Cellular neuroscience
Postnatal appearance of 5-HT2A receptors on fast flexor and slow extensor rat motor neurons

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Abstract

Motor neurons to the slowly contracting extensor soleus muscle in behaving rats begin to fire tonically in the 2nd week after birth. In the adult, tonic firing becomes predominant and appears to arise from plateau potentials under monoaminergic control. In the present work, motor neurons to slowly contracting extensor soleus and rapidly contracting extensor digitorum longus, a physiological flexor muscle, were retrogradely labeled with fluorescent dextran and examined for immunoreactivity to 5-HT2A receptors in 1 and 2 week old and adult rats. No reactivity was detected at 1 week. At 2 weeks, reactivity was detected on 67% slowly contracting extensor soleus (16 of 24) and 19% extensor digitorum longus (11 of 57) motor neurons. In the adult, the intensity of staining was higher and the percentage of labeled motor neurons 79 for slowly contracting extensor soleus (34 of 43) and 31 for extensor digitorum longus (11 of 35). On slowly contracting extensor soleus motor neurons, labeling appeared more often on soma and dendrites than on dendrites only, whereas on extensor digitorum longus motor neurons, labeling appeared more often on dendrites only. These results are consistent with the hypothesis that serotonergic innervation contributes to the appearance and subsequent increase in tonic firing of rat slowly contracting extensor soleus motor neurons in postnatal development.

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Subjects

One day to 2 week old male and female Wistar rats and adult male Wistar rats weighing ∼300g (B&K Universal, Sollentuna, Sweden) were used. Care was taken to minimize suffering of the animals and to keep the number of experimental animals as low as possible. The experiments were approved by the Norwegian Animal Research Authority in accordance with Norway’s Animal Protection Act and overseen by the veterinarian responsible for the animal house at the department. The Norwegian legislation

5-HT2A receptors appear on SOL and EDL motor neurons between postnatal days 7 and 14

Injection of TMR-dextran into EDL and SOL muscles on postnatal days 1–3 resulted in retrograde labeling of EDL and SOL motor neurons, the column of SOL motor neurons being located more caudally than the EDL column (Fig. 1). On postnatal day 7, neither SOL motor neurons (Fig. 2B) nor EDL motor neurons (not shown) displayed immunoreactivity for 5-HT2A receptors. But cell structures around the central canal were labeled (Fig. 2B inset). On postnatal day 14, the antibody labeled both SOL and EDL

Discussion

Three findings stand out from this work. First, SOL and EDL motor neurons begin to display detectable 5-HT2A receptors between postnatal days 7 and 14. Second, many more SOL than EDL motor neurons display such receptors. Third, on SOL motor neurons, the receptors appear more often on cell body and dendrites than on dendrites only, whereas on EDL motor neurons, they appear more often on dendrites only.

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    1

    Present address: Department of Orthopaedics, Lund University Hospital, Lund, Sweden.

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