Behavioral effects of local injection of 6-hydroxydopamine in the medial ventral tegmentum in the rat. Possible role of the mesolimbic dopamingergic system

Emotional memory impairments induced by AAV-mediated overexpression of human α-synuclein in dopaminergic neurons of the ventral tegmental area

Parkinson's disease (PD) is associated with extensive degeneration of dopaminergic neurons originating in the substantia nigra pars compacta, but neuronal loss is also found in the ventral tegmental area (VTA). The VTA projects to areas involved in cognitive and emotional processes, including hippocampus, amygdala, nucleus accumbens and prefrontal cortex, and has thus been proposed to play a role in emotional memory impairments in PD. Since the formation of α-synuclein inclusions throughout the central nervous system is a pathological hallmark of PD, we studied the progressive effects of α-synuclein overexpression in the VTA on motor functions, emotional behaviour and emotional memory. Adeno-associated viral (AAV) vectors encoding either human α-synuclein or green fluorescent protein (GFP) were injected stereotactically into the VTA, and behaviour was monitored 3 and 8 weeks following AAV injection. At week 8, there was a 22% reduction of TH+ neurons in the VTA. We demonstrate that α-synuclein overexpression in dopaminergic neurons of the VTA induced mild motor deficits that appeared 3 weeks following AAV-α-synuclein injection and were aggravated at week 8. No depressive- or anxiety-like behaviours were found. To address emotional memory, we used the passive avoidance test, a one-trial associative learning paradigm based on contextual conditioning which requires minimal training. Interestingly, emotional memory impairments were found in α-synuclein overexpressing animals at week 8. These findings indicate that α-synuclein overexpression induces progressive memory impairments likely caused by a loss of function of mesolimbic dopaminergic projections.

Animal models of ADHD

Studies employing animal models of attention-deficit/hyperactivity disorder (ADHD) present clear inherent advantages over human studies. Animal models are invaluable tools for the study of underlying neurochemical, neuropathological and genetic alterations that cause ADHD, because they allow relatively fast, rigorous hypothesis testing and invasive manipulations as well as selective breeding. Moreover, especially for ADHD, animal models with good predictive validity would allow the assessment of potential new therapeutics. In this chapter, we describe and comment on the most frequently used animal models of ADHD that have been created by genetic, neurochemical and physical alterations in rodents. We then discuss that an emerging and promising direction of the field is the analysis of individual behavioural differences among a normal population of animals. Subjects presenting extreme characteristics related to ADHD can be studied, thereby avoiding some of the problems that are found in other models, such as functional recovery and unnecessary assumptions about aetiology. This approach is justified by the theoretical need to consider human ADHD as the extreme part of a spectrum of characteristics that are distributed normally in the general population, as opposed to the predominant view of ADHD as a separate pathological category.

Akathisia after mild traumatic head injury

The authors describe the case of a 13-year-old boy who exhibited progressive disabling motor restlessness, torticollis, urinary symptoms, and confusion following a fall from a bicycle. The differential diagnosis of this striking symptom complex in this clinical context can be problematic. In this case, the symptoms ultimately appeared most consistent with severe akathisia resulting from a single administration of haloperidol used at an outside hospital to sedate the patient prior to a head CT scan. The literature on akathisia in pediatric patients, and especially in patients following acute head injury, is reviewed, with suggestions for an approach to these symptoms in this clinical setting.

The restless legs syndrome

The restless legs syndrome (RLS) is one of the commonest neurological sensorimotor disorders at least in the Western countries and is often associated with periodic limb movements (PLM) during sleep leading to severe insomnia. However, it remains largely underdiagnosed and its underlying pathogenesis is presently unknown. Women are more affected than men and early-onset disease is associated with familial cases. A genetic origin has been suggested but the mode of inheritance is unknown. Secondary causes of RLS may share a common underlying pathophysiology implicating iron deficiency or misuse. The excellent response to dopaminegic drugs points to a central role of dopamine in the pathophysiology of RLS. Iron may also represent a primary factor in the development of RLS, as suggested by recent pathological and brain imaging studies. However, the way dopamine and iron, and probably other compounds, interact to generate the circadian pattern in the occurrence of RLS and PLM symptoms remains unknown. The same is also the case for the level of interaction of the two compounds within the central nervous system (CNS). Recent electrophysiological and animals studies suggest that complex spinal mechanisms are involved in the generation of RLS and PLM symptomatology. Dopamine modulation of spinal reflexes through dopamine D3 receptors was recently highlighted in animal models. The present review suggests that RLS is a complex disorder that may result from a complex dysfunction of interacting neuronal networks at one or several levels of the CNS and involving numerous neurotransmitter systems.

Animal models of acute drug-induced akathisia - a review

Akathisia is a complex neurobehavioural side effect of neuroleptics and some other drugs which is characterised by subjective report and objective manifestations of restlessness. Its pathophysiology is poorly understood and there are many limitations to its investigation in humans. This paper reviews the various attempts that have been made in modelling acute akathisia in animals. Homologous as well as isomorphic models have been attempted, but most models are partial as they reproduce either the subjective or the objective features of the syndrome. None of the available models has been fully validated. Neuroleptic-induced defecation in the rat, even though constrained by a lack of symptom similarity and thereby face validity, has been most studied as a model of subjective akathisia. Rat models of restlessness, in particular those involving the use of serotonergic drugs or lesions of the ventral tegmentum or medial prefrontal cortex, are interesting partial models that should be further investigated. Neuroleptic-induced akathisia is observed in primates and has been modelled in dogs, and these should be studied further for their validation. It is also necessary to consider the subtypes of akathisia in the attempts to develop these models.

Electrophysiological characterization of GABAergic neurons in the ventral tegmental area

GABAergic neurons in the ventral tegmental area (VTA) play a primary role in local inhibition of mesocorticolimbic dopamine (DA) neurons but are not physiologically or anatomically well characterized. We used in vivo extracellular and intracellular recordings in the rat VTA to identify a homogeneous population of neurons that were distinguished from DA neurons by their rapid-firing, nonbursting activity (19.1 +/- 1.4 Hz), short-duration action potentials (310 +/- 10 microseconds), EPSP-dependent spontaneous spikes, and lack of spike accommodation to depolarizing current pulses. These non-DA neurons were activated both antidromically and orthodromically by stimulation of the internal capsule (IC; conduction velocity, 2.4 +/- 0.2 m/sec; refractory period, 0.6 +/- 0.1 msec) and were inhibited by stimulation of the nucleus accumbens septi (NAcc). Their firing rate was moderately reduced, and their IC-driven activity was suppressed by microelectrophoretic application or systemic administration of NMDA receptor antagonists. VTA non-DA neurons were recorded intracellularly and showed relatively depolarized resting membrane potentials (-61.9 +/- 1.8 mV) and small action potentials (68.3 +/- 2.1 mV). They were injected with neurobiotin and shown by light microscopic immunocytochemistry to be multipolar cells and by electron microscopy to contain GABA but not the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Neurobiotin-filled dendrites containing GABA received asymmetric excitatory-type synapses from unlabeled terminals and symmetric synapses from terminals that also contained GABA. These findings indicate that VTA non-DA neurons are GABAergic, project to the cortex, and are controlled, in part, by a physiologically relevant NMDA receptor-mediated input from cortical structures and by GABAergic inhibition.

