ntain­ ing cells of the central nervous system has emerged much more slowly. This rela­ tively slower rate of progress appears to have resulted in part from the fact that methods for the analysis of the catecholamine pathways required techniques not previously available, and in part from the fact that once these techniques were available and in widespread use they produced a body of cytological and physiologi­ cal data that indicated that catecholamine neuron systems have properties that differ dramatically from those of "classical" central systems. In this chapter, we seek to review the present status of the central dopamine neuron systems with particular reference to their structure and function. We also place in perspective those structural and functional properties of these neuron systems that set them at variance with certain other well-defined neuronal circuits.

Central Catecholamine Neuron Systems: Anatomy and Physiology of the Dopamine Systems

1. Bullfrog lumbar sympathetic neurones were voltage-clamped in vitro through twin micro-electrodes. Four different outward (K(+)) currents could be identified: (i) a large sustained voltage-sensitive delayed rectifier current (I(K)) activated at membrane potentials more positive than -25 mV; (ii) a calcium-dependent sustained outward current (I(C)) activated at similar positive potentials and peaking at +20 to +60 mV; (iii) a transient current (I(A)) activated at membrane potentials more positive than -60 mV after a hyperpolarizing pre-pulse, but which was rapidly and totally inactivated at all potentials within its activation range; and (iv) a new K(+) current, the M-current (I(M)).2. I(M) was detected as a non-inactivating current with a threshold at -60 mV. The underlying conductance G(M) showed a sigmoidal activation curve between -60 and -10 mV, with half-activation at -35 mV and a maximal value (G(M)) of 84+/-14 (S.E.M.) nS per neurone. The voltage sensitivity of G(M) could be expressed in terms of a simple Boltzmann distribution for a single multivalent gating particle.3. I(M) activated and de-activated along an exponential time course with a time constant uniquely dependent upon voltage, maximizing at approximately 150 ms at -35 mV at 22 degrees C.4. Instantaneous current-voltage (I/V) curves were approximately linear in the presence of I(M), suggesting that the M-channels do not show appreciable rectification. However, the time- and voltage-dependent opening of the M-channels induced considerable rectification in the steady-state I/V curves recorded under both voltage-clamp and current-clamp modes between -60 and -25 mV. Both time- and voltage-dependent rectification in the voltage responses to current injection over this range could be predicted from the kinetic properties of I(M).5. It is suggested that I(M) exerts a strong potential-clamping effect on the behaviour of these neurones at membrane potentials subthreshold to excitation.

M-currents and other potassium currents in bullfrog sympathetic neurones

1. Pulse synchronous bursts of multi-unit sympathetic activity was recorded from median or peroneal muscle nerve fascicles in fourteen subjects resting in the recumbent position. The neural activity was quantitated in terms of burst incidence, i.e. the number of bursts in the mean voltage neurogram/100 heart beats, during successive rest periods of 2-4 min.2. For each individual the burst incidence was fairly constant between different rest periods but the mean burst incidence varied widely between individuals, the range being from less than 10 to more than 90 bursts/100 heart beats.3. Simultaneous double nerve recordings were made on one subject from median and peroneal nerves and on eight subjects from the two peroneal nerves. There was always close similarity between the two records in such experiments regardless of which muscles the nerve fascicles innervated. When analysed separately the difference in burst incidence between the two sides ranged from 0.7 to 5.1 bursts/100 heart beats. The findings suggest that sympathetic neurones destined to skeletal muscles are subjected to a homogenous central drive and that contributions to the activity from ganglionic or segmental sources are of lesser importance.4. On seven subjects repeated recordings at rest were made with intervals of 3 weeks-21 months between recordings. In each subject mean burst incidences were similar in all recordings (range of differences 0.5-11.2 bursts/100 heart beats) suggesting an individually constant level of sympathetic activity in muscle nerves.5. For each individual the variability of burst amplitudes in the mean voltage neurogram was described by burst amplitude spectra. Most subjects had a relatively larger proportion of small than high amplitude bursts, but there was a tendency for more even amplitude distributions in subjects with high burst incidence. The finding may be an indication of interindividual differences in the average number of impulses/burst.6. It is concluded that the multi-unit recording technique can be used for comparisons of the level of muscle nerve ;sympathetic tone' between different subjects.

The variability of muscle nerve sympathetic activity in resting recumbent man.

1. A slow synaptic potential recorded in neurones of sympathetic ganglia of bullfrog, the late slow e.p.s.p., is probably mediated by a peptide resembling luteinizing hormone-releasing hormone (LHRH), because (1). a LHRH-like peptide is contained in preganglionic nerve terminals and is released upon nerve stimulation, (2) application of LHRH to ganglion cells mimics the effects of the natural transmitter for the late slow e.p.s.p., and (3) the pharmacological properties of LHRH and the natural transmitter for the late slow e.p.s.p. are similar. 2. Neurones in frog sympathetic ganglia are also depolarized by substance P. The substance P receptors, unlike the LHRH receptors or the post-synaptic receptors for the late slow e.p.s.p., are not blocked by antagonist of LHRH. No cross-desensitization was found between the substance P-induced response and the LHRH-induced response or the late slow e.p.s.p. 3. Substance P-like immunoreactivity is contained in bundles of axons passing through the ganglia. This distribution is distinct from the distribution of LHRH-like immunoreactivity in preganglionic nerve terminals. Thus, substance P receptors and substance P-containing fibers are distinct from the post-synaptic receptors and preganglionic fibers responsible for the late slow e.p.s.p. 4. Physiological and anatomical evidence is presented which indicates that the peptidergic transmitter for the late slow e.p.s.p. can diffuse for many micrometres before acting on ganglion cells.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1982.sp014228

Peptidergic transmission in sympathetic ganglia of the frog.

