Emperor penguin body surfaces cool below air temperature.

Emperor penguins Aptenodytes forsteri are able to survive the harsh Antarctic climate because of specialized anatomical, physiological and behavioural adaptations for minimizing heat loss. Heat transfer theory predicts that metabolic heat loss in this species will mostly depend on radiative and convective cooling. To examine this, thermal imaging of emperor penguins was undertaken at the breeding colony of Pointe Géologie in Terre Adélie (66°40' S 140° 01' E), Antarctica in June 2008. During clear sky conditions, most outer surfaces of the body were colder than surrounding sub-zero air owing to radiative cooling. In these conditions, the feather surface will paradoxically gain heat by convection from surrounding air. However, owing to the low thermal conductivity of plumage any heat transfer to the skin surface will be negligible. Future thermal imaging studies are likely to yield further insights into the adaptations of this species to the Antarctic climate.

Dopaminergic terminals in the rat cortex.

The destruction of ascending noradreniergic pathways by bilateral microinjections of 6-hydroxydopamnine made laterally to the pedunculus cerebellaris superior completely abolished the in vitro synthesis of [(3)H]norepinephrine from L-[(3)H]tyrosine in slices and in synaptosomes of the rat cortex. However, normal [(3)H]dopamine synthesis could still be observed in both cortical preparations from animals with lesions. These results provide the first biochemical support for the existence of dopaminergic terminals independent of noradrenergic terminals in the rat cortex.

Dopamine modulates temporal dynamics of feedforward inhibition in rat prefrontal cortex in vivo.

Midbrain dopamine (DA) neurons project to pyramidal cells and interneurons of the prefrontal cortex (PFC). At the microcircuit level, interneurons gate inputs to a network and regulate/pattern its outputs. Whereas several in vitro studies have examined the role of DA on PFC interneurons, few in vivo data are available. In this study, we show that DA influences the timing of interneuron firing. In particular, DA had a reductive influence on interneuron spontaneous firing, which in the context of the excitatory response of interneurons to hippocampal electrical stimulation, lead to a temporal focalization of the interneuron response. This suggests that the reductive influence of DA on interneuron excitability is responsible for filtering out weak excitatory inputs. The increase in the temporal precision of interneuron firing is a mechanism by which DA can modulate the temporal dynamics of feedforward inhibition in PFC circuits and can thereby influence cognitive information processing.

Relationships between the prefrontal cortex and the basal ganglia in the rat: physiology of the cortico-nigral circuits.

The prelimbic/medial orbital areas (PL/MO) of the rat prefrontal cortex are connected to substantia nigra pars reticulata (SNR) through three main circuits: a direct nucleus accumbens (NAcc)-SNR pathway, an indirect NAcc-SNR pathway involving the ventral pallidum (VP) and the subthalamic nucleus (STN), and a disynaptic cortico-STN-SNR pathway. The present study was undertaken to characterize the effect of PL/MO stimulation on SNR cells and to determine the contribution of these different pathways. The major pattern of responses observed in the SNR was an inhibition preceded by an early excitation and followed or not by a late excitation. The inhibition resulted from the activation of the direct NAcc-SNR pathway because it disappeared after acute blockade of the glutamatergic cortico-striatal transmission by CNQX application into the NAcc. The late excitation resulted from the activation of the indirect NAcc-VP-STN-SNR pathway via a disinhibition of the STN because it disappeared after either CNQX application into the NAcc or blockade of the GABAergic striato-pallidal transmission by bicuculline application into the VP. The early excitation, which was markedly decreased after blockade of the cortico-STN transmission by CNQX application into the STN, resulted from the activation of the disynaptic cortico-STN-SNR pathway. Finally, the blockade of the cortico-STN-VP circuit by CNQX application into STN or VP modified the influence of the trans-striatal circuits on SNR cells. This study suggests that, in the prefrontal cortex-basal ganglia circuits, the trans-subthalamic pathways, by their excitatory effects, participate in the shaping of the inhibitory influence of the direct striato-nigral pathway on SNR neurons.