Evidence for N-methyl-D-aspartate receptor mediation of cocaine induced corticosterone release and cocaine conditioned stimulant effects

The role of the N-methyl-D-aspartate (NMDA) receptors in cocaine conditioning and sensitization of locomotor activity was studied in four groups of Sprague-Dawley rats. A sub-motoric dose of the NMDA antagonist MK-801 (0.1 mg/kg, i.p.) was employed using a novel dual-compartment Pavlovian drug conditioning paradigm. The animals were placed sequentially in two different test environments in which locomotor activity was monitored. In the first compartment, the animals always received a non-drug test for 20 min. Upon completion of this test, the animals received either saline, cocaine (10 mg/kg i.p.), MK-801 or MK-801 plus cocaine depending on group assignment and were then placed immediately into the second compartment and again tested for 20 min. A total of six non-drug and six drug tests were conducted every other day over a 12-day period. Across all drug/saline treatment and post-treatment tests for conditioning, there were no statistical differences in locomotor activity among the saline and drug treatment groups in the non-drug test environment. In the drug/saline associated environment, however, cocaine had a reliable stimulant effect on locomotion when administered alone or in combination with MK-801. Following a 1-day and again after 21-days of withdrawal, all animals were administered a non-drug test for conditioning in which no injections were administered. On both tests, all groups had equivalent activity levels in the non-drug environment. In the drug/saline environment, only the cocaine group of the three drug treatment groups exhibited conditioned hyperlocomotion. Importantly, MK-801 blocked conditioned hyperlocomotion in the combined cocaine+MK-801 group. MK-801 did not alter serum or brain cocaine concentration or the cocaine effects on dopamine metabolism in limbic brain tissue. The co-administration of MK-801 with cocaine, however, blocked the corticosterone release effect of cocaine. Thus, the NMDA receptor site appears critical for cocaine induced conditioning and for corticosterone release.

Ethanol acts via the ventral tegmental area to influence hippocampal physiology

Ethanol selectively alters hippocampal dentate physiology, in part by increasing recurrent inhibition and suppressing long-term potentiation (LTP), a result of ethanol modulation of subcortical inputs. One of these inputs includes the ventral tegmental area (VTA) in the midbrain, whose neurons have been shown to discharge faster following systemic ethanol. To further understand how subcortical inputs regulate hippocampal physiology and their modulation by ethanol, we studied the effects of acute intoxicating levels of ethanol on VTA facilitation of the perforant path to dentate (PPD) responses. Furthermore, to test the role of the VTA on known pharmacological effects of ethanol on hippocampal physiology, we studied the effects of disruption of the VTA-dentate inpute on ethanol actions on recurrent inhibition. Stimulation of the perforant path produced well-characterized evoked responses in the ipsilateral dentate gyrus. Whereas VTA stimulation had no effect on PPD population EPSPs, VTA conditioning markedly increased perforant path-evoked PS amplitudes (140%). The maximum facilitation was observed at VTA conditioning intervals of 30-40 ms. PS amplitudes returned to baseline levels immediately following cessation of VTA conditioning. Intraperitoneal injections of ethanol (1.2 g/kg) markedly decreased VTA facilitation of PPD PS amplitudes. Lesions of the VTA blocked the ethanol-mediated increase in PPD paired-pulse inhibition. These results demonstrate that, to a great extent, the effects of intoxicating doses of ethanol on hippocampal physiology are mediated by remote pharmacological effects on the ventral tegmental area, whose direct or indirect influences on dentate physiology are described.

Unilateral damage to the ventral tegmental area facilitates feeding induced by stimulation of the contralateral ventral tegmental area

Unilateral lesions of the ventral tegmental area (VTA) facilitated feeding induced by electrical stimulation of the homologous VTA tissue in the contralateral hemisphere. The lesions shifted the function relating latency to begin feeding to stimulation frequency to the left simultaneously causing a reduction of frequency threshold for feeding reaction. Facilitation of feeding was immediate (with a peak on the 2nd postlesion day) and in some animals persisted up to the end of the 2-week experimental period. No facilitation of VTA stimulation-induced feeding was found in the control animals in which comparable lesions were performed in the contralateral lateral hypothalamus or the antero-dorsal thalamus which suggests that the effect was site specific. Individual differences in the magnitude and duration of the facilitatory effect on feeding may be related to the variability in the medio-lateral localization of the lesions. The results are interpreted in terms of compensatory increase in the dopaminergic transmission and/or decrease of the GABA-ergic inhibitory tone in the contralateral hemisphere after unilateral lesion to the mesencephalic dopaminergic systems. A possible involvement of the noradrenergic transmission is also discussed.

Differential temporal dynamics of serum and brain cocaine: relationship to behavioral, neuroendocrine and neurochemical cocaine induced responses

Rats administered cocaine 10 mg/kg i.p. exhibited hyperlocomotion which occurred within 5 min post injection. Subsequently, rats were injected with 10 mg/kg cocaine and brain and serum concentrations of cocaine were measured at 5, 10, and 20 min post injection. Within this time frame, cocaine concentration in limbic brain tissue was maximal at 5 min post injection and then declined substantially by 20 min post injection. In contrast, serum cocaine concentration increased from 5 to 20 min post injection. Neurochemical effects of cocaine upon limbic dopamine turnover and plasma corticosterone had a time course similar to serum cocaine. Brain cocaine concentrations paralleled the locomotor stimulant cocaine response whereas cocaine neuroendocrine effects paralleled serum cocaine concentrations. These findings point to the importance of brain cocaine concentration determinations in neurobehavioral studies of cocaine.

Apomorphine sensitization effects: evidence for environmentally contingent behavioral reorganization processes

Apomorphine-induced behavioral sensitization was investigated with a Pavlovian conditioning paradigm. Rats were administered apomorphine (2.0 mg/kg SC) daily for 7 days either paired or unpaired with a 10-min test environment placement. Initially, apomorphine induced hypolocomotion, but by treatment day 5, hyperlocomotion developed. Utilizing a videoimage analysis program which quantitated angular movement, it was determined that the increase in locomotion induced by repeated apomorphine treatment was due to an increase in rotational locomotion. Critically, rotation per se did not increase, but rather wide angle rotation toward the periphery of the test environment increased. Furthermore, a directional bias of rotation developed and stabilized which was unrelated to the animal's initial asymmetry bias. This emergence of a new locomotion pattern in conjunction with hyperlocomotion pointed to the need to reconceptualize behavioral sensitization phenomena into a new framework consistent with a progressive change in behavioral structure. Behavioral reorganization is presented as an alternative formulation to that of behavioral sensitization, as a drug-environment interactive process which is more compatible with the behavioral dynamics that emerge with repeated intermittent dopaminergic psychostimulant drug treatment.

