[Effects of oral cannabis and dronabinol on driving capacity]. / Effets du cannabis oral et du dronabinol sur la capacité à conduire.

Two retrospective epidemiologic studies have shown that cannabis is the main psychoactive substance detected in the blood of drivers suspected of driving under the influence of psychotropic drugs. An oral administration double-blind crossover study was carried out with eight healthy male subjects, aged 22 to 30 years, all occasional cannabis smokers. Three treatments and one placebo were administered to all participants at a two week interval: 20 mg dronabinol, 16.5 mg D9-tétrahydrocannabinol (THC) and 45.7 mg THC as a cannabis milk decoction. Participants were asked to report the subjective drug effects and their willingness to drive under various circumstances on a visual analog scale. Clinical observations, a psychomotor test and a tracking test on a driving simulator were also carried out. Compared to cannabis smoking, THC, 11-OH-THC and THC-COOH blood concentrations remained low through the whole study (<13.1 ng THC/mL,<24.7 ng 11-OH-THC/mL and<99.9 ng THC-COOH/mL). Two subjects experienced deep anxiety symptoms suggesting that this unwanted side-effect may occur when driving under the influence of cannabis or when driving and smoking a joint. No clear association could be found between these adverse reactions and a susceptibility gene to propensity to anxiety and psychotic symptoms (genetic polymorphism of the catechol-O-methyltransferase). The questionnaires have shown that the willingness to drive was lower when the drivers were assigned an insignificant task and was higher when the mission was of crucial importance. The subjects were aware of the effects of cannabis and their performances on the road sign and tracking test were greatly impaired, especially after ingestion of the strongest dose. The Cannabis Influence Factor (CIF) which relies on the molar ratio of active and inactive cannabinoids in blood provided a good estimate of the fitness to drive.

Treatment of secondary peritonitis: is a less expensive broad-spectrum antibiotic as effective as a carbapenem?

BACKGROUND: In this retrospective study, the possibility of using amoxicillin/clavulanate for empirical therapy of secondary peritonitis as an alternative to imipenem was explored. METHODS: All secondary peritonitis cases at our institution between 1998 and 2000 were included. Susceptibility to imipenem and amoxicillin/clavulanate of microorganisms isolated in peritoneal fluid and success rates were compared. Therapeutic failure was defined as death, necessity of repeated surgical intervention, or clinical deterioration with persistent positive cultures. RESULTS: Seventy-six cases of secondary peritonitis with 156 microorganisms were found. One hundred and forty-nine (98%) were susceptible to imipenem versus 124 (82%) to amoxicillin/clavulanate (p = 0.0001). Thirteen therapeutic failures occurred in 52 patients treated with amoxicillin/clavulanate (25%) versus 3 out of 8 (38%) with imipenem (p = 0.43). The proportion of organisms resistant to amoxicillin/clavulanate in therapeutic failures was greater in nosocomial versus community-acquired secondary peritonitis (p = 0.041). CONCLUSION: Despite its better in vitro bacteriological activity, clinical efficacy of imipenem was identical to amoxicillin/clavulanate. The use of amoxicillin/clavulanate instead of imipenem would save 889 Euro per case.

Delta(9)-THC, 11-OH-Delta(9)-THC and Delta(9)-THCCOOH plasma or serum to whole blood concentrations distribution ratios in blood samples taken from living and dead people.

The recreational use and abuse of Cannabis is continuously increasing in Switzerland. Cannabinoids are very often detected alone or in combination with other drugs in biological samples taken from drivers suspected of driving under the influence of drugs. Moreover, they are also frequently found in blood specimens from people involved in various medico-legal events, e.g. muggings, murders, rapes and working accidents as well. In order to assess the influence of Cannabis exposure on man behavior and performances, it is often needed to estimate the time of Cannabis use. For that purpose two mathematical models have been set up by Huestis and coworkers. These models are based on cannabinoids concentrations in plasma. Because plasma samples are rarely available for forensic determinations in our laboratory, it could be useful to assess the time-laps since Cannabis use through these models from whole blood values. One prerequisite to the use of these models from whole blood values is the knowledge of the plasma to whole blood concentrations distribution ratios of cannabinoids. In this respect, the Delta(9)-THC, 11-OH-Delta(9)-THC and Delta(9)-THCCOOH concentrations were measured in plasma and whole blood taken from eight volunteers who smoke Cannabis on a regular basis. Cannabinoids levels were also determined in "serum" and whole blood samples taken from six corpses. The values of the plasma to whole blood distribution ratios were found to be very similar and their individual coefficient of variation relatively low suggesting that plasma levels could be calculated from whole blood concentrations taken into account a multiplying factor of 1.6. The data obtained postmortem suggest that the distribution of cannabinoids between whole blood and "serum" is scattered over a larger range of values than those determined from living people and that more cannabinoids (mean value of the serum/whole blood concentrations ratios=2.4) can be recovered from the "serum" fraction. The successful use of the mathematical models of Huestis and coworkers may, therefore, rely in part upon the selection of the appropriate blood sample, i.e. plasma. When plasma is not available, whole blood values could be considered with some caution taken into account a multiplying factor of 1.6 to calculate plasma concentrations from blood values. In the case of blood samples taken after death, the use of these models to assess the time of Cannabis use is not recommended.