Dendritic arborizations of the rat substantia nigra pars reticulata neurons: spatial organization and relation to the lamellar compartmentation of striato-nigral projections.

The cerebral cortex provides a major source of inputs to the basal ganglia. As has been well documented, the topography of corticostriatal projections subdivides the striatum into a mosaic of functionally distinct sectors. How information flow from these striatal sectors remains segregated or not within basal ganglia output nuclei has to be established. Electrophysiologically identified neurons of the rat substantia nigra pars reticulata were labeled by juxtacellular injection of Neurobiotin, and the spatial organization of their dendritic arborizations was analyzed in relation to the projection fields of individual striatal sectors. Thirty-nine nigral neurons located in the projection territory of the distinct striatal sensorimotor sectors were reconstructed. The data show that the dendritic arborizations of nigral neurons conform to the geometry of striato-nigral projections. Like striatal projections, the arborizations formed a series of curved laminas enveloping a dorsolaterally located core. Although dendritic fields of the neurons lying in the laminae were flat, those located in the core were spherical or cylindrical, thereby conforming to the shape of the striatal projection fields. This remarkable alignment between the dendritic arborizations of nigral neurons and the projection fields from individual striatal districts supports the concept of a parallel architecture of the striato-nigral circuits. However, pars reticulata neurons usually extend part of their dendrites within adjacent striatal projection fields, thereby ensuring a continuum between channels. The extension of the dendritic arborizations within the striatal projection fields suggests that nigral neurons integrate the information that is relevant for the completion of the specific motor behavior they control.

Segregation and convergence of information flow through the cortico-subthalamic pathways.

Cortico-basal ganglia circuits are organized in parallel channels. Information flow from functionally distinct cortical areas remains segregated within the striatum and through its direct projections to basal ganglia output structures. Whether such a segregation is maintained in trans-subthalamic circuits is still questioned. The effects of electrical stimulation of prefrontal, motor, and auditory cortex were analyzed in the subthalamic nucleus as well as in the striatum of anesthetized rats. In the striatum, cells (n = 300) presenting an excitatory response to stimulation of these cortical areas were located in distinct striatal territories, and none of the cells responded to two cortical stimulation sites. In the subthalamic nucleus, both prefrontal and motor cortex stimulations induced early and late excitatory responses as a result of activation of the direct cortico-subthalamic pathway and of the indirect cortico-striato-pallido-subthalamic pathway, respectively. Stimulation of the auditory cortex, which does not send direct projection to the subthalamic nucleus, induced only late excitatory responses. Among the subthalamic responding cells (n = 441), a few received both prefrontal and motor cortex (n = 19) or prefrontal and auditory cortex (n = 10) excitatory inputs, whereas a larger number of cells were activated from both motor and auditory cortices (n = 48). The data indicate that the segregation of cortical information flow originating from prefrontal, motor, and auditory cortices that occurred in the striatum is only partly maintained in the subthalamic nucleus. It can be proposed that the existence of specific patterns of convergence of information flow from these functionally distinct cortical areas in the subthalamic nucleus allows interactions between parallel channels.

Anatomical evidence for direct connections between the shell and core subregions of the rat nucleus accumbens.

