Neuropharmacology and neurochemistry of canine narcolepsy

It is believed that narcolepsy involves abnormalities of rapid eye movement (REM) sleep, especially of REM sleep atonia. Compelling evidence suggests that the regulation of REM sleep and REM sleep atonia involves a reciprocal interaction of cholinergic and monoaminergic systems. Using our canine model of narcolepsy and a pharmacological approach, we have previously demonstrated a similar interaction in the regulation of cataplexy. Global activation of cholinergic or monoaminergic transmission aggravates or suppresses canine cataplexy, respectively. We have also identified the subtypes of monoaminergic and cholinergic receptors specifically involved in this interaction. Cataplexy is aggravated by activation of the cholinergic system via M2 stimulation, as well as deactivation of the catecholaminergic systems by either blockade of postsynaptic alpha-1b receptors or stimulation of alpha-2 or D2 inhibitory autoreceptors. These pharmacological results correspond to previously identified neurochemical abnormalities in canine narcolepsy, such as significant increases in M2 receptors in the pons, alpha-1 receptors in the amygdala, alpha-2 receptors in the locus coeruleus and D2 receptors in the amygdala and nucleus accumbens, when compared to control animals. Using local perfusion of active compounds, we have further demonstrated that cholinoceptive sites in the pontine reticular formation, as well as in the basal forebrain, are involved in the regulation of cataplexy. Although the specific sites of action of the monoaminergic compounds remain unknown, the results of our pharmacological and neurochemical studies to date suggest that a widespread hyperactivity of cholinergic systems within the central nervous system together with a hypoactivity of catecholaminergic systems underlie the pathophysiology of narcolepsy.

Cholinergic regulation of cataplexy in canine narcolepsy in the pontine reticular formation is mediated by M2 muscarinic receptors

Both rapid eye movement sleep and cataplexy in the narcoleptic canine have been shown to increase after both systemic and local administration of cholinergic agonists in the pontine reticular formation. Furthermore, binding studies indicate an increase in the number of M2 muscarinic receptors in the pontine reticular formation of narcoleptic canines. In the present study we have investigated the receptor subtypes involved in mediating the cholinergic stimulation of cataplexy, as defined by brief periods of hypotonia induced by emotions, within the pontine reticular formation of narcoleptic canines. Specific cholinergic and monoaminergic agonists and antagonists, and excitatory or inhibitory amino-acid neurotransmitter receptor agonists, were perfused through microdialysis probes implanted bilaterally in the pontine reticular formation of narcoleptic canines, and cataplexy was monitored using the Food-Elicited Cataplexy Test and recordings of electroencephalogram, electrooculogram and electromyogram. In narcoleptic canines, bilateral perfusion with oxotremorine (M2 muscarinic) (10(-5)-10(-3) M) in the pontine reticular formation produced a dose-dependent increase in cataplexy, which reached complete muscle atonia (status cataplecticus) during the highest concentration. In control canines bilateral perfusion with oxotremorine (10(-5)-10(-3) M) did not produce any cataplectic attacks, but did produce muscle atonia after the highest concentration. Bilateral perfusion with either McN-A-343 (M1 muscarinic) or nicotine (both 10(-5)-10(-3) M) did not have any effect on cataplexy in either narcoleptic or control canines. The increase in cataplexy in narcoleptic canines produced by local perfusion with carbachol (10(-4) M) followed by equimolar perfusion with a muscarinic antagonist was rapidly reversed by atropine (muscarinic) and gallamine (M2 muscarinic), partially reversed by 4-DAMP (M3/M1 muscarinic) and completely unaffected by pirenzepine (M1 muscarinic). Bilateral perfusion with excitatory, glutamatergic receptor agonists N-methyl-D-aspartate, AMPA (both at 10(-4)-10(-3) M) and kainic acid (10(-5)-10(-4) M) did not have any effect on cataplexy, whereas bilateral perfusion with the inhibitory GABAergic receptor agonist muscimol (10(-4)-10(-3) M) produced a moderate increase in cataplexy in the narcoleptic canines. Bilateral perfusion with numerous monoaminergic compounds, BHT-920 (alpha-2 agonist), yohimbine (alpha-2 antagonist), propranolol (beta antagonist) and prazosin (alpha-1 antagonist), did not have any effect on cataplexy. These findings demonstrate that cholinergic regulation of cataplexy in the narcoleptic canine at the level of the pontine reticular formation is mediated by M2, and possibly M3, muscarinic receptors. The effects of muscimol indicate that the stimulation of cataplexy might be elicited by local neuronal inhibition.