[Cannabis and automobile driving]. / Cannabis et conduite automobile.

Cannabis use has increased considerably during the last 15 years. One of the major problems dealing with cannabis use is driving under the influence of drugs. With the exception of ethyl alcohol, the majority of the epidemiological studies have shown that cannabis is the most frequently detected substance in people suspected of driving under the influence of drugs. Experimental studies are therefore needed to assess cannabis effects on driving capability. Many studies indicate that cannabis impairs psychomotor performance. This impairment becomes obvious when high doses of cannabis are taken, when ethyl alcohol or other drugs are simultaneously ingested, or when sustained attention is needed. Moreover, cannabis effects are qualitatively different from those observed after ethyl alcohol consumption. In forensic practice, cannabis impairment of driving performance must be related to cannabinoids blood concentrations. To facilitate the interpretation of cannabinoids blood levels, several models were set up recently. These models must be further improved in order to fit in with all circumstances of cannabis use.

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.

Three-dimensional distribution of nigrostriatal neurons in the rat: relation to the topography of striatonigral projections.

Functional regions of the rat striatum related to identified cortical territories were injected ionophoretically with wheat germ agglutinin coupled to horseradish peroxidase. Coronal serial sections were cut throughout the substantia nigra. The distributions of labelled striatal projections and nigrostriatal neurons were studied. Using software developed in our laboratory, three-dimensional reconstructions were calculated which confirmed and extended the organizational scheme of striatonigral projections already reported by our group. These projections were organized as a set of longitudinal lamellae spatially organized so as to segregate the flow of information emanating from striatal regions affiliated to sensorimotor and associative-limbic cortical areas. In addition, the relationship between the striatonigral projections and the nigrostriatal neurons was studied by three-dimensional reconstruction. For each striatal injection site, two populations of retrogradely labelled nigral neurons could be discriminated by their position with respect to the striatal projection field. The first one occupied a proximal position, in register with the labelled striatal projections, while the second was more distal. The populations of proximal neurons which innervate different functional striatal sectors were segregated both mediolaterally, dorsoventrally and rostrocaudally, while the populations of distal neurons were more scattered and showed a lesser degree of spatial segregation. The organization of these two populations with respect to the striatal projection fields suggests that the substantia nigra might control the flow of cortical information through the striatum via two different modalities, based respectively on a closed nigrostriatal loop involving the proximal neurons, and an open loop involving the distal ones.

Prefrontal cortex-basal ganglia circuits in the rat: involvement of ventral pallidum and subthalamic nucleus.

The core of the nucleus accumbens (NAcc core) is the principal input structure to the basal ganglia circuitry for the prelimbic and medial orbital areas (PL/MO) of the prefrontal cortex. As is now well recognized in the rat, the main basal ganglia output of this prefrontal channel is the dorsomedial part of the substantia nigra pars reticulata (SNR) and not the ventral pallidum as previously suggested. There is evidence suggesting that the ventral pallidum is rather involved with the subthalamic nucleus (STN) in an indirect NAcc-SNR pathway. Indeed, we have recently shown that the NAcc core sends an inhibitory input to the lateral ventral pallidum (VPl), which projects to the medial STN. In the present study, we injected biocytin into the medial STN, at a site where neurons presented an inhibitory response to VPl stimulation. This produced anterogradely labelled fibres in the medial SNR and in the VPl. Furthermore, the stimulation of the VPl induced an inhibition in a majority of the STN cells identified, by the antidromic activation method, as projecting to SNR (76.6%) and/or back to the VPl (72.7%). In conclusion, these data further demonstrate the existence of an indirect striato-nigral pathway in the PL/MO channel and indicate that VPl is involved in an inhibitory feedback circuit, which modulates the discharge of medial STN. These results indicate that the medial STN is implicated in the limbic/cognitive functions of the basal ganglia.

