Pharmacological properties of cholinoceptive neurones in the medulla and pons of the cat

Retrograde labeling of neurons in the brain stem following injections of [3H]choline into the rat spinal cord

In an attempt to identify cholinergic neurons in the brain stem which project to the spinal cord, [3H]choline (100, 20, 10, 5 or 1 microCi) was injected into the upper cervical spinal cord in 55 rats. The animals were killed 20 h later and the brains processed for autoradiography of diffusible substances. At all doses of [3H]choline, cells were consistently, retrogradely labeled in the medical medullary reticular formation, the lateral vestibular nucleus, the dorsolateral pontine tegmentum and the red nucleus. The retrogradely labeled cells were found to be moderately to darkly stained for acetylcholinesterase. Injection of [3H]noradrenaline (50 microCi) into the upper cervical spinal cord resulted in retrograde labeling of cells in the locus coeruleus, subcoeruleus and the ventrolateral pontine tegmentum, that correspond in position to the neurons of the A6, A7 and A5 catecholamine cell groups, respectively. Injection of [3H]serotonin (20 microCi) into the upper cervical spinal cord was associated with retrograde labeling of cells in the raphe pallidus, obscurus and magnus nuclei that correspond in position to those of the B1, B2 and B3 serotonin cell groups, respectively. Injection of True Blue into the upper cervical spinal cord was followed by retrograde labeling of a large number of cells located in the areas where cells were retrogradely labeled by [3H]choline, [3H]noradrenaline and [3H]serotonin, and additionally, in the solitary tract nucleus, the lateral, parvicellular medullary reticular formation, the caudal and oral pontine reticular formation, the mesencephalic reticular formation and the superior colliculus. These results indicate that from the cervical spinal cord, [3H]choline selectively retrogradely labels a certain population of non-monaminergic, acetylcholinesterase-positive cells localized in the medial medullary, and secondarily the dorsolateral pontine, reticular formation, the lateral vestibular nucleus, and the red nucleus.

Dimethylphenylpiperazinium-induced vomiting: nicotinic mediation in area postrema

In unanesthetized cats the emetic action of dimethylphenylpiperazinium (DMPP) was investigated, after it was injected into the cerebral ventricles through chronically implanted cannulae. DMPP injected in 0.2-2.0 mg doses into the cerebral ventricle produced dose-dependent vomiting, which was abolished after ablation of area postrema. However, copper sulfate given intragastrically evoked vomiting in cats with an ablated area postrema. Further, the emetic response to ICV DMPP and to intragastric copper sulfate was depressed or abolished in cats pretreated with ICV reserpine. The emetic response to ICV DMPP, but not that caused by intragastric copper sulfate, was potentiated in cats pretreated with ICV 5,6-dihydroxytryptamine. Ganglionic blocking agents, mecamylamine and hexamethonium, injected ICV prevented the vomiting elicited by ICV DMPP. On the other hand, selected anti-muscarinic drugs, alpha and beta adrenergic antagonists, dopamine antagonists, antihistamines and a 5-hydroxytryptamine (5-HT) antagonist all injected into the cerebral ventricles had virtually no effect on the vomiting induced by DMPP. It is postulated that DMPP evokes vomiting by its action on nicotinic receptors of nerve cells within the area postrema but not on catecholaminergic, serotonergic, or cholinergic receptors. Finally, 5-HT and acetylcholine could also be involved in the inhibition of the complex mechanisms underlying the central regulation of vomiting.

Nicotine-induced salivation in cats: effects of various drugs

The effect of an intracerebroventricular injection of an antimuscarinic drug, ganglionic blocking agent, alpha and beta adrenergic blocking drug, antiserotonin agent or antihistamine upon salivation produced by nicotine similarly injected in the cat was investigated. Atropine and hexamethonium abolished the salivation evoked by nicotine. On the other hand, salivation induced by nicotine was not significantly altered by yohimbine, practolol, methysergide and antazoline. It is concluded that the salivation produced by nicotine is mediated via central receptors which have mixed nicotinic and muscarinic properties.

Central nicotinic receptors: vomiting, ear twitching and panting

Vomiting, ear twitching and panting produced by either nicotine or dimethylphenylpiperazinium (DMPP) administered intracerebroventricularly (ICV) to the unanesthetized cat were studied and compared. On a molar basis, nicotine evoked stronger effects overall in terms of the three responses studied, but the DMPP-induced vomiting and ear twitching responses, while weaker, were of longer duration. No significant differences were found in the duration of panting evoked by these nicotinic agonists. The nicotinic blocker hexamethonium injected ICV abolished the vomiting and ear twitching caused by either ICV nicotine or ICV DMPP, but vomiting and ear twitching persisted following the ICV administration of the muscarinic blocker, atropine. Both hexamethonium and atropine depressed or abolished the panting response evoked by either ICV nicotine or ICV DMPP. In cats with ablations of the area postrema, nicotine and DMPP injected ICV did not produce the vomiting response, but ear twitching and panting still occurred. It is concluded that the ICV injection of nicotine or DMPP evoked vomiting and ear twitching in the cat by way of an action on the central nicotinic receptors. However, panting produced by these nicotine agonists is mediated by the mimicking action of acetylcholine on central receptors having mixed nicotinic and muscarinic properties.

