Genetic knockout and pharmacological blockade studies of the 5-HT7 receptor suggest therapeutic potential in depression

The affinity of several antidepressant and antipsychotic drugs for the 5-HT7 receptor and its CNS distribution suggest potential in the treatment of psychiatric diseases. However, there is little direct evidence of receptor function in vivo to support this. We therefore evaluated 5-HT7 receptors as a potential drug target by generating and assessing a 5-HT7 receptor knockout mouse. No difference in assays sensitive to potential psychotic or anxiety states was observed between the 5-HT7 receptor knockout mice and wild type controls. However, in the Porsolt swim test, 5-HT7 receptor knockout mice showed a significant decrease in immobility compared to controls, a phenotype similar to antidepressant treated mice. Intriguingly, treatment of wild types with SB-258719, a selective 5-HT7 receptor antagonist, did not produce a significant decrease in immobility unless animals were tested in the dark (or active) cycle, rather than the light, adding to the body of evidence suggesting a circadian influence on receptor function. Extracellular recordings from hypothalamic slices showed that circadian rhythm phase shifts to 8-OH-DPAT are attenuated in the 5-HT7 receptor KO mice also indicating a role for the receptor in the regulation of circadian rhythms. These pharmacological and genetic knockout studies provide the first direct evidence that 5-HT7 receptor antagonists should be investigated for efficacy in the treatment of depression.

A role for 5-ht6 receptors in retention of spatial learning in the Morris water maze

This study investigates the effect of intracerebroventricular administration of a 5-ht6 antisense oligonucleotide (AO) complementary to bases 1-18 of the rat 5-ht6 cDNA initiation sequence (Mol. Pharmacol. 43 (1993) 320) (1.5 microg twice daily for six days) and i.p. injection of a selective 5-ht6 receptor antagonist Ro 04-6790 (10 or 30 mg/kg once daily for three days) on acquisition and retention in the Morris water maze. Neither the 5-ht6 AO (which reduced cortical [3H]-LSD binding sites by 10-16%) nor Ro 04-6790 affected acquisition, but both enhanced retention of the learned platform position such that rats spent significantly longer searching the trained platform position than any other area during the probe tests. Furthermore, neither AO nor Ro 04-6790 had any effect on the time taken to reach a raised visible platform, indicating that visual acuity was unimpaired. In addition, AO reduced both food consumption and body weight and the later effect was also seen following Ro 04-6790, suggesting a role for the 5-ht6 receptor in the regulation of feeding. Hence, while the underlying mechanism remains unclear, enhanced retention of spatial learning following both AO and 5-ht6 antagonist administration strongly indicate a role for this receptor in memory processes.

Immunohistochemical localisation of the 5-HT2C receptor protein in the rat CNS

5-HT2C receptor mRNA has a widespread distribution in the human and rat CNS but the absence of a specific high affinity ligand has made autoradiographic localisation of the receptor difficult. By raising polyclonal antibodies against the rat 5-HT2C receptor protein this study reports the immunohistochemical distribution of this receptor in the rat CNS. A sephadex purified 5-HT2C antiserum visualised a single immunopositive band (54 kDa) in Western blots of membranes prepared from several rat brain regions and caused intense membrane immunofluorescence in HEK 293 cells transfected with h5-HT2C cDNA, which were both attenuated by incubation with the antigenic peptide sequence (200-300 microM). 5-HT2C-like immunoreactivity was located on neurones throughout the CNS. The most abundant 5-HT2C-like immunoreactive cell bodies were in the anterior olfactory nucleus, medial and intercalated amygdaloid nuclei, hippocampus layers CA1 to CA3, laterodorsal and lateral geniculate thalamic nuclei, caudate-putamen and several areas of the cortex (including piriform and frontal), consistent with this receptor being located postsynaptic to serotonergic neurones. Immunopositive neurones were also found in the dorsal raphé, suggesting that 5-HT2C receptors may be on some serotonergic neurones. The overall distribution of 5-HT2C-like immunoreactivity complements previous findings with conventional radioligands and agrees well with reported levels of 5-HT2C receptor mRNA.

Pindolol-insensitive [3H]-5-hydroxytryptamine binding in the rat hypothalamus; identity with 5-hydroxytryptamine7 receptors

Pindolol-insensitive [3H]-5-hydroxytryptamine ([3H]-5-HT) binding to rat hypothalamic membranes was pharmacologically and functionally characterized to resolve whether this procedure selectively labels 5-HT7 receptors. Consistent with a previous report, 3 microM and not 100 nM pindolol was required to occupy fully 5-HT1A and 5-HT1B receptors. Remaining [3H]-5-HT binding was saturable (KD, 1.59+/-0.21 nM; Bmax, 53.8+/-3.1 fmol x mg protein(-1)). Displacement of [3H]-5-HT with metergoline and 5-CT revealed shallow Hill slopes (<0.5) but seven other compounds had slopes >0.8 and pKi values and the rank order of affinity were significantly correlated (r = 0.81 and 0.93, respectively) with published [3H]-5-HT binding to rat recombinant 5-HT7 receptors. In the presence of pindolol, 5-HT-enhanced accumulation of [32P]-cyclic AMP was unaffected by the 5-HT4 antagonist RS39604 (0.1 microM) or the 5-ht6 antagonist Ro 04-6790 (1 microM) but significantly attenuated by mesulergine (250 nM), ritanserin (450 nM) or methiothepin (200 nM) which have high affinity for the 5-HT7 receptor. Intracerebroventricular pretreatment with the serotonergic neurotoxin 5,7-dihydroxytryptamine, 5,7-DHT, elevated the [3H]-5-HT Bmax 2 fold, indicating that the hypothalamic 5-HT7 receptor is post-synaptic to 5-HT nerve terminals and regulated by synaptic 5-HT levels. These results suggest that, in the presence of 3 microM pindolol, [3H]-5-HT selectively labels hypothalamic binding sites consistent with functional 5-HT7 receptors.

