Altered exploratory behaviour after 6-OHDA lesion to the dorsal noradrenergic bundle

Uncertainty, epistemics and active inference

Biological systems—like ourselves—are constantly faced with uncertainty. Despite noisy sensory data, and volatile environments, creatures appear to actively maintain their integrity. To account for this remarkable ability to make optimal decisions in the face of a capricious world, we propose a generative model that represents the beliefs an agent might possess about their own uncertainty. By simulating a noisy and volatile environment, we demonstrate how uncertainty influences optimal epistemic (visual) foraging. In our simulations, saccades were deployed less frequently to regions with a lower sensory precision, while a greater volatility led to a shorter inhibition of return. These simulations illustrate a principled explanation for some cardinal aspects of visual foraging—and allow us to propose a correspondence between the representation of uncertainty and ascending neuromodulatory systems, complementing that suggested by Yu & Dayan (Yu & Dayan 2005 Neuron46, 681–692. (doi:10.1016/j.neuron.2005.04.026)).

The role of the septo-hippocampal system and its noradrenergic afferents in behavioural responses to none-reward

Our experiments were designed with two purposes: (i) to examine the effects on one behaviour of differing interventions in the septo-hippocampal system; (ii) to compare these effects with those of minor tranquillizers. The behaviour studied (in rats) is extinction in the alley after continuous (CRF) or partial (PRF) reinforcement. Minor tranquillizers and large septal lesions produce three effects: (1) resistance to extinction is increased after CRF; (2) resistance to extinction is decreased after PRF; (3) the partial reinforcement extinction effect (PREE) is abolished. Small septal lesions fractionate this syndrome: either effect (1) or an actual increase in the size of the PREE is produced by medial septal lesions abolishing hippocampal theta; effects (2) and (3), but not (1), are produced by lateral septal lesions sparing theta. Dorso-medial fornix section, abolishing theta, reproduces the effects of medial septal lesions. Fimbrial section, sparing theta, reproduces some of the effects of lateral septal lesions. Minor tranquillizers produce a rise in the threshold for septal driving of hippocampal theta specifically at 7.7 Hz. This effect is reproduced by blockade of noradrenergic transmission or destruction of the dorsal noradrenergic bundle with 6-hydroxydopamine. This lesion reproduces all three behavioural changes listed above. These results suggest a model for the role of the septo-hippocampal system and its noradrenergic inputs in the PREE. This model is compared with other approaches to the septo-hippocampal system.

Depletion of brain norepinephrine does not reduce spontaneous ambulatory activity of rats in the home cage

6-Hydroxydopamine (6-OHDA) lesions of brain noradrenergic neurons and terminals were made in rats to assess the importance of forebrain norepinephrine (NE) for mediating circadian patterns of spontaneous ambulatory activity that rats show in the home cage. 6-OHDA was injected intracranially into the fibers of the ascending noradrenergic dorsal and ventral bundle pathways or infused into the lateral ventricle or both. Rats living in a 12/12 h light/dark cycle exhibit a marked increase in ambulatory activity during the dark period in comparison to the light period and a 'W-shaped' pattern of activity during the 12 h of the dark phase. Results showed that near-total depletion of brain NE did not impair the capacity to generate normal patterns of spontaneous ambulatory activity that occur in the home cage. In the animals that sustained the most complete NE lesions, the amounts of activity generated at times of peak activity were exaggerated in comparison to the control animals, which is consistent with the possibility that NE in the brain exerts a moderating influence on behavior.

Noradrenergic receptor mechanisms in neophobia

We have previously demonstrated that depletion of forebrain norepinephrine (NE) led to an attenuation of neophobia in a novel environment, as defined by a greater preference for novel food over familiar food. To study further the role of forebrain NE in neophobia we chronically infused noradrenergic receptor ligands or forskolin into the lateral ventricles of sham and 6-hydroxydopamine dorsal bundle lesioned rats. Chronic NE infusions into lesioned animals reversed the lesion-induced shift in relative food preference. The beta receptor agonist isoproterenol had moderate effects similar to those of NE in lesioned and sham animals. Phenylephrine, an alpha-1 agonist, was without effect. Forskolin, an adenylate cyclase activator, mimicked the effects of NE infusions. These data suggest a role for noradrenergic stimulation of adenylate cyclase in neophobia.