Cholinergic mechanisms in canine narcolepsy--II. Acetylcholine release in the pontine reticular formation is enhanced during cataplexy

Cataplexy in the narcoleptic canine has been shown to increase after local administration of carbachol into the pontine reticular formation. Rapid eye movement sleep has also been shown to increase after local administration of carbachol in the pontine reticular formation, and furthermore, acetylcholine release in the pontine tegmentum was found to increase during rapid eye movement sleep in rats. Therefore, in the present study we have investigated acetylcholine release in the pontine reticular formation during cataplexy in narcoleptic canines. Extracellular acetylcholine levels were measured in the pontine reticular formation of freely moving narcoleptic and control Doberman pinschers using in vivo microdialysis probes. Cataplexy was induced by the Food-Elicited Cataplexy Test and monitored using recordings of electroencephalogram, electrooculogram and electromyogram. Basal levels of acetylcholine in the microdialysis perfusates were approximately 0.5 pmol/10 min in both control and narcoleptic canines. Local perfusion with tetrodotoxin (10(-5) M) or artificial cerebrospinal fluid without Ca2+ produced a decrease, while intravenous injections of physostigmine (0.05 mg/kg) produced an increase in acetylcholine levels, indicating that the levels of acetylcholine levels measured are derived from neuronal release. During cataplexy induced by the Food-Elicited Cataplexy Test, acetylcholine levels increased by approximately 50% after four consecutive tests in narcoleptic canines, but did not change after four consecutive tests in control canines. Motor activity and feeding behavior, similar to that occurring during a Food-Elicited Cataplexy Test, had no effect on acetylcholine levels in the narcoleptic canines.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic mechanisms in canine narcolepsy--I. Modulation of cataplexy via local drug administration into the pontine reticular formation

Cataplexy in the narcoleptic canine has been shown to increase after systemic administration of cholinergic agonists. Furthermore, the number of cholinergic receptors in the pontine reticular formation of narcoleptic canines is significantly elevated. In the present study we have investigated the effects of cholinergic drugs administered directly into the pontine reticular formation on cataplexy, as defined by brief episodes of hypotonia induced by emotions, in narcoleptic canines. Carbachol and atropine were perfused through microdialysis probes implanted bilaterally in the pontine reticular formation of freely moving, narcoleptic and control Doberman pinschers. Cataplexy was quantified using the Food-Elicited Cataplexy Test, and analysed using recordings of electroencephalogram, electrooculogram and electromyogram. Cataplexy was characterized by a desynchronized electroencephalogram and a drop in electromyogram and electrooculogram activity. In narcoleptic canines, both unilateral and bilateral carbachol (10(-5) to 10(-3) M) produced a dose-dependent increase in cataplexy, which resulted in complete muscle tone suppression at the highest concentration. In control canines, neither bilateral nor unilateral carbachol (10(-5) to 10(-3) M) produced cataplexy, although bilateral carbachol, did produce muscle atonia at the highest dose (10(-3)). The increase in cataplexy after bilateral carbachol (10(-4) M) was rapidly reversed when the perfusion medium was switched to one containing atropine (10(-4) M). Bilateral atropine (10(-3) to 10(-2) M) alone did not produce any significant effects on cataplexy in narcoleptic canines; however, bilateral atropine (10(-2) M) did reduce the increase in cataplexy produced by systemic administration of physostigmine (0.05 mg/kg, i.v.). These findings demonstrate that cataplexy in narcoleptic canines can be stimulated by applying cholinergic agonists directly into the pontine reticular formation. The ability of atropine to inhibit locally and systemically stimulated cataplexy indicates that the pontine reticular formation is a critical component in cholinergic stimulation of cataplexy. Therefore, it is suggested that the pontine reticular formation plays a significant role in the cholinergic regulation of narcolepsy.

