Understanding the Role of GPCR Heteroreceptor Complexes in Modulating the Brain Networks in Health and Disease

The introduction of allosteric receptor-receptor interactions in G protein-coupled receptor (GPCR) heteroreceptor complexes of the central nervous system (CNS) gave a new dimension to brain integration and neuropsychopharmacology. The molecular basis of learning and memory was proposed to be based on the reorganization of the homo- and heteroreceptor complexes in the postjunctional membrane of synapses. Long-term memory may be created by the transformation of parts of the heteroreceptor complexes into unique transcription factors which can lead to the formation of specific adapter proteins. The observation of the GPCR heterodimer network (GPCR-HetNet) indicated that the allosteric receptor-receptor interactions dramatically increase GPCR diversity and biased recognition and signaling leading to enhanced specificity in signaling. Dysfunction of the GPCR heteroreceptor complexes can lead to brain disease. The findings of serotonin (5-HT) hetero and isoreceptor complexes in the brain over the last decade give new targets for drug development in major depression. Neuromodulation of neuronal networks in depression via 5-HT, galanin peptides and zinc involve a number of GPCR heteroreceptor complexes in the raphe-hippocampal system: GalR1-5-HT1A, GalR1-5-HT1A-GPR39, GalR1-GalR2, and putative GalR1-GalR2-5-HT1A heteroreceptor complexes. The 5-HT1A receptor protomer remains a receptor enhancing antidepressant actions through its participation in hetero- and homoreceptor complexes listed above in balance with each other. In depression, neuromodulation of neuronal networks in the raphe-hippocampal system and the cortical regions via 5-HT and fibroblast growth factor 2 involves either FGFR1-5-HT1A heteroreceptor complexes or the 5-HT isoreceptor complexes such as 5-HT1A-5-HT7 and 5-HT1A-5-HT2A. Neuromodulation of neuronal networks in cocaine use disorder via dopamine (DA) and adenosine signals involve A2AR-D2R and A2AR-D2R-Sigma1R heteroreceptor complexes in the dorsal and ventral striatum. The excitatory modulation by A2AR agonists of the ventral striato-pallidal GABA anti-reward system via targeting the A2AR-D2R and A2AR-D2R-Sigma1R heteroreceptor complex holds high promise as a new way to treat cocaine use disorders. Neuromodulation of neuronal networks in schizophrenia via DA, adenosine, glutamate, 5-HT and neurotensin peptides and oxytocin, involving A2AR-D2R, D2R-NMDAR, A2AR-D2R-mGluR5, D2R-5-HT2A and D2R-oxytocinR heteroreceptor complexes opens up a new world of D2R protomer targets in the listed heterocomplexes for treatment of positive, negative and cognitive symptoms of schizophrenia.

Maternal creatine supplementation affects the morpho-functional development of hippocampal neurons in rat offspring

Creatine supplementation has been shown to protect neurons from oxidative damage due to its antioxidant and ergogenic functions. These features have led to the hypothesis of creatine supplementation use during pregnancy as prophylactic treatment to prevent CNS damage, such as hypoxic-ischemic encephalopathy. Unfortunately, very little is known on the effects of creatine supplementation during neuron differentiation, while in vitro studies revealed an influence on neuron excitability, leaving the possibility of creatine supplementation during the CNS development an open question. Using a multiple approach, we studied the hippocampal neuron morphological and functional development in neonatal rats born by dams supplemented with 1% creatine in drinking water during pregnancy. CA1 pyramidal neurons of supplemented newborn rats showed enhanced dendritic tree development, increased LTP maintenance, larger evoked-synaptic responses, and higher intrinsic excitability in comparison to controls. Moreover, a faster repolarizing phase of action potential with the appearance of a hyperpolarization were recorded in neurons of the creatine-treated group. Consistently, CA1 neurons of creatine exposed pups exhibited a higher maximum firing frequency than controls. In summary, we found that creatine supplementation during pregnancy positively affects morphological and electrophysiological development of CA1 neurons in offspring rats, increasing neuronal excitability. Altogether, these findings emphasize the need to evaluate the benefits and the safety of maternal intake of creatine in humans.

