The origin of the reticulo-olivary projection in the rat: a retrograde horseradish peroxidase study

Spontaneous synchronized neural activity in decerebrate gallamine-paralysed cats

In decerebrate cats paralysed with gallamine, over a period of several days there develops a remarkable synchronization of discharge in widely different motor nerves throughout the body, including intercostal nerves and limb nerves. These discharges are also in synchrony with slow waves approximately 100 ms in duration in the inferior olive. The slow waves and discharges are at first irregular and only weakly synchronized, but become increasingly strongly synchronized and by about the fourth day exhibit a strong 6-8 Hz rhythm. The degree of synchronization is greater the lower the end-tidal CO2 concentration. Transection of the spinal cord at a high cervical level breaks the synchrony and may abolish the discharge in the nerves, but the slow waves in the inferior olive continue rhythmically. It was shown, however, that gallamine injected subdurally at cervical vertebra 7 or lumbar vertebra 7 has a direct excitatory action on the spinal cord. Slow waves in the inferior olive are elicited by gallamine in the decerebrate, spinalized and decerebellectomized cat, and therefore must originate in the brainstem. Gallamine is known to act directly on olivary neurons and the slow waves may originate in the inferior olive, but further experiments are needed to determine what other structures it affects. The condition of the cat a few days after decerebration and paralysis resembles the clinical condition of reticular reflex myoclonus and it is suggested that the genesis of the myoclonus may be similar in the two conditions.

Connections of a vagal communicating branch in the ferret. II. Central projections

There is increasing interest in the central mechanisms involved in the regulation of gastrointestinal function. The ferret is becoming widely used for research in this area. However, knowledge of the brain stem organization of this species is inadequate. As part of an on-going study designed to provide information regarding the site of termination of abdominal afferents, the central connections of a supradiaphragmatic vagal communicating branch were determined in the ferret through the use of the horseradish peroxidase (HRP) tracing technique. The branch was exposed using a thoracotomy and HRP crystals were applied to the cut ventral end of the branch. Following a 72 hour survival period, the animals were reanesthetized and perfused. The brain stem was removed and processed using the tetramethylbenzidine method. Afferent terminals were found bilaterally in the nucleus of the solitary tract (nTS), area postrema (AP), the dorsal motor nucleus of the vagus (DMV), the external cuneate nucleus (ECN) and the principal subnucleus of the inferior olive (IOP). This is the first study of a brain stem projection of a specific vagal branch in this species, and demonstrates the similarities and differences which exist between the ferret and other species.

Cell bodies of origin of serotonin-immunoreactive afferents to the inferior olivary complex of the rat

Previous studies have used immunohistochemistry to localize serotonin to distinct olivary nuclei in several mammalian species. However, the location of the cell bodies of origin for the serotoninergic projection to the inferior olive in any of these species was unknown. In the present study, a paradigm which combines transport of horseradish peroxidase (HRP) and serotonin immunohistochemistry (PAP) was used to identify the cell bodies of origin of this afferent system to the inferior olivary complex of the rat. Cells which contain both retrogradely transported HRP and brown cytoplasmic staining indicating that they are serotoninergic cells that project to the inferior olivary complex are found exclusively in an area dorsal to the rostrolateral dorsal accessory olive within the nucleus reticularis paragigantocellularis. Neurons within this nucleus were also found to be a source of serotoninergic afferents to the cerebellum and spinal cord of the rat. This raises the possibility that individual serotonin-immunoreactive neurons within this nucleus may project to all 3 areas. Future studies will be designed to address this possibility. No double-labeled cells were observed within any of the raphe nuclei.

Localization of glutamic-acid-decarboxylase-immunoreactive axon terminals in the inferior olive of the rat, with special emphasis on anatomical relations between GABAergic synapses and dendrodendritic gap junctions

Immunocytochemical and electron microscopic methods were used to examine the GABAergic innervation of the inferior olivary nucleus in adult rats. This neuronal system was visualized with an antibody against glutamic acid decarboxylase (GAD, EC 4.1.1.15), the GABA-synthesizing enzyme. A GAD-positive reaction product was encountered only in short segments of preterminal axons and in axon terminals. Their relative number per unit area of neuropil was very similar in all olivary subnuclei. Despite this homogeneity in density, obvious intraregional differences existed. Some regions were strongly immunoreactive (the "c" subgroup, the beta nucleus, and the mediolateral outgrowth of the medial accessory olive), whereas others were weakly labeled (the dorsomedial cell column and the central zones of the medial accessory and principal olives). The strongly immunoreactive areas contained the largest and most intensively labeled axon terminals. Areas of weak labeling were filled with small, weakly immunoreactive nerve terminals. Thus, variations in size and in intensity of labeling create a specific pattern of GABA innervation, revealed by an almost continuous gradient between the above-mentioned extremes. The GAD-positive axon terminals established conventional synapses with dendrites (94% of the samples) or with cell bodies (6%). The vast majority of these synapses were type II (84%) and only a small proportion formed type I synaptic contacts (16%), regardless of the nature of the postsynaptic element. Immunoreactive terminals were also involved in the complex synaptic arrangements--the glomeruli, which characterize the olivary neuropil. Within these formations, olivary neurons were electrotonically coupled through dendrodendritic gap junctions. There was a constant association between GAD-positive axon terminals and small dendritic appendages linked by gap junctions. This association was revealed not only by the systematic presence of immunolabeled terminals directly apposed to the dendritic appendages but, more importantly, by the frequent presence of type II synapses straddling both elements. These synapses were in close proximity to the low-resistance pathways represented by the gap junctions. The strategic location of these GABA synapses is discussed in relation to recent findings indicating the possibility of a synaptic modulation of the electrical coupling: the release of GABA, by increasing nonjunctional membrane conductance, could shunt the coupling between olivary neurons. The functional decoupling of selected gap junctions would be responsible for the spatial organization of the olivary electrotonic coupling.