The neuroanatomical basis of anxiety

This review details the neural systems that are important in anxiety-related behaviours. In particular, the role of the amygdaloid complex, Papez circuit, septohippocampal formation and raphe nuclei are described and discussed. Evidence is gathered from a variety of experimental approaches. These include behavioural assessment of anxiety in animals after intracerebral injection of pharmacological agents and following lesions of discrete brain nuclei and selective neurotransmitter pathways. Further evidence is provided by functional brain mapping studies applied to animals and humans. It is proposed that the neural systems recruited in different experimental conditions of anxiety may differ, supporting the notion that clinical anxiety exists in several forms. This has implications for the identification of new anxiolytic treatments. In particular, the findings suggest that approaches aimed at identifying new anxiolytic agents must take into account both the distribution of receptors for the drug and the neuronal systems activated by the experimental protocol.

Lesions of the dorsal noradrenergic bundle simultaneously enhance and reduce responsivity to novelty in a food preference test

In conclusion, DNAB lesions have been shown to have two, apparently contradictory effects in a food preference test: to increase neophobia to a novel environment, and to increase the tendency to eat novel food in a novel environment. It has been suggested that these two effects occur because, although NA has a common action on neuronal firing in terminal fields, the dissociable consequences reflect the different functions of areas in receipt of these noradrenergic afferents. In addition, it has been shown that DNAB lesions, VNAB lesions, and a benzodiazepine, chlordiazepoxide, all have somewhat different behavioral effects in the food preference test. Taken together with the lack of correlation between the various behavioral measures used in these experiments, this suggests that neophobia does not reflect a single behavioral process, such as anxiety, or reactivity to novelty.

Substantia nigra lesions attenuate the development of hypertension and behavioural hyperreactivity in spontaneously hypertensive rats

The possible relation between changes in behaviour and the development of hypertension was investigated. Depletion of striatal dopamine by lesions in the substantia nigra of Spontaneously Hypertensive Rats (SHR) was associated with an inhibition of the development of hypertension. In the open field a decrease in rearing score was found with no effect on other parameters. Rearing activity was significantly correlated with blood pressure as well as with striatal dopamine content. Blood pressure was weakly, although significantly, correlated with striatal dopamine content. Neither blood pressure nor striatal dopamine content was significantly correlated with ambulation activity. In normotensive Wistar-Kyoto rats a decrease was also found in rearing activity after nigra lesions, although this effect was less pronounced. Antihypertensive treatment of SHR with captopril or hydralazine did neither affect striatal dopamine levels nor open-field behaviour. Induction of renal hypertension or DOCA-salt hypertension in Wistar rats did not influence brain dopamine or behaviour. The results support the suggestion that brain dopamine systems may play a role in the development of hypertension in SHR as well as in the changes in behaviour observed in these rats. Changes in behaviour do not appear to be mediated by changes in blood pressure per se.

Suppressant effect of REM sleep deprivation on neophobia in normal rats and in rats with selective DSP-4 induced damage of locus coeruleus neurons

The influence of REM sleep deprivation (REMD) on open field behavior of normal and locus coeruleus (LC)-damaged animals was investigated under the assumption that REMD suppresses neophobia in rats. REMD (for 24 or 72 hr, water tank technique) produced marked changes in behavior of rats encountering a novel object (white cube) in the center of the open field. REMD induced an increase in activity of treated rats; latency to the object approach was shorter, the number of center entries, time spent in object exploration, frequency of ambulation and rearing were significantly higher than in controls, also defecation was nearly abolished. LC-damage (using DSP-4, a selective central noradrenergic neurotoxin) induced neophobic-like reactions manifested by significantly prolonged latency, tendency to decreased object exploration, center entries and reduction of ambulation and rearing. This "neophobic" behavior of DSP-4 rats was counteracted by REMD as well as by subchronic, but not acute treatment, with antidepressant oxaprotiline (2 X daily for 8 days, 10 mg/kg, IP). The results provide strong support for antineophobic activity of REMD. In addition, they indicate possible similarity of REMD and subchronic oxaprotiline action on neophobia-like behavior in rats with damaged LC-neurons.

The role of noradrenaline in tuning and dopamine in switching between signals in the CNS

Neuronal catecholaminergic activity modulates central nervous function. Specifically noradrenaline can exert a tuning or biassing function whereby the signal to noise ratio is altered. Dopamine activity may promote switching between inputs and outputs of information to specific brain regions. It has been ten years since evidence for a tuning function was advanced for noradrenaline and in the last 5 years the switching hypothesis for dopamine has been tentatively put forward. Recent studies are reviewed to show that while catecholamine activity contributes to neural interactions in separate brain regions that give rise to the organization of different functions, their working principles may be common between species and independent of the nucleus of origin. Behavioral examples are discussed and an attempt is made to integrate this with evidence from intracellular recording studies. It is suggested that the tuning principle in noradrenergic systems is particularly important for the formation of associations and neural plasticity (interference control) and that the switching principle of dopaminergic systems modulates the timing, time-sharing and initiation of responses (program-control).