A search was made in sympathetic ganglia of the bullfrog for a noncholinergic, nonaminergic transmitter that is released by a distinct group of preganglionic axons. These initiate a late slow excitatory postsynaptic potential which lasts for many minutes. The most promising candidate for the role of transmitter is a peptide that resembles luteinizing hormone-releasing hormone (LHRH; luliberin). The reasons are: (i) LHRH (1 muM) and some of its analogs cause a slow depolarization of ganglion cells. (ii) Radioimmunoassays established that 100-800 pg of a LHRH-like substance is contained in the lumbar chain of sympathetic ganglia. (iii) The LHRH-like material is specifically distributed in those spinal nerves that contain axons that initiate the slow noncholinergic synaptic responses. (iv) Five days after ipsilateral preganglionic axons are cut, 95% of the LHRH-like substance disappears from ganglia, while the LHRH immunoreactivity triples in the spinal nerves proximal to the cut region. (v) About 0.6% of the LHRH-like material within ganglia can be collected from the perfusate after 30 min of incubation in isotonic KC1; this release is Ca dependent. (vi) The candidate for transmitter has several chemical characteristics of a peptide and has a Mr near 1000.

https://www.pnas.org/content/pnas/76/3/1501.full.pdf

A peptide as a possible transmitter in sympathetic ganglia of the frog

The sympathetic ganglion of the toad consists of two kinds of neurons, the sB and sC neurons of which axons are the B and C fibers, respectively. The sB neurons receive a monosegmental innervation of preganglionic B fibers whereas the sC neurons receive mono- or multisegmental innervation of preganglionic C fibers. The pre- and postganglionic neurons are divided into a fast (sB neuron) and slow (sC neuron) conduction system. The conduction velocity of the preganglionic axons is proportional to that of the synaptically connected postganglionic neuron axons in the sB and sC neuron systems. In the sB neuron system, the conduction velocity of the preganglionic axons is faster than that of the postganglionic fibers. The relation is reversed in the sC neuron system. The antidromic and orthodromic responses as well as the electrical membrane properties of sC neurons were analyzed and compared with those of the sB neurons. In both neurons, the time constant of the synaptic potential decay was always longer than that of the membrane. The synaptic potentials of sB and sC neurons were abolished at the membrane potential levels of −10 mv and −7 mv on the average, respectively, which indicates that the equilibrium potentials of these neurons are almost the same. The changes in the excitability of the soma as well as axon of the sB and sC neurons after a single response and the response of the sB and sC neurons to a long cathodal current were analyzed and compared.

Studies on sympathetic B and C neurons and patterns of pregnaglionic innervation.

THE calcium ions in the membrane appear to be involved in some way with the unknown physico-chemical reactions which occur in the membrane and which may be responsible for the changes in the membrane permeability or the membrane potential of excitable tissues1,2. Inasmuch as the membrane is composed of lipoproteins which possess negative phosphoric sites of phospholipids, it is likely that the calcium ions are bound to these sites. The experiment recorded here, therefore, was projected to determine whether or not calcium ions could actually bind to phospholipids, and if so, whether the binding of calcium ions would be affected by monovalent cations or other substances.

Binding of calcium ion to lecithin film.

Autonomic and Enteric Ganglia

Shanes and Bianchi1 have demonstrated that stimulation of frog sartorius muscle accelerates the release of calcium-45 loaded on muscles, although Harris2 had failed to observe this effect. Koketsu and Miyamoto3 recently confirmed an increase in the output of calcium-45 during muscle activity. An important problem remains, however, whether acceleration of calcium output during activity is caused by the mechanism specific of the contractile system or actually is responsible for the electrical membrane phenomena. Hodgkin and Keynes4 have reported that no change in calcium efflux can be detected during stimulation of squid axon, concluding that calcium movement during nervous activity is wholly inward. Furthermore, the 30-fold greater influx of calcium per impulse in muscle than in nerve despite the near equality of the resting influxes has been noted as consistent with a special role for calcium in contraction5.

Release of calcium-45 from frog nerves during electrical activity.

FROG Ranvier's nodes immersed in potassium-rich media are capable of producing action potentials when the membrane is hyperpolarized by anodal currents1. This observation was confirmed in frog sartorius muscles by one of us, who found that action potentials were restored by anodal polarization in frog sartorius muscles which depolarized in calcium-free media2. The so-called ‘hyper-polarizing response’ was also observed in muscle fibres soaked in both potassium-rich and calcium-free media. This experimental evidence seems to suggest that removal of calcium from the membrane in these two solutions might be responsible for the loss of membrane excitability, as suggested in a previous communication2. The rate of output of calcium-45 loaded on muscles, therefore, was studied in both calcium-free and potassium-rich solutions.

K. Koketsu

Papers

Studies on sympathetic B and C neurons and patterns of pregnaglionic innervation.

Binding of calcium ion to lecithin film.

Autonomic and Enteric Ganglia

Release of calcium-45 from frog nerves during electrical activity.

Significance of Membrane Calcium in Calcium-free and Potassium-rich Media