Pressor responses to central injection of H2 antagonists not caused by GABA blockade

Further evidence for the involvement of histamine H2 receptors in the control of defensive behaviour generated in the midbrain tectum

The dorsal periaqueductal grey matter (dPAG) and inferior colliculus (IC) have been implicated in the control of defence reactions. Electrical and chemical stimulation of these structures induces escape behaviour, usually accompanied by autonomic responses and decreased pain sensitivity. Recently, we presented evidence for an involvement of histamine in the generation and organization of such defensive reactions in the midbrain tectum. In this study we have used the open field test to assess the effects of microinjections of histamine (40 nmol), dimaprit (10 nmol) and ranitidine (50 nmol) into the midbrain tectum of rats. Dimaprit is an agonist and ranitidine an antagonist of H2 histamine receptors. Immediately after the injections the animals were tested in an open field for 60 min. In an additional groups of rats, dimaprit was injected 15 min before the microinjections of ranitidine into either the dPAG or the IC. The results show that whereas histamine preceded by saline did not cause any apparent behavioural changes, ranitidine led to a behavioural reaction, with clear signs of fear, which was blocked by previous injection of dimaprit. Injections of only dimaprit had no apparent effects. The present results suggest that H2 receptors may be involved in the control of defensive behaviour following activation of the neural substrates of fear in the dPAG and IC.

Interaction of ethanol and L-glutamate in the guinea pig ileum myenteric plexus

The guinea pig ileum longitudinal muscle myenteric plexus has recently been shown to contain receptors for excitatory amino acids like L-glutamate which are pharmacologically similar to the N-methyl-D-aspartate (NMDA) receptor subtype in the central nervous system (CNS). The present study utilized the longitudinal muscle myenteric plexus preparation to determine whether the reported ability of acute ethanol treatment to inhibit NMDA receptor activation in mammalian CNS preparations also occurs in the periphery. In the absence of Mg2+, L-glutamate (3-100 microM) induced transient contractions in longitudinal muscle myenteric plexus that could be blocked by atropine. Contractile responses to L-glutamate were completely blocked by D,L-2-amino-5-phosphonovalerate (APV; 100 microM) and Mg2+ (600 microM). Preincubation with ethanol (30-100 mM) for 2 min inhibited contractions to L-glutamate by up to 50% and caused additive inhibition with 100 microM Mg2+. Ethanol (65 mM) inhibition of L-glutamate (60 microM) contractions increased from 30% after a 2 min preincubation to a maximum of 60% following 10 min. Ethanol (65 mM) inhibited contractions induced by acetylcholine (0.1 microM), 5-hydroxytryptamine (0.1 microM) or histamine (0.3 microM), by no more than 10% suggesting that impairment of smooth muscle or cholinergic neuronal activity were not likely responsible for the 40% inhibition of L-glutamate contractions seen with ethanol. A previously identified contractile response to ethanol (10-300 mM), occurring immediately after addition to the longitudinal muscle myenteric plexus preparation, was still present in Mg2+ deficient buffer.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intraventricular histamine and H2 receptor antagonists on intraocular pressure

Severe ocular hypertension has been reported in a chronic glaucoma patient following use of histamine H2 receptor antagonists for treatment of peptic ulcer. Subsequent studies, however, have failed to demonstrate a significant action of topical or intravenously administered H2 blockers on intraocular pressure (IOP) in humans. In this study, cimetidine and ranitidine were administered into the cerebral ventricles of unanesthetized New Zealand White rabbits. Both drugs caused prolonged increases in IOP at a dose of 1 umol. Maximal elevations of IOP occurred approximately 20 min after drug injections and averaged 5-8 mmHg above pre-drug values. In contrast, histamine (0.3 and 1.0 umol) produced biphasic effects on IOP when given by intracerebroventricular (i.c.v.) injection. These data suggest that central mechanisms may mediate the actions of some histamine receptor agonists and antagonists on IOP.