Influence of ventral tegmental area (A10 region) on flight behaviour elicited by hypothalamic stimulation in the cat

The influence of the ventral tegmental area (VTA) (dopaminergic A10 group neurons) on flight behaviour, induced by hypothalamic stimulation, was studied in the cat. Co-stimulation of hypothalamus and VTA (ipsi- or contralateral) induced an increase of the flight latency. Slow-motion analysis of flight behaviour showed that this increase was due to the augmentation of the fixation latency (FL) whereas the upright latency (UL) was not modified. Sulpiride injection (50 mg/kg i.p.) provoked the disappearance of VTA effect without affecting the basal behavioural sequence. The results show that DA A10 group neurons increase the attentive component of the flight reaction, suggesting a possible influence of the DA A10 system on the mechanisms modulating focused attention in the animal.

Effects of intracerebral dopaminergic grafts on behavioural deficits induced by neonatal 6-hydroxydopamine lesions of the mesotelencephalic dopaminergic pathway

The functional capabilities of dopamine neuron-rich grafts implanted into the accumbens and striatal regions in neonatal rats were evaluated in a series of behavioural tests. The ascending mesotelencephalic dopaminergic system of three-day-old rat pups was bilaterally lesioned by injecting 6-hydroxydopamine at the level of the lateral hypothalamus. Five days later a suspension containing dopaminergic neurons obtained from embryonic day 14 mesencephali was injected bilaterally into the striatal complex. The functional effects of such grafts were evaluated using behavioural tests for which it was known that the performance of the animals is changed following the lesion of the mesotelencephalic pathway and for which the influence of dopaminergic grafts implanted into adult hosts have previously been described. The dopamine-rich grafts compensated for the modifications of the locomotor responsiveness to amphetamine and apomorphine induced by neonatal dopamine depletion. However, the grafts were unable to restore more complex behaviours such as hoarding for food pellets, schedule-induced polydipsia and learning behaviours. Moreover, the neonatal transplants induced additional deficits such as catalepsia, nocturnal hyperactivity and day-time hyperactivity during food deprivation. It was concluded that, at least in the present paradigm, the implantation into neonatal brain does not lead to any greater functional recovery than that observed after implantation during adulthood.

Effects of mediodorsalis thalamic nucleus lesions on vigilance and attentive behaviour in cats

In this study performed on 10 cats we analysed the effects of limited lesions of nucleus medialis dorsalis of the thalamus (MD) on behaviour and on some specific electrocorticographic (ECoG) patterns, known from previous works to accompany various states of waking attentiveness. The animals were tested during 90 min in three distinct behavioural situations; a neutral one (NS) where they simply explored their environment and then usually went to sleep; a second one (FA) where they could watch a mouse (but not catch it), which favoured 'focussed' attentiveness accompanied by the development of rhythmic ECoG activities in the anterior frontoparietal cortex ('beta rhythms' at 40 Hz); a third one (EX) with a hidden mouse whose appearance the cat was waiting for ('expectancy') which rather than favouring the development of beta rhythms elicited the appearance of another rhythmic activity dominating in somatic area SI, 'mu rhythms' at 14 Hz. The duration of each waking behaviour and its ECoG concomitant as well as that of slow wave and of paradoxical sleep were compared in each cat before and after lesion. Our results showed that MD lesions situated in the posterior part of the nucleus tended to increase the time occupied by focussed attentive behaviour and the accompanying beta rhythms. On the other hand, anteriorly located MD lesions elicited a concomitant reduction of both manifestations, behavioural and electrocortical. No such contrasting effects could be systematically noticed for the durations of expectancy, of slow sleep and of paradoxical sleep. These findings were discussed considering previous data showing that at least two systems project upon MD, that play distinct--in a way even antagonistic--roles in focussed attention upon a target. Both originate from the ventral tegmental mesencephalic area; one reaches MD through the ventral striatum (nucleus accumbens), the other one through the amygdala. The MD nucleus thus receiving contrasting information may participate in a final adjustment of the attentive state of the animal to its environment.

SPECT findings in Parkinson's disease associated with dementia

Dementia in Parkinson's disease is thought to be attributable not only to subcortical lesions but also to cortical alterations, especially frontal lobe dysfunction. To evaluate cortical function, the regional cerebral blood flow (rCBF) was estimated of 13 demented and 13 non-demented age matched patients with Parkinson's disease compared with that of 10 age matched controls using I-123 iodoamphetamine single photon emission tomography (IMP-SPECT). The rCBF of the nondemented Parkinson's patients showed no significant differences from that of the control subjects. In the demented patients, the bilateral frontal and parietal and left temporal regional blood flow was significantly less than in the controls. Four demented patients showed isolated frontal hypoperfusion, 8 showed fronto-parietal hypoperfusion, and 1 showed isolated parietal hypoperfusion. Frontal hypoperfusion was therefore present in 12 of the 13 demented patients, and this finding agrees with the frontal lobe dysfunction hypothesis. Parietal rCBF had a significant positive correlation with cortical functions such as calculation and language ability in the MMSE scores. The parietal and temporal reduction in rCBF probably reflects the presence of Alzheimer pathology, cortical Lewy body disease, or both.

The electrophysiological actions of dopamine and dopaminergic drugs on neurons of the substantia nigra pars compacta and ventral tegmental area

The dopaminergic neurons of the substantia nigra pars compacta and ventral tegmental area play a crucial role in regulating movement and cognition respectively. Several lines of evidence suggest that a degeneration of dopaminergic cells in the substantia nigra produces the symptoms of Parkinson's disease. On the other hand, a hyperactivity of the dopaminergic transmission in the brain induces dyskinesia, dystonia and psychosis. It is also well established that the euphoric and rewarding responses evoked by drugs of addiction, such as amphetamine and cocaine, are mediated by central dopamine systems. Electrophysiological experiments which study the activity of single dopaminergic neurons in the ventral mesencephalon have shown that dopamine and dopaminergic drugs reduce the firing frequency of these cells. This is due to the stimulation of D2-D3 autoreceptors and to a hyperpolarization of the membrane produced by an increase in potassium conductance. In addition, substances which increase the release (amphetamine), the synthesis (levodopa) or block the uptake (cocaine, nomifensine, amineptine) of dopamine in the brain inhibit the firing activity of the dopaminergic cells throughout dopamine-mediated mechanisms. In this review, we will briefly examine the literature concerning the physiological and behavioural responses caused by dopamine and dopaminergic agents on the dopaminergic neurons of the ventral mesencephalon. Our conclusion suggests that the electrophysiological actions of dopamine and dopamine-related drugs on dopaminergic cells in the ventral mesencephalon might be indicative of the pharmacological effects of these agents on the brain.