Heterozygosity at the canarc-1 locus can confer susceptibility for narcolepsy: induction of cataplexy in heterozygous asymptomatic dogs after administration of a combination of drugs acting on monoaminergic and cholinergic systems

Narcolepsy is a genetically determined disorder of sleep characterized by excessive daytime sleepiness and abnormal manifestations of REM sleep that affects both humans and animals. Although its exact pathophysiologic mechanisms remain undetermined, recent experiments have demonstrated that in both humans and canines, susceptibility genes are linked with immune-related genes. A striking difference, however, is that the genes thought to be involved in the human pathology are autosomal dominant, whereas canine narcolepsy in Dobermans is transmitted as a single autosomal recessive gene with full penetrance (canarc-1). In this study, we have examined the development of narcoleptic symptoms in homozygous narcoleptic, heterozygous, and control Dobermans. Animals were behaviorally observed until 5 months of age and then treated at weekly intervals with cataplexy-inducing compounds that act on cholinergic or monoaminergic systems (alone and in combination). Our data indicate that cataplexy can be induced in 6-month-old asymptomatic heterozygous animals, but not in control canines, with a combination of drugs that act on the monoaminergic and cholinergic systems. This demonstrates that disease susceptibility may be carried by heterozygosity at the canarc-1 locus. Our data further suggest that cataplexy, a model of REM sleep atonia, is centrally regulated by a balance of activity between cholinergic and monoaminergic neurons.

A Schizosaccharomyces pombe gene that promotes sexual differentiation encodes a helix-loop-helix protein with homology to MyoD

Nitrogen starvation of Schizosaccharomyces pombe induces a differentiated state in which haploid cells mate and sporulate. esc1+, a newly isolated S.pombe cDNA that promotes this sexual differentiation, encodes a putative transcription factor with a helix-loop-helix (HLH) motif similar to those of the human MyoD and Myf-5 myogenic differentiation inducers. Disruption of esc1+ in wild-type cells leads to a decrease in the efficiency of sexual conjugation, an early step in sexual differentiation. The disruption was also able partially to substitute for cAMP, an inhibitor of differentiation, to suppress the lethal, constitutive differentiation induced by the pat1 mutation. Conversely, overexpression of this cDNA conferred partial resistance to cAMP-mediated inhibition of differentiation. Transcription from this novel gene was induced early in response to nitrogen starvation and is largely independent of the ste11+ gene product, which is required for the differentiation-specific expression of other genes. Thus, this MyoD/Myf-5-like protein appears to promote sexual differentiation by modulating responses to decreases in cAMP, a part of the nitrogen starvation signal that induces differentiation.

Chasing the elusive animal model of late-phase bronchoconstriction: studies in dogs, guinea pigs and rats

Antigen inhalation in sensitized dogs, guinea pigs and rats resulted in a marked, late-phase, eosinophil-rich, influx of inflammatory cells into the bronchial lumen. Attempts to demonstrate an associated late-phase bronchoconstriction were disappointing. We were unable to demonstrate a late-phase bronchoconstriction in either rats or dogs, even when dogs were pretreated with metyrapone to reduce blood cortisol levels. In ovalbumin-sensitized guinea pigs, challenged with low doses of ovalbumin, we observed an immediate bronchoconstriction, a late-phase bronchopulmonary eosinophilia but no late-phase bronchoconstriction. However, inhalation of very high doses of antigen in mepyramine-treated sensitized guinea pigs did induce a moderate late-phase bronchoconstriction.

Monoaminc and metabolite levels in the cerebrospinal fluid of hibernating and euthermic marmots

Cerebrospinal fluid from yellow-bellied marmots, Marmota flaviventris, was analysed for monoamine and monoamine metabolite content during euthermia and deep hibernation. Dopamine (DA) levels were decreased, while DA metabolite levels, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were dramatically increased in hibernating marmots. Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5HIAA) levels were also greatly enhanced during hibernation while norepinephrine (NE) levels were only moderately increased. These findings demonstrate that cerebrospinal monoamine levels are dynamically altered during hibernation, such that DA versus 5-HT and NE levels undergo opposite changes. Therefore, these data indicate that DA, 5-HT and NE neuronal systems are differentially altered during hibernation in mammals.