The nucleus reticularis dorsalis: a region sensitive to physostigmine

Microinjection of physostigmine (3 micrograms) into the nucleus reticularis dorsalis of the rat produced hypertension. Little or no increases in blood pressure were found at the other areas of the medullary reticular formation. The pressor effect following injection of physostigmine into the nucleus reticularis dorsalis was sensitive to atropine (1 microgram) but resistant to hexamethonium (5 micrograms), similarly injected. Atropine (1 microgram) bilaterally injected into the nucleus decreased pressor responses to intravenous physostigmine (100 micrograms/kg). These data may indicate that there are muscarinic receptors responsible for pressor effects in the nucleus reticularis dorsalis. The medullary cholinoceptors may be involved in the systemic physostigmine effect on blood pressure.

Cholinoceptive properties of respiratory neurones in the rat medulla

The effects of iontophoretically applied acetylcholine, the acetylcholine agonists nicotine and muscarine, and the antagonists atropine, dihydro-beta-erythroidine (DH beta E) and mecamylamine, together with the excitatory amino acids, glutamate and D,L-homocysteic acid (DLH) were examined on the activity of respiratory-related neurones in the rat medulla and were compared with effects on non-respiratory brain stem neurones. Most neurones were excited by acetylcholine and no inhibitory responses were seen. Glutamate and DLH also excited but there was a trend for the phasic activity of respiratory neurones to be converted to a tonic discharge. Nicotine also excited most neurones to which it was applied and these responses were blocked by DH beta E but not by atropine. Muscarine also caused excitation and these responses were blocked by atropine but not by DH beta E. Both antagonists blocked acetylcholine-induced excitation but had no effect on responses to glutamate or DLH. Mecamylamine was without effect. It is concluded that the proportion of cholinoceptive respiratory neurones in the rat brain stem is similar to that for non-respiratory neurones. It seems likely that both nicotinic and muscarinic receptors are present on the majority of respiratory neurones and that both contribute to the response produced by iontophoretically-applied acetylcholine.

Unit study in cat claustrum of the effects of iontophoretic neurotransmitters and correlations with the effects of activation of some afferent pathways

Glutamate (Glut), acetylcholine (ACh) and dopamine (DA) were iontophoretically applied on cat claustral neurons. Glut did not affect all the neurons; ACh had both excitatory and inhibitory effects, while DA was prevalently inhibitory. An analysis was made of the time-course of excitatory and inhibitory responses on the basis of the mean firing rate variations during and after ACh and DA release. Three types of responses are described for each drug: short lasting inhibition, long lasting inhibition and long lasting excitation. The experimental data were statistically elaborated. The effects of ACh and of DA were compared with those of activation obtained by sensorial peripheric and thalamic stimulations. ACh could be supposed to be the transmitter of most of the inhibitory terminals of these sensitive afferences to the claustrum.

Cholinergic excitation of mammalian hippocampal pyramidal cells

Responses of CA1 pyramidal neurons to ACh were recorded with intracellular microelectrodes utilizing the in vitro guinea pig hippocampal slice preparation. ACh was delivered by drop or iontophoretic application to stratum oriens or stratum radiatum. Threshold dose for drop application was 1 mM. An initial hyperpolarization of 3.1 +/- 1.8 (S.D.) mV associated with a decrease in membrane input resistance (RN) of 21 +/- 9% (S.D.) occurred in about half the cells. This result is consistent with a presynaptic action of ACh mediated through excitation of inhibitory interneurons. This interpretation was supported by recordings of cholinergic excitatory responses from presumed interneurons, and repetitive spontaneous IPSPs from pyramidal neurons during the hyperpolarization. ACh evoked a slow depolarization (14.3 +/- 10.8 (S.D.) mV) accompanied by a peak increase in apparent input resistance (Ra) of about 60% in the majority of cells. Large increases in spike frequency were associated with these events but action potential shape was unchanged. Plots of Ra versus membrane potential following ACh application revealed that Ra increases were proportionately higher at depolarized membrane potential levels (less than or equal to -70 mV) in some neurons. In these cells Ra was increased significantly at -60 mV (28%), but only 6% at -75 mV. These results are consistent with the conclusion that ACh reduces a voltage-dependent gK, distinct from delayed rectification. ACh also induced a non-voltage-dependent increase in Ra in some cells. ACh-evoked changes in Ra were long-lasting and gave rise to alterations in firing mode, with development of burst generation. ACh also transiently blocked after hyperpolarizations which followed spike trains in pyramidal neurons and presumed interneurons, an action which may be related to effects on a Ca2+-activated gK.