Manipulation of the amygdala noradrenergic system impairs extinction of passive avoidance

In a consolidation paradigm, noradrenergic manipulation of the rat amygdala results in disruption of extinction for passive avoidance. These results are discussed in light of a recent controversy concerning the role of central norepinephrine systems in general extinction behavior. The discussion is cast primarily within the framework of rodent natural life history, the presence of innate behavioral strategies, and the organization of information processing as an integration of experiential attributes.

Destruction of the locus coeruleus or the dorsal NE bundle does not alter the release of punished responding by ethanol and chlordiazepoxide

The hypothetical involvement of central noradrenergic projections in the manifestation of "anxiety" and the "anti-anxiety" effects of alcohol and benzodiazepines was tested in an operant conflict situation by examining the effects of destruction of this noradrenergic system on response rates. Rats were trained on a Geller-Seifter conflict test [9] modified for incremental shock [21], in which two food-reinforced lever press components were separated by a time-out. Responses during the "food-alone" component (RI) were reinforced on a random interval schedule-30 sec; responses during the "time-out" component were never reinforced; and responses during the conflict component (CONFLICT) were continuously reinforced with both food and shock. This shock strength increased incrementally with each successive shock during the conflict period. Each session consisted of two cycles of a 5 min RI period, a two min time out, and a two min CONFLICT period presented in succession. Both ethanol (0.5 to 1.0 g/kg) and chlordiazepoxide (5 and 10 mg/kg) produced a significant increase in punished responding during the CONFLICT component. Ethanol, but not chlordiazepoxide (CDP) also significantly decreased responding during the RI component. Virtual total destruction of the dorsal noradrenergic projection with 6-hydroxydopamine (6-OHDA), or destruction of the locus coeruleus itself, failed to significantly alter baseline responding or the release of punished responding to ethanol or CDP. These results do not support the hypothesis that the locus coeruleus projections have an essential role in "anxiety" or in the "anxiety-reducing" properties of ethanol or the benzodiazepines.

A comparative study of the behavioral effects of the locus coeruleus and the dorsal noradrenergic bundle lesions in the rat

The effects of bilateral lesions of the dorsal noradrenergic bundle (DB) or of the locus coeruleus (LC) on the rat's behavior in different anxiogenic behavioral situations have been studied. The DB rats defecate less but ambulate more than shams in the open-field (O.F.); these data suggest a decrease in the reactivity of these animals to novelty. Furthermore, the LC rats have a behavior identical to that of shams in the O.F. We note moreover that the DB rats do not habituate to a novel stimulus. During the Henderson test, the behavioral inhibition of all lesioned animals seems to be less important than that of shams. These results are discussed in relation to existing hypotheses of the DB function. The lesions of DB induce a decrease in the noradrenaline (NA) cortical level and in the catecholamines level in hypothalamus. The lesions of LC produce 30% loss of forebrain NA. These results reveal a discrepancy between the effects induced by the lesioning of DB fibers and those produced by lesions of LC, which originate in the DB. The presence of non-noradrenergic elements, fibers of passage, which do not travel with the DB fibers or terminals in the LC region may be elements for interpretation.

Effect of local 6-OHDA and 5,6-DHT injections into the rat olfactory bulb on neophobia and learned aversion to a novel food

This study investigated the role of the noradrenergic (NA) and the serotoninergic (5-HT) centrifugal fibers on the olfactory bulb (OB). For this purpose, behavioral responses largely depending on olfactory information were measured in rats after discrete bilateral injections into the OB with either 6-hydroxydopamine (6-OHDA) or 5,6-dihydroxytryptamine (5,6-DHT) solutions. The evaluation of the level of endogenous 5-HT in the OB was measured by high performance liquid chromatography. The NA fibers appeared lesioned after the 6-OHDA treatment only, whereas the 5-HT level was significantly reduced in 5,6-DHT treated bulbs only (34% of the control value). The animals were fed on usual stock-diet (S) and had the choice between S and a novel food (V) on neophobia test, and again on the following day after aversive conditioning (0.9 mEq/kg LiCl i.p.). The behavioral results showed that the drug administrations were followed by a significant effect in one case only: the intake of V was significantly enhanced in 6-OHDA treated animals during neophobia test. Thus, neophobia was reduced by 6-OHDA. The data are interpreted as follows: (1) the lesion of the bulbar NA or 5-HT fibers does not induce total anosmia; and (2) the NA innervation of the OB could play a role in the modulation of behavioral responses to biological odorants.