Maternal dietary loads of alpha-tocopherol increase synapse density and glial synaptic coverage in the hippocampus of adult offspring

An increased intake of the antioxidant α-Tocopherol (vitamin E) is recommended in complicated pregnancies, to prevent free radical damage to mother and fetus. However, the anti-PKC and antimitotic activity of α-Tocopherol raises concerns about its potential effects on brain development. Recently, we found that maternal dietary loads of α-Tocopherol through pregnancy and lactation cause developmental deficit in hippocampal synaptic plasticity in rat offspring. The defect persisted into adulthood, with behavioral alterations in hippocampus-dependent learning. Here, using the same rat model of maternal supplementation, ultrastructural morphometric studies were carried out to provide mechanistic interpretation to such a functional impairment in adult offspring by the occurrence of long-term changes in density and morphological features of hippocampal synapses. Higher density of axo-spinous synapses was found in CA1 stratum radiatum of α-Tocopherol-exposed rats compared to controls, pointing to a reduced synapse pruning. No morphometric changes were found in synaptic ultrastructural features, i.e., perimeter of axon terminals, length of synaptic specializations, extension of bouton-spine contact. Glia-synapse anatomical relationship was also affected. Heavier astrocytic coverage of synapses was observed in Tocopherol-treated offspring, notably surrounding axon terminals; moreover, the percentage of synapses contacted by astrocytic endfeet at bouton-spine interface (tripartite synapses) was increased. These findings indicate that gestational and neonatal exposure to supranutritional tocopherol intake can result in anatomical changes of offspring hippocampus that last through adulthood. These include a surplus of axo-spinous synapses and an aberrant glia-synapse relationship, which may represent the morphological signature of previously described alterations in synaptic plasticity and hippocampus-dependent learning.

Physical exercise and environment exploration affect synaptogenesis in adult-generated neurons in the rat dentate gyrus: possible role of BDNF

A brief training in a pool maze, with or without cognitive tasks, modifies the synaptogenesis and maturation of newborn neurons in adult rat dentate gyrus. These types of trainings have many aspects, including physical activity and exploration. Therefore, to evaluate whether physical exercise and environment exploration are able to affect synapse formation and the maturation of adult-generated neurons, GFP-retrovirus infusion was performed on rats which, on the fourth day after injection, were housed under running conditions or allowed to explore an enriched environment briefly in the absence of exercise for the following three days. Afterward, at the end of the trainings, electrophysiological and morphological studies were conducted. Considering that neurotrophic factors increase after exercise or environment exploration, hippocampal BDNF levels and TrkB receptor activation were evaluated. In this study, we show that both spontaneous physical activity and enriched environment exploration induced synaptogenesis and T-type voltage-dependent Ca(2+) currents in very immature neurons. Hippocampal BDNF levels and TrkB receptor activation were determined to be increasing following physical activity and exploration. A possible contribution of BDNF signaling in mediating the observed effects was supported by the use of 7-8-dihydroxyflavone, a selective TrkB agonist, and of ANA-12, an inhibitor of TrkB receptors.

Bdnf expression in rat skeletal muscle after acute or repeated exercise

Brain derived growth factor (BDNF) gene of rat has a complex structure: at least four 5' untranslated exons regulated by different promoters and one 3' exon containing the encoding region. BDNF is expressed by skeletal muscles in an activity-dependent manner. In this study, BDNF mRNA was analysed by RT-PCR in the soleus muscle following a single (acute) session of running or a training of five days of running (repetitive exercise). Moreover, the expression of the exons was quantitatively analysed by real time RT-PCR. Finally, muscle BDNF protein level was evaluated by western blotting. BDNF mRNA was found to increase over the second day after acute exercise; on the other hand, two peaks (2 and 24 hours after the last session, respectively) in BDNF mRNA level were found after repetitive exercise, but it was similar to that of controls 6 hours after the last session. BDNF protein level progressively increased also after the mRNA went back to the basal level, so suggesting that it cumulates within the cell after acute exercise, whereas it followed the mRNA level time course after repetitive exercise. These results point to the following conclusions: BDNF mRNA is up-regulated by activity, but this response is delayed to the second day after acute exercise; repetitive exercise transiently depresses the expression of BDNF mRNA, so that the over-expression due to the previous day's exercise completely disappears 6 hours after the last exercise session.

