Agu Pert

Autoradiographic evidence for nicotine receptors on nigrostriatal and mesolimbic dopaminergic neurons

Rats received unilateral injections of 6-hydroxydopamine into the medial forebrain bundle, resulting in an ipsilateral loss of striatal dopamine and of dopaminergic perikarya. A concomitant reduction of displaceable tritiated nicotine binding was observed autoradiographically in the ipsilateral substantia nigra, ventral tegmental area, striatum, nucleus accumbens, and olfactory tubercle. Thus, nicotine receptors appear to be located on nigrostriatal and mesolimbic dopaminergic neurons at the level of perikarya and terminals.

Autoradiographic distribution of nicotine receptors in rat brain.

The autoradiographic visualisation of 90%-specific tritiated nicotine binding to slide-mounted sections of rat brain is reported. Tritiated nicotine bound with high affinity (nanomolar Kd) and was selectively displaced by nicotinic agonists (e.g. L-nicotine approximately ACh greater than D-nicotine). The strikingly discrete distribution pattern obtained deviates from that of alpha-bungarotoxin, and suggests several possible roles for nicotinic transmission in the brain.

Afferents to brain stem nuclei (brain stem raphe, nucleus reticularis pontis caudalis and nucleus gigantocellularis) in the rat as demonstrated by microiontophoretically applied horseradish peroxidase

Using a retrograde tracer technique with microiontophoretically applied horseradish peroxidase (HRP), afferent projections to the brain stem raphe nuclei (BR, raphe magnus, pallidus and obscurus) and to two adjacent reticular nuclei, nucleus reticularis pontis caudalis (nRPC) and nucleus gigantocellularis (nGC) were identified. The most striking difference between the afferent projections to the BR and the adjacent nuclei as determined by this method is that afferents to the BR originate primarily from structures rostral to the pons, especially the mesencephalic central gray and the dorsal and ventral tegmentum. In contrast, the two reticular nuclei studied (nGC and nRPC) received afferent projections within or caudal to the pons-medulla. For example, the nGC receives prominent afferent projections from the gray matter of the spinal cord. In addition, evidence for interconnections between all of the adjacent nuclei (BR, nGC and nRPC) was found. Such afferent projections are compatible with the notion that the brain stem raphe nuclei may serve as connections within the brain stem for a descending system, while the nGC may be a relay in a feedback loop between the spinal cord and the reticular formation.

Opiate receptor affinities and behavioral effects of enkephalin: structure-activity relationship of ten synthetic peptide analogues.

Abstract Synthetic met- and leu-enkephalin bind to rat brain opiate receptors with 1 2 and 1 7 the affinity of morphine. The aromatic hydroxyl moiety of the tyrosine residue is critical for receptor binding. Intracranial microinjection of met-enkephalin requires very high doses to produce an evanescent, naloxone reversible analgesia and stuperous immobility, presumably because of its rapid enzymatic degradation. Leu-enkephalin fails to elicit analgesia.

Innervation of substantia nigra neurons by cholinergic afferents from pedunculopontine nucleus in the rat: neuroanatomical and electrophysiological evidence

Dopaminergic neurons of the substantia nigra pars compacta are excited by nicotine and acetylcholine, and possess both high-affinity nicotine binding sites and intense acetylcholinesterase activity, consistent with a cholinoceptive role. A probable source of cholinergic afferents is the pedunculopontine nucleus, which forms part of a prominent group of cholinergic perikarya located caudal to the substantia nigra in the tegmentum. Although pedunculopontine efferents, many of them cholinergic, project to the substantia nigra pars compacta, it has not been established whether they terminate in this structure. In the first experiment, which combined retrograde tracing with immunohistochemical visualization of cholinergic neurons, cholinergic cells in and around the pedunculopontine nucleus were found to send projections to the substantia nigra. This projection was almost completely ipsilateral. Subsequent experiments employed anaesthetized rats; kainate was microinfused into tegmental sites in order to stimulate local cholinergic perikarya, and concurrently, extracellular recordings were made of single dopaminergic neurons in the substantia nigra. Consistent with our anatomical findings, unilateral microinfusion of kainic acid in or near the pedunculopontine nucleus increased the firing rate of dopaminergic neurons situated remotely in the ipsilateral substantia nigra. The kainate-induced excitation of nigral dopaminergic neurons was dose-related and was prevented by intravenous administration of the centrally-acting nicotinic cholinergic antagonist mecamylamine. These results suggest that cholinergic perikarya in the vicinity of the pedunculopontine tegmental nucleus innervate dopaminergic neurons in the substantia nigra pars compacta via nicotinic receptors.

Comparative localization of neurotensin receptors on nigrostriatal and mesolimbic dopaminergic terminals

Neurotensin (NT), a brain-gut peptide, possesses many biological actions similar to those reported for neuroleptics. Moreover, it has been shown that NT alters dopaminergic activity of both nigrostriatal and mesolimbic pathways. We now report that NT receptors are located on dopaminergic cell bodies in both systems. However, the proportion of NT receptors on presynaptic dopaminergic terminals appears to be different. NT receptor sites are mainly found on presynaptic dopaminergic terminals in the caudate-putamen while they are pre- and post-synaptically located in the nucleus accumbens and the olfactory tubercule. NT receptors differential localization in these two pathways could be used as a model to study the comparative physiology of various dopaminergic brain systems.