Modification of 5-HT2 receptor mediated behaviour in the rat by oleamide and the role of cannabinoid receptors

Oleamide (cis-9,10-octadecenoamide) is an endogenous brain lipid which has been suggested to induce sleep in experimental animals. The mechanism of action is unclear but shares many of the characteristics of endogenous cannabinoids such as anandamide and has been shown to enhance in vitro responses to 5-HT and GABA. In the present study we investigated the effects of oleamide on two motor behaviours, back muscle contractions (BMC) and wet-dog shakes (WDS) induced in rats by treatment with the 5-HT2 receptor agonist DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride). We then examined the potential involvement of CB1 cannabinoid receptors in the responses to oleamide and the mechanism of interaction between CB1 and 5-HT2 receptors. Oleamide and the cannabinoid receptor agonist HU210 (6aR)-trans-3-(1,1-dimethylheptyl)6a,7,10,10a-tetrahydro-1-h ydroxy-6,6-dimethyl-6H-dibenzo[b,d]pyran-9-methanol) produced a hypolocomotion which was prevented by the CB1 antagonist SR141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-me thyl-1H-pyrazole-3-carboxamide hydrochloride). Despite having no effect alone, oleamide and HU210 potentiated BMC induced by treatment with DOI. SR141716A alone did not affect the response to DOI but it blocked the potentiations caused by oleamide or HU210. WDS were unaffected by oleamide and slightly reduced by HU210. In vitro, oleamide and HU210 enhanced the high affinity binding of 5-HT to 5-HT2 receptors on rat cerebral cortex membranes labelled with 3H-ketanserin. Neither agent, however, altered 5-HT-stimulated phosphoinositide hydrolysis in rat cerebral cortex slices. Oleamide occupied CB1 cannabinoid receptors on rat brain membranes labelled with 3H-CP55940 with an IC50 of 10 microM. The data presented are consistent with oleamide acting via a cannabinoid recognition site to enhance 5-HT2 receptor function in vivo. The mechanism of the modulation is still unclear but it does not appear to involve a potentiation of 5-HT2 receptor-stimulated phosphoinositide hydrolysis.

Antisense oligonucleotide-induced reduction in 5-hydroxytryptamine7 receptors in the rat hypothalamus without alteration in exploratory behaviour or neuroendocrine function

The effect of a 5-hydroxytryptamine7 (5-HT7) receptor-directed antisense oligonucleotide on rat behaviour and neuroendocrine function was investigated. Six days of intracerebroventricular 5-HT7 antisense oligonucleotide treatment significantly reduced [3H]5-HT binding to hypothalamic 5-HT7 receptors, whereas cortical 5-HT2C density remained unchanged. In rats on a food-restricted diet, both antisense and mismatch oligonucleotides reduced food intake and body weight compared with that in vehicle-treated controls by day 4 of administration. 5-HT7 antisense oligonucleotide administration did not affect exploratory or locomotor activity in photocell activity monitors on day 4 or elevated plus-maze behaviour on day 6 of intracerebroventricular treatment. 5-HT7 antisense oligonucleotide did not affect plasma corticosterone or prolactin levels or 5-HT turnover in either 5-HT cell body or terminal areas. These data demonstrate that intracerebroventricular 5-HT7 antisense oligonucleotide administration selectively reduced rat hypothalamic 5-HT7 receptor density without affecting any of the biochemical or behavioural measures. The results suggest that this antisense protocol could be a valuable tool to investigate central 5-HT7 receptor functions, and that this receptor is not critical for the control of neuroendocrine function or food intake.

Effect of chronic m-CPP on locomotion, hypophagia, plasma corticosterone and 5-HT2C receptor levels in the rat

1. The present study examined 5-HT2C receptor agonist-induced behavioural tolerance and 5-HT2C receptor down-regulation in adult rat brain. The effect of chronic subcutaneous infusion of the 5-HT2C receptor agonist, m-chlorophenylpiperazine (m-CPP, 10 mg kg(-1), day(-1)), for 14 days was examined on daily food intake, the ability of acute m-CPP (2.5 mg kg(-1), i.p.) to induce hypolocomotion in a novel arena and elevate plasma corticosterone levels and on ex vivo cortical [3H]-mesulergine binding and hippocampal 5-HT2C receptor protein levels. 2. Before chronic infusion, m-CPP (2.5 mg kg(-1), i.p.) attenuated the number of turns and rears made in a novel open field arena. In contrast, while m-CPP still elicited this hypolocomotion following 14 days, saline infusion, no such hypolocomotion occurred in rats given chronic m-CPP (10 mg kg(-1) day(-1)), indicating that almost complete tachyphylaxis of this behaviour occurred with chronic 5-HT2C receptor agonist injection. 3. During chronic infusion of m-CPP, rats consumed less food per day than saline-treated controls. Acute challenge with m-CPP following two weeks, treatment still attenuated food intake over the next four hours (by 43% and 30%, respectively from that on the previous day) in saline and m-CPP infusion groups, showing that only partial tolerance to 5-HT2C receptor agonist-induced hypophagia occurred. 4. In naive home cage rats, plasma corticosterone was elevated in a dose-dependent manner 35 min after m-CPP injection (0.5, 1 and 3 mg kg(-1), i.p.) but levels were comparable to control values 16 h after m-CPP (2, 5 and 10 mg kg(-1), i.p.). Sixteen hours after a single m-CPP injection (2.5 mg kg(-1), i.p.), plasma corticosterone levels were comparable in a group of rats which had received 14 days infusion of m-CPP or saline. However, following a similar acute m-CPP injection (2.5 mg kg(-1), i.p., - 16 h) in rats previously infused for 14 days with m-CPP, plasma corticosterone levels were lower than those in a separate group which received no chronic infusions (but only acute m-CPP injection), even though the plasma m-CPP levels were comparable in both groups. The data are consistent with the proposal that chronic m-CPP induced some down-regulation of hypothalamic 5-HT2C receptors which contribute, in a tonic manner, to plasma corticosterone secretion under the conditions investigated. 5. Chronic m-CPP infusion reduced the amount of [3H]-mesulergine binding (by 27%, without altering the KD) in membranes prepared from parietal/occipital/temporal cortex (under conditions to exclude binding to 5-HT2A receptors) and 5-HT2C receptor protein-like immunoreactive levels measured by radioimmunoassay in the hippocampus by 38%, confirming that 5-HT2C receptor down-regulation had occurred. 6. Even after 14 days m-CPP infusion only partial behavioural tolerance and 5-HT2C receptor down-regulation were observed, which may vary in different brain regions of the rat. Thus the hypophagia produced by m-CPP may involve activation of 5-HT2C receptors in the hypothalamus, where there is a greater receptor reserve or which are more resistant to agonist-induced down-regulation than 5-HT2C receptors in limbic areas (striatum and nucleus accumbens) mediating m-CPP-induced hypolocomotion.