Behavioral effects of lesions of the central noradrenergic bundles in the rat

Rats with bilateral lesions of the dorsal noradrenergic bundle (DB) or of the ventral noradrenergic bundle (VB) were studied in different behavioral test situations. All lesioned animals defecate less than sham operated animals in the open-field (OF) or in the conditioning apparatus described by Henderson [16]. These data suggest a decrease of emotional reactivity in lesioned animals. However, the DB rats' level of exploration was higher than that of VB rats. No effect on the amplitude of the startle response has been shown after lesioning. The lesions of the dorsal noradrenergic bundle induce a decrease in cortical noradrenaline hypothalamic catecholamines. The lesions of the ventral noradrenergic bundle induce a decrease in hypothalamic catecholamines without change in the cortex. These results do not support the postulation [22] that the dorsal bundle and the ventral bundle play an opposite role in behavior. Yet, a selective participation of each bundle is suggested in modulating responses to novel environments and anxiogenic situations.

Evidence against a role of the rat's dorsal noradrenergic bundle in selective attention and place memory

In Experiment 1, 6-hydroxydopamine-induced lesions of the dorsal noradrenergic bundle (DNB) in rats did not impair either acquisition of non-delayed alternation, retention of non-delayed alternation, or performance of alternation with delays in a T-maze, whether or not the goal arms of the maze were visually distinctive. These results were in contrast with those of a previous report indicating that DNB lesions impair learning of spatial alternation. In Experiment 2, the lack of a reliable effect of DNB lesions on learning performance of spatial alternation was confirmed. However, the rats with DNB lesions showed an impairment of spontaneous alternation. The negative results of the present study do not support either the hypothesis that depletion of forebrain noradrenaline impairs selective attention or the hypothesis that such depletion induces an amnesia for past places. On the other hand, the finding of impaired spontaneous alternation is consistent with previous observations suggesting that depletion of forebrain noradrenaline impairs habituation of fear reactions.

T-maze learning, spontaneous activity and food intake recovery following systemic administration of the noradrenaline neurotoxin, DSP4

Following systemic administration of the noradrenaline (NA) neurotoxin, DSP4 (50 mg/kg), rats were found to be retarded in the rate at which they acquired the "right-turn" running response in a modified T-maze choice situation, as measured by the total number of errors per session and median latency to reach the goal box. Desipramine (DMI, 20 mg/kg), injected 30 min before DSP4 blocked the acquisition retardation. DSP4 was found to have a short-lasting effect upon spontaneous motor activity, while food and water intake recovery was complete within 7 days of the injection. Both the NA-accumulation data and endogenous NA concentrations indicated profound NA, but not 5-hydroxytryptamine (5-HT) and dopamine (DA), depletions in the cortex, hippocampus and cerebellum. These data seem to confirm the role of the locus coeruleus-noradrenaline (LC-NA) system in an instrumental learning situation.

The role of the ventral noradrenergic bundle in relation to endorphins in the control of core temperature, open-field and ingestive behaviour in the rat

Discrete, bilateral, radiofrequency destruction of the ventral noradrenergic bundle (VB) resulted in a pronounced fall in levels of noradrenaline in the hypothalamus but not in the cortex. On days 4 and 12, but not 28, post-surgery, VB-lesioned rats were hyperactive (rearing and ambulation) upon exposure to a novel open-field space. This hyperactivity was greatly attenuated by naloxone, which did not significantly modify sham activity. These data suggest that the VB may be involved in the control of locomotor-exploratory activity via an interaction with an endorphinergic system. On day 4, but not 12 or 25, VB-lesioned rats displayed a significant elevation in core temperature (Tc). No difference in the hyperthermia elicited by introduction into the open-field was, however, seen between VB-lesioned and sham rats on day 4. In both groups, this rise in Tc was strongly attenuated by naloxone. These data indicate that the VB may be involved in the control of Tc but that it does not mediate novelty-stress evoked hyperthermia, for which endorphins are primarily responsible. Within 7 days post-surgery, VB-lesioned rats developed an enhancement of daily food intake which led to a slight obesity. From day 15 onward, a hyperdipsia was also seen in VB-lesioned rats. Naltrexone reduced the food and water intake of both sham and VB-lesioned animals but failed to totally block this hyperphagia. It is suggested that the VB is involved in the regulation of daily ingestive behaviour and that endorphins do not exclusively mediate the VB-lesion induced hyperphagia.

