Introduction to the session on modulators

&wropharmaco/ogy Vol. 26, No. 7B, pp. 1019-1026, Printed in Great Britain

INTRODUCTION

0028-3908/87

1987

$3.00 + 0.00

Pergamon Journals Ltd

TO THE SESSION ON MODULATORS A. G. KARCZMAR

Department of Pharmacology, Loyola University Medical Center, Maywood, Illinois, U.S.A. Key words: modulation, presynaptic, postsynaptic, acetylcholine, catecholamines, GABA, sensitization, desensitization, neurotransmitters, peptides, second messenger.

EARLYCONCEPTS OF MODULATION It is difficult to assign the first formulation of the concept of modulation to any particular investigator or group of investigators. Certain aspects of this concept were raised, without using the term modulation by W. Feldberg and Martha Vogt in 1948. At that time, they demonstrated, by means of the bioassay for acetylcholine (ACh), that regions of the brain rich in ACh alternated with regions poor in ACh; very heu-

ristically, they suggested that cholinergic transmission alternated in the central nervous system with other neurochemical transmissions and they postulated that there was an interaction (modulation?) in the brain between various neurotransmitter systems. A neurochemical correlate of this finding was presented in this laboratory (Glisson, Karczmar and Barnes, 1972) as it was demonstrated that anticholinesterase-induced accumulation of ACh led to changes in the levels and presumably the turnover of the catecholaminergic and indolaminergic systems. This kind of interaction cannot be, however, considered in the modern sense of modulation, unless this interaction between more than one transmitter is envisaged as occurring at a single synaptic junction, rather than taking place in a circuit composed of a number of synapses; this possibility was not thought of either by Feldberg and Vogt (1948) or subsequenty by others. It must be added that in the 195Os, B. B. Brodie clearly envisaged the possibility that more than one neurotransmitter may be released from one single synaptic nerve terminal (B. B. Brodie, personal communication, 1955). Thus released, several neurotransmitters may interact at a single syanpse and this interaction may result in the modulation of the primary transmission in the synapse in question, as described below; indeed, such processes have been described at this Symposium, for example in B. S. Bunney’s description of central dopamine-peptide interactions. Close to this concept of modulation, the modification of the synaptic response at a particular single synapse was demonstrated by Eccles in the 1960s. In fact, at that time, he had shown modulations, although again not referred to by this particular term, as depending on two kinds of mechanisms, presynaptic and postsynaptic. As shown in Fig. 1, the postsynaptic modulatory mechanism arises when two presynap-

tic afferents to a single neuron are stimulated, more or less simultaneously, evoking an interaction between the inhibitory postsynaptic potential and the excitatory postsynaptic potential. It must be stated that this case can be considered as that of modulation of as primary response which is excitatory in nature; this example would not, perhaps, constitute modulation if both potentials were considered to be primary responses at the synapse in question, this point will be discussed in more detail when a definition of modulation is presented. The other modulatory mechanism clearly envisaged by Eccles was that involving presynaptic inhibitory transmission (E&es, 1964a, b). The example of interaction is shown in Fig. 1 which illustrates how the depolarization of a nerve terminal by an afferent fiber, which is presynaptic with regard to this nerve terminal, diminishes the excitatory postsynaptic potential, the primary response, by diminishing the release from the nerve terminal of a transmitter. In the 1950s other investigators, including ourselves, were also interested in the problems of the control of a primary postsynaptic response, and the mechanism that was explored was the process of desensitization demonstrated at the neuromyal junction by Thesleff (1955) as well as by the present author (Karczmar and Howard, 1955; Karczmar, 1957). Desensitization was envisaged and what was referred to subsequently as sensitization (Karczmar, 1967) as processes that safeguard transmission, at least at the neuromyal junction, affording a protection and ensuring the maintenance of function (Fig. 2; Karczmar, Nishi and Blaber, 1972); parenthetically, the term “modulation” was employed at that time. It should be added that the processes of densensitization and sensitization appear today to depend on a specialized function of the receptor, very possibly arising at a specific subunit of the receptor (Raftery, Conti-Tronconi and Dunn, 1985), the essential effect being the allosteric modulation demonstrated with respect to the y-aminobutyric acid (GABAergic) receptor by Costa and his associates; several papers at this Symposium concern the allosteric mechanism of the facilitation of transmission.