Distribution of the 5-hydroxytryptamine2C receptor protein in adult rat brain and spinal cord determined using a receptor-directed antibody: effect of 5,7-dihydroxytryptamine

A synthetic peptide, corresponding to the N-terminal decapeptide (+Y11C12) of the rat 5-hydroxytryptamine2C (5-HT2C) receptor protein was used to produce a sheep polyclonal antiserum. Western blot analysis showed that the resultant antibody G241 recognised two membrane proteins, one (55 kDa) approximating the molecular mass of the 5-HT2C receptor (52 kDa) and a second (63 kDa), which may be a glycosylated form of the receptor protein. HEK 293 cells transfected with human 5-HT2C cDNA displayed intense cell surface immunoreactivity with the 5-HT2C antiserum, which was completely prevented by incubating the antibody with the synthetic 5-HT2C peptide (10 microM), whilst neither non-immune serum nor untransfected cells displayed any immunoreactivity. A radioimmunoassay was developed to quantify the regional distribution of 5-HT2C-like immunoreactivity (LI) in the adult rat brain. The choroid plexus contained five-fold higher levels of 5-HT2C-LI than any brain region but high levels were found in the frontal cortex, septum, hypothalamus, and striatum, intermediate levels in the thalamus and midbrain, and lower levels in brainstem, cerebellum, and spinal cord. In rat cortical membranes, the B(max) value from [3H]-mesulergine binding was ten-fold lower than 5-HT2C-LI levels determined by radioimmunoassay, which may reflect measurement of internalised receptor protein by radioimmunoassay which is not detected with conventional 5-HT2C ligands. Ten days after depletion of 5-HT with the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), there was a significant increase in 5-HT2C-LI in the choroid plexus and the ventral cervical spinal cord, suggesting that receptors therein are located post-synaptic to destroyed serotonergic nerve terminals. In contrast, the significant reduction in 5-HT2C-LI observed in the midbrain, brainstem, and dorsal thoracic spinal cord following 5,7-DHT implies that 5-HT2C receptors may be located on 5-HT nerve terminals in these regions.

Effect of neuropeptides on cognitive function

Recent evidence indicates that, in addition to the involvement of cholinergic and other neurotransmitter systems, various neuropeptides that occur in cortical and subcortical brain regions have a role in cognitive behavior. This evidence results largely from behavioral studies in rodents and other animals, following peptide administration and only in a very few cases from similar studies in human subjects. Several neuropeptides studied appear to enhance or produce changes conducive to improvement in cognitive performance and these include vasopressin, corticotrophin-releasing hormone (CRH), somatostatin, substance P, neuropeptide Y, and thyrotrophin-releasing hormone (TRH), while one peptide, galanin, has been reported to inhibit cognitive processes. Of those neuropeptides that improve performance, only TRH has been shown recently to attenuate the memory impairment of human subjects and Alzheimer patients treated with an anticholinergic drug, and this review describes a series of complimentary studies in adult and aged rodents that contribute to our understanding of the possible mechanisms involved in the role of TRH in cognition.

Activation of 5-HT2B receptors in the medial amygdala causes anxiolysis in the social interaction test in the rat

In a recent study, we reported the presence of neurones expressing 5-HT2B receptor protein in the medial amygdaloid nucleus of the adult rat brain. In the present study, bilateral micro-injection of the 5-HT2B receptor agonist 1-[5-(2-thienylmethoxy)-1H-3-indolyl]propan-2-amine hydrochloride (BW 723C86, 0.09 and 0.93 nmol, 5 min pretest) into the medial amygdaloid nuclei increased the total interaction time of a pair of male rats in the social interaction test, to a comparable extent to chlordiazepoxide (5 mg/kg p.o., 1 hr pretest) without altering locomotor activity; indicative of anxiolytic activity. The increase in social interaction was prevented by pretreatment with the 5-HT2C/2B receptor antagonist N-(1-methyl-5-indoyl)-N'-(3-pyridyl) urea hydrochloride (SB 200646A, at 2 but not 1 mg/kg p.o., 1 hr pretest), which did not alter behaviour when given alone. Intra-amygdala BW 723C86 (0.09, 0.31 and 0.93 nmol, 5 min pretest) did not significantly alter the number of punished responses made when the same rats were examined seven days later in a Vogel punished drinking test, although chlordiazepoxide (5 mg/kg p.o., 1 hr pretest) produced the expected anxiolytic profile. The results are consistent with the proposal that activation of 5-HT2B receptors in the medial amygdala induces anxiolysis in the social interaction model but has little effect on behaviour in a punished conflict model of anxiety. These data suggest that serotonergic neurotransmission in this nucleus may selectively affect specific kinds of anxiety generated by different animal models.

Evidence for expression of the 5-hydroxytryptamine-2B receptor protein in the rat central nervous system

The 5-hydroxytryptamine-2B receptor is the most recent addition to the 5-hydroxytryptamine-2 family of G-protein-coupled receptors. In the rat stomach fundus, 5-hydroxytryptamine-2B receptor activation causes contraction; however, its distribution and function in the rat CNS are unclear. By performing immunohistochemistry with an antiserum raised against the N-terminus of the 5-hydroxytryptamine-2B receptor protein, this study identifies receptor expression in longitudinal and circular smooth muscle in the rat stomach fundus and in neurons in discrete nuclei in the cerebellum, lateral septum, dorsal hypothalamus and medial amygdala. The potential function of this receptor in the CNS is discussed.

Increased 5-HT2C receptor responsiveness occurs on rearing rats in social isolation

To investigate whether isolation rearing alters 5-hydroxytryptamine2C (5-HT2C) receptors, the effect of the serotonin agonist m-chlorophenylpiperazine (mCPP) was examined on elevated plus-maze behaviour, plasma corticosterone and brain 5-HT2C receptor protein levels in rats. There was no distinction between behaviour or corticosterone levels in drug-free isolates or socially housed rats exposed to the elevated plus-maze. The anxiogenic response to mCPP (decrease in open arm entry and time and an increase in stretch attend postures) on the elevated plus-maze was greater in isolation than in socially reared controls without any concomitant difference in the hypolocomotor effect of mCPP in the two groups. mCPP produced a greater elevation in plasma corticosterone in isolates than in group-housed controls. Hippocampal 5-HT2C receptor protein-like immunoreactive levels were significantly lower following mCPP than saline only in rats reared in isolation. These results indicate that increased 5-HT2C receptor responsiveness accompanies isolation-rearing and may contribute to the enhanced response to stress and the increased neophobia seen in this animal model of trait anxiety/depression. In isolation reared rats, rapid down-regulation of supersensitive 5-HT2C receptors may occur in the hippocampus following 5-HT agonist challenge.

