Endogenous levels of tryptophan, serotonin and 5-hydroxyindole acetic acid in the developing brain of the cat

Differential development of the dual serotoninergic fiber system in the cerebral cortex of the cat

The changes in distribution and density of the serotoninergic innervation of the cerebral cortex were studied in kittens from birth (PO) to 60 days of age (P59). Three cortical areas were sampled: prefrontal, primary auditory, and primary visual areas. Two systems of serotoninergic axons were demonstrated by immunocytochemical techniques: the fine axon system characterized by small fusiform varicosities up to 1 micron in diameter, and the beaded axon system, the fibers of which have round varicosities up to 5 microns in diameter. The density of the two types of fibers across the cortical layers at different ages was measured with a semiautomatic computerized system. In all three areas, the density of fine axons increased steadily from birth, although the pattern of innervation changed from an even distribution at PO to a distinct concentration of the fibers in layers I-III by week 2 in the prefrontal cortex and by week 3 in auditory and visual cortices. By contrast, the beaded axons first appeared in the cortex at week 2 for the prefrontal cortex, at week 3 in auditory and visual areas. Initially, these fibers were distributed throughout all cortical layers and were of much lower density than the fine axons. At later ages the beaded axons became confined to layers I-III where they gradually increased in number, and from week 4, they formed pericellular arrays which were only observed in the prefrontal and auditory cortices, not in visual cortex. These findings provide further evidence for the existence of two parallel subsystems of serotoninergic axons which are different not only in their morphology and nuclear origin, but also in their development. Our finding that the two serotoninergic fiber systems arrive in the cortex in two different stages suggests that they have differential roles in development. The late formation of the pericellular arrays indicates that the formation of the specific connections made by the beaded fibers could be dependent on a certain degree of maturity of the target neurons.

Unmasking of a neonatal somatovesical reflex in adult cats by the serotonin autoreceptor agonist 5-methoxy-N,N-dimethyltryptamine

In neonatal kittens, micturition is induced by a spinal somatovesical reflex pathway that is activated by the mother cat licking the perigenital region of the kitten. The somatovesical reflex pathway disappears about the time of weaning and is replaced by a vesicovesical reflex pathway that produces micturition via a supraspinal reflex pathway that is activated by distension of the urinary bladder. Furthermore, stimulation of the perigenital region in adult cats actually inhibits the supraspinal vesicovesical micturition reflex. Spinalization prompts the return of the somatovesical reflex, immediately in weaned kittens but over a course of days to weeks in adult cats. The purpose of the present experiments was to determine if the somatovesical reflex could be demonstrated acutely, and reversibly, in adult cats with an intact spinal cord via pharmacological suppression of the serotonergic system. The serotonergic system was suppressed by the intravenous administration of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), a serotonin agonist that inhibits the firing of serotonergic neurons via activation of inhibitory somatodendritic autoreceptors. 5-MeODMT in low doses (20-50 micrograms/kg) abolished inhibition of the bladder produced by either light tactile stimulation of the perigenital region or by electrical stimulation of the pudendal nerve, which carries the afferent fibers from the perigenital region, in 9 of 10 adult cats. Furthermore, in 8 of the 10 cats, the bladder inhibition was reversed to an excitation of variable amplitudes in each cat. Higher doses of 5-MeODMT (100-1000 micrograms/kg) abolished spontaneous bladder activity but did not inhibit perigenital-induced bladder contractions in those 8 animals in which the drug unmasked the excitatory somatovesical reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes of serotonin and 5-hydroxyindoleacetic acid levels in specific regions of the pigeon central nervous system

Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels were determined in the visual Wulst, optic lobes, retina, cerebellum and brainstem of the pigeon during embryonic and posthatching periods. 5-HT content increased during development in almost all regions. 5-HIAA content generally showed the highest values within the second posthatching week and then decreased to reach adult values. The high 5-HT turnover (as indicated by high (5-HIAA/5-HT ratio) observed over the first posthatching week suggests a possible role of 5-HT on developmental processes which occur in pigeon visual areas over the same time.

Maturation of pyrogen-elicited fever in the kitten

The febrile response to the endotoxin Salmonella typhosa was studied in developing kittens. We found that kittens younger than 30 days of age generated only a small rise in temperature in response to a standardized endotoxin challenge that consistently causes fever in adult cats. Some degree of febrile response was present at birth, but the dose of pyrogen necessary to elicit a fever was 10-15 times greater than that required in the adult. There was a gradual increase in both the magnitude and duration of fever as a function of age with the largest change occurring after 30 days of age. There was a direct relationship between the ability of the kitten to maintain its body temperature (Tbo) at the room ambient (Ta) and the magnitude of the elicited fever. However, increasing the Ta to thermoneutral (Ta = 30-32 degrees C) did not enhance the thermal response indicating that the failure to elicit the fever is not due to passive effects of Ta. These data suggest that the febrile response to an endotoxin develops over the first 6-7 weeks of the kitten's life and are discussed in relation to other physical variables of development.