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.

Activation of muscarinic receptors stimulates GABA release in the rat globus pallidus.

The effect of carbachol on the spontaneous release of 3H-GABA was investigated on rat globus pallidus (GP) slices. Carbachol dose-dependently enhanced the release of 3H-GABA. The carbachol (5 x 10(-4) M) induced 3H-GABA release is mediated by muscarinic receptors since atropine (10(-6) M), pirenzepine (10(-6) M) and AF-DX384MS (10(-6) M) abolished the effect. An indirect carbachol effect mediated by dopaminergic and glutamatergic afferents was ruled out since the effect was not blocked by either D1 (SCH23390 10(-6) M) and D2 (sulpiride 10(-5) M) receptor antagonists or by ionotropic glutamate receptor antagonists (CNQX 10(-6) M and 10(-5) M, MK801 10(-6) M). A direct effect is further evidenced by the persistence of the carbachol effect in the presence of tetrodotoxin (5 x 10(-7) M). Surprisingly the carbachol effect was not abolished by lowering the Ca2+ concentration of the superfusion medium or by increasing concomitantly the Mg2+ concentration. The involvement of a GABA transporter can partially explain this latter result, as nipecotic acid (10(-3) M) blocked the effect by only 50%. Carbachol stimulated the accumulation of 3H-phosphoinositides in pallidal slices, an effect that was antagonized by atropine (10(-6) M), pirenzepine (10(-6) M), and AF-DX384MS (10(-6) M). These results suggest that the activation of muscarinic receptors localized on striatopallidal terminals stimulates the release of GABA in the globus pallidus through inositol phosphate hydrolysis.

The lamellar organization of the rat substantia nigra pars reticulata: segregated patterns of striatal afferents and relationship to the topography of corticostriatal projections.

The striatonigral pathway provides one of the most direct routes for information flow through the basal ganglia system. Via this pathway information from sensory, motor and associative areas of the cerebral cortex are routed to a variety of thalamocortical and brainstem networks involved in the organization of motor behaviour. In a previous analysis of the rat substantia nigra pars reticulata we have shown that the nigral cells which project to thalamus, tectum and tegmentum are topographically ordered along a series of curved laminae. Extending these observations, the present study examined how striatal regions related to particular areas of the cerebral cortex innervate the lamellar keyboard of nigral output neurons. For this purpose, small microiontophoretic injections of wheat germ agglutinin conjugated to horseradish peroxidase were performed in the striatum and the distribution of retrogradely-labelled cells in the cerebral cortex and anterogradely-labelled axons in the substantia nigra were conjointly examined. The results indicate that with the exception of the striatal region related to the allocortex, all the various components of the striatal functional mosaic are represented in the substantia nigra pars reticulata. This representation is organized under the form of longitudinal bands which compose a series of curved laminae enveloping a core located dorsolaterally in the substantia nigra. The striatal mapping in substantia nigra pars reticulata is such that the projections of the auditory and visual compartments are confined to the most ventral lamina. More dorsally, an ordered representation of the body is achieved by the nigral lamination. The oral and perioral body parts are centred on the dorsolateral core and the more distal parts of the face and limbs are progressively set out in more peripheral laminae. In the region affiliated to the prefrontal cortex, the dorsal cingulate district innervate a ventromedial lamina, the prelimbic/insular district lie dorsal to it. Projections from lateral orbital and insular compartments extend laterally along the dorsal margin of the pars reticulata. Since the "onion-like" distribution of striatal inputs is precisely the form observed in the distribution of nigral efferent neurons, the present observations favour the view that the nigral lamination underlies formation of specific input-output channels of processing. Evidence is considered that these channels are specialized for particular classes of movements or behaviours and integrate the various information relevant to the completion of these movements or behaviours.

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.

Muscarinic receptor stimulation increases the spontaneous [3H]GABA release in the rat substantia nigra through muscarinic receptors localized on striatonigral terminals.

