Dopamine stimulates rat cortical somatostatin release.

Release of somatostatin from slices of rat frontal cortex was studied. Increasing the potassium ion concentration in the medium from 6 mM to 55 mM resulted in a significantly increased release of somatostatin. Dopamine increased the release of somatostatin from cortex and this effect of dopamine was blocked by haloperidol and other dopaminergic antagonists. Other catecholamines as well as serotonin, histamine and acetylcholine failed to stimulate the release of somatostatin. The stimulatory effect of dopamine on release of somatostatin from cortical slices provides an approach for examination of the receptor properties and function of dopamine in this brain region.

Striatal met-enkephalin concentration increases following nigrostriatal denervation.

Following specific lesion of the nigrostriatal dopaminergic pathways in rat brain, striatal met-enkephalin on the lesioned side increased to 245% of that on the non-lesioned side. This increase was evident only after a lag period of 7 days and the increase was maintained for at least 2 months after lesion. By contrast, there was no change in striatal somatostatin or vasoactive intestinal polypeptide concentration, indicating that the effect was not a generalised one. Levels of all three of these neuropeptides were unchanged in frontal cortex. These findings support the concept of a dopaminergic-enkephalinergic functional interrelationship in the striatum. In addition, the findings provide evidence that, following destruction of nigrostriatal dopaminergic neurons, not only is there a gradually developing postsynaptic dopamine receptor supersensitivity but also a compensatory alteration in the enkephalinergic system.

Heart conditioned medium elicits post-lesion muscarinic receptor recovery in vivo.

Intraseptal administration of heart conditioned medium (HCM) stimulates the growth of injured cholinergic fibers into iris implants placed in the anterodorsal hippocampus. The aims of the present report were to monitor muscarinic cholinergic receptor concentrations during denervation and central innervation of the peripheral tissue targets, and to evaluate the effect of HCM on these changes. For 0, 4, 8, 16 and 28 days after mechanical injury to septohippocampal axons, animals received either intraseptal injections of HCM or control vehicle. Binding of [3H]quinuclidinylbenzilate ([3H]QNB) within iris implants was used as an index of cholinergic muscarinic receptor concentration. The results indicate that within the iris implants: (1) a dramatic drop in the number of muscarinic receptors is observed 4 days after denervation; (2) under control conditions, central cholinergic innervation is not associated with muscarinic receptor recovery; and (3) after administration of HCM, muscarinic receptor levels begin to increase within two weeks and approach the pre-lesion endogenous concentration following 28 days of treatment. These results support the hypothesis that trophic factors may facilitate the restoration of effective, appropriate connections between nerve fibers and their targets.

Heart conditioned medium promotes central cholinergic regeneration in vivo.

The aim of the present study was to evaluate the effect of a proposed cholinergic growth factor, heart conditioned medium (HCM), on the regeneration of the cholinergic septo-hippocampal pathway in rats. At 0, 4, 8 and 16 days after mechanical injury to septo-hippocampal axons, animals received either repeated intraseptal injections of HCM or control vehicles. Recovery of choline acetyltransferase (CAT) activity in iris implants placed in the hippocampus was used as an index of cholinergic regeneration. The results indicate that CAT activity in iris implants from animals receiving HCM was significantly greater than in control implants. Thus, this is the first report of the in vivo activity of heart conditioned medium as a cholinergic growth factor.

Properties of dopamine agonist and antagonist binding sites in mammalian retina.

Retinal homogenates of calf, rat, rabbit and Cebus appella and Macaca mulata monkeys were found to contain stereospecific binding sites for the dopamine antagonist [3H]spiroperidol. In further studies with calf and rat retina, stereospecific binding sites were also found for the dopamine agonist [3H]ADTN (2-amino-6,7,-dihydroxy-1,2,3,4-tetrahydronapththalene). The [3H]spiroperidol binding sites in calf retina were pharmacologically similar to the dopaminergic spiroperidol binding sites previously demonstrated to be present in striatum. However, calf and rabbit retina contained less than 1/10 the concentration of [3H]spiroperidol binding sites found in striatum. Saturation studies and Scatchard analyses showed a single class [3H]spiroperidol binding sites with Kd (apparent dissociation constant) = 0.3 and 0.2 nM and Bmax (binding site number) = 38 and 24 fmol/mg protein in calf retina and rabbit retina respectively. Rates of [3H]spiroperidol association and dissociation were also evaluated in calf retina. Drug specificity for [3H]ADTN binding in calf retina resembled that previously reported for striatal [3H]ADTN binding and thus differed from retinal [3H]spiroperidol binding. Calf retinal [3H]ADTN binding sites had a Kd = 9 nM and Bmax = 113 +/- 12 fmol/mg protein. Thus, the total number of [3H]ADTN sites in retina was at least twice that of [3H]spiroperidol sites. Guanine nucleotides (GTP and Gpp (NH)p) but not ATP reduced the affinity of the dopamine agonist ADTN for [3H]spiroperidol binding, and also reduced the specific binding of [3H]ADTN itself up to a maximal value of about 50% of control binding. Saturation studies of calf retinal [3H]ADTN binding confirmed that Gpp(NH)p-displaceable sites were a discrete saturable subset of stereospecific [3H]ADTN sites with Kd = 9 nM and Bmax = 50 +/- 6 fmol/mg protein. The Gpp(NH)p insensitive sites had a Kd = 9 nM and Bmax = 63 +/- 7 fmol/mg protein. It is proposed that although [3H]ADTN sites differ pharmacologically from [3H]spiroperidol sites, since [3H]spiroperidol sites are guanine nucleotide-sensitive and similar in number to the guanine nucleotide-sensitive class of [3H]ADTN sites, they may possibly be related to these sites as well as to adenylate cyclase. In addition, retina contains guanine nucleotide-insenstive [3H]ADTN sites, possibly presynaptic and probably not coupled to adenylate cyclase.

