Michael D. Gershon

Serotonin: Synthesis and Release from the Myenteric Plexus of the Mouse Intestine

After injection of its radioactive precursor, 5-hydroxytryptophan, radioactive serotonin was biosynthesized and bound in the myenteric plexus of the mouse intestine. Addition of nonradioactive serotonin to preparations in vitro caused a net release of radioactive serotonin from the plexus. This release appeared to result from activity in the intramural nervous system of the intestine. A neurotransmitter role between sensory and motor neurons in the peristaltic reflex pathway is suggested as a working hypothesis to explain the action of serotonin.

The role of the raphe nuclei in the regulation of ponto-geniculo-occipital wave activity

Abstract A study has been made of the effect of brain stem lesions upon ponto-geniculo-occipital PGO) wave activity. The results include the following. (1) Midline lesions, in the region of the raphe nuclei, caused a transient increase in waves having the characteristics of PGO REM . After 1–4 days these waves were suppressed during wakefulness but continued to appear during slow wave sleep, as well as during REM sleep. (2) The disturbance in the regulation of PGO REM was greatest in the animals with large lesions, involving several raphe nuclei. (3) Unilateral parasagittal cuts, 1.5 mm lateral to the midline in the region of the raphe nuclei, caused changes in PGO wave activity similar to those following midline lesions of comparable extent. (4) Pentobarbital, administered several days after a raphe lesion, gave rise to a transient increase in PGO wave activity. The recordings resembled those obtained s shortly after the lesion was made. (5) The onset of drug-induced wave activity, following treatment with PCPA, was accelerated in animals with midline lesions of the raphe area. (6) Stimulation of the pacemaker region of the pons, on the side ipsilateral to an extensive parasagittal cut, consistently evoked lateral geniculate responses during slow wave sleep, as well as during REM sleep. Stimulation of the pacemaker on the opposite side of the pons evoked lateral geniculate responses only during REM sleep. These results support the hypothesis that 5-HT containing neurons, having their perikarya in the raphe nuclei, play an important role in the normal their perikarya in the raphe nuclei, play an important role in the normal regulation of PGO REM .

Eye movement potentials in the oculomotor and visual systems of the cat: A comparison of reserpine induced waves with those present during wakefulness and rapid eye movement sleep

A study has been made of the ponto-geniculo-occipital (PGO) waves associated with eye movement in the cat. Three charactersitcs serve to distinguish the PGO waves normally present during wakefulness (PGOw) from those recorded during REM (PGOREM): (1) cortical PGOW are attenuated in amplitude in the dark and abolished by lateral optic nerve section, whereas cortical PGOREM are unaltered by these procedures; (2) cortical PGOREM often contain an initial surface negative component never present in cortical PGOW; and (3)_lateral geniculate PGOW are invariably much smaller in amplitude than PGOREM recorded from the same electrode. The observation that PGOW are reduced in amplitude in the dark provides further support for the notion that these potentials correspond to lambda waves Stimulation of the pontine reticular formation evokes responses within the visual system which resemble spontaneous PGO waves. During wakefulness the evoked responses have the characteristics of PGOW and during REM they have the characteristics of PGOREM. The results suggest that PGOW and PGOREM are closely related, but not identical, phenomena, both dependent upon the activity of pacemakers localized in the pons. The PGO waves induced by reserpine (PGORes) resemble PGOREM, rather than PGOW, despite the fact that animals remain awake when the waves are present. Throughout the period when during induced waves appear, similar potentials are evoked by stimulation of the pontine reticular formation. Thus, reserpine appears to act by opening the gating mechanisms which normally confines PGOREM to episodes of REM.

Uptake of serotonin by intrinsic neurons of the myenteric plexus grown in organotypic tissue culture.