The nucleus accumbens is thought to subserve different aspects of adaptive and emotional behaviors. The anatomical substrates for such actions are multiple, parallel ventral striatopallidal output circuits originating in the nucleus accumbens shell and core subregions. Several indirect ways of interaction between the two subregions and their associated circuitry have been proposed, in particular through striato-pallido-thalamic and dopaminergic pathways. In this study, using anterograde neuroanatomical tracing with Phaseolus vulgaris-leucoagglutinin and biotinylated dextran amine as well as single-cell juxtacellular filling with neurobiotin, we investigated the intra-accumbens distribution of local axon collaterals for the identification of possible direct connections between the shell and core subregions. Our results show widespread intra-accumbens projection patterns, including reciprocal projections between specific parts of the shell and core. However, fibers originating in the core reach more distant areas of the shell, including the rostral pole (i.e. the calbindin-poor part of the shell anterior to the core) and striatal parts of the olfactory tubercle, than those arising in the shell and projecting to the core. The latter projections are more restricted to the border region between the shell and core. The density of the fiber labeling within both the shell and core was very similar. Moreover, specific intrinsic projections within shell and core were identified, including a relatively strong projection from the rostral pole to the rostral shell, reciprocal projections between the rostral and caudal shell, as well as projections within the core that have a caudal-to-rostral predominance. The results of the juxtacellular filling experiments show that medium-sized spiny projection neurons and medium-sized aspiny neurons (most likely fast-spiking) contribute to these intra-accumbens projections. While such neurons are GABAergic, the intrastriatal projection patterns indicate the existence of lateral inhibitory interactions within, as well as between, shell and core subregions of the nucleus accumbens.

Indirect nucleus accumbens input to the prefrontal cortex via the substantia nigra pars reticulata: a combined anatomical and electrophysiological study in the rat.

The nucleus accumbens is a major component of the ventral striatum through which most of the limbic affiliated cortical areas gain access to the basal ganglia circuitry. In this study, the organization of the pathways linking the nucleus accumbens to the thalamus, via the substantia nigra pars reticulata, was examined in the rat using anatomical and electrophysiological methods. Use of anterograde and retrograde transport of wheatgerm agglutinin conjugated to horseradish peroxidase has established that the core of the nucleus accumbens innervates a dorsal region of the substantia nigra pars reticulata which projects to subfields of the mediodorsal and ventral medial thalamic nuclei. These subfields consist of the rostral pole of the mediodorsal nucleus with the exception of its central segment and a region of the ventral medial nucleus, medial to the mammillothalamic tract. Confirming the existence of a nucleus accumbens nigrothalamic link, we have observed that electrical or chemical stimulation of the nucleus accumbens induces an inhibition of the spontaneous discharges of the nigral cells which project to the mediodorsal and ventral medial thalamic nuclei. Finally, the cortical projections of the thalamic subfields involved in the nucleus accumbens nigrothalamic circuit were determined using the anterograde and retrograde axonal transport of wheatgerm agglutinin conjugated with horseradish peroxidase. These subfields innervate mainly the prelimbic and to a lesser degree the orbital areas of the prefrontal cortex. The present data show that the substantia nigra pars reticulata is a major link between the core of the nucleus accumbens and the prefrontal cortex and provide further evidence for the concept of a parallel architecture in the basal ganglia thalamocortical circuits of the ventral striatum.

Basal ganglia and processing of cortical information: functional interactions between trans-striatal and trans-subthalamic circuits in the substantia nigra pars reticulata.

The substantia nigra pars reticulata (SNR), a major output station of basal ganglia, receives information from the cerebral cortex through three main pathways, i.e. a direct inhibitory trans-striatal pathway, an indirect excitatory trans-striatal pathway that involves the pallidum and the subthalamus and a direct excitatory trans-subthalamic pathway. In order to determine how cortical information flow originating from functionally distinct cortical areas and processed through the trans-striatal and trans-subthalamic pathways is integrated within the SNR, the responses induced by electrical stimulation of prefrontal, motor and auditory cortex in SNR cells were analyzed in anesthetized rats. Further confirming that direct striato-nigral pathways related to these functionally distinct cortical areas are organized in parallel channels, stimulation of the prefrontal, motor and auditory cortex induced an inhibitory response on distinct subpopulations of SNR cells. Within a given channel, the direct trans-striatal and the trans-subthalamic pathways converge on a large number of nigral cells. In addition, the present study reveals that nigral cells receiving an inhibitory input from a given cortical area through the direct trans-striatal pathway can also receive an excitatory input from a functionally distinct cortical area through the trans-subthalamic pathways. Such a convergence mainly occurred between the direct striato-nigral pathway issued from the auditory cortex and the trans-subthalamic pathways issued from the motor cortex. These data reveal the existence of a converging influence of trans-subthalamic and direct striato-nigral pathways not only within but also across channels. Within a given cortico-basal ganglia channel, the trans-subthalamic pathways likely contribute to the temporal shaping of the striato-nigral inhibition and thus of the disinhibition of the related nigral target nuclei in the thalamus and mesencephalon. Across channels, the specific interactions between trans-subthalamic and direct striato-nigral pathways could contribute to prevent inhibition of subpopulations of nigral cells implicated in competing functions.