Modulation of calcitonin gene-related peptide release from dorsal thoraco-lumbar spinal cord slices by 5-HT3 receptors

The release of endogenous calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) from slices of dorsal and ventral thoraco-lumbar spinal cord was examined using a fixed volume incubation technique and radioimmunoassay. Incubation with potassium (25-100 mM) produced a dose-related increase in basal CGRP-LI in the supernatant of dorsal slices which was calcium dependent and release was also evoked by incubation with capsaicin (10 microM). Pre-incubation with the 5-HT3 agonist 2-methyl-5-hydroxytryptamine (10(-5) M) attenuated potassium-induced release from dorsal spinal cord slices, which was prevented by additional pre-incubation with the 5-HT3 antagonist ondansetron (10(-8) M). In contrast, the low level of CGRP-LI released from ventral spinal cord slices was not significantly enhanced by incubation with either potassium or capsaicin.

Behavioural effects of scopolamine and the TRH analogue RX77368 on radial arm maze performance in the rat

Effects of repeated intracerebroventricular administration of the thyrotrophin-releasing hormone (TRH) analogue, RX77368 (3,3'-dimethyl-TRH, 2 μg, once daily), on a scopolamine-induced performance deficit in an eight-arm radial maze were evaluated in adult rats. Scopolamine (0.3 mg/kg i.p.-30 min) pre-treatment produced a significant deficit in the number of unrepeated arm entries and total arm entries and increased the percentage of incorrect arm entries and the total time on the maze, compared with saline-treated controls. Prior treatment with RX77368 (40 min before maze testing) produced a partial but significant attenuation of the scopolamine-induced performance deficit on the maze during the first five trials but RX77368 also enhanced maze performance during the same period when given alone. These results suggest that the observed scopolamine-induced performance deficit on the radial arm maze partly results from a reduction in locomotion and maze exploration rather than solely impairment of memory, and that RX77368 treatment may improve radial maze performance by increasing arousal and exploratory behaviour in rats rather than directly enhancing cognition.

Galanin fails to alter both acquisition of a two trial per day water maze task and neurochemical markers of cholinergic or serotonergic neurones in adult rats

The co-existence of galanin with acetylcholine in ventral forebrain neurones combined with evidence that galanin attenuates cholinergic function and is present in senile plaques in Alzheimer's disease all implicate this neuropeptide in the regulation of cognition. This study simultaneously examines the effect of galanin on acquisition in a Morris water maze and post-training markers of cholinergic and serotonergic forebrain neurones thought to be involved in cognition. Synthetic porcine galanin (10(-9) to 10(-6) M) produced dose-related inhibition of atropine sensitive indirectly-evoked contractions of an isolated guinea-pig ileum which was unaffected by naloxone (10(-7) M). This confirmed the bioactivity of synthetic galanin, which reduces acetylcholine, but not opiate, release from the ileal myenteric plexus. Galanin pretreatment (1 or 10 micrograms i.c.v., -15 min) failed to alter acquisition of a Morris water maze task (2 trials per day) in Hooded Lister rats. Following behavioural acquisition, five days of galanin administration did not alter choline acetyltransferase activity, thyrotrophin-releasing hormone-like immunoreactivity or 5-hydroxytryptamine levels or turnover in the frontal cortex, hippocampus or septum, although dopamine levels were significantly elevated in the frontal cortex. These findings suggest that galanin does not affect acquisition in a simple visual-spatial task which taxes reference more than working memory and questions the assumption that a cholinergic mechanism is the major contributor to previously reported cognitive effects of galanin.

The serotoninergic bulbospinal system and brainstem-spinal cord content of serotonin-, TRH-, and substance P-like immunoreactivity in the aged rat with special reference to the spinal cord motor nucleus

The 5-hydroxytryptamine (5HT) containing bulbospinal pathway was studied with immunohistochemical (IF) and chemical techniques in 2-3 and 30 months old male Sprague-Dawley rats. The coexisting neuropeptides substance P (SP), thyrotropin-releasing hormone (TRH) and galanin were also analysed. Furthermore, the expression of mRNA encoding aromatic L-amino acid decarboxylase (AADC), prepro-TRH, and preprotachykinin (prepro-SP) was analysed with in situ hybridization (ISH) in the midline raphé nuclei inthe lower brainstem. The results showed a decreased number of axonal 5HT fibers with a normal morphology in the ventral horn of the aged rat lumbosacral spinal cord, and several 5HT immunoreactive (IR) fibers with an aberrant morphology, suggestive of axonal degeneration, were intermingled. This was evident in both the dorsal and ventral horn of the spinal cord. The 5HT-IR fibers with an aberrant morphology usually also contained TRH-and/or SP- and/or galanin-like immunoreactivity (LI) in the ventral horn. These signs of degeneration were clearly less evident in the thoracic and cervical spinal cord segments. Moreover, these changes varied between aged litter-mates. This was in agreement with behavioural signs of motor disturbances, present in about 40% of the aged rats and which in all cases were confined to the hindlimbs. Chemical analyses disclosed significantly lower levels of TRH-LI and, in particular, SP-LI in both the ventral and dorsal quandrants of the spinal cord in the aged rat compared to young adults. The differences were largest in the lumbar regions of the spinal cord. Corresponding analysis of 5HT and 5-hydroxyindoleacetic acid (5HIAA) in the same tissue specimens revealed largely unaltered levels of 5HT and a slight increase in 5HIAA, indicating the possibility of an increased 5HT turnover in the aged rat spinal cord. Neurons in nucleus raphé obscurus and nucleus raphé pallidus were immunoreactive to 5HT, and after pretreatment with colchicine to TRH-, SP-, and galanin-LI as well. There was no obvious difference in number of labeled cells, or labeling intensity, between colchicine-treated young adult and aged rats, although, in the corresponding region of medulla oblongata, chemical analysis disclosed significantly lower levels of 5HT, TRH, and, in particular, SP in untreated aged rats. In contrast, in situ hybridization analysis revealed increased mRNA levels encoding prepro-TRH and prepro-SP in old rats, while mRNA content encoding AADC mRNA was similar in young adult and aged rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of calcitonin gene-related peptide in rat and rabbit spinal cords: effect of intrathecal 5,7-dihydroxytryptamine

Calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) was measured in selected regions of the cervical, thoracic, and lumbar spinal cord of untreated rabbits and, following intrathecal injection of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), in the thoracolumbar cord in rats using a sheep antiserum raised against tyrosine0 calcitonin gene-related peptide28-37. In the cervical, thoracic, and lumbar segments of the rabbit spinal cord, CGRP-LI levels were 15-50-fold higher in the dorsal than in the ventral grey region in the same segment. The only segmental variation in CGRP-LI levels was in the dorsal white region, where levels in the thoracic cord were lower than those in cervical or lumbar segments. Within individual spinal segments, the pattern of distribution of CGRP-LI in the rabbit spinal cord was analogous to that in other species previously examined, including rat, human, and cat spinal cord. Intrathecal injection of 5,7-DHT, which caused 85-91% depletion of 5-hydroxytryptamine and 5-hydroxyindoleacetic acid from the thoracolumbar ventral spinal cord, did not affect choline acetyltransferase activity, which is colocalized with CGRP in motoneurones in this spinal cord region. In contrast, intrathecal 5,7-DHT produced a threefold increase in CGRP-LI in the ventral thoracolumbar cord, suggesting that spinal motoneurones selectively increase production of CGRP 10 days after neurotoxin-induced denervation of bulbospinal raphe neuronal input.

Thyrotropin-releasing hormone (TRH)-like immunoreactivity in the grey monkey (Macaca fascicularis) spinal cord and medulla oblongata with special emphasis on the bulbospinal tract

The distribution of thyrotropin-releasing hormone (TRH)-like immunoreactivity (LI) has been studied in the grey monkey (Macaca fascicularis) spinal cord and medulla oblongata by the use of indirect immunofluorescence and the peroxidase-antiperoxidase (PAP) technique. Furthermore, double-labeling experiments were performed in order to study colocalization of 5-hydroxytryptamine (5-HT)- and substance P-LI. A dense innervation of TRH-immunoreactive (IR) varicose fibers was found in the ventral horn motor nuclei, in the region surrounding the central canal, in the intermediolateral cell column, and in the dorsal horn laminae II and III. In addition, cell bodies harboring TRH-LI were found in the dorsal horn laminae II-IV. In the ventral horn, many of the large cell bodies and their proximal dendrites were totally encapsulated by TRH-IR fibers. From double-labeled sections a high degree of coexistence could be established between TRH-/5-HT-LI, TRH-/substance P-LI, and 5-HT-/substance P-LI in fibers in the motor nuclei; as a consequence, a large proportion of these fibers should harbor TRH-/5-HT-/substance P-LI. A coexistence between TRH-/5-HT-LI could also be demonstrated in the intermediolateral cell column. However, no unequivocal coexistence could be found between TRH-/substance P-LI and 5-HT-/substance P-LI in this region. In the dorsal horn, no clear coexistence could be encountered for any of the above indicated combinations. Electron microscopic analysis of material from the lumbar lateral motor nucleus demonstrated TRH-IR terminals making synapses with large cell bodies and dendrites. In addition, contacts lacking synaptic specializations could also be verified. In the medulla oblongata, with the use of the PAP technique, a large number of cell bodies containing TRH-LI were encountered in the midline raphe nuclei and in nucleus reticularis lateralis. A similar distribution pattern could be found for 5-HT-LI, but no cell bodies containing substance P-LI could be seen in these regions. Chemical analysis of specimens from cervical, thoracic, and lumbar spinal cord revealed higher concentrations of TRH- and 5-HT-LI in the ventral quadrants, whereas substance P-LI dominated in the dorsal quadrants. Thus, the concentrations of TRH-, 5-HT-, and substance P-LI was in accordance with the observed regional variation in density of IR-fibers and varicosities found in the spinal cord. We have shown that TRH-LI has a distribution in the monkey spinal cord and medulla oblongata similar to that previously demonstrated in other species.(ABSTRACT TRUNCATED AT 400 WORDS)

Ventral horn neuropeptides modulate the release of noradrenaline from tissue slices of rat brainstem and ventral thoracic spinal cord

The release of endogenous noradrenaline (NA) from slices of adult rat brainstem and ventral thoracic spinal cord was investigated using a fixed-volume incubation technique and HPLC with electrochemical detection. Incubation with potassium (15-50 mM) produced a dose-related increase in basal NA release that was calcium dependent. The potassium-evoked release of NA from spinal cord or brainstem slices was potentiated according to dose by preincubation with either (a) the selective alpha 2-adrenoceptor antagonist idazoxan (10(-6)-10(-4) M) or (b) the thyrotrophin-releasing hormone (TRH) analogue RX 77368 (pGlu-His-3,3'-dimethyl ProNH2; 10(-5) and 10(-4) M). Incubation of spinal cord slices with the NA uptake inhibitor maprotiline (1 microM) enhanced the effect of idazoxan but inhibited that of RX 77368. The effects of RX 77368 and potassium alone (15 mM) on NA release from both spinal cord and brainstem slices were reduced to basal levels with tetrodotoxin (10(-7) M). Similarly, preincubation of spinal cord, but not brainstem, slices with the insect neuropeptide proctolin (10(-4) M) significantly attenuated the potassium- or RX 77368-induced release of NA, whereas substance P (3 X 10(-5) and 1 X 10(-4) M) had no effect on either tissue. These results suggest that changes in NA release in the spinal cord and brainstem may mediate some of the actions of neuropeptides in ventral spinal cord, although the peptides may not be acting directly on the noradrenergic nerve terminals in these tissues.