Synaptically-silent immature neurons show gaba and glutamate receptor-mediated currents in adult rat dentate gyrus

The fate of adult-generated neurons in dentate gyrus is mainly determined early, before they receive synapses. In developing brain, classical neurotransmitters such as GABA and glutamate exert trophic effects before synaptogenesis. In order for this to occur in adult brain as well, immature non-contacted cells must express functional receptors to GABA and glutamate. In this investigation, patch-clamp recordings were used in adult rat dentate gyrus slices to assess the presence and analyze the characteristics of GABA- and glutamate-evoked currents in highly immature, synaptically-silent granule cells. Whole-cell patch-clamp recordings showed that all the analyzed cells responded to puff application of GABA and most of them responded to glutamate. Currents evoked by GABA were mediated exclusively by GABAA receptors and those elicited by glutamate were mediated by NMDA and AMPA/Kainate receptors. GABAA receptor-mediated currents were reduced by furosemide, which suggests that synaptically-silent immature neurons express high-affinity, alpha4-subunit-containing GABAA receptors. Gramicidin-perforated-patch recordings showed that GABAA receptor-mediated currents exerted a depolarizing effect due to high intracellular chloride concentration. Synaptically-silent immature cells shared morphological and electrophysiological properties with GFP-expressing, 7-day-old adult-generated granule layer cells, indicating that they could be in the first week of life, the period of maximal newborn cell death. Moreover, the presence of functional GABA and glutamate receptors was confirmed in these GFP-expressing cells. Present findings are mostly consistent with previous data obtained in female mice undergoing spontaneous activity and in transgenic mice, except for some inconsistencies about the presence of functional glutamatergic receptors. We speculate that adult-generated, non-contacted granule cells may be able to sense activity-related variations of GABA and glutamate extracellular levels. This condition is necessary, even if not sufficient, for these neurotransmitters to have a direct role in addressing cell survival.

The role of sensory input in motor neuron sprouting control

Primary sensory neurons project to motor neurons directly or through interneurons and affect their activity. In our previous paper we showed that intramuscular sprouting can be affected by changing the sensory synaptic input to motor neurons. In this work, motor axon sprouting within a peripheral nerve (extramuscular sprouting) was induced by nerve injury at such a distance from muscle so as not to allow nerve-muscle trophic interactions. Two different procedures were carried out: (1) sciatic nerve crush and (2) sciatic nerve crush with homosegmental ipsilateral L3-L5 dorsal rhizotomy. The number of regenerating motor axons innervating extensor digitorum longus muscle was determined by in vivo muscle tension recordings and an index of their individual conduction rate was obtained by in vitro intracellular recordings of excitatory postsynaptic end-plate potentials in muscle fibers. The main findings were: (1) there are more regenerated axons distally from the lesion than parent axons proximally to the lesion (sprouting at the lesion); (2) sprouting at the lesion was negatively affected by homosegmental ipsilateral dorsal rhizotomy; (3) the number of motor axons innervating extensor digitorum longus muscle extrafusal fibers counted proximally to the lesion increased following nerve injury and regeneration but this did not occur when sensory input was lost. A transient innervation of extrafusal fibers by gamma motor neurons may explain the increase of motor axons counted proximally to the lesion.

Glucose-6-phosphate dehydrogenase supports the functioning of the synapses in rat cerebellar cortex

This study investigates heterogeneous glucose-6-phosphate dehydrogenase (G6PD) expression in the rat cerebellar cortex. G6PD activity and its electrophoretic pattern, evaluated on the cerebellar homogenate, were found to be similar to those of other brain areas. However, histochemical and immunohistochemical analyses revealed that the highest expression of G6PD activity and protein was in Purkinje's cells, followed by the molecular and granular layers. Electron microscopy analysis showed that, in Purkinje's cells, the G6PD reaction products were concentrated in the neurites while in the basket cells in the cell body. The granules showed a weaker activity everywhere. The quantitative distribution of G6PD is discussed in the light of the neurochemical function of these cells.

Alpha-tocopherol controls cell proliferation in the adult rat dentate gyrus

The effect of alpha-tocopherol on cell proliferation and proliferated cell survival was investigated in the dentate gyrus of adult rats. Adult rats were supplemented with alpha-tocopherol, injected with 5-bromo-2'-deoxyuridine (BrdU), that is incorporated into DNA during the S-phase, and killed at different time after BrdU injection. The number of newborn cells decreased after alpha-tocopherol supplementation, confirming the hypothesis that alpha-tocopherol is able to depress cell proliferation in vivo. Most newborn cells die within few days; more newborn cells survive in alpha-tocopherol-treated rats, suggesting the hypothesis that alpha-tocopherol decreases cell death.

