The differential projections from the dorsal raphe and median raphe nuclei of the midbrain were autoradiographically traced in the rat brain after 3H-proline micro-injections. Six ascending fiber tracts were identified, the dorsal raphe nucleus being the sole source of four tracts and sharing one with the median raphe nucleus. The tracts can be classified as those lying within the medial forebrain bundle (dorsal raphe forebrain tract and the median raphe forebrain tract) and those lying entirely outside (dorsal raphe arcuate tract, dorsal raphe periventricular tract, dorsal raphe cortical tract, and raphe medial tract). The dorsal raphe forebruin tract lies in the ventrolateral aspect of the medial forebrain bundle (MFB) and projects mainly to lateral forebrain areas (e.g., basal ganglion, amygdala, and the pyriform cortex). The median raphe forebrain tract lies in the ventromedial aspect of the MFB and projects to medial forebrain areas (e.g., cingulate cortex, medial septum, and hippocampus). The dorsal raphe cortical tract lies ventrolaterally to the medial longitudinal fasciculus and projects to the caudate-putamen and the parieto-temporal cortex. The dorsal raphe periuentricular tract lies immediately below the midbrain aqueduct and projects rostrally to the periventricular region of the thalamus and hypothalamus. The dorsal raphe arcuate tract curves laterally from the dorsal raphe nucleus to reach the ventrolateral edge of the midbrain and projects to ventrolateral geniculate body nuclei and the hypothalamic suprachiasmatic nuclei. Finally, the raphe medial tract receives fibers from both the median and dorsal raphe nuclei and runs ventrally between the fasciculus retroflexus and projects to the interpeduncular nucleus and the midline mammillary body.Further studies were done to test whether the fiber tracts travelling in the MFB contained 5-HT. Unilateral (left) injections of 5,7-dihydroxytryptamine (5 pgm/400 nl) 18 days before midbrain raphe microinjections of 3H-proline produced a reduction in the grain concentrations in all the ascending fibers within the MFB. Furthermore, pharmacological and behavioural evidence was obtained to show that the 5-HT system had been unilaterally damaged; these animals displayed preferential ipsilateral turning in a rotameter which was strongly reversed to contralateral turning after 5-hydroxytryptophan administration.The results show that DR and MR nuclei have numerous ascending projections whose axons contain the transmitter 5-HT. The results agree with the neuroanatomical distribution of the 5-HT system previously determined biochemically, histochemically, and neurophysiologically. The midbrain serotonin system seems to be organized by a series of fiber pathways. The fast transport rate in these fibers was found to be about 108 mm/day.

Calcium waves represent a widespread form of intercellular communication. Although they have been thought for a long time to require gap junctions, we recently demonstrated that mouse cortical astrocytes use an extracellular messenger for calcium wave propagation. The present experiments identify ATP as a major extracellular messenger in this system. Medium collected from astrocyte cultures during (but not before) calcium wave stimulation contains ATP. The excitatory effects of medium samples and of ATP are blocked by purinergic receptor antagonists and by pretreatment with apyrase; these same purinergic receptor antagonists block propagation of electrically evoked calcium waves. ATP, applied at the concentration measured in medium samples, evokes responses that are qualitatively and quantitatively similar to those evoked by those medium samples. These data implicate ATP as an important transmitter between CNS astrocytes.Key words: glia; astrocytes; calcium waves; ATP; extracellular signal; suramin; apyrase; purinergic; electrical stimulation Intercellular calcium waves, i.e., rises in intracellular free calcium that propagate between neighboring cells, occur widely among different cell types throughout the animal kingdom (Cornell-Bell et al., 1990;Boitano et al., 1992;Enomoto et al., 1992;Demer et al., 1993;Nathanson et al., 1995;C ao et al., 1997;Young et al., 1996;Jørgensen et al., 1997;Newman and Z ahs, 1997). Although we are at an early stage of understanding the f unction of such waves, one must consider that glial calcium waves may provide an information-processing system operating in parallel with neuronal circuits within the nervous system. There is clear evidence of interaction between glial calcium waves and neurons; neuronal activity can directly evoke glial calcium waves (Dani et al., 1992), and glial calcium waves can directly evoke calcium transients and electrical activity in neurons (Nedergaard, 1994;Parpura et al., 1994;Hassinger et al., 1995;Newman and Z ahs, 1998). Understanding how glial calcium waves could contribute to information processing requires an understanding of the mechanisms underlying calcium wave propagation.For some years, glial calcium waves have been thought to propagate through gap junctions (Boitano et al., 1992;Charles et al., 1993;Sanderson et al., 1994;Sneyd et al., 1994Sneyd et al., , 1995. We demonstrated recently that an extracellular communication system can provide a dominant path for glial calcium wave propagation (Hassinger et al., 1996), because calcium waves can propagate between physically separated astrocytes, and the extent and direction of calcium wave propagation are significantly influenced by movement of the extracellular medium. T wo subsequent publications now confirm that astrocytes do not absolutely require functional gap junction coupling for calcium wave propagation (Guan et al., 1997;Naus et al., 1997). Taken together, the newer literature supports the idea that substance(s) released from astrocytes can activate receptor systems on adjacent astroc...

To understand the mechanism of the sequential restriction of multipotency of stem cells during development, we have established culture conditions that allow the differentiation of neuroepithelial precursor cells from embryonic stem (ES) cells. A highly enriched population of neuroepithelial precursor cells derived from ES cells proliferates in the presence of basic fibroblast growth factor (bFGF). These cells differentiate into both neurons and glia following withdrawal of bFGF. By further differentiating the cells in serum-containing medium, the neurons express a wide variety of neuron-specific genes and generate both excitatory and inhibitory synaptic connections. The expression pattern of position-specific neural markers suggests the presence of a variety of central nervous system (CNS) neuronal cell types. These findings indicate that neuronal precursor cells can be isolated from ES cells and that these cells can efficiently differentiate into functional post-mitotic neurons of diverse CNS structures.

The introduction of high-resolution time lapse imaging and molecular biological tools has changed dramatically the rate of progress towards the understanding of the complex structure-function relations in synapses of central spiny neurons. Standing issues, including the sequence of molecular and structural processes leading to formation, morphological change, and longevity of dendritic spines, as well as the functions of dendritic spines in neurological/psychiatric diseases are being addressed in a growing number of recent studies. There are still unsettled issues with respect to spine formation and plasticity: Are spines formed first, followed by synapse formation, or are synapses formed first, followed by emergence of a spine? What are the immediate and long-lasting changes in spine properties following exposure to plasticity-producing stimulation? Is spine volume/shape indicative of its function? These and other issues are addressed in this review, which highlights the complexity of molecular pathways involved in regulation of spine structure and function, and which contributes to the understanding of central synaptic interactions in health and disease.