Studies on the somatotopy of the trigeminal system in the mallard, Anas platyrhynchos L. III. Afferents and organization of the nucleus basalis.

The ascending projections from the principal sensory nucleus V (PrV) have been studied by tracing degeneration after lesions in the PrV and by injections of HRP into the projection zone of PrV. The quintofrontal tract arises from PrV, ascends into the forebrain, and terminates in the ipsilateral and the contralateral nucleus basalis (NB). The contralateral fibers decussate in the tegmentum at the level of the trochlear-oculomotor nuclei. NB is a laminar nucleus lying over the rostral part of the paleostriatal complex. Dorsally NB is bounded by the neostriatum. NB consists of small neurons. In the dorsal part of NB, these neurons are arranged in vertical columns; the afferents ascend through these columns, and clusters of degenerated boutons are found around the cells. It is possible to distinguish regions in NB receiving ophthalmic, maxillary, mandibular, or glossopharyngeal afferents. The rostral part of NB receives an exclusive ipsilateral projection; the intermediate part, a bilateral projection; and the caudal part, a contralateral projection, with the exception of the most caudal area, which also receives a bilateral projection. It is not clear whether NB should be considered a thalamic, a telencephalic, or even a pallial structure. The hypothesis that the columnar organization of the NB is a prerequisite to preserve a precise somatotopy of the tactile system of the oral region is discussed. In this respect the organization of NB can be compared to that of layer IV of the somatosensory (SI) cortex of mammals. Knowledge of the structure and functions of the peripheral tactile sense system opens the possibility of subdividing the NB into functional units.

Cholinergic stimulation of the ventrolateral striatum elicits mouth movements in rats: pharmacological and regional specificity.

Cholinomimetic drugs are known to induce changes in perioral behavior in rodents, characterized primarily by "purposeless" chewing movements, but little is known about their central sites of action. Using observational methods, the effects of direct microinfusion of a mixture of physostigmine and acetylcholine (PS/Ach, 0, 0.5, 2.5, 5.0 micrograms of each in 0.5 microliter saline) into the ventrolateral striatum (VLS) were assessed. Cholinergic stimulation of this region produced a dose-dependent induction of mouth movements, characterized by chewing movements, jaw opening and closing, tongue protrusions and jaw tremors. These movements were not directed toward any stimulus. In some rats, cholinergic stimulation of the VLS also induced stereotyped self-biting, although this effect was less prominent and of shorter duration. Induction of mouth movements by cholinergic stimulation of the VLS was blocked by prior administration of atropine, either systemically (50 mg/kg) or directly into the VLS (10 micrograms). Systemic administration of methylatropine (50 mg/kg) did not block the mouth movements. Pretreatment with haloperidol (2.5 micrograms into VLS) had no effect on PS/Ach-induced mouth movements. Infusion of PS/Ach (0, 2.5, 5.0 micrograms) into the dorsolateral or ventromedial striatum did not produce significant changes in oral behavior, although the level of mouth movements was somewhat higher at the medial site. The three sites studied were also differentiated with respect to spontaneous motor behaviors (locomotion and rearing) following direct cholinergic stimulation. These findings are considered as further evidence for the role of the ventrolateral striatum in oral motor behavior.

Topographical organization of the projections from physiologically identified areas of the motor cortex to the striatum in the rat.

The present study was undertaken to determine in the rat the topography of the neostriatal projections originating from the motor cortex. For that purpose, anterograde tracers (Phaseolus vulgaris leucoagglutinin: PHA-L; wheat germ agglutinin conjugated to horseradish peroxidase: WGA-HRP) were deposited in discrete cortical sites physiologically identified by microstimulation. Five major motor areas were considered in this study: the rostral (RFL) and caudal (CFL) forelimb areas, the hindlimb (HL) area, the vibrissae motor-frontal eye field (V-FEF) region and the jaw, lips and tongue (JLT) area (according to the nomenclature of Neafsey et al.). The results indicate that functionally different regions of the motor cortex project to different sectors of the caudate putamen (CPU). All 3 distinct limb areas RFL, CFL and HL project to the dorsolateral quarter of the CPU, V-FEF area projects to the dorsomedial quarter, whereas the JLT area projects to the ventrolateral quarter. The pattern of terminal labeling is relatively consistent, whatever the cortical area in which the tracer is deposited. This pattern is characterized by the presence of two or more labeled bands which are obliquely oriented along a ventrolateral-dorsomedial axis. Control experiments were also undertaken in which a retrograde tracer (WGA-HRP) was deposited in various neostriatal loci. The results are congruent with the findings of the anterograde study and further indicate that a given neostriatal sector receives projections from cytoarchitectonically different but functionally related regions of the neocortex. The somatotopic features of both motor and somatosensory corticostriatal projections appear to be in register. In addition, the striatal distribution of motor cortical fibers was compared in 6 experimental cases to the compartmental subdivision of the striatum in patches and matrix, following immunohistochemical localization of calbindin 28 kDa. The calbindin-immunoreactivity is extremely weak in the dorsolateral sector but is higher in the central and ventrolateral parts of the CPU. In these deep striatal regions receiving fibers from V-FEF, JLT and, to a lesser extent, from the limb areas, the cortical fibers are mostly directed to the matrix. The band-like organization of the projection from the motor cortex is correlated to the patch-matrix organization. The patches correspond to the bands of low density of terminal fibers and the matrix to the bands of high terminal density. The present results provide an anatomical basis to both electrophysiological and behavioral observations suggesting that functional distinctions can be established between subregions of the striatum.