1019

DEFINITION OF MODULATION With this introduction

in mind, a specific definition

1020

A. G. KARCZMAR

Fig. 1. Composite diagram showing specimen records of the various types of responses associated with synaptic action. On the right side is an intracellular electrode with specimen records of inhibitory postsynaptic potentials (IPSPs) (downward deflections) and excitatory postsynaptic potentials (EPSPs) (upward deflections) and their interactions, there being two examples of spike inhibition in the lower row. Excitatory and inhibitory synapses are shown on the neuron, as is a presynaptic inhibitory ending abutting on the excitatory fiber, which is seen to be depolarized (intracellular recording; note the electrode) as shown by the differences in the intracellular and extracellular records in the upper right traces for one, two and four volleys in the PBST nerve. In the upper left quadrant are shown the effects of this presynaptic depolarization (primary afferent depolarization; PAD) on its excitatory synaptic action (EPSP), as recorded intracellularly. Note the large diminution of the excitatory potentials in the lower row. In the upper row the presynaptic inhibition depressed the excitatory potential so that it often failed to generate a spike, as it regularly did in the control (C record, the arrow). In the lower left quadrant is seen the resultant diminution of the reflect spike discharge recorded in the ventral roots, the first being the control response. PBST = combined posterior biceps-semitendinosus. [From Eccles (1963). Legend modified from that for Fig. 20, Eccles (1964a). See also Fig. 92 (Eccles, 1964b). With permission.]

Fig. 2. Cholinergic transmission and the feedback system at the postsynaptic neuromyal site. A number of processes which facilitate and inhibit transmission are indicated by pluses and by minuses, respectively. It can be seen from the figure how certain events produce feedback of negative or positive nature. For instance, when acetylcholine is released in quantities such that a block may arise, its presence in the synaptic cleft induces tolerance phenomena (desensitization), which act in the opposite direction. If accumulation of acetylcholine occurs stepwise, the adaptation will also militate against blockade by acetylcholine. Other feedback mechanisms can be seen. [Legend modified from that for Fig. 2, Karczmar et al. (1972). From Karczmar ef al. (1972). With permission.]

Introduction to the session on modulators of modulation which stems particularly from concepts and experiments carried out over many years by Koketsu (198 1; Koketsu and Karczmar, 1986; Koketsu and Akasu, 1986) may be presented as follows: modulation consists of a change in the primary synaptic response in a manner independent of the neurotransmitter which evokes this primary transmission. The definition may be amplified by dwelling on the teleological aspects of the modulation. As shown above, modulation may be regarded as having as its goal the maintenance of transmission and its protection against excesses that may arise from pharmacological treatment or from activity. In a more general sense, the modulation may be regarded as concerning the conversion of a simple all-or-nothing synaptic relay system, and this was the image of transmission at the time of the first demonstration of chemical synaptic transmission, to a subtle, digital and feedback control of transmission that allows it to react appropriately to manifold inputs and to environmental changes and manipulations. This then leads to the description of the mechanisms available for the modulatory process as defined above. TYPES AND MECHANISMS

OF MODULATION

First of all, it should be clearly stated that, as already known to Eccles, modulations can be localized either at the pre- or postsynaptic membranes (Table 1). Furthermore, the modulatory processes can occur either at quiescent or at active synapses. In the first case, they regulate phenomena characterizing the quiescent synapses, such as the miniature potentials and channel noise. In the second case, they occur in the course of synaptic activity and they regulate the synaptic response as it develops. It must be finally added, that both physiological and pharmacological, intrinsic and extrinsic mechanism may participate in modulation. The specific mechanisms that are involved in presynaptic modulation should be discussed. As shown in Table 2 a number of intrinsic and extrinsic mechanisms must be considered. First of all, the very synaptic activity constitutes an intrinsic mechanism whether through the mechanisms listed in Table 2 or mechanisms not as yet known. For instance, the activity as it deals with the fluxes of calcium and the action potential generated by ionic fluxes, leads to the facilitation of synaptic transmission and to what was referred to as the synaptic excitatory cycle (Hinman, Jacobs and Karczmar, 1976). The intrinsic presynaptic mechan-

