Chemical transmission in the central nervous system

Burnstock and the legacy of the inhibitory junction potential and P2Y1 receptors

The synaptic event called the inhibitory junction potential (IJP) was arguably one of the more important discoveries made by Burnstock and arguably one of his finer legacies. The discovery of the IJP fundamentally changed how electromechanical coupling was visualised in gastrointestinal smooth muscle. Its discovery also set in motion the search for novel inhibitory neurotransmitters in the enteric nervous system, eventually leading to proposal that ATP or a related nucleotide was a major inhibitory transmitter. The subsequent development of purinergic signalling gave impetus to expanding the classification of surface receptors for extracellular ATP, not only in the GI tract but beyond, and then led to successive phases of medicinal chemistry as the P2 receptor field developed. Ultimately, the discovery of the IJP led to the successful cloning of the first P2Y receptor (chick P2Y1) and expansion of mammalian ATP receptors into two classes: metabotropic P2Y receptors (encompassing P2Y1, P2Y2, P2Y4, P2Y6, P2Y11–14 receptors) and ionotropic P2X receptors (encompassing homomeric P2X1–P2X7 receptors). Here, the causal relationship between the IJP and P2Y1 is explored, setting out the milestones reached and achievements made by Burnstock and his colleagues.

The role of histaminergic-noradrenergic axis in naloxone-induced withdrawal symptoms in mice

The effects of histamine antagonists on naloxone-precipitated withdrawal symptoms were studied in morphine-dependent mice. Chlorpheniramine (0.5-10 mg/kg), a H1-blocker, given 1P 30 min before naloxone challenge produced a dose-dependent potentiation of withdrawal body weight loss, burrowing, and hypothermia, but did not influence either jumping or wet-dog shakes. On the other hand, cimetidine (10-100 mg/kg), a H2-blocker, produced dose-dependent potentiation of withdrawal hypothermia and jumping. Cimetidine was without effect on wet-dog shakes, burrowing, and body weight loss. The effect of chlorpheniramine was investigated in mice injected with 6-hydroxydopamine (6-OHDA) intracerebrally to examine whether histamine-mediated effects are some-how linked to noradrenergic pathways. Intracerebral injection of 6-OHDA in 5-day-old mice pups resulted in hyperlocomotion by the end of 30 days before initiation of morphine dependence. Mice pretreated with 6-OHDA developed a higher degree of naloxone-induced withdrawal jumping than nontreated mice. 6-OHDA (50 micrograms) lesions completely blocked the potentiating effect of chlorpheniramine on burrowing, hypothermia, and even reversed the effect on body weight loss. These findings suggest that both histamine H1- and H2-receptors may be involved in the expression of precipitated withdrawal in morphine-dependent mice and histamine receptors function as modulators of noradrenergic neurotransmission.

Neurohypophysial hormones and brain function: the neurophysiological effects of oxytocin and vasopressin

An increasing body of evidence suggests that the neurohypophysial hormones, in addition to their classical actions, may also act as neurotransmitters. They have a widespread but discontinuous distribution in the CNS; apart from their presence in the magnocellular nuclei they may be found in the hippocampus, amygdala, septum, substantia nigra, brainstem and spinal cord. They exert profound effects on behavior, particularly on memory, a function frequently ascribed to the hippocampus, amygdala and septum; on memory consolidation, internal reward and self stimulation functions frequently ascribed to brainstem and diencephalic aminergic systems including the substantia nigra and on sensory and autonomic responses which involve the medulla and spinal cord. When applied to the CNS they alter multiple unit activity in certain regions, particularly the hippocampus and cells which contain neural lobe hormones appear to be able to drive other cells synaptically. Finally application of the hormones can profoundly affect the activity of single nerve cells in just those parts of the CNS where, on the basis of their behavioral actions, they might be expected to act.

The effects of physostigmine during the acquisition of avoidance behaviour as a function of the intersession interval

Rats were trained and tested on an avoidance task in a shuttle box. The change in the performance of the control rats over two sessions was found to be a U-shaped function of the interval between the sessions. The change in performance of rats injected with physostigmine prior to the second session was also found to be a U-shaped function of the intersession interval, although the drug was shown to impair avoidance behaviour. These results are consistent with those of Hamburg (1967) and of Biederman (1970), and support the general contention that cholinergic mechanisms in the brain are involved in the control of avoidance and escape behaviour in the rat. They do not, however, necessarily support the hypothesis advanced by Deutsch (1969, 1971) to describe a biochemical basis of learning and memory, especially if it is used to explain the effects of cholinesterase inhibitors on avoidance behaviour in the shuttlebox.

Mechanism of histamine-induced antidiuretic response

1. In dogs anaesthetized with alpha-chloralose, intracerebroventricular (i.c.v.) injection of histamine induced antidiuresis and increase in jugular vein blood antidiuretic hormone (ADH) level but no change in urinary electrolytes. The mechanism of the histamine-induced antidiuretic response was analysed by the use of pharmacological agents.2. Histamine (i.c.v.) in 1-20 mug doses produced a variable effect on urine outflow as well as on the blood ADH concentration; however, higher doses (25-500 mug) of histamine elicited a dose-dependent antidiuretic response with concomitant rise in blood ADH titre.3. Repeated administration of high doses of i.c.v. histamine (400 mug) elicited a diminishing antidiuretic response which was not observable after the fourth dose, thus exhibiting tachyphylaxis. The antidiuretic response to histamine could be restored by central administration of noradrenaline (500 mug).4. Central pretreatment with mepyramine (5 mg) prevented the histamine-induced antidiuresis. Atropine (2 mg i.c.v.) was ineffective in blocking the antidiuretic effect of histamine. A diuretic response to histamine (400 mug i.c.v.) was obtained in phenoxybenzamine (i.c.v.) pretreated animals; this response could be blocked by i.c.v. injection of propranolol. Tetrabenazine pretreatment prevented the antidiuretic response to histamine.5. The results of the study lead us to conclude that histamine releases central catecholamines which activate the central adrenergic mechanism for the release of antidiuretic hormone.