John D. Connor

ZINC IN MATURING RAT BRAIN: HIPPOCAMPAL CONCENTRATION AND LOCALIZATION

—Alterations in the localization and concentration of zinc in the hippocampus and in other regions of rat brain were studied during postnatal maturation. Histochemical observations indicated increasing levels of zinc in the hippocampal mossy fibre layer at about 20 days of age. Between 18 and 22 days, hippocampal levels of zinc increased by 35 per cent to reach adult levels (121 ng/mg of protein). After subcellular fractionation, more than one‐half of the hippocampal zinc was found in the first pellet (P1; 1085 g– 10 min), where large boutons would be expected. Autoradiographic evidence, revealing the sequential location of positron tracks first in the granular cell layer and later in the mossy fibre layer, raises the possibility of axoplasmic transport of zinc from granule cell perikarya to their terminal boutons. Our data suggest that zinc may be involved in the maturation and function of the mossy fibre pathway.

Postmortem stability of norepinephrine, dopamine, and serotonin in rat brain.

INVESTIGATORS have attempted in recent years to correlate brain dysfunctions, such as Parkinson’s disease (FAHN et al., 1971) and mental depression (BESKOW et al., 1976), with aberrant concentrations of biogenic amines in human brains postmortem. Interpretation of neurochemical data from human tissue is complicated by a potential for marked alterations in the concentrations of labile substances after death. Amine putative neurotransmitters, norepinephrine (NE), dopamine (DA), and serotonin (5-HT), are labile in aqueous solution at body or room temperature and physiological pH. It seems probable that their rates of degradation would be even greater in a milieu which includes catabolic enzymes. As human brain tissue cannot usually be analyzed until a relatively long time after brain death, i t is important to have reasonable estimates of the degree to which amine values change between the time of death and tissue assay. Changes in concentration, if they occur, could be due to a number of variables inherent in work with human tissue, including: (1) time between brain death and autopsy; (2) temperature at which the body is stored before autopsy; (3) time between removal of the brain from the cranium and freezing or assay. Information on stability postmortem of brain monoamines is scattered, and often contradictory. BERTLER & ROSENGREN (1959) found that NE concentrations in whole rabbit brain removed from the skull immediately after death fell to 84 and 47% of control at 1 and 3 h, respectively. In contrast to a continuing decline for NE, DA concentrations fell to 74% of control at 1 h, then decreased only slightly to 71% at 3 h. MCGEER & MCGEER (1962) reported that in whole rat brain kept at room temperature for various intervals, NE declined little up to 12 h postmortem; DA concentrations were altered marginally at 3 h. but decreased sharply to low levels 6 h after death. GRABARTIS et al., (1965) measured changes in NE in mouse and rat brain up to 4 h after extirpation. They reported that NE in mouse whole brain declined gradually to 32% of control after 4 h; in rat whole brain, NE was 47% of control after 4 h. Later time points and other amines were not examined. V~GEL et al. (1968) measured NE, DA and 5-HT concentrations in whole brains of rats killed by a cervical blow and kept at 4 or 20°C for 16 h before the brains were removed from the skulls. Only one timepoint was considered. Catecholamine concentrations were ap-

Serotonergic properties of β-phenethylamine in rats

Current research interest in β-phenethylamine (PEA) derives, in part, from a possible role for this endogenous compound in paranoid schizophrenia. Systemic administration of PEA caused a behavioral syndrome in rats (simultaneous forepaw padding, side-to-side headweaving or head tremor and splayed hindlimbs) which reflects activation of serotonin (5-HT) receptors in the central nervous system. This syndrome in rats was also produced by large doses of amphetamine, a drug structurally related to PEA, which can induce a behavioral state in man similar to paranoid schizophrenia. Reduction of endogenous 5-HT concentrations by p-chlorophenylalanine or 5,7-dihydroxytryptamine prevented the 5-HT behavioral syndrome caused by amphetamine, but not by PEA. The presumed 5-HT receptor antagonists, methysergide and mianserine, blocked these behavioral responses to both amphetamine and PEA. Therefore, it seems most likely that PEA, in addition to its catecholaminergic actions, produces serotonergic effects in rats by a direct 5-HT agonist action. This is in contrast to amphetamine which apparently causes the same serotonergic behavioral effects by releasing endogenous 5-HT.

Influence of Adrenergic Blocking Drugs on Central Angiotensin Effects

The central effects of angiotensin in unanes thetized rats were studied before and after administration of adrenergic blocking drugs. Intraventricular injections of prevented angiotensin-induced drinking, sympathetic stimulation, and vasopressin release. Administration Phentolamine of various β-adrenergic blocking drugs, either intraarterially or intraventricularly, did not alter central angiotensin activity in a manner which could be correlated with β-adrenergic blockade. Local anesthetics inhibited central angiotensin pressor and drinking effects. Centrally administered phentolamine or tetracaine appear to be useful tools for further studies of the actions of angiotensin on the central nervous system.

Hippocampal mediation of shaking behavior induced by electrical stimulation of the perforant path in the rat

Perforant path stimulation in awake, unrestrained rats caused frequency-dependent shaking behavior in the absence of detectable epileptiform activity. Granule cell discharge, monitored by evoked field potentials, was necessary for the induction of shaking behavior. Direct stimulation of the granule cell layer also caused shakes. Intracerebroventricular injections of kainic acid destroyed hippocampal CA3 pyramidal cells and blocked the shake response. These results indicate that hippocampal granule cell discharge mediates stimulation-induced shaking behavior, and, that epileptiform activity is not required for the induction of shaking behavior by electrical stimulation.