isms also involve changes in the synthesis of transmitter; these may be again generated by activity or by other processes not very clearly understood at this time. For instance, the activity of choline acetyltransferase (CAT) is affected by transmission and repetitive stimulation of cholinergic synapses as discussed at length by Tucek (1978, 1984). Also, second messengers, whether components of the phosphatydil inositol cascade or cyclic nucleotides, contribute to the synthesis of neurotransmitters as well as to the processes of release and the facilitation or inhibition of release as proposed by Greengard and his associates (Nestler and Greengard, 1984); indeed, processes of phosphorylation of proteins evoked through the second messengers induce conformational changes in neuronal, including presynaptic, membranes. Many of these intrinsic mechanisms of presynaptic modulation depend on the presence of neurotransmitters and bioactive substances present either in the nerve terminal which activates the primary process of the transmission or in its immediate vicinity, in glial cells or in the blood (see below, and Tables 4 and 5). Thus, coexistence and releasability of neurotransmitters, such as catecholamines and GABA, at cholinergic synapses underlies the regulation of the relase of ACh at these synapses; in fact, ACh itself contributes to the control of its release by automodulatory processes similar to the action of dopamine on autoreceptors at dopaminergic synapses. The facilitatory and inhibitory effects of ACh at autoreceptors were postulated for the first time by George Koelle (1963) as he proposed the percussive role for ACh. Subsequently, inhibitory muscarinic nerve terminal receptors were shown to exist both at the periphery and the CNS (Polak, 1965; Szerb, 1977; Koketsu and Karczmar, 1986; Karczmar, 1986). Table 2 lists a number of extrinsic phenomena that also regulate the release of transmitter. Drugs may indeed affect the synthesis of neurotransmitters and second messengers, as well ionic fluxes; what the Table shows is the effect of neurotransmitters exogenously supplied or neurotransmitter-like drugs on the release of neurotransmitter. It must be stressed that the sensitivity of the nerve terminal to drugs was demonstrated early in the Bejing laboratories of Feng and others (Feng and Li, 1941); subsequently, W. F. Riker and F. Standaert (Standaert and Riker, 1967), stressed the importance of these effects at motor and ganglionic nerve terminals and suggested that they implicate the nerve terminal as a regulatory site of transmission. The specific mechanisms that underlie postsynaptic modulation are shown in Table 3 (Koketsu and Karcz-

Table I. Types of modulation INTRINSIC

(ENDOGENOUS)

and ENTRINSIC PRESYNAPTIC

SpontaneousRelease Evoked

1021

Release

POSTSYNAPTIC

(PHARMACOLOGICAL)

1022

A. G.

tiCZtdAR

Table 2. Presynaptic GENERAL INTRINSIC

modulations

ACTIONS MECHANISMS

Activity Ca2+ and Other Ions Transmitter

Synthesis and Turnover

Second Messengers Neurotransmitters EXTRINSIC

and Phosphorylations and Modulators MECHANISMS

Drugs, Exogenously Applied Transmitters and Modulators. and Transmitter-like Agents SPECIAL

EFFECTS

ON NERVE

TERMINAL

ENDOGENOUS

AND RELEASE

OF TRANSMITTER

SUBSTANCES

GABA. Catecholamines, ACh. Prostaglandins. Enkephalins and Endorphins, 5-Hydroxytryptamine, Histamine EXOGENOUS

SUBSTANCES

Drugs Including Cholinomimetics and Hvdroxvanilininiums

mar, 1986; Koketsu and Akasu, 1986). The early example of modulation proposed by Eccles, i.e. the interaction between depolarizations and hyperpolarizations, is shown in the Table as intrinsically or extrinsically acting neurotransmitters and modulators, such as catecholamines, GABA, etc., which affect the postsynaptic resting potential making it less or more amenable to primary transmission. Another type of modulation has been described at this Symposium, namely the allosteric modulation of the receptor; the effects in question are listed in the Table, as those involving receptor sensitivity. The increase and decrease of sensitivity may in turn relate to such a phenomena as the binding of the receptor and the number of functional receptor-channel macromolecules, as described by Koketsu (1981). The increase and decrease in the number of receptors, i.e. up- and down-regulation may be considered as a related aspect