Failure to replicate the dorsal bundle extinction effect: telencephalic norepinephrine depletion does not reliably increase resistance to extinction but does augment gustatory neophobia

Depletion of telencephalic noradrenaline (Ne), caused by lesion of the dorsal tegmental bundle, has been reported to increase persistence of non-reinforced responding in various operant tasks. This has been referred to as the dorsal bundle extinction effect (DBEE). In an effort to reproduce this effect, rats receiving 6-hydroxydopamine lesions of the dorsal Ne bundle (DB-6-OHDA) were compared to controls during extinction of a continuous food rewarded (CRF) lever-press response. While the lesion group showed an increase in responding during initial extinction, no significant difference in resistance to extinction using a 2-min non-response criterion was obtained. Moreover, no differences in reinforced response rates were observed with CRF, fixed ratio (FR-15, FR-30, FR-60) or variable interval (VI-30, VI-60, VI-120 s) schedules of reinforcement. In order to test the hypothesis that the DBEE is dependent on time of behavioral testing after surgery, subsequent experiments were performed where rats began CRF operant training 5, 17, 31 or 110 days post-lesion. No differences in resistance to extinction were observed between lesion and control rats at any post-lesion interval. Neonatal treatment with 6-OHDA which permanently lesions forebrain Ne terminals also failed to prolong extinction. Finally, when both DB-6-OHDA and neonatal rats were given a choice between water and saccharin the lesioned animals exhibited a neophobic reaction whereby they drank significantly less saccharin. We conclude that while the DBEE is not a reliably reproducible phenomenon other effects of forebrain Ne lesions, such as neophobia, appear to be robust.

Enhanced neophobia but normal plasma corticosterone levels in rats with dorsal noradrenergic bundle lesions

The effects of dorsal noradrenergic bundle (DNB) lesions on plasma corticosterone levels were determined in male albino rats. DNB lesions did not affect baseline plasma corticosterone levels. Furthermore, the increased corticosterone levels produced by various environmental manipulations did not differ between control and DNB lesioned groups. However, the lesioned group did exhibit longer latencies to eat familiar food in novel environments, as well as to eat novel foods in a familiar environment. Latencies to eat novel food in novel environments did not differ between the two groups and this was attributed to a "ceiling" effect. These endocrinological data fail to support the hypothesis that the enhanced "neophobia" observed in DNB lesioned rats is due to an increase in the intensity of the emotional reaction to novel stimuli. The data do not preclude the possibility, however, that the enhanced neophobic reactions reflect impaired habituation to these stimuli.

Lesions of the dorsal noradrenergic bundle and rewarded running: the role of pretraining

Local injection of 6-hydroxydopamine was used to selectively destroy the dorsal ascending noradrenergic bundle (DB), producing 75% loss of hippocampal noradrenaline. Lesioned and control rats were trained to run in a straight alley for food reward with or without pretraining (handling and habituation to the apparatus). Lesioned rats ran more slowly than controls only if they had not been pretrained. This result may explain previous discrepancies in the literature; it is discussed in relation to existing hypotheses of DB function.

Behavior during hippocampal microinfusions. I. Norepinephrine and diversive exploration

Adult male Sprague-Dawley rats were surgically implanted with guide cannulas in the anterodorsal hippocampal formation. After recovery from surgery they were administered bilateral infusions into the dentate gyrus of D,L-norepinephrone (NE) at the rate of 0.025 microliter/min throughout a 40-min session in a holeboard/activity apparatus. The computerized holeboard system measured the animals' locomotor activity, stimulus responsivity, and response to novelty, and permitted the reconstruction and analysis of their sequential patterns of movement. The NE infusions failed to affect overall locomotion or holepoking, although group means were slightly elevated. Rearing frequency and duration were significantly increased, as were the number of different holes poked per 5-min epoch and the amount of time in and number of entries into the center region of the holeboard. The novel object reaction of NE-infused rats, measured as the increase in poke duration into holes with novel stimuli, was indistinguishable from that of saline-infused rats. The NE-infused rats exhibited more varied, widespread spatial distributions of sequential patterns of locomotor activity. A dose of the beta-adrenergic antagonist propranolol that was minimally effective when infused by itself blocked all of NE's affects when co-infused in the same solution. Infusions of the noradrenergic releasing agent tyramine mimicked NE's actions, whereas infusions of NE's relatively inactive stereoisomer D-NE or infusions into the overlying lateral ventrical failed to do so. Histological examination of dye-infused brains and microspectrofluorimetry of NE-infused brains treated using the Falck-Hillarp technique for the formaldehyde-induced fluorescence of monoamines indicated that spread of the infusate was confined to the dentate gyrus of the anterodorsal hippocampal formation. The behavioral profile of the NE-infused rats suggests a role for the noradrenergic input to the hippocampal formation in spontaneous environmental reconnaissance and the diversification of stimulus sampling-'diversive' exploration, as opposed to the inspection or 'specific' exploration of unfamiliar stimuli.