Muscle reinnervation in hypothyroid rats

Reinnervation of extensor digitorum longus muscle following crush of sciatic nerve was studied in rats made hypothyroid after weaning. In vitro intracellular recordings of muscle cell postsynaptic potentials were carried out; moreover twitch and tetanus following direct muscle stimulation and nerve stimulation were recorded. Frequency of miniature end-plate potentials (mepps) may be regarded as an index of presynaptic mechanism in regenerated nerve endings: when regenerating axons reach the muscle, the frequency of spontaneous acetylcholine quantal release is very low and increases in subsequent weeks. No significant differences were noted in miniature end-plate potential frequency between muscles of normal and hypothyroid rats at the same time from denervation; mepp amplitude was higher in hypothyroids, in accordance with the smaller muscle fibre diameters. Regenerating nerve fibres entering the muscle extensively sprout, giving rise to a number of nerve endings which exceeding the number of muscle cells, are subsequently withdrawn; correspondingly, muscle cells are transiently polyinnervated and the number of polyinnervated muscle cells peaks decreases subsequently approximating zero. The percentage of polyinnervated cells peaked sooner in hypothyroid rats than in controls and afterwards decreased; a tail of polyinnervation was found at long term. Tension recording experiments showed a shorter time of reinnervation of muscles in hypothyroid rats, but no difference in regeneration rate could be argued. These findings suggest an influence of thyroid hormones in the stabilization of motor innervation of reinnervated muscle, but not in nerve regeneration process.

Spatial learning affects immature granule cell survival in adult rat dentate gyrus

Neurogenesis occurs throughout life in mammalian dentate gyrus. The effect of learning on newborn cell survival was studied in rat. Rats were trained on a hippocampus-dependent spatial learning task by using Morris water maze. Neurogenesis was evaluated by 5-bromo-2'deoxyuridine administered before learning. Several newborn cells expressed the immature neuron marker TOAD-64. The main findings were as follows: (1) the survival of newborn cells was enhanced by learning at early stage of differentiation; (2) the newborn cells saved by learning were mainly located in the rostral part of external blade of granule cell layer and (3) there was a correlation between the actual individual learning and newborn cell survival.

Are there proliferating neuronal precursors in adult rat dorsal root ganglia?

The origin of new neurons in dorsal root ganglia of adult rat was investigated using an experimental model in which postnatal neurogenesis naturally occurring is enhanced and restricted in a brief period of life. Possible mitotic origin of new neurons was investigated by means of 5-bromo-2-deoxyuridine, anti-NF 200 antibody was used to detect if proliferated cells showed a neuronal phenotype. The results suggest that postnatal neurogenesis in dorsal root ganglia could depend only in part on precursor proliferation and that normally new neurons derive from the late differentiation of postmitotic cells.

Rat motor neuron plasticity induced by dorsal rhizotomy

The control of peripheral structural plasticity of motor neurons by primary sensory neurons was studied in rat extensor digitorum longus (EDL) muscle. Polyinnervation of muscle fibers, sprouting and the motor neuron peripheral field size following L4 dorsal root cutting were evaluated using three different approaches: intracellular recording of end plate potentials, histochemical demonstration of sprouting and polyinnervation and in vivo recording of nerve-evoked twitch. Nodal sprouting was found in rhizotomized rats but not in controls and consistently muscle polyinnervation appeared. The muscle portion innervated by L3 ventral root was relatively reduced and that innervated by L5 was relatively enlarged: a trend to caudal shift of muscle innervation arose in rhizotomized rats. A control of motor neuron plasticity by primary sensory neurons is suggested.

Postnatal proliferation of DRG non-neuronal cells in vitamin E-deficient rats

Changes in the number of satellite cells in neuron body sheaths in dorsal root ganglia (DRGs) were studied from 1 to 5 months of age in control and in vitamin E-deficient rats; furthermore, the satellite cell proliferation rate was detected in the same groups of animals with immunohistochemistry for 5-bromo-2'-deoxyuridine (BrdU). The number of satellite cells in sheaths of DRG neurons increased in the period of life considered both in control and in vitamin E-deficient rats. Satellite cell proliferation was observed in both groups, but its rate was found to be higher in vitamin E-deficient rats. The results obtained in control rats confirm that mitotic ability is retained by satellite cells in adulthood and show that at least some of newborn satellite cells add to the pre-existing population. The results obtained in vitamin E-deficient rats suggest that a faster turnover in satellite cell population takes place in these animals and support the idea that vitamin E could be an exogenous factor controlling cell proliferation.