Contribution of an α1-adrenergic receptor subtype to the expression of the “ventral tegmental area syndrome”

Bilateral electrolytic lesions of the rat ventral tegmental area, a mesencephalic structure containing the cell bodies of ascending dopaminergic neurons, induce a behavioural syndrome characterized by a permanent locomotor hyperactivity. Acute intraperitoneal injections of prazosin, an alpha 1-adrenergic receptor antagonist, at a dose (0.5 mg/kg) which does not affect locomotor activities of control animals, abolished locomotor hyperactivities of lesioned rats. Antagonists of other monoaminergic receptors (propranolol, ritanserin, yohimbine), and also another antagonist of alpha 1-adrenergic receptors, 2-(2',6'-dimenthoxyphenoxyethyl)-aminomethyl-1,4-benzodioxan (WB4101) were ineffective. Comparisons of autoradiograms of brain slices incubated in the presence of 1 nM [3H]prazosin or 10 nM [3H]WB4101 indicated clear topographical differences. [3H]Prazosin labelling is present in the septum and in layer III of the cerebral cortex but absent in the striatum. [3H]WB 4101 labelling is diffuse in the superficial layers of the cerebral cortex and present in the striatum. In addition, intraperitoneal injection of WB4101 displaces, only weakly, [3H]prazosin binding in layer III of the cerebral cortex (-18%) while it decreases by 50% [3H]prazosin binding in the more superficial cortical layers. These observations strongly suggest that the binding site labelled by [3H]prazosin is different from alpha 1A- and alpha 1B-adrenergic receptor subtypes labelled by [3H]WB4101. Finally, it is proposed that the prazosin-induced blockade of the locomotor hyperactivity exhibited by ventral tegmental area lesioned animals is linked to the previously demonstrated regulatory role of noradrenergic neurons on cortical dopamine transmission.

The dopamine theory of attention deficit hyperactivity disorder (ADHD)

Clinical, animal and neuroanatomical studies of differential isomer and dosage effects of CNS stimulant medications on behaviour are reviewed. Wender's hypothesis that an underlying biochemical abnormality and a disorder of reinforcement was the primary deficit in "MBD" children is restated in terms of a disorder of polysynaptic dopaminergic circuits, between prefrontal and striate centres. Wender's notion of a disorder of reinforcement is broadened to include a disorder of planning and correction of behaviour, including capacity for cortical control of automatic instinctual motor programmes. The dopamine hypothesis of Attention Deficit Hyperactivity Disorder (ADHD) is examined from the point of view of differential dose effects of CNS stimulant medications, and theories of neural control. Clinical, animal and neuropharmacological studies are reviewed. Implications of the findings for understanding clinical and side effects in ADHD children of stimulants are discussed.

The acquisition of self-stimulation of the medical prefrontal cortex following exposure to escapable or inescapable footshock

The effect of acute stress on the acquisition of an instrumental action reinforced by electrical stimulation of the medial prefrontal cortex (MPC) was investigated by exposing rats to either escapable, inescapable or no footshock prior to daily self-stimulation training sessions. Treatment with inescapable footshock did not affect the number of sessions required for acquisition of MPC self-stimulation but did increase the rate of responding over acquisition sessions compared with the no-shock group. When the treatment footshock was escapable, however, both a facilitation in acquisition, as indexed by a reduction in the number of sessions to criterion, and an increase in the rate of MPC self-stimulation was found. These data were interpreted as offering evidence for the operation of a dopaminergic mechanism in the acquisition of MPC self-stimulation. Further, they indicate, contrary to the reported effects of footshock on self-stimulation of other brain areas, that exposure to acute stress has a facilitatory effect on the rate of self stimulation of the MPC.

Dopamine in the lateral hypothalamus may be involved in the inhibition of locomotion related to food and water seeking

Experiments were conducted in male rats to assess the motor effects of bilateral intraperifornical microinjections of sulpiride, dopamine (DA) and other drugs. Sulpiride increased locomotion of the animals in all the experiments reported here. DA (10 micrograms) administered 5 minutes before sulpiride (8 micrograms) reduced the motor stimulant effect of the neuroleptic from 1601.3 +/- 337.6 to 742.5 +/- 180.4 counts/30 min. SCH 23390 (15 micrograms), haloperidol (2.5 micrograms) and atropine (18 micrograms) did not modify the locomotion level of animals acclimated to the actimeters. After carbachol (5 micrograms) the animals attained a level of hyperactivity (1459.5 +/- 146.5 counts/30 min) similar to that induced by sulpiride (1595.7 +/- 365.7 counts/30 min) in the same experiment. In other experiments DA (10 micrograms) administered 30 min before sulpiride again blocked the effect of 8 micrograms of sulpiride, and reduced the initial hyperactivity of food- and water-deprived animals previously familiarized with the actimeters (922.4 +/- 49.38 counts/15 min under saline, vs. 544 +/- 29 counts/15 min under DA). The same DA dose did not modify the initial spontaneous activity of nonfamiliarized nonfood-deprived rats (508.9 +/- 96.1 after saline vs. 520.9 +/- 47.1 after DA). These results suggest the presence of cells in the lateral hypothalamus involved in the control of locomotion. These experiments also suggest that locomotion triggered by the LH may be exploratory behavior essential to the search for water and food. As a corollary, DA in the LH appears to be involved not only in the inhibition of feeding and drinking but also in the inhibition of exploratory and food- and water-directed locomotion.

The influence of dopaminergic A10 neurons on the motor pattern evoked by substantia nigra (pars compacta) stimulation

The influence of the mesolimbic-mesocortical dopaminergic (DA) system on the motor pattern evoked by substantia nigra pars compacta (SNpc) stimulation was studied. Electrical stimulation of the A10 group of neurons caused an inhibitory effect preferentially directed towards the orientation movement. Sulpiride administration at low dosages (50 mg/kg i.p.) did not modify this movement at the basal condition, but abolished the increase of its duration induced by ventral tegmental area (VTA) co-stimulation. Mesolimbic activation opposes the effects of SNpc stimulation, restraining the animal in its antero-posterior axis by means of orientation movement inhibition. The results suggest a role of the DA mesolimbic-mesocortical system in the maintenance of focused attention.

Facilitatory effect of ventral tegmental area A10 region on the attack behaviour in the cat: possible dopaminergic role in selective attention

Lateral hypothalamus (LH) stimulation in cats which do not spontaneously attack rats, produces an attack pattern which may be divided into 3 main stages: the first, defined as exploratory time (ET), begins with an environmental search and culminates in orienting towards the prey; in the second, defined as attack time (AT), the cat stalks the rat; the last is the biting stage in which the cat seizes and kills the prey by biting its head and neck. The effects of ventral tegmental area (VTA) stimulation on the latency of the whole sequence and on the different stages of the attack pattern were studied. VTA activation resulted in a significant decrease of biting latency, due to the reduction of exploratory time. Moreover, a significant period of prey fixation, seldom present during LH stimulation alone, was observed after VTA-LH co-stimulation. Sulpiride injection caused the disappearance of VTA effects on the predatory pattern. The results indicate that VTA activation induces a decrease in behaviour related to exploration of the environment, and an increase in the focusing of attention on the prey, which seems an important component in the regulation of the predatory pattern. Pharmacological evidence indicates that the VTA effect is mediated by the mesolimbic-mesocortical dopaminergic (DA) system.