Transmitter sensitivities among various types of cultured brain stem neurons

We prepared cultures of dissociated cells from the lower brain stem of 14- to 15- day-old rat fetuses, and studied how neuronal activities of various types of neurons were influenced by application of various transmitters or transmitter-like substances. Unit discharges of the cultured neurons were recorded extracellularly, using suction micropipette, while the test substances were applied iontophoretically to the surface membrane of the cell body. We selected for testing 4 types of neurons with the typical appearance, i.e. multipolar, pyramidal, polygonal and fusiform one. Each type of neuron had specific sensitivities to acetylcholine and leucine-enkephalin. Amino acids and substance P acted to 4 types of neurons as either unanimous excitant or inhibitor. Monoamines and leucine-enkephalin had both inhibitory and facilitatory effect on unit discharges, although the inhibitory effect was dominant. These findings suggest that there is a specific relation between morphological properties of the brain stem neurons and their sensitivities to transmitters.

Time course effects of soman on acetylcholine and choline levels in six discrete areas of the rat brain

The time course of changes in rat brain levels of acetylcholine (ACh) and choline (Ch) was investigated following a single SC injection of soman (0.9 LD50, 120 micrograms/kg) to understand the relationship between central neurotransmitter alteration and soman toxicity. Of the animals exposed to the dose of soman, 46% died within 24 h, with maximum mortality occurring during the first 40 min following soman administration. In a second group, surviving rats were killed at various times after treatment by a beam of focused microwave radiation to the head, and ACh and Ch levels were determined by gas chromatography-mass spectrometry. Soman produced a maximal ACh elevation in the brain stem at 20 min (34.4%), in cerebellum at 40 min (51.9%), in cortex and striatum at 2 h (320.3% and 35.2%, respectively), and in hippocampus and midbrain at 3 h (94.5% and 56.8%, respectively). ACh levels remained above normal approximately 30 min in the brain stem; 2 h in the midbrain, cerebellum, and striatum; 8 h in the cortex; and 16 h in the hippocampus. Ch levels were elevated in all areas except the striatum. Ch maxima occurred at 10-40 min and returned to control levels approximately 3 h after injection. Results suggest that perturbation of ACh levels due to soman was not uniform throughout the brain and that soman toxicity may reflect ACh changes in multiple areas, rather than changes in any given area. These data further suggest a possible relationship between elevated Ch levels and soman toxicity.

Central sites and mechanisms of action of nicotine

Research conducted in this laboratory over the last ten years has been directed towards determining possible CNS sites and mechanisms by which nicotine is producing its psychopharmacological effects. To accomplish these goals, a drug discrimination paradigm was utilized in which rats were trained to detect nicotine using a two-lever operant procedure. In this situation nicotine acted as a discriminative stimulus (DS) to correct lever responding. In other words rats had to be able to differentiate nicotine's effects from saline in order to obtain a food reinforcement. The ability of nicotine to exert its DS effects appear to be dependent upon a stimulation of central nicotinic-cholinergic receptors which are stereospecific to (-)-nicotine. Interestingly, the nicotine DS could not be mimicked or potentiated by elevating brain acetylcholine levels centrally suggesting that the receptor action was non-cholinergic. Additional studies indicated that nicotine is acting at both reticular formation and hippocampus sites. The hippocampal site of nicotine action also appears to be dependent on a dopamine neuron connection as well.

Biphasic responses to acetylcholine in mammalian reticulospinal neurons

Acetylcholine (ACh) responses were elicited by ionophoresis from neurons, located in the medial pontine reticular formation, which were antidromically identified as having axons projecting in the reticulospinal tracts. Most neurons were silent at rest and could be caused to discharge at a regular, slow rate by a constant application of glutamate. ACh altered this slow rate of firing in 28 of 29 cells but showed three different patterns of effect: approximately one-third were excited, one-third were inhibited, and one-third showed biphasic inhibition-excitation. The ACh responses were not sensitive to atropine. These observations suggest that reticulospinal neurons have ACh receptors mediating both inhibition and excitation, perhaps located on different portions of the same neuron.