The effect of DSP-4 on some positively reinforced operant behaviors in the rat

The effects of a new neurotoxin for noradrenergic (NA) neurons, DSP-4, on the acquisition, retention and extinction of positively reinforced operant responses were studied in rats. The acquisition of an L-shaped runway task for food reward, of a simultaneous light-dark discrimination in a Y-maze task and of a lever press response for food was not affected in DSP-4 treated animals. No deficit in retention behavior was found in the Y-maze task after DSP-4 treatment. The DSP-4 treated animals showed increased resistance to extinction in the L-shaped runway. However, the extinction of the lever press response for food (CRF) was unaffected in the same animals. In accordance with previous results, DSP-4 resulted in a widespread depletion of NA in cortex, hippocampus, cerebellum and spinal cord. One week after the DSP-4 administration, histochemical studies at the light microscopic level indicated that the cell bodies of the locus coeruleus remained intact in DSP-4 treated animals. Desipramine pretreatment provided only partial protection from the neurotoxic effects of DSP-4. The results are discussed in relation to the mechanism by which DSP-4 produced the long-term depletion of central NA ans with reference to the role of central NA neurons in behavior.

Conspecific odor preferences of male albino rats are reversed by intracerebral 6-hydroxydopamine

The odor of pine shavings from the nest of a female and her litter attracted sexually experienced male Sprague-Dawley rats tested in a two-choice situation. The preference persisted after surgery in controls treated with bilateral intracerebral injections of saline-ascorbic vehicle into the vicinity of the ascending noradrenergic bundle. In contrast, 7 out of 8 animals receiving bilateral injections of the neurotoxin 6-hydroxydopamine (6-OHDA), preferred the odor of clean pine to pine nest odor after surgery, indicating preference reversal. 6-hydroxydopamine also reduced olfactory cortex norepinephrine (NE) concentrations by 85%. Pretreatment with intracerebral injection of amphetamine, a catecholamine uptake inhibitor and releaser, prevented 6-OHDA-induced preference reversal in 7 out of 8 animals and limited NE reduction to 38% of concentrations measured in amphetamine-pretreated vehicle-injected controls. The data implicate central NE in the modulation of responses to conspecific odors.

Mesolimbicocortical dopamine terminal fields are necessary for normal locomotor and investigatory exploration in rats

Rats explore a novel open field or novel object less after denervation of mesolimbicocortical dopaminergic terminal fields produced by bilateral 6-hydroxydopamine (6-OHDA) microinjections into the anterolateral hypothalamus after pretreatment with desmethylimipramine (DMI). These behavioral deficits were correlated with complete or nearly complete loss of fluorescent dopaminergic (DA) terminals in the nucleus accumbens, olfactory tubercle, dorsal bed nucleus of the stria terminalis, lateral septal nucleus and the deep layers of the frontal and piriform cortices. There were also fewer A10, medial A9, and A8 DA fluorescent cells after the 6-OHDA-DMI injections; this suggests retrograde degeneration of the cells of origin of the mesolimbicocortical DA system. When the DMI pretreatment was omitted, identical bilateral 6-OHDA microinjections also produced severe loss of norepinephrine (NE) fibers in the neocortex, hippocampus, lateral hypothalamus and ventral bed nucleus of the stria terminalis. The addition of this noradrenergic damage did not change the exploratory deficits observed after mesolimbicocortical DA denervation alone. Systemic administration of the DA agonist apomorphine, but not the adrenergic agonist clonidine, to the 6-OHDA-DMI rats repaired the deficits in exploration of a novel open field or novel object. The increased locomotion in a novel open field and investigation of a novel object produced by apomorphine in 6-OHDA-DMI rats were blocked by the DA antagonist, pimozide. This is evidence that apomorphine restored exploratory responses by stimulating dopaminergic receptors. The exploratory responses produced by apomorphine were also blocked by testing rats in a familiar open field or with a familiar novel object. This is evidence that apomorphine facilitates exploratory responding to novel stimuli by 6-OHDA-DMI rats, but that the same dose of apomorphine does not increase activity when 6-OHDA-DMI rats are confronted by familiar stimuli. We conclude: (1) that mesolimbicocortical dopaminergic terminals are necessary for normal exploratory behavior in rats; and (2) that DA released by these terminals may facilitate optimal sensorimotor integration in these terminal fields during spontaneous exploratory behavior.