Differential effects of ascending neurons containing dopamine and noradrenaline in the control of spontaneous activity and of evoked responses in the rat prefrontal cortex.

The medial prefrontal cortex receives converging projections from the mediodorsal thalamic nucleus, dopaminergic cells from the ventral tegmental area dn noradrenergic cells from the locus coeruleus. Stimulation of the ventral tegmental area inhibits the spontaneous activity of prefrontal cortical neurons and blocks the excitatory response evoked by stimulation of the mediodorsal thalamic nucleus (10 Hz). The aim of the present study was to compare the influence of dopaminergic and noradrenergic afferents on the spontaneous and evoked activity of medial prefrontal cortical neurons. In ketamine-anaesthetized rats, repetitive stimulation (20 Hz, 10 s) of the locus coeruleus produced a long-lasting post-stimulus inhibition (mean duration: 45 s) of the spontaneous activity of 56% of the tested cells. This effect was decreased markedly following selective destruction of the ascending noradrenergic pathways (local 6-hydroxy-dopamine injection) or depletion of cortical catecholamines by alpha-methyl-para-tyrosine pretreatment, suggesting that these inhibitory responses are mediated by noradrenergic neurons. The excitatory response to mediodorsal thalamus nucleus stimulation (10 Hz) could still be evoked during the post-stimulus inhibitory period induced by locus coeruleus stimulation (20 Hz, 10 s) resulting in the enhancement of signal-to-noise ratio. On the other hand, a population of prefrontal cortex neurons (26%) was found to be reproducibly activated by noxious tail pinch. This evoked response was still present during the post-stimulus inhibitory period induced by locus coeruleus stimulation but was completely suppressed during stimulation of the ventral tegmental area (10 Hz). In conclusion, these results indicate that the dopaminergic and noradrenergic systems exert a completely distinct control of information transfer in the medial prefrontal cortex.

Inhibitory effects of ventral tegmental area stimulation on the activity of prefrontal cortical neurons: evidence for the involvement of both dopaminergic and GABAergic components.

The medial prefrontal cortex of the rat receives dopamine and non-dopaminergic projections from the ventral tegmental area. Both electrical stimulation of the ventral tegmental area and local application of dopamine induce an inhibition of the spontaneous activity of most prefrontal cortical neurons, including efferent neurons. In the present study, the techniques of extracellular recording and microiontophoresis were used in anesthetized rats in order to determine whether these dopamine- and ventral tegmental area-induced inhibitory responses involve GABAergic components. Prefrontal cortex output neurons were identified by antidromic activation from subcortical structures. The inhibitory responses evoked by the local application of dopamine were blocked by the iontophoretic application of the D2 antagonist sulpiride, and the GABAA antagonist bicuculline in 89 and 57% of the cases, respectively. In addition, sulpiride and bicuculline abolished the inhibition induced by ventral tegmental area stimulation in 54 and 51% of the prefrontal cortical cells tested, respectively. The implication of a non-dopaminergic mesocortical system in the ventral tegmental area-induced inhibition was further analysed using rats pre-treated with alpha-methylparatyrosine to deplete dopamine stores. The proportion of prefrontal cortical cells inhibited by ventral tegmental area stimulation was markedly reduced (39%) in alpha-methylparatyrosine-treated rats, when compared to controls (86%). Remaining ventral tegmental area-induced inhibition was no longer affected by sulpiride, but in all cases blocked by the local microiontophoretic application of bicuculline. The present results suggest that: (1) the dopamine-induced inhibition of prefrontal cortex neurons could involve cortical GABAergic interneurones; (2) the non-dopaminergic mesocortical system exerts also an inhibitory influence on prefrontal cortical cells and appears to be GABAergic.

