Morphology of sympathetic preganglionic neurons in the thoracic spinal cord of the cat: an intracellular horseradish peroxidase study

The intermediolateral nucleus: an 'open' or 'closed' nucleus?

The sympathetic preganglionic neurons located in the intermediolateral nucleus (IML) that project to the superior cervical ganglion of the rabbit were observed to have two major dendritic orientations after retrograde labeling with horseradish peroxidase. One projection extends longitudinally within IML. The second projection courses medially and presents a triangular shape in horizontal sections. The labeled processes that project medially arise from cells in IML and project through the intercalated nucleus towards the central autonomic area and follow the contour of the central canal. Medially oriented dendrites intruding into other areas of the intermediate grey matter show that IML is an 'open' rather than a 'closed' nucleus as has been recently suggested. The location and distribution of the sympathetic preganglionic neurons projecting to the superior cervical ganglion in the rabbit are compared with those reported for other species.

Fluorescent carbocyanine dyes allow living neurons of identified origin to be studied in long-term cultures

A prerequisite for many studies of neurons in culture is a means of determining their original identity. We needed such a technique to study the interactions in vitro between a class of spinal cord neurons, sympathetic preganglionic neurons, and their normal target, neurons from the sympathetic chain. Here, we describe how we use two highly fluorescent carbocyanine dyes, which differ in color but are otherwise similar, to identify neurons in culture. The long carbon chain carbocyanine dyes we use are lipid-soluble and so become incorporated into the plasma membrane. Neurons can be labeled either retrogradely or during dissociation. Some of the labeled membrane gradually becomes internalized and retains its fluorescence, allowing identification of cells for several weeks in culture. These dyes do not affect the survival, development, or basic physiological properties of neurons and do not spread detectably from labeled to unlabeled neurons. It seems likely that cells become retrogradely labeled mainly by lateral diffusion of dye in the plane of the membrane. If so, carbocyanine dyes may be most useful for retrograde labeling over relatively short distances. An additional feature of carbocyanine labeling is that neuronal processes are brightly fluorescent for the first few days in culture, presumably because dye rapidly diffuses into newly inserted membrane. We have used carbocyanine dyes to identify sympathetic preganglionic neurons in culture. Our results indicate that preganglionic neurons can survive in the absence of their target cells and that several aspects of their differentiation in the absence of target appear normal.

Three types of sympathetic preganglionic neurones with different electrophysiological properties are identified by intracellular recordings in the cat

Intracellular recordings were obtained from sympathetic preganglionic neurones (SPN) of the third thoracic segment in cats. Based on differences in their active and passive electrophysiological properties, three different types of SPNs were discerned: Type A neurones had a high resting membrane potential (RMP) (-60 to -86 mV) and a low input resistance (RN) 12-23 M omega). Action potentials of these neurones had a pronounced IS-SD inflexion and a prominent shoulder in their falling phase. Spikes were rarely generated from the on-going synaptic activity. Type B neurones had a lower RMP (-48 to -65 mV) and a higher RN (21-37 M omega). Their action potentials were characterized by an after-depolarization; they showed a slight IS-SD inflexion and a less pronounced shoulder in their falling phase. The after-depolarization was abolished by membrane hyperpolarization in a time dependent way. A hyperpolarization of at least 50 ms duration was required for its abolition. The after-depolarization was also abolished during repetitive discharges. In most of these neurones spikes were generated at irregular intervals and low rates (0.06-4.6 spikes/s) from the synaptic activity. Type C neurones were similar to type B neurones, but their action potentials did not show the after-depolarization. Additionally, spikes were generated at fairly regular intervals and rather high rates (0.8-6.5 spikes/s). The rate of spike repolarization of all neurones was markedly increased by hyperpolarization and decreased by membrane depolarization. Current-voltage curves of some type B and C neurones showed a marked rectification upon membrane hyperpolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological properties of sympathetic preganglionic neurons in the cat spinal cord in vitro

Intracellular recordings were obtained from sympathetic preganglionic neurons of the intermedio-lateral nucleus of the adult cat in slices of upper thoracic spinal cord maintained in vitro. The neurons were identified by their antidromic responses to stimulation of various ipsilateral sites. Sites from which antidromic responses could be evoked were the white ramus, the ventral root, the ventral root exit zone, the white matter between the latter and the outer edge of the tip of the ventral horn, the lateral edge of the ventral horn. Resting membrane potential was --61.3 +/- 1.6 mV (mean +/- SEM), input resistance 67.5 +/- 3.7 M omega, time constant 11.5 +/- 1.2 ms. The amplitude of the action potential generated by antidromic or direct stimulation was 77.4 +/- 2.3 mV. Threshold for direct spikes was 18.2 +/- 1.8 mV. The action potential had an average duration of 3.03 +/- 0.16 ms. It showed a prominent "hump" on the falling phase. The action potential had a tetrodotoxin (TTX)-sensitive and a TTX-resistant component. The latter was abolished by cobalt. Tetraethylammonium, cesium and barium prolonged the action potential duration which acquired a plateau-shape. A prolonged after-hyperpolarization (AHP) followed the sympathetic preganglionic neuron spike. Following a single spike, AHP duration and peak amplitude were 2.8 +/- 0.3 s and 16.6 +/- 0.7 mV, respectively. The AHP was abolished by cesium or barium, but enhanced by tetraethylammonium. An AHP followed the TTX-resistant spike. EPSPs and IPSPs could be generated by focal stimulation. The EPSP triggered spikes when threshold (15.0 +/- 2.0 mV) was reached.(ABSTRACT TRUNCATED AT 250 WORDS)