Dorsal bundle extinction effect: motivation or attention?

Male albino Wistar rats were injected bilaterally with 4 micrograms of 6-hydroxydopamine into the dorsal noradrenergic bundle to deplete forebrain noradrenaline to less than 5% of control values. Acquisition learning of a fixed interval schedule or a continuously reinforced schedule was not altered but resistance to extinction was seen after food reinforced training on either schedule but not after water reinforced training. A possible increase in food motivation was tested by the use of preloading with free food prior to a fixed interval session but both control and lesioned rats reacted similarly to this manipulation thus appearing to exclude an increase in food motivation. An attentional explanation is proposed and tested by the demonstration that resistance to extinction does not occur after a partially (variable ratio 4), as opposed to a continuously, reinforced schedule. Further evidence in favour of an attentional mechanism comes from the finding that on both a fixed interval and a continuously reinforced schedule the lesion has to be present during the acquisition phase to result in subsequent resistance to extinction. Intact animals trained on either schedule and subsequently subjected to the lesion failed to show an increased resistance to extinction.

Theories of the dorsal bundle extinction effect

Selective destruction of the noradrenaline systems in the rat brain using the neurotoxin 6-hydroxydopamine has been found to cause resistance to extinction in a number of behavioural situations. Several theories concerning the behavioural mechanism altered by the lesion, and hence about the role of noradrenaline in normal brain functioning, are proposed and evaluated. Theories suggesting a role for noradrenaline in activity, perseveration, internal inhibition, frustrative non-reward, motivation, or secondary reinforcement, fail to explain all the available evidence and direct tests of each theory fails to support its predictions. A model which suggests that noreadrenaline is involved in attentional behaviour, specifically in filtering out or learning to ignore irrelevant environmental stimuli, is successful in explaining all available data and direct tests of the lesioned rats' attentional capacity serve to confirm many of the predictions of an attentional theory of the dorsal bundle extinction effect.

The effect of bilateral ventral noradrenaline bundle lesions on lever pressing for food in rats

Food reward has been associated with activation of noradrenergic mechanisms in the brain. Using rats trained to press a lever for food reward, we have investigated the effects of 6-hydroxydopamine lesions, which severly depleted hypothalamic noradrenaline, on the willingness of the rats to press the lever for food reward. We found that performance in the food-rewarded task was significantly impaired following such lesions, and that this was especially marked when the task was made more difficult. From our results we suggest that ventral noradrenaline bundle lesions can decrease the rewarding nature of food, thus making the animals less willing to work for food reward.

The dorsal bundle extinction effect:dependence on subtle changes in acquisition

Destruction of the ascending noradrenergic innervation to the forebrain in rats by intracerebral injection of the selective neurotoxin 6-hydroxydopamine (4 microgram in 2 microliter injected bilaterally into the dorsal bundle in the mesencephalon) was found to cause resistance to extinction of a continuously reinforced lever press response. However, this effect occurred only if the lesion were present during acquisition training on the reinforced schedule and not if intact animals were trained and the lesion inflicted after completion of acquisition training and just prior to the extinction phase. Thus, the behavioural effect that manifests itself during extinction appears to be due to subtle changes in the acquisition learning process. This is consistent with the predictions of an attentional theory of noradrenergic function and appears to exclude most other suggested explanations of the dorsal bundle extinction effect.

