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Circadian involvement in Parkinson's disease (PD) and more specifically in nigro-striatal dopamine (NSD) function is of increasing interest to the neurosciences. Given that bright light therapy is of therapeutic value in PD, possible mechanisms underlying retinal involvement in this phenomenon was explored further by administering anti-Parkinsonian chemotherapies into the vitreus humour directly adjacent to the retina. 2 microl of a 100 mM solution of L-Dopa significantly improved motor function in the later stages of degeneration and during the day while the injection of 2 microl of a 10 mM solution of the melatonin receptor antagonist ML-23 improved motor function in the early stages of PD and during the dark phase of the light/dark cycle. The results suggest that the function of nigral cells is regulated by a more global system embracing circadian physiology that extends from the retina to the pineal. Furthermore, the induction of PD is characterised by an imbalance between melatonin and dopamine (DA) whereby this ratio is elevated at least 6 to 1 in favour of melatonin. The commonly observed treatment failures and side effects of DA replacement therapy probably result from increasing endogenous DA without taking parallel melatonin dysfunction into account. The proposed integrated function of the NSD and circadian systems may permit therapeutic targeting at a level which is safer, more effective and without the side effects of systemically administered regimens of DA replacement.

Rat hippocampal pyramidal cells were studied for their response to serotonin applied iontophoretically and to stimulation of the midbrain raphe nuclei. Ninety-two per cent of the cells studied were inhibited by serotonin. Fourty-eight per cent of the cells responded by inhibition to dorsal and median raphe stimulation. The inhibitory response to raphe stimulation was absent when the rats were pretreated with p-chlorophenylalanine (PCPA), a serotonin synthesis inhibitor; PCPA effects were alleviated by 5-HTP or 5-HT administration. The response to raphe stimulation was blocked by methysergide and cyproheptadine. The responses to raphe stimulation were potentiated by chlorimipramine, a serotonin reuptake blocker. These data satisfy several of the criteria required to identify serotonin as the inhibitory neuro-transmitter for the raphe-hippocampal pathway.

Melatonin, and its analogs 6-chloro- and 6-fluoromelatonin inhibited in a dose-dependent way (10(-8)-10(-5) M) labeled prostaglandin (PG) E2, PGF2 alpha, thromboxane (Tx) B2 and 6-keto-PGF1 alpha production from [14C]arachidonate by rat medial basal hypothalamus (MBH). 5-Methoxytryptamine also depressed arachidonate metabolism; at 10(-8) M concentrations the effect of 5-methoxytryptamine on PGE2, PGF2 alpha and TxB2 synthesis (93-96% inhibition), and 6-keto-PGF1 alpha (75% inhibition) was greater than that observed for melatonin (51-56% and 44% inhibition, respectively). Neither 6-hydroxymelatonin nor serotonin affected MBH cyclo-oxygenase pathway in vitro.

A quantitative gas chromatography-mass spectrometry method has been used to determine 5-methoxytryptamine (5-MT) in the CNS and pineal gland of various species. The mean ± S.D. levels of 5-MT in sheep, pig and cow pineal glands was 545 ± 180, 228 ± 119 and 117 ± 48 pmol/g, respectively. The postmortem levels of 5-MT in the pig pineal gland was stable for 2 h after death but decreased by more than 90% 24 h after death. In the CNS, 5-MT was found only in the sheep hypothalamus (28 ± 4 pmol/g). Analysis of the rat pineal gland and CNS failed to detect any 5-MT at the limit of sensitivity of the method which is at variance with previously reported results.

