Morphological and physiological properties of neostriatal neurons: An intracellular horseradish peroxidase study in the rat

The physiological and morphological (light and electron microscopic) properties of four categories of neostriatal neurons (two types of medium spiny cells and two types of aspiny cells) were analyzed using the technique of intracellular recording and intracellular labeling with horseradish peroxidase. All of the neurons in this study had excitatory responses following stimulation of the cortex and substantia nigra except for the large aspiny neuron for which only substantia nigra inputs were tested. Morphologically, these neurons differed with respect to the size and shape of their somata, density and distribution of dendritic spines and distribution of their axons and axon collaterals. Ultrastructurally, observed somatic differences included the quantity and distribution of organelles and conformation of the nuclear envelope.The axons of one type of medium spiny neuron and the large aspiny neuron were myelinated. Unmyelinated axon collaterals arose from the axons of both types of medium spiny neurons and formed synapses on the dendritic shafts and possibly with the necks of spines of other neostriatal neurons. The parent axons of the most common type of medium spiny neurons were followed to the globus pallidus and, in some cases, to the internal capsule.     

A morphological analysis of neurons in the lateral parabrachial nucleus: an intracellular horseradish peroxidase study in the rat

Neurons in the lateral parabrachial nucleus (LPB) of the rat were analyzed by the intracellular horseradish peroxidase method. Sixteen LPB neurons were successfully labeled with HRP injected intracellularly. HRP-labeled LPB neurons were divided into type I (7 neurons) and type II (9 neurons) LPB neurons. Type I LPB neurons, which were activated antidromically by stimulation of the ipsilateral posteromedial ventral nucleus of the thalamus, had long dendrites and a long axon. Type II LPB neurons, which were not activated antidromically by the stimulation of the thalamus, had short dendrites and a short axon. It was concluded that type I LPB neurons were projection neurons, while type II LPB neurons were local circuit neurons.     

Localization of motoneurons innervating the muscle of the hard palate in the rat: A horseradish peroxidase study

Using the horseradish peroxidase-technique, the myotopical arrangement of motoneurons innervating the transverse palatine muscle in the rat was studied. It appears that this muscle is innervated by axons from cells located in the ipsilateral intermediate subnucleus of the facial motor nucleus. By nerve transection and electrophysiological experiments it is shown that the transverse palatine muscle is innervated by the inferior as well as the superior buccolabial branch of the facial nerve.     

The distribution of infrahyoid motoneurons in the cat: a retrograde horseradish peroxidase study

Summary The distribution of cell bodies and the peripheral course of axons of infrahyoid motoneurons were examined in the cat by the retrograde horseradish peroxidase method after application of the enzyme to the peripheral nerve branches supplying the infrahyoid muscles. Infrahyoid motoneurons were observed to constitute a slender cell column, which extended from a level of the caudal part of the hypoglossal nucleus usually to the most caudal level of the C1 cord segment, or occasionally to the lower levels of the C2 cord segment. The cell column was located immediately lateral to that of motoneurons of the spinal accessory nerve. In the cell column, thyrohyoid motoneurons were distributed in the medulla oblongata; sternohyoid motoneurons were located somewhat more cranially than sternothyroid motoneurons in the medulla oblongata and cervical cord. However, the level of craniocaudal distribution of thyrohyoid, sternohyoid or sternothyroid motoneurons highly overlapped. The experiments involving severance of the hypoglossal and/or cervical nerves indicated that axons of thyrohyoid and sternohyoid motoneurons passed via the roots of both hypoglossal and C1 nerves, that axons of sternohyoid motoneurons passed via the C1 nerve roots, and that axons of infrahyoid motoneurons innervating the conjugated part of the sternohyoid and sternothyroid muscles passed usually via the C1 nerve roots, or occasionally via the roots of both C1 and C2 nerves.     

Identification of motoneurons supplying the tensor veli palatini muscle in the guinea pig and cat: An horseradish peroxidase study

Identification of motoneurons supplying the tensor veli palatini muscle was attempted by the horseradish peroxidase (HRP) method in the guinea pig and cat. After HRP injection into the tensor veli palatini muscle, HRP-labeled neurons were seen in the regions closely medial to the cluster of the lateral pterygoid motoneurons in the dorsolateral division of the trigeminal motor nucleus. In the guinea pig the tensor veli palatini motoneurons were observed at the level of the whole rostrocaudal extent of the trigeminal motor nucleus, while in the cat they were seen at the level of the rostral two-thirds of the nucleus.     