Characterization of the 5-HT receptor subtypes involved in the motor behaviours produced by intrathecal administration of 5-HT agonists in rats

1. The motor behavioural effects of intrathecal injections of 5-hydroxytryptamine (5-HT) and a variety of 5-HT receptor agonists were examined in adult Wistar rats to establish; (a) which 5-HT receptor subtype/s elicit each behaviour and (b) whether these receptors are located within the spinal cord. 2. Intrathecal injection of 5-methoxy-N,N'-dimethyltryptamine (5-MeODMT), (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) or 2,5-dimethoxy-alpha,4-dimethylbenzene ethamine hydrochloride (DOM) produced dose-related back muscle contractions (BMC) and wet dog shakes (WDS) which were both markedly attenuated by intraperitoneal pretreatment with either ritanserin (1 mg kg-1), ketanserin (0.16 mg kg-1) or mianserin (0.6 mg kg-1) indicating the involvement of 5-HT2 receptors in both these motor behaviours. Both fluoxetine (1-20 mg kg-1, i.p.) and high doses of 5-HT (50 micrograms) following fluoxetine (5 mg kg-1, i.p.) also elicited BMC, further confirming the involvement of 5-HT in this behaviour. 3. Intrathecal 5-carboxamidotryptamine (5-CT) evoked a marked wet-dog shake response without producing any BMC. Intrathecal pretreatment with 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) enhanced, while in contrast 2-methyl-5-HT pretreatment attenuated, 5-HT agonist-induced BMC without affecting WDS. These data suggest that the spinal 5-HT2 receptors mediating BMC are positively modulated by 5-HT1A but negatively influenced by 5-HT3 receptor activation and may be of a different subtype to the supra-spinal 5-HT2 receptors which elicit WDS. 4. A contrast, reciprocal forepaw treading, lateral head weaving, flat body posture and Straub-tail were evoked by 5-MeODMT, 8-OH-DPAT or 5-CT but not by DOI or DOM indicating that these behaviours were not produced by 5-HT2 receptor activation alone. Ritanserin (1 mg kg- 1, i.p.) or ketanserin (0.16mgkg-1, i.p.) pretreatment reduced the reciprocal forepaw treading induced by high intrathecal doses of either 5-MeODMT (25.pg) or 5-CT (50,ug) suggesting that this behaviour may be facilitated by 5-HT2 receptor activation. 5. Intrathecal injection of 5-HT (0.05-50pg, after systemic fluoxetine, 5mg kg 1, i.p.), or 1-(3-chlorophenyl) piperazine (mCPP) produced dose-related forepaw-licking and grooming, neither of which were attenuated by ketanserin (0.16 mgkg-1, i.p.) pretreatment suggesting these behaviours may be mediated by 5-HT1c receptors. In contrast, 2-methyl-5-HT (50 and 100pg) produced sideward tail-flicks, not evoked by any other 5-HT agonist and could therefore be mediated by spinal 5-HT3 receptor activation. 6. These data provide behavioural evidence for the existence of spinal 5-HT2 receptors which produce a novel motor behaviour, BMC. Ligand binding studies and dose-response studies with a range of selective 5-HT antagonists are required to establish whether BMC and WDS are mediated by different subtypes of 5-HT2 receptors.

Acute and chronic effects of intrathecal galanin on behavioural and biochemical markers of spinal motor function in adult rats

The spinal motor effects of galanin, which co-exists with 5-hydroxytryptamine (5-HT) and thyrotrophin-releasing hormone (TRH) in bulbospinal raphe neurones innervating spinal motoneurones, were examined by administering this neuropeptide through indwelling intrathecal cannulae to conscious adult Wistar rats. The acute effect of intrathecal galanin on spontaneous motor behaviour and the motor behaviours (back muscle contractions and wet-dog shakes) elicited by intrathecal injection of the non-selective 5-HT receptor agonist, 5-methoxy-N, N'-dimethyltryptamine (5-MeODMT) or the TRH analogue, RX 77368 analogue, RX 77368 (pGlu-His-3,3'-dimethyl-ProNH2), respectively, and the chronic effect of galanin on neurochemical markers for bulbospinal raphe neurones and spinal motoneurones were determined. Intrathecal galanin (0.1 to 10 micrograms) did not produce any notable motor behaviours when given alone, but pretreatment with the neuropeptide (0.1 micrograms) significantly attenuated both the number of wet-dog shakes and the amount of forepaw-licking induced by RX 77368, without affecting 5-MeODMT-induced back muscle contractions. Repeated intrathecal galanin administration (1 microgram, twice daily for 5 d) significantly elevated 5-HT (but not 5-hydroxyindoleacetic acid) and substance P-like immunoreactive (LI) levels and choline acetyltransferase (ChAT) activity in the dorsal, but not in the ventral, portion of the thoraco-lumbar spinal cord. In contrast, chronic intrathecal galanin did not alter the TRH- or calcitonin gene-related peptide (CGRP)-LI levels in either spinal cord region.(ABSTRACT TRUNCATED AT 250 WORDS)

5-Hydroxytryptamine, substance P, and thyrotropin-releasing hormone in the adult cat spinal cord segment L7: immunohistochemical and chemical studies

The terminal projections of the descending 5-hydroxytryptamine (5-HT) bulbospinal pathway and the coexistence among 5-HT-, substance P (SP)-, and thyrotropin-releasing hormone (TRH)-like immunoreactivities (LI) in fibers innervating the L7 segment in the cat spinal cord were studied quantitatively and semiquantitatively by use of the indirect double-staining immunofluorescence technique. The content of 5-HT, SP, and TRH in different parts of the spinal cord was determined by use of radioimmunoassay (RIA) (SP and TRH) and high-performance liquid chromatography with electrochemical detection (HPLC-ECD) (5-HT). For all three substances studied, immunoreactive (IR) axon terminals were found in all parts of the gray matter, but with clear regional variation in the density of innervation. Thus, all three substances showed a dense innervation in the motor nucleus, particularly in the ventral part of the nucleus, while the superficial dorsal horn was very densely innervated by SP-IR fibers (laminae I and II) and TRH-IR fibers (laminae II and III). In the motor nucleus, the studied substances coexisted to a very high degree, but some 5-HT-IR fibers (about 10%) lacked peptide-LI and some SP-IR fibers (about 10%) lacked 5-HT-LI while virtually all TRH-IR fibers also contained 5-HT-LI. In the superficial dorsal horn (laminae I-III), no coexistence was detected, while other parts of the gray matter displayed various degrees of coexistence in between those found in the motor nucleus and laminae I-III. The quantitative analysis of IR varicosities in the motor nucleus suggested that the unilateral L7 motor nucleus is innervated by about 55-110 x 10(6) 5-HT-IR nerve terminals, which may indicate as many as 4,000 boutons per descending 5-HT cell body in the brain stem only with this restricted projection. When combing these results with the biochemical data, it could be calculated that the concentration of 5-HT in IR varicosities is about 3-6 x 10(-3) M, while the corresponding figures for SP and TRH was 0.3-0.5 x 10(-3) M and 0.1-0.2 x 10(-3) M, respectively. In cats subjected to spinal cord transection at the lower thoracic level, all 5-HT-IR fibers in the L7 segment had disappeared 44 days after the lesion, indicating a strict suprasegmental origin of 5-HT-IR fibers in this segment.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative behavioural and biochemical effects of repeated intrathecal administration of thyrotrophin-releasing hormone (TRH) or two analogues of TRH in adult rats

