Neurophysiological events during the head critical period for prothoracicotropic hormone release in fourth-instar larvae of Manduca sexta

The initiation of larval moulting in the tobacco hornworm, Manduca sexta, involves an endocrine cascade that begins with the release of the cerebral prothoracicotropic hormone (PTTH) from the retrocerebral corpora allata. During the fourth larva instar of Manduca when the gated release of PTTH occurs, the in situ electrical activity of the nervi corporis cardiaci (NCC I + II), the paired nerve trunks from the brain that innervate the corpora cardiaca and more distal corpora allata, undergoes a distinct change in firing pattern. The initial change in pattern, from a steady firing, was a brief, intense burst of relatively large units followed by a short period (3-10 min) of completely suppressed activity. Next, bursting activity of several different units resumed, and depending upon the preparation, this event was either long or short in duration. This unique bursting pattern occurred at about the head critical period for PTTH release and was expressed by the brain-corpus cardiacum-allatum both in situ and in vitro. The complexity of the action potential patterns suggests that a variety of neurosecretory cell types may be involved in a coordinated, transient inhibition of electrical activity of the NCC I + II that is followed by selective disinhibition of a specific set of cerebral neurones that, based upon the release of PTTH from the corpus cardiacum-allatum preparations, culminates in PTTH release. That this neurophysiological event occurs at a precise time and occurs in vitro suggests that a pattern generator for the unique bursting firing and a clock mechanism controlling this event reside within the brain-corpus cardiacum-allatum. The significance of these results in relation to the neuroendocrinology of larval mounting is discussed.