Position of the ventral pallidum in the rat prefrontal cortex-basal ganglia circuit.

The ventral pallidum receives major inputs from the nucleus accumbens, a striatal region related to the prefrontal cortex. The ventral pallidum, through its projections to the mediodorsal nucleus of the thalamus, has been considered as the main output structure of the prefrontal-basal ganglia circuits. However, as shown recently, the ventral pallidum also sends efferents to the subthalamic nucleus and the substantia nigra, suggesting that it could participate in intrinsic basal ganglia circuits. The aim of the present investigation was to determine the position of the ventral pallidum in the prefrontal-basal ganglia circuit originating from the prelimbic and medial orbital areas. Following injections of biocytin (an anterograde tracer) into the region of the core of the nucleus accumbens receiving excitatory inputs from the prelimbic and medial orbital areas, axonal terminal fields were observed in a delineated dorsal region of the ventral pallidum. When the biocytin injections were made into this ventral pallidal region, anterogradely labelled fibres were observed in both the dorsomedial substantia nigra pars reticulata and the medial subthalamic nucleus, but not in the mediodorsal nucleus of the thalamus. Confirming these anatomical observations, electrical stimulation of the core of the nucleus accumbens induced an inhibition of the spontaneous activity (D=34.9+/-13.3 ms, L=9.2+/-3.3 ms) in 46.5% of the ventral pallidal cells. Among these responding cells, 43% were antidromically driven from the subthalamic nucleus, 30% from the substantia nigra pars reticulata and only 6% from the mediodorsal nucleus of the thalamus. These data demonstrate that the region of the ventral pallidum involved in the prefrontal cortex-basal ganglia circuit originating from the prelimbic and medial orbital areas represents essentially a ventral subcommissural extension of the external segment of the globus pallidus since it exhibits similar extrinsic connections and functional characteristics. In conclusion, in this prelimbic and medial orbital channel, the ventral pallidum cannot be considered as a major output structure but is essentially involved in intrinsic basal ganglia circuits.

Effect of a functional impairment of corticostriatal transmission on cortically evoked expression of c-Fos and zif 268 in the rat basal ganglia.

The activity-dependent induction of immediate-early genes is commonly used to map activated neuronal networks. In a previous analysis of the cortico-basal ganglia circuits, we have shown that a cortical stimulation produces Fos protein expression in the striatum and the subthalamic nucleus, with a pattern which conforms to the anatomical organization of cortical projections [Sgambato V. et al. (1996) Neuroscience 81, 93-112]. In the present study, we examined the effects of a unilateral blockade of the corticostriatal transmission on c-fos and zif 268 messenger RNA expression evoked in the substantia nigra pars reticulata and the subthalamic nucleus following stimulation of the ipsilateral motor cortex. The blockade of the corticostriatal pathway was performed either by an excitotoxic striatal lesion or by an application of the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione within the striatum. After application of the glutamate receptor antagonist, which prevented the cortical stimulation activating the GABAergic striatonigral pathway, the induction of both c-fos and zif 268 messenger RNAs was facilitated in the ipsilateral substantia nigra pars reticulata. In the subthalamic nucleus ipsilateral to the application of 6-cyano7-nitroquinoxaline-2,3-dione, the cellular discharges evoked by stimulation of the cortex were considerably shortened as a result of the blockade of the disinhibitory striato-pallido-subthalamic circuit. However, a strong expression of immediate-early genes was still induced by the cortical stimulation. By contrast, after unilateral kainate lesion of the striatum, the cortical stimulation was no longer able to induce c-fos and zif 268 messenger RNA expression in the ipsilateral subthalamic nucleus and in the substantia nigra pars reticulata bilaterally. The lack of immediate-early gene induction strongly contrasted with the neuronal discharges evoked in these nuclei by the cortical stimulation. Comparison between the cortically evoked neuronal activities and the pattern of immediate-early gene expression suggests that the induction of immediate-early genes in the basal ganglia mainly reflects the level of synaptic activity rather than the frequency of discharge of the postsynaptic neurons. Moreover, the results stress that modifications of immediate-early gene expression observed in the basal ganglia after an acute or a chronic interruption of the corticostriatal transmission are not superimposable.