Bombesin: Receptor distribution in brain and effects on nociception and locomotor activity

The radioreceptor assay was used to examine the distribution of bombesin receptors in the rat brain. The highest concentrations of receptors appeared to be associated with limbic forebrain and midbrain structures such as the hippocampus, amygdala, hypothalamus and the periaqueductal gray matter. The caudate-putamen of the extrapyramidal motor system and the forebrain also exhibited high bombesin binding. Intraventricular injections of bombesin (0.1, 1.0 and 10 microgram) produced a dose-dependent increase in locomotor activity in rats. Injections of bombesin into the periaqueductal gray matter produced an antinociceptive reaction in the hot-plate as well as the tail-flick test. This apparent analgesia was not antagonized by naloxone.

Electrophysiological actions of nicotine on substantia nigra single units.

1 Extracellular recordings of single unit activity were made in the substantia nigra (SN) of chloral hydrate‐anaesthetized rats. 2 Dopaminergic neurones of the pars compacta (SNC) were stimulated by (—)‐nicotine bitartrate (1.0 mg k−1) given subcutaneously (s.c.). This action was prevented by the secondary amine mecamylamine HCl (2.0 mg kg−1 i.v.) but not by a ganglion‐blocking dose of the bisquaternary compound chlorisondamine Cl (0.1 mg kg−1 i.v.). Mecamylamine reduced the spontaneous activity of dopaminergic neurones. 3 Nicotine, when administered intravenously (2–128 μg kg−1 cumulative dose), also stimulated dopamine cells and this action was dose‐related. 4 Nicotine, administered intravenously, (2–128 μg kg−1 cumulative dose) markedly excited non‐dopamine cells in the pars reticulata (SNR) in a dose‐related manner. In rats pretreated with chlorisondamine (0.1 mg kg−1 i.v.), nicotine induced a small excitatory or depressant action, but the marked excitation was not seen. Mecamylamine (2 mg kg−1 i.v.) completely prevented the actions of nicotine. 5 The results are consistent with a direct excitatory action of nicotine on dopaminergic neurones of the substantia nigra pars compacta. The pronounced excitatory action of systemically administered nicotine on non‐dopamine cells of the pars reticulata appears to be of peripheral origin.

Focal application of alcohols elevates extracellular dopamine in rat brain: a microdialysis study

Dopaminergic systems are thought to play a major role in the stimulant and reinforcing properties of drugs of abuse, including ethanol. The present study describes the effects of local perfusion with ethanol (and other alcohols) on extracellular dopamine in the striatum and nucleus accumbens. Following the establishment of basal dopamine levels (2-3 h), various concentrations of ethanol in artificial CSF (0.01-10% v/v) were slowly perfused through a microdialysis probe. Each dose of ethanol was found to increase dopamine concentrations in both the striatum and nucleus accumbens. This increase was dose-related in the striatum. The exclusion of calcium and inclusion of 12.5 mM magnesium in the perfusion medium prevented, or greatly attenuated the ethanol-induced dopamine (DA) release. Thus, the release of DA by ethanol is exocytotic in nature and involves calcium-dependent processes. The other alcohols tested, namely methanol and butanol, demonstrated a structure-activity relationship together with ethanol, in their ability to increase extracellular DA. The relative potency was butanol greater than ethanol greater than methanol. The diffusion of ethanol into the brain tissue was investigated following perfusion through the probe. Relatively low concentrations of ethanol were found in striatal tissue during perfusion and they declined rapidly with time, following the removal of ethanol from the perfusate. The concentrations of ethanol achieved in brain tissue following focal application through the microdialysis probe were relevant to human intoxication.

Apparent antioxidant effect of l-deprenyl on hydroxyl radical formation and nigral injury elicited by MPP+ in vivo

Using a modified microdialysis procedure, we confirmed that intrastriatal administration of 1-methyl-4-phenylpyridinium ion (MPP+) induced a sustained overflow of dopamine accompanied by increased formation of hydroxyl free radicals (.OH) as reflected by salicylate hydroxylation. Pretreatment with l-deprenyl (selegiline 60 pmol, intrastriatal perfusion) significantly decreased the .OH formation elicited by MPP+ (75 nmol). There was a small decrease of dopamine efflux and an insignificant change of the ratio of 3,4-dihydroxyphenylacetic acid (DOPAC)/dopamine following l-deprenyl pretreatment. These in vivo findings support prior in vitro data that an unique antioxidant property of l-deprenyl may be independent of its inhibition of type B monoamine oxidase. In addition, intranigral co-administration of l-deprenyl (4.2 nmol) with MPP+ (4.2 nmol) completely protected nigral neurons from probable oxidative injuries induced by MPP+ (4.2 nmol), as reflected by a near 50% loss of striatal dopamine ipsilateral to the side of infusion of drug into the substantia nigra. This apparent neuroprotective effect of l-deprenyl on midbrain nigral neurons was also confirmed by histological findings. The present in vivo data clearly demonstrate that l-deprenyl can protect nigral neurons against dopamine neurotoxicity produced by MPP+, as suggested by an earlier in vitro study. Thus, l-deprenyl can preserve the function of MPP(+)-damaged nigral neurons perhaps by its apparent antioxidant property in addition to its blockade of the bioactivation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to toxic pyridinium metabolites by type B monoamine oxidase.