Stressful environmental stimuli increase extracellular DOPAC levels in the prefrontal cortex of hypoemotional (Roman high-avoidance) but not hyperemotional (Roman low-avoidance) rats. An in vivo voltammetric study

The effects of a variety of stressful environmental situations on dopamine metabolism in the prefrontal cortex (as assessed by in vivo voltammetry with carbon fiber electrodes) have been compared in two genetically selected lines of rat (Roman high (RHA/Verh) and low (RLA/Verh) avoidance) which differ drastically in their level of emotionality. Heart rate was continuously monitored in these animals (via chronically implanted subcutaneous electrodes) so as to index the emotional reaction to the stressors. An electrochemical signal corresponding to the oxidation of dihydroxyphenylacetic acid (DOPAC) was recorded in the deeper laminae of the anteromedial prefrontal cortex in both lines of rats. Under normal conditions, this signal was stable for at least 4 h and its amplitude was similar in both lines. Introduction of the animals into an unfamiliar environment (30 min), application of a mild tail pinch (10 min) or of a high-intensity loud noise (30 min) or immobilization (20 min) were all associated with an increase in extracellular cortical DOPAC levels in the hypoemotional RHA/Verh line but not in the hyperemotional RLA/Verh line. Similarly, forced locomotion on a rotarod (40 min) provoked a dramatic increase in the amplitude of the cortical DOPAC oxidation peak in RHA/Verh rats and only a mild increase in this parameter in RLA/Verh rats. In RHA/Verh rats, tolerance to this increase was observed when animals were subjected to forced locomotion every day for 5 days. All of the stressful situations investigated provoked an immediate augmentation of heart rate which resumed gradually after cessation of the stressful stimulus; the magnitude and duration of this increase were much greater in RLA/Verh than in RHA/Verh rats. Moreover, in all stress situations, RLA/Verh but not RHA/Verh rats showed behavioral signs of emotional response e.g. defecation, freezing and self-grooming. It is concluded that the increase in cortical dopamine metabolism induced by stress is not connected to the emotional reaction caused by the aversive nature of the stressor but may rather reflect a heightened attention of the animal or activation of cognitive processes in an attempt to cope with the stressor.

Neuropeptide Y in the rat nucleus accumbens: ultrastructural localization in aspiny neurons receiving synaptic input from GABAergic terminals

The ultrastructure, afferent input, and sites of termination of neurons containing neuropeptide Y-like immunoreactivity (NPY-LI) were examined in the adult rat nucleus accumbens by using the peroxidase-antiperoxidase (PAP) method. The NPY-LI was seen in sparsely distributed, spindle-shaped perikarya having cross-sectional diameters of 15-20 microns. These perikarya exhibited highly invaginated nuclear membranes and thin rims of cytoplasm containing Golgi lamellae, dense-core vesicles, and other organelles. A few large, principally aspiny, dendrites also showed NPY-LI. The dendrites received synaptic input from unlabeled terminals forming both symmetric and asymmetric junctions. Immunolabeling for NPY was evident in other processes that were not clearly differentiated as dendrites or axons. These were seen primarily near glial processes and the basal laminae of blood vessels. A few myelinated and many unmyelinated axons and axon terminals also were labeled for NPY. These terminals contained numerous, small (40-60 nm), clear and one or more large (80-100 nm) dense core vesicles. Forty-seven percent (27 out of 57) of the terminals containing NPY-LI formed symmetric junctions with unlabeled dendrites or dendritic spines. The remainder lacked recognizable densities within single planes of section. The neurons exhibiting NPY-LI in the nucleus accumbens were characterized further with respect to their afferent input from terminals labeled for the GABA-synthesizing enzyme, glutamic acid decarboxylase (GAD). Immunogold labeling of a rabbit antiserum against NPY and PAP labeling for a sheep antiserum to GAD were sequentially applied to the same sections. The GAD-labeled terminals formed symmetric junctions primarily with the more numerous unlabeled dendrites. However, a few synaptic junctions also were detected between the GAD-labeled terminals and dendrites showing immunogold labeling for NPY. We conclude (1) that in the rat nucleus accumbens, NPY-LI is found principally in neurons of the aspiny type and (2) that the output from these presumably intrinsic neurons to other neighboring neurons or blood vessels is at least partially modulated by GABA.

Inhibitory effect of the ventral tegmental A10 region on the hypothalamic defence reaction: evidence for a possible dopaminergic mediation

The influence of A10 region neurons of the ventral tegmental area (VTA) on the defence reaction evoked by stimulation of the ventromedial hypothalamic nucleus (VMH) was studied in the cat. The latency of the hissing in the defence reaction increased when the VTA was stimulated both ipsi- and contralaterally. A sulpiride (50mg/kg i.p.) injection totally abolished the VTA-provoked increase of the hissing latency without affecting the basal response.

Tail-pinch stress increases extracellular DOPAC levels (as measured by in vivo voltammetry) in the rat nucleus accumbens but not frontal cortex: antagonism by diazepam and zolpidem

The effect of a tail-pinch stress on dopamine metabolism in the nucleus accumbens and frontal cortex was investigated in the awake unrestrained rat by measuring extracellular 3,4-dihydroxyphenylacetic acid (DOPAC) levels through the use of in vivo differential pulse voltammetry. Mild tail pressure for 8 min caused a large (maximal effect + 70%) and sustained (more than 2 h) increase in the amplitude of the DOPAC oxidation peak in the nucleus accumbens but not in the prefrontal cortex. A similar increase in DOPAC levels was observed in the nucleus accumbens postmortem 1 h after tail-pinch stress. The tail-pinch induced increase in extracellular DOPAC levels in the nucleus accumbens was antagonized by pretreatment with diazepam (5 mg/kg i.p.) or zolpidem (5 mg/kg i.p.), a novel non-benzodiazepine hypnotic possessing anxiolytic properties. These results suggest that in contrast to other stressors, tail-pinch selectively activates dopaminergic systems projecting to the nucleus accumbens.

Locomotor behavior following kindling in three different brain sites

Spontaneous and drug-induced interictal and post-ictal locomotor behavior of rats was investigated following electrical 'kindling' of different limbic structures at 3 brain sites which differ in the relative amount of innervation from dopamine cells of the ventral tegmentum: nucleus accumbens (N.Acc.), amygdala (AMYG), and dorsal hippocampus (DHPC). Kindling produced decreases in spontaneous post-ictal locomotion that did not appear to depend on the site of stimulation, but produced region specific attenuations in spontaneous interictal behavior as well as amphetamine-induced interictal and post-ictal responses. The most dramatic decreases were seen in N.Acc. kindled animals and the least dramatic were seen in rats kindled via DHPC. These data suggest that the mesolimbic dopamine system may participate in the changes in locomotion seen following kindled seizures, and may also provide a model for the study of human post-ictal and interictal behavior.