Prefrontal cortex inputs of the nucleus accumbens-nigro-thalamic circuit.

The functional organization of the cortico-nucleus accumbens-substantia nigra pars reticulata circuit was investigated in the rat using combined anatomical and electrophysiological approaches. The nucleus accumbens neurons which project to the substantia nigra pars reticulata are located in a circumscribed region of the core immediately adjacent and extending dorsally to the anterior commissure. As shown by retrograde and anterograde transports of wheatgerm agglutinin conjugated to horseradish peroxidase, the region of the nucleus accumbens related to the substantia nigra was found to receive bilateral inputs from restricted areas of the medial and lateral prefrontal cortex, i.e., prelimbic/medial orbital and dorsal agranular insular areas. The electrical stimulation of these medial and lateral prefrontal cortical areas induced excitatory responses in nucleus accumbens neurons projecting to the dorsomedial substantia nigra pars reticulata. Interestingly, an important proportion (61%) of the nucleus accumbens-nigral cells responding to the stimulation of the lateral prefrontal cortex were also excited by the stimulation of the medial prefrontal cortex, demonstrating the existence of a convergent influence of these cortical areas on single nucleus accumbens cells. Furthermore, the present data also show that the stimulation of the medial prefrontal cortex results in a powerful inhibition of the tonic firing of the substantia nigra pars reticulata neurons. In conclusion, this study reveals the existence of a functional link between the prefrontal cortex (prelimbic/medial orbital and agranular insular areas) and the nucleus accumbens neurons which innervate the dorsomedial region of the substantia nigra pars reticulata. Since the dorsomedial region of substantia nigra pars reticulata is known to project to subfields of the mediodorsal and ventromedial thalamic nuclei related to the prefrontal cortex, the present data further demonstrate the existence of a prefrontal-nucleus accumbens-thalamo-cortical circuit involving the substantia nigra pars reticulata.

Inhibition of hippocampoprefrontal cortex excitatory responses by the mesocortical DA system.

The prefrontal cortex (PFC) receives dopaminergic (DA) afferents from the ventral tegmental area (VTA) and excitatory inputs from the hippocampal formation. The present electrophysiological experiments performed in anaesthetized rats demonstrate that activation of the mesocortical DA system blocked the spontaneous activity of PFC neurones and the excitatory responses induced by hippocampal stimulation. These data indicate that there is a convergence in the effects of hippocampal and DA mesocortical afferents on common PFC neurones. Furthermore, a population of PFC neurones responding to hippocampal stimulation could be identified as projecting to either the nucleus accumbens and/or the VTA, suggesting that the hippocampus exerts an indirect influence on these two structures through the PFC.

Mediodorsal thalamic evoked responses in the rat prefrontal cortex: influence of the mesocortical DA system.