Identification of motoneurons innervating the tensor tympani muscle in the rabbit: a retrograde horseradish peroxidase study

Vasoactive intestinal polypeptide (VIP) in the concentration of 1 microM or less markedly and selectively increased the amplitude and duration of the muscarinic slow excitatory postsynaptic potential (EPSP) without appreciably affecting the nicotinic fast EPSP or the non-cholinergic EPSP of guinea pig inferior mesenteric ganglion cells. The membrane depolarization evoked by the muscarinic agonist, acetyl-beta-methylcholine in these neurons was similarly enhanced by VIP. Our results suggest that the peptide may be a neuromodulator effective in enhancing the sensitivity of postsynaptic muscarinic receptors to acetylcholine in the vertebrate sympathetic ganglia.     

The distribution and origin of the ipsilateral descending limb of the brachium conjunctivum. An autoradiographic and horseradish peroxidase study in the rat

Summary The distribution, organization and origin of the ipsilateral descending limb of the Brachium Conjunctivum (B.C.), have been studied in the rat by using anterograde and retrograde tracing techniques. After injections of tritiated leucine/proline into the lateral cerebellar nucleus, covering both its medial part, corresponding to the dorsolateral hump (DLH) of Goodman et al. (1963) and its lateral part, (designated here as the lateral dentate, LD), and the neighboring interposed nucleus (NI), emerging fibres are numerous and leave laterally from the B.C. On the contrary, injections restricted to LD reveal very few such fibers. Within the lateral parvocellular reticular formation (LPRF) terminal labelling is heavy, and moderate to sparse within the adjacent trigeminal complex. Rostro-caudally, silver grain accumulation within the LPRF extends from the level of the motor trigeminal nucleus (VM) to the pyramidal decussation, exhibiting a cephalocaudal decrease of grain density. Within the trigeminal complex, labelling occurs in the caudal VM, the dorsal portion of the principal sensory nucleus, and within and around the trigeminal spinalis oralis. In addition, the area surrounding the VM (in part corresponding to the supratrigeminal region of Lorente de Nó 1922, 1933) is moderately labelled. After injections of HRP into various levels of the ipsilateral descending B.C.'s projection field, retrogradely labelled cells are numerous within the DLH. A slightly lesser amount of labelled cells are found in the lateral half of the NI, primarily concerning the nucleus interpositus posterior. Within the LD, only a few labelled cells are observed: these are mainly restricted to the dorsal portion at rostral levels of the nucleus. The results obtained by both the anterograde and retrograde studies suggest an absence of a topographic organization within this descending B.C. component. The possible functional meaning of these results is discussed.Abbreviations B.C. Brachium conjunctivum - DLH Dorsolateral hump - FTN Fibers of the trigeminal nerve - IO Inferior olive - LD Lateral dentate - LPRF Lateral parvocellular reticular formation - NI Interposed nucleus - NIA Nucleus interpositus anterior - NIP Nucleus interpositus posterior - NVII Facial nerve - PD Pyramidal decussation - TB Trapezoid body - VM Trigeminal motor nucleus - VPS Trigeminal principal sensory nucleus - VSC Trigeminal spinalis caudalis subnucleus - VSI Trigeminal spinalis interpolaris subnucleus - VSO Trigeminal spinalis oralis subnucleus - VII Facial nucleus - XII Hypoglossal nucleus This work was submitted as partial fulfillment for the degree of Doctorat de 3ème cycle at the Université Pierre et Marie Curie     

Postsynaptic dorsal column neurons in the cat: a study with retrograde transport of horseradish peroxidase

Summary In situ hybridization histochemistry and RNA blots were used to study the expression of glutamic acid decarboxylase (GAD) mRNA in rats with or without a unilateral lesion of midbrain dopamine neurons. Two populations of GAD mRNA positive neurons were found in the intact caudate-putamen, substantia nigra and fronto-parietal cortex. In caudate-putamen, only one out of ten of the GAD mRNA positive neurons expressed high levels, while in substantia nigra every second of the positive neurons expressed high levels of GAD mRNA. Relatively few, but intensively labelled neurons were found in the intact fronto-parietal cerebral cortex. In addition, one out of six of the GAD mRNA positive neurons in the fronto-parietal cortex showed a low labeling. On the ipsilateral side, the forebrain dopamine deafferentation induced an increase in the number of neurons expressing high levels of GAD mRNA in caudateputamen, and a decrease in fronto-parietal cortex. A smaller decrease was also seen in substantia nigra. However, the total number of GAD mRNA positive neurons were not significantly changed in any of these brain regions. The changes in the levels of GAD mRNA after the dopamine lesion were confirmed by RNA blot analysis. Hence, midbrain dopamine neurons appear to control neuronal expression of GAD mRNA by a tonic down-regulation in a fraction of GAD mRNA positive neurons in caudate-putamen, and a tonic up-regulation in a fraction of GAD mRNA positive neurons in fronto-parietal cortex and substantia nigra.     