Ultrastructural localization of tyrosine hydroxylase in rat nucleus accumbens

Immunocytochemical localization of tyrosine hydroxylase (TH) was used to determine the ultrastructural morphology and synaptic associations of catecholaminergic terminals in the nucleus accumbens of the rat. The brains were fixed by vascular perfusion with 4% paraformaldehyde and 0.2% glutaraldehyde. Coronal sections cut with a vibrating microtome were incubated with rabbit antiserum to TH then immunocytochemically labeled by the peroxidase-antiperoxidase method. Immunoreactivity for the enzyme was found within unmyelinated axons and axon terminals. These terminals contained either all small clear or combined small clear and large dense core vesicles. Approximately 40% of the labeled terminals formed symmetric synapses with unlabeled proximal or distal dendritic shafts. The dendrites showed a spare distribution of spines. Axosomatic synapses and axonal associations of the TH-containing terminals also were detected. The recipient perikarya were usually 10-20 micrometers in diameter and contained an indented nucleus and abundant cytoplasm. The content of large dense vesicles and synaptic associations with somata and proximal dendrites suggest that a certain proportion of the TH-containing terminals within the nucleus accumbens are morphologically distinct from catecholaminergic terminals within the dorsal striatum. These differences are discussed in relation to neuropeptides and functions of the dopaminergic mesolimbic and nigrostriatal pathways.

Dopamine-rich transplants in rats with 6-OHDA lesions of the ventral tegmental area. I. Effects on spontaneous and drug-induced locomotor activity

In order to investigate the relative contribution of dopaminergic projections to the nucleus accumbens and prefrontal cortex in the regulation of spontaneous and drug-induced locomotor activity, separate groups of rats were prepared with 6-OHDA lesions of the ventral tegmental area alone, or additional grafts of dopamine-rich tissue reinnervating either the nucleus accumbens or medial prefrontal cortex. A fourth unoperated group served as normal controls. The lesions induced no change in spontaneous, daytime activity, but increased overnight activity. The lesioned rats were also hyperactive to apomorphine, while the activational effects of amphetamine were blocked. Grafts of dopamine-rich tissue, whether into the prefrontal cortex or nucleus accumbens, resulted in a significant normalization of both drug responses towards control levels. Neither graft influenced overnight hyperactivity, whereas spontaneous daytime activity was increased above both control and lesion levels by the accumbens grafts alone. The results are interpreted as suggesting that dopaminergic projections to prefrontal cortex and nucleus accumbens are similarly rather than antagonistically involved in the regulation of drug-induced locomotor activation.

The possible participation of a dopaminergic system in mutilating behavior in rats with forelimb deafferentation

Partial rhizotomy (section of dorsal roots C5 to Th 1 included) leads to abnormal mutilating behavior in the rat, presumably due to pain sensation in the deafferented limb. As dopamine (DA) has been shown to play a role in analgesia, destruction and stimulation of the ventral tegmental area (VTA) were used to check whether they induced an increase or a decrease in the mutilating behavior respectively. Destruction of DA neurons located in the VTA was performed by local administration of 6-hydroxydopamine whereas activation was achieved either by imposed electrical stimulation or by chronic oral administration of D-amphetamine sulfate. These 3 treatments accelerated the onset of the mutilating behavior and induced an extension of the mutilating wounds, D-amphetamine sulfate producing the most pronounced acceleration in their development. The results obtained after destruction of DA-VTA neurons fit well with the working hypothesis. The increase of the mutilating behavior rather than a decrease subsequent to the other treatments could have been caused by the hyperactivity and increase of chewing behavior induced by the less specific stimulating techniques.

Hippocampal damage: effects on dopaminergic systems of the basal ganglia

Although the research topics discussed in this chapter cover a substantial range of areas, methods, and behaviors, there is a consistent central issue: the alterations of brain and behavior subsequent to hippocampal lesions. The initial hypothesis that important secondary changes occur in the basal ganglia after hippocampal lesions has been sustained, at least in part, but other and unexpected results have been obtained that both clarify and mystify at the same time. The restoration of normal locomotion to animals with hippocampal damage by the application of a DAi agonist, DPI, to n. accumbens certainly supports our general idea, but the fact that membrane and transmitter changes in accumbens are transitory needs to be explained and perhaps related to alterations in DA receptor populations. The fact that intra-accumbens DPI can restore ACTH-induced excessive grooming at 28 days after hippocampal damage supports the idea that progressive changes are occurring in DA receptors in n. accumbens, but we need to know their nature more precisely. To do this, however, will probably require advances in our general knowledge of central DA receptors. One of the more interesting aspects of our observations is that the intra-accumbens administration of DPI restores normal behavioral patterns in several types of behaviors, especially locomotion and the excessive grooming response. These two behaviors are not closely linked in nature because large changes can occur in them independently, given appropriate conditions for testing. On the basis of available evidence both of these behaviors seem linked to activities in forebrain DA systems. This would further support the idea that the hippocampus modulates DA activities in basal ganglia systems. The results that suggest the presence of multiple sites for the central induction of grooming were also unexpected. As evidence mounts in support of this idea, it is likely that the one located in n. accumbens is more influenced by hippocampal destruction than the region(s) affected by icv neuropeptide administration. The interactions that take place in the basal ganglia can be extensive. Afferents from the entirety of the neocortical surface, the limbic system, and the projections from the brainstem DA cell groups meet in the striatum and ventral striatum. The efferents from this region not only reach motor cortical and subcortical sites but also project back onto diffusely projecting monoaminergic cells of the brainstem.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of dopamine in locomotor activity and learning

The discovery that the brain contains neurons utilizing dopamine (DA) as their transmitter has led to studies of the behavioral function of these neurons. Changes in overall level of activity of DA neurons appear to produce parallel changes in locomotor activity. Additionally, DA neurons seem to mediate in part the effects of biologically significant (reinforcing) stimuli on learning. One way in which reinforcing stimuli produce learning is to increase the incentive motivational (response-eliciting) properties of neutral stimuli associated with them; also, reinforcing stimuli maintain the incentive motivational properties of previously conditioned incentive stimuli. Normal DA functioning appears to be required for the establishment and maintenance of incentive learning in naive animals. Previous incentive learning in trained animals can influence behavior for a time even when the function of DA neurons is disrupted; however, with continued testing in the absence of normal DA functioning, previously established conditioned incentive stimuli cease to influence behavior. From these observations and recent physiological, anatomical and biochemical studies of DA systems it is suggested that the biological substrate of DA-mediated incentive learning is a heterosynaptic facilitation of muscarinic cholinergic synapses. This model has important clinical implications since it has been suggested that DA hyperfunctioning underlies the development of schizophrenia.