Table 3. Postsynaptic EFFECTS

modulations

ON RESTING

Catecholamines, EFFECTS

POTENTIAL

5-HT. ATP, GABA,

ON ACTION

ACh

POTENTIAL

Action on G,, and G, ACh, Catecholamines. ATP. Polypeptides EFFECTS

ON RECEPTOR-CHANNEL Endogenous

Catecholamines,

5-HT. Histamine, Exogenous

Blockers. Neurotoxins. DOWNDamage,

ATP, Polypeptides,

ACh

Agents

Sensitizers

and Desensitizers

AND UP-REGULATION

Drugs and Neurotoxins, EFFECTS

SENSITIVITY

Agents

Endogenous

ON CATABOLIC

Endogenous

and Exogenous

TROPHIC

ACTIONS

ENZYMES Agents

Substances

of receptor sensitivity and this effect may be caused by nerve damage, activity, or finally, pharmacological manipulations such as, in the case of choline& synapses, the use of anticholinesterases. The receptor sensitivity will be, of course, changed when the receptor or the channel is affected by drugs, such as receptor and channel blockers; this effect may occur also through the activity of endogenous blockers and sensitizers or facilitators. It must be stressed here that desensitization is an all pervasive phenomenon that occurs following the action of all transmitters, including excitatory amino acids, ACh or inhibitory neurotransmitters, such as GABA or catecholamines. In fact, in order to identify in this laboratory the transmitter involved in the postsynaptic response to presynaptic stimulation, it was demonstrated that this response was desensitized by prior superfusion or electrophoretic application of the putative neurotransmitter used as a pharmacological substance (Dun and Karczmar, 1979). It is of interest that desensitization, particularly of cholinergic synapses through anticholinesterases or depolarizing blocking substances, such as succinylcholine, is very difficult to reverse by pharmacological and, presumably, endogenous means; in fact, in these laboratories, some 30-40 neuromyally active substances were evaluated in an attempt to antagonize neuromyal desensitization, due to anticholinesterases and accumulation of ACh, and only a few such agents were found to exert this action Ohta, Akasu and Karczmar, 1987). Another postsynaptic regulatory effect, which is less well known and less appreciated than those already referred to, and is listed in Table 3, concerns the postsynaptic spike or action potential. A number of substances, whether acting intrinsically or extrinsically as pharmacological agents, can significantly affect the action potential (Koketsu and Karczmar, 1986; Koketsu and Akasu, 1986). The changes in the action potential and related phenomena, such as after-

1023

Introduction to the session on modulators

etc., affect the excitability and the refractory period of the neuron. Furthermore, changes in the pattern or duration of the action potential affect the release of the transmitter from the nerve terminal of the neuron in question. Finally, the changes in the action potential may reflect changes in the membrane, other than those in the resting potential, which may affect the responses of the postsynaptic membrane to presynaptic stimulation. Still other factors that may prove increasingly important in the future are the trophic factors that may be present in, and released from, the nerve terminals or generated elsewhere and made available for synaptic modulation. While these factors, similar to the nerve growth factor of Levi-Montalcini (1964) may relate to neuronal and/or tissue growth (see also Karczmar, 1946) rather than to transmission and similar factors may regulate distribution and organization and perhaps generation, of receptors (Peng, 1987; Karczmar, 1987) and thus affect transmission. In the case of still another item listed in Table 3 is stressed the obvious, namely that many substances may affect the enzymes present in the synapse that regulate transmission by terminating the action of the neurotransmitters at the postsynaptic receptor or protect the nerve terminal from undue action of the transmitter (Koelle, 1963). While catabolic enzymes may be less important than uptake in nerve terminals in terminating the action of catecholamine transmitters, the enzymes such as cholinesterases are very effective in terminating the action of transmitters such as ACh. Again, it must be stressed that, besides drugs, endogenous substances may affect the catabolic enzymes; many years ago, the late Theodore Koppanyi and the author, demonstrated, without identifying them chemically, that naturally-occurring inhibitors of acetylcholinesterase inhibitors are present at peripheral cholinergic synapses (Karczmar and Koppanyi, 1954).