Physiological changes accompanying senescence in the ephemeral daylily flower

The daylily flower, Hemerocallis hybrid cv Cradle Song, develops from the opening bud to full senescence in 36 hours. Unlike other ephemeral flowers studied to date, it does not respond to ethylene, but other senescence phenomena are similar. There was a small respiration climacteric coinciding with early flower senescence, and it was also observed in isolated petals and petal slices. Cycloheximide abolished the climacteric and delayed senescence in all three systems. Petal apparent free space increased from 30% at bud opening to 38% at the onset of senescence, and sugar efflux increased from 0.2 to 2.8 milligrams per gram of fresh weight per hour during the same period. A sharp increase in ion efflux from 0.8 to 4.0 micromoles of NaCl equivalents per gram of fresh weight per hour, coinciding with the climacteric, was abolished by cycloheximide. Uptake of radiolabeled inorganic phosphate by petal slices from 100 micromolar solution increased during onset of senescence from 6 to 10 nmoles per gram of fresh weight per hour. Half was esterified; of this, 14% went into ATP, and the cellular energy charge remained high at 0.86 during senescence. The proportion incorporated into phospholipid (2.2%) did not change during senescence, but the proportion in phosphatidyl choline increased and in phosphatidyl glycerol decreased during senescence. The general phosphate ester pattern in presenescent slices closely resembled that in other plant tissues except that phospholipid precursors were more prominent (approximately 20% of total organic (32)P versus 5%). In senescent slices, the proportion of hexose phosphates decreased from 40 to 15% of total organic (32)P and that of phospholipid precursors increased to approximately 50%, suggesting that phospholipid synthesis was blocked early in senescence.

Effects of intranigral substance P and neurokinin A injections on extracellular dopamine levels measured with microdialysis in the striatum and frontoparietal cortex of rats

Extracellular levels of dopamine (DA) and its metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), in the striatum and frontoparietal (sensorimotor) cortex in halothane-anesthetized rats were analyzed simultaneously using in vivo microdialysis. Basal DA levels, measured from the microdialysis perfusate, were 6.4 +/- 0.8 nM (n = 15) in the striatum and 0.9 +/- 0.1 nM (n = 15) in the frontoparietal cortex. Subcutaneous injections of d-amphetamine (2 mg/kg) increased DA levels 10-fold in the striatum and fivefold in the cortex. Injections of substance P (0.07 nmol/0.2 microliters) into the substantia nigra pars reticulata (SNR) increased DA and DOPAC levels approximately 30% in the ipsilateral striatum and approximately 50% in the ipsilateral frontoparietal cortex. Injections of neurokinin A (0.09 nmol/0.2 microliter) into the SNR increased DA and DOPAC levels approximately 30% in the ipsilateral striatum but did not significantly affect DA levels in the ipsilateral frontoparietal cortex, although DOPAC levels were increased by approximately 50%. It is suggested that striatal and cortical DA release is regulated differently by nigral substance P and neurokinin A terminals.

Striatal dopamine and glutamate release: effects of intranigral injections of substance P

The levels of extracellular striatal dopamine and glutamate were measured simultaneously in halothane-anaesthetized rats using microdialysis. Unilateral injections of substance P (0.07 nmol) into the substantia nigra, pars reticulata enhanced the levels of dopamine and glutamate in the ipsilateral striatum. Intranigral injections of neurokinin A (0.09 nmol) enhanced the levels of striatal dopamine, and intranigral injections of gamma-aminobutyric acid (300 nmol) or dynorphin A (0.5 nmol) decreased the levels of striatal dopamine, but none of these had any effect on the levels of striatal glutamate. Local perfusion with the dopamine agonists apomorphine (D1/D2), SKF 38393 (D1) or pergolide (D2) (each at 10(5) M) decreased the levels of striatal dopamine and enhanced the levels of striatal glutamate. In unilateral 6-hydroxydopamine-lesioned rats, basal striatal glutamate levels were decreased bilaterally. Furthermore, on the denervated side intranigral substance P stimulation of striatal glutamate levels was enhanced, while on the intact side intranigral substance P stimulation of striatal dopamine and glutamate levels was similar to that seen in normal rats. These findings suggest that striatonigral substance P provides a stimulatory regulation of ipsilateral striatal glutamate release. Furthermore, it is indicated that striatal glutamate release can also be regulated by dopamine terminals.