Investigations on the CNS sites of action of the discriminative stimulus effects of arecoline and nicotine

The role of the dorsal hippocampus (DH) and mesencephalic reticular formation (MRF) in mediating the discriminative stimulus (DS) effects of nicotine and arecoline was assessed. In rats trained to discriminate nicotine (1.14 mg/kg) from saline, peripherally administered nicotine generalized to injection of nicotine, but not arecoline, directly into the DH and MRF. The stimulus effect of centrally administered nicotine was antagonized by peripherally administered mecamylamine, but not atropine. Response rate decreases were also observed after nicotine injection into either site. In rats trained to discriminate arecoline (1.74 mg/kg) from saline, peripherally administered arecoline did not generalize to the direct injection of arecoline into the DH and MRF. However, a decrease in response rates was observed after arecoline injection into either site. Thus, The DH and MRF are important in mediating the DS effect of nicotine but not arecoline.

A cholinergic mechanism involved in the neuronal excitation by H+ in the respiratory chemosensitive structures of the ventral medulla oblongata of rats in vitro

The mechanism of neuronal excitation by H+ in the medullary chemosensitive structures was analyzed in brains slices of the rat in vitro. Responses of neurons to H+ in the ventral surface layer were compared with responses to various transmitter substances. Neurons excited by H+ were always also excited by acetylcholine (ACH). ACh increased the activity of 70% of superficial ventral medullary neurons. Effects of noradrenaline and serotonin on the activity of neurons were largely opposite to that of H+. Cholinergic blocking agents like atropine, hexamethonium and mecamylamine depressed the H+-elicited excitation of neurons. The cholinesterase inhibitor, eserine, increased the neuronal activity. In the presence of eserine, a solution of low pH caused further increase in discharge of most neurons. The low pH solution prolonged and augmented the excitatory action of ACh on the ventral medullary neurons. It is concluded that the H+-elicited excitation of neurons in the "chemosensitive" structures is dependent upon intact cholinergic transmission in the surface layer. This may be interpreted as resulting from facilitation and/or prolongation of such a chemical transmission by H+.

A cholinergic mechanism involved in the respiratory chemosensitivity of the medulla oblongata in the cat

1. Cholinomimetic and adrenomimetic substances were tested on the chemosensitive zones of the ventral surface of the medulla oblongata using a plexiglas ring method. Tidal volume and respiratory frequency, arterial pressure and heart frequency were observed. 2. The increase of ventilation and the depression of arterial blood pressure by locally applied acetylcholine could be blocked by previous local application of atropine. It is therefore assumed that the acetylcholine receptors have muscarinic properties. 3. Nicotine in a small dose raises arterial pressure and with higher doses a drop is observed. The responses of respiration and of arterial pressure to nicotine were blocked by previous intravenous administration of hexamethonium. 4. Local application of atropine in the caudal (L) and rostral (M) chemosensitive zones reduced resting ventilation and the slope of the ventilatory response to CO2-inhalation. Physostigmine in these areas enhanced resting ventilation leaving unchanged the slope of the ventilatory response to CO2-inhalation. 5. With high concentrations of (L)-noradrenaline and (L)-adrenaline a slight increase of arterial pressure was seen while serotonin caused a drop. 6. These results together with those of Fukuda and Loeschcke (1978) suggest that a cholinergic transmission in the surface layer of the ventral medulla is a component in the respiratory and circulatory control systems.

Effects of excitatory and inhibitory amino acids on phasic respiratory neurons

The responsiveness of phasically active brainstem respiratory neurons to several amino acids was investigated in cats under Dial anesthesia. Four-barreled microelectrodes were used to extrude iontophoretically the putative neurotransmitters L-glutamate, L-asparatate, glycine, and gamma-aminobutyric acid (GABA), L-glutamate and L-aspartate caused increased activity when applied to either inspiratory or expiratory neurons and appeared to be equal in efficacy. Likewise, GABA and glycine depressed ongoing phasic neural activity of both inspiratory and expiratory units. In this case, however, the dosage of GABA required to produce a given depression was significantly less than the required dosage of glycine. These findings support the hypothesis that L-glutamate and/or L-aspartate may act as excitatory neurotransmitter agents at the synapses of brainstem respiratory neurons and conversely, GABA may act as the natural inhibitory neurotransmitter.

Brainstem acetylcholine sensitive neurons activated by cutaneous impulses in cats

In order to determine the cholinoceptive mechanism associated with cutaneous inhibition of jaw-closing and lumbar motoneurons, the area related to the inhibition produced by stimulation of the superficial radial nerve was identified by a lesion within the pontomedullary reticular formation and effects of drugs upon neurons were studied within this area. The cutaneous inhibition, as tested by the inhibition of monosynaptic reflex activity of jaw-closing and that of hindlimb spinal motoneurons was completely abolished by lesion of the medial portion of the pontomedullary reticular formation, but was little affected by lesions of the lateral portion. Intravenously administered physostigmine (0.15--0.30 mg/kg) excited 11 of 21 neurons whereas electrophoretic ACh (90 nA) excited 26 and inhibited 4 of 96 brainstem neurons located in this area. Eight of 11 physostigmine, and 4 of 26 ACh excited neurons were reticulospinal neurons with axonal conduction velocities of 20--40 m/sec. From results presented here together with those reported previously, these physostigmine sensitive and ACh excited brainstem neurons, reticulospinal and non-reticulospinal neurons, could be cholinoceptive interneurons of the polysnaptic inhibitory pathway, from the superficial radial nerve to jaw-closing and hindlimb spinal motoneurons.

