Serotonin regulates synaptic connections in the dentate molecular layer of adult rats via 5-HT1a receptors: evidence for a glial mechanism.

The present study sought to verify effects of 5-HT on synaptic density at the ultrastructural level, to determine whether the 5-HT1a receptor is important for the maintenance of synaptic connections and to obtain evidence implicating S100 beta in the apparent neurotrophic actions of 5-HT. Reduction of hippocampal 5-HT with para-chloroamphetamine (PCA) resulted in a significant decline in the synaptic density of the dentate molecular layer. Reduction of norepinephrine with DSP-4 produced a slight decrease in the number of molecular layer synapses, but this difference was not statistically different from control values. 5-HT1a antagonist treatment resulted in a decline in synaptic density comparable to that observed following PCA treatment. These observations suggest that 5-HT functions to maintain synaptic connections in the dentate molecular layer via a 5-HT1a mechanism. To determine whether the change in synaptic density was due to the action of 5-HT on neuronal receptors or astrocytic receptors, a monoclonal antibody against S100 beta was infused into the lateral ventricle for seven days. Controls received infusions of normal goat serum. Half of the rats from the anti-S100 beta and control groups also received daily injections of NAN-190. Anti-S100 beta infusion resulted in a significant (p < 0.01) decrease in synapses compared to serum controls. Concomitant NAN-190 administration did not enhance synapse loss in the anti-S100 beta group. The results of this study suggest that the maintenance of synaptic connections in the dentate molecular layer is influenced by S100 beta levels that are controlled by 5-HT stimulation of astrocytic 5-HT1a receptors.

Electrical stimulation of the anterior ethmoidal nerve produces the diving response.

Stimulation of the upper respiratory tract usually produces apnea, but it can also produce a vagally mediated bradycardia and a sympathetically mediated increase in peripheral vascular resistance. This cardiorespiratory response, often called the diving response, is usually initiated by nasal stimulation. The purpose of this research was to investigate the anterior ethmoidal nerve (AEN) that innervates the nasal mucosa of muskrats (Ondatra zibethicus). Electrical stimulation of the AEN (typically 50 Hz, 100 micros and 500 microA) produced immediate and sustained bradycardia and cessation of respiration similar to that of the diving response. Heart rate (HR) significantly decreased from 264+/-18 to 121+/-8 bpm, with a concurrent 4.2+/-0.9 s apnea, during the 5 s stimulation period. BP decreased from 97.9+/-4.8 to 91.2+/-6.4 mmHg. Using estimations from (1) cross-sectional areas of AEN trigeminal ganglion cells labeled with WGA-HRP, and (2) electron microscopic analysis of the AEN, we found that approximately 65% of the AEN is composed of unmyelinated C-fibers. In addition, 72.4% of myelinated fibers from the nerves that innervate the nasal passages were of small diameter (<6 microm, presumably Adelta fibers). Thus, the AEN of the muskrat contains a high concentration of small diameter fibers (89.8%). We conclude that electrical stimulation of small diameter fibers within the AEN of muskrats can produce the cardiovascular and respiratory responses similar to that of the diving response.

Synaptogenesis in the postnatal rat fascia dentata is influenced by 5-HT1a receptor activation.

Neonatal serotonin (5-HT) depletion or 5-HT1a receptor blockade results in a significant and permanent reduction in the number of dentate granule cell dendritic spines. The purpose of this study was to determine whether the loss of spines was accompanied by a reduction in molecular layer synaptic profile density. Rat pups were treated with parachloroamphetamine (PCA), 5,7-DHT or the 5-HT1a receptor antagonist NAN-190. The synaptic profile density (profiles/100 microm2) of the dentate molecular layer was estimated on P14, P21 and P60. Molecular layer synaptic profile density the was significantly reduced in each treatment group on P14 and P21. By P60, the NAN-190 and PCA groups had reached control values, but the 5,7-DHT group remained significantly lower than control. The most dramatic changes were observed among synapses terminating on dendritic spines. Numbers of profiles forming simple spine contacts were significantly reduced by all treatments at P14 and P21, but returned to normal by P60 in the PCA and NAN-190 groups. Simple spine synapses in the 5,7-DHT group remained significantly below control, but numbers of complex spine synapses were higher than either control or the other treatment groups at each age. These results indicate that the loss of dendritic spines observed following 5-HT depletion or 5-HT1a antagonist treatment is accompanied by a decrease in synaptic profile density. This effect appears to be a retardation of synaptogenesis since recovery occurs once 5-HT1a receptor activity resumes. Data from the 5,7-DHT group shows that complex spine synapse formation may represent an effort to attain some degree of functional compensation when synaptogenesis is slowed.