potential

hyperpolarization,

Table 4. Occurrence and

PRESENCE OF A MULTITUDE OF BIOGENIC, BIOACTIVE SUBSTANCES IN NERVOUS TISSUE It is important to note that the armamentarium needed for the intrinsic modulations discussed above and listed in Tables 2 and 3, is available in the synapses and/or nervous and satellite tissues. A ganglion which, as noted above, cannot be considered today as a simple relay system but indeed as a convenient model for the brain itself (Karczmar, 1986), is a good example of a neuronal site where the tools needed for synaptic modulation are present. As shown in Tables 4 and 5, the ganglion, its satellite cells or the neighboring blood, are very rich in substances involved in presynaptic and postsynaptic modulation. Table 4 lists nine bioactive peptides that were identified by 1986 in the sympathetic ganglia. In addition to the peptides, a number of non-peptide transmitter substances and bioactive modulators are present in the ganglia (Table 5). The list of substances is growing and, of course, it is even more extended in the case of the brain. All the substances in question are involved in the ganglia in pre- and postsynaptic modulation, and ACh itself, as already alluded to, acts in the ganglion and elsewhere as a modulator acting on presynaptic autoreceptors (Koketsu and Karczmar, 1986); furthermore, modulatory effects of the substances listed in Tables 4 and 5, which were demonstrated for the ganglion, occur also in the central nervous system (Karczmar, 1986).

SPECIFIC GLUTAMINERGIC

EXAMPLE: MODULATION OF TRANSMISSION AT MOTONEURONS

A relevant example of pre- and postsynaptic modulation that arises through the action of ACh at the thoracolumbar spinal motoneurons of the rat was described recently in this laboratory by Dun and Jiang (1982). Figures 3 and 4 illustrate that, at this particular locus where the excitatory amino acid,

localization of ueotides in svmoathetic aanalia References

Peptide

Localization

Sympathetic ganglia

Species

BOM CCK DYN ENK

NF NF PGN NF PGN

CMG iMG, CMG CMG IMG, CMG, SCG SCG

Rat Guinea pig Guinea pig, Rat Guinea pig, Rat Rat, Guinea pig

GRH LH-RH SOM

SIF SIF NF PGN PGN

SCG, IMG, CMG Paravertebral CMG Paravertebral SCG, IMG, CMG

Guinea pig, Rat Bullfrog Rat Bullfrog Guinea pig, Rat

SP

NF NF

IMG IMG, CMG, SCG

Cat Guinea pig, Cat

SCG

Rat

Schultzberg and Dalsgaard (1983) Larsson and Rehfeld (I 979) Vincent et al. (1984) Schultzberg et al. (1979) DiGiulio et ol. (1978) Schultzberg et al. (1979) Schultzberg et al. (1979) Kondo (unpublished) Kondo et al. (1983) Jan and Kuffler (1979) Hokfelt et al. (1977a), Leranth et al. (1980) Lundberg et al. (1980) Hokfelt et al. (1977a). Konishi et al. (1979). Dun and Jiang (1982) Robinson et al. (1980)

IMG, CMG, SCG IMG, CMG

Guinea pig, Rat Guinea pig

Hokfelt et 01. (1977b) Hokfelt et al. (1977b)

VIP

(intrinsic) NF PGN

Abbreviations used: inferior mesenteric ganglia (IMG); coeliac inferior mesenteric ganglia complex (CMG); superior cervical ganglia (SCG); small intensely fluorescent cells (SIF); nerve fibers (NF); principal postganglionic neurons (PGN); bombesin (BOM); cholecystokinin (CCK); dynorphin (DYN); enkephalin (ENK); gastrin releasing hormone (GRH); Iuteiniting hormone-releasing hormone (LH-RH); somatostatin (SOM); substance P (SP); vasoactive intestinal peptide (VIP).

A. G. KARCZMAR

1024 Table 5. Bioactive

non-peptide

substances

present in the sympathetic

ganglia

Entities Substance CAs 5-HT ACh PGE, GABA CAMP cGMP

Preganglionic

SIF cell

Primary

Glia circulation

+

+ +

+ + +?

new

Selected references Eranko and Harkonen (1965) Verhofstad ef al. (1981)

+

+ (paras.) +?

f?

Trevisani et al. (1982) Bertilsson el al. (1976) McAfee er al. (1971) see Greengard (I 976)

+ + +

+

Abbreviations used: catecholamines (CAs); serotonin cyclic AMP and GMP (CAMP and cGMP).