The precise origin of the tectospinal pathway in three common laboratory animals: a study using the horseradish peroxidase method.

The horseradish peroxidase tracing method has been used to study the cells of origin of the tectospinal projections in the opossum, the tree shrew, and the cat. The present data show that only those collicular neurons which occupy the deep (ventral to the stratum opticum) tectal laminae send axons to the cervical spinal cord. In particular, layer IV contains the greatest number of spinal projecting neurons. Our results also reveal that while only the large sized collicular neurons project upon the cervical spinal cord in the opossum and the tree shrew, neurons comprising several different size categories do so in the cat. We thus suggest that several different descending channels exist over which the superior colliculus can influence the neck musculature in the cat.     

The relationship of dorsal root afferents to motoneuron somata and dendrites in the adult bullfrog: a light and electron microscopic study using horseradish peroxidase

The relationship of lumbar dorsal root afferents to lateral motor column motoneurons was studied using anterograde injury filling of dorsal roots and retrograde injury filling of ventral roots with horseradish peroxidase. At the light microscopic level, horseradish peroxidase labelled dorsal root axons were observed to separate into a medial division of large diameter axons which enter the dorsal funiculus and a lateral division of small diameter axons which form a compact bundle in the dorsolateral funiculus which may be homologous to the mammalian tract of Lissauer. Within the spinal gray, primary afferents terminate in two distinct regions. The more ventral of these terminal fields, which receives collaterals of primary afferent axons in the dorsal funiculus, overlaps the dendritic arborizations of the lateral motor column motoneurons. Some axons leave the ventral terminal field to enter the dorsal lateral motor column. Here they terminate on the primary dendrites and somata of lateral motor column motoneurons. At the electron microscopic level, labelled primary afferent terminals were seen to synapse upon lateral motor column motoneuron dendrites as well as upon the somata of dorsally positioned lateral motor column motoneurons. These terminals contain small spherical vesicles and occasional dense-cored vesicles. The synaptic specializations are characterized by a small amount of postsynaptic material. The lateral motor column may be divided into dorsal and ventral portions on the basis of the primary afferent distribution and this is in accord with functional, physiological and developmental data.     

Harmaline induced tremor III. A combined simple units,horseradish peroxidase,and 2-deoxyglucose study of the olivocerebellar system in the rat

Summary Purkinje cells were recorded extracellularly and mapped in the cerebellar cortex of the rat under tremogenic doses of harmaline. Four différent types of responses were encountered, of which two were considered as being responsible for the harmaline tremor. The latter had a regular firing pattern of complex spikes at 5 to 10 Hz and were mostly found in the vermis. Their number decreased in the more lateral region of the cerebellar cortex until they eventually disappeared. Horseradish peroxidase was injected into all the areas of the cerebellar cortex containing Purkinje cells with harmaline-induced activity. Labeled neurons were in all cases traced to the medial accessory olive. The metabolic activity of the inferior olive under harmaline was measured with 2-deoxyglucose. Increased labeling was only found in the medial accessory olive. Such an increase was demonstrated as being due to a direct effect of the drug on the inferior olivary neurons, indicating that the medial accessory olive is responsible for the harmaline tremor in the rat. Our results point out that, in the rat, there is an inverse relationship between serotoninergic innervation of a region in the inferior olivary nucleus and that with harmaline sensitivity, therefore a serotoninergic mechanism hypothesis for the harmaline tremor needs further investigation.Assistant from University Paris VII     

Synaptic structure and axon collaterals of type B neurons in bullfrog sympathetic ganglia: intracellular horseradish peroxidase (HRP)-labeling study