Electrocortical changes induced by the perfusion of noradrenaline, acetylcholine and their antagonists directly into the dorsal raphé nucleus of the cat

1 The electrocortical changes induced by the perfusion of drugs directly into the dorsal raphé nucleus of the cat encéphale isolé preparation have been studied. 2 (-)-Noradrenaline (NA), (-)-adrenaline, or (-)-isoprenaline (Isop) produced intermittent or sustained electrocortical desynchronization during the perfusion period. 3 These changes were markedly attenuated or completely abolished by the prior perfusion of (+/-)-sotalol or (-)-propranolol, but not by equimolecular concentrations of (+)-propranolol. 4 The effects of NA or Isop were also blocked by phentolamine, whereas phenoxybenzamine either potentiated the responses to NA and Isop or mimicked the effects of these catecholamines. 5 The effect of dopamine was similar to that induced by NA, but could not be elicited at all of the perfusion sites where NA was effective. It could be blocked by (+/-)-sotalol or (-)-propranolol and also by the prior perfusion of fusaric acid. 6 Acetylcholine (ACh) increased, or initiated, electrocortical synchronization. These effects could be antagonized by sensory stimulation, the prior perfusion of atropine, or the simultaneous perfusion of NA at the same site. 7 Lignocaine, induced prolonged electrocortical desynchronization, behavioral alerting and an increased responsiveness to sensory stimulation. 8 The results have been discussed in relation to the possible involvement of inhibitory beta-adrenoceptors and facilitatory cholinoceptors (muscarinic) in the functioning of the dorsal raphé nucleus.

Observations on the pharmacology of cholinoceptive neurones in the rat brain stem

The pharmacology of spontaneously active cholinoceptive neurones in the brain stem of rats anaesthetized with urethane has been investigated using microiontophoresis to administer muscarinic and nicotinic agonists and antagonists. 2. Acetylcholine (ACh) excited most cells but occasionally depressed their activity. Muscarine, and the muscarinic agonists methacholine and bethanechol produced prolonged excitation or inhibition of cells whereas nicotine produced prolonged excitations but no inhibitions. 3 Atropine selectively antagonized ACh excitations and both excitation and inhibition of neuronal activity produced by muscarine and muscarinic agonists, but not the excitations produced by nicotine, glutamate or DL-homocysteic acid. 4 Dihydro-beta-erythroidine (DHBE) and tubocurarine antagonized both ACh and nicotine excitations but not those induced by glutamate or DL-homocysteic acid. Inhibitions by ACh or muscarine were not affected. 5 It is concluded that excitations of cholinoceptive neurones in the rat brain stem may be mediated by activation of both muscarinic and nicotinic receptors whereas inhibitions are mediated by activation of a muscarinic receptor.

Acetylcholine-sensitive cells in the caudal medulla of the rat: distribution, pharmacology and effects of pentobarbitone

1. The distribution of cholinoceptive and non-cholinoceptive cells in various nuclei of the caudal medulla of the rat is described. 2. The nature of the responses of cells of the paramedian reticular nucleus and of the perihypoglossal nuclei to electrophoretically applied acetylcholine (ACh) was investigated. 3. In unanaesthetized decerebrate preparations ACh responses were usually of a "fast onset-fast offset" nature. Dihydro-beta-erythroidine was a more effective antagonist than atropine. 4. In rats anaesthetized with barbiturate nearly all the tach responses showed a slower onset and prolonged action. Atropine was the more effective antagonist. 5. The synaptic responses of cells of the paramedian reticular and perihypoglossal nuclei to stimulation of glossopharyngeal, superior laryngeal, lingual and hypoglossal nerves were investigated. It is concluded that ACh does not mediate the responses at the level of these nuclei.

The sensitivity of paramedian reticular neurones to acetylcholine

1. Of paramedian reticular neurones a significantly higher proportion of those antidromically activated from the cerebellum than of those orthodromically activated from this source were excited by acetylcholine. 2. Receptors for acetylcholine were of the muscarinic type. 3. No differences were found in the proportions of cholinoceptive and non-cholinoceptive cells responding to stimulation of cranial and limb nerves or to changes in blood pressure either spontaneous or induced. 4. Either a cholinergic pathway to paramedian reticular cells projecting to the cerebellum was not activated in these experiments or the receptors for acetylcholine are not located at synapses. An association between muscarinic receptors and acetylcholinesterase may be present with cells of this area.