Neuronal dye coupling in the developing rat fascia dentata.

The present study describes dye coupling among neurons of the developing rat fascia dentata following impalement and intracellular filling with Neurobiotin. The number of neuronal impalements resulting in dye-coupled cells decreases from P14 to P120. The most rapid decline in dye coupling was observed between P14 and P21, the beginning of the most active period of synaptogenesis in the dentate molecular layer. Dye coupling between granule cells and axo-axonic interneurons (chandelier cells) accounts for about 10% of the dye-coupled neuronal population acquired in slices from P14 and P21 rats and declines to less than 5% by P60 and P120. Our data suggest that dye coupling is related reciprocally to the number of synapses formed on granule cells. Thus the relationship of dye coupling to synaptic density in the developing fascia dentata is similar to that reported in studies of the aging fascia dentata. Also the observation of axo-axonic interneurons coupled to granule cells at all ages suggests an interesting neuronal arrangement with the potential of limiting granule cell discharge to discrete neuronal assemblies in response to perforant path input.

Transient reduction in hippocampal serotonergic innervation after neonatal parachloroamphetamine treatment.

This study examined the effects of parachloroamphetamine on neonatal forebrain serotonergic (5-HT) innervation. Rat pups were treated with PCA on P3 and P4. Significant reductions in 5-HT content were observed in the hippocampal formation, frontal cortex and entorhinal cortex on P5 and P7. By P14, neocortical 5-HT had returned to normal levels while hippocampal 5-HT values remained less than control. Hippocampal 5-HT content reached normal range by P21. High affinity 5-HT uptake in hippocampal synaptosomal preparations was similarly reduced on P5 and P7 suggesting that 5-HT terminals were being lesioned by PCA. 5-HT uptake recovered significantly by P14 perhaps reflecting the extraordinary plasticity of the 5-HT projections in the neonate. However, in contrast to the complete restoration of hippocampal 5-HT content, 5-HT uptake values remained significantly less than control. No change in 5-HT content was observed in either the hypothalamus or midbrain raphe at any age studied. Thus, the rapid onset of effects, regional selectivity and transient reduction of 5-HT levels recommend the use of PCA in studies of the role of 5-HT in hippocampal development.

Visual responses and connectivity in the turtle pretectum.

1. Using an isolated turtle brain preparation, we made extracellular spike recordings in the dorsal midbrain during visual stimulation. Single units were isolated by their response to a slow-moving full-field visual pattern imaged on the contralateral retina. This stimulus elicits responses from the basal optic nucleus (BON) and the cerebellar cortex using a similar preparation. Direction and speed tuning were then analyzed, as well as the size and position of the receptive field. 2. In one brain stem region, anterior to the optic tectum and deep to the dorsal surface, all of the visually responsive neurons were direction sensitive (DS) to contralateral retinal stimulation. The location and properties of these cells indicate that they are in the mesencephalic lentiform nucleus (nLM). Anterograde transport of intravitreally injected horseradish peroxidase revealed that this pretectal nucleus receives direct input from the contralateral eye. 3. All but 2 of the 48 cells of the nLM were strongly DS. The most effective stimulus was a slowly moving complex visual pattern that drifted nasally in the contralateral visual field. Brief flashes of spots, patterns, or diffuse light were much less effective. Receptive fields were large and usually (9 of 13 cells) centered in the superior visual field near the horizon and nasal to the blind spot. 4. The visual responses of nLM cells were compared to those of cells in the superficial layers of the optic tectum. In contrast to nLM, the responses of tectal cells were heterogeneous and frequently not DS. Neither tectum or nLM cells had much spontaneous spike activity during darkness or stationary patterns. On the other hand, visual responses of nLM cells were very similar to those of the BON, where neurons also had low spontaneous activity, preferred slow-moving patterns, and were DS. However, nLM and BON exhibit different distributions of preferred directions. Most nLM cells preferred temporal-to-nasal motion, whereas BON cells preferred almost any direction, although few preferred the nasal direction. nLM cell responses were not affected by removal of the ventral brain stem including the BON. 5. The visual properties of nLM cells recorded in vitro were very similar to those that were recorded in intact turtles.(ABSTRACT TRUNCATED AT 400 WORDS)