The effect of muscarinic agonists on the spontaneous release of [3H]GABA was investigated in vitro on rat substantia nigra slices. Acetylcholine (5 x 10(-5) M) in the presence of eserine (5 x 10(-5) M) induced a 12.3% increase of the spontaneous release of [3H]GABA. Similarly, carbachol (5 x 10(-4) M) enhanced by 9% the release of [3H]GABA. This effect was Ca(2+)-dependent, it was abolished in the presence of 0.4 mM Ca2+ and enhanced from 9 to 17% when Ca(2+)-concentration of the superfusion medium was increased from 1.3 to 2.4 mM. The carbachol effect was mediated by muscarinic receptors since it was abolished by atropine (2 x 10(-6) M). The pharmacologically M2 muscarinic receptor subtypes seems to be involved as the carbachol-induced effect was abolished by AF-DX384MS (10(-6) M), an M2 antagonist and was only partially reversed by pirenzepine (10(-5) and 10(-4) M), an M1 antagonist which at these doses also block the M2 receptors. The absence of effect of SCH23390 (10(-6) M) a D1 antagonist as well as the lack of effect of CNQX (10(-5) M) and dizocilpine maleate (10(-6) M), two glutamate antagonists, on the carbachol-induced effect indicated that neither dopamine (through D1 receptors) nor glutamate (through ionotropic receptors) were involved in the response. In addition, the persistence of the carbachol-induced effect in the presence of tetrodotoxin (2 x 10(-7) M) suggests a direct muscarinic-mediated modulation of [3H]GABA. The localization of muscarinic receptors on striatonigral fibres was confirmed by autoradiographic studies showing a decrease of [3H]pirenzepine binding in the substantia nigra after a unilateral striatal lesion induced by kainic acid injection. This latter result provides evidence of the presence of M1 receptors on striatonigral terminals as the concentration of [3H]pirenzepine used (10 nM) is M1-selective. These results indicate a cholinergic modulation of GABA release in the rat substantia nigra mediated by muscarinic receptors localized on striatonigral terminals. The involvement of the m4 muscarinic receptor subtype that have a M1/M2 pharmacology is discussed.

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.

Stimulation by nicotine of the spontaneous release of [3H]gamma-aminobutyric acid in the substantia nigra and in the globus pallidus of the rat.

The effect of (-)-nicotine on the spontaneous release of [3H]gamma-aminobutyric acid ([3H]GABA) was studied in vitro in rat substantia nigra (SN) and globus pallidus (GP) slices. In both structures, nicotine (10(-4) M) elicited a transient increase of [3H]GABA release lasting no more than 2.5 min. At the peak of the effect, a 18.5% and 25% increase of [3H]GABA was observed in GP and SN slices, respectively. At lower concentration (10(-5) M), nicotine produced a small but significant transient increase (+8%) in GP slices whereas this concentration was ineffective in SN slices. Pempidine (10(-5) M) totally antagonized the 10(-4) M nicotine-induced effect in SN and GP. The increase of [3H]GABA release elicited by 10(-4) M nicotine was abolished when Ca2+ concentration in the superfusion medium was lowered from 2.4 to 0.4 mM. To investigate a possible dopaminergic (DA) link in the response, we examined the sensitivity of the nicotine-induced effect to DA D1 (SCH23390) and D2 (sulpiride) receptor antagonists. In SN, SCH23390 (10(-6) M) abolished the 10(-4) M nicotine-induced effect. In GP, sulpiride (10(-5) M) failed to modify the response. Moreover, SCH23390 partially reversed the nicotine-induced effect (-37%) in GP. Taken together these results indicate that nicotine differentially modulate the [3H]GABA release in SN and GP. In SN, the nicotine-induced [3H]GABA release appears to be mediated by DA neurons. In GP, only a part of the nicotinic response involved a DA link. A possible direct stimulation of nicotinic receptors localized on striato-pallidal terminals is discussed.

Evidence that biocytin is taken up by axons.

Unexpected anterograde labeling is systematically observed in the pontine nuclei following iontophoretic injection of biocytin in the substantia nigra pars reticulata. Control experiments using WGA-HRP have led to deny the existence of a nigro-pontine pathway. The possibility that biocytin is taken up by fibers of passage has been tested. Deposits of biocytin in the corpus callosum result in a massive axonal labeling of this fibrous system. This study, in contrast to previous reports, stresses that biocytin is easily taken up and transported by axons. Hence, this tracer has to be used with careful controls when injected in structures crossed by fibrous tracts.