Increases in rat striatal extracellular dopamine and vacuous chewing produced by two sigma receptor ligands.

The present studies were carried out to analyze the neurochemical and behavioral effects of peripheral sigma ligand administration in the rat. Based upon previous studies which showed an increase in turning behavior following unilateral intranigral administration of sigma ligands, we determined the effects of two sigma ligands, 1,3-di-o-tolylguanidine (DTG) and (+)-pentazocine, on extracellular dopamine levels in the rat striatum. Dopamine levels were monitored via microdialysis in awake freely moving animals following i.p. injection of the ligands. Both DTG (1 and 3 mg/kg) and (+)-pentazocine (10 mg/kg) produced a significant (30-50%) increase in extracellular dopamine. Given the relatively high concentration of sigma receptors in brain nuclei involved in facial and mouth movements, we have also determined the effects of the two sigma ligands on facial movements. Both ligands produced a significant increase in vacuous chewing movements, suggesting that studies on the consequences of sigma receptor activation may have relevance to animal models of human dystonia and/or dyskinesia.

The sites of origin of projection fibers from the cerebral cortex to the jaw region of the striatum of the rat.

Electrical microstimulation of the jaw region of the rat striatum (SJR) has been reported to provoke clear electromyographic activity in the anterior digastric muscle but not in the masseter muscle (Neurosci. Lett., 252 (1998) 79-82). Thus, in the present study, we examined the sites of origin of cortico-SJR fibers by the retrograde labeling. The SJR, identified by electrical microstimulation, was injected electrophoretically with cholera toxin B subunit. In the cerebral cortex ipsilateral to the injection, there existed two foci of retrograde labeling: One focus was centered on the lateral part of the sensorimotor area, while the other on the insular cortical area around the middle cerebral artery. These foci appeared to correspond to the reported two cortical areas, where two different types of rhythmical jaw movements were induced by repetitive electrical stimulation (Jap. J. Oral Biol., 32 (1990) 57-68).

Neuronal activity in monkey ventral striatum related to the expectation of reward.

Projections from cortical and subcortical limbic structures to the basal ganglia are predominantly directed to the ventral striatum. The present study investigated how the expectation of external events with behavioral significance is reflected in the activity of ventral striatal neurons. A total of 420 neurons were studied in macaque monkeys performing in a delayed go-no-go task. Lights of different colors instructed the animal to do an arm-reaching movement or refrain from moving, respectively, when a trigger light was illuminated a few seconds later. Task performance was reinforced by liquid reward in both situations. A total of 60 ventral striatal neurons showed sustained increases of activity before the occurrence of individual task events. In 43 of these neurons, activations specifically preceded the delivery of reward, independent of the movement or no-movement reaction. In a series of additional tests, these activations were time locked to the subsequent reward, disappeared within a few trials when reward was omitted, and were temporally unrelated to mouth movements. Changes in the appetitive value of the reward liquid modified the magnitude of activations, suggesting a possible relationship to the hedonic properties of the expected event. Activations also occurred when reward was delivered in a predictable manner outside of any behavioral task. These data suggest that neurons in the ventral striatum are activated during states of expectation of individual environmental events that are predictable to the subject through its past experience. The prevalence of activations related to the expectation of reward suggests that ventral striatal neurons have access to central representations of reward and thereby participate in the processing of information underlying the motivational control of goal-directed behavior.

Blockade of adenosine A2A receptors antagonizes parkinsonian tremor in the rat tacrine model by an action on specific striatal regions.

Acute administration of the acetylcholinesterase inhibitor tacrine to rats induces tremulous jaw movements which can be used as a valuable model of parkinsonian tremor. In the present study, the number of tremor episodes and jaw movements were evaluated to assess the effects of the selective A2A antagonists SCH 58261 and SCH BT2 on tremorgenesis. SCH 58261 dose-dependently, and maximally at 5 mg/kg, reduced the number of both tremor episodes (-35%) and jaw movements (-50%), induced in rats by tacrine (2.5 mg/kg ip). Since adenosine A2A receptors are largely expressed throughout the striatum, chronic cannulae were implanted in the rat dorsomedial (DMS) and ventrolateral striatum (VLS) to investigate whether A2A antagonists could act at this level. Infusion of SCH BT2 (5 microg/microl), a water-soluble analogue of SCH 58261, in VLS antagonized both tremor episodes (-68%) and jaw movements (-76%) elicited by tacrine (2.5 mg/kg ip), whereas SCH BT2 infusion in DMS was less effective in blocking jaw movements (-50%) and did not significantly affect the number of tremor episodes. Taken together, the results of this study indicate that A2A antagonists effectively reduce the magnitude of tremulous jaw movements induced in rats by acute tacrine, mainly by an action in VLS and suggest that A2A antagonists might be used as specific agents against parkinsonian tremor.