locus coeruleus (LC) or the dorsal noradrenergic bundle (DNAB), prior to the measurement of locomotor and rearing activity induced by D-amphetamine. The increased locomotor activity induced by D-amphetamine (1.8 mg/kg) was significantly attenuated by both the LC and the DNAB lesions. The stimulatory effect of the 7.2 mg/kg dose of amphetamine was attenuated by the LC lesion, whereas the DNAB lesion potentiated this effect. The LC lesion also attenuated rearing induced by the 7.2 mg/kg dose of amphetamine. These results suggest some involvement of central noradrenergic neurons in the activity induced by amphetamine in the rat.     

Mesial prefrontal cortical lesions and timidity in rats. II. Reactivity to novel stimuli

An earlier set of experiments suggested that mesial prefrontal cortical (MFC) lesions in rats enhanced timidity. It was uncertain whether this increased timidity was a general phenomenon, or was restricted to fear of bright, open spaces. The experiments reported here measured behavioral reactivity to a variety of stimuli, under situations where light/dark differences were minimized. It was found that MFC rats were slowed in leaving an open field to enter a small box. In the open field, MFC subjects showed signs of enhanced reactivity, but only when the field was novel and the subjects unhandled. When allowed to choose between four alleys containing varying stimuli, brain-damaged rats avoided novel objects and complex stimuli, but spent more time than controls in contact with other rats in the apparatus. In a test of food neophobia, MFC subjects were not neophobic in a familiar test environment, but did avoid the experimenter more than controls. Finally, duration of barbiturate anesthesia was shortened by MFC lesions, but only under conditions of high novelty. It is concluded that MFC lesions produce a timidity which is not restricted to photophobia.     

Visceral afferent bias on cortical processing: role of adrenergic afferents to the basal forebrain cholinergic system

Intraperitoneal epinephrine enhances the cerebral auditory evoked potential (AEP), an effect that is dependent on the basal forebrain cortical cholinergic system. The present study examined the hypothesis that ascending noradrenergic projections from brainstem autonomic substrates to the basal forebrain cholinergic system represent an essential component of the ascending pathway mediating this effect of epinephrine. Epinephrine again enhanced the AEP in rats, and this effect was attenuated by infusion of the selective alpha1 adrenergic antagonist terazosin into the basal forebrain. Moreover, infusions of the selective alpha1 adrenergic agonist phenylephrine into the basal forebrain mimicked the priming effects of epinephrine. Results support the hypothesis that noradrenergic afferents to the basal forebrain cholinergic system represent a component of an ascending visceral afferent system.     

Comportement alimentaire,rythmes circannuels pondéral et d'hibernation chez le hamster d'europe porteur de lésions des faisceaux noradrénergiques ascendants

Bilateral electrolytic lesions were made in the ascending noradrenergic bundles of an hibernator, European hamster, at various periods of the year. The most posterior and ventral lesions are followed by cheek-pouch hoarding behavior with hyperphagia and obesity only when done in the periods where the hamsters fatten. In the other periods hyperphagia and obesity are absent or moderate. Circannual rhythm is not abolished but the operated animals are not in phase with their controls. Hibernation is facilitated in the lesioned animals especially at 15°C. In all cases lesions were followed by a fall in brain NE. These results are discussed in comparison with the actual hypothesis of body weight regulation, rhythms and hibernation.     

Food intake, aggression, and fear behavior in the mother rat: control by neural systems concerned with milk ejection and maternal behavior

Mother rats eat more, are more aggressive, and show less fear behavior (freezing) than during other stages of the reproductive cycle. Electrolytic lesions in the peripeduncular area of the lateral midbrain made nursing mother rats eat less and interact peacefully with male intruders. This midbrain area forms part of the ascending milk-ejection pathway, so it seems plausible that the suckling stimulus maintains hyperphagia and aggression in mother rats. Because no alteration in fear behavior was observed in mothers with lesions, it was predicted that the reduction in freezing was related primarily to maternal responsiveness to pup cues other than suckling. In line with this hypothesis, it was found that the experimental induction of maternal behavior in ovariectomized, hormone-treated females was associated with a significant decrease in fear behavior, with no concomitant changes in food intake or aggression.     