Evidence for selective loss of brain dopamine- and histamine-stimulated adenylate cyclase activities in rabbits with aging.

Dopamine-stimulated adenylate cyclase activity in striatum and both dopamine- and histamine-stimulated adenylate cyclase activity in hypothalamus, frontal cortex and anterior limbic cortex declined by about 50% as rabbits aged from 5.5 months to 5.5 years of age. These changes were primarily in maximal response to amine although an additional component involving decreased affinity in the case of dopamine may also be present. In contrast, dopamine-stimulated adenylate cyclase of retina and both basal and guanyl-5'-yl-imidodiphosphate (Gpp(NH)p)-stimulated activity in these regions were not altered with age. There was no measurable decrease in the old animals in either dopamine or norepinephrine concentration in striatum, anterior limbic cortex or retina, or in choline acetylase activity or [3H]quinuclidinylbenzilate binding in striatum, anterior limbic cortex or frontal cortex. It is proposed that selective age-dependent decreases in transmitter receptors coupled to adenylate cyclases occur in the absence of or independent from neuronal cell loss, as evidenced by the retention of the other biochemical markers.

Evidence for loss of brain [3H]spiroperidil and [3H]ADTN binding sites in rabbit brain with aging.

[3H]Spiroperidol and [3H]2-amino-6,7-dihydroxyl-1,2,3,4,-tetrahydronaphthalene hydrochloride (ADTN) binding were measured in various central nervous system regions of 5 month and 5.5 year old rabbits. In striatum, young animals had a 38% higher number of [3H]spiroperidol binding sites and a 140% higher number of [3H]ADTN binding sites than did the older animals. In frontal cortex and anterior limbic cortex there were respectively 42% and 26% more [3H]spiroperidol binding sites in the young animals. There was no change in the binding site number or affinity for [3H]spiroperidol in retina with aging. Pharmacological characterization demonstrated that [3H]spiroperidol binds to a dopamine receptor in striatum and to a serotonin receptor in cortex.

Retina contains guanine nucleotide sensitive and insensitive classes of dopamine receptors.

Calf retina contains high affinity, stereospecific and saturable binding sites, characterized as dopaminergic, for 3H-spiropheridol (Bmax from Scatchard analysis = 38 +/- 4 fmoles/mg protein) and for 3H-ADTN (2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene). The 3H-ADTN sites include a discrete and saturable sub-set of sites sensitive to guanine nucleotide inhibition (Bmax = 50 +/- 6 fmoles/mg protein) and a separate saturable sub-set of sites insensitive to guanine nucleotides (Bmax = 63 +/- 7 fmoles/mg protein). The binding of agonists to 3H-spiroperidol sites is also inhibited by guanine nucleotides.

Aging and monoamine receptors in brain.

Biochemical evidence is presented for selective decreases in biogenic amine receptor systems with age in the rabbit. Dopamine-stimulated adenylate cyclase activity in striatum, hypothalamus, frontal cortex, and anterior limbic cortex declined by about 50% as rabbits aged from less than 1 to 5 years of age. Similar decreases were found for histamine-stimulated activity in hypothalamus and the cortical regions. These changes were in maximal response rather than in affinity for amine. In contrast, dopamine-stimulated adenylate cyclase of retina and both basal and Gpp(NH)p-stimulated activity in these regions were not altered with age. In addition, with age the number of binding sites for [3H]spiroperidol, a dopamine antagonist, decreased by 30--40% without change in ligand affinity in striatum and limbic cortex. These changes in striatum and cortex occurred in the absence of decreases in either dopamine concentration or choline acetylase activity. It is proposed that selective age-dependent decreases in the functional number of biogenic amine receptors occur in the absence of, or independent from neuronal cell loss, possibly by a mechanism of desensitization. These changes occurred in brain regions that in man are thought to be of importance in the age-related loss of cerebral function.

Biogenic amine-stimulated adenylate cyclase and spiroperidol-binding sites in rabbit brain: evidence for selective loss of receptors with aging.

Evidence for selective decreases in biogenic amine receptor function with age in the rabbit has been obtained. Dopamine-stimulated adenylate cyclase activity in the striatum (caudate-putamen) of rabbit brain declined by about 50 percent as rabbits aged from less than 1 to 4 to 5 years of age. Similar decreases in transmitter-stimulated adenylate cyclase activity were found for histamine as well as for dopamine and norepinephrine in hypothalamus, frontal cortex and anterior limbic cortex. Isoproterenol-stimulated activity was also decreased with age in frontal cortex. These changes appeared to represent decreases in maximal response and not alteration in affinity for amine. In contrast, dopamine-stimulated adenylate cyclase of retina and transmitter-independent (basal or Gpp(NH)p-stimulated) activity in each of the regions studied were not altered with age. Dopamine receptors in striatum directly assessed by measurement of [3H]-spiroperidol binding revealed a comparable decrease in the number of binding sites without change in ligand affinity. Preliminary data also indicated decreased spiroperidol binding sites in the cortical regions of older animals. These changes in striatum and cortex were evident in the absence of decreases in either dopamine content or choline acetylase activity, an activity presumed to be present in neurons containing dopamine receptors. It is proposed that selective age-dependent decreases in postsynaptic biogenic amine receptor content occur in the absence of, or independent from, neuronal cell loss, possibly by a mechanism involving receptor desensitization. These changes occur in the animal model in those brain regions which in man are thought to be of importance in the loss of cerebral function that is found with senscence.