The effect of repeated intrathecal injection of thyrotrophin-releasing hormone (TRH) and two analogues of TRH, C-terminally modified RX 77638 and N-terminally modified CG 3509, were examined on behavioural (wet-dog shakes and forepaw licking) and biochemical markers for spinal motoneurones, bulbospinal raphe nerve terminals and the pituitary-thyroid axis in rats. Saline (10 microliters washed in with 15 microliters), TRH (20 micrograms), RX 77368 (2 micrograms) or CG 3509 (2 micrograms) were administered intrathecally (twice daily for 3 or 5 days), after which levels of plasma-free thyroxine and thyroid-stimulating hormone (TSH) were measured and the dorsal and ventral portions of the thoracolumbar spinal cord, brainstem and hypothalamus were assayed for TRH- and calcitonin gene-related peptide (CGRP)-like immunoreactivity, levels of indoleamines and the activity of choline acetyltransferase (ChAT). Behavioural tolerance developed rapidly with consecutive injections of RX 77368, such that wet-dog shakes were significantly reduced and forepaw-licking tended to be decreased by the third intrathecal injection. Five, but not 3, days of administration of RX 77368 selectively elevated levels of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid in the ventral spinal cord, where these substances are principally located in bulbospinal raphe nerve terminals. The time course of the change in indoleamines suggests that administration of TRH peptides elevated the synthesis, rather than the release, of 5-HT from these nerve terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of 5-HT2 receptors in the behaviours produced by intrathecal administration of selected 5-HT agonists and the TRH analogue (CG 3509) to rats

1. The behavioural effects of the intrathecal injection of a thyrotrophin-releasing hormone (TRH) analogue L-orotyl-L-histidyl-prolineamide (CG 3509, 0.5 micrograms), the non-selective 5-HT1 and 5-HT2 receptor agonist 5-methoxy-N,N'-dimethyltryptamine (5-MeODMT, 2-100 micrograms) and the selective 5-HT2 receptor agonist 2,5-dimethoxy-alpha,4-dimethyl-benzene ethamine hydrochloride (DOM, 2-25 micrograms) were compared with the response of systemically administered 5-MeODMT (2 mg kg-1, i.p.) in rats, to establish whether the agonist-induced behaviours were mediated by bulbospinal 5-HT1 or 5-HT2 receptors. 2. Intrathecal injection of 5-MeODMT or DOM produced dose-related back muscle contractions (a previously undocumented behaviour) and wet-dog shakes which were both markedly attenuated by ritanserin pretreatment (1 mg kg-1, i.p.) indicating the involvement of 5-HT2 receptors. In contrast, reciprocal forepaw treading, flat body posture and Straub-tail were evoked by 5-MeODMT but not by DOM indicating that these behaviours were not produced by 5-HT2 receptor activation alone. However, as ritanserin pretreatment reduced the reciprocal forepaw treading induced by intrathecal 5-MeODMT, this behaviour may be facilitated by 5-HT2 receptor activation. 3. Intrathecal 5,7-dihydroxytryptamine (5,7-DHT, 2 x 150 micrograms) treatment decreased thoraco-lumbar spinal cord 5-HT (-95%) and potentiated the back muscle contractions produced by intrathecal DOM injection without altering the wet-dog shake behaviour. None of the components of the 5-HT syndrome produced by 5-MeODMT (2 mg kg-1, i.p.), with the exception of a small increase in wet-dog shakes, was significantly altered by intrathecal 5,7-DHT (which reduced thoraco-lumbar spinal cord 5-HT by 84%). Taken together these data suggest that the only 5-HT agonist-induced behaviour mediated by the activation of 5-HT2 receptors located postsynaptic to bulbospinal 5-hydroxytryptaminergic (5-HTergic) neurones was back muscle contractions. 4. The wet-dog shake and forepaw licking behaviors produced by intrathecal CG 3509 (0.5 micrograms) were attenuated when ritanserin was administered intrathecally 30 min before, but not when it was given at the same time as CG 3509 and neither behaviour was altered by intrathecal 5,7-DHT. This suggests that bulbospinal 5-HTergic neurones are not involved in the production of these TRH analogue-induced behaviours and that the 5-HT2 receptors which mediate these behaviours are not located in the spinal cord.

A comparison of the motor behaviours produced by the intrathecal administration of thyrotrophin-releasing hormone and thyrotrophin-releasing hormone analogues in the conscious rat

The behaviours evoked by the intrathecal injection of thyrotrophin-releasing hormone and a variety of analogues (RX77368, CG3509 and CG3703) were examined in conscious rats and the spread of injectate at the peak of the behavioural response was determined using 14C-labelled RX77368. The number of wet-dog shakes observed following intrathecal injection of thyrotrophin-releasing hormone, RX77368, CG3509 and CG3703 was linearly related to log10 dose (0.01-200 micrograms) in the first 6 min with the relative potencies being 1:7:10:60 respectively. The thyrotrophin-releasing hormone analogues also produced a marked forepaw-licking behaviour, but this did not increase with dose. Intrathecal or intraperitoneal pretreatment with prazosin (0.5 microgram and 1 or 2 mg/kg, respectively) attenuated both the wet-dog shake and forepaw-licking behaviours normally produced by the thyrotrophin-releasing hormone peptides. Following intrathecal [14C]RX77368 the radioactivity was principally restricted to the spinal cord with only limited amounts rostral to the rhombencephalon. These results imply that a tonically active bulbospinal noradrenergic pathway facilitates both thyrotrophin-releasing hormone-induced behaviours via alpha 1-adrenoceptors.