Ontogenesis of unit activity in the raphe dorsalis of the behaving kitten: its relationship with the states of vigilance

The relationship between the spontaneous unit activity in the raphe dorsalis (RD), and the sleep-wakefulness cycles, was analyzed in the cat from birth to 40 days of age. Electrodes for polygraphic sleep monitoring were implanted under anesthesia, and unit recordings were obtained from bundles of microwires positioned in the RD area in kittens of different ages. Attention was paid only to units with slow firing in wakefulness (W) (1-6 spikes/s), and two types of discharge patterns during this state were obtained: a 'regular' type, whose discharge in W had the same characteristics of regularity as those described for the adult under the same conditions, was always found inside the RD. An 'irregular' type was always found in sites outside the RD. Injections of different doses of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT i.m.) induced a transient decrease in the firing rate of the regular type of cells, and no change for the units of the irregular type, suggesting that the regular neurons were of serotoninergic nature. Whereas the cells of the irregular type exhibited an increase of discharge frequency in active or paradoxical sleep (AS-PS), those which fired in a clock-like manner during W exhibited a rate of discharge which progressively decreased in quiet or slow wave sleep (QS-SWS) and even more in AS-PS. Such a pattern was qualitatively close to the adult one at all ages, but the discharge rate in AS was significantly higher during the first and second weeks of life than later on. The observation that these serotoninergic neurons exhibited at birth an adult-like pattern of discharge during W, indicates that during ontogenesis there was no direct relationship between the RD activity and the behavioral output. It is proposed that the RD neuronal discharge would be largely under genetic influences, and that the maturation of sleep regulations at the brainstem and mesencephalic levels is achieved only after the second week of postnatal age.

The maturation of 5-hydroxytryptamine and 5-hydroxyindole acetic acid in the gerbil (Meriones unguiculatus) brain

5-Hydroxytryptamine (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) stores have been estimated in gerbil cerebral hemispheres and brainstem between one day postpartum and adulthood. Although patterns of 5-HT and 5-HIAA maturation were similar in part to those of the rat and mouse, there was significant difference in the pattern of postnatal 5-HIAA.

Roles for serotonin in neuroembryogenesis

Possible non-transmitter roles for 5-HT in different phases of early neuroembryogenesis have been discussed based upon experimental evidence from the rat and chick. Fluorescence histochemical studies have demonstrated sites of uptake and synthesis of 5-HT in the chick embryo during the first few days of incubation. These sites are located in discrete regions of the notochord and floor plate of the neural tube as well as in extra-neural regions such as the somites and primitive gut. The 5-HT patterns are distinctly different from those observed for the uptake and synthesis of norepinephrine in embryos of the same age. Spatio-temporal changes in the distribution of these sites during closure of the neural tube suggest a role for 5-HT in various aspects of neural tube development. Moreover, the non-overlapping localization of 5-HT and norepinephrine raises the possibility that these two amines may exert different and perhaps cooperative influences on early neurogenic processes in the chick. In the rat, autoradiographic and biochemical studies concerning the consequences of 5-HT depletion in the embryo for development of different brain regions have provided evidence that 5-HT acts as a "differentiation signal" regulating the time of neuronal genesis in those cell populations which will eventually receive 5-HT innervation. Although the details of this system are as yet unknown, these studies suggest that 5-HT (and possibly the other monoamine transmitters) may actually "mold" the construction of their own circuitry during neurogenesis. Further, the ability of drugs and stress to interact with this process during that period of gestation when the monoamines are required as humoral signals suggests that maternal influences can interfere with ontogeny of this circuitry during pre- and possibly postnatal development. It is not yet clear whether the data in chicks and rats can be directly analogized from the one species to the other. Nevertheless, the evidence that sites of 5-HT uptake and/or synthesis are present during the earliest phases of neurogenesis in the chick and the observation that 5-HT depletion can alter the time of genesis of 5-HT target cells in the rat provide a new context for the consideration of possible actions of 5-HT prior to its role as a neurotransmitter substance.

Serotoninergic development in the postnatal rat brain

Various characteristics of the developing serotoninergic system in the brain of rats aged 1 to 28 days were studied biochemically. The levels of the precursor amino acid tryptophan showed a maximal increase in the blood, brain and cerebrospinal fluid (CSF) during the 7th and 10th postnatal days. The development of tryptophan hydroxlyase activity measured in vivo by means of 5-hydroxytryptophan (5-HTP) accumulation after NSD 1015 was closely related to the 5-hydroxytryptamine (5-HT) levels at the various ages. 5-HTP accumulation and 5-HT levels increased most markedly after the second postnatal week. 5-Hydroxyindoleacetic acid (5-HIAA) levels were found to increase rapidly in the brain but somewhat more slowly in the CSF during the second week of postnatal development. Regional studies of 5-HTP accumulation after NSD 1015, 5-HT and 5-HIAA levels indicated a caudal to rostral way of maturation. The disappearance of 5-HT was measured after inhibition of tryptophan hydroxylase with H 22/54. The half-life generally decreased in the various brain parts with advancing age, and in the younger animals the shortest half-life was found in the most caudal brain parts. At 28 days of age the half-life was similar in all brain parts studied. These results indicate the existence of an adult like nerve impulse flow in the 5-HT neurons in the brain stem region of the newborn rats. The results from this investigation clearly indicate that th maturation of the different biochemical parameters of the 5-HT pathways develop in a caudal to rostral direction. The study also supports the view that tryptophan hydroxylase may be the limiting step in the development of the serotoninergic system.