(5-HT); acetylcholine

+

glutamate, serves as the primary transmitter, ACh which is not the primary neurotransmitter at this site, acts as a pre- and postsynaptic modulator. Indeed, ACh acts here on the resting potential of the postsynaptic membrane according to the modulatory mechanism discussed above (see Table 3) as well as inhibiting presynaptically the release of glutamate, thus regulating the glutamate-generated excitatory postsynaptic potential. The Figures illustrate also that the inhibitory presynaptic effect of ACh is muscarinic

(ACh); prostaglandin

E, (PGE,);

y-aminobutyric

acid (GABA);

rather than nicotinic, thus resembling the inhibitory effects of ACh exerted at presynaptic sites elsewhere in the CNS (see above). Where does the ACh needed for the effects in question come from? The evidence for this is lacking. While it may be unlikely that ACh is released in this case from presynaptic afferents to the motoneuron, ACh is present, as is well known, in the motoneuron collaterals as it participates in the transmission to the Renshaw cells and serves as a transmitter at other

>

--I= 5

-11Oms

5mV

m,llOnN

lL!M

ATROPINE

A

A

:Ch

ACh

ACh

301

I 1OmV I.5nA

+Ch .5 mM

WASH

A

A

Fig. 3. Depolarization and suppression of EPSPs by ACh in a motoneuron of a rat. A: individual EPSPs evoked by stimulation of dorsal rootlets (arrowhead) before (a) and after (b) pressure ejection of ACh; a and b correspond to the time indicated on recording B. Two hyperpolarizing electrotonic potentials immediately following a and b are shown to the right. B: slow chart recording showing EPSPs (upward deflections) and hyperpolarizing electrotonic potentials (downward deflections) induced by hyperpolarizing current pulses which are represented by downard deflections of lower tracing. Acetylcholine, applied by pressure ejection (arrowheads, 15 msec pulse duration, 40 psi) evoked a slow depolarization and a concomitant decrease of EPSPs. A depression of EPSPs could still be demonstrated when the muscarinic depolarization was nullified by returning the membrane potential to the resting level (recording shown in the middle). Atropine effectively antagonized both the depolarization and synaptic depression induced by ACh. C: antidromic spike evoked by stimulation of ventral rootlets. D: membrane depolarizations induced by pressure ejection of glutamate (arrowheads, 100 mM, 100 msec pulse duration, 40 psi) in a motoneuron. The glutamate response was not affected by ACh (0.5 mM), applied by superfusion. Recordings in A, B and C were taken from the same motoneuron, whereas recordings in D were from a different motoneuron. [From Jiang and Dun (1986). With permission.]

Introduction

to the se!rsion on modulators

(a)

-1

1Omr

5mV

(b) 301

A ACh

I 1OmV

A ACh

Fig. 4. Depolarization and suppression of spontaneous EPSPs by ACh in an unidentified ventral horn neuron. A: fast recording showing individual spontaneous EPSP. B: slow chart recordings showing membrane depolarization and suppressions of spontaneous EPSPs by pressure ejection of ACh (arrowheads, 4 msec pulse duration, 40 psi). [From Jiang and Dun (1986). With permission.]

interneurons of the ventral cord; it may diffuse from these synapses to the sites in question and it may be present in effective concentrations in the local circulation, subserving the circuitry in question. CONCLUSIONS As presented here, many mechanisms are available for the modulation and regulation of primary transmission processes. In parallel, bioactive substances that are capable of subserving the mechanisms in question are present synaptically and extrasynaptically, for instance, in the blood, interstitial fluid, etc. In fact, it was readily demonstrated for the CNS and, even more conclusively for ganglia (Koketsu and Karczmar, 1986; Koketsu and Akasu, 1986), that these substances and probably other substances as yet not identified, can modulate and control peripheral, spinal and supraspinal transmission. Much of the evidence has been presented at this Symposium, for example, by the FIDIA group and by J. M. Musaccio and M. A. Simmonds with regard to GABA and related sites, and by Jenner and Bunney with regard to the modulation of dopamine receptors. These regulations afford central transmission the subtle control of its function and its supercomputer ability to respond, nanosecond by nanosecond and picoamp by picoamp, to changing signals from the environment. REFERENCES Bertilsson L.. Suria A. and Costa E. (1976) y-Aminobutyric acid in rat superior cervical ganglion. Nature, Land. 260: 540-541. Di Giulio A. M., Yang H.-Y. T., Lutold B., Fratta W., Hong J. and Costa E. (1978) Characterization of enkephalinlike material extracted from sympathetic ganglia. Neuropharmacology 17: 989-992. Dun N. J. and Karczmar A. G. (1979) Action of substance P on sympathetic neurons. Neuropharmacol. 18: 2 15-2 18. Dun N. J. and Jiang Z. G. (1982) Non-cholinergic excitatory transmission in inferior mesenteric ganglia of the guinea-pig: possible mediation by substance P. J. Phys-

1025

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A. G. KARCZMAR

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