Neurogenesis in the adult rat dentate gyrus is enhanced by vitamin E deficiency

Neurogenesis occurs throughout adult life in rat dentate gyrus. Factors and mechanisms of adult neurogenesis regulation are not well known. Vitamin E deficiency has been found to deliver a neurogenetic potential in rat dorsal root ganglia. To determine whether the role of tocopherols in adult neurogenesis may be generalized to the central nervous system, changes in adult rat dentate gyrus neurogenesis were investigated in vitamin E deficiency. Neurogenesis was quantitatively studied by determination of the density of 5-bromo-2'-deoxyuridine (BrdU)-labeled cells and by determination of the total number of cells in the granule cell layer. The BrdU-labeled cells were immunocytochemically characterized by demonstration of neuronal marker calbindin D28K. The following results were found: (1) the volume of the granule layer increased in controls from 1 to 5 months of age, mainly due to cell density decrease; (2) the volume increased by a similar amount in vitamin E-deficient rats, mainly because of an increase in cell number; (3) BrdU-positive cells were more numerous in vitamin E-deficient rats in comparison to age-matched controls; (4) the increase in proliferated cells was located in the hilus and in the plexiform layer. This study confirms that neurogenesis occurs within adult dentate gyrus and demonstrates that this process is enhanced in vitamin E deficiency. This finding indicates that vitamin E may be an exogenous factor regulating adult neurogenesis.

Control of neuron outgrowth by NMDA receptors

The role of N-methyl-D-aspartic acid receptors (NMDARs) of glutamate on neuritogenesis was studied in cultured neurons of chick embryo spinal cord using the NMDAR non-competitive antagonist dizocilpine maleate (MK-801). No cell population was fully prevented from neuritogenesis by MK-801. Different aspects of neuritogenesis were quantitatively evaluated. Neurite initiation, elongation and branching were depressed by MK-801. Inhibition was dose-dependent and reversible. A loss of responsiveness of neuritogenesis to MK-801 was found during the second day of treatment at a concentration of 10 microM, but not at higher concentrations. Our findings support the idea that Ca2+ influx through NMDAR associated channels is one of the possible triggers of a cascade resulting in neuritogenesis. The effects of NMDAR blocking on neuritogenesis occurred before synaptogenesis, suggesting a role of excitatory aminoacids in neuron morphological differentiation at early stages of development. Scanning electron microscopy confirmed a reduction in neurite tree complexity in MK-801 treated cells and showed a production of filopodium-like processes in some of these cells.

Enlargement of motoneuron peripheral field following partial denervation with or without dorsal rhizotomy

In partially denervated skeletal muscle, spared motor fibres sprout, enlarging motor unit size. Neuritogenesis and sprouting are known to depend on the synaptic input to the neurons. This suggests that spared motoneuron reaction to partial muscle denervation might be controlled by primary sensory neurons which directly or indirectly project to motoneurons. In two groups of rats, different surgical procedures were carried out: partial denervation of the extensor digitorum longus muscle without or with homolateral dorsal rhizotomy. Spared motoneuron peripheral field was evaluated by nerve-evoked tension measures. Following partial muscle denervation, spared motoneurons enlarged their projection peripheral field five to six times, innervating most of the denervated portion of the muscle. When dorsal rhizotomy was carried out together with partial denervation, the enlargement of the motoneuron's peripheral field occurred later; however, the peripheral field size was the same or greater than that found in partially denervated muscles without dorsal rhizotomy in the long term. Excitatory postsynaptic potential recordings at neuromuscular junctions consistently showed that innervation of denervated muscle cells by spared motoneurons was impaired when the dorsal roots were cut. Finally, in both groups of operated rats an increase in motor unit number occurred early after surgery, anticipating a process normally occurring in the same age range. These findings are consistent with the idea that sensory input trans-synaptically controls motoneuron peripheral field size.