Neurochemical basis of the dorsal bundle extinction effect

Injection of 6-hydroxydopamine into the mesencephalon of the rat has been found to cause resistance to extinction on continuously reinforced schedules. The neurochemical basis of this effect was investigated by using another concentration of 6-hydroxydopamine and by another position of injection. Severe depletion of forebrain noradrenaline was found after these injections with no change in dopamine, serotonin, cholinergic or GABAergic parameters in any brain area measured. The noradrenergic nature of the effect was further shown by the reversal fo the usual behavioural effect following pretreatment with a noradrenaline uptake inhibitor (desimipramine, 25 mg/kg 30 min prior to intracerebral injection of 6-hydroxydopamine). This rules out non-specific damage caused by the 6-hydroxydopamine as the neurochemical basis of the dorsal bundle extinction effect. Failure to find resistance to extinction after either kainic acid or 5-7 dihydroxytryptamine injection seems also to exclude respectively cell body loss at the injection site or damage to serotonergic systems. It is concluded that the dorsal bundle extinction effect is noradrenergic in nature.

Effects of lesions in the ventral noradrenergic bundle on behavior and response to psychotropic drugs in rats

Bilateral lesions of the ventral noradrenergic bundle (VB) decreased concentration of noradrenaline within the mesendiencephalon but not in the cortex. Lesioned rats showed increased activity measured in the open field test. Cataloptogenic effects of chlorpromazine and haloperidol were almost completely abolished in VB-lesioned animals. The stereotypy induced by both--amphetamine and apomorphine was, however, unchanged. It is supposed that lesions of the VB lead to increased activity in dopaminergic neurons in the brain.

The effects of dorsal bundle injections of 6-hydroxydopamine on avoidance responding in rats

1 The effect of injection of 6-hydroxydopamine (6-OHDA) into the fibres of the dorsal noradrenergic bundle on acquisition, retention and extinction of active avoidance in rats were examined. 2 6-OHDA injections severely depleted noradrenaline in all forebrain areas assayed, with the interesting exception of the septum. No significant effect on dopamine concentrations in various forebrain regions was found. 3 Acquisition and retention of active avoidance was not altered by the lesion. Marked resistance to extinction was seen when the unconditioned stimulus (shock) was removed. 4 A comparison with work by other authors in which both forebrain noradrenaline and dopamine were depleted suggest that the alteration in extinction seen in both studies is a noradrenergic effect, whereas the deficits in acquisition and retention found previously are dopaminergic in origin.

The locus coeruleus noradrenergic system--evidence against a role in attention, habituation, anxiety and motor activity

Lesions of the locus coeruleus system were induced by combined stereotaxic injections of 6-OH-dopamine to the ascending fibres and just lateral to the locus coeruleus itself, to deplete the noradrenaline content of both the cerebral and cerebellar cortices. A group of rats with cortical noradrenaline concentrations of less than 30 ng/g were compared with a group with lesser destruction of the system (mean noradrenaline concentration 71 +/- 44 ng/g) and with controls (mean noradrenaline 347 +/- 58 ng/g). Lesioned rats showed normal motor activity and exploration (assessed with a holeboard) and showed normal habituation of these behaviours. The lesioned rats gave no evidence of increased susceptibility to distracting auditory stimuli whilst licking for water, and the groups did not differ in their rate of habituation to these stimuli, or in dishabituation. In a social interaction test, lesioned animals showed a decrease in social contacts in an unfamiliar situation (interpreted as a response to anxiety) of similar magnitude to that seen in the control group. In this test, lesioned animals engaged in more 'aggressive' behaviour (boxing and wrestling) than did the controls. These findings are incompatible with hypotheses that the locus coeruleus system is an integral part of the physiological mechanisms which control gross motor behaviour, attention, habituation or anxiety. Together with previous findings with the benzodiazepines, the results with the social interaction test make it unlikely that the benzodiazepines exert their anxiolytic effects by inhibiting the locus coeruleus system.

Parameters of the dorsal bundle extinction effect: previous extinction experience

Lesion to the dorsal noradrenergic bundle using the selective neurotoxin 6-hydroxydopamine which depleted telencephalic noradrenaline to less than 5% of control values was found to cause prolonged responding in extinction of a continuously reinforced (CRF) operant lever pressing response (a further replication of the dorsal bundle extinction effect). The parameters involved in causing this effect). The parameters involved in causing this effect were investigated, particularly the role of previous extinction experience. Although resistance to extinction was seen the first time animals were placed in extinction, it disappeared when they were retrained on CRF and extinguished a second time. Experience of extinction as part of the acquisition process, brought about by training on a successive visual discrimination, also prevented the development of over-responding in subsequent extinction. These results are discussed in the context of related demonstrations of the absence of the dorsal bundle extinction effect after partially reinforced acquistion training.