Organization and physiology of thesubstantia nigra

Summary A comprehensive review of the literature on the anatomy, electrophysiology and pharmacology of thesubstantia nigra is presented. A diagram is developed taking into account the interneuronal interactions of neurotransmitters and receptors that control firing rates and neurotransmitter releases. The central features of the diagram are a positive dopaminergic feedforward process and a positive feedback mechanism mediated by extrasynaptic substance P diffusing from striatal terminals to dopaminergic dendrites of thezona compacta neurons. This loop can enhance the transmission of information from thestriatum through thepars reticulata output neurons. The loop is controlled at the level of zona compacta neurons by a negative feedback supported by the dendritic release of dopamine and boosted by pedunculopontine activation mediated by muscarinic receptors. The output of the loop is controlled by two negative feedforward processes, both involving GABAergic striatonigral afferents. Application of the model to pharmacological studies of diverse behaviors including seizures, turning, and conditioned behaviors reveals unforseen relationships and may offer insights into, and directions for, further analysis of the mechanisms and functions involved.     

Human isolated small intestine: motor responses of the longitudinal muscle to field stimulation and exogenous neuropeptides

Summary (1) Longitudinal muscle strips from the human small intestine (jejunum/ileum) responded to electrical field stimulation (1–50 Hz) with frequency-related primary contractions which were largely atropine- (3 M) sensitive. When the tone was raised by addition of galanin (0.3 – 1 M), prostaglandin (PG) E₂ (1–10 M) or neurokinin A (NKA, 0.1 M), a frequency-related relaxation was evident which was potentiated by atropine. All the responses to field stimulation were abolished by tetrodotoxin (1 M), thus indicating their neural origin. (2) The atropine-sensitive primary contraction to field stimulation was virtually abolished by omega conotoxin fraction GVIA (CTX, 0.1–0.3 M) while the relaxations were CTX-resistant. The field stimulation-induced relaxations, which were observed in the presence of atropine and guanethidine (3 M), were also unaffected by apamin (0.1 M). (3) NKA and substance P (SP) produced a concentration- (1 nM–1 M for both peptides) related contraction, NKA being about 53 times more potent than SP. [Pro⁹]SP sulphone and [MePhe⁷]-NKB, selective agonists of the NK-1 and NK-3 receptor, respectively, were barely effective. On the other hand, [\Ala⁸]NKA(4–10), a selective NK-2 receptor agonist, had a potent contractile activity, similar to that of NKA. (4) Galanin (1 nM–1M) produced an atropine- and tetrodotoxin-resistant concentration-related contraction of longitudinal muscle of human isolated small intestine. The response to galanin did not show any sign of fading and was particularly suitable to study the evoked relaxations. (5) Calcitonin gene-related peptide (CGRP) (10–100 nM) consistently inhibited the nerve-mediated contractions of strips from the ileum while the effect on the jejunum was less pronounced. Vasoactive intestinal polypeptide (VIP, 0.1–1 M) inhibited nerve-mediated contractions both in the ileum and the jejunum. (6) These experiments indicate that both cholinergic excitatory and non-adrenergic non-cholinergic nerves affect motility of the longitudinal muscle of the human small intestine. Furthermore, several neuropeptides produce potent motor effects, the contractile response to tachykinins being apparently mediated by activation of NK-2 receptors.     

Differential effects of phosphoramidon and captopril on NK1 receptor-mediated plasma extravasation in the rat trachea

We sought to confirm the identity of the tachykinin receptor subtype that mediates plasma extravasation in the rat trachea, and assess the respective contributions of neutral endopeptidase (NEP) and angiotensin-converting enzyme (ACE) in regulating this tachykinin-induced response. To achieve these aims, we determined the relative potencies of several natural tachykinins and receptor-selective synthetic agonists, both before and after inhibiting NEP with phosphoramidon and ACE with captopril. We also determined the effects of these peptidase inhibitors, and the NK-1 receptor antagonist L-703,606, on the plasma extravasation produced by capsaicin, which releases tachykinins endogenously from sensory nerve endings. We found that the rank order of potency for producing plasma extravasation in the rat trachea was NK-1 receptor agonist ([Sar⁹, Met(O₂)¹¹] SP)>substance P>neurokinin A> neurokinin B. The NK-2 ([Nle¹⁰]NKA (4–10)) and NK-3 ([MePhe⁷]NKB) receptor agonists were without effect. We observed no change in the relative potencies of these peptides after giving rats phosphoramidon or captopril, which suggests that the different peptide potencies are not simply the consequence of different rates of enzymatic degradation. Nevertheless, the responses to substance P and neurokinin A were clearly potentiated in rats given phosphoramidon, indicating that NEP effectively degrades tachykininsin vivo. No significant potentiation was evident for any peptide in rats given captopril. Similarly, the plasma extravasation produced by capsaicin was potentiated in rats given phosphoramidon, but not in those given captopril. Pretreating rats with L-703,606 abolished the response to capsaicin. We conclude from these observations that NK-1 receptors mediate tachykinin-induced plasma extravasation in the rat trachea, and that NEP regulates this response with little or no contribution from ACE.     