Dissociable effects of lesions to the dorsal or ventral noradrenergic bundle on the acquisition, performance, and extinction of aversive conditioning

In six experiments we studied the effects of lesions to either the dorsal or ventral noradrenergic bundle on the acquisition and extinction of the conditioned emotional response (CER) as measured in a conditioned suppression paradigm. Infusions of the neurotoxin 6-hydroxydopamine (6-OHDA) into the trajectory of the dorsal noradrenergic ascending bundle (DNAB) impaired the acquisition of on-the-baseline and off-the-baseline conditioned suppression. The acquisition impairment for on-the-baseline conditioning was also shown to still be present when training did not commence until 8 weeks following central noradrenergic depletion. However, in rats previously trained on the CER, DNAB lesions did not affect performance. There was also a small resistance to extinction following on-, but not off-the-baseline conditioning. The acquisition impairment was shown not to be because of an altered sensitivity to the footshock. In contrast, infusions of 6-OHDA into the ventral noradrenergic ascending bundle (VNAB) had no effect upon the acquisition of the CER in an on-the-baseline procedure, but retarded its extinction to a much greater extent. The results here are discussed in terms of other acquisition deficits shown by rats with DNAB lesions, and with reference to Gray's "anxiety" and Mason's "selective attention" theories of locus coeruleus function.     

Comparative effects of lesion of the basolateral nucleus and lateral nucleus of the amygdaloid body on neophobia and conditioned taste aversion in the rat

The effects of bilateral lesions of the basolateral and lateral nuclei of the amygdala on the neophobic response and LiCl-conditioned taste aversion to a saccharin solution were studied in rats. Compared to intact animals, rats with basolateral lesions did not exhibit neophobia to the novel stimulus, while rats with lateral lesions demonstrated an initial preference to the sweet solution over water. The LiCl-induced aversion was suppressed after basolateral lesions and was unchanged after lateral lesions. It is concluded that these two amygdaloid nuclei play an important but distinct role in neophobia and conditioned taste aversion.     

Catecholaminergic mechanisms underlying neurohypophysial hormone responses to unconditioned or conditioned aversive stimuli in rats

Oxytocin release from the neurohypophysis is facilitated by systemic cholecystokinin octapeptide (CCK) administration and noxious stimuli. Oxytocin release after CCK administration is mediated by A2 noradrenergic neurones while the release after noxious stimuli appears to be mediated by A1 noradrenergic neurones. On the other hand, facilitation of vasopressin release after noxious stimuli is not dependent upon noradrenergic neurones but on dopamine receptors. Environmental stimuli previously paired with noxious stimuli (conditioned fear stimuli) or novel environmental stimuli facilitate oxytocin release and suppress vasopressin release. These neuroendocrine responses to conditioned fear stimuli, but not to novel stimuli, are impaired by central noradrenaline depletion or i.c.v. adrenoceptor antagonists. These data suggest that there are at least two types of stress responses in neuroendocrine systems, one noradrenaline dependent, and one noradrenaline independent. It is also suggested that noradrenergic neurones are functionally heterogeneous in the control of oxytocin release.     

Collateral sprouting and reduced activity of the rat mesocortical dopaminergic neurons after selective destruction of the ascending noradrenergic bundles

The properties of the mesocortical dopaminergic neurons projecting to the pregenual and anterior supragenual cortices were examined 3–6 months after the degeneration of ascending noradrenergic pathways caused by bilateral multiple or single microinjections of 6-hydroxydopamine made laterally to the pedunculus cerebellaris superior. In all rats and in all cortical areas examined, noradrenaline levels were reduced by more than 75%. A similar decrease in noradrenaline levels was obtained in the ventral tegmental area. As indicated by the increases in cortical levels of dopamine and in [³H]dopamine specific uptake sites as well as by histochemical analysis, these lesions induced a collateral sprouting of the mesocortical dopaminergic neurons. The intensity of the effect varied from one animal to another and even from one anteromedial hemicortex to another. When present, the increase in dopamine levels was observed in all the cortical areas investigated. As suggested by the decreased ratio of the amount of dihydroxyphenylacetic acid to dopamine in the cortex, the activity of the mesocortical dopaminergic neurons was reduced in the rats with lesions. This effect was even seen in rats in which the cortical levels of dopamine were only slightly increased. Both the collateral sprouting and the reduced activity of the mesocortical dopaminergic neurons were related to the degeneration of the noradrenergic neurons and not to a non-specific effect of 6-hydroxydopamine, since both phenomena did not occur in rats pretreated with desipramine, a treatment which prevented the decline in noradrenaline levels.Thus, a lesion of the ascending noradrenergic pathways can lead to sprouting of dopaminergic neurons in the cortex and a reduced activity of these dopaminergic neurons. The respective role of the disappearance of the noradrenergic innervation in the cerebral cortex and in the ventral tegmental area in the collateral sprouting and in the reduced activity of the mesocortical dopaminergic neurons is discussed.     