Response of nigral dopamine neurons to actue and prolonged morphine treatment: effect of exposure to cold, physostigmine and nicotine

The effects of morphine and acute exposure to cold on nigral dopamine (DA) neurons and possible interactions with cholinergic systems were studied by histochemical microfluorimetry in normal and partially morphine-tolerant mice. Morphine (40 mg/kg), cold (4 degrees C), nicotine (1 mg/kg) and physiostigmine (0.25 mg/kg) elicited a rapid rise and subsequent decrease in the fluorescence intensity of DA nerve cells with certain differences in time course. Fastest changes with a peak at 5 min, a marked subsequent drop below control levels and return towards control intensity after 40 min were seen after physostigmine. Antagonisms between various treatments were noted. DA responses correlated well with the time course of behavioural effects, especially after physostigmine. After 3 1/2 days of morphine treatment, the locomotor and analgesic effects of the drug were reduced. At this stage, the initial increase in fluorescence intensity after morphine and the biphasic pattern caused by physostigmine were delayed without any change in response magnitude. Responses to cold and nicotine remained unaltered both in magnitude and time course. Thus, partial tolerance affected the response of nigral DA neurons to some but not all funtional conditions and thereby markedly changed the interaction with cholinergic systems. The difference between physostigmine and nicotine suggests that neuronal circuits including mustcarinic mechanisms are possibly more susceptible to alteration.

Effects of nicotine given into the brain of fowls

1 The effects of nicotine, given into the IIIrd ventricle of adult conscious fowls (Gallus domesticus) or infused into various brain regions of conscious young chicks, were tested on behaviour, electrocortical activity, respiratory rate and body temperature. Its effects given intraventricularly or applied externally to the brain-stem of anaesthetized fowls were also examined.2 After intraventricular nicotine, fowls squatted for 3 to 5 min with eyes closed, electrocortical activity resembling that during sleep but with superimposed spike activity. Following this, fowls reawakened and tachypnoea developed, together with partial abduction of the wings from the trunk, the back becoming horizontal and the tail flexed. These effects were prevented by pempidine.3 Intraventricular nicotine suppressed or, less commonly, reduced operant key-pecking, an effect unrelated linearly to dose.4 Intraventricular nicotine given to fowls anaesthetized with chloralose produced brief apnoea, followed by increased amplitude of respiratory excursion for about 5 minutes. Respiratory rate accelerated slightly but tachypnoea did not develop. Nicotine applied directly to the ventral brain-stem increased respiratory amplitude in three out of seven fowls.5 In anaesthetized fowls, intraventricular nicotine raised blood pressure for 2 to 3 min, an effect prolonged up to 70 min by acute bilateral vagotomy, whereas pressor effects of intravenous nicotine were extended merely two to three fold. Dividing the spinal cord at C2 prevented pressor effects of intraventricular nicotine; those of intravenous nicotine were unaltered.6 In young chicks, nicotine infused into the diencephalon, telencephalon and myelencephalon induced effects similar to those observed immediately after intraventricular nicotine, i.e. chicks squatted with closed eyes but recovered within 3 to 5 minutes. Simultaneously, electrocortical activity changed from an alert to the sleep pattern, usually with superimposed ;spike' activity. Tachypnoea and associated postural changes did not develop. Pempidine prevented the behavioural and electrocortical effects of nicotine.

Effects of cholinomimetic agents given into the brain of fowls

1 Effects of cholinomimetic agents, given into the IIIrd ventricle of adult fowls (Gallus domesticus) or infused into the hypothalamus of young chicks, were tested on behaviour, respiratory rate and body temperature.2 Carbachol evoked behavioural and electrocortical arousal but lacked postural and respiratory effects. Contrariwise, pilocarpine increased respiratory rate and induced postural changes, i.e. abduction of the wings, but lacked other behavioural effects and did not alter electrocortical activity. Benzoylcholine induced tachypnoea, postural changes and brief electrocortical arousal. Acetylcholine was ineffective unless given with physostigmine, when electrocortical arousal, postural changes and tachypnoea developed. Methacholine induced tachypnoea and postural changes.3 Effects of carbachol and pilocarpine were prevented by hyoscine and those of benzoylcholine by pempidine; hyoscine and pempidine were required together to prevent the effects of methacholine and to attenuate those of acetylcholine with physostigmine.