Effects of capsaicin,tachykinins, calcitonin gene-related peptide and bradykinin in the pig iris sphincter muscle

Summary Capsaicin-sensitive sensory neurons of the rabbit iris, by releasing tachykinins, exert a major role in the control of pupil motility in response to various noxious stimuli. However, the contribution of sensory innervation to the regulation of iris smooth muscle tone in other mammals species is not known. We have studied the effects produced by electrical field stimulation, capsaicin, substance P, neurokinin A, calcitonin gene-related peptide (CGRP), and bradykinin in the isolated iris sphincter muscle of the pig.Capsaicin (10 M): a) contracted the isolated sphincter muscle and; b) released immunoreactivity for substance P (SP-LI) and CGRP (CGRP-LI) from this preparation. These two effects were no longer observed at the second exposure to the drug. Electrical field stimulation (10 Hz, 60 V, 0.5 ms for 5 s) produced a biphasic contractile response. The rapid component was inhibited by atropine (1 M), while the delayed response was blocked by previous exposure to capsaicin (10 M).Substance P and neurokinin A consistently produced contraction of the pig iris sphincter muscle, substance P being more potent than neurokinin A. CGRP induced a contractile response in more than 50% of the preparations. The tachykinin antagonist [D-Argl, D-Trp^7,9, Leu¹¹-substance P (3 M) blocked: a) the effect of substance P (1 nM); b) the delayed response to electrical field stimulation and; c) reduced by more than 50% response to capsaicin. Bradykinin (10 M) failed to release either SP-LI or CGRP-LI. The contractile response evoked by bradykinin was unaffected by in vitro pretreatment with capsaicin (10 M).The existence in the pig iris of capsaicin-sensitive sensory fibres releasing neuropeptides and thus regulating sphincter muscle tone is proposed. Send offprint requests to Dr. P. Geppetti at the above address     

Effect of ω-conotoxin on cholinergic and tachykininergic excitatory neurotransmission to the circular muscle of the guinea-pig colon

The aim of this study was to compare the stimulus-response characteristics of the cholinergic and tachykininergic excitatory transmission to the circular muscle of the guinea-pig proximal colon and their susceptibility to inhibition by the N-type calcium channel blocker -conotoxin (CTX). All experiments were performed in the presence of guanethidine (3 M), indomethacin (10 M), L-nitroarginine (L-NOARG, 30 M) and apamin (0.1 M). In the presence of the tachykinin receptor antagonists, FK 888 (10 M) and GR 94800 (3 M), to block NK₁ and NK₂ receptors, respectively, electrical field stimulation (EFS) produced frequency-dependent atropine- (1 M) sensitive contractions. In the presence of atropine (1 M), EFS produced tachykininergic contractions which were abolished by the combined administration of FK 888 (10 M) and GR 94 800 (3 M). The maximal responses produced by cholinergic and tachykininergic neurotransmission ranged between 80 and 100% of the maximal contractile response to 80 mM KCI. The frequency of stimulation, pulse width and voltage required to produce 50% of the maximal cholinergic and tachykininergic contraction were not different from each other, although cholinergic transmission appeared more efficient in producing twitch contractions in response to single pulse EFS. Furthermore, cholinergic transmission was more efficient than tachykininergic transmission in producing contraction in response to short periods of EFS.CTX (0.1 M for 30 min) produced a large and comparable rightward shift of the cholinergic and tachykininergic frequency-response curve (19 and 17 fold increase in the frequency of stimulation producing 50% of the maximal response, respectively) and markedly depressed (51 and 43% inhibition, respectively) the maximal concentrations response. CTX failed to affect the contraction of the colon produced by submaximally effective concentrations of the muscarinic receptor agonist, methacholine (0.1–0.3 M) and those produced by the tachykinin NK₁ and NK₂ receptor selective agonists [Sar⁹] substance P sulfone and [\Ala⁸] neurokinin A (4–10) (1–3 nM).The present findings demonstrate that the cholinergic and tachykininergic components of the excitatory transmission to the circular muscle of the guinea-pig colon are activated at comparable intensities of nerve stimulation and are both inhibited, in a qualitatively and quantitatively comparable manner, by CTX at the prejunctional level. These findings are consistent with the idea that acetylcholine and tachykinins are co-released from the same population of enteric motoneurones which innervate the circular muscle of the colon. Correspondence to: C. A. Maggi at the above address     