Amygdalar lateralization in fear conditioning: evidence for greater involvement of the right amygdala

The relative contribution of left and right amygdalae in the acquisition and retention of fear conditioning was investigated in rats. Pretraining bilateral electrolytic lesions blocked the acquisition of conditioned fear to tone and context, whereas unilateral lesions induced partial impairments with no left-right amygdala differences. In contrast, posttraining bilateral and unilateral lesions produced significant deficits in the retention of conditioned fear to tone and context. Although no left-right difference was observed to tone, the right amygdala lesions generated greater deficits in contextual fear than the left amygdala lesions. These results indicate that fear conditioning is partially disrupted with unilateral amygdalar lesions, but that the right amygdala has greater involvement than the left amygdala when conditioning occurs under a normal brain state.     

The effects of lesions of lateral tegmental noradrenergic neurons on components of sexual behaviour and pseudopregnancy in female rats

Lesions (electrolytic and neurotoxic, using 6-hydroxydopamine) have been made of the projections of medullary noradrenergic neurons in the ascending ventral noradrenergic pathway in female rats. The lesions caused major depletion of noradrenaline in sub-cortical structures, notably the hypothalamus. In behavioural tests, these lesions selectively impaired sexual receptivity, measured as lordosis responses, but did not abolish proceptivity (soliciting). The impaired behaviour is critically dependent on somatosensory stimulation generated by the male during a mount. So, too, is pseudopregnancy since stimulation of the uterine cervix during coitus is essential for the prolongation of luteal life which underlies it. The ability to induce a pseudopregnant state was also abolished by the same ventral noradrenergic pathway lesion.These data, together with supportive evidence from experiments using procaine anaesthesia, have led us to suggest an important function of subcortically projecting noradrenergic neurons in the central transmission and/or processing of somatosensory information in neuroendocrine systems. We also present data suggesting that the effects of noradrenergic neuron activity are dependent on dopaminergic mechanisms more directly concerned with the control of motivated behaviour and anterior pituitary function.     

An analysis of the behavior elicited by stimulation of the dorsal pons in rat

Dorsal pontine intracranial self-stimulation (ICSS) sites were electrically stimulated in rats. Only stimulation-induced oral behavior and not locomotor behavior was observed. The predominant elicited behavior was a fragmentary type of forepaw grooming. In some cases the stimulation-induced forepaw grooming was changed to drinking by the forced presentation of a drinking spout in the perioral region. Neither the dorsal pontine stimulation-induced oral behavior nor the ICSS was disrupted by six-hydroxydopamine lesions of ascending noradrenergic projections from locus coeruleus to widespread forebrain areas. It is suggested that the mesencephalic nucleus of the trigeminal nerve, with its connections to the motor trigeminal nucleus, may mediate dorsal pontine stimulation-induced behavior and ICSS.     

Fear conditioning to tone, but not to context, is attenuated by lesions of the insular cortex and posterior extension of the intralaminar complex in rats

C. Shi and M. Davis (1999) recently reported that combined lesions of the posterior extension of the intralaminar complex (PINT) and caudal insular cortex (INS) block acquisition but not expression of fear-potentiated startle to discrete conditioned stimuli (CSs) and a footshock unconditioned stimulus (US) and proposed that PINT-INS projections to the amygdala constitute the essential US pathways involved in fear conditioning. The present study further tested this hypothesis by examining whether PINT-INS lesions block fear conditioning (as measured by freezing) to diffuse-context and discrete-tone CSs, and whether posttraining lesions with continued CS-US training result in extinction to the CSs. Posttraining lesions resulted in a selective attenuation of tone conditioning, but context conditioning was unaffected by pre- and posttraining lesions. These results do not support the view that the PINT-INS represent the essential US pathway in fear conditioning.     