The myenteric plexus contains axons, not found elsewhere in the peripheral nervous system, which are distinguished by a specific, high affinity transport system for serotinin (5-HT). This study was undertaken to determine the location of the cell bodies of origin of these axons. Vagotomy decreased uptake of [3H]5-HT and tritiated norepinephrine ([3H]NE) by the myenteric plexus. However, while examination by histofluorescence revealed the presence of descending vagal adrenergic fibers, no evidence was found for the presence or accumulation of 5-HT above a vagal ligature. Vagus nerves thus contain adrenergic but not serotonergic axons. The gut was also denervated of all extrinsic axons by growth of intestinal explants in organotypic tissue culture for 3 weeks. Uptake of [3H]5-HT persisted while uptake of [3H]NE was lost. Light and quantitative electron microscopic radioautography revealed that, as in intact gut, the elements of the cultures responsible for uptake of [3H]5-HT were axons distinguished by varicosities containing large dense cored vesicles. In conclusion, these experiments establish that the mammalian gut contains intrinsic neurons which selectively take up 5-HT. The capacity of these neurons for 5-HT uptake may be influenced by the vagus nerves.

Separately developing axonal uptake of 5-hydroxytryptamine and norepinephrine in the fetal ileum of the rabbit

Uptake of 5-hydroxytryptamine (5-HT) by adult and fetal rabbits ileum was studied. The adult myenteric plexus accumulated tritium when incubated with tritiated 5-HT. However, in addition to labeled 5-HT, tritiated 5-hydroxyindole acetic acid and, when monoamine oxidase (MAO) was inhibited, 5-HT-o-glucuronide were found in the tissue. Two uptake processes differing in affinity could be defined. Only the high affinity process was saturable. Fetal ileum took up tritiated 5-HT but glucuronidation did not occur when MAO was inhibited. The uptake of tritiated 5-HT by the fetal ileum was due to a single, saturable, temperature sensitive (Q10 at 27-37 degress C = 2.4) process inhibited by ouabain. It was identical to the high affinity uptake found in adult tissue. This specific high affinity uptake could be found as early as the 16th day of gestation, 5-8 days before uptake of norepinephrine (NE) begins. Light and electron microscope radioautography revealed that the uptake of 5-HT was primarily into axons and a characteristic structure called the expanded process, both in the myenteric plexus. Both contained dense-cored vesicles. Axons were not labeled by tritiated NE until after 24 days and the expanded process was never labeled by tritiated NE. This study shows that uptake of 5-HT is a property of distinct system of axons in the mammalian myenteric plexus which develops prior to adrenergic axons during ontogeny.

An analysis of the effect of reserpine upon ponto-geniculo-occipital wave activity in the cat*

Abstract The effect of reserpine upon ponto-geniculo-occipital wave activity has been studied, using implanted electrodes, in a series of experiments on unrestrained cats. Five stages of action of reserpine have been defined. Stage 1: Drug induced waves (PGORes) resembling the waves normally present during REM sleep (PGOREM), appear and gradually increase in frequency, but may be suppressed by arousing stimuli. Stage 2: PGORes occur with a very regular frequency of 15–25 waves min and cannot be suppressed by arousal. Animals are heavily sedated, but awake. Stage 3: PGORes become modulated in frequency and amplitude. Both cortical synchrony and a low voltage cervical EMG are associated with enhanced wave activity. Stage 4: PGORes gradually become inhibited during periods of behavioral arousal. Stage 5: REM sleep returns, but REM-type PGO waves continue to appear during slow wave sleep, several minutes before the onset of REM sleep. Regulation of PGORes, during stages 3 and 4, can be accounted for by postulating that this activity is inhibited by arousal. Regulation of PGORes during stage 5 suggests the existence of an additional influence, which is maximal at the end of each REM sleep episode and declines progressively to become minimal in the minutes immediately preceding the next period of REM sleep.