The inhibitory modulation of guinea-pig intestinal peristalsis caused by capsaicin involves calcitonin gene-related peptide and nitric oxide

The effect of capsaicin-induced stimulation of afferent neurons on peristalsis and the possible neural mediators involved in this action were examined in the guinea-pig isolated ileum. The intraluminal pressure threshold for eliciting peristaltic waves was used to quantify facilitation (decrease in threshold) or inhibition (increase in threshold) of peristalsis. Capsaicin (0.1–1 M) caused an initial short-lasting stimulation of peristalsis followed by a prolonged inhibition of peristaltic activity. Capsaicin (1 M) was ineffective when the gut segments had been pretreated with 3.3 M capsaicin, which is indicative of an afferent neuron-dependent action of the drug. In contrast, the abolition of peristalsis caused by a high concentration of capsaicin (33 M) was fully reversible on removal and reproducible on readministration of capsaicin, a feature characteristic of a nonspecific depression of smooth muscle excitability. Baseline peristalsis and the excitatory/inhibitory effect of capsaicin (1 M) on peristalsis remained unaltered by a combination of the tachykinin NK₁ receptor antagonist ( + )-(2S, 3S)-3-(2-methoxybenzylamino)-2-phenyl piperidine (CP-99,994; 0.3 M) and the tachykinin NK₂ receptor antagonist L(-)-N-methyl-N[4-acetylamino-4-phenyl-piperidino-2-(3,4-dichlorophenyl)butyl]-benzamide (SR-48,968; 0.1 M). Further experiments, performed in the presence of a low concentration of atropine (10 nM) showed that the catcitonin gene-related peptide (CGRP) antagonist human -catcitonin gene-related peptide (8–37) [hCGRP (8–37); 10 M] attenuated the delayed inhibitory effect of capsaicin on peristalsis, but did not influence baseline peristaltic activity and the capsaicin-induced facilitation of peristalsis. Blockade of nitric oxide (NO) synthesis by N ^G-nitro-l-arginine methylester (l-NAME, 300 M) facilitated baseline peristaltic activity and reduced the delayed inhibition of peristalsis caused by capsaicin (1 M) without affecting the initial peristalsis-stimulating action of capsaicin. The effects of l-NAME were prevented by l-arginine (1 mM). The data of the current study indicate that capsaicin-sensitive afferent neurons do not participate in the neural pathways subserving peristalsis in the guinea-pig small intestine, but modulate peristaltic activity upon stimulation with capsaicin. The initial stimulant action of capsaicin on peristalsis is independent of tachykinins acting via NK₁ or NK₂ receptors, while the delayed capsaicin-induced depression of peristalsis involves CGRP and NO.     

The effects of capsaicin upon electrogenic ion transport in rat descending colon

Summary Preparations of rat descending colon mucosa have been used to record changes in short circuit current (SCC) under voltage clamp conditions. When added to the basolateral compartment capsaicin (8-methyl-N-vanillyl-6-nonenamide, 0.1-3 M) caused an initial transient increase in SCC, followed by a more prolonged reduction in SCC, that lasted for 20–30 min.Repeated applications of 3 M capsaicin caused desensitisation of the initial secretory response. The antisecretory effects (i. e. reduction in SCC from the original baseline) remained, although they were significantly reduced. In some preparations described as non-responders, 3 M capsaicin did not elicit a secretory response. No desensitisation of the remaining antisecretory responses was observed in these tissues; in fact these reductions in SCC were consistently larger than those from tissues which responded with a secretory response.Tetrodotoxin (100 nM), hexamethonium (10 M), and yohimbine (50 M) had no significant effect upon either secretory or antisecretory responses. Ruthenium red (10 M) abolished the secretory response to 3 M capsaicin, but had no effect upon the antisecretory responses. Pretreatment of the tissues with 1 M substance P(SP) resulted in significant desensitisation to the peptide and abolished the secretory response to 3 M capsaicin. The antisecretory responses remained, and were significantly larger compared with responses from control tissues. Send offprint requests to H. M. Cox at present address     