Concomitant depression of locus coeruleus neurons and of flexor reflexes by an alpha 2-adrenergic agonist in rats: a possible mechanism for an alpha 2-mediated muscle relaxation

The alpha 2-agonist tizanidine, clinically used as an antispastic drug, also strongly reduces polysynaptic flexor reflexes. The hypothesis was tested that the noradrenergic coerulespinal system exerts a tonic facilitation on spinal reflexes and that the depressant effects of tizanidine may be explained by an alpha 2-mediated autoinhibition of the tonic activity of locus coeruleus neurons, resulting in a disfacilitation of the spinal reflexes. The following results support this working hypothesis: (1) systemic injections of tizanidine markedly decreased the spontaneous activity of locus coeruleus neurons, but not of non-locus coeruleus neurons. The alpha 2-antagonist yohimbine reversed this effect. (2) The time course of diminished locus coeruleus activity paralleled that of depressed flexor reflexes. (3) Flexor reflexes were also markedly depressed by the alpha 1-adrenergic antagonist prazosin, administered alone, which is in line with the proposition that the noradrenergic system exerts a tonic facilitation on spinal neurons by way of alpha 1-adrenergic receptor activation. (4) Flexor reflexes were facilitated by conditioning microstimulation of locus coeruleus neurons, and this effect was reversed by prazosin. (5) Flexor reflexes significantly diminished in size following placement of an irreversible lesion in the ipsilateral locus coeruleus. Although these results strongly support the above hypothesis regarding a descending modulatory function of the descending locus coeruleus system on spinal reflexes, possible additional mechanisms, perhaps also involving the ascending projection of the locus coeruleus to supraspinal motor structures, remain to be elucidated.     

The intra-cortical trajectory of the coeruleo-cortical projection in the rat: A tangentially organized cortical afferent

Cortical and sub-cortical lesions in the rat were used to analyze the intracortical trajectory of the noradrenergic axons, which were visualized by aldehyde-induced catecholamine histofluorescence and by immunohistochemistry using an antibody directed against rat dopamine-β-hydroxylase. Following subcortical lesions there is a slowly progressive reduction in the density of cortical noradrenergic axons, indicating that they undergo asynchronous anterograde degeneration. By 2 weeks after transection of the dorsal noradrenergic bundle, no dopamine β-hydroxylase-immunoreactive fibers are detectable in the ipsilateral cortex. Neither transection of the cingulum bundle, nor parasagittal incisions through the dorsal cortex lateral to the cingulum, diminished the noradrenergic innervation of medial or dorso-lateral cortex. A cortical lesion medial to the cingulum bundle markedly reduced the density of noradrenergic fibers in cingulate cortex caudal to the lesion, but did not affect the innervation of dorso-lateral cortex. In contrast, dorso-lateral frontal incisions and decortication (frontal lobotomy) produced a marked ipsilateral decrease in the noradrenergic fiber density throughout the remaining dorso-lateral cortex, while sparing the innervation of cingulate and infra-rhinal cortex.These results demonstrate that the dorso-lateral cortex is innervated by noradrenergic fibers in the medial forebrain bundle that reach the frontal pole, turn dorsally over the anterior portion of the forceps minor and continue caudally within the deep layers of frontal and dorso-lateral cortex, supplying the noradrenergic innervation throughout their trajectory. The medial cortex is innervated by a separate group of noradrenergic fibers that ascend through the septum, curve over the genu of the corpus callosum, and run caudally in the supracallosal stria.The present results show that the cingulum bundle is not a major intra-cortical noradrenergic pathway and does not provide branches that contribute significantly to the innervation of dorsal or lateral cortex. Thus the medial and lateral cortex can be selectively and differentially denervated of noradrenergic fibers and a coarse topographic order exists in the noradrenergic innervation of cortex. Since noradrenergic fibers travel long distances within the cortical grey matter, a small lesion of frontal cortex can have far-reaching effects on the innervation of distant, more caudal regions of cortex. The coeruleocortical projection has properties that differ from those of the best characterized cortical afferents and may be a useful model for the study of other ascending monoamine systems. The tangential, intracortical trajectory of the noradrenergic fibers would confer upon the coeruleo-cortical system the capacity to modulate neuronal activity simultaneously through a vast expanse of neocortex. A formulation of cortical organization is presented which integrates the tangential organization of the coeruleo-cortical projection with the concept of columnar organization of cortex.     