Organization of the ascending projections from the ventral tegmental area: a multiple fluorescent retrograde tracer study in the rat

The projections from the ventral tegmental area of Tsai (VTA) to the frontal cortex (FC), lateral septum (LS), were investigated in the rat by means of the double retrograde fluorescent tracer technique. True blue and fast blue were used in combination with nuclear yellow as retrograde tracers. After combined injections placed into two different terminal fields, many singly and some doubly labeled neurons were seen in the midbrain. In all cases the labeled cells were observed in the ipsilateral VTA, while after injections placed into the LS and Acc some fluorescent neurons were also seen in the contralateral VTA. The patterns of distribution of the labeled neurons showed a topographic organization of the VTA efferent pathways. However, some degree of overlapping was evident in the distribution of cells retrogradely labeled from different terminal fields. The number of the doubly labeled neurons varied according to the sites of combined injections, but in each experiment it never exceeded 10% of the total number of labeled perikarya. Doubly labeled neurons were particularly numerous after combined injections placed into the FC, LS, or LH; on the contrary, very few doubly labeled cells were observed after combined injections placed into the CPu and LS or LH. The organization of the ascending VTA projections suggests that they are probably integrated into different anatomical sets.

Catecholamine alterations in basal ganglia after hippocampal lesions

Rats were given sham, cortical, or hippocampal lesions and sacrificed 7 or 28 days following surgery. Levels of norepinephrine, dopamine, and the major dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid (HVA), were assayed in 3 brain regions. At day 7 there was a decrease in dopamine utilization and a decrease in norepinephrine levels in the nucleus accumbens after hippocampal damage but both of these measures returned to normal levels by day 28. In the neostriatum HVA levels decreased at day 7 after hippocampal damage. The utilization of dopamine in the neostriatum was decreased at day 28 in animals that received neocortical lesions but this was not observed in animals with hippocampal destruction. No effects of any lesion at any day were found in the olfactory tubercle region, the third brain region analyzed. It is thought that the removal of hippocampal and neocortical input to the basal ganglia influences catecholamine function reflected in the loss and subsequent recovery of dopamine utilization.

Dopamine and homovanillic acid concentrations in striatal and limbic regions of human brain

In an attempt to further define the dopaminergic nature of the limbic nucleus accumbens from the morphologically similar striatal caudate and putamen, the levels of dopamine (DA), homovanillic acid (HVA), and HVA/DA ratios, an index of dopamine turnover, were measured in these three structures of human brain. The levels of dopamine in the accumbens (2.49 ng/mg), caudate (2.39 ng/mg), and putamen (3.00 ng/mg) were similar. The homovanillic acid concentration in the accumbens (7.44 ng/mg) and putamen (6.54 ng/mg) were comparable, while its concentration was considerably lower in the caudate (3.61 ng/mg). The most striking difference between the limbic accumbens and the striatum was observed in the HVA/DA ratio. This index of turnover was significantly higher in the accumbens (3.64) when compared to the caudate (1.80), and was 59% higher than that found in the putamen (2.53). The data provide evidence for differences in dopamine activity in the mesolimbic versus the nigrostriatal pathways.

Cholinergic action in the nucleus accumbens: modulation of dopamine and acetylcholine release

The action of oxotremorine and acetylcholine on the release of dopamine and acetylcholine from tissue slices of the rat nucleus accumbens was studied. Oxotremorine significantly enhanced the release of [14C]-dopamine evoked by 34 mMK+ and the EC50 for this action was 1.5 X 10(-7)M. A maximal enhancement (30%) for this effect was reached at 2 X 10(-7)M oxotremorine. A further enhancement of dopamine release occurred at concentrations of oxotremorine greater than 10(-4)M. The action of oxotremorine on [14C]-dopamine release was calcium-dependent and blocked by atropine (10(-4) M) but not mecamylamine (up to 10(-4) M). Oxotremorine affected [3H]-acetylcholine release differentially, inhibiting the K+-evoked release of [3H]-acetylcholine at concentrations greater than 10(-5) M. The IC50 for this process was 4.3 X 10(-5) M. Acetylcholine (8 X 10(-4) M) showed a similar pattern of action to oxotremorine: it enhanced the K+-evoked release of [14C]-dopamine (50%) and inhibited the K+-evoked release of [3H]-acetylcholine (30%). The mechanism of action of oxotremorine on dopamine release is discussed in terms of a presynaptic receptor-mediated process.

Hyperactivity and hypoactivity produced by lesions to the mesolimbic dopamine system

Spontaneous locomotor activity and the locomotor response to amphetamine and apomorphine were studied in rats subjected to either radiofrequency (RF), 6-hydroxydopamine (6-OHDA) or both RF and 6-OHDA lesions of the mesolimbic dopamine (DA) system. Large 6-OHDA lesions of the ventral tegmental area (VTA) or of the nucleus accumbens (N.Acc.) produced hypo-activity in the open field, a complete blockade of the locomotor stimulating effects of D-amphetamine and a profound supersensitive response to apomorphine as measured by a significant increase in locomotor activity as compared to sham-operated animals. In contrast, smaller 6-OHDA lesions of the VTA produced significant increases in spontaneous daytime and nocturnal activity with the biggest effect occurring at the lowest dose. RF lesions to the VTA produced even greater hyperactivity which was blocked by the addition of a 6-OHDA lesion to the N.Acc. The rats with RF lesions to VTA alone that were spontaneously hyperactive remained hyperactive after injection of amphetamine, whereas apomorphine produced a significant decrease in this hyperactivity. In contrast, the rats with the combined RF lesion and N.Acc. 6-OHDA lesion showed a blockade of the locomotor stimulating effects of D-amphetamine and a potentiated response to apomorphine identical to that observed with a N.Acc. lesion alone. All lesion groups revealed massive depletion of DA in the N.Acc. and anterior striatum with significantly greater depletions in those groups showing hypoactivity and hypo-responsiveness to amphetamine. All groups except the N.Acc. 6-OHDA alone group showed significant depletions of DA in the posterior striatum. Thus, limited destruction of the mesolimbic DA system can produce hyperactivity, but more extensive destruction of this system in the region of the N.Acc. and anterior striatum can reverse this hyperactivity and produce a hypo-responsiveness to the locomotor stimulating effects of amphetamine. These results suggest an essential role for dopamine in the expression of spontaneous and stimulant-induced activity. Furthermore, the much larger increase in spontaneous activity in the RF-VTA lesion group as compared to the VTA-6-OHDA groups suggests the presence of an, as yet unidentified, powerful inhibitory influence to the mesolimbic DA system within the midbrain tegmentum.

Bilateral infarction of the anterior cingulate gyri and of the fornices. Report of a case

A 70-year-old woman had complex behavioural changes of sudden onset. The symptoms consisted of indifference, docility and inappropriate urination, but predominantly in a lack of attention. She was unable to maintain the attention necessary to perform a goal-directed activity and she was distracted by any stimulus, such as a sound, an object, or a word, which might induce behaviour irrelevant to the preconceived activity. She also exhibited confabulatory-amnestic syndrome. Neuropathological examination of the brain revealed infarcts in th territories of both anterior cerebral arteries. The rostral part of the anterior cingulate gyrus (Acg), small areas of the adjacent medial prefrontal cortex, and the underlying white matter were destroyed bilaterally. Infarction involved the deep territory of the left anterior cerebral artery, with a bilateral lesion of the fornices. This cingulate damage was more restricted than the Acg lesions reported in some cases of akinetic-mutism, which extended more caudally, but was presumably larger than the lesions created in psychosurgery. The impairment of attention was analyzed according to the possible roles of the cingulate and of the fornix lesions as causing a dysfunction between the frontal lobes and the hippocampal formations.