Polymorphic variation as a driver of differential neuropeptide gene expression

The regulation of neuropeptide gene expression and their receptors in a tissue specific and stimulus inducible manner will determine in part behaviour and physiology. This can be a dynamic process resulting from short term changes in response to the environment or long term modulation imposed by epigenetically determined mechanisms established during life experiences. The latter underpins what is termed ‘nature and nurture, or ‘gene × environment interactions’. Dynamic gene expression of neuropeptides or their receptors is a key component of signalling in the CNS and their inappropriate regulation is therefore a predicted target underpinning psychiatric disorders and neuropathological processes. Finding the regulatory domains within our genome which have the potential to direct gene expression is a difficult challenge as 98% of our genome is non-coding and, with the exception of proximal promoter regions, such elements can be quite distant from the gene that they regulate. This review will deal with how we can find such domains by addressing both the most conserved non-exonic regions in the genome using comparative genomics and the most recent or constantly evolving DNA such as repetitive DNA or retrotransposons. We shall also explore how polymorphic changes in such domains can be associated with CNS disorders by altering the appropriate gene expression patterns which maintain normal physiology.     

Central Effects of Neuropeptide K on Water and Food Intake in the Rat

The present study investigated the effect on water and food intake in the rat of the intracerebroventricular (ICV) injection of neuropeptide K (NPK), the N-terminally extended form of neurokinin A. NPK inhibited water deprivation-induced water intake even at 31.2 ng/rat. At higher doses, it inhibited also water intake induced by ICV angiotensin II or by subcutaneous hypertonic NaCl, and food-associated drinking, the threshold dose being 125 ng/rat. In response to 125 ng/rat, food intake following 16 h food deprivation was not reduced. NPK inhibited food intake only at 500 ng/rat, a dose that evoked excessive grooming in treated animals. Thus NPK is a potent inhibitor of water deprivation-induced drinking and at higher doses it exerts a general antidipsogenic effect towards several dipsogenic determinants, without affecting food intake. On the other hand, it inhibits food intake only at high doses, 500 ng/rat or more, but this inhibition might be just related to the intense grooming evoked. The effects of NPK on ingestive behavior are markedly different from those of neurokinin A, which selectively inhibits osmotic drinking and food-associated drinking. These differences suggest that NPK itself may exert its effects on the central nervous system, not necessarily through the conversion to neurokinin A.     

Mast cells in citric acid-induced cough of guinea pigs

It was demonstrated previously that mast cells play an important role in citric acid (CA)-induced airway constriction. To investigate the role of mast cells in CA-induced cough, three experiments were carried out in this study. In the first experiment, 59 guinea pigs were employed and we used compound 48/80 to deplete mast cells, cromolyn sodium to stabilize mast cells, MK-886 to inhibit leukotriene synthesis, pyrilamine to antagonize histamine H(1) receptor, methysergide to antagonize serotonin receptor, and indomethacin to inhibit cyclooxygenase. In the second experiment, 56 compound 48/80-pretreated animals were divided into two parts; the first one was used to test the role of exogenous leukotriene (LT) C(4), while the second one to test the role of exogenous histamine in CA-induced cough. Each animal with one of the above pretreatments was exposed sequentially to saline (baseline) and CA (0.6 M) aerosol, each for 3 min. Then, cough was recorded for 12 min using a barometric body plethysmograph. In the third experiment, the activation of mast cells upon CA inhalation was investigated by determining arterial plasma histamine concentration in 17 animals. Exposure to CA induced a marked increase in cough number. Compound 48/80, cromolyn sodium, MK-886 and pyrilamine, but not indomethacin or methysergide, significantly attenuated CA-induced cough. Injection of LTC(4) or histamine caused a significant increase in CA-induced cough in compound 48/80-pretreated animals. In addition, CA inhalation caused significant increase in plasma histamine concentration, which was blocked by compound 48/80 pretreatment. These results suggest that mast cells play an important role in CA aerosol inhalation-induced cough via perhaps mediators LTs and histamine.