Regeneration of adrenergic axons in rat sciatic nerve: Effect of a conditioning lesion

Abstract The rat sciatic nerve was crushed at the level of the mid-thigh (testing lesion), and the rate of outgrowth of the regenerating adrenergic axons was examined in the nerve. Two methods were used: (1) counts of axons containing norepinephrine demonstrable by histofluorescence; (2) measurement of the specific uptake of [3H] d,l -norepinephrine in vitro. In order to determine whether a prior nerve lesion had any effect on the outgrowth rate, all of the animals were subjected, two weeks prior to the testing lesion, to either a transection of the tibial branch of the sciatic nerve at the level of the ankle (conditioning lesion) or a sham operation. Counts of fluorescent axons made 3 days after the testing lesion showed that at 1.40 mm proximal to the lesion, the numbers of axons in both the conditioning-lesion group and the sham-operated group were not significantly different from non-lesioned animals. At a level 0.35 mm proximal to the testing lesion, counts in both groups were about 75% greater than at 1.40 mm, indicating that axonal sprouting had occurred. At 1.40 mm distal to the testing lesion, the numbers of axons had decreased by 9% in the sham-operated group, and by 24% in the group that had been subjected to a conditioning lesion (P The specific uptake of [3H] d,l -norepinephrine was measured 3 and 7 days after the testing lesion. The axonal component of uptake showed an approximately exponential decay with distance from the testing lesion; the leading edge of axonal uptake advanced at a rate of 3.9 ± 0.5 mm/day (S.E.) in the sham-operated group compared to 1.8 ± 0.6 mm/day (S.E.) in the conditioning-lesion group (P

Brain stem serotonin depletion and Ponto-Geniculo-Occipital wave activity in the cat treated with reserpine☆

Abstract In the first part of this study the time course of depletion of serotonin (5 HT) was determined in several regions of the brain following treatment with reserpine (0.75 mg kg , i.p.). The results were compared with the temporal course of ponto-geniculo-occipital wave activity (PGORes) induced by similar drug treatment. These experiments revealed a good correlation between the initial decrease in 5 HT throughout the brain and the appearance of PGORes. During recovery, however, 5 HT levels remained low at the time when regulation of PGO wave activity began to return to normal. In the second part of this study, the effect upon PGO wave activity of infusions of reserpine into the ventricular and subarachnoid spaces was investigated. Infusions of reserpine phosphate (250 μg) into the fourth ventricle caused changes in wave activity similar to those induced by systemic administration of the drug. In contrast, infusions into several different regions of the subarachnoid space caused only minor changes in PGO wave activity. 5 HT levels, measured at that time when PGORes appeared following infusion of reserpine into the fourth ventricle, were significantly depressed only in the pons. These results are consistent with the hypothesis that monoaminergic neurons exert a tonic inhibitory influence at the brain stem level which serves to regulate or gate REM-type PGO waves.

Studies of the interaction of 5-hydroxytryptamine and the perivascular innervation of the guinea-pig caecum.

1 The action and interaction of noradrenaline (NA), 5‐hydroxytryptamine (5‐HT) and the sympathetic innervation was studied in the isolated taenia of the guinea‐pig caecum. 2 Addition of 5‐HT led to a contraction of the taenia while addition of NA or perivascular nerve stimulation resulted in relaxation. Responses to 5‐HT or perivascular nerve stimulation were abolished by tetrodotoxin. Tetrodotoxin did not affect responses to applied NA. Hexamethonium and hyoscine converted the 5‐HT response to a relaxation and augmented the relaxation which followed low frequency perivascular nerve stimulation. Hexamethonium and hyoscine did not affect the dose‐response relationship for NA. 3 Fatigue of mechanical responses of the taenia to perivascular nerve stimulation was accelerated when nerves were stimulated in the presence of 5‐HT or α‐methyl‐p‐tyrosine (α‐MPT). These two agents were additive in this action. 4 Reserpine, 6‐hydroxydopamine and α‐MPT all reduced the NA content of the taenia. However, only after 6‐hydroxydopamine could adrenergic activity be related to NA content. 5 Segments of taenia were incubated with either tritiated NA or 5‐HT. An increased rate of release of radioactivity followed perivascular nerve stimulation after incubation with either substance. This release did not occur when tissue was taken from animals given reserpine or 6‐hydroxydopamine. 6 It is concluded that 5‐HT activates neural elements exclusively while NA has a direct effect on smooth muscle. 5‐HT can apparently be taken up by adrenergic axons, and appears to enter the releasable neurotransmitter pool. Since none of the actions characteristic of 5‐HT are seen when it is released by adrenergic axons as a false neurotransmitter, the released amine probably fails to reach neuronal receptors for 5‐HT.