Type B neurons of the bullfrog sympathetic ganglia were examined to confirm the existence of axon collaterals and the distribution of synaptic contacts using the intracellular horseradish peroxidase (HRP) labeling method. The mean diameter of the perikarya was 60.8 (+/- 11.5 standard deviation; n = 36) X 43.8 (+/- 11.3) microns and the mean diameter of the initial segments of axons was 6.0 (+/- 1.8; n = 36) microns. Axon collaterals were found in 6 cells among 36 examined. They branched from axons at 61-167 microns from the perikaryon of origin. Short-axon collaterals containing vesicles (diameter: about 70 nm) were also observed to protrude from the stem axons. Spine-like processes were observed from the cell soma, axon hillock and the initial segment of the axon. They enclosed synaptic axon varicosities, or extended into the extracellular space without any synaptic contact. Serial sections revealed 171 axon varicosities in contact with a single ganglion cell; 32 (18.7%) varicosities were seen on the somata. 66 (38.6%) on the axon hillock and 73 (42.7%) on the initial segment of the axon which extended 100 microns from the perikaryon. Synaptic terminals were also found on the axon as far as 494 microns from the cell body of origin. These findings would provide a morphological basis for interaction between bullfrog sympathetic neurons at pre- or postsynaptic sites.     

Brainstem projections to lumbar motoneurons in rat--I. An ultrastructural study using autoradiography and the combination of autoradiography and horseradish peroxidase histochemistry

In 11 rats the descending projections from the ventrolateral medullary medial reticular formation, the medullary raphe nuclei and the area of the nucleus coeruleus and subcoeruleus to lumbar motoneuronal cell groups were studied by means of electron microscopical autoradiography after [3H]leucine injections in the respective brainstem areas. The distribution of the transported radioactivity in the autoradiographs was determined using the circle method [Williams (1977), in Practical Methods in Electron Microscopy, Vol. 6, pp. 85-173] which showed that the vast majority of the silver grains was located over terminal profiles. In the motoneuronal cell groups six different types of terminals were distinguished. After injections in the ventrolateral medial reticular formation the majority of the silver grains was located over F-type terminal profiles while many fewer silver grains were found over S- and G-types. After injections in the raphe nuclei and the adjoining medial reticular formation approximately equal numbers of silver grains were found over F- and G-type terminals while fewer were found over S-type. A small proportion of silver grains was present over C-type terminals and only after injections in the ventrolateral medial reticular formation. After [3H]leucine injections in the area of the nucleus coeruleus and subcoeruleus the majority of silver grains were located over E- and S-type terminals whereas relatively few were located over F-type terminals. The E-type terminal, which has not been described before in the motoneuronal cell groups, is characterized by the fact that it contains relatively small vesicles and occasionally elongated or canaliculi-like structures. In the three groups of experiments approximately 40-50% of the labelled S- and F-type terminal profiles established synaptic contacts, but only approximately 10% of the labelled E- and G-type terminal profiles did so. In all cases these synaptic contacts were established mainly with proximal dendrites. In the autoradiographs some of the silver grains were concentrated into clusters. The vast majority of these clusters, consisting of six or more silver grains, were centred over terminal profiles. The differential distribution of these clusters over the different types of terminal profiles in the various experiments was roughly the same as found by means of the circle method. In two rats [3H]leucine injections in the ventrolateral medial reticular formation were combined with horseradish peroxidase injections in the ipsilateral hindleg muscles, resulting in retrograde labelling of the corresponding motoneurons as visualized by means of the tetramethyl benzidine incubation method.(ABSTRACT TRUNCATED AT 400 WORDS)     

The afferent connections of the inferior olivary complex in rats: A study using the retrograde transport of horseradish peroxidase