Changes in Neostriatal Dopamine Concentrations in Response to Levodopa Infusions

Abstract: Levodopa was infused under various circumstances of pretreatment into the ear veins of unanesthetized rabbits. Concentrations of neostriatal dopamine formed in response to levodopa administration were determined. The aim was to characterize the temporal relationship between the concentrations of levodopa in plasma and dopamine in the neostriatum. When plasma levodopa was maintained constant by i.v. infusion, the concentration of neostriatal dopamine reached a plateau by 1 h. Increases in dopamine were proportional to the amount of precursor in plasma. The tissue half‐life of this dopamine in normal rabbits was not more than 15 min. Half‐lives of comparable duration for striatal dopamine were calculated from rabbits treated chronically with levodopa, and from rabbits with monoamine‐depleting lesions. The results show that the concentration of dopamine in rabbit neostriatum correlates closely with the concentration of levodopa in plasma. Concurrent analyses of neocortical tissues indicate that the neostriatum may not be different from other brain regions with regard to dopamine storage mechanisms. Interpretation of the results in terms of the clinical use of levodopa suggests that the durations of short‐term effects (measured in h) of the drugs are paralleled by changes in concentration of brain dopamine.

Serotonin agonist actions of p-chlorophenylalanine.

Abstract p -Chlorophenylalanine (pCPA), in combination with a monoamine oxidase inhibitor, caused a behavioural syndrome shown previously to be due to serotonin receptor activation. p -Chlorophenyl-ethylamine (pCPEA), a decarboxylation product of p CPA, also caused the behavioural syndrome. Decarboxylase inhibition prevented the syndrome after p CPA administration, but not after p CPEA. Prior depletion of brain serotonin to 18% of control prevented the syndrome caused by p CPA (plus monoamine oxidase inhibitor), or by pPCEA alone. Subsequent replacement of serotonin in depleted rats by 5-hydroxytryptophan restored the behavioural effects of p CPA and p CPEA. The results show that p CPA (in combination with an inhibitor of monoamine oxidase) caused serotonin receptor activation. The behavioural syndrome is probably elicited by displacement of presynaptic serotonin by p CPEA. As p CPA apparently has a long biological half-life, continued synthesis of p CPEA may cause anomalous behavioural effects not correlated with serotonin depletion. This possibility should be considered in behavioural studies with p CPA, especially during early time periods when serotonin concentrations are not yet reduced.

Methionine enkephalin-induced shaking behavior in rats: dissociation from brain serotonin mechanisms.

Abstract Intracerebroventricular injection of methionine enkephalin (met-enkephalin) caused dosedependent shakes in rats. The role of endogenous 5-hydroxytryptamine (5-HT) in met-enkephalin-induced shakes was investigated since shaking behavior has been associated with activation of 5-HT receptors. Met-enkephalin-induced shakes were not affected significantly by pretreatment with p-chlorophenylalanine, 5,7-dihydroxytryptamine, or methysergide. Naloxone inhibited the shakes caused by met-enkephalin. The results suggest that met-enkephalin elicits shaking behavior by a mechanism independent of central 5-HT.

Para-halogenated phenethylamines: Similar serotonergic effects in rats by different mechanisms☆

The serotonin (5-HT) behavioral syndrome in rats served as a model to test for possible acute serotonergic effects of para-halogenated phenethylamines. p-Chloro-, p-chloro-beta-methyl-, p-fluoro-, p-bromo-, and p-iodophenethylamine produced the same 5-HT behavioral syndrome as did p-chloroamphetamine, but unlike the latter did not deplete brain 5-HT 3 days after injection. Pretreatment of rats with the 5-HT depletor p-chlorophenylalanine (pCPA)( prevented the serotonergic effects of both chloro-derivatives, and partially prevented the effects of bromo- and iodophenethylamine. 5-hydroxytryptophan restored the behavioral responses to these compounds in pCPA-pretreated rats. pCPA treatment did not prevent the behavioral effects of p-fluorophenethylamine. Similarly, zimelidine, a 5-HT uptake inhibitor, prevented the serotonergic behavioral effects of all compounds tested except p-fluorophenethylamine. Taken as a group, para-halogenated phenethylamines are short-acting serotonergic compounds which, unlike p-chloroamphetamine, do not produce long-lasting depletion of brain 5-HT. p-Chlorophenethylamine and its beta-methyl analog apparently activate central 5-HT receptors indirectly, i.e., by 5-HT release; p-fluorophenethylamine is a direct 5-HT agonist. The p-bromo- and p-iodo-derivatives apparently possess both properties.

Monoamine concentrations in brain regions of the developing rabbit

Growth patterns in selected areas of rabbit brain were studied neurochemically by assaying for norepinephrine, dopamine and serotonin from the neonatal to adult stages of development. These measurements showed that the various monoamines reached adult concentrations at different rates within a brain region; each region had a unique pattern of amine development. Changes in content were most marked in the first 2 weeks of life. Concentration increases were pronounced for norepinephrine and serotonin in the hypothalamus and for dopamine in the caudate nucleus. By contrast, norepinephrine concentrations in the pons‐medulla and colliculi were relatively stable. The marked changes and differences in amine concentrations of maturing rabbit brain provide basic data on monoaminergic innervation of discrete regions and indicate the rabbit merits consideration in neurochemical and physiologic studies on the development of neurotransmission.