The prefrontal cortex (PFC) receives a dopaminergic (DA) innervation from the ventral tegmental area (VTA) and is reciprocally connected with the mediodorsal thalamic nucleus (MD). The present study was performed in anaesthetized rats to determine the influence of the mesocortical DA system on excitatory responses evoked in the PFC by stimulation of MD: (1) short latency (< 4 ms) responses resulting from activation of the MD-PFC pathway; (2) long latency responses (> 10 ms) resulting from activation of recurrent collaterals of PFC neurons projecting to MD. Local DA application and VTA stimulation did not affect short latency responses but blocked long latency responses. These results suggest that the mesocortical DA system inhibits excitations by recurrent collaterals of the PFC-MD neurones but not the excitatory MD inputs to the PFC.

Selectivity of the hippocampal projection to the prelimbic area of the prefrontal cortex in the rat.

Afferent connections of the medial and lateral prefrontal cortex of the rat arising from the hippocampal formation were investigated using iontophoretic application of the fluorescent tracer, Fluoro-gold. Our results demonstrate that the projection which originates in the temporal part of the CA1 hippocampal field and in the prosubiculum is restricted to the prelimbic area of the prefrontal cortex.

Effect of noxious tail pinch on the discharge rate of mesocortical and mesolimbic dopamine neurons: selective activation of the mesocortical system.

The effects of noxious tail pinch on the activity of mesocortical and mesolimbic dopamine (DA) neurons located in the ventromedial mesencephalic tegmentum were analyzed in ketamine-anesthetized rats. The great majority of mesocortical DA neurons responded to tail pinch, either by an excitation (65%), or by an inhibition (25%). In contrast, most DA neurons projecting either to the nucleus accumbens or the septum remained unaffected. These results demonstrate that noxious tail pinch selectively influences the firing rate of mesocortical DA neurons.

Electrophysiological identification of mesencephalic ventromedial tegmental (VMT) neurons projecting to the frontal cortex, septum and nucleus accumbens.

The electrophysiological properties of neurons located in the mesencephalic ventromedial tegmentum (VMT) and the organization of the efferents of these neurons to the frontal cortex, the septum, the nucleus accumbens and the head of the striatum were studied in ketamine-anesthetized rats. The projections of the VMT cells were determined through use of the antidromic activation method. Our results show that VMT projections to different target areas originate mainly from different VMT neurons. However, in some cases single VMT neurons were found to send axon collaterals to two different areas. Three branching patterns were observed: septum-cortex, septum--nucleus accumbens and septum--striatum. The occasional observation of temporally distinct antodromic responses from a single area was considered to result from activation of different branches of the arborizing axon. The distribution of antidromic response latencies for VMT projections to each structure is discussed in relation to the question of dopaminergic versus non-dopaminergic mesolimbic and mesocortical systems.

Effects of ventro-medial mesencephalic tegmentum (VMT) stimulation on the spontaneous activity of nucleus accumbens neurones: influence of the dopamine system.

The effects of VMT-stimulation (100-500 microA, 0.6 ms; 1 Hz) on the spontaneous activity of neurones in the nucleus accumbens were analyzed in ketamine-anaesthetized rats. On spontaneously active cells (firing greater than 0.5 spikes/s), 3 types of responses were observed: either inhibition (36%), excitation (5%) or a composite sequence of excitation followed by inhibition (12%). Moreover, 14% of silent nucleus accumbens neurones were excited by single pulse VMT-stimulation. Finally, 3% of nucleus accumbens neurones recorded were driven antidromically by VMT-stimulation. Destruction of dopamine (DA) projections by 6-hydroxydopamine prevented the inhibitory responses to VMT stimulation in the great majority of cells studied, without affecting the excitatory responses. After systemic administration of haloperidol or sulpiride, the inhibitory responses to VMT stimulation were attenuated markedly, whilst the excitatory responses were, however, maintained. These results suggest that the inhibitory, but not the excitatory, effects of VMT-stimulation on nucleus accumbens neurones may be mediated by an activation of the mesolimbic DA system.