Effect of hippocampal lesions produced by intracerebroventricular kainic acid on alcohol drinking in the rat

Anatomical "circuitry" in the brain mediating alcohol drinking revealed by THP-reactive sites in the limbic system

The involvement of aldehyde adducts in the etiology of alcoholism continues to be supported by a number of experimental findings. These metabolites are synthesized endogenously from a condensation reaction of a biogenic aldehyde with a catechol- or indole-amine and act in the brain to augment or suppress the drinking of ethyl alcohol. When given by the intracerebroventricular route in an animal which does not prefer alcohol, certain tetrahydro-isoquinolines and beta-carbolines can augment significantly the voluntary intake of alcohol even in aversive concentrations. This paper describes the historical background and current status of the "Multiple Metabolite" theory of alcoholism. The recent identification of anatomical structures in the limbic-midbrain, limbic-forebrain of the Sprague-Dawley rat, which mediate changes in the intake of alcohol induced by tetrahydropapaveroline (THP) is also described. When injected in a low dose of 25 ng in a specific site, over a 3-day period, THP induces persistent increases in the intake of alcohol even in aversive concentrations. These THP-reactive sites comprise the substantia nigra, reticular formation, medial lemniscus, zona incerta, medial forebrain bundle, nucleus accumbens, olfactory tubercle, lateral septal nucleus, preoptic area, stria terminalis, and rostral hippocampus. A higher dose of 250 ng THP microinjected at homologous loci tends to inhibit the rat's self-selection of alcohol or exert no effect on drinking. Morphological mapping of histologically identified sites sensitive to THP revealed a distinct "circuitry" of neuronal structures overlapping both dopaminergic and enkephalinergic pathways. This "circuit" extends from the tegmental-nigral area of the midbrain rostrally to structures within the limbic-forebrain. When a THP-reactive structure, the N. accumbens, was lesioned by either of two neurotoxins, 6-hydroxydopamine or 5.7-dihydroxytryptamine, the rats' preference for alcohol increased sharply. This suggests that impairment of transmitter release, denervation supersensitivity or other perturbation of receptor function within this and other structures play a part in the aberrant drinking of alcohol. It is envisaged that a dopamine-enkephalin link underlies the mechanism for the onset, maintenance and permanency of alcohol preference generated by an aldehyde adduct. Finally, the "Two-Channel, Brain Metabolite" theory of alcoholism proposes that the transitory presence of an endogenously formed aldehyde adduct within cells of the brain causes a permanent perturbation of normal receptor processes and transmitter activity within synapses of specific structures of the limbic system. This theory thus explains the nature of the rewarding properties of alcohol as well as its complex addictive liability which is physiologically irreversible.

Isoquinolines, beta-carbolines and alcohol drinking: involvement of opioid and dopaminergic mechanisms

Two classes of amine-aldehyde adducts, the tetrahydroisoquinoline (TIQ) and beta-carboline (THBC) compounds, have been implicated in the mechanism in the brain underlying the addictive drinking of alcohol. One part of this review focuses on the large amount of evidence unequivocally demonstrating not only the corporeal synthesis of the TIQs and THBCs but their sequestration in brain tissue as well. Experimental studies published recently have revealed that exposure to alcohol enhances markedly the endogenous formation of condensation products. Apart from their multiple neuropharmacological actions, certain adducts when delivered directly into the brain of either the rat or monkey, to circumvent the brain's blood-barrier system, can evoke an intense and dose-dependent increase in the voluntary drinking of solutions of alcohol even in noxious concentrations. That the abnormal intake of alcohol is related functionally to opioid receptors in the brain is likely on the basis of several distinct lines of evidence which include: the attenuation of alcohol drinking by opioid receptor antagonists; binding of a TIQ to opiate receptors in the brain; and marked differences in enkephalin values in animals genetically predisposed to the ingestion of alcohol. Finally, it is proposed that the dopaminergic reward pathways which traverse the meso-limbic-forebrain systems of the brain more than likely constitute an integrative anatomical substrate for the adduct-opioid cascade of neuronal events which promote and sustain the aberrant drinking of alcohol.

Anatomical localization in hippocampus of tetrahydro-beta-carboline-induced alcohol drinking in the rat

Guide cannulae for unilateral or bilateral micro-injection were implanted stereotaxically into the dorsal hippocampus of the male adult Sprague-Dawley rat. Following post-operative recovery, the animal's individual preference for ethyl alcohol in concentrations from 3-30% (v/v) was tested over a 9-day period by a three-bottle, two-choice technique. Following this pre-screen, 3.0 microliter of 1,2,3,4-tetrahydro-beta-carboline (TH beta C) hydrochloride, a benzodiazepine receptor antagonist, was infused in a concentration of 25-200 ng into the hippocampus of each unrestrained rat twice a day for three to six days. After the first two days of infusion, the 9-day preference test for alcohol drinking was begun and continued identically as in the earlier test. A third alcohol preference test during which no injections were given was conducted at an interval of two weeks following the second. The micro-injection of TH beta C into certain sites in the hippocampus enhanced alcohol consumption from 0.5-2.0 g/kg during the 9-day test interval. The magnitude of this elevated intake was dependent on the site of infusion and was more pronounced when intermediate concentrations of 7-12% alcohol were offered to the rat. At sites in coronal planes encompassing AP 3.0 and AP 3.5, the micro-injection of TH beta C enhanced alcohol drinking significantly in 75% of the animals; however, when delivered at sites in coronal planes AP 1.0 through AP 2.5, TH beta C augmented alcohol drinking significantly in 15% of the rats.(ABSTRACT TRUNCATED AT 250 WORDS)