The effects of elevating gamma-amino butyrate content in the substantia nigra on the behaviour of rats

Gabaculine, a specific inhibitor of GABA transaminase, was injected bilaterally into the substantia nigra of rats. One day after injection, GABA was increased 11-fold in the nigra, 6-fold in thalamus and pons-medulla, and 2-fold in pallidum. 5 h after operation, rats showed continuous sniffing and head movement. This behaviour was blocked by a small dose of picrotoxin injected bilaterally into the nigra, but haloperidol (i.p.) was less effective. One day after injection, rats showed high ambulation and this ambulation was blocked by high doses of picrotoxin. On the second day, GABA contents in all regions were less than twice the control level and behaviour had returned to normal. Rats with gabaculine injected into the pallidum or medulla did not show changes of behaviour as seen in rats with injections into the substantia nigra at any of the times. Striatum dopamine turnover was slightly but significantly decreased at 5 h but not at 24 h after intra-nigral injection with gabaculine. The results suggest that gabaculine-induced sniffing and head movement were mediated by nigral GABAergic synapses and were independent of any dopaminergic system, and that the high ambulation at 24 h after operation may have been due to a non-specific effect of abnormal GABA elevation in thalamus and/or nigra.

Behavioral effects of lesions in the A10 dopaminergic area of the rat

Experiments have been carried out with 150 rats in order to study some psychophysiological functions of the mesencephalocortico limbic dopaminergic A10 group. Lesions in the A10 area were made by using 6-hydroxydopamine (6-OHDA) local injections; 2 small volumes of injections were used at the same concentration (2 mug/1 mul or 1 mug/0.5 mul). In a first experiment the effects of these two injections were tested on locomotor activity measured in a circular corridor, 10 and 30 days after surgery. Injections provoked hyperactivity, mainly during nocturnal basal activity periods, but not during initial exploratory activity periods. The larger the injection, the more important the hyperactivity was. The larger injections induced important food spillage evidence through the wire floor of the home cage and perturbation in a passive avoidance learning. There was no change in body weight or in amount of ingested food. In a second experiment, the effects of local injection of 6-OHDA in the other CA structures or bundles situated in or near the ventral tegmental area were tested. Injections in the substantia nigra compacta, in the noradrenergic ventral bundle, in the dorsal periventricular system-tegmental radiations did not provoke locomotor hyperactivity. In a third experiment, a possible role of the median raphe (MR) nucleus in the A10-lesion induced hyperactivity was tested: first, radiofrequency MR lesions were made and no durable significant hyperactivity was recorded; secondly, 6-OHDA (1 mug/0.5 mul) was injected into the A10 area and activity was measured 10 days later: these injections provoked significant hyperactivity during the nocturnal basal and the diurnal basal activity periods. It might be concluded that neither the neighboring CA fibers nor the MR were directly involved in the ventral tegmental -- 6-OHDA lesions syndrome. Anatomical controls by using the Fink-Heimer silver impregnating method have demonstrated, first, that the 6-OHDA injections did not destroy fibers other than catecholaminergic and secondly, that the degenerations are found in the forebrain and cortical limbic A10 projections. Hypotheses are made about a possible general inhibitory role of the A10 in behavior, in the sence of selective and attentive arousal processes, often impaired in some mental illnesses.

Silver impregnation of dopaminergic systems after radiofrequency and 6-OHDA lesions of the rat ventral

Nauta and Fink-Heimer silver impregnation techniques were used to study the anterior degeneration produced by radiofrequency (RF) or 6-OHDA lesions in the medial and lateral ventral mesencephalic tegmentum (VMT), substantia nigra and dorsalis tegmental decussation (DTD) in rats. Both Nauta and Fine-Heimer impregnating methods showed that RF lesion of the VNT produced degeneration in three major pathways: a ventral pathway corresponding to the fasciculus medialis prosencephali (FMP), an intermediate pathway projecting to the ventral thalamus, and a dorsal pathway to the medio-dorsal thalamus and to the nucleus lateralis habenulae. In addition, the Fink-Heimer method demonstrated prejections of the dopaminergic A10 and A9 cell group in the VMT to the nucleus caudatus after RF or 6-OHDA lesions. Projections to nucleus accumbens, tuberculum olfactorium, stria terminalis, and cortex frontalis were observed only after 6-OHDA lesion of the A10 cell group. Degeneration in cortex cinguli and entorhinalis was seen mainly after 6-OHDA lesion of the A9 cell group. The limbic forebrain cortical projections of the A10 group provide a coherent anatomical basis for the behavioral syndrome provoked by RF and 6-OHDA lesions in the VMT.

The role of nerve-growth factor (NGF) in the central nervous system

NGF is a protein that stimulates growth and differentiation of sympathetic and sensory components of the peripheral nervous system. The purpose of this review is to examine the evidence that NGF has similar activity in the central nervous system. First, the primary mode of interaction of NGF with the nerve cell will be discussed, and the possibility that such an interaction takes place in the brain will be examined. Recent studies have demonstrated that NGF promotes regenerative sprouting of damaged catecholamine-containing neurons in the brain. The next part of the paper reviews this literature, and other findings that indicate or contraindicate a role of NGF in brain maturation of maintenance. The final part of this paper suggests specific avenues for future research in this area, and presents conclusions regarding the literatureon brain activity of NGF to date.

Dopaminergic innervation of the rat prefrontal cortex: a fluorescence histochemical study

After the destruction of the noradrenergic ascending pathways, the localization of frontal cortical fields receiving fibers from the dopaminergic mesocortical system has been studied in rats, using a glyoxylic-paraformaldehyde method. The dopaminergic innervation was distributed in two main areas. The area of highest density was a medial field which spread in the medial cortex anterior and dorsal to the genu of the corpus callosum. It did not reach the shoulder region except in the foremost part of the frontal lobe where dopaminergic fibers were scattered in the whole cortex, including the molecular layer. A deep sulcal field was situated between the dorsal bank of the rhinal sulcus and the lateral cortex above it. In addition, a moderately dense band of dopaminergic fibers was observed between the corpus callosum and the anterior commissura, beside the accumbens nucleus. Similar data were obtained with dopamine uptake experiments on reserpine-treated but otherwise normal animals. The frontal areas receiving dopaminergic innervation coincide strikingly with the 'prefrontal cortex' as defined by neuroanatomical studies, which is assumed to be more or less equivalent to the prefrontal cortex of primates and derives direct projections from the amygdala. The functional implications of these findings are discussed.