Further studies on the mode of action of psychotomimetic drugs: antagonism of the excitatory actions of 5-hydroxytryptamine by methylated derivatives of tryptamine

1 The actions of 5-methoxytryptamine (5-MeOT), N,N-dimethyltryptamine (DMT), 5-hydroxy-N,N-dimethyltryptamine (bufotenine, 5-HODMT) and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), and their interactions with 5-hydroxytryptamine (5-HT), acetylcholine, (-)-noradrenaline, and glutamate were studied by microiontophoresis on single neurones in the brain stem of rats anaesthetized with urethane or decerebrate cats.2 Like D-lysergic acid diethylamide (LSD 25) the three psychotomimetic derivatives (DMT, 5-HODMT, 5-MeODMT) specifically antagonized 5-HT excitations of single neurones, but the non-psychotomimetic 5-MeOT had no antagonistic effects.3 In contrast to LSD 25, the psychotomimetic tryptamines only rarely antagonized glutamate effects, indicating that the excitatory 5-HT receptors and the glutamate receptors on the same neurones may be closely related spatially, but are separate.4 The methylated tryptamine derivatives were able to mimic the actions of 5-HT on neurones. The non-psychotomimetic 5-MeOT was most potent in this respect, while the other three derivatives which are psychotomimetic, were less active.5 The 5-HT mimicking actions of 5-MeOT were the same in rats pretreated with p-chlorophenylalanine or reserpine as in untreated rats. It therefore seems that the 5-HT mimicking actions are unlikely to be due to release of 5-HT, but are due to direct actions on 5-HT receptors.6 The evidence presented supports the hypothesis that LSD-like psychotomimetics act by an antagonism of 5-HT in the lower brain stem, and is not compatible with the suggestion that the psychotomimetic action of these drugs is related to 5-HT receptor stimulation.

Effects of biogenic amines on central thermoresponsive neurones in the rabbit

1. Single unit activities were recorded with five-barrelled micropipettes from the thermo-responsive neurones in the preoptic area and the mid-brain reticular formation in urethanized rabbits. 5-hydroxytryptamine (5-HT), noradrenaline (NA) and acetylcholine (ACh) were applied micro-iontophoretically to the immediate vicinity of the recording cells.2. Out of seventeen warm-responsive neurones recorded in the preoptic area, fifteen neurones responded to 5-HT with the increase in firing rate and two showed no response. Thirteen out of seventeen warm-units decreased their firing rate in response to application of NA and four were not affected. ACh had no effect on any of the warm-units examined.3. Six out of seven cold-units in the preoptic area were depressed by 5-HT, while NA excited five of six units studied. None of the cold-units were influenced by ACh.4. These results are in good agreement with the changes in rectal temperature produced by 5-HT and NA micro-injected into the hypothalamus in rabbits.5. In the mid-brain reticular formation, 5-HT excited all of fourteen cold-responsive neurones. Of these, eight cold-units were depressed and six were unaffected by NA, while ACh excited six units and had no effect on eight units. All of the five warm-responsive units were inhibited by 5-HT and none were influenced by NA. Thus, the responses of reticular thermoresponsive neurones to 5-HT and NA were opposite to those of the preoptic thermo-responsive neurones.

Modification of the responses of brain stem neurones to transmitter substances by anaesthetic agents

1. The effects of microiontophoretic applications of acetylcholine (ACh), (-)-noradrenaline ((-)-NA) and 5-hydroxytryptamine (5-HT) have been investigated on spontaneously active brain stem neurones in decerebrate unanaesthetized rats and in rats anaesthetized with either tribromoethanol, urethane or pentobarbitone.2. Four types of responses to both (-)-NA and 5-HT were seen. These were: simple excitation; excitation preceded by a short-lasting inhibition; short-lasting inhibition and prolonged inhibition. Three types of responses to ACh were seen: an excitation with long latency of onset; excitation with short latency of onset, resembling the response to an excitant amino acid, and a short-lasting inhibitory response.3. The types of responses to microiontophoretically applied ACh, (-)-NA or 5-HT in anaesthetized and unanaesthetized animals were similar.4. The number of ACh excitatory responses with short latency of onset were significantly reduced in the pentobarbitone-anaesthetized group and a small but significant increase in the number of 5-HT inhibitory effects were observed in each anaesthetized group of animals.5. A significantly greater proportion of slower firing neurones (less than 10 spikes/s) were found in the pentobarbitone-anaesthetized animals.6. The effects of microiontophoretically applied and i.v. administered pentobarbitone were studied on spontaneously active neurones which responded consistently to ACh and a control agonist.7. Pentobarbitone administered by either route reduced the firing rate of most neurones studied and was shown to antagonize specifically the excitation of neurones by exogenously applied ACh.8. It is suggested that postsynaptic antagonism of endogenously released ACh may be a contributing factor in the mechanism of action of pentobarbitone.