Neuron and glial cells in neocortex after methylazoxymethanol treatment in early development

The quantitative changes were investigated in neuron and glia density in the different cortical layers of the frontal cortex of 3 and 12 month old mice, exposed to methylazoxymethanol on embryonic day 13 (MAM13). No loss of cortical neurons was found between young and adult animals. MAM exposure on the 13th day of development induced a neuron density decrease throughout on the entire cortical depth and did not produce changes in the density of glial cells with respect to the controls and to age. Consequently, at 3 months of age we observe a glia/neuron ratio greater than that of controls and at 12 months a similar value. In the neocortex of MAM-mice at this numerical uniformity of glial cell density, did not correspond to a similar proportional composition: the frequency of the astrocytes is lower, adapting to the decreased neuron density; the greater oligodendrocyte percentage may be related to disturbed layering and to the hyperinnervation of the hypoplastic cortex; the microglia shows a trend similar to that of the controls. These results, together with those of other studies, suggest that prenatal exposure to MAM causes a cortical compensatory response regulating glial cells proliferation.

Developmental changes in innervation of rat extensor digitorum longus muscle

Motor neurons and ventral root motor fibres innervating extensor digitorum longus (EDL) muscle in rats of 0.5-3.5 months of age were studied by HRP-retrograde labelling and in vivo muscle tension recording, respectively. EDL nucleus size increased with age but motor neurons number and size did not change. Twitch and tetanus tension increased with age proportionally to muscle mass. At 0.5 months of age, but not at subsequent ages, the muscle was incompletely innervated functionally. The contribution to EDL muscle innervation came from L3, L4 (pre-eminently), and L5 ventral roots. The number of motor fibres running in L3-L5 ventral roots and innervating EDL muscle increased from 41.7 +/- 2.2 (mean +/- S.E.M.) at 0.5 months to 68.6 +/- 1.9 at 3.5 months (P < 0.001). The greatest changes in m.w., in tension and in number of alpha nerve fibres innervating the muscle occurred from 0.5 to 1.5 months of age; afterwards changes occurred, but at a slower rate. HRP labels all neurons innervating EDL muscle, while tension recordings allow the counting of alpha nerve fibres (not gamma) running in ventral roots; these differences may account for the different results obtained.

Motor nerve sprouting induction by a nerve explant in normal and vitamin E-deficient rats

Sprouting induction by a peripheral degenerating nerve has been evaluated in normal and vitamin E-deficient rats. A piece of sural nerve was grafted close to the peroneal nerve of the same animal: at one and two months after grafting thin unmyelinated axons were visible in the graft and they were sometimes functionally active; when nerve explant was frozen before grafting, sprouting induction did not take place either in controls or in vitamin E-deficient animals. No difference was noted in sprouting induction between the two groups, while degeneration showed a different time course. Some hypotheses of possible stimuli of sprouting induction are discussed.

Effects of proanthocyanidin on normal and reinnervated rat muscle

Proanthocyanidin-A2, a catechic dimer extracted from the bark of Aesculus hippocastanum L., was tested on peripheral nerve regeneration. Reinnervation of EDL and soleus muscles following traumatic nerve damage was investigated in rats by using "in vivo" tension recording technique. Muscle contraction force (twitch and tetanus), the number of motor units and the time course of twitch (time to peak and half relaxation time), were observed. The results obtained do not show that the time course of EDL and soleus muscles reinnervation is different in Proanthocyanidin-A2-treated rats in comparison to control animals. On the contrary, results point out an increase in EDL and soleus muscle mass, both in denervated and in undenervated treated rats compared to corresponding controls. Moreover, consistently with this finding, an increase in their contraction force was found. These data show that Proanthocyanidin-A2 exerts a trophic effect on muscle.

Response of fast muscle innervation to hypothyroidism

The early period of motor innervation development is characterized by multiple innervation of muscle cells. This transitory state in rat extensor digitorum longus (edl) muscle is normally concluded at weaning when a 1:1 ratio between nerve endings and muscle cells is reached. Motor innervation of edl muscle in rats made hypothyroid after weaning was studied in three ways: electrophysiology (intracellular recordings of muscle postsynaptic potentials) was carried out to study neuromuscular transmission; silver impregnation of terminal axons to observe sprouting; force production in twitch and tetanus following direct muscle stimulation and nerve stimulation. A number of multiply innervated muscle cells was found in hypothyroid rats following two months of treatment. This finding seems to be related to the appearance of nodal sprouting in motor axons. No sign of denervated end-plates was found. Twitch and tetanus tension were smaller than in controls, but they were bigger when referred to unitary muscle mass. Time course of twitch, particularly half relaxation, was slowed in muscles of hypothyroid rats. These findings suggest that plastic processes occur in muscle innervation of rats made hypothyroid after weaning. Therefore, thyroid hormones play a role in stabilizing motor innervation not only during development, but also in adults.