Effect of intrathecal proctolin administration on the behaviour evoked by the thyrotrophin-releasing hormone (TRH) analogue (RX 77368) and the indoleamine, TRH, substance P and calcitonin gene-related peptide levels and choline acetyltransferase activity in the rat spinal cord

The behavioral effects and the biochemical changes produced following either a single or repeated intrathecal injection respectively, of the insect peptide proctolin (Arg-Tyr-Leu-Pro-Thr-OH) have been compared with the effects of a stable analogue of thyrotrophin-releasing hormone (TRH) in rats. Intrathecal proctolin (1-100 micrograms) did not produce any marked behavioural effects on its own, while intrathecal TRH analogue (RX 77368, 0.5 microgram) administration produced wet-dog shakes and forepaw-licking behaviours. Proctolin (10 micrograms) significantly attenuated the wet-dog shake and forepaw-licking behaviours evoked by intrathecal RX 77368 administration when it was given 30 min before, but not when given in combination with RX 77368. Repeated intrathecal proctolin administration (10 micrograms twice daily for 5 days) significantly reduced the 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid and TRH levels in the ventral, but not in the dorsal, horn of the spinal cord nor in the brainstem, and elevated hypothalamic TRH without affecting plasma free thyroxine levels when compared with values in saline-treated controls. Repeated proctolin injection did not alter substance P levels in any brain region examined, nor did it affect the choline acetyltransferase activity or the calcitonin gene-related peptide-like immunoreactive levels in the ventral horn of the spinal cord, both of which are principally located in motoneurones in this cord region.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal effects of chronic intrathecal administration of the thyrotrophin-releasing hormone analogue (CG 3509) in rats

The effect of repeated intrathecal administration of a thyrotrophin-releasing hormone (TRH) analogue (CG 3509; 2 micrograms twice daily for 5 days) on wet-dog shake (WDS) and forepaw-licking (FPL) behaviours and spinal cord TRH and indoleamine levels and choline acetyltransferase (ChAT) activity was examined in adult rats. A rapid behavioural tolerance developed to repeated intrathecal injections of CG 3509; WDS and FPL behaviours were reduced by 57% and 34%, respectively, following the fifth injection and remained reduced at the ninth injection. Repeated CG 3509 administration selectively elevated ChAT activity and the level of 5-hydroxytryptamine (5-HT) in the ventral but not in the dorsal horn of the spinal cord, while 5-hydroxyindoleacetic acid (5-HIAA) and TRH levels were unaltered in either region. As ventral horn ChAT activity is principally located within motoneurones this data implies that TRH exerts a trophic-like influence on mature rat motoneurones in vivo. The results also suggest that long-term intrathecal TRH administration may decrease the release of 5-HT from bulbospinal raphe neurones.

Involvement of catecholaminergic neurones and alpha-adrenoceptors in the wet-dog shake and forepaw licking behaviour produced by the intrathecal injection of an analogue of thyrotrophin-releasing hormone (CG 3509)

Intrathecal injection of the analogue of TRH, CG 3509, into conscious rats produced dose-related wet-dog shakes and forepaw licking, which showed a bell-shaped relationship of intensity to dose. Pretreatment with alpha-MPT intraperitoneally, markedly reduced levels of noradrenaline and dopamine in the spinal cord and brainstem and attenuated both CG 3509-induced responses, while intrathecal treatment with DSP4 selectively reduced noradrenaline in the spinal cord without affecting either behaviour. Since denervation supersensitivity may develop following treatment with DSP4, these results are not inconsistent with a proposal that bulbospinal noradrenergic neurones modulate the behaviour induced by CG 3509. Wet-dog shakes and forepaw licking induced by CG 3509 were reduced by pretreatment with phenoxybenzamine or prazosin, suggesting that a tonic noradrenergic pathway may facilitate both behavioural responses through alpha 1-adrenoceptors. Methoxamine, combined with CG 3509 partially attenuated the wet dog shake behaviour, but methoxamine produced marked hindlimb jerking which might physiologically antagonise wet-dog shakes. Concomitant administration of clonidine and CG 3509 potently reduced wet-dog shakes in a dose-related manner but did not significantly affect forepaw licking, while idazoxan did not significantly affect either response. The latter findings imply that alpha 2-adrenoceptors play different roles in modulating the two behavioural responses and the possible synaptic location of the receptors is discussed. Taken together these results suggest that CG 3509 may release noradrenaline from bulbospinal neurones regulating motor function.

Regional distribution of substance P- and thyrotrophin-releasing hormone-like immunoreactivity and indoleamines in the rabbit spinal cord

The distribution of thyrotrophin-releasing hormone (TRH), substance P, and the indoleamines [5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA)] has been examined in selected regions of the thoracic and lumbar spinal cord of the rabbit using sensitive radioimmunoassays for the first two and HPLC with electrochemical detection for the indoleamines. The levels of TRH- and substance P-like immunoreactivity (TRH-I and SP-I, respectively) were greatest in the ventral and dorsal grey matter, respectively. The level of TRH-I in most thoracic regions was greater than that in equivalent lumbar regions, but the only segmental difference in SP-I was in the ventral grey matter, where the lumbar segment contained more immunoreactivity. 5-HT and 5-HIAA were more evenly distributed than either peptide and showed no segmental variation in levels in equivalent regions, but the ventral grey matter contained significantly higher levels of 5-HT and had a greater 5-HT/5-HIAA ratio than all other regions. The absolute levels and the overall distribution of SP-I, TRH-I, and indoleamines in the thoracolumbar cord of the rabbit was very similar to that previously reported in both rats and humans, and the possible functional role of the peptides and indoleamines in spinal neurones is discussed.

The effects of alfentanil and selected narcotic analgesics on the rate of action potential discharge of medullary respiratory neurones in anaesthetized rats

The effects of intravenous injections of alfentanil, fentanyl, phenoperidine or morphine on respiratory and peak inspiratory air flow rate, the diaphragm electromyogram (EMG), the activity recorded extracellularly from respiratory neurones located in the ventral respiratory group and the cardiovascular system were examined in anaesthetized rats. Alfentanil produced dose-dependent changes in peripheral and central respiratory parameters, which were prevented by naloxone pretreatment. Minimal effects were produced on the cardiovascular system. The bradypnoea was principally due to a prolongation of the inspiratory phase and was accompanied by a comparable decrease in the peak inspiratory air flow rate. Alfentanil prolonged the discharge duration of inspiratory neurones such that it still maintained a strict phase correlation with the diaphragm EMG, but changes in firing frequency were inconsistent and negligible. The action on expiratory neuronal discharge was analogous to that on inspiratory neuronal discharge but delayed in onset. Hypotension produced by morphine limited the dose used but the respiratory responses to morphine and other selected narcotic analgesics were otherwise similar to that of alfentanil, differing mainly in time-course and magnitude. From the respiratory parameters assessed, the order of duration of effect was morphine greater than phenoperidine greater than fentanyl greater than alfentanil and the relative potencies were 0.1, 0.5, 2.5 and 1 respectively. The selective prolongation of inspiration and the immediate action on inspiratory neurones suggests that systemically administered narcotic analgesics may alter the mechanisms within the central respiratory rhythm generator which determine the cessation of inspiration.