Intranigral substance P stimulation of striatal dopamine release is inhibited by spantide II: a new tachykinin antagonist without apparent neurotoxicity

The effects of intranigral injections of Spantide II, a novel tachykinin antagonist, on extracellular dopamine, and dihydroxyphenylacetic acid (DOPAC) levels in the rat striatum were studied using in vivo microdialysis. The ability of Spantide II to inhibit intranigral substance P or neurokinin A stimulation of striatal dopamine levels was also studied. A unilateral injection (all substances were injected in a volume of 0.2 microliter) of Spantide II (0.7 nmol) into the substantia nigra, pars reticulata (SNR) of halothane anaesthetized rats produced a short-lasting decrease in dopamine levels in the ipsilateral striatum. Striatal DOPAC levels showed no change after Spantide II. A unilateral injection of substance P (0.07 nmol) into the SNR produced an increase in ipsilateral striatal dopamine levels, which was prevented when substance P was co-administered with Spantide II (0.7 nmol). A unilateral injection of neurokinin A (0.09 nmol) into the SNR produced an increase in ipsilateral striatal dopamine levels, which was not modified when neurokinin A was co-administered with Spantide II (0.7 nmol). Immunohistochemical analysis using antisera to tyrosine hydroxylase and neuropeptide K, as well as Cresyl violet staining, revealed that intranigral injections of Spantide II (0.7 nmol) did not produce significant damage in the substantia nigra. The results indicate that Spantide II is not 'neurotoxic' when injected intranigrally, and that it is a selective antagonist of substance P in the substantia nigra. Furthermore, the reduction of striatal dopamine levels after intranigral Spantide II injections suggests that the nigrostriatal dopamine projection is tonically stimulated by striatonigral substance P.

Striatonigral GABA, dynorphin, substance P and neurokinin A modulation of nigrostriatal dopamine release: evidence for direct regulatory mechanisms

The striatonigral pathway contains several neurotransmitters which may regulate the activity of the nigrostriatal dopamine projection in the rat. This was investigated by measuring extracellular dopamine levels in the striatum, using microdialysis, after injections of GABA (300 nmol/0.2 microliters), dynorphin A (0.5 nmol/0.2 microliters), substance P (0.07 mnol/0.2 microliters) or neurokinin A (0.09 nmol/0.2 microliters) into the ipsilateral substantia nigra, pars reticulata (SNR). Intranigral injections of GABA or dynorphin A inhibited, while intranigral injections of substance P or neurokinin A stimulated dopamine levels in the ipsilateral striatum. In rats with ibotenic acid lesions (2.5 micrograms/0.5 microliters) in the SNR, intranigral injections of GABA or dynorphin A inhibited, while intranigral injections of substance P or neurokinin A stimulated dopamine levels in the ipsilateral striatum. These responses were not significantly different than those in unlesioned rats. Analysis of the intranigral lesion with in situ hybridization revealed a heavy loss of glutamic acid decarboxylase mRNA expression in the SNR and a significant loss of tyrosine hydroxylase (TH) mRNA expression in the SNC. Immunohistochemical analysis revealed a disappearance of TH-Like immunoreactivity (LI) im dendrites in the SNR, a considerable loss of TH-LI cell bodies in the SNC and a restricted loss of neuropeptide K-LI in the SNR around the tip of the injection cannula. Furthermore, lesioned rats rotated ipsilateral to the lesion after apomorphine (1 mg/kg, s.c.), indicating that the basal ganglia output mediated via the SNR GABA neurons was impaired on the lesioned side. Analysis of the striatum revealed that a dense TH-LI fiber network could still be seen on the lesioned side. Furthermore, basal and amphetamine stimulated extracellular dopamine levels in the striatum on the lesioned side were not significantly depleted. This indicates that the ascending nigrostriatal dopamine projection was functionally intact on the lesioned side. These findings indicate that intranigral GABA, dynorphin A, substance P and neurokinin A modulation of ipsilateral striatal dopamine release is mediated via direct action on the nigrostriatal projection. Thus, it is suggested that the striatonigral pathway, which contains GABA, dynorphin, substance P and neurokinin A, exerts a direct regulatory effect on the activity of the nigrostriatal dopamine projection.