Regeneration of unmyelinated peripheral axons in vitamin E-deficient rats

Unmyelinated axons of normal and regenerated sciatic nerve were counted in controls and vitamin E-deficient rats. No significant change in the number of unmyelinated axons of uninjured nerve was found in the vitamin E deficiency in comparison to controls (12961 +/- 1591 and 12450 +/- 1290, respectively, mean +/- SEM). In regenerated nerve of control rats the number of unmyelinated axons was higher than in uninjured nerve (16971 +/- 1854 and 20786 +/- 1574 at 1 and 2 months after crush, respectively). In vitamin E-deficient rats the increase in number of unmyelinated axons was greater than in corresponding controls (21880 +/- 662) at 1 month after lesion, but the number returned to value found in uninjured nerve at 2 months after lesion (12536 +/- 659). These results suggest that sprouting at lesion may be enhanced but some regenerated axons does not survive at long term in vitamin E-deficiency.

Regeneration of motor nerves in bilobalide-treated rats

Bilobalide, a terpene extracted from the leaves of the Ginkgo biloba tree, has been proposed to exert trophic and protective effects on neurons and on Schwann's cells in various neuropathies. The reinnervation of the extensor digitorum longus muscle following traumatic nerve damage was investigated in rats by using electrophysiological and histological techniques. Evaluation parameters included the membrane resting potential of muscle cells, the spontaneous quantal release of acetylcholine, and the percentage of muscle cells receiving multiple innervation. The percentage of muscle cells receiving multiple innervation reached a peak more rapidly in treated animals and declined subsequently to values lower than those found in controls. These data suggest that the rearrangement of regenerated innervation occurs more rapidly in bilobalide-treated animals.

Nodal and terminal sprouting by regenerating nerve in vitamin E-deficient rats

The increased number of poly-innervated cells in normal and reinnervated extensor digitorum longus (edl) muscle of vitamin E-deficient rats suggests enhanced sprouting by motor neurons in conditions of decreased protection against lipid peroxidation. End-plates and terminal axons were observed by a combined technique that shows both end-plate acetylcholinesterase area and axons. Quantitative observations of nodal and terminal sprouting in normally innervated and reinnervated edl muscles of vitamin E-deficient rats were carried out. Branch points of nerve terminal within end-plates were also observed. Three main results were obtained. First, a notable increase of both terminal and nodal sprouting was found in reinnervated muscles of normal and vitamin E-deficient rats; moreover, a relative increase in the number of nodal sprouts occurs in the long run. Second, in muscles of uninjured, vitamin E-deficient rats, nodal and terminal sprouting and branching within end-plate was greater than in controls. Third, nodal sprouting by regenerating axons was more affected by vitamin E-deficiency than terminal sprouting and branching within end-plates.

[Changes due to age in the regenerating axons of the adult rat]

Muscle reinnervation after nerve crush was observed in rats at different ages with a combined technique that simultaneously demonstrates nerve endings and endplates. At early times of reinnervation the amount of sprouting was higher in older rats than in younger rats; according to this finding an enhanced number of polyinnervated endplates was found in older rats. A similar enhancement of sprouting and polyinnervation was observed during muscle reinnervation of vitamin E deficient rats, supporting the proposed analogy between vitamin E deficiency and aging.

[Progression of multiple innervation of reinnervated rat muscle treated with bilobalide]

During motor nerve regeneration a transitory polyinnervation of muscle cells occurs, which represents a phase of rearrangement of the recovered innervation. Bilobalide, a terpene extrated from Ginkgo biloba leaves, was proposed to affect some aspects of nervous system development and regeneration. In this work, influence of bilobalide on polyinnervation in reinnervated extensor digitorum longus muscle was studied, through electrophysiological and histological techniques. The muscle was denervated crushing the sciatic nerve and it was examined at 1 or 2 months after denervation. The polyinnervated muscle cells in controls reached 24% at 1 month and thus the percentage decreased. In muscles of bilobalide treated rats the number of polyinnervated cells was decreased at both times.