Cell-free preparations from superior and inferior colliculi of very young rats (1–3 days old) contained adenylate cyclase systems which were highly responsive to serotonin. The response to serotonin declined markedly during early development and was very low at maturity. Adenylate cyclase activity in the 10, 000 × g particulate fraction from colliculi of newborn rats was significantly stimulated by 0.05 μ M serotonin. Half-maximal activation was produced with less than 1 μ M serotonin. Maximal stimulation of collicular adenylate cyclase was about 80% above basal enzyme activity and occured with approximately 50 μ M serotonin. Tryptamine and several derivatives of serotonin produced responses which were comparable to that obtained with serotonin; 5-methoxytryptamine was uniformly the most active compound tested. Norepinephrine or dopamine produced much smaller increases in adenylate cyclase activity. Stimulation of collicular adenylate cyclase by serotonin was significantly but incompletely blocked by serotonin antagonists, including D -lysergic acid diethylamide ( D -LSD), 2-bromo- D -lysergic acid diethylamide, methysergide, 1-methyl-8β-carbobenzyloxy-aminomethyl-10α-ergoline and cyproheptadine. Chlorpromazine also produced partial blockade. In contrast, L -lysergic acid diethylamide, haloperidol, propranolol, phenoxybenzamine and morphine were ineffective as serotonin blocking agents. Of the compounds which produced a partial blockade of serotonin action, D -LSD, cyproheptadine and chlorpromazine were themeselves capable of stimulating adenylate cyclase activity. These results are consistent with the existence of multiple receptors in rat brain which are capable of interacting with indoleamines.

Recent evidence has identified directly muscarinic acetylcholine receptor (m-ACh R) and nicotinic acetylcholine receptor (n-ACh R) in the brain utilizing receptor binding assay. Several studies suggest that release of dopamine (DA) in the striatum is regulated by presynaptic receptors present on dopaminergic terminals. In the present study, the effects of cholinergic drugs on [3H]DA release were examined using micropunched tissue and synaptosomes obtained from rat striatum. ACh (5 x 10(-4) M) significantly increased spontaneous [3H]DA release, and the overflow was partially inhibited by D-tubocurarine (1 mM) but not atropine. Nicotine, lobeline, coniine and spartein, nicotinic agonists, significantly increased spontaneous and 25 mM K + evoked [3H]DA release almost in a dose-dependent manner. In contrast, oxotremorine (2 x 10(-4) M), muscarinic agonist, did not any change in [3H]DA efflux. Furthermore, the metabolites of [3H]DA were separated by column chromatography. The main metabolite of [3H]DA in the spontaneous release from rat striatal synaptosomes was [3H]DOPAC (3,4-dihydroxyphenylacetic acid). Lobeline (5 x 10(-5) M) accelerated the outflow of [3H]DOPAC and [3H]OMDA metabolites (O-methylated and deaminated metabolites). These results could give rise to the suggestion that there was n-ACh R on the dopaminergic nerve terminals in the striatum and n-ACh R might have related to a directly excitatory effect on the DA release.

The effect of several pineal methoxyindoles on 45Ca2+-uptake was examined in a crude synaptosome preparation from adult rat brains. 5-Methoxytryptol, 5-methoxytryptamine, melatonin and 6-chloromelatonin (10(-8)-10(-6) M) depressed significantly the K+-stimulated increase in Ca2+-uptake, without affecting the basal, unstimulated Ca2+-uptake by synaptosomes. At 10(-6) M concentration the following order of potency was found: 6-chloromelatonin greater than or equal to 5-methoxytryptamine greater than 5-methoxytryptophol greater than or equal to melatonin. Serotonin did not affect significantly either basal or stimulated Ca2+-uptake.

Recent work by ourselves and others has indicated that serotonin (5-HT) acts as a regulator of neuronal growth in fetal tissue, probably through an action on the high affinity 5-HT receptors known to be present. In order for our hypothesis to be correct, these receptors must be shown to be functional in fetal tissue. Furthermore, since 5-HT has a dual role in neuronal development, these receptors must be functional both in the region of the serotonergic cell body (brainstem) and in projection areas (forebrain). We have tested the functional status of fetal 5-HT receptors by testing their ability to adapt to changes in the level of 5-HT. Pregnant rats were treated with saline, p-chlorophenylalanine (a 5-HT depletor) or 5-methoxytryptamine (a 5-HT agonist) and the characteristics of the high affinity 5-HT receptors in the rat pups determined using a binding assay with [3H]5-HT. Our results show that both forebrain and brainstem receptors respond to alterations in transmitter level in a manner similar to adult brain. Thus, fetal 5-HT receptors are functional and capable of playing a role in neuronal development.