Intranigral substance P modulation of striatal dopamine: interaction with N-terminal and C-terminal substance P fragments

The effects of unilateral injections of two substance P fragments, the N-terminal substance P (1-7) (SP1-7) and the C-terminal substance P (6-11) (SP6-11) into the substantia nigra, pars reticulata on dopamine (DA) release in the ipsilateral striatum of halothane-anaesthetized rats were studied using microdialysis. SP1-7 and SP6-11 were also tested for their ability to modify the DA stimulation produced by intranigral injections of SP or neurokinin A (NKA). In addition, the SP antagonist Spantide I was tested for its ability to modify the DA stimulation produced by an intranigral injection of SP1-7. Intranigral injections of SP1-7 (0.001-5.0 nmol) inhibited DA release after low doses (0.001-0.01 nmol), but stimulated DA release after high doses (0.1-5.0 nmol). Striatal dihydroxyphenylacetic acid (DOPAC) levels increased moderately after high doses of SP1-7 (1.0-5.0 nmol). Intranigral injections of SP6-11 (0.01-5.0 nmol) inhibited DA release, but enhanced striatal DOPAC levels, dose-dependently. SP1-7 (0.01-0.1 nmol), but not SP6-11 (0.1 nmol), blocked the stimulation of striatal DA release produced by intranigral SP (0.07 nmol). Neither SP1-7 (0.1 nmol) nor SP6-11 (0.1 nmol) could modify the stimulation of striatal DA release produced by intranigral NKA (0.09 nmol). The increase in DA release after a high dose of SP1-7 (1.0 nmol) was not modified by co-administration with Spantide I (0.07 nmol).(ABSTRACT TRUNCATED AT 250 WORDS)

The substance P(1-7) fragment is a potent modulator of substance P actions in the brain

The neuropeptide, substance P (SP), produces a spectrum of behavioural effects. When given locally into the substantia nigra, SP induces dopamine release in the ipsilateral striatum and produces contralateral rotation in a dose-dependent, but bell-shaped, manner. Similar dose-response relationships have been observed for SP and other peptides in different bioassays. To test whether SP fragmentation is responsible for this phenomenon, SP(1-7), which is the main SP fragment in rat CNS, was injected intranigrally. SP(1-7) was found to act as a very potent antagonist against the SP-induced responses and was formed locally in the nigra after SP injection. It is proposed that SP(1-7) is an endogenous modulator of SP actions. Generation of peptide fragments, which retain receptor affinity but not efficacy, may be a general mechanism for autoregulation in peptidergic systems.

The effects of intranigral GABA and dynorphin A injections on striatal dopamine and GABA release: evidence that dopamine provides inhibitory regulation of striatal GABA neurons via D2 receptors

The effects of injections of gamma-aminobutyric acid (GABA) and dynorphin A into the substantia nigra, pars reticulata on the levels of extracellular dopamine (DA) and GABA in the ipsilateral striatum of halothane-anaesthetized rats were studied using microdialysis. The effects of intranigral injections of substance P and neurokinin A were also studied. Intranigral GABA (300 nmol) or dynorphin A (0.5 nmol) injections produced a simultaneous decrease in DA and increase in GABA levels, while intranigral substance P (0.07 nmol) or neurokinin A (0.09 nmol) injections produced an increase in DA but had no effect on GABA levels. DA agonists, apomorphine (D1/D2), SKF 38393 (D1) and pergolide (D2) were applied locally by perfusing them through the microdialysis probe, each at a concentration of 10(-5) M. All 3 agonists decreased the levels of DA in the striatum. However, while apomorphine and SKF 38393 increased, pergolide decreased the levels of GABA in the striatum. The increase in striatal GABA produced by intranigral injections of GABA (300 nmol) was reversed by local perfusion with pergolide (10(-5) M), but was not reversed by local perfusion with SKF 38393 (10(-5) M). These findings suggest that D1 and D2 receptors differentially regulate striatal GABA release, and are stimulatory and inhibitory, respectively. Furthermore, it is suggested that nigrostriatal DA functions as an inhibitory modulator of striatal GABA neurons, acting via D2 receptors.

