Ganglion cells from chick retina display multiple functional nAChR subtypes.

We have examined the properties of nicotinic acetylcholine receptors in embryonic chick retinal ganglion cells. Ganglion cells, identified according to morphological and physiological criteria, displayed spontaneous or induced action potentials. In 94/99 cells acetylcholine pulses evoked responses. In current clamp mode, acetylcholine provoked membrane depolarization and triggered action potentials. Under voltage clamp conditions, acetylcholine evoked inward currents that were readily blocked by d-tubocurarine. Antagonists specific for homomeric (alpha-bungarotoxin) and heteromeric (dihydro-beta-erythroidine) receptors revealed that ganglion cells express multiple functional receptor subtypes. These findings demonstrate that ACh modulates the electrical activity of these cells and is likely to mediate synaptic transmission. The presence of multiple receptor subtypes may contribute to processing and transmission of information in the retina.

Comparative distribution of nicotinic receptor subtypes during development, adulthood and aging: an autoradiographic study in the rat brain.

The distribution in the rat brain of high affinity nicotinic heteromeric acetylcholine receptors and of low affinity nicotinic, alpha7-containing, homomeric receptors was studied using in vitro light microscopic autoradiography. As ligands, we used [3H]epibatidine, or [125I]epibatidine, and [125I]alpha-bungarotoxin, respectively. In adult animals, the two types of binding sites were widely distributed in many different brain structures, including the brainstem, cerebellum, mesencephalic structures, limbic system and cortex, but their anatomical distribution differed markedly. Only in rare instances could a co-localization be observed, for example in the superficial layer of the superior colliculus. In developing animals, both types of labeling were strongly expressed during embryonic and postnatal phases. Their distributions were qualitatively similar to those observed in adult animals, with a few noticeable exceptions in the cerebral cortex, hippocampus and brain stem. In aging animals, neither the distribution nor the density of nicotinic binding sites was significantly altered. Our conclusions are the following. (a) There is little overlap in the distribution of heteromeric and alpha7-containing homomeric nicotinic receptors in the rat brain. (b) The abundance of neuronal nicotinic receptors during embryonic and postnatal development suggests that they may play a role in the establishment of neuronal connectivity. (c) The expression of neuronal nicotinic receptors is unaltered in middle aged animals, suggesting that in the rat these receptors do not play any major role in aging process.

Reduced binding of oxytocin in the rat brain during aging.

Central oxytocin (OT) receptors were labelled in 3-month-old and 20-month-old rats with an iodinated OT antagonist. Comparison of the autoradiograms by quantitative image analysis revealed in the old animals a significant reduction of binding in three regions; the number of labelled OT receptors was decreased by 90% in the head of the caudate putamen, by 68% in the olfactory tubercle, and by 41% in the ventromedial hypothalamic nucleus. Previous studies had shown that the expression of OT receptors in the olfactory tubercle and in the ventromedial hypothalamic nucleus was dependent upon gonadal steroids. Therefore we hypothesize that the reduced number of OT receptors in the latter two structures of aged rats was the consequence of the 4-fold decrease of plasma testosterone that we found in this age. Another mechanism may be responsible for the marked reduction of OT receptors in the caudate putamen.

Axotomy induces the expression of vasopressin receptors in cranial and spinal motor nuclei in the adult rat.

8-L-Arginine vasopressin ([Arg8]VP) receptors are expressed transiently in the rat facial nucleus during the perinatal period. Electrophysiological studies suggest that at least part of these receptors is located on facial motoneurones. In the present study we report that, in the adult rat, unilateral section of a facial nerve results in a massive and transient reexpression of [Arg8]VP receptors in the deeferented facial nucleus. Data were obtained by quantitative film autoradiography. During the first 2 postoperative weeks, binding of an iodinated ligand selective for V1a-type receptors increased about 10-fold. Maximal levels of binding were maintained for 1-2 weeks and then started to decrease. Binding was not strictly restricted to the facial nucleus but included the neuropile between motoneuronal pools and the perifacial area, which may indicate a dendritic localization of [Arg8]VP receptors. To investigate whether other motor nuclei also react to axotomy by up-regulating [Arg8]VP receptors, we sectioned either a hypoglossal nerve or a sciatic nerve. Two weeks after surgery, the hypoglossal nucleus or sciatic motoneuronal pools ipsilateral to the lesion were intensely labeled with the iodinated ligand. In contrast, nerve section had no effect on oxytocin binding sites in facial, hypoglossal, or sciatic motor nuclei. The results suggest that [Arg8]VP receptor expression in motor nuclei may depend upon neuromuscular contacts and, thus, that [Arg8]VP may be involved in the establishment of neuromuscular connections during development and in their reestablishment after nerve injury.

Efficiency of work performance and contraction velocity in isotonic tetani of frog sartorius.

Mechanical work, ATP, ADP, PC, free creatine and lactate concentrations were determined on IAA poisoned frog sartorii tetanically stimulated in humidified N2 at 10 degrees C in isotonic conditions (0.25 or 0.45 Po). Tetanus duration was 0.35 s, number of tetani was varied from 0 (rest) to 25 (exhaustion). The mechanical work performed per mole ATP + PC split (W*P) amounted on the average to 16.7 kJ/mol. It was observed, however, that W*P increased from about 13 to about 24 kJ/mol with decreasing ATP concentration from about 2 (resting value) to about 1 mumol/g and that this decrease in ATP was associated with a decrease of the shortening (and relaxation) speed of the muscle to about 30% of the values observed on the first tetanus. It is concluded that the thermodynamic efficiency of muscle contraction, calculated from the ratio of WP* (measured) to the thermodynamic affinity (free energy change) of ATP hydrolysis (estimated) increases from about 0.3 to about 0.5 with decreasing ATP concentration and shortening speed.