Retrograde transport of horseradish peroxidase (HRP) was used to define the origin of afferents to the inferior olivary complex (IOC) in rats. Using both ventral and dorsal surgical approaches to the brainstem, HRP was injected into the IOC through a micropipette affixed to the tip of a 1-μl Hamilton syringe. After a 2-day postoperative survival, animals were sacrificed by transcardiac perfusion with a 1% paraformaldehyde-1.25% gluteraldehyde solution, and brains were processed according to the DeOlmos protocol (1977), using o-dianisidine as the chromogen. Labeled cells were found at many levels of the nervous system extending from lumbar spinal cord to cerebral cortex. This wide-ranging input from numerous regions clearly underscores the complexity of the IOC and its apparent involvement in several functions. Within the spinal cord, labeled neurons were identified from cervical to lumbar but not at sacral levels. These neurons were found contralaterally in the neck region of the dorsal horn and in the medial portions of the intermediate gray. In the caudal brainstem, reactive cells in the dorsal column nuclei, the spinal trigeminal nucleus, and the subnucleus y of the vestibular complex were observed primarily contralateral to the injection sites. Labeling within the gigantocellular, magnocellular, ventral, and lateral reticular nuclei and the nucleus prepositus hypoglossi was primarily ipsilateral. Reactive neurons in the medial and inferior vestibular nuclei were predominantly ipsilateral or contralateral to HRP injections into the caudal or rostral IOC, respectively. The dentate and interposed nuclei of the cerebellum contained small, lightly labeled neurons primarily contralateral to the injection site, while the fastigial nuclei contained a few relatively large, heavily labeled cells bilateral to caudal olivary injections. Ipsilaterally labeled mesencephalic regions included the periaqueductal gray, interstitial nucleus of Cajal, rostromedial red nucleus, ventral tegmental area, medial terminal nucleus of the accessory optic tract, nucleus of the optic tract, and the lateral deep mesencephalic nucleus. The caudal part of the pretectum and small cells of the stratum profundum of the superior colliculus were labeled predominantly contralateral to the injection. In the caudal diencephalon labeled neurons were most numerous within the nucleus of Darkschewitsch and the subparafascicular nucleus, primarily ipsilateral to olivary injections. Scattered reactive neurons were also found within the ipsilateral zone incerta. With the exception of the zona incerta, all labeled mesencephalic and diencephalic nuclei had some bilateral representation of labeled cells. No labeled neurons were identified within the basal ganglia, while numerous reactive cells were found bilaterally within layer V of the frontal and parietal cerebral cortex.     

The striatonigral projection and nigrotectal neurons in the rat. A correlated light and electron microscopic study demonstrating a monosynaptic striatal input to identified nigrotectal neurons using a combined degeneration and horseradish peroxidase procedure

In a light and electron microscopic study of the substantia nigra of the rat, the distribution and morphology of nigrotectal neurons and the pattern of termination of striatonigral fibres have been examined following the placement of horseradish peroxidase injections in the superior colliculus and kainic acid lesions in the dorsal striatum. In confirmation of previous findings, nigrotectal neurons which had been identified by the retrograde transport of horseradish peroxidase from the superior colliculus had mainly medium sized somata, varied from fusiform to stellate in shape and were found in mainly ventral regions of the rostral two-thirds of the substantia nigra pars reticulata. On electron microscopic examination, single and multiple (from two to six) degenerating striatonigral boutons were found in synaptic contact with the soma, proximal mainstem dendrites and small dendrites (but mainly on small dendrites) of labelled nigrotectal and unlabelled nigral neurons in the ventral region of the pars reticulata. In addition, a small number of degenerating striatonigral boutons formed axoaxonic synapses with degenerating or normal boutons which were presynaptic to nigral dendrites. Almost all of the identified striatonigral synapses were of the symmetrical type, although a few degenerating boutons established asymmetrical synaptic contacts on unlabelled dendrites. These findings provide evidence of a monosynaptic input from the dorsal striatum to nigrotectal projection neurons in the substantia nigra and thus demonstrate the existence of a bineuronal pathway from the striatum through the substantia nigra to the superior colliculus. The possible significance of the pattern of termination of striatonigral fibres in the substantia nigra is discussed with reference to the known dendritic arborization of nigral neurons.     

Ultrastructural and electrophysiological properties of accessory abducens nucleus motoneurones: An intracellular horseradish peroxidase study in the cat

The physiological and morphological (light and electron microscopy) properties of six retractor bulbi motoneurones were analysed using the technique of intracellular recording and intracellular labelling with horseradish peroxidase. The retractor bulbi motoneurones were identified by antidromic invasion and orthodromic responses following stimulation of trigeminal afferents were studied. Two of these motoneurones were examined ultrastructurally. Terminal boutons forming synapses with labelled soma, labelled proximal and distal dendrites were characterized. Serial sections allowed the axon hillock to be analyzed and the initial segment of a presumed motoneurone to be observed in the section where the injected motoneurone was described. The ultrastructure of unidentified elements observed in the accessory abducens nucleus is stressed.     

Upper cervical inspiratory neurons in the rat: an electrophysiological and morphological study

Upper cervical inspiratory neurons form a distinct neuronal column located near the lateral edge of the intermediate grey matter in the rostral spinal segments. Previous studies conducted in cats have demonstrated synaptic inputs to these neurons from several respiratory related regions of the medulla, and long descending axonal projections mainly towards the motoneurons supplying the intercostal muscles. The aim of this study was to examine the electrophysiological and morphological properties of this propriospinal system in the rat. Extracellular recordings were made from 127 cervical inspiratory units, mainly in the C₁ and C₂ segments. Eighty-two percent could be antidromically activated from the C₇/C₈ border. No evidence of monosynaptic connection was obtained by cross-correlating the activity of some of these units with the discharge of the phrenic nerve. Intracellular recordings were made from seven neurons, three of which were labelled with biotinamide (neurobiotin). Long survival times after intracellular injections (up to 23 h) resulted in staining of axons for long distances, at least to the C₅ segment. Each of the three labelled axons issued only one short collateral which arborized in the region of the phrenic nucleus. These results demonstrate that upper cervical inspiratory neurons in the rat have features similar to those previously described in the cat, including only a limited projection to the phrenic nucleus. In addition, this study provides the first morphological identification of these neurons.     

The motor innervation of the single-bellied digastric muscle in the rabbit: A retrograde horseradish peroxidase study

The digastric muscle of the rabbit consists of a single anterior belly which inserts onto the lower jaw. Horseradish peroxidase was injected into the muscle and into subcutaneous regions overlying the lower jaw to determine the sites of origin of the motor innervation to both the digastric muscle and the platysma muscles. After digastric muscle injection, labelled cells were found in the ipsilateral retrotrigeminal nucleus as well as in the intermediate subnucleus of the main facial nucleus on both sides. Subcutaneous injections produced labelling which was found bilaterally in the intermediate subnucleus and in the ventromedial portion of the medial subnucleus. These results are interpreted in relation to the common embryological origin of these two muscles and their innervation.     

Dendritic and axonic fields of purkinje cells in developing and X-irradiated rat cerebellum. a comparative study using intracellular staining with horseradish peroxidase

Intracellular staining of cerebellar Purkinje cells with horseradish peroxidase was achieved in normal developing rats (8–13 days old), in normal adult rats and in adult rats in which the cerebellum had been degranulated by X-ray treatment. The mono- and multiple innervation of Purkinje cells by climbing fibres was electrophysiologically determined and correlated with their dendritic pattern and axonal field.In immature rats, considerable variations in dendritic arborization were observed between cells at the same age, according to their position in the vermis. In adult X-irradiated animals, a large variety of dendritic shapes was found, confirming previous anatomical data, but no obvious correlation was found between the morphology of the dendrites of Purkinje cells and their synaptic investment by climbing fibres.As regards the axonal field, the adult branching pattern of recurrent axon collaterals was almost established by postnatal day 8, except for some cells which exhibited richer recurrent collaterals. On the other hand, in X-irradiated animals, profuse plexuses were the rule and they originated either from one collateral stem, or from several collaterals, also independently of the number of afferent climbing fibres. The existence of these enlarged recurrent collateral plexuses can be explained by the persistence of an immature stage, and certainly also by collateral sprouting following the largely impaired innervation of the terminal field during development.These results emphasize the role of the cellular interactions that occur during Purkinje cell growth in the formation of both its axonal and dendritic fields.     

Identification and distribution of the spinal and hypophyseal projection neurons in the paraventricular nucleus of the rat. A light and electron microscopic study with the horseradish peroxidase method

Summary The distribution of labeled neurons in the paraventricular nucleus of the hypothalamus (PVN) was studied following injections of horseradish peroxidase (HRP) into the spinal cord (C8 to T1) or the hypophysis in the rat. Injections were also made in the spinal cord in another group of animals, which were subjected to water deprivation for a period of 3 days, and the PVN of these animals was examined with the electron microscope.Spinal projection neurons (paraventriculospinal tract, PVST, neurons) formed two groups; the dorsal and the ventral groups. They were located within the parvocellular part of the PVN and fused into one at the caudal level. The neurons of the dorsal group were well assembled whereas those of the ventral group were intermingled with paraventriculohypophyseal tract (PVHT) neurons, which were concentrated in the magnocellular part. Electron microscopic observations revealed that HRP-labeled neurons after spinal injections did not contain neurosecretory granules and that they were not affected by water deprivation. On the other hand, neurons containing a number of neurosecretory granules displayed a significant degree of dilatation of the endoplasmic reticulum as the result of water deprivation. These neurons contained no HRP granules.The present findings suggest that the PVST neurons are distinct from the PVHT neurons and that the neuronal groups of both systems form different cell columns within the nucleus.Abbreviations C caudal - D dorsal - Mgc magnocellular part - NH neurohypophysis - PVHT paraventriculohypophyseal tract - PVN paraventricular nucleus - PVST paraventriculospinal tract - R rostral - SC spinal cord - V ventral - III third ventricle