Blood pressure effects obtained by drugs applied to the ventral surface of the brain stem

1. In cats anaesthetized with pentobarbitone sodium the effect on arterial blood pressure was examined of substances applied bilaterally to the exposed ventral surface of the brain stem by means of Perspex rings placed lateral to the pyramids and caudal to the trapezoid bodies. Routinely, atropine methyl nitrate, which does not pass the blood-brain barrier, was injected I.V.2. The cholinomimetic substances carbachol and physostigmine, and the amino acids glycine and GABA, caused a fall in arterial blood pressure.3. Atropine produced a small but definite rise in arterial blood pressure, antagonized the depressor effects of the cholinomimetic substances, but not those of the amino acids.4. Strychnine, leptazol and tubocurarine, caused a rise in arterial blood pressure.5. The depressor and pressor effects are due to changes in vasomotor tone. They are central effects brought about by penetration of the substances into the brain tissue from the ventral surface of the brain stem. They are not due to their absorption into the blood stream.6. The depressor effects of the cholinomimetic substances may imitate the action of cholinergic neurones, and those of the amino acids that of central inhibitory neurones ending on cells near the ventral surface of the brain stem and exerting an inhibitory influence on vasomotor tone. The pressor effects of strychnine and tubocurarine may in part result from ;disinhibition', i.e. from an antagonistic action produced by these drugs on the amino acids released from the central inhibitory neurones.

Cholinergic mechanisms in the rat somatosensory cerebral cortex

1. The responses of identified cells in the rat cerebral cortex to cholinomimetic and anticholinergic substances has been investigated.2. Acetylcholine and muscarinic agonists have an excitatory action on 80% of pyramidal tract cells. This response is found especially on cells responding to specific thalamic stimulation and the burst of spikes evoked from this site can sometimes be blocked by the iontophoresis of atropine. This strongly suggests an excitatory transmitter function for acetylcholine in a specific thalamocortical pathway.3. Experiments on non-pyramidal tract cells have detected a muscarinic depression of some cells, and a nicotinic excitation of some cells above a depth of 600 mu in the cortex.4. It is suggested that the increased release of acetylcholine from the cortex produced by atropine administration may be due to an excess of muscarinic inhibitory over excitatory synapses in the cortex.

Short-latency excitation of brain stem neurones in the rat by acetylcholine

A fast excitatory response to acetylcholine (ACh) which has not previously been reported, has been found in the rat brain stem. Micro-iontophoretic applications of ACh to single brain stem neurones in unanaesthetized rats excited 81% and inhibited 3% of neurones studied. Two types of excitatory response were distinguished by their time course. Type I ACh excitation of neurones was of long latency resembling that previously reported in various parts of the brain. Type II excitation was of short latency, similar to that of micro-iontophoretically applied glutamate ions and to ACh excitation of Renshaw cells.

Effects of muscarine given into the brain of fowls

1. The effects of muscarine, given intraventricularly, in adult conscious fowls (Gallus domesticus) or microinfused into various brain regions of conscious young chicks, were tested on behaviour, electrocortical activity and respiratory rate. Its effects given intraventricularly or intravenously to anaesthetized fowls were also examined.2. After intraventricular injection, muscarine elicited immediate behavioural and electrocortical arousal; body temperature was unaffected. After a delay of 30-40 min, tachypnoea developed together with postural changes which included partial abduction of the wings away from the trunk, the back and tail becoming horizontal. These effects were prevented by intravenous or intraperitoneal atropine or hyoscine, but not by pempidine or methylatropine, and were potentiated by physostigmine. Hyoscine given intraventricularly or intravenously did not affect electrocortical activity.3. Intraventricular muscarine given to anaesthetized adult fowls produced brief apnoea. On return of respiration, amplitude of respiratory excursion was diminished for about 5 min; tachypnoea did not develop. Blood pressure also rose briefly. With larger doses of intraventricular muscarine, large amplitude electromyographic potentials developed in the dorsal neck muscles followed later by side-to-side neck movements.4. Muscarine given intravenously to anaesthetized adult fowls, raised blood pressure and perfusion pressure in a perfused hind limb, an effect most likely due to secretion of adrenal medullary catecholamines; these pressor effects were prevented by pempidine and phenoxybenzamine. Given directly to the perfused hind limb, muscarine lowered perfusion pressure.5. In young chicks, muscarine microinfused into the diencephalon or myelencephalon elicited intense bilateral electrocortical alerting associated with periods of alternating violent motor activity and quiescence. Microinfusion of muscarine into the telencephalon induced ipsilateral electrocortical desynchronization without affecting behaviour. These effects of muscarine were prevented by intravenous, intraperitoneal or intracerebral hyoscine, but once its effects were established could be antagonized only with difficulty; pempidine did not prevent these effects. Microinfusions of muscarine into the brain did not affect posture, respiration or temperature.