Harmaline-induced tremor and impairment of learning are both blocked by dizocilpine in the rabbit

Abstract

Harmaline is known to produce tremors and retard acquisition of the rabbit's nictitating membrane response. These actions have been demonstrated to depend on the ability of harmaline to activate the inferior olive which gives rise to climbing fibers that project directly onto Purkinje cells in cerebellar cortex. However, the precise receptor systems involved in harmaline's actions remain unknown. This study examined the role of the NMDA receptor in harmaline's actions. Harmaline (10 mg/kg, s.c.) produced intense tremors and impaired the acquisition of conditioned responses. Both of these effects of harmaline were significantly blocked by the prior administration of the noncompetitive NMDA channel blocker, dizocilpine (0.01 mg/kg, s.c., given 20 min prior to the administration of harmaline). This dose of dizocilpine had no effect on acquisition of conditioned responses when given alone. A higher dose of dizocilpine (0.1 mg/kg s.c.) completely blocked the tremorogenic effects of harmaline (10 mg/kg, s.c.). Dizocilpine had no effect on motor behavior when given alone. It was suggested that the blockade of harmaline's actions by dizocilpine may be occurring at NMDA channels within the inferior olive. Regardless of the site of action, these data demonstrate that harmaline's ability to activate the inferior olivary nucleus depends on the normal activity of the NMDA receptor.

Introduction

It is known that harmaline, a β-carboline, retards the acquisition of conditioned responses (CRs) during Pavlovian conditioning 7, 31, produces generalized tremors [16], and at higher doses exerts an excitotoxic effect on cerebellar Purkinje cells [17]. All three of these effects have been attributed to harmaline's ability to activate neurons in the inferior olive, which give rise to climbing fibers whose innervation of cerebellar cortex is restricted to Purkinje cells. The precise receptor(s) through which harmaline produces an activation of the inferior olive remains unknown [27].

A possible clue to harmaline's actions was provided by studies employing ibogaine, a β-carboline related to harmaline, that is also both tremorogenic and neurotoxic [17]. It was reported that ibogaine's excitotoxic effects on cerebellar Purkinje cells was markedly attenuated by dizocilpine (MK-801), a noncompetitive NMDA channel blocker [18]. Although glutamate has been considered to be a principal transmitter employed by climbing fibers 17, 37, its excitation of Purkinje cells is mediated by non-NMDA receptors 1, 19. However, the inferior olive does contain a high density of NMDA receptors 15, 23which demonstrate a specific binding of [³H]dizocilpine or NMDA-sensitive [³H]glutamate. This suggests that the blockade of ibogaine's (and by inference harmaline's) neurotoxic effects by dizocilpine might have been due to blockade of NMDA receptors in the inferior olivary nucleus [18]. Such a conclusion presupposes that dizocilpine was preventing the activation of the inferior olive by ibogaine.

The present study examined whether harmaline's ability to induce tremors and retard learning would also be antagonized by dizocilpine, an outcome that would be expected if dizocilpine was able to block harmaline-induced activation of the inferior olive and if harmaline's ability to induce tremor and retard learning were both dependent on activation of the inferior olive. Three experiments were carried out. First we examined the ability of dizocilpine to block the tremorogenic effects of harmaline. Second, because dizocilpine had been reported to retard CR acquisition during Pavlovian conditioning of the rabbit's nictitating membrane (NM) response 3, 21we carried out a dose–response study to establish a dose of dizocilpine that had no effect on learning. Finally, in a third experiment, we determined whether this neutral dose of dizocilpine would block the retardant effects of harmaline on learning.