Lesions in the bed nucleus of the stria terminalis disrupt corticosterone and freezing responses elicited by a contextual but not by a specific cue-conditioned fear stimulus

The bed nucleus of the stria terminalis (BNST) is believed to be a critical relay between the central nucleus of the amygdala (CE) and the paraventricular nucleus of the hypothalamus in the control of hypothalamic-pituitary-adrenal (HPA) responses elicited by conditioned fear stimuli. If correct, lesions of CE or BNST should block expression of HPA responses elicited by either a specific conditioned fear cue or a conditioned context. To test this, rats were subjected to cued (tone) or contextual classical fear conditioning. Two days later, electrolytic or sham lesions were placed in CE or BNST. After 5 days, the rats were tested for both behavioral (freezing) and neuroendocrine (corticosterone) responses to tone or contextual cues. CE lesions attenuated conditioned freezing and corticosterone responses to both tone and context. In contrast, BNST lesions attenuated these responses to contextual but not tone stimuli. These results suggest CE is indeed an essential output of the amygdala for the expression of conditioned fear responses, including HPA responses, regardless of the nature of the conditioned stimulus. However, because lesions of BNST only affected behavioral and endocrine responses to contextual stimuli, the results do not support the notion that BNST is critical for HPA responses elicited by conditioned fear stimuli in general. Instead, the BNST may be essential specifically for contextual conditioned fear responses, including both behavioral and HPA responses, by virtue of its connections with the hippocampus, a structure essential to contextual conditioning. The results are also not consistent with the hypothesis that BNST is only involved in unconditioned aspects of fear and anxiety.     

Lithium chloride and avoidance of novel places

Rats were exposed to a distinctive chamber (chamber A, part of a two-chamber apparatus), which was novel for half of the rats but familiar for the other half. Each rat was subsequently injected with lithium chloride or saline. In a test trial conducted 24 hr later, all rats were given a choice between chamber A and a second chamber (B), which was novel for all rats. The main result was that the group made familiar with chamber A and then given lithium showed a significant preference for that side or an avoidance of the novel side, a "spatial neophobia." A second experiment confirmed the spatial neophobia effect and demonstrated that it was not dependent on the particular conditioning procedure used in the first experiment. The spatial neophobia effect was related to similar effects in the taste aversion literature, and to the results of research on lithium-induced decreases in exploratory behavior.     

Lesions of the amygdala, but not of the cerebellum or red nucleus, block conditioned fear as measured with the potentiated startle paradigm

Rats were given 10 light-shock pairings on 2 successive days. At 24-48 hr following training, groups of rats received bilateral transection of the cerebellar peduncles, bilateral lesions of the red nucleus (which receives most of the cerebellar efferents), or bilateral lesions of the central nucleus of the amygdala. Control rats were sham operated. At 3-4 days after surgery, the rats were tested for potentiated startle (increased acoustic startle in the presence of the light previously paired with shock). Potentiated startle was blocked by lesions of the central nucleus of the amygdala. Transection of the cerebellar peduncles or lesions of the red nucleus did not block potentiated startle. A second experiment in which a visual prepulse test was used indicated that the blockade of potentiated startle observed in the animals with amygdala lesions could not be attributed to optic tract damage. A third experiment demonstrated that the absence of potentiation in the animals with amygdala lesions was not simply due to a lowered startle level ceiling, because these animals could show increased startle with increased stimulus intensity and with administration of strychnine, a drug that increases startle. Taken together, the results are consistent with the hypothesis that the amygdala is involved in fear conditioning, because potentiated startle is a measure of conditioned fear.