Effects of intranigral substance P and neurokinin A on striatal dopamine release--II. Interactions with bicuculline and naloxone

The functional roles of striatonigral neurokinins were studied by analysing the effects of intranigral injections of substance P and neurokinin A on the extracellular levels of dopamine and dihydroxyphenylacetic acid in the striatum, as measured by in vivo microdialysis in rats. An opioid antagonist, naloxone, and a GABAergic antagonist, bicuculline, were tested and analysed for their ability to modify the neurokinin effects. Unilateral injections of substance P (0.07 nmol) or neurokinin A (0.09 nmol) into the substantia nigra, pars reticulata of halothane anaesthetized rats produced long-lasting increases in ipsilateral striatal dopamine and dihydroxyphenylacetic acid levels. Intranigral injections of naloxone (30 and 300 nmol) produced short-lasting decreases in striatal dopamine, concomitant with an increase in dihydroxyphenylacetic acid. Intranigral injections of 7.0 nmol bicuculline produced an increase, while 70 nmol produced a decrease in striatal dopamine, however, both doses produced an increase in dihydroxyphenylacetic acid. When co-administered intranigrally, the high dose of naloxone (300 nmol) completely blocked the dopamine stimulation of substance P (0.07 nmol), but only moderately inhibited that of neurokinin A (0.09 nmol). The high dose of bicuculline (70 nmol) completely blocked the dopamine stimulation of neurokinin A, but only moderately inhibited that of substance P. Naloxone (30 and 300 nmol) enhanced the dihydroxyphenylacetic acid response to substance P, while bicuculline (70 nmol) inhibited the dihydroxyphenylacetic acid response to neurokinin A. These findings complement and extend the findings in the preceding paper, demonstrating that intranigral substance P and neurokinin A stimulate striatal dopamine via different neuronal mechanisms. We suggest that opioid drugs have a greater influence over substance P while GABAergic drugs have a greater influence over neurokinin A.

Effects of intranigral substance P and neurokinin A on striatal dopamine release--I. Interactions with substance P antagonists

The functional role of striatonigral neurokinins were studied by analysing the effects of intranigral injections of substance P and neurokinin A on the extracellular level of dopamine and dihydroxyphenylacetic acid in the striatum, as measured by in vivo microdialysis in rats. Two substance P antagonists, substance P D-Pro2 D-Trp7,9 and substance P D-Arg1 D-Trp7,9 Leu11 were tested and analysed for their ability to block the neurokinin effects. Unilateral injections of substance P (0.00007-7.0 nmol injected in 0.2 microliter) as well as neurokinin A (0.009-9.0 nmol) into the substantia nigra, pars reticulata of halothane anaesthetized rats produced long-lasting increases in ipsilateral striatal dopamine and dihydroxyphenylacetic acid levels. The dose-response relationship for substance P on dopamine was biphasic, with maximal effects occurring after the middle dose (0.007-0.07 nmol). The dose-response relationship for neurokinin A was monophasic. Intranigral injections of substance P D-Pro2 D-Trp7,9 (0.07-0.7 nmol) or substance P D-Arg1 D-Trp7,9 Leu11 (0.07-0.7 nmol) produced a decrease in striatal dopamine, but an increase in striatal dihydroxyphenylacetic acid. At a low dose (0.07 nmol) substance P D-Pro2 D-Trp7,9 enhanced the dopamine increase produced by intranigral substance P (0.07 nmol) or neurokinin A (0.09), while at a high dose (0.7 nmol) it blocked both substance P and neurokinin A effects. Both doses of substance P D-Arg1 D-Trp7,9 Leu11 (0.07 and 0.7 nmol) blocked the substance P- but not the neurokinin A-induced increase in striatal dopamine. Immunohistochemical analysis revealed that high doses of substance P (7.0 nmol) and neurokinin A (0.9 and 9.0 nmol), as well as substance P D-Pro2 D-Trp7,9 and substance P D-Arg1 D-Trp7,9 Leu11 (0.07 and 0.7 nmol), induced a restricted loss of tyrosine hydroxylase in dendrites and cells, and neuropeptide K in terminals, at the site of injection. Further analysis shows that co-administration of substance P (0.07 nmol) or neurokinin A (0.09 nmol) did not modify the extent of the depletion of both immunoreactivities induced by substance P D-Arg1 D-Trp7,9 Leu11 (0.7 nmol). The extent of the effect produced by substance P D-Arg1 D-Trp7,9 Leu11 (0.7 nmol) was, however, smaller than the spread of intranigral injection of [125I]Bolton-Hunter-labelled substance P D-Arg1 D-Trp7,9 Leu11, and it is suggested that the "neurotoxic" effects of the substance P antagonists are not primarily involved in their abilities to inhibit striatal dopamine release and block the stimulation of dopamine after intranigral substance P and neurokinin A.(ABSTRACT TRUNCATED AT 400 WORDS)