Reticulo-olivary circuits: an intracellular HRP study in the rat

Intracellular recordings were obtained from medullary reticular neurons subsequent to electrical stimulation of the ipsilateral or contralateral inferior cerebellar peduncle (ICP) and/or the midbrain. After recording physiological data, the neurons were intracellularly injected with horseradish peroxidase (HRP). Thirty-four HRP filled neurons were subjected to light microscopic analysis. They could be divided into two general groups: those which extend dendritic processes into the neuropil of the inferior olivary complex (n = 19); and those that have no anatomical relationship to the inferior olive (n = 15). These two populations of reticular neurons differ in their distribution, morphological characteristics and physiological responses. Neurons which extend dendritic processes into the inferior olive are located within 200 microns of the dorsal border of this nuclear complex, between the exiting fibers of the XIIth nerve and the raphe. The cell bodies are located in the nucleus reticularis gigantocellularis and are fusiform or multipolar in shape. Their dendrites extend for long distances in the mediolateral direction; are thin and relatively spine-free except at their distal tips where spines and varicose appendages are evident. Physiologically, midbrain stimulation elicits a fast rising hyperpolarization which is identified as an inhibitory postsynaptic potential. However, only rarely is a response observed subsequent to stimulation of either the ipsilateral or contralateral ICP. Dendrites from 4 neurons from the first group of reticular cells were analyzed at the ultrastructural level. Based on random and serial thin sections, the following features were noted: they contain numerous mitochondria when compared to olivary dendrites; they contribute to the postsynaptic elements within olivary synaptic clusters (glomeruli); and they exhibit focal clusters of synaptic vesicles although conventional synaptic complexes have not been observed. Reticular neurons of the second group, those that do not extend dendritic processes into the inferior olive, are located either lateral to the XIIth nerve or at distances greater than 200 microns from the dorsal border of the inferior olivary complex. Their cell bodies and dendrites are comparable morphologically to the reticular neurons whose dendrites do arborize in the inferior olive. However, rarely are the distal tips of their dendrites characterized by spines or varicose appendages. Physiologically, this population of reticular neurons respond to midbrain stimulation with a low amplitude, short latency depolarizing potential which is interrupted by a hyperpolarizing potential.(ABSTRACT TRUNCATED AT 400 WORDS)

The identification of some sources of afferent input to the rat nucleus basalis magnocellularis by retrograde transport of horseradish peroxidase

Neurons of the nucleus basalis magnocellularis (NBm) of the rat are contained within the ventromedial globus pallidus and adjacent internal capsule. Horseradish peroxidase injection limited to the ventromedial globus pallidus result in sparse neuronal labeling in a variety of brainstem, thalamic and hypothalamic nuclei, and the basal nuclei identified after NBm injections. Thus, these contiguous regions have comparable subcortical inputs. By contrast, only NBm injections yielded a large number of labeled neurons in layer V of NBm cholinergic neurons. in addition to the reciprocity observed between NBm and frontal cortex, the ventral tegmental area and NBm likewise appear to be reciprocally connected.

Substance P and serotonin immunoreactivity in the rat inferior olive

The present study analyzes in detail the distribution of substance P and serotonin immunoreactivity in the inferior olivary complex of the rat using the indirect antibody peroxidase-antiperoxidase technique. The entire dorsal accessory olive contains an abundance of substance P fibers and varicosities. In addition, the dorsal cap of Kooy, nucleus Beta, parts b and c of the medial accessory olive, and the dorsomedial cell column contain numerous substance P immunoreactive elements. Fibers and varicosities positive for serotonin are present throughout the entire dorsal accessory olive. In addition, the rostral medial accessory olive contains an abundance of serotonin immunostaining. Scattered serotonin immunoreactive elements are present in the principal olive in particular in the area immediately subjacent to the lateral aspect of the dorsal accessory olive. Only scant serotonin immunolabeling is present in the dorsal cap of Kooy, nucleus Beta and the dorsomedial cell column. Speculations on